- Email: [email protected]
InAs strained-layer superlattice long wavelength infrared detectors
Infrared Physics & Technology 59 (2013) 18–21
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Heterojunction-based GaSb/InAs strained-layer superlattice long wavelength infrared detectors Sumith Bandara ⇑, Neil Baril, Patrick Maloney, Curtis Billman, Eric Nallon, Tiffany Shih, Joseph Pellegrino, Meimei Tidrow U.S. Army RDECOM CERDEC NVESD, Fort Belvoir, VA 22060, United States
h i g h l i g h t s " Design parameters for strained layer superlattice long-wave infrared detector are investigated. " Advantages of Heterojunction-based design are discussed. " Optimization of doping profile in hetero-junction based superlattice long-wave infrared detector are investigated.
a r t i c l e
i n f o
Article history: Available online 28 December 2012 Keywords: Long-wave Infrared detectors Strained layer superlattice Heterojunction
a b s t r a c t Design parameters for the heterojunction-based strained layer superlattice (SLS) long-wave infrared (LWIR) detector are investigated so that it operates at a lower bias voltage with lower dark current and higher photo response. At typical operating temperatures (T 77 K), the dark current of GaSb/InAs SLS LWIR detectors is dominated by the Shockley–Read–Hall (SRH) generation–recombination (g–r) process in the space-charge (depletion) region. In order to suppress this dark current, a wide bandgap region next to the absorber layer has been included in recent SLS designs. A series of heterojunction-based LWIR SLS detectors with various doping and barrier profiles have been designed and characterized. The significance of the doping profile and thickness of the wide-bandgap layer in optimization of the heterojunction-based SLS detector performance are exhibited from the modeling and experimental results of these devices. Published by Elsevier B.V.
1. Introduction Unlike those made from bulk materials, strained layer superlattice (SLS) long-wave infrared (LWIR) detectors exhibit flexibility in band alignments, material combinations, layer thicknesses, and carrier doping profiles. They also offer the potential for larger effective mass than bulk semiconductors such as mercury cadmium telluride (HgCdTe or MCT) for the equivalent bandgap (to suppress diode tunneling currents), and the possibility of Auger suppression in both the conduction and valence bands [1,2]. A typical LWIR SLS detector consists of a P+-p-N+ photodiode with a slightly p-type doped absorber layer [3–5]. The signal-to-noise ratios (SNRs) of these photodiodes are determined not only by minority carrier electron lifetimes, but also by the carrier doping density of the absorber layer. In an HgCdTe LWIR detector, the doping density of the absorber layer is minimized to suppress Auger recombination and band-to-band tunneling processes. Therefore, the HgCdTe detec⇑ Corresponding author. E-mail address: [email protected] (S. Bandara). 1350-4495/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.infrared.2012.12.003
tor’s absorber layer achieves longer minority carrier lifetimes, which leads to a very low dark current and high SNR. However, in bandgap-engineered SLS photodiodes, Auger recombination is suppressed by design, generation and recombination currents are reduced by inclusion of wider bandgap depletion layers, and band-to-band tunneling is weak due to high effective masses. Therefore, SLSs whose performance is limited by defect-mediated generation and recombination in their absorber layers can achieve higher SNRs by increasing the doping density of their absorber layers, thereby reducing minority carrier concentrations without a significant impact on minority carrier lifetimes [3–5]. 2. Homojunction versus heterojunction Analysis comparing detectors with SLS ‘‘heterojunction’’ designs to those having ‘‘homojunction’’ designs suggests that heterojunction-based devices have significantly lower generation–recombination (g–r) dark currents. Homojunction designs of SLS LWIR detectors include a lightly p-doped absorber layer sandwiched between heavily doped p-type and n-type contact layers, i.e., P+-p-N+
S. Bandara et al. / Infrared Physics & Technology 59 (2013) 18–21
diodes [6–8]. A typical absorber layer consists of several hundred periods of InAs/GaSb superlattice with InSb interfaces, for strain balancing to a GaSb substrate, or InAs/GaInSb strained-layer superlattices, where the GaInSb composition is designed for strain-balancing against InAs. During detector operation, a p–n junction is created at the interface between the absorber and n-type contact layer, and the photodiode operates under low reverse bias voltage. Due to high doping density of the n-contact layer, the depletion layer almost entirely exists within the absorber layer near the p–n junction. An analysis of experimental measurements shows that the dark currents of these devices are dominated by g–r over diffusion at the typical operating temperatures (T 77 K) [9]. Impurities, defects, and interface states located within the space-charge region, can act as g–r centers of the Shockley–Read–Hall type which result in a substantial junction dark current. Heterojunction designs of SLS LWIR detector include an additional wide bandgap region between the absorber and the N+-contact layers [4,5,10,11]. Therefore, the p–n junction is created at the interface between the absorber and the wide-gap layer, or at the wide-gap and the N+ contact layer, depending on the type of doping in the wide-gap barrier. In either case, most of the depletion region of the heterojunction SLS lies within the wide-gap layer. It has been shown that by moving the space-charge region to the wider bandgap layer, the dark current associated with g–r process can be suppressed, i.e., reducing the overall dark current. Thus, engineering the band-profile of the heterojunction SLS LWIR detectors plays a critical role in developing high-sensitivity mid- and longwavelength infrared detectors.
