Selenium passivation of GaAs with Se/NH4OH solution

GaAs surfaces were passivated with selenium using Se/NH4OH solution. Selenium powder of 99.8% purity was dissolved in NH4OH and the GaAs substrates were immersed in this solution. The extent of passivation was studied by electrical measurements (current-voltage characteristics) of the Schottky diodes. The experimental results indicated that the surface properties were substantially improved without requiring a succeeding Na2S treatment. The degradation of the ideality factor, n, of the Schottky diodes was efficiently retarded by this selenium passivation. The surface chemistry of the passivated GaAs surface was investigated with X-ray photoelectron spectroscopy (XPS). The XPS data indicated that both the oxidation of the GaAs surface (formation of Ga2O3 and As-2 O-3) and segregation of elemental arsenic (As-0) at the surface were suppressed or retarded by this passivation. This corresponds well to retardation of degradation of the n value. As2Se3 was observed in the Se/NH4OH-passivated surface, which suggests that the segregated As was removed by chemical reaction between As and Se to produce As2Se3. There is a possibility of the formation of Ga2Se3 (Ga-Se bond), though it is not observed in our XPS data. The formation of As-Se and/or Ga-Se bonds is suggested as the reason for suppression of oxidation of the GaAs surface. When the Se-passivated surface was exposed to air ambient, the total amount of selenium and the amount pf As2Se3 decreased. This may be due to oxidation of As2Se3 to form As2O3 The extent of Se/ NH4OH passivation and the degree of degradation due to air exposure are similar with those of the (Na2Se/NH4 OH + Na2S) passivation reported by Sandroff et al. [J. Appl. Phys. 67 (1990) 586].