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B7.2.1

Bandgap engineering of ZnO transparent conducting films

K. Matsubara, H. Tampo, A. Yamada, P. Fons, K. Iwata, K. Sakurai, and S. Niki, National Institutes of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, 305-8568, Japan ABSTRACT Low resistivity and transparent Al doped ZnMgO films were deposited on glass substrates by a pulsed laser deposition system. For up to 32 atm% of Mg content, segregation of a MgO phase was not observed. The bandgap of these films could be widened to about 4 eV with increasing Mg content. The relation between bandgap and resistivity was found to be a trade-off; i.e. the larger the bandgap, the higher the resistivity. The maximum bandgap among films with an electrical resistivity of less than 10-3 Ω cm was 3.94 eV. The average optical transmittance of these films was more than 90 % for wavelengths λ between 400 and 1100 nm. The transmittance around λ = 330 nm was still 50 %.

INTRODUCTION As the band lineup of Cu[InGa]Se2 (CIGS), CdS and ZnO is expected to effect the performance of CIGS thin film solar cells, bandgap engineering of ZnO films is important [1]. Bandgap engineering of ZnO films using a solid solution of ZnO with other materials such as MgO has been studied by several groups [2-4]. However in these studies, ZnMgO was treated as a semiconductor, not as a transparent conducting oxide. We propose the use of the bandgap modified ZnMgO as a transparent conducting film to control the band lineup in CIGS solar cells. As a first step, we have tried to produce low resistivity and transparent impurity doped ZnMgO films. Among several methods to deposit ZnO transparent conducting films, pulsed laser deposition (PLD) is a promising technique to deposit the films with low damage and to survey materials. We have succeeded in producing low resistivity, highly transparent ZnO films by PLD [5,6] and we have employed PLD system in this work.

EXPERIMENTS

Downloaded from https://www.cambridge.org/core. La Trobe University, on 02 Dec 2018 at 16:57:28, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-763-B7.2

B7.2.2

Al-doped and undoped ZnMgO films were deposited by a PLD system. Quartz glass and Corning 7059 glass were used as substrates; the thickness of the substrates was 0.7 mm. The substrate was heated to 200 C by a lamp heater located behind the substrate holder. Films were deposited in an oxygen ambient at a pressure of 1.5 x 10-5 torr. More details of the PLD system can be found in Ref.[5]. The target preparation procedure follows; ZnO (purity: 99.999 %) powder and MgO (purity: 99.99 %) powder were mixed. To make the Al doped target, Al2O3 (purity 99.999 %) powder was also incorporated. The mixed powder was ground, pressed into a pellet and sintered at 950 C in air. The Mg content of the undoped ZnMgO films was determined by inductively coupled plasma emission spectroscopy (ICP) by dissolving the films in HCl solution. Independent calibrations of ICP measurement were done using Zn and