Blue-emitting CaS:Cu,F phosphor fabricated by thermal incorporation of Cu and F into ALD-grown CaS thin film

Abstract

II-VI compounds doped with luminescent centers have been traditionally used as phosphor materials for display devices. Recently, thin film process of the phosphors need to be developed due to the stringent demands on both low voltage field emission display (FED) and full color thin film electroluminescent displays (TFELD). Since 1980s, very reliable Mn-doped zinc sulfide (ZnS) thin film phosphor have been successfully used for monochrome thin film EL and thin film color phosphors adopting alkaline-earth sulfides, such as calcium sulfide (CaS) and strontium sulfide (SrS), as host materials have shown promising results [1–4]. For practical use of these phosphors, however, there have been many problems to be solved. Especially, the preparation of CaS-based phosphors is more difficult compared to ZnS, because of their chemical instability and hygroscopic nature resulting in non-stoichiometric composition and poor crystallinity at a preparation temperature lower than 800 ◦C [1]. In recent years, stoichiometric CaS films could be grown using atomic layer deposition (ALD) [5]. ALD is a self-limiting deposition process utilizing surface saturation by alternating chemisorption of the precursors and surface reaction. However, a major obstacle in the use of ALD process for luminescent center doped CaS thin film has been the lack of suitable precursors for some dopants such as Cu and Eu. The addition of dopants in physically deposited host materials has been accomplished by thermal evaporation of compound sources, such as Cu2S, CuCl2, EuF3, etc., followed by a high temperature annealing [6]. However, it was difficult to maintain very low evaporation rate resulting in the concentration of luminescent centers lower than a few % [7]. The present work reports an approach to prepare a more efficient blue emitting CaS:Cu,F thin film phosphor by doping luminescent centers using a drivein anneal. Cu-doping was accomplished by chemical vapor deposition (CVD) of very thin Cu film containing considerable amount of F and the subsequent