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Near-IR Emitting Si Nanocrystals Fabricated by Thermal Annealing of SiNx/Si3N4 Multilayers

Near-IR Emitting Si Nanocrystals Fabricated by Thermal Annealing of SiNx/Si3N4 Multilayers applied sciences Article Near-IR Emitting Si Nanocrystals Fabricated by Thermal Annealing of SiN /Si N Multilayers x 3 4 1 , 2 3 3 , 4 3 D. M. Zhigunov * , A. A. Popov , Yu. M. Chesnokov , A. L. Vasiliev , A. M. Lebedev , 3 3 5 5 I. A. Subbotin , S. N. Yakunin , O. A. Shalygina and I. A. Kamenskikh Center for Photonics and Quantum Materials, Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, 121205 Moscow, Russia Valiev Institute of Physics and Technology, Russian Academy of Sciences, Yaroslavl Branch, Universitetskaya 21, 150007 Yaroslavl, Russia; imiraslab4@yandex.ru National Research Centre “Kurchatov Institute”, pl. Akademika Kurchatova 1, 123182 Moscow, Russia; chessyura@yandex.ru (Y.M.C.); a.vasiliev56@gmail.com (A.L.V.); lebedev.alex.m@gmail.com (A.M.L.); i.a.subbotin@gmail.com (I.A.S.); s.n.yakunin@gmail.com (S.N.Y.) Moscow Institute of Physics and Technology (State University), MIPT, Institutskiy per. 9, Dolgoprudny, 141701 Moscow Region, Russia Faculty of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory 1, 119991 Moscow, Russia; olga@vega.phys.msu.ru (O.A.S.); ikamenskikh@bk.ru (I.A.K.) * Correspondence: d.zhigunov@skoltech.ru Received: 1 October 2019; Accepted: 2 November 2019; Published: 6 November 2019 Abstract: Silicon nanocrystals in silicon nitride matrix are fabricated by thermal annealing of SiN /Si N multilayered thin films, and characterized by transmission electron microscopy, X-ray x 3 4 reflectivity and di raction analysis, photoluminescence and X-ray photoelectron spectroscopy techniques. Si nanocrystals with a mean size of about 4 nm are obtained, and their properties are studied as a function of SiN layer thickness (1.6–2 nm) and annealing temperature (900–1250 C). The e ect of coalescence of adjacent nanocrystals throughout the Si N barrier layers is observed, 3 4 which results in formation of distinct ellipsoidal-shaped nanocrystals. Complete intermixing of multilayered film accompanied by an increase of nanocrystal mean size for annealing temperature as high as 1250 C is shown. Near-IR photoluminescence with the peak at around 1.3–1.4 eV is detected and associated with quantum confined excitons in Si nanocrystals: Photoluminescence maximum is red shifted upon an increase of nanocrystal mean size, while the measured decay time is of order of microsecond. The position of photoluminescence peak as compared to the one for Si nanocrystals in SiO matrix is discussed. Keywords: nanocrystals; silicon; superlattice; photoluminescence; X-ray di raction; HRTEM; EFTEM; XPS 1. Introduction Following the early studies on porous silicon photoluminescence (PL) [1], optical properties of silicon nanocrystals (Si NCs) in solid matrices have been a focus of thorough research. Bright luminescence at room temperature from Si NCs, not observable for bulk silicon, appears very intriguing from the point of view of optoelectronic applications [2]. Di erent host matrices for Si NCs such as silicon oxide, nitride, and carbide are examined [3–5], while the most intense luminescence is typically observed for Si NCs in SiO matrix due to the smallest concentration of PL quenching centers [6]. At the same time, enhanced electron transport properties of Si NCs array in matrices with lower band gap (silicon nitride or carbide) are preferable for the photovoltaic applications [4]. To achieve a precise control of the size of Si NCs multilayered thin films can be fabricated using so-called superlattice Appl. Sci. 2019, 9, 4725; doi:10.3390/app9224725 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 4725 2 of 9 (SL) approach [7–9]. This approach allows one to limit the maximum size of Si NCs by deposition of Si-rich layers, wherein Si NCs are self-organized during high temperature annealing, alternating with stoichiometric barrier layers, which should remain unchanged [9,10]. However, as shown recently, the stability of multilayers against annealing depends on the thickness of layers and treatment temperature, thus partial or total intermixing of layered structure may occur, and Si NCs with the size exceeding the thickness of initial of Si-rich layers may be formed [11,12]. The question about the origin of observed luminescence is especially complicated in the case of silicon nitride-based superlattices, since a large number of radiative defects are found in SiN + 0 films, such as N , N or K-centers [6,13]. PL maximum attributed to Si NCs in Si N matrix is 4 2 3 4 detected in a broad range of photon energies, generally from 1.5 to 3 eV [4,9,13–15]. One of the main arguments in favor of quantum confinement e ect, which is responsible for such PL, is a blue shift of peak energy with decreasing Si NCs size. At the same time, similar PL spectra are observed for SiN films without Si NCs, whereas corresponding PL peak might be shifted in a wide range from about 1.6 to 2.6 eV by changing Si/N ratio [16,17]. A characteristic lifetime of such defect-related PL is of order of nanoseconds [18], while Si NC-related PL is known to possess much longer decay times in a microsecond range [3,12]. Hence, time-resolved PL measurements are important for the correct identification of PL source, which are often not presented in papers dedicated to Si NC luminescence in Si N [9,10,13]. 3 4 In the present study we demonstrate the formation of silicon nitride-embedded Si NCs, which emit in near-IR region (peak at 1.3–1.4 eV) with a characteristic PL lifetime in a microsecond timescale. In this spectral region, no contribution from radiative defects is expected. Similar near-IR luminescence with the peaks in the range 1.4–1.7 eV was shown recently from annealed B-doped SiN /Si N multilayer x 3 4 films with Si quantum dots produced by magnetron sputtering; however, no time-resolved PL data were provided [10]. 2. Materials and Methods 2.1. Sample Fabrication Silicon nitride films SiN with di erent x values were deposited by low frequency (55 kHz) discharge plasma enhanced chemical vapor deposition (LF PECVD). Monosilane SiH and ammonia NH were used as components of gas mixture. Gas flow ratio k = [NH ]/[SiH ] defined the stoichiometry 3 3 4 coecient x in deposited SiN materials. Gas mixture with k = 5 and 1.2 were used for deposition of nearly stoichiometric Si N barriers and silicon rich SiN layers (x  0.85), respectively [19]. In total, 3 4 x 18 pairs of layers were deposited on substrates at a power density of 0.2 W/cm . Pressure of gas mixture in plasma-chemical reactor was 250 Pa, and the deposition temperature was 380 C. For the fabricated samples the thicknesses of SiN /Si N bilayers are equal to 2/2.25  0.2 nm (SN1) and 1.6/1.75 3 4 0.2 nm (SN2), as measured by means of transmission electron microscopy (TEM). Prepared samples were subsequently annealed in a tube furnace under nitrogen atmosphere during 1 h at the following temperatures (T ): 900, 1150 and 1250 C, in order to form Si nanocrystals during phase separation ann of silicon-rich SiN layers: y SiN = (y x) Si + x SiN (1) x y where y is supposed to be close to stoichiometric 1.33 value [20]. 2.2. Sample Characterization The cross-section specimens for TEM were studied in a Titan 80–300 TEM/STEM (FEI, USA) at an accelerating voltage of 300 kV. The energy filtered TEM (EFTEM) measurements were performed in the low loss region of the electron energy loss spectrum with a 2 eV energy slit around the bulk Si plasmon loss peak (16.7 eV), which is commonly used to visualize a sample superlattice structure. High-resolution TEM (HRTEM) images showing individual Si nanocrystals were obtained at Scherzer defocus value. Phase separation of SiN was studied by X-ray photoelectron spectroscopy (XPS) performed on the x Appl. Sci. 2019, 9, 4725 3 of 9 ESCA branch of the NanoPES station at the Kurchatov synchrotron radiation source (National Research Center Kurchatov Institute) with Al K excitation (1486.61 eV). Sample surface sputtering was carried out by the 1.5-keV Ar ions bombardment in order to remove organic contaminations and native oxide. To study superlattice structure stability against thermal annealing, angular dependences of the intensity of X-ray reflectivity (XRR) were measured in step mode 2/! on X-ray di ractometer with a rotating anode (Rigaku SmartLab, Japan) using the characteristic Cu K 1 line (radiation energy 8.048 keV). In-plane grazing incidence X-ray di raction (XRD) studies were conducted on BM25B beamline at the European Synchrotron Radiation Facility (ESRF) using 22.025 keV radiation energy (incidence angle 0.1 ). Photoluminescence (PL) was excited by a He-Cd laser (h = 3.81 eV, 20 mW exc power) focused into a spot with 1 mm diameter. The PL signal was measured using a 500 mm focal length spectrometer (SOLAR MS 350, Belarus) equipped with a CCD camera with a sensitivity range of 200–1100 nm. The spectra were taken at room temperature and corrected for the system response, as well as for spectrometer dispersion by multiplying by  . For time-resolved PL measurements pulsed Nd:YAG laser excitation (h = 2.33 eV, pulse duration 34 ps, laser pulse fluence ~3 mJ/cm , repetition exc rate 10 Hz) was used. The PL signal was collected by means of intensified CCD (PI-MAX Gen III, Princeton Instruments) coupled to a 500 mm focal length imaging spectrograph (Princeton Instruments SpectraPro 2500i, USA). All PL spectra were detected within 1 s gate width taking various delays after excitation pulse onset in the range from 0 to 40 s. The PL decays for di erent emission energy (1.5 and 2 eV) are plotted using the PL intensity at a chosen photon energy values as a function of delay time. 3. Results and Discussion 3.1. TEM Results Figure 1 shows HRTEM and EFTEM images of SN1 and SN2 films annealed at T = 1150 C. ann Silicon nanocrystals can be seen as sets of lattice fringes (circled) in HRTEM images for both samples (see Figure 1a,d). Bright regions in EFTEM images (see Figure 1b,e) correspond to Si, while the dark ones correspond to Si N , thus multilayered structure is also seen. At the same time, it is obvious, 3 4 that size of Si NCs is not precisely restricted to SiN layer thickness. An e ect of coalescence of neighboring nanocrystals throughout Si N barrier layers can be observed, thus their shape starts to 3 4 be non-spherical (see Figure 1a,d). The mean diameter of Si NCs, which is defined as half sum of ellipse major axes in case of non-spherical particles, was estimated from HRTEM images. According to the statistical observations for more than 100 nanocrystals, their most frequently detected mean diameter was 2 nm, as shown in Figure 1c,f, which correlates well with SiN layer thickness (2 and 1.6 nm). At the same time, coalescent ellipsoidal-shaped nanocrystals with the mean diameter up to 10 and 9 nm were formed in case of SN1 and SN2 samples, respectively. Hence, the overall mean size of Si NCs was calculated to be 3.8  2.3 nm for SN1 and 3.4  1.7 nm for SN2 sample, which reflects the reduction of SiN layer thickness for the latter sample. Formation of nanocrystals with a non-spherical shape was also observed for annealed SiN monolayered films [21], while similar e ect of uncontrollable growth of Si NCs has been demonstrated recently for annealed SiN /SiO x 2 hetero-superlattices [12]. Appl. Sci. 2019, 9, 4725 4 of 9 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 9 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 9 Figure 1. (a) HRTEM image, (b) EFTEM image, and (c) size distribution of Si NCs for SiN /Si N Figure Figure 1. 