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Role of Strain-Induced Microscale Compositional Pulling on Optical Properties of High Al Content AlGaN Quantum Wells for Deep-Ultraviolet LED

Role of Strain-Induced Microscale Compositional Pulling on Optical Properties of High Al Content... A systematic study was carried out for strain-induced microscale compositional pulling effect on the structural and optical properties of high Al content AlGaN multiple quantum wells (MQWs). Investigations reveal that a large tensile strain is introduced during the epitaxial growth of AlGaN MQWs, due to the grain boundary formation, coalescence and growth. The presence of this tensile strain results in the microscale inhomogeneous compositional pulling and Ga segregation, which is further confirmed by the lower formation enthalpy of Ga atom than Al atom on AlGaN slab using first principle simulations. The strain-induced microscale compositional pulling leads to an asymmetrical feature of emission spectra and local variation in emission energy of AlGaN MQWs. Because of a stronger three-dimensional carrier localization, the area of Ga segregation shows a higher emission efficiency compared with the intrinsic area of MQWs, which is benefit for fabricating efficient AlGaN-based deep-ultraviolet light-emitting diode. Keywords: AlGaN, DUV, MQWs, Strain, Compositional pulling Introduction LEDs with high efficiency and low cost are highly and AlGaN-based deep-ultraviolet (DUV) light-emitting- immediately desired. diodes (LEDs) have various applications including steri- Numerous studies have focused on improving the lization and disinfection, water and air purification, performance of AlGaN-based DUV LEDs in the past medical diagnostics, high density optical recording, infor- decades [1, 2, 7–10]. However, at present, the efficiency mation sensing, bio-chemistry, and security [1–3]. Espe- and power of AlGaN-based DUV LEDs are still rela- cially after the outbreak of COVID-19 pandemic, DUV tively low compared with their visible counterparts that LEDs have rapidly received expanding academic and constructed by InGaN and GaN. Most of the reported industrial interests in the field of global public health [4, external quantum efficiency (EQE) of AlGaN-based DUV 5]. Moreover, according to the International Minamata LEDs is below 20% [11, 12]. Another problem about the Convention on Mercury, most of traditional DUV light AlGaN-based DUV LEDs is the commonly observed sources that contain toxic mercury were prohibited in multiple or asymmetrical emission, even in the high-per- 2020 [6]. Therefore, the solid-state AlGaN-based DUV formance devices [11, 13–15]. Similar phenomenons are generally observed in the InGaN alloy system and have been widely ascribed to the local Indium cluster induced *Correspondence: jinchaili@xmu.edu.cn; chycn@xmu.edu.cn by compositional segregation [16–19]. However, few Fujian Key Laboratory of Semiconductor Materials and Applications, CI studies concern the mechanism contributing to the mul- Center for OSED, College of Physical Science and Technology, Xiamen University, Xiamen 361005, China tiple or asymmetrical spectra of AlGaN DUV materials. © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Lu et al. Nanoscale Research Letters (2022) 17:13 Page 2 of 11 It is known that large strain would be introduced into The structure and crystalline quality of MQWs were the AlGaN materials and devices during the epitaxial characterized by a high-resolution X-ray diffractometer growth and cooling down processes due to the large lat- (HRXRD, PANalytical X’pert PRO MRD Holland) with tice and thermal mismatch between AlGaN and sapphire an X-ray wavelength of 0.154056 nm using Cu Kα radia- substrate [20]. The existence of large strain further influ - tion, and a scanning/transmission electron microscope ences the incorporation of Al and Ga atom into the lat- system (Thermo Scientific Talos F200X S/TEM). The sur - tice, which is known as compositional pulling effect. The face morphology of AlGaN MQWs was characterized by compositional pulling effect was commonly observed in SEM using Carl Zeiss FE-SEM SIGMA HD system, and the InGaAsP and InGaN alloy system [21, 22]. Previous AFM using the Seiko SPA400 system. CL spectra and studies find a bilayer nature of AlGaN film grown on the images were obtained by using an electron gun (Orsay sapphire due to the compositional pulling effect, in which Physics “Eclipse” FEB Column) to excite the MQWs and a compositional transition region and a compositional the emitted light was dispersed by a 320 mm focal-length uniform region were observed [23, 24]. However, most monochromator (Horiba Jobin Yvon iHR320) through an of these researches about compositional pulling are focus optical fiber. Raman spectra were collected by a Raman on the AlGaN epilayer or heterojunction [25]. Far less microscope (WITec alpha 300RA) with a 488 nm laser. attention has been paid to the influence on the properties To explore the influence of strain in the epitaxy of of the multiple quantum wells (MQWs), which is more AlGaN, the first principle simulations were carried out critical for the fabrication of DUV LEDs and commercial using the Vienna ab-initio simulation package (VASP) applications. in the frame work of DFT [26, 27]. The Perdew–Burke– In this paper, we conduct systematic study for the Ernzerhof (PBE) generalized gradient approximation strain-induced microscale compositional pulling effect (GGA) was used for the exchange–correlation interac- on the structural and optical properties of high Al con- tions among the electrons [28]. Ga-3d electrons were tent AlGaN MQWs combining characterization and treated as part of valence electrons. A 6 × 6 × 2 Monk- simulation. Real-time monitoring curve of metal organic horst–Pack grid of k points was used for sampling the vapor phase epitaxy (MOVPE) was analyzed to deter- Brillouin zone, and a cutoff energy of 520  eV was used mine the growth process of MQWs. Atomic force micro- to expand the electronic wavefunctions, which was suf- scope (AFM), X-ray diffraction (XRD) and transmission ficient for the plane wave basis to achieve energy con - electron microscope (TEM) were performed to char- vergence results. The geometry optimizations were acterize the structure and crystalline quality of MQWs. performed by using the conjugate gradient algorithm –3 –4 Microscopic Cathodoluminescence (CL) and Raman with convergence energy of 1 × 10   eV and 1 × 10   eV spectra were used to investigate the strain-induced for ions and electrons, respectively. An Al Ga N slab 0.5 0.5 microscale compositional pulling effect and correspond - model generated by 4a × 4b × 3c primitive cells was ing optical properties. To explore the influence of strain constructed for the simulation. A vacuum layer about in the epitaxy and Ga segregation of AlGaN, first princi - 25  Å was applied, which was determined to be suffi - ple simulations based on the frame work of density-func- ciently large to avoid interaction between neighboring tional theory (DFT) were also conducted. supercells. For the un-strained slab model, the lattice parameters and atomic coordinates were based on the bulk structure, which was optimized by relaxing all the degrees of freedom. And then, the tensile strain was Experimental and Simulation Methods applied based on the un-strained slab model. At the step The high Al content AlGaN MQWs was grown on c-plane of adsorption simulation, the Al Ga N under layer was sapphire substrate via MOVPE in a vertical Thomas Swan 0.5 0.5 fixed and the additional adsorbents were allowed to relax system (3 × 2 inch CCS Aixtron). The source precur - to minimize the total energy of the system. sors were trimethylaluminum (TMA), trimethylgallium (TMG) and ammonia (NH ). Silane (SiH ) was used as 3 4 Results and Discussion n-type dopant source and hydrogen (H ) as the carrier To determine the growth process of sample, real-time gas. Figure  1a shows the schematic diagram of the sam- monitoring curve recorded during the MOVPE growth ple structure. First, a thin AlN buffer layer was deposited was first discussed, as shown in Fig.  1b. In the initial step, on sapphire as the nucleation layer. Subsequently, a high- an approximately 20 nm buffer layer was grown for nucle - quality AlN layer was grown by the pulsed atomic layer ation and the reflected signal was enhanced gradually due epitaxy method (PALE), followed by AlN/Al Ga N 0.5 0.5 to the higher refractive index of AlN than the sapphire superlattice (SL) layer, undoped Al Ga N layer and 0.5 0.5 substrate. By introducing the PALE method, a remarkable Si-doped n-type Al Ga N layer. Finally, 10 periods 0.5 0.5 increased amplitude appeared after the growth of buffer Al Ga N/Al Ga N MQWs were grown. 