The purpose of this study is to investigate the role of strain in the accumulation of crystalline defects created by ion irradiation. Previous studies state that strained Si1xGex is more easily amorphized by ion irradiation than unstrained, bulk Si in a periodic superlattice structure; however, the reason for preferential amorphization of the strained Si1xGex layer in the periodic structure of strained and unstrained layers is not well understood. In this study, various ion irradiations will be carried out on SiGe strained layer superlattices grown on (100)-orientation bulk Si by low temperature molecular beam epitaxy. The samples under investigation are 50 nm surface Si0:8Ge0:2/bulk Si and 50 nm surface Si/60 nm Si0:8Ge0:2/bulk Si. Defects will be created in both surface and buried SiGe strained layers by medium and high energy light ion irradiation. The amount of permanently displaced atoms will be quantified by channeling Rutherford backscattering spectrometry. The amorphization model, the path to permanent damage creation, of bulk Si and surface strained SiGe will be investigated. The strain in surface and buried Si0:8Ge0:2 layers will be measured by comparison to bulk Si with Rutherford backscattering spectrometry by a novel technique, channeling analysis by multi-axial Rutherford backscatter- ing spectrometry, and the limitations of measuring strain by this technique will be explored. Results of this study indicated that the amorphization model, the number of ion collision cascades that must overlap to cause permanent damage, of strained Si0:8Ge0:2 is similar to that of bulk Si, suggesting that point defect recombination is less efficient in strained Si0:8Ge0:2. Additionally, a surface strained Si0:8Ge0:2 is less stable under ion irradiation than buried strained Si0:8Ge0:2. Repeated analysis by multi-axial channeling Rutherford backscattering spectrometry, which requires high fluence of 2 MeV He ions, proved destructive to the surface strained Si0:8Ge0:2 layer.

The refraction properties of photonic crystal lattices offers methods to control the beam steering of light through use of non-linear dispersion contours. In this thesis new photonic crystal structures, such as the square and triangular superlattices, that provide novel refractive properties are analyzed. The property difference between rows in these structures is the hole radius Δr. The difference in hole sizes leads to observation of the superlattice effect, that is, a change in the refractive index Δn between opposite rows of holes. The index difference becomes a function of the size of the smaller r2 hole area or volume due to the addition of the higher index background material compared to the larger r1 holes. The difference in hole radii Δr = r1 - r2 is referred to as the static superlattice strength and is designated by the ratio of r2/r1. The superlattice strength increases as the ratio of r2/r1 decreases. The hole size modulation creates modified dispersion contours that can be used to fabricate advanced beam steering devices through the introduction of electro-optical materials and a controlled bias. A discussion of these superlattice structures and their optical properties will be covered, followed by both static and dynamic tunable device constructions utilizing these designs. Also, static tuning of the devices through the use of atomic layer deposition, as well as active tuning methods utilizing liquid crystal (LC) infiltration, sealed LC cells, and the addition of electro-optic material will be discussed. Also in this thesis we present designs to implement a simpler demonstration of cloaking, the carpet cloak, in which a curved reflective surface is compressed into a flat reflective surface, effectively shielding objects behind the curve from view with respect to the incoming radiation source. This approach eliminates the need for metallic resonant elements. These structures can now be fabricated using only high index dielectric materials by the use of electron beam lithography and standard cleanroom technologies. The design method, simulation analysis, device fabrication, and near field optical microscopy (NSOM) characterization results are presented for devices designed to operate in the 1400-1600nm wavelength range. Improvements to device performance by the deposition/infiltration of linear, and potentially non-linear optical materials, were investigated.

