Effects of thickness and interlayer on optical properties of AlN films at room and high temperature
Electrical and Computer Engineering
This paper investigates the systematic influence of thickness (136-412 nm) and temperature (300-860 K) on the refractive index and the band-edge of aluminum nitride (AlN) films. The combination of x-ray diffraction, spectroscopic ellipsometry (SE), and transmittance measurements at 300 K shows that the increase of epilayer thickness or the introduction of an AlN interlayer can improve the crystal quality. This is observed as an enlargement of the grain size, a reduction of the Urbach binding energy, and strain with a corresponding increase in the refractive index and bandgap. Moreover, the expected reduction in the bandgap and the increase of the refractive index are observed at elevated temperatures by SE. The temperature dependence of the refractive index at 632.8 nm and the bandgap were well understood and modeled using a quadratic nonlinear equation and the Bose-Einstein equation, respectively. This high-temperature phenomenological and quantitative analysis suggests that the reduction of the bandgap with temperature is more significant in thinner or noninterlayer films than expected due to the corresponding stronger electron-phonon interactions involved with larger Urbach binding energies. The thickest AlN film in this work (with an epilayer thickness of 412.9 nm and an interlayer thickness of 20.69 nm) has the smallest strength of the average electron-phonon coupling (407 meV) in the temperature range 300-860 K. On the other hand, the temperature-dependent variation of the refractive index in the transparent region is more rapid as the film thickness decreases owing to the high correlation between temperature-dependent bandgap and refractive index. These observations are critical when designing AlN-based device structures that can operate well above room temperature.
Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films
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