Positive charge in SOS heterostructures with interlayer silicon oxide

The continuous transfer of (001)Si layers 0.2–1.7 $\mu$m thick by implanted hydrogen to the $c$-sapphire surface during direct bonding at high temperatures of 300–500$^{\circ}$C is demonstrated for the first time. The formation of an intermediate silicon-oxide layer SiO$_ x$ during subsequent heat treatments at 800–1100$^{\circ}$C, whose increase in thickness (up to 3 nm) correlates with an increase in the positive charge $Q_i$ at the heterointerface to $\sim$1.5 $\times$ 10$^{12}$ cm$^{-2}$ in contrast to the negative charge at the SiO$_{x}$/Al$_{2}$O$_{3}$ ALD heterointerface. During silicon-layer transfer to sapphire with a thermal silicon-dioxide layer, $Q_i$ decreases by more than an order of magnitude to 5 $\times$ 10$^{10}$ cm$^{-2}$ with an increase in the SiO$_2$ thickness from 50 to 400 nm, while the electron and hole mobilities barely differ from the values in bulk silicon. Based on these results, a qualitative model of the formation of positively charged oxygen vacancies in a 5-nm sapphire layer near the bonding interface is proposed.