3. Material growth In this study, four heterojunction based SLS structures were grown on n-type GsSb substrates in a molecular beam epitaxy chamber. Intended designs of all four structures were the same except the thickness and doping of the wide-gap of each layer. Fig. 1 shows the details of the device growth parameters. Each structure consists of a 0.3 Be-doped GaSb buffer layer, a 0.5 lm thick p-type bottom-contact, a 2.5 lm thick LWIR absorber followed by a
19
wide-bandgap barrier and capped with a 0.43 lm thick n-type top-contact layer. The absorber layer, which consists of p-type (p = 1 1016 cm 3) 400-period (44 Å, 21 Å)-InAs/GaSb superlattice, is designed to have the cut-off wavelength of kc = 9.5 lm. The material of the wide-gap barriers is (42 Å, 12 Å)-InAs/AlSb superlattice, which has bandgap equivalent to 3 lm. The thicknesses and doping of the barriers in four devices are SLS-A: 0.432 lm with n = 1 1016 cm 3, SLS-B: 0.108 lm with p = 1 1016 cm 3, SLS-C: 0.162 lm with p = 1 1016 cm 3 and SLS-D: 0.432 lm p = 1 1016 cm 3. The bottom contact layers are 0.5 lm thick and have the same material as the absorber layer with heavy p-type doping P+ = 5 1017 cm 3. The top contact layers are 0.5 lm thick and have the same material as the barrier layer with heavy n-type doping N+ = 5 1017 cm 3. The wafers were processed into semi-etch test detectors with diameters ranging from 100 lm to 400 lm using standard wetchemical etching technique, i.e., the test detector mesas were isolated by etching just past the p–n junction in the wide-gap barrier. Ring-shaped metal contacts were deposited on the top-contacts allowing for a clear area for optical measurements via top illumination. After the processing, the samples were wire-bonded on to chip-carriers and loaded into a cryostat for optical and electric characterization.
4. Experimental data and discussions Fig. 2A shows normalized spectral responsivities measured at the peak bias voltages and Fig. 2B shows the behaviors of the responsivities as a function of applied bias voltage. All the responsivities shown in Fig. 2 are measured at temperature T = 77 K. Four wafers investigated in this study had the same absorber layer nominally but were grown at different times in different MBE reactors. Therefore, small deviations from the nominal structural parameters resulted in variations in cut-off wavelengths, which is apparent from Fig. 2A. However, the significance of this study is the behavior of responsivity as a function of applied bias voltage. As shown in Fig. 2B, SLS-A turns on at zero bias voltage and reaches a peak value immediately, i.e., Vb 0.02 V. In contrast, SLS-D requires Vb 0.6 V bias to turn on and Vb > 2 V bias to reach its peak
Fig. 1. Illustration of the band diagram and the growth parameters of the SLS detectors under study.
20
S. Bandara et al. / Infrared Physics & Technology 59 (2013) 18–21
Fig. 2. Normalized spectral responsivities measured at the peak bias voltages (A) and the behaviors of the responsivities as a function of applied bias voltage (B).