0.4 0.6 0.5 0.5 Lu  et al. Nanoscale Research Letters (2022) 17:13 Page 3 of 11 Fig. 1 a Schematic of Al Ga N/Al Ga N MQWs epitaxial structure. b Real-time monitoring curve of the complete growth process for the 0.4 0.6 0.5 0.5 sample according to MOVPE. c SEM image, d AFM surface morphology, e cross-sectional TEM image, and f HRXRD (0002) ω/2θ scan of the AlGaN MQWs layer, which indicates the ending of the coalescence of barrier is about 3.0 nm and 10.1 nm, respectively, which the initial nucleating islands and the growth of high qual- agree well with the growth parameters and the design ity AlN layer with a gradually smoothed surface. Subse- expectation. A series of satellite peaks until fourth order quently, the intensity of interference oscillation further can be resolved from the HRXRD (0002) ω/2θ scan increased during the growth of SLs and undoped AlGaN. curve, as shown in Fig. 1e, further demonstrating the for- After that, the oscillation intensity is maintained steady mation of sharp interface and good periodicity of MQWs. and uniform until the end of the growth process for By analyzing the satellite peaks via Vegard’s law [29], the n-AlGaN and MQWs. This monitoring curve indicates period thickness of MQWs was further determined to that our epitaxial strategy leads to a two-dimensional be 12.1  nm, which is similar to that obtained from the layer-by-layer growth and a smooth surface of sample, analyses of HRTEM. Because only 10 periods of quantum which can be further confirmed by the AFM results. As wells were grown, the intensity of satellite peaks is rela- shown in Fig.  1c, well-defined steps and terraces can be tively low. The 0th satellite peak cannot be resolved from observed on the smooth surface, suggesting that the step- the diffraction peak of n-type Al Ga N layer due to the 0.5 0.5 flow growth mode has occurred. The root-mean-square close Al composition. (RMS) roughness value of the surface is just 0.8 nm, dem- Figure  2a shows the room temperature CL spectrum onstrating the atomically flat surface and good crystal of MQWs. Evidently, the spectrum shows an intense quality of MQWs. CL emission near 281  nm, with an obvious asymmetri- Figure 1d illustrates the cross-sectional HRTEM image cal feature. One dominant peak located at 280.5  nm of the MQWs structure. Evidently, the abrupt interfaces (4.432  eV, namely P1) and a shoulder peak centered at and good periodicity between the well and the barrier 286.4 nm (4.341 eV, namely P2) were identified by fitting layers can be clearly observed. The width of well and the CL spectrum with Gaussian function, as shown by Lu et al. Nanoscale Research Letters (2022) 17:13 Page 4 of 11 Fig. 2 a Experimental and Gaussian-fitted CL spectra of MQWs. Black arrows indicate the wavelength used for the mapping. b CL spectra collected in Position A, B and C, and monochromatic CL mapping images taken in the same area at 285 K using the wavelengths of c 276 nm, d 281 nm and e 286 nm the dash-dot lines in Fig.  2a. To figure out the origin of distribution, we collected the single CL spectrum in three these two emission peaks, spatially resolved monochro- different positions in Fig.  2d (Position A, B, and C), and matic CL images were conducted at 276 nm, 281 nm, and the spectra are shown in Fig.  2b. From the center (Posi- 286  nm, respectively (as black arrows indicate). Based tion A), to the edge (Position C) of the hexagonal area, on the monochromatic CL mapping images (Fig.  2c–e), the emission peaks redshift about 2.9 nm (45 meV) from one can find that the higher energy emission around 281.5  nm (4.416  eV) to 284.4  nm (4.371  eV), combining 276  nm, which is mainly composed of P1, is uniformly with an intensity enhancement about 2.22 times. These come from the whole area of MQWs. However, for the observations demonstrate that the asymmetrical spec- lower energy emission around 281 nm, which is contrib- trum comes from two different types of emission. P1 uted by both P1 and P2, the emission intensity shows an comes from the intrinsic emission of MQWs, and P2 may inhomogeneous microscale distribution. As indicated by have different origin. the white arrows, some areas with boundary-like feature Considering epitaxial growth mechanism of group- show much more intense emission. And the most intense III nitride materials, we speculate that the origin of P2 emission comes from the edge of a hexagonal area, as may be related to the structural evolution in the growth illustrated by the red dash line. With further increasing process. As we known, due to the large lattice and ther- the wavelength of CL mapping to 286  nm, the P1 com- mal mismatch between group-III nitride and sapphire ponent is reduced and P2 component becomes domi- substrate, it is difficult to grow nitride material directly nant. Hence, the intensity around the boundary-like area on the sapphire. A special two-step growth technology and the edge of hexagonal area is increased ulteriorly. is commonly used to grow high-quality nitride material To further understand this inhomogeneous microscale with smooth surface, as indicated in Fig.  3a–j. Firstly, a Lu  et al. Nanoscale Research Letters (2022) 17:13 Page 5 of 11 Fig. 3 a–e Optical micrographs of the MQWs sample with the focus points changed from the sapphire surface to the MQWs surface. One can observe that the grain size is similar with the emission pattern in the monochromatic CL mapping images. f–j The schematic of two-step epitaxial growth process for nitride material low temperature buffer layer is deposited on the sapphire wavelength of P2 suggests that it may be originated in for the nucleation, then the temperature is increased the segregation with higher Ga composition instead of above 1000 °C for the further nucleation and crystalliza- with higher Al composition. Moreover, the dislocations tion, followed by the coalescence of nucleation islands in AlGaN alloy are basically nonradiative [36, 37]. Again, through lateral growth. Finally, the growth mode changes this is not consistent with our CL observation, which from three-dimensional to quasi-two-dimensional shows an obvious enhancement of P2 emission. Based growth [30, 31]. By gradually changing the focus posi- on the above analysis, one can exclude the dislocation- tions from the sapphire layer to the MQWs layer, the typ- induced segregation for our AlGaN MQWs. ical optical micrographs representing two-step epitaxial Recently, some researches have demonstrated that processes were collected (Fig.  3a–e). From these optical during the growth of AlGaN layer, strong compres- micrographs, one can clearly observe the coalescence of sive strain can pull Ga atom from the AlGaN epilayer islands and grains, as well as the growth-mode transfor- to reduce the strain energy, leading to the growth of mation. A lot of mosaic grains were introduced into the AlGaN alloy with high Al content [23, 38]. As an anal- material during growth, which could be the origin of P2 ogy, if there exists a strong tensile strain field, Al atom emission because they share the similar boundary-like may also be pulled out from AlGaN, resulting in a or hexagonal-shape-like features. Moreover, one also can higher Ga composition. As illustrated in Fig. 4a–c, a lot observe that the grain size (about 10 μm) and density in of grain boundaries are produced in the epitaxial layer the opical micrographs are similar with the emission pat- due to the three-dimensional growth mode in the initial terns in the monochromatic CL mapping images, which stage. During the grain