A great deal of work is currently devoted to the development of new semiconductor alloys that can expand the range of material properties and device applications. Although group‐III nitride alloys are attractive materials owing to the wide range of tunable direct bandgaps and other suitable properties, the material choices are limited to only a few binary and ternary alloys. This situation is due in part to the limitations of the conventional deposition methods, such as chemical vapor deposition, requiring high temperature and high pressure to synthesize a wider range of metastable alloys. In this context, the synthesis of previously unreported quaternary nitride alloys including Mg—(InGaMg)N—is presented. These alloys, with a tunable bandgap and good crystallinity, extend the group of the materials that may be suitable for the fabrication of optoelectronic devices. The method is based on the conventional plasma‐assisted molecular beam epitaxy (PA‐MBE), using a flux‐modulation technique to enable the incorporation of all elements reaching the growth surface. In addition to detailed experimental characterization of structural and optical properties of the Mg containing nitride alloys, computations on their electronic band structures are also carried out.

Tunnel junctions are indispensable elements of multi junction solar cells. The fabrication of InGaN tunnel junctions requires the growth of degenerately doped n- and p-type layers. While highly doped n-type InGaN films have been demonstrated, the growth of degenerately p-doped InGaN films and the fabrication of high indium fraction InGaN tunnel junctions is still to be demonstrated. We present an investigation of the effect of Mg doping on the InGaN crystal properties over a large range of Mg fluxes and InN mole fractions in the range from 30% to 40%, using multiple characterization techniques. InGaN thin films were grown on GaN/sapphire templates and doped with Mg using plasma-assisted molecular beam epitaxy (PAMBE). We have found that the Mg concentration in the film increases linearly with the Mg beam equivalent pressure (BEP) at first, followed by a saturation at similar to 4 x 10(21) cm(-3) similar to the Mg doping behavior reported for GaN. The growth rate of the alloy changes by more than 50% with the changes in the surface availability of Mg. These effects can be explained through the saturation of the atomic sites available for incorporation in the case of Mg concentration saturation and by the passivation of the free nitrogen radicals in the case of the growth rate variation. The incorporation of In and Ga depends on the flux ratio (Phi(In) + Phi(Ga))/(Phi(Mg)) at the growth surface and it is shown that the decrease of this ratio below a threshold of similar to 2000 causes the almost complete loss of In and the formation of a new quaternary wide band gap semiconductor alloy (InGaMg)N.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTPolypeptide-Assisted Organization of π-Conjugated Polymers into Responsive, Soft 3D NetworksCornelia Rosu*†‡∥, Christopher J. Tassone#, Ping-Hsun Chu‡, Paul L. Balding§∥, Andrew Gorman†, Jeff L. Hernandez§, Michael Hawkridge∇, Anirban Roy¶, Ioan I. Negulescu⊥, Paul S. Russo†§∥, and Elsa Reichmanis*‡§†∥View Author Information‡ § † ∥ †School of Materials Science and Engineering, ‡School of Chemical and Biomolecular Engineering, §School of Chemistry and Biochemistry, ∥Georgia Tech Polymer Network, GTPN, Georgia Institute of Technology, Atlanta, Georgia 30332, United States# Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Stanford, California 94025, United States∇ PANalytical, 117 Flanders Road, Westborough, Massachusetts 01851, United States¶ Anasys Instruments, 325 Chapala Street, Santa Barbara, California 93101, United States⊥ Department of Textile Apparel Design & Merchandising, Louisiana State University, Baton Rouge, Louisiana 70803, United States*Cornelia Rosu; Email: [email protected]*Elsa Reichmanis; Email: [email protected]Cite this: Chem. Mater. 2017, 29, 12, 5058–5062Publication Date (Web):June 8, 2017Publication History Received17 May 2017Revised6 June 2017Published online12 June 2017Published inissue 27 June 2017https://doi.org/10.1021/acs.chemmater.7b02035Copyright © 2017 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views831Altmetric-Citations3LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (5 MB) Get e-AlertsSupporting Info (4)»Supporting Information Supporting Information SUBJECTS:Chemical structure,Crystal structure,Organic polymers,Polymers,X-ray scattering Get e-Alerts

