Hitoshi Takane1,Itsuhiro Kakeya1,Hirokazu Izumi2,Takeru Wakamatsu1,Yuki Isobe1,Kentaro Kaneko3,Katsuhisa Tanaka1
Kyoto University1,Hyogo Prefectural Institute of Technology2,Ritsumeikan University3
Hitoshi Takane1,Itsuhiro Kakeya1,Hirokazu Izumi2,Takeru Wakamatsu1,Yuki Isobe1,Kentaro Kaneko3,Katsuhisa Tanaka1
Kyoto University1,Hyogo Prefectural Institute of Technology2,Ritsumeikan University3
Rutile-type wide and ultrawide band-gap oxide semiconductors, such as r-GeO<sub>2</sub> and r-SnO<sub>2</sub>, are emerging materials for high-power electronics and deep ultraviolet optoelectronics devices [1,2]. Recently, their alloy (r-Ge<sub>x</sub>Sn<sub>1–x</sub>O<sub>2</sub>) has also gained attentions due to the possibility to modulate its bandgap and electrical properties by tuning the Ge:Sn compositional ratio [2-4]. In addition, Sn-rich r-Ge<sub>x</sub>Sn<sub>1–x</sub>O<sub>2</sub> exhibits excellent n-type conductivity, which is favorable for future device-oriented researches [2,3]. However, details of its electrical properties including carrier transport mechanisms have not been elucidated yet.<br/>In this study, we investigated low-temperature electron transport properties of Sn-rich r-Ge<i><sub>x</sub></i>Sn<sub>1–<i>x</i></sub>O<sub>2</sub> thin films (<i>x</i>=0.28 and 0.41). Temperature-dependent Hall effect measurements suggest that the hopping conduction is dominant at low temperatures (< 100 K) in both r-Ge<sub>0.41</sub>Sn<sub>0.59</sub>O<sub>2</sub> and r-Ge<sub>0.28</sub>Sn<sub>0.72</sub>O<sub>2</sub>. Temperature-dependent resistivity measurements indicate that the hopping mechanism at lower temperatures (<i>T</i> ≤ 15 K) is explained by the Efros-Shklovskii variable-range hopping (ES VRH), that is, hopping over the states within the Coulomb gap. In the temperature region, both r-Ge<sub>0.41</sub>Sn<sub>0.59</sub>O<sub>2</sub> and r-Ge<sub>0.28</sub>Sn<sub>0.72</sub>O<sub>2</sub> exhibit both negative and positive magnetoresistances. Based on the theory of quantum interference that includes effects of both frozen- and free-electron spins [5], it is found that the negative and positive components are attributed to the quantum interference and field-induced spin-alignment, respectively, and the transport mechanism at <i>T</i> ≤ 15 K for both r-Ge<sub>0.41</sub>Sn<sub>0.59</sub>O<sub>2</sub> and r-Ge<sub>0.28</sub>Sn<sub>0.72</sub>O<sub>2</sub> corresponds to the ES VRH with large number of scattering centers. The results are coincident with those obtained by the temperature dependence of resistivity. At <i>T</i> ≥ 30 K, the negative magnetoresistance induced by the quantum-interference effect decreases as the temperature increases. Moreover, at <i>T</i> ≥ 150 K, both r-Ge<sub>0.41</sub>Sn<sub>0.59</sub>O<sub>2</sub> and r-Ge<sub>0.28</sub>Sn<sub>0.72</sub>O<sub>2</sub> show temperature-independent positive magnetoresistance induced by the Lorentz force. It is considered that these results indicate a mixture of the Mott VRH and thermally activated band conduction (<i>T</i> < 100 K) and almost pure thermally activated band conduction (<i>T</i> ≥ 150 K).<br/>We believe that our experimental and analytical results provide a basis for the development of not only r-Ge<i><sub>x</sub></i>Sn<sub>1–<i>x</i></sub>O<sub>2</sub>, but also all the other rutile-type wide and ultrawide band-gap oxide semiconductors.<br/>This work was, in part, supported by JSPS KAKENHI under Grant Number 21H01811 and 20H02606.<br/>[1] S. Chae et al., <i>Appl. Phys. Lett.</i> 118, 260501 (2021).<br/>[2] H. Takane et al., <i>Phys. Rev. Mater.</i> 6, 084604 (2022).<br/>[3] Y. Nagashima et al., <i>Chem. Mater.</i> 34, 10842 (2022).<br/>[4] F. Liu et al., <i>Commun. Mater.</i> 3, 69 (2022).<br/>[5] A. V. Shumilin et al., <i>Phys. Rev. B</i> 85, 115203 (2012).