LIQUID-PHASE EPITAXY ON ALXGA1-XAS SURFACES PASSIVATED IN SULFIDE SOLUTIONS

A study has been made of the possibility of using sulfide passivation of a surface in order to achieve repeated growth by liquid-phase epita xy on epitaxial layers of AlxGa1-xAs solid solutions which have been e xposed to air. The results show that the surface passivation in sodium sulfide solutions does indeed make possible a repeated growth on AlxG a1-xAs, at aluminum concentrations up to x = 0.8. The quality of the l ayer which is grown depends in a definite way on the passivation condi tions: the higher the aluminum content in the substrate, the more conc entrated the sodium sulfide solution required to achieve a high-qualit y layer. Passivation in ammonium sulfide solutions does not support re peated growth. The etching of GaAs and AlxGa1-xAs in sodium sulfide so lutions has been studied in order to optimize the passivation conditio ns. The etching process is nonuniform, slowing down as time elapses, u ntil the process comes to a complete halt. A mechanism is proposed for the observed effects. This mechanism is based on chemical reactions w hich occur at the surface of a semiconductor in the sulfide solutions. An examination of these reactions shows that the state of the surface during the passivation is determined by two competing processes: the etching of the semiconductor in the basic medium and the formation of a passivating coating as a result of the chemisorption of S-2 anions o n the surface.