1. ( a (a ) ) H H RTEM RTEM imag imag e, e, ( b (b ) ) EFTEM EFTEM ima ima ge, ge, and and (c (c ) ) si si ze ze di di st st ribution ribution of of Si Si NC NC s s for for Si Si N N x/Si x/Si 3N 3N 4 4(2/2. (2/2. x 25 3 25 4 (2/2.25 nm) SN1 sample; (d) HRTEM image, (e) EFTEM image, and (f) size distribution of Si NCs for nm) SN1 sample; (d) HRTEM image, (e) EFTEM image, and (f) size distribution of Si NCs for nm) SN1 sample; (d) HRTEM image, (e) EFTEM image, and (f) size distribution of Si NCs for SiN SiN /Si x/Si N 3N4 (1.6 (1.6/ /1.75 1.75 nm) nm) SN2 SN2 sample. sample. SiNx/Si3N4 (1.6/1.75 nm) SN2 sample. x 3 4 3.2. X-ray Studies 3.2. X-ray Studies 3.2. X-ray Studies As results from XRR measurements show in Figure 2, our samples annealed at 1150 C may As As rr esu esu lts lts fr fr oo m m XR XR R R me me asuremen asuremen ts ts sho sho w w in in Figure Figure 2, 2, our our s s ample ample ss anne anne aled aled at at 1150 1150 °° C C ma ma y y be be be considered as still multilayered films with some point distortions. Indeed, pronounced Bragg con con si si der der ed ed as as stil stil l l m m ul ul ti ti lay lay ered ered fil fil ms ms w w ith ith some some po po int int dis dis to to rtions rtions . . Ind Ind eed, eed, pro pro no no unced unced Br Br ag ag g g peaks peaks peaks (at around 1.1 and 1.5 degree for SN1 and SN2 sample, respectively) indicate the presence of (a (a t t ar ar ound ound 1. 1. 1 1 an an d d 1.5 1.5 de de gree gree fo fo r r SS N1 N1 and and SN2 SN2 ss am am ple, ple, re re spect spect ively) ively) in in dic dic aa te te th th e e presen presen ce ce of of superlattices, superlattices, whose whose periods periods ar ar eeestimated estimated as as 3.9 3.9 nm nm ((SN1) SN1) and and 3.1 3.1nm nm (SN2 (SN2), ), which which mmatches atches wel well l superlattices, whose periods are estimated as 3.9 nm (SN1) and 3.1 nm (SN2), which matches well those obtained by TEM measurements (4.25 and 3.35 nm, respectively). At the same time, relatively those those obtained obtaineby d by TEM TEM measur measuremen ementsts (4.25 (4.25 and and 3.35 3.35 nm, nm, rrespect espectively). ively). At Atth the e ssame ame time time, , rel relatively atively large width of Bragg peaks also points out at the multilayered structure imperfections, which are large larwidth ge width of Bragg of Brag peaks g peaks alsoalso points points out out at the at multilayer the multilayered ed structur struct e ure imperfections, imperfections which , whi ar ch e most are mo mo st st likel likel yy cc oal oal ee scent scent nn an an ocrysta ocrysta ls ls . . In In turn, turn, an an incr incr ea ea se se of of anne anne aling aling tem tem per per ature ature up up to to 1250 1250 °° C C likely coalescent nanocrystals. In turn, an increase of annealing temperature up to 1250 C resulted in result result ed ed in in de de struct struct ion ion oo f f SS L L struct struct u u re, re, as as evid evid ent ent fro fro m m Bra Bra gg gg pe pe aks aks d d isapp isapp ea ea rr ing ing for for bo bo th th S S N1 N1 and and destruction of SL structure, as evident from Bragg peaks disappearing for both SN1 and SN2 samples SN2 samples (see Figure 2). Such a temperature-dependent SL structure breakdown was observed in SN2 samples (see Figure 2). Such a temperature-dependent SL structure breakdown was observed in (see Figure 2). Such a temperature-dependent SL structure breakdown was observed in our previous our previous study of SiOx/SiO2 and SiOxNy/SiO2 multilayered films with 1.5 nm thick Si-rich layers, our previous study of SiOx/SiO2 and SiOxNy/SiO2 multilayered films with 1.5 nm thick Si-rich layers, study of SiO /SiO and SiO N /SiO multilayered films with 1.5 nm thick Si-rich layers, and explained x 2 x y 2 and explained by the gain in the Gibbs free energy for a mixed system as compared with initial and explained by the gain in the Gibbs free energy for a mixed system as compared with initial by the gain in the Gibbs free energy for a mixed system as compared with initial unmixed one [11]. unmixed unmixed one one [11]. [11]. At At th th e e ss am am e e time time , ,it it is is wort wort h h not not ing ing th th at at si si m m ilar ilar S S iN iN x/S x/S i3 iN 3N 4 4mult mult ilayered ilayered struc struc tures tures At the same time, it is worth noting that similar SiN /Si N multilayered structures even with ultrathin x 3 4 even even with with u u ltrathin ltrathin (1 (1 nn m) m) SS i3 iN 3N 4 4 bb arriers arriers rem rem ai ai nn stab stab le le aft aft er er high high tem tem pe pe ra ra ture ture tre tre atmen atmen t t as as (1 nm) Si N barriers remain stable after high temperature treatment as demonstrated recently [9,10]. 3 4 demonstrated recently [9,10]. Hence, we can assume that the most probable reason for the effect of Si demonstrated recently [9,10]. Hence, we can assume that the most probable reason for the effect of Si Hence, we can assume that the most probable reason for the e ect of Si NC coalescence demonstrated NC coalescence demonstrated in Figure 1 is the small enough thickness of SiNx layers (1.6–2 nm), NC coalescence demonstrated in Figure 1 is the small enough thickness of SiNx layers (1.6–2 nm), in Figure 1 is the small enough thickness of SiN layers (1.6–2 nm), leading to an appearance of distinct leading to an appearance of distinct intermixing regions. This trend is in full agreement with our leading to an appearance of distinct intermixing regions. This trend is in full agreement with our intermixing regions. This trend is in full agreement with our previous observations of SL structure previous observations of SL structure intermixing for SiOx and SiNx-based multilayers with ultrathin previous observations of SL structure intermixing for SiOx and SiNx-based multilayers with ultrathin intermixing for SiO and SiN -based multilayers with ultrathin Si-rich layers [11,22,23]. x x Si Si -- rich rich l ay lay er er s s [11,2 [11,2 2,23]. 2,23]. Figure Figure 2. 2. XRR XRR sc sc ans f ans f or ( or ( aa ) )S S N1 N1 and and (b (b ) SN ) SN 2 2 samples anne samples anne ale ale d d at at d d iff iff eren eren t t temper temper atur atur es es . . Figure 2. XRR scans for (a) SN1 and (b) SN2 samples annealed at di erent temperatures. Appl. Sci. 2019, 9, 4725 5 of 9 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 9 The formation of Si NC with an increase of annealing temperature was additionally studied by Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 9 means of XRD and XPS techniques. XRD scans of SN2 sample are shown in Figure 3. As can be The formation of Si NC with an increase of annealing temperature was additionally studied by seen, 900 C annealing temperature is obviously not enough to form nanocrystals, since XRD pattern means of XRD and XPS techniques. XRD scans of SN2 sample are shown in Figure 3. As can be seen, The formation of Si NC with an increase of annealing temperature was additionally studied by constitute of a broad band around the position of Si (111) di raction peak, which is a signature of 900 °C annealing temperature is obviously not enough to form nanocrystals, since XRD pattern means of XRD and XPS techniques. XRD scans of SN2 sample are shown in Figure 3. As can be seen, amorphous silicon nanoclusters. In contrast, for higher annealing temperatures relatively narrow constitute of a broad band around the position of Si (111) diffraction peak, which is a signature of 900 °C annealing temperature is obviously not enough to form nanocrystals, since XRD pattern peak is observed, which is a typical XRD pattern for Si NCs [9]. Remarkably, the peak becomes amorphous silicon nanoclusters. In contrast, for higher annealing temperatures relatively narrow constitute of a broad band around the position of Si (111) diffraction peak, which is a signature of narrpeak ower is with obser the ved, rise wh of ich annealing is a typical temperatur XRD pattern e fr f om or S 1150 i NCs C [9] to . Rema 1250rkC, ablconsequently y, the peak becom the es mean amorphous silicon nanoclusters. In contrast, for higher annealing temperatures relatively narrow size narrower of Si NCs with incr th eases e rise o by f an about nealing 30% temaccor perature ding fro to m the 1150 estimation °C to 1250 °C, by con means sequent of ly Scherr the mean er equation size peak is observed, which is a typical XRD pattern for Si NCs [9]. Remarkably, the peak becomes of Si NCs increases by about 30% according to the estimation by means of Scherrer equation (the (the same trend is reproduced for SN1 sample). This fact is consistent with SL structure breakdown narrower with the rise of annealing temperature from 1150 °C to 1250 °C, consequently the mean size same trend is reproduced for SN1 sample). This fact is consistent with SL structure breakdown at at 1250 C demonstrated by XRR, hence the formation of Si NCs with a larger mean size can be of Si NCs increases by about 30% according to the estimation by means of Scherrer equation (the 1250 °C demonstrated by XRR, hence the formation of Si NCs with a larger mean size can be expected, same trend is reproduced for SN1 sample). This fact is