Chemical mechanical polishing of gallium nitride with collodial silica has been demonstrated. Removal rates and ultimate surface roughnesses have been found to depend on sample polarity ͑either A or B faced͒ as well as the pH of the polishing fluid and the initial surface morphology. High quality A-faced samples exhibited no polishing action while B-faced material was readily removed. Removal rates at room temperature were found to vary from 0.4 to 1.1 m/h resulting in surface roughnesses as low as 1.1 nm root mean square roughness ͑RMS͒ over 400 m 2 decreasing to 0.4 nm RMS over 25 m 2 . Investigations into the mechanism of polishing reveal both a chemical and mechanical aspect to polishing GaN with colloidal silica.Gallium nitride devices have attracted widespread interest recently as demand increases for highly efficient short wavelength emitters and high power transistor devices. To date, however, the majority of GaN devices developed must be produced using heteroepitaxial techniques on either sapphire or silicon carbide substrates. Large-scale high quality wafers of GaN are not yet readily available even though their usage as homoepitaxial growth substrates is widely considered to be necessary for the next generation of GaN devices. Despite the lack of availability of true bulk substrates for device applications, several techniques exist for generating quasi-bulk substrates including the removal of thick hydride vapor phase epitaxy ͑HVPE͒ GaN layers from their original sapphire growth substrate and high pressure equilibrium growth techniques which produce small single crystal platelets. 1 Both methods require mechanical surface polishing to remove surface features such as hillocks and pits from growth as well as mechanical bowing resulting also from growth and thermal expansion stresses. Polishing with diamond or alumina particles, both harder than GaN, leaves these surfaces ͑and their subsurfaces͒ decorated with scratches and defects. The observations presented here that GaN can be polished damage-free with colloidal silica are the result of an earlier study on processing GaN vertical cavity surface emitting lasers ͑VCSELs͒, 2 devices which are intolerant to the damage produced by abrasive polishing since scattering losses by surfaces and interfaces are proportional to roughness.At a first look, polishing GaN with a much softer colloidal silica material appears to be an exercise in futility, but numerous studies of polishing materials with SiO 2 particles have revealed a curious trend with regard to damage-free removal of material during polishing. Surfaces of silicon, 3,4 GaAs, 5 tungsten, 6 aluminum, interlayer dielectrics ͑ILDs͒, 7 and sapphire 8 each polish at room temperature at high rates activated by the combination of both mechanical polishing and chemical etching. A detailed study of chemical mechanical polishing of sapphire by Gutsche and Moody 8 revealed that the stock removal rate depended on the pH of the stabilized silica suspension, the temperature of the polishing fluid, and the concentratio...

Films of GaN and ZnO can be separated from the substrates on which they are grown by the use of a laser-assisted debonding process in which a pulsed laser is shone through the substrate and absorbed in the film. Experience shows that the success in separating the films intact and damage free depends sensitively on the laser parameters used and the physical and geometric properties of the films. In this contribution, the mechanics of the laser-assisted debonding of GaN films are presented and used to construct process maps that delineate the conditions for damage-free film separation. The key variable is the nondimensional group ΩEp/(dp2τ), where Ω is a lumped material constant, Ep is the laser pulse energy, dp is the diameter of the illuminated area and τ is the laser pulse length. Experimental observations of UV/excimer laser assisted debonding of GaN films from sapphire substrates are used to illustrate the types of deformations and cracking modes on which the process maps are based.

The use of a low-temperature layer of GaN formed by hydride vapor-phase epitaxy (HVPE) as a template to grow high-quality HVPE films is demonstrated. Using layers formed by reacting GaCl and NH3 at 550 °C and annealed at a growth temperature of 1050 °C, thick films of GaN can be grown by HVPE with fewer than 108 dislocations per cm2. Dislocation densities measured by high-resolution x-ray diffraction, atomic-force microscopy step termination density and plan-view transmission electron miscroscopy reveal that ∼23 μm films have dislocation densities of ∼6×107 cm−2. Obtaining high-quality single-crystal character films was found to be dependent on several factors, most importantly, the rate of temperature increase to growth temperature and the layer thickness.

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Thick films of (112¯0)-oriented GaN have been grown on Ti-coated metal organic chemical vapor deposition templates using hydride vapor phase epitaxy. Significant reductions in crack density were observed enabling 240μm thick films to be grown on sapphire. The use of Ti interlayers was shown to generate significant fractions of voids at the interlayer regrowth interface facilitating void-assisted separation on cooling. Ti metal layers annealed under optimal conditions were found to produce a TiN nanomask suitable for lateral overgrowth during HVPE. An estimate of the void size required to allow spontaneous delamination of the substrate at the TiN–GaN interface is discussed with reference to growth conditions.