Transparent p-n Heterojunction Thin Film Diodes
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Transparent p-n Heterojunction Thin Film Diodes M. K. Jayaraj, A. D. Draeseke, J. Tate, R. L. Hoffman1, and J. F. Wager1 Department of Physics, Oregon State University, Corvallis, OR 97331 USA. 1 Department of Electrical and Computer Engineering, Oregon State University, Corvallis, OR 97331 USA. ABSTRACT Transparent p-n heterojunction diodes are fabricated using p-type CuYO2:Ca and n-type ZnO:Al thin films on a glass substrate coated with indium-tin oxide (ITO). The contact between the n-ZnO:Al / p-CuYO2:Ca heterojunction is found to be rectifying, while the ITO / ZnO:Al contact is ohmic. The typical ratio of forward to reverse current is 15 in the range -3 to 3V. The diode current-voltage characteristics are dominated by the flow of space charge limited current, which is ascribed to the existence of an insulating ZnO interfacial layer. The diode structure has a total thickness of 0.85 µm and an optical transmission of 40%-50% in the visible region. INTRODUCTION The best known transparent conducting oxides (TCOs), such as doped ZnO, In2O3 or SnO2, are n-type. Because of their excellent properties, these films are used in many optoelectronic applications that require metallic-like conductivity and optical transparency. The use of TCOs in the fabrication of active elements in optoelectronic devices has not been possible until now because of the lack of TCOs that exhibit p-type conductivity. The first report of p-type conductivity in transparent CuAlO2 films [1] has attracted much attention, and several similar compounds that share its delafossite structure have been reported with varying conductivity and transparency [2-5]. CuAlO2 and other materials that share the same structure have never been used in p-n junctions or other optoelectronic devices. All-oxide transparent p-n junctions and light-emitting diodes have been successfully fabricated using ZnO and SrCu2O2 [6,7]. SrCu2O2 does not have the delafossite structure, but does feature monovalent Cu in a layered structure. It also requires less stringent growth conditions than CuAlO2. Rectifying behavior in other oxide structures has also been reported, including p-NiO / i-NiO / i-ZnO / n-ZnO [8] and n-ZnO / p-ZnO [9], although well-characterized p-ZnO has proven elusive. Bipolarity has been reported in the CuInO2 delafossite system, with the promise of an oxide p-n homojunction [10]. We have recently focused on materials with the delafossite structure and have developed increasingly conductive p-type delafossite thin films of CuYO2:Ca [3], CuScO2 [2] and CuCrO2:Mg [5], with conductivities of 1, 30 and 220 Scm-1, respectively. Intercalation of oxygen to form CuScO2+x and CuYO2+x:Ca phases is possible because of the larger size of the Sc and Y cations. Such intercalation is not possible for isostructural CuAlO2 because the smaller size of the Al cation shrinks the lattice to the point where oxygen cannot readily penetrate. Oxygen intercalation is likewise impossible for the CuCrO2:Mg compound; however, Mg doping is particularly effective in this case. Here we report t
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