THE ELECTROCHEMICAL OXIDATION OF SILICON AND FORMATION OF POROUS SILICON IN ACETONITRILE

The photoelectrochemical oxidation and dissolution of silicon has been investigated in the absence of water and oxygen. The etch rate and photocurrent for n-Si in an anhydrous, HF-acetonitrile solution were directly proportional to light intensity. Four electrons were transferred per silicon oxidized, with a quantum yield greater than 3.3 due to electron injection. The anodic dissolution of p-Si, as Si(IV) without H-2 gas at up to 1.4 A/cm2, yielded a novel porous structure which exhibited electroluminescence and photoluminescence. Noninterconnected pores were formed perpendicular to the surface, and were 1 to 2 mum in diameter, spaced 2 to 3 mum apart, and over 225 mum long. The profusion of micropores and quantum size structures (<100 nm), normally found with porous silicon produced in aqueous electrolytes, were not detected by transmission electron microscopy and scanning electron microscopy analyses. A mechanism for the oxidative dissolution of silicon is proposed. The luminescence was a function of the pore length and appears to be related to the presence of a dihydride surface. The absence of water and oxygen shows that siloxene is not involved in the luminescence. The inability to detect obvious quantum structures, and the insensitivity of the luminescence to porous shapes tends to support the a-SiH(x) alloy or surface passivation mechanism of luminescence.