We demonstrate that two-photon spectroscopy is a powerful tool to study the intrinsic optical, interfacial and electronic properties of thin film semiconductors. The emission properties of CdS films formed by close-spaced vapor transport (CSVT) on calcium fluoride, by spray pyrolysis on pyrex, and by pulsed-laser deposition (PLD) on glass were investigated. The films were excited with ultrashort (200 fs) laser pulses at 1.54 eV at room temperature. Though the impinging photon energy is far below the bandgap of CdS (2.45 eV), the excitation caused green bandgap emission due to two-photon absorption. Notably, the emission revealed the intrinsic properties of the films independent of preparation method, doping and substrate. The influence of the substrate/CdS interface on the emission was probed by comparing the spectra measured at face and rear. The PLD films revealed a clear dependence on the experimental geometry by shifting the back emission 40 meV towards lower energies with respect to the front emission. In contrast to this, the emission of films formed by spray pyrolysis and CSVT did not show geometry dependence. Additionally, we show that two-photon spectroscopy is capable of probing bandgap shrinkage. The knowledge of the latter is very useful for the design of laser cavities.

Date Published: 21 May 2002
PDF: 8 pages
Proc. SPIE 4650, Photodetector Materials and Devices VII, (21 May 2002); doi: 10.1117/12.467667

Published in SPIE Proceedings Vol. 4650:
Photodetector Materials and Devices VII
Gail J. Brown; Manijeh Razeghi, Editor(s)