Y. Rajakarunanayake

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.

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.

Intersubband absorption in Si1−xGex/Si superlattices for long wavelength infrared detectors

We have calculated the absorption strengths for intersubband transitions in n-type Si1–xGex/Si superlattices. These transitions can be used for the detection of long-wavelength infrared radiation. A significant advantage in Si1–xGex/Si superlattice detectors is the ability to detect normally incident light; in Ga1–xAlxAs/GaAs superlattices intersubband absorption is possible only if the incident light contains a polarization component in the growth direction of the superlattice. We present detailed calculations of absorption coefficients, and peak absorption wavelengths for [100], [111], and [110] Si1–xGex/Si superlattices. Peak absorption strengths of about 2000–6000 cm^–1 were obtained for typical sheet doping concentrations ([approximately-equal-to]10^12 cm^–2). Absorption comparable to that in Ga1–xAlxAs/GaAs superlattice detectors, compatibility with existing Si technology, and the ability to detect normally incident light make these devices promising for future applications.

Band offset of the ZnSe/ZnTe superlattices: A fit to photoluminescence data by k.p theory

The ZnSe–ZnTe superlattices have attracted considerable attention as possible blue/green light emitters. Although these superlattices have been successfully fabricated and show intense photoluminescence, there are many basic issues about this system which still remain unresolved. The most important of them is the value of the valence band offset between ZnSe and ZnTe. We have studied the band structure of ZnSe–ZnTe superlattices. Our calculations are based on second order k·p theory and include the effects of strain and spin-orbit splitting on the superlattice band structure. We have investigated the dependence of the superlattice band gap on the valence band offset. Based on the assumption that the photoluminescence from the superlattice corresponds to a bound exciton at a Te1 isoelectronic center in ZnSe, we have fit the experimental photoluminescence data with k·p theory to obtain the best value of the valence band offset. The value we find is 0.97±0.10 eV. Alternatively, assuming that the photoluminescence was due to band-to-band transitions we obtain a valence band offset of 1.20±0.13 eV. We have also calculated the superlattice band gap as a function of the constituent material layer thicknesses for the first valence band offset quoted. We expect these results to be important in gaining an understanding of the value of the valence band offset, and the nature of the photoluminescence from this system.

Band alignment of Zn1-xCdxTe/ZnTe and ZnTe1-xSex/ZnTe strained layer superlattices

We present photoluminescence spectra from CdZnj_Te /ZnTe and ZnSe,,Tei_ /ZnTe strained layer superlattices grown by MBE, and analyze the band alignments and strain effects. Our results are based on fitting the dominant photoluminescence peaks to the superlattice band structure obtamed by k •theory. We find that the valence band offset of the CdZniTe /ZnTe system is quite small. On the other hand, the photoluminescence data from the ZnSeTei_ /ZnTe superlattices suggest that the band alignment is type II, with a large valence band offset. We also investigate the band gap bowing in the ZnSeTej_ alloys, and determine the individual components of the bowing in valence and conduction bands. Based on our results for band alignments, we evaluate the prospects for minority carrier injection in wide bandgap heterostructures based on ZnSe, ZnTe, and CdTe.

Interband tunneling in InAs/GaSb/AlSb heterostructures

Abstract 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.

Characterization of CdSe/ZnTe heterojunctions

Abstract We have measured the valence band offset in a cubic CdSe/ZnTe (100) heterojunction by X-ray photoelectron spectroscopy (XPS). Our preliminary result, based on analysis of one heterojunction, is 0.65±0.08 eV. The electrical characteristics of a doped n-CdSe/p-ZnTe heterojunction appear to be dominated by traps. We have attempted to verify the band offset through capacitance-voltage measurements, but have been unable to extract a consistent value for the flat band voltage from scans taken at different temperatures and frequencies.

Excitons in II-VI heterostructures

Abstract The properties of excitons are examined in two model II-VI heterostructures: the Type-I CdTe/ZnTe system with small valence band offset and the Type-II ZnTe/ZnSe system. The strain induced band offset in the CdTe/ZnTe heterojunction is included to provide the confinement for the heavy hole state. It is found that the large enhancements of exciton binding energy and oscillator strength in the CdTe/ZnTe system are similar to what one finds in systems with a much larger valence band offset. For the ZnTe/ZnSe system, it is found that strong confinement of electrons and holes by the large band offsets can give rise to a very large exciton binding energy for thin heterojunction layers. Also, the mismatch in dielectric constants induces an image charge at the interface, which modifies significantly the exciton Hamiltonian in an asymmetric structure.

InAs/GaSb/AlSb: The Material System of Choice for Novel Tunneling Devices

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.