boundary formation and coales- further confirm the fundamental origin of P2 emission. cence, the step edges of two adjacent grains get closer Similar multiple or asymmetrical spectra and inhomo- with each other, and a short-range attractive interac- geneous emission distributions were commonly observed tion between two step edges becomes strong enough to in the InGaN alloy system. This phenomenon has been build up a tensile strain. With the continuous growth mainly ascribed to the local Indium cluster induced by of grains, the quasi-two-dimensional growth mode is compositional segregation [16–19], or the compositional established and the tensile strain field also reaches a pulling effect caused by mismatch strain [22, 32]. For steady-state value, finally extending through the epitax - AlGaN alloy system, the Al composition has been found ial layer [39, 40]. Therefore, based on these understand - to segregate around dislocation lines between the crystal ings of strain generation mechanism, one can draw grain boundaries due to the non-conforming orienta- a conclusion that the observed local variation in the tion of crystal columns including tilt and twist [33–35]. emission energy and intensity is most probably caused In our AlGaN-based MQWs sample, the longer emission by the tensile strain-induced microscale compositional Lu et al. Nanoscale Research Letters (2022) 17:13 Page 6 of 11 Fig. 4 a Schematic of the tensile strain field in the epitaxial layer. b, c Mechanism of the tensile strain generation and extension with the grain boundary formation and growth. d Optical image of the hexagonal area of MQWs. The seven points indicate the positions for taking Raman spectra. e Raman spectra, f Raman shift of E (GaN-like) mode and calculated stress of the seven positions pulling and Ga segregation. And one can determine to determine the biaxial stress σ of the AlGaN with the xx the composition variation according to the commonly following relationship [44]: known relationship between composition and band gap �ω −1 −1 of Al Ga N as follows [41]: σ = cm GPa , (2) x 1−x xx 4.3 E (x) = (1 − x)E (GaN) + xE (AlN) − bx(1 − x), g g g where ∆ω represents the phonon frequency shift with (1) respect to that in the unstressed AlGaN. According to where E (GaN) is the band gap of GaN (~ 3.5  eV), previous work [45], the phonon frequency of unstressed E (AlN) is the band gap of AlN (~ 6.1 eV) and b (~ 1  eV ) AlGaN with Al composition similar to our MQWs (0.47 is the bowing parameter. The corresponding Al composi - −1 average Al composition) is located in 593.3  cm , then tion of Position A and Position C is 0.4475 and 0.4295, the biaxial stress of Point 1 to 7 can be evaluated accord- and the difference value is about 0.0180 according to the ing to the above relation. As can be seen from Fig.  4e, f, emission energies. all the Raman frequencies of seven points are located in To verify our deduction, the microscale strain field dis - the lower frequency side compared with the un-strained tribution of a hexagonal area of MQWs was character- frequency, indicating that the sample experiences a ized by the Raman measurement. As shown in Fig. 4d, e, large tensile stress. Moreover, from Point 1 to Point 4 the Raman spectra of seven positions from the outside to (similar position with Position C in the CL images), the the center of a hexagonal area were collected. The spec - −1 Raman frequency shifted from 585.9 to 583.5  cm , and −1 tra show typical E (GaN-like) mode around 585  cm the tensile stress increased from 1.72 to 2.28 GPa. From that allowed by Raman selection rule [42]. As has been Point 4 to Point 7 (similar position with Position A in reported, the Raman shift of E (GaN-like) mode is sen- the CL images), the Raman frequency shifted back to sitive to the biaxial strain condition of AlGaN [43]. And −1 585.9  cm , and tensile stress decreased to the similar the E (GaN-like) phonon frequency shift Δω can be used value of Position 1. Raman results clearly demonstrate Lu  et al. Nanoscale Research Letters (2022) 17:13 Page 7 of 11 −1 that the boundary area between two grains experiences a small shift about 1.11  cm for E (GaN-like) mode a larger tensile stress compared with grain area itself. As based on previous reports [42, 45]. This value is smaller −1 discussed above, the additional tensile stress will further than the experimentally observed shift value (2.4  cm ), pull out Al atom from AlGaN and result in a higher Ga indicating that the observed shift of E (GaN-like) mode composition, finally leading to the redshift of emission is mostly contributed from the additional tensile stress. energy. In spite of a lot of works have reported the strain- It is worth noting that the tensile stress itself can intro- induced compositional pulling effect, there still lack of duce a reduction in band gap in AlGaN directly, and the a detail analysis about the inside mechanism. To further relation between the band gap E and the stress follows provide a clear understanding of this phenomenon, the the formula [46]: formation enthalpies of the Al/Ga atom adsorbed onto Al Ga N surface under different tensile strain levels 0.5 0.5 −4 −8 2 E = E (0) + 3.6 × 10 P − 1.76 × 10 P , (3) g g were evaluated by performing first principle total-energy calculation, using the following formula [47]: where E (0) is the band gap without stress, P is the stress value of material. According to this relationship, we can E = (E − E ) − �n µ f tot clean i i calculate the stress-induced band gap difference ΔE = (E − E ) − �n µ − �n µ , tot Ga Ga clean Al Al (4) between Point 4 and Point 7 (or Point 1) which is only about 0.2  meV. That is much smaller than the emission where E and E represent the total energies of the tot clean energy shift observed in the CL spectra (45 meV), mean- absorbed and clean Al Ga N surface, n denotes the 0.5 0.5 ing that the stress-induced band gap change is not the difference between the number of atoms in the absorbed direct and main factor of emission energy shift, which and clean Al Ga N surface, and µ is the chemical 0.5 0.5 should be the strain-induced microscale compositional potentials of Ga and Al atom, respectively. Figure  5a–c pulling. On the other hand, the variation in AlGaN com- shows the simulation model and results. Under the un- position also can lead to the shift of E (GaN-like) mode. strained condition, the formation enthalpy of adding Al It is difficult to distinguish the contributions from com - atom is smaller about 0.44  eV than adding Ga atom on position or stress for the Raman shift. However, we still the top of N ad-layer. This behavior indicates that Al atom can make a qualitative analysis based on the tendency of is much easier to be incorporated into the lattice, which E (GaN-like) along with composition [42, 45]. According is agreed with the nature of low surface migration of Al to the CL emission results, the corresponding Al com- atom [48]. When the tensile strain is applied into the position of center and edge of a hexagonal area is about AlGaN slab, the formation enthalpies of both Al and Ga 0.4475 and 0.4295, and the difference value is about atoms increase greatly due to the introduction of strain 0.0180. This little variation in composition can introduce energy. Most importantly, the enthalpy increment of Al Fig. 5 a Al Ga N slab model generated by 4a × 4b × 3c primitive cells for the simulation. b Schematic of Al Ga N slab model with Al atom 0.5 0.5 0.5 0.5 or Ga atom adsorbed onto the surface with N ad-layer. c Formation enthalpy of Al atom and Ga atom adsorbed onto the Al Ga N surface as a 0.5 0.5 function on the tensile strain level Lu et al. Nanoscale Research Letters (2022) 17:13 Page 8 of 11 atom (1.19 eV at 1% strain) is much larger than Ga atom of MQWs. Thus, we further measured the temperature- (0.33 eV at 1% strain), which leads to a higher formation dependent CL spectra of Position A and C in a range enthalpy about 0.42  eV of Al atom than Ga atom. These of 285  K to 93  K, as shown in Fig.  6a, b, respectively. In results confirm that under a tensile strain, the incorpora - order to compare the peak positions and shape, all CL tion of