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Abstract
hv photonics Article Structural, Optical and Electrical Characterizations of Midwave Infrared Ga-Free Type-II InAs/InAsSb Superlattice Barrier Photodetector 1 , 2 2 , 3 2 2 4 U. Zavala-Moran , M. Bouschet , J. P. Perez , R. Alchaar , S. Bernhardt , 4 1 2 , I. Ribet-Mohamed , F. de Anda-Salazar and P. Christol * IICO, Univ. Autónoma de San Luis Potosí, Av. Karakorum 1470, San Luis Potosí CP 78210, Mexico; ulises.zavala-moran@ies.univ-montp2.fr (U.Z.-M.); francisco.deanda@uaslp.mx (F.d.A.-S.) IES, Univ. Montpellier, CNRS, F-34000 Montpellier, France; maxime.bouschet@ies.univ-montp2.fr (M.B.); perez@ies.univ-montp2.fr (J.P.P.); alchaar@ies.univ-montp2.fr (R.A.) LYNRED, BP 21, 38113 Veurey-Voroize, France ONERA, Chemin de la Hunière, F-91761 Palaiseau, France; sylvie.bernhardt@onera.fr (S.B.); isabelle.ribet@onera.fr (I.R.-M.) * Correspondence: christol@ies.univ-montp2.fr y This paper is an extended version from our paper, U. Zavala-Moran; R. Alchaar; J. P. Perez; J. B. Rodriguez; M. Bouschet; V. H. Compean; F. de Anda; P. Christol Antimonide-based Superlattice Infrared Barrier Photodetectors. In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 45–51; https://doi.org/10.5220/0009004900450051. Received: 24 August 2020; Accepted: 17 September 2020; Published: 18 September 2020 Abstract: In this paper, a full set of structural, optical and electrical characterizations performed on midwave infrared barrier detectors based on a Ga-free InAs/InAsSb type-II superlattice, grown by molecular beam epitaxy (MBE) on a GaSb substrate, are reported and analyzed. a Minority carrier lifetime value equal to 1 s at 80 K, carried out on dedicated structure showing photoluminescence peak position at 4.9 m, is extracted from a time resolved photoluminescence measurement. Dark 5 2 current density as low as 3.2 10 A/cm at 150 K is reported on the corresponding device exhibiting a 50% cut-o wavelength around 5 m. A performance analysis through normalized spectral response and dark current density-voltage characteristics was performed to determine both the operating bias and the di erent dark current regimes. Keywords: midwave infrared quantum detector; barrier structure; ga-free type-II superlattice 1. Introduction Today, high performance, high speed cooled photodetectors operating in the midwave infrared (MWIR) spectral domain between 3 m and 5 m are of great interest for specific applications such as cancer diagnosis, gas analysis, astronomy, search and rescue in harsh environments and night vision. The maximum operating temperature of a semiconductor IR photodetector is usually determined by its dark current, which increases exponentially with temperature. Therefore, in order to maintain a high signal-to-noise ratio (or a low dark current value) of the focal plane arrays (FPAs) of IR photodetectors, it is necessary to reduce the operating temperature down to cryogenic temperatures (typically around 80–100 K), involving the implementation of a cryocooler inducing significant restrictions in terms of weight, compactness and energy autonomy. Taking into account these constraints is essential to generate a new class of applications using high performance handheld thermal imagers in embedded systems, for future civil and defense applications. Photonics 2020, 7, 76; doi:10.3390/photonics7030076 www.mdpi.com/journal/photonics Photonics 2020, 7, 76 2 of 14 Consequently, improving the temperature operation, without damaging the performance of the detectors, is currently one of the main challenges investigated by the cooled IR detector community in order to satisfy Size, Weight and Power (SWaP) criteria [1,2]. The InSb (Indium Antimonide) and MCT (Mercury Cadmium Telluride) photodetectors are the leading technologies in the MWIR domain where the presence of a strong CO absorption line at 4.25 m splits the MWIR window into two spectral domains usually called MWIR blue-band and MWIR red-band. The commercial InSb FPAs operate at 80–90 K in the full MWIR spectral domain with a cut-o wavelength at 5.4 m [3] while MCT FPAs can reach operation temperatures up to 120 and 150 K with 5 and 4.2 m cut-o wavelengths, respectively [4]. At the end of 2000 s, InAsSb XBn photodetector structures were proposed [5,6] and impressive results were obtained allowing typical operation temperatures as high as 150 K with dark current 7 2 density as low as 3 10 A/cm [7]. With a cuto wavelength around 4.2 m at 150 K, such an IR system currently commercially available [8], covers only the MWIR blue-band. One of the main advantages of Sb-based type-II superlattice (T2SL) structures is the possibility to adjust the bandgap by tailoring the layer thicknesses and the period composition, while also ensuring the lattice matching with a GaSb substrate. To extend the cut-o wavelength up to 5 m, one can consider an InAs/GaSb T2SL [9,10]. Even if this new technology begins to be commercially available [11], Ga-containing T2SL devices su er from a low minority carrier lifetime (100 ns in the MWIR) due to the presence of Ga-related native defects [12]. As a consequence, such T2SL detectors exhibit temperature operation lower than 110 K for a 5 m cut-o [13]. An alternative to this technology could be the Ga-free InAs/InAs Sb T2SL structures [14]. 1-x Indeed, an impressive minority carrier lifetime value higher than 3 s at 80 K in the MWIR domain has been measured [15] and results on Ga-free T2SL detectors have recently been reported by research groups [16–23]. Although this new kind of detector technology operating in the full MWIR domain has recently reached significant performances, it still requires improvements in terms of dark current density values, turn on voltage, quantum eciency and operation temperature. In order to complete results previously reported [24], this paper describes the structural, optical and electrical measurements allowing the assess of the performance of the MWIR Ga-free T2SL detector, fabricated by molecular beam epitaxy (MBE) on a GaSb substrate. 2. Materials and Methods In this section, fundamental concepts dealing both with molecular beam epitaxy (MBE) of a T2SL structure on a GaSb substrate and design of MWIR Ga-free T2SL structure and XBn unipolar barrier detector are detailed. First of all, a choice in terms of superlattice (SL) period (p) and antimony composition (x) has to be made to address MWIR broadband domain but also to optimize absorption coecient (proportional to wave functions overlap). InAs/InAsSb SL can be strained balanced on GaSb by setting the average lattice parameter of one period of the SL equal to the lattice parameter of GaSb. Consequently, InAsSb and InAs layer thicknesses (t and t ) as functions of the antimony composition (x) and SL InAsSb InAs period (p) can be calculated by using Equations (1) and (2): t = ((a a )/(a a )) (p/x) (1) InAsSb GaSb InAs InSb InAs t + t = p (2) InAs InAsSb where a = 6.0954 A; a = 6.0584 A and a = 6.4794 A stands for the lattice parameters of the GaSb InAs InSb binary compounds. The MBE growth conditions of strained balanced SL structure were studied considering dedicated samples (Figure 1a). Such samples consist of a 3 m thick InAs/InAsSb SL layers sandwiched between two GaSb layers that provide carriers barrier confinement. To assess optimal T2SL growth conditions, structural and optical measurements were performed. Structural characterizations were Photonics 2020, 7, 76 3 of 14 Photonics 2020, 7, x FOR PEER REVIEW 3 of 15 made of high-resolution x-ray di raction (HRXRD) scans and atomic force microscopy (AFM) was used to investigate the surface morphology. To perform optical characterizations, the samples were characterizations, the samples were placed in a cryostat that allows a precise control of the placed in a cryostat that allows a precise control of the temperature from 10 K to 300 K. To obtain temperature from 10 K to 300 