Extended short wavelength infrared nbn photodetectors based on type II InAs/AlSb/GaSb superlattices with an AlAsSb/GaSb superlattice barrier

Transcription

1 Extended short wavelength infrared nbn photodetectors based on type II InAs/AlSb/GaSb superlattices with an AlAsSb/GaSb superlattice barrier A. Haddadi, R. Chevallier, A. Dehzangi, and M. Razeghi 1,a) 1 Center for Quantum Devices, Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois, 60208, USA Abstract Extended short wavelength infrared nbn photodetectors based on type II InAs/AlSb/GaSb superlattices on GaSb substrate have been demonstrated. An AlAs 0.10 Sb 0.90 /GaSb H structure superlattice design was used as the large bandgap electron barrier in these photodetectors. The photodetector is designed to have a 100% cut off wavelength of ~2.8 μm at 300K. The photodetector exhibited a room temperature (300 K) peak responsivity of 0.65 A/W at 1.9 μm, corresponding to a quantum efficiency of 41% at zero bias under front side illumination, without any anti reflection coating. With an R A of 78 Ω cm 2 and a dark current density of A/cm 2 under -400mV applied bias at 300 K, the nbn photodetector exhibited a specific detectivity of cm Hz 1/2 /W. At 150 K, the photodetector exhibited a dark current density of A/cm 2 and a quantum efficiency of 50%, resulting in a detectivity of cm Hz 1/2 /W. a) Corresponding author: razeghi@eecs.northwestern.edu 1

2 The extended short wavelength infrared (eswir) spectral range from 1.7 to 2.5 μm has many applications, including astronomy, earth sciences, and advanced optical communications. A camera that is capable of eswir imaging will produce a higher quality image than conventional short wavelength infrared (SWIR) cameras because roughly half of the available SWIR emission from the night sky falls between 1.7 to 2.5 μm. Up to now, a variety of material systems have addressed parts of this region; each of those material systems has its own advantages and disadvantages. For example, photodetectors based on In x Ga 1-x As compounds have shown high performance when nearly lattice matched to InP ( 1.7 μm cut off wavelength), but their performance reduces rapidly at longer wavelength due to mismatch induced defects. 1 Similarly, photodetectors based on mercury cadmium telluride (MCT) can cover the eswir spectral range; however, they require relatively complex material growth and device fabrication processes that significantly increase the production costs. In contrast, type II superlattices (T2SLs) are a developing material system that has recently demonstrated eswir spectral region coverage. 2,3 This material system has many intrinsic advantages such as low growth and manufacturing costs, great flexibility in bandgap engineering, the possibility for auger recombination suppression 4, and excellent material uniformity over large areas 5 which make it an attractive choice for infrared detection and imaging over the whole infrared spectrum. 6 However, while initial simple p i n homojunction eswir photodiodes based on T2SLs have been demonstrated, 2,3 the dark current density and specific detectivity of these photodetectors do not yet provide the ultra low noise that the T2SLs material system offers. Generation recombination (GR) is the dominant mechanism of dark 2

3 current generation in these p i n homojunction eswir photodetectors when operating at low temperatures (< 250 K). 2,3 However, performance can be dramatically improved by adopting a heterojunction based photodetector structure, such as nbn 7, which can significantly reduce the GR based dark current and, ultimately, total dark current density of the device. In this letter, we present the demonstration of eswir nbn photodetectors based on InAs/AlSb/GaSb type II superlattices with wide bandgap AlAs 0.10 Sb 0.90 /GaSb superlattice based electron barriers. The AlAs 0.10 Sb 0.90 layer in the AlAsSb/GaSb superlattices is lattice matched to GaSb substrate and has antimony (Sb) atom in common with GaSb that provides a great deal of flexibility in the superlattice design, and unlike M 8 and W superlattices 9 it does not require any special interface design or strain balancing. Because of the common anion rule of band lineups, the valence band offset between AlAs 0.10 Sb 0.90 and GaSb is small; thus, it provides little possibility to engineer the valence band. However, the electron quantum well in this superlattice is deep ( 1.19 ev 13,14 ) and thus this superlattice can be tuned to make the wide bandgap electron barriers necessary for eswir nbn photodetectors. Furthermore, using an AlAs 0.10 Sb 0.90 /GaSb superlattice as the electron barrier results in a smoother surface morphology compared to the case in which the barrier consists of a simple ternary AlAs 0.10 Sb 0.90 layer. The growth conditions for AlAs 0.10 Sb 0.90 /GaSb superlattices are exactly the same as for InAs/AlSb/GaSb T2SL and the heterostructure can be experimentally realized without any difficulty, such as an increase in strain or interface problems. All of these makes AlAs 0.10 Sb 0.90 /GaSb superlattices an ideal wide bandgap addition to the 6.1 Å family of materials. We call 3

