Reactive Sputtering of Magnesium Hydride Thin Films for Photovoltaic Applications
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1210-Q03-15
Reactive sputtering of magnesium hydride thin films for photovoltaic applications C. Platzer-Björkman1,2, T. Mongstad1, S. Karazhanov1, J.P. Mæhlen1, E. Marstein1, and A. Holt1 1
2
Institute for Energy Technology, NO-2027 Kjeller, Norway Uppsala University, Solid State Electronics, Box 534, SE-751 21 Uppsala Ph: +47-63606463, email: [email protected]
ABSTRACT Deposition of MgHx (MgH2 + Mg) thin films is performed using RF reactive sputtering in argon-hydrogen plasma. Films are characterized using x-ray diffraction (XRD), scanning electron microscopy, optical and resistivity measurements. Formation of crystalline MgH2 is confirmed by XRD, but the formation of some metallic Mg in the films could not be avoided. Increased H/Mg ratio by deposition at high hydrogen flow or high total pressure gives films that oxidize within days or weeks. Deposition at elevated substrate temperature results in improved crystallinity and stability. Initial studies of MgHx for silicon surface passivation are presented. INTRODUCTION Metal hydride materials have been thoroughly studied for hydrogen storage, battery and switchable windows applications. In these cases low stability is an important requirement, as it enables hydrogenation/ dehydrogenation at reasonable temperatures and pressures. Due to the insulating or semiconducting nature of many metal hydrides, they were recently proposed for application in photovoltaic devices [1]. The high hydrogen content in these materials could possibly be beneficial for passivation in silicon-based devices and also used as antireflective coating. Another possibility, based on the suitable and large span of band gap energies for the metal hydride material group, could be hydride-based multijunction devices. Both applications would require stable materials as well as favorable electrical and optical properties. MgH2 is an insulator with a band gap of 5.6 eV [2] that has been studied for hydrogen storage purposes due to the light weight, high hydrogen content and large availability of Mg. However, the hydrogenation and dehydrogenation processes are slow, in particular for large particle sizes [3]. Thin films of MgH2 have been prepared by hydrogenation of Pd capped Mg films, and by activated reactive evaporation. In the case of hydrogenation of metallic Mg with a thin palladium cap, completely transparent films with surprisingly low resistivity are obtained [4]. In-situ deposition of MgH2 was reported using activated reactive evaporation of magnesium with an atomic hydrogen source [5]. In that case, insulating, amorphous films with a brown/transparent appearance were obtained. By modeling the optical properties using effective medium theory it was concluded that the films consisted of metallic magnesium particles in magnesium hydride. About 10 vol.% of metallic Mg could not be prevented for in-situ grown films. In this work we present initial experimental studies of in-situ deposition of metal hydride thin films using RF sputtering of magnesium in a hydrogen containing plas
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