A chemically driven insulator–metal transition in non-stoichiometric and amorphous gallium oxide

Abstract

Insulator-metal transitions are well known in transition metal oxides, but inducing an insulator-metal transition in the oxide of a main group element is a major challenge. Here we report the observation of an insulator-metal transition, with a conductivity jump of seven orders of magnitude, in highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO 1.2 at a temperature around 670 K. We demonstrate through experimental studies and density-functional-theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga 2 O 3 within the metastable oxide matrix -in chemical terms by a disproportionation. This novel mechanism -an insulator-metal transition driven by a heterogeneous solid state reactionopens up a new route to achieve metallic behaviour in oxides that are expected to exist only as classic insulators.Insulator-metal transitions belong to the most fascinating phenomena in condensed-matter physics 1,2 . Since Mott's landmark work 3,4 it has been known that in crystalline solids strong electron-electron interactions can cause an insulator-metal transition. One example is crystalline Cr-doped vanadium oxide, (V 1-x Cr x ) 2 O 3 , which shows a Mott transition from a paramagnetic Mott insulator to a strongly correlated metal upon an increase in pressure, a lowering of temperature, or a decrease in dopant level 5,6 . In non-crystalline solids structural disorder can also lead to an insulator-metal transition on account of Anderson localization 7 .As shown by Anderson 7 and Mott 8 , in any non-crystalline material the lowest states in the conduction band are localized, i.e. they are electron traps. Only for energies above the mobility edge, E c , do states become non-localized or extended. If the Fermi energy E F is below the mobility edge, states at the Fermi level are localized and the material is an electronic insulator. If, however, the number of electrons increases and the Fermi energy rises above the mobility edge, the material becomes metallic (Anderson transition). As transition metals change their valence state easily, most examples of insulator-metal transitions concern transition metal compounds 4-6,9-11 . The above considerations do not, however, exclude the possibility of inducing an insulator-metal transition in a simple binary oxide of a main group element, even without doping. Instead, large deviations from the ideal stoichiometry, i.e. high defect concentrations, provide a high concentration of electronic defects (self-doping). And if, in addition, the oxide is amorphous, there are two phenomena, strong structural disorder and strong chemical disorder, which could result in an insulator-metal transition.Here we report such a case: Highly non-stoichiometric, amorphous gallium oxide with an approximate chemical composition GaO 1.2 shows an unprecedented insulator-metal transition, with a jump in conductivity of ca. 7 orders of magnitude at temperatures as high as 670 K. We show that this in...

Thermodynamics, structure and kinetics in the system Ga–O–N

“…These calculations show that gallium interstitials, Ga I , and oxygen vacancies, V O , both produce defect states in the band gap of Ga 2 O 3 . For the gallium interstitial we find inside the band gap two new localized states/bands that are occupied by the three valence electrons of the extra Ga atom [64]. For an oxygen vacancy we find a color centre occupied by two electrons.…”

“…These new materials show an unprecedented insulatormetal transition, with a jump in conductivity of ca. 7 orders of magnitude at temperatures as high as 670 K [64]. We could show that this insulator-metal transition is chemically induced by an internal redox reaction (a disproportionation), namely the crystallization of stoichiometric b-Ga 2 O 3 within the amorphous oxide matrix, which becomes as a result even more non-stoichiometric.…”

On the ammonolysis of Ga2O3: An XRD, neutron diffraction and XAS investigation of the oxygen-rich part of the system Ga2O3–GaN

“…The resulting gallium oxynitride is anion deficient and contains several percent of nitrogen. In these new materials an insulator-metal transition can be observed during crystallization of stoichiometric b-Ga 2 O 3 [22].…”

Structural-relaxation-driven electron doping of amorphous oxide semiconductors by increasing the concentration of oxygen vacancies in shallow-donor states

“…[1][2][3][4][5] A notable characteristic of amorphous oxides, relative to their crystalline counterparts, is that these materials contain structural disorder-related defects (for example, free volume) in addition to non-stoichiometric defects (for example, oxygen vacancies), which significantly affect the corresponding electrical properties. 3,[6][7][8] Moreover, the degree of structural disorder in amorphous metal oxides is always likely to decrease to form more stable structures because internal atomic rearrangement occurs even below the glass transition temperature (T g ). This process is known as structural relaxation (SR) [9][10][11] and results in continuous changes in the electrical properties.…”