On thallium incorporation in GaInTlAs layers grown on InP by low temperature molecular beam epitaxy

The growth of GaInTlAs alloys on InP has been attempted by solid source molecular beam epitaxy. Thallium incorporation in Ga_1-xIn_xAs matrices was studied as a function of growth temperature, thallium flux and arsenic overpressure. At high temperatures (>350°C) thallium segregates on the surface and evaporates whereas at intermediary temperatures (270°C-350°C) thallium segregates into droplets at the surface. Only in the low temperature range (180°C-270°C) thallium can be incorporated in some conditions, leading to mirror like surfaces. The thallium incorporation was evidenced by Secondary Ion Mass Spectrometry (SIMS) and Rutherford Backscattering Spectroscopy (RBS). Up to 18% Tl content was incorporated into a Ga_1-xIn_xAs matrix. For these high Tl concentrations, Tl droplets are avoided and Tl incorporation is achieved only when using high arsenic pressures. This limits surface adatom diffusion and leads to amorphous, twinned or polycrystalline materials. Thus, single-crystalline layers have been grown successfully only for low Tl contents (4%) when the V/III beam equivalent pressure (BEP) ratio is chosen as low as possible (~4) while avoiding thallium segregation and droplet formation