Photoluminescence (PL), Raman spectroscopy, and x-ray diffraction are employed to demonstrate the co-existence of a biaxial and a hydrostatic strain that can be present in GaN thin films. The biaxial strain originates from growth on lattice-mismatched substrates and from post-growth cooling. An additional hydrostatic strain is shown to be introduced by the presence of point defects. A consistent description of the experimental results is derived within the limits of the linear and isotropic elastic theory using a Poisson ratio $\nu =0.23\mathrm{}\pm 0.06$ and a bulk modulus $B=200\mathrm{}\pm 20$ GPa. These isotropic elastic constants help to judge the validity of published anisotropic elastic constants that vary greatly. Calibration constants for strain-induced shifts of the near-band-edge PL lines with respect to the $E_2$ Raman mode are given for strain-free, biaxially strained, and hydrostatically contracted or expanded thin films. They allow us to extract differences between hydrostatic and biaxial stress components if present. In particular, we determine that a biaxial stress of one GPa would shift the near-band-edge PL lines by 27±2 meV and the $E_2$ Raman mode by 4.2±0.3 ${\mathrm{cm}}^{-1\mathrm{}}$ by use of the listed isotropic elastic constants. It is expected from the analyses that stoichiometric variations in the GaN thin films together with the design of specific buffer layers can be utilized to strain engineer the material to an extent that greatly exceeds the possibilities known from other semiconductor systems because of the largely different covalent radii of the Ga and the N atom.

Organometallic vapor phase lateral epitaxy and coalescence of GaN layers originating from GaN stripes deposited within 3-μm-wide windows spaced 3 μm apart and contained in ${\mathrm{SiO}}_{\mathrm{2}}$ masks on GaN/AlN/6H–SiC(0001) substrates are reported. The extent and microstructural characteristics of the lateral overgrowth were a strong function of stripe orientation. A high density of threading dislocations, originating from the interface of the underlying GaN with the AlN buffer layer, were contained in the GaN grown in the window regions. The overgrowth regions, by contrast, contained a very low density of dislocations. The coalesced layers had a rms surface roughness of 0.25 nm.

The microstructure and the lateral epitaxy mechanism of formation of homoepitaxially and selectively grown GaN structures within windows in ${\mathrm{SiO}}_{\mathrm{2}}$ masks have been investigated by transmission electron microscopy (TEM) and scanning electron microscopy. The structures were produced by organometallic vapor phase epitaxy for field emission studies. A GaN layer underlying the ${\mathrm{SiO}}_{\mathrm{2}}$ mask provided the crystallographic template for the initial vertical growth of the GaN hexagonal pyramids or striped pattern. The ${\mathrm{SiO}}_{\mathrm{2}}$ film provided an amorphous stage on which lateral growth of the GaN occurred and possibly very limited compliancy in terms of atomic arrangement during the lateral growth and in the accommodation of the mismatch in the coefficients of thermal expansion during cooling. Observations with TEM show a substantial reduction in the dislocation density in the areas of lateral growth of the GaN deposited on the ${\mathrm{SiO}}_{\mathrm{2}}$ mask. In many of these areas no dislocations were observed.

Reverse bias current–voltage measurements of ∼100-μm-diameter gold Schottky contacts deposited on as-received, n-type $\mathrm{ZnO}\text{(0001̄)}$ wafers and those exposed for 30 min to a remote 20% ${\mathrm{O}}_2\text{/80\%}$ He plasma at $\text{525±20\hspace{0.05em}°}\mathrm{C}$ and cooled either in vacuum from 425 °C or the unignited plasma gas have been determined. Plasma cleaning resulted in highly ordered, stoichiometric, and smooth surfaces. Contacts on as-received material showed μA leakage currents and ideality factors >2. Contacts on plasma-cleaned wafers cooled in vacuum showed $\text{∼36±1\hspace{0.17em}}\mathrm{nA}$ leakage current to −4 V, a barrier height of $\text{0.67±0.05\hspace{0.17em}}\mathrm{eV},$ and an ideality factor of $\text{1.86±0.05.}$ Cooling in the unignited plasma gas coupled with a 30 s exposure to the plasma at room temperature resulted in decreases in these parameters to ∼20 pA to −7 V, $\text{0.60±0.05\hspace{0.17em}}\mathrm{eV},$ and $\text{1.03±0.05,}$ respectively. Differences in the measured and theoretical barrier heights indicate interface states. (0001) and $\text{(0001̄)}$ are used in this letter to designate the polar zinc- and oxygen-terminated surfaces, respectively.