Coherent transport in Nb/delta-doped-GaAs hybrid microstructures

Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is presented. The structures fabrication procedure is based on delta-doped layers grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap layer to protect the active semiconductor layers during ex situ transfer. The superconductor is then sputter deposited in situ after thermal desorption of the protective layer. Two types of structures in particular will be discussed, i.e., a reference junction and the engineered one that contains an additional insulating AlGaAs barrier inserted during the growth in the semiconductor. This latter configuration may give rise to controlled interference effects and realizes the model introduced by de Gennes and Saint-James in 1963. While both structures show reflectionless tunneling-dominated transport, only the engineered junction shows additionally a low-temperature single marked resonance peaks superimposed to the characteristic Andreev-dominated subgap conductance. The analysis of coherent magnetotransport in both microstructures is successfully performed within the random matrix theory of Andreev transport and ballistic effects are included by directly solving the Bogoliubov-de Gennes equations. The impact of junction morphology on reflectionless tunneling and the application of the employed fabrication technique to the realization of complex semiconductor-superconductor systems are furthermore discussed.Comment: 9 pages, 8 figures, invited review paper, to be published in Mod. Phys. Lett.

Asymmetric quantum well structures for enhanced infrared photon absorption

Compared to inter-band transition for photon absorption in a quantum wells, intra-band (or inter-subband) transitions in heterojunction (GaAs/InP) quantum wells can provide access to a broader range of wavelengths for detector design, specifically detectors operating in the mid infrared region of spectrum (4-12 [micro]m) and beyond is possible. These quantum wells not only provide great flexibility in optimizing the Eigen energy levels or wavefunctions, and inter-subband optical matrix elements determining the corresponding transition probability, but also allow controlling electron-phonon scattering rates and thus electron lifetime. The research presented in this dissertation investigates asymmetric quantum well structures formed through III-V semiconductor material system such as AlGaAs/ InxGa(1-x)As/InyGa(1-y) As/AlGaAs that can further improve the responsivity through higher carrier mobility. Asymmetry is introduced by using multiple materials to form the well region. The advantage of exploring stepped quantum well structure stems from experimental evidence that such structures are capable of absorbing normal incidence and thus eliminates the requirement of incorporating additional optical coupling schemes such as grating structures. An important contribution of this research is the development of an analytical model to analyze single or multiple quantum well structures to quantify photon absorption. The physical model developed in this work is based on non-equilibrium Green's function (NEGF), Fermi's golden rule and quantum mechanical wave impedance concept. The approach has two distinct advantages. First, it is accurate, easily programmable and yet computationally efficient. Second, it facilitates quantifying the broadening of states resulting from both photon absorption and tunneling, which provides important insight for improving detection efficiency. Instead of being presented through calculations, such broadening has been simply assumed in previously reported works. The method developed in this researcIncludes bibliographical references (pages 112-113)

Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published versio

Terahertz and mid-infrared photodetectors based on intersubband transitions in novel materials systems

The terahertz (THz) and mid-infrared (MIR) spectral regions have many potential applications in the industrial, biomedical, and military sectors. Yet, a wide portion of this region of the electromagnetic spectrum (particularly the THz range) is still relatively unexplored, due mainly to the absence of suitable sources and photodetectors, related to the lack of practical semiconductor materials with adequately small band gap energies. Intersubband transitions (ISBTs) between quantized energy states in quantum heterostructures provide tunable wavelengths over a broad spectral range including the THz region, by choosing appropriate layer thicknesses and compositions. This work focuses on the development of THz and MIR Quantum Well Infrared Photodetectors (QWIPs) based on ISBTs in GaN/AlGaN and Si/SiGe heterostructures. Due to their large optical phonon energies, GaN materials allow extending the spectral reach of existing far-infrared photodetectors based on GaAs, and may enable higher-temperature operation. In the area of MIR optoelectronic devices, I have focused on developing QWIPs based on ISBTs in Si/SiGe heterostructures in the form of on strain-engineered nanomembranes. Due to their non-polar nature, these materials are free from reststrahlen absorption and ultrafast resonant electron/phonon scattering, unlike traditional III-V semiconductors. Therefore, Si/SiGe quantum wells (QWs) are also promising candidates for high-temperature high-performance ISB device operation (particularly in the THz region), with the additional advantage of direct integration with CMOS technology. In this thesis work, numerical modeling is used to design the active region of the proposed devices, followed by sample fabrication and characterization based on lock-in step-scan Fourier transform infrared spectroscopy. Three specific QWIP devices have been developed. The first is a III-nitride THz QWIP based on a novel double-step QW design in order to alleviate the material limitations provided by the intrinsic electric fields of GaN/AlGaN heterostructures. Next, I have developed a THz GaN/AlGaN QWIP grown on semi-polar (202 ̅1 ̅) GaN, where the detrimental effects of the internal fields are almost completely eliminated. Finally, I have demonstrated a Si/SiGe MIR QWIP based on a novel fabrication approach, where nanomembrane strain engineering is used to address the materials quality issues normally found in SiGe QWs. Promising photodetector performance is obtained in all cases.2017-06-21T00:00:00

