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Now showing items 1 - 14 of 14

  • Controlled Nucleation of GaN Nanowires Grown with Molecular Beam Epitaxy

    Bertness, Kris A.   Sanders, Aric W.   Rourke, Devin M.   Harvey, Todd E.   Roshko, Alexana   Schlager, John B.   Sanford, Norman A.  

    The location of GaN nanowires is controlled with essentially perfect selectivity using patterned SiN(x) prior to molecular beam epitaxy growth. Nanowire growth is uniform within mask openings and absent on the mask surface for over 95% of the usable area of a 76 mm diameter substrate. The diameters of the resulting nanowires are controlled by the size of the mask openings. Openings of approximately 500 nm or less produce single nanowires with symmetrically faceted tips.
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  • Lateral variations in self-assembled InGaAs quantum dot distributions

    Roshko, Alexana   Harvey, Todd E.   Hyland, Brit L.   Lehman, Susan Y.   Cobry, Keith D.  

    The lateral uniformity of self-assembled InGaAs quantum dots grown by molecular beam epitaxy (MBE) was assessed as a function of growth conditions. Variations in the dot density and height were determined from atomic force micrographs. Growth rate had a large influence on lateral uniformity. The most uniform dot distributions were grown at low rates, 0.15 monolayers/s (ML/s). Dots deposited at a rate of 1.15 ML/s had large variations in both height and density. These variations decreased as the dot density increased; however, they remained larger than those of dots deposited slowly. The lateral uniformity of dots deposited quickly also improved for the top layer of dots in stacked layers, even though these layers had decreased dot densities. There were negligible differences in the lateral height and density uniformities of dots as functions of continuous versus pulsed growth, wafer diameter and mole fraction of In. Published by Elsevier B.V.
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  • Selective Area Growth and Structural Characterization of GaN Nanostructures on Si(111) Substrates

    Roshko, Alexana   Brubaker, Matt   Blanchard, Paul   Harvey, Todd   Bertness, Kris A.  

    Selective area growth (SAG) of GaN nanowires and nanowalls on Si(111) substrates with AIN and GaN buffer layers grown by plasma-assisted molecular beam epitaxy was studied. For N-polar samples filling of SAG features increased with decreasing lattice mismatch between the SAG and buffer. Defects related to Al-Si eutectic formation were observed in all samples, irrespective of lattice mismatch and buffer layer polarity. Eutectic related defects in the Si surface caused voids in N-polar samples, but not in metal-polar samples. Likewise, inversion domains were present in N-polar, but not metal-polar samples. The morphology of Ga-polar GaN SAG on nitride buffered Si(111) was similar to that of homoepitaxial GaN SAG.
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  • Injection-level-dependent internal quantum efficiency and lasing in low-defect GaN nanowires

    Schlager, John B.   Sanford, Norman A.   Bertness, Kris A.   Roshko, Alexana  

    Measurements of temperature-dependent and time-resolved photoluminescence (PL) on individual GaN nanowires revealed PL lifetimes and values of internal quantum efficiency (IQE) that increased with excitation fluence. With sufficient injection levels, radiative recombination dominated within the nanowire temperature range of 75 K to 175 K, as indicated by the T(3/2) temperature dependence of the free-exciton PL lifetimes for this bulk material. The IQE was close to unity here. Free-carrier recombination became more significant as temperatures increased toward room temperature, but excitonic recombination remained important with ultrashort excitation pulse fluences as high as 190 mu J/cm(2). The IQE at room temperature fell to a value between 3% and 30% depending on the nature of the recombination, and, considering both excitonic and free-carrier recombination, the effective IQE was roughly 15%. Temperature-dependent measurements of lasing thresholds in optically pumped nanowires showed lower thresholds at temperatures where excitonic radiative recombination was strong, indicating a possible persistence of excitoniclike behavior with high injected carrier densities at temperatures below T = 170 K. [doi:10.1063/1.3553418]
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  • The role of Si in GaN/AlN/Si(111) plasma assisted molecular beam epitaxy:polarity and inversion

    Roshko, Alexana   Brubaker, Matthew   Blanchard, Paul   Harvey, Todd   Bertness, Kris  

