Observation of Interdiffusion in ZnO/CuInSe 2 Heterostructures and its Effect on Film Properties
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Observation of Interdiffusion in ZnO/CuInSe2 Heterostructures and its Effect on Film Properties Ralf Hunger, Paul Fons, Kakuya Iwata, 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 directly on epitaxial CuInSe2 (001) (CIS) by radical-source molecular beam epitaxy (RS-MBE). The substrate-film interdiffusion was investigated dependent on the ZnO growth temperature. Secondary Ion Mass Spectroscopy (SIMS) profiles indicate the mutual temperature-activated diffusion of Zn and In at a growth temperature of 440ºC which is absent at 250ºC. Zn indiffusion into the CIS substrate leads to characteristic changes in the photoluminescence (PL) properties, whereas the In outdiffusion into the growing ZnO film causes an increased carrier concentration. INTRODUCTION One of the key elements to high-efficiency Cu(In,Ga)Se2 solar cells appears to be a chemical bath deposition (CBD) of a CdS or ZnS buffer layer [1,2]. An essential effect is claimed to be the indiffusion of Cd or Zn into the Cu-deficient CIGS surface layer, occupying CdCu (ZnCu) sites, and thereby generating a buried p-n homojunction [1]. A similar mechanism was reported for the exposure of Cu(In,Ga)Se2 surfaces to Zn vapor at elevated temperatures and subsequent completion of the solar cell with a direct CIGS/ZnO interface [3]. Such a process omits the CBD of a buffer layer and allows for an all-vacuum in-line fabrication process with reduced fabrication costs. We have investigated the direct growth of ZnO on epitaxial CuInSe2(001) by radical-source molecular beam epitaxy (RS-MBE, [4]). In RS-MBE, the Zn is supplied from an elemental Zn effusion cell, therefore Zn exposure steps can be readily employed. In this paper, we focus on interdiffusion phenomena occuring during ZnO growth in dependence of the growth temperature. EXPERIMENTAL CIS(001) substrates with a cation ratio [Cu]/[In]§ and 0.91 were grown epitaxially on undoped GaAs(001) by MBE at Tsub=450ºC [5]. The RHEED pattern during growth indicated the c-axis oriented epitaxial growth of CIS (c-axis perpendicular to the GaAs(001) surface) and the occurance of twinning on {112}, typical for the epitaxy of In-rich CIS on GaAs [6]. The CIS epilayers had a thickness of ~0.6 µm which is well above the critical thickness for latticemismatch relaxation of the CIS/GaAs heterosystem [7]. Hence, the CIS(001) substrates are considered to be essentially strain-relaxed. Onto CIS(001) substrates with [Cu]/[In]§=Q2ILOPVZHUHGHSRVLWHGE\56-MBE at growth temperatures Tgr of 250ºC and 440ºC. In RS-MBE, oxygen is supplied as an atomic beam from a RF-plasma source which is equipped with an ion trap. Zn is supplied from a conventional
H8.21.1
effusion cell [4]. The ZnO/CuInSe2 heterostructures were characterized by Secondary Ion Mass Spectroscop
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