Growth and characterization of GaTlAs

The growth of GaTlAs has been attempted by gas source molecular beam epitax y using a variety of growth conditions. At substrate temperatures greater t han or equal to 275 degrees C a two-phase mixture of metallic Tl droplets i n a matrix of GaAs was produced, similar to that often observed in the InTl As material system. However. at low temperatures (similar to 200 degrees C) , a specular morphology was obtained with incorporation of 5 at. % Tl as me asured by electron microprobe analysis (EMPA). Increasing the Tl flux for f ilms grown at a substrate temperature of 200 degrees C produced layers cont aining as high as 15 at. % Tl, although with slightly rougher surface morph ologies. Backscattered electron imaging showed no phase contrast, indicatin g there was no spatial variation in the distribution of Tl throughout the f ilm, nor was there any evidence of metallic Tl droplets from powder x-ray d iffraction scans. EMPA also indicated a much higher As/group III ratio, wit h values up to 3/2 obtained. Films grown at 200 degrees C showed no evidenc e of oxidation as is normally associated with the Tl containing materials. Cross-sectional transmission electron microscopy revealed the material to b e amorphous. A large shift in the absorption edge was observed, but is beli eved to be due to the incorporation of large amounts of As rather than the Tl. (C) 2000 American Vacuum Society. [S0734-2101 (00)01405-6].