AlSb Barriers Research Articles

High radiation tolerance of InAs∕AlSb high-electron-mobility transistors

In As ∕ Al Sb -based high-electron-mobility transistors (HEMTs) were irradiated with 2MeV protons. Radiation damage caused the source-drain current Ids to decrease nearly linearly with fluence Φ at a rate of Δ[Ids(Φ)∕Ids(0)]∕ΔΦ≈7×10−16cm2. Radiation-induced decreases in Ids have been observed for other HEMT material systems, and have been attributed to high-efficiency defect-induced scattering of carriers out of the two-dimensional electron gas. However, in the InAs∕AlSb system the rate of decrease of Ids is about 140 times less than that for typical GaAs∕AlGaAs HEMTs. An explanation is presented in which the high radiation tolerance of InAs∕AlSb HEMTs is related to carrier reinjection and the unusually large energy offset between the AlSb barriers and the InAs quantum well.

Andreev scattering, Zener tunneling, and anomalous ac Josephson effect in near-ballistic quasi-two-dimensional weak links

The competition and cooperation of Andreev scattering and normal scattering in superconducting heterojunctions lead to the formation of Andreev-Josephson bands (AJB), whose energies vary periodically with the phase difference between the pair potentials. We compute these bands for a quasi-two-dimensional electron gas in an InAs channel between an AlSb substrate and superconducting Nb stripes separated by an AlSb barrier. In such systems ac Josephson currents with frequency $2{\ensuremath{\omega}}_{J}$ have been observed, where ${\ensuremath{\omega}}_{J}=2eV∕\ensuremath{\hbar}$ is the canonical Josephson frequency at voltage $V$. In analogy with semiconductor physics Kroemer has reasoned that Zener tunneling between the AJB should be responsible for the extra oscillations of the (anomalous) ac Josephson currents. From the time-dependent Bogoliubov-de Gennes equations we calculate the ac Josephson current density ${\stackrel{P\vec}{j}}_{J}$ and, perturbation theoretically, the Zener-tunneling current density ${\stackrel{P\vec}{j}}_{ZT}$. The total current density $\stackrel{P\vec}{j}={\stackrel{P\vec}{j}}_{J}+{\stackrel{P\vec}{j}}_{ZT}$ contains a strong component that oscillates with $2{\ensuremath{\omega}}_{J}$.

Large negative persistent photoconductivity in InAs∕AlSb quantum wells

Negative persistent photoconductivity in backgated InAs∕AlSb quantum wells under red and blue light emitting diode illumination is observed to depend on both illumination energy and intensity. During these experiments the electron sheet density is varied by one order of magnitude from 5.6×1011 to 5.8×1010cm−2. This behavior is attributed to the role of optically excited holes in the AlSb barrier layers near the InAs quantum well. Furthermore, a long relaxation time in the electron sheet density is observed and attributed to the slow movement of electrons from the InAs quantum well to ionized deep levels in the AlSb barriers.

Near-infrared intersubband transitions in delta-doped InAs∕AlSb multi-quantum wells

Intersubband transitions (ISBTs) in narrow InAs∕AlSb multiple quantum wells (MQWs) were investigated for well widths, d, ranging from 5 nm down to 1.8 nm with 10, 20, or 60 periods. In order to observe a strong ISBT signal, a heavy silicon doping was made in each InAs quantum well. Delta doping was employed for the narrowest wells to prevent silicon incorporation into the AlSb barrier layers. As the well width decreased, the ISBT signal of the MQWs decreased. However, it persisted down to d=2.1nm with a sheet doping density in each quantum well of 9×1012cm−2 and 60 periods. The ISBT signal observed for d=2.1nm was peaked at an energy of 650 and 670 meV at 300 and 77 K, respectively. These are the highest energy values ever observed for ISBTs in InAs∕AlSb MQWs.

