Photoluminescence measurements in the phase transition region for CdS thin films

Abstract

CdS polycrystalline thin films were grown by the chemical bath deposition technique at 80 °C onto glass substrates. The films grow in the cubic crystalline structure as determined by x-ray diffraction analysis. After thermal annealing in S2 and Ar atmospheres, the CdS changes from the metastable zinc blende phase to a stable wurtzite one. The cubic-to-hexagonal transition temperature has been determined to be 370 °C, as seen by the photoluminescence spectra and the x-ray diffraction patterns of the different samples. These spectra show the well-known green emission band of the CdS centered at 2.4 eV for the as-grown sample, which shifts to 2.25 eV for the sample annealed at 365 °C just before the phase transition takes place. For the sample annealed at 374 °C, an abrupt blueshift of the green band occurs going back to an energy value of 2.4 eV, when the crystalline phase transition occurs.

Photoluminescence study of polycrystalline photovoltaic CdS thin film layers grown by close-spaced sublimation and chemical bath deposition

“…In general CdCl 2 treatment seems to suppress the development of orange and yellow luminescences depending on the substrate used. Some authors associate the yellow luminescence peak observed at 2.11 eV, 43,47 those between 2.04-2.11 eV, 44 and the green band at 2.20 eV with the formation of deep donor levels formed by Cd i states. 39,43 CdS layer grown by CBD typically appears amorphous in a metastable cubic crystal structure 41 and in a mixed cubic and hexagonal structure 48 depending on processing parameters.…”

“…[41][42][43][44][45][46] Depending on the growth method and CdS grain size the red luminescence is found centered at 1.67, 44 43,46 It is believed that the red luminescence is caused by transitions of electrons trapped in surface states to the valence band, 39,40 and this effect is therefore correlated with the accumulation of crystallographic defects in CdS layers grown at low deposition temperatures. Annealing of smallgrained CdS films increases grain size, decreases the number of grain boundaries, heals lattice defects, and reduces strain in the layer.…”

“…The appearance of the Several studies report about the appearance of a PL feature located at 2.25 eV for CdS films grown by CBD 43,45 and by sputtering, 38 respectively. Agata et al 39 find this band centered around 2.2 eV for evaporated CdS films.…”

“…It is believed that this luminescence band is caused by a transition from a donor level ͑e.g., Cd i ͒ to the valence band. 39,43 As mentioned above, CdS films convert during heat treatment from a cubic into a hexagonal lattice structure. The lattice conversion is accompanied by rearrangement of donor and acceptor states as well as by an increase in CdS band gap provided these films are annealed at temperatures above 300°C.…”

Detailed Analysis of Five Aspects Addressed to Minimize Costs and Waste in the Chemical Bath Deposition of CdS Films Using the Cd<i>B</i>–<i>A</i>C<sub>6</sub>H<sub>5</sub>O<sub>7</sub>–<i>A</i>OH–(NH<sub>2</sub>)<sub>2</sub>CS System

“…This modification consisted, basically, in the establishment of the use of cadmium-chloride (CdCl 2 ) as Cd 2+ ions source, and in the use of potassium-hydroxide (KOH) and pH 10 ammonia buffer-composed of ammonium-chloride (NH 4 Cl) and NH 4 OH-instead of only NH 4 OH. This modification was based on some observations derived from the synthesis of CdS in zeolites [36] and from the experience acquired [37][38][39][40] with the use of the ammonia-containing formulation employed by the group made up by Zelaya-Angel and collaborators [37][38][39][40][41][42][43][44][45][46][47][48][49][50][51], which basically differs from the standard chemical formulation employed by Nagao and Watanabe [10] in the use of CdCl 2 instead of cadmium-nitrate (Cd(NO 3 ) 2 ).…”