Mapping CZT charge transport parameters with collimated X-Ray and gamma-ray beams
It is desirable to characterize the charge production and transport properties of CZT wafers prior to fabrication into detectors. This allows rejection of undesirable material early in the production cycle as well as acceptance of less-than-perfect material that may be adequate for particular applications. Significant production cost reduction and increased yield will result. We report on the development of techniques to achieve these aims with equipment that may be used in small research groups and companies. Collimators made of high-Z material produce 30-micron to 3-mm pencil beams of X-rays and gamma-rays from radioactive sources and X-ray generators. These are scanned precisely acrosswafers prepared with simple planar contacts that are read out by a single electrode, the anode in our case. With irradiation on the cathode side, the spatial variation of the signal is mostly due to variations in electron production and transport. Therefore, the position-dependence and bias dependence of the signal’s spectral properties and event rate may be analyzed to determine and map the charge production and electron trapping lifetime, and the 3-D dependence of these parameters. Grain boundaries and other regions of degraded charge production an transport are readily identified. The technique is described in detail, and examples of scanning results are presented.