Structural studies have been performed using transmission electron microscopy and diffraction on Ill-V ternary and quaternary alloy epitaxial layers. The results revealed that 'natural superlattice' structures had spontaneously formed in some of the layers by phase separation and/or atomic ordering which occurred at the surface during growth. The structure of these natural superlattices and the growth conditions for their occurrence are described and their effect on the electrical and optical properties of the layers, e.g. bandgap narrowing, and possible mechanisms of formation are briefly discussed.

The electrical quality of lnAs films grown on GaAs substrates by MBE is found to be optimum for growth temperatures close to 490 "C. The Hall mobility for such samples is80000cm2V-'s-' at 77 K for film thicknesses of 5 pm but falls to about 10 000 cm2 V -' s -' at a thickness of 0.05 pm. The carrier concentration in the bulk of the films is believed to be less than 10'5cm-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 x 10'8cm-3 and 15000cm2V-'s-' at 77 K respectively for a growth temperature of 350 "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 200000cm2V-' 5 -' 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 f 0.0003m. with a pressure coefficient of ' +2.0% kbar-'. 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 x 10'ecm-3 where the mobility is 2000 cm2 V -' s -' )

Nodestructive optical methods, based on measurements of the 'plasma edge' and the Moss-Burstein shift, are investigated as contactless reflection and transmission spectra of undoped and Sidoped lnAs 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 x 10'' c t r 3 . For n above 4 x I O ' ' 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.

In this letter we report on the growth of high quality InSb by molecular beam epitaxy that has been optimized using reflection high energy electron diffraction. A 4.8 μm InSb layer grown on GaAs at a growth temperature of 395 °C and a III/V incorporation ratio of 1:1.2 had an x-ray rocking curve of 158 arcsec and a Hall mobility of 92 300 cm2 V−1 at 77 K. This is the best material quality obtained for InSb nucleated directly onto GaAs reported to date.

Raman scattering by coupled plasmon-phonon modes is studied with Sidoped InAs epilayers grown by MBE with carrier concentrations from 7.5 x l o ' ' c m -3 to 4 x iO'9cm-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.

Phonon energies in inAs,,Sb, 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"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°C and compositions near t h e 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.

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Molecular beam epitaxial growth of a normally homogeneous InAs0.5Sb0.5 alloy below 430 °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.