K. To obtain photoluminescence (PL) spectra and then to evaluate the photoluminescence (PL) spectra and then to evaluate the band gap energy of the T2SL structure, band gap energy of the T2SL structure, the samples were optically excited at 50 W/cm with a 784 nm the samples were optically excited at 50 W/cm with a 784 nm laser diode modulated at 133 kHz. laser diode modulated at 133 kHz. The luminescence signal was analyzed with a Nexus 870 FT-IR The luminescence signal was analyzed with a Nexus 870 FT-IR system equipped with an MCT detector system equipped with an MCT detector (12 µ m cut-off wavelength). The whole optical path was (12 m cut-o wavelength). The whole optical path was under atmospheric conditions. In addition, under atmospheric conditions. In addition, minority carrier lifetime for InAs/InAsSb SL was minority carrier lifetime for InAs/InAsSb SL was extracted from time resolved photoluminescence extracted from time resolved photoluminescence (TRPL) measurements. For such measurements, the (TRPL) measurements. For such measurements, the samples were excited by a 1.55 m laser pulse samples were excited by a 1.55 µ m laser pulse (10 ns) at a repetition rate of 149.8 kHz, used to generate (10 ns) at a repetition rate of 149.8 kHz, used to generate excess carriers. The power of the laser was excess carriers. The power of the laser was tunable, and measurements were done at different laser tunable, and measurements were done at di erent laser pulse fluences. The photoluminescence signal pulse fluences. The photoluminescence signal was detected with a fast HgCdTe photodiode (4 ns was detected with a fast HgCdTe photodiode (4 ns temporal resolution, cut-o wavelength 8 m) and temporal resolution, cut-off wavelength 8 µ m) and analyzed with a Yokogawa oscilloscope. analyzed with a Yokogawa oscilloscope. InAs (1.4nm)/InAs Sb (4.1 nm) T2SL 0.65 0.35 InAs C1 electron miniband VH1 heavy hole miniband VL1 light hole miniband I<f If >I =62% InAsSb C1 VH1 VH1 C1 (a) (b) Figure 1. (a) Schematic cross section of the type-II superlattice (T2SL) structure dedicated to structural Figure 1. (a) Schematic cross section of the type-II superlattice (T2SL) structure dedicated to structural and optical measurements. (b) Schematic band diagram and first electron and hole minibands of and optical measurements. (b) Schematic band diagram and first electron and hole minibands of Ga- Ga-free (a) T2SL structures. On the lower part, the fundamental electron (C1) and heavy hole (VH1) free (a) T2SL structures. On the lower part, the fundamental electron (C1) and heavy hole (VH1) presence probability densities are reported. presence probability densities are reported. With a type II-b InAs/InAs Sb band o set [25] where electrons are confined in the binary layer 1-x With a type II-b InAs/InAs1-xSbx band offset [25] where electrons are confined in the binary layer (InAs) while holes are confined in the alloy one (InAsSb), the quantized miniband energies of the (InAs) while holes are confined in the alloy one (InAsSb), the quantized miniband energies of the strain balanced InAs/InAs Sb T2SL have been calculated with nextnano commercial software [17]. 1-x strain balanced InAs/InAs1-xSbx T2SL have been calculated with nextnano commercial software [17]. From these data, it appears that x = 0.35 and 5 p (nm) 6 are required to reach, at 150 K, a cut-o From these data, it appears that x = 0.35 and 5 ≤ p (nm) ≤ 6 are required to reach, at 150 K, a cut-off wavelength ( ) around 5 m together with a wave function overlap between 55% and 66% (Figure 1b). co wavelength (λco) around 5 µ m together with a wave function overlap between 55% and 66% (Figure An important feature of the Ga-free T2SL material structure is the possibility of implementing it in 1b). a nBn [18] or pBn [21] MWIR barrier detector structure as absorbing layer (AL) associated with a high An important feature of the Ga-free T2SL material structure is the possibility of implementing it band gap barrier material. The main objective of the barrier structure is to reduce the contribution of in a nBn [18] or pBn [21] MWIR barrier detector structure as absorbing layer (AL) associated with a the Shockley-Read-Hall (SRH) recombination current, thus the generation–recombination (GR) current high band gap barrier material. The main objective of the barrier structure is to reduce the to the dark current of the detector. For that, it is required to control the electric field zone by confining contribution of the Shockley-Read-Hall (SRH) recombination current, thus the generation– it in the barrier material instead of in the absorbing zone structure. Therefore, SRH processes occur recombination (GR) current to the dark current of the detector. For that, it is required to control the in the high band gap material, instead of the IR absorbing layer. In addition, the barrier layer (BL) electric field zone by confining it in the barrier material instead of in the absorbing zone structure. plays a similar role as the space charge zone in the pn structure by blocking the majority carriers and Therefore, SRH processes occur in the high band gap material, instead of the IR absorbing layer. In allowing the transfer of the minority ones. Consequently, this kind of device is called a bariode. When addition, the barrier layer (BL) plays a similar role as the space charge zone in the pn structure by the bariode is correctly designed, GR dark current is eliminated and the dark current is di usion blocking the majority carriers and allowing the transfer of the minority ones. Consequently, this kind of device is called a bariode. When the bariode is correctly designed, GR dark current is eliminated and the dark current is diffusion current (Jdiff) limited whatever the operation temperature, improving the performances of the detector compared to a pn junction photodiode [5,6]. The diffusion current density is given by Equation (3): Jdiff(T) ∝ q × (ni /Nd) × (L/τdiff) (3) Photonics 2020, 7, 76 4 of 14 current (J ) limited whatever the operation temperature, improving the performances of the detector di compared to a pn junction photodiode [5,6]. The di usion current density is given by Equation (3): Photonics 2020, 7, x FOR PEER REVIEW 4 of 15 J (T)/ q (n /N ) (L/ ) (3) di i d di where q stands for the charge of the electron, ni for the intrinsic carrier density, Nd for the doping where q stands for the charge of the electron, n for the intrinsic carrier density, N for the doping level i d level of the T2SL AL, L for the thickness of the AL and τdiff for the minority carrier lifetime. of the T2SL AL, L for the thickness of the AL and for the minority carrier lifetime. di An AlAs0.09Sb0.91 alloy with a lattice parameter equal to the one of the GaSb substrate (Figure 2), An AlAs Sb alloy with a lattice parameter equal to the one of the GaSb substrate (Figure 2), 0.09 0.91 may a priori be considered as a good candidate for barrier layer material when combined with a Ga- may a priori be considered as a good candidate for barrier layer material when combined with a free InAs/InAsSb T2SL absorbing layer. Indeed, due to the intrinsic n-type doping of the T2SL Ga-free InAs/InAsSb T2SL absorbing layer. Indeed, due to the intrinsic n-type doping of the T2SL absorber, holes are the minority carriers and AlAsSb barrier layer will block majority carriers, absorber, holes are the minority carriers and AlAsSb barrier layer will block majority carriers, meaning meaning the electrons. the electrons. Figure 2. Bandgap energy vs. lattice constant for some III-V semiconductors showing the InAs/InAsSb Figure 2. Bandgap energy vs. lattice constant for some III-V semiconductors showing the InAs/InAsSb T2SL structure and the AlAsSb ternary alloy lattice-matched to the GaSb substrate. T2SL structure and the AlAsSb ternary alloy lattice-matched to the GaSb substrate. Figure 3a shows the stacking of the Ga-free T2SL nBn unipolar barrier photodetector. From bottom Figure 3a shows the stacking of the Ga-free T2SL nBn unipolar barrier photodetector. From to top, the structure consists of a 400 nm Te-doped (n-type) GaSb bu er layer, which is followed by a bottom to top, the structure consists of a 400 nm Te-doped (n-type) GaSb buffer layer, which is 100 nm thick n-type doped InAs/InAs Sb T2SL and by non-intentionally doped (nid) 3 m thick 0.65 0.35 followed by a 100 nm thick n-type doped InAs/InAs0.65Sb0.35 T2SL and by non-intentionally doped absorption layer (AL) made of the same T2SL structure. A barrier layer (BL) is then made from 100 nm (nid) 3 µ m thick absorption layer (AL) made of the same T2SL structure. A barrier layer (BL) is then nid AlAs Sb and finally, the contact layer (CL) of the structure is composed of an 80 nm thick 0.09 0.91 made from 100 nm nid AlAs0.09Sb0.91 and finally, the contact layer (CL) of the structure is composed n-type doped T2SL. The AL and BL are undoped, and the residual dopings are expected to be n-type of an 80 nm thick n-type doped T2SL. The AL and BL are undoped, and the residual dopings are 16 3 16 3 and p-type at 10 cm and 5.10 cm , respectively. 