The exploitation of band structure engineering as a means of designing optimized quantum structures with novel properties has led to a revolution in semiconductor electro-optics. This chapter reviews several specific device classes for which this remarkable degree of control over the interband and intersubband optical and electronic properties may be used to a particular advantage. Improved growth techniques yielding atomic-level control coupled with rapid advances in device-fabrication technologies have encouraged a trend toward increasingly complex structural configurations. The finite element method capability allows us to go beyond conventional bandgap or band-structure engineering, to a more general approach that may be designated wavefunction engineering. The primary obstacle blocking the progress of such important mid-wave infrared (IR) technologies as chemical sensing and IR countermeasures is the lack of inexpensive, convenient, and reliable sources capable of emitting high powers at non-cryogenic operating temperatures. The chapter illustrates the new opportunities for IR electrooptical devices based on antimonide quantum heterostructures.

We have developed a fundamental theory for electro-optical and nonlinear optical processes involving intersubband transitions in conduction valleys with arbitrary effective-mass anisotropy and orientation. New classes of amplitude and phase modulators, as well as frequency converters employing L-valley interactions in GaSb/AlSb-based quantum well systems are proposed. Key advantages of these devices include operation at normal incidence (enabled by the L-valley effective-mass anisotropy and tilted growth direction with respect to the L-valley constant energy ellipsoids) and adaptability to a wide range of infrared wavelengths (as short as 1.5 mu m, made possible by the large band offsets). The electro-optic coefficients and second-order nonlinear susceptibilities derived from our modeling are larger than any measured or predicted to date for conventional Gamma-valley processes.

We have grown Bi1-x Sbx alloy thin films on CdTe(111)B over a wide range of Sb concentrations (0≤x≤0.183) using MBE. We have observed several differences with the bulk system. The 3.5 and 5.1% Sb alloys show semiconducting behavior, and the Sb concentration with the maximum bandgap is shifted to a lower Sb concentration, from 15% in bulk to 9%. The power factor S2/ρ (where S is thermoelectric power(TEP) and ρ electrical resistivity) peaks at a significantly higher temperature (250K) than previously reported for the bulk alloy (80K). The magnetotransport properties of Bi1-x,Sbx thin films (x = 0, 0.09, and 0.16) and Bi/CdTe superlattices have been determined by applying the Quantitative Mobility Spectrum Analysis (QMSA) and multicarrier fitting to the magneticfield- dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. The calculated S values are in good agreement with experimental results. The structural stability of bulk Bi is studied using the local density linear muffin-tin orbital method. It is shown that the internal displacement changes the Bi electronic structure from a metal to a semimetal, in qualitative agreement with a Jones-Peierls-type transition. The total energy is calculated to have a double well dependence on the internal displacement, and to provide a stabilization of the trigonal phase. We show that an increase of the trigonal shear angle leads to a semimetal-semiconductor transition in Bi.

We report an experimental study of the nonlinear optical properties of HgTe-CdTe superlattices grown by MBE and Pb1−xSnxSe grown by hot wall epitaxy. Nondegenerate four-wave mixing has been employed to measure third-order nonlinear susceptibilities at 10.6 μm as a function of temperature, laser intensity, and difference frequency. The non-linearity is believed to be due to modulation of the free carrier temperature and density by the optical beams. The HgTe-CdTe results are compared to theoretical calculations based on free carrier contributions to the susceptibility, and the agreement with experiment is quite good. In 1-μm-thick Pb1−xSnxSe layers, multiple internal reflections of the light within the sample is found to enhance the nonlinear signal by an order of magnitude.

For modern semiconductor heterostructures containing multiple populations of distinct carrier species, conventional Hall and resistivity data acquired at a single magnetic field provide far less information than measurements as a function of magnetic field. However, the extraction of reliable and accurate carrier densities and mobilities from the field-dependent data can present a number of difficult challenges, which were never fully overcome by earlier methods, such as the multicarrier fit, the mobility-spectrum analysis of Beck and Anderson, and the hybrid mixed-conduction analysis. More recently, to overcome the limitations of those methods, several research groups have contributed to development of the quantitative mobility-spectrum analysis (QMSA), which is now available as a commercial product. The algorithm is analogous to a fast Fourier transform in that it transforms from the magnetic-field (B) domain to the mobility (μ) domain. The QMSA converts the field-dependent Hall and resistivity data into a visually meaningful transformed output, comprising the conductivity density of electrons and holes in the mobility domain. In this article, we apply QMSA to both synthetic and real experimental data that are representative of modern multilayer HgCdTe structures. We discuss such features as the accuracy of the extraction of individual layer conductivities and average mobilities, reconstruction of the carrier mobility distribution within a particular layer, the resolution of two carriers with similar mobilities, and limits of the sensitivity.

High density plasma etching of mercury cadmium telluride using CH4/H2/Ar plasma chemistries is investigated. Mass spectrometry is used to identify and monitor etch products evolving from the surface during plasma etching. The identifiable primary etch products are elemental Hg, TeH2, and Cd(CH3)2. Their relative concentrations are monitored as ion and neutral fluxes (both in intensity and composition), ion energy and substrate temperature are varied. General insights are made into surface chemistry mechanisms of the etch process. These insights are evaluated by examining etch anisotropy and damage to the remaining semiconductor material. Regions of process parameter space best suited to moderate rate, anisotropic, low damage etching of HgCdTe are identified.

We hake fabricated Bi/Sb superlattice alloys that are artificially ordered on the atomic scale using molecular-beam epitaxy. We observe that by changing the superlattice period thickness. the electronic structure can be "tuned" from a semimetal, through zero gap, to a narrow-gap semiconductor, These unique properties, which are distinct from those in random alloys. are believed to be a consequence of ordered atomic configurations.

