Determination of Absorption Coefficients and Thermal Diffusivity of Modulated Doped GaAlAs/GaAs Heterostructure by Photothermal Deflection Spectroscopy

We report in this study theoretical and experimental studies of photothermal deflection spectroscopy (PDS) for planar doped heterostructures AlGaAs/GaAs. The PDS spectra at T=300K are recorded by varying the wavelength of the excited radiation for different modulation frequencies. They show two different regions related to the fundamental electronic transitions from GaAs and Si doped AlGaAs. The DX levels have been observed approximately at 0.150 eV below the conduction band edge of Si doped Al_0.33Ga_0.67As (Eg=1.835 eV at 300 K). We give in this study a method of measuring the absorption coefficient of AlGaAs/GaAs heterostructures from the amplitude measurement of PDS signal. The optical absorption coefficient found was between 10 and 10⁵ cm^-1 in the energy range 1-2.5eV for samples. The PDS data are compared with those given by conventional transmission technique. The evolution of PDS amplitude spectra by varying the modulation frequency from 5 to 100 Hz (usually called depth profile analysis) and using He-Ne laser probe beam shows that the PDS signal amplitude increases with decreasing the modulated frequency. A numerical procedure is applied to fit the measured amplitude of the PDS signal. The value of thermal diffusivity (D_s=0. 27cm²s^-1) shows a very good agreement with results given by spectroscopic ellipsometry.