Chemical mechanical polishing of ͑0001͒ GaN has been demonstrated with sodium-hypochlorite-based solutions. Slurries including alumina abrasive provide an efficient means of planarization for both the Ga-and N-face that does not induce significant crystalline damage. Removal rates were found to be ϳ50 nm/min and were equivalent for both polarities. Additionally, a fine polishing method was developed that includes abrasive-free solutions of either sodium hypochlorite for the N-face or a mixture of sodium hypochlorite and citric acid for the Ga-face. With this fine polishing step, scratch-free surfaces were achieved with a root-mean-square roughness of 0.5-0.6 nm.Gallium-nitride-based structures are recognized as one of the most promising materials for short wavelength optoelectronic devices and high-power, high-frequency electronic devices. However, the potential of this material has been limited by the lack of a suitable lattice matched substrate for the epitaxially grown device layers. This has led to the development of bulk GaN substrates. Homoepitaxial devices fabricated on bulk substrates can exhibit an order of magnitude lower dislocation density compared to heteroepitaxially grown devices and have been shown to exhibit superior performance. 1,2 With the development of these substrates, surface preparation techniques must also be investigated to provide atomically smooth, damage-free surfaces, such as chemical mechanical polishing ͑CMP͒. Additionally, alternative processes that may further expand GaN technologies, including wafer bonding and layer transfer techniques, often require planarization steps creating a need for a well-controlled GaN CMP process. [3][4][5] CMP uses a combination of chemical and mechanical reactions to remove material leaving a planarized, damage-free surface. Ideally, material removal is achieved by chemically altering the surface to a mechanically weaker form; this material is then abraded from the surface leaving the bulk undisturbed. Planarization occurs due to the acceleration of both mechanical grinding and chemical transformation at the high points. Previous GaN CMP studies experimented with sodium or potassium hydroxide with silica abrasive slurries. 6,7 Although these slurries were able to achieve smooth surfaces over small areas on the N-face, the Ga-face showed no alteration. This was attributed to the fact that the Ga-face is more chemically inert than the N-face, making material removal by CMP of this face difficult. Additionally, the damage induced to the surface of the material by the KOH or NaOH solutions was not addressed. In this study, planarization of bulk GaN is carried out with a sodiumhypochlorite-based slurry to obtain a damage-free process that can be used for either polarity. Sodium hypochlorite is a strongly oxidizing agent that has been successfully used to polish other III-V materials. 8-10 Experimental 1 ϫ 1 cm GaN substrates were provided by Kyma Technologies. As-received, the Ga-face was ground so that it was not specular and the N-face was epiready po...

To investigate the mechanisms of hydrogen-induced blistering in III–V materials, a standard splitting dose of 5×1016H2+∕cm2 at 150keV was implanted into InP substrates cooled to −20°C. Substrate cooling during the implantation improved the reproducibility of this approach by limiting hydrogen mobility during ion implantation. The implant profile and defect structure of unbonded wafers were studied for various annealing schedules with double-axis x-ray diffraction and transmission electron microscopy. It was found that exfoliation was greatly facilitated by a combined lower-temperature (150°C) “defect nucleation” step, followed by a higher-temperature anneal (300°C). The nucleation of defects in the lower-temperature regime, which did not occur if the initial anneal was conducted only at higher temperatures, was attributed to defect trapping of hydrogen. This annealing sequence presents a means by which to (i) improve the interfacial bond strength at low temperatures while the “nucleation” occurs and (ii) promote efficient exfoliation at high temperature.