4 this design an H structure superlattice because of the deep wells, as shown in the inset of Figure 1. With those advantages, the H structure superlattice was inserted as the electron barrier between an n contact layer and an n type eswir absorption region to create a nbn photodetector for this study. The eswir absorption region and n contact were chosen to have 8/1/5/1 mono layers (MLs) of InAs/GaSb/AlSb/GaSb and 5/1/5/1 MLs of InAs/GaSb/AlSb/GaSb, respectively. It was expected that absorption region and n contact would have cut off wavelengths of ~2.5 and ~1.9 μm, respectively, at 150 K. The electron barrier H structure superlattice is 300 nm thick consisting of 5/2 MLs of AlAs 0.10 Sb 0.90 /GaSb, respectively, with a bandgap energy of ~1 ev (equal to a 1 μm cut off wavelength) at 150 K. The H structure superlattices barrier needs to be thick enough so that there is a negligible electron tunneling through it and the barrier should be high enough so there is a negligible thermal excitation of majority carriers over it and a negligible absorption inside the electron barrier near the cut off of the photodetector. The heterojunction between the electron barrier and the eswir absorption layer should have a large energy discontinuity in the conduction band and near zero energy discontinuity in the valence band. From the empirical tight binding model (ETBM) calculations, the band discontinuity between the barrier and absorption region is 20 mev in the valence band and 546 mev in the conduction band. The presence of this wide bandgap barrier not only reduces the GR based dark current but also reduces the trap assisted and band to band tunneling. The complete photodetector structure is shown in Figure 1. 4

5 Figure 1. Schematic diagram the eswir nbn photodetector with the inset showing the superlattice band alignment of the H structure electron barrier. The colored rectangles in the inset represent the forbidden gap of component materials. The dashed lines represent the effective conduction and valence bands of the H structure superlattice. After selecting the proper superlattice design for each part of the photodetector structure (see Figure 1), the device was grown using a solid source molecular beam epitaxy (SSMBE) reactor equipped with group III SUMO cells and group V valved crackers. The material was grown on a Te doped n type (10 17 cm -3 ) GaSb wafer. The epitaxial growth of the photodetector was started by growing a 100 nm thick GaSb buffer layer to smooth out the surface, then, a 500 nm n doped InAs 0.91 Sb 0.09 buffer layer (10 18 cm -3 ), followed by a 500 nm thick n type eswir contact (10 18 cm -3 ), a 1 μm thick n type eswir absorption region (10 14 cm -3 ), a 300 nm thick electron barrier, and a 200 nm thick SWIR n contact (10 18 cm -3 ). Unlike the eswir bottom n contact which shares the same superlattice design with the absorption region, the top SWIR n contact superlattice design has larger bandgap energy to eliminate absorption by the top n 5

6 contact near the spectral cut off of the photodetector in front side illumination configuration. Silicon (Si) was used as the n type dopant. Figure 2. (a) The atomic force microscopy image of a m 2 surface area of the grown material, for the nbn device with RMS roughness value of 1.21 Å. (b) High-resolution X-ray diffraction (HR- XRD) of the material grown for the nbn device. The quality of the material was assessed after epitaxial growth using atomic force microscopy (AFM) and high resolution X ray diffraction (HR XRD). The sample exhibits good surface morphology with clear atomic steps and a small surface roughness of 1.21 Å over a μm 2 area which indicates no structural degradation is created by the addition of the H structure superlattice barrier (Figure 2-a). 2,3 The satellite peaks in the HR XRD scan showed the overall periods of the eswir absorption region, the electron barrier, and the top SWIR n contact were about 52, 22, and 43 Å, respectively. The lattice mismatch to the GaSb substrate of the three superlattices was less than 1500 ppm, which is in agreement with the superlattice design (Figure 2-b). After structural characterization, the wafer was processed into a set of unpassivated mesa isolated nbn photodetectors with device areas ranging from to cm 2 using standard photo lithographic processing techniques. 15 The photodetectors were left unpassivated but in order to minimize the surface leakage special 6

7 attention was paid to ensure the surface were kept clean and minimal surface oxidization occurred. Then, the sample was wire bonded to a 68 pin leadless ceramic chip carrier (LCCC). Finally, it was loaded into a cryostat for both optical and electrical characterization at temperatures ranging from 150 to 300 K. Figure 3. Saturated 150 and 300 K quantum efficiency spectra of the nbn photodetector in front-side illumination configuration without any anti-reflection coating. Inset: The variation of the 100% cut-off wavelength of the device vs. temperature. The nbn photodetectors were front side illuminated using either calibrated 1000 C blackbody source or a Bruker IFS 66v/S Fourier transform infrared (FTIR) spectrometer. No anti reflection (AR) coating was applied to the nbn photodetectors. The optical performance of the devices is shown in Figure 3. The photodetectors exhibit a 100% cut off wavelength of ~2.5 m at 150 K (Figure 3 inset) as predicted from the band structure calculations; the device responsivity then peaks at 0.65 A/W, corresponding to quantum 7

