Abstract
The Segall-Mahan theory is employed to calculate both one and two longitudinal-optical (LO) phonon sidebands of free excitons of zinc oxide (ZnO) in a wide temperature range. The energy spacing from the zero-phonon line to 1 LO and 2 LO phonon sidebands deviates gradually from their characteristic LO phonon energy with increaseing the temperature. The experimental results are good agreement with the theoretical calculation. Only one adjustable parameter is taken into account in this calculation, which determines the range of values of the hole effective mass in ZnO. OCIS codes: 160.0160, 160.4670. doi: 10.3788/COL201008S1.0221. Zinc oxide (ZnO) has received much attention in recent years due to its unique physical properties and its potential technological applications, such as blue and ultraviolet (UV) light emitters and UV detectors [1−5] . Although significant investigation has been conducted on ZnO [6−11] , some fundamental problems regarding its optical properties remain unclear. To obtain a proper understanding of its optical properties, the excitonic state must be taken into account. Based on Green’s function method, the Segall-Mahan theory considered the effects of exciton-phonon interactions at all orders, including the line broadening, shift, and asymmetry [12−14] , and was adopted to calculate phonon-assisted radiative recombination of free excitons in ZnO. In this letter, we study theoretically and experimentally the temperature dependence of the phonon replica of free excitons in ZnO. The ZnO samples used study were commercial bulk crystals (Commercial Crystal Laboratories). A 325-nm He-Cd laser with approximate 35-mW output was employed to illuminate the Zn-terminated (0001) surface of the sample at a tilted angle of approximate 45 ◦ . The signal was collected by an SPEX 750M monochromator and amplified by standard lock-in techniques. In the Segall-Mahan theory, the absorption coefficient for the process involving one phonon or two phonons can be calculated by [12−14]
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