Ga atom into the lattice is more thermodynami- spectra were normalized. In Position A, there is only cally stable and favorable than Al atom. Therefore, the one peak, which can be attributed to P1, i.e., the intrinsic mechanism of strain-induced microscale compositional emission of MQWs. In Position C, there is also only one pulling effect and the origin of lower Al composition, i.e., peak at high temperature and can be attributed to P2, i.e., the Ga segregation, in MQWs are theoretically explained. the emission originated from compositional pulling and It is important to explore the role of strain-induced Ga segregation. Interestingly, with temperature lower microscale compositional pulling on the optical proper- than 157  K, another new peak appears in the longer ties that have great influence on the quantum efficiency wavelength side of P2. For convenience, we denote this Fig. 6 Normalized temperature-dependent CL spectra taken in the a Position A and b Position C. c Variation in P1, P2 and P3 energy with temperature, which were extracted by Gaussian fitting. d Temperature dependence of the integrated CL intensity of Position A and C. The inset shows the normalized 285/93 K CL integral intensity ratio of three positions Lu  et al. Nanoscale Research Letters (2022) 17:13 Page 9 of 11 new peak as P3. Figure  6c shows the variation in P1, P2 Position A. This is because that the strain-induced local and P3 peak energy as a function of temperature, which Ga segregation can provide stronger three-dimensional were extracted from spectra by Gaussian fitting. carrier localization to enhance the radiation recombi- As the temperature increases, P1 exhibits a redshift nation rate and suppress the outflow of carriers toward from 4.404  eV at 93  K to 4.400  eV at 115  K firstly, then nonradiative centers, finally boost the overall lumines - blue shifts to 4.425  eV at 192  K, finally redshifts again cence efficiency of MQWs [53, 54]. It has been reported to 4.415  eV at 285  K, shows a typical “S-shape” temper- that using the misoriented sapphire substrate can ature-dependent behavior. For P2, as the temperature promote the formation of step bunches in the AlGaN increases from 93 to 225  K, the energy blue shifts from MQWs, thus introduce Ga composition inhomogene- 4.368 to 4.389 eV, then redshifts to 4.379 eV at 285 K. The ity and locally varied potential minima, finally enhance “S-shape” behavior is commonly observed in the InGaN the DUV luminescence [55]. Enlighted by these works, epilayer [49], InGaN-based MQWs [50], and AlGaN we believe that it is also possible to further increase epilayer [51]. In general, this phenomenon is explained the proportion of the high efficiency P2 emission by by the carrier recombination dynamic with localization introducing more step bunches and grain coalescence effect [50]. For the first redshift process at low tempera - boundaries via using misoriented sapphire substrate. ture, the radiative recombination process is dominant and the carrier lifetime increases with temperature, hence the carriers have more opportunity to relax down Conclusions into the lower energy tail states, leading to the redshift In summary, a systematic study that combined charac- with increasing temperature. For the blueshift process at terization and simulation was carried out for the strain- higher temperature, the carrier lifetimes decrease greatly induced microscale compositional pulling effect on the and the dissociation rate increases, so the carriers recom- structural and optical properties of high Al content bine quickly before reaching the lower energy tail states, AlGaN MQWs. The CL spectrum of the MQWs dem - resulting in the observed blueshift. Further increas- onstrates a typical asymmetrical feature. Microscopic ing temperature to room temperature, the nonradiative CL mapping shows that the asymmetrical emission is recombination becomes dominant process and the car- caused by two different type of emissions, which have rier lifetimes decrease to a constant, then the blueshift inhomogeneous spatial distributions and diverse ori- behavior is reduced. Besides, due to the thermal distur- gins. Microscopic Raman spectra reveal that a large bance-induced delocalization, the localization carriers tensile strain is introduced during the epitaxial growth become the free carriers, which enhances the temper- of AlGaN MQWs, due to the grain boundary formation, ature-induced band gap shrinkage, finally the emission coalescence and growth. The presence of this tensile energy redshift again with increasing temperature. This strain results in the microscale inhomogeneous compo- analysis shows that the second inflection point of tem - sitional pulling and Ga segregation, which is further con- perature-dependent emission energy represents the delo- firmed by the lower formation enthalpy of Ga atom than calization of most carriers. For P2, the temperature of Al atom on AlGaN slab using first principle simulations. this inflection point between blueshift and final redshift This strain-induced microscale compositional pulling is higher than P1, which demonstrates that the stronger leads to the asymmetrical feature of emission spectra carrier localization centers exist around the Position C. and a local variation in the emission energy and intensity As for the P3 at low temperature, at present, we are still of AlGaN MQWs. Moreover, the area of Ga segregation not quite sure about its exact origin, but it may come shows a higher emission efficiency compared with the from the emission of bound exciton around the area of intrinsic area of MQWs due to stronger three-dimen- grain boundary [52]. sional carrier localization, which is benefit for fabricat - The PL or CL intensity ratio between the low and ing high-performance AlGaN-based DUV LEDs. high temperature is closely related to the internal quan- tum efficiency (IQE) and often used to estimate the Abbreviations efficiency. Figure  6d shows the temperature depend- MQWs: Multiple quantum wells; DUV: Deep-ultraviolet; LED: Light-emitting ence of the integrated CL intensity of Position A and C, diode; EQE: External quantum efficiency; MOVPE: Metal organic vapor phase epitaxy; AFM: Atomic force microscope; XRD: X-ray diffraction; TEM: Transmis- and the intensity is normalized with that at the lowest sion electron microscope; CL: Cathodoluminescence; DFT: Density-functional temperature. In the whole range of temperature, Posi- theory; TMA: Trimethylaluminum; TMG: Trimethylgallium; NH : Ammonia; SiH : 3 4 tion C exhibits a higher intensity ratio than Position A. Silane; H : Hydrogen; PALE: Pulsed atomic layer epitaxy method; SL: Superlat- tice; VASP: Vienna ab-initio simulation package; PBE: Perdew–Burke–Ernzerhof; The normalized 285/93  K ratio of Position C is about GGA : Generalized gradient approximation; RMS: Root-mean-square; IQE: 1.7 times higher compared with Position A, indicating Internal quantum efficiency. that Position C has a higher quantum efficiency than Lu et al. Nanoscale Research Letters (2022) 17:13 Page 10 of 11 Acknowledgements achieved by improving light-extraction efficiency. Appl Phys Express Not applicable. 10:031002. https:// doi. org/ 10. 7567/ APEX. 10. 031002 13. Wang T-Y, Tasi C-T, Lin C-F, Wuu D-S (2017) 85% internal quantum effi- Authors’ contributions ciency of 280-nm AlGaN multiple quantum wells by defect engineering. SL and ZL designed the experiments, worked on the characterization, made Sci Rep 7:14422. https:// doi. org/ 10. 1038/ s41598- 017- 14825-8 the simulations, and co-wrote the manuscript. WL, DL and SL grown the 14. Dong P, Yan J, Wang J et al (2013) 282-nm AlGaN-based deep ultraviolet MQWs sample. DC, KH, NG and YZ checked the experiment and simulation light-emitting diodes with improved performance on nano-patterned data. JL, HC and JK co-wrote the manuscript. All authors read and approved sapphire substrates. Appl Phys Lett 102:241113. https:// doi. org/ 10. the final manuscript.1063/1. 48122 37 15. Hirayama H, Norimatsu J, Noguchi N et al (2009) Milliwatt power 270 nm- Funding band AlGaN deep-UV LEDs fabricated on ELO-AlN templates. Phys Status This work was supported by NSFC (61874090, 62074133, 61974124, 62174141, Solidi 6:S474–S477. https:// doi. org/ 10. 1002/ pssc. 20088 0959 and 61874091) of China, Key scientific and technological program of Xiamen 16. Jinschek JR, Erni R, Gardner NF et al (2006) Local indium segregation and (3502Z20191016 and 3502ZCQ20191001). bang gap variations in high efficiency green light emitting InGaN/GaN diodes. 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Role of Strain-Induced Microscale Compositional Pulling on Optical Properties of High Al Content AlGaN Quantum Wells for Deep-Ultraviolet LED