This thesis deals with infrared studies of impurity states, ultrafast carrier dynamics as well as coherent intersubband polarizations in semiconductor quantum structures such as quantum wells and superlattices, based on the GaAs/AlGaAs material system. In the first part it is shown that the 2pz confined impurity state of a semiconductor quantum well develops into an excited impurity band in the case of a superlattice. This is studied by following theoretically the transition from a single to a multiple quantum well or superlattice by exactly diagonalizing the three-dimensional Hamiltonian for a quantum well system with random impurities. Intersubband absorption experiments, which can be nearly perfectly reproduced by the theory, corroborate this interpretation, showing that at low temperatures in the low doping density regime all optical transitions originate from impurity transitions. These results also require reinterpretation of previous experimental data. The relaxation dynamics of interminiband transitions in doped GaAs/AlGaAs superlattices in the mid-IR are studied. This involves single-color pump-probe measurements to explore the dynamics at different wavelengths, which is performed with the Rossendorf freeelectron laser (FEL), providing picosecond pulses in a range from 3-200 µm and are used for the first time within this thesis. In these experiments, a fast bleaching of the interminiband transition is observed followed by thermalization and subsequent relaxation, whose time constants are determined to be 1-2 picoseconds. This is followed by an additional component due to carrier cooling in the lower miniband. In the second part, two-color pump-probe measurements are performed, involving the FEL as the pump source and a table-top broad-band tunable THz source for probing the transmission changes. These measurements allow a separate specification of the cooling times after a strong excitation, exhibiting time constants from 230 ps to 3 ps for different excitation densities and miniband widths. In addition, the dynamics of excited electrons within the minibands is explored and their contribution quantitatively extracted from the measurements. Intersubband absorption experiments of photoexcited carriers in single quantum well structures, measured directly in the time-domain, i.e. probing coherently the polarization between the first and the second subband, are presented. From the data we can directly extract the density and temperature dependence of the intersubband dephasing time between the two lowest subbands, ranging from 50 up to 400 fs. This all optical approach gives us the ability to tune the carrier concentration over an extremely wide range which is not accessible in a doped quantum well sample. By varying the carrier density, many-body effects such as the depolarization and their influence on the spectral position as well as on the lineshape on the intersubband dephasing are studied. Also the difference of excitonic and free-carrier type excitation is discussed, and indication of an excitonic intersubband transition is found.

Made available in DSpace on 2014-06-11T19:23:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-04-11Bitstream added on 2014-06-13T19:29:34Z : No. of bitstreams: 1 simon_ra_me_bauru.pdf: 585170 bytes, checksum: 22e864a53f831e4bf21be6c6b81ed2f3 (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Na área de pesquisa em sistemas de elétrons fortemente correlacionados, o modelo das super-redes Hubbard tem sido utilizada para explicar e prever comportamentos de heteroestruturas, como multicamadas magnéticas, que apresentam propriedades diferentes e incomuns, comparadas com as dos sistemas homogêneos análogos, E.G., ordenamentos magnéticos e de carga. Neste contexto, foram estudados nesta dissertação os efeitos da introdução de um novo valor , 'T IND. C', para o parâmetro de hopping de elétron nas interfaces de camadas diferentes da super-rede, ou seja, introduziu-se uma modulação na condução entre as camadas. Variou-se 'T IND. C'entre 0.1T e T, onde T é o parâmetro de hopping entre sítios da mesma camada. Para a lacuna de carga encontramos um comportamento dependente da densidade eletrônica na rede. Em geral, momentos magnéticos locais e a ocupação eletrônica nos sítios, apresentam uma distribuição mais uniforme para 'T IND. C'=0.1T. No entanto, nas densidades onde ocorre um aumento abrupto na lacuna de carga com a diminuição de 'T IND. C', essa uniformidade é mais pronunciada, possivelmente devido a uma comensurabilidade entre a distribuição dos elétrons na cadeia e a estrutura da super-rede. Para as funções de correlação de carga, que foram estudadas através do fator de estrutura, encontramos em alguns casos uma dependência em 'T IND. C' do período do ordenamento correspondente. / In the research field of strongly correlated electron systems, the Hubbard superlattice model has been used for explain and predict the behavior of heterostructures, such as magnetic multilayers, whose unusual properties differ from the properties of the analogous homogeneous counterpart. In this context, we have studied here the effects of introducing a new value for the hopping parameter, 'T IND. C', for electrons between different layers, E.G., we introduced a modulation in the electronic conduction between layers. We consider 'T IND. C' in the range 0.1T to T, where T is the hopping parameter for sites inside the layers. For the charge gap, we found a behavior that depends on the electronic density in the lattice. Local magnetic and electronic occupation on sites, generally, show a greater uniformity with decreasing 'T IND. C'. However, for electronic densities where is an abrupt increase in the charge gap for decreasing 'T IND. C', this uniformity is enhanced, probably due to a commensurability between the electronic distribution in the chain and the underlying superlattice structure. For the charge correlation functions, that were studied through their structure factor, we found in some cases a dependency in 'T IND. C' of the correlations oscillation period.