16 −3 16 −3 expected to be n-type and p-type at 10 cm and 5.10 cm , respectively. Figure 3b, displaying the simulated band diagram of the Ga-free T2SL nBn unipolar barrier Figure 3b, displaying the simulated band diagram of the Ga-free T2SL nBn unipolar barrier detector at T = 150 K and V = 0 V, reveals both an accumulation layer in the AL (due to the polarities of detector at T = 150 K and V = 0 V, reveals both an accumulation layer in the AL (due to the polarities AL and BL) and a low valence band o set between AL and BL which should not impede the transit of of AL and BL) and a low valence band offset between AL and BL which should not impede the transit holes from the AL to the CL. of holes from the AL to the CL. Such a detector structure was studied by electrical and electro-optical measurements. The T2SL devices were placed in a probe station in order to perform capacitance-voltage (C-V) measurements at Ga-free T2SL nBn MWIR detector a frequency f = 1 MHz and dark current density-voltage (J-V) measurements (under 0 degree field of view) for di erent operating temperatures. For 3that, a KEITHLEY 6517A Electrometer was used Barrier to both apply the bias voltage and read the current density (ratio of current and area of the device) Layer T = 150K delivered by the device. In addition, the samples 2 were wire bounded onto a pin LCC, placed in a LN2-cooled cryostat and the non-calibrated spectral photoresponse (PR) of the detector V was = 0V measured bias using a FTIR spectrometer. Absorbing Layer hh Contact Layer -1 0.0 0.2 0.4 3.0 3.2 Position (µm) (b) (a) Energy (eV) Photonics 2020, 7, x FOR PEER REVIEW 4 of 15 where q stands for the charge of the electron, ni for the intrinsic carrier density, Nd for the doping level of the T2SL AL, L for the thickness of the AL and τdiff for the minority carrier lifetime. An AlAs0.09Sb0.91 alloy with a lattice parameter equal to the one of the GaSb substrate (Figure 2), may a priori be considered as a good candidate for barrier layer material when combined with a Ga- free InAs/InAsSb T2SL absorbing layer. Indeed, due to the intrinsic n-type doping of the T2SL absorber, holes are the minority carriers and AlAsSb barrier layer will block majority carriers, meaning the electrons. Figure 2. Bandgap energy vs. lattice constant for some III-V semiconductors showing the InAs/InAsSb T2SL structure and the AlAsSb ternary alloy lattice-matched to the GaSb substrate. Figure 3a shows the stacking of the Ga-free T2SL nBn unipolar barrier photodetector. From bottom to top, the structure consists of a 400 nm Te-doped (n-type) GaSb buffer layer, which is followed by a 100 nm thick n-type doped InAs/InAs0.65Sb0.35 T2SL and by non-intentionally doped (nid) 3 µ m thick absorption layer (AL) made of the same T2SL structure. A barrier layer (BL) is then made from 100 nm nid AlAs0.09Sb0.91 and finally, the contact layer (CL) of the structure is composed of an 80 nm thick n-type doped T2SL. The AL and BL are undoped, and the residual dopings are 16 −3 16 −3 expected to be n-type and p-type at 10 cm and 5.10 cm , respectively. Figure 3b, displaying the simulated band diagram of the Ga-free T2SL nBn unipolar barrier detector at T = 150 K and V = 0 V, reveals both an accumulation layer in the AL (due to the polarities of AL and BL) and a low valence band offset between AL and BL which should not impede the transit Photonics 2020, 7, 76 5 of 14 of holes from the AL to the CL. Ga-free T2SL nBn MWIR detector Photonics 2020, 7, x FOR PEER REVIEW 5 of 15 Barrier Layer T = 150K Figure 3. (a) Schematic diagram of the nBn device structure dedicated to electrical measurements. (b) Calculated energy band diagrams of the Ga-free nBn barrier detector structure at 150 K and 0 V. V = 0V bias Such a detector structure was studied by electrical and electro-optical measurements. The T2SL Absorbing Layer devices were placed in a probe station in order to perform capacitance-voltage (C-V) measurements at a frequency f = 1 MHz and dark current density-voltage (J-V) measurements (under 0 degree field hh of view) for different operating temperatures. For that, a KEITHLEY 6517A Electrometer was used to Contact Layer -1 both apply the bias voltage and read the current density (ratio of current and area of the device) 0.0 0.2 0.4 3.0 3.2 delivered by the device. In addition, the samples were wire bounded onto a pin LCC, placed in a Position (µm) LN2-cooled cryostat and the non-calibrated spectral photoresponse (PR) of the detector was (b) (a) measured using a FTIR spectrometer. Figure 3. (a) Schematic diagram of the nBn device structure dedicated to electrical measurements. 3. Results (b) Calculated energy band diagrams of the Ga-free nBn barrier detector structure at 150 K and 0 V. 3. Results 3.1. Fabrication and Characterizations of Ga-Free T2SL Structure 3.1. Fabrication In this woand rk, Characterizations all samples were of groGa-Fr wn on ee T2SL an n-typ Structur e GaSeb Te-doped (100) substrate by solid source MBE equipped with valve crackers set up to produce As2 and Sb2 species. The quality of the structures In this work, all samples were grown on an n-type GaSb Te-doped (100) substrate by solid source grown were evaluated in terms of HRXRD, AFM, PL and TRPL measurements. MBE equipped with valve crackers set up to produce As and Sb species. The quality of the structures 2 2 An example of HRXRD spectrum of such a T2SL structure, with p = 5.3 nm, x = 34.5%, is shown grown were evaluated in terms of HRXRD, AFM, PL and TRPL measurements. in Figure 4. The presence of numerous and intense satellite peaks (SL ± 1, ±2, ±3) is a signature of the An example of HRXRD spectrum of such a T2SL structure, with p = 5.3 nm, x = 34.5%, is shown crystallographic structure’s quality. Their angular separation allows the calculation of the period in Figure 4. The presence of numerous and intense satellite peaks (SL 1, 2, 3) is a signature of thickness of the T2SL while the antimony composition x of the InAs/InAs1-xSbx T2SL structure the crystallographic structure’s quality. Their angular separation allows the calculation of the period together with the lattice mismatch of the structure with the GaSb substrate, which are extracted from thickness of the T2SL while the antimony composition x of the InAs/InAs Sb T2SL structure together 1-x x 0th order peak-substrate angular separation Δθ through Equation (4): with the lattice mismatch of the structure with the GaSb substrate, which are extracted from 0th order peak-substrate angular separation D through Equation (4): Δa/a = (sin(θsubstrate)/sin(θsubstrate + Δθ)) − 1 (4) where θsubstrate stands for the angle in degrees of the substrate peak measured by HRXRD. Da/a = (sin( )/sin( + D)) 1 (4) substrate substrate In order to check whether