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Abstract

A systematic study was carried out for strain-induced microscale compositional pulling effect on the structural and optical properties of high Al content AlGaN multiple quantum wells (MQWs). Investigations reveal that a large tensile strain is introduced during the epitaxial growth of AlGaN MQWs, due to the grain boundary formation, coalescence and growth. The presence of this tensile strain results in the microscale inhomogeneous compositional pulling and Ga segregation, which is further confirmed by the lower formation enthalpy of Ga atom than Al atom on AlGaN slab using first principle simulations. The strain-induced microscale compositional pulling leads to an asymmetrical feature of emission spectra and local variation in emission energy of AlGaN MQWs. Because of a stronger three-dimensional carrier localization, the area of Ga segregation shows a higher emission efficiency compared with the intrinsic area of MQWs, which is benefit for fabricating efficient AlGaN-based deep-ultraviolet light-emitting diode. Keywords: AlGaN, DUV, MQWs, Strain, Compositional pulling Introduction LEDs with high efficiency and low cost are highly and AlGaN-based deep-ultraviolet (DUV) light-emitting- immediately desired. diodes (LEDs) have various applications including steri- Numerous studies have focused on improving the lization and disinfection, water and air purification, performance of AlGaN-based DUV LEDs in the past medical diagnostics, high density optical recording, infor- decades [1, 2, 7–10]. However, at present, the efficiency mation sensing, bio-chemistry, and security [1–3]. Espe- and power of AlGaN-based DUV LEDs are still rela- cially after the outbreak of COVID-19 pandemic, DUV tively low compared with their visible counterparts that LEDs have rapidly received expanding academic and constructed by InGaN and GaN. Most of the reported industrial interests in the field of global public health [4, external quantum efficiency (EQE) of AlGaN-based DUV 5]. Moreover, according to the International Minamata LEDs is below 20% [11, 12]. Another problem about the Convention on Mercury, most of traditional DUV light AlGaN-based DUV LEDs is the commonly observed sources that contain toxic mercury were prohibited in multiple or asymmetrical emission, even in the high-per- 2020 [6]. Therefore, the solid-state AlGaN-based DUV formance devices [11, 13–15]. Similar phenomenons are generally observed in the InGaN alloy system and have been widely ascribed to the local Indium cluster induced *Correspondence: jinchaili@xmu.edu.cn; chycn@xmu.edu.cn by compositional segregation [16–19]. However, few Fujian Key Laboratory of Semiconductor Materials and Applications, CI studies concern the mechanism contributing to the mul- Center for OSED, College of Physical Science and Technology, Xiamen University, Xiamen 361005, China tiple or asymmetrical spectra of AlGaN DUV materials. © The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Lu et al. Nanoscale Research Letters (2022) 17:13 Page 2 of 11 It is known that large strain would be introduced into The structure and crystalline quality of MQWs were the AlGaN materials and devices during the epitaxial characterized by a high-resolution X-ray diffractometer growth and cooling down processes due to the large lat- (HRXRD, PANalytical X’pert PRO MRD Holland) with tice and thermal mismatch between AlGaN and sapphire an X-ray wavelength of 0.154056 nm using Cu Kα radia- substrate [20]. The existence of large strain further influ - tion, and a scanning/transmission electron microscope ences the incorporation of Al and Ga atom into the lat- system (Thermo Scientific Talos F200X S/TEM). The sur - tice, which is known as compositional pulling effect. The face morphology of AlGaN MQWs was characterized by compositional pulling effect was commonly observed in SEM using Carl Zeiss FE-SEM SIGMA HD system, and the InGaAsP and InGaN alloy system [21, 22]. Previous AFM using the Seiko SPA400 system. CL spectra and studies find a bilayer nature of AlGaN film grown on the images were obtained by using an electron gun (Orsay sapphire due to the compositional pulling effect, in which Physics “Eclipse” FEB Column) to excite the MQWs and a compositional transition region and a compositional the emitted light was dispersed by a 320 mm focal-length uniform region were observed [23, 24]. However, most monochromator (Horiba Jobin Yvon iHR320) through an of these researches about compositional pulling are focus optical fiber. Raman spectra were collected by a Raman on the AlGaN epilayer or heterojunction [25]. Far less microscope (WITec alpha 300RA) with a 488 nm laser. attention has been paid to the influence on the properties To explore the influence of strain in the epitaxy of of the multiple quantum wells (MQWs), which is more AlGaN, the first principle simulations were carried out critical for the fabrication of DUV LEDs and commercial using the Vienna ab-initio simulation package (VASP) applications. in the frame work of DFT [26, 27]. The Perdew–Burke– In this paper, we conduct systematic study for the Ernzerhof (PBE) generalized gradient approximation strain-induced microscale compositional pulling effect (GGA) was used for the exchange–correlation interac- on the structural and optical properties of high Al con- tions among the electrons [28]. Ga-3d electrons were tent AlGaN MQWs combining characterization and treated as part of valence electrons. A 6 × 6 × 2 Monk- simulation. Real-time monitoring curve of metal organic horst–Pack grid of k points was used for sampling the vapor phase epitaxy (MOVPE) was analyzed to deter- Brillouin zone, and a cutoff energy of 520  eV was used mine the growth process of MQWs. Atomic force micro- to expand the electronic wavefunctions, which was suf- scope (AFM), X-ray diffraction (XRD) and transmission ficient for the plane wave basis to achieve energy con - electron microscope (TEM) were performed to char- vergence results. The geometry optimizations were acterize the structure and crystalline quality of MQWs. performed by using the conjugate gradient algorithm –3 –4 Microscopic Cathodoluminescence (CL) and Raman with convergence energy of 1 × 10   eV and 1 × 10   eV spectra were used to investigate the strain-induced for ions and electrons, respectively. An Al Ga N slab 0.5 0.5 microscale compositional pulling effect and correspond - model generated by 4a × 4b × 3c primitive cells was ing optical properties. To explore the influence of strain constructed for the simulation. A vacuum layer about in the epitaxy and Ga segregation of AlGaN, first princi - 25  Å was applied, which was determined to be suffi - ple simulations based on the frame work of density-func- ciently large to avoid interaction between neighboring tional theory (DFT) were also conducted. supercells. For the un-strained slab model, the lattice parameters and atomic coordinates were based on the bulk structure, which was optimized by relaxing all the degrees of freedom. And then, the tensile strain was Experimental and Simulation Methods applied based on the un-strained slab model. At the step The high Al content AlGaN MQWs was grown on c-plane of adsorption simulation, the Al Ga N under layer was sapphire substrate via MOVPE in a vertical Thomas Swan 0.5 0.5 fixed and the additional adsorbents were allowed to relax system (3 × 2 inch CCS Aixtron). The source precur - to minimize the total energy of the system. sors were trimethylaluminum (TMA), trimethylgallium (TMG) and ammonia (NH ). Silane (SiH ) was used as 3 4 Results and Discussion n-type dopant source and hydrogen (H ) as the carrier To determine the growth process of sample, real-time gas. Figure  1a shows the schematic diagram of the sam- monitoring curve recorded during the MOVPE growth ple structure. First, a thin AlN buffer layer was deposited was first discussed, as shown in Fig.  1b. In the initial step, on sapphire as the nucleation layer. Subsequently, a high- an approximately 20 nm buffer layer was grown for nucle - quality AlN layer was grown by the pulsed atomic layer ation and the reflected signal was enhanced gradually due epitaxy method (PALE), followed by AlN/Al Ga N 0.5 0.5 to the higher refractive index of AlN than the sapphire superlattice (SL) layer, undoped Al Ga N layer and 0.5 0.5 substrate. By introducing the PALE method, a remarkable Si-doped n-type Al Ga N layer. Finally, 10 periods 0.5 0.5 increased amplitude appeared after the growth of buffer Al Ga N/Al Ga N MQWs were grown. 