    The microstructure, polarity and Si distribution in AlN/GaN layers grown by plasma assisted molecular beam epitaxy on Si(111) was assessed by scanning transmission electron microscopy. Samples grown under both metal-and nitrogen-rich conditions contained defects at the AlN/Si interface which suggest formation of an Al-Si eutectic. Correlated with this, interfacial segregation of Si was found in the samples. It is proposed that Si is dissolved in a eutectic layer floating on the AlN surface under metal-rich conditions. This Si is then incorporated into the film if the growth becomes nitrogen-rich, either intentionally or due to plasma source transients. These Si-rich layers appear to induce inversion of the nitride from nitrogen-to metal-polarity, and uncontrolled variations in the Si concentration cause occasional nonuniformity in the resulting inversion. (C) 2019 The Japan Society of Applied Physics
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  • Electron-Enhanced Atomic Layer Deposition of Boron Nitride Thin Films at Room Temperature and 100 degrees C

    Sprenger, Jaclyn K.   Sun, Huaxing   Cavanagh, Andrew S.   Roshko, Alexana   Blanchard, Paul T.   George, Steven M.  

    Electron-enhanced atomic layer deposition (EE-ALD) was used to deposit boron nitride (BN) thin films at room temperature and 100 degrees C using sequential exposures of borazine (B3N3H6) and electrons. Electron-stimulated desorption (ESD) of hydrogen surface species and the corresponding creation of reactive dangling bonds are believed to facilitate borazine adsorption and reduce the temperature required for BN film deposition. In situ ellipsometry measurements showed that the BN film thickness increased linearly versus the number of EE-ALD cycles at room temperature. Maximum growth rates of similar to 3.2 angstrom/cycle were measured at electron energies of 80-160 eV. BN film growth was self-limiting versus borazine and electron exposures, as expected for an ALD process. The calculated average hydrogen ESD cross section was sigma =3D 4.2 X 10(-17) cm(2). Ex situ spectroscopic ellipsometry measurements across the similar to 1 cm(2) area of the BN film defined by the electron beam displayed good uniformity in thickness. Ex situ X-ray photoelectron spectroscopy and in situ Auger spectroscopy revealed high purity, slightly boron-rich BN films with C and O impurity levels <3 at. %. High-resolution transmission electron microscopy (HR-TEM) imaging revealed polycrystalline hexagonal and turbostratic BN with the basal planes approximately parallel to the substrate surface. Ex situ grazing incidence X-ray diffraction measurements observed peaks consistent with hexagonal BN with domain sizes of 1-2 nm. The BN EE-ALD growth rate of similar to 3.2 angstrom/cycle is close to the distance of 3.3 angstrom between BN planes in hexagonal BN. The growth rate and HR-TEM images suggest that approximately one monolayer of BN is deposited for every BN EE-ALD cycle. TEM and scanning TEM/electron energy loss spectroscopy measurements of BN EE-ALD on trenched wafers also showed preferential BN EE-ALD on the horizontal surfaces. This selective deposition on the horizontal surfaces suggests that EE-ALD may enable bottom-up filling of vias and trenches.
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  • Toward Discrete Axial p-n Junction Nanowire Light-Emitting Diodes Grown by Plasma-Assisted Molecular Beam Epitaxy

    Brubaker, Matt D.   Blanchard, Paul T.   Schlager, John B.   Sanders, Aric W.   Herrero, Andrew M.   Roshko, Alexana   Duff, Shannon M.   Harvey, Todd E.   Bright, Victor M.   Sanford, Norman A.   Bertness, Kris A.  

    In this paper we investigate axial p-n junction GaN nanowires grown by plasma-assisted molecular beam epitaxy (MBE), with particular attention to the effect of Mg doping on the device characteristics of individual nanowire light-emitting diodes (LEDs). We observe that a significant fraction of single-nanowire LEDs produce measurable band-gap electroluminescence when a thin AlGaN electron blocking layer (EBL) is incorporated into the device structure near the junction. Similar devices with no EBL typically yield below-detection-limit electroluminescence, despite diode-like I-V characteristics and optically measured internal quantum efficiencies (IQEs) of similar to 1%. I-V measurements of the p-regions in p-n junction nanowires, as well as nanowires doped with Mg only, indicate low p-type conductivity and asymmetric Schottky-like p-contacts. These observations suggest that imbalanced carrier injection from the junction and p-contact can produce significant nonradiative losses.
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  • Comparison of convergent beam electron diffraction and annular bright field atomic imaging for GaN polarity determination

    Roshko, Alexana   Brubaker, Matt D.   Blanchard, Paul T.   Bertness, Kris A.   Harvey, Todd E.   Geiss, Roy H.   Levin, Igor  

