Eric P. Carlson

OPTICAL ACTIVATION OF BE IMPLANTED INTO GAN

Single crystalline (0001) gallium nitride layers were implanted with beryllium. Photoluminescence (PL) measurements were subsequently performed as a function of implantation dose and annealing temperature. One new line in the PL spectra at 3.35 eV provided strong evidence for the presence of optically active Be acceptors and has been assigned to band–acceptor (eA) recombinations. The determined ionization energy of 150±10 meV confirmed that isolated Be has the most shallow acceptor level in GaN. Co-implantation of nitrogen did not enhance the activation of the Be acceptors.

X-ray photoelectron spectroscopy analysis of GaN/(0001)AlN and AlN/(0001)GaN growth mechanisms

The mechanisms of growth of GaN on AlN and AlN on GaN via gas source-molecular beam epitaxy with NH3 as the nitrogen source have been investigated using x-ray photoelectron spectroscopy, low energy electron diffraction, and Auger electron spectroscopy. The growth of GaN on AlN at low temperatures (650–750 °C) occurs via a Stranski–Krastanov 2D→3D type mechanism with the transition to 3D growth occurring at ≈10–15 A. The mechanism changes to Frank van der Merwe (FM)/layer-by-layer growth above 800 °C. The growth of AlN on GaN occurred via a FM layer-by-layer mechanism within the 750–900 °C temperature range investigated. We propose a model based on the interaction of ammonia and atomic hydrogen with the GaN/AlN surfaces which indicates that the surface kinetics of hydrogen desorption and ammonia decomposition are the factors that determine the GaN growth mechanism.

X-ray and Raman analyses of GaN produced by ultrahigh-rate magnetron sputter epitaxy

Thick films of GaN were studied by x-ray diffraction and Raman spectroscopy. The GaN thick films were deposited on (0001) sapphire using ultrahigh-rate magnetron sputter epitaxy with typical growth rates as high as 10–60 μm/min. The width of the x-ray rocking curve from the (0002) reflection for the sample produced by this technique is ∼300 arcsec, which is unprecedented for GaN produced by a sputtering-type process. Our recent sample shows an x-ray rocking curve width of 240 arcsec. Only allowed modes were observed in the polarized Raman spectra. The background free carrier concentration is lower than 3×1016 cm−3. The phonon lifetime of the Raman E2(2) mode of the sputtered GaN was comparable to that of bulk single crystal GaN grown by sublimation. The quality of the film was uniform across the wafer. The film was thermally stable upon annealing in N2 ambient. The x-ray and Raman analyses revealed that the sputtered GaN films are of high crystalline quality.

Ohmic contact formation to doped GaN

Ohmic contact strategies for n- and p-type GaN have been investigated electrically, chemically, and microstructurally using transmission line measurements, high-resolution EELS and cross-sectional TEM, respectively. The contributions to contact performance from work function differences, carrier concentrations, annealing treatments, and interface metallurgy have been examined. The contact materials of Ti, TiN, Au, and Au/Mg were deposited via electron beam evaporation; Al was deposited via thermal evaporation. As-deposited Al and TiN contacts to highly doped n-GaN were ohmic, with room-temperature specific contact resistivities of 8.6×10 −5 Ω cm 2 and 2.5×10 −5 Ωcm 2 respectively. The Ti contacts developed low-resistivity ohmic behavior as a result of annealing; TiN contacts also improved with further heat treatment. For p-GaN, Au became ohmic with annealing, while Au/Mg contacts were ohmic in the as-deposited condition. The performance, structure, and composition of different contact schemes varied widely from system to system. An integrated analysis of the results of this study is presented below and coupled with a discussion of the most appropriate contact systems for both n- and p-type GaN.

