310 Search Results

Small changes in the chemical potential at a semiconductor interface can result in dramatic changes to the space-charge layer that underpins applications in the electronic and photovoltaic industries as well as in photoelectrochemical cells for fuel production. There has hence been great interest in techniques that directly probe the space-charge layer, yet many fail at the semiconductor–electrolyte interface due to the potential drop in the electric double-layer region of the electrolyte. This article demonstrates that photovoltages, obtained from straightforward cyclic voltammetry measurements, provide an experimental and quantitative approach for characterizing the semiconductor–electrolyte interface. Key parameters accessible through this approach includemore » the flat-band potential (E_fb), the fraction of the total potential that drops across the space-charge layer (γ_sc) and the electric double layer, as well as the surface recombination lifetime (τ_s). Here, we report photovoltage measurements for p-type Si(111) photoelectrodes in contact with electrolytes containing redox-active species with a range of known reduction potentials that exceed the 1.1 eV bandgap. In tetrabutylammonium [NBu₄]⁺ electrolyte, the flat-band potential determined for hydrogen-terminated (p-Si–H), methyl-terminated (p-Si–CH₃), and chemically oxidized (p-Si–cSiO_x) surfaces were −0.02, −0.31, and 0.30 V vs Fc^+/0, respectively, agreeing well with expected shifts arising from surface dipole modifications. The quantitative analysis also reveals that 67% of the applied bias drops across the space-charge layer for p-Si–H, 73% for p-Si–CH₃, and only 44% for p-Si–cSiO_x. The remaining potential drop is attributed to the interfacial surface layer, which consists of a molecular dipole or oxide overlayer, and the Helmholtz layer within the electrolyte. When the larger [NBu₄]⁺ electrolyte was replaced with Li⁺, the flat-band position showed minimal changes, but the fraction of the potential drop across the space-charge layer increased significantly, consistent with the small cation altering the structure of the electric double layer.« less

Chalcohalide semiconductors are rapidly gaining traction as stable, biocompatible materials for energy conversion applications. While the solid-state synthesis of bulk chalcohalides is relatively well-developed, the colloidal chemistry of these materials is still in its early stages. Colloidal semiconductors are often advantageous in device fabrication due to the cost effectiveness of solution processing. Thus, we aim to increase the utility of chalcohalides in device fabrication by establishing solution phase chemistry of promising compositions. We show that silyl hot-injection is a versatile and effective method of making colloidal PnChI (Pn = Sb, Bi; Ch = S, Se) and Sn₂PnS₂I₃ (Pn = Sb,more » Bi) chalcohalides of tunable sizes and compositions. Furthermore, we demonstrate the preparation of mixed-pnictide chalcohalides through direct hot-injection and/or postsynthetic cation exchange, the latter being one of the few reported instances in chalcohalides. Additionally, we use the thiocyanate heat-up approach in combination with density functional theory to study halide mixing in quaternary tin chalcohalides. By pushing the limits of each synthetic technique, we have designed more soluble chalcohalides with tunable compositions while also gaining a better understanding of the efficacy of each procedure in respect to thin film and subsequent device fabrication. In addition to size and composition tuning, silyl hot-injection can help facilitate the future development and wide-scale application of chalcohalide-based devices by expanding the selection of solution-processable chalcohalides.« less

Here, we present a machine learning (ML) method for efficient computation of vibrational thermal expectation values of physical properties from first principles. Our approach is based on the nonperturbative frozen phonon formulation in which stochastic Monte Carlo algorithm is employed to sample configurations of nuclei in a supercell at finite temperatures based on a first-principles phonon model. A deep-learning neural network is trained to accurately predict physical properties associated with sampled phonon configurations, thus bypassing the time-consuming ab initio calculations. To incorporate the point-group symmetry of the electronic system into the ML model, group-theoretical methods are used to develop amore » symmetry-invariant descriptor for phonon configurations in the supercell. We apply our ML approach to compute the temperature dependent electronic energy gap of silicon based on density functional theory (DFT). We show that, with less than a hundred DFT calculations for training the neural network model, an order of magnitude larger number of sampling can be achieved for the computation of the vibrational thermal expectation values. Our work highlights the promising potential of ML techniques for finite temperature first-principles electronic structure methods.« less

