【Epitaxy Papers】Anomalous hopping regime at the Mott transition from disordered semiconductor to disordered metal in Si-doped Ga₂O₃

      Researchers from the Slovak Academy of Sciences have published a dissertation titled "Anomalous hopping regime at the Mott transition from disordered semiconductor to disordered metal in Si-doped Ga₂O₃" in Physical Review B.

Abstract

      In a non-degenerate n-type semiconductor, the electron conduction at low temperatures is due to the Efros-Shklovskii variable range hopping in a narrow donor-energy band separated from the conduction band by the donor ionization energy. As the donor density reaches the critical density for the Mott transition (n_c), the donor ionization energy turns to zero and the electrons released from the donors form a degenerate gas in the conduction band. Usually, the Fermi energy of the degenerate gas exceeds the Anderson mobility edge and the Mott transition is therefore accompanied by a change of the hopping conductance to the metallic conductance. Here we study the highly-Si-doped β-Ga₂O₃ semiconducting films by measuring the conductivity, Hall effect, Seebeck coefficient, and optical absorption spectrum, as well as by structural and chemical characterization. We vary the density of the electrically-active Si donors (n) from low values up to n = n_c (we show that n_c = 5.7×10 ¹⁸ cm⁻³) and slightly above. We observe the donor-density-driven Mott transition from the non-degenerate disordered semiconductor to a degenerate disordered metal with Fermi energy well below the mobility edge. At low temperatures on the semiconducting side of the Mott transition (n < n_c), we observe the Efros-Shklovskii hopping with characteristic energy (1 −n/ n_c)², as predicted by theory. Similarly, the donor ionization energy tends to diminish for n→n_c and it is likely proportional to the same factor. On the metallic side of the Mott transition (n - n_c) the low-temperature transport changes from the Efros-Shklovskii hopping to the Mott variable range hopping, manifesting the closure of the Efros-Shklovskii Coulomb gap at Mott transition. At high temperatures, the conductance still exhibits localized transport for both n> n_c and n < n_c, but surprisingly, in the latter case with a linear temperature dependence as n approaches n_c. We identify this transport regime as the anomalous variable range hopping of the non-degenerate conduction electrons moving in disorder which does not originate from Si doping. Our optical data indeed show that the conduction-band states are localized by extra disorder due to the oxygen vacancies which are confirmed also by the chemical composition data. Finally, the room-temperature Seebeck data confirm the non-degenerate electron transport and also indicate merging of the donor band into the conduction band at Mott transition.

DOI：

https://doi.org/10.1103/khqm-bvwf