Fig. 3. (left) Illustrates the schematic band diagram for a SLS with a p-type absorber and n-wide-gap barrier followed by a N+-contact, i.e., similar to SLS-A. (right) Illustrates the schematic band diagrams at low and high bias voltages for a SLS with a p-type absorber and p-wide-gap barrier followed by a N+-contact, i.e., similar to SLS-B, C and D.
value. SLS-B and SLS-C show intermediate behavior, with SLS-C requiring higher bias voltages than SLS-B. Responsivity versus bias voltage data in Fig. 2B can be explained using a modeled upward or downward conduction band bending at the interface between the absorber and the wide-bandgap barrier and its dependence on the doping profile, wide-bandgap layer thickness, and the operating bias voltage. Fig. 3(left) illustrates the schematic band diagram for a SLS with a p -type absorber and n -wide-gap barrier followed by a N+-contact, i.e., similar to SLS-A. As shown in the figure, the p–n junction of SLS-A is located at the interface between the absorber and the wide-bandgap barrier, and the depletion layer starts in the absorber layer and spreads to the wide-gap barrier. Therefore the downward bended conduction band at the edge of the absorber layer allows photoexcited electrons to be easily transported to the N+ contact, without the aid of an additional bias voltage. Fig. 3(right) illustrates the schematic band diagram for a SLS with a p -type absorber and p -wide-gap barrier followed by a N+-contact, i.e., similar to SLSB, C and D. As shown in Fig. 3(right), p–n junction is located at the interface between the wide-bandgap barrier and the N+-contact, and the depletion layer starts in the N+-contact layer and spreads in the wide-gap barrier towards the absorber layer. If the depletion layer is thinner than the barrier layer, the conduction band bends upward at the absorber/barrier interface and the photoexcited electrons need to overcome an additional barrier in the wide-bandgap layer to transport to N+ contact. As bias voltage increases, the additional potential barrier disappears and depletion layer reaches the absorber/barrier interface. This explains the higher turn-on and peak bias voltage requirements shown in Fig. 2B for
Fig. 4. Dark current densities of the four devices measured at T = 77 K temperature as a function of applied bias voltage. The arrows indicate the applied bias voltages required to peak the responsivities of each detector.
SLS-D, which has the thickest p barrier. As barrier thickness decreases, the required turn-on and peak bias voltages decrease, i.e., in SLS-B and SLS-C in Fig. 2B. Modeling shows good agreement between the estimated bias voltages required to deplete the entire wide-barrier region and the measured bias voltages to reach the peak responsivities of SLS-B, SLS-C and SLS-D. Fig. 4 shows the dark current densities of the four devices measured at temperature T = 77 K as a function of applied bias voltage. The behavior of the dark current as a function of the bias voltage is to some extent complicated; however, it generally follows a trend
21
S. Bandara et al. / Infrared Physics & Technology 59 (2013) 18–21
Table 1 Dark currents normalized to a single cut-off wavelength knc = 9.5 lm at the operating bias voltages. Experimentally measured saturation bias voltages are compared with the calculations. Device
Cut-off w.l (kc) (lm)
Saturation bias (V)
Dark current @ T = 77 K (A/cm2)
Normalized dark current @ k = 9.5 lm (A/cm2)
Description of the wide-gap barrier
SLS-A SLS-B SLS-C SLS-D
9.8 8.5 8.8 9.3
0.05 0.15 0.35 2.0
8.98E 1.07E 1.06E 1.06E
4.89E 1.11E 5.15E 1.62E
0.432 lm 0.108 lm 0.162 lm 0.432 lm
05 05 05 05
similar to the behavior of the responsivity. Detector performance of the four SLSs can be evaluated by comparing dark current densities at the peak responsivity bias voltages. However, direct comparison of the dark current densities is erroneous because of the difference between the cut-off wavelengths. Table 1 shows dark currents normalized to a single cut-off wavelength knc = 9.5 lm by using a factor exp( hc/kT(kc knc)) where h is the Planck’s constant, c is the velocity of light, k is the Boltzmann’s factor, T is the operating temperature and kc is the cut-off wavelength. According to Table 1, SLS-D, the structure with the thickest p-doped barrier, shows the lowest normalized dark current at the operating bias voltage, and as the barrier thickness decreases (SLS-C and SLS-B) normalized dark current increases. It is also noticeable that SLSA, the structure with the thickest n-doped barrier, shows moderately lower dark current compared to the thinnest p-type barrier structure (SLS-B). As shown in Fig. 4 the steep increase in dark current densities of SLS-B and SLS-C suggests tunneling through the barriers, compared to thick barrier SLS-A and SLS-D.
