This thesis reports a detailed characterization of indium arsenide antimonide (InAs1- xSbx) photodetectors grown on gallium arsenide (GaAs) substrate by molecular beam epitaxy. A combination of polyimide and sulphur and a single layer of polyimide were used as passivation films in this study. Two different epilayer structures were used for assessing the detector performance and comparing the above passivation layers. For the first structure, the optical measurements revealed that Sb mole fraction was 0.13 and the cut-off wavelength was around 4.1 &micro / m at 80 K. The Sb mole fraction of the second structure was 0.2, and the 77 K cut-off wavelength was 4.8 &micro / m. Detailed electrical and optical characterizations were performed on 33x33 &micro / m2 test diodes. The photodiodes yielded peak detectivities of ~3.65x1010 and ~1.22x1010 cmHz1/2/W at 80 K for the first and second structures, respectively. Considerable 1/f noise current related with trap-assisted tunneling mechanism was observed in both structures at 80 K. Dark current modeling study showed that the dark current was dominated by the shunt and trap-assisted tunneling mechanisms throughout the entire reverse bias voltages for both detectors. The dark current analysis of variable area detectors fabricated with both epilayer structures revealed that the reverse bias current was mainly generated by the surface leakage in small sized (33x33 &micro / m2) detectors for both passivation layers. However, the surface current contribution was observed to be lower for the double layer (polyimide and sulphur) passivated detectors suggesting that it is a better passivation technique.

abstract: Infrared photodetectors, used in applications for sensing and imaging, such as military target recognition, chemical/gas detection, and night vision enhancement, are predominantly comprised of an expensive II-VI material, HgCdTe. III-V type-II superlattices (SLs) have been studied as viable alternatives for HgCdTe due to the SL advantages over HgCdTe: greater control of the alloy composition, resulting in more uniform materials and cutoff wavelengths across the wafer; stronger bonds and structural stability; less expensive substrates, i.e., GaSb; mature III-V growth and processing technologies; lower band-to-band tunneling due to larger electron effective masses; and reduced Auger recombination enabling operation at higher temperatures and longer wavelengths. However, the dark current of InAs/Ga1-xInxSb SL detectors is higher than that of HgCdTe detectors and limited by Shockley-Read-Hall (SRH) recombination rather than Auger recombination. This dissertation work focuses on InAs/InAs1-xSbx SLs, another promising alternative for infrared laser and detector applications due to possible lower SRH recombination and the absence of gallium, which simplifies the SL interfaces and growth processes. InAs/InAs1-xSbx SLs strain-balanced to GaSb substrates were designed for the mid- and long-wavelength infrared (MWIR and LWIR) spectral ranges and were grown using MOCVD and MBE by various groups. Detailed characterization using high-resolution x-ray diffraction, atomic force microscopy, photoluminescence (PL), and photoconductance revealed the excellent structural and optical properties of the MBE materials. Two key material parameters were studied in detail: the valence band offset (VBO) and minority carrier lifetime. The VBO between InAs and InAs1-xSbx strained on GaSb with x = 0.28 - 0.41 was best described by Qv = ÄEv/ÄEg = 1.75 ± 0.03. Time-resolved PL experiments on a LWIR SL revealed a lifetime of 412 ns at 77 K, one order of magnitude greater than that of InAs/Ga1-xInxSb LWIR SLs due to less SRH recombination. MWIR SLs also had 100's of ns lifetimes that were dominated by radiative recombination due to shorter periods and larger wave function overlaps. These results allow InAs/InAs1-xSbx SLs to be designed for LWIR photodetectors with minority carrier lifetimes approaching those of HgCdTe, lower dark currents, and higher operating temperatures. / Dissertation/Thesis / Ph.D. Electrical Engineering 2012

