J. H. English

Effects of interface layer sequencing on the transport properties of InAs/AlSb quantum wells: Evidence for antisite donors at the InAs/AlSb interface

Data are presented on the role of the InAs/AlSb interface in determining the electron transport in AlSb/InAs/AlSb quantum wells grown by molecular‐beam epitaxy. Because both anion and cation change across an InAs/AlSb interface, it is possible to grow such wells with two different types of interfaces, one with an InSb‐like bond configuration, the other AlAs‐like. Electron mobility and concentration were found to depend very strongly on the manner in which the quantum well’s interfaces were grown, indicating that high mobilities are seen only if the bottom interface is InSb‐like. An As‐on‐Al sites antisite defect model is postulated for bottom AlAs‐like interfaces. Such antisites were used in subsequent samples as donors in modulation‐doped high‐mobility InAs/AlSb quantum wells.

Molecular‐beam epitaxy growth of tilted GaAs/AlAs superlattices by deposition of fractional monolayers on vicinal (001) substrates

We report the successful growth of GaAs/AlAs superlattices having interface planes tilted with respect to the substrate surface plane. The amount of tilt and the superlattice period may be controlled by adjusting the growth parameters. The tilted superlattices (TSL’s) were produced by depositing fractional monolayer superlattices (GaAs)m(AlAs)n, with p=m+n≂1, on vicinal (001) substrates. We demonstrate the growth of quantum wirelike structures produced by placing short sections of TSL between horizontal layers of AlAs. Variations of the TSL period and tilt, both on uniform surfaces and on surfaces containing defects, yield insight to the growth kinetics and to the influence of variations in the growth parameters during molecular‐beam epitaxy growth.

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.

Interface roughness scattering in InAs/AlSb quantum wells

We present a study of interface roughness scattering in not‐intentionally‐doped AlSb/InAs/AlSb quantum wells grown by molecular beam epitaxy on [001] GaAs semi‐insulating substrates. The low‐temperature mobility is found to be limited by interface roughness scattering for well widths below 100 A. The measured mobilities are well accounted for by Gold’s theoretical treatment [A. Gold, Phys. Rev. B 35, 723 (1987)], once it is suitably modified to account for the band nonparabolicity of InAs. The experimental electron density dependence of the mobility indicates a lateral correlation length for the interface roughness Λ≊62 A for interface fluctuations approximately 1 monolayer high. We believe this roughness scale is characteristic of the bottom (InAs‐on‐AlSb) interface.

Pulsed laser deposition of oriented In2O3 on (001) InAs, MgO, and yttria‐stabilized zirconia

Oriented In2O3 films have been grown on (001) InAs, MgO, and YSZ substrates using pulsed laser deposition. The films in each case displayed a cube‐on‐cube orientation relation with respect to the substrate, as determined by in situ RHEED analysis and x‐ray θ–2θ measurements. X‐ray rocking curve full width at half‐maximum values as low as 1.3°, 1.5°, and 0.29° have been obtained for In2O3 layers on InAs, MgO, and YSZ, respectively. An oriented native surface oxide layer was employed to provide an appropriate epitaxial template for aligned growth of In2O3 on InAs substrates, while growths were carried out directly on MgO and YSZ. The not intentionally doped films displayed resistivities on the order of 10−4 Ω cm, with Hall mobilities of 50 cm2/V s measured for In2O3 deposited on YSZ substrates.

Electrical properties and band offsets of InAs/AlSb n-N isotype heterojunctions grown on GaAs

The MBE growth and selected properties of InAs/AlSb n‐N isotype heterojunctions on n+‐GaAs substrates are described. Because of a large conduction‐band offset (1.35 eV), these junctions behave like Schottky barriers, with excellent rectification characteristics, despite the presence of a very high density (>107 cm−2) of threading dislocations resulting from the large lattice mismatch (7%) between AlSb and the GaAs substrate. The forward I‐V characteristics, corrected for series resistance, exhibit a large nonideality factor of about 1.8, suggesting that the main current flow is along a defect path, presumably related to the misfit dislocations. Reverse C‐V characteristics exhibit a perfectly linear 1/C2 vs V plot, from which a conduction‐band offset of 1.35±0.05 eV is deduced. This value is in excellent agreement with the value predicted from the known band offsets in InAs/GaSb and GaSb/AlSb.

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.

Electrically tunable Fabry-Perot mirror using multiple quantum well index modulation

We describe a novel surface normal Fabry–Perot multiple quantum well index reflection modulator which may be tuned electrically. The Fabry–Perot etalon, composed of two AlAs/Ga1−xAlxAs quarter‐wavelength grating mirrors separating a multiple quantum well GaAs/Ga1−xAlxAs active medium, is formed in a single growth by molecular beam epitaxy. Contrast ratios of up to 8:1 at 873 nm have been measured.

Growth of InAs-AlSb quantum wells having both high mobilities and high concentrations

Low-temperature mobilities in InAs-AlSb quantum wells depend sensitively on the buffer layer structures. Reflection high energy electron diffraction and x-ray diffraction show that the highest crystalline quality and best InAs transport properties are obtained by a buffer layer sequence GaAs → AlAs → AlSb → GaSb, with a final GaSb layer thickness of at least 1 μm. Using the improved buffer scheme, mobilities exceeding 600,000 cm2/Vs at 10 K are routinely obtained. Modulation δ-doping with tellurium has yielded electron sheet concentrations up to 8 × 1012 cm−2 while maintaining mobilities approaching 100,000 cm2/Vs at low temperatures.

Remotely-doped graded potential well structures

Abstract We present a new method to obtain high-mobility three-dimensional electron gas systems. We have achieved control of carrier density and of carrier profile by growth of the first remotely-doped parabolic potential well structures. Computer-controlled molecular beam epitaxy is used to grow a layer of ultra-fine superlattices with a programmable composition gradient. This produces conduction-band potentials which, in the absence of doping, are equivalent to the potential profiles of fixed charge distributions. When conduction electrons are introduced into these graded wells through remote doping of the barrier regions, they distribute themselves in such a way as to produce a uniform chemical potential at thermal equilibrium. We illustrate through computer simulations employing Fermi statistics that electrons introduced into a wide parabolic potential well distribute themselves uniformly. More significantly, the carrier distribution in the well is remarkably insensitive to the dopant sheet charge in the barrier, the more so at lower temperatures. We have fabricated remotely-doped graded potential well structures of the proposed type by molecular beam epitaxy. These structures exhibit the above effects. Measured mobilities of such three-dimensional electron gases grown using the GaAs/Al x Ga 1−x As system are higher than those of bulk-doped GaAs doped to give the same uniform electron concentration.