Improved Transparent Conducting Oxides Through Modulation-Doped ZnO/ZnMgO Thin Films

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ZnO is a member of the unique class of materials known as transparent conducting oxides (TCOs). TCOs are currently used for many applications including flat panel displays, solar cells, and energy efficient windows. Of particular interest is the possibility of developing materials that have high electron mobilities, such that conductivities may be increased without loosing transparency in the visible spectrum. Modulation doping was chosen as a possible technique to achieve this outcome. ZnMgO:Al films were grown and characterized as a potential barrier layer in a modulation doped structure. High quality films, rocking curve FWHM ~1- 2o, were grown on c-plane sapphire substrates. The ability of Mg to increase the band gap of ZnO up to a value of 3.76 eV was confirmed. Aluminum was used as a donor in ZnMgO, and maximum carrier concentration levels of ~1 x 1020 cm-3 were achieved. The wide band gap semiconductor ZnMgO:Al was determined to be a suitable choice as a barrier layer in a modulation doped structure. A one dimensional Schrodinger/Poisson simulation program was used to investigate the influence of the parameters in a modulation doped ZnO/ZnMgO:Al structure on the film properties. The optimum electrical properties were achieved when the active ZnO layer and barrier ZnMgO:Al layer were both in the range of 2-5 nm. The optimum thickness for the ZnMgO spacer layer was calculated to be 1.5 nm. Mobilities as high as 145 cm2/Vs were predicted for the optimum structures, compared to ~30 cm2/Vs in monolithic ZnO films. The maximum sheet electron density that could be transferred from the doped to the undoped layers was predicted to be ~1013 cm-2. Multilayer structures were grown and characterized. Following the trends predicted from the multilayer simulations, a five period multilayer with ZnO and ZnMgO:Al layers of 5 nm had a mobility of ~33 cm2/Vs and a resistivity of 1.44 x 10-3 W cm, compared to a multilayer with 20 nm thick layers which had a mobility of ~23 cm2/Vs and a resistivity of 2.45 x 10-3 W cm. The experimental results were in reasonable agreement with the predictions of the above simulation.

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  • 10/01/2018
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