Thermoelastic and Optical Properties of Thick Boride Templates on Silicon for Nitride Integration Applications

Abstract

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.

Comparative study of the electronic structure, phonon spectra, and electron-phonon interaction of ZrB2and TiB2

“…[22][23][24] Figure 4 shows the experimentally measured photoemission (PES) 28 and x-ray photoemission (XPS) 27 spectra of ZrB 2 compared with the calculated energy distribution of total DOS. The calculated DOS has been broadened to account for life-time effects and for the experimental resolution.…”

“…Experimentally the electronic structure of ZrB 2 and TiB 2 has been investigated by means of photoemission spectroscopy, 27,28 point contact spectroscopy, 33 x-ray absorption spectroscopy, 28,59 and optical spectroscopy. [22][23][24] Figure 4 shows the experimentally measured photoemission (PES) 28 and x-ray photoemission (XPS) 27 3). The major peak a close to the Fermi energy is derived by Zr 4d states.…”

“…27,28,59. The optical spectra of ZrB 2 was investigated experimentally by several authors, [22][23][24] however, there is neither theoretical investigation of the x-ray absorption spectra or the optical properties of the transition metal diborides.…”

“…Presently, a number of experimental studies exist dealing with the physical properties of ZrB 2 and TiB 2 such as electric transport properties, 7,[13][14][15][16][17] the de Haas-van Alphen (dHvA) measurements of the Fermi surface, 18-21 optical [22][23][24] and electron-energyloss spectra, 25,26 x-ray absorption and photoemission spectra, 27,28 magnetic susceptibility 29,30 and NMR measurements, 31 the phonon density of states, 32 and electron-phonon interaction. 32,33 First-principles calculations of the electronic structure of diborides have been widely presented.…”

Infrared spectroscopic ellipsometry study of sulfur-doped In0.53Ga0.47As ultra-shallow junctions

“…20 Since the infrared wavelengths are not sensitive to the surface, the thickness of the surface layer was kept fixed at $2 ffiffi ffi 2 p times the RMS value from AFM measurements. 21 We found that neglecting the surface layer from the model introduces an error of up to 20% in thickness and resistivity values of $3-4 nm shallow junctions. As a result, up to 6% error in R s can be observed.…”