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.

This thesis deals with the deposition of epitaxial chalcogenide (Ge2Sb2Te5 (GST225), GeTe and Sb2Te3) thin films and superlattice (SL) arrangement based on GeTe-Sb2Te3 using pulsed laser deposition (PLD) technique on (111)-oriented Si substrates. The thin films are characterized using in-situ RHEED, XRD, SEM, AFM and TEM. The epitaxial trigonal GST225 films with out-of-plane c-plane orientation were grown in 2D growth mode. For the first group of the films (substrate-target distance (dts) of ~7.5 cm), the epitaxial window was observed from 200 °C to 300 °C. By varying laser frequency, deposition rate as high as 42 nm/ min can be achieved. The deposition with a slight reduction of dts to ~6 cm (second group) at moderate Ts of 220 °C results in the epitaxial films with heterogeneous vacancy structures (coexisting metastable phases. i.e. with random and ordered vacancies, and stable trigonal phase). Thermal annealing (at 220 °C) leads to a phase transformation towards a pure trigonal phase. The epitaxial Sb2Te3 films with out-of-plane (0001) oriented trigonal structure were grown at Ts from 140 to 280 °C in 2D growth mode. The optimum Ts in terms of deposition rate and film quality was determined to be 240 °C. The epitaxial growth of Sb2Te3 thin films is initiated by the self-organized formation of a Sb/Te single-atomic passivation layer on the Si surface. The growth of GeTe was initialized by the formation of an ultra-thin amorphous layer. The films were predominantly grown in the mix of 2D and 3D growth modes. The deposited films possesses trigonal structure out-of-plane (0001)-orientated on Si(111). By employing a 2D-bonded Sb2Te3 as a seeding layer on Si(111), the epitaxial window of GeTe can be extended especially towards the lower temperature regime, up to 145 °C. Additionally, the surface topography can be significantly improved, indicating that the films are grown in 2D growth mode on the buffered substrate. The epitaxial SLs can be grown starting at Ts = 140 °C. Each layer of the SLs, i.e. Sb2Te3 and GeTe layer, was grown in 2D growth mode. An intermixing of GeTe and Sb2Te3 layers occurred at a higher temperature deposition. Studies on local structure of 140 °C-deposited SL showed that the SL consists of Ge-rich Ge(x+y)Sb(2–y)Tez and Sb2Te3 units intercalated by Van der Waals gaps with the inhomogeneity of layer thickness across the SL. The obtained results demonstrate the feasibility of PLD for deposition of good quality epitaxial chalcogenide thin films and SL structure on Si(111).

De nombreuses filières de détecteurs coexistent dans le domaine infrarouge (longueur d’onde entre 1µm et 50µm). Chacune possède ses avantages et ses inconvénients (coût, performance, compacité …). Certaines filières sont bien établies et disponibles commercialement alors que d’autres sont encore émergentes. La filière super-réseaux (SR) est une filière récemment commercialisée. Elle repose sur l’empilement périodique de semiconducteurs (InAs/GaSb), donnant un détecteur quantique capable de détecter des longueurs d’ondes comprises entre 1 et 32µm typiquement.L’objectif de cette thèse est d’évaluer le potentiel de la filière super-réseaux en cryostat opérationnel à l’aide de fonctions de mérite orientées système qui tiennent compte du packaging entourant le détecteur. Nous nous concentrerons sur la Fonction de Transfert de Modulation (FTM), décrivant la résolution du système ainsi que sur le rapport Bruit Spatial Fixe Résiduel sur Bruit Temporel (BSFR/BT), décrivant la stabilité temporelle de la qualité image.Ce travail a ainsi permis de confirmer deux promesses des SR en moyen infrarouge : d’une part, la grande stabilité temporelle de la qualité image et d’autre part le faible nombre de pixels clignotants. Par ailleurs, les bancs de mesures de FTM et de stabilité temporelle développés au cours de la thèse pourront être adaptés pour caractériser d’autres filières dans les mêmes conditions de mesure. / Many photodetector technologies coexist in the infrared domain (wavelength between 1µm and 50µm). Each of them comes with its assets and drawbacks (cost, performance, compactness, etc.). Some technologies are well established and available while others are still under development. The superlattice technology (SL) recently made its way into the market. It is based on the periodic stack of semiconductors (InAs/GaSb), giving a quantum detector capable of detecting wavelengths between 1 and 32µm typically. Like other quantum infrared detectors, superlattice photodetectors need to be cooled at cryogenic temperature to maximize their electro-optical performance.The objective of this thesis is to evaluate the potential of the SLSL in IDDCA using system-oriented figures of merit, which also take into account the packaging of the detector. The present work is focused on the Modulation Transfer Function, which describes the system resolution and the ratio Residual Fixed Pattern Noise over Temporal Noise (RPFN/TN), which evaluates the temporal stability of the image quality.This present work successfully confirmed two promises of the SL in midwave infrared : the excellent stability of the image quality and the low flickering pixel count. Besides, the experimental benches developped (MTF and temporal stability alike) can be adapted to perform similar measurements with other technologies.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18052号 / 理博第3930号 / 新制||理||1567(附属図書館) / 30910 / 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, 准教授 芝内 孝禎, 教授 石田 憲二 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

