Epitaxial growth of CuScO2 thin films on sapphire a-plane substrates by pulsed laser deposition

Abstract

An epitaxial film of CuScO2, a transparent oxide semiconductor with a delafossite structure, was grown on an α−Al2O3(112¯0) substrate by a pulsed laser deposition method using a single-phase Cu2Sc2Oδ target. A two-dimensional x-ray reciprocal space mapping measurement revealed that the film was single phase with a rhombohedral crystal structure. The film showed six-fold rotational symmetry in the basal plane, indicating that the film had a twinned domain structure. The epitaxial growth of CuScO2[3R](0001) thin films on α−Al2O3(112¯0) substrates is caused by the uniaxial locked epitaxy mechanism along the ⟨1¯21¯0⟩ direction of the film, and the orientation relationships of the film with respect to the substrate were CuScO2[3R](0001)∕∕α−Al2O3(112¯0) and CuScO2[3R][1¯21¯0]∕∕α−Al2O3[88¯01]. The optical transmittance of the film was larger than 65% in the visible/near-infrared regions, while the energy gap for direct allowed transition was estimated as 3.7 eV. The resistivity of the film, 9.3×106Ωcm at room temperature, significantly decreased to 4.0 Ωcm after both substituting Mg2+ ions for Sc3+ and intercalating excess oxygen. The Mg-doped CuScO2+X(0001) thin film showed optical transmittance of larger than 65% in the visible region, and the Seebeck coefficient was positive, indicating a p-type conductivity.

Optical and electrical properties of CuScO2 epitaxial films prepared by combining two-step deposition and post-annealing techniques

“…CSO thin films were deposited by a PLD apparatus with an oxygen radical source (ULVAC: MB95-5005) using a Cu 2 Sc 2 O 5 pellet as a target [14][15][16]. A KrF excimer laser beam (l=248 nm, pulse duration=30 ns) was used for the ablation.…”

“…We reported on the epitaxial growth of CSO[3R] thin films with a 50-nm thickness prepared at relatively low deposition rates by a pulsed laser deposition (PLD) method [14,15]. However, it was possible to easily grow grains with different orientations and other phases in CSO thin films by increasing the film thickness using a long deposition time or a high laser repetition frequency [16].…”

Electronic structure of CuAlO2 and CuScO2 delafossites under pressure

“…CuAlO 2 and CuScO 2 delafossites are wide gap semiconductors behaving as p-type transparent conductors, with applications in transparent electronic and optoelectronic devices [1][2][3]. At atmospheric pressure they can occur in rhombohedral (3R) and hexagonal (2H) crystalline phases [4], although the 3R form predominates in nature.…”

First principles study of CuAlO2 doping with S

“…However, the conductivity of these TCOs is too low for the application in the transparent p-n junction, so to develop the good-performance p-type TCOs is necessary and urgent for the development of photo-electricity devices. Now, most of the stable p-type TCOs contain Cu + iron [14][15][16][17][18][19][20], which have a moderate band gap to ensure the high transparency, and their transmittance is higher than 80% [21]. Unfortunately, these p-type TCOs have a low conductivity [22] which restricts their applications in the transparent p-n junctions.…”

“…It is known that a good p-type TCOs should have three properties: high transparency, high hole conductivity and high doped ability, which indicates that the good p-type should have large optical gap, small hole effective masses and high valence band maximum (VBM) with respect to vacuum level. The CuAlO 2 [23] is one of the p-type TCOs, and its band gap and effective masses are 3.5 eV [20] [24] because the impurities Mg can induce the holes and change the position of VBM. If we use the S atoms to substitute O atoms, how do the properties of CuAlO 2 change?…”