A self-consistent investigation of the semimetal–semiconductor transition in InAs/GaSbquantum wells under external electric fields

We investigate the phase transitions in InAs/GaSb quantum wells sandwiched between twowide-gap AlSb barrier layers under an external electric field perpendicular to interfaces.The Schrödinger and the Poisson equations are solved self-consistently to derive the subbanddispersions, the potential profile, the electron charge distribution in the InAs layer,and the hole charge distribution in the GaSb layer. The Burt–Foreman envelopefunction theory and the scattering matrix method are used to solve the Schrödingerequation in the framework of the eight-band model, including the spin-splitting of subbands in our calculation. We have found that ina thick InAs/GaSb quantum well, which has been investigated experimentallyby Cooper et al (1998 Phys. Rev. B 57 11915), under low external electric fields,two electron levels stay below the highest hole level at zero in-plane wavevectork‖ = 0. Then, the anticrossings of electron and hole levels produce several minigaps in thein-plane dispersions, inside which the states of other subbands exist. As a result, thesystem is in a semimetal phase. With increasing external electric field, the semimetal phasechanges to semiconductor phase with only one hybridization gap. When all electronlevels become higher than the hole levels at higher electric fields, the system has asemiconducting gap.

Landau level structures and semimetal-semiconductor transition in strained InAs/GaSb quantum wells

Using Burt's envelope function theory and the scattering matrix method, we investigate the hybridized electron-hole Landau levels in strained InAs/GaSb quantum wells sandwiched between wide-gap AlSb barrier layers under electric and a quantizing magnetic fields applied perpendicular to interfaces. At zero magnetic field, in the structures studied here, the lowest electron level in the InAs layer lies below the highest heavy-hole level in the GaSb layer. With increasing magnetic field, the electron levels move up and the heavy-hole levels move down, producing anticrossings and gaps in the Landau level structures. We have found that the Landau level structures depend strongly on the lattice-mismatched strain and the applied voltage. As a result, in the region before anticrossings, the g factor of the lowest electron Landau level has a larger value for the quantum well structure grown on GaSb than that for the structure grown on InAs, while in the region after anticrossings the situation reverses for the g factor. Under low magnetic field, the difference between the electron g factors for the structures grown on different substrates is found to be as large as 10 for zero bias and decreases significantly with increasing bias. When all electron levels become higher than hole levels at high magnetic fields, the semimetal-semiconductor transition occurs. The critical magnetic field ${B}_{c}$ for the phase transition in structures grown on InAs is found to be lower than that in structures grown on GaSb. It is also obtained that a positive voltage biased across the InAs/GaSb well essentially decreases ${B}_{c}.$ Therefore, for a fixed magnetic field, the semimetal-semiconductor transition can be controlled by a bias.

InAs/InAsP composite channels for antimonide-based field-effect transistors

We report the growth and transport characteristics of stepped InAs/InAs1−xPx quantum wells with AlSb barriers. Electron mobilities and carrier concentrations in these composite stepped quantum wells were studied as a function of growth temperature and phosphorus content. For InAs1−xPx grown at 430 °C substrate temperature (nominal x=0.2), a high 22 500 cm2/V s electron mobility was observed, while 7100 cm2/V s mobility was observed in a single strained InAs1−xPx quantum well layer. Heterostructure field-effect transistors fabricated using the composite quantum wells exhibited increased breakdown voltage and a 14:1 reduction in source-drain dc conduction when compared to a similar InAs-channel device.

Transport properties in asymmetric InAs/AlSb/GaSb electron-hole hybridized systems

Transport properties in asymmetric InAs/GaSb and InAs/AlSb/GaSb heterostructures sandwiched by AlGaSb layers were studied. For the InAs/GaSb structures, partially compensated quantum Hall effects arising from the electron-hole hybridization were observed. By changing the thickness of each layer, the energy positions of the conduction band and the valence band can be controlled independently. In InAs/AlSb/GaSb structures, we tried to control the hybridization strength by varying the AlSb barrier thickness. Magnetoresistance measurements, in which the magnetic field was applied perpendicular and parallel to the surface, and cyclotron resonance (CR) measurements show that there is a clear transition from the electron-hole-hybridized system to the electron-hole-independent system. The transition point appeared at around the 2-nm-barrier thickness in all experiments, suggesting a dominant role of the overlap of electron and hole wave functions. According to the absorption peak shapes in the CR measurements, the hybridized system can be further classified as strongly hybridized or weakly hybridized. Only in the weakly hybridized system was the Shubnikov--de Haas--like oscillation due to the short-range scattering by the hole potential observed in the electron CR absorption peak.

