We have achieved solid phase epitaxy of thin amorphous Si layers on GaAs using in situ plasma processing and subsequent annealing. High-resolution transmission electron microscopy of the SiO2/Si/GaAs structure shows that a Si layer ≂20 Å thick epitaxially crystallizes on GaAs after annealing at ≂570 °C in N2. Metal-oxide-semiconductor capacitors fabricated on these structures confirm the high quality of these interfaces. By comparing a high- frequency (100 kHz) capacitance-voltage curve with a quasi-static one, interface state densities as low as 4×1012 eV−1/cm−2 were measured on both n- and p-type GaAs.