A method for routine characterisation of the hole concentration in p-type cadmium mercury telluride

Abstract

Recently there has been growth of interest in the fabrication of photodiodes in CMT, for use in electronically scanned hybrid focal plane arrays. Most devices are currently fabricated using a p-type wafer with an n-type surface layer introduced either by ion implantation or diffusion. In both types of junction the &,A product is dependent on the net doping concentration of the p-type substrate. In ion-implanted junctions for example the performance can be limited even at zero bias by the junction tunnelling current if the substrate doping is too high. It has been determined that a carrier concentration of 1 x 1Ol6 cme3 or less is required to produce implanted detectors with high zero bias resistance-junction area, R,,A, products. (l) Consequently an accurate, and rapid, method of determining the carrier concentration of p-type CMT is required. A common method of determining the net carrier concentrations in routine characterisation of semiconductors is to measure the Hall coefficient, RH, at a single temperature in the exhaustion range of the extrinsic conduction region. This method, using a spot measurement at 77K, has been found to be satisfactory for n-type CMT in which carrier freeze-out is not observed. However, p-type CMT is much more difficult to characterise by Hall effect measurements because of the high electron-to-hole mobility ratio, b, (500 at 80K) and a relatively high intrinsic carrier density in this narrow gap semiconductor. As a result the mixed conduction region in lightly doped p-type samples extends below 80K and may overlap the freeze-out region so that no exhaustion region is observed. Determination of the net doping concentrations in p-type material has therefore required the interpretation of Hall data obtained over a range of temperatures extending down to about 20K. The interpretation may be further complicated since oxidation of the surface of the samples can lead to an n-type skin which shunts the sample at low temperatures and in extreme cases can cause a p-type sample to appear in n-type. To overcome these difficulties the possibility of using a measurement of bulk conductivity to characterise the hole concentration has been investigated. The two band expressions for conductivity, 0, and low field Hall coefficient, RH are: