AlSb Barriers Research Articles

The effect of Landau quantization on cyclotron resonance in a non-parabolic quantum wells

Electron cyclotron resonance is studied in very deep quantum wells consisting of InAs between AlSb barriers. High electron mobility and strong conduction band non-parabolicity together with the small effective mass and the large effective g factor of this material enables the authors to observe simultaneous cyclotron transitions between adjacent sets of spin-split Landau states. Their experiments resolve spin-conserving transitions involving two or three different Landau levels depending on the filling factor. The results are compatible with a single-particle model.

Surface donor contribution to electron sheet concentrations in not-intentionally doped InAs-AlSb quantum wells

The electron concentration in not-intentionally doped InAs/AlSb quantum wells is found to depend sensitively on the top AlSb barrier thickness even for barriers as thick as 100 nm. The carrier concentration increases as the thickness of this barrier is decreased. The analysis of the dependence of concentration on top barrier thickness indicates that the Fermi level is pinned at the surface of the sample, 850±50 meV below the conduction band edge of the AlSb top layer. Surface donors are the main contribution to the high carrier concentrations in these not-intentionally doped wells.

Effects of barrier thicknesses on the electron concentration in not-intentionally doped InAs–AlSb quantum wells

We present the results of a study of the dependence of electron concentration in molecular-beam-epitaxy-grown not-intentionally doped InAs–AlSb quantum-well structures on the top barrier thickness in single-well structures and on the internal barrier thickness in multiwell structures. A contribution from surface donors was found to be dominant in single wells with thin top barriers. The bulk donor concentration in the AlSb barriers varied from 2 × 1015 to 1016 cm−3. Interface donors at the InAs–AlSb heterointerface were also found, with an energetic location below the Fermi level in the wells. They are primarily responsible for the increase of the carrier concentration with increasing temperature.

High-transconductance InAs/AlSb heterojunction field-effect transistors with delta -doped AlSb upper barriers

The authors report the fabrication and temperature-dependent characterization of InAs/AlSb quantum-well heterojunction field-effect transistors (HFETs). Devices with electron sheet concentrations of 3.8*10/sup 12/ cm/sup -2/ and low-field electron mobilities of 21000 cm/sup 2//V-s have been realized through the use of Te delta -doping sheets in the upper AlSb barrier. One device with a 2.0- mu m gate length showed a peak extrinsic transconductance of 473 mS/mm at room temperature. Gate leakage current, operating current density, and extrinsic transconductance were found to decrease with decreasing temperature. >

The negative differential resistance characteristics of double-barrier interband tunneling structures

The tunneling mechanism responsible for negative differential resistance in GaSb/AlSb/GaSb/AlSb/InAs double-barrier interband tunneling structures was studied systematically. It was found that the peak current results from resonant interband tunneling and that the spacers make a significant contribution to the valley current. Furthermore, the optimal thicknesses for the GaSb well and AlSb barriers were predicted to be 65 and 10 Å, respectively, which agrees fairly well with experimental results. Our theoretical results give some useful design principles for this type of interband tunneling device.

Studies of the tunneling currents in the InAs/AlSb/GaSb single-barrier interband tunneling diodes grown on GaAs substrates

This paper studies the tunneling currents in the single-barrier InAs/AlSb/GaSb interband tunneling structures, with an emphasis on correlating with the bandedge alignment. The WKB approximation combined with the k⋯p two-band model are used to analyze the interband tunneling currents versus the barrier widths to obtain the energy level in the AlSb barrier through which the peak tunneling currents occur. The obtained energy level (0.48 ± 0.1 eV above the valence band maximum of the AlSb) agrees with the valence-band offset between AlSb and GaSb. A good agreement of the measured peak voltages and the C − V data with the predicted values by the theory using a self-consistent Schrödinger-Poisson solver is obtained, with a value of 0.15–0.2 eV for the InAs/GaSb band overlap.

Interband tunneling in InAs/GaSb/AlSb heterostructures

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 4.2×10 4 to 8.0×10 4 A/cm 2 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 1.6×10 5 A/cm 2 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.

Growth and characterization of high current density, high-speed InAs/AlSb resonant tunneling diodes

High quality resonant tunneling diodes have been fabricated from the InAs/AlSb material system (InAs quantum well and cladding layers, AlSb barriers) on (100)GaAs substrates. A diode with a 6.4-nm-thick InAs quantum well and 1.5-nm-thick AlSb barriers yielded a room-temperature peak current density of 3.7×105 A cm−2 and peak-to-valley current ratio of 3.2. This corresponds to an available current density of 2.6×105 A cm−2, which is comparable to that of the best In0.53Ga0.47As/AlAs diodes grown on lattice-matched substrates and is three times higher than that of the best GaAs/AlAs diode reported to date. These results were obtained in spite of a 7.2% lattice mismatch between the InAs epilayers and the GaAs substrates, which leads to a measured threading dislocation density of roughly 109 cm−2. The experimental peak voltage and current density are in good agreement with theoretical calculations based on a stationary-state transport model with a two-band envelope function approximation.