consistent with SL structure breakdown at expected, in a similar way as demonstrated previously in case of SiO N /SiO SL annealed at the same x y 2 in a similar way as demonstrated previously in case of SiOxNy/SiO2 SL annealed at the same 1250 °C demonstrated by XRR, hence the formation of Si NCs with a larger mean size can be expected, temperature [24]. The mean size of Si NCs grows together with the thickness of SiN layers: As results temperature [24]. The mean size of Si NCs grows together with the thickness of SiNx layers: As results in a similar way as demonstrated previously in case of SiOxNy/SiO2 SL annealed at the same from comparative XRD measurements (see inset in Figure 3), on the average Si NCs in SN1 sample are from comparative XRD measurements (see inset in Figure 3), on the average Si NCs in SN1 sample temperature [24]. The mean size of Si NCs grows together with the thickness of SiNx layers: As results about 14% larger than those in SN2 sample (4.1  0.1 nm vs. 3.6  0.1 nm), according to calculations are about 14% larger than those in SN2 sample (4.1 ± 0.1 nm vs. 3.6 ± 0.1 nm), according to calculations from comparative XRD measurements (see inset in Figure 3), on the average Si NCs in SN1 sample using Scherrer equation [9], which agrees well with the mean size estimations performed by analyzing using Scherrer equation [9], which agrees well with the mean size estimations performed by are about 14% larger than those in SN2 sample (4.1 ± 0.1 nm vs. 3.6 ± 0.1 nm), according to calculations HRTEM images. analyzing HRTEM images. using Scherrer equation [9], which agrees well with the mean size estimations performed by analyzing HRTEM images. Figure 3. XRD scans around the position of Si (111) diffraction peak (10.3° for photon energy used) Figure 3. XRD scans around the position of Si (111) di raction peak (10.3 for photon energy used) for for SN2 sample annealed at different remperatures. Inset: XRD scans of SN1 and SN2 samples SN2Figure sample 3.annealed XRD scans at arou di er nd ent the remperatur position ofes. Si Inset: (111) di XRD ffracti scans on peak of SN1 (10.3 and ° forSN2 photon samples energannealed y used) at ann  ealed at 1150 °C. 1150for C. SN2 sample annealed at different remperatures. Inset: XRD scans of SN1 and SN2 samples annealed at 1150 °C. XPS spectra of SN1 sample before and after annealing at the characteristic energy of Si 2p core XPS spectra of SN1 sample before and after annealing at the characteristic energy of Si 2p core levels levels are presented in Figure 4. The formation of Si nanoclusters (Tann = 900 °C) and Si NCs (Tann = XPS spectra of SN1 sample before and after annealing at the characteristic energy of Si 2p core are presented in Figure 4. The formation of Si nanoclusters (T = 900 C) and Si NCs (T = 1150 C) ann ann 1150 °C) can be attributed to an appearance of a shoulder at 99.7 eV, which corresponds to Si-Si bonds levels are presented in Figure 4. The formation of Si nanoclusters (Tann = 900 °C) and Si NCs (Tann = can be attributed to an appearance of a shoulder at 99.7 eV, which corresponds to Si-Si bonds (Si peak). (Si peak). In turn, evidence of a significant phase separation within SiNx layers at Tann = 1150 °C 1150 °C) can be attributed to an appearance of a shoulder at 99.7 eV, which corresponds to Si-Si bonds In turn, evidence of a significant phase separation within SiN layers at T = 1150 C follows out of x ann 4+ follows 0 out of an appearance of Si peak with a characteristic energy 101.7 eV, which corresponds to (Si peak). In turn, 4+ evidence of a significant phase separation within SiNx layers at Tann = 1150 °C an appearance of Si peak with a characteristic energy 101.7 eV, which corresponds to Si-N bonds in Si-N bonds in Si3N4 matrix [25]. 4+ follows out of an appearance of Si peak with a characteristic energy 101.7 eV, which corresponds to Si N matrix [25]. 3 4 Si-N bonds in Si3N4 matrix [25]. Figure 4. XPS spectra of SN1 sample (as prepared and annealed at different temper atures) measured in the energy region of Si 2p levels. Figure 4. XPS spectra of SN1 sample (as prepared and annealed at different temperatures) measured Figure 4. XPS spectra of SN1 sample (as prepared and annealed at di erent temperatures) measured in in the energy region of Si 2p levels. the energy region of Si 2p levels. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 9 3.3. Photoluminescence Measurements PL spectra of SN1 and SN2 samples annealed at 1150 °C are shown in Figure 5a. An observed PL peak shift may be associated with the size-dependent quantum confinement effect for excitons in Appl. Sci. 2019, 9, 4725 6 of 9 Si NCs. Spectra are characterized by maxima in the near IR range (885–940 nm), which is not typical for Si NCs in Si3N4 matrix, as mentioned above. Nevertheless, luminescence in this spectral region was also observed recently from similar SiNx/Si3N4 SLs with Si NCs, but produced by different 3.3. Photoluminescence Measurements methods [10,23]. Moreover, bare theory of quantum confinement predicts the reduction of an energy PL spectra of SN1 and SN2 samples annealed at 1150 C are shown in Figure 5a. An observed PL gap (i.e., red shift) for Si nanocrystals with the decrease of confinement barriers, i.e., when peak shift may be associated with the size-dependent quantum confinement e ect for excitons in Si nanocrystals are incorporated into Si3N4 matrix instead of SiO2 [4]. Thus, PL with lower peak energies NCs. Spectra are characterized by maxima in the near IR range (885–940 nm), which is not typical for Si from Si NCs in silicon nitride as compared with silicon oxide matrix (~1.7 eV at most) is indeed NCs in Si N matrix, as mentioned above. Nevertheless, luminescence in this spectral region was also justified. At the same time, the majority of experimental data on Si NC luminescence in Si3N4 reveals 3 4 observed recently from similar SiN /Si N SLs with Si NCs, but produced by di erent methods [10,23]. the PL maximum at relatively hix gh photon energies (above 1.6–1.7 eV), which is blue-shifted as 3 4 Moreover, bare theory of quantum confinement predicts the reduction of an energy gap (i.e., red shift) compared to a typical emission of Si NCs in SiO2 characterized by a maximum at about 1.2–1.7 eV for Si nanocrystals with the decrease of confinement barriers, i.e., when nanocrystals are incorporated [2,26]. From the theoretical point of view, this discrepancy is usually solved by empirical modification into of thSi e th N eoret matrix ical par instead amete ofrs SiO to achi [4].eve Thus, a bet PL ter with fit wi lower th thpeak e expener eriment giesal fr d om ataSi [4]. NCs On in th sil e icon other nitride hand, 3 4 2 as compared with silicon oxide matrix (~1.7 eV at most) is indeed justified. At the same time, the in silicon rich nitrides there are numerous matrix-related radiative defects having strong emission majority lines, wh of ich experimental makes it diff dat ica ult onto Si dis NC tinguish luminescence the cont inribution Si N reveals of Si the NCPL s, e maximum specially whe at relatively n short- 3 4 high photon energies (above 1.6–1.7 eV), which is blue-shifted as compared to a typical emission of Si wavelength excitation is used [13,15]. With respect to SL structures, matrix-related luminescence NCs signal in intensi SiO characterized ty may be re by duced a maximum by decreasing at about 1.2–1.7 the thickness eV [2,26o ].f Fr Si om 3N4the bar theor riers, etical as demo pointn of stra view ted, this discrepancy is usually solved by empirical modification of the theoretical parameters to achieve a recently by an example of SiOxNy/Si3N4 hetero-SLs [6]. Indeed, in general, luminescence from Si NCs better in SiNfit x/Si with 3N4 SLs the experimental with the peakdata energy at about [4]. On the other 1.7 eV or below hand, in silicon is obrich served, wh nitridesen Si ther3 eN ar 4 e bar numer riers has ous matrix-related radiative defects having strong emission lines, which makes it dicult to distinguish the thickness under 2 nm [9,10,23]. On the opposite, in case of thicker barriers or SiNx monolayered the film contribution s with Si NCs of Sith NCs, e poespecially sition of PL when peak short-wavelength is usually fouexcitation nd in the is spect used ra [13 l r,ange 15]. W above ith respect 1.7 eto V SL structures, matrix-related luminescence signal intensity may be reduced by decreasing the thickness [13,16,27]. Taking into account a number of radiative defects in SiNx, which emit at such high photon of energie Si N s,barriers, an attem as ptdemonstrated to attribute cert recently ain PL band by an s to example Si NC emi of SiO ssion Nin /Si SiN N x seem heter s o-SLs highly [6qu ]. Indeed, estionable in 3 4 x y 3 4 general, luminescence from Si NCs in SiN /Si N SLs with the peak energy at about 1.7 eV or below in this case, as emphasized elsewhere [17]. x 3 4 is observed, To confwhen irm that Si N exc barriers iton recom hasbination the thickness in Siunder NCs 2 is nm the[ 9 ori ,10 gi ,23 n ].of On obthe served opposite, PL for in our case S of L 3 4 thicker barriers or SiN monolayered films with Si NCs the position of PL peak is usually found in structures, time-resolved PL measurements were carried out. The results for SN1 sample are shown the in F spectral igure 5b range for above two d1.7 ifferent eV [13 ph ,16 ot ,27 on ]. energ Taking ies. into Bot account h curves a number are char ofacter radiative ized defects by a stiretc n SiN hed , which emit at such high photon energies, an attempt to attribute certain PL bands to Si NC emission in exponential decay: SiN seems highly questionable in this case, as emphasized elsewhere [17]. I = I0 exp [− (t/τ) ] (2) 1.0 SN1 0.8 SN2 0.6 0.4 0.2 0.