Semiconductor photoelectrochemistry is a dynamic and interdisciplinary field at the forefront of research in solar fuels, energy conversion, and catalysis. Here, this Perspective captures the collective insights from the second Gerischer Electrochemistry Today Symposium, held at Colorado State University in Fort Collins, CO, in August 2024, which convened leading researchers, early-career scientists, and industry partners to define the critical next steps for the field. Through interactive sessions, technical talks, panel discussions, and training initiatives─including a Semiconductor Electrochemistry Bootcamp─the symposium emphasized three pillars of advancement: (i) facilitating the exchange of new ideas in semiconductor electrochemistry and charge separation; (ii) fostering themore » development of future researchers, research topics, and participation in the semiconductor workforce; and (iii) building community. This Energy Focus distills key themes from the meeting and identifies major knowledge gaps in the following areas: mechanisms of charge separation and recombination, role of defects and disorder, dynamic and operando characterization methods, interfacial chemistry and surface passivation, theoretical and modeling limitations, and standardization and benchmarking. The inclusive and collaborative structure of the symposium enabled the generation of this comprehensive report that will serve as a roadmap for fundamental and applied research in the rapidly evolving field of semiconductor electrochemistry over the next decade.« less

Extended x-ray absorption fine structure (EXAFS) measurements of single-crystal Ge/GeSn radial heterostructure nanowires are used to examine the effects of composition on both short-range order (SRO) and longer-range disorder in GeSn alloys. GeSn has prompted significant interest because it can achieve a direct band gap for sufficient Sn concentrations beyond the equilibrium solid solubility limit in an all-group IV system. Short-range order in this material is particularly interesting as it has been predicted to affect the band gap independent of average composition or strain effects. By independently controlling the Sn composition and GeSn thickness during chemical vapor deposition of misfittingmore » GeSn shells around ultrathin, elastically compliant, Ge core nanowires, the elastic misfit strain in the GeSn is minimized for Sn compositions over the studied range ≈Ge_0.96Sn_0.04 to Ge_0.88Sn_0.12. The degree of SRO was found to decrease with increasing Sn composition. Additionally, damping of the EXAFS signal was observed as the Sn content increased, particularly for increasingly distant atomic shells about the absorbing atom, even for scattering paths not involving Sn atoms. This result is quantified as an increase in the mean-squared relative displacement parameters of the shells. These measurements reveal the accommodation of local strain due to the presence of the highly size-mismatched Sn atoms in the Ge diamond cubic lattice (≈14%), which may have effects on the band structure of the material in addition to the influence of short-range atomic order. Comparison among the nanowire samples allows for calculation of the topological rigidity parameter, a^∗∗, for the first-neighbor bond lengths. Furthermore, these exhibit chemically distinct values for Ge-Ge, Ge-Sn, and Sn-Sn, and they are consistent with the value a^∗∗ = 0.75 ± 0.07 confirming the general applicability of the model to alloys with both large amounts of natural misfit strain and the potential for short-range order.« less

The coupled electronic states in two-dimensional (2D) and three-dimensional (3D) double quantum dot (DQD) systems are investigated using a phenomenological model applied to InAs/GaAs heterostructures. The single-band k • p effective potential approach previously proposed by our group is employed to numerically calculate the energy spectrum and spatial localization of a single electron, serving as an indicator of the coupling strength within the binary system. For identical quantum dots (QDs) in a DQD, the electronic states exhibit ideal coherence. We systematically vary the DQD geometry and the strength of the confinement potential (via an applied electric field) to examine themore » effects of symmetry breaking and the sensitivity of electron localization in both identical and nearly identical DQDs. Our results show that coherence in DQDs is highly sensitive to these subtle variations. This sensitivity can be harnessed to detect changes in the surrounding environment, such as fluctuations in chemical or electrical properties that affect the DQD system.« less

