1029 Effect of the modulation duty cycle on the amplitude of photoreflectance Esam Al-Arfaj, R. Glosser, Kambiz Alavi, and E.A. Beam III Abstract: We have observed the behavior of photoreflectance (PR) spectra from a set of a molecular-beam-epitaxy-grown heterostructures as a function of the modulation duty cycle. This was done over a range of frequencies extending from 10–1000 Hz. By varying the duty cycle of the pump beam at different frequencies, we are able to find the duty cycle that gave the largest amplitude. While standard PR techniques use a 50% duty cycle, we have found that at low frequencies (10–30 Hz) the optimal value is between 30–40% in these particular sets of experiments resulting in as much as a 33% increase in amplitude. The change in the optimal values of the duty cycle from sample to sample was qualitatively correlated to the deep levels sample. PACS Nos.: 78.20.Ci, 78.40., 78.40.Fy Résumé : Nous avons étudié le spectre du facteur de réflexion (PR) sur un ensemble d’hétérostructures, développées par faisceau moléculaire épitaxial (MBE), en fonction de la modulation du cycle d’opération. Ceci a été complété sur un domaine de fréquences allant de 10 à 1000 Hz. En variant la fréquence du cycle d’opération du faisceau pompe, nous sommes capables de trouver le cycle d’opération qui donne la plus grande amplitude. Alors que les techniques PR usuelles utilisent un cycle d’opération de 50 %, nous trouvons qu’à basse fréquence [10 - 30 Hz] la valeur optimale est entre 30 % et 40 % pour ces ensembles particuliers d’expériences, résultant en une augmentation allant jusqu’à 33 % en amplitude. La variation du cycle d’opération optimal d’un échantillon à l’autre était qualitativement correlé à la profondeur des niveaux. [Traduit par la Rédaction] Introduction Photoreflectance (PR), a contactless form of electromodulation, is a powerful tool for characterizing semiconductor materials [1]. Conventional PR is done with a pump beam, usually a laser, that is modulated either by a mechanical chopper or by an acousto-optic modulator (AOM). Shen et al. [2] among others [3–7] have shown PR to be a useful tool in studying the interface and surface charges that control the electric-field distribution in semiconductors. Received 4April 2005.Accepted 27 July 2005. Published on the NRC Research Press Web site at http://cjp.nrc.ca/ on 6 October 2005. E. Al-Arfaj 1,2 and R. Glosser. Department of Physics, University of Texas at Dallas, Richardson, TX 75083, USA. K. Alavi. Department of Electrical Engineering, University of Texas at Arlington, Arlington, TX 76019, USA. E.A. Beam III. TriQuint Semiconductors, Richardson, TX 75083, USA. 1 Corresponding author (e-mail: esaam@yahoo.com). 2 Present Address: Department of Physics, Umm-AlQura University, Makkah Saudi Arabia. Can. J. Phys. 83: 1029–1034 (2005) doi: 10.1139/P05-052 © 2005 NRC Canada