The Lifetime of Polar-Optical Modes in Semiconductors
- PDF / 188,303 Bytes
- 12 Pages / 612 x 792 pts (letter) Page_size
- 48 Downloads / 165 Views
1221-CC06-05
The Lifetime of Polar-Optical Modes in Semiconductors Brian. K. Ridley1, Angela Dyson2 1 2
School of Computing Science and Electronic Engineering, University of Essex, Colchester, UK. Department of Physics, University of Hull, Hull, UK.
ABSTRACT An enduring problem in the engineering of high-power semiconductor devices is how to mitigate the effect of heating. Heating means the proliferation of phonons, and phonons, interacting with electrons directly affect the electronic performance of the device. Nowhere is this more evident than the role of hot polar-optical phonons in reducing the drift velocity in the channel of an HFET and hence reducing its performance at high frequencies. The task of describing hotphonon effects is complicated by the coupling to plasma modes. We present a theory of coupled plasmon-phonon modes in GaN, how they interact with electrons and how their lifetime becomes density-dependent. Raman scattering in bulk material shows a reduction of lifetime with increasing density and we offer an explanation for this in terms of the frequency dependence of the anharmonic decay mechanism. Hot-phonon effects, however, involve modes with wavevectors beyond those probed by Raman scattering. By adopting a single-pole approximation for these modes we have obtained the lifetime dependence on wave vector, electron temperature and density.
1. INTRODUCTION The creation of a large non-equilibrium population of phonons by hot electrons is a wellknown and well-studied phenomenon in both bulk and quasi-2D semiconductors [1]. Its principal effect is to reduce the rate of loss of electron energy to the lattice and thence to the thermal reservoir. This reduction is caused by the increased probability of the electron absorbing a phonon. When the effect is strong, the phonon temperature approaches and may equal the electron temperature, and when this happens the rate of loss of energy is entirely determined by the lifetime of the phonon. Thus, the phonon lifetime becomes a critical parameter in the theory of hot-phonon effects, ass important as the emission and absorption rates associated with the electron-phonon interaction. Awareness of its importance for III-nitride devices has stimulated the measurement of Raman-scattering data for its dependence on lattice temperature in AlN [2], GaN [3] and InN [4]. The hot-phonon effect is naturally associated with a high density of electrons. This means that, where the phonons involved are the polar LO modes (the usual case in practice), the role of the plasma modes of the electron gas cannot be neglected. It is well-known that the electronphonon interaction must be described in terms of coupled plasmon-phonon modes, which results in the bare interaction being screened or anti-screened, depending on wave-vector[5].
Most accounts of hot-phonon effects contain the assumption of a phonon lifetime that is simply determined by lattice-anharmonic interactions at the lattice temperature., and therefore a constant. That this assumption is incorrect has been amply shown
Data Loading...