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 Photon Energy (eV) Figure 5. (a) PL spectra of fabricated samples annealed at 1150 C; (b) PL decay curves for SN1 sample Figure 5. (a) PL spectra of fabricated samples annealed at 1150 °C; (b) PL decay curves for SN1 sample (points) and corresponding approximation using stretched exponential function (lines). (points) and corresponding approximation using stretched exponential function (lines). To confirm that exciton recombination in Si NCs is the origin of observed PL for our SL structures, with the time constant τ reducing from 1.84 to 0.35 μs upon increase of photon energy. Such reduction time-resolved PL measurements were carried out. The results for SN1 sample are shown in Figure 5b as well as relatively long PL lifetime (in a microsecond range) and non-exponential parameter β ~ for two di erent photon energies. Both curves are characterized by a stretched exponential decay: 0.5–0.6 are typical for quantum confined Si NCs-related PL [3,28]. At the same time, characteristic time constant seems to be several times smaller than that for SiOx-embedded Si NCs [3]. The latter I = I exp [ (t/) ] (2) may be explained by a more defective nature of SiNx matrix as compared to SiOx one [6]. Defects like with the time constant  reducing from 1.84 to 0.35 s upon increase of photon energy. Such reduction as well as relatively long PL lifetime (in a microsecond range) and non-exponential parameter ~ 0.5–0.6 are typical for quantum confined Si NCs-related PL [3,28]. At the same time, characteristic time Norm. PL Intensity (a.u.) Appl. Sci. 2019, 9, 4725 7 of 9 constant seems to be several times smaller than that for SiO -embedded Si NCs [3]. The latter may be explained by a more defective nature of SiN matrix as compared to SiO one [6]. Defects like dangling x x bonds act as luminescence quenching centers, increasing the nonradiative recombination rate and, hence, reducing the measured PL lifetime. 4. Conclusions In the present study, we fabricated Si NCs in Si N matrix by high temperature annealing of 3 4 SiN /Si N multilayered films deposited by low frequency discharge PECVD. Formation of Si NCs is x 3 4 confirmed by HRTEM, EFTEM, XRD and XPS measurements. It is shown that superlattice structure remains stable in general after 1150 C annealing with some point distortions such as coalescent nanocrystals. Size of these distinct ellipsoidal-shaped nanocrystals exceeds more than twice the period of multilayered structure. In turn, an increase of annealing temperature up to 1250 C led to superlattice breakdown and Si NC mean size growth, in full analogy with the previous observations of the similar e ect in SiO N /SiO multilayered films annealed under the same conditions. Fabricated samples x y 2 are characterized by a photoluminescence signal with the peak in near IR range (1.3–1.4 eV), while time-resolved PL measurements allowed us to ascribe this PL to a recombination of quantum confined excitons in Si NCs. In our opinion, the main reason for such low-energy luminescence, which is not typical for SiN -embedded Si NCs, is a negligible contribution from radiative defects in matrix due to small enough thickness of Si N barrier layers. Our results are found to agree well with the predictions 3 4 of quantum confinement theory, which states a redshift of confined energy upon reduction of potential barrier height by changing a wide band gap SiO matrix to a lower band gap Si N . 2 3 4 Author Contributions: Conceptualization, D.M.Z. and I.A.K.; investigation, Y.M.C., A.M.L, I.A.S. and O.A.S.; resources, A.A.P.; writing—original draft preparation, D.M.Z.; writing—review and editing, A.L.V. and A.M.L.; supervision, D.M.Z., S.N.Y. and A.L.V. Funding: This work was financially supported by the Ministry of Science and Higher Education of the Russian Federation, project No. RFMEFI58117X0026. Silicon nitride film deposition has been performed under the State Program 0066-2019-0003. D.M.Z. gratefully acknowledges the support from the Russian Foundation for Basic Research (grant No. 18-32-20217). Acknowledgments: We are grateful to Maria Vila Santos at the ESRF for providing assistance in using beamline BM25B. XRR and electron microscopy measurements have been carried out using the equipment of Resource Centers of the X-ray techniques and Probe and Electron Microscopy of NRC “Kurchatov Institute”. Conflicts of Interest: The authors declare no conflict of interest. References 1. Kovalev, D.; Heckler, H.; Polisski, G.; Koch, F. Optical Properties of Si Nanocrystals. Phys. Status Solidi B 1999, 215, 871–932. [CrossRef] 2. Takeoka, S.; Fujii, M.; Hayashi, S. Size-dependent photoluminescence from surface-oxidized Si nanocrystals in a weak confinement regime. Phys. Rev. B 2000, 62, 16820–16825. [CrossRef] 3. Linnros, J.; Lalic, N.; Galeckas, A.; Grivickas, V. Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO . J. Appl. Phys. 1999, 86, 6128–6134. [CrossRef] 4. Conibeer, G.; Green, M.; Cho, E.-C.; König, D.; Cho, Y.-H.; Fangsuwannarak, T.; Scardera, G.; Pink, E.; Huang, Y.; Puzzer, T.; et al. Silicon quantum dot nanostructures for tandem photovoltaic cells. Thin Solid Films 2008, 516, 6748–6756. [CrossRef] 5. Wan, Z.; Huang, S.; Green, M.; Conibeer, G. Residual stress study of silicon quantum dot in silicon carbide matrix by Raman measurement. Phys. Status Solidi C 2011, 8, 185–188. [CrossRef] 6. Zelenina, A.; Sarikov, A.; Gutsch, S.; Zakharov, N.; Werner, P.; Reichert, A.; Weiss, C.; Zacharias, M. Formation of size-controlled and luminescent Si nanocrystals from SiO N /Si N hetero-superlattices. J. Appl. Phys. x y 3 4 2015, 117, 175303. [CrossRef] Appl. Sci. 2019, 9, 4725 8 of 9 7. Heitmann, J.; Kovalev, D.; Schmidt, M.; Yi, L.X.; Scholz, R.; Eichhorn, F.; Zacharias, M. Synthesis and size control of Si nanocrystals by SiO/SiO superlattices and Er doping. MRS Proc. 2002, 737, F1.6. [CrossRef] 8. Volodin, V.A.; Arzhannikova, S.A.; Gismatulin, A.A.; Kamaev, G.N.; Antonenko, A.K.; Cherkova, S.G.; Cherkov, A.G.; Kochubei, S.A.; Popov, A.A.; Robert, S.; et al. Laser pulse crystallization and optical properties of Si/SiO and Si/Si N multilayer nano-heterostructures. Proc. SPIE 2012, 8700, 870008. [CrossRef] 2 3 4 9. So, Y.-H.; Huang, S.; Conibeer, G.; Green, M.A. Formation and photoluminescence of Si nanocrystals in controlled multilayer structure comprising of Si-rich nitride and ultrathin silicon nitride barrier layers. Thin Solid Films 2011, 519, 5408–5412. [CrossRef] 10. Chen, X.; Yang, W.; Yang, P.; Yuan, J.; Zhao, F.; Hao, J.; Tang, Y. Size-controlled Si quantum dots embedded in B-doped SiN /Si N superlatice for Si quantum dot solar cells. J. Mater. Sci. Mater. Electron. 2017, 28, x 3 4 1322–1327. [CrossRef] 11. Zhigunov, D.M.; Sarikov, A.; Chesnokov, Y.M.; Vasiliev, A.L.; Zakharov, N.; Kashkarov, P.K. Thickness and temperature depending intermixing of SiO /SiO and SiO N /SiO superlattices: Experimental observation x 2 x y 2 and thermodynamic modeling. Appl. Phys. Lett. 2016, 108, 223102. [CrossRef] 12. Zelenina, A.; Sarikov, A.; Zhigunov, D.M.; Weiss, C.; Zakharov, N.; Werner, P.; López-Conesa, L.; Estradé, S.; Peiró, F.; Dyakov, S.A.; et al. Silicon nanocrystals in SiN /SiO hetero-superlattices: The loss of size control after thermal annealing. J. Appl. Phys. 2014, 115, 244304. [CrossRef] 13. Sain, B.; Das, D. Tunable photoluminescence from nc-Si/a-SiN :H quantum dot thin films prepared by ICP-CVD. Phys. Chem. Chem. Phys. 2013, 15, 3881–3888. [CrossRef] [PubMed] 14. Lin, C.-H.; Uen, W.-Y.; Lan, S.-M.; Huang, Y.-C.; Liao, S.-M.; Li, Z.-Y.; Yang, T.-N.; Ku, C.-T.; Chen, M.-C.; Huang, Y.-H. Luminescence mechanisms of silicon-rich nitride films fabricated by atmospheric pressure chemical vapor deposition in N and H atmospheres. J. Appl. Phys. 2009, 105, 053107. [CrossRef] 2 2 15. Wang, M.; Li, D.; Yuan, Z.; Yang, D.; Que, D. Photoluminescence of Si-rich silicon nitride: Defect-related states and silicon nanoclusters. Appl. Phys. Lett. 2007, 90, 131903. [CrossRef] 16. Delachat, F.; Carrada, M.; Ferblantier, G.; Grob, J.-J.; Slaoui, A. Properties of silicon nanoparticles embedded in SiN deposited by microwave-PECVD. Nanotechnology 2009, 20, 415608. [CrossRef] [PubMed] 17. Kistner, J.; Chen, X.; Weng, Y.; Strunk, H.P.; Schubert, M.B.; Werner, J.H. Photoluminescence from silicon nitride—No quantum e ect. J. Appl. Phys. 2011, 110, 023520. [CrossRef] 18. Giorgis, F.; Vinegoni, C.; Pavesi, L. Optical absorption and photoluminescence properties of a-Si N :H 1x films deposited by plasma-enhanced CVD. Phys. Rev. B 2000, 61, 4693–4698. [CrossRef] 19. Korchagina, T.T.; Marin, D.V.; Volodin, V.A.; Popov, A.A.; Vergnat, M. Structure and optical properties of SiN :H films with Si nanoclusters produced by low-frequency plasma-enhanced chemical vapor deposition. Semiconductors 2009, 43, 1514–1520. [CrossRef] 20. Molinari, M.; Rinnert, H.; Vergnat, M. Evolution with the annealing treatments of the photoluminescence mechanisms in a-SiN : H alloys prepared by reactive evaporation. J. Appl. Phys. 2007, 101, 123532. [CrossRef] 21. Liao, W.; Zeng, X.; Wen, X.; Chen, X.; Wang, W. Annealing and excitation dependent photoluminescence of silicon rich silicon nitride films with silicon quantum dots. Vacuum 2015, 121, 147–151. [CrossRef] 22. Sarikov, A.; Zhigunov, D. Thermodynamic mechanism of the intermixing of multilayered structures in the SiO /SiO superlattices with nanometer thick layers. Mater. Today Commun. 2017, 13, 163–169. [CrossRef] 23. Zhigunov, D.M.; Martyshov, M.N.; Forsh, P.A.; Kamenskikh, I.A.; Yakunin, S.N.; Kashkarov, P.K. Structure-related current transport and photoluminescence in SiO N and SiN based superlattices with Si x y x nanocrystals. Phys. Status Solidi A Appl. Mater. 2017, 214, 1700040. [CrossRef] 24. Hartel, A. Structural and Optical Properties of PECVD Grown Silicon Nanocrystals Embedded in SiO N x y Matrix. Ph.D. Thesis, Albert-Ludwigs-Universität Freiburg im Breisgau, Freiburg, Germany, 5 August 2013. 25. Zhigunov, D.M.; Kamenskikh, I.A.; Lebedev, A.M.; Chumakov, R.G.; Logachev, Y.A.; Yakunin, S.N.; Kashkarov, P.K. X-ray reflectivity and photoelectron spectroscopy of superlattices with silicon nanocrystals. JETP Lett. 2017, 106, 517–521. [CrossRef] 26. Heitmann, J.; Müller, F.; Zacharias, M.; Gösele, U. Silicon nanocrystals: Size matters. Adv. Mater. 2005, 17, 795–803. [CrossRef] Appl. Sci. 2019, 9, 4725 9 of 9 27. Zelenina, A.; Dyakov, S.A.; Hiller, D.; Gutsch, S.; Trouillet, V.; Bruns, M.; Mirabella, S.; Löper, P.; López-Conesa, L.; López-Vidrier, J.; et al. Structural and optical properties of size controlled Si nanocrystals in Si N matrix: The nature of photoluminescence peak shift. J. Appl. Phys. 2013, 114, 184311. [CrossRef] 3 4 28. Timoshenko, V.Y.; Lisachenko, M.G.; Shalygina, O.A.; Kamenev, B.V.; Zhigunov, D.M.; Teterukov, S.A.; Kashkarov, P.K.; Heitmann, J.; Schmidt, M.; Zacharias, M. Comparative study of photoluminescence of undoped and erbium-doped size-controlled nanocrystalline Si/SiO multilayered structures. J. Appl. Phys. 2004, 96, 2254–2260. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Near-IR Emitting Si Nanocrystals Fabricated by Thermal Annealing of SiNx/Si3N4 Multilayers