5. Conclusions In summary, dark currents of SLS detectors have been reduced the by introducing heterojunction-based designs and by moving the p–n junction away from the absorber/barrier interface to the barrier/contact interface. Also, the operating bias voltage can be tuned by varying the thickness of the wide-gap barrier or by changing the doping type. Although the thick p-type barrier shows the lowest dark current, it requires a higher bias voltage to operate. Lowering the thickness of the barrier reduces the operating bias voltage, however it could enhance unwanted tunneling currents through the barrier. N-type thick barrier does not show any signs of tunneling currents, but it puts the p–n junction right at the edge of the barrier. Therefore, careful engineering is required to adjust the band and doping profile for heterojunction-based LWIR SLS
05 04 05 05
barrier barrier barrier barrier
n-type p-type p-type p-type
detectors to operate at lower bias voltage with lower dark current and higher photo response. Acknowledgments The authors gratefully acknowledge D. Ting for many helpful discussions, A. Liu, J. Fastenau, Y. Qiu and D. Loubychev for material growth. References [1] C.H. Grein, P.M. Young, M.E. Flatte, H. Ehrenreich, Long wavelength IlnAs/ lnGaSb infrared detectors: optimization of carrier lifetimes, J. Appl. Phys. 78 (12) (1995) 15. [2] W.E. Tennant, Rule 07 revisited: still a good heuristic predictor of p/n HgCdTe photodiode performance?, J Electron. Mater. 39 (7) (2010) 1030. [3] S. Bandara, P. Maloney, N. Baril, J. Pellegrino, M. Tidrow, Doping dependence of minority carrier lifetime in long-wave Sb-based type II superlattice infrared detector materials, Opt. Eng. 50 (6) (2011) 061015. [4] B.-M. Nguyen, D. Hoffman, E.K.-W. Huang, P.-Y. Delaunay, M. Razeghi, Background limited long wavelength infrared type-II InAs/GaSb superlattice photodiodes operating at 110 K, Appl. Phys. Lett. 93 (12) (2008) 123502. [5] P.Y. Delaunay, B.-M. Nguyen, M. Razeghi, Background limited performance of long wavelength infrared focal plane arrays fabricated from type-II InAs/GaSb M-structure superlattice, SPIE 7298 (2009) 72981Q-1. [6] F. Fuchs, U. Weimer, W. Pletschen, J. Schmitz, E. Ahlswere, M. Walter, J. Wagner, P. Koidl, High-performance InAs/Ga1-xInxSb superlattice infrared photodiodes, Appl. Phys. Lett. 71 (22) (1997) 3251. [7] C.J. Hill, A. Soibel, S.A. Keo, J.M. Mumolo, S.D. Gunapala, D.R. Rhiger, R.E. Kvaas, S.F. Harris, Infrared imaging arrays based on superlattice photodiodes, Proc. SPIE 6940 (2008) 69400C. [8] E.H. Aifer, I. Vurgaftmana, C.L. Canedy, J.H. Warnera, E.M. Jacksonb, J.G. Tischlera, J.R. Meyers, Recent progress in W-structured type-II superlattice photodiodes, Proc. SPIE 6479 (2007) 64790Y1. [9] J. Pellegrino, R. DeWames, Minority carrier lifetime characteristics in type II InAs/GaSb LWIR superlattice n+pp+ photodiodes, SPIE 7298 (2009) 72981U-1. [10] D.Z. Ting, C.J. Hill, A. Soibel, S.A. Keo, J.M. Mumolo, J. Nguyen, S.D. Gunapala, A high-performance long wavelength superlattice complementary barrier infrared detector, Appl. Phys. Lett 95 (2009) 023508. [11] David R. Rhiger, Robert E. Kvaas, Sean F. Harris, Borys P. Kolasa, Cory J. Hill, David Z. Ting, Characterization of barrier effects in superlattice LWIR detectors, Proc. SPIE 7660 (2010) 76601N-10.
