R L Williams

Observation and control of the amphoteric behaviour of Si-doped InSb grown on GaAs by MBE

The MBE growth and doping of heteroepitaxial layers of InSb on GaAs (100) are investigated. The layers are assessed by low-field Hall and magnetoresistivity measurements and high-field Shubnikov-de Haas studies together with infrared transmission, and TEM. The mechanism for silicon incorporation is investigated as a function of growth temperature. At low growth temperatures ( approximately=340 degrees C) silicon acts only as a donor and can produce electron concentrations up to 3*1018 cm-3 with 77 K mobilities identical to those found with bulk material. Although higher concentrations than 3*1018 cm-3 can be achieved; auto-compensation appears to occur in those samples. The 77 K mobilities achieved for less heavily doped samples (>40000 cm2 V-1 s-1 for n=1.2*1017 cm-3 for samples grown at 340 degrees C) are the highest low-temperature mobilities yet reported for n-type InSb films of approximately=1 mu m thickness grown on GaAs. However, higher growth temperatures ( approximately=420 degrees C) combined with constant silicon flux are found to simultaneously decrease electron concentration and mobility measured at 77 K although the structural quality as assessed by TEM remains unchanged. Analysis of the observed behaviour in terms of the Brooks-Herring model of ionised impurity scattering, modified for nonparabolicity, suggests that silicon is acting amphoterically with compensation ratios (NA/ND) reaching 0.5 at the higher temperatures. The effect of the interface between GaAs and InSb (lattice mismatch=14%) on the electrical properties is studied by introducing doping slabs of thickness approximately=1300 AA at various distances (d) between the interface (d=0 mu m) and the surface (d approximately=1.5 mu m) of the epilayer. A series of peaks not periodic in reciprocal field (1/B) are found at low fields with B parallel to the slabs and are interpreted as arising from the diamagnetic depopulation of the large number of subbands occupied as a result of the considerable thickness of the slabs. Be doping at 2*1019 cm-3 was demonstrated and, as with silicon, the bulk mobility corresponding to this hole concentration was achieved.

Subband dependent mobilities and carrier saturation mechanisms in thin Si doping layers in GaAs in the high density limit

Shubnikov-de Haas and persistent photoconductivity measurements have been performed as a function of hydrostatic pressure to study the saturation of the free electron concentration and the mobilities of the individual subbands at high doping densities in very thin sheets (2, 5, 10 nm) of silicon donors in MBE GaAs. The samples were grown at very low temperature (400 degrees C) in order to limit dopant diffusion, and silicon concentrations were close to the solubility limit at this temperature. As has been shown previously with spike-doped GaAs(Si), the relative occupancies and the mobilities of the lower subbands are very sensitive to the spreading of the dopant distribution. A routine was developed for the analysis of the Fourier transforms of the complex pattern of Shubnikov-de Haas peaks in order to provide quantitative values for the mobilities of the individual subbands. The results of this analysis are compared with values deduced from the magnetic field dependence of the resistivity and Hall effect. On applying hydrostatic pressures of the order of 15 kbar in the dark, a decrease of the free electron concentration of a factor of two was observed. This was accompanied by an increase in the mobility of all the subbands due to the change in the charge state of the silicon donors in the doping slab. With the two thinnest slabs the mobility at ambient pressure is so low in the i=0 subband that Shubnikov-de Haas peaks from this subband could not be detected at fields up to 15 T, although strong peaks could be observed from the higher order subbands. After illumination of the thinnest sample at high pressure the measured free electron concentration is not restored to the zero pressure value. One possibility is that the missing electrons occupy a localized non-metastable Si state resonant with the conduction band rather than a DX centre. The pressure coefficient of the carrier density yields an extrapolated position for the energy level for the Si localized state of 270+or-10 meV above the Gamma -conduction band edge at ambient pressure.

A transmission electron microscopy and reflection high‐energy electron diffraction study of the initial stages of the heteroepitaxial growth of InSb on GaAs (001) by molecular beam epitaxy

In situ reflection high‐energy electron diffraction and cross‐sectional and plan‐view transmission electron microscopy have been used to investigate the initial stages of InSb growth on GaAs(001), by molecular‐beam epitaxy. Growth of the InSb commences with the formation of rectangular‐based islands, having flat tops and sloping sides, with facets on certain planes of types {111} and {113}. The islands show near normal lattice spacings, with no significant straining. As deposition proceeds, islands coalesce and, after the equivalent of 40 monolayers of deposition, form a connected network. Complete coverage of the GaAs substrate is achieved after ≂300 monolayers of deposition. This places a lower limit on the thickness of InSb layers, which may be considered in the design of optoelectronic devices.

