Investigation of Nucleation and Initial Stage of Gan Growth by Atomic Force Microscopy and X-Ray Diffraction
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The importance of the III-V nitride compounds for blue and ultraviolet optoelectronics was noted decades ago by Pankov et al. [1]. Their success in growing GaN on sapphire by VPE and the demonstration of MIS light emitting diodes highlighted the promise of the wide bandgap material family. The delay in achieving commercial successes was caused by lack of bulk GaN or structurally and thermally matching hetero-substrates as well as difficulty of p-type doping. The technical breakthroughs made by Yoshida et al.[2], and Akasaki et al.[3,4] have brought about an acceleration in progress of blue and UV LEDs, LDs, detectors and transistors[5,6,7]. Current device development requires the growth of high quality GaN on mismatched heterosubstrates such as sapphire or 6H-SiC. This is a challenge in the view of conventional III-V compounds epitaxy where the lattice mismatch is in a range of 10--104-. In the case of growing GaN on (0001) sapphire substrate, the lattice mismatch is 16 % after 30 degree rotation of the GaN around c-axis relative to the sapphire. By using a AIN or GaN buffer layer, grown at low temperature[2,3], prior to the high temperature growth of GaN made the surface morphology of the film smooth. In addition, it improved the electrical and optical properties of the material drastically even though the dislocation density is still in the range of 10-101°/cm2 [8]. In this paper we will summarize our investigation on the nucleation and initial stage of MOCVD growth of GaN on c-plane sapphire substrate by atomic force microscopy, x-ray diffraction and photoluminescence. 93 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
EXPERIMENT MOCVD growth of GaN is carried out in a vertical, water-cooled, stainless steel chamber. The temperature was monitored using a pyrometer. The measured temperature was found to change as much as 20 0C with a 2000 sccm variation of the gas injection flow rate. The actual substrate surface temperature is different from the measured one because of the low thermal conductivity of the sapphire, and dependent on gas injection rate, substrate rotation speed and chamber pressure. To obtain comparable data, the chamber pressure was fixed at 100 Torr, and the gas injection rate and the disc rotation speed were kept constant. The growth proceess involved three steps: 1) Low temperature deposition of a GaN buffer layer, 2) crystallization of the buffer layer at 1000IC, and 3) growth of the GaN epitaxial films at approximately 1000TC. Atomic force microscopy, x-ray diffraction and photoluminescence measurements were used to investigate nucleation, annealing effect, evolution of the crystal structure, and optical properties of the materials. Rigaku and Philips x-ray diffractometers were used to evaluate structural properties by 0-20 scan, double crystal rocking curve analysis, --circle scans of GaN (112) and sapphire (113) reflections as well as )--scan of GaN (101) reflection. A Digital Instruments Dimension 5000 AFM was used to examine the surface morphology. RESULTS The incli
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