Temperature Dependent Hall Measurements on CdGeAs 2
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performed at West Virginia University using a system based on a typical Keithley Instruments Hall effect set-up. Indium contacts were soldered to the samples using the standard Van der Pauw geometry. Since CdGeAs 2 is a highly anisotropic crystal, specially matched sets of samples were fabricated from each boule for Hall analysis. Hall measurements performed on the sample that had the c-axis perpendicular to the plane of the sample (ci) allowed measurement of the carrier concentration, the mobility and the resistivity along the a-axis. The other sample had the c-axis in the plane of the sample, parallel to one edge (c, i), allowing the resistivity of both the a- and c-axes to be determined directly. For each matched set, the a-axis resistivities agreed reasonably well, with the worse case differing by a factor of two. It is reasonable to assume this variation is due primarily to carrier concentration variations within a boule, and thus the resistivity ratio coupled with the a-axis mobility determines the c-axis mobility, giving a complete set of electrical properties for each direction in the crystal. In addition to the electrical data, infrared absorption measurements were performed at room temperature from 220 jtm using a Nicolet Magna-IR 550 FTIR spectrometer. SAMPLE CHARACTERIZATION AND ANALYSIS Representative data from the temperature dependent Hall measurements made on the ci samples of CdGeAs 2 are plotted in Figure 1. Hall effect measurements of this type indicated ptype material in all but one case. Shown in Figure 1 are the carrier concentration and mobility measured for sample 49. The room temperature carrier concentration for this particular sample was _-lxlO15 cm 3 with the effect of carrier "freeze-out" easily seen. The maximum hole mobility was seen to be -225 cm 2/V-sec occurring -190 K. Also of note, the Hall coefficient changes signs, from positive to negative, at about 350 K, indicating that intrinsic carrier concentration effects are becoming important near this temperature. The activation energy of the acceptors can be 103 determined from the low-temperature 1017 (+.) *o (carrier concentration data, where the intrinsic electrons have not yet become 0 C? 1016 20 important. The data was fit to the full charge balance equation, based on one "•"1015 AAAAAAAAAA 11 acceptor level and a fully ionized 0 * A shallow donor with no assumptions 102 F, A B 1014 about the relative concentrations of * S t1' impurities. [5] Due to the large S1013 activation energy of the acceptors, S .. . A complete ionization would only occur at temperatures where conduction is U 1012 -0 A dominated by intrinsic electrons. With the constraint of incomplete ionization, 10' the fitting procedure could only 10"1 lO)0 150 200 250 300 350 400 4:50 determine the compensation of the material, i.e. the ratio of the number of Temperature (K) donors to the number of acceptors. Table I contains information on all of Figure 1. Typical electrical properties of the samples that were measured, cadmium germanium arsenide.
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Table L Sele
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