the epilayer has achieved a good lattice match with the substrate, typical Δa/a 0.1% was required for the samples. where stands for the angle in degrees of the substrate peak measured by HRXRD. substrate Figure 4. High-resolution X-ray di raction HRXRD pattern of a p = 5.3 nm Ga-free InAs/InAs Sb 0.655 0.345 Figure 4. High-resolution x-ray diffraction HRXRD pattern of a p = 5.3 nm Ga-free InAs/InAs0.655Sb0.345 T2SL structure grown on a GaSb substrate. T2SL structure grown on a GaSb substrate. The surface morphology was also routinely investigated by AFM root-mean-square (RMS) surface roughness measurements. Typical 10 × 10 μm scan area highlights well-defined atomic steps (Figure 5) associated with RMS surface roughness only equals to 0.12 nm, that is to say, less than one monolayer in the case of Sb-based materials. Energy (eV) Photonics 2020, 7, 76 6 of 14 In order to check whether the epilayer has achieved a good lattice match with the substrate, typical Da/a 0.1% was required for the samples. The surface morphology was also routinely investigated by AFM root-mean-square (RMS) surface roughness measurements. Typical 10 10 m scan area highlights well-defined atomic steps (Figure 5) Photonics 2020, 7, x FOR PEER REVIEW 6 of 15 associated with RMS surface roughness only equals to 0.12 nm, that is to say, less than one monolayer Photonics 2020, 7, x FOR PEER REVIEW 6 of 15 in the case of Sb-based materials. Figure 5. 10 × 10 mm atomic force microscopy (AFM) scan of an InAs/InAsSb T2SL sample. Clear Figure 5. 10 10 mm atomic force microscopy (AFM) scan of an InAs/InAsSb T2SL sample. Clear monolayer steps can be observed. Figure 5. 10 × 10 mm atomic force microscopy (AFM) scan of an InAs/InAsSb T2SL sample. Clear monolayer steps can be observed. monolayer steps can be observed. PL measurements were performed from 50 to 250 K. PL spectra, presented in Figure 6, display PL measurements were performed from 50 to 250 K. PL spectra, presented in Figure 6, display a a shift in PL peak from 4.9 µ m to 5 µ m in the temperature range (77–150 K). Such a trend strengthens PL measurements were performed from 50 to 250 K. PL spectra, presented in Figure 6, display shift in PL peak from 4.9 m to 5 m in the temperature range (77–150 K). Such a trend strengthens the choice of the InAs/InAsSb T2SL period and antimony composition suitable for addressing MWIR a shift in PL peak from 4.9 µ m to 5 µ m in the temperature range (77–150 K). Such a trend strengthens the choice of the InAs/InAsSb T2SL period and antimony composition suitable for addressing MWIR broadband domain. At 77 K, the full width at half maximum (FWHM) is equal to 36 meV, higher than the choice of the InAs/InAsSb T2SL period and antimony composition suitable for addressing MWIR broadband domain. At 77 K, the full width at half maximum (FWHM) is equal to 36 meV, higher than the one reported by E. H. Steenbergen et al. at around 30 meV [26]. broadband domain. At 77 K, the full width at half maximum (FWHM) is equal to 36 meV, higher than the one reported by E. H. Steenbergen et al. at around 30 meV [26]. the one reported by E. H. Steenbergen et al. at around 30 meV [26]. Energy (eV) 0.344 0.318 0.295 0.276 0.258 0.243 0.230 0.218 0.207 0.197 Energy (eV) 2.5 48K 0.263 4.72 0.344 0.318 0.295 0.276 0.258 0.243 0.230 0.218 0.207 0.197 @77K: 4.89 µm 68K 2.5 0.259 4.79 48K 78K 0.263 4.72 0.256 4.85 2.0 @77K: 4.89 µm 68K 90K 0.259 4.79 0.252 4.92 110K 78K 0.256 4.85 2.0 90K 130K 0.249 4.99 0.252 4.92 110K 150K 0.245 5.06 1.5 130K 0.249 4.99 170K 0.242 5.13 @150K: 5.03 µm 150K 190K 0.245 5.06 1.5 0.238 5.21 170K 210K 0.242 5.13 @150K: 5.03 µm 1.0 190K 230K 0.235 5.29 0.238 5.21 210K 250K 0.231 5.37 1.0 230K 0.235 5.29 0.228 5.45 250K 0.231 5.37 0.5 0.224 5.53 0.228 5.45 0.221 5.62 0.5 Eg 0.224 5.53 Einstein Oscillator model fit 0.217 5.71 0.0 0.221 5.62 Eg 3.6 3.9 4.2 4.5 4.8 5.1 5.4 5.7 6.0 6.3 Einstein Oscillator model fit 0 30 60 90 120 150 180 210 240 270 0.217 5.71 0.0 Wavelength (µm) 3.6 3.9 4.2 4.5 4.8 5.1 5.4 5.7 6.0 6.3 Temperature (K) 0 30 60 90 120 150 180 210 240 270 Wavelength (µm) Temperature (K) (a) (b) (a) (b) Figure 6. (a) Photoluminescence (PL) spectra of the Ga-free InAs/InAs Sb T2SL structure between 0.63 0.37 Figure 6. (a) Photoluminescence (PL) spectra of the Ga-free InAs/InAs0.63Sb0.37 T2SL structure between 48 and 250 K. (b) Ga-free T2SL bandgap variation as a function of the temperature. The red line presents 48 and 250 K. (b) Ga-free T2SL bandgap variation as a function of the temperature. The red line Figure 6. (a) Photoluminescence (PL) spectra of the Ga-free InAs/InAs0.63Sb0.37 T2SL structure between a fit curve using Einstein oscillator equation. presents a fit curve using Einstein oscillator equation. 48 and 250 K. (b) Ga-free T2SL bandgap variation as a function of the temperature. The red line presents a fit curve using Einstein oscillator equation. Using data from Figure 6a, the bandgap energy Eg(T), given by Equation (5), was plotted as a function of temperature (Figure 6b). Using data from Figure 6a, the bandgap energy Eg(T), given by Equation (5), was plotted as a function of temperature (Figure 6b). Eg(T) = Epeak(T) − KBT/2 (5) Eg(T) = Epeak(T) − KBT/2 (5) The continuous red curve in Figure 6b represents the Einstein Oscillator model used by Webster et al. [27] and given by the Equation (6). The continuous red curve in Figure 6b represents the Einstein Oscillator model used by Webster et al. [27] and given by the Equation (6). Eg(T) = E0 − α × (TE/(exp(TE/T) − 1)) (6) Eg(T) = E0 − α × (TE/(exp(TE/T) − 1)) (6) where α is the slope of high temperature linear asymptote, E0 the energy gap at 0 K, TE the Einstein temperature and T the absolute temperature. Computing the best fit from Equation (5), values of E0 where α is the slope of high temperature linear asymptote, E0 the energy gap at 0 K, TE the Einstein temperature and T the absolute temperature. Computing the best fit from Equation (5), values of E0 Intensity (a. u.) Intensity (a. u.) Energy (eV) Energy (eV) Wavelength (µm) Wavelength (µm) Photonics 2020, 7, 76 7 of 14 Using data from Figure 6a, the bandgap energy Eg(T), given by Equation (5), was plotted as a function of temperature (Figure 6b). E (T) = E (T) K T/2 (5) peak B The continuous red curve in Figure 6b represents the Einstein Oscillator model used by Webster et al. [27] and given by the Equation (6). E (T) = E (T /(exp(T /T) 1)) (6) 0 E E where is the slope of high temperature linear asymptote, E the energy gap at 0 K, T the Einstein 0 E Photonics 2020, 7, x FOR PEER REVIEW 7 of 15 temperature and T the absolute temperature. Computing the best fit from Equation (5), values of E = 258 meV, = 2.013 10 meV/K and T = 117.6 K were extracted. Such fitting of the bandgap as 0 E −4 = 258 meV, α = 2.013 × 10 meV/K and TE = 117.6 K were extracted. Such fitting of the bandgap as a a function of temperature will be useful to analyze dark current density measurements performed on function of temperature will be useful to analyze dark current density measurements performed on devices [28]. devices [28]. TRPL measurements were performed on T2SL structures to extract the minority carrier lifetime. TRPL measurements were performed on T2SL structures to extract the minority carrier lifetime. A typical TRPL signal measured at 90 K is reported in Figure 7. Following the approach of A typical TRPL signal measured at 90 K is reported in Figure 7. Following the approach of Donetsky Donetsky et al. [29], a lifetime value as high as 1.1 s was extracted from these measurements, et al. [29], a lifetime value as high as 1.1 µ s was extracted from these measurements, a decade higher a decade higher than values related to InAs/GaSb MWIR T2SLs [12,29,30]. Such a lifetime value than values related to InAs/GaSb MWIR T2SLs [12,29,30]. Such a lifetime value validates both the validates both the MBE growth procedure and the choice of Ga-free T2SL structure as AL for the MBE growth procedure and the choice of Ga-free T2SL structure as AL for the device. However, device. However, because heavy holes are mainly confined in the InAsSb layer, vertical transport will because heavy holes are mainly confined in the InAsSb layer, vertical transport will have to be have to be investigated. Very recent results show that heavy hole mobility is strongly temperature investigated. Very recent results show that heavy hole mobility is strongly temperature dependant dependant [31,32]. [31,32]. Figure Figure 7. 