Heterostructure engineering of quantum dots-in-a-well infrared photodetectors

Three of the most important characteristics of third-generation imaging systems are high operating temperature, multispectral operation, and large format arrays. The quantum dot infrared photodetector technology, owing to the three-dimensional confinement of carriers, the richness of the electronic spectra in quantum dots, and the mature III-V based fabrication technology, satisfy these requirements. This work focuses on quantum dots-in-a-well (DWELL) detectors in which InAs quantum dots are embedded in a compressively strained InGaAs-GaAs quantum well. Barriers separating two stacks of quantum dots can be GaAs, AlGaAs or a combination of different materials, with \u27smart barriers\u27. Motivation for this work is to improve the understanding and the performance of DWELL detectors to achieve high temperature operation and high signal to noise ratio for these detectors for given wavelength requirements, at applied biases compatible with CMOS technology. This aim has been pursued on three fronts: barrier designs, device designs and material systems. Smart barriers, such as resonant tunneling barriers have been demonstrated to improve the signal to noise ratio of the detector by reducing the dark current significantly, while keeping the photocurrent constant. A systematic experimental study has been conducted for understanding the effect of different types of transitions on the properties of DWELL detectors, which showed that bound to quasibound (B-Q) type of transitions optimize the device performance at moderate bias levels. The performance of B-Q type of architectures has been substantially improved by the use of confinement enhancing (CE) barriers that combine the advantages of high energy barriers, such as low dark current and high signal to noise ratio, with those of low energy barriers, such as high responsivity and longer peak wavelengths at low bias operation. A new type of detector, a quantum dot based quantum cascade detector, has been proposed and implemented. QD-QCD exhibits a strong photovoltaic action, leading to strong performance at zero bias, by the virtue of internal electric field generated by the quantum cascade action in the barrier. The zero bias operation, combined with record low photoconductive gains for any quantum dot detectors, makes QD QCD very attractive for focal plane array applications. For improved understanding, theoretical modeling of quantum dot strain, based on atomistic valence force field method as well as transport simulations of general heterostructure detectors with drift-diffusion model have been developed. The transport simulation results indicate the presence of a strong space charge region forming between the highly n-doped contact regions and non-intentionally doped barrier regions, which makes the internal electric field highly nonlinear in space. This has been verified by systematic experiments, in which effects of this electric field nonlinearity on the device parameters have been studied. This work would enable a device designer to choose different device parameters such as spectral response position and shape, photoconductive gain, response, signal to noise ratio, dark current levels, activation energies etc. This knowledge has been utilized in demonstrating highly sensitive FPAs, as well as high operating temperature imaging (at 140K) with DWELL detectors. State of the art performance has been obtained from different devices at different wavelengths, such as such as a detectivity of 4x1011 cm.Hz1/2W-1 at 77K in a bound to quasibound device with a cutoff wavelength of 8.5 μm, which is higher than that obtained from state of the art QWIPs. Although the dark current levels are substantially lower than standard QWIPs, and background limited photodetection is at much higher temperature, the focal plane array sensitivities are lower than those of the state of the art QWIPs, by around 10 mK, due to lower quantum efficiency (a factor of 2-3) and higher photoconductive gain. This difference can be eliminated by the use of gratings or shape engineering through the use of submonolayer quantum dots and with smaller photoconductive gains with DWELL detectors

