Anodization of Layered Semiconductors: A Representation of the Layer Periodicity

Abstract

Anodization of layered semiconductors, especially Bi2Te3 and GaSe, has been done to have a new representation of the layer periodicity which the layered semiconductors possess as their peculiar property. Constant current anodization (c.c.a.) in a solution of sodium borate in ethylene glycol (GSE) at pH 7 has been mainly adopted since the periodic surface condition for the successive monolayer film formation can be maintained by c.c.a, and GSE has been found to be an excellent electrolyte for the representation of the layer periodicity in these semiconductors. The layer periodicity has been represented by the periodically varying structures observed in the cell voltage vs. time (Vc-t) and its second derivative (--d2Vc/dt2-t) curves for Bi2Te3, Bi2Se3, GaTe, and GaSe, and in the in situ ellipsometric off-null signal vs. time (AI-t) curves for Bi2Tes in c.c.a. Unit line shapes of the Vc-t and the surface coverage vs. time (0-t) curves derived from AI-t curves have been investigated to understand the anodization process for the monolayer film formation in Bi2Te3 where the two-dimensional nucleation and growth, which may result from a suppressed growth in the perpendicular direction to the cleavage surface by the potential barrier at the layer boundary, are taking place with the slow step of competing rate-determining processes with comparable rates. The competing rate-determining processes have been also recognized by the current transients in the potentiostatic anodization of Bi2Te8 in GSE on the analogy of the electrocrystallization study on nickel reported by Fleischmann et al. It has turned out in c.c.a, of GaSe in GS~ that insoluble nonpassive films grow two dimensionally in the initial stage of anodization and transform into passive films. Another interesting observation has been made in the transient 'behaviors of --d2Vc/dt2-t characteristics.They have been classified into five types, depending on the kind of electrolytes and semiconductor samples used, or on being related to the current efficiency, the rate-determining process and/or transformation from three-dimensional to two-dimensional growth of passive films.Anodization of semiconductors has been applied to measurements on the depth profile of the carrier concentration (1) and of the alloy composition of GaAsz-xPx (2) and for control of thickness of epitaxial layer on GaAs (3), utilizing the property that thickness of anodically grown films is highly and easily controllable. As a new approach of application, we have shown in the previous paper (4) that periodically varying structures reflecting the layered structure of the layered semiconductors were observed in the cell voltage vs. time (Vc-t) curves when anodization was carried out under a proper constant current condition (c.c.c.) with the use of a proper electrolyte as shown in Fig. 1 for Bi2Tes, and these periodic structures have enabled us to count the number of layers with a mono!ayer thickness of around 10A since the unit line shape from which the periodic structure in the Vc-t curves is construc...

The Photoelectrochemistry of Gallium Selenide

“…In the present study we describe the basic electrochemical and photoelectrochemical behaviour of p-type GaSe electrodes such as current potential curves in the dark and under illumination, the interaction with redox systems, the potential a n d wavelength dependence of the quantum yield of photocurrents, the effects of crystal orientation etc. The anodic oxidation of GaSe under conditions where unsoluble films are formed o n the surface was recently studied by Moritani et al [6].…”

“…[6] the unit cell which expands over two layers [7]. T h e layers a r e loosely bound together by van der Waals forces.…”