InAsxSb1-x (Indium Arsenide Antimonide) is an important low bandgap semiconductor whose high quality growth on GaAs or Si substrates is indispensible for low cost, large format infrared focal plane arrays (FPAs). Quantum well infrared photodetector (QWIP) technology, relying on mature semiconductors, is also promising for the above purpose. While AlGaAs/GaAs has been the standard material system for QWIPs, the search for alternative materials is needed for better performance. This thesis reports a detailed investigation of molecular beam epitaxy grown mid-wavelength infrared InAsxSb1-x photodiodes on alternative substrates, and long wavelength infrared InP/InGaAs QWIPs. In the first part of the study, InSb and InAs0.8Sb0.2 photodiodes grown on Si and GaAs substrates are investigated to reveal the performance degrading mechanisms due to large lattice mismatch. InAs0.8Sb0.2/GaAs photodiodes yield peak detectivities of 1.4&times / 1010 and 7.5&times / 108 cmHz&frac12 / /W at 77 K and 240 K, respectively, showing that the alloy is promising for both cooled and near room temperature detectors. Under moderate reverse bias, 80 K RoA product limiting mechanism is trap assisted tunneling, which introduces considerable 1/f noise. InSb/Si photodiodes display peak 77 K detectivity as high as ~1&times / 1010 cmHz 1/2/W and reasonably high peak quantum efficiency in spite of large lattice mismatch. RoA product of detectors at 80 K is limited by Ohmic leakage with small activation energy (25 meV). Bias and temperature dependence of 1/f noise is in reasonable agreement with Kleinpenning&rsquo / s mobility fluctuation model, confirming the validity of this approach. The second part of the study concentrates on InP/In0.53Ga0.47As QWIPs, and 640&times / 512 FPA, which to our knowledge, is the largest format InP/InGaAs QWIP FPA reported. InP/InGaAs QWIPs yield quantum efficiency-gain product as high as 0.46 under moderate bias. At 70 K, detector performance is background limited with f/2 aperture up to ~3 V bias where peak responsivity (2.9 A/W) is thirty times higher than that of the Al0.275Ga0.725As/GaAs QWIP with similar spectral response. Impact ionization in InP/InGaAs QWIPs does not start until the average electric-field reaches 25 kV/cm, maintaining high detectivity under moderate bias. The 640&times / 512 InP/InGaAs QWIP FPA yields noise equivalent temperature difference of ~40 mK at an FPA temperature as high as 77 K and reasonably low NETD even with short integration times (t). 70 K NETD values of the FPA with f/1.5 optics are 36 and 64 mK under &ndash / 0.5 V (t=11 ms) and &ndash / 2 V (t=650 Rs) bias, respectively. The results clearly show the potential of InP/InGaAs QWIPs for thermal imaging applications requiring short integration times. Keywords: Cooled infrared detectors, InAsSb, QWIP, focal plane array.

abstract: High-performance III-V semiconductors based on ternary alloys and superlattice systems are fabricated, studied, and compared for infrared optoelectronic applications. InAsBi is a ternary alloy near the GaSb lattice constant that is not as thoroughly investigated as other III-V alloys and that is challenging to produce as Bi has a tendency to surface segregate and form droplets during growth rather than incorporate. A growth window is identified within which high-quality droplet-free bulk InAsBi is produced and Bi mole fractions up to 6.4% are obtained. Photoluminescence with high internal quantum efficiency is observed from InAs/InAsBi quantum wells. The high structural and optical quality of the InAsBi materials examined demonstrates that bulk, quantum well, and superlattice structures utilizing InAsBi are an important design option for efficient infrared coverage. Another important infrared material system is InAsSb and the strain-balanced InAs/InAsSb superlattice on GaSb. Detailed examination of X-ray diffraction, photoluminescence, and spectroscopic ellipsometry data provides the temperature and composition dependent bandgap of bulk InAsSb. The unintentional incorporation of approximately 1% Sb into the InAs layers of the superlattice is measured and found to significantly impact the analysis of the InAs/InAsSb band alignment. In the analysis of the absorption spectra, the ground state absorption coefficient and transition strength of the superlattice are proportional to the square of the electron-hole wavefunction overlap; wavefunction overlap is therefore a major design parameter in terms of optimizing absorption in these materials. Furthermore in addition to improvements through design optimization, the optical quality of the materials studied is found to be positively enhanced with the use of Bi as a surfactant during molecular beam epitaxy growth. A software tool is developed that calculates and optimizes the miniband structure of semiconductor superlattices, including bismide-based designs. The software has the capability to limit results to designs that can be produced with high structural and optical quality, and optimized designs in terms of maximizing absorption are identified for several infrared superlattice systems at the GaSb lattice constant. The accuracy of the software predictions are tested with the design and growth of an optimized mid-wave infrared InAs/InAsSb superlattice which exhibits superior optical and absorption properties. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2016