京都大学 / 0048 / 新制・論文博士 / 博士(工学) / 乙第12860号 / 論工博第4107号 / 新制||工||1609(附属図書館) / 31540 / (主査)教授 北野 正雄, 教授 川上 養一, 准教授 酒井 道 / 学位規則第4条第2項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The research of III-nitride wide-bandgap semiconductor devices, such as laser diodes (LDs), ultra-violet (UV) light-emitting diodes (LEDs), and high electron mobility transistors (HEMTs), has recently increased. Numerous opportunities exist for performance improvement in the wide bandgap semiconductor device structure, including material selection, compound compositions, polarization effects, and layer thicknesses. On the other hand, they can make optimization more challenging. It still takes a lot of resources to analyze and test complicated structures in a systematic manner. This dissertation creates a new path for device design by using TCAD and machine learning to deliver quick forecasts of III-nitride semiconductor device performance. The dissertation includes three major components. In Chapter 2, the TCAD-assisted HEMT device design is discussed. We demonstrate the performance improvement of using the new material BAlN as an interlayer in GaN/AlGaN HEMT devices and compare the various interlayer design alternatives for HEMTs. In chapter 3, we propose asymmetrical p-AlGaN/i-InGaN/n-AlGaN tunnel junctions (TJs) by combining machine learning (ML) with TCAD calculations. The resistances for 22254 various TJ structures were predicted by the model, which creates a tool for real-time TJ resistance prediction. Based on our TJ predictions, we proposed asymmetric TJ with higher Al content in the p-layer and lower TJ resistance. In Chapter 4, using the stacked XGBoost/LightGBM algorithm, we thoroughly examined the superlattice (SL) electron blocking layer (EBL) for AlGaN deep ultra-violet (DUV) LEDs. Based on the ML model, we suggest a low Al-content SL-EBL (1 nm/5 nm Al0.7Ga0.3N/Al0.58Ga0.42N) that is simpler, experimentally realizable and can greatly improve carrier transport. Additionally, we examine the prediction data and show how the composition and thickness affect the improvement of the IQE. The work contributes to the advancement of using SL-EBLs for high-efficiency DUV LEDs by providing methodical research on SL-EBLs. This dissertation presents novel approaches to the design of electrical and optical wide bandgap semiconductor devices, which opens up a new avenue for future research. It is possible that it might be used in a broad variety of fields, including illumination, sensing, disinfection, and power devices.

[pt] Os semicondutores III-V são amplamente investigados para a fabricação de fotodetectores de infravermelho baseados em pontos quânticos (QWIPs); no entanto, o comprimento de onda de operação é limitada pelo bandoffset dos materiais que permitem transições de infravermelho de comprimento de onda maior que 3,1 um. Para comprimentos de onda mais curto do que 1,7 um transições banda a banda são facilmente empregadas. Assim, em QWIPs III-V, o intervalo entre 1,7 e 3,1 um não pode ser alcançado tanto por transições banda-banda ou por transições intrabanda. Nesta tese uma estrutura de superrede especialmente desenhada é proposta a fim de detectar a radiação dentro desta faixa proibida. A estrutura proposta consiste numa superrede com um poço quântico central mais amplo, o qual gera uma modulação no contínuo criando minibandas e minigaps para energias acima da parte inferior da banda de condução do poço quântico, incluindo no contínuo. Com esta abordagem, a limitação de ter estados ligados apenas com energias abaixo a barreira não se mantém e é possível detectar energias mais elevadas do que o limite imposto pelo bandoffset dos materiais. Simulações teóricas para a estrutura foram realizados e medidas de absorção, corrente de escuro, e fotocorrente foram realizadas mostrando picos em 2,1 um, em estreita concordância com o valor teoricamente esperado. / [en] III-V semiconductors are extensively investigated for fabrication of quantum well infrared photodetectors (QWIPs); however the operation wavelength is limited by the bandoffset of the materials allowing infrared transitions for wavelength larger than 3.1 um. For wavelength shorter than 1.7 um band to band transitions are easily employed. Thus, in III-V QWIPs, the range between 1.7 and 3.1 um cannot be reached either by band-to-band or by intraband transitions. In this thesis a specially designed superlattice structure is proposed in order to detect radiation within this forbidden range. The structure proposed consists of a superlattice with a wider central quantum well, which generates a modulation in the continuum creating minibands and minigaps for energies above the bottom of the conduction band of the quantum well, including in the continuum. With this approach the limitation of having bound states only with energies below the barrier no longer holds and it is possible to detect energies higher than the limit imposed by the bandoffset of the materials. Theoretical simulations for the structure were performed and absorption, dark current, and photocurrent measurements were carried out showing peaks at 2.1 um, in close agreement with the theoretically expected value.

