Advanced Semiconductor Heterostructures

The following sections are included:

• PREFACE

• CONTENTS

We present a simple semianalytical model, which allows comprehensive analysis of the LO-phonon assisted electron relaxation in quantum well intersubband semiconductor lasers. Examples of scattering rate tailoring in type-I double quantum well heterostructures and analysis of the subband depopulation process in type-II heterostructures illustrate applicability of the model.

InAs/GaAs quantum dot devices have the potential to be the leading technology for infrared detection and emission, which are necessary for many military and domestic applications. Quantum dot infrared photodetectors yield higher operating temperatures, lower dark currents, and more wavelength tunability. They also permit the detection of normal-incidence light. Quantum dot infrared sources are also expected to yield higher operating temperatures, in addition to lower threshold currents and higher modulation bandwidths. After a brief review of the history of infrared detection and emission, the optical and electrical characteristics of self-organized In(Ga)As/GaAs quantum dots grown by molecular beam epitaxy are discussed, followed by results for the quantum dot detectors and emitters that have been developed at the University of Michigan, Ann Arbor.

In this chapter, we present our work on the development of coherent THz sources based on intersubband transition in quantum-well structures. The main focus is on electrically pumped or quantum-cascade structures, which have been quite successful in generating coherent radiation at mid-infrared frequencies. Relevant issues, such as various depopulation intersubband scattering rates, the role of complex phonon spectra, and coherent vs. incoherent tunneling are discussed in details. Optically pumped sources, including optical parametric amplifiers, and both intersubband and interband pumped THz emitters, are also investigated for their feasibility in generating coherent THz radiations.

The design of room-temperature, InGaAsSb/AlGaAsSb diode lasers has evolved from the first double-heterojunction lasers described in 1980 that operated in the pulsed-current mode to presentday continuous-wave (CW), high-power, quantum-well diode lasers. We discuss in detail recent results from type-I-heterostructure, GaSb-based CW room-temperature diode lasers. The devices operate within the wavelength range of 1.8 to 2.7 μm, providing output powers up to several Watts. We analyze the factors limiting device performance.

This account presents a survey of recent advances in quantum-dot research and technology and highlights trends of key significance as they relate to the emerging applications of quantum-dot technology in biology.

We have investigated the problem of electron runaway at strong electric fields in polar semiconductors focusing on the nanoscale nitride-based heterostructures. A transport model which takes into account the main features of electrons injected in short devices under high electric fields is developed. The electron distribution as a function of the electron momenta and coordinate is analyzed. We have determined the critical field for the runaway regime and investigated this regime in detail. The electron velocity distribution over the device is studied at different fields. We have applied the model to the group-III nitrides: InN, GaN and AIN. For these materials, the basic parameters and characteristics of the high-field electron transport are obtained. We have found that the transport in the nitrides is always dissipative. However, in the runaway regime, energies and velocities of electrons increase with distance which results in average velocities higher than the peak velocity in bulk-like samples. We demonstrated that the runaway electrons are characterized by the extreme distribution function with the population inversion. A three-terminal heterostructure where the runaway effect can be detected and measured is proposed. We also have considered briefly different nitride-based small-feature-size devices where this effect can have an impact on the device performance.

The inverse Nottingham Effect (INE) cooling involves emission of electrons above the Fermi level into the vacuum. Our scheme involves the use of a Double Barrier Resonant Tunneling (DBRT) section positioned between the surface and the vacuum for a much increased emission, and to provide energy selectivity for assuring cooling, without surface structuring such as tips and ridges leading to current crowding and additional heating. Unlike resonant tunneling from contact-to-contact, where barrier heights and thicknesses are controlled by the choice of heterojunctions, the work function at the surface dictates the barrier height for tunneling into the vacuum. The calculated field emission via resonant tunneling gives at least two orders of magnitude greater than without resonance, however, without work function lowering, the large gain happens at fairly high field. The use of resonance to enhance cooling by INE results in an important byproduct, an efficient cold-cathode field emitter for vacuum electronics.

The single electron transistor (SET) is a nanometer-size device that turns on and off again every time one electron is added to it. In this article, the physics of the SET is reviewed. The consequences of confining electrons to a small region of space are that both the charge and energy are quantized. We review how the charge states and energy states of the confined electrons, sometimes called an artificial atom, are measured, and how the precision of these measurements depends on temperature. We also discuss the coupling of electrons inside the artificial atom to those in the leads of the SET, which results in the Kondo effect. We review measurements of the Kondo effect, which demonstrate that the Anderson Hamiltonian provides a quantitative description of the SET.

Hot electron distributions within the active region of quantum well lasers lead to gain suppression, reduced quantum efficiency, and increased diffusion capacitance, greater low-frequency roll-off and high-frequency chirp. Recently, “tunnel injection lasers” have been developed to minimize electron heating within the active quantum well region by direct injection of cool electrons from the separate confinement region into the lasing subband(s) through a tunneling barrier. Tunnel injection lasers, however, also present a rich physics of transport and scattering, and a correspondingly rich set of challenges to simulation and device optimization. In this work, some of the fundamental physics of carrier capture and transport that should be addressed for optimization of such lasers is elucidated using Schrödinger Equation Monte Carlo (SEMC) based quantum transport simulation. In the process, qualitative limitations of the Golden-Rule of scattering in this application are pointed out by comparison. Specifically, a Golden-Rule-based analysis of the carrier injection into the active region of the ideal tunnel injection laser would suggest approximately uniform injection of electrons among the nominally degenerate quantum well states from the separate confinement region states. However, such an analysis ignores (via a random-phase approximation among the final states) the basic real-space transport requirement that injected carriers still must pass through the wells sequentially, coherently or otherwise, with an associated attenuation of the injected current into each subsequent well due to electron-hole recombination in the prior well. Transport among the wells then can be either thermionic, or, of theoretically increasing importance for low temperature carriers, via tunneling. Coherent resonant tunneling between wells, however, is sensitive to the potential drops between wells that split the energies of the lasing subbands and (further) localizes the electron states to individual wells. In this work such transport issues are elucidated using Schrödinger Equation Monte Carlo (SEMC) based quantum transport simulation.

We present calculations of the acoustic phonon spectra for a variety of quantum dots and consider the cases where the quantum dots are both free-standing and embedded in a selection of different matrix materials — including semiconductors, plastic, and water. These results go beyond previous calculations for free-standing quantum dots and demonstrate that the matrix material can have a large effect on the acoustic phonon spectrum and consequently on a variety of phonon-assisted transitions in quantum-dot heterostructures.

Since the introduction of the man-made superlattices and quantum well structures, the field has taken off and developed into Quantum Slab, QS; Quantum Wire, QW; Quantum Dot, QD; and Nanoelectronics. This rapidly expanding field owes its success to the development of epitaxially grown heterojunctions and heterostructures to confine carriers in injection lasers. Meanwhile, the advancement of lithography allows potentials to be applied in nanoscale dimension leading to the possibility of quantum confinement without heterostructures. Actually, quantum states in the inversion layer of field effect transistors, FETs, formed by the application of a large gate voltage appeared several years before the introduction of the superlattices and quantum wells. The quantum Hall effect was first discovered in the Si inversion layer. This chapter, Multipole-Electrode Heterojunction Hybrid Structure, MEHHS, discusses hybrid structures of heterojunctions and applied potentials via multipole-electrodes for a much wider variety of structures for future quantum devices. The technology required to fabricate these electrodes, to some degree, is routinely used in the double-gate devices. Few specific examples are detailed here, hopefully, to stimulate a rapid adoption of a hybrid system for the formation of quasi-discrete states for quantum devices.