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Heterojunction-based GaSb/InAs strained-layer superlattice long wavelength infrared detectors Sumith Bandara ⇑, Neil Baril, Patrick Maloney, Curtis Billman, Eric Nallon, Tiffany Shih, Joseph Pellegrino, Meimei Tidrow U.S. Army RDECOM CERDEC NVESD, Fort Belvoir, VA 22060, United States
h i g h l i g h t s " Design parameters for strained layer superlattice long-wave infrared detector are investigated. " Advantages of Heterojunction-based design are discussed. " Optimization of doping profile in hetero-junction based superlattice long-wave infrared detector are investigated.
a r t i c l e
i n f o
Article history: Available online 28 December 2012 Keywords: Long-wave Infrared detectors Strained layer superlattice Heterojunction
a b s t r a c t Design parameters for the heterojunction-based strained layer superlattice (SLS) long-wave infrared (LWIR) detector are investigated so that it operates at a lower bias voltage with lower dark current and higher photo response. At typical operating temperatures (T 77 K), the dark current of GaSb/InAs SLS LWIR detectors is dominated by the Shockley–Read–Hall (SRH) generation–recombination (g–r) process in the space-charge (depletion) region. In order to suppress this dark current, a wide bandgap region next to the absorber layer has been included in recent SLS designs. A series of heterojunction-based LWIR SLS detectors with various doping and barrier profiles have been designed and characterized. The significance of the doping profile and thickness of the wide-bandgap layer in optimization of the heterojunction-based SLS detector performance are exhibited from the modeling and experimental results of these devices. Published by Elsevier B.V.
1. Introduction Unlike those made from bulk materials, strained layer superlattice (SLS) long-wave infrared (LWIR) detectors exhibit flexibility in band alignments, material combinations, layer thicknesses, and carrier doping profiles. They also offer the potential for larger effective mass than bulk semiconductors such as mercury cadmium telluride (HgCdTe or MCT) for the equivalent bandgap (to suppress diode tunneling currents), and the possibility of Auger suppression in both the conduction and valence bands [1,2]. A typical LWIR SLS detector consists of a P+-p-N+ photodiode with a slightly p-type doped absorber layer [3–5]. The signal-to-noise ratios (SNRs) of these photodiodes are determined not only by minority carrier electron lifetimes, but also by the carrier doping density of the absorber layer. In an HgCdTe LWIR detector, the doping density of the absorber layer is minimized to suppress Auger recombination and band-to-band tunneling processes. Therefore, the HgCdTe detec⇑ Corresponding author. E-mail address: [email protected] (S. Bandara). 1350-4495/$ - see front matter Published by Elsevier B.V. http://dx.doi.org/10.1016/j.infrared.2012.12.003
tor’s absorber layer achieves longer minority carrier lifetimes, which leads to a very low dark current and high SNR. However, in bandgap-engineered SLS photodiodes, Auger recombination is suppressed by design, generation and recombination currents are reduced by inclusion of wider bandgap depletion layers, and band-to-band tunneling is weak due to high effective masses. Therefore, SLSs whose performance is limited by defect-mediated generation and recombination in their absorber layers can achieve higher SNRs by increasing the doping density of their absorber layers, thereby reducing minority carrier concentrations without a significant impact on minority carrier lifetimes [3–5]. 2. Homojunction versus heterojunction Analysis comparing detectors with SLS ‘‘heterojunction’’ designs to those having ‘‘homojunction’’ designs suggests that heterojunction-based devices have significantly lower generation–recombination (g–r) dark currents. Homojunction designs of SLS LWIR detectors include a lightly p-doped absorber layer sandwiched between heavily doped p-type and n-type contact layers, i.e., P+-p-N+
S. Bandara et al. / Infrared Physics & Technology 59 (2013) 18–21
diodes [6–8]. A typical absorber layer consists of several hundred periods of InAs/GaSb superlattice with InSb interfaces, for strain balancing to a GaSb substrate, or InAs/GaInSb strained-layer superlattices, where the GaInSb composition is designed for strain-balancing against InAs. During detector operation, a p–n junction is created at the interface between the absorber and n-type contact layer, and the photodiode operates under low reverse bias voltage. Due to high doping density of the n-contact layer, the depletion layer almost entirely exists within the absorber layer near the p–n junction. An analysis of experimental measurements shows that the dark currents of these devices are dominated by g–r over diffusion at the typical operating temperatures (T 77 K) [9]. Impurities, defects, and interface states located within the space-charge region, can act as g–r centers of the Shockley–Read–Hall type which result in a substantial junction dark current. Heterojunction designs of SLS LWIR detector include an additional wide bandgap region between the absorber and the N+-contact layers [4,5,10,11]. Therefore, the p–n junction is created at the interface between the absorber and the wide-gap layer, or at the wide-gap and the N+ contact layer, depending on the type of doping in the wide-gap barrier. In either case, most of the depletion region of the heterojunction SLS lies within the wide-gap layer. It has been shown that by moving the space-charge region to the wider bandgap layer, the dark current associated with g–r process can be suppressed, i.e., reducing the overall dark current. Thus, engineering the band-profile of the heterojunction SLS LWIR detectors plays a critical role in developing high-sensitivity mid- and longwavelength infrared detectors.
