Material properties of Cd1 x Mg x O alloys synthesized by radio ...

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APPLIED PHYSICS LETTERS 103, 041902 (2013) Material properties of Cd 12x Mg x O alloys synthesized by radio frequency sputtering Guibin Chen, 1,2 K. M. Yu, 1,a) L. A. Reichertz, 1,3 and W. Walukiewicz 1 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA 2 Department of Physics and Jiangsu Key Laboratory for Chemistry of Low Dimensional Materials, Huaiyin Normal University, Jiangsu 223300, People’s Republic of China 3 RoseStreet Labs Energy, Phoenix, Arizona 85034, USA (Received 27 February 2013; accepted 2 July 2013; published online 22 July 2013) We have studied structural, electrical, and optical properties of sputter deposited ternary CdMgO alloy thin films with total Mg concentration as high as 44%. We found that only a fraction (50%–60%) of Mg is incorporated as substitutional Mg contributing to the modification of the electronic structures of the alloys. The electrical and optical results of the Cd 1 x Mg x Oalloysare analyzed in terms of a large upward shift of the conduction band edge with increasing Mg concentration. With the increase of the intrinsic bandgap, appropriately doped Cd-rich CdMgO alloys can be potentially useful as transparent conductors for photovoltaics. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4816326] Metal oxides have been extensively studied over the last two decades. 1–6 The reason for the wide interest in these materials is that they can be highly conductive and transparent over a wide wavelength region at the same time. Wide gap oxides such as In 2 O 3 :Sn (ITO) 7–10 or ZnO:Al (AZO) 11–13 are widely used as transparent contacts on thin film solar cells. An important limitation in the applications of these oxides is that they have significant long wavelength absorption and thus cannot be used for photovoltaics that utilize the infrared part of the solar spectrum. The poor infrared transmittance of these materials can be attributed to the relatively strong ionized impurity scattering resulting in low mobility and large free carrier absorption. In addition, due to the high electron concentration in these materials (10 21 /cm 3 ) their plasma reflection edge typically occurs at wavelengths
041902-2 Chen et al. Appl. Phys. Lett. 103, 041902 (2013) FIG. 1. (220) x-ray diffraction peaks from a series of Cd 1 x Mg x O alloys with different substitutional Mg concentrations. substitution of Cd with Mg. The amount of substitutional Mg in Cd 1 x Mg x O can be derived from the shift of the diffraction peak using Vegaard’s law. Here we use the values of 4.212 Å and 4.6958 Å for the lattice parameter of MgO and CdO, respectively. Figure 2(a) compares the Mg concentrations determined by XRD with those measured by RBS. We note that RBS measures the total Mg concentration whereas peak shifts in XRD arise from lattice parameter change due to the substitution of Cd with Mg. Since the electronic band structure is affected by the composition of substitutional Mg, therefore, in the following, the samples will be identified by the composition obtained from the XRD. Figure 2(a) shows that a significant fraction of the total Mg is incorporated on nonsubstitutional sites, most likely in the forms of Mg interstitials and Mg or MgO precipitates. In ZnMgO Mg interstitials can increase the lattice parameter of the alloy and hence partially compensate the reduction of the lattice parameter due to Mg substitution. As a result, if significant amount of Mg interstitials are present the XRD measurements may underestimate the substitutional Mg. However, we found that as the FIG. 2. (a) Dependence of the substitutional Mg concentration on the total Mg concentration. (b) Grain size of the Cd 1 x Mg x O films as a function of Mg concentration. The dashed line connecting the data is a polynomial fit to the data. FIG. 3. Carrier concentration (n) and mobility (l) as a function of Mg concentration, x, in the Cd 1 x Mg x O films. Mg composition increases, the electron concentration decreases in the alloy (Fig. 3), and this does not support the presence of significant amount of Mg interstitial donors. This suggests that Mg interstitials are not the dominating defects (
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