A novel InAs quantum wire system

We report here a new technique in patterning high quality low-dimensional electrons in single InAs quantum wells. Grown by molecular beam epitaxy, the single InAs quantum well is sandwiched between AlSb barriers and capped by a thin layer of InAs. When the InAs cap layer is patterned by electron beam lithography and selectively removed, electrons are induced in the InAs quantum well below due to the different surface Fermi level pinning voltages on the exposed AlSb layer from the InAs cap. One-dimensional quantum wires can thus be conveniently defined by lithography and nm-shallow etching. We demonstrate that these one-dimensional electrons posses a long elastic mean free path (>1.4 μm) and a long coherence length (>3 μm) at 2 K .

Reduction of the unintentional background electron density in AlSb/InAs/AlSb quantum wells

InAs quantum wells clad by AlSb barriers contain an unintentional electron density of approximately 1×10 12 cm −2. This report concerns the compensation of these electrons by p-type modulation doping with Be. The electron concentration in the InAs quantum well decreases linearly with increasing Be doping as expected from lever rule arguments. Hall measurements show that the electron mobility increases with increasing electron concentration at temperatures between 10 and 300 K. These findings need to be considered in the design of field-effect transistors based on this material system.

Intersubband absorption in n-doped InAs/AlSb multiple-quantum-well structures

Well-thickness dependence of intersubband absorption energies is investigated in n-doped InAs/AlSb multiple-quantum-well structures grown by molecular-beam epitaxy. Decreasing the InAs well thickness from 18 to 9 monolayers, the absorption peak shifts from 3.31 μm (375 meV) to 1.98 μm (627 meV). These absorption energies are found to be larger than those of the corresponding spatially indirect band gap between the electron ground state in the InAs well and the heavy-hole ground state in the AlSb barrier. Intersubband transition energies are calculated self-consistently using multiband k⋅p calculation combined with Poisson equation, and compared with the experimental results.

Characterization of one-dimensional quantum channels in InAs/AlSb

We report the magnetoresistance characteristics of one-dimensional electrons confined in a single InAs quantum well sandwiched between AlSb barriers. As a result of a novel nanofabrication scheme that utilizes a 3nm-shallow wet chemical etching to define the electrostatic lateral confinement, the system is found to possess three important properties: specular boundary scattering, a strong lateral confinement potential, and a conducting channel width that is approximately the lithography width. Ballistic transport phenomena, including the quenching of the Hall resistance, the last Hall plateau, and a strong negative bend resistance, are observed at 4K in cross junctions with sharp corners. In a ring geometry, we have observed Aharonov-Bohm interference that exhibits characteristics different from those of the GaAs counterpart due to the ballistic nature of electron transport and the narrowness of the conducting channel width.

Energy Relaxation Time and Microwave Noise in InAs/AlSb/GaSb/GaAs Heterostructures

Experimental technique based on microwave noise measurements is used to evaluate energy dissipation by hot electrons in a type II heterostructure consisting of InAs and GaSb with a thin AlSb barrier layer between them and containing a two-dimensional electron gas (2DEG). The energy relaxation time is estimated at 80 K and 300 K lattice temperatures. The energy relaxation time is almost independent of electric field and lattice temperature.

Barrier roughness effects in resonant interband tunnel diodes

Peak current densities of InAs/AlSb/GaSb/AlSb/InAs resonant interband tunneling diodes (RITD) grown by molecular beam epitaxy have been measured as a function of the growth temperature. The growth procedures were designed to produce nominally identical AlSb tunneling barriers. The variations observed in the peak current for positive bias are consistent with the barrier on the substrate side of the RITD becoming effectively thicker for diodes grown at high temperatures. Plan-view in situ scanning tunneling microscopy (STM) measurements indicate that smoother AlSb barriers are grown at high temperature. The growth temperature dependence of the peak current density and STM results are consistent, because tunneling is highly dependent on barrier thickness. While the high and low temperature growths were designed to have the same barrier thickness, the large current flowing through the thin areas of a rough barrier result in an effectively thinner barrier compared to the smooth one.

