Photoelectron Holographic Study for Atomic Site Occupancy for Si Dopants in Si-Doped κ-Ga2O3(001).

We investigated atomic site occupancy for the Si dopant in Si-doped κ-Ga2O3(001) using photoelectron spectroscopy (PES) and photoelectron holography (PEH). From PES and PEH, we found that the Si dopant had one chemical state, and three types of inequivalent Si substitutional sites (SiGa) were formed. The ratios for the inequivalent tetrahedral, pentahedral, and octahedral SiGa sites were estimated to be 55.0%, 28.1%, and 16.9%, respectively. Higher (lower) ratios for the three inequivalent SiGa sites may come from a lower (higher) formation energy. The Tetra (Octa) SiGa site has the highest (lowest) ratio of the three SiGa sites since it has the lowest (highest) formation energy. We suggest that the tetrahedral SiGa site is due to the active dopant site, whereas the pentahedral and octahedral SiGa sites can be attributed to the inactive dopant sites for Si-doped κ-Ga2O3(001).

Non-Equilibrium Synthesis of Highly Active Nanostructured, Oxygen-Incorporated Amorphous Molybdenum Sulfide HER Electrocatalyst.

Molybdenum sulfide emerged as promising hydrogen evolution reaction (HER) electrocatalyst thanks to its high intrinsic activity, however its limited active sites exposure and low conductivity hamper its performance. To address these drawbacks, the non-equilibrium nature of pulsed laser deposition (PLD) is exploited to synthesize self-supported hierarchical nanoarchitectures by gas phase nucleation and sequential attachment of defective molybdenum sulfide clusters. The physics of the process are studied by in situ diagnostics and correlated to the properties of the resulting electrocatalyst. The as-synthesized architectures have a disordered nanocrystalline structure, with nanodomains of bent, defective S-Mo-S layers embedded in an amorphous matrix, with excess sulfur and segregated molybdenum particles. Oxygen incorporation in this structure fosters the creation of amorphous oxide/oxysulfide nanophases with high electrical conductivity, enabling fast electron transfer to the active sites. The combined effect of the nanocrystalline pristine structure and the surface oxidation enhances the performance leading to small overpotentials, very fast kinetics (35.1 mV dec-1 Tafel slope) and remarkable long-term stability for continuous operation up to -1 A cm-2. This work shows possible new avenues in catalytic design arising from a non-equilibrium technique as PLD and the importance of structural and chemical control to improve the HER performance of MoS-based catalysts.

Multi-layered hierarchical nanostructures for transparent monolithic dye-sensitized solar cell architectures.

Monolithic dye-sensitized solar cell (DSC) architectures hold great potential for building-integrated photovoltaics applications. They indeed benefit from lower weight and manufacturing costs as they avoid the use of a transparent conductive oxide (TCO)-coated glass counter electrode. In this work, a transparent monolithic DSC comprising a hierarchical 1D nanostructure stack is fabricated by physical vapor deposition techniques. The proof of concept device comprises hyperbranched TiO2 nanostructures, sensitized by the prototypical N719, as photoanode, a hierarchical nanoporous Al2O3 spacer, and a microporous indium tin oxide (ITO) top electrode. An overall 3.12% power conversion efficiency with 60% transmittance outside the dye absorption spectral window is demonstrated. The introduction of a porous TCO layer allows an efficient trade-off between transparency and power conversion. The porous ITO exhibits submicrometer voids and supports annealing temperatures above 400 °C without compromising its optoelectronical properties. After thermal annealing at 500 °C, the resistivity, mobility, and carrier concentration of the 800 nm-thick porous ITO layer are found to be respectively 2.3 × 10-3 Ω cm-1, 11 cm2 V-1 s-1, and 1.62 × 1020 cm-3, resulting in a series resistance in the complete device architecture of 45 Ω. Electrochemical impedance and intensity-modulated photocurrent/photovoltage spectroscopy give insight into the electronic charge dynamic within the hierarchical monolithic DSCs, paving the way for potential device architecture improvements.

Molecular Beam Epitaxy of ß-(InxGa1-x)2O3 on ß-Ga2O3 (010): Compositional Control, Layer Quality, Anisotropic Strain Relaxation, and Prospects for Two-Dimensional Electron Gas Confinement.

In this work, we investigate the growth of monoclinic ß-(InxGa1-x)2O3 alloys on top of (010) ß-Ga2O3 substrates via plasma-assisted molecular beam epitaxy. In particular, using different in situ (reflection high-energy electron diffraction) and ex situ (atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy) characterization techniques, we discuss (i) the growth parameters that allow for In incorporation and (ii) the obtainable structural quality of the deposited layers as a function of the alloy composition. In particular, we give experimental evidence of the possibility of coherently growing (010) ß-(InxGa1-x)2O3 layers on ß-Ga2O3 with good structural quality for x up to ≈ 0.1. Moreover, we show that the monoclinic structure of the underlying (010) ß-Ga2O3 substrate can be preserved in the ß-(InxGa1-x)2O3 layers for wider concentrations of In (x ≤ 0.19). Nonetheless, the formation of a large amount of structural defects, like unexpected (102̅) oriented twin domains and partial segregation of In is suggested for x > 0.1. Strain relaxes anisotropically, maintaining an elastically strained unit cell along the a* direction vs plastic relaxation along the c* direction. This study provides important guidelines for the low-end side tunability of the energy bandgap of ß-Ga2O3-based alloys and provides an estimate of its potential in increasing the confined carrier concentration of two-dimensional electron gases in ß-(InxGa1-x)2O3/(AlyGa1-y)2O3 heterostructures.

Interfacial Properties of the SnO/κ-Ga2O3 p-n Heterojunction: A Case of Subsurface Doping Density Reduction via Thermal Treatment in κ-Ga2O3.

The interfacial properties of a planar SnO/κ-Ga2O3 p-n heterojunction have been investigated by capacitance-voltage (C-V) measurements following a methodological approach that allows consideration of significant combined series resistance and parallel leakage effects. Single-frequency measurements were carried out in both series- and parallel-model measurement configurations and then compared to the dual-frequency approach, which permits us to evaluate the depletion capacitance of diode independently of leakage conductance and series resistance. It was found that in the bias region, where the dissipation factor was low enough, they give the same results and provide reliable experimental C-V data. The doping profile extracted from the C-V data shows a nonuniformity at the junction interface that was attributed to a depletion of subsurface net donors at the n-side of the diode. This attribution was corroborated by doping profiles and carrier distributions in the n and p sides of the heterojunction obtained from the simulation of the measured C-V data by the Synopsys Sentaurus-TCAD suite. Hall effect measurements and Hg-probe C-V investigation on single κ-Ga2O3 layers, either as-grown or submitted to thermal treatments, support the hypothesis of the subsurface donor reduction during the SnO deposition. This study can shed light on the subsurface doping density variation in κ-Ga2O3 due to high-temperature treatment. The investigation of the SnO/κ-Ga2O3 heterointerface provides useful hints for the fabrication of diodes based on κ-Ga2O3. The methodological approach presented here is of general interest for reliable characterization of planar diodes.

Tackling Disorder in γ-Ga2 O3.

Ga2 O3 and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga2 O3 offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ-Ga2 O3 presents a particular challenge across synthesis, characterization, and theory due to its inherent disorder and resulting complex structure-electronic-structure relationship. Here, density functional theory is used in combination with a machine-learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ-Ga2 O3 . The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by photoluminescence excitation spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step toward exploring how their electronic structure can be understood in terms of local coordination and overall structure.