Influences of temperature and transport properties on the birefringence of CdGeAs2

We have directly measured the birefringence Delta n of CdGeAs2 using polarized light interference spectra obtained in transmittance from 2.4 to 18 mu m over a temperature range of 14-450 K. Four different samples, exhibiting a wide range of free carrier concentrations, were studied. It was found that free carriers can have a significant effect on the room-temperature birefringence. At 14 K, however, the data from all samples were virtually identical. The temperature dependence of Delta n was obtained from a sample with low carrier concentration (approximate to 10(15)/cm(3)) and high resistivity (9 Ohm cm cm). Its temperature derivative d(Delta n)/dT was determined at 50 K increments over the entire 14-450 K temperature range, and was found to vary from 2.6x10(-5)/K at 100 K to 5.1x10(-5)/K at 400 K. A least-squares fit to the data yielded a temperature coefficient for an of 8.2x10(-8)/K. Hall-effect measurements indicate that the resistivity, mobility, and carrier concentration all exhibit a significant temperature dependence. The resistivity for one sample increased by a factor of 10(8) when cooled from room temperature to 14 K. Our results suggest that previously published Delta n values were probably obtained from samples with relatively high carrier concentrations, and that our results more accurately describe the true intrinsic birefringence of the material. The value of Delta n decreases by 0.0004 per 10(16) carriers a factor of 10 less than suggested by theory but still significantly large. The heavy to light hole effective mass ratio in the uppermost valence band was found to be 7.1.