D. A. Collins

Study of interface asymmetry in InAs–GaSb heterojunctions

We present reflection high energy electron diffraction, secondary ion mass spectroscopy, scanning tunneling microscopy and x‐ray photoelectron spectroscopy studies of the abruptness of InAs–GaSb interfaces. We find that the interface abruptness depends on growth order: InAs grown on GaSb is extended, while GaSb grown on InAs is more abrupt. We first present observations of the interfacial asymmetry, including measurements of band alignments as a function of growth order. We then examine more detailed studies of the InAs–GaSb interface to determine the mechanisms causing the extended interface. Our results show that Sb incorporation into the InAs overlayer and As exchange for Sb in the GaSb underlayer are the most likely causes of the interfacial asymmetry.

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.

Large peak current densities in novel resonant interband tunneling heterostructures

We have observed negative differential resistance (NDR) and large peak current densities in a novel resonant interband tunneling structure grown by molecular beam epitaxy in the InAs/GaSb/AlSb material system. The structure consists of a thin AlSb barrier layer displaced from an InAs(n)/GaSp(p) interface. NDR is readily observable at room temperature with peak current densities greater than 105 A/cm2. The enhancement in peak current density relative to a structure with no AlSb barrier is consistent with the existence of a quasi‐bound state in the region between the barrier and the InAs/GaAs interface. Furthermore, we demonstrate that by growing the AlSb layer on either the InAs or GaSb side of the interface, the quasi‐bound state can be localized in either material.

Measurement of the valence band offset in novel heterojunction systems: Si/Ge (100) and AlSb/ZnTe (100)

We have used x-ray photoelectron spectroscopy to measure the valence band offset in situ for strained Si/Ge (100) heterojunctions and for AlSb/ZnTe (100) heterojunctions grown by molecular-beam epitaxy. For the Si/Ge system, Si 2p and Ge 3d core level to valence band edge binding energies and Si 2p to Ge 3d core level energy separations were measured as functions of strain, and strain configurations in all samples were determined using x-ray diffraction. Our measurements yield valence band offset values of 0.83±0.11 eV and 0.22±0.13 eV for Ge on Si (100) and Si on Ge (100), respectively. If we assume that the offset between the weighted averages of the light-hole, heavy-hole, and spin-orbit valence bands in Si and Ge is independent of strain, we obtain a discontinuity in the average valence band edge of 0.49±0.13 eV. For the AlSb/ZnTe (100) heterojunction system, we obtain a value of –0.42±0.07 eV for the valence band offset. Our data also suggest that an intermediate compound, containing Al and Te, is formed at the AlSb/ZnTe (100) interface.

Scanning tunneling microscopy of InAs/GaSb superlattices: Subbands, interface roughness, and interface asymmetry

Scanning tunneling microscopy and spectroscopy is used to characterize InAs/GaSb superlattices, grown by molecular‐beam epitaxy. Roughness at the interfaces between InAs and GaSb layers is directly observed in the images, and a quantitative spectrum of this roughness is obtained. Electron subbands in the InAs layers are resolved in spectroscopy. Asymmetry between the interfaces of InAs grown on GaSb compared with GaSb grown on InAs is seen in voltage‐dependent imaging. Detailed spectroscopic study of the interfaces reveals some subtle differences between the two in terms of their valence‐band onsets and conduction‐band state density. These differences are interpreted in a model in which the GaSb on InAs interface has an abrupt InSb‐like structure, but at the InAs on GaSb interface some Sb grading occurs into the InAs overlayer.

Demonstration of resonant transmission in InAs/GaSb/InAs interband tunneling devices

We have performed a theoretical and experimental analysis of current transport in InAs/GaSb/InAs interband tunneling devices as a function of GaSb layer width. Our results demonstrate that current transport in these devices occurs not through simple ohmic conduction, as had been previously proposed, but via light‐hole‐like resonances in the GaSb valence band formed due to the imperfect matching of InAs conduction‐band and GaSb valence‐band wave functions at the InAs/GaSb interfaces. These resonances produce a strong dependence of the current‐voltage characteristics on GaSb layer width that is both predicted theoretically and observed experimentally. Our results also suggest that coupling between InAs conduction‐band and GaSb heavy‐hole valence‐band states is relatively unimportant in these devices. In addition, we have been able to obtain peak current densities of ∼9×104 A/cm2, significantly higher than any previously reported current densities for this structure.

Modeling of novel heterojunction tunnel structures

We have implemented a simple model that allows realistic yet rapid simulation of conventional as well as interband resonant tunneling devices. Using this model we have studied GaAs/AlAs asymmetric triple barrier structures and found that coherence between the quasibound states in the two quantum wells should be observable in the I–V characteristics of the devices. We have also examined InAs–GaSb–InAs broken‐gap interband tunnel devices and found that, despite the absence of classically forbidden barrier regions, a resonant tunneling process is involved in producing the observed negative differential resistance. Furthermore, we have found that maximum peak current densities should be found in devices with GaSb layer thicknesses corresponding to a single, rather than a multiple transmission resonance peak in the broken‐gap region.

X‐ray photoelectron spectroscopy investigation of the mixed anion GaSb/InAs heterointerface

X‐ray photoelectron spectroscopy has been used to measure levels of anion cross‐incorporation and to study interface formation for the mixed anion GaSb/InAs heterojunction. Anion cross‐incorporation was measured in 20 A thick GaSb layers grown on InAs, and 20 A thick InAs layers grown on GaSb for cracked and uncracked sources. It was found that significantly less anion cross‐incorporation occurs in structures grown with cracked sources. Interface formation was investigated by studying Sb soaks of InAs surfaces and As soaks of GaSb surfaces as a function of cracker power and soak time. Exchange of the group V surface atoms was found to be an increasing function of both cracker power and soak time. We find that further optimization of current growth parameters may be possible by modifying the soak time used at interfaces.

p‐type doping of gallium antimonide grown by molecular beam epitaxy using silicon

We report the first effective p‐type doping of molecular beam epitaxy (MBE) grown GaSb using silicon. The samples were grown by molecular beam epitaxy and characterized by Hall‐effect measurements and photoluminescence. Room‐temperature hole concentrations ranging from 4.0×1015 to 4.3×1018 cm−3 were obtained. Photoluminescence (PL) spectra undergo considerable broadening with increasing doping concentration, consistent with an impurity banding picture. Furthermore, the MBE‐grown samples display only one of the two PL features found in a melt‐grown substrate and no other satellites, suggesting higher material purity. This may be a direct benefit from the use of an antimony cracker, ultrahigh vacuum conditions, and high‐purity elemental sources. The short‐period strained‐layer superlattice buffering scheme employed may have also contributed to better structural quality.

Effect of interface composition and growth order on the mixed anion InAs/GaSb valence band offset

We have used x‐ray photoelectron spectroscopy (XPS) to measure the dependence of the InAs/GaSb valence band offset on both interface composition and growth order. Molecular beam epitaxy was used to grow InAs‐on‐GaSb and GaSb‐on‐InAs interfaces with both InSb‐like and GaAs‐like interface compositions. Analysis of XPS core level separations showed no dependence of the valence band offset on interface composition; however, a 90 meV increase in the valence band offset was observed for InAs grown on GaSb compared to GaSb grown on InAs. This difference is attributed to the extended nature of the InAs‐on‐GaSb interface. Results from analysis of an intentionally extended GaSb‐on‐InAs interface were consistent with this conclusion.