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

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 mum/h were achieved by growing CdTe at around 770 degreesC and with the source temperature at 827 degreesC. 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 (40 0)CdTe peak down to 59 arcsec are obtained for similar to 30 mum 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 degreesC are p-type and low resistive [rho less than or equal to 20 Omega cm at room temperature (RT)], whilst at temperatures > 650 degreesC the layers are n-type. In this case, resistivity similar to 10(6) Omega cm are obtained for growth between 675 degreesC and 700 degreesC, but lower values result above 700 degreesC, However, resistivity in the 10(4)-10(5) Omega cm range are achieved in the latter case by increasing the epilayer thickness up to 30 mum. 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 accepters 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 mu tau approximate to 4.9 x 10(-6)cm(2)/V for this material. Such low value arises from a large electron trapping in CdTe epilayers, consistently with the similar to 10(18)cm-(3) density of compensated (positively charged) donors. (C) 2001 Elsevier Science B.V. All rights reserved.