CdTe/MgCdTe double heterostructures (DHs) integrated with a heavily-doped a-Si:H layer as the hole contact was demonstrated a record open-circuit voltage (VOC) of 1.11 V and an active-area efficiency of 20% in 2016. Despite this significant progress, some of the underlying device physics has not been fully understood. The first part of this dissertation reports a systematic study of the CdTe/MgCdTe DH devices. The CdTe/MgCdTe DHs are grown on InSb(001) substrates. The vertical transport mechanisms across the CdTe and InSb heterovalent interface are investigated with N-CdTe/n-InSb and N-CdTe/p-InSb heterostructures. A transport model including tunneling through CdTe barrier and InSb interband transition is developed to explain the different temperature dependent current-voltage characteristics of these two heterostructures. Different p-type layers are integrated with the CdTe/MgCdTe DHs to form solar cells with different VOC values and efficiencies. The low VOC of devices with ZnTe:Cu and ZnTe:As hole contacts is attributed to the low built-in voltage and reduced minority carrier lifetime in the CdTe absorber, respectively. The critical requirements for reaching high VOC values are analyzed. A novel epitaxial lift-off technology for monocrystalline CdTe is developed using a water-soluble and nearly lattice-matched MgTe sacrificial layer grown on InSb substrate. The freestanding CdTe/MgCdTe DH thin films obtained from the lift-off process show improved optical performance due to enhanced light extraction efficiency and photo-recycling effect. This technology enables the possible development of monocrystalline CdTe thin-film solar cells and 1.7/1.1-eV MgCdTe/Si or MgCdTe/Cu(InGa)Se2 tandem solar cells. The monocrystalline CdTe thin-film solar cells and 1.7-eV MgCdTe DH solar cells have been demonstrated with a power conversion efficiency of 9.8% and an active-area efficiency as high as 15.2%, respectively. Additionally, a study of the radiation effects on CdTe DHs under 68-MeV proton irradiation is performed and showed their superior radiation tolerance. All these findings indicate that the monocrystalline CdTe thin-film solar cells are reasonably expected to have low weight, high-efficiency and high power density, ideal for space applications.

Ge1-xSnx and SiyGe1-x-ySnx materials are being researched intensively for applications in infra-red optoelectronic devices. Due to their direct band gap these materials may in-fact be the enabling factor in the commercial realization of silicon photonics/group IV photonics and the integration of nanophotonics with nanoelectronics. However the synthesis of these meta-stable semiconductor alloys, with a range of Sn-compositions, remains the primary technical challenge. Highly specialized epitaxial growth methods must be employed to produce single crystal layers which have sufficient quality for optoelectronic device applications. Up to this point these methods have been unfavorable from a semiconductor manufacturing perspective. In this work the growth of high-quality Si-Ge-Sn epitaxial alloys on Ge-buffered Si (100) using an industry-standard reduced pressure chemical vapor deposition reactor and a cost-effective chemistry is demonstrated. The growth kinetics are studied in detail in-order to understand the factors influencing layer composition, morphology, and defectivity. In doing so breakthrough GeSn materials and device results are achieved including methods to overcome the limits of Sn-incorporation and the realization of low-defect and strain-relaxed epitaxial layers with up to 20% Sn.

P and n-type doping methods are presented in addition to the production of SiGeSn ternary alloys. Finally optically stimulated lasing in thick GeSn layers and SiGeSn/GeSn multiple quantum wells is demonstrated. Lasing wavelengths ranging from 2-3 µm at temperatures up to 180K are realized in thick layers. Whereas SiGeSn/GeSn multiple quantum wells on a strain-relaxed GeSn buffers have enabled the first reported SiGeSn/GeSn multiple quantum well laser operating up to 80K with threshold power densities as low as 33 kW/cm2.

