Time-Resolved Reflectivity Studies of Electric Field Effects in III-Nitride Semiconductors
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Time-Resolved Reflectivity Studies of Electric Field Effects in III-Nitride Semiconductors M. Wraback1, H. Shen1, A.V. Sampath1, C.J. Collins1, G.A. Garrett1, W.L. Sarney1, Y. Fedyunin2, J. Cabalu2, and T.D. Moustakas2 1 U.S. Army Research Laboratory, Sensors and Electron Devices Directorate, AMSRD-ARL-SEEM, 2800 Powder Mill Road, Adelphi, MD 20783 2 ECE Department, Photonics Center, Boston University, 8 St. Mary’s Street, Boston MA, 02215 ABSTRACT Comparison of room temperature, frequency degenerate and nondegenerate femtosecond pump-probe reflectivity measurements reveal an additional component in the transient reflectivity (∆R) decays for near bandgap probe in GaN that is attributed to a reduction of bandedge broadening due to screening of internal electric fields by photogenerated carriers. Pump-probe measurements with pulses spectrally centered at 363 nm show a ∆R component with a nanosecond decay time that is essentially absent from data obtained from nondegenerate experiments performed under identical pumping conditions, but with a probe at 385 nm far enough from the bandedges to minimize the electric field contribution and monitor primarily the carrier lifetime in the band states. The observation of significantly shorter decay times for 385 nm probe suggests that the carriers leave the band states on a picosecond time scale, but the slow decay of the field screening observed with 363 nm probe implies that they recombine from trap states at longer times. In addition, this field screening provides a mechanism for generation and detection of strain pulses in strained GaN and AlGaN epilayers that is used to measure the longitudinal sound velocity in bulk GaN and AlGaN with up to 0.4 Al content. INTRODUCTION Time-resolved optical spectroscopy has proven to be a valuable tool in the study of carrier dynamics and evaluation of material quality in nitride semiconductors. Transient optical data in bulk wurtzite, c-plane-oriented material may be complicated, however, by the presence of electric fields associated with surface band bending [1], spontaneous polarization effects [2], and piezoelectric contributions related to incomplete relaxation of interface strain [3], in conjunction with a shallow carrier generation depth (≤0.1 µm) due to strong resonant absorption of the incident light pulses. Such an electric field would produce a broadening of the bandedges due to electroabsorption effects, and screening of the field by photogenerated electron-hole pairs can cause a reduction in this broadening that is manifested optically as photoinduced bleaching of the probe absorption [4]. Since the generation of the electron-hole pairs also creates bleaching due to band filling effects, it is difficult to distinguish these two phenomena and their time evolution for degenerate pump-probe experiments employing pulses with near bandgap photon energies. In this paper, we show that a combination of degenerate and nondegenerate pump-probe reflectivity experiments in GaN allows one to separate the electric field screening co
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