abstract: Sb-based type-II superlattices (T2SLs) are potential alternative to HgCdTe for infrared detection due to their low manufacturing cost, good uniformity, high structural stability, and suppressed Auger recombination. The emerging InAs/InAsSb T2SLs have minority carrier lifetimes 1-2 orders of magnitude longer than those of the well-studied InAs/InGaSb T2SLs, and therefore have the potential to achieve photodetectors with higher performance. This work develops a novel method to measure the minority carrier lifetimes in infrared materials, and reports a comprehensive characterization of minority carrier lifetime and transport in InAs/InAsSb T2SLs at temperatures below 77 K. A real-time baseline correction (RBC) method for minority carrier lifetime measurement is developed by upgrading a conventional boxcar-based time-resolved photoluminescence (TRPL) experimental system that suffers from low signal-to-noise ratio due to strong low frequency noise. The key is to modify the impulse response of the conventional TRPL system, and therefore the system becomes less sensitive to the dominant noise. Using this RBC method, the signal-to-noise ratio is improved by 2 orders of magnitude. A record long minority carrier lifetime of 12.8 μs is observed in a high-quality mid-wavelength infrared InAs/InAsSb T2SLs at 15 K. It is further discovered that this long lifetime is partially due to strong carrier localization, which is revealed by temperature-dependent photoluminescence (PL) and TRPL measurements for InAs/InAsSb T2SLs with different period thicknesses. Moreover, the PL and TRPL results suggest that the atomic layer thickness variation is the main origin of carrier localization, which is further confirmed by a calculation using transfer matrix method. To study the impact of the carrier localization on the device performance of InAs/InAsSb photodetectors, minority hole diffusion lengths are determined by the simulation of external quantum efficiency (EQE). A comparative study shows that carrier localization has negligible effect on the minority hole diffusion length in InAs/InAsSb T2SLs, and the long minority carrier lifetimes enhanced by carrier localization is not beneficial for photodetector operation. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2016

All-optical time-resolved measurement techniques provide a powerful tool for investigating critical parameters that determine the performance of infrared photodetector and emitter semiconductor materials. Narrow-bandgap InAs/GaSb type-II superlattices (T2SLs) have shown great promise as a next generation source of these materials, due to superior intrinsic properties and versatility. Unfortunately, InAs/GaSb T2SLs are plagued by parasitic Shockley-Read-Hall recombination centers that shorten the carrier lifetime and limit device performance. Ultrafast pump-probe techniques and time-resolved differential transmission measurements are used here to demonstrate that Ga-free InAs/InAsSb T2SLs and InAsSb alloys do not have this same limitation and thus have significantly longer carrier lifetimes. Measurements at 77 K provided minority carrier lifetimes of 9 μs and 3 μs for an unintentionally doped mid-wave infrared (MWIR) InAs/InAsSb T2SL and InAsSb alloy, respectively; a two order of magnitude increase compared to the 90 ns minority carrier lifetime measured in a comparable MWIR InAs/GaSb T2SL. Through temperature-dependent lifetime measurements, the various carrier recombination processes are differentiated and the dominant mechanisms identified for each material. These results demonstrate that these Ga-free materials are viable options over InAs/GaSb T2SLs for potentially improved infrared photodetectors. In addition to carrier lifetimes, the drift and diffusion of excited charge carriers through the superlattice growth layers (i.e. vertical transport) directly affects the performance of photodetectors and emitters. Unfortunately, there is a lack of information pertaining to vertical transport, primarily due to difficulties in making measurements on thin growth layers and the need for non-standard measurement techniques. However, all-optical ultrafast techniques are successfully used here to directly measure vertical diffusion in MWIR InAs/GaSb T2SLs. By optically generating excess carriers near one end of a MWIR T2SL and measuring the transit time to a thin, 2 lower-bandgap superlattice placed at the other end, the time-of-flight of vertically diffusing carriers is determined. Through investigation of both unintentionally doped and p-type superlattices at 77 K, the vertical hole and electron diffusion coefficients are determined to be 0.04±0.03 cm2/s and 4.7±0.5 cm2/s, corresponding to vertical mobilities of 6±5 cm2/Vs and 700±80 cm2/Vs, respectively. These measurements are, to my knowledge, the first direct measurements of vertical transport properties in narrow-bandgap superlattices. Lastly, the widely tunable two-color ultrafast laser system used in this research allowed for the investigation of nonlinear optical properties in narrow-bandgap semiconductors. Time-resolved measurements taken at 77 K of the nondegenerate two-photon absorption spectrum of bulk n-type GaSb have provided new information about the nonresonant change in absorption and two-photon absorption coefficients in this material. Furthermore, as the nondegenerate spectrum was measured over a wide range of optical frequencies, a Kramers-Kronig transformation allowed the dispersion of the nondegenerate nonlinear refractive index to be calculated.