3. Material growth In this study, four heterojunction based SLS structures were grown on n-type GsSb substrates in a molecular beam epitaxy chamber. Intended designs of all four structures were the same except the thickness and doping of the wide-gap of each layer. Fig. 1 shows the details of the device growth parameters. Each structure consists of a 0.3 Be-doped GaSb buffer layer, a 0.5 lm thick p-type bottom-contact, a 2.5 lm thick LWIR absorber followed by a
19
wide-bandgap barrier and capped with a 0.43 lm thick n-type top-contact layer. The absorber layer, which consists of p-type (p = 1 1016 cm 3) 400-period (44 Å, 21 Å)-InAs/GaSb superlattice, is designed to have the cut-off wavelength of kc = 9.5 lm. The material of the wide-gap barriers is (42 Å, 12 Å)-InAs/AlSb superlattice, which has bandgap equivalent to 3 lm. The thicknesses and doping of the barriers in four devices are SLS-A: 0.432 lm with n = 1 1016 cm 3, SLS-B: 0.108 lm with p = 1 1016 cm 3, SLS-C: 0.162 lm with p = 1 1016 cm 3 and SLS-D: 0.432 lm p = 1 1016 cm 3. The bottom contact layers are 0.5 lm thick and have the same material as the absorber layer with heavy p-type doping P+ = 5 1017 cm 3. The top contact layers are 0.5 lm thick and have the same material as the barrier layer with heavy n-type doping N+ = 5 1017 cm 3. The wafers were processed into semi-etch test detectors with diameters ranging from 100 lm to 400 lm using standard wetchemical etching technique, i.e., the test detector mesas were isolated by etching just past the p–n junction in the wide-gap barrier. Ring-shaped metal contacts were deposited on the top-contacts allowing for a clear area for optical measurements via top illumination. After the processing, the samples were wire-bonded on to chip-carriers and loaded into a cryostat for optical and electric characterization.
4. Experimental data and discussions Fig. 2A shows normalized spectral responsivities measured at the peak bias voltages and Fig. 2B shows the behaviors of the responsivities as a function of applied bias voltage. All the responsivities shown in Fig. 2 are measured at temperature T = 77 K. Four wafers investigated in this study had the same absorber layer nominally but were grown at different times in different MBE reactors. Therefore, small deviations from the nominal structural parameters resulted in variations in cut-off wavelengths, which is apparent from Fig. 2A. However, the significance of this study is the behavior of responsivity as a function of applied bias voltage. As shown in Fig. 2B, SLS-A turns on at zero bias voltage and reaches a peak value immediately, i.e., Vb 0.02 V. In contrast, SLS-D requires Vb 0.6 V bias to turn on and Vb > 2 V bias to reach its peak
Fig. 1. Illustration of the band diagram and the growth parameters of the SLS detectors under study.
20
S. Bandara et al. / Infrared Physics & Technology 59 (2013) 18–21
Fig. 2. Normalized spectral responsivities measured at the peak bias voltages (A) and the behaviors of the responsivities as a function of applied bias voltage (B).
