Shubnikov-de Haas-like oscillations of the tunnel current were observed at 4.2 K in GaAs/AlAs/GaAs single-barrier heterostructures with 50-, 35-, and 25-Angstrom-thick barriers and 600-Angstrom spacer layers. An accumulation layer was formed at the emitter side of the barriers under external bias. Analysis of the experimental data showed that the concentration of two-dimensional (2D) electrons in the accumulation layer could not be determined from the oscillation period. Only the total number of 2D states on the emitter side of the barrier, below the Fermi level of the emitter bulk, could be determined.

Hall measurements were made on selectively Si-doped InSb, GaSb/InSb, and GaAs/InSb heterostructures grown by molecular beam epitaxy using cracker sources. A cap layer had a pronounced influence especially on the low temperature conduction. The measurements showed an apparent increase in free carrier concentration and reduction of mobility due to compensation and/or parallel conduction. The best mobility, 38 700 cm2/V s at room temperature, was measured in a 2-μm-thick InSb layer capped with GaSb and doped with a concentration of 3×1016 cm−3 within 1000 Å from the cap interface. The variation of carrier concentration with temperature in doped samples revealed singularities at 50 and 90 K due to conduction from an unintentional p-background doping.

The authors have investigated InSb layers grown heteroepitaxially on GaAs(100) substrates by molecular beam epitaxy (MBE). The dependence of electron mobilities on the MBE-growth conditions was investigated. The best room temperature mobility, 55000 cm2 V-1 s-1 for a 2 mu m thick layer, was obtained for a growth temperature of 420 degrees C with an antimony over indium ratio of 1.4. The 14.6% lattice mismatch between epilayer and substrate gives rise to threading dislocations and microtwins, as evidenced by transmission electron microscopy. The defects are shown to reduce the mobility for thin samples. One of the most interesting results of the work is the evidence of an electron accumulation layer at the InSb(100) surface. This result is obtained from temperature-dependent Hall measurements which exhibited two singularities in the carrier concentration versus temperature plot. Calculations of the Hall constant considering parallel conduction is successfully used to model this temperature dependence. The MBE-grown InSb layers are shown to have an unintentional acceptor background. The authors also investigated n-type doping using silicon. It is shown that the measured low temperature carrier concentrations and mobilities in undoped samples are considerably influenced by compensation effects.

Advances in high-frequency resonant-tunneling-diode (RTD) oscillators are described. Oscillations up to a frequency of 420 GHz have been achieved in the GaAs/AlAs system. Recent results obtained with In0.53Ga0.47As/AlAs and InAs/AlSb RTDs show a greatly increased power density and indicate the potential for fundamental oscillations up to about 1 THz. These results are consistent with a lumped-element equivalent circuit model of the RTD. The model shows that the maximum oscillation frequency of the GaAs/AlAs RTDs is limited primarily by series resistance, and that the power density is limited by low peak-to-valley current ratio.

Oscillations have been obtained at frequencies from 100 to 712 GHz in InAs/AlSb double-barrier resonant-tunneling diodes at room temperature. The measured power density at 360 GHz was 90 W cm−2, which is 50 times that generated by GaAs/AlAs diodes at essentially the same frequency. The oscillation at 712 GHz represents the highest frequency reported to date from a solid-state electronic oscillator at room temperature.

We summarize improvements to the performance of oscillators based on double-barrier resonant-tunneling diodes and their relationship to developments in three material systems. Higher frequencies, and more recently higher output powers, have resulted from these materials developments, so that today waveguide oscillators produce output power of up to a milliwatt at lower frequencies and about one microwatt near 400 GHz. The basic concepts of resonant-tunneling oscillators are described, and the ways in which new materials contribute to improved device characteristics are discussed.

The nearly lattice—matched InAs/GaSb/AlSb system offers tremendous flexibility in designing novel heterostructures due to the wide range of available band alignments. We have recently exploited this advantage to demonstrate several different devices exhibiting negative differential resistance (NDR) based on interband tunneling. These devices show a wide range of different characteristics including very high peak current densities (1.6 × 105 A/cm2) or peak to valley current ratios (20:1 at 300K and 88:1 at 77K). We have also studied “traditional” double barrier (resonant) tunneling in the InAs/GaSb/AlSb system. In particular, extremely high peak current densities in InAs/AlSb double barrier devices have been exploited to fabricate oscillators operating at the highest frequencies yet reported. Two and three terminal tunneling devices in this material system show great promise for use in high frequency analog and digital applications.

