Infrared emission from zinc sulfide: Rare-earth doped thin films

Abstract

Infrared (IR) electroluminescent (EL) thin film phosphors were radio frequency magnetron sputter deposited by cosputtering of an undoped ZnS target together with ZnS: 1.5 mole % ErF3 or ZnS: 1.5 mole % NdF3 targets. The ZnS:ErF3 and ZnS:NdF3 thin film phosphors were annealed in a N2 ambient at temperatures ranging from 350 to 475 °C for 1 h to increase radiance. The maximum EL radiance observed was 28 μW/cm2 at 1550 nm for ZnS:ErF3, and 26 μW/cm2 at 910 nm and 15 μW/cm2 at 1060 nm for ZnS:NdF3 (at 40 V above the threshold voltage) after a 425 °C, 1 h anneal in nitrogen. For anneals above 425 °C visible emission increased, while near infrared (NIR) emission from both ZnS:ErF3 and ZnS:NdF3 was either constant or decreased. For ZnS:ErF3, the 1550 nm NIR peak decreased, but the 990 nm peak remained constant in intensity. The crystallinity of ZnS was improved by annealing, and these results are consistent with the postulate that residual defects limit the acceleration of “hot” electrons for anneals at ⩽425 °C. Under these conditions, hot electrons only have sufficient energy to excite Er+3 into the lower lying I413/2 excited state which leads to 1550 nm NIR emission. With increasing annealing temperatures, hot electrons can excite from the I415/2 ground state into higher energy excited states (e.g., the F47/2 state for 990 nm emission). The NIR emissions from ZnS:NdF3 at 910 and 1060 nm originate from the same excited state and both peaks exhibited maximum NIR intensities after annealing at 425 °C. While the emission spectra from Er were independent of annealing temperature, peak shifts were observed for Nd. These shifts were discussed in terms of the nephelauxetic effect and hybridization of the 5d–4f orbitals.

Effects of processing parameters on electroluminescence of rf magnetron sputter deposited ZnS:ErF3

“…Annealing phosphor films at 425°C for 1 h resulted in a 60% increase in the mean brightness, consistent with the effects of postdeposition annealing reported by Kale et al 20 Hsu et al 21 reported that annealing at temperatures 300°C increased the EL brightness and reduced the concentration of F in ZnS : TbF 3 ACTFELDs. They attributed the increased brightness to a reduction in F and improved film crystallinity.…”

8.3: Flexible Infrared Emitting ZnS:ErF[sub 3] Alternating Current Thin Film Electroluminescent Devices

“…After the phosphor was deposited, dopant activation was performed by annealing the multilayer stack at 425 o C in UHP N 2 for 1 hour. This annealing temperature was found to be optimal, and to significantly increase the infrared output of RF magnetron sputtered ZnS:ErF 3 phosphor films [4,5]. The order of annealing and deposition of the top insulator and could be reversed without noticeable detriment to device performance.…”

Enhanced outcoupling of electroluminescence from ZnS:ErF3 thin films by a photonic crystal

“…[2][3][4] Applications of near-infrared emitters include telecommunications, phototherapy, chemical detection, friend-foe identification, and other night-vision technologies. ACTFEL displays exhibit high contrast, wide viewing angle, fast response time, low power, long operating life, crisp images, and the capability for very high resolution, which have all led to commercially available visible light emitting ACTFEL displays 1 while prototype devices emitting in the near infrared have been reported.…”