This work evaluates a new process route to making float zone (Fz)-quality silicon wafers using a combination of computational fluid dynamics (CFD) modeling and technoeconomic analysis. Our analysis finds that the new process competes with Czochralski (Cz)-grown wafers on a levelized cost of energy system level. The new process also decreases embodied energy and carbon of silicon photovoltaics (PV) by ~6x circumventing the energy-costly Siemens process used in polycrystalline silicon (poly-Si) production plants to generate feedstock for Fz and Cz boules. Instead of using poly-Si from the Siemens process to feed crystallization, the new process uses the high-purity, trichlorosilane (TCS)more » precursor gas to grow a poly-Si feed rod in-situ during a modified Fz1,2 boule growth process. The gas-to-boule float zone process enables opportunity to produce high-purity (low metals and oxygen content), uniformly doped single crystal silicon boules and wafers with high bulk lifetimes (τ_bulk > 15 ms) to enable higher efficiency cells (>27 %) with fewer known degradation mechanisms than Czochralski (Cz)-grown wafers. These benefits reduce the levelized cost of electricity (LCOE) of PV-produced electricity. Here we show the results of our CFD and chemical modeling of the process to prove feasibility and economic viability.« less

Local resistance imaging can provide information on nm-scale carrier distribution in semiconductor devices. Scanning spreading resistance microscopy (SSRM), an atomic force microscopy-based nm-scale resistance mapping technique, has been developed for carrier delineation in Si microdevices. We report on the development and validation of SSRM on CdTe materials, by testing on molecular beam epitaxy (MBE) grown CdTe films. The probe/CdTe contact resistance was suppressed sufficiently below sample's spreading resistance by pressing the probe into the sample with ∼mN contact force and applying a large sample/probe forward bias voltage (V_s), which was understood by analyzing current-voltage (I-V) involving a serially connected insulatingmore » top layer with underlying spreading resistance. The carrier concentration as deduced from the resistance measurement, using a single mobility value, is consistent with Hall measurement with a standard deviation of 14% based on a set of MBE films with carrier concentrations in the range of 10¹⁵–10¹⁶/cm³. The doping polarity was readily identified by flipping V_s polarity, where the resistance with reverse V_s is orders of magnitude larger than forward V_s. While focusing on the SSRM technique validation, we also show an example on an As-doped Cd(Se,Te) polycrystalline thin film of a high-performance CdTe solar cell, which illustrates the local resistance nonuniformity with up to two orders of magnitude differences, indicating if local mobility is roughly constant, local carrier concentration can have significant nonuniformity.« less

In this paper, we investigate, computationally and experimentally, the phase stability, electronic structure properties, and the propensity for n-type doping of In₂X₂O₇ (X = Si, Ge) ternary oxides. This family of materials contains promising novel wide-gap semiconductors based on their estimated high n-type Baliga figures of merit and acceptable thermal conductivity for power electronics applications. Here, we predict that both In₂Si₂O₇ and In₂Ge₂O₇ are n-type dopable, with Zr providing between 10¹⁶ and above 10²¹ cm⁻³ net donor concentrations under O-poor conditions, depending on the chemistry, structure (ground-state thortveitite or high-pressure pyrochlore), and synthesis temperature. To verify our predictions, we synthesizemore » Zr-doped In₂Ge₂O₇ in the thortveitite structure and measure its electrical properties. Initial thin-film growth and annealing lead to polycrystalline thin films with bandgaps over 4 eV and confirm Zr doping predictions by achieving electron concentrations at 10¹⁴–10¹⁶ cm⁻³ even under O-rich conditions. While future epitaxial growth development is still needed, this study establishes In₂X₂O₇ as promising n-type wide-gap semiconductors for power electronic applications.« less

Long-range Coulomb forces give rise to correlated insulating states when charge particles populate a moiré superlattice at certain fractional filling factors. Such behavior is characterized by a broken translation symmetry wherein particles spontaneously form a Wigner crystal. Here, focusing on the experimental findings of Xu et al. [Nature (London) 587, 214 (2020)], we present a theory that captures the correlated insulating state of a fractionally filled moiré superlattice through the energy shift and change in oscillator strength of the exciton absorption resonance. The theory shows that the experimental findings can only be supported if the electrons reside in a charge-orderedmore » state (i.e., electrons are not randomly distributed among the sites of the moiré superlattice). Furthermore, we explain why the energy shifts of exciton resonances are qualitatively different in cases where the superlattice is nearly empty compared with a superlattice whose sites are doubly occupied.« less