Investigation of the influence of the well and the barrier thicknesses in GaSb/AlSb/GaSb/AlSb/InAs double-barrier interband tunneling structures

The tunneling currents of GaSb/AlSb/GaSb/AlSb/InAs double-barrier interband tunneling (DBIT) structures were studied experimentally by varying the thickness of the well and the barrier layers systematically. The optimal thicknesses for the GaSb well and the AlSb barriers were found to be 6.5 and 1.0 nm, respectively, to obtain a high peak current density (19 kA/cm/sup 2/), with a large peak-to-valley ratio of 4. The high peak current in the DBIT structure shows the strong effect of the resonant coherence of the wave function across the double barrier. For the case of a small GaSb well width (3 nm), a drastic reduction of the peak current was observed, an effect suggesting that the electron-wave function in the InAs couples primarily to the quantized light hole state in the GaSb well.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the effect of the barrier widths in the InAs/AlSb/GaSb single-barrier interband tunneling structures

The dependence of the interband tunneling current on AlSb barrier widths is studied in the InAs/AlSb/GaSb single-barrier diode structures. The experimental results show that the peak current density displays an exponential dependence on the barrier width. The Wentzel-Kramers-Brillouin approximation combined with the k⋅p two-band model were used in analyzing the energy level in the AlSb barrier through which the peak tunneling currents occur. The energy level thus obtained (0.48±0.10 eV above the valence band edge of the AlSb) agrees with the valence-band offset (0.40±0.15 eV) between the AlSb and the GaSb obtained by x-ray photoemission measurement reported by Gualtieri et al. [Appl. Phys. Lett. 49, 1037 (1986)]. By adjusting the barrier width properly, we obtained a high peak current density of 24 kA/cm2 (with a peak-to-valley ratio of 1.4) and a high peak-to-valley ratio of 4.5 (with a peak current density of 3.5 kA/cm2) at room temperature. In addition, the peak-current voltages for different AlSb barrier widths were calculated and compared with the measured results.

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.

InAs/AlSb/GaSb single-barrier interband tunneling diodes with high peak-to-valley ratios at room temperature

We have fabricated an InAs/AlSb/GaSb single-barrier interband tunneling diode by molecular beam epitaxy. In this structure, a large tunneling current can be obtained by taking the advantage of the large heterojunction-conduction band to valence band overlap (0.15 eV) between InAs and GaSb which offers flexible designs of the AlSb barrier thickness and the doping concentrations. We have obtained a negative differential resistance with a peak-to-valley current ratio as high as 4.7 and a peak current density of 3.5 kA/cm2 at room temperature with a 1.5-nm-thick AlSb barrier. The current transport mechanism in this tunneling structure will be discussed according to the I-V characteristics as a function of temperature.

InAs-AlSb heterostructure field-effect transistors fabricated using argon implantation for device isolation

We have fabricated planar heterostructure field-effect transistors with InAs quantum well channels and AlSb barrier layers. Argon implantation was used to form a damaged layer, which resulted in partial device to device isolation. A 1.5 μm gate device had a room-temperature extrinsic transconductance of 208 mS/mm.

Interband tunneling in single-barrier InAs/AlSb/GaSb heterostructures

Negative differential resistance due to interband tunneling has been observed at room temperatures for the first time in polytype heterostructures of InAs/AlSb/GaSb. The peak-to-valley ratio is about 1.7:1 (5.5:1 at 77 K) for an AlSb barrier width of 2.5 nm. The peak current density is studied as a function of barrier width and compared to calculations based on the two-band model.

Electron concentrations and mobilities in AlSb/InAs/AlSb quantum wells

We present data on the electron concentrations and mobilities in deep (≊1.3 eV) AlSb/InAs/AlSb quantum wells grown by molecular-beam epitaxy. High electron sheet concentrations of the order 1012 cm−2, found in the not-intentionally doped wells, indicate the presence of a deep donor in the AlSb barriers. Typical mobilities are between 22 000 and 28 000 cm2/V s at room temperature, increasing with decreasing temperature, and leveling out below 50 K at values between 175 000 and 330 000 cm2/V s. The temperature-independent low-temperature mobilities indicate a nonthermal scattering mechanism, possibly interface roughness scattering. Under illumination the wells exhibit a strong negative photoconductivity, which is explained as a natural consequence of the band structure of the wells.

Growth and transport properties of (Ga,Al)Sb barriers on InAs

We have investigated the transport properties of (Ga,Al)Sb–InAs heterostructures grown by molecular-beam epitaxy with the Al composition in the range of 0.3–0.5 where the alloy serves as a potential barrier for electrons in InAs. Field effect capacitors using a thick (Ga,Al)Sb barrier demonstrate the modulation of carrier densities in InAs by the application of an electric field. In the region of thin (Ga,Al)Sb, where tunneling becomes dominant, a novel negative resistance appears in InAs/(Ga,Al)Sb/InAs single-barrier structures due to the specific band alignment of this system. The results have been further elucidated under a magnetic field.

Electron transport of (Al,Ga)Sb/InAs heterojunctions prepared by molecular beam epitaxy

Molecular beam epitaxial growth of (Al,Ga)Sb/InAs hetero-structures is described. Electron transport studies indicate that these heterojunctions are of high quality. Shubnikov-de Haas measurement of the AlSb/InAs/GaSb quantum well shows a two-dimensional electron gas of concentration 2×1012cm2 and a low-temperature mobility approaching 105cm2/Vs. Low-temperature capacitance/voltage measurement indicates that the thin AlSb barrier is a classical Mott-type barrier.

GaSb/AlSb multiquantum well structures: Molecular beam epitaxial growth and narrow-well photoluminescence

Multiquantum well structures of GaSb wells down to 12-Å width, separated by 120-Å AlSb barriers, were grown by molecular beam epitaxy, and their low-temperature photoluminescence was studied. Even though GaSb should become an indirect-gap semiconductor for well widths below about 90 Å, the direct-gap luminescence persists to the narrowest wells. The shift of photon energy with well width indicates strong nonparabolicity effects; it is in good agreement with Bastard’s simple model of quantum wells in nonparabolic semiconductors. For wells less than 45 Å wide, the direct-gap luminescence is accompanied by a strong and very broad lower-energy luminescence peak, which appears to be related to electron accumulation in the X valleys, but the exact mechanism for which is not clear. A two-electron transition model is proposed.