Gregory N. Henderson

Ballistic electron transport in semiconductor heterostructures and its analogies in electromagnetic propagation in general dielectrics

A comprehensive set of analogies between ballistic electron wave propagation in semiconductors (arbitrary kinetic energy and effective mass) and electromagnetic propagation in general dielectrics (arbitrary permittivity and permeability) is established. The expressions for electron wave propagation, reflection, and refraction are developed and shown to have the same functional form as in electromagnetics, if analogous definitions of electron wave phase and amplitude refractive indexes are used. The reflectivity characteristics such as total internal reflection (critical angle) and zero reflectivity (Brewster angle) are analyzed as a function of material parameters for both general dielectrics and semiconductor materials. The critical angle and Brewster angle results are then applied to electron wave propagation in Ga/sub 1-x/Al/sub x/As, where it is shown that all interfaces in this material will have both a critical angle and a Brewster angle due to differing effective masses across the interface. >

Optical transitions to above‐barrier quasibound states in asymmetric semiconductor heterostructures

An asymmetric semiconductor electron wave Fabry–Perot interference filter has been designed with two above‐barrier quasibound states for optical transitions. The upper state was designed to have a spatial confinement lifetime greater than three times that of the lower state (which was designed to be less than 100 fs). Such lifetime ratios and magnitudes, which are nearly impossible for below‐barrier states, satisfy the criteria required for achieving population inversion. Furthermore, the transitions were designed to have large dipole matrix elements. Absorption measurements at multiple temperatures were used to demonstrate the first bound‐to‐quasibound transitions in an asymmetric structure. The experimental energies and dipole matrix elements are in agreement with calculated values. This type of structure could represent the basis for a new room‐temperature infrared semiconductor laser.

Quantum interference effects in semiconductors: a bibliography

The bibliography has been compiled as an introduction and study guide to this field. The papers listed describe the extensive theoretical and experimental results that have been obtained on quantum interference effects and discuss possible application areas. Works of a fundamental nature concerning phenomena that are basic to all semiconductor behavior have not been included. Articles on the properties and band structure of semiconductors, which are essential to a complete understanding of quantum interference effects, have not been included. Conference papers, though frequently very important, have not been included to conserve space. The papers are listed alphabetically according to the first authors surname. As in the compilation of any bibliography, numerous valuable and pertinent articles have probably been inadvertently omitted. >

Ballistic electron emission testing of semiconductor heterostructures

Abstract In ballistic electron emission microscopy (BEEM) and spectroscopy, ballistic electrons are injected into a sample using a scanning tunneling microscope to probe the electrical properties of buried interfaces. In this communication, a method is proposed that uses the BEEM technique to observe the electron wave optical properties of semiconductor heterostructures. This method provides a three-terminal configuration for characterizing electron wave devices that overcomes many of the limitations encountered in other two- and three-terminal measurement techniques. Specifically, the method provides an injector, which is well isolated from the heterostructure, that injects a collimated beam of ballistic carriers with a precisely controlled energy distribution. These carriers accurately probe the quantum transmittance of a voltage-tunable electron wave interference structure, which can be designed with a light doping to minimize impurity and electron-electron scattering. A general procedure is presented for analyzing this experimental configuration based on a combination of the models used to describe BEEM and ballistic electron transport in semiconductors. Using this procedure, BEEM testing of an electron wave energy filter is modeled and clear quantum interference effects are predicted. This BEEM configuration should allow for the precise characterization of a wide range of ballastic electron transport effects such as quantum reflections from interfaces and electron wave interference effects, phenomena that are presently of wide interest.

Low‐temperature scanning tunneling microscope for ballistic electron emission microscopy and spectroscopy

Design details and initial results are presented for a low‐temperature scanning tunneling microscope specifically intended for measurements of ballistic‐carrier transmittance through heterostructures. The basic design is of the Besocke type, modified for ballistic electron emission microscopy and spectroscopy (BEEM). This instrument is the first to acquire BEEM spectra below 77 K. Salient features are (1) operation in a liquid‐helium storage Dewar to below 6 K, (2) a lateral positioning range of 5 mm at low temperature, and (3) lateral drift rate less than 0.2 nm/h at the lowest temperatures. For BEEM spectroscopy, the microscope’s high positional stability allows extended signal‐averaging at a single location on the sample.