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applied sciences Article Near-IR Emitting Si Nanocrystals Fabricated by Thermal Annealing of SiN /Si N Multilayers x 3 4 1 , 2 3 3 , 4 3 D. M. Zhigunov * , A. A. Popov , Yu. M. Chesnokov , A. L. Vasiliev , A. M. Lebedev , 3 3 5 5 I. A. Subbotin , S. N. Yakunin , O. A. Shalygina and I. A. Kamenskikh Center for Photonics and Quantum Materials, Skolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, 121205 Moscow, Russia Valiev Institute of Physics and Technology, Russian Academy of Sciences, Yaroslavl Branch, Universitetskaya 21, 150007 Yaroslavl, Russia; imiraslab4@yandex.ru National Research Centre “Kurchatov Institute”, pl. Akademika Kurchatova 1, 123182 Moscow, Russia; chessyura@yandex.ru (Y.M.C.); a.vasiliev56@gmail.com (A.L.V.); lebedev.alex.m@gmail.com (A.M.L.); i.a.subbotin@gmail.com (I.A.S.); s.n.yakunin@gmail.com (S.N.Y.) Moscow Institute of Physics and Technology (State University), MIPT, Institutskiy per. 9, Dolgoprudny, 141701 Moscow Region, Russia Faculty of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory 1, 119991 Moscow, Russia; olga@vega.phys.msu.ru (O.A.S.); ikamenskikh@bk.ru (I.A.K.) * Correspondence: d.zhigunov@skoltech.ru Received: 1 October 2019; Accepted: 2 November 2019; Published: 6 November 2019 Abstract: Silicon nanocrystals in silicon nitride matrix are fabricated by thermal annealing of SiN /Si N multilayered thin films, and characterized by transmission electron microscopy, X-ray x 3 4 reflectivity and di raction analysis, photoluminescence and X-ray photoelectron spectroscopy techniques. Si nanocrystals with a mean size of about 4 nm are obtained, and their properties are studied as a function of SiN layer thickness (1.6–2 nm) and annealing temperature (900–1250 C). The e ect of coalescence of adjacent nanocrystals throughout the Si N barrier layers is observed, 3 4 which results in formation of distinct ellipsoidal-shaped nanocrystals. Complete intermixing of multilayered film accompanied by an increase of nanocrystal mean size for annealing temperature as high as 1250 C is shown. Near-IR photoluminescence with the peak at around 1.3–1.4 eV is detected and associated with quantum confined excitons in Si nanocrystals: Photoluminescence maximum is red shifted upon an increase of nanocrystal mean size, while the measured decay time is of order of microsecond. The position of photoluminescence peak as compared to the one for Si nanocrystals in SiO matrix is discussed. Keywords: nanocrystals; silicon; superlattice; photoluminescence; X-ray di raction; HRTEM; EFTEM; XPS 1. Introduction Following the early studies on porous silicon photoluminescence (PL) [1], optical properties of silicon nanocrystals (Si NCs) in solid matrices have been a focus of thorough research. Bright luminescence at room temperature from Si NCs, not observable for bulk silicon, appears very intriguing from the point of view of optoelectronic applications [2]. Di erent host matrices for Si NCs such as silicon oxide, nitride, and carbide are examined [3–5], while the most intense luminescence is typically observed for Si NCs in SiO matrix due to the smallest concentration of PL quenching centers [6]. At the same time, enhanced electron transport properties of Si NCs array in matrices with lower band gap (silicon nitride or carbide) are preferable for the photovoltaic applications [4]. To achieve a precise control of the size of Si NCs multilayered thin films can be fabricated using so-called superlattice Appl. Sci. 2019, 9, 4725; doi:10.3390/app9224725 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 4725 2 of 9 (SL) approach [7–9]. This approach allows one to limit the maximum size of Si NCs by deposition of Si-rich layers, wherein Si NCs are self-organized during high temperature annealing, alternating with stoichiometric barrier layers, which should remain unchanged [9,10]. However, as shown recently, the stability of multilayers against annealing depends on the thickness of layers and treatment temperature, thus partial or total intermixing of layered structure may occur, and Si NCs with the size exceeding the thickness of initial of Si-rich layers may be formed [11,12]. The question about the origin of observed luminescence is especially complicated in the case of silicon nitride-based superlattices, since a large number of radiative defects are found in SiN + 0 films, such as N , N or K-centers [6,13]. PL maximum attributed to Si NCs in Si N matrix is 4 2 3 4 detected in a broad range of photon energies, generally from 1.5 to 3 eV [4,9,13–15]. One of the main arguments in favor of quantum confinement e ect, which is responsible for such PL, is a blue shift of peak energy with decreasing Si NCs size. At the same time, similar PL spectra are observed for SiN films without Si NCs, whereas corresponding PL peak might be shifted in a wide range from about 1.6 to 2.6 eV by changing Si/N ratio [16,17]. A characteristic lifetime of such defect-related PL is of order of nanoseconds [18], while Si NC-related PL is known to possess much longer decay times in a microsecond range [3,12]. Hence, time-resolved PL measurements are important for the correct identification of PL source, which are often not presented in papers dedicated to Si NC luminescence in Si N [9,10,13]. 3 4 In the present study we demonstrate the formation of silicon nitride-embedded Si NCs, which emit in near-IR region (peak at 1.3–1.4 eV) with a characteristic PL lifetime in a microsecond timescale. In this spectral region, no contribution from radiative defects is expected. Similar near-IR luminescence with the peaks in the range 1.4–1.7 eV was shown recently from annealed B-doped SiN /Si N multilayer x 3 4 films with Si quantum dots produced by magnetron sputtering; however, no time-resolved PL data were provided [10]. 2. Materials and Methods 2.1. Sample Fabrication Silicon nitride films SiN with di erent x values were deposited by low frequency (55 kHz) discharge plasma enhanced chemical vapor deposition (LF PECVD). Monosilane SiH and ammonia NH were used as components of gas mixture. Gas flow ratio k = [NH ]/[SiH ] defined the stoichiometry 3 3 4 coecient x in deposited SiN materials. Gas mixture with k = 5 and 1.2 were used for deposition of nearly stoichiometric Si N barriers and silicon rich SiN layers (x  0.85), respectively [19]. In total, 3 4 x 18 pairs of layers were deposited on substrates at a power density of 0.2 W/cm . Pressure of gas mixture in plasma-chemical reactor was 250 Pa, and the deposition temperature was 380 C. For the fabricated samples the thicknesses of SiN /Si N bilayers are equal to 2/2.25  0.2 nm (SN1) and 1.6/1.75 3 4 0.2 nm (SN2), as measured by means of transmission electron microscopy (TEM). Prepared samples were subsequently annealed in a tube furnace under nitrogen atmosphere during 1 h at the following temperatures (T ): 900, 1150 and 1250 C, in order to form Si nanocrystals during phase separation ann of silicon-rich SiN layers: y SiN = (y x) Si + x SiN (1) x y where y is supposed to be close to stoichiometric 1.33 value [20]. 2.2. Sample Characterization The cross-section specimens for TEM were studied in a Titan 80–300 TEM/STEM (FEI, USA) at an accelerating voltage of 300 kV. The energy filtered TEM (EFTEM) measurements were performed in the low loss region of the electron energy loss spectrum with a 2 eV energy slit around the bulk Si plasmon loss peak (16.7 eV), which is commonly used to visualize a sample superlattice structure. High-resolution TEM (HRTEM) images showing individual Si nanocrystals were obtained at Scherzer defocus value. Phase separation of SiN was studied by X-ray photoelectron spectroscopy (XPS) performed on the x Appl. Sci. 2019, 9, 4725 3 of 9 ESCA branch of the NanoPES station at the Kurchatov synchrotron radiation source (National Research Center Kurchatov Institute) with Al K excitation (1486.61 eV). Sample surface sputtering was carried out by the 1.5-keV Ar ions bombardment in order to remove organic contaminations and native oxide. To study superlattice structure stability against thermal annealing, angular dependences of the intensity of X-ray reflectivity (XRR) were measured in step mode 2/! on X-ray di ractometer with a rotating anode (Rigaku SmartLab, Japan) using the characteristic Cu K 1 line (radiation energy 8.048 keV). In-plane grazing incidence X-ray di raction (XRD) studies were conducted on BM25B beamline at the European Synchrotron Radiation Facility (ESRF) using 22.025 keV radiation energy (incidence angle 0.1 ). Photoluminescence (PL) was excited by a He-Cd laser (h = 3.81 eV, 20 mW exc power) focused into a spot with 1 mm diameter. The PL signal was measured using a 500 mm focal length spectrometer (SOLAR MS 350, Belarus) equipped with a CCD camera with a sensitivity range of 200–1100 nm. The spectra were taken at room temperature and corrected for the system response, as well as for spectrometer dispersion by multiplying by  . For time-resolved PL measurements pulsed Nd:YAG laser excitation (h = 2.33 eV, pulse duration 34 ps, laser pulse fluence ~3 mJ/cm , repetition exc rate 10 Hz) was used. The PL signal was collected by means of intensified CCD (PI-MAX Gen III, Princeton Instruments) coupled to a 500 mm focal length imaging spectrograph (Princeton Instruments SpectraPro 2500i, USA). All PL spectra were detected within 1 s gate width taking various delays after excitation pulse onset in the range from 0 to 40 s. The PL decays for di erent emission energy (1.5 and 2 eV) are plotted using the PL intensity at a chosen photon energy values as a function of delay time. 3. Results and Discussion 3.1. TEM Results Figure 1 shows HRTEM and EFTEM images of SN1 and SN2 films annealed at T = 1150 C. ann Silicon nanocrystals can be seen as sets of lattice fringes (circled) in HRTEM images for both samples (see Figure 1a,d). Bright regions in EFTEM images (see Figure 1b,e) correspond to Si, while the dark ones correspond to Si N , thus multilayered structure is also seen. At the same time, it is obvious, 3 4 that size of Si NCs is not precisely restricted to SiN layer thickness. An e ect of coalescence of neighboring nanocrystals throughout Si N barrier layers can be observed, thus their shape starts to 3 4 be non-spherical (see Figure 1a,d). The mean diameter of Si NCs, which is defined as half sum of ellipse major axes in case of non-spherical particles, was estimated from HRTEM images. According to the statistical observations for more than 100 nanocrystals, their most frequently detected mean diameter was 2 nm, as shown in Figure 1c,f, which correlates well with SiN layer thickness (2 and 1.6 nm). At the same time, coalescent ellipsoidal-shaped nanocrystals with the mean diameter up to 10 and 9 nm were formed in case of SN1 and SN2 samples, respectively. Hence, the overall mean size of Si NCs was calculated to be 3.8  2.3 nm for SN1 and 3.4  1.7 nm for SN2 sample, which reflects the reduction of SiN layer thickness for the latter sample. Formation of nanocrystals with a non-spherical shape was also observed for annealed SiN monolayered films [21], while similar e ect of uncontrollable growth of Si NCs has been demonstrated recently for annealed SiN /SiO x 2 hetero-superlattices [12]. Appl. Sci. 2019, 9, 4725 4 of 9 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 9 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 9 Figure 1. (a) HRTEM image, (b) EFTEM image, and (c) size distribution of Si NCs for SiN /Si N Figure Figure 1. 