Matching Items (6)

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Study of CdTe/MgxCd1-xTe double heterostructures and their application in high efficiency solar cells and in luminescence refrigeration

Description

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

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.

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Date Created
  • 2016

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Synthesis and properties of Sn-based group IV alloys

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Sn-based group IV materials such as Ge1-xSnx and Ge1-x-ySixSny alloys have great potential for developing Complementary Metal Oxide Semiconductor (CMOS) compatible devices on Si because of their tunable band structure

Sn-based group IV materials such as Ge1-xSnx and Ge1-x-ySixSny alloys have great potential for developing Complementary Metal Oxide Semiconductor (CMOS) compatible devices on Si because of their tunable band structure and lattice constants by controlling Si and/or Sn contents. Growth of Ge1-xSnx binaries through Molecular Beam Epitaxy (MBE) started in the early 1980s, producing Ge1-xSnx epilayers with Sn concentrations varying from 0 to 100%. A Chemical Vapor Deposition (CVD) method was developed in the early 2000s for growing Ge1-xSnx alloys of device quality, by utilizing various chemical precursors. This method dominated the growth of Ge1-xSnx alloys rapidly because of the great crystal quality of Ge1-xSnx achieved. As the first practical ternary alloy completely based on group IV elements, Ge1-x-ySixSny decouples bandgap and lattice constant, becoming a prospective CMOS compatible alloy. At the same time, Ge1-x-ySixSny ternary system could serve as a thermally robust alternative to Ge1-ySny binaries given that it becomes a direct semiconductor at a Sn concentration of 6%-10%. Ge1-x-ySixSny growths by CVD is summarized in this thesis. With the Si/Sn ratio kept at ~3.7, the ternary alloy system is lattice matched to Ge, resulting a tunable direct bandgap of 0.8-1.2 eV. With Sn content higher than Si content, the ternary alloy system could have an indirect-to-direct transition, as observed for Ge1-xSnx binaries. This thesis summarizes the development of Ge1-xSnx and Ge1-x-ySixSny alloys through MBE and CVD in recent decades and introduces an innovative direct injection method for synthesizing Ge1-x-ySixSny ternary alloys with Sn contents varying from 5% to 12% and Si contents kept at 1%-2%. Grown directly on Si (100) substrates in a Gas-phase Molecular Epitaxy (GSME) reactor, both intrinsic and n-type doped Ge1-x-ySixSny with P with thicknesses of 250-760 nm have been achieved by deploying gas precursors Ge4H10, Si4H10, SnD4 and P(SiH3)3 at the unprecedented low growth temperatures of 190-220 °C. Compressive strain is reduced and crystallinity of the Ge1-x-ySixSny epilayer is improved after rapid thermal annealing (RTA) treatments. High Resolution X-ray Diffraction (HR-XRD), Rutherford Backscattering Spectrometry (RBS), cross-sectional Transmission Electron Microscope (XTEM) and Atomic Force Microscope (AFM) have been combined to characterize the structural properties of the Ge1-x-ySixSny samples, indicating good crystallinity and flat surfaces.

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Date Created
  • 2019

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Characterization of MBE-grown semiconductor materials for photovoltaic applications

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The research described in this dissertation involved the use of transmission electron microscopy (TEM) to characterize II-VI and III-V compound semiconductor quantum dots (QDs) and dilute-nitride alloys grown by molecular

The research described in this dissertation involved the use of transmission electron microscopy (TEM) to characterize II-VI and III-V compound semiconductor quantum dots (QDs) and dilute-nitride alloys grown by molecular beam epitaxy (MBE) and intended for photovoltaic applications. The morphology of CdTe QDs prepared by the post-annealing MBE method were characterized by various microscopy techniques including high-resolution transmission electron microscopy (HR-TEM), and high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM). Extensive observations revealed that the of QD shapes were not well-defined, and the QD size and spatial distribution were not determined by the amount of CdTe deposition. These results indicated that the formation of II-VI QDs using a post-annealing treatment did not follow the conventional growth mechanism for III-V and IV-IV materials. The structural properties of dilute-nitride GaAsNx films grown using plasma-assisted MBE were characterized by TEM and HAADF-STEM. A significant amount of the nitrogen incorporated into the dilute nitride films was found to be interstitial, and that fluctuations in local nitrogen composition also occurred during growth. Post-growth partial relaxation of strain resulted in the formation of {110}-oriented microcracks in the sample with the largest substitutional nitrogen composition. Single- and multi-layered InAs QDs grown on GaAsSb/GaAs composite substrates were investigated using HR-TEM and HAADF-STEM. Correlation between the structural and optoelectronic properties revealed that the GaAsSb barrier layers had played an important role in tuning the energy-band alignments but without affecting the overall structural morphology. However, according to both XRD measurement and electron microscopy the densities of dislocations increased as the number of QD layers built up. An investigation of near-wetting layer-free InAs QDs incorporated with AlAs/GaAs spacer layers was carried out. The microscopy observations revealed that both embedded and non-embedded near-wetting layer-free InAs QDs did not have well-defined shapes unlike conventional InAs QDs. According to AFM analysis and plan-view TEM characterization, the InAs QDs incorporated with spacer layers had smaller dot density and more symmetrical larger sizes with an apparent bimodal size distribution (two distinct families of large and small dots) in comparison with conventional InAs QDs grown without any spacer layer.

