W. I. Wang

Resonant tunneling in AlSb/InAs/AlSb double-barrier heterostructures

We report the first observations of resonant tunneling in the AlSb/InAs material system, with a maximum peak‐to‐valley current ratio of 1.8:1 at room temperature and 9:1 at 77 K. The large AlSb/InAs barrier height of 1.8 eV for electrons and high‐mobility InAs will be advantageous in device applications. In particular, the small electron effective mass in InAs makes it possible to demonstrate quantum effects in a 24 nm well, the longest coherence distance reported for double‐barrier tunneling structures. We estimate that an AlSb/InAs resonant tunneling transistor can significantly outperform similar devices based on AlGaAs/GaAs.

Heterojunction field‐effect transistors based on AlGaSb/InAs

We have fabricated the first InAs‐channel field‐effect transistor, which shows a transconductance of 180 mS/mm at 1 V drain‐source bias (77 K). An improved buffer layer could significantly improve the device performance. In addition, we propose a new broken‐gap heterojunction field‐effect transistor based on these materials that could provide an order of magnitude higher transconductance compared to existing device configurations based on AlGaAs/GaAs.

High electron mobility AlGaN/GaN heterostructures grown on sapphire substrates by molecular-beam epitaxy

High-quality AlGaN/GaN heterostructures have been grown by ammonia gas-source molecular-beam epitaxy on sapphire substrates. Incorporation of a low-temperature-grown AlN interlayer during the growth of a thick GaN buffer is shown to substantially increase the mobility of the piezoelectrically induced two-dimensional electron gas (2DEG) in unintentionally doped AlGaN/GaN heterostructures. For an optimized AlN interlayer thickness of 30 nm, electron mobilities as high as 1500 cm2/V s at room temperature, 10 310 cm2/V s at 77 K, and 12 000 cm2/V s at 0.3 K were obtained with sheet densities of 9×1012 cm−2 and 6×1012 cm−2 at room temperature and 77 K, respectively. The 2DEG was confirmed by strong and well-resolved Shubnikov–de Haas oscillations starting at 3.0 T. Photoluminescence measurements and atomic force microscopy revealed that the densities of native donors and grain boundaries were effectively reduced in the AlGaN/GaN heterostructures incorporating low-temperature-grown AlN interlayers.

Resonant interband coupling in single-barrier heterostructures of InAs/GaSb/InAs and GaSb/InAs/GaSb

A new mechanism for negative differential resistance due to electron/light hole coupling has been observed using broken gap heterostructures of InAs/GaSb/InAs and GaSb/InAs/GaSb. The best peak‐to‐valley ratio is about 2:1 (3.7:1 at 77 K) for a GaSb layer width of 10 nm. The peak current density of 4.2 kA cm−2 and the peak voltage of 300 mV are consistent with the interpretation of these experiments as interband coupling between the InAs conduction band and the GaSb valence (light hole) band.

Negative differential resistance in AlGaSb/InAs single‐barrier heterostructures at room temperature

We have observed for the first time negative differential resistance at room temperature in a single‐barrier tunneling heterostructure. A typical InAs/AlGaSb/InAs structure exhibits a current peak of 2.1×103 A cm−2 at 0.28 V and a peak to valley ratio of 1.6:1. We attribute the observation of room‐temperature negative differential resistance to the favorable band alignment in the AlGaSb/InAs material system, which appears promising for device applications of single‐barrier tunneling.

High-breakdown-voltage AlSbAs/InAs n-channel field-effect transistors

InAs channel field-effect transistors of 1- mu m gate length were grown by molecular beam epitaxy and observed to operate at channel electric fields (20 kV/cm) higher than previously demonstrated and several times greater than the threshold for impact ionization in bulk InAs. Voltage gains on the order of 10 were observed with transconductances as high as 414 mS/mm and output conductances as low as 33 mS/mm. These voltage gains are comparable to those of GaAs-based devices and are the highest observed for InAs channel devices. The results demonstrate the potential for practical room-temperature operation of InAs FETs.<<ETX>>

Resonant tunneling of holes in AlSb/GaSb/AlSb double‐barrier heterostructures

We have made the first observations of resonant tunneling in the AlSb/GaSb material system. Double‐barrier p‐type heterostructures exhibit two distinct features in their current‐voltage characteristics, indicating resonant tunneling via confined valence‐band states. The measured energy level positions are consistent with a substantial valence‐band offset of approximately 0.4 eV.

Molecular‐beam epitaxial growth of metastable Ge1−xSnx alloys

Substrate‐stabilized, metastable, single‐crystal Ge1−xSnx films can be grown by molecular‐beam epitaxy (MBE). We have grown for the first time single crystal Ge1−xSnx alloys on lattice matched GaSb (with x=0.5) and InP (with x=0.26) substrates up to a thickness of 0.3 μm. Reflection high‐energy electron diffraction (RHEED) observations and x‐ray measurements show that even at very small lattice mismatch (less than 0.05%), single crystal Ge1−xSnx films cannot be grown thicker than 0.3 μm. Our x‐ray results suggest that the critical thickness of α‐Sn and Ge1−xSnx single crystal films is mainly determined by a phase transition mechanism, and the dislocation generation equivalent critical thickness is an overestimate. Under practical MBE growth conditions, it is very difficult to grow thick films, due to the sensitivity of the critical thickness to composition fluctuations. We have shown that even under an exact lattice match between substrate and film, the critical film thickness is limited.

Effects of carrier mass differences on the current‐voltage characteristics of resonant tunneling structures

We show that the current‐voltage characteristics of resonant tunneling structures are drastically influenced by the difference of electronic effective masses between the electrodes and the quantum well. In particular, if the mass in the well is larger than that in the emitter, the current peak is shifted to lower voltages, relative to the more conventional case of equal masses. This situation is illustrated experimentally with GaAs/AlAs/GaAs heterostructures, in which Γ electrons emitted from GaAs tunnel resonantly through the AlAs X point, where the mass is considerably heavier.

Resonant tunneling through X‐valley states in GaAs/AlAs/GaAs single‐barrier heterostructures

Clear negative differential resistance has been observed in a GaAs/AlAs/GaAs single‐barrier heterostructure due to the presence of a quasi‐bound state associated with the X‐point profile. This surprising result is due to the fact that although the Γ‐point profile of this heterostructure is a simple single tunneling barrier, the X‐point profile actually constitutes a quantum well some 0.3 eV deep lying about 0.2 eV above the Γ point of GaAs. The experimental evidence is a sharp cutoff in conductance at about 0.36 V bias, characteristic of tunneling via a confined state.