abstract: The research described in this dissertation has involved the use of transmission electron microcopy (TEM) to characterize the structural properties of II-VI and III-V compound semiconductor heterostructures and superlattices. The microstructure of thick ZnTe epilayers (~2.4 µm) grown by molecular beam epitaxy (MBE) under virtually identical conditions on GaSb, InAs, InP and GaAs (100) substrates were compared using TEM. High-resolution electron micrographs revealed a highly coherent interface for the ZnTe/GaSb sample, and showed extensive areas with well-separated interfacial misfit dislocations for the ZnTe/InAs sample. Lomer edge dislocations and 60o dislocations were commonly observed at the interfaces of the ZnTe/InP and ZnTe/GaAs samples. The amount of residual strain at the interfaces was estimated to be 0.01% for the ZnTe/InP sample and -0.09% for the ZnTe/GaAs sample. Strong PL spectra for all ZnTe samples were observed from 80 to 300 K. High quality GaSb grown by MBE on ZnTe/GaSb (001) virtual substrates with a temperature ramp at the beginning of the GaSb growth has been demonstrated. High-resolution X-ray diffraction (XRD) showed clear Pendellösung thickness fringes from both GaSb and ZnTe epilayers. Cross-section TEM images showed excellent crystallinity and smooth morphology for both ZnTe/GaSb and GaSb/ZnTe interfaces. Plan-view TEM image revealed the presence of Lomer dislocations at the interfaces and threading dislocations in the top GaSb layer. The defect density was estimated to be ~1 x107/cm2. The PL spectra showed improved optical properties when using the GaSb transition layer grown on ZnTe with a temperature ramp. The structural properties of strain-balanced InAs/InAs1-xSbx SLs grown on GaSb (001) substrates by metalorganic chemical vapor deposition (MOCVD) and MBE, have been studied using XRD and TEM. Excellent structural quality of the InAs/InAs1-xSbx SLs grown by MOCVD has been demonstrated. Well-defined ordered-alloy structures within individual InAs1-xSbx layers were observed for samples grown by modulated MBE. However, the ordering disappeared when defects propagating through the SL layers appeared during growth. For samples grown by conventional MBE, high-resolution images revealed that interfaces for InAs1-xSbx grown on InAs layers were sharper than for InAs grown on InAs1-xSbx layers, most likely due to a Sb surfactant segregation effect. / Dissertation/Thesis / Ph.D. Physics 2012

