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 -' )

For the first time a diamond anvil cell high-pressure technique has been used for far-infrared laser magneto-optical experiments. A shallow-deep transition was observed lor shallow sulphur and silicon donors in high-purity GaAs at pressures of 25 and 30 kbar respectively. This transition is similar to that previously observed at 9 kbar for Ge residual donors. The deduction of A , symmetry tor tne deep electronic slates involved results from the observation of anticrossing effects. Large 'chemical shills' between deep states of different impurities provide evidence lor their strong localizalion. Fast electron capture onto these deep states at low temperature demonstrates their non-DX-like character. Present results and similar experiments with lnSb show that three kinds of donor states, shallow, deep A, and DX-like. are generally expected to exist in iil-V compounds and alloys.