Investigation of MBE grown polycrystalline CdTe films on the Medipix readout chip

Cadmium Telluride (CdTe) films are directly deposited on a CMOS (complementary metal-oxide-semiconductor) based readout chip as sensor layer for X-ray detection. This is performed by using a modified Molecular Beam Epitaxy (MBE) setup with a carbon collimator enabling growth rates up to 10 mu m/h. To obtain a good contacting behaviour of the 25-50 mu m thick CdTe films, Te and Sb2Te3 are additionally evaporated during the process. The investigation of polycrystalline sensor layers deposited at 400 degrees C with SEM (scanning electron microscopy) and XRD (X-ray diffraction) reveals a columnar growth of the individual grains oriented predominantly in (111). By PES (photoelectron spectroscopy) measurements the chemical composition of the different layers is identified in a depth profile and changes in work function along the contact structure are observed. Detector properties reveal a linear behaviour of the count rate with increasing radiation intensity as well as sensibility to holes and electrons. Spatial resolution measurements result in a resolution of 5 lp/mm, which is a mandatory requirement for medical applications.