Electrical Properties of Stannic Oxide Single Crystals

Abstract

The Hall coefficient and electrical resistivity have been measured on single-crystal specimens of «-type Sn02 between 80 and 900°K. Data were obtained on samples in the "as grown" state, as well as on crystals which were thermally equilibrated in oxygen. A single-donor-level analysis on heat-treated crystals with donor concentrations (ND) between 1X10 16 and 5X10 17 cm" 3 resulted in values of the "density-of-states" effective mass m (JNr) =0.22m. Values of the donor ionization energy ED were found to decrease with increasing ND. At infinite dilution ED has an estimated value of 0.15 eV. Room-temperature thermoelectric-power measurements resulted in calculated values of m {N) between 0.12 and 0.18m. Antimony-doped crystals of Sn02 with ND>6X10 18 cm -8 appeared to be degenerate above 80°K. The low-temperature Hall mobility was found to decrease with decreasing donor concentration. A qualitative treatment of the data appears consistent with the hypothesis of impurity-level transport. Mobility above 300°K was analyzed by considering polar-optical modes of vibration as being the dominant lattice scattering mechanism. Both the perturbation and intermediate-coupling theories were in reasonable agreement with experimental values using a Debye temperature ®r^500°K.

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

“…4), and that oxygen vacancies are found in significant concentrations, rising above 10 19 cm −3 for T > 1000 K (using the PBE0 and B97-2 functionals). With the PBE0 and B97-2 functionals, in this range of E F the dominant charge state for the V O is neutral, but at T > 1000 K we find that approximately 2% of the vacancies are thermally ionized, resulting in electron concentrations ranging up to 10 18 cm −3 at T = 1500 K. With the BB1k functional, as the transition level is shallower, a greater proportion of the vacancies are thermally ionized (at T = 1200 K, 27% are in the + and 65% are in the 2+ state), and we find that n 0 and [V O ] are close in value and increase to just above 10 18 cm −3 at T = 1500 K. From these results, 054604-9 we see that, despite such a deep transition level, in reducing conditions significant intrinsic charge carrier concentrations can arise at temperatures above 800 K, in good agreement with experiment [72][73][74]. For T < 800 K, the concentrations we calculate are somewhat lower than experiment; the source of this discrepancy may be residual n-type activity from impurities or other defects which could dominate at reduced temperatures.…”

“…If, instead, this level can be attributed to V O , which becomes more dominant once the hydrogen donors are removed, then our calculated value is also in good agreement with this result. We note that a level at 0.28-0.35 eV has been observed in many n-type samples [75,[164][165][166] (as has the ∼0.15 eV level) [73][74][75]167], and that, in a study on SnO 2 nanoparticles [168], a switching of activation energy from 0.11 to 0.35 eV was observed in the largest nanoparticle when changing from low to high temperature.…”

“…A clear increase in conductivity as P O 2 is decreased can be observed when temperature T > 1000 K, indicating the presence of a relatively deep donor [34]. From these data, a donor activation energy of 0.15 eV in the dilute limit has been derived [72][73][74], or possibly at ∼0.3 eV [75], but the majority of computational studies indicates that the most likely donor, the oxygen vacancy, is deeper with the (2+/0) transition occurring at about 1 eV below the conduction band minimum (CBM) [20,41,43].…”

Grain-boundary-limited transport in semiconducting SnO2 thin films: Model and experiments

“…͑4͒. The mobilities are typically an order of magnitude lower than the values measured in single-crystalline samples of similar carrier density, 10,11 which already points to the importance of the grain boundaries in limiting the carrier transport. First, we will describe the results for the respective samples, mainly in terms of the energy band diagrams.…”

Epitaxial SnO2 thin films grown on (1̄012) sapphire by femtosecond pulsed laser deposition

“…Figure 4 shows that the electron concentration in the SnO 2 film is in the order of 10 19 cm Ϫ3 , which agrees with the value of bulk single crystal SnO 2 . 24 Studies of undoped single crystal bulk SnO 2 showed that the Hall mobility of electrons at room temperature, for an electron concentration of about 10 19 cm Ϫ3 , is around 90 cm 2 /V s. 24 In the present work, the mobility at room temperature is about 37 cm 2 /V s. Planar defects, such as CSPs and antiphase boundaries as well as interfaces, have been shown to act as scattering centers in nonstoichiometric rutile TiO 2 , 25 which significantly reduces the conductivity with respect to perfect crystal. The main defects observed in the SnO 2 thin film fabricated by fPLD are antiphase boundaries ͑coherent CSPs͒ and misfit dislocation at the interface.…”