7. TRP TRPL L si signal gnal of of the the G Ga a-fr -free ee InAs InAs/ /I InAs nAs0.63Sb Sb 0.37 T2 T2SL SL ststr ruuctur cture eat at 990 0 K K. . A A m minority inority carrier carrier 0.63 0.37 li lifetime fetime of of1 1.1 .1 µs s iis s extracte extracted. d. The Table 1 summarizes the structural and optical baseline values routinely used as a quality The Table 1 summarizes the structural and optical baseline values routinely used as a quality indicator to evaluate the performance of Ga-free InAs/InAsSb T2SL structures. indicator to evaluate the performance of Ga-free InAs/InAsSb T2SL structures. Table 1. Structural and optical criteria used to evaluate Ga-free T2SL structures grown by MBE. Table 1. Structural and optical criteria used to evaluate Ga-free T2SL structures grown by MBE. HRXRD HRXRD AFM AFM PL PL PL PL TRPL TRPL Da/a RMS @ 150 K FWHM Lifetime a/a RMS peak @ 150 K FWHM Lifetime peak 0.1% 0.15 nm 5 µm 0.30 meV 800 ns 0.1% 0.15 nm 5 m 0.30 meV 800 ns 3.2. Fabrication and Characterizations of Ga-Free T2SL Barrier Detector From epitaxial T2SL structures, circular mesa nBn photodetectors with diameters from 60 µ m to 310 µ m (Figure 8) were fabricated using standard photolithography. Mesa photodetecors were realized by wet etching using citric acid solution and polymerized photoresist was used to protect the mesa surface from ambient air. Metallization was ensured on the n-GaSb substrate and on the n- type T2SL cap layer. The barrier detector devices were then characterized by electrical and electro- optical measurements. Photonics 2020, 7, 76 8 of 14 3.2. Fabrication and Characterizations of Ga-Free T2SL Barrier Detector From epitaxial T2SL structures, circular mesa nBn photodetectors with diameters from 60 m to 310 m (Figure 8) were fabricated using standard photolithography. Mesa photodetecors were realized by wet etching using citric acid solution and polymerized photoresist was used to protect the mesa surface from ambient air. Metallization was ensured on the n-GaSb substrate and on the n-type T2SL cap layer. The barrier detector devices were then characterized by electrical and electro-optical measurements. Photonics 2020, 7, x FOR PEER REVIEW 8 of 15 Photonics 2020, 7, x FOR PEER REVIEW 8 of 15 Figure 8. Top view of a processed sample, blind diodes (C) and photodetectors (p) with several Figure 8. Top view of a processed sample, blind diodes (C) and photodetectors (p) with several Figure 8. Top view of a processed sample, blind diodes (C) and photodetectors (p) with several diameters from 60 µ m up to 310 µ m. diameters from 60 m up to 310 m. diameters from 60 µ m up to 310 µ m. Figure 9 shows non-calibrated PR spectra recorded at different bias (from −60 mV to −1.50 V) Figure 9 shows non-calibrated PR spectra recorded at di erent bias (from 60 mV to 1.50 V) and Figure 9 shows non-calibrated PR spectra recorded at different bias (from −60 mV to −1.50 V) and at 150 K. The bias-dependent PR signal increases with increasing reverse bias and starts to at 150 K. The bias-dependent PR signal increases with increasing reverse bias and starts to saturate at and at 150 K. The bias-dependent PR signal increases with increasing reverse bias and starts to saturate at −220 mV. In addition, the PL peak at 5.03 µ m is in good agreement with the 50% cut-off 220 mV. In addition, the PL peak at 5.03 m is in good agreement with the 50% cut-o wavelength saturate at −220 mV. In addition, the PL peak at 5.03 µ m is in good agreement with the 50% cut-off wavelength (co) extracted from the spectral PR. ( ) extracted from the spectral PR. co wavelength (co) extracted from the spectral PR. Energy (eV) Energy (eV) -1.50V 0.620 0.496 0.413 0.354 0.310 0.276 0.248 0.225 0.207 0.191 -1.50V -1.40V 0.620 0.496 0.413 0.354 0.310 0.276 0.248 0.225 0.207 0.191 0.35 -1.40V 0.35 -1.30V -1.30V = 5.03 µm cutoff -1.20V PR spectra = 5.03 µm cutoff -1.20V PR spectra -1.10V 0.30 PL spectrum -1.10V 0.30 PL spectrum -1.00V -1.00V -0.90V 150K -0.90V 150K 0.25 -0.80V 0.25 -0.80V -0.70V -0.70V -0.60V -0.60V 0.20 -0.50V 0.20 -0.50V -0.40V -0.40V -0.30V 0.15 -0.30V 0.15 -0.28V -0.28V -0.26V -0.26V -0.24V 0.10 -0.24V 0.10 -0.22V -0.22V -0.20V -0.20V 0.05 -0.18V 0.05 -0.18V -0.16V -0.16V -0.14V 0.00 -0.14V -0.12V 0.00 -0.12V 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 -0.10V 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 -0.10V -0.06V Wavelength (µm) -0.06V Wavelength (µm) Figure 9. Photoresponse (PR) at di erent biases and photoluminescence (PL) measurements at 150 K. Figure 9. Photoresponse (PR) at different biases and photoluminescence (PL) measurements at 150 Figure 9. Photoresponse (PR) at different biases and photoluminescence (PL) measurements at 150 K. K. Dark current density was measured and Figure 10 shows the dark current density-voltage (J-V) characteristics carried out for a 210 m diameter detector at di erent temperatures, from 77 to 270 K. Dark current density was measured and Figure 10 shows the dark current density-voltage (J-V) Dark current density was measured and Figure 10 shows the dark current density-voltage (J-V) At the expected temperature operation of 150 K and bias equal to 395 mV, the dark current density is characteristics carried out for a 210 µ m diameter detector at different temperatures, from 77 to 270 K. characteristics carried out for a 210 µ m diameter detector at different temperatures, from 77 to 270 K. 5 2 equal to 3.24 10 A/cm . Below 110 K, the dark current is limited by the photonic current due to the At the expected temperature operation of 150 K and bias equal to −395 mV, the dark current density At the expected temperature operation of 150 K and bias equal to −395 mV, the dark current density −5 2 is equal to 3.24 × 10 A/cm . Below 110 K, the dark current is limited by the photonic current due to −5 2 is equal to 3.24 × 10 A/cm . Below 110 K, the dark current is limited by the photonic current due to the experimental set-up. The shapes of the J-V characteristics are in accordance with those related the experimental set-up. The shapes of the J-V characteristics are in accordance with those related recently reported XBn Ga-free MWIR diodes [16–23]. recently reported XBn Ga-free MWIR diodes [16–23]. Uncalibred Quantum Efficiency (a. u.) Uncalibred Quantum Efficiency (a. u.) Photonics 2020, 7, 76 9 of 14 experimental set-up. The shapes of the J-V characteristics are in accordance with those related recently Photonics 2020, 7, x FOR PEER REVIEW 9 of 15 Photonics 2020, 7, x FOR PEER REVIEW 9 of 15 reported XBn Ga-free MWIR diodes [16–23]. 