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).