The impact of interdiffusion on strain energy in the GaN-sapphire system was studied. Gallium nitride epitaxial layers were grown on (0001) Al2O3 by low-pressure MOCVD at 850 °C, V/III ratio = 3600, P= 100 Torr using TEGa and NH3 sources in a N2 carrier. The ∼0.3 µm thick films were then annealed at growth temperature in N2 for a period of 30 to 120 min. The Al and Ga diffusion coefficients at 850 °C were estimated as DAl = 3.98 × 10–17 cm2/s and DGa = 4.81 × 10–17 cm2/s from SIMS depth profile data. A model was developed to predict the strain energy and describe its relaxation behavior. The calculations assume a gradual transition of lattice parameter near the interface rather than an abrupt transition. The effect of interdiffusion on the strain energy and predicted critical thickness were clearly demonstrated. The estimated strain energy of the first layer was only 176 J/mole, compared to 30 kJ/mole assuming an abrupt interface. For an anneal time of 120 min at 850 °C the model predicts that dislocations are formed when the strain energy reaches 10 kJ/mole after the 4th layer is added, compared to their formation after the 1st layer is formed when an abrupt interface is assumed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A hydride - metal organic vapor phase epitaxy (H-MOVPE) system was developed that has the ability to alternate between MOCVD and HVPE in the same reactor. This technique was used to fabricate freestanding GaN substrates by depositing thick GaN on closely lattice-matched LiGaO2 (LGO) and LiAlO2 (LAO) wafers. Additionally, this deposition system was used to deposit single-crystal GaN on Si (111) by a two-stage process with a low-temperature MOCVD step followed by a low-temperature HVPE step. The surface morphology was studied by AFM and SEM, while the structural quality was analyzed by XRD and TEM. The chemical composition was investigated by AES, ESCA and SIMS.

Free-standing GaN substrates were fabricated by hydride-metal organic vapor phase epitaxy (H-MOVPE) on closely lattice-matched LiGaO2 substrates. The key to obtaining GaN films on LiGaO2 was the initial surface nitridation step. Nitriding and cooling processes were found to be critical film–substrate self-separation. The GaN surface morphology of the GaN was determined by AFM; the structural quality was analyzed by XRD; the chemical composition was investigated by AES, ESCA, and SIMS. Raman spectroscopy was applied for film and substrate characterization.

Self-separating single crystal GaN substrates were grown by hydride-metalorganic vapor phase epitaxy (H-MOVPE) on closely lattice-matched LiGaO2 substrates. A critical process for obtaining high quality GaN films on LiGaO2 was the initial surface nitridation of the oxide substrate with NH3. This nitriding process and the cooling schedule after growth were critical to achieving film-substrate self-separation. The as-grown single crystal GaN substrate exhibited a flat surface without any mechanical or chemical treatment. No cracks or residual strain were observed. Different characterization techniques were used to assess the quality of the substrates and films quality. The surface morphology was examined by AFM and the structural quality was analyzed by XRD; the chemical composition was investigated by AES, ESCA, SNMS and SIMS were used to determine composition profiles near the surface. Micro Raman spectroscopy was applied for film and substrate characterization.