270K 270K 250K 250K 230K 230K -1 -1 210K 210K 190K 190K -2 -2 10 170K 170K 160K 160K -3 -3 10 155K 10 155K 150K 150K -4 -4 145K 10 145K 140K 140K -5 -5 10 130K 10 130K 120K photonic 120K -6 photonic -6 10 110K 110K 100K 100K -7 -7 90K 10 90K 77K 77K -8 -8 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 Bias (V) Bias (V) Figure 10. Dark current density-voltage (J-V) characteristics of a Ga-free T2SL nBn MWIR detector for Figure 10. Dark current density-voltage (J-V) characteristics of a Ga-free T2SL nBn MWIR detector for Figure 10. Dark current density-voltage (J-V) characteristics of a Ga-free T2SL nBn MWIR detector for temperatures from 77 to 270 K. temperatures from 77 to 270 K. temperatures from 77 to 270 K. To complete electrical characterizations, C-V measurements were performed on the devices. From To complete electrical characterizations, C-V measurements were performed on the devices. To complete electrical characterizations, C-V measurements were performed on the devices. these measurements, typical 1/C data as a function of voltage obtained at T = 150 K are shown From these measurements, typical 1/C data as a function of voltage obtained at T = 150 K are shown From these measurements, typical 1/C data as a function of voltage obtained at T = 150 K are shown in Figure 11. Using the Equation (7) [33], the extracted slopes allow to reaching of the residual in Figure 11. Using the Equation (7) [33], the extracted slopes allow to reaching of the residual carrier 15 3 in Figure 11. Using the Equation (7) [33], the extracted slopes allow to reaching of the residual carrier carrier concentration (N ) both in the nid p-type BL (5.2 10 cm ) and in the nid n-type AL res 15 −3 15 −3 concentration (Nres) both in the nid p-type BL (5.2 × 10 cm ) and in the nid n-type AL (3.2 × 10 cm ) 15 −3 15 −3 concentra 15tion (N 3 res) both in the nid p-type BL (5.2 × 10 cm ) and in the nid n-type AL (3.2 × 10 cm ) (3.2 10 cm ) of the nBn device. of the nBn device. of the nBn device. (A/C) = (2/(q" " )) ((V V)/N ) (7) res 2 0 SL d (A/C) = (2/(qϵ ϵ )) × ((V −V)/N ) (7) 0 SL d res (A/C) = (2/(qϵ ϵ )) × ((V −V)/N ) (7) 0 SL d res where A stands for the active area of the photodetector, Vd for the diffusion potential, q for the charge wher where e A A stands stands for for the the active active ar area ea of of the the photodetector photodetector, , V Vd for for th the e d di iffusion usion potential, potential, q q for for the the char charge ge carrier, ε0 for the vacuum permittivity and εSL for the relative permittivity of the InAs/InAsSb T2SL carrier carrier, , " ε0 for for the the vacuum vacuum permittivity permittivity and and " εSL for for th the e rel relative ative permittivity permittivity of of the the InAs InAs/ /InAsSb InAsSb T2SL T2SL 0 SL (εSL = 15.15). ((" εSL = 15.15 = 15.15). ). SL C(V) C(V) T = 150K T = 150K 15 0 15 0 10 -2 -1 0 1 -2 -1 0 1 Tension (V) Tension (V) 15 -3 15 -3 3.2x10 cm 3.2x10 cm 15 -3 15 -3 5.2x10 cm 5.2x10 cm -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Voltage (V) Voltage (V) Figure 11. Capacitance-voltage measurement of a Ga-free T2SL nBn detector at T = 150 K. 1/C 2 (V) Figure 11. Capacitance-voltage measurement of a Ga-free T2SL nBn detector at T = 150 K. 1/C 2(V) Figure 11. Capacitance-voltage measurement of a Ga-free T2SL nBn detector at T = 150 K. 1/C (V) characteristic and in inset, surface capacitance C (V) curve. characteristic and in inset, surface capacitance Csu surf rf (V) curve. characteristic and in inset, surface capacitance Csurf (V) curve. Current Density (A/cm²) Current Density (A/cm²) 2 2 2 2 2 2 (A/C) (cm /F) (A/C) (cm /F) 2 -4 pF/cm .10 2 -4 pF/cm .10 Photonics 2020, 7, x FOR PEER REVIEW 10 of 15 Photonics 2020, 7, 76 10 of 14 4. Discussion 4. Discussion Several analyses can be made concerning the obtained results. To explain the behavior of the PR Several analyses can be made concerning the obtained results. To explain the behavior of the PR spectrum as a function of the bias applied (Figure 9), a band diagram of the nBn device was spectrum as a function of the bias applied (Figure 9), a band diagram of the nBn device was considered. considered. In addition to the band diagram calculated at 150 K and V = 0 V (Figure 3b), Figure 12 In addition to the band diagram calculated at 150 K and V = 0 V (Figure 3b), Figure 12 reports two reports two more band diagrams calculated at −100 mV and −500 mV. At V = 0 V, the band diagram more band diagrams calculated at 100 mV and 500 mV. At V = 0 V, the band diagram highlights the highlights the presence of a potential barrier blocking the minority heavy hole carriers. Even, at a bias presence of a potential barrier blocking the minority heavy hole carriers. Even, at a bias operation equal operation equal to −100 mV, this potential barrier remains, penalizing the quantum efficiency. At V = to 100 mV, this potential barrier remains, penalizing the quantum eciency. At V = 500 mV, the bias −500 mV, the bias is high enough to suppress the potential barrier allowing the transport of a hole is high enough to suppress the potential barrier allowing the transport of a hole minority carrier to the minority carrier to the top contact layer. The maximum value of the photo-generated carriers is then top contact layer. The maximum value of the photo-generated carriers is then collected. The vanishing collected. The vanishing of the potential barrier starts at V = −220 mV, explaining the saturation of of the potential barrier starts at V = 220 mV, explaining the saturation of the PR signal recorded. This the PR signal recorded. This analysis shows that the choice of the AlAsSb BL and the T2SL CL with analysis shows that the choice of the AlAsSb BL and the T2SL CL with their corresponding dopings is their corresponding dopings is probably not optimal. probably not optimal. a) b) Ga-free T2SL nBn MWIR detector Ga-free T2SL nBn MWIR detector 3 3 Barrier Barrier T = 150K Layer Layer T = 150K 2 2 V = -500mV bias V = -100mV bias 1 1 Absorbing Layer Absorbing Layer 0 0 Contact Contact Layer Layer -1 -1 0.4 0.0 0.0 0.2 3.0 3.2 0.2 0.4 3.0 3.2 Position (µm) Position (µm) (a) (b) Figure 12. Calculated band diagrams of the Ga-free T2SL nBn barrier structure at 150 K and (a) 100 mV, Figure 12. Calculated band diagrams of the Ga-free T2SL nBn barrier structure at 150 K and (a) −100 (b) 500 mV. mV, (b) −500 mV. An arrhenius’s plot can be extracted from J-V characteristic curves (Figure 10). Such a graph An arrhenius’s plot can be extracted from J-V characteristic curves (Figure 10). Such a graph (Figure 13) shows an activation energy Ea = 225 meV at a high temperature, which is approximately (Figure 13) shows an activation energy Ea = 225 meV at a high temperature, which is approximately the energy of T2SL bandgap. Such a value is in accordance with a di usion current (Equation (3)) the energy of T2SL bandgap. Such a value is in accordance with a diffusion current (Equation (3)) regime. At temperatures lower than 140 K, the device is GR dark current density limited, evidencing regime. At temperatures lower than 140 K, the device is GR dark current density limited, evidencing the presence of an unwanted electric field in the T2SL AL. The presence of the electric field in the AL the presence of an unwanted electric field in the T2SL AL. The presence of the electric field in the AL may be suppressed by a better control of doping levels during the MBE growth, both for BL and AL. may be suppressed by a better control of doping levels during the MBE growth, both for BL and AL. 5 2 Moreover, a dark current value equal to 3.24 10 A/cm at 150 K (Figure 10) has to be improved. −5 2 Moreover, a dark current value equal to 3.24 × 10 A/cm at 150 K (Figure 10) has to be improved. Indeed, such a result, compared to the MCT state of the art of photodiode limited by di usion dark Indeed, such a result, compared to the MCT state of the art of photodiode limited by diffusion dark current [34], is 20 times higher at the corresponding cut-o wavelength and remains slightly superior current [34], is 20 times higher at the corresponding cut-off wavelength and remains slightly superior to the most recent results reported on Ga-free T2SL detectors [18–23]. To lower the dark current, the N to the most recent results reported on Ga-free T2SL detectors [18–23]. To lower the dark current, the x product must be as high as possible (Equation (3)). A complementary study to determine the di Nd x diff product must be as high as possible (Equation (3)). A complementary study to determine the optimal Nd x product is necessary and planned. di optimal Nd x τdiff product is necessary and planned. Electrical results and spectral PR at 150 K may be jointly and closely analyzed in order