Calorimetric measurement of absorption loss in orientation-patterned GaP and GaAs

Materials grown by vapor phase techniques such as chemical vapor deposition or hydride vapor phase epitaxy (HVPE) often exhibit very low losses which are difficult to quantify by simple transmission measurements. The measurement of extremely low absorption coefficients can be carried out by laser calorimetric or thermal rise techniques, which determine the absorption coefficients by measuring the temperature increase caused by the absorbed laser radiation. We report here on results of measuring absorption coefficients of bulk HVPE-grown orientation-patterned GaAs (OP-GaAs) and GaP (OP-GaP) crystals using one of the methods of laser calorimetry, called transient calorimetry. In our setup, the sample under test is attached to a high-conductivity copper holder and placed in a vacuum chamber. A 2-micron cw laser beam is transmitted through the sample and the temperature rise in the sample is measured and, through the calorimeter calibration process, related to the power absorbed in the sample. The absorbed power, Pa, is a function of the total attenuation coefficient αtot , the length of the sample, and the laser power Po, defined as Pa = Po exp (-αtot l), where total attenuation αtot is the sum of absorption and scattering: αtot = αabs + αscat. Since scattered light does not cause heating, the calorimetric technique is only applicable to determining αabs. By this technique we have measured 2-micron absorption coefficients in OP-GaAs and OP-GaP as low as 0.007 cm-1.