0.4 0.6 0.5 0.5 Lu  et al. Nanoscale Research Letters (2022) 17:13 Page 3 of 11 Fig. 1 a Schematic of Al Ga N/Al Ga N MQWs epitaxial structure. b Real-time monitoring curve of the complete growth process for the 0.4 0.6 0.5 0.5 sample according to MOVPE. c SEM image, d AFM surface morphology, e cross-sectional TEM image, and f HRXRD (0002) ω/2θ scan of the AlGaN MQWs layer, which indicates the ending of the coalescence of barrier is about 3.0 nm and 10.1 nm, respectively, which the initial nucleating islands and the growth of high qual- agree well with the growth parameters and the design ity AlN layer with a gradually smoothed surface. Subse- expectation. A series of satellite peaks until fourth order quently, the intensity of interference oscillation further can be resolved from the HRXRD (0002) ω/2θ scan increased during the growth of SLs and undoped AlGaN. curve, as shown in Fig. 1e, further demonstrating the for- After that, the oscillation intensity is maintained steady mation of sharp interface and good periodicity of MQWs. and uniform until the end of the growth process for By analyzing the satellite peaks via Vegard’s law [29], the n-AlGaN and MQWs. This monitoring curve indicates period thickness of MQWs was further determined to that our epitaxial strategy leads to a two-dimensional be 12.1  nm, which is similar to that obtained from the layer-by-layer growth and a smooth surface of sample, analyses of HRTEM. Because only 10 periods of quantum which can be further confirmed by the AFM results. As wells were grown, the intensity of satellite peaks is rela- shown in Fig.  1c, well-defined steps and terraces can be tively low. The 0th satellite peak cannot be resolved from observed on the smooth surface, suggesting that the step- the diffraction peak of n-type Al Ga N layer due to the 0.5 0.5 flow growth mode has occurred. The root-mean-square close Al composition. (RMS) roughness value of the surface is just 0.8 nm, dem- Figure  2a shows the room temperature CL spectrum onstrating the atomically flat surface and good crystal of MQWs. Evidently, the spectrum shows an intense quality of MQWs. CL emission near 281  nm, with an obvious asymmetri- Figure 1d illustrates the cross-sectional HRTEM image cal feature. One dominant peak located at 280.5  nm of the MQWs structure. Evidently, the abrupt interfaces (4.432  eV, namely P1) and a shoulder peak centered at and good periodicity between the well and the barrier 286.4 nm (4.341 eV, namely P2) were identified by fitting layers can be clearly observed. The width of well and the CL spectrum with Gaussian function, as shown by Lu et al. Nanoscale Research Letters (2022) 17:13 Page 4 of 11 Fig. 2 a Experimental and Gaussian-fitted CL spectra of MQWs. Black arrows indicate the wavelength used for the mapping. b CL spectra collected in Position A, B and C, and monochromatic CL mapping images taken in the same area at 285 K using the wavelengths of c 276 nm, d 281 nm and e 286 nm the dash-dot lines in Fig.  2a. To figure out the origin of distribution, we collected the single CL spectrum in three these two emission peaks, spatially resolved monochro- different positions in Fig.  2d (Position A, B, and C), and matic CL images were conducted at 276 nm, 281 nm, and the spectra are shown in Fig.  2b. From the center (Posi- 286  nm, respectively (as black arrows indicate). Based tion A), to the edge (Position C) of the hexagonal area, on the monochromatic CL mapping images (Fig.  2c–e), the emission peaks redshift about 2.9 nm (45 meV) from one can find that the higher energy emission around 281.5  nm (4.416  eV) to 284.4  nm (4.371  eV), combining 276  nm, which is mainly composed of P1, is uniformly with an intensity enhancement about 2.22 times. These come from the whole area of MQWs. However, for the observations demonstrate that the asymmetrical spec- lower energy emission around 281 nm, which is contrib- trum comes from two different types of emission. P1 uted by both P1 and P2, the emission intensity shows an comes from the intrinsic emission of MQWs, and P2 may inhomogeneous microscale distribution. As indicated by have different origin. the white arrows, some areas with boundary-like feature Considering epitaxial growth mechanism of group- show much more intense emission. And the most intense III nitride materials, we speculate that the origin of P2 emission comes from the edge of a hexagonal area, as may be related to the structural evolution in the growth illustrated by the red dash line. With further increasing process. As we known, due to the large lattice and ther- the wavelength of CL mapping to 286  nm, the P1 com- mal mismatch between group-III nitride and sapphire ponent is reduced and P2 component becomes domi- substrate, it is difficult to grow nitride material directly nant. Hence, the intensity around the boundary-like area on the sapphire. A special two-step growth technology and the edge of hexagonal area is increased ulteriorly. is commonly used to grow high-quality nitride material To further understand this inhomogeneous microscale with smooth surface, as indicated in Fig.  3a–j. Firstly, a Lu  et al. Nanoscale Research Letters (2022) 17:13 Page 5 of 11 Fig. 3 a–e Optical micrographs of the MQWs sample with the focus points changed from the sapphire surface to the MQWs surface. One can observe that the grain size is similar with the emission pattern in the monochromatic CL mapping images. f–j The schematic of two-step epitaxial growth process for nitride material low temperature buffer layer is deposited on the sapphire wavelength of P2 suggests that it may be originated in for the nucleation, then the temperature is increased the segregation with higher Ga composition instead of above 1000 °C for the further nucleation and crystalliza- with higher Al composition. Moreover, the dislocations tion, followed by the coalescence of nucleation islands in AlGaN alloy are basically nonradiative [36, 37]. Again, through lateral growth. Finally, the growth mode changes this is not consistent with our CL observation, which from three-dimensional to quasi-two-dimensional shows an obvious enhancement of P2 emission. Based growth [30, 31]. By gradually changing the focus posi- on the above analysis, one can exclude the dislocation- tions from the sapphire layer to the MQWs layer, the typ- induced segregation for our AlGaN MQWs. ical optical micrographs representing two-step epitaxial Recently, some researches have demonstrated that processes were collected (Fig.  3a–e). From these optical during the growth of AlGaN layer, strong compres- micrographs, one can clearly observe the coalescence of sive strain can pull Ga atom from the AlGaN epilayer islands and grains, as well as the growth-mode transfor- to reduce the strain energy, leading to the growth of mation. A lot of mosaic grains were introduced into the AlGaN alloy with high Al content [23, 38]. As an anal- material during growth, which could be the origin of P2 ogy, if there exists a strong tensile strain field, Al atom emission because they share the similar boundary-like may also be pulled out from AlGaN, resulting in a or hexagonal-shape-like features. Moreover, one also can higher Ga