Jasprit Singh

Role of strain and growth conditions on the growth front profile of InxGa1−xAs on GaAs during the pseudomorphic growth regime

Theoretical and experimental studies are presented to understand the initial stages of growth of InGaAs on GaAs. Thermodynamic considerations show that, as strain increases, the free‐energy minimum surface of the epilayer is not atomically flat, but three‐dimensional in form. Since by altering growth conditions the strained epilayer can be grown near equilibrium or far from equilibrium, the effect of strain on growth modes can be studied. In situ reflection high‐energy electron diffraction studies are carried out to study the growth modes and surface lattice spacing before the onset of dislocations. The surface lattice constant does not change abruptly from that of the substrate to that of the epilayer at the critical thickness, but changes monotonically. These observations are consistent with the simple thermodynamic considerations presented.

Role of interface roughness and alloy disorder in photoluminescence in quantum‐well structures

A formalism to study the effect of alloy disorder and interface roughness on the linewidths of excitonic emission spectra in quantum‐well structures is developed. The study includes the cases where the alloy forms (a) the barrier region, (b) the well region, and (c) both the barrier and well regions of the quantum‐well structures, and demonstrates the importance of alloy quality in all three cases. The relative importance of the effects of alloy disorder and interface roughness on the excitonic linewidths is discussed. As an illustration, the formalism is applied to AlGaAs/GaAs, InP/InGaAs, and InAlAs/InGaAs quantum‐well structures and the results compared with the available experimental data.

Charge control and mobility studies for an AlGaN/GaN high electron mobility transistor

A charge control model and a mobility model are developed for the Al–GaN/GaN high electron mobility transistor (HEMT) device. The model addresses issues of how piezoelectric effect and interface roughness influence device properties. We find that even small amount of interface roughness has very strong effect on the two-dimensional electron gas properties. Low-lying electronic states are strongly localized and transport through these states is not described by Born approximation but by phonon-assisted hopping. At low temperature the effects of localization are quite important and we use the Kubo formula to study this transport. Results for charge control and mobility are presented as a function of Al composition in the AlGaN/GaN HEMT.

Semiconductor Device Physics and Design

Semiconductor Device Physics and Designprovides a fresh and unique teaching tool. Over the last decade device performances are driven by new materials, scaling, heterostructures and new device concepts. Semiconductor devices have mostly relied on Si but increasingly GaAs, InGaAs and heterostructures made from Si/SiGe, GaAs/AlGaAs etc have become important. Over the last few years one of the most exciting new entries has been the nitride based heterostructures. New physics based on polar charges and polar interfaces has become important as a result of the nitrides. Nitride based devices are now used for high power applications and in lighting and display applications. For students to be able to participate in this exciting arena, a lot of physics, device concepts, heterostructure concepts and materials properties need to be understood. It is important to have a textbook that teaches students and practicing engineers about all these areas in a coherent manner. Semiconductor Device Physics and Designstarts out with basic physics concepts including the physics behind polar heterostructures and strained heterostructures.Important devices ranging from p-n diodes to bipolar and field effect devices is then discussed. An important distinction users will find in this book is the discussion presented on device needs from the perspective of various technologies. For example, how much gain is needed in a transistor, how much power, what kind of device characteristics are needed. Not surprisingly the needs depend upon applications. The needs of an A/D or D/A converter will be different from that of an amplifier in a cell phone. Similarly the diodes used in a laptop will place different requirements on the device engineer than diodes used in a mixer circuit. By relating device design to device performance and then relating device needs to system use the student can see how device design works in real world. This book is comprehensive without being overwhelming.The focus was to make this a useful text book so that the information contained is cohesive without including all aspects of device physics. The lesson plans demonstrated how this book could be used in a 1 semester or 2 quarter sequence.

Absorption, carrier lifetime, and gain in InAs-GaAs quantum-dot infrared photodetectors

Quantum-dot infrared photodetectors (QDIPs) are being studied extensively for mid-wavelength and long-wavelength infrared detection because they offer normal-incidence, high-temperature, multispectral operation. Intersubband absorption, carrier lifetime, and gain are parameters that need to be better characterized, understood, and controlled in order to realize high-performance QDIPs. An eight-band k/spl middot/p model is used to calculate polarization-dependent intersubband absorption. The calculated trend in absorption has been compared with measured data. In addition, a Monte-Carlo simulation is used to calculate the effective carrier lifetime in detectors, allowing the calculation of gain in QDIPs as a function of bias. The calculated gain values can be fitted well with experimental data, revealing that the gain in these devices consists of two mechanisms: photoconductive gain and avalanche gain, where the latter is less dominant at normal operating biases.