8 efficiency (QE) of 41% for a 1 μm thick absorption region. At 300 K, the sample shows a 100% cut off wavelength at ~2.8 m; the device responsivity then peaks at 0.82 A/W, corresponding to a quantum efficiency of 50%. The device QE spectrum reaches to its saturation point under -400 mv applied bias voltage. The decreased quantum efficiency at shorter wavelengths is caused by partial absorption of the short wavelength light in the top SWIR n contact where it does not contribute to the photo current of the device. This issue can be addressed by thinner top contacts or larger bandgap energies. Figure 4. (a) Dark current density vs. applied bias voltage characteristic of the photodetectors as a function of temperature. (b) Arrhenius plot of the dark current density of the photodetector under mv applied bias. The green and red lines represent the expected diffusion and generationrecombination (G-R) limits, respectively. Figure 4(a) presents the dark current density vs. applied bias voltage characteristics of the nbn photodetector at different temperatures ranging from 150 to 300 K. At 150 K, the 8

9 photodetector exhibits a dark current density of A/cm 2 under -400mV applied bias, whereas at room temperature (T = 300 K), the dark current density at -400mV is A/cm 2. In T2SL based homojunction photodiodes operating in the SWIR region, 2,3 the diffusion current at temperatures below 200 K is typically several orders of magnitude lower than the GR current, while at room temperature it is several orders higher. An Arrhenius plot of the dark current density versus inverse temperature (1/T) from 150 to 300 K under -400 mv applied bias voltage is shown in Figure 4(b). The dark current of this nbn photodetector is diffusion limited at operating temperatures above 180 K, and G R current limited below this temperature, with both G R and diffusion current being equal at 180 K (the cross over temperature, T 0 ). In contrast, the cross over temperature of our previous T2SL based homojunction SWIR photodiodes 2,3 was very close to or higher than room temperature. The nbn device architecture has enabled us to push T 0 to significantly lower operating temperatures which leads to lower dark current than that of homojunction T2SL based photodiode operating at the same temperature. Alternately, this device can operate at a higher temperature with the same dark current. In addition to the intrinsic improvement of the electrical performance, the nbn photodetector design also lends itself well to a shallow etched device geometry. The mesa etch can terminates at the wide bandgap H structure superlattice barrier and the smaller bandgap eswir absorption region does not need to be etched, thus avoiding surface leakage. This can lead to more uniform in focal plane arrays. Figure 4(b) also suggests that there is still room for further improvement of the low temperature (< 180 K) electrical performance by further suppressing G R current. 9

10 Figure 5. Specific detectivity (D * ) spectrum of the device at (a) 150 K and (b) 300 K. at -400 mv applied bias. The device is front-side illuminated, without any anti-reflection coating. The specific detectivity is calculated based on the equation in the inset, where R i is the device responsivity, J is the dark current density, R A is the differential resistance area product, k b is the Boltzmann constant, and T is the operating temperature. After performing both electrical and optical characterization of the nbn photodetector, the specific detectivity (D * ) was calculated. The device exhibits a saturated dark current shot noise limited specific detectivity of cm Hz 1/2 /W under -400 mv of applied bias at 150 K (Figure 5 a). At 300 K, the photodetector exhibits a specific detectivity of cm Hz 1/2 /W under -400 mv applied bias (Figure 5 b) for the same background condition. In summary, we have reported the design, growth, and characterization of high performance eswir nbn photodetectors based on InAs/AlSb/GaSb type II superlattices. An AlAs 0.10 Sb 0.90 /GaSb wide-bandgap H structure superlattice design was used as the 10

11 electron barrier. These nbn photodetectors exhibit 100% cut off wavelengths of ~2.5 and ~2.8 m at 150 and 300 K, respectively. The devices achieve saturated quantum efficiency values of 41% and 50% at 150 and 300 K, respectively, under front side illumination and without any AR coating. At 150 K, the photodetectors exhibit a dark current density of A/cm 2 under 400 mv applied bias providing a specific detectivity of cm Hz 1/2 /W. At 300 K, the dark current density reaches A/cm 2 under -400 mv bias, providing a specific detectivity of cm Hz 1/2 /W. The H structure superlattice based electron barrier design in combination with nbn photodetector architecture has made it possible for T2SL based eswir to operate with lower dark current densities. Moreover, using digital readout integrated circuits (digital ROIC) that have unlimited charge integration capability this photodetector could be used for high performance room temperature imaging. This will open the possibility of incorporating T2SL based eswir photodetectors in high performance infrared imaging systems and makes type II superlattices a viable candidate for making high performance infrared imagers and a possible replacement for current state of the art technologies. This material is based on research sponsored by Air Force Research Laboratory (AFRL) under agreement number FA

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