Thermoelastic and optical properties of thick boride templates on silicon for nitride integration applications

We present a comparative study of the structural, thermoelastic, and optical properties of ZrB₂ films grown on silicon with the corresponding bulk ZrB₂ behavior. Thick ZrB₂ films (up to 500 nm) with device quality morphological and structural properties were grown on Si(111) for potential integration of GaN with Si substrates. HR-XRD was used to analyze the thickness and temperature dependence of the films' strain state. The data indicate that at room temperature a residual tensile strain of ∼0.5% persists in all samples independent of thickness. When the films are heated back to the growth temperature of 900 °C, two distinct behaviors are observed: thinner films (∼200 nm) follow the thermal expansion of the Si substrate, which results in a tensile strain at the growth temperature. Thicker films (∼400 nm) are fully relaxed at 900 °C and thus decoupled from the substrate. These strain behaviors imply that hybrid ZrB₂/Si(111) templates are better matched to GaN than any other known substrate. Comparison of the mismatch strains between sapphire, SiC, and bulk ZrB₂ substrates with GaN films over a broad temperature range (20-900 °C) illustrates the superior structural and thermal characteristics of hybrid ZrB₂/Si(111) templates for nitride integration. Measurements of the ZrB₂ dielectric function ε(ω) and its reflectivity R(ω) were conducted in the 0.2-7 eV range on thin films and compared for the first time with density functional theory simulations. The dielectric function displays a typical metallic Drude behavior across the wide IR range, with a reflectivity approaching unity at the operational wavelengths of GaN-based intersubband devices. The characteristic Drude plasma energy and lifetimes are compared with those obtained from transport measurements in the isostructural MgB₂ analogue. A detailed electronic structure analysis is also used to identify the interband transitions responsible for characteristic features observed at 2.5, 4.3, and 5.7 eV in the spectrum. Collectively our studies pave the way for understanding key optical and thermoelastic design parameters in novel conductive and reflective buffer layers for improved performance in nitride LEDs, fully integrated with silicon.