Electron wave diffraction by semiconductor gratings: Rigorous analysis and design parameters

An exact rigorous coupled‐wave analysis has been developed to model ballistic electron wave diffraction by gratings with periodic effective mass and/or potential energy variations. Design expressions have been derived to calculate diffracted angles, to identify evanescent orders, and to identify the Bragg condition. Design expressions for Bragg regime (up to 100% diffraction efficiency in a single order) and Raman–Nath regime (high diffraction efficiency divided among multiple orders) diffraction are presented along with example Ga1−xAlxAs grating designs. Design procedures for ballistic electron switches, multiplexers, spectrometers, and electron waveguide couplers are described.

Nanostructure optical emitters based on quasibound electron energy levels

Abstract Given two energy states (levels) in a quantum well formed by two potential barriers of finite thickness, elementary quantum mechanics tells us that the lower energy state is more tightly bound than the upper state. This produces a longer spatial confinement lifetime in the lower state than in the upper state. This ratio of lifetimes is opposite to that needed for laser action between these states. Furthermore, the lifetime of the lower energy state must be significantly shorter than the electron scattering time for the upper state. These facts have blocked the development of lasers based on these transitions. However, in this paper we report experimental and analytical results on a versatile type of semiconductor heterostructure that overcomes these difficulties. Unlike previous devices, this structure relies on an optical transition between two states which are both above-barrier quasibound states in the ‘classical’ continuum. The oscillator strength is large and the operation of the device clearly demonstrates coherent electron wave behavior. Such structures could represent the basis for a new room-temperature infrared semiconductor laser.

Diffraction of ballistic electrons by semiconductor gratings: Rigorous analysis, approximate analyses, and device design

A rigorous coupled-wave analysis is developed to model ballistic electron diffraction by semiconductor gratings with periodic effective mass and for potential energy variations. This analysis includes expressions for diffracted angles, evanescent and propagating orders, the Bragg condition, and diffraction efficiencies. Two approximate diffraction regimes, Bragg and Raman-Nath, are defined in which the rigorous coupled-wave equations (RCWEs) can be solved analytically, and the approximations required, the approximate solutions, and the restrictions placed on the grating parameters for each regime are given. It is shown that both the Bragg regime and the Raman-Nath regime are achievable with physically fabricated semiconductor grating structures. In addition, it is shown that both narrow and broad angular and energy selectivities can be achieved through control of the effective thickness of the grating. These results are used in the design of a two-dimensional electron gas (2-DEG) switch and a 2-DEG broadcast device. >

Quantum well mid-infrared lasers based on above-barrier transitions

A possible laser device is designed with the use of classically free quasibound electron states. An asymmetric semiconductor electron wave Fabry-Perot interference filter is designed with an upper electron state having much stronger confinement than the lower electron state. This structure also allows for direct current pumping of the upper state and rapid depletion of the lower state under the presence of a field. Spectroscopy experiments demonstrate the existence of the upper quasibound state in a test structure. This laser filter structure, designed for infrared gain with current pumping, is combined with a special injector filter for room temperature narrow energy current injection into the upper lasing state. A stack of 54 periods of this electrically pumped structure is placed within a waveguide geometry. A laser device is fabricated by etching mesa structures from 50 to 100 micrometers wide. End cleaved facets serve as reflectors for mesas from 2 to 5 mm long. Tests are performed on these devices to determine their electrical properties and suitability for lasing.

Use of Classically Free Quasibound States for Infrared Emission

A possible laser device is designed with the use of classically free quasibound electron states. An asymmetric semiconductor electron wave Fabry-Perot interference filter is designed with an upper electron state having much stronger confinement (235f6 lifetime) than the lower electron state (76f s lifetime). This structure also allows for direct current pumping of the upper state and rapid depletion of the lower state under the presence of a field. Experiments demonstrate the existence of the upper quasibound state in this structure. Another structure, designed for infrared gain with current pumping, has improved parameters over the structure used in the spectroscopy measurement.