Formation of the Conductive Layer near the Surface of Semi-Insulating GaAs Covered with Oxide Film

Thin conductive layer is formed near the surface of i-GaAs doped with Cr, when the surface of i-GaAs is covered with SiO₂ or phospho-silicate glass (PSG) grown by means of thermal decomposition of alchoxyl silicate and phosphate. The thickness of the layer increases with increasing duration and temperature of heat treatment. This layer shows n-type conduction, and the concentration and mobility of electron in this layer are 1∼2×10¹⁷ cm^-3 and about 2500 cm²/V·sec, respectively. It is concluded that the cause of the conductive layer formation is a decrease of the concentration of Cr which compensates undesirable donors in i-GaAs, as a result of the diffusion of Cr into the oxide film. The formation of such conductive layers can be prevented by covering i-GaAs with sputtered SiO₂ from fused quartz or Si₃N₄. The activation energy of diffusion coefficient of Cr in i-GaAs is estimated to be 3.6 eV.