Dynamics of LO phonons in InN studied by subpicosecond time-resolved Raman spectroscopy
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Dynamics of LO phonons in InN studied by subpicosecond timeresolved Raman spectroscopy K.T. Tsen Department of Physics, Arizona State University, Tempe, AZ 85287
ABSTRACT The phonon dynamics of both the A1 ( LO) and the E1 ( LO) phonons in InN has been studied by timeresolved Raman spectroscopy on a subpicosecond time scale. From the temperature-dependence of their lifetimes, we demonstrate that both phonons decay primarily into a large wavevector TO phonon and a large wavevector TA/LA phonon consistent with the accepted phonon dispersion relationship for wurtzite InN. Their lifetimes have been found to 17 −3 to 0.25 ps, at the highest density decrease from 2.2 ps, at the low electron-hole pair density of 5 x10 cm 19
−3
of 2 x10 cm . Our experimental findings demonstrate that carrier-density dependence of LO phonon lifetime is a universal phenomenon in polar semiconductors. Key words: InN, time-resolved Raman spectroscopy; LO phonon lifetimes
1. INTRODUCTION The accumulation of a large non-equilibrium phonon population results partly from an efficient electron-phonon coupling and partly from a relatively long phonon lifetime. The lifetime of LO phonons in semiconductors will therefore play an important role in determining the performance of an electronic device. Indium nitride (InN),Gallium nitride (GaN), aluminum nitride (AlN) and their alloys have long been considered to be very promising materials for device applications [1-3]. Recent progress in material growth techniques has led to the production of high-quality single-crystal InN layers [4-8]. Monte Carlo simulations [9-11] have indicated that InN exhibits the highest peak overshoot velocity and that the velocity overshoot in InN lasts for the longest distance among the nitride-based materials. This prediction is in fact confirmed by Liang et al [12-14]. in subpicosecond transient Raman measurements of electron transport in an extremely high quality InN thick layer grown on GaN. It is well known [15] that the presence of non-equilibrium longitudinal optical (LO) phonons will significantly reduce the electron drift velocity in electronic devices. In this paper, we report direct measurements of the lifetimes of both the A1 ( LO) and the E1 ( LO) phonons in a high quality InN thick film grown on GaN by using sibpicosecond time-resolved Raman spectroscopy. Our experimental results demonstrate that both phonons have a lifetime of τ = ( 2.5 ± 0.2) ps at T = 10K. In addition, our data suggest that these LO phonons decay primarily into a large wavevector TO phonon and a large wavevector TA/LA phonon. New developments in crystal growth technique have recently produced high quality, single crystal InN that has a bandgap of about 0.7 eV at room temperature [4-8]. This indicates that its ternary compound In x Ga1− x N not only has great potential for white light generation but also is suitable for applications in solar cells. InN also has been theoretically predicted [9-12] and experimentally demonstrated [12,13] to have enormously large transient electr
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