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We report the growth of epitaxial wurtzite AlScN thin films on Si (111) substrates with a wide range of Sc concentrations using ultra-high vacuum reactive sputtering. Sc alloying in AlN enhances piezoelectricity and induces ferroelectricity, and epitaxial thin films facilitate systematic structure-based investigations of this important and emerging class of materials. Two main effects are observed as a function of increasing Sc concentration. First, increasing crystalline disorder is observed together with a structural transition from wurtzite to rocksalt at ∼30 at% Sc. Second, nanoscale compositional segregation consistent with spinodal decomposition occurs at intermediate compositions, before the wurtzite to rocksalt phase boundary is reached. Lamellar features arising from composition fluctuations are correlated with polarization domains in AlScN, suggesting that composition segregation can influence ferroelectric properties. The present results provide a route to the creation of single crystal AlScN films on Si (111), as well as a means for self-assembled composition modulation. 

Core–shell Ge/GeSn nanowires provide a route to dislocation-free single crystal germanium-tin alloys with desirable light emission properties because the Ge core acts as an elastically compliant substrate during misfitting GeSn shell growth. However, the uniformity of tin incorporation during reduced pressure chemical vapor deposition may be limited by the kinetics of mass transfer to the shell during GeSn growth. The balance between Sn precursor flux and available surfaces for GeSn nucleation and growth determines whether defects are formed and their type. On the one hand, when the Sn precursor delivery is insufficient, local variations in Sn arrival rate at the nanowire surfaces during GeSn growth produce asymmetries in shell growth that induce wire bending. This inhomogeneous elastic dilatation due to the varying composition occurs via deposition of Sn-poor regions on some of the {112} sidewall facets of the nanowires. On the other hand, when the available nanowire surface area is insufficient to accommodate the arriving Sn precursor flux, Sn-rich precipitate formation results. Between these two extremes, there exists a regime of growth conditions and nanowire densities that permits defect-free GeSn shell growth. 

Solar water splitting using photoelectrochemical cells (PEC's) is a promising pathway toward clean and sustainable storage of renewable energy. Practical realization of solar-driven synthesis of hydrogen and oxygen integrating light absorption and electrolysis of water has been challenging because of (1) the limited stability of good photovoltaic materials under the required electrochemical conditions, and (2) photovoltaic efficiency losses due to light absorption by catalysts, the electrolyte, and generated bubbles, or reflection at their various interfaces. Herein, we evaluate a novel integrated solar water splitting architecture using efficient silicon heterojunction photovoltaic cells that avoids such losses and exhibits a solar-to-hydrogen (STH) efficiency in excess of 10%. Series-connected silicon Heterojunction with Intrinsic Thin layer (HIT) cells generate sufficient photovoltage for unassisted water splitting, with one of the cells acting as the photocathode. Platinum is deposited on the back (dark) junction of this HIT cell as the catalyst for the hydrogen evolution reaction (HER). The photocathode is protected from corrosion by a TiO 2 layer deposited by atomic layer deposition (ALD) interposed between the HIT cell and the Pt, enabling stable operation for >120 hours. Combined with oxygen evolution reaction (OER) catalysts deposited on a porous metal dark anode, these PEC's achieve stable water splitting with a record high STH efficiency for an integrated silicon photosynthesis device. 

The hole transport layer (HTL) is one of the key components in planar perovskite solar cells. This study reports a new kind of HTL fabricated using atomic layer deposition (ALD). By alloying TiO₂with IrO_x, it is demonstrated that TiO₂, a well‐known electron‐selective contact and electron transport layer in photovoltaic devices, can behave as an HTL with an appropriately high work function. Perovskite Cs_0.17FA_0.83Pb(I_0.83Br_0.17)₃solar cells including this new hole transport material achieve a power conversion efficiency of 15.8% under AM 1.5G simulated solar irradiation compared to a 14.3% efficiency for otherwise‐identical devices incorporating a more standard NiO HTL layer. These results suggest the promise of transition metal oxide alloys synthesized by ALD as hole contact materials for optoelectronic devices, including advanced photovoltaics.

 

Reactive ion etching (RIE) used to fabricate high‐aspect‐ratio (HAR) nano/microstructures is known to damage semiconductor surfaces which enhances surface recombination and limits the conversion efficiency of nanostructured solar cells. Here, defect passivation of ultrathin Al₂O₃‐coated Si micropillars (MPs) using different surface pretreatment steps is reported. Effects on interface state density are quantified by means of electrochemical impedance spectroscopy which is used to extract quantitative capacitance–voltage and conductance–voltage characteristics from HAR dielectric–semiconductor structures which would otherwise suffer from high gate leakage currents if tested using solid‐state metal–insulator–semiconductor structures. High‐temperature thermal oxidation to form a sacrificial oxide on RIE‐fabricated Si MPs, followed by atomic layer deposition of 4 nm thick Al₂O₃after removal of the sacrificial layer produces an interface trap density (D_it) as low as 1.5 × 10¹¹cm⁻²eV⁻¹at the mid‐gap energy of silicon. However, a greatly reduced mid‐gapD_it(2 × 10¹¹cm⁻²eV⁻¹) is possible even with a simple air annealing procedure having a maximum temperature of 400 °C.