Magneto-optical and transport studies of ultrahigh mobility films of InAs grown on GaAs by molecular beam epitaxy

InAs layers of very high electrical quality are grown on GaAs substrates by molecular beam epitaxy (MBE). The observation of sharp cyclotron resonance and donor lines (linewidths approximately=1 cm-1) in far-infrared magneto-optical studies suggest that the low-temperature mobilities in the bulk of the films are in the range 200 000-300 000 cm2 V-1 s-1 with an electron concentration of approximately=2*1014 cm-3. A strong but broad cyclotron resonance line and the Shubnikov-de Haas effect are observed from a two-dimensional electron gas (2DEG) at the surface or GaAs interface (nS approximately=1*1012 cm-2 and mu s approximately=20 000 cm2 V-1 s-1). As a consequence of parallel conduction from the low mobility layer the Hall mobility measured from a 5 mu m thick sample is 80 000 cm2 V-1 s-1 at 77 K and that in a 2 mu m sample is only 30 000 cm2 V-1 s-1. The width of bulk cyclotron resonance and impurity lines depend only weakly on thickness and consequently scattering from dislocations generated by the misfit at the GaAs/InAs interface is not thought to affect the bulk mobility strongly down to film thicknesses of 1 mu m. The parallel conduction from the 2DEG also produces a large magnetoresistance. Please note - the first author name has been corrected from Homes to Holmes

MBE growth and quantum transport measurements of spike-doped InSb and InAs

Molecular beam epitaxy is used to prepare high-mobility films of InSb and InAs either homoepitaxially or heteroepitaxially on GaAs substrates. Silicon donors and beryllium acceptors can be introduced at high concentrations ( approximately 1019 cm-3), although low-temperature growth (<or=300 degrees C) must be employed in the case of silicon in InSb to avoid compensating amphoteric behaviour. Atomic plane doping of these impurities is studied by quantum transport measurements. Up to five sub-bands are occupied at high doping levels. Little or no diffusion of silicon away from the doping plane is found provided that the growth temperatures are kept low.

Local vibrational mode spectroscopy of Si donors and Be acceptors in MBE InAs and InSb studied by infrared absorption and Raman scattering

Samples of InAs and InSb either singly or doubly doped with Si and Be impurities have been studied using FTIR absorption spectroscopy and Raman scattering. Localized vibrational modes of 28SiIn donors and 9BeIn acceptors have been identified at 359 and 435 cm-1 respectively in InAs. The two impurities give corresponding lines at 316 and 414 cm-1 in InSb. Comparisons are made with the behaviour of the same impurities in compensated of p-type GaAs. Strong resonance effects relating to the incident photon energy are found in the Raman spectra of 20SiIn donors.

RHEED intensity oscillations observed during the MBE growth of InSb (100)

RHEED intensity oscillation data recorded during the molecular beam epitaxial (MBE) growth of InSb are presented for the first time. The effects of changing the substrate temperature and the antimony to indium flux ratio (J(Sb4)/JIn) are investigated. RHEED oscillations are most pronounced for antimony-rich growth, with flux ratios close to unity. Antimony-induced RHEED intensity oscillation data are also presented. RHEED oscillations can now be used to calibrate growth rates and effective flux ratios during the MBE growth of InSb.

Observation of coupled LO phonon-intersubband plasmon modes in GaSb/InAs quantum wells by resonant Raman scattering

Two new peaks are observed in resonant Raman scattering from GaSb/InAs quantum wells grown on (001) GaAs by MBE. These two lines are assigned to the coupled LO phonon-intersubband plasmon modes originating from the InAs wells. The lower-frequency branch of the coupled system (L- mode) lies between the LO and the TO frequencies of InAs, and the line intensity depends strongly on the two-dimensional carrier concentration and the laser excitation energies (resonant near the E1 gap of InAs). The L+ line is weak and relatively broad. Its frequency increases with increasing carrier concentration in the InAs wells and is not observed when the carrier concentration is low.

Parallel and Perpendicular Field Magnetotransport Studies of MBE Grown GaAs Doping Superlattices and Slab Doped InSb Formed by Selective Doping with Silicon

Selective doping in the form of slabs or spikes of dopants offers new possibilities for lowdimensional structures. The results for spike-doped superlattices in GaAs can be interpreted in terms of a nearly free-electron model with the electrons occupying a Fermi surface which has ‘neck’ and ‘belly’ orbits for the magnetic field applied perpendicular to the doping planes and with magnetic break-down occurring at high fields with B parallel to the doping planes. The carrier concentration is varied by applying hydrostatic pressure and the change in orbit area with carrier concentration is consistent with this model.