Growth of Ta₂SnO₆ Films, a Candidate Wide-Band-Gap p-Type Oxide

Abstract

In an effort to discover a high-mobility p-type oxide, recent computational studies have focused on Sn2+-based ternary oxides. Ta2SnO6 has been suggested as a potentially useful p-type material based on the prediction of simultaneously high hole mobility and a wide range of synthesis conditions over which it is the energetically favored phase. In this study, we synthesized this material epitaxially for the first time and evaluated its properties experimentally. We measured the band gap to be 2.4 eV and attempted to substitutionally dope titanium for tantalum (TiTa ’) and potassium for tin (KSn ’) but found that both doped and undoped films were insulating. Amorphous Ta2SnO6 films were also grown via thermal atomic layer deposition (ALD) at 175 °C. Electrical characterization of the ALD-fabricated amorphous films found them to be insulating with an optical band gap of 2.24 eV. Density functional theory calculations indicate that, under MBE growth conditions, oxygen vacancies have a negative energy of formation in crystalline Ta2SnO6 when the Fermi energy lies near the valence band edge. These oxygen vacancies would lead to compensation of holes generated by TiTa ’ or KSn ’ dopants, which is consistent with our observations. We conclude that the direct growth of epitaxial p-type Ta2SnO6 films using MBE-accessible growth conditions is thwarted by the spontaneous formation of oxygen vacancies. While our growth conditions do not yield p-type films, we calculate that there are conditions under which Ta2SnO6 is the thermodynamically stable phase and spontaneous formation of compensating defects does not occur, motivating further studies with different synthesis techniques.

Epitaxial Synthesis of a Vertically Aligned Two-Dimensional van der Waals Crystal: (110)-Oriented SnO

“…As for many candidate p -type oxides, one challenge of synthesizing SnO is isolating the intermediate oxidation state, which has a limited range of stability (i.e., suppressing reduction to β-Sn or oxidation to SnO 2 ). , This problem has previously been overcome by using the SnO molecular beam generated by an SnO 2 source. ,,, From thermodynamic calculations, the molecular beam produced by heating SnO 2 (or a two-phase mixture of SnO 2 (s) + Sn (l)) should be 99.9% SnO (g) molecules, and this has been observed experimentally using mass spectrometry . By starting with an SnO molecular beam, the phase has been kinetically stabilized to grow (001)-oriented SnO films by MBE, and we employ the same strategy here to grow (110)-oriented SnO films.…”

“…Even within the relatively narrow range of oxygen chemical potentials for which p -type suboxides are stable, spontaneous formation of compensating defects can prevent the formation of mobile holes. For example, oxygen vacancies are often low-energy defects that can trap would-be holes preventing p -type conduction, and such compensating defects become increasingly favorable as more holes are introduced, which can result in their spontaneous formation at useful carrier concentrations. ,,− Another major challenge is interfacial traps that pin the Fermi energy. These can cause films with promising hole mobilities by Hall measurements to become useless for electric devices .…”

“…In this material, filled Sn 5s orbitals hybridize with O 2p orbitals to form the top of the valence band, resulting in much more dispersion, meaning a lower effective mass and a higher mobility. Furthermore, unlike many candidate p -type oxides for which appreciable hole doping has proven difficult or impossible due to spontaneous formation of compensating defects ,,− or interfacial traps, holes with mobility >5 cm 2 /(V·s) have been demonstrated in SnO using an electric field effect, intrinsic, and extrinsic dopants. − Using field effect, hole mobilities as high as 10.8 cm 2 /(V·s) have been measured in nanocrystalline SnO with processing temperatures <200 °C . Hall effect measurements have revealed room-temperature hole mobilities as high as 30 cm 2 /(V·s) in bulk, polycrystalline SnO and as high as 21 cm 2 /(V·s) in epitaxial SnO .…”

Tuning of hole carrier density in p-type α-SnWO4 by exploiting oxygen defects

“…In this regard, precise tuning of the carrier densities of Sn 2+ -based p-type oxide semiconductors is highly necessary. However, intentional control over the hole carrier density remains a challenging undertaking 17,[19][20][21][22] . Especially, Sn 2+ -based oxides in the thin-film form still show the insulating properties [19][20][21][22] .…”

“…However, intentional control over the hole carrier density remains a challenging undertaking 17,[19][20][21][22] . Especially, Sn 2+ -based oxides in the thin-film form still show the insulating properties [19][20][21][22] . This is likely because oxygen vacancies form spontaneously to compensate for the positive charge 19,20,23 .…”