Anthony Rice

Surface preparation and homoepitaxial deposition of AlN on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition

Chemical surface treatments were conducted on mechanically polished (MP) and chemomechanically polished (CMP) (0001)-oriented single crystalline aluminum nitride (AlN) substrates to determine a surface preparation procedure for the homoepitaxial deposition of AlN epitaxial layers by metalorganic chemical vapor deposition. MP AlN substrates characterized by atomic force microscopy exhibited 0.5 nm rms roughness and polishing scratches, while CMP AlN substrates exhibited 0.1 nm rms roughness and were scratch-free. X-ray photoelectron spectroscopy analysis of MP and CMP AlN substrates indicated the presence of a surface hydroxide layer composed of mixed aluminum oxide hydroxide and aluminum trihydroxide. Wet etching with sulfuric and phosphoric acid mixtures reduced the amount of surface hydroxide. Ammonia annealing at 1250 °C converted the substrate hydroxide layer to AlN and increased the rms roughness of MP and CMP AlN substrates to 2.2 nm and 0.2 nm, respectively. AlN epitaxial layers were deposited at 1...

Influence of gallium supersaturation on the properties of GaN grown by metalorganic chemical vapor deposition

A thermodynamic supersaturation model for gallium (Ga) was developed to describe GaN growth characteristics in low-pressure metalorganic chemical vapor deposition. The model takes into account the simplified GaN chemical reaction that occurs at the growth interface, Ga+NH3=GaN+3/2H2. The supersaturation was varied in two ways: (1) by the V/III ratio and (2) by the choice of the diluent gas. Two diluent gases were considered: H2, a commonly used diluent gas, and N2, a reaction inert gas. The choice of the diluent played a role in the degree of Ga supersaturation; since H2 is the product in the GaN formation, the addition of hydrogen significantly lowered the supersaturation. Atomic force microscopy revealed that surface morphology was associated with the different Ga supersaturation and the Burton–Cabrera–Frank model was used to relate it to the observed spiral size and terrace width. In addition to growth morphology, the degree of Ga supersaturation also influenced the carrier compensation level in n-type...

Growth and Characterization of AlN and AlGaN Epitaxial Films on AlN Single Crystal Substrates

AlN and AlGaN epitaxial films were deposited by metal organic chemical vapor deposition on single crystal AlN substrates processed from AlN boules grown by physical vapor transport. Structural, chemical, and optical characterization demonstrated the high crystalline quality of the films and interfaces.

Strain in Si doped GaN and the Fermi level effect

Using high resolution x-ray diffraction and Hall effect measurements, we found that the tensile strain caused by dislocation inclination in Si doped GaN became immeasurable when carbon codoping was used to compensate the free carriers. This result suggested that the tensile strain is related to free carrier concentration instead of Si concentration. Such an effect could be explained by the Fermi level effect on the surface-mediated dislocation climb governed by Ga vacancies, whose concentration is strongly influenced by the Fermi level position. This phenomenon is possibly similar to the well-known Fermi level effect in GaAs and GaP systems.

Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN

We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescence measurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracte...

Direct determination of the silicon donor ionization energy in homoepitaxial AlN from photoluminescence two-electron transitions

We report on the identification of a two-electron transition for the shallow donor silicon in homoepitaxial aluminum nitride (AlN). One c-oriented sample was analyzed by low temperature photoluminescence spectroscopy on multiple excitation spots. We find a unique correlation of one single emission band, 76.6 meV below the free excitonic emission, with the luminescence of excitons bound to neutral silicon proving the identity as a two-electron transition. The assignment is confirmed by temperature dependent photoluminescence investigations. We find a donor ionization energy of (63.5 ± 1.5) meV for silicon in AlN.

Strain relaxation by pitting in AlN thin films deposited by metalorganic chemical vapor deposition

Strain relaxation mechanisms were investigated in epitaxial AlN layers deposited on (0001)-oriented AlN substrates by metalorganic chemical vapor deposition. It was revealed that epitaxial AlN layers under tensile strain can exhibit micro-cracks and nano-pits. A correlation existed between the amount of strain and number of pits in localized areas. Pit densities as high as 1010 cm−2 were observed in areas where the tensile strain reached ∼0.4%, while unstrained areas of the film showed step flow growth. These nano-pits occurred as a strain relaxation mechanism and were not related to intrinsic defects, such as threading dislocations or inversion domains.

Simultaneous growth of a GaN p∕n lateral polarity junction by polar selective doping

Differences in surface energy between the Ga-polar orientation and the N-polar orientation of GaN translate into a completely different behavior for the incorporation of intentional and unintentional impurities. Oxygen is found to be an impurity with higher concentrations in N-polar films than in Ga-polar films and is the cause for the high n-type carrier concentration observed in N-polar films. We fabricated a lateral p∕n junction in GaN by the simultaneous growth of p- and n-type regions, utilizing the doping selectivity of the two different polar domains. The N-polar domains were n type with a carrier concentration of 1.7×1019cm−3, predominantly due to high oxygen incorporation, while Ga-polar domains were p type with a carrier concentration of 1.7×1017cm−3. No significant difference in Mg incorporation was observed between the two polar domains. This junction showed the characteristics that define a p∕n junction: current rectification, electroluminescence, and the photovoltaic effect.

On the strain in n-type GaN

It was demonstrated that Ge has the same effect as Si on the strain evolution in n-type GaN as measured by x-ray diffraction. Dislocation inclination, which causes tensile strain in n-type GaN, was clearly observed by transmission electron microscopy where Ge doping was introduced during epitaxial growth. This result is explained by the Fermi level effect model that indicates dislocation inclination due to the climbing process through Ga vacancies. Therefore, there is no dependence of dislocation inclination on dopant species.

X-ray characterization of composition and relaxation of AlxGa1―xN(0 ≤ x ≤ 1 ) layers grown on GaN/sapphire templates by low pressure organometallic vapor phase epitaxy

A characterization procedure was developed to determine the alloy composition and strain state of AlxGa1−xN/GaN(0<x<1) heterostructures deposited on c-plane sapphire substrates by low pressure organometalic vapor phase epitaxy. Motivated by a method suggested by Bowen and Tanner for separating the contributions of strain and composition in cubic crystals, we extended the technique to the case of hexagonal crystals by first principles derivation from elastic strain theory. The technique was evaluated using double-axis and triple-axis reciprocal space maps of 200 and 30 nm AlxGa1−xN layers. The procedure did not require absolute lattice parameter measurements and relied instead on relative measurements of the layer and substrate peak positions. Symmetric and asymmetric reflections of the film and substrate were measured in the double-axis configuration with ω–2θ scans. From the peak separation, the strained lattice parameters were determined. Assuming biaxial strain and linear variation of the relaxed latti...