Fig. 3. (left) Illustrates the schematic band diagram for a SLS with a p-type absorber and n-wide-gap barrier followed by a N+-contact, i.e., similar to SLS-A. (right) Illustrates the schematic band diagrams at low and high bias voltages for a SLS with a p-type absorber and p-wide-gap barrier followed by a N+-contact, i.e., similar to SLS-B, C and D.
value. SLS-B and SLS-C show intermediate behavior, with SLS-C requiring higher bias voltages than SLS-B. Responsivity versus bias voltage data in Fig. 2B can be explained using a modeled upward or downward conduction band bending at the interface between the absorber and the wide-bandgap barrier and its dependence on the doping profile, wide-bandgap layer thickness, and the operating bias voltage. Fig. 3(left) illustrates the schematic band diagram for a SLS with a p -type absorber and n -wide-gap barrier followed by a N+-contact, i.e., similar to SLS-A. As shown in the figure, the p–n junction of SLS-A is located at the interface between the absorber and the wide-bandgap barrier, and the depletion layer starts in the absorber layer and spreads to the wide-gap barrier. Therefore the downward bended conduction band at the edge of the absorber layer allows photoexcited electrons to be easily transported to the N+ contact, without the aid of an additional bias voltage. Fig. 3(right) illustrates the schematic band diagram for a SLS with a p -type absorber and p -wide-gap barrier followed by a N+-contact, i.e., similar to SLSB, C and D. As shown in Fig. 3(right), p–n junction is located at the interface between the wide-bandgap barrier and the N+-contact, and the depletion layer starts in the N+-contact layer and spreads in the wide-gap barrier towards the absorber layer. If the depletion layer is thinner than the barrier layer, the conduction band bends upward at the absorber/barrier interface and the photoexcited electrons need to overcome an additional barrier in the wide-bandgap layer to transport to N+ contact. As bias voltage increases, the additional potential barrier disappears and depletion layer reaches the absorber/barrier interface. This explains the higher turn-on and peak bias voltage requirements shown in Fig. 2B for
Fig. 4. Dark current densities of the four devices measured at T = 77 K temperature as a function of applied bias voltage. The arrows indicate the applied bias voltages required to peak the responsivities of each detector.
SLS-D, which has the thickest p barrier. As barrier thickness decreases, the required turn-on and peak bias voltages decrease, i.e., in SLS-B and SLS-C in Fig. 2B. Modeling shows good agreement between the estimated bias voltages required to deplete the entire wide-barrier region and the measured bias voltages to reach the peak responsivities of SLS-B, SLS-C and SLS-D. Fig. 4 shows the dark current densities of the four devices measured at temperature T = 77 K as a function of applied bias voltage. The behavior of the dark current as a function of the bias voltage is to some extent complicated; however, it generally follows a trend
21
S. Bandara et al. / Infrared Physics & Technology 59 (2013) 18–21
Table 1 Dark currents normalized to a single cut-off wavelength knc = 9.5 lm at the operating bias voltages. Experimentally measured saturation bias voltages are compared with the calculations. Device
Cut-off w.l (kc) (lm)
Saturation bias (V)
Dark current @ T = 77 K (A/cm2)
Normalized dark current @ k = 9.5 lm (A/cm2)
Description of the wide-gap barrier
SLS-A SLS-B SLS-C SLS-D
9.8 8.5 8.8 9.3
0.05 0.15 0.35 2.0
8.98E 1.07E 1.06E 1.06E
4.89E 1.11E 5.15E 1.62E
0.432 lm 0.108 lm 0.162 lm 0.432 lm
05 05 05 05
similar to the behavior of the responsivity. Detector performance of the four SLSs can be evaluated by comparing dark current densities at the peak responsivity bias voltages. However, direct comparison of the dark current densities is erroneous because of the difference between the cut-off wavelengths. Table 1 shows dark currents normalized to a single cut-off wavelength knc = 9.5 lm by using a factor exp( hc/kT(kc knc)) where h is the Planck’s constant, c is the velocity of light, k is the Boltzmann’s factor, T is the operating temperature and kc is the cut-off wavelength. According to Table 1, SLS-D, the structure with the thickest p-doped barrier, shows the lowest normalized dark current at the operating bias voltage, and as the barrier thickness decreases (SLS-C and SLS-B) normalized dark current increases. It is also noticeable that SLSA, the structure with the thickest n-doped barrier, shows moderately lower dark current compared to the thinnest p-type barrier structure (SLS-B). As shown in Fig. 4 the steep increase in dark current densities of SLS-B and SLS-C suggests tunneling through the barriers, compared to thick barrier SLS-A and SLS-D.