Process Routes for Low Defect-Density GaN on Various Substrates Employing Pendeo-Epitaxial Growth Techniques

GaN films have been grown on 6H-SiC substrates employing a new form of selective lateral epitaxy, namely pendeo-epitaxy. This technique forces regrowth to start exclusively on sidewalls of GaN seed structures. Both discrete pendeo-epitaxial microstructures and coalesced single crystal layers of GaN have been achieved. SEM and TEM analysis are used to evaluate the morphology of the resulting GaN films. Process routes leading to GaN pendeo-epitaxial growth using silicon substrates have also been achieved and the preliminary results are discussed.

Photoluminescence characterization of Mg implanted GaN

Single crystalline (0001) gallium nitride layers, capped with a thin epitaxial aluminum nitride layer, were implanted with magnesium and subsequently annealed in vacuum to 1150--1300 C for 10--60 minutes. Photoluminescence (PL) measurements showed the typical donor acceptor pair (DAP) transition at 3.25 eV after annealing at high temperatures, which is related to optically active Mg acceptors in GaN. After annealing at 1300 C a high degree of optical activation of the implanted Mg atoms was reached in the case of low implantation doses. Electrical measurements, performed after removing the AlN-cap and the deposition of Pd/Au contacts, showed no p-type behavior of the GaN samples due to the compensation of the Mg acceptors with native n-type defects.

Optical characterization of high quality GaN produced by high rate magnetron sputter epitaxy

The thick films of GaN were investigated using X-ray diffraction, micro-Raman spectroscopy and photoluminescence spectroscopy. The thick films of GaN were prepared on (0001) sapphire using high rate magnetron sputter epitaxy with growth rates as high as 10–60 m/min. The width of the X-ray rocking curve ((0002) reflection) for the sample produced by this method is ∼300 arc-sec. Only the allowed modes were observed in the polarized Raman spectra. The background electron concentration is lower than 3×10 16 cm −3 , which was determined from the Raman spectra. The phonon lifetime determined from Raman E 2 (2) mode was 1.6 ps, which is comparable to that of bulk single crystal GaN grown by sublimation (1.4 ps). The full-width-at-half-maximum of the near band-edge photoluminescence peak obtained at 77K is ∼100 meV.

Pendeo-epitaxial growth of GaN on SiC and silicon substrates via metalorganic chemical vapor deposition

Pendeo-epitaxial lateral growth (PE) of GaN epilayers on (0001) 6H-silicon carbide and (111) Si substrates has been achieved. Growth on the latter substrate was accomplished through the use of a 3C-SiC transition layer. The coalesced PE GaN epilayers were characterized using scanning electron diffraction, x-ray diffraction and photoluminescence spectroscopy. The regions of lateral growth exhibited {approximately} 0.2{degree} crystallographic tilt relative to the seed layer. The GaN seed and PE epilayers grown on the 3C-SiC/Si substrates exhibited comparable optical characteristics to the GaN seed and PE grown on 6H-SiC substrates. The near band-edge emission of the GaN/3C-SiC/Si seed was 3.450 eV (FWHM {approximately} 19 meV) and the GaN/6H-SiC seed was 3.466 eV (FWHM {approximately} 4 meV).

Selection, Growth, and Characterization of Gate Insulators on Mocvd Gallium Nitride for the Use in High Power Field Effect Devices

Metal-insulator-semiconductor (MIS) capacitors have been fabricated on n-type GaN (0001) films using thermally grown Ga 2 O 3 , remote plasma enhanced chemical vapor deposited (RPECVD) SiO 2 , and molecular beam epitaxy (MBE) AIN as the gate insulator and Al as the gate electrode. Each GaN epitaxial layer was grown by organometallic chemical vapor deposition (OMCVD) on a 6H-SiC(0001) substrate on which was previously deposited a 1000A buffer layer of AIN. Nitrogen-free polycrystalline films of Ga 2 O 3 were grown on the GaN. Capacitancevoltage measurements of capacitors fabricated from this oxide showed distinct depletion and accumulation regions with significant leakage. The AIN and SiO 2 capacitors demonstrated better electrical characteristics than the Ga 2 O 3 because of lower leakage. The RPECVD SiO 2 /GaN heterostructures, in particular, showed good agreement with the curves calculated for an ideal oxide and a small amount of hysteresis.