J. R. Söderström

New negative differential resistance device based on resonant interband tunneling

We propose and demonstrate a novel negative differential resistance device based on resonant interband tunneling. Electrons in the InAs/AlSb/GaSb/AlSb/InAs structure tunnel from the InAs conduction band into a quantized state in the GaSb valence band, giving rise to a peak in the current-voltage characteristic. This heterostructure design virtually eliminates many of the competing transport mechanisms which limit the performance of conventional double-barrier structures. Peak-to-valley current ratios as high as 20 and 88 are observed at room temperature and liquid-nitrogen temperature, respectively. These are the highest values reported for any tunnel structure.

Growth and characterization of InAs/Ga1−xInxSb strained‐layer superlattices

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 A/25 A, 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.

Two‐band modeling of narrow band gap and interband tunneling devices

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.

Growth and characterization of ZnTe films grown on GaAs, InAs, GaSb, and ZnTe

We report the successful growth of ZnTe on nearly lattice-matched III-V buffer layers of InAs (0.75%), GaSb (0.15%), and on GaAs and ZnTe by molecular beam epitaxy. In situ reflection high-energy electron diffraction measurements showed the characteristic streak patterns indicative of two-dimensional growth. Photoluminescence measurements on these films show strong and sharp features near the band edge with no detectable luminescence at longer wavelengths. The integrated photoluminescence intensity from the ZnTe layers increased with better lattice match to the buffer layer. The ZnTe epilayers grown on high-purity ZnTe substrates exhibited stronger luminescence than the substrates. We observe narrow luminescence linewidths (full width at half maximum ~ 1–2 A) indicative of uniform high quality growth. Secondary-ion mass spectroscopy and electron microprobe measurements, however, reveal substantial outdiffusion of Ga and In for growths on the III-V buffer layers.

InAs/AlSb double-barrier structure with large peak-to-valley current ratio: a candidate for high-frequency microwave devices

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.<<ETX>>

Demonstration of large peak‐to‐valley current ratios in InAs/AlGaSb/InAs single‐barrier heterostructures

We report large peak-to-valley current ratios in InAs/AlxGa1−xSb/InAs single-barrier tunnel structures. The mechanism for single-barrier negative differential resistance (NDR) has been proposed and demonstrated recently. A peak-to-valley current ratio of 3.4 (1.2) at 77 K (295 K), which is substantially larger than what has been previously reported, was observed in a 200-A-thick Al0.42Ga0.58Sb barrier. A comparison with a calculated current-voltage curve yields good agreement in terms of peak current and the slope of the NDR region. The single-barrier structure is a candidate for high-speed devices because of expected short tunneling times and a wide NDR region.

Experimental observation of negative differential resistance from an InAs/GaSb interface

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 8.2×10^4 A/cm^2 and 4.2×10^4 A/cm^2 were measured for structures with and without undoped spacer layers at the heterointerface, respectively.

Observation of large peak-to-valley current ratios and large peak current densities in AlSb/InAs/AlSb double-barrier tunnel structures

We report improved peak‐to‐valley current ratios and peak current densities in InAs/AlSb double‐barrier, negative differential resistance tunnel structures. Our peak‐to‐valley current ratios are 2.9 at room temperature and 10 at liquid‐nitrogen temperatures. Furthermore, we have observed peak current densities of 1.7×105 A/cm2. These figures of merit are substantially better than previously reported values. The improvements are obtained by adding spacer layers near the barriers, thinner well regions, and thinner barriers.

NEGATIVE DIFFERENTIAL RESISTANCE DUE TO RESONANT INTERBAND TUNNELING OF HOLES

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.

Large peak-to-valley current ratios in triple barrier heterostructures

Abstract We have studied the effect of a thin AlAs barrier in the middle of an GaAs/AlAs double barrier heterostructure on peak-to-valley current ratios. The addition of a 3 monolayer barrier in the middle of the GaAs well increased the peak-to-valley current ratios by over a factor of 5 at 77 K and increased the number of negative differential resistance regions from 2 to 3. The 3 monolayer middle barrier caused only a slight change in the peak current density hence the improvement in device performance was attributed to a decrease in the nonresonant valley currents. At 300 K the introduction of the middle barrier improved the peak-to-valley current ratios by only a factor of two indicating that the valley currents being suppressed by the middle barrier have a substantial non-thermionic component.