Mobility in SnO₂:F Thin Polycrystalline Films: Grain Boundary Effect and Scattering in the Grain Bulk

Abstract

Basic electronic properties relevant to the carrier mobility were studied in tin oxide thin films doped with fluorine, prepared by atmospheric pressure chemical vapor deposition. Electrical resistivity, Hall and Seebeck effects, plasma and collision frequencies were measured (the last two by using multiangle spectral ellipsometry) and analyzed for films with carrier concentrations from 1.8×1020 to 5.6×1020 cm−3. Scanning over the sample area of resistivity (four-point probe method) and Seebeck coefficient (thermoprobe) monitored uniformity of electronic properties in plane. Ellipsometry was used to check uniformity over the film thickness. Films with a thickness above 400 nm demonstrated high spatial uniformity and were used for further studies.Effective mass was determined from combined Hall and plasma frequency measurements and was found to be independent of carrier concentration, which indicates a parabolic band spectrum. Its value was very close to the literature data. In films with carrier concentration ≥3×1020 cm−3 the Hall mobility was very close to the optical mobility calculated based on collision frequency and effective mass values. This indicates a very small contribution of grain boundaries to the total resistivity of films. Thus the measured mobility is close to the electron mobility in the grain bulk.The scattering parameter value derived from thermopower measurements along with the temperature independent mobility indicated that electron scattering by impurity ions screened by free carriers is the dominating scattering mechanism. Theoretical estimates of mobility are very close to the highest measured mobility values (≥ 30 cm2/Vs) if the spatial dispersion of the dielectric constant is taken into account. Comparison of differently processed films showed that compensation of donor dopant with uncontrolled acceptor centers significantly impacted mobility.

Transparent Conducting Oxides: Texture and Microstructure Effects on Charge Carrier Mobility in MOCVD-Derived CdO Thin Films Grown with A Thermally Stable, Low-Melting Precursor

“…Therefore, it can be concluded that GBS is not a dominant process, even in high-quality epitaxial CdO films with modest carrier concentrations and small grain sizes, 2 × 10 20 cm -3 and ∼100 nm, respectively. This conclusion is in agreement with recent results suggesting that GBS should be insignificant in TCO films based on alternative, carrier mean-free-path arguments. ,,,, According to Zhang and Ma, a TCO with a useful conductivity will be degenerate with a high carrier concentration, limiting the carrier mean-free-path to a value much smaller than the grain size of typical films. The formulation for carrier mean-free-path is given in eq 8. , …”

“…This conclusion is in agreement with recent results suggesting that GBS should be insignificant in TCO films based on alternative, carrier mean-free-path arguments. 17,18,21,23,24 According to Zhang and Ma, 18 a TCO with a useful conductivity will be degenerate with a high carrier concentration, limiting the carrier mean-free-path to a value much smaller than the grain size of typical films. The formulation for carrier mean-freepath is given in eq 8.…”

“…Carrier scattering mechanisms and hypothesized limits to TCO film mobility have been the subject of extensive discussion. − Mobile carriers in TCOs arise from lattice defects created through nonstoichiometry (oxygen vacancies or cation interstitials) or aliovalent doping (e.g., Sn 4+ on an In 3+ site). These point defects are generally ionized and scatter carriers strongly.…”

“…While it is generally accepted that ionized impurity scattering (IIS) is the dominant scattering mechanism in TCOs, the roles of neutral impurity scattering (NIS) and grain boundary scattering (GBS) are poorly understood. Dismissed as unimportant in some analyses, − ,, examples can also be found where NIS − and GBS ,,, are reported to have pronounced effects on carrier mobilities. Given the large mobility and simple face-centered cubic crystal structure, CdO is an ideally suited model thin-film material for closer examination of these effects.…”

Tuning the Properties of Transparent Oxide Conductors. Dopant Ion Size and Electronic Structure Effects on CdO-Based Transparent Conducting Oxides. Ga- and In-Doped CdO Thin Films Grown by MOCVD

“…Similar to Y- and Sc-doped CdO, , the mobilities and conductivities of CGO films are essentially independent of temperature in the low-temperature region (<100 K), suggesting that neutral impurity scattering (NIS) and/or ionized impurity scattering (IIS) processes are dominant. ,– In the high-temperature region (>100 K), the mobility and conductivity decrease with increasing temperature, suggesting that lattice vibration scattering (LVS), which is temperature-dependent, has now become an important scattering contributor. – The importance of grain boundary scattering (GBS) is an incompletely resolved mechanistic issue in most CdO-based TCOs. It has been argued that GBS is insignificant for highly degenerate TCOs because the carrier mean free paths (determined optically) are typically much smaller than the grain sizes of typical films. ,,– …”