5. Conclusions In summary, dark currents of SLS detectors have been reduced the by introducing heterojunction-based designs and by moving the p–n junction away from the absorber/barrier interface to the barrier/contact interface. Also, the operating bias voltage can be tuned by varying the thickness of the wide-gap barrier or by changing the doping type. Although the thick p-type barrier shows the lowest dark current, it requires a higher bias voltage to operate. Lowering the thickness of the barrier reduces the operating bias voltage, however it could enhance unwanted tunneling currents through the barrier. N-type thick barrier does not show any signs of tunneling currents, but it puts the p–n junction right at the edge of the barrier. Therefore, careful engineering is required to adjust the band and doping profile for heterojunction-based LWIR SLS
05 04 05 05
barrier barrier barrier barrier
n-type p-type p-type p-type
detectors to operate at lower bias voltage with lower dark current and higher photo response. Acknowledgments The authors gratefully acknowledge D. Ting for many helpful discussions, A. Liu, J. Fastenau, Y. Qiu and D. Loubychev for material growth. References [1] C.H. Grein, P.M. Young, M.E. Flatte, H. Ehrenreich, Long wavelength IlnAs/ lnGaSb infrared detectors: optimization of carrier lifetimes, J. Appl. Phys. 78 (12) (1995) 15. [2] W.E. Tennant, Rule 07 revisited: still a good heuristic predictor of p/n HgCdTe photodiode performance?, J Electron. Mater. 39 (7) (2010) 1030. [3] S. Bandara, P. Maloney, N. Baril, J. Pellegrino, M. Tidrow, Doping dependence of minority carrier lifetime in long-wave Sb-based type II superlattice infrared detector materials, Opt. Eng. 50 (6) (2011) 061015. [4] B.-M. Nguyen, D. Hoffman, E.K.-W. Huang, P.-Y. Delaunay, M. Razeghi, Background limited long wavelength infrared type-II InAs/GaSb superlattice photodiodes operating at 110 K, Appl. Phys. Lett. 93 (12) (2008) 123502. [5] P.Y. Delaunay, B.-M. Nguyen, M. Razeghi, Background limited performance of long wavelength infrared focal plane arrays fabricated from type-II InAs/GaSb M-structure superlattice, SPIE 7298 (2009) 72981Q-1. [6] F. Fuchs, U. Weimer, W. Pletschen, J. Schmitz, E. Ahlswere, M. Walter, J. Wagner, P. Koidl, High-performance InAs/Ga1-xInxSb superlattice infrared photodiodes, Appl. Phys. Lett. 71 (22) (1997) 3251. [7] C.J. Hill, A. Soibel, S.A. Keo, J.M. Mumolo, S.D. Gunapala, D.R. Rhiger, R.E. Kvaas, S.F. Harris, Infrared imaging arrays based on superlattice photodiodes, Proc. SPIE 6940 (2008) 69400C. [8] E.H. Aifer, I. Vurgaftmana, C.L. Canedy, J.H. Warnera, E.M. Jacksonb, J.G. Tischlera, J.R. Meyers, Recent progress in W-structured type-II superlattice photodiodes, Proc. SPIE 6479 (2007) 64790Y1. [9] J. Pellegrino, R. DeWames, Minority carrier lifetime characteristics in type II InAs/GaSb LWIR superlattice n+pp+ photodiodes, SPIE 7298 (2009) 72981U-1. [10] D.Z. Ting, C.J. Hill, A. Soibel, S.A. Keo, J.M. Mumolo, J. Nguyen, S.D. Gunapala, A high-performance long wavelength superlattice complementary barrier infrared detector, Appl. Phys. Lett 95 (2009) 023508. [11] David R. Rhiger, Robert E. Kvaas, Sean F. Harris, Borys P. Kolasa, Cory J. Hill, David Z. Ting, Characterization of barrier effects in superlattice LWIR detectors, Proc. SPIE 7660 (2010) 76601N-10.