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Date Created
  • 2014

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Material properties of MBE Grown ZnTe, GaSb and their heterostructures for optoelectronic device applications

Description

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

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.

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Date Created
  • 2012

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Monolithic Heterovalent Integration of Compound Semiconductors and Their Applications

Description

Compound semiconductors tend to be more ionic if the cations and anions are further apart in atomic columns, such as II-VI compared to III-V compounds, due in part to the

Compound semiconductors tend to be more ionic if the cations and anions are further apart in atomic columns, such as II-VI compared to III-V compounds, due in part to the greater electronegativity difference between group-II and group-VI atoms. As the electronegativity between the atoms increases, the materials tend to have more insulator-like properties, including higher energy band gaps and lower indices of refraction. This enables significant differences in the optical and electronic properties between III-V, II-VI, and IV-VI semiconductors. Many of these binary compounds have similar lattice constants and therefore can be grown epitaxially on top of each other to create monolithic heterovalent and heterocrystalline heterostructures with optical and electronic properties unachievable in conventional isovalent heterostructures.

Due to the difference in vapor pressures and ideal growth temperatures between the different materials, precise growth methods are required to optimize the structural and optical properties of the heterovalent heterostructures. The high growth temperatures of the III-V materials can damage the II-VI barrier layers, and therefore a compromise must be found for the growth of high-quality III-V and II-VI layers in the same heterostructure. In addition, precise control of the interface termination has been shown to play a significant role in the crystal quality of the different layers in the structure. For non-polar orientations, elemental fluxes of group-II and group-V atoms consistently help to lower the stacking fault and dislocation density in the II-VI/III-V heterovalent heterostructures.

This dissertation examines the epitaxial growth of heterovalent and heterocrystalline heterostructures lattice-matched to GaAs, GaSb, and InSb substrates in a single-chamber growth system. The optimal growth conditions to achieve alternating layers of III-V, II-VI, and IV-VI semiconductors have been investigated using temperature ramps, migration-enhanced epitaxy, and elemental fluxes at the interface. GaSb/ZnTe distributed Bragg reflectors grown in this study significantly outperform similar isovalent GaSb-based reflectors and show great promise for mid-infrared applications. Also, carrier confinement in GaAs/ZnSe quantum wells was achieved with a low-temperature growth technique for GaAs on ZnSe. Additionally, nearly lattice-matched heterocrystalline PbTe/CdTe/InSb heterostructures with strong infrared photoluminescence were demonstrated, along with virtual (211) CdZnTe/InSb substrates with extremely low defect densities for long-wavelength optoelectronic applications.

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  • 2019

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Characterization of HgCdTe and HgCdSe materials for third generation infrared detectors

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HgCdTe is the dominant material currently in use for infrared (IR) focal-plane-array (FPA) technology. In this dissertation, transmission electron microscopy (TEM) was used for the characterization of epitaxial HgCdTe epilayers

HgCdTe is the dominant material currently in use for infrared (IR) focal-plane-array (FPA) technology. In this dissertation, transmission electron microscopy (TEM) was used for the characterization of epitaxial HgCdTe epilayers and HgCdTe-based devices. The microstructure of CdTe surface passivation layers deposited either by hot-wall epitaxy (HWE) or molecular beam epitaxy (MBE) on HgCdTe heterostructures was evaluated. The as-deposited CdTe passivation layers were polycrystalline and columnar. The CdTe grains were larger and more irregular when deposited by HWE, whereas those deposited by MBE were generally well-textured with mostly vertical grain boundaries. Observations and measurements using several TEM techniques showed that the CdTe/HgCdTe interface became considerably more abrupt after annealing, and the crystallinity of the CdTe layer was also improved. The microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures grown by MBE was investigated. Many inclined {111}-type stacking faults were present throughout the thin ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Lattice parameter measurement and elemental profiles indicated that some local intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd, Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively. Initial observations of CdTe(211)B/GaAs(211) heterostructures indicated much reduced defect densities near the vicinity of the substrate and within the CdTe epilayers. HgCdTe epilayers grown on CdTe(211)B/GaAs(211) composite substrate were generally of high quality, despite the presence of precipitates at the HgCdTe/CdTe interface. The microstructure of HgCdSe thin films grown by MBE on ZnTe/Si(112) and GaSb(112) substrates were investigated. The quality of the HgCdSe growth was dependent on the growth temperature and materials flux, independent of the substrate. The materials grown at 100°C were generally of high quality, while those grown at 140°C had {111}-type stacking defects and high dislocation densities. For epitaxial growth of HgCdSe on GaSb substrates, better preparation of the GaSb buffer layer will be essential in order to ensure that high-quality HgCdSe can be grown.

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Date Created
  • 2011