composition. As illustrated in Fig. 4a–c, a lot observe that the grain size (about 10 μm) and density in of grain boundaries are produced in the epitaxial layer the opical micrographs are similar with the emission pat- due to the three-dimensional growth mode in the initial terns in the monochromatic CL mapping images, which stage. During the grain boundary formation and coales- further confirm the fundamental origin of P2 emission. cence, the step edges of two adjacent grains get closer Similar multiple or asymmetrical spectra and inhomo- with each other, and a short-range attractive interac- geneous emission distributions were commonly observed tion between two step edges becomes strong enough to in the InGaN alloy system. This phenomenon has been build up a tensile strain. With the continuous growth mainly ascribed to the local Indium cluster induced by of grains, the quasi-two-dimensional growth mode is compositional segregation [16–19], or the compositional established and the tensile strain field also reaches a pulling effect caused by mismatch strain [22, 32]. For steady-state value, finally extending through the epitax - AlGaN alloy system, the Al composition has been found ial layer [39, 40]. Therefore, based on these understand - to segregate around dislocation lines between the crystal ings of strain generation mechanism, one can draw grain boundaries due to the non-conforming orienta- a conclusion that the observed local variation in the tion of crystal columns including tilt and twist [33–35]. emission energy and intensity is most probably caused In our AlGaN-based MQWs sample, the longer emission by the tensile strain-induced microscale compositional Lu et al. Nanoscale Research Letters (2022) 17:13 Page 6 of 11 Fig. 4 a Schematic of the tensile strain field in the epitaxial layer. b, c Mechanism of the tensile strain generation and extension with the grain boundary formation and growth. d Optical image of the hexagonal area of MQWs. The seven points indicate the positions for taking Raman spectra. e Raman spectra, f Raman shift of E (GaN-like) mode and calculated stress of the seven positions pulling and Ga segregation. And one can determine to determine the biaxial stress σ of the AlGaN with the xx the composition variation according to the commonly following relationship [44]: known relationship between composition and band gap �ω −1 −1 of Al Ga N as follows [41]: σ = cm GPa , (2) x 1−x xx 4.3 E (x) = (1 − x)E (GaN) + xE (AlN) − bx(1 − x), g g g where ∆ω represents the phonon frequency shift with (1) respect to that in the unstressed AlGaN. According to where E (GaN) is the band gap of GaN (~ 3.5  eV), previous work [45], the phonon frequency of unstressed E (AlN) is the band gap of AlN (~ 6.1 eV) and b (~ 1  eV ) AlGaN with Al composition similar to our MQWs (0.47 is the bowing parameter. The corresponding Al composi - −1 average Al composition) is located in 593.3  cm , then tion of Position A and Position C is 0.4475 and 0.4295, the biaxial stress of Point 1 to 7 can be evaluated accord- and the difference value is about 0.0180 according to the ing to the above relation. As can be seen from Fig.  4e, f, emission energies. all the Raman frequencies of seven points are located in To verify our deduction, the microscale strain field dis - the lower frequency side compared with the un-strained tribution of a hexagonal area of MQWs was character- frequency, indicating that the sample experiences a ized by the Raman measurement. As shown in Fig. 4d, e, large tensile stress. Moreover, from Point 1 to Point 4 the Raman spectra of seven positions from the outside to (similar position with Position C in the CL images), the the center of a hexagonal area were collected. The spec - −1 Raman frequency shifted from 585.9 to 583.5  cm , and −1 tra show typical E (GaN-like) mode around 585  cm the tensile stress increased from 1.72 to 2.28 GPa. From that allowed by Raman selection rule [42]. As has been Point 4 to Point 7 (similar position with Position A in reported, the Raman shift of E (GaN-like) mode is sen- the CL images), the Raman frequency shifted back to sitive to the biaxial strain condition of AlGaN [43]. And −1 585.9  cm , and tensile stress decreased to the similar the E (GaN-like) phonon frequency shift Δω can be used value of Position 1. Raman results clearly demonstrate Lu  et al. Nanoscale Research Letters (2022) 17:13 Page 7 of 11 −1 that the boundary area between two grains experiences a small shift about 1.11  cm for E (GaN-like) mode a larger tensile stress compared with grain area itself. As based on previous reports [42, 45]. This value is smaller −1 discussed above, the additional tensile stress will further than the experimentally observed shift value (2.4  cm ), pull out Al atom from AlGaN and result in a higher Ga indicating that the observed shift of E (GaN-like) mode composition, finally leading to the redshift of emission is mostly contributed from the additional tensile stress. energy. In spite of a lot of works have reported the strain- It is worth noting that the tensile stress itself can intro- induced compositional pulling effect, there still lack of duce a reduction in band gap in AlGaN directly, and the a detail analysis about the inside mechanism. To further relation between the band gap E and the stress follows provide a clear understanding of this phenomenon, the the formula [46]: formation enthalpies of the Al/Ga atom adsorbed onto Al Ga N surface under different tensile strain levels 0.5 0.5 −4 −8 2 E = E (0) + 3.6 × 10 P − 1.76 × 10 P , (3) g g were evaluated by performing first principle total-energy calculation, using the following formula [47]: where E (0) is the band gap without stress, P is the stress value of material. According to this relationship, we can E = (E − E ) − �n µ f tot clean i i calculate the stress-induced band gap difference ΔE = (E − E ) − �n µ − �n µ , tot Ga Ga clean Al Al (4) between Point 4 and Point 7 (or Point 1) which is only about 0.2  meV. That is much smaller than the emission where E and E represent the total energies of the tot clean energy shift observed in the CL spectra (45 meV), mean- absorbed and clean Al Ga N surface, n denotes the 0.5 0.5 ing that the stress-induced band gap change is not the difference between the number of atoms in the absorbed direct and main factor of emission energy shift, which and clean Al Ga N surface, and µ is the chemical 0.5 0.5 should be the strain-induced microscale compositional potentials of Ga and Al atom, respectively. Figure  5a–c pulling. On the other hand, the variation in AlGaN com- shows the simulation model and results. Under the un- position also can lead to the shift of E (GaN-like) mode. strained condition, the formation enthalpy of adding Al It is difficult to distinguish the contributions from com - atom is smaller about 0.44  eV than adding Ga atom on position or stress for the Raman shift. However, we still the top of N ad-layer. This behavior indicates that Al atom can make a qualitative analysis based on the tendency of is much easier to be incorporated into the lattice, which E (GaN-like) along with composition [42, 45]. According is agreed with the nature of low surface migration of Al to the CL emission results, the corresponding Al com- atom [48]. When the tensile strain is applied into the position of center and edge of a hexagonal area is about AlGaN slab, the formation enthalpies of both Al and Ga 0.4475 and 0.4295, and the difference value is about atoms increase greatly due to the introduction of strain 0.0180. This little variation in composition can introduce energy. Most importantly, the enthalpy increment of Al Fig. 5 a Al Ga N slab model generated by 4a × 4b × 3c primitive cells for the simulation. b Schematic of Al Ga N slab model with Al atom 0.5 0.5 0.5 0.5 or Ga atom adsorbed onto the surface with N ad-layer. c Formation enthalpy of Al atom and Ga atom adsorbed onto the Al Ga N surface as a 0.5 0.5 function on the tensile strain level Lu et al. Nanoscale Research Letters (2022) 17:13 Page 8 of 11 atom (1.19 eV at 1% strain) is much larger than Ga atom of MQWs. Thus, we further measured the temperature- (0.33 