“…Single crystals of different substances (e.g., alkali halides, group Ha fluorides, mica, quartz, sapphire, spinel, berillia) were investigated as cand idates for such substrates (68,69); however, sapphire (a-Al z 0 3) is virtually the only substance to have survived in this regard . Pioneering studies of Manasevit and co-workers'?"'?"…”