Atomic layer deposition of electroluminescent ZnS, SrS, and BaS thin films

Jarkko Ihanus
Abstract & Cover

The light emitted by flat panel displays (FPD) can be generated in many different ways,  such as for example alternating current thin film electroluminescence (ACTFEL), liquid  crystal display (LCD), light emitting diode (LED), or plasma display panel (PDP)  technologies. In this work, the focus was on ACTFEL devices and the goal was to  develop new thin film processes for light emitting materials in ACTFEL devices. The  films were deposited with the atomic layer deposition (ALD) method, which has been  utilized in the manufacturing of ACTFEL displays since the mid-1980s. The ALD  method is based on surface-controlled self-terminated reactions and a maximum of one  layer of the desired material can be prepared during one deposition cycle. Therefore, the  film thickness can be controlled simply by adjusting the number of deposition cycles. In  addition, both large areas and deep trench structures can be covered uniformly.  During this work, new ALD processes were developed for the following thin film  materials: BaS, CuxS, MnS, PbS, SrS, SrSe, SrTe, SrS1-xSex, ZnS, and ZnS1-xSex. In  addition, several ACTFEL devices were prepared where the light emitting material was  BaS, SrS, SrS1-xSex, ZnS, or ZnS1-xSex thin film that was doped with Ce, Cu, Eu, Mn, or  Pb. The sulfoselenide films were made by substituting the elemental selenium for sulfur  on the substrate surface during film deposition. In this way, it was possible to replace a  maximum of 90% of the sulfur with selenium, and the XRD analyses indicated that the  films were solid solutions. The polycrystalline BaS, SrS, and ZnS thin films were  deposited at 180-400, 120-460, and 280-500 °C, respectively, and the processes had a  wide temperature range where the growth rate of the films was independent of the  deposition temperature. The electroluminescence studies showed that the doped  sulfoselenide films resulted in low emission intensity. However, the emission intensities  and emission colors of the doped SrS, BaS, and ZnS films were comparable with those  found in earlier studies. It was also shown that the electro-optical properties of the  different ZnS:Mn devices were different as a consequence of different ZnS:Mn  processes. Finally, it was concluded that because the higher deposition temperature  seemed to result in a higher emission intensity, the thermal stability of the reactants has  a significant role when the light emitting materials of ACTFEL devices are deposited  with the ALD method.  

Source of Information
FinALD40 exhibition material,
University of Helsinki, Department of Chemistry, Laboratory of Inorganic Chemistry
(Helsinki, Finland)
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