LaCuOS, a layered oxysulfide built from alternating [La2O2] oxide and [Cu2S2] sulfide layers, was reported in 2000 (Ueda et al., Appl. Phys. Lett. 77, 2701) as a transparent p-type semiconductor with a ~3.1 eV gap, Sr-tunable p-type conductivity, and sharp near-edge photoluminescence. The report left the microscopic origin of dark coloration and of defect-related broad visible luminescence unidentified, and left hole-density limits, defect control, and device demonstration as future work. Twenty-five years of follow-up have quantitatively closed several of these questions: room-temperature excitons with ~50 meV binding energy are established, degenerate p-type conductivity up to ~10^2 S/cm is achieved, and transparent p-n diodes and excitonic blue LEDs exist in the LaCuOCh (chalcogen) family. What remains open is the defect core of the original question: which native defects (Cu vacancies, S vacancies, Cu2+-related or hydrogen/impurity centers) cause the coloration and any sub-gap absorption/emission in LaCuOS, what are their concentrations, charge states, and energy levels in the gap, and how can their populations be reproducibly controlled - questions for which a quantitative defect-thermodynamics treatment exists for the selenide analogue LaCuOSe but not for LaCuOS itself.

Background

A transparent conducting oxide (TCO) is a material that combines electrical conductivity with high visible transparency; finding a p-type (hole-conducting) TCO is far harder than the ubiquitous n-type ones such as Sn-doped In2O3, because the top of the valence band of most oxides is deep, localized on oxygen 2p states, so holes are heavy and easily self-trapped. The design concept of 'chemical modulation of the valence band', introduced by Kawazoe and co-workers for the delafossite-type oxide CuAlO2 (a layered oxide in which Cu atoms sit on linearly coordinated O-Cu-O units; Nature 389, 939-942 (1997), DOI 10.1038/40087), replaces oxygen 2p at the valence-band maximum with more extended Cu 3d states hybridized with chalcogenide p states, raising hole mobility while keeping the band gap wide. Ueda, Inoue, Hirose, Kawazoe and Hosono extended this design to oxysulfides in 2000 with LaCuOS (Appl. Phys. Lett. 77, 2701, DOI 10.1063/1.1319507), a layered compound of the ZrCuSiAs structure type in which fluorite-type [La2O2] insulating oxide layers alternate with anti-fluorite-type [Cu2S2] conductive sulfide layers; the wide gap (~3.1 eV) comes mainly from the oxide layers and the p-type conduction from the Cu3d-S3p valence band in the sulfide layers. Radio-frequency-sputtered La1-xSrxCuOS films (x = 0, 0.05) showed >=70% visible transparency, p-type conduction (positive Seebeck coefficient, i.e. a positive thermoelectric voltage signaling hole conduction) with conductivities 1.2x10^-2 and 2.6x10^-1 S cm^-1, the higher value already reflecting the degenerate (metallic-like) state induced by 5% Sr2+ substituting for La3+, and a sharp photoluminescence peak at the absorption edge whose excitonic character the quoted 3.1 eV gap estimate neglected. The report left open, among other items: the microscopic origin of the coloration of dark or oxidized samples and of defect-related broad visible luminescence, tentatively ascribed to intrinsic defects such as vacancies or Cu2+-related centers; the exciton binding energy; the achievable limits of hole concentration and mobility; reproducible control of sample color and defect populations; and functional transparent devices. Subsequent work by the same and other groups answered much of this. The sharp emission and absorption peaks are room-temperature excitons (bound electron-hole pairs) stabilized by the two-dimensional electronic structure; in epitaxial LaCuOS1-xSex solid-solution films the exciton binding energy is about 50 meV for the whole composition range including LaCuOS, and spin-orbit-split exciton levels and their quantum beats (coherent oscillations between two coupled exciton states, observable as a periodic modulation of the emission intensity) have been measured. Epitaxial growth, heavy Mg or Cu-vacancy doping (hole densities up to 2.7x10^20 cm^-3 in sputtered Sr-doped films and up to 1.7x10^21 cm^-3 in LaCuOSe:Mg, with a hole effective mass of 1.6 +/- 0.2 electron masses and mobility of a few cm^2/Vs), and record p-type conductivities of order 10^2 S cm^-1 in LaCuOS films (2023) are now established, and transparent p-n heterojunction diodes and an excitonic blue light-emitting diode based on p-LaCuOSe/n-InGaZn5O8 have been demonstrated. First-principles work showed that Cu vacancies, not the nominal divalent dopant, dominate the high hole density in LaCuOSe, and hybrid density-functional-theory (DFT) calculations - DFT with a fraction of exact exchange, needed for correct gap and defect-level positions - have given the full defect chemistry (formation energies and charge-state transition levels) for LaCuOSe, explaining its doping anomalies and sub-gap absorption. The corresponding quantitative defect physics for LaCuOS itself is missing: the one dedicated Cu-off-stoichiometry study of LaCu1-xSO found a broad sub-gap absorption near 2.1 eV and, from (non-hybrid) DFT, attributed it to in-gap states of charge-compensating sulfur vacancies, but no experimental defect identification (for example electron paramagnetic resonance, a magnetic-resonance technique sensitive to unpaired electrons and hence to paramagnetic centers such as Cu2+, or positron annihilation spectroscopy, which detects vacancy-type defects through changes in the positron-annihilation signature) and no formation-energy/transition-level study of native defects in LaCuOS has appeared.

Problem Statement

For the layered transparent p-type oxysulfide semiconductor LaCuOS (band gap about 3.1 eV, ZrCuSiAs-type structure with alternating [La2O2] and [Cu2S2] layers), determine the microscopic origin of the native-defect-related optical and electrical signatures left unresolved by the original 2000 report and its successors. Specifically: (i) identify which defect species - for example copper vacancies, sulfur vacancies, Cu2+-related centers, or hydrogen/impurity-related centers - give rise to the dark coloration (sub-band-gap optical absorption) of non-stoichiometric, oxidized, or hydrogen-exposed LaCuOS samples and to any broad visible sub-gap luminescence reported for LaCuOS films; (ii) determine their charge states, typical concentrations, and electronic levels within the band gap, quantitatively; and (iii) establish how those defect populations can be reproducibly controlled (film color, defect-related absorption and emission, and the achievable p-type hole concentration and mobility) through synthesis stoichiometry, precursor treatment, and post-annealing conditions, so that defect engineering, rather than incidental off-stoichiometry, governs the transparent-conducting and luminescent properties of LaCuOS.

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