Control of Optical and Electrical Properties of ZnO Films for Photovoltaic Applications
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Control of Optical and Electrical Properties of ZnO Films for Photovoltaic Applications Ralf Hunger, Kakuya Iwata, Paul Fons, Akimasa Yamada, Koji Matsubara, Shigeru Niki, Ken Nakahara1, Hidemi Takasu1 National Institute of Advanced Industrial Science and Technology (AIST), Energy Electronics Institute, Thin Film Solar Cells Group, Tsukuba, Ibaraki 305-8568, Japan 1 ROHM Co Ltd., Optical Device R&D Divison, Kyoto, Japan ABSTRACT ZnO films were grown by radical-source molecular beam epitaxy (RS-MBE) on sapphire and glass substrates, and they were characterized in terms of Hall mobility and optical transmission. Undoped ZnO films exhibit a low intrinsic defect density and optical properties close to bulk ZnO. By Ga doping, a resistance ρ as low as 2×10-4 Ωcm could be achieved. Balancing high conductivity and low transmission losses due to free carrier absorption in the infrared, the optimum was obtained for ρ=3.4×10−4 Ωcm, electron mobility µe=37 cm2/Vs and an average transmission T of 96% in the wavelength range 400-1100 nm. Polycrystalline growth on glass yields slightly reduced but still good film quality (µe=30 cm2/Vs, T=90%). By the incorporation of Mg, conducting Mg0.3Zn0.7O films with an increased band gap up to ~ 4eV were realized. INTRODUCTION In this study we explore the potential of radical-source molecular beam epitaxy (RS-MBE) for the growth of highly conductive and transparent ZnO window layers. In contrast to the conventionally employed sputtering techniques where highly energetic particles are produced, the RS-MBE growth of ZnO using an atomic oxygen beam can be considered as a "soft" deposition technique which is potentially advantageous for the deposition of window layers in solar cells [1]. In order to investigate the effect of extrinsic impurity doping on ZnO film properties, we have first developed a growth technique to reduce the native defect density of non-doped ZnO films, achieving electron densities ne < 1017cm-3 and mobilities µe >100 cm2/Vs. In a second step, we have employed Ga as extrinsic dopant in order to control the carrier concentration. In order to enable band gap and band offset engineering for ZnO-based TCO films, we have in addition implemented the incorporation of Mg during growth and fabricated MgxZn1-xO films. Finally, the application of the growth process to polycrystalline substrates is investigated by comparative growth experiments on glass substrates. EXPERIMENTAL The RS-MBE system was installed in a UHV chamber. ZnO was grown from an elemental Zn effusion cell (7N) and an O* radical source fed by an O2 flow of 0.3 sccm (6N). The O* radicals were produced from a 300W RF-plasma and the exit of charged ions was prevented by an electrostatic ion trap operated at 300V. For the standard deposition process, both-side polished single crystalline a-plane sapphire substrates (i.e. Al2O3(1120 )) with a thickness of 330 µm were employed. By this growth process single-crystalline c-axis oriented ZnO(0001) films are grown epitaxially [2]. In some experiments, Corning® 7059 glass substr
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