Daniel A. Gulino

Residual stress in GaN epilayers grown on silicon substrates

GaN films on AlN buffer layers were grown on (111) silicon substrates in a vertical rotating disk metal–organic chemical-vapor deposition reactor (CVD, Inc.). The dependence of residual stress in GaN films on the V/III molar flow ratio was studied. The crystalline quality of GaN films was analyzed by x-ray diffractometry, and the x-ray lattice parameter method was used to determine the residual stress in the films by measuring the c-axis and a-axis strain separately. The x-ray results show that the residual stress decreases with an increasing V/III molar flow ratio if all other growth parameters are kept constant.

Residual stress in GaN films grown by metalorganic chemical vapor deposition

We have studied the growth of gallium nitride (GaN) on (0001) sapphire substrates in a commercial (CVD Equipment Corp.) metalorganic chemical vapor deposition reactor. High quality epitaxial GaN films were obtained by deposition at 1000 °C from trimethylgallium and ammonia precursors (1.5 slpm total flow rate with V/III molar flow ratio of 5800) on a 24-nm-thick GaN buffer layer grown at 500 °C. X-ray rocking-curve measurements of films grown under these conditions showed a full width at half maximum of 0.28°. The x-ray lattice parameter method was used to determine the residual stress in GaN films by measuring the c-axis and a-axis strains separately. The results indicate that residual stress in GaN film grown on sapphire is compressive and is reduced as the buffer layer thickness increases.

Dual-layer coatings for polyimide composite thermo-oxidative stability

Abstract Dual-layer coatings consisting of layers of both aluminum oxide and silicon dioxide have been deposited by plasma-enhanced chemical vapor deposition onto polymide-based composites for thermo-oxidative protection. The silica was deposited from tetraethoxysilane, and the alumina was deposited from aluminum triisopropoxide. Deposition temperatures ranged between 573 and 598 K, and film thicknesses ranged from 100 to 200 nm. In all cases, alumina was deposited first, followed by the silica. Results indicate that such coatings improve the stability of these composites at elevated temperatures (to 673 K) for up to 300 h exposure to ambient air at 101.3 kPa (1 atm). Film systems with an alumina layer of intermediate thickness (150 nm) gave the greatest degree of resistance to thermo-oxidative mass loss (about half the mass loss rate of an uncoated control sample). Comparisons with earlier work with silica-only coatings showed the silica coating to be the best of all coatings studied, but there is considerable variation in the batch-to-batch performance of uncoated composite.

Effects of Susceptor Geometry on GaN Growth on SI(111) with a New Mocvd Reactor

High quality GaN films on AIN buffer layers were grown on Si(111) with a new, commercial, two-injector vertical rotating disk MOCVD reactor (CVD, Inc.). It was found that the geometry of the susceptor greatly affected the structural quality of the epilayers on Si. For the original susceptor geometry, though single crystal GaN films could be obtained, the films were dark gray in appearance with a rough morphology, and the best x-ray rocking curve FWHM was 2.33°. After modifying the susceptor geometry, transparent, mirror-like single crystal GaN films were obtained with the best x-ray rocking curve FWHM being 0.24°. Photoluminescence (PL) and infrared reflectance (IR) spectra of the grown films were compared. The film growth rate was found to increase with decrease of the growth pressure. A 2-D simulation of the flow, heat transfer, and chemical species transport in the reactor showed a more symmetric flow, larger velocity gradient, and lower upward velocity with the modified susceptor, which may be the main reason for the improvement of the structural quality of the films.