1. ( a (a ) ) H H RTEM RTEM imag imag e, e, ( b (b ) ) EFTEM EFTEM ima ima ge, ge, and and (c (c ) ) si si ze ze di di st st ribution ribution of of Si Si NC NC s s for for Si Si N N x/Si x/Si 3N 3N 4 4(2/2. (2/2. x 25 3 25 4 (2/2.25 nm) SN1 sample; (d) HRTEM image, (e) EFTEM image, and (f) size distribution of Si NCs for nm) SN1 sample; (d) HRTEM image, (e) EFTEM image, and (f) size distribution of Si NCs for nm) SN1 sample; (d) HRTEM image, (e) EFTEM image, and (f) size distribution of Si NCs for SiN SiN /Si x/Si N 3N4 (1.6 (1.6/ /1.75 1.75 nm) nm) SN2 SN2 sample. sample. SiNx/Si3N4 (1.6/1.75 nm) SN2 sample. x 3 4 3.2. X-ray Studies 3.2. X-ray Studies 3.2. X-ray Studies As results from XRR measurements show in Figure 2, our samples annealed at 1150 C may As As rr esu esu lts lts fr fr oo m m XR XR R R me me asuremen asuremen ts ts sho sho w w in in Figure Figure 2, 2, our our s s ample ample ss anne anne aled aled at at 1150 1150 °° C C ma ma y y be be be considered as still multilayered films with some point distortions. Indeed, pronounced Bragg con con si si der der ed ed as as stil stil l l m m ul ul ti ti lay lay ered ered fil fil ms ms w w ith ith some some po po int int dis dis to to rtions rtions . . Ind Ind eed, eed, pro pro no no unced unced Br Br ag ag g g peaks peaks peaks (at around 1.1 and 1.5 degree for SN1 and SN2 sample, respectively) indicate the presence of (a (a t t ar ar ound ound 1. 1. 1 1 an an d d 1.5 1.5 de de gree gree fo fo r r SS N1 N1 and and SN2 SN2 ss am am ple, ple, re re spect spect ively) ively) in in dic dic aa te te th th e e presen presen ce ce of of superlattices, superlattices, whose whose periods periods ar ar eeestimated estimated as as 3.9 3.9 nm nm ((SN1) SN1) and and 3.1 3.1nm nm (SN2 (SN2), ), which which mmatches atches wel well l superlattices, whose periods are estimated as 3.9 nm (SN1) and 3.1 nm (SN2), which matches well those obtained by TEM measurements (4.25 and 3.35 nm, respectively). At the same time, relatively those those obtained obtaineby d by TEM TEM measur measuremen ementsts (4.25 (4.25 and and 3.35 3.35 nm, nm, rrespect espectively). ively). At Atth the e ssame ame time time, , rel relatively atively large width of Bragg peaks also points out at the multilayered structure imperfections, which are large larwidth ge width of Bragg of Brag peaks g peaks alsoalso points points out out at the at multilayer the multilayered ed structur struct e ure imperfections, imperfections which , whi ar ch e most are mo mo st st likel likel yy cc oal oal ee scent scent nn an an ocrysta ocrysta ls ls . . In In turn, turn, an an incr incr ea ea se se of of anne anne aling aling tem tem per per ature ature up up to to 1250 1250 °° C C likely coalescent nanocrystals. In turn, an increase of annealing temperature up to 1250 C resulted in result result ed ed in in de de struct struct ion ion oo f f SS L L struct struct u u re, re, as as evid evid ent ent fro fro m m Bra Bra gg gg pe pe aks aks d d isapp isapp ea ea rr ing ing for for bo bo th th S S N1 N1 and and destruction of SL structure, as evident from Bragg peaks disappearing for both SN1 and SN2 samples SN2 samples (see Figure 2). Such a temperature-dependent SL structure breakdown was observed in SN2 samples (see Figure 2). Such a temperature-dependent SL structure breakdown was observed in (see Figure 2). Such a temperature-dependent SL structure breakdown was observed in our previous our previous study of SiOx/SiO2 and SiOxNy/SiO2 multilayered films with 1.5 nm thick Si-rich layers, our previous study of SiOx/SiO2 and SiOxNy/SiO2 multilayered films with 1.5 nm thick Si-rich layers, study of SiO /SiO and SiO N /SiO multilayered films with 1.5 nm thick Si-rich layers, and explained x 2 x y 2 and explained by the gain in the Gibbs free energy for a mixed system as compared with initial and explained by the gain in the Gibbs free energy for a mixed system as compared with initial by the gain in the Gibbs free energy for a mixed system as compared with initial unmixed one [11]. unmixed unmixed one one [11]. [11]. At At th th e e ss am am e e time time , ,it it is is wort wort h h not not ing ing th th at at si si m m ilar ilar S S iN iN x/S x/S i3 iN 3N 4 4mult mult ilayered ilayered struc struc tures tures At the same time, it is worth noting that similar SiN /Si N multilayered structures even with ultrathin x 3 4 even even with with u u ltrathin ltrathin (1 (1 nn m) m) SS i3 iN 3N 4 4 bb arriers arriers rem rem ai ai nn stab stab le le aft aft er er high high tem tem pe pe ra ra ture ture tre tre atmen atmen t t as as (1 nm) Si N barriers remain stable after high temperature treatment as demonstrated recently [9,10]. 3 4 demonstrated recently [9,10]. Hence, we can assume that the most probable reason for the effect of Si demonstrated recently [9,10]. Hence, we can assume that the most probable reason for the effect of Si Hence, we can assume that the most probable reason for the e ect of Si NC coalescence demonstrated NC coalescence demonstrated in Figure 1 is the small enough thickness of SiNx layers (1.6–2 nm), NC coalescence demonstrated in Figure 1 is the small enough thickness of SiNx layers (1.6–2 nm), in Figure 1 is the small enough thickness of SiN layers (1.6–2 nm), leading to an appearance of distinct leading to an appearance of distinct intermixing regions. This trend is in full agreement with our leading to an appearance of distinct intermixing regions. This trend is in full agreement with our intermixing regions. This trend is in full agreement with our previous observations of SL structure previous observations of SL structure intermixing for SiOx and SiNx-based multilayers with ultrathin previous observations of SL structure intermixing for SiOx and SiNx-based multilayers with ultrathin intermixing for SiO and SiN -based multilayers with ultrathin Si-rich layers [11,22,23]. x x Si Si -- rich rich l ay lay er er s s [11,2 [11,2 2,23]. 2,23]. Figure Figure 2. 2. XRR XRR sc sc ans f ans f or ( or ( aa ) )S S N1 N1 and and (b (b ) SN ) SN 2 2 samples anne samples anne ale ale d d at at d d iff iff eren eren t t temper temper atur atur es es . . Figure 2. XRR scans for (a) SN1 and (b) SN2 samples annealed at di erent temperatures. Appl. Sci. 2019, 9, 4725 5 of 9 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 9 The formation of Si NC with an increase of annealing temperature was additionally studied by Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 9 means of XRD and XPS techniques. XRD scans of SN2 sample are shown in Figure 3. As can be The formation of Si NC with an increase of annealing temperature was additionally studied by seen, 900 C annealing temperature is obviously not enough to form nanocrystals, since XRD pattern means of XRD and XPS techniques. XRD scans of SN2 sample are shown in Figure 3. As can be seen, The formation of Si NC with an increase of annealing temperature was additionally studied by constitute of a broad band around the position of Si (111) di raction peak, which is a signature of 900 °C annealing temperature is obviously not enough to form nanocrystals, since XRD pattern means of XRD and XPS techniques. XRD scans of SN2 sample are shown in Figure 3. As can be seen, amorphous silicon nanoclusters. In contrast, for higher annealing temperatures relatively narrow constitute of a broad band around the position of Si (111) diffraction peak, which is a signature of 900 °C annealing temperature is obviously not enough to form nanocrystals, since XRD pattern peak is observed, which is a typical XRD pattern for Si NCs [9]. Remarkably, the peak becomes amorphous silicon nanoclusters. In contrast, for higher annealing temperatures relatively narrow constitute of a broad band around the position of Si (111) diffraction peak, which is a signature of narrpeak ower is with obser the ved, rise wh of ich annealing is a typical temperatur XRD pattern e fr f om or S 1150 i NCs C [9] to . Rema 1250rkC, ablconsequently y, the peak becom the es mean amorphous silicon nanoclusters. In contrast, for higher annealing temperatures relatively narrow size narrower of Si NCs with incr th eases e rise o by f an about nealing 30% temaccor perature ding fro to m the 1150 estimation °C to 1250 °C, by con means sequent of ly Scherr the mean er equation size peak is observed, which is a typical XRD pattern for Si NCs [9]. Remarkably, the peak becomes of Si NCs increases by about 30% according to the estimation by means of Scherrer equation (the (the same trend is reproduced for SN1 sample). This fact is consistent with SL structure breakdown narrower with the rise of annealing temperature from 1150 °C to 1250 °C, consequently the mean size same trend is reproduced for SN1 sample). This fact is consistent with SL structure breakdown at at 1250 C demonstrated by XRR, hence the formation of Si NCs with a larger mean size can be of Si NCs increases by about 30% according to the estimation by means of Scherrer equation (the 1250 °C demonstrated by XRR, hence the formation of Si NCs with a larger mean size can be expected, same trend is reproduced for SN1 sample). This fact is consistent with SL structure breakdown at expected, in a similar way as demonstrated