Theory of photoluminescence line shape due to interfacial quality in quantum well structures

We have developed a simple theory to understand the role of interfacial quality in the line shape of photoluminescence spectra in quantum wells. The interface is described in terms of microscopic fluctuations δ1 and δ2, where δ1 is the local fluctuation in the well width and δ2 is the lateral correlated extent of the fluctuation. We make use of Lifshitz theory of disordered alloys to determine the probability of distribution of fluctuations in the well size over the extent of the optical probe, i.e., the exciton. The line shape is then calculated from this probability distribution. Both δ1 and δ2 are found to be important in controlling the linewidths in quantum wells. The use of this quantitative theory to characterize the microscopic nature of interfaces is discussed.

Direct measurement of auger recombination in In0.1Ga0.9N/GaN quantum wells and its impact on the efficiency of In0.1Ga0.9N/GaN multiple quantum well light emitting diodes

The Auger recombination coefficient in In0.1Ga0.9N/GaN quantum wells, emitting at 407 nm has been determined from large signal modulation measurements on lasers in which these quantum wells form the gain region. A value of 1.5×10−30 cm6 s−1 is determined for the Auger coefficient at room temperature, which is used to analyze the reported efficiency characteristics of 410 nm In0.1Ga0.9N/GaN quantum wells light emitting diodes. The calculated efficiencies agree remarkably well with the measured ones. It is apparent that Auger recombination is largely responsible for limiting device efficiencies at high injection currents.

Tunneling injection lasers: a new class of lasers with reduced hot carrier effects

In conventional quantum-well lasers, carriers are injected into the quantum wells with quite high energies. We have investigated quantum-well lasers in which electrons are injected into the quantum-well ground state through tunneling. The tunneling injection lasers are shown to have negligible gain compression, superior high-temperature performance, lower Auger recombination and wavelength chirp, and better modulation characteristics when compared to conventional lasers. The underlying physical principles behind the superior performance are also explored, and calculations and measurements of relaxation times in quantum wells have been made. Experimental results are presented for lasers made with a variety of material systems, InGaAs-GaAs-AlGaAs, InGaAs-GaAs-InGaAsP-InGaP, and InGaAs-InGaAsP-InP, for different applications. Both single quantum-well and multiple quantum-well tunneling injection lasers are demonstrated.

In(Ga)As/GaAs self-organized quantum dot lasers: DC and small-signal modulation properties

Self-organized growth of InGaAs/GaAs strained epitaxial layers gives rise to an ordered array of islands via the Stranski-Krastanow growth mode, for misfits >1.8%. These islands are pyramidal in shape with a base diagonal of /spl sim/20 nm and height of /spl sim/6-7 nm, depending of growth parameters. They therefore exhibit electronic properties of zero-dimensional systems, or quantum dots. One or more layers of such quantum dots can be stacked and vertically coupled to form the gain region of lasers. We have investigated the properties of such single-layer quantum dot (SLQD) and multilayer quantum dot (MLQD) lasers with a variety of measurements, including some at cryogenic temperatures. The experiments have been complemented with theoretical calculations of the electronic properties and carrier scattering phenomena in the dots. Our objective has been to elucidate the intrinsic behavior of these devices. The lasers exhibit temperature independent threshold currents up to 85 K, with T/sub 0//spl les/670 K. Typical threshold currents of 200-/spl mu/m long room temperature lasers vary from 6 to 20 mA. The small-signal modulation bandwidths of ridge waveguide lasers are 5-7.5 GHz at 300 K and increased to >20 GHz at 80 K. These bandwidths agree well with electron capture times of /spl sim/30 ps determined from high-frequency laser impedance measurements at 300 K and relaxation times of /spl sim/8 ps measured at 18 K by differential transmission pump-probe experiments. From the calculated results we believe that electron-hole scattering intrinsically limits the high-speed performance of these devices, in spite of differential gains as high as /spl sim/7/spl times/10/sup -14/ cm/sup 2/ at room temperature.

Theoretical studies of the effect of strain on the performance of strained quantum well lasers based on GaAs and InP technology

A discussion is presented of the use of strain to improve the performance of quantum well laser structures. The deformation potential theory is used to study the effect of strain produced by the addition of excess indium on the conduction band and valence band properties. Full-band mixing effects are retained in the calculations. Using a numerical technique developed to study laser parameters in arbitrary quantum well structures, the authors study the effect of strain on the threshold current density and polarization dependence. Dramatic improvements are found due to the strain-induced band-structure changes. Optimization results are presented which show that single quantum well structures have the best performance. >