Spin dynamics in semiconductors

This article reviews the current status of spin dynamics in semiconductors which has achieved a lot of progress in the past years due to the fast growing field of semiconductor spintronics. The primary focus is the theoretical and experimental developments of spin relaxation and dephasing in both spin precession in time domain and spin diffusion and transport in spacial domain. A fully microscopic many-body investigation on spin dynamics based on the kinetic spin Bloch equation approach is reviewed comprehensively.Comment: a review article with 193 pages and 1103 references. To be published in Physics Reports

Modeling techniques for quantum cascade lasers

Quantum cascade lasers are unipolar semiconductor lasers covering a wide range of the infrared and terahertz spectrum. Lasing action is achieved by using optical intersubband transitions between quantized states in specifically designed multiple-quantum-well heterostructures. A systematic improvement of quantum cascade lasers with respect to operating temperature, efficiency and spectral range requires detailed modeling of the underlying physical processes in these structures. Moreover, the quantum cascade laser constitutes a versatile model device for the development and improvement of simulation techniques in nano- and optoelectronics. This review provides a comprehensive survey and discussion of the modeling techniques used for the simulation of quantum cascade lasers. The main focus is on the modeling of carrier transport in the nanostructured gain medium, while the simulation of the optical cavity is covered at a more basic level. Specifically, the transfer matrix and finite difference methods for solving the one-dimensional Schr\"odinger equation and Schr\"odinger-Poisson system are discussed, providing the quantized states in the multiple-quantum-well active region. The modeling of the optical cavity is covered with a focus on basic waveguide resonator structures. Furthermore, various carrier transport simulation methods are discussed, ranging from basic empirical approaches to advanced self-consistent techniques. The methods include empirical rate equation and related Maxwell-Bloch equation approaches, self-consistent rate equation and ensemble Monte Carlo methods, as well as quantum transport approaches, in particular the density matrix and non-equilibrium Green's function (NEGF) formalism. The derived scattering rates and self-energies are generally valid for n-type devices based on one-dimensional quantum confinement, such as quantum well structures

Excitations of Quasi-Particles in Nanostructured Systems

The excitation of quasiparticles, like the investigated excitons and plasmons here, are the optically most prominent responses of materials. In nanostructured system, the sample quality is crucial for quantitative investigations of these optical excitations. We used electron beam evaporation, nano-second laser dewetting, and electron metalorganic chemical vapor deposition techniques to prepare well-defined and “clean” transmission electron microscopy (TEM) samples. Electron energy-loss microscopy (EELS) performed in STEM mode was employed to investigate the structural and electro-optical properties. Quantifit software was used to analyze the EELS spectra quantitatively in terms of inelastic scattering probability, energy and lifetime. We found that the ferroplasmon originates from induced excitation by the Ag’s intrinsic dipole mode at low energy, and it has a redshift with increasing particle size. Because the bimetallic system is associated with one dipole mode only, the ferroplasmons is strongly dependent on geometry. Disc-skirt AgCo nanostructures also show ferroplasmons because plasmon excitation mode of Ag disc is similar in geometry to Ag spherical, while the nanotriangles and nanobowties did not show a ferroplasmon. The bulk plasmon (BP) did not have a significate change from the pure metals to the metals in the bimetallic systems, indicating that the electron density did not change through the contact of the metals. In semiconductors, high binding energy excitons were detected universally at room temperature by EELS for the first time. The states associated with these excitons were identified as molecular states. The singlet S0 state can be directly excited to the triplet T1 state by electrons, even though the transition is forbidden optically. The conclusion on molecular states was based on the fact that this excitation can be bleached with time, and recovered in minutes. Bandbending was observed when the semiconductor is in contacting with Au nanoparticles. This exciton has a signal reduction and blue shift introduced by the band bending. The higher energy exciton can be excited from the S0 state to the singlet S1 state when the band bending is large enough. The distribution of the point defects can be mapped with high precision through mapping the intensity of the exciton

Electrostatic Effects in III-V Semiconductor Based Metal-optical Nanostructures

The modification of the band edge or emission energy of semiconductor quantum well light emitters due to image charge induced phenomenon is an emerging field of study. This effect observed in quantum well light emitters is critical for all metal-optics based light emitters including plasmonics, or nanometallic electrode based light emitters. This dissertation presents, for the first time, a systematic study of the image charge effect on semiconductor–metal systems. the necessity of introducing the image charge interactions is demonstrated by experiments and mathematical methods for semiconductor-metal image charge interactions are introduced and developed