Understanding the thermal influence on gallium nitride (GaN) single crystal substrates is pivotal for enhancing GaN-based optoelectronic devices. Recent research has comprehensively characterized how these substrates perform at elevated temperatures, shedding light on their dielectric function and thermo-optic coefficients across significant spectral ranges.
Gallium nitride (GaN) is highly valued for its wide bandgap, exceptional thermal conductivity, and low dielectric constant, facilitating its use in various applications such as power electronics and optoelectronic devices. Notably, the introduction of silicon doping has been instrumental, yielding N-type GaN and optimizing its electronics performance. With operating temperatures extending up to 600 °C and beyond for some applications, fully grasping GaN substrates' optical properties under thermal stress is imperative.
Traditional studies have largely concentrated on GaN thin films, which have inherent challenges, including defects from thermal mismatches. This study aims to fill the knowledge gap concerning GaN single crystal substrates, particularly how their optical properties evolve with temperature, making strides toward more efficient device designs.
Using spectroscopic ellipsometry, the researchers analyzed GaN substrates across the wavelength range of 250 nm to 1600 nm at temperatures between 298 K and 873 K. This method allowed them to isolate the intrinsic optical behaviors of GaN by eliminating issues associated with the thin film's characteristics.
The results revealed systematic temperature-dependent behaviors of the dielectric function, as the exciton transitions modified significantly with rising temperatures. Interestingly, the empirical Varshni expression could effectively describe the exciton transitions, demonstrating the complex interplay of thermal effects on the material's optical characteristics.
Crucially, findings indicated for the first time the thermo-optic coefficients across the spectrum, parameterized using the Sellmeier model. This analytic approach provides valuable insights, significantly advancing the existing GaN optical properties knowledge and offering guidance for optimizing device designs.
Research showed consistent behaviors of the dielectric function, with notable shifts attributed to lattice vibrations (phonons) and thermal expansion. Changes within the optical spectrum illustrated how increasing temperatures resulted in bandgap shrinkage, influencing both dielectric functions and refractive indices.
The linearity observed in the thermo-optic coefficients, particularly from 400 nm to 1600 nm, indicates predictable optical responses important for device engineering. The findings highlighted the distinct characteristics of GaN substrates compared to thin films—they display regular and less extreme temperature dependencies. This characteristic may offer improved performance and reliability for devices operating at high temperatures.
Overall, the comprehensive assessment of the dielectric function and thermo-optic coefficients of silicon-doped GaN substrates contributes to developing next-generation optoelectronic devices, fostering enhanced performance and efficiency.