ara tes

queou 0.75 V porou istic 1 m ss of s ace la se wi he loc letion 441 A ived Since holes in the valence band are required for anodic oxida- tentials are referenced. Prior to immersion in the electrolyte, the working electrode was immersed in an etchant 3:1:1 ociety /$20.0 Downlotion, n-type electrodes do not etch in the dark under “normal ” deple-tion conditions. At highly positive potentials, however, tunneling of carriers across the depletion layer can occur and the resulting holes in the valence band enable etching to take place.26 Since tunneling is sensitive to the surface condition of the semiconductor, the etching process that can take place under anodic bias may be strongly af-fected by crystallographic defects and surface impurities.27 Conse-quently, anodic etching of n-type semiconductors is often localized, thereby giving rise to the formation of porous sub-surface micro-structures. This paper describes an investigation of the anodic formation of nanoporous InP in KOH electrolytes with concentrations in the range 2–5 mol dm−3. The anodic processes were studied electroana-lytically using potential sweep and constant potential techniques un-der a variety of conditions, and the resulting porous structures were characterized using transmission electron microscopy TEM. H2SO4:H2O2:H2O for 4 min and then rinsed in deionized water. “Standard ” n-InP working electrodes had a carrier concentration of 3 1018 cm−3. In experiments on the effect of carrier concentra-tion, electrodes with lower 5 1017 cm−3, supplied by Lucent Technologies, grown by the vertical gradient freeze method and higher 6 1018 cm−3 values were used where indicated in the text. All of the electrochemical experiments were carried out at room temperature and in the absence of light. A CH Instruments model 650A Electrochemical Workstation in-terfaced to a Personal Computer PC was employed for cell param-eter control and for data acquisition. The surfaces of the anodized samples were examined using a Veeco Explorer atomic force micro-scope AFM. Electron transparent sections for plan-view and cross-sectional TEM examination were prepared by thinning to electron transparency using standard focused ion beam FIB milling procedures28 in a FEI 200 FIB workstation. The TEM characteriza-tion was performed using a JEOL 2000FX TEM operating at 200 kV