Picosecond Raman Studies of Electron-Phonon Interactions in the Wide Bandgap Semiconductor GaN
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the non-equilibrium phonon population with Raman scattering. The photoexcited electron-hole pair density was estimated to be 3xl016 cm- 3 from the fitting of time-integrated luminescence spectrum[4]. Polarized Raman scattering experiments were carried out in a variety of scattering geometries as specified below. The sample was kept in contact with a cold finger tip of a closecycled refrigerator. The temperature of the sample was estimated to be T _ 25K. The scattered signal was collected and analyzed with a standard Raman setup with a CCD detection system. EXPERIMENTAL RESULTS AND DISCUSSIONS Figs. l(a) and 2(a) show two polarized Stokes Raman spectra of a GaN sample taken at T=25K and in Z(X,X)Z, Y(X,X)Y scattering geometries, respectively; where X= (100), Y= (010) and Z= (001). Similar Raman spectra were reported by Azuhata et al.[5] at T= 300K. In Fig. l(a), the sharp peak around 757 cm- 1 comes from scattering of light by the Eg phonon mode of sapphire; the shoulder close to 741 cm- 1 belongs to the AI(LO) phonon mode; On the other hand, in Fig. 2(a), the sharp peak around 574 cm- 1 corresponds to the E 2 mode; the peak centered about 538 cm-1 represents the AI(TO) mode; the small structure around 422 cm- 1 is from the Alg mode of sapphire. 10-
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Fig. 1: Polarized Stokes Raman spectrum (a) and anti-Stokes Raman spectrum (b) for a GaN sample taken at T=25K, and in the scattering geometry Z(X,X)Z. For clarity, the anti-Stokes Raman signal has been multiplied by a factor of 50. The appearance of the A1 (LO) mode in the anti-Stokes spectrum is an indication of strong electron-LO phonon interaction in GaN.
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Fig. 2: Polarized Stokes Raman spectrum (a) and anti-Stokes Raman spectrum (b) for a GaN sample taken at T=25K and in a scattering geometry Y(X,X)Y. For clarity, the anti-Stokes signal has been multiplied by a factor of 10. The fact that no observable phonon modes in the anti-Stokes Raman spectrum suggests that neither E2 nor A,(TO) phonon modes interacts strongly with electrons.
E
K Fig. 3: A diagram showing how the strength of electron-phonon interactions can be probed by picosecond Raman spectroscopy. Here CB, VB represent conduction and valence bands, respectively.
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As depicted in Fig. (3), one way[6] to study electron-phonon interactions in semiconductors is to first generate electron-hole pairs with sufficient kinetic energy (>> ho)LO); the high energy electrons then relax to the bottom of the conduction band by emitting all types of high frequency phonons. By monitoring this thermalization process with Raman spectroscopy at very low temperatures, information such as the relative strength of the intera
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