Ultraviolet Emitting SrS:Te Thin Films
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Ultraviolet Emitting SrS:Te Thin Films J.M. Fitz-Gerald 1 , J. Hoekstra1 , J.D.Fowlkes2 , P.D. Rack2 1 University of Virginia, Dept of Materials Science and Engineering, 116 Engineer's Way Charlottesville, VA 22904-4745 2 University of Tennessee, Department of Materials Science and Engineering, 603 Dougherty Hall, Knoxville, TN 37996-2200 ABSTRACT In the bulk SrS has an indirect bandgap of ~ 4.32 eV. When SrS is doped with tellurium, ultraviolet emission occurs at 360 nm (for singlet) and 400nm (for Te-Te dimers) due to recombination from bound exciton states. In this paper we discuss the ultraviolet emission of pulsed laser deposited thin films of SrS:Te grown at room temperature on Si. Deposited film thickness ranged from 0.1 – 1.5 µm, with optimized films grown at ~0.5 µm to avoid peeling and cracking. Te doping was incorporated by both ion implantation and conventional diffusion of deposited Te capping films. The characteristics of the ultraviolet emission will be discussed along with results from microstructural, chemical and optical characterization techniques. INTRODUCTION The development of semiconductor based ultraviolet (UV) light sources is of critical importance for miniaturized ultraviolet light sources, which have application in biological agent detection, non- line-of-sight covert communications, water purification, equipment/personnel decontamination, and white light generation. Currently, a significant amount of research is being performed to extend the III-V nitride blue lasers and light emitting diodes into the ultraviolet region. This research is focused on increasing the aluminum content of III-V alloys and necessarily increasing the semiconductor bandgap. The main obstacle with this approach is the limited concentration of p-type dopant that can be introduced in high aluminum content ternary and quaternary III-V nitride alloys. In this paper we will discuss preliminary work (on SrS:Te) investigating tellurium doped alkaline earth sulfide materials (SrS:Te, CaS:Te, and MgS:Te) as an alternative material for UV and deep UV (DUV) solid-state light sources. Figure 1 shows the alkaline earth sulfide bandgap versus the experimental (BaS:Te and SrS:Te) and calculated (CaS:Te and MgS:Te) emission wavelengths for so-called Te singlet and Te-Te dimer bound exciton emission. The calculations suggest that tunable solid-state emission at wavelengths from 430 to 265 nm is possible with this new class of materials. The theory of bound exciton emission is not new, as the idea of an isoelectronic trap was first introduced by Thomas et al. in 1965 when they used this theory to describe the photoluminescence observed in GaP:N. 1,2 They suggested that even though nitrogen had the same valence as phosphorus (isoelectronic), nitrogen acted as an electron trap in the GaP lattice. It was reasoned that because nitrogen is significantly more electronegative than phosphorus, electrons are preferentially trapped at nitrogen sites. Subsequently, the N-electron complex has a net negative charge, which sets up a shor
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