In this thesis, we investigate the collective electron dynamics in single and coupled superlattice systems under the influence of a DC electric field. Firstly, we illustrate that Bloch oscillations suppress electron transport and the resulting charge domains form self-sustained current oscillations. Upon the application of a tilted magnetic field, stochastic web structures are shown to form in the phase space of the electron trajectory. This occurs only when the Bloch and cyclotron frequencies are commensurate allowing the electrons to demonstrate chaotic unbounded trajectories, leading to an increase in transport. The charge domain dynamics also present additional peaks during such resonances. The rapid changes in the dynamical states found is an example of non-KAM chaos. We show then the amplitude and frequency of current oscillations in a single superlattice can be controlled. Secondly, two models are designed to mutually couple two semiconductor superlattices by a common resistive load. We examine the effects of coupling strength and frequency detuning on the collective current dynamics. The devices are considered to be arranged together on a single substrate as well as on individual substrates. Large AC power is witnessed during anti-phase and in-phase synchronization between current oscillations. Finally, two superlattices are coupled through a resonance circuit incorporating single mode resonances from external influences in the circuit. In this system, chaotic current dynamics are induced with regions of chaos separating different regions of synchronization. High frequency oscillations with minimal phase difference cause the largest power generation. In all three coupling models high frequency components are found in the Fourier power spectra. The power generated in the coupled systems is found greater and at times more than double the power generated in the autonomous superlattice. Thus this thesis provides innovative methods of enhancing and controlling powerful high-frequency signals. This effectively gives manipulation over the intensity of the electromagnetic radiation produced by the superlattice.

<p>Energy harvesting is an interesting topic for today since we face running out of energy source, a serious problem in the world. Thermoelectric devices are a good candidate. They can convert heat (i.e. temperature gradient) to electricity. This result leads us to use them to harvest waste heat from engines or in power plants to generate electricity. Moreover, thermoelectric devices also perform cooling by applied voltage to device. This process is clean, which means that no greenhouse gases are emitted during the process. However, the converting efficiency of thermoelectrics are very low compare to a home refrigerator. The thermoelectric figure of merit (ZT_m) is a number which defines the converting efficiency of thermoelectric materials and devices. ZT_m is defined by Seebeck coefficient, electrical conductivity and thermal conductivity. To improve the converting efficiency, nanolaminated materials are good candidate.</p><p> </p><p>This thesis studies TiN/ScN artificial nanolaminates, or superlattices were grown by reactive dc magnetron sputtering from Ti and Sc targets. For TiN/ScN superlattice, X-ray diffraction (XRD) and reciprocal space map (RSM) show that we can obtain single crystal TiN/ScN superlattice. X-ray reflectivity (XRR) shows the superlattice films have a rough surface, supported by transmission electron microscopy (TEM). Also, TiN/ScN superlattices grew by TiN as starting layer has better crystalline quality than ScN as starting layer. The electrical measurement shows that our superlattice films are conductive films.</p><p> </p><p>Ca-Co-O system for inherently nanolaminated materials were grown by reactive rf magnetron sputtering from Ca/Co alloy target. The XRD shows we maybe get the [Ca₂CoO₃]_xCoO₂ phase, so far. The energy dispersive X-ray spectroscopy (EDX) reported that our films have Al conmination. We also discovered unexpected behavior when the film grown at high temperature showed larger thickness instead of thinner, which would have been expected due to possible Ca evaporation. The Ca-Co-O system requires further studies.</p>

<p>Understanding lithium-ion insertion/extraction mechanisms in battery electrode materials is of crucial importance in developing new materials with better cycling performance. In this thesis, these mechanisms are probed for two different potential cathode materials by a combination of electrochemical and single-crystal X-ray diffraction studies. The materials investigated are V₆O₁₃and cubic LiMn₂O₄spinel.</p><p>Single-crystal X-ray diffraction studies of lithiated phases in the Li_xV₆O₁₃ system (x=2/3 and 1) exhibit superlattice phenomena and an underlying Li⁺ ion insertion mechanism which involves the stepwise addition of Li⁺ions into a two-dimensional array of chemically equivalent sites. Each successive stage in the insertion process is accompanied by a rearrangement of the Li⁺ ions together with an electron redistribution associated with the reduction of specific V-atoms in the structure. This results in the formation of electrochemically active sheets in the structure. A similar mechanism occurs in the LiMn₂O₄ delithiation process, whereby lithium is extracted in a layered arrangement, with the Mn atoms forming charge-ordered Mn³⁺/Mn⁴⁺ layers.</p><p>Lithium-ion insertion/extraction processes in transition-metal oxides would thus seem to occur through an ordered two-dimensional arrangement of lithium ions extending throughout the structure. The lithium ions and the host structure rearrange cooperatively to form superlattices through lithium and transition-metal ion charge-ordering. A picture begins to emerge of a universal two-dimensional lithium-ion insertion/extraction mechanism analogous to the familiar staging sequence in graphite.</p>