to define the operating bias and to explain the di erent dark current regimes as a function of bias. For this purpose, Figure 14a–c show the normalized PR, the dark current density and the R A product, respectively. R d d represents the di erential resistance, calculated from the derivative of the voltage over the current and A is the device area. The normalized PR values were extracted from the spectral photoresponse measurements at di erent biases and at 4 m (Figure 9). Energy (eV) Energie (eV) Photonics 2020, 7, 76 11 of 14 Photonics 2020, 7, x FOR PEER REVIEW 11 of 15 Temperature (K) 290 232 193 166 145 129 116 105 97 89 83 77 Data -1 J_d J_GR J : E = 225 meV -2 diff a -3 -4 J : E = 112 meV GR a -5 photonic currrent T = 145 K cross 40 50 60 70 80 90 100 110 120 130 140 150 -1 1/KT (eV ) Photonics 2020, 7, x FOR PEER REVIEW 12 of 15 Figure 13. Arrhenius’s plot extracting from J-V curves at 350 mV. Figure 13. Arrhenius’s plot extracting from J-V curves at −350 mV. Electrical results and spectral PR at 150 K may be jointly and closely analyzed in order to define a) Normalized PR (a.u.) V V V the operating bias and to explain the different dark current GRregimes as a function of bias. For this OP ON purpose, Figure 14a– 1. c 0show the normalized PR, the dark current density and the RdA product, respectively. Rd represents the differential resistance, calculated from the derivative of the voltage over the current and A is the device area. The normalized PR values were extracted from the spectral 0.8 photoresponse measurements at different biases and at 4 µ m (Figure 9). By examining the shape of the displayed curves at 150 K, we can identify three main dark current 0.6 regimes. The first significant bias value, Von (turn on bias), is located at the first RdA minimum at −200 mV. At this bias, thermal-generated holes can reach the CL and the normalized PR is higher than 0.4 4 2 90%. Next, the RdA peak at −395 mV, equal to 2.5 × 10 Ω cm , corresponds to the operating bias Vop 150K (operating bias). The photoresponse is maximized and the device is fully turned on at this bias. As a 0.2 consequence, it is not necessary to increase the bias among Vop when the photodetector is operating. The depletion region is still confined within the barrier until Vop and the C-V measurement permits -2 determination of the reduced carrier concentration in the BL (Figure 11). Then, the next RdA b) J (A/cm ) minimum at −580 mV indicates that the GR current starts to appear. At this bias, the barrier is fully -3 depleted and the depletion region reaches the absorber, which explains the appearance of the electric field-related GR current. We define VGR = −580 mV and at higher bias, the reduced carrier -4 concentration in the AL is determined with C-V measurement (Figure 11). Results obtained show that the observed operating bias value (−395 mV) is higher than the -5 expected one to satisfy SWAP criteria. This drawback could be due to a misalignment between the AL and BL valence bands, impeding the flow of minority carriers. To circumvent it, future design -6 improvement is required. 150K -7 -8 5 c) R A (.cm ) 150K -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 Voltage (V) Figure 14. Stacking of the experimental (a) normalized photoresponse, (b) dark current density, (c) Figure 14. Stacking of the experimental (a) normalized photoresponse, (b) dark current density, Differential Resistance Area product (RdA )product as a function of the voltage at 150 K for the Ga- (c) Di erential Resistance Area product (RdA) product as a function of the voltage at 150 K for the free T2SL MWIR nBn photodetector. Ga-free T2SL MWIR nBn photodetector. 5. Conclusions In conclusion, we have reported a set of structural, optical and electrical characterizations performed on MWIR Ga-free InAs/InAsSb T2SL barrier quantum detectors grown by MBE on a GaSb substrate in order to evaluate their performances and the way to improve them. The photodetector under study showed a cut-off wavelength around 5.03 µ m at 150 K. The dark current measurements were then analyzed to identify different current mechanisms in the structure and to estimate the operating bias. The operating bias (−380 mV) was higher than expected for SWAP conditions and this Current Density (A/cm²) Photonics 2020, 7, 76 12 of 14 By examining the shape of the displayed curves at 150 K, we can identify three main dark current regimes. The first significant bias value, V (turn on bias), is located at the first R A minimum at on 200 mV. At this bias, thermal-generated holes can reach the CL and the normalized PR is higher than 4 2 90%. Next, the R A peak at 395 mV, equal to 2.5 10 W cm , corresponds to the operating bias V op (operating bias). The photoresponse is maximized and the device is fully turned on at this bias. As a consequence, it is not necessary to increase the bias among V when the photodetector is operating. op The depletion region is still confined within the barrier until V and the C-V measurement permits op determination of the reduced carrier concentration in the BL (Figure 11). Then, the next R A minimum at 580 mV indicates that the GR current starts to appear. At this bias, the barrier is fully depleted and the depletion region reaches the absorber, which explains the appearance of the electric field-related GR current. We define V = 580 mV and at higher bias, the reduced carrier concentration in the AL GR is determined with C-V measurement (Figure 11). Results obtained show that the observed operating bias value ( 395 mV) is higher than the expected one to satisfy SWAP criteria. This drawback could be due to a misalignment between the AL and BL valence bands, impeding the flow of minority carriers. To circumvent it, future design improvement is required. 5. Conclusions In conclusion, we have reported a set of structural, optical and electrical characterizations performed on MWIR Ga-free InAs/InAsSb T2SL barrier quantum detectors grown by MBE on a GaSb substrate in order to evaluate their performances and the way to improve them. The photodetector under study showed a cut-o wavelength around 5.03 m at 150 K. The dark current measurements were then analyzed to identify di erent current mechanisms in the structure and to estimate the operating bias. The operating bias ( 380 mV) was higher than expected for SWAP conditions and this issue could be reduced by both adjusting the valence band alignment between the absorber and barrier layers and by optimizing their residual doping levels during the MBE growth. At this operating 5 2 bias, a dark current density equal to 3.24 10 A/cm was extracted from J(V) measurement at 150 K. Compared to the dark current state of the art of photodetector limited by di usion current, this value has to be lowered by optimizing the N x product. It will be the main subject of d di forthcoming studies. Author Contributions: U.Z.-M., M.B., R.A. and J.P.P. fabricated the structures and devices; U.Z.-M., M.B., R.A., and S.B. performed the measurements; U.Z.-M., M.B., J.-P.P., I.R.-M., F.d.A.-S. and P.C. analyzed the data; U.Z.-M., J.P.P., F.d.A.-S., and P.C. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This work was partially funded by the French “Investment for the Future” program (EquipEx EXTRA, ANR 11-EQPX-0016) and by the French ANR under project HOT-MWIR (ANR-18-CE24-0019-01). Conflicts of Interest: The authors declare no conflict of interest. References 1. Manissadjian, A.; Rubaldo, L.; Rebeil, Y.; Kerlain, A.; Brellier, D.; Mollard, L. Improved IR detectors to swap heavy systems for SWaP. In Proceedings of the SPIE Infrared Technology and Applications XXXVIII, Baltimore, MD, USA, 31 May 2012; Volume 8353, p. 835334. 2. Reibel, Y.; Taalat, R.; Brunner, A.; Rubaldo, L.; Augey, T.; Kerlain, A.; Péré-Laperne, N.; Manissadjian, A.; Gravrand, O.; Castelein, P.; et al. Infrared SWAP Detectors: Pushing the limits. In Proceedings of the SPIE Infrared Technology and Applications XLI, Baltimore, MD, USA, 11 June 2015; Volume 9451, p. 945110. 