Study of CaF<sub>2</sub>- Doped PbTe Thin Films Grown by Molecular Beam Epitaxy

Abstract

We present here a study on the electrical and structural properties of p-type PbTe films doped with CaF2. The layers were grown by molecular beam epitaxy on freshly cleaved (111) BaF2 substrates. The doping level was monitored by the CaF2 solid source cell temperature (TCaF2), which varied from 500 to 1150 °C. The films with low doping level, TCaF2 ≤ 1010 °C, exhibited flat surfaces with crystalline quality close to the undoped PbTe sample. In contrast, samples with high levels of doping (TCaF2 > 1010 °C) presented CaF2 agglomerates on the surface and a worse crystal quality. The hole density at 77 K versus TCaF2 oscillated between 1.3 × 1017 and 3.6 × 1017 cm-3 and did not exhibit a systematic behavior as the fluoride supply is raised. The results indicate that CaF2 is not an effective p-type dopant for PbTe, due to the abscence of a resonant level close to the valence band or to compensation of extrinsic dopant levels.