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Electric Field Induced Heating and Energy Relaxation in GaN T. A. Eckhause, Ö. Süzer, Ç. Kurdak Department of Physics, University of Michigan, Ann Arbor, MI 48109 F. Yun, and H. Morkoç Department of Electrical Engineering, Virginia Commonwealth University, Richmond, VA 23284 ABSTRACT We report results of an investigation of electric field induced heating at low temperature in GaN 3-dimensional electron gas films grown on sapphire substrates. The excess noise of the electron gas in a patterned GaN film, while the substrate is held at low temperature, is used to determine the electron temperature. We calculate the rate of power dissipation and compare our results with a calculation of acoustic deformation potential scattering processes in GaN. We discuss the existence of a thermal boundary resistance between the GaN film and the sapphire substrate. INTRODUCTION The considerable interest in GaN and GaN-based heterostructures for use in green through ultra-violet, and high-power applications[1,2] has motivated our study of energy relaxation processes in GaN. The emission of acoustic and optical phonons by a hot thermalized electron gas sets the energy relaxation time, which in turn determines the characteristic time for hot electrons to relax to the lattice temperature. At low temperatures we expect the dominant method of cooling to be the emission of acoustic phonons through acoustic deformation potential scattering. At higher temperatures (above 100 K) we expect the faster emission of polar optical phonons to dominate electron cooling. Knowledge of the energy relaxation time is of fundamental importance and is also relevant to device design. In high power devices understanding the processes that govern the cooling of hot carriers is useful in assessing device performance. Measurements of luminescence [3], measurements of white noise[4, 5], Subnikov-de Haas measurements [6], and blackbody radiation measurements [7] are commonly used to determine the temperature of an electron gas in excess of the lattice temperature. We have used the high frequency (up to 100 kHz) Johnson noise as an intrinsic thermometer of the electron temperature, while the substrate was held in a 4.2 K bath. In addition to the study of excess high frequency noise, we have examined low frequency, or 1/f noise [8], and compared their value between samples with different electron densities. The microscopic origin of 1/f noise in this material is not well understood. EXPERIMENT We begin with two 3-dimensional electron gases (3DEGs) grown by molecular beam epitaxy on sapphire substrates. We will discuss two samples, A and B. Sample A is a high electron density 3DEG GaN film grown on c-plane sapphire substrate, with a 40 nm AlN buffer layer. The 1.1 µm thick film is intentionally doped with Si. Sample B,

I11.25.1

Sample A Sample B

Electron Density 4.75 x 1018 cm-3 4.0 x 1017 cm-3

Mobility (cm2/V.s)

Rsquare (Ω)

Thickness

89.0 143

217 1817

0.68 µm 0.60 µm

Table 1: The characteristics of the high and density samples, A and B respectively. The den