abstract: InAs/InAsSb type-II superlattices (T2SLs) can be considered as potential alternatives for conventional HgCdTe photodetectors due to improved uniformity, lower manufacturing costs with larger substrates, and possibly better device performance. This dissertation presents a comprehensive study on the structural, optical and electrical properties of InAs/InAsSb T2SLs grown by Molecular Beam Epitaxy. The effects of different growth conditions on the structural quality were thoroughly investigated. Lattice-matched condition was successfully achieved and material of exceptional quality was demonstrated. After growth optimization had been achieved, structural defects could hardly be detected, so different characterization techniques, including etch-pit-density (EPD) measurements, cathodoluminescence (CL) imaging and X-ray topography (XRT), were explored, in attempting to gain better knowledge of the sparsely distributed defects. EPD revealed the distribution of dislocation-associated pits across the wafer. Unfortunately, the lack of contrast in images obtained by CL imaging and XRT indicated their inability to provide any quantitative information about defect density in these InAs/InAsSb T2SLs. The nBn photodetectors based on mid-wave infrared (MWIR) and long-wave infrared (LWIR) InAs/InAsSb T2SLs were fabricated. The significant difference in Ga composition in the barrier layer coupled with different dark current behavior, suggested the possibility of different types of band alignment between the barrier layers and the absorbers. A positive charge density of 1.8 × 1017/cm3 in the barrier of MWIR nBn photodetector, as determined by electron holography, confirmed the presence of a potential well in its valence band, thus identifying type-II alignment. In contrast, the LWIR nBn photodetector was shown to have type-I alignment because no sign of positive charge was detected in its barrier. Capacitance-voltage measurements were performed to investigate the temperature dependence of carrier densities in a metal-oxide-semiconductor (MOS) structure based on MWIR InAs/InAsSb T2SLs, and a nBn structure based on LWIR InAs/InAsSb T2SLs. No carrier freeze-out was observed in either sample, indicating very shallow donor levels. The decrease in carrier density when temperature increased was attributed to the increased density of holes that had been thermally excited from localized states near the oxide/semiconductor interface in the MOS sample. No deep-level traps were revealed in deep-level transient spectroscopy temperature scans. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2015

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

abstract: The first part of this dissertation reports the study of the vertical carrier transport and device application in InAs/InAs1-xSbx strain-balanced type-II superlattice. It is known that the low hole mobility in the InAs/InAs1-xSbx superlattice is considered as the main reason for the low internal quantum efficiency of its mid-wave and long-wave infrared photodetectors, compared with that of its HgCdTe counterparts. Optical measurements using time-resolved photoluminescence and steady-state photoluminescence spectroscopy are implemented to extract the diffusion coefficients and mobilities of holes in the superlattices at various temperatures from 12 K to 210 K. The sample structure consists of a mid-wave infrared superlattice absorber region grown atop a long-wave infrared superlattice probe region. An ambipolar diffusion model is adopted to extract the hole mobility. The results show that the hole mobility first increases from 0.2 cm2/Vs at 12 K and then levels off at ~50 cm2/Vs as the temperature exceeds ~60 K. An InAs/InAs1-xSbx type-II superlattice nBn long-wavelength barrier infrared photodetector has also been demonstrated with a measured dark current density of 9.5×10-4 A/cm2 and a maximum resistance-area product of 563 Ω-cm2 at 77 K under a bias of -0.5 V. The Arrhenius plot of the dark current density reveals a possible high-operating-temperature of 110 K.The second part of the dissertation reports a lift-off technology using a water-soluble sacrificial MgTe layer grown on InSb. This technique enables the seamless integration of materials with lattice constants near 6.5 Å, such as InSb, CdTe, PbTe, HgTe and Sn. Coherently strained MgTe with a lattice constant close to 6.5 Å acts as a sacrificial layer which reacts with water and releases the film above it. Freestanding CdTe/MgxCd1-xTe double-heterostructures resulting from the lift-off process show increased photoluminescence intensity due to enhanced extraction efficiency and photon-recycling effect. The lifted-off thin films show smooth and flat surfaces with 6.7 Å root-mean-square roughness revealed by atomic-force microscopy profiles. The increased photoluminescence intensity also confirms that the CdTe/MgxCd1-xTe double-heterostructures maintain the high optical quality after epitaxial lift-off. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第22238号 / 理博第4552号 / 新制||理||1654(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 松田 祐司, 教授 石田 憲二, 教授 寺嶋 孝仁 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM