R A Stradling

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.

Electrical and magneto-optical of MBE InAs on GaAs

The electrical quality of InAs films grown on GaAs substrates by MBE is found to be optimum for growth temperatures close to 490 degrees C. The Hall mobility for such samples is 80000 cm2 V-1 s-1 at 77 K for film thicknesses of 5 mu m but falls to about 10000 cm2V-1s-1 at a thickness of 0.05 mu m. The carrier concentration in the bulk of the films is believed to be less than 1015 cm-3. The carrier concentration rises and the mobility falls as the growth temperature is varied on either side of this optimum value, reaching 2.5 *1016 cm-3 and 15000 cm2 V-1 s-1 at 77 K respectively for a growth temperature of 350 degrees C. Extremely sharp free-carrier cyclotron resonance and shallow donor lines are observed from the bulk of the film in far-infrared magneto-optical measurements, together with a very broad but strong cyclotron resonance line from an electron accumulation layer believed to be at the surface. The width of the cyclotron resonance line is consistent with a bulk mobility of the order of 200000 cm2 V-1 s-1 and the decrease in Hall mobility, together with the apparent increase in carrier concentration with decreasing film thickness, can be explained by the parallel conductance from the two-dimensional electron gas at the surface. There is no evidence for a significant reduction in mobility from the high density of threading dislocations caused by the mismatch with the GaAs substrate. The sharpness of the cyclotron resonance allows an accurate value for the band edge effective mass to be determined of 0.0236+or-0.0003 me with a pressure coefficient of +2.0% kbar-1. The donor lines are sufficiently sharp that central cell structure due to two different donor contaminants can be detected, and these donors are thought to be sulphur and selenium originating from the As source material. Certain of the transitions detected are too energetic to be from the shallow donors and these are thought to arise from singly ionized double donors which may be arsenic antisites. Silicon is found to act as a donor dopant up to high concentrations (6*1019 cm-3 where the mobility is 2000 cm2 V-1 s-1).

Molecular beam epitaxial growth of InAsSb strained layer superlattices. Can nature do it better

Molecular beam epitaxial growth of a normally homogeneous InAs0.5Sb0.5 alloy below 430u2009°C results in its coherent phase separation into platelets of two different alloy compositions with tetragonally distorted crystal lattices. This produces a ‘‘natural’’ strained layer superlattice (n‐SLS) with clearly defined interfaces modulated in the [001] growth direction. A description of the n‐SLS growth mode in InAsSb is outlined, and the optical response of a n‐SLS structure, which extends to 12.5 μm−considerably further than that of a homogeneous InAs0.5Sb0.5 layer (8.9 μm)−is reported.

SUPPRESSION OF AUGER RECOMBINATION IN ARSENIC-RICH INAS1-XSBX STRAINED LAYER SUPERLATTICES

Room-temperature pump-probe transmission experiments have been performed on an arsenic-rich InAs/InAs1-xSbx strained layer superlattice (SLS) above the fundamental absorption edge near 10 mu m, using a ps far-infrared free-electron laser. Measurements show complete bleaching at the excitation frequency, with recovery times which are found to be strongly dependent on the pump photon energy. At high excited carrier densities, corresponding to high photon energy and interband absorption coefficient, the recombination is dominated by Auger processes, A direct comparison with identical measurements on epilayers of InSb, of comparable room-temperature band gap, shows that the Auger processes have been substantially suppressed in the superlattice case as a result of both the quantum confinement and strain splittings in the SLS structure, In the nondegenerate regime, where the Auger lifetime scales as tau(aug)(-1)=C1Ne2, a value of C-1 some 100 times smaller is obtained for the SLS structure. The results have been interpreted in terms of an 8x8 k . p SLS energy band calculation, including the full dispersion for both k in plane and k parallel to the growth direction. This is the strongest example of room-temperature Auger suppression observed to date for these long-wavelength SLS alloy compositions and implies that these SLS materials may be attractive for applications as room-temperature mid-IR diode lasers

Infrared reflection and transmission of undoped and Si-doped InAs grown on GaAs by molecular beam epitaxy

Nondestructive optical methods, based on measurements of the plasma edge and the Moss-Burstein shift, are investigated as contactless alternatives to Hall measurements for determining carrier concentrations. Infrared reflection and transmission spectra of undoped and Si-doped InAs grown on GaAs by MBE are studied. A curve-fitting procedure is developed to fit the reflectivity spectra with or without phonon-plasmon coupling. The range of carrier concentrations over which these optical methods can provide useful characterization is evaluated. The effective mass determined from plasma edge measurements agrees well with the simple Kane model for n below 2.7*1019 cm-3. For n above 4*1019 cm-3, the sample effective mass deviates considerably from the simple Kane model. Excitonic structure in the absorption edge is reported for high-purity undoped samples.

Preparation of InSb substrates for molecular beam epitaxy

Several chemical cleaning procedures for InSb(100) substrates were studied. Nomarski microscopy, x‐ray photoelectron spectroscopy, and Auger electron spectroscopy were used to assess their credibility for use in molecular beam epitaxial growth of high quality films. The most satisfactory substrate surface was prepared using a modified CP4A etchant (HNO3:CH3COOH:HF:DI H2O at 2:1:1:10). This etchant was found to produce a flat surface with low defect density and a passivating layer consisting mainly of easily removable Sb oxide.

Raman scattering in InAs1-xSbx alloys grown on GaAs by molecular beam epitaxy

Phonon energies in InAs1-xSbx ternary alloys, grown on GaAs by molecular beam epitaxy, have been studied by Raman scattering. The microstructure for this alloy system depends strongly on the growth temperature. For growth temperatures above 400 degrees C, transmission electron microscopy (TEM) shows the alloy epilayer to be homogeneous. Raman spectra of these homogeneous InAsSb alloys show a strong InAs-like longitudinal optical (LO) phonon line, as well as an InSb-like LO line, throughout the composition range. The frequency of the InAs-like LO phonon varies linearly with composition. For growth temperatures below 400 degrees C and compositions near the middle of the range, an interleaved platelet structure, arising from phase separation, is observed in TEM. Effects of phase separation in these alloys have been observed in the Raman spectra.

Excitonic photoluminescence in high-purity InAs MBE epilayers on GaAs substrates

Temperature- and excitation-dependent photoluminescence measurements have been carried out on 0.7-5 mu m thick heteroepitaxial InAs layers grown by molecular beam epitaxy (MBE). Excitonic photoluminescence with linewidths down to 5 meV reveals the high optical quality of the epilayers despite the 7% mismatch between the InAs and the GaAs substrates. Peaks at 403 and 391 meV, which quench rapidly with increasing temperature, are attributed to bound excitons, and a sharp (7 meV FWHM) intense line at 417 meV is tentatively attributed to free excitonic recombination. A broad 18 meV wide band (peaking at 378 meV) which blue shifts with increasing excitation, characteristic of a donor-acceptor pair transition band, is reported for the first time in InAs.

Raman scattering by plasmon-phonon modes in highly doped n-InAs grown by molecular beam epitaxy

Raman scattering by coupled plasmon-phonon modes is studied with Si-doped InAs epilayers grown by MBE with carrier concentrations from 7.5*1017 cm-3 to 4*1019 cm-3. Unexpectedly, an unscreened LO line is observed throughout the whole carrier concentration range together with a low frequency (L-) line arising from wavevector dependent LO phonon-plasmon coupling. The frequency of the L- branch lies between the LO and TO phonon frequencies and approaches the TO frequency asymptotically from the high-frequency side as the carrier concentration increases. This behaviour is attributed to competition between screening (dominant in the high-density limit) and large-wavevector induced decoupling. The L+ branch of the plasmon-phonon system is observed for the first time in Raman experiments with InAs.

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.