Substrate Preparation and Impact of Polarity

The y-LiA.102 substrates used in this study were grown by a modified Czochralski method at the Institute for Crystal Growth in Berlin, Germany, and were chemo-mechanically polished. The peak-to-valley (P/V) roughness and root-mean-square (RMS) roughness of the polished substrates have values of 1.0 nm and 0.08 nm, respectively, across a 3 x 3 area. These values compare favorably with those obtained for state-of-the-art polished SiC or AI2O3 substrates. 

Substrate preparation deserves utmost attention for obtaining reproducibly high-quality epilayers. Usually, the substrate is subject to chemical preparation before it is loaded into the growth reactor. For example, for SiC(OOOl) substrates, a dramatic improvement of the surface quality is attained upon annealing the wafers to about 1600 °C in flowing H2 [36]. Etched SiC wafers exhibit a well-ordered stepped surface. 

Since LAO is attacked by various acids and even water, well-established cleaning processes used for other substrates are not applicable. In fact, we have not yet found any effective method, such as thermal or chemical etching, to improve the surface morphology of LAO. The as-received substrates are thus only degreased sequentially in trichloroethylene (10 min), acetone (5 min), and methanol (5 min) in an ultrasonic bath. Finally, the substrate is dipped briefly (30-60 s) in deionized water. The LAO substrate is then fixed with In onto a Si wafer clipped to a Mo holder. Prior to growth, the 

An annealing step at 750 °G desorbs these products and excess Ga remaining on the surface. However, for LAO, this procedure was found to merely induce an excessive roughening of the substrate, and the structural properties of GaN films grown on such a substrate were actually degraded. 

More important, however, is the fact that in between these etch pits, the surface efface A remains relatively smooth, whereas face B becomes clearly corrugated. Note, however, that a longer treatment or higher temperatures tend to obscure this difference due to the resulting excessive roughening of both faces. 

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