On hydrogen transport VPE-grown CdTe epilayers for fabrication of 1–100 keV X-ray detectors

Abstract

The hydrogen transport vapour-phase epitaxy growth of CdTe on ZnTe/GaAs hybrid substrates for the realisation of detector-grade epilayers is reported. Growth rates close to 10 μm/h were achieved by growing CdTe at around 770°C and with the source temperature at 827°C. The crystalline structure of the as-grown epilayers was investigated by double crystal X-ray diffraction measurements. Despite the large epilayer/substrate lattice misfit, the crystalline perfection of CdTe layers rapidly improves with the thickness; indeed, FWHM values of the (4 0 0)CdTe peak down to 59 arcsec are obtained for ∼30 μm thick samples; this value compares well with results published in the literature for best epitaxial CdTe. Correspondingly, mirror-like and nearly featureless surfaces were observed. CdTe samples grown below 650°C are p-type and low resistive [ ρ≤20 Ω cm at room temperature (RT)], whilst at temperatures >650°C the layers are n-type. In this case, resistivity ∼10 6 Ω cm are obtained for growth between 675°C and 700°C, but lower values result above 700°C. However, resistivity in the 10 4–10 5 Ω cm range are achieved in the latter case by increasing the epilayer thickness up to 30 μm. This behaviour is ascribed to the occurrence of Ga-related donors in CdTe, the latter originating by the diffusion of Ga atoms from the GaAs substrate. These donors are almost compensated by residual acceptors in CdTe, giving rise to the medium/high resistivity values observed for some of the samples. The detection capability of the material has been demonstrated by time-of-flight (TOF) measurements performed on a device made by an n-CdTe epilayer. The analysis of the TOF collected charge as a function of the applied reverse voltage gives μτ≈4.9×10 −6 cm 2/V for this material. Such low value arises from a large electron trapping in CdTe epilayers, consistently with the ∼10 18 cm −3 density of compensated (positively charged) donors.