Growth and characterization of GaTlAs

The growth of GaTlAs has been attempted by gas source molecular beam epitaxy using a variety of growth conditions. At substrate temperatures ⩾275 °C a two-phase mixture of metallic Tl droplets in a matrix of GaAs was produced, similar to that often observed in the InTlAs material system. However, at low temperatures (∼200 °C), a specular morphology was obtained with incorporation of 5 at. % Tl as measured by electron microprobe analysis (EMPA). Increasing the Tl flux for films grown at a substrate temperature of 200 °C produced layers containing as high as 15 at. % Tl, although with slightly rougher surface morphologies. Backscattered electron imaging showed no phase contrast, indicating there was no spatial variation in the distribution of Tl throughout the film, nor was there any evidence of metallic Tl droplets from powder x-ray diffraction scans. EMPA also indicated a much higher As/group III ratio, with values up to 3/2 obtained. Films grown at 200 °C showed no evidence of oxidation as is normally associated with the Tl containing materials. Cross-sectional transmission electron microscopy revealed the material to be amorphous. A large shift in the absorption edge was observed, but is believed to be due to the incorporation of large amounts of As rather than the Tl.