previously in case of SiO N /SiO SL annealed at the same x y 2 in a similar way as demonstrated previously in case of SiOxNy/SiO2 SL annealed at the same 1250 °C demonstrated by XRR, hence the formation of Si NCs with a larger mean size can be expected, temperature [24]. The mean size of Si NCs grows together with the thickness of SiN layers: As results temperature [24]. The mean size of Si NCs grows together with the thickness of SiNx layers: As results in a similar way as demonstrated previously in case of SiOxNy/SiO2 SL annealed at the same from comparative XRD measurements (see inset in Figure 3), on the average Si NCs in SN1 sample are from comparative XRD measurements (see inset in Figure 3), on the average Si NCs in SN1 sample temperature [24]. The mean size of Si NCs grows together with the thickness of SiNx layers: As results about 14% larger than those in SN2 sample (4.1  0.1 nm vs. 3.6  0.1 nm), according to calculations are about 14% larger than those in SN2 sample (4.1 ± 0.1 nm vs. 3.6 ± 0.1 nm), according to calculations from comparative XRD measurements (see inset in Figure 3), on the average Si NCs in SN1 sample using Scherrer equation [9], which agrees well with the mean size estimations performed by analyzing using Scherrer equation [9], which agrees well with the mean size estimations performed by are about 14% larger than those in SN2 sample (4.1 ± 0.1 nm vs. 3.6 ± 0.1 nm), according to calculations HRTEM images. analyzing HRTEM images. using Scherrer equation [9], which agrees well with the mean size estimations performed by analyzing HRTEM images. Figure 3. XRD scans around the position of Si (111) diffraction peak (10.3° for photon energy used) Figure 3. XRD scans around the position of Si (111) di raction peak (10.3 for photon energy used) for for SN2 sample annealed at different remperatures. Inset: XRD scans of SN1 and SN2 samples SN2Figure sample 3.annealed XRD scans at arou di er nd ent the remperatur position ofes. Si Inset: (111) di XRD ffracti scans on peak of SN1 (10.3 and ° forSN2 photon samples energannealed y used) at ann  ealed at 1150 °C. 1150for C. SN2 sample annealed at different remperatures. Inset: XRD scans of SN1 and SN2 samples annealed at 1150 °C. XPS spectra of SN1 sample before and after annealing at the characteristic energy of Si 2p core XPS spectra of SN1 sample before and after annealing at the characteristic energy of Si 2p core levels levels are presented in Figure 4. The formation of Si nanoclusters (Tann = 900 °C) and Si NCs (Tann = XPS spectra of SN1 sample before and after annealing at the characteristic energy of Si 2p core are presented in Figure 4. The formation of Si nanoclusters (T = 900 C) and Si NCs (T = 1150 C) ann ann 1150 °C) can be attributed to an appearance of a shoulder at 99.7 eV, which corresponds to Si-Si bonds levels are presented in Figure 4. The formation of Si nanoclusters (Tann = 900 °C) and Si NCs (Tann = can be attributed to an appearance of a shoulder at 99.7 eV, which corresponds to Si-Si bonds (Si peak). (Si peak). In turn, evidence of a significant phase separation within SiNx layers at Tann = 1150 °C 1150 °C) can be attributed to an appearance of a shoulder at 99.7 eV, which corresponds to Si-Si bonds In turn, evidence of a significant phase separation within SiN layers at T = 1150 C follows out of x ann 4+ follows 0 out of an appearance of Si peak with a characteristic energy 101.7 eV, which corresponds to (Si peak). In turn, 4+ evidence of a significant phase separation within SiNx layers at Tann = 1150 °C an appearance of Si peak with a characteristic energy 101.7 eV, which corresponds to Si-N bonds in Si-N bonds in Si3N4 matrix [25]. 4+ follows out of an appearance of Si peak with a characteristic energy 101.7 eV, which corresponds to Si N matrix [25]. 3 4 Si-N bonds in Si3N4 matrix [25]. Figure 4. XPS spectra of SN1 sample (as prepared and annealed at different temper atures) measured in the energy region of Si 2p levels. Figure 4. XPS spectra of SN1 sample (as prepared and annealed at different temperatures) measured Figure 4. XPS spectra of SN1 sample (as prepared and annealed at di erent temperatures) measured in in the energy region of Si 2p levels. the energy region of Si 2p levels. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 9 3.3. Photoluminescence Measurements PL spectra of SN1 and SN2 samples annealed at 1150 °C are shown in Figure 5a. An observed PL peak shift may be associated with the size-dependent quantum confinement effect for excitons in Appl. Sci. 2019, 9, 4725 6 of 9 Si NCs. Spectra are characterized by maxima in the near IR range (885–940 nm), which is not typical for Si NCs in Si3N4 matrix, as mentioned above. Nevertheless, luminescence in this spectral region was also observed recently from similar SiNx/Si3N4 SLs with Si NCs, but produced by different 3.3. Photoluminescence Measurements methods [10,23]. Moreover, bare theory of quantum confinement predicts the reduction of an energy PL spectra of SN1 and SN2 samples annealed at 1150 C are shown in Figure 5a. An observed PL gap (i.e., red shift) for Si nanocrystals with the decrease of confinement barriers, i.e., when peak shift may be associated with the size-dependent quantum confinement e ect for excitons in Si nanocrystals are incorporated into Si3N4 matrix instead of SiO2 [4]. Thus, PL with lower peak energies NCs. Spectra are characterized by maxima in the near IR range (885–940 nm), which is not typical for Si from Si NCs in silicon nitride as compared with silicon oxide matrix (~1.7 eV at most) is indeed NCs in Si N matrix, as mentioned above. Nevertheless, luminescence in this spectral region was also justified. At the same time, the majority of experimental data on Si NC luminescence in Si3N4 reveals 3 4 observed recently from similar SiN /Si N SLs with Si NCs, but produced by di erent methods [10,23]. the PL maximum at relatively hix gh photon energies (above 1.6–1.7 eV), which is blue-shifted as 3 4 Moreover, bare theory of quantum confinement predicts the reduction of an energy gap (i.e., red shift) compared to a typical emission of Si NCs in SiO2 characterized by a maximum at about 1.2–1.7 eV for Si nanocrystals with the decrease of confinement barriers, i.e., when nanocrystals are incorporated [2,26]. From the theoretical point of view, this discrepancy is usually solved by empirical modification into of thSi e th N eoret matrix ical par instead amete ofrs SiO to achi [4].eve Thus, a bet PL ter with fit wi lower th thpeak e expener eriment giesal fr d om ataSi [4]. NCs On in th sil e icon other nitride hand, 3 4 2 as compared with silicon oxide matrix (~1.7 eV at most) is indeed justified. At the same time, the in silicon rich nitrides there are numerous matrix-related radiative defects having strong emission majority lines, wh of ich experimental makes it diff dat ica ult onto Si dis NC tinguish luminescence the cont inribution Si N reveals of Si the NCPL s, e maximum specially whe at relatively n short- 3 4 high photon energies (above 1.6–1.7 eV), which is blue-shifted as compared to a typical emission of Si wavelength excitation is used [13,15]. With respect to SL structures, matrix-related luminescence NCs signal in intensi SiO characterized ty may be re by duced a maximum by decreasing at about 1.2–1.7 the thickness eV [2,26o ].f Fr Si om 3N4the bar theor riers, etical as demo pointn of stra view ted, this discrepancy is usually solved by empirical modification of the theoretical parameters to achieve a recently by an example of SiOxNy/Si3N4 hetero-SLs [6]. Indeed, in general, luminescence from Si NCs better in SiNfit x/Si with 3N4 SLs the experimental with the peakdata energy at about [4]. On the other 1.7 eV or below hand, in silicon is obrich served, wh nitridesen Si ther3 eN ar 4 e bar numer riers has ous matrix-related radiative defects having strong emission lines, which makes it dicult to distinguish the thickness under 2 nm [9,10,23]. On the opposite, in case of thicker barriers or SiNx monolayered the film contribution s with Si NCs of Sith NCs, e poespecially sition of PL when peak short-wavelength is usually fouexcitation nd in the is spect used ra [13 l r,ange 15]. W above ith respect 1.7 eto V SL structures, matrix-related luminescence signal intensity may be reduced by decreasing the thickness [13,16,27]. Taking into account a number of radiative defects in SiNx, which emit at such high photon of energie Si N s,barriers, an attem as ptdemonstrated to attribute cert recently ain PL band by an s to example Si NC emi of SiO ssion Nin /Si SiN N x seem heter s o-SLs highly [6qu ]. Indeed, estionable in 3 4 x y 3 4 general, luminescence from Si NCs in SiN /Si N SLs with the peak energy at about 1.7 eV or below in this case, as emphasized elsewhere [17]. x 3 4 is observed, To confwhen irm that Si N exc barriers iton recom hasbination the thickness in Siunder NCs 2 is nm the[ 9 ori ,10 gi ,23 n ].of On obthe served opposite, PL for in our case S of L 3 4 thicker barriers or SiN monolayered films with Si NCs the position of PL peak is usually found in structures, time-resolved PL measurements were carried out. The results for SN1 sample are shown the in F spectral igure 5b range for above two d1.7 ifferent eV [13 ph ,16 ot ,27 on ]. energ Taking ies. into Bot account h curves a number are char ofacter radiative ized defects by a stiretc n SiN hed , which emit at such high photon energies, an attempt to attribute certain PL bands to Si NC emission in exponential decay: SiN seems highly questionable in this case, as emphasized elsewhere [17]. I = I0 exp [− (t/τ) ] (2) 1.0 SN1 0.8 SN2 0.6 0.4 0.2 0.