Thermoelectric Properties of ZnO Thin Films Grown by Metal-Organic Chemical Vapor Deposition
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the top n-GaN layers. Moreover, high resolution x-ray diffraction (HRXRD) measurements were performed using a Philips X’Pert MRD triple-axis diffractometer equipped with a four crystals Ge (220) monochromator in the incident beam optics and a Cu sealed anode and room temperature photoluminescence (PL) analyses by utilizing deep ultraviolet (DUV) spectroscopy (excitation at 248nm), to investigate the effect of structural and optical properties of the samples, respectively. DISCUSSION Figures 1 (a) and (b) illustrate the electrical conductivity and mobility of unintentionally doped thin film ZnO samples, respectively. It should be noted that thin film ZnO samples are unintentionally doped that lead to an enhancement of the carrier density and consequently the electrical conductivity. It can be clearly observed that electrical conductivity and mobility increase gradually as the carrier density increases. The highest electrical conductivity 366Ω -1cm-1 and mobility 23cm2V-1s-1 are found at 9.89×1019cm-3. Additionally, the electrical conductivity and mobility exhibit a sudden increase starting at 4×1019cm-3, which could be attributed to impact of point defects, such as oxygen vacancy and/or Zn interstitial. This is because slight variation in the oxygen content results in compositional defects similar to oxygen vacancies or zinc interstitials, which act as donors and these defects results in scattering of electrons, therefore induces electrical conductivity increment or decrement. For instance, as the scattering centers increase the electrical conductivity decreases since mobility decreases at the same time as well. a)
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Measured mobility Polar optical-phonon Piezoelectric
Acoustic-phonon Ionized impurity Total
Piezoelectric
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-1 -1
Mobility (cm V s )
Acoustic-phonon
Polar optical-phonon Ionized impurity Total
1E18
1E19
1E20 -3
Carrier Density (cm )
Figure 1: (a) Electrical conductivity and (b) mobility of thin film ZnO as function of carrier density at 300K (The dashed line is guided to the eye). x and y axes are logarithmic scale for both figures. In Figure 1 (b), the influences of different scattering mechanisms to the total mobility are depicted by dashed curves. The mobility is measured between the values of 5 to 20cm2V-1s-1, which are relatively poor compared to thin film GaN at similar carrier densities, however these values are in agreement with other reported results [18,19]. The contribution of different scattering mechanisms on ZnO mobility is presented in Figure 1 (b) using the Boltzmann transport equation with a variational
method. The list of material properties to calculate the electron scattering mechanisms including polar optical-phonon scattering, ionized impurity scattering, acoustic-phonon scattering, and piezoelectric interactions are also determined in reference [20]. It can be observed that electron mobility data dramatically increases and becomes closer to the theoretical limit as the carrier density increases. However, the measure
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