High-Resolution Quadrature Photopyroelectric Spectroscopy of a-Si:H Thin Films Deposited on Silicon Wafers

Abstract

The recently developed photothermal technique of quadrature photopyroelectric spectroscopy (Q-PPES) has been applied to measurements of amorphous Si thin films deposited on crystalline Si substrates. Direct, meaningful comparisons have been made between purely optical transmission in-phase (IP-PPES) spectra, and purely thermal-wave sub-gap spectra with the use of a novel noncontacting PPES instrument to record lock-in in-phase and quadrature spectra, respectively. FT-IR transmission spectra have also been obtained for a comparison with this IP-PPES optical method. The results of the present work showed that the FT-IR method performs the worst in terms of spectral resolution of thin films and sub-bandgap defect/impurity absorptions inherent in the Si wafer substrate. The optical IP-PPES channel, however, albeit more sensitive than the FT-IR technique, fails to resolve spectra from surface films thinner than 2100 Å, but is sensitive to sub-bandgap absorptions. The thermal-wave Q-PPES channel is capable of resolving thin-film spectra well below 500 Å thick and exhibits strong signal levels from the crystalline Si sub-bandgap absorptions. Depending on the surface thin-film orientation toward, or away from, the direction of the incident radiation, the estimated minimum mean film thickness resolvable spectroscopically by Q-PPES is either 40 Å or 100 Å, respectively.

Measurement of thermal diffusivity of air using photopyroelectric interferometry

“…[8][9][10] Furthermore, both results are in good agreement with literature values lying in the range of 0.19-0.228 cm 2 s Ϫ1 . [13][14][15] The standard deviation of ␣ g is about 10% for the singlebeam method, but it is only approximately 1.3% for the dualbeam destructive interferometric measurement. The SNR and the measurement precision of thermal-wave interferometry using transparent exterior walls is comparable with results using a single-ended TWRC cavity with opaque external wall (ϳ1%).…”

“…1 Earlier attempts to eliminate this direct signal component from PPE spectroscopic investigations include lock-in quadrature detection. This technique has been successful in suppressing the direct transmitted PPE signal by up to two orders of magnitude, 14,15 but problems remain with uncompensated detector noise and signal drift from the lock-in IP to quadrature ͑Q͒ channel, especially related to the dc heating of the PVDF transducer.…”