José Manuel Atalaia Rosa will defend his doctoral thesis “Red-emitting Phosphor Fabrication by Atomic Layer Deposition” on 29 April 2022 at 12:00 in Aalto University School of Electrical Engineering, Department of Electronics and Nanoengineering, in lecture hall AS2 and online in Zoom.
Opponent: Prof. Ola Nilsen, University of Oslo, Norway
Supervisor: Prof. Harri Lipsanen, Aalto University School of Electrical Engineering
Abstract:
Atomic layer deposition (ALD) is a promising fabrication technology used to solve the difficult prerequisites for the development of novel materials. The excellent fabrication control at the atomic level allowed by ALD, provides ideal conditions for the development of high-quality thin films. Recently, the atomic control capabilities of ALD are being exploited to create mixed-anionic compounds, which take part in the next-generation of hybrid materials. The controlled addition of different anionic species in a thin film may unlock new and enhanced material functionalities. This thesis explores the ability to change the properties of well-known red-emitting phosphors by controlling the inclusion of anions during the ALD growth. These phosphor materials can be used in many other advanced optoelectronic applications.
This work starts by studying the structural and luminescent properties of two red-emitting CaS:Euphosphors grown by ALD. The difference between the samples relies on the growth of the dopant element. In one sample, Eu is grown on the CaS matrix by reacting Eu(thd)3 with H2S originating CaS:Eu phosphor, whereas in the other sample Eu(thd)3 reacted with O3 forming CaS:EuOphosphor. It was shown that the emission wavelengths of CaS:EuO and CaS:Eu were ~625.8 nm and ~647 nm, respectively. Therefore, the addition of O2- ions in the CaS:Eu phosphor results in a blue-shift of 21.1 nm in relation to the sample without oxygen. Further, the studies of photoluminescent and structural properties for Y2O3−xSx:Eu thin film were carried out. Here the Eu ions were grown by pulsing either O3 or H2S. The Y2O3−xSx:Eu thin film grown with Eu(thd)3/O3 sequence emitted a red/pink colour with emission spectra between ~550 nm and 720 nm. Instead,the use of Eu(thd)3/H2S sequence led to a violet/blue emission with the wavelength lower than 500nm. As shown in these two studies, ALD can be used as an efficient method to control anionic inclusion and manipulate the properties of inorganic phosphors. This effect can be useful in the development of optoelectronic components, such as electroluminescent (EL) devices.
Using a similar recipe developed in the Y2O3−xSx:Eu study, a Y2O3:Eu thin film was grown by ALDat 300 °C. The objective of developing Y2O3:Eu thin film at relatively low temperature was to address the lack of efficient red-emitting EL devices. In this study, the doping level of Y2O3:Euphosphor was optimized, and the structural and optical properties of the film were addressed. TheY2O3:Eu thin film was then used as a phosphor material in a multi-layered red-emitting EL device. The electrical performance of the EL device was determined using electroluminescent measurements and showed a maximum luminance of ~40 cd/m2 at a driving frequency of 1 kHz and an efficiency of ~0.28 lm/W. Overall, the results presented in this work support the idea that rare-earth materials grown by ALD have tremendous potential in the advancement of photonic and optoelectronic applications.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 764951.