eV at 1% strain), which leads to a higher formation dependent CL spectra of Position A and C in a range enthalpy about 0.42  eV of Al atom than Ga atom. These of 285  K to 93  K, as shown in Fig.  6a, b, respectively. In results confirm that under a tensile strain, the incorpora - order to compare the peak positions and shape, all CL tion of Ga atom into the lattice is more thermodynami- spectra were normalized. In Position A, there is only cally stable and favorable than Al atom. Therefore, the one peak, which can be attributed to P1, i.e., the intrinsic mechanism of strain-induced microscale compositional emission of MQWs. In Position C, there is also only one pulling effect and the origin of lower Al composition, i.e., peak at high temperature and can be attributed to P2, i.e., the Ga segregation, in MQWs are theoretically explained. the emission originated from compositional pulling and It is important to explore the role of strain-induced Ga segregation. Interestingly, with temperature lower microscale compositional pulling on the optical proper- than 157  K, another new peak appears in the longer ties that have great influence on the quantum efficiency wavelength side of P2. For convenience, we denote this Fig. 6 Normalized temperature-dependent CL spectra taken in the a Position A and b Position C. c Variation in P1, P2 and P3 energy with temperature, which were extracted by Gaussian fitting. d Temperature dependence of the integrated CL intensity of Position A and C. The inset shows the normalized 285/93 K CL integral intensity ratio of three positions Lu  et al. Nanoscale Research Letters (2022) 17:13 Page 9 of 11 new peak as P3. Figure  6c shows the variation in P1, P2 Position A. This is because that the strain-induced local and P3 peak energy as a function of temperature, which Ga segregation can provide stronger three-dimensional were extracted from spectra by Gaussian fitting. carrier localization to enhance the radiation recombi- As the temperature increases, P1 exhibits a redshift nation rate and suppress the outflow of carriers toward from 4.404  eV at 93  K to 4.400  eV at 115  K firstly, then nonradiative centers, finally boost the overall lumines - blue shifts to 4.425  eV at 192  K, finally redshifts again cence efficiency of MQWs [53, 54]. It has been reported to 4.415  eV at 285  K, shows a typical “S-shape” temper- that using the misoriented sapphire substrate can ature-dependent behavior. For P2, as the temperature promote the formation of step bunches in the AlGaN increases from 93 to 225  K, the energy blue shifts from MQWs, thus introduce Ga composition inhomogene- 4.368 to 4.389 eV, then redshifts to 4.379 eV at 285 K. The ity and locally varied potential minima, finally enhance “S-shape” behavior is commonly observed in the InGaN the DUV luminescence [55]. Enlighted by these works, epilayer [49], InGaN-based MQWs [50], and AlGaN we believe that it is also possible to further increase epilayer [51]. In general, this phenomenon is explained the proportion of the high efficiency P2 emission by by the carrier recombination dynamic with localization introducing more step bunches and grain coalescence effect [50]. For the first redshift process at low tempera - boundaries via using misoriented sapphire substrate. ture, the radiative recombination process is dominant and the carrier lifetime increases with temperature, hence the carriers have more opportunity to relax down Conclusions into the lower energy tail states, leading to the redshift In summary, a systematic study that combined charac- with increasing temperature. For the blueshift process at terization and simulation was carried out for the strain- higher temperature, the carrier lifetimes decrease greatly induced microscale compositional pulling effect on the and the dissociation rate increases, so the carriers recom- structural and optical properties of high Al content bine quickly before reaching the lower energy tail states, AlGaN MQWs. The CL spectrum of the MQWs dem - resulting in the observed blueshift. Further increas- onstrates a typical asymmetrical feature. Microscopic ing temperature to room temperature, the nonradiative CL mapping shows that the asymmetrical emission is recombination becomes dominant process and the car- caused by two different type of emissions, which have rier lifetimes decrease to a constant, then the blueshift inhomogeneous spatial distributions and diverse ori- behavior is reduced. Besides, due to the thermal distur- gins. Microscopic Raman spectra reveal that a large bance-induced delocalization, the localization carriers tensile strain is introduced during the epitaxial growth become the free carriers, which enhances the temper- of AlGaN MQWs, due to the grain boundary formation, ature-induced band gap shrinkage, finally the emission coalescence and growth. The presence of this tensile energy redshift again with increasing temperature. This strain results in the microscale inhomogeneous compo- analysis shows that the second inflection point of tem - sitional pulling and Ga segregation, which is further con- perature-dependent emission energy represents the delo- firmed by the lower formation enthalpy of Ga atom than calization of most carriers. For P2, the temperature of Al atom on AlGaN slab using first principle simulations. this inflection point between blueshift and final redshift This strain-induced microscale compositional pulling is higher than P1, which demonstrates that the stronger leads to the asymmetrical feature of emission spectra carrier localization centers exist around the Position C. and a local variation in the emission energy and intensity As for the P3 at low temperature, at present, we are still of AlGaN MQWs. Moreover, the area of Ga segregation not quite sure about its exact origin, but it may come shows a higher emission efficiency compared with the from the emission of bound exciton around the area of intrinsic area of MQWs due to stronger three-dimen- grain boundary [52]. sional carrier localization, which is benefit for fabricat - The PL or CL intensity ratio between the low and ing high-performance AlGaN-based DUV LEDs. high temperature is closely related to the internal quan- tum efficiency (IQE) and often used to estimate the Abbreviations efficiency. Figure  6d shows the temperature depend- MQWs: Multiple quantum wells; DUV: Deep-ultraviolet; LED: Light-emitting ence of the integrated CL intensity of Position A and C, diode; EQE: External quantum efficiency; MOVPE: Metal organic vapor phase epitaxy; AFM: Atomic force microscope; XRD: X-ray diffraction; TEM: Transmis- and the intensity is normalized with that at the lowest sion electron microscope; CL: Cathodoluminescence; DFT: Density-functional temperature. In the whole range of temperature, Posi- theory; TMA: Trimethylaluminum; TMG: Trimethylgallium; NH : Ammonia; SiH : 3 4 tion C exhibits a higher intensity ratio than Position A. Silane; H : Hydrogen; PALE: Pulsed atomic layer epitaxy method; SL: Superlat- tice; VASP: Vienna ab-initio simulation package; PBE: Perdew–Burke–Ernzerhof; The normalized 285/93  K ratio of Position C is about GGA : Generalized gradient approximation; RMS: Root-mean-square; IQE: 1.7 times higher compared with Position A, indicating Internal quantum efficiency. that Position C has a higher quantum efficiency than Lu et al. Nanoscale Research Letters (2022) 17:13 Page 10 of 11 Acknowledgements achieved by improving light-extraction efficiency. Appl Phys Express Not applicable. 10:031002. https:// doi. org/ 10. 7567/ APEX. 10. 031002 13. Wang T-Y, Tasi C-T, Lin C-F, Wuu D-S (2017) 85% internal quantum effi- Authors’ contributions ciency of 280-nm AlGaN multiple quantum wells by defect engineering. SL and ZL designed the experiments, worked on the characterization, made Sci Rep 7:14422. https:// doi. org/ 10. 1038/ s41598- 017- 14825-8 the simulations, and co-wrote the manuscript. 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Journal

Nanoscale Research Letters – Springer Journals

Published: Jan 15, 2022

Keywords: AlGaN; DUV; MQWs; Strain; Compositional pulling

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