High electron mobility in epitaxial SnO2−x in semiconducting regime

Abstract

We investigated the electronic transport properties of epitaxial SnO2−x thin films on r-plane sapphire substrates. The films were grown by pulsed laser deposition technique and its epitaxial growth direction was [101] and the in-plane alignment was of SnO2−x [010]//Al2O3[12̄10]. When the SnO2−x films were grown in the oxygen pressure of 30 mTorr, we have found the electron mobility of the 30 nm thick SnO2−x thin films strongly dependent on the thicknesses of the fully oxidized insulating SnO2 buffer layer. When the buffer layer thickness increased from 100 nm to 700 nm, the electron mobility of values increased from 23 cm2 V−1 s−1 to 106 cm2 V−1 s−1 and the carrier density increased from 9 × 1017 cm−3 to 3 × 1018 cm−3, which we attribute to reduction of large density of dislocations as the buffer layer thickness increases. In addition, we studied the doping dependence of the electron mobility of SnO2−x thin films grown on top of 500 nm thick insulating SnO2 buffer layers. The oxygen vacancy doping level was controlled by the oxygen pressure during deposition. As the oxygen pressure increased to 47.5 mTorr, the carrier density was found to decrease to 9.1 × 1016 cm−3 and the electron mobility values to 13 cm2 V−1 s−1, which is consistent with the dislocation limited transport properties. We also checked the conductance change of the SnO2−x during thermal annealing cycles, demonstrating unusual stability of its oxygen. The correlation between the electronic transport properties and microstructural defects investigated by the transmission electron microscopy was drawn. The excellent oxygen stability and high electron mobility of low carrier density SnO2−x films demonstrate its potential as a transparent oxide semiconductor.

Structural and plasmonic characteristics of sputtered SnO2:Sb and ZnO:Al thin films as a function of their thickness

“…This dependence is usually explained by scattering from charged dislocations, which become screened better by more carriers and thus the electron mobility increases with the carrier concentration [39,40]. Similar reduction in electron mobility at low carrier densities has been observed in SnO 2-x and ZnO epilayers [41,42]. For the AZO samples, with higher carrier concentrations, the mobility varies approximately proportional to n -2/3 that corresponds to the prediction for ionized impurity scattering [38,43].…”

High mobility approaching the intrinsic limit in Ta-doped SnO2 films epitaxially grown on TiO2 (001) substrates

“…Hall mobility (μ H ) is a key parameter in determining the performance of such devices, and the μ H values of bulk SnO 2 single crystals are in the range of 70 to 260 cm 2 V −1 s −1 at room temperature 9-11 . However, SnO 2 thin films show a rather low μ H of less than 100 cm 2 V −1 s −1 even in well-optimized epitaxial films 12,13 , which limits the practical use of SnO 2 .The lower μ H in SnO 2 epitaxial thin films is primarily attributable to the lack of lattice-matched substrates. Thus far, corundum Al 2 O 3 and rutile TiO 2 have been widely used as the substrates for the epitaxial growth 14,15 of SnO 2 .…”

“…Indeed, it was reported that μ H of the undoped SnO 2 film with (001) orientation on TiO 2 (001) was limited to a rather small value 16 , that is, ~40 cm 2 V −1 s −1 . To overcome the above-mentioned difficulty, very thick self-buffer layers 12,13 have been employed to grow high-μ H epitaxial SnO 2 films on Al 2 O 3 .Another important factor for achieving high μ H is to control the carrier density (n e ) because carriers play two competing roles in μ H ; an increase in n e enhances the screening of the Coulomb scattering potential and thus increases μ H , whereas an increased amount of dopants suppresses μ H owing to impurity scattering. To date, much effort has been made to grow undoped [13][14][15][16][17][18] or heavily doped [19][20][21][22] SnO 2 films on a wide variety of substrates.…”

“…Hall mobility (μ H ) is a key parameter in determining the performance of such devices, and the μ H values of bulk SnO 2 single crystals are in the range of 70 to 260 cm 2 V −1 s −1 at room temperature 9-11 . However, SnO 2 thin films show a rather low μ H of less than 100 cm 2 V −1 s −1 even in well-optimized epitaxial films 12,13 , which limits the practical use of SnO 2 .…”

“…Indeed, it was reported that μ H of the undoped SnO 2 film with (001) orientation on TiO 2 (001) was limited to a rather small value 16 , that is, ~40 cm 2 V −1 s −1 . To overcome the above-mentioned difficulty, very thick self-buffer layers 12,13 have been employed to grow high-μ H epitaxial SnO 2 films on Al 2 O 3 .…”