We report the experimental observation of negative differential resistance (NDR) at room temperature from a structure consisting of a single InAs(n)/GaSb(p) interface. The peak current densities ranged from 4.2×104 to 8.0×104 A/cm2 depending on how the structure is doped. The mechanism that causes the NDR is similar to that of an Esaki tunnel diode. We have also observed NDR at room temperature in a second class of novel devices. These structures consist of a thin layer of AlSb displaced from a single InAs(n)/GaSb(p) interface. NDR with peak current densities greater than 1.6×105 A/cm2 is seen in these structures. We attribute the increase in peak current densities with the addition of the AlSb barrier to the formation of a quasi-bound state between the AlSb layer and the InAs/GaSb interface. This quasi-bound state forms either in the conduction band of InAs or the valence band of GaSb, depending on where the AlSb barrier is placed and leads to a resonate enhancement of the current in the structures.

We report the first observation of resonant tunneling in the InSb/AlxIn1−xSb material system. Five samples with InSb quantum well thicknesses ranging from 70 to 110 Å and Al0.5In0.5Sb barrier thicknesses ranging from 22 to 36 Å were grown by molecular beam epitaxy on GaAs(100) substrates at a temperature of 420 °C. The best sample, which had 22-Å-thick barriers and a 110-Å-thick quantum well, displayed a peak-to-valley current ratio of 1.4(3.9) at room temperature (77 K) with a corresponding peak current density of 3.6×104 A/cm2. Transmission electron microscopy revealed threading dislocations, misfit dislocations, and microtwins in the barrier region.

High quality resonant tunneling diodes have been fabricated from the InAs/AlSb material system (InAs quantum well and cladding layers, AlSb barriers) on (100)GaAs substrates. A diode with a 6.4-nm-thick InAs quantum well and 1.5-nm-thick AlSb barriers yielded a room-temperature peak current density of 3.7×105 A cm−2 and peak-to-valley current ratio of 3.2. This corresponds to an available current density of 2.6×105 A cm−2, which is comparable to that of the best In0.53Ga0.47As/AlAs diodes grown on lattice-matched substrates and is three times higher than that of the best GaAs/AlAs diode reported to date. These results were obtained in spite of a 7.2% lattice mismatch between the InAs epilayers and the GaAs substrates, which leads to a measured threading dislocation density of roughly 109 cm−2. The experimental peak voltage and current density are in good agreement with theoretical calculations based on a stationary-state transport model with a two-band envelope function approximation.

We report on current transport through AlAsGaAs double quantum well heterostructures s a function of the width of the barrier separating the two wells. We compare the current-voltage characteristics of a 108 Å quantum well with those of devices consisting of two 54 Å wells separated by a thin AlAs barrier. For the thinnest central barriers the peak current densities in the structures are unaffected, and there is a sharp increase in the triple barriers' peak-to-valley current ratio as compared to the double barrier diode. When the middle barrier's thickness is increased beyond a certain width there is a dramatic drop in the number and intensity of the resonances indicating that the coherent interaction between the states in the two wells is lost.

We report the successful growth of InAs/Ga1−xInxSb strained-layer superlattices, which have been proposed for far-infrared applications. The samples were grown by molecular beam epitaxy, and characterized by reflection high-energy electron diffraction, x-ray diffraction, and photoluminescence. Best structural quality is achieved for superlattices grown on thick, strain-relaxed, GaSb buffer layers on GaAs substrates at fairly low substrate temperatures (<400 °C). Photoluminescence measurements indicate that the energy gaps of the strained-layer superlattices are smaller than those of InAs/GaSb superlattices with the same layer thicknesses, in agreement with the theoretical predictions of Smith and Mailhiot [J. Appl. Phys. 62, 2545 (1987)]. In the case of a 37 Å/25 Å, InAs/Ga0.75In0.25Sb superlattice, an energy gap of 140±40 meV (≊9 μm) is measured. This result demonstrates that far-infrared cutoff wavelengths are compatible with short superlattice periods in this material system.

We report the successful growth of InAs/Ga1−xInxSb strained-layer superlattices by molecular-beam epitaxy. The superlattices are grown on thick, strain-relaxed InAs or GaSb buffer layers on (100)-oriented GaAs substrates. A short-period, heavily strained superlattice at the GaAs interface is found to improve the structural quality of the buffer layer. Arsenic incorporation in nominally pure GaSb layers is found to depend strongly on substrate temperature and As-background pressure. Best strained-layer superlattice structural quality is achieved for samples grown at fairly low substrate temperatures (<400 °C). Photoluminescence measurements indicate that the energy gaps of the strained-layer superlattices are smaller than those of InAs/GaSb superlattices with the same layer thicknesses, in agreement with the theoretical predictions of Smith and Mailhiot [J. Appl. Phys. 62, 2545 (1987)]. Far-infrared photoluminescence is observed from a 37/25 Å, InAs/Ga0.75In0.25Sb superlattice, demonstrating that far-infrared cutoff wavelengths are compatible with short superlattice periods in this material system.

