The Effect of Hydrogen Carrier Gas on the Morphological Evolution and Material Properties of GaN on Sapphire
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surface with quasi-2-dimensional uniform growth [3]. The two-step nucleation procedure has enabled the growth of GaN with smooth morphology and far superior electrical and optical properties than GaN directly nucleated on sapphire. Subsequently Nakamura et al. also proposed the use of low temperature GaN buffer layer which is reported to be functionally identical to the AIN buffer layer [4]. Using a variety of microscopy techniques, subsequent studies on the growth mechanism revealed an evolution of the LT buffer layer starting from its deposition through the temperature ramp and anneal. These studies showed that the degree of crystallinity, the average grain size and root mean square (RMS) roughness of the buffer layer increased at the end of the process [5, 6, 7]. While such ex-situ studies revealed detailed but static information about the crystal structures and surface morphology of the growing film, an in-situ diagnostic capability in an MOCVD reactor will provide a more dynamic picture to the growth evolutionary processes. Using a pyrometer monitoring the infrared (IR) transmission through the sapphire substrate and GaN epitaxial film, Nakamura et al. was able to correlate the pyrometric output with the surface roughness of the GaN [8]. The observed decay and recovery of the IR intensity was supportive of the above mentioned model of growth evolution. Here, a real-time in-situ optical reflectance monitoring set-up has been implemented which has allowed us to make the same observations, as well as to dynamically explore the effects of various growth parameters. In addition to the parameters which have been reported 143 Mat. Res. Soc. Symp. Proc. Vol. 482 01998 Materials Research Society
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Time (sec) Figure 1: Reflectance (-) GaN on sapphire.
and temperature --- -) taces of a typical two-step growth of
in the literature, namely the buffer layer thickness and growth temperature, V/III ratio, nitridization and etc, we have found that the concentration of hydrogen in the gas composition exerts a pronounced effect which has not been previously addressed. EXPERIMENT Growth of GaN is conducted in an inductively heated rotating disk reactor (RDR) with ammonia (NH 3 ) and trimethylgallium (TMGa) as precursors. Hydrogen (H2 ) is used as the carrier gas as well as the make-up flow to supplement the total flow rate required to maintain a matched laminar flow pattern in an RDR. Temperature is monitored by an IR
pyrometer calibrated by the melting of Ge at 934°C. C-plane (0001) sapphire substrates are degreased and thermally treated in-situ in H2 at 1100 0 C for 10 minutes before the LT GaN buffer deposition. The temperatures for the buffer deposition and growth are kept fixed at 5601C and 1075°C respectively for this series of study. The susceptor is rotated at 1200 rpm. The rotating speed in combination with the reactor pressure and susceptor temperature determine the t
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