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 Photon Energy (eV) Figure 5. (a) PL spectra of fabricated samples annealed at 1150 C; (b) PL decay curves for SN1 sample Figure 5. (a) PL spectra of fabricated samples annealed at 1150 °C; (b) PL decay curves for SN1 sample (points) and corresponding approximation using stretched exponential function (lines). (points) and corresponding approximation using stretched exponential function (lines). To confirm that exciton recombination in Si NCs is the origin of observed PL for our SL structures, with the time constant τ reducing from 1.84 to 0.35 μs upon increase of photon energy. Such reduction time-resolved PL measurements were carried out. The results for SN1 sample are shown in Figure 5b as well as relatively long PL lifetime (in a microsecond range) and non-exponential parameter β ~ for two di erent photon energies. Both curves are characterized by a stretched exponential decay: 0.5–0.6 are typical for quantum confined Si NCs-related PL [3,28]. At the same time, characteristic time constant seems to be several times smaller than that for SiOx-embedded Si NCs [3]. The latter I = I exp [ (t/) ] (2) may be explained by a more defective nature of SiNx matrix as compared to SiOx one [6]. Defects like with the time constant  reducing from 1.84 to 0.35 s upon increase of photon energy. Such reduction as well as relatively long PL lifetime (in a microsecond range) and non-exponential parameter ~ 0.5–0.6 are typical for quantum confined Si NCs-related PL [3,28]. At the same time, characteristic time Norm. PL Intensity (a.u.) Appl. Sci. 2019, 9, 4725 7 of 9 constant seems to be several times smaller than that for SiO -embedded Si NCs [3]. The latter may be explained by a more defective nature of SiN matrix as compared to SiO one [6]. Defects like dangling x x bonds act as luminescence quenching centers, increasing the nonradiative recombination rate and, hence, reducing the measured PL lifetime. 4. Conclusions In the present study, we fabricated Si NCs in Si N matrix by high temperature annealing of 3 4 SiN /Si N multilayered films deposited by low frequency discharge PECVD. Formation of Si NCs is x 3 4 confirmed by HRTEM, EFTEM, XRD and XPS measurements. It is shown that superlattice structure remains stable in general after 1150 C annealing with some point distortions such as coalescent nanocrystals. Size of these distinct ellipsoidal-shaped nanocrystals exceeds more than twice the period of multilayered structure. In turn, an increase of annealing temperature up to 1250 C led to superlattice breakdown and Si NC mean size growth, in full analogy with the previous observations of the similar e ect in SiO N /SiO multilayered films annealed under the same conditions. Fabricated samples x y 2 are characterized by a photoluminescence signal with the peak in near IR range (1.3–1.4 eV), while time-resolved PL measurements allowed us to ascribe this PL to a recombination of quantum confined excitons in Si NCs. In our opinion, the main reason for such low-energy luminescence, which is not typical for SiN -embedded Si NCs, is a negligible contribution from radiative defects in matrix due to small enough thickness of Si N barrier layers. Our results are found to agree well with the predictions 3 4 of quantum confinement theory, which states a redshift of confined energy upon reduction of potential barrier height by changing a wide band gap SiO matrix to a lower band gap Si N . 2 3 4 Author Contributions: Conceptualization, D.M.Z. and I.A.K.; investigation, Y.M.C., A.M.L, I.A.S. and O.A.S.; resources, A.A.P.; writing—original draft preparation, D.M.Z.; writing—review and editing, A.L.V. and A.M.L.; supervision, D.M.Z., S.N.Y. and A.L.V. Funding: This work was financially supported by the Ministry of Science and Higher Education of the Russian Federation, project No. RFMEFI58117X0026. Silicon nitride film deposition has been performed under the State Program 0066-2019-0003. D.M.Z. gratefully acknowledges the support from the Russian Foundation for Basic Research (grant No. 18-32-20217). Acknowledgments: We are grateful to Maria Vila Santos at the ESRF for providing assistance in using beamline BM25B. XRR and electron microscopy measurements have been carried out using the equipment of Resource Centers of the X-ray techniques and Probe and Electron Microscopy of NRC “Kurchatov Institute”. Conflicts of Interest: The authors declare no conflict of interest. References 1. Kovalev, D.; Heckler, H.; Polisski, G.; Koch, F. Optical Properties of Si Nanocrystals. Phys. Status Solidi B 1999, 215, 871–932. [CrossRef] 2. Takeoka, S.; Fujii, M.; Hayashi, S. Size-dependent photoluminescence from surface-oxidized Si nanocrystals in a weak confinement regime. Phys. Rev. B 2000, 62, 16820–16825. [CrossRef] 3. Linnros, J.; Lalic, N.; Galeckas, A.; Grivickas, V. Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO . J. Appl. Phys. 1999, 86, 6128–6134. [CrossRef] 4. Conibeer, G.; Green, M.; Cho, E.-C.; König, D.; Cho, Y.-H.; Fangsuwannarak, T.; Scardera, G.; Pink, E.; Huang, Y.; Puzzer, T.; et al. Silicon quantum dot nanostructures for tandem photovoltaic cells. Thin Solid Films 2008, 516, 6748–6756. [CrossRef] 5. Wan, Z.; Huang, S.; Green, M.; Conibeer, G. Residual stress study of silicon quantum dot in silicon carbide matrix by Raman measurement. Phys. Status Solidi C 2011, 8, 185–188. [CrossRef] 6. Zelenina, A.; Sarikov, A.; Gutsch, S.; Zakharov, N.; Werner, P.; Reichert, A.; Weiss, C.; Zacharias, M. Formation of size-controlled and luminescent Si nanocrystals from SiO N /Si N hetero-superlattices. J. Appl. Phys. x y 3 4 2015, 117, 175303. [CrossRef] Appl. Sci. 2019, 9, 4725 8 of 9 7. Heitmann, J.; Kovalev, D.; Schmidt, M.; Yi, L.X.; Scholz, R.; Eichhorn, F.; Zacharias, M. Synthesis and size control of Si nanocrystals by SiO/SiO superlattices and Er doping. MRS Proc. 2002, 737, F1.6. [CrossRef] 8. Volodin, V.A.; Arzhannikova, S.A.; Gismatulin, A.A.; Kamaev, G.N.; Antonenko, A.K.; Cherkova, S.G.; Cherkov, A.G.; Kochubei, S.A.; Popov, A.A.; Robert, S.; et al. Laser pulse crystallization and optical properties of Si/SiO and Si/Si N multilayer nano-heterostructures. Proc. SPIE 2012, 8700, 870008. [CrossRef] 2 3 4 9. So, Y.-H.; Huang, S.; Conibeer, G.; Green, M.A. Formation and photoluminescence of Si nanocrystals in controlled multilayer structure comprising of Si-rich nitride and ultrathin silicon nitride barrier layers. Thin Solid Films 2011, 519, 5408–5412. [CrossRef] 10. Chen, X.; Yang, W.; Yang, P.; Yuan, J.; Zhao, F.; Hao, J.; Tang, Y. Size-controlled Si quantum dots embedded in B-doped SiN /Si N superlatice for Si quantum dot solar cells. J. Mater. Sci. Mater. Electron. 2017, 28, x 3 4 1322–1327. [CrossRef] 11. Zhigunov, D.M.; Sarikov, A.; Chesnokov, Y.M.; Vasiliev, A.L.; Zakharov, N.; Kashkarov, P.K. Thickness and temperature depending intermixing of SiO /SiO and SiO N /SiO superlattices: Experimental observation x 2 x y 2 and thermodynamic modeling. Appl. Phys. Lett. 2016, 108, 223102. [CrossRef] 12. Zelenina, A.; Sarikov, A.; Zhigunov, D.M.; Weiss, C.; Zakharov, N.; Werner, P.; López-Conesa, L.; Estradé, S.; Peiró, F.; Dyakov, S.A.; et al. Silicon nanocrystals in SiN /SiO hetero-superlattices: The loss of size control after thermal annealing. J. Appl. Phys. 2014, 115, 244304. [CrossRef] 13. Sain, B.; Das, D. Tunable photoluminescence from nc-Si/a-SiN :H quantum dot thin films prepared by ICP-CVD. Phys. Chem. Chem. Phys. 2013, 15, 3881–3888. [CrossRef] [PubMed] 14. Lin, C.-H.; Uen, W.-Y.; Lan, S.-M.; Huang, Y.-C.; Liao, S.-M.; Li, Z.-Y.; Yang, T.-N.; Ku, C.-T.; Chen, M.-C.; Huang, Y.-H. Luminescence mechanisms of silicon-rich nitride films fabricated by atmospheric pressure chemical vapor deposition in N and H atmospheres. J. Appl. Phys. 2009, 105, 053107. [CrossRef] 2 2 15. Wang, M.; Li, D.; Yuan, Z.; Yang, D.; Que, D. Photoluminescence of Si-rich silicon nitride: Defect-related states and silicon nanoclusters. Appl. Phys. Lett. 2007, 90, 131903. [CrossRef] 16. Delachat, F.; Carrada, M.; Ferblantier, G.; Grob, J.-J.; Slaoui, A. Properties of silicon nanoparticles embedded in SiN deposited by microwave-PECVD. Nanotechnology 2009, 20, 415608. [CrossRef] [PubMed] 17. Kistner, J.; Chen, X.; Weng, Y.; Strunk, H.P.; Schubert, M.B.; Werner, J.H. Photoluminescence from silicon nitride—No quantum e ect. J. Appl. Phys. 2011, 110, 023520. [CrossRef] 18. Giorgis, F.; Vinegoni, C.; Pavesi, L. Optical absorption and photoluminescence properties of a-Si N :H 1x films deposited by plasma-enhanced CVD. Phys. Rev. B 2000, 61, 4693–4698. [CrossRef] 19. Korchagina, T.T.; Marin, D.V.; Volodin, V.A.; Popov, A.A.; Vergnat, M. Structure and optical properties of SiN :H films with Si nanoclusters produced by low-frequency plasma-enhanced chemical vapor deposition. Semiconductors 2009, 43, 1514–1520. [CrossRef] 20. Molinari, M.; Rinnert, H.; Vergnat, M. Evolution with the annealing treatments of the photoluminescence mechanisms in a-SiN : H alloys prepared by reactive evaporation. J. Appl. Phys. 2007, 101, 123532. [CrossRef] 21. Liao, W.; Zeng, X.; Wen, X.; Chen, X.; Wang, W. Annealing and excitation dependent photoluminescence of silicon rich silicon nitride films with silicon quantum dots. Vacuum 2015, 121, 147–151. [CrossRef] 22. Sarikov, A.; Zhigunov, D. Thermodynamic mechanism of the intermixing of multilayered structures in the SiO /SiO superlattices with nanometer thick layers. Mater. Today Commun. 2017, 13, 163–169. [CrossRef] 23. Zhigunov, D.M.; Martyshov, M.N.; Forsh, P.A.; Kamenskikh, I.A.; Yakunin, S.N.; Kashkarov, P.K. Structure-related current transport and photoluminescence in SiO N and SiN based superlattices with Si x y x nanocrystals. Phys. Status Solidi A Appl. Mater. 2017, 214, 1700040. [CrossRef] 24. Hartel, A. Structural and Optical Properties of PECVD Grown Silicon Nanocrystals Embedded in SiO N x y Matrix. Ph.D. Thesis, Albert-Ludwigs-Universität Freiburg im Breisgau, Freiburg, Germany, 5 August 2013. 25. Zhigunov, D.M.; Kamenskikh, I.A.; Lebedev, A.M.; Chumakov, R.G.; Logachev, Y.A.; Yakunin, S.N.; Kashkarov, P.K. X-ray reflectivity and photoelectron spectroscopy of superlattices with silicon nanocrystals. JETP Lett. 2017, 106, 517–521. [CrossRef] 26. Heitmann, J.; Müller, F.; Zacharias, M.; Gösele, U. Silicon nanocrystals: Size matters. Adv. Mater. 2005, 17, 795–803. [CrossRef] Appl. Sci. 2019, 9, 4725 9 of 9 27. Zelenina, A.; Dyakov, S.A.; Hiller, D.; Gutsch, S.; Trouillet, V.; Bruns, M.; Mirabella, S.; Löper, P.; López-Conesa, L.; López-Vidrier, J.; et al. Structural and optical properties of size controlled Si nanocrystals in Si N matrix: The nature of photoluminescence peak shift. J. Appl. Phys. 2013, 114, 184311. [CrossRef] 3 4 28. Timoshenko, V.Y.; Lisachenko, M.G.; Shalygina, O.A.; Kamenev, B.V.; Zhigunov, D.M.; Teterukov, S.A.; Kashkarov, P.K.; Heitmann, J.; Schmidt, M.; Zacharias, M. Comparative study of photoluminescence of undoped and erbium-doped size-controlled nanocrystalline Si/SiO multilayered structures. J. Appl. Phys. 2004, 96, 2254–2260. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Published: Nov 6, 2019

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