We report the successful growth of InAs/Ga_(1−x)In_xSb strained‐layer superlattices by molecular‐beam epitaxy. The superlattices are grown on thick, strain‐relaxed InAs or GaSb buffer layers on (100)‐oriented GaAs substrates. A short‐period, heavily strained superlattice at the GaAs interface is found to improve the structural quality of the buffer layer. Arsenic incorporation in nominally pure GaSb layers is found to depend strongly on substrate temperature and As‐background pressure. Best strained‐layer superlattice structural quality is achieved for samples grown at fairly low substrate temperatures (<400 °C). Photoluminescence measurements indicate that the energy gaps of the strained‐layer superlattices are smaller than those of InAs/GaSb superlattices with the same layer thicknesses, in agreement with the theoretical predictions of Smith and Mailhiot [J. Appl. Phys. 62, 2545 (1987)]. Far‐infrared photoluminescence is observed from a 37/25 A, InAs/Ga_(0.75)In_(0.25)Sb superlattice, demonstrating that far‐infrared cutoff wavelengths are compatible with short superlattice periods in this material system.

Negative differential resistance (NDR) in InAs/AlSb/InAs/AlSb/InAs double-barrier structures with peak-to-valley current (PVC) ratios as large as 11 at room temperature and 28 at 77 K is reported. This is a large improvement over previous results for these materials and is also considerably better than those obtained for the extensively studied GaAs/AlGaAs material system. The peak current density was also improved by reducing the barrier thickness, and values exceeding 10/sup 5/ A/cm/sup 2/ have been observed. These results suggest that InAs/AlSb structures are interesting alternatives to conventional GaAs/AlGaAs structures in high-frequency devices. NDR in a InAs/AlSb superlattice double-barrier structure with a lower PVC ratio than in the solid barrier case has also been observed. This result indicates that valley current contributions arising from X-point tunneling are negligible in these structures, consistent with the large band offset.< >

We have observed negative differential resistance at room temperature from devices consisting of a single interface between n-type InAs and p-type GaSb. InAs and GaSb have a type II staggered band alignment; hence, the negative differential resistance arises from the same mechanism as in a p+-n+ tunnel diode. Room-temperature peak current densities of 8.2×104 A/cm2 and 4.2×104 A/cm2 were measured for structures with and without undoped spacer layers at the heterointerface, respectively.

The nearly lattice-matched InAs/GaSb/A1Sb system offers tremendous flexibility in designing novel heterostructures due to its wide range of band alignments. We have recently exploited this advantage to demonstrate a new class of negative differential resistance (NDR) devices based on interband tunneling. We have also studied "traditional" double barrier (resonant) and single barrier NDR tunnel structures in the InAs/GaSb/AlSb system. Several of the interband and resonant tunneling structures display excellent peak current densities (as high as 4 x 1O A/cm2 ) and/or peak-to-valley current ratios (as high as 20:1 and 88:1 at 300 K and 77 K, respectively), offering great promise for high frequency and logic applications.

A two-band transfer matrix method has been developed to study tunneling currents in narrow gap and interband tunnel structures. This relatively simple model gives good agreement with recently reported experimental results for InAs/AlSb/InAs/AlSb/InAs double-barrier heterostructures and InAs/AlSb/GaSb/AlSb/InAs resonant interband tunneling devices, and should be useful in the design of new interband tunneling devices.

The current-voltage (I-V) behavior of a GaSb(p)/AlSb/InAs/AlSb/GaSb(p) resonant interband tunneling (RIT) heterostructure is analyzed experimentally and theoretically. The structure has been successfully grown on a (100)-oriented GaAs substrate by molecular-beam epitaxy, demonstrating that more exotic lattice-matched substrates (such as InAs or GaSb) are not required for RIT devices. Theoretical simulations of I-V behavior are developed, employing a two-band tight-binding model. Experimental I-V curves show pronounced negative differential resistance, with a peak-to-valley current ratio of 8.3 at 300 K. Good agreement is observed between measured and calculated peak current densities, consistent with light-hole tunneling through the confined InAs conduction-band state.