Effects of Dislocation on Carrier Transport in α-Ga₂O₃ on M-Plane Sapphire Substrate

Gallium oxide (Ga₂O₃) has been gaining attentions as a promising material for next-generation power devices due to its large bandgap. Among the five polymorphs in Ga₂O₃, thermally the most stable β-Ga₂O₃ with bandgap of 4.8 eV has been intensively investigated, and its MOSFET with high performance was demonstrated [1]. Recently, in addition, notable progress of devices based on meta-stable α-Ga₂O₃ with bandgap of 5.6 eV [2] has been also reported [3,4]. For α-Ga₂O₃, however, further studies are needed in order to understand its basic physical properties. In particular, there are dislocations with high density of 10¹⁰-10¹¹ cm^-² in α-Ga₂O₃ films on sapphire substrates due to their large lattice mismatch [5,6], and relationships between electrical properties and such defects in α-Ga₂O₃ are remained to be elucidated. Additionally, it is also interesting that α-Ga₂O₃ on m-plane sapphire showed higher mobility than that on c-plane sapphire [7].<br/>In this study, we prepared Si-doped α-Ga₂O₃ film on m-plane sapphire substrate with different carrier concentrations of 3.5×10¹⁷ (#1), 1.6×10¹⁸ (#2), and 1.4×10¹⁹ cm^-³ (#3) at room temperature by mist chemical vapor deposition. The three samples were investigated by temperature-dependent Hall effect measurements in a temperature range of 20-100K.<br/>First, we analyzed #1, which is a non-degenerate one, using the charge neutrality equation and the Boltzmann transport equation in relaxation time approximation. Its temperature-dependent carrier concentration and mobility was simultaneously and self-consistently fitted. In #1, dislocation scattering with estimated dislocation density (<i>N</i>_dis) of 4.9×10¹⁰ cm^-² was dominant, and the donor density (<i>N</i>_D) of 1.84×10¹⁸ cm^-³ was significantly compensated by acceptor centers such as dislocations and other impurities/defects. In order to evaluate <i>N</i>_dis in #1, X-ray diffraction skew-symmetric w scan was conducted and <i>N</i>_dis of about 10¹⁰ cm^-² was confirmed. For #2, an ionized energy of donor was nearly zero, suggesting that <i>N</i>_D of #2 overlapped or exceeded the <i>N</i>_D where the Mott metal-insulator transition would occur. Finally, we analyzed #3. Its carrier concentration was completely independent on temperature, showing #3 to be degenerate. Fitting its temperature-dependent mobility by a model arranged for degenerate semiconductors revealed that mobility in #3 was determined by ionized impurity scattering instead of dislocation scattering. This is because charged dislocations were strongly screened by carriers in degenerate one. The marked effects of dislocation on electron transport is inevitable and the reduction of its density is an essential issue for the device applications. We analyzed the maximum dislocation density with which the dislocation scattering does not become the dominant scattering process. It was 10⁶-10⁷ cm^-2 and the details will be shown at the symposium.<br/>This work was, in part, supported by MIC research and development (JPMI00316).<br/>[1] Higashiwaki et al., APL 103, 123511 (2013).<br/>[2] Segura et al., PRMater. 1, 024604 (2017).<br/>[3] Oda et al., APEX 9, 021101 (2016).<br/>[4] Press Release material from FLOSFIA Inc.<br/>[https://flosfia.com/struct/wp-content/uploads/79cd9d2dfa54a771f642e008cc4f9cb0.pdf]<br/>[5] Kaneko et al., JJAP 51, 020201 (2012).<br/>[6] Jinno et al., Sci. Adv. 7, eabd5891 (2021).<br/>[7] K. Akaiwa et al., PSS A 217, 1900632 (2020).