We have observed a large increase in the magnetoresistance (MR) of molecular beam epitaxy grown Bi thin films, which were subjected to a postannealing procedure 3 °C below the Bi melting point. We have achieved an increase in the MR by a factor of 2560 at helium temperatures compared with of 343 for an as-grown film. The enhancement of the MR in the annealed films is due to higher electron and hole mobilities (μe≈1×106 cm2/V s at 5 K) relative to those of the as-grown films (μe≈9×104 cm2/V s at 5 K). The enhancement of the mobility in the annealed films is also supported by the observation of Shubnikov–de Haas oscillations.

We report artificially atomic-scale ordered superlattice alloy systems, new scheme to pursue high-ZT materials. We have fabricated Bi/Sb superlattice alloys that are artificially ordered on the atomic scale using MBE, confirmed by the presence of XRD superlattice satellites. We have observed that the electronic structure can be modified from semimetal, through zero-gap, to semiconductor by changing the superlattice period and sublayer thicknesses using electrical resistivity, thermopower, and magneto-transport measurements. InSb/Bi superlattice alloys have also been prepared and studied using XRD and thermopower measurements, which shows that their thermoelectric transport properties can be modified in accordance with structural modification. This superlattice alloy scheme gives us one more tool to control and tune the electronic structure and consequently the thermoelectric properties.

The magnetotransport properties of Bi1-xSbx thin films (x = 0, 0.09 and 0.16) and Bi/CdTe superlattices have been determined by applying the quantitative mobility spectrum analysis (QMSA) and multicarrier fitting to the magnetic-field-dependent resistivities and Hall coefficients, using algorithms which account for the strong anisotropy of the mobilities. The experimentally derived electron and hole densities and mobilities are then used as the input to a nonparabolic and anisotropic band model that derives the thermal occupations and relaxation times for L-valley electrons and holes as well as T-valley and H-valley holes. This allows a calculation of the thermoelectric properties for comparison with experimental results. While the data for a Bi thin film and Bi/CdTe superlattices are reproduced quite well without any adjustable parameters, for Bi1-xSbx thin films a non-uniform doping profile must be assumed.

An extension of the quantitative mobility spectrum analysis (QMSA) procedure, which determines free electron and hole densities and mobilities from magnetic-field-dependent Hall and resistivity measurements, to materials exhibiting anisotropic conduction is presented. As test cases, the fully computer-automated procedure is used to analyze magnetotransport data from Bi thin films and Bi/CdTe superlattices. Using the results of the QMSA procedure, the thermoelectric properties of these films can be accurately modeled. As a second exmaple, an electron mobility anisotropy ratio of ≈4.5 is derived from the QMSA treatment of the Hall data for bulk Si samples.

Two different approaches, a photoconductive response technique and a correlation of lasing thresholds with theoretical threshold carrier concentrations have been used to determine Auger lifetimes in InAs/GaInSb quantum wells. For energy gaps corresponding to 3.1–4.8 μm, the room-temperature Auger coefficients for seven different samples are found to be nearly an order-of-magnitude lower than typical type-I results for the same wavelength. The data imply that at this temperature, the Auger rate is relatively insensitive to details of the band structure.

We demonstrate a new mechanism for annealing silicon that does not involve the direct application of heat as in conventional thermal annealing or pulsed laser annealing. A laser pulse focused to high power on a small surface spot of a neutron-transmutation-doped silicon slab is shown to anneal regions far outside the illuminated spot where no heat was directly deposited. Electrical activation of donors throughout the slab was uniform and comparable to that of thermally annealed control samples. We conjecture that the annealing was caused by mechanical energy introduced by the laser pulse.

We have successfully grown BiSb alloy thin films on CdTe(lll)B over a wide range of Sb compositions using molecular beam epitaxy. It is well known that small bandgap (similar to 18 meV) bulk BiSb alloys are good n-type thermoelements at liquid nitrogen temperature. We have observed that the power factor (S(2)sigma) for MBE-grown 1 mu m thick BiSb thin films grown on CdTe(lll) peak at a significantly higher temperature (250K) than previous results for the bulk alloy (80K), possibly due to an enhanced bandgap. For doping experiments we used the group IV(VI) element Sn(Te) as an acceptor(donor). Thermoelectric Power (TEP) and electrical resisitivity were studied in the range of temperatures 2-300 K. Doping Sn into the BiSb system causes the TEP to change sign (from negative to positive), and the maximum value of the TEP can be controlled with the Sn dopant concentration.

We demonstrate an optimized quantitative mobility spectrum analysis (QMSA) technique for determining free electron and hole densities and mobilities from magnetic-field-dependent Hall and resistivity data. The procedure is applied to an In1−xGaxAs–In1−xAlxAs single quantum well, GaAs–AlAs asymmetric double quantum wells, and Hg1−xCdxTe epitaxial thin films containing multiple carrier species. The results illustrate the reliability, versatility, and sensitivity of the analysis, which is fully computer automated following input of the magnetic-field-dependent data. QMSA is found to be a suitable standard tool for the routine electrical characterization of semiconductor material and device transport properties.

We propose an all-optical limiter based on the thermally induced intervalley transfer of electrons from Γ-valley states with forbidden normal-incidence intersubband interactions to L-valley states which absorb strongly. Detailed modeling of the device performance in the short-pulse regime (⩽100 ns) yields that the output intensity for a limiter with only 10% insertion loss at low excitation levels will remain clamped over a dynamic range of 25–40.