A INTRODUCTION

While anodic oxidation of crystalline Si under constant current in aqueous or ethanolic solutions of HF is without question the most common porous Si preparative method [1], an intrinsically easier and less frequently employed technique is the use of an open-circuit chemical stain-etch consisting essentially of HF, nitric acid, and water [2]. Stain films have also been studied for over thirty years [3], and it is important to note that as early as 1960 electro-chemiluminescence from this specific type of porous Si was observed during anodic oxidation under bias [3]. These stain films on Si were initially exploited in the 1960s to probe p-n junctions [4], but the discovery of visible light emission in electrochemically-etched porous Si [5] stimulated a renaissance in the study of visible light emission from this material as well [6, 7]. While criticised for non-uniform etch profiles, extremely thin (~10 ² nm) porous layers [8], and relatively lower quantum yields of photoluminescence [9], a number of systematic studies regarding the formation and structural/optical properties of stain etch-derived porous Si have been reported.

B MECHANISM

As with the anodic oxidation of Si under bias, a key component to stain etch routes to porous Si formation is hole (h ⁺) generation. In most stain etch methods employing HF and HNO ₃, initially NO is produced (EQN (1)) which serves as a hole (h ⁺) injector [9]:

(1)

(2)

(3)

(4)

In...