R. Magno

Photoluminescence studies of self‐assembled InSb, GaSb, and AlSb quantum dot heterostructures

Photoluminescence (PL) spectroscopy has been performed on a set of self‐assembled InSb, GaSb, and AlSb quantum dot (QD) heterostructures grown on GaAs. Strong emission bands with peak energies near 1.15 eV and linewidths of ∼80 meV are observed at 1.6 K from 3 monolayer (ML) InSb and GaSb QDs capped with GaAs. The PL from a capped 4 ML AlSb QD sample is weaker with peak energy at 1.26 eV. The PL bands from these Sb‐based QD samples shift to lower energy by 20–50 meV with decreasing excitation power density. This behavior suggests a type II band lineup. Support for this assignment, with electrons in the GaAs and holes in the (In,Ga,Al)Sb QDs, is found from the observed shift of GaSb QD emission to higher energies when the GaAs barrier layers are replaced by Al0.1Ga0.9As.

Molecular beam epitaxial growth of InSb, GaSb, and AlSb nanometer‐scale dots on GaAs

Thin layers of InSb, GaSb, and AlSb were grown on GaAs substrates by molecular beam epitaxy. Atomic force microscopy was used to examine surface morphology as a function of growth temperature and monolayer coverage. For each material, conditions were found which resulted in Stranski–Krastanov growth with the strain‐induced formation of nanometer‐scale dots. Relatively uniform distributions of dots form in a temperature window near the congruent sublimation temperature for both InSb and GaSb. In the case of InSb, deposition of 2 monolayers at 430 °C produced a surface with 3×109/cm2 dots with heights of 58±5 A and diameters of 600±50 A.

NANOSTRUCTURE PATTERNS WRITTEN IN III-V SEMICONDUCTORS BY AN ATOMIC FORCE MICROSCOPE

An atomic force microscope has been used to pattern nanometer-scale features in III–V semiconductors by cutting through a thin surface layer of a different semiconductor, which is then used as an etch mask. Cuts up to 10 nm deep, which pass through 2–5 nm thick epilayers of both GaSb and InSb, have been formed. Lines as narrow as 20 and 2 nm deep have been made. Selective etchants and a 5 nm GaSb etch mask are used to transfer patterns into an InAs epilayer. The results are promising for applications requiring trench isolation, such as quantum wires and in-plane gated structures.

Phonons in self‐assembled (In,Ga,Al)Sb quantum dots

Quantum dots of InSb, GaSb, and AlSb were grown on GaAs substrates by molecular beam epitaxy and characterized by atomic force microscopy and Raman spectroscopy. There is a clear correlation between the observation of quantum dots by atomic force microscopy and a phonon mode at an energy a few wavenumbers below the longitudinal optic phonon energy for thick (In,Ga,Al)Sb layers. In the case of nominally AlSb quantum dots, a two‐mode behavior is observed and attributed to the segregation of Ga into the AlSb during growth.

Self‐assembled InSb and GaSb quantum dots on GaAs(001)

Quantum dots of InSb and GaSb were grown on GaAs(001) by molecular‐beam epitaxy. In situ scanning tunneling microscopy measurements taken after 1–2 monolayers of InSb or GaSb growth reveal the surface is a network of anisotropic ribbon‐like platelets. These platelets are a precursor to quantum dot growth. Transmission electron microscopy measurements indicate that the quantum dots are coherently strained. Quantum dots of InSb and GaSb capped by GaAs exhibit strong luminescence near 1.1 eV.

Stranski-Krastanov growth of InSb, GaSb, and AlSb on GaAs: structure of the wetting layers

Abstract Thin layers of InSb, GaSb and AlSb were grown on GaAs(0 0 1) by molecular beam epitaxy and characterized in situ with scanning tunneling microscopy. All three materials exhibit a Stranski-Krastanov growth mode. Distinct wetting layers and self-assembled quantum dots are present after deposition of one to four monolayers of (In,Ga,Al)Sb. The wetting layers consist of anisotropic, ribbon-like structures oriented along the [1 1 0] direction, with characteristic separations of 40–50 A. The initial GaAs surface reconstruction affects both the wetting layer structure and the quantum dot density.

Growth of InAsSb-channel high electron mobility transistor structures

We discuss the molecular beam epitaxial growth of the random alloy InAsSb for use as the channel in high electron mobility transistors (HEMTs). Room-temperature mobilities of 22000cm2∕Vs have been achieved at a sheet carrier density of 1.4×1012∕cm2. This is a marked improvement over the mobility of 13000cm2∕Vs at the same carrier density obtained in previous attempts to grow the InAsSb channel using a digital alloy procedure [J. B. Boos, M. J. Yang, B. R. Bennett, D. Park, W. Kruppa, R. Bass, Electron. Lett. 35, 847 (1999)]. We have also implemented different barriers and buffer layers to enhance the transport properties and overall performance of the HEMT structure.

Monolithic integration of resonant interband tunneling diodes and high electron mobility transistors in the InAs/GaSb/AlSb material system

InAs/AlSb high electron mobility transistors (HEMTs) and resonant interband tunneling diodes (RITDs) with AlSb barriers and GaSb wells were grown in a single heterostructure by molecular beam epitaxy. The resulting HEMTs exhibit excellent dc and microwave performance at low drain voltages, with an intrinsic unity-current-gain cutoff frequency of 220 GHz. The RITD performance is comparable to RITDs grown directly on InAs substrates, with peak current densities above 104 A/cm2 and peak-to-valley ratios near 11 for 15 A AlSb barriers. The results represent an important step toward the fabrication of high-speed, low-power logic circuits in this material system.

Nonvolatile reprogrammable logic elements using hybrid resonant tunneling diode–giant magnetoresistance circuits

We have combined resonant interband tunneling diodes (RITDs) with giant magnetoresistance (GMR) elements so that the GMR element controls the switching current and stable operating voltage points of the hybrid circuit. Parallel and series combinations demonstrate continuous or two-state tunability of the subsequent RITD-like current–voltage characteristic via the magnetic field response of the GMR element. Monostable–bistable transition logic element operation is demonstrated with a GMR/RITD circuit in both the dc limit and clocked operation. The output of such hybrid circuits is nonvolatile, reprogrammable, and multivalued.

Narrow band gap InGaSb, InAlAsSb alloys for electronic devices

Solid source molecular beam epitaxy has been used to grow random alloy quaternary InAlAsSb and ternary InGaSb alloys with a 6.2A lattice constant for use in electronic devices such as p-n junctions and heterojunction bipolar transistors (HBTs). Several p-n hetrojunctions composed of p-type InGaSb and one of several different n-type InAlAsSb alloys have been fabricated and show good rectification with ideality factors near one. In addition, several of these alloys have been used to make an n-p-n HBT that has demonstrated a dc current gain of 25.