Thermoelastic and Optical Properties of Thick Boride Templates on Silicon for Nitride Integration Applications R. Roucka, † V. R. D’Costa, ‡ Y.-J. An, † M. Canonico, § J. Kouvetakis, † J. Menéndez, ‡ and A. V. G. Chizmeshya* ,† Department of Chemistry and Biochemistry, Arizona State UniVersity, Tempe, Arizona 85287; Department of Physics, Arizona State UniVersity, Tempe, Arizona 85287; and Freescale Semiconductor, Tempe, Arizona 85284 ReceiVed September 6, 2007. ReVised Manuscript ReceiVed NoVember 19, 2007 We present a comparative study of the structural, thermoelastic, and optical properties of ZrB 2 films grown on silicon with the corresponding bulk ZrB 2 behavior. Thick ZrB 2 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 ZrB 2 /Si(111) templates are better matched to GaN than any other known substrate. Comparison of the mismatch strains between sapphire, SiC, and bulk ZrB 2 substrates with GaN films over a broad temperature range (20–900 °C) illustrates the superior structural and thermal characteristics of hybrid ZrB 2 /Si(111) templates for nitride integration. Measurements of the ZrB 2 dielectric function ǫ(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 MgB 2 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. 1. Introduction Transition-metal diborides such as ZrB 2 , TaB 2 , and HfB 2 are of great interest because they share a layered hexagonal structure with MgB 2 , the high-temperature superconductor discovered in 2001, 1 and also because their lattice parameters are very close to those of AlN and GaN, which makes them attractive as substrates for the growth of nitride semiconduc- tors. 2 Our prior work in this area has demonstrated proof of principle growth of pure ZrB 2 layers and HfB 2 /ZrB 2 hetero- structures on Si(111) and provided a preliminary account of technologically relevant optical and structural properties. Additionally, we have obtained compositional and lattice constant tuning by growth of Zr 1-x Hf x B 2 alloy layers directly on Si(111), and this raises opportunities of developing unique nitride device technologies on cheap Si substrates. 3,4 The design of structures based on boride layers requires a thorough understanding of their thermoelastic behavior, including parameters such as critical thickness and thermal expansion of ZrB 2 /Si(111) films. Accordingly, we have concentrated our efforts to develop methodologies that lead to routine and reproducible growth of thick ZrB 2 /Si(111) films (up to 500 nm). These samples were then utilized to determine the temperature dependence of the lattice param- eters from ambient up to the growth temperature of 900 °C, which overlaps with the lower range for nitride deposition. In our prior work we examined the dependence of room temperature lattice dimensions on composition for Hf x Zr 1-x B 2 alloys 4 and found that a constant residual tensile strain remains after cooling from 900 °C. We have also demon- * To whom correspondence should be addressed. † Department of Chemistry and Biochemistry, Arizona State University. ‡ Department of Physics, Arizona State University. § Freescale Semiconductor. (1) Nagamatsu, J.; Nakagawa, N.; Muranaka, T.; Zenitani, Y.; Akimitsu, J. Nature (London) 2001, 410, 682463. (2) Kamiyama, S.; Takanami, D.; Tomida, Y.; Iida, K.; Kawashima, T.; Fukui, S.; Iwaya, M.; Kinoshita, H.; Matsuda, T.; Yasuda, T.; Otani, S.; Amano, H.; Akasaki, I. Phys. Status Solidi A 2003, 200, 67–70. (3) (a) Hu, C.-H.; Chizmeshya, A. V. G.; Tolle, J.; Kouvetakis, J.; Tsong, I. S. T. J. Cryst. Growth 2004, 267, 554–563. (b) Tolle, J.; Kouvetakis, J.; Kim, D.-W.; Mahajan, S.; Bell, A.; Ponce, F. A.; Tsong, I. S. T.; Kottke, M. L.; Chen, Z. D. Appl. Phys. Lett. 2004, 84, 3510–3512. (4) Roucka, R.; An, Y.-J.; Chizmeshya, A. V. G.; Tolle, J.; Kouvetakis, J.; D’Costa, V. R.; Menendez, J.; Crozier, P. Appl. Phys. Lett. 2006, 89, 242110. 1431 Chem. Mater. 2008, 20, 1431–1442 10.1021/cm702547p CCC: $40.75  2008 American Chemical Society Published on Web 01/05/2008