Investigation of ohmic contact to P-type CdTe:P using ac and dc techniques

The effect of ohmic contact to P-type CdTe:P single crystal has been studied for different back contacts: Au and graphite doped with Cu, Te and CdCl2. The ac electrical measurements were performed at room temperature in the frequency range 10−1 Hz to 106 Hz. The results were compared with dc measurements at the same temperature. Al/CdTe:P Schottky diodes were fabricated using thermal evaporation technique. The same applies for Au back contacts while for the others, Cu, Te and CdCl2, powder forms were mixed in fixed portions with graphite. In ac and dc measurements, it was found that Cu electrodes give the lowest contact resistance which is nearly frequency independent in a frequency range 10−1 Hz to about 103 Hz. However, the plot of the imaginary component (Z″) versus the real component (Z′) of the complex ac impedance yields two semi-circles in the case Cu and Te back contacts with less defined two semi-circles in the case of CdCl2. For Au back contact a single semi-circle was constructed with a single relaxation time in the order of 0.04 ms. The two semi-circles correspond to the bulk material (imperfections) at high frequency region, and the electrode interface between CdTe:P and the contact electrode. This may indicate that a P+ layer has been formed in the interface between the material and the back-contact layer. However, a good correlation was found between the ac and dc measurements for all back contacts.