The effect of rapid thermal annealing on the optical and structural properties of GaAsBi/GaAs quantum wells (QWs) is investigated. The photoluminescence (PL) spectra of the samples are measured at 80 K and room temperature before and after rapid thermal annealing, to ascertain any improvement in the optical quality of the material. The impact of annealing temperature on QW interface quality, layer composition, and thicknesses are studied with x-ray diffraction. For a 60 second annealing time, the low temperature peak PL intensity increases to a maximum of 1.8 times the original intensity at an annealing temperature of 500 °C. Validating this optimum annealing temperature, the room temperature PL peak intensity is seen to increase by 2.2 times. The peak position exhibits a minor blueshift of 15 meV throughout the 450–700 °C temperature range, while annealing at 750 °C produces a blue-shift on the order of 100 meV, indicating out-diffusion of bismuth from the QW. Degradation of the QW interfaces with annealing temperatures above 550 °C is observed. The composition and thickness of the QWs remained constant up to 700 °C. Significant out-diffusion of bismuth and QW thinning are observed at an annealing temperature of 750 °C.

High quality Bi2Te3 and Sb2Te3 topological insulators films were epitaxially grown on GaAs (111) substrate using solid source molecular beam epitaxy. Their growth and behavior on both vicinal and non-vicinal GaAs (111) substrates were investigated by reflection high-energy electron diffraction, atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy. It is found that non-vicinal GaAs (111) substrate is better than a vicinal substrate to provide high quality Bi2Te3 and Sb2Te3 films. Hall and magnetoresistance measurements indicate that p type Sb2Te3 and n type Bi2Te3 topological insulator films can be directly grown on a GaAs (111) substrate, which may pave a way to fabricate topological insulator p-n junction on the same substrate, compatible with the fabrication process of present semiconductor optoelectronic devices.

GaAsBi/GaAs/AlGaAs separate confinement heterostructures are grown using an asymmetric temperature profile due to the low optimal growth temperature of GaAsBi; the bottom AlGaAs barrier is grown at 610 °C, while the GaAsBi quantum well and the top AlGaAs barrier are grown at 320 °C. Cross-sectional transmission electron microscopy and room temperature photoluminescence measurements indicate that this approach results in samples with excellent structural and optical properties. The high quality of the low temperature AlGaAs barrier is attributed to the presence of Bi on the surface as indicated by a (1 × 3) surface reconstruction persisting throughout the low temperature growth.

Correlation between structural defects and appearance of multiple cathodoluminescence and photoluminescence peaks of InxGa1-xN grown nominally with x=0.1 and increasing layer thickness (100 nm to 1000 nm) is discussed. Cathodoluminescence studies were performed on the cross-section samples earlier characterized by electron microscopy including Z-contrast microscopy. Strained and relaxed layers with different In concentrations were observed for InGaN layers above the critical layer thickness. Stacking faults appear at high density in the relaxed layer which also roughens, created a saw-tooth surface profile due to V-shaped pits. Large domains of closely separated stacking faults (polytype-like) were observed. In Z-contrast microscopy stacking faults in upper/lower part of the layer appear with higher/lower brightness, suggesting different amount of In incorporation in agreement with x-ray and RBS results. Only thin, strained InGaN layers showed single band-edge CL peaks. Multiple CL peaks appear in the relaxed, defective portion of the InGaN layers. By comparison with GaN samples where structural defects are associated with CL peak shifts, we postulate that defects, their type and distribution are main contributors to the multiple peaks observed for InGaN samples. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

GaN 1 − x As x alloys grown across the composition range by low temperature molecular beam epitaxy have great technological potential for photovoltaic applications owing to their strong absorption coefficient and wide tunability of band gap and band edges. We found that amorphous GaN1−xAsx alloys that are formed for the compositions x, in the range of x∼0.3–0.7 are stable up to 700 °C. This is surprising since growth of GaN1−xAsx above 400 °C results in phase segregation. At annealing temperatures higher than 700 °C the alloy phase segregates into GaAs:N and GaN:As. The relative size of the nanocrystals depends on the initial film composition and annealing conditions.

Temperature-dependent thermopower and Hall-effect measurements, combined with model calculations including all of the relevant elastic-and inelastic-scattering mechanisms, are used to quantify the role of charged line defects on electron transport in n-type InN films grown by molecular-beam epitaxy. Films with electron concentrations between 4 x 10(17) and 5 x 10(19) cm(-3) were investigated. Charged point and line defect scattering produce qualitatively different temperature dependences of the thermopower and mobility, allowing their relative contribution to the scattering to be evaluated using charge neutrality at the measured electron concentration. Both charge state possibilities for the dislocations [positively charged (donors) or negatively charged (acceptors)], were considered. The 100-300 K temperature dependence of the mobility and the 200-320 K temperature dependence of the thermopower can be modeled well with either assumption. The dislocation density was independently measured by plan-view and cross-sectional transmission electron microscopy and corresponds well with the values obtained from transport modeling.

Transmission Electron Microscopy shows that the InN samples doped with either increasing or constant Mg concentration follow a cation or anion substrate polarity. In-polar samples change growth polarity when the Mg concentration is >10 cm. N-polar samples have much higher density of planar defects than In-polar samples and their presence leads to a decrease in dislocation density. In the N-polar samples equally spaced planar defects are observed for Mg concentration >10 cm. Three different polytypes (2H, 3C and 4H) were observed in this type of samples. A band of planar defects with thick layers of a cubic material (3C) is observed for Mg concentration >10 cm. At this Mg concentration only n-type conductivity was reported earlier.

Transmission Electron Microscopy shows that the InN samples doped with either increasing or constant Mg concentration follow a cation or anion substrate polarity. In-polar samples change growth polarity when the Mg concentration is >10(19) cm(-3). N-polar samples have much higher density of planar defects than In-polar samples and their presence leads to a decrease in dislocation density. In the N-polar samples equally spaced planar defects are observed for Mg concentration >10(19) cm(-3). Three different polytypes (2H, 3C and 4H) were observed in this type of samples. A band of planar defects with thick layers of a cubic material (3C) is observed for Mg concentration >10(20) cm(-3). At this Mg concentration only n-type conductivity was reported earlier. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A new alloy system, the GaN1-xAsx alloys in the whole composition range was successfully synthesized using the non-equilibrium low temperature molecular beam epitaxy method. The alloys are amorphous in the composition range of 0.17 < x < 0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition and sharp, well defined optical absorption edges. The bandgap energy varies in a broad energy range from similar to 3.4 eV in GaN to similar to 0.8 eV at x similar to 0.85. The reduction of the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x > 0.2, and to the upward movement of the valence band for alloys with x < 0.2. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Using low temperature MBE, we have shown that it is possible to grow amorphous GaN1-xAsx layers with a variable As content (0 < x < 0.8) on both crystalline (sapphire and silicon) and amorphous (glass and Pyrex glass) substrates. Despite the fact that the samples with high As content are amorphous, we observe a gradual continuous decrease of bandgap from similar to 3.4 to similar to 0.8 eV with increase in As content. To the best of our knowledge this is the first demonstration of homogeneous amorphous GaN-based alloys over a wide composition range. The large band gap range of the amorphous phase of GaNAs covers much of the solar spectrum. The amorphous nature of the GaNAs alloys is particularly advantageous since low cost substrates such as glass and Pyrex glass can be used for solar cell fabrication. (C) 2010 Elsevier B.V. All rights reserved.

Using plasma-assisted molecular beam epitaxy (PA-MBE) high quality InxGa1-xN layers with x in the range from 25 to 31% have been grown on silicon (111) substrates. The polarity of the layers has been found to impact the incorporation of in with Ga polar butters promoting the deposition of uniform composition InGaN. We have achieved films with indium fraction up to 31% and rocking curve width of 538 arcsec. Residual donor concentration as low as similar to 1.2 x 10(18) cm(-3) was measure in these films suggesting that p-typed doping with Mg can be achieved. The presence of AIN layers and the increasing thickness of the GaN buffer do not appear to have a significant contribution to the series resistance of the structure. The investigation of the InGan layers by X-ray diffraction did not reveal any significant phase separation occurring the MBE deposition although the photoluminescence spectrum exhibits low energy features that would require further investigation. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report the growth of ternary aluminum gallium nitride (AlGaN) layers on AlN/sapphire template/substrates by digitally alloyed modulated precursor flow epitaxial growth (DA-MPEG), which combined an MPEG AlN sub-layer with a conventional metalorganic chemical vapor deposition (MOCVD)-grown GaN sub-layer. The overall composition in DA-MPEG Al x Ga1−x N was controlled by adjustment of the growth time (i.e., the thickness) of the GaN sub-layer. As the GaN sub-layer growth time increased, the Al composition in AlGaN decreased to 50%, but the surface morphology of the AlGaN layer became rough, and a three-dimensional structure with islands appeared for the DA-MPEG AlGaN with relatively thick GaN sub-layers, possibly resulting from the Ga adatom surface migration behavior and/or the strain built up from lattice mismatch between AlN and GaN sub-layers with increasing GaN sub-layer growth time. Through strain analysis by high-resolution x-ray diffraction, reciprocal space mapping, and scanning transmission electron microscopy, it was found that there was compositional inhomogeneity in the DA-MPEG AlGaN with AlN and GaN binary sub-layers for the case of the layer with relatively thick GaN sub-layers.