Picosecond Photoinduced Reflectivity Studies of GaN Prepared by Lateral Epitaxial Overgrowth
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CT The pump-probe technique has been used to perform room temperature studies of the photoinduced changes in the reflectivity ∆R associated with exciton and carrier dynamics in GaN prepared by lateral epitaxial overgrowth. For resonant excitation of cold excitons, the ∆R decay possesses a 720 ps component attributed to the free exciton lifetime in this high quality material. For electrons with small excess energy (< 50 meV), the strong increase in the ∆R decay rate with decreasing excitation density suggests that screening of the Coulomb interaction may play an important role in the processes of carrier relaxation and exciton formation. The faster decay times at a given carrier density observed for hot (> 100 meV) electron relaxation are attributed to electron-hole scattering in conjunction with the screened electron-LO phonon interaction. INTRODUCTION Gallium Nitride (GaN) has become an important material for ultraviolet light emitters and detectors, as well as high power, high frequency electronic devices. It has been demonstrated that growth of GaN on sapphire by lateral epitaxial overgrowth (LEO) [1-3] greatly reduces the threading dislocation density in this material. A device lifetime of more than 10000 h at room temperature has been reported for cw operation of InGaN multiple quantum well laser diodes employing LEO GaN grown by metalorganic chemical vapor deposition (MOCVD) [1], and a significant decrease of p-n junction reverse leakage current in LEO materials has also been observed [3]. However, little is known about the room temperature dynamics of resonantly created excitons and free carriers with low excess energy crucial to an understanding of device performance in this high quality GaN. In this paper we present a time-resolved pump-probe study of exciton and free carrier dynamics in LEO GaN in which photoinduced changes in reflectivity ∆R are monitored on a picosecond timescale. The ∆R transients were obtained as a function of excitation intensity for three cases: (i) resonantly created excitons; (ii) electrons with low excess energy; and (iii) electrons with sufficient excess energy to emit longitudinal optical (LO) phonons. The data obtained from these measurements provides information about exciton lifetimes in this high quality material, the dynamics of exciton formation and screening in the presence of cold electron-hole pairs, and the cooling of a hot electron distribution due to carrier-carrier scattering and the partially screened electron-LO phonon interaction.
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EXPERIMENTAL CONSIDERATIONS The sample employed in this study was a ~5 µm thick undoped GaN LEO film on an undoped GaN/sapphire substrate. 15 µm-wide SiO2 stripes deposited by plasma enhanced chemical vapor deposition (PECVD) were patterned with 3 µm windows between the stripes. The growth of the LEO GaN upon this material was accomplished by rotating-disk MOCVD at a growth pressure of 100 torr and a growth temperature of 1070° C. Visible/near infrared laser pulses derived from the signal beam of a 250 kHz Ti:sapphire regene
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