Quantitative in-situ photopyroelectric spectroscopy of optoelectronic quantum structures. Theory and experiment with Al0.6Ga0.4As/GaAs quantum wells

Quantitative photopyroelectric (PPE) optical absorption coefficient spectra of an (Al, Ga)As/GaAs multiple quantum well were obtained in-situ on the semi-insulating GaAs substrate completely non-intrusively in the region 900-1080 nm and in the presence of significant absorption form the substrate. The two spectra were decoupled numerically by using a rigorous coupled electromagnetic/thermal-wave photopyroelectric model and optical reflectance and transmittance spectra obtained independently from the PPE spectra. The role of thermal contact resistance at the sample-pyroelectric interface was examined and found to be important for quantitative analysis. An iterative numerical technique matching the theory to the data with the quantum well optical absorption coefficient as the variable parameter yielded quantitative spectra with ca. 10% experimental uncertainty. Superior PPE resolution of absorption coefficients due to the quantum well superlattice to the purely optical spectroscopies was observed, thus allowing ultra-thin film in-situ quantitative spectroscopy in the GaAs sub-bandgap region for the first time.