Polycrystalline CdS/CdTe solar cells continue to dominate the thin-film photovoltaics industry with an achieved record efficiency of over 22% demonstrated by First Solar, yet monocrystalline CdTe devices have received considerably less attention over the years. Monocrystalline CdTe double-heterostructure solar cells show great promise with respect to addressing the problem of low Voc with the passing of the 1 V benchmark. Rapid progress has been made in driving the efficiency in these devices ever closer to the record presently held by polycrystalline thin-films. This achievement is primarily due to the utilization of a remote p-n heterojunction in which the heavily doped contact materials, which are so problematic in terms of increasing non-radiative recombination inside the absorber, are moved outside of the CdTe double heterostructure with two MgyCd1-yTe barrier layers to provide confinement and passivation at the CdTe surfaces. Using this design, the pursuit and demonstration of efficiencies beyond 20% in CdTe solar cells is reported through the study and optimization of the structure barriers, contacts layers, and optical design. Further development of a wider bandgap MgxCd1-xTe solar cell based on the same design is included with the intention of applying this knowledge to the development of a tandem solar cell constructed on a silicon subcell. The exploration of different hole-contact materials—ZnTe, CuZnS, and a-Si:H—and their optimization is presented throughout the work. Devices utilizing a-Si:H hole contacts exhibit open-circuit voltages of up to 1.11 V, a maximum total-area efficiency of 18.5% measured under AM1.5G, and an active-area efficiency of 20.3% for CdTe absorber based devices. The achievement of voltages beyond 1.1V while still maintaining relatively high fill factors with no rollover, either before or after open-circuit, is a promising indicator that this approach can result in devices surpassing the 22% record set by polycrystalline designs. MgxCd1-xTe absorber based devices have been demonstrated with open-circuit voltages of up to 1.176 V and a maximum active-area efficiency of 11.2%. A discussion of the various loss mechanisms present within these devices, both optical and electrical, concludes with the presentation of a series of potential design changes meant to address these issues.

CdTe/MgxCd1-xTe double heterostructures (DHs) have been grown on lattice matched InSb (001) substrates using Molecular Beam Epitaxy. The MgxCd1-xTe layers, which have a wider bandgap and type-I band edge alignment with CdTe, provide sufficient carrier confinement to CdTe, so that the optical properties of CdTe can be studied. The DH samples show very strong Photoluminescence (PL) intensity, long carrier lifetimes (up to 3.6 μs) and low effective interface recombination velocity at the CdTe/MgxCd1 xTe heterointerface (~1 cm/s), indicating the high material quality. Indium has been attempted as an n-type dopant in CdTe and it is found that the carriers are 100% ionized in the doping range of 1×1016 cm-3 to 1×1018 cm-3. With decent doping levels, long minority carrier lifetime, and almost perfect surface passivation by the MgxCd1-xTe layer, the CdTe/MgxCd1-xTe DHs are applied to high efficiency CdTe solar cells. Monocrystalline CdTe solar cells with efficiency of 17.0% and a record breaking open circuit voltage of 1.096 V have been demonstrated in our group.

Mg0.13Cd0.87Te (1.7 eV), also with high material quality, has been proposed as a current matching cell to Si (1.1 eV) solar cells, which could potentially enable a tandem solar cell with high efficiency and thus lower the electricity cost. The properties of Mg0.13Cd0.87Te/Mg0.5Cd0.5Te DHs and solar cells have been investigated. Carrier lifetime as long as 0.56 μs is observed and a solar cell with 11.2% efficiency and open circuit voltage of 1.176 V is demonstrated.

The CdTe/MgxCd1-xTe DHs could also be potentially applied to luminescence refrigeration, which could be used in vibration-free space applications. Both external luminescence quantum efficiency and excitation-dependent PL measurement show that the best quality samples are almost 100% dominated by radiative recombination, and calculation shows that the internal quantum efficiency can be as high as 99.7% at the optimal injection level (1017 cm-3). External luminescence quantum efficiency of over 98% can be realized for luminescence refrigeration with the proper design of optical structures.

This work demonstrates novel nBn photodetectors including mid-wave infrared (MWIR) nBn photodetectors based on InAs/InAsSb type-II superlattices (T2SLs) with charge as the output signal, and visible nBn photodetectors based on CdTe with current output. Furthermore, visible/MWIR two-color photodetectors (2CPDs) are fabricated through monolithic integration of the CdTe nBn photodetector and an InSb photodiode.

The MWIR nBn photodetectors have a potential well for holes present in the barrier layer. At low voltages of < −0.2 V, which ensure low dark current <10-5 A/cm2 at 77 K, photogenerated holes are collected in this well with a storage lifetime of 40 s. This charge collection process is an in-device signal integration process that reduces the random noise significantly. Since the stored holes can be readout laterally as in charge-coupled devices, it is therefore possible to make charge-output nBn with much lower noise than conventional current-output nBn photodetectors.

The visible nBn photodetectors have a CdTe absorber layer and a ZnTe barrier layer with an aligned valence band edge. By using a novel ITO/undoped-CdTe top contact design, it has achieved a high specific detectivity of 3×1013 cm-Hz1/2/W at room temperature. Particularly, this CdTe nBn photodetector grown on InSb substrates enables the monolithic integration of CdTe and InSb photodetectors, and provides a platform to study in-depth device physics of nBn photodetectors at room temperature.

