Origin of High Mobility in Molybdenum-Doped Indium Oxide

Abstract

Molybdenum-doped indium oxide (IMO) thin films prepared by aerosol-assisted chemical vapor deposition (AACVD) show significantly improved charge carrier mobilities as compared to nominally undoped films prepared by the same technique. The basis for this very unusual behavior has been investigated by density functional theory calculations using a hybrid Hamiltonian, mobility modeling, X-ray photoemission, and X-ray absorption spectroscopies. In contrast to previous claims that Mo acts as a three-electron donor, it is shown that substitutional Mo traps two electrons in localized states falling within the bulk bandgap and thus Mo is a simple one-electron donor. At the same time, there is very little hybridization of Mo 4d states with In 5s states at the bottom of the conduction band. This results in conduction that is spatially separated to some degree from the donors, giving rise to significantly reduced ionized impurity scattering, enhancing the carrier mobility. This is in contrast to Sn-doped In2O3 where the conduction band minimum has significant Sn 5s character, resulting in regular ionized impurity scattering.

Charge mobility increase in indium-molybdenum oxide thin films by hydrogen doping

“…5(a). For as-deposited "IMO ArOx" films (deposited without hydrogen) E g = 3.8 eV, in agreement with that previously reported for IMO films [9]. Unlike the effect reported for ITO [23], the addition of hydrogen in the sputtering gas does not change appreciably the optical properties of IMO films: transmittance is not altered and E g does not show systematic variations as a function of the H 2 -content in the sputtering gas.…”

“…These experimental facts may be explained by the reduction from Mo 6+ to Mo 4+ , with the consequent Mo-O distance accommodation and the ordering of the In 2 O 3 lattice in the Mo neighborhood. After annealing, the obtained Mo-O and Mo-In distances are in agreement with those reported for IMO films prepared by aerosol-assisted chemical vapor deposition [9].…”

“…Higher mobility in IMO is attainable because the d-states of Mo do not hybridize with the s-states of In [8,9] resulting in charge carriers effective mass similar to that in undoped In 2 O 3 .…”

“…In a first step, the presence of Mo 6+ , detected by X-ray photoemission spectroscopy (XPS), in IMO films deposited at high substrate temperature, was considered to explain the observed increase in mobility [2,6] in contrast with those prepared at room temperature. Nevertheless, a clear correlation between Mo 4+ content and charge carrier concentration was finally established [18,9].…”

Transparent conducting n-type ZnO:Sc – synthesis, optoelectronic properties and theoretical insight

“…10 While Al-doped and Ga-doped ZnO have received much attention as extrinsically n-type doped ZnO systems in recent years, reports on other trivalent dopants are reported considerably less often. In particular, Sc-doped ZnO (SZO) has shown promise as an alternative TCO material with low resistivity and high visible transmittance, 4,11 however with relatively few deposition studies having been carried out and still fewer theoretical studies developing an understanding of this material.…”