Fabrication of monolithically-integrated InAlAs/InGaAs/InP HEMTs and InAs/AlSb/GaSb resonant interband tunneling diodes

The integration of InAs/AlSb/GaSb resonant interband tunneling diodes (RITDs) with InAlAs/InGaAs/InP high electron mobility transistors (HEMTs) is reported. The integrated devices exhibit nearly identical performance to discrete control devices from DC through microwave frequencies. RITDs with peak current densities of 24.5 kA/cm/sup 2/ and peak voltages of 0.12 V have been demonstrated for devices with 1.2-nm thick AlSb barriers. HEMTs with 0.2-/spl mu/m gates have been fabricated, and f/sub t/s of 127 GHz and f/sub max/ of 183 GHz have been obtained. To the authors' knowledge, this is the first report of the monolithic integration of RITDs with HEMTs on InP.

Resonant interband tunnel diodes with AlGaSb barriers

The peak current density of InAs/AlSb/GaSb/AlSb/InAs resonant interband tunneling diodes has been enhanced by replacing the AlSb barriers with Al1−xGaxSb that has a narrower band gap. The devices were grown by molecular beam epitaxy and tested at room temperature. Diodes with nominally identical 7-ML-thick ternary alloy barriers with x=0.35 are found to have peak current densities three times larger than those with AlSb barriers. The peak-to-valley current ratio decreases by only one third from 18 for the AlSb diodes to 12 for diodes with the ternary alloy barriers.

Proton irradiation of InAs/AlSb/GaSb resonant interband tunneling diodes

Room temperature current–voltage measurements have been made on InAs/AlSb/GaSb resonant interband tunnel diodes irradiated with 2 MeV protons to determine the effect of displacement damage on the negative resistance peak current Ip and the peak-to-valley current ratio P/V. Diodes with 5 and 13 ML AlSb barrier thickness were irradiated and measured several times until the total fluences reached 1×1015 and 2×1014 H+/cm2, respectively. The current due to radiation-induced defects has a nonlinear voltage dependence, with a large increase occurring in the voltage range between the negative resistance peak and the valley. Ip increased <50% while a large increase in the valley current decreased the P/V ratios to about 2.

Comparative investigation of interface effect on transmission function of electrons and holes in InAs/AlSb-based heterostructures

The influence of interface types in InAs/AlSb based heterostructures are considered for both electrons and holes. First calculations based on the transfer matrix method and including AlAs, InSb interface types or some of their combinations show the existence of more quantized energy hole levels than can be expected in the case when no interface effects are considered. The consideration is simplified by assuming the interfaces as individual intermediate layers formed between the InAs–quantum well and AlSb barriers. While for electrons such an intermediate layer of AlAs modifies the designed barriers or quantum wells of the heterostructure, it creates narrow barriers for holes leading to the appearance of a miniband type of energy levels.

Tunnel switch diode based on AlSb/GaSb heterojunctions

We report on tunnel switch diodes based on AlSb barriers and GaSb p–n junctions grown by molecular beam epitaxy. These were the devices with thyristor like switching in the GaSb/AlSb system. The characteristic “S” shaped current–voltage curve was found to occur for structures with AlSb barriers less than 300 Å thick. The switching voltage and current density exhibited less sensitivity to barrier and epilayer thickness than was predicted by the punch-through model. The results were correlated with drift diffusion simulations which have been modified to account for the presence of a tunneling contact.

Crystallographic Features and Subband Transitions of Al/InAs Metal-Semiconductor Junctions

Abstract To study the nature of metal-semiconductor junction, metal-clad InAs quantum wells with aluminum deposited on 15-nm thick InAs grown on (100) GaAs with AlSb barrier were prepared by an in-situ method (aluminum was grown immediately by molecular beam epitaxy) and by an ex-situ method (aluminum was deposited after exposing InAs surface in ambient condition by selective etching). Cross sectional TEM studies indicated that the Al grains in the clad aluminum layer were randomly arranged in the ex-situ grown junctions, while, in the in-situ samples, the aluminum grains were found to be well oriented to the underlying InAs. When the interface is viewed along [Oil], three observed crystallographic relationships for in-situ Al growth are 1. [011]AI // [011]InAs and (100)AI // (100)InAs as shown in Fig. 1(a): the aluminum grains of facecentered cubic (fee) structure are perfectly aligned with the fee InAs lattice when they grow;