Band edge evolution of transparent Zn M2III O4 (MIII=Co, Rh, Ir) spinels

ZnMIII 2 O4 (MIII = Co, Rh, Ir) spinels have been recently identified as promising p-type semiconductors for transparent electronics. However, discrepancies exist in the literature regarding their fundamental optoelectronic properties. In this paper, the electronic structures of these spinels are directly investigated using soft/hard x-ray photoelectron and x-ray absorption spectroscopies in conjunction with density functional theory calculations. In contrast to previous results, ZnCo2O4 is found to have a small electronic band gap with forbidden optical transitions between the true band edges, allowing for both bipolar doping and high optical transparency. Furthermore, increased d-d splitting combined with a concomitant lowering of Zn s/p conduction states is found to result in a ZnCo2O4 (ZCO) < ZnRh2O4 (ZRO) ≈ ZnIr2O4 (ZIO) band gap trend, finally resolving long-standing discrepancies in the literature

Accelerated optimization of transparent, amorphous zinc-tin-oxide thin films for optoelectronic applications

In the last decade, transparent amorphous oxide semiconductors (TAOS) have become an essential component of many electronics, from ultra high resolution displays to solar cells. However, these disordered oxides typically rely on expensive component metals like indium to provide sufficient charge carrier conduction, and their optoelectronic properties are not as predictable and well-described as those of traditional, crystalline semiconductors. Herein we report on our comprehensive study of the amorphous zinc-tin-oxide (a-ZTO) system for use as an indium-free, n-type TAOS. Using a combination of high-throughput co-deposition growth, high resolution spectral mapping, and atomistic calculations, we explain the development of disorder-related subgap states in SnO2-like a-ZTO and optical bandgap reduction in ZnO-like a-ZTO. In addition, we report on a composition-induced electronic and structural transition in ZnO-like a-ZTO resulting in an exceptionally high figure of merit, comparable to that of amorphous indium-gallium-zinc-oxide. Our results accelerate the development of a-ZTO and similar systems as indium-free TAOS materials

Distinction between Intrinsic and X-ray-Induced Oxidized Oxygen States in Li-Rich 3d Layered Oxides and LiAlO2

Resonant inelastic X-ray scattering (RIXS) at the O K-edge is considered a prime technique to identify bulk oxidized oxygen formation, but its fundamental interpretation is not straightforward. In this study, we intentionally induce RIXS signatures of oxidized oxygen upon beam exposure in LiAlO2 polymorphs that are easily distinguished because of their wide band gaps. After careful consideration of beam exposure effects on Li[Li0.144Ni0.136Mn0.544Co0.136]O2 (LR-NMC), we conclude that oxidized oxygen features are inherent at high states of charge and are lost upon aggressive beam exposure. The extracted oxidized oxygen line shapes from our X-ray irradiation studies for both LiAlO2 (induced) and LR-NMC (inherent) are found to have an additional oxidized oxygen RIXS feature not observed in O2 gas studies. This study highlights the unique insight of O K-edge RIXS into determining the nature and stability of oxidized oxygen states

Revisiting the charge compensation mechanisms in LiNi0.8Co0.2-: YAlyO2 systems

Oxygen participation, arising from increased transition metal-oxygen covalency during delithiation, is considered essential for the description of charge compensation in conventional layered oxides. The advent of high-resolution mapping of the O K-edge resonant inelastic X-ray scattering (RIXS) provides an opportunity to revisit the onset and extent of oxygen participation. Combining RIXS with an array of structural and electronic probes for the family of Ni-rich LiNi0.8Co0.2-yAlyO2cathodes, we identify common charge compensation regimes that are assigned to formal transition metal redox (&lt;4.25 V) and oxygen participation through covalency (&gt;4.25 V). From O K-edge RIXS maps, we find the emergence of a sharp RIXS feature in these systems when approaching full delithiation, which has previously been associated with lattice oxidized oxygen in alkali-rich systems. The lack of transition metal redox signatures and strong covalency at these high degrees of delithiation suggest this RIXS feature is similarly attributed to lattice oxygen charge compensation as in the alkali-rich systems. The RIXS feature's evolution with state of charge in conventional layered oxides is evidence that this feature reflects the depopulation of occupied O 2p states associated with oxygen participation

Influence of Polymorphism on the Electronic Structure of Ga2O3

The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy efficient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations