Effect of Tl content on the growth of TlGaAs films by low-temperature molecular-beam epitaxy

Abstract

TlGaAs layers with various Tl contents have been grown on GaAs(001) substrates by solid-source molecular-beam epitaxy at a substrate temperature of 210 °C and an As/Ga beam equivalent pressure ratio of 100. The Tl content was characterized by Rutherford backscattering spectrometry, secondary-ion mass spectrometry, and electron microprobe analysis. Nomarski microscope observation revealed that the surfaces of the TlxGa1−xAs layers were mirror like until a Tl content of x=7.0% while Tl droplets appeared on the surfaces of samples with a Tl content of x=7.7%. X-ray diffraction (XRD) curves showed clearly resolved epi-layer peaks until a Tl content of x=6.4%. We estimated the epitaxial thickness hepi, at which single-crystalline growth is replaced by polycrystalline growth, by two different methods using the XRD data. In one method we measured the change in diffracted intensity of the epi-layer peak after progressive gradual etching of the TlGaAs layers grown. The other was on the basis of a simulation of the diffraction curves of the as-grown samples within dynamical diffraction theory. The values of hepi estimated by the two methods agreed well and showed a remarkable decrease with an increase in Tl content. Cross-sectional transmission electron microscopy confirmed the reliability of the estimated value of hepi. In addition, the lattice mismatch was analyzed utilizing both symmetric and asymmertic XRD data. The analysis indicated that the lattice constant of an imaginary zinc-blende type crystal of stoichiometric TlAs (aTlAs) is slightly smaller than that of InAs (aInAs).