Effects of co-dopants on the microstructure and electroluminescence of ZnS:Mn thin film phosphors

The objective of this study is to investigate the effects of the co-dopants of KCl and Ga₂S₃ and post-deposition annealing on the microstructure and electroluminescence (EL) properties of ZnS:Mn thin film phosphors. ZnS:Mn thin films are deposited by radio frequency (RF) magnetron sputtering from ZnS and Mn targets onto pre-deposited indium tin oxide (ITO) and aluminum titanium oxide (ATO) layers on Corning 7059 glass. Argon at 20mTorr is the sputtering ambient. The substrates are held at 180°C during deposition. Co-dopants are thermally evaporated after the ZnS:Mn films, and diffused into the ZnS:Mn films by ex situ annealing between 600°C and 800°C for 5 minutes in a nitrogen ambient. Brightness versus the applied voltage, luminous efficiency, and photoluminescence (PL) are used to characterize the EL devices. The figures of merit are the threshold voltage V_th, at which luminescence is first detected, B₄₀ and η₄₀, the brightness and efficiency at 40V above the threshold voltage, respectively.; In the as-deposited ZnS:Mn phosphor, the microstructure is heavily defected with two different grain morphologies: a roughly 100nm layer of equiaxed fine grains at the insulator/phosphor interface and columnar grains with an average diameter of 89nm in the rest of the film. The EL properties of as-deposited films are poor, with a V_th of 125V, B₄₀ of 48.7nits, and a η₄₀ of 0.2275lm/W. Annealing at 700°C for 5 minutes raises B₄₀ to 99.6nits and η₄₀ to 0.4463lm/W, with little change in V_th.; In KCl doped ZnS:Mn samples, after 5 minutes of annealing at 700°C, SIMS indicates a uniform distribution of K and a complete diffusion of Cl throughout the phosphor. KCl co-doping enhances grain growth by improving dislocation motion, and the columnar grain size increases from 132nm to 187nm. EL properties are improved, with a B₄₀ of 252nits and η ₄₀ of 0.9879lm/W. A slight increase in V_th is observed.; In ZnS:Mn samples with Ga₂S₃, the grain growth is less than that in undoped ZnS:Mn. Energy dispersive spectrometry (EDS) data show Ga segregation to grain boundaries and triple points. Decreases of 40V in V_th and 10nits in B₄₀ are observed from ZnS:Mn,Ga ₂S₃ samples annealed at 800°C. It is postulated that Ga₂S₃ creates a range of shallow donor states at the interface close to the conduction band, which causes lower threshold voltages and “leaky” turn-on properties. The low EL brightness is attributed to the low threshold voltage. The lack of reduction of the defects in the microstructure of ZnS:Mn,Ga₂S₃ during anneal is another reason for the poor EL properties.; When samples are doped with Ga₂S₃ first followed by KCl, much better EL properties are observed than from samples doped with KCl first followed by Ga₂S₃ (B₄₀ of 120nits versus 64.2nits). (Abstract shortened by UMI.)...