Formation Mechanism of Microporous Siucon: Predictions and Experimental Results
- PDF / 1,301,262 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 35 Downloads / 226 Views
FORMATION MECHANISM OF MICROPOROUS SILICON: PREDICTIONS AND EXPERIMENTAL RESULTS V. LEHMANN* AND U.GOSELE** *Siemens, Dept. ZFE BT ACM 42,8000 Munich 83, Otto-Hahn-Ring 6, Germany **Duke Univ., School of Engineering, Durham, NC 27706, USA ABSTRACT Recently we presented a formation mechanism for micro-porous silicon which is based on a depletion of holes in the porous region due to quantum confinement. This theory allows predictions concerning the dependence of the porous morphology on the formation conditions. It is the purpose of this work to check whether these predictions are in accordance with experimental observations. INTRODUCTION The basic conditions for electrochemical pore formation in a homogeneous electrode are a passive state of the pore walls and an active state, which promotes dissolution, at the pore tip. Dissolution of a silicon electrode in hydrofluoric acid will occur if holes (h+) are present at the surface. Consequently a surface area which is depleted of holes will be passivated. Hole depletion will only occur if any hole that reaches the interface is immediately consumed in the dissolution reaction. This requires that the chemical reaction is not limited by mass transfer in the electrolyte. This condition is fulfilled if the current density is below a critical value Jps which is given by the following equation, in which Jps is in A/cm 2 , c in wt.% HF, T in K, Ea = 0.345 eV, k = 8.6171 x 10-5 eV/K and A = 3300 x (wt.% HF) -3/2 A/cm2,(1): Jps = A e (-Ea/kT) c 3/2 For current densities >Jps the reaction is limited by ionic mass transfer which will lead to a surface charge of holes and to a smoothing of the electrode surface (electropolishing). For current densities
Data Loading...
