N. R. Perkins

THERMALLY STABLE PTSI SCHOTTKY CONTACT ON N-GAN

Platinum silicide (PtSi) and Pt Schottky contacts on n-GaN have been investigated and compared. The PtSi contacts were formed on n-GaN by annealing a multilayer structure of Pt/Si with the appropriate thickness ratio at 400 °C for 1 h in forming gas. The barrier height of the as-formed PtSi contacts was found to be 0.87 eV capacitance–voltage (C–V), and remained unchanged after further annealing at 400 and 500 °C. Upon annealing at 600 °C for 1 h, the barrier height decreased to 0.74 eV (C–V), but the diodes remained well-behaved. The as-deposited Pt yielded a barrier height of 1.0 eV (C–V). Upon annealing at 400 °C for 1 h, the Pt diodes degraded and most of the diodes did not survive additional annealing at 400 °C for longer times. The electrical measurements and the Rutherford backscattering spectrometry results indicated that PtSi contacts are thermally much more stable than Pt contacts on GaN.

Photoluminescence of erbium-implanted GaN and in situ-doped GaN:Er

The photoluminescence of in situ-doped GaN:Er during hydride vapor phase epitaxy was compared to an Er-implanted GaN sample. At 11 K, the main emission wavelength of the in situ-doped sample is shifted to shorter wavelengths by 2.5 nm and the lifetime is 2.1±0.1 ms as compared to 2.9±0.1 ms obtained for the implanted sample. The 295 K band edge luminescence of the in situ-doped sample was free of the broad band luminescence centered at 500 nm which dominated the spectrum of the implanted sample. Reversible changes in the emission intensity of the in situ-doped sample upon annealing in a N 2 versus a NH 3 / H 2 ambient indicate the probable role of hydrogen in determining the luminescence efficiency of these samples.

Scanning tunneling microscopy and tunneling luminescence of the surface of GaN films grown by vapor phase epitaxy

We report scanning tunneling microscopy (STM) images of surfaces of GaN films and the observation of luminescence from those films induced by highly spatially localized injection of electrons or holes using STM. This combination of scanning tunneling luminescence with STM for GaN surfaces and the ability to observe both morphology and luminescence in GaN is the first step to investigate possible correlations between surface morphology and optical properties.

Variation of GaN valence bands with biaxial stress and quantification of residual stress

Low-temperature reflectance data on epitaxial GaN thin-film samples covering the widest range of tensile and compressive stress (−3.8–3.5 kbar) thus far explicitly show the nonlinear behavior of the B–A and C–A splittings versus the energy of the A exciton. Lineshape ambiguities that hindered previous interpretations have been resolved with reciprocal-space analysis, allowing us to obtain band parameters such as ΔSO=17.0±1meV with increased confidence.

Room temperature epitaxy of Pd films on GaN under conventional vacuum conditions

Pd films deposited at room temperature have been found to grow epitaxially on GaN grown by metalorganic vapor phase epitaxy (MOVPE). The Pd films were deposited on GaN substrates cleaned by chemicals only, and in a conventional e‐beam evaporation system with a vacuum of ∼1×10−7 Torr. MeV 4He backscattering spectrometry and the Read x‐ray camera were used to evaluate the Pd films. The effects of various chemical etchants—such as aqua regia, HCl:H2O, and HF:H2O—on the epitaxial quality of the Pd films have also been investigated. Ni and Pt films deposited on GaN in a similar manner were also found to be epitaxial.

Halide Vapor Phase Epitaxy of Gallium Nitride Films on Sapphire and Silicon Substrates

A major limitation of the current technology for GaN epitaxy is the availability of suitable substrates matched in both lattice constant and thermal expansion coefficient. One alternative for the development of GaN substrates rests in the application of halide vapor phase epitaxy (HVPE) to produce GaN films at high growth rates. In this paper, the authors describe the growth of thick GaN films via the HVPE technique on (0001) sapphire and (111) Si substrates. At a temperature of 1,030 C, films are grown at rates between 70 and 90 {micro}m/hr, yielding total thicknesses exceeding 200 {micro}m on sapphire. DCXRD measurements of GaN/sapphire indicate FWHM values less than 220 arcsec on 180 {micro}m thick films. Room temperature PL measurements of GaN/sapphire indicate strong emission at 3.41 eV, with a FWHM value of 65 meV. Moreover, no detectable deep level emission was found in room temperature PL measurement. Under optimized conditions, films are morphologically smooth and optically clear. The GaN morphology appears to be a strong function of the initial nucleation conditions, which in turn are strongly affected by the partial pressure of GaCl. HVPE growth on (111) Si substrates is accomplished using an AlN MOVPE buffer layer.

Effect of reactor geometry and growth parameters on the uniformity and material properties of GaNsapphire grown by hydride vapor-phase epitaxy

Abstract The effects of flow rate variation and geometry on the growth rate, growth uniformity and crystal quality were investigated in a horizontal gallium nitride vapor-phase epitaxy reactor. The effects of these parameters, were studied through the comparison of numerical model predictions to experimentally observed values. Gas-phase reactions between Groups III and V sources and deposition of material on the wall are shown to lead to reduced overall growth rates and may be responsible for inferior crystal quality. A low ammonia concentration is correlated with the deposition of polycrystalline films. A low V III ratio and ammonia concentration lead to inferior crystalline quality and increased yellow luminescence. An optimum HVPE growth process requires selection of reactor geometry and operating conditions to minimize gas-phase reactions and wall deposition while providing a uniform reactant distribution across the substrate.

A near‐field scanning optical microscopy study of the photoluminescence from GaN films

We have achieved spatially resolved photoluminescence from GaN films using a near‐field scanning optical microscope (NSOM). GaN films grown by hydride vapor phase epitaxy (HVPE) and metal‐organic vapor phase epitaxy (MOVPE) on sapphire substrates have been studied. We have performed spatial scans of topography, band edge, and yellow luminescence signals. Atomic force microscopy measurements were also made and compared with the NSOM topography. We have found spatial variations in photoluminescence characteristics at the submicron scale for both HVPE and MOVPE GaN. The observed enhancement of yellow luminescence at multiatomic step edges on the HVPE GaN surface suggests that the yellow luminescence is associated with chemical impurities incorporated during the growth of GaN films.

Photoemission spectroscopy studies of the surface of GaN films grown by vapor phase epitaxy

Surfaces of GaN films have been investigated with photoemission spectroscopy. The measured valence band is in good agreement with band structure calculations and correlates well with tunneling luminescence measurements performed on the same samples. The effect of N depletion on band structure is explored, clarifying disagreements in previous photoemission measurements.

Ohmic contacts to n-GaN using PtIn2

A new metallization scheme has been developed to form Ohmic contacts to n-GaN. Contacts were fabricated by sputtering the intermetallic compound, PtIn2 on metal–organic vapor phase epitaxy grown n-GaN (n∼5×1017 cm−3) with some of the contacts subjected to rapid thermal annealing. Contacts in the as-deposited state exhibited nearly Ohmic behavior with a specific contact resistance of 1.2×10−2 Ω cm2. Contacts subjected to rapid thermal annealing at 800 °C for 1 min exhibited linear current–voltage characteristics and had specific contact resistances less than 1×10−3 Ω cm2. Auger depth profiling and glancing angle x-ray diffraction were used to examine the interfacial reactions of the PtIn2/n-GaN contacts. Consistent with estimated phase diagram information, the results from Auger depth profiling and glancing angle x-ray diffraction indicated the formation of (InxGa1−x)N at the contact interface, which could be responsible for the Ohmic behavior of PtIn2 contacts.