Determining the nonparabolicity factor of the CdO conduction band using indium doping and the Drude theory

Abstract

Abstract. Due to their high intrinsic electron mobility, CdO-based materials are gaining interest as transparent conductive oxides. By creating model dielectric functions based on the Drude theory, accurate fits to the measured transmittance and reflectance of CdO and CdO:In thin-films were achieved without using a frequency dependent Drude damping parameter. Difference in the model between undoped and In-doped CdO showed that the Burstein-Moss shift is not the only mechanism which improves the transparency in In-doped samples. Comparing the Drude analysis with Hall measurements revealed a nonlinear relationship between the free-electron effective mass and the carrier concentration, an effect which is caused by the nonparabolicity of the CdO conduction band. Analysis of 50 CdO:In thin-films grown by pulsed filtered cathodic arc showed the nonparabolicity factor was C = (0.5 ± 0.2) eV −1 and the band-edge effective mass was (0.16 ± 0.05)me. Knowledge of the effective mass allows for optical measurements of carrier mobility, which was less than or equal to the Hall measured mobility in these films due to the large electron mean-free-path compared to the grain size.

Structural, optical, and electrical properties of indium-doped cadmium oxide films prepared by pulsed filtered cathodic arc deposition

“…It is extremely important to note that the Tauc relation assumes a parabolic conduction band. However, in our previous study, nonparabolicity of the conduction band in heavily doped arc-grown CdO films was clearly observed [37]. Therefore, the bandgap of CdO:In films is also calculated using the derivative of the transmittance spectra [38,39].…”

“…Based on Jain's model, in a heavily doped semiconductor, the bandgap renormalization can be described by [14,42] [21,22,37], the relative dielectric constant is 21.9 [43], and the values of Λ and N b are 1 [42]. The three terms in equation (6) represent the exchange energy of the majority carriers, the correlation energy, and the impurity interaction energy, respectively.…”

Dopant-induced band filling and bandgap renormalization in CdO : In films

“…For these CdO and CdO:In samples, ω p was obtained by simultaneously fitting the NIR transmittance and reflectance of each film, and n was taken from Hall measurements [10]. Using Pisarkiewicz's model [20], it has been observed that the nonparabolicity factor and the band edge effective mass of arc-grown CdO:In films are about 0.5±0.2 eV -1 and 0.16±0.05 m 0 , respectively [10]. The obtained values are in line with the results using the alpha approximation which was derived from Kane's two band k.p model [21].…”

“…Under most circumstances, it has been ignored that the Tauc relation is based on parabolic valence and conduction bands. As such, the Tauc relation is extensively misused to determine the optical bandgap in doped CdO since the conduction band exhibits clear nonparabolicity [4,5,10]. Thus, optical bandgaps obtained from Tauc plots may add controversy when studying the bandgap shift.…”

Transparent conductive Nd-doped ZnO thin films

“…The measured transmittance spectra of films were fitted by the calculated transmittance from a dielectric model using Scout commercial software, which includes several built-in optical models [33][34][35]. An interband transition model of the absorption edge and the extended Drude free electron model for the free carriers were used, as applied previously for different doped ZnO thin films [30][31][32][33].…”