Nitridation of Sapphire Substrate Using Remote Plasma Enhanced-Ultrahigh Vacuum Chemical Vapor Deposition At Low Tempera
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crystal structures between the sapphire substrate and the GaN epitaxial layer[l]. The quality of the GaN epitaxial layer was greatly improved with the two-step growth method. More improvements in the electrical, optical properties and surface morphology of the GaN epitaxial layer were realized with the nitridation of the sapphire surface prior to the nucleation layer growth[2-4]. But those properties were degraded probably due to the protrusions formed on the sapphire surface when the nitridation time was too long[2-4]. In this experiment, the protrusion density on the sapphire surface was found to be closely related to the substrate temperature. This suggests that the compressive thermal stress generated in the nitridated layer in the cooling step critically affects the formation of protrusions. It is possible to achieve a nitridation of the sapphire surface without protrusions at a low temperature. The crystallinity of the GaN overlayer grown on the nitridated sapphire surface at 450 *C was found to be improved as the nitridation time increased, indicating that the growth of the GaN overlayer is enhanced by the nitridation of the sapphire surface. EXPERIMENT The RPE-UHVCVD system used in this experiment was built to investigate the growth of a high quality GaN epitaxial layer at low temperatures. Reactor and loadlock chambers were evacuated by two turbo molecular pumps to obtain an ultrahigh vacuum pressure of about 10-9 torr. Reactive nitrogen species were obtained by cracking the nitrogen gas of 6-nine purity in the RF-ICP cracking cell attached to the top of the 81 Mat. Res. Soc. Symp. Proc. Vol. 482 01998 Materials Research Society
reactor. The distance between the plasma source and the substrate could be varied by a sample manipulator. The sapphire (0001) substrate was cleaned using organic solvents (TCE, acetone and methanol) in an ultrasonic environment, etched with acid (phosphoric acid:sulfuric acid=l:3), rinsed with DI water in an ultrasonic environment, dried with 6-nine purity nitrogen gas, and loaded into the reactor. Sapphire (0001) was exposed to activated nitrogen species under various conditions; RF power, substrate temperature, N2 flow rate, process pressure, nitridation time and distance of plasma source-to-substrate as shown in Table I. During the cooling process after nitridation, the reactor was maintained with a high pressure of nitrogen to prohibit the dissociation of nitrogen from the nitridated surface. The nitridated sapphire surface was investigated with XPS, AFM, and x-ray specular reflectivity. A GaN overlayer was grown on the nitridated sapphire (0001) plane. The growth conditions are summarized in Table I. The crystallinity of the GaN overlayer was measured by taking an x-ray 0/20 scan, and a 6-rocking curve. The surface morphology
of the GaN overlayer was investigated with AFM. The crystal structure of GaN and the in-plane epitaxial relationship between GaN and the substrate were investigated by an x-ray in-plane momentum transfer study. Table I. The experimental conditions GaN
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