Growth Mechanism of thin Film Wide-Gap Semiconductors by Chemical Bath Deposition Technique
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MODEL DEVELOPMENT FOR SIMULATION OF CBD THIN FILM GROWTH Fig. 1 shows typical growth curves of thin films deposited by the CBD technique obtained in our laboratory using the recipes reported in a review paper [1]. The behavior seen here, together with previous studies reported for chemically deposited semiconductor thin films [1, 11] define the general characteristics of CBD growth process. There is an initial induction period during which the various chemical equilibria are established in the bath and an initial monolayer of the metal chalcogenide is formed (also called nucleation or incubation period). This is followed by the creation of an initial monolayer of the semiconductor on the substrate, which acts as a catalytic surface for the condensation of the metal ions and chalcogenide ions leading to the growth phase. The growth assumes a maximum rate at a certain time depending on the bath parameters and finally reaches a terminalphase at which the film stops growing due to depletion of constituent ions for film formation in the bath. It is generally observed that the induction period decreases with increase in temperature and concentration of ions in the bath. It is possible to arrive at an optimum temperature-concentration combination to achieve a maximum film thickness for a given duration of deposition.
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duration of deposition (h) Figure 1: Experimental growth curves of thin films of different semiconductor materials obtained using CBD technique. The initial concentration of metal ions in the bath and the deposition temperature are given. In the deposition of II-VI compounds of CdS, CdSe, ZnSe, ZnS, etc, the metal ions are complexed with various ligands simultaneously - NH 3, citrate, triethanolamine, etc. However, depending on the instability constants, the dissociation of one of these complexes may be responsible for the rate-determining step. This permits us to assume a first order reaction rate kinetics for the dynamics involved in CBD:
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dC(t) =1) = .kC(t)(1 dt Here k = A exp(-E/RT), A is the Arrhenius constant; Ea, the activation energy; R, the universal gas constant; and T, the temperature. It is assumed that the metal ions, M2, released from the complex instantly react with S2 or Se 2 ions available in the bath to form the compound MX (X = S, Se). The un-reacted amount C(t) of the metal ions at time t resulting from an initial concentration Ci is obtained integrating eqn. (1): C(t) = Ci exp(-kt) (2) Consequently, the reacted or used-up concentration C5 (t) of the metal ions in the chemical reaction M2, + X2- _ MX is: C, (t) = Cj [I - exp(-kt)] (3). If we consider ion-by-ion condensation of metal ions and chalcogenide ions on the substrate [9, 10] resulting in (i) the formation of compact specularly reflective thin film (/) of MX and (ii) precipitation of MX (p) in the bulk of the deposition bath,
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