Fig. 1: Set of RF power and nitrogen flow parameters for a fixed value of sensor output voltage U.

In Fig. 2 the GaN growth rate vs. nitrogen flow is plotted for U = 1.5 V and U = 2.4 V. For each point the gallium flux was adjusted to keep the surface covered by 2 ML of Ga. As seen, the growth rate does not change when the plasma parameters vary along the red (U = 1.5 V) and the orange (U = 2.4 V) curves in Fig. 1. On the contrary, the growth rate monotonically increases with intensity of light emitted by nitrogen plasma as shown in Fig. 3. Since under gallium-rich conditions growth is controlled by nitrogen flux, results shown in Figs. 2-3 clearly indicate that the optical sensor output signal is a direct measure of the amount of active nitrogen species available for growth. Optical spectra of the plasma were measured to check how intensity of light emitted by atomic and molecular nitrogen species depend on nitrogen flow and RF power of the cell.

Fig. 2: GaN growth rate vs. nitrogen flow for U=1.5 V and U=2.4 V.

Fig. 3: GaN growth rate vs. sensor output voltage U for nitrogen flow of 2.5 sccm.

A procedure is presented allowing to find an optimal conditions of the plasma cell for high-quality GaN growth. Under these conditions the nitrogen flux contains maximum amount of excited metastable molecules and minimal amount of ionic and atomic nitrogen species to minimize GaN lattice damage, even at high growth rates.

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