Electrical and Optical Properties of Cr2O3 Films Prepared by Chemical Vapour Deposition

Abstract

Polycrystalline Cr2O3 films are prepared by normal pressure chemical vapour deposition (CVD), and electrical and optical properties of the films are investigated. The films are transparent from 800 to 1000 nm wavelength, and absorbing for wavelengths shorter than 800 nm. The optical band gap energy is Eopt = 2.98 to 3.09 eV for films formed at different substrate temperatures. The electrical conductivity σ of the films is from 1 × 10−2 to 2.5 × 10−3 S cm−1 at 500 K. The films are p‐type semiconducting. At temperatures higher than 500 K. the conduction is due to small polaron hopping. For temperatures lower than 500 K, a T−1/4 dependence of σT1/2 is found, which is attributed to variable‐range hopping.

Conducting mechanism in the epitaxialp-type transparent conducting oxideCr2O3:Mg

“…This suggests that there is only one dominant activation mechanism. A previous paper for undoped Cr 2 O 3 also found no significant dependence of the Seebeck coefficient on temperature up to 450 K [30], which is supported by our data on Seebeck measurements with a higher temperature differential (see Fig. 9 inset).…”

“…This dependency is consistent with many findings for p-type oxides, where a suppression of compensating defects such as oxygen vacancies V O or an increase in hole creating oxygen interstitials O i leads to improved conductivity [25]. Although Cr 2 O 3 is known as an oxygen excess oxide [24,30], suggesting oxygen interstitial formation, we do not believe that these defects make a significant contribution to the conductivity. The solubility of MgO in Cr 2 O 3 is related to the oxygen partial pressure and the growth temperature [47].…”

“…Obtained W values range from 210-300 ± 5 meV, in line with previous reports of ∼250 meV for nonstoichiometric Cr 2 O 3 [21,30]. Equation (3) is valid once the hopping activation energy is the dominant, limiting term in the conduction.…”

“…There is uncertainty in the literature over the exact conduction mechanism in Cr 2 O 3 . Band conduction [29], SPH [21], and variable range hopping [30] have been suggested. Indeed, it is heavily discussed in the literature what model is applicable for p-type TCOs.…”

Band-structure calculations for the 3dtransition metal oxides inGW

“…The optical characterizations are, however, also subject to uncertainties if the minimum band gap is indirect or optically forbidden, or when excitonic effects, including also internal d-d transitions, cause strong subgap absorption. Thus, a given absorption onset could signify the indirect/forbidden band gap (e.g., in a bulk sample), the direct-allowed band gap (e.g., in a thin-film where phonon-assisted and disorderinduced transitions are too weak to contribute sufficiently to the absorption), the threshold for exciton generation (e.g., in wide gap systems like SiO 2 with large exciton binding energies [20]), or the excitation of internal d-d transitions (e.g., in Cr 2 O 3 the absorption bands observed around 2.1 and 2.8 eV [49] coincide with well-known d-d internal excitations of octahedral Cr +III [78]). These considerations need to be taken into account when interpreting optical measurements to in terms of band gap energies.…”