John Mangum

Development of GaInP Solar Cells Grown by Hydride Vapor Phase Epitaxy

We demonstrate the growth of homojunction GaInP solar cells by dynamic hydride vapor phase epitaxy for the first time. Simple unpassivated n-on-p structures grown in an inverted configuration with gold back reflectors were analyzed. Short wavelength performance varied strongly with emitter thickness, since collection in the emitter was limited by the lack of surface passivation. Collection in the base increased strongly with decreasing doping density, in the range 1 × 10¹⁶ − 5 × 10¹⁷ cm⁻³. Optical modeling indicated that, in our best device, doped ∼1 × 10¹⁶ cm⁻³, almost 94% of photons that passed through the emitter were collected. Modeling also indicated that the majority of collection occurs in the depletion region with this design, suggesting that nonradiative recombination there might limit device performance. In agreement with this observation, the experimental dark <italic>J–V</italic> curve exhibited an ideality factor near <italic>n</italic> = 2. Thus, limitation of deep level carrier traps in the material is a path to improved performance. Preliminary experiments indicate that a reduced V/III ratio, which potentially affects the density of these presumed traps, improves cell performance. With reduced V/III ratio, we demonstrate a ∼13% efficient GaInP cell measured under the 1-sun AM1.5G spectrum. This cell had an antireflective coating, but no front surface passivation.

Enhanced piezoelectric response of AlN via CrN alloying

Since AlN has emerged as an important piezoelectric material for a wide variety of applications, efforts have been made to increase its piezoelectric response via alloying with transition metals that can substitute for Al in the wurtzite lattice. Herein, we report density functional theory calculations of structure and properties of the Cr-AlN system for Cr concentrations ranging past the wurtzite-rocksalt transition point. By studying the different contributions to the longitudinal piezoelectric coefficient, we propose that the physical origin of the enhanced piezoelectricity in Cr

High-fraction brookite films from amorphous precursors

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Correlative Raman spectroscopy and focused ion beam for targeted phase boundary analysis of titania polymorphs

Al

Understanding crystallization pathways leading to manganese oxide polymorph formation

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Stabilization of wide band-gap p-type wurtzite MnTe thin films on amorphous substrates

N alloys is the increase of the internal parameter

Theory-Guided Synthesis of a Metastable Lead-Free Piezoelectric Polymorph

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Design of Metastable Tin Titanium Nitride Semiconductor Alloys

of the wurtzite structure upon substitution of Al with the larger Cr ions. Among a set of wurtzite-structured materials, we have found that Cr-AlN has the most sensitive piezoelectric coefficient with respect to alloying concentration. Based on these results, we propose that Cr-AlN is a viable piezoelectric material whose properties can be tuned via Cr composition; we support this proposal by combinatorial synthesis experiments, which show that Cr can be incorporated in the AlN lattice up to 30\% before a detectable transition to rocksalt occurs. At this Cr content, the piezoelectric modulus

Enhanced Piezoelectric Response of AlN via CrN Alloying

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Redox-Mediated Stabilization in Zinc Molybdenum Nitrides

is approximately four times larger than that of pure AlN. This finding, combined with the relative ease of synthesis, may propel Cr-AlN as the prime piezoelectric material for applications such as resonators and acoustic wave generators.