Hydrogen transport vapour growth and properties of thick CdTe epilayers for RT X-ray detector applications

The morphological, structural, and electrical properties of thick (8÷50 µm) CdTe epilayers grown on ZnTe/(100)GaAs by H2 transport vapour phase epitaxy (H2T-VPE) are reported. Using a CdTe powder source temperature of 827°C and a substrate temperature of 764°C optimised CdTe epilayers were grown at atmospheric pressure with a smooth and almost featureless surface morphology. Mapping of the high resolution X-ray diffraction intensity in the vicinity of the CdTe (400) reciprocal lattice point demonstrates that the samples are single crystalline and have negligible mosaicity contribution, supporting the epilayer high crystalline quality. Calibrated secondary ion mass spectrometry (SIMS) elemental depth profiles indicate that at the relatively high growth temperature of H2T-VPE a complete interdiffusion between the thin ZnTe buffer and the overgrown CdTe crystal occur: ZnTe acts as a sacrificial layer ensuring the homogeneous (100)-nucleation of CdTe on (100)GaAs during early growth stages. Strong diffusion of Ga and As atoms from the substrate is also observed. The RT electrical properties of epilayers thinner than ∼30 µm are determined by Ga-related donor centres, leading to intermediate (1e2÷1e4 Ω⋅cm) resistivity values and electron concentrations which decrease monotonously with increasing epilayer thickness in the 1e14-1e11 cm-3 range. For thicker samples the material turns to p-type conductivity with a net hole concentration ~1e13 cm-3. Finally, the unintentional incorporation of H into the epilayers at concentrations up to 6e20 cm-3 is demonstrated by SIMS and ascribed to the H2T-VPE process.