Furthermore, the visible/MWIR 2CPD has been developed by the monolithic integration of the CdTe nBn and an InSb photodiode through an n-CdTe/p-InSb tunnel junction. At 77 K, the photoresponse of the 2CPD can be switched between a 1-5.5 μm MWIR band and a 350-780 nm visible band by illuminating the device with an external light source or not, and applying with proper voltages. Under optimum conditions, the 2CPD has achieved a MWIR peak responsivity of 0.75 A/W with a band rejection ratio (BRR) of 52 dB, and a visible peak responsivity of 0.3 A/W with a BRR of 18 dB. This 2CPD has enabled future compact image sensors with high fill-factor and responsivity switchable between visible and MWIR colors.

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.

There has been recent interest in demonstrating solar cells which approach the detailed-balance or thermodynamic efficiency limit in order to establish a model system for which mass-produced solar cells can be designed. Polycrystalline CdS/CdTe heterostructures are currently one of many competing solar cell material systems. Despite being polycrystalline, efficiencies up to 21 % have been demonstrated by the company First Solar. However, this efficiency is still far from the detailed-balance limit of 32.1 % for CdTe. This work explores the use of monocrystalline CdTe/MgCdTe and ZnTe/CdTe/MgCdTe double heterostructures (DHs) grown on (001) InSb substrates by molecular beam epitaxy (MBE) for photovoltaic applications.

Undoped CdTe/MgCdTe DHs are first grown in order to determine the material quality of the CdTe epilayer and to optimize the growth conditions. DH samples show strong photoluminescence with over double the intensity as that of a GaAs/AlGaAs DH with an identical layer structure. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a carrier lifetime of up to 179 ns for a 2 µm thick CdTe layer, which is more than one order of magnitude longer than that of polycrystalline CdTe films. MgCdTe barrier layers are found to be effective at confining photogenerated carriers and have a relatively low interface recombination velocity of 461 cm/s. The optimal growth temperature and Cd/Te flux ratio is determined to be 265 °C and 1.5, respectively.

Monocrystalline ZnTe/CdTe/MgCdTe P-n-N DH solar cells are designed, grown, processed into solar cell devices, and characterized. A maximum efficiency of 6.11 % is demonstrated for samples without an anti-reflection coating. The low efficiency is mainly due to the low open-circuit voltage (V_oc), which is attributed to high dark current caused by interface recombination at the ZnTe/CdTe interface. Low-temperature measurements show a linear increase in V_oc with decreasing temperature down to 77 K, which suggests that the room-temperature operation is limited by non-radiative recombination. An open-circuit voltage of 1.22 V and an efficiency of 8.46 % is demonstrated at 77 K. It is expected that a coherently strained MgCdTe/CdTe/MgCdTe DH solar cell design will produce higher efficiency and V_oc compared to the ZnTe/CdTe/MgCdTe design with relaxed ZnTe layer.

Photodetectors in the 1.7 to 4.0 μm range are being commercially developed on InP substrates to meet the needs of longer wavelength applications such as thermal and medical sensing. Currently, these devices utilize high indium content metamorphic Ga1-xInxAs (x > 0.53) layers to extend the wavelength range beyond the 1.7 μm achievable using lattice matched GaInAs. The large lattice mismatch required to reach the extended wavelengths results in photodetector materials that contain a large number of misfit dislocations. The low quality of these materials results in a large nonradiative Shockley Read Hall generation/recombination rate that is manifested as an undesirable large thermal noise level in these photodetectors. This work focuses on utilizing the different band structure engineering methods to design more efficient devices on InP substrates. One prospective way to improve photodetector performance at the extended wavelengths is to utilize lattice matched GaInAs/GaAsSb structures that have a type-II band alignment, where the ground state transition energy of the superlattice is smaller than the bandgap of either constituent material. Over the extended wavelength range of 2 to 3 μm this superlattice structure has an optimal period thickness of 3.4 to 5.2 nm and a wavefunction overlap of 0.8 to 0.4, respectively. In using a type-II superlattice to extend the cutoff wavelength there is a tradeoff between the wavelength reached and the electron-hole wavefunction overlap realized, and hence absorption coefficient achieved. This tradeoff and the subsequent reduction in performance can be overcome by two methods: adding bismuth to this type-II material system; applying strain on both layers in the system to attain strain-balanced condition. These allow the valance band alignment and hence the wavefunction overlap to be tuned independently of the wavelength cutoff. Adding 3% bismuth to the GaInAs constituent material, the resulting lattice matched Ga0.516In0.484As0.970Bi0.030/GaAs0.511Sb0.489superlattice realizes a 50% larger absorption coefficient. While as, similar results can be achieved with strain-balanced condition with strain limited to 1.9% on either layer. The optimal design rules derived from the different possibilities make it feasible to extract superlattice period thickness with the best absorption coefficient for any cutoff wavelength in the range.  

