These semiconductors are used as solid state ionisation chambers. The detector signals and thus performance are determined by the charge transport properties of the devices. The charge transport across the bulk as well as the transport across the interface of the electrical contact to the readout system need be optimised for a good quality detector. Intrinsic and extrinsic defects, impurities, radiation induced damage, surface processing and contact fabrication play an important role.
Facilities & techniques
Fabrication and processing facilities for the production of prototype detector structures from raw material are available, including polishing equipment, photolithography, metal deposition by thermal evaporation. Ion implantation can be used in collaboration with the Surrey Ion beam centre on campus.
Standard spectroscopy tests using radioisotope sources at various temperatures, X-ray irradiations can be performed using a variety of standard X-ray tubes up to 160 kVp. Photo induced current transient spectroscopy and particle induced current/charge pulse shape analysis is used to extract carrier mobilities, life time and trapping effects at defects. These studies are complemented by optical methods, like photoluminescence mapping at variable temperatures, infra red and birefringence microscopy, Pockel’s imaging and photocurrent measurements.
The group also has a high reputation in the application of ion beam induced charge (IBIC) microscopy performed at the Surrey microbeam line, which images the variation of charge transport properties like the mobility lifetime product with micrometre spatial resolution.
Materials under investigation
Research in radiation detectors focuses on the development of new wide bandgap semiconductor materials for radiation detector applications. A particular emphasis of the research is in material characterisation and charge transport studies, especially in II-VI semiconductors (CdTe, CdZnTe, CdMnTe) and in diamond. Other wide bandgap materials such as GaAs, GaN, InP, HgI2 and TlBr are also studied.
The study of II-VI semiconductors for radiation imaging applications concentrates on cadmium telluride (CdTe), cadmium zinc telluride (CdZnTe) and cadmium manganese telluride (CdMnTe). Activities are focused on material characterisation and charge transport properties in bulk crystalline material. Working closely with both material growth specialists and detector end-users, we have developed an exceptional range of diagnostic and semiconductor physics facilities optimised for II-VI materials characterisation.
The group is a member of the 'HEXITEC' Basic Technology project to develop CdZnTe pixel imaging detectors.
Diamond detector research concentrates on charge transport and radiation hardness studies of both polycrystalline and single-crystal synthetic diamond. With the recent availability of single-crystal synthetic diamond, many interesting applications now exist for diamond radiation detectors, e.g. as radiation hard tracking detectors or as tissue-equivalent medical dosimeters. Research at Surrey mainly investigates the charge transport and trapping phenomena in synthetic diamond film. Fabrication of high quality ohmic contacts on diamond is also investigated, e.g. using novel graphitised contacts formed by high fluence ion implantation. Our work makes extensive use of the ion beam analysis and ion implantation facilities hosted in the University's Ion Beam Centre. We also have a 213nm YAG laser for near band edge PL studies in diamond.
Facilities and techniques
Techniques developed in our laboratories include room temperature photoluminescence (PL) imaging and sub bandgap infra-red microscopy. PL imaging provides a fast contactless method for initial assessment of material quality, showing dislocations, twins and other mechanical features. Our PL mapping system uses a 25 mW 633 nm HeNe laser which can image wafers over a 1 inch x 1 inch area. The spatial resolution of the system is <100 microns. IR microscopy is a complementary technique for measurement of tellurium precipitates and other features in bulk material. Using a silicon CCD, our system has a field of view of typically 400 um x 400um capable of imaging Te precipitates smaller than 5 um. The sample holder is mounted on a XY stage, allowing composite images up to 10 mm x 10 mm to be acquired.
The group is a leading international centre for the application of ion beam techniques for semiconductor detector characterisation. The group has extensively developed the use of Ion Beam Induced Charge (IBIC) imaging for micron-resolution imaging of charge collection efficiency (CCE) and charge transport in wide bandgap semiconductor materials.