3. Klipstein, P.; Aronov, D.; Ben Ezra, M.; Barkai, I.; Berkowicz, E.; Brumer, M.; Fraenkel, R.; Glozman, A.; Grossman, S.; Jacobsohn, E.; et al. Recent progress in InSb based quantum detectors in Israel. Infrared Phys. Technol. 2013, 59, 172–181. [CrossRef] 4. Rogalski, A.; Kopytko, M.; Martyniuk, P.; Hu, W. Comparison of performance limits of HOT HgCdTe photodiodes with 2D material infrared photodetectors. Opto-Electron. Rev. 2020, 28, 82–92. Photonics 2020, 7, 76 13 of 14 5. Maimon, S.; Wicks, G.W. nBn detector, an infrared detector with reduced dark current and higher operating temperature. Appl. Phys. Lett. 2006, 89, 151109. [CrossRef] 6. Klipstein, P. “XBn” barrier photodetectors for high sensitivity and high operating temperature infrared sensors. In Proceedings of the SPIE Defense and Security Conference, Orlando, FL, USA, 21 March 2008; Volume 6940, p. 69402. 7. Klipstein, P.; Klin, O.; Grossman, S.; Snapi, N.; Lukomsky, I.; Aronov, D.; Yassen, M.; Glozman, A.; Fishman, T.; Berkowicz, E.; et al. XBn barrier photodetectors based on InAsSb with high operating temperatures. Opt. Eng. 2011, 50, 061002. [CrossRef] 8. Klipstein, P. XBnn and XBpp infrared detectors. J. Cryst. Growth 2015, 425, 351–356. [CrossRef] 9. Taalat, R.; Rodriguez, J.-B.; Delmas, M.; Christol, P. Influence of the period thickness and composition on the electro-optical properties of type-II InAs/GaSb midwave infrared superlattice photodetectors. J. Phys. D Appl. Phys. 2013, 47, 015101. [CrossRef] 10. Razeghi, M.; Nguyen, B.-M. Band gap tunability of Type II Antimonide-based superlattices. Phys. Procedia 2010, 3, 1207–1212. [CrossRef] 11. Höglund, L.; Naureen, S.; Diel, W.; Smuk, S.; Ivanov, R.; Delmas, M.; Evans, D.; Rihtnesberg, D.; Almqvist, S.; Becanovic, S.; et al. Type-II superlattice SWaP IDDCA production at IRnova. In Proceedings of the SPIE Infrared Technology and Applications XLVI, Online Only, CA, USA, 16 June 2020; Volume 11407, p. 114070. 12. Svensson, S.; Donetsky, D.; Wang, D.; Hier, H.; Crowne, F.; Belenky, G. Growth of type II strained layer superlattice, bulk InAs and GaSb materials for minority lifetime characterization. J. Cryst. Growth 2011, 334, 103–107. [CrossRef] 13. Chen, G.; Haddadi, A.; Hoang, A.-M.; Chevallier, R.; Razeghi, M. Demonstration of type-II superlattice MWIR minority carrier unipolar imager for high operation temperature application. Opt. Lett. 2015, 40, 45–47. [CrossRef] 14. Schuler-Sandy, T.; Klein, B.; Casias, L.; Mathews, S.; Kadlec, C.; Tian, Z.; Plis, E.; Myers, S.; Krishna, S. Growth of InAs–InAsSb SLS through the use of digital alloys. J. Cryst. Growth 2015, 425, 29–32. [CrossRef] 15. Olson, B.V.; Shaner, E.A.; Kim, J.K.; Klem, J.F.; Hawkins, S.D.; Murray, L.M.; Prineas, J.P.; Flatté, M.E.; Boggess, T.F. Time-resolved optical measurements of minority carrier recombination in a mid-wave infrared InAsSb alloy and InAs/InAsSb superlattice. Appl. Phys. Lett. 2012, 101, 092109. [CrossRef] 16. Rhiger, D.R.; Smith, E.P.; Kolasa, B.P.; Kim, J.K.; Klem, J.F.; Hawkins, S.D. Analysis of III–V Superlattice nBn Device Characteristics. J. Electron. Mater. 2016, 45, 4646–4653. [CrossRef] 17. Durlin, Q.; Perez, J.P.; Rossignol, R.; Rodriguez, J.B.; Cerutti, L.; Delacourt, B.; Rothman, J.; Cervera, C.; Christol, P. InAs/InAsSb superlattice structure tailored for detection of the full midwave infrared spectral domain. In Proceedings of the SPIE Quantum Sensing and Nano Electronics and Photonics XIV, San Francisco, CA, USA, 27 January 2017; Volume 10111, p. 1011112. 18. Ting, D.Z.; Soibel, A.; Khoshakhlagh, A.; Rafol, S.B.; Keo, S.; Höglund, L.; Fisher, A.M.; Luong, E.M.; Gunapala, S.D. Mid-wavelength high operating temperature barrier infrared detector and focal plane array. Appl. Phys. Lett. 2018, 113, 021101. [CrossRef] 19. Michalczewski, K.; Tsai, T.Y.; Martyniuk, P.; Wu, C.H. Demonstration of HOT photoresponse of MWIR T2SLs InAs/InAsSb photoresistors. Bull. Pol. Acad. Sci. Tech. Sci. 2019, 67, 141–145. 20. Ariyawansa, G.; Duran, J.; Reyner, C.; Scheihing, J. InAs/InAsSb Strained-Layer Superlattice Mid-Wavelength Infrared Detector for High-Temperature Operation. Micromachines 2019, 10, 806. [CrossRef] 21. Wu, D.; Li, J.; Dehzangi, A.; Razeghi, M. Mid-wavelength infrared high operating temperature pBn photodetectors based on type-II InAs/InAsSb superlattice. AIP Adv. 2020, 10, 025018. [CrossRef] 22. Wu, D.; Li, J.; Dehzangi, A.; Razeghi, M. High Performance InAs/InAsSb Type-II Superlattice Mid-Wavelength Infrared Photodetectors with Double Barrier. Infrared Phys. Technol. 2020, 109, 103439. [CrossRef] 23. Deng, G.; Chen, D.; Yang, S.; Yang, C.; Yuan, J.; Yang, W.; Zhang, Y. High operating temperature pBn barrier mid-wavelength infrared photodetectors and focal plane array based on InAs/InAsSb strained layer superlattices. Opt. Express 2020, 28, 17611. [CrossRef] 24. Zavala-Moran, U.; Alchaar, R.; Perez, J.; Rodriguez, J.; Bouschet, M.; Compean, V.; De Anda, F.; Christol, P. Antimonide-based Superlattice Infrared Barrier Photodetectors. In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology, Valletta, Malta, 27–29 February 2020; Volume 1, pp. 45–51. Photonics 2020, 7, 76 14 of 14 25. Krizman, G.; Carosella, F.; Philippe, A.; Ferreira, R.; Rodriguez, J.B.; Perez, J.P.; Christol, P.; de Vaulchier, L.-A.; Guldner, Y. InAs/InAsSb type 2 superlattices band parameters determination via magnetoabsorption and kp modeling. In Proceedings of the SPIE Photonic West Conference, San Francisco, CA, USA, 2 March 2020; Volume 11274, p. 1127408. 26. Steenbergen, E.; Massengale, J.; Ariyawansa, G.; Zhang, Y.-H. Evidence of carrier localization in photoluminescence spectroscopy studies of mid-wavelength infrared InAs/InAs1 Sb type-II superlattices. J. Lumin. 2016, 178, 451–456. [CrossRef] 27. Webster, P.T.; Riordan, N.A.; Liu, S.; Steenbergen, E.H.; Synowicki, R.A.; Zhang, Y.-H.; Johnson, S.R. Measurement of InAsSb bandgap energy and InAs/InAsSb band edge positions using spectroscopic ellipsometry and photoluminescence spectroscopy. J. Appl. Phys. 2015, 118, 245706. [CrossRef] 28. Cervera, C.; Jaworowicz, K.; Aït-Kaci, H.; Chaghi, R.; Rodriguez, J.-B.; Ribet-Mohamed, I.; Christol, P. Temperature dependence performances of InAs/GaSb superlattice photodiode. Infrared Phys. Technol. 2011, 54, 258–262. [CrossRef] 29. Donetsky, D.; Belenky, G.; Svensson, S.; Suchalkin, S. Minority carrier lifetime in type-II InAs/GaSb strained-layer superlattices and bulk HgCdTe materials. Appl. Phys. Let. 2010, 97, 052108. [CrossRef] 30. Delmas, M.; Rodriguez, J.-B.; Christol, P. Electrical modeling of InAs/GaSb superlattice mid-wavelength infrared pin photodiode to analyze experimental dark current characteristics. J. Appl. Phys. 2014, 116, 113101. [CrossRef] 31. Tsai, C.-Y.; Zhang, Y.; Ju, Z.; Zhang, Y.-H. Study of vertical hole transport in InAs/InAsSb type-II superlattices by steady-state and time-resolved photoluminescence spectroscopy. Appl. Phys. Lett. 2020, 116, 201108. [CrossRef] 32. Casias, L.K.; Morath, C.P.; Steenbergen, E.H.; Umana-Membreno, G.A.; Webster, P.T.; Logan, J.V.; Kim, J.K.; Balakrishnan, G.; Faraone, L.; Krishna, S. Vertical carrier transport in strain-balanced InAs/InAsSb type-II superlattice material. Appl. Phys. Lett. 2020, 116, 182109. [CrossRef] 33. Cervera, C.; Rodriguez, J.-B.; Chaghi, R.; Aït-Kaci, H.; Christol, P. Characterization of midwave infrared InAs/GaSb superlattice photodiode. J. Appl. Phys. 2009, 106, 024501. [CrossRef] 34. Tennant, W.E.; Lee, N.; Zandian, M.; Piquette, E.; Carmody, M. MBE HgCdTe Technology: A Very General Solution to IR Detection, Described by “Rule 07”, a Very Convenient Heuristic. J. Electron. Mater. 2008, 37, 1406–1410. [CrossRef] © 2020 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/).
Journal
Photonics – Multidisciplinary Digital Publishing Institute
Published: Sep 18, 2020