GaAs, InP, GaSb, and InAs were investigated for layer transfer to other III-V substrates using hydrogen ion exfoliation and wafer bonding to develop III-V based wafer bonded templates for subsequent epitaxial growth of device structures. High-resolution X-ray diffraction was proven to be particularly helpful in this investigation enabling nondestructive testing of the initial implantation profile as well as the strain relief ͑associated with the diffusion of hydrogen and other point defects͒ after various annealing sequences. The kinetics of exfoliation for many different III-V materials ͑including GaAs, InP, InAs, and GaSb͒ showed similar dependence on the processing temperature relative to the material melting temperature ͑and other materials parameters͒, i.e., lower melting temperature materials required lower temperature processing to retain the implanted hydrogen for exfoliation. In all of these cases, a multiple annealing sequence was shown to produce the most efficient exfoliation.

Wafer bonding and hydrogen implantation exfoliation techniques have been used to fabricate a thin InP template layer on GaAs with intermediate silicon nitride bonding layers. This template layer was used to directly compare subsequent metal organic vapor phase epitaxial growth of InGaAs∕InAlAs quantum-well structures on these wafer-bonded templates to growth on a standard InP substrate. Chemical mechanical polishing of the bonded structure and companion InP substrates was assessed. No effects from the coefficient of thermal mismatch are detected up to the growth temperature, and compositionally equivalent structures are grown on the wafer-bonded InP template and the bare InP substrate. However, after growth dislocation, loops can be identified in the InP template layer due to the ion implantation step. These defects incur a slight mosaic tilt but do not yield any crystalline defects in the epitaxial structure. Low-temperature photoluminescence measurements of the InGaAs grown on the template structure and the InP substrate exhibit near-band-edge luminescence on the same order; this indicates that ion implantation and exfoliation is a viable technique for the integration of III-V materials.

The effect of different heat-treatments during the hydrogen-induced exfoliation of a thin InP layer to control surface roughness was investigated. Hydrogen implantation was carried out by implanting InP with 5 ϫ 10 16 H 2 + /cm 2 at 150 keV. Exfoliation and transfer of the layer to a GaAs substrate occurred at either 150 or 300°C. Exfoliation at 150°C produced smoother surfaces, 2.8 nm, compared to the sample exfoliated at 300°C, 8.5 nm. This change in surface morphology exhibits a much larger impact than the effect of implantation energy ͑i.e., straggle͒ on the surface morphology. The difference is attributed to the ability to trap hydrogen and form extended defects parallel to the surface at the lower temperature. Additionally, the exfoliation depth of the lower temperature specimen is slightly deeper than the specimen exfoliated at 300°C, indicating that the hydrogen diffuses towards the peak damage region at the higher temperatures.

A transfer-enhanced semiconductor substrate has been demonstrated, integrating a thin InP layer on a Si handle substrate with an imbedded porous silicon layer. A Si wafer was anodized to create a low-density porous surface layer. A thin layer of InP was transferred to the Si handle substrate through wafer bonding and hydrogen-induced exfoliation. High-resolution X-ray diffraction, atomic force microscopy, and transmission electron microscopy characterization showed the structure to have high surface and crystalline quality for epitaxial deposition after undergoing chemical mechanical polishing. Layer transfer capability was demonstrated with a similar structure by mechanical fracture through the porous Si layer.

Specular and diffuse x-ray reflectivity measurements were employed for wafer bonding studies of surface and interfacial reactions in ∼800 Å thick SiN films deposited on III–V substrates. CuKα1 radiation was employed for these measurements. The as-deposited films show very low surface roughness and uniform, high density SiN. Reflectivity measurements show that an oxygen plasma treatment converts the nitride surface to a somewhat porous SiOx layer (67 Å thick, at 80% of SiO2 density), with confirmation of the oxide formation from x-ray photoelectron spectroscopy. Reactions at the bonded interface of two oxygen plasma treated SiN layers were examined using a bonded structure from which one of the III–V wafers is removed. Reflectivity measurements of bonded structures annealed at 150°C and 300°C show an increase in the SiOx layer density and thickness and even a density gradient across this interface. The increase in density is correlated with an increase in bond strength, where after the 300°C anneal, a high interfacial bond strength, exceeding the bulk strength, was achieved.

No abstract