Optical Properties of Magnesium-Zinc Oxide for Thin Film Photovoltaics

Abstract

Motivated by their utility in CdTe-based thin film photovoltaics (PV) devices, an investigation of thin films of the magnesium-zinc oxide (Mg_xZn_1-xO or MZO) alloy system was undertaken applying spectroscopic ellipsometry (SE). Dominant wurtzite phase MZO thin films with Mg contents in the range 0 ≤ x ≤ 0.42 were deposited on room temperature soda lime glass (SLG) substrates by magnetron co-sputtering of MgO and ZnO targets followed by annealing. The complex dielectric functions ε of these films were determined and parameterized over the photon energy range from 0.73 to 6.5 eV using an analytical model consisting of two critical point (CP) oscillators. The CP parameters in this model are expressed as polynomial functions of the best fitting lowest CP energy or bandgap E₀ = E_g, which in turn is a quadratic function of x. As functions of x, both the lowest energy CP broadening and the Urbach parameter show minima for x ~ 0.3, which corresponds to a bandgap of 3.65 eV. As a result, it is concluded that for this composition and bandgap, the MZO exhibits either a minimum concentration of defects in the bulk of the crystallites or a maximum in the grain size, an observation consistent with measured X-ray diffraction line broadenings. The parametric expression for ε developed here is expected to be useful in future mapping and through-the-glass SE analyses of partial and complete PV device structures incorporating MZO.

Keywords: (Mg,Zn)O; CdTe photovoltaics; optical properties; spectroscopic ellipsometry.

Figure 1

X-ray diffraction patterns measured with Cu-Kα radiation (wavelength: 0.154056 nm) for Mg_xZn_1−xO thin films deposited with different Mg contents on room temperature soda lime glass substrates and annealed at 250 °C for 120 min. The samples are labelled according to their bandgaps.

Figure 2

Crystalline size applying the Scherrer equation to the widths of the diffraction peaks for the MZO samples of Figure 1. The error bars are the standard deviations arising from independent measurements from the (002), (101), (102), and (103) diffractions, the four most visible peaks in Figure 1. The line is a guide for the eye.

Figure 3

Bandgap of MZO thin films as determined from NIR-UV spectroscopic ellipsometry (SE) analysis plotted as a function of Mg atomic fraction as determined by energy dispersive X-ray spectroscopy (EDS) (squares). Over this range of Mg content, the MZO films exhibit a dominant wurtzite phase. Error bars on the Mg content arise from the standard deviation of multiple measurements whereas error bars on the bandgap arise from the confidence limits in the SE analysis. The latter are in the range of ±0.001 to ±0.003 eV, smaller than the data point size. The solid blue line is the best fitting quadratic expression. Experimental data from Refs. [16] (triangles) and [40] (circles) and a theoretical result from Ref. [41] incorporating a bowing parameter (black line) are also shown.

Figure 4

(a) Ellipsometric spectra (ψ, Δ) for a Mg_xZn_1−xO sample fabricated on a soda lime glass substrate with power levels of 240 W and 500 W applied to the ZnO and MgO targets, respectively, yielding a Mg content of x = 0.32 ± 0.06 and a bandgap of E₀ = E_g = 3.690 ± 0.002 eV. The experimental data (points) are plotted along with the best fitting simulation (lines). The plot corresponds to results for a single representative location with coordinates (3.75, 4.69) (in cm), measured such that the center of the 15 cm × 15 cm sample is (0, 0). Here the Mg_xZn_1−xO bulk layer and effective thicknesses are ~225.6 nm and 232.0 nm, respectively. The parameters defining the complex dielectric function spectra from the optical model appear in Figure 5 with best polynomial fits versus E₀ = E_g reported in Table 2. (b) The structural model and associated parameters corresponding to the best fit in (a) are shown.

Figure 5

Best fitting variable parameters of lower energy CP (a) amplitude, (b) broadening, and (c) exponent, as well as the (d) Urbach slope representing the MZO dielectric function according to Equations (1) and (2).

Figure 6

Analytically determined complex dielectric function (ε = ε₁ − iε₂) spectra for hypothetical Mg_xZn_1−xO films with different Mg contents x and bandgap energies E₀ = E_g. (a) photon energy of ε₁; (b) photon energy of ε₂. These spectra were determined from Equations (1) and (2) using the bandgap and linked E_t value, seven fixed parameters, and four polynomial descriptions as presented in Table 2. The polynomial expressions in Table 2 enable linking the additional four parameters to the bandgap, enabling the dielectric function spectra to be specified by the single bandgap variable.