Recently a new materials platform consisting of semiconductors grown on GaSb and InAs substrates with lattice constants close to 6.1 A was proposed by our group for various electronic and optoelectronic applications. This materials platform consists of both II-VI (MgZnCdHg)(SeTe) and III-V (InGaAl)(AsSb) compound semiconductors, which have direct bandgaps spanning the entire energy spectrum from far-IR (~0 eV) up to UV (~3.4 eV). The broad range of bandgaps and material properties make it very attractive for a wide range of applications in optoelectronics, such as solar cells, laser diodes, light emitting diodes, and photodetectors. Moreover, this novel materials system potentially offers unlimited degrees of freedom for integration of electronic and optoelectronic devices onto a single substrate while keeping the best possible materials quality with very low densities of misfit dislocations. This capability is not achievable with any other known lattice-matched semiconductors on any available substrate. In the 6.1-A materials system, the semiconductors ZnTe and GaSb are almost perfectly lattice-matched with a lattice mismatch of only 0.13%. Correspondingly, it is expected that high quality ZnTe/GaSb and GaSb/ZnTe heterostructures can be achieved with very few dislocations generated during growth. To fulfill the task, their MBE growth and material properties are carefully investigated. High quality ZnTe layers grown on various III-V substrates and GaSb grown on ZnTe are successfully achieved using MBE. It is also noticed that ZnTe and GaSb have a type-I band-edge alignment with large band offsets (delta_Ec=0.934 eV, delta_Ev=0.6 eV), which provides strong confinement for both electrons and holes. Furthermore, a large difference in refractive index is found between ZnTe and GaSb (2.7 and 3.9, respectively, at 0.7 eV), leading to excellent optical confinement of the guided optical modes in planar semiconductor lasers or distributed Bragg reflectors (DBR) for vertical-cavity surface-emitting lasers. Therefore, GaSb/ZnTe double-heterostructure and ZnTe/GaSb DBR structure are suitable for use in light emitting devices. In this thesis work, experimental demonstration of these structures with excellent structural and optical properties is reported. During the exploration on the properties of various ZnTe heterostructures, it is found that residual tensile strains exist in the thick ZnTe epilayers when they are grown on GaAs, InP, InAs and GaSb substrates. The presence of tensile strains is due to the difference in thermal expansion coefficients between the epilayers and the substrates. The defect densities in these ZnTe layers become lower as the ZnTe layer thickness increases. Growth of high quality GaSb on ZnTe can be achieved using a temperature ramp during growth. The influence of temperature ramps with different ramping rates in the optical properties of GaSb layer is studied, and the samples grown with a temperature ramp from 360 to 470 C at a rate of 33 C/min show the narrowest bound exciton emission peak with a full width at half maximum of 15 meV. ZnTe/GaSb DBR structures show excellent reflectivity properties in the mid-infrared range. A peak reflectance of 99% with a wide stopband of 480 nm centered at 2.5 um is measured from a ZnTe/GaSb DBR sample of only 7 quarter-wavelength pairs.

A dual chamber molecular beam epitaxy (MBE) system was rebuilt for the growth of 6.1 Angstrom II-VI and III-V compound semiconductor materials that are to be used in novel optoelectronic devices that take advantage of the nearly continuous bandgap availability between 0 eV and 3.4 eV. These devices include multijunction solar cells and multicolor detectors. The MBE system upgrade involved the conversion of a former III-V chamber for II-VI growth. This required intensive cleaning of the chamber and components to prevent contamination. Special features including valved II-VI sources and the addition of a cold trap allowed for the full system to be baked to 200 degrees Celsius to improve vacuum conditions and reduce background impurity concentrations in epilayers. After the conversion, the system was carefully calibrated and optimized for the growth of ZnSe and ZnTe on GaAs (001) substrates. Material quality was assessed using X-ray diffraction rocking curves. ZnSe layers displayed a trend of improving quality with decreasing growth temperature reaching a minimum full-width half-maximum (FWHM) of 113 arcsec at 278 degrees Celsius. ZnTe epilayer quality increased with growth temperature under Zn rich conditions attaining a FWHM of 84 arcsec at 440 degrees Celsius. RHEED oscillations were successfully observed and used to obtain growth rate in situ for varying flux and temperature levels. For a fixed flux ratio, growth rate decreased with growth temperature as the desorption rate increased. A directly proportional dependence of growth rate on Te flux was observed for Zn rich growth. Furthermore, a method for determining the flux ratio necessary for attaining the stoichiometric condition was demonstrated.