Answer to Question #9651 Submitted to "Ask the Experts"
Category: Instrumentation and Measurements
The following question was answered by an expert in the appropriate field:
A major part of the bulk or volume leakage associated with many of these semiconductor materials arises from imperfections and impurities that appear in the crystalline structures. Minority carrier diffusion across the detector junction will produce much of the volume leakage current. Thermal effects also may increase volume leakage; as temperatures increases so does charge volume leakage. There is also leakage associated with surface phenomena. It is very important to keep surfaces as free of contaminants as possible during manufacture of the crystalline semiconductors to minimize potential leakage associated with such contaminants. Additionally, the properties of the surface electrodes have an effect on leakage. For example a CdTe detector with a Pt/CdTe/Pt electrode configuration will generally exhibit most of its charge leakage as a result of holes being injected from the anode. There is evidence that the extent of leakage appears to be more closely associated with the detector perimeter than with the surface area. Also, the selection of crystals with specific crystalline planes available for electrode placement may influence the leakage.
As you know, CdTe and related materials have the propensity for building up stored charge associated with the trapping of low mobility holes produced by radiation interactions, and this has very negative consequences for radiation detection and spectral measurements. In order to improve charge collection to minimize tailing effects caused by the hole trapping it is necessary to increase applied voltage to higher values than would otherwise be required. Unfortunately, the degree of leakage increases with increasing voltage, and additional measures are then necessary to yield acceptable leakage levels.
One technique that has been used to reduce surface leakage is to use a guard ring on the detector surface. The guard ring operates in a fashion similar to guard rings in other detector types, such as ionization chambers. The guard ring is typically concentric about the cathode electrode and separated by a small gap from it. A paper by K. Nakazawa et al. describes the use of a guard ring with a CdTe detector.
A paper by Kosyachenko et al. is available on the internet and describes the use of a combination of ohmic and high barrier Shottky contact electrodes in a Ni/CdTe/Ni configuration as a means for reducing leakage and improving performance. A paper by T. Ozaki et al. (Thermal treatment of CdTe surfaces for radiation detectors, Nuclear Instruments and Methods in Physics Research Section A, 380 (1-2):141-144; 1996) discusses a process for heating the CdTe to 200-300oC prior to electrode placement as a means for reducing leakage current; the electrode configuration used was In/CdTe/Au.
The actual magnitude of the leakage current will depend on the physical characteristics of the detector—its facial area and thickness, the nature of the electrodes, the particular crystal plane faces that are used for the electrodes, contaminants present, applied voltage, temperature, and other factors. At relatively high applied voltages, such as 500 volts, leakage current densities on the order of 20 nA cm-2 would be common while at a low applied voltage of perhaps 15 volts such current density might be reduced to about 1 to 2 nA cm-2. Use of some of the leakage reduction techniques noted above may reduce the leakage currents by more than an order of magnitude.
You can find more discussion of charge leakage and other related topics in a variety of sources if you perform a simple literature search as well as search of Internet resources. Knoll’s textbook, Radiation Detection and Measurement, 4th ed., Wiley, 2010 is an excellent source of information on many topics, including semiconductor detectors and their properties.
Good luck in your pursuits.
George Chabot, PhD, CHP