QUANTUM-SIZE EFFECTS IN THE STUDY OF CHEMICAL SOLUTION DEPOSITION MECHANISMS OF SEMICONDUCTOR-FILMS

Chemical solution deposited (CD) CdSe films possess a nanocrystalline structure and exhibit quantum size effects due to the small crystal si ze. This results in a blue shift of the optical spectra. It has been o bserved that there is a certain critical ratio between the complexing agent (nitrilotriacetate) and Cd concentrations used in preparing the films, denoted as R(c), above which there is a pronounced red shift of the optical spectra of the films. Using X-ray diffraction and electro n microscopy, this red shift was correlated with an increase in crysta l size. This sharp change suggested a changeover in the CD mechanism. Optical absorption spectra and laser scattering measurements of the de position solutions in the absence of selenosulfate showed that Cd(OH)( 2) was present in solutions below R(c) (often only after an induction period during which the solution pH increased), but not above R(c), al though a visible Cd(OH)2 suspension was not apparent under normal depo sition conditions, even below R(c). X-ray photoelectron spectroscopy i ndicated the presence of adsorbed colloidal Cd(OH)(2) on the glass sub strates only under conditions where Cd(OH)(2) was also present in solu tion. It is proposed that, below R(c), the CD mechanism is initiated o n the Cd(OH)(2) colloidal particles adsorbed on the substrate while, a bove R(c), deposition occurs directly on the substrate by initial CdSe formation, without any mediation by Cd(OH)(2). The change in crystal size at R(c) is explained by the change in mechanism. Similar behavior was obtained for CdS and PbSe, showing the generality of the conclusi ons.