    A comparison of two electron microscopy techniques used to determine the polarity of GaN nanowires is presented. The techniques are convergent beam electron diffraction (CBED) in TEM mode and annular bright field (ABF) imaging in aberration corrected STEM mode. Both measurements were made at nominally the same locations on a variety of GaN nanowires. In all cases the two techniques gave the same polarity result. An important aspect of the study was the calibration of the CBED pattern rotation relative to the TEM image. Three different microscopes were used for CBED measurements. For all three instruments there was a substantial rotation of the diffraction pattern (120 or 180 degrees) relative to the image, which, if unaccounted for, would have resulted in incorrect polarity determination. The study also shows that structural defects such as inversion domains can be readily identified by ABF imaging, but may escape identification by CBED. The relative advantages of the two techniques are discussed.
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  • Comparison of convergent beam electron diffraction and annular bright field atomic imaging for GaN polarity determination

    Roshko, Alexana   Brubaker, Matt D.   Blanchard, Paul T.   Bertness, Kris A.   Harvey, Todd E.   Geiss, Roy H.   Levin, Igor  

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  • UV LEDs based on p-i-n core-shell AlGaN/GaN nanowire heterostructures grown by N-polar selective area epitaxy

    Brubaker, Matt D.   Genter, Kristen L.   Roshko, Alexana   Blanchard, Paul T.   Spann, Bryan T.   Harvey, Todd E.   Bertness, Kris A.  

    Ultraviolet light-emitting diodes fabricated from N-polar AlGaN/GaN core-shell nanowires (NWs) with p-i-n structure produced electroluminescence at 365 nm with similar to 5x higher intensities than similar GaN homojunction LEDs. The improved characteristics were attributed to localization of spontaneous recombination to the NW core, reduction of carrier overflow losses through the NW shell, and elimination of current shunting. Poisson-drift-diffusion modeling indicated that a shell Al mole fraction of x =3D 0.1 in AlxGa1-xN effectively confines electrons and injected holes to the GaN core region. AlGaN overcoat layers targeting this approximate Al mole fraction were found to possess a low-Al-content tip and high-Al-content shell, as determined by scanning transmission electron microscopy. Photoluminescence spectroscopy further revealed the actual Al mole fraction to be NW diameter-dependent, where the tip and shell compositions converged towards the nominal flux ratio for large diameter NWs.
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  • UV LEDs based on p-i-n core-shell AlGaN/GaN nanowire heterostructures grown by N-polar selective area epitaxy

    Brubaker, Matt   Genter, Kristen   Roshko, Alexana   Blanchard, Paul   Spann, Bryan   Harvey, Todd   Bertness, Kris  

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  • Characterization of Sub-Monolayer Contaminants at the Regrowth Interface in GaN Nanowires Grown by Selective-Area Molecular Beam Epitaxy

    Blanchard, Paul   Brubaker, Matt   Harvey, Todd   Roshko, Alexana   Sanford, Norman   Weber, Joel   Bertness, Kris A.  

    While GaN nanowires (NWs) offer an attractive architecture for a variety of nanoscale optical, electronic, and mechanical devices, defects such as crystal polarity inversion domains (IDs) can limit device performance. Moreover, the formation of such defects during NW growth is not fully understood. In this study, we use transmission electron microscopy (TEM) and atom probe tomography (APT) to investigate the effects of sub-monolayer contamination at the regrowth interface in GaN NWs grown by selective-area molecular beam epitaxy (MBE). TEM energy dispersive X-ray spectroscopy (EDS) and APT independently identified Al and O contamination localized at the regrowth interface in two of the three growth runs examined. The Al and O concentrations were each estimated to be on the order of 11% of an ideal c-plane monolayer in the most severely contaminated case. The amount of contamination correlated with the number of crystal polarity inversion domain defects (IDs) across the growth runs. A growth run in which the pre-regrowth HF vapor etch step was replaced by HCl immersion showed the smallest quantity of O and no measurable Al. In addition, many of the NWs examined from the HCl-treated growth run turned out to be free of IDs. These results suggest that sub-monolayer contamination introduced during processing contributes to defect formation in MBE-grown GaN NWs.
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  • Polarity-Controlled GaN/AlN Nucleation Layers for Selective-Area Growth of GaN Nanowire Arrays on Si(111) Substrates by Molecular Beam Epitaxy

    Brubaker, Matt D.   Duff, Shannon M.   Harvey, Todd E.   Blanchard, Paul T.   Roshko, Alexana   Sanders, Aric W.   Sanford, Norman A.   Bertness, Kris A.  

    We have demonstrated dramatic improvement in the quality of selective-area GaN nanowire growth by controlling the polarity of the underlying nucleation layers. In particular, we find that N-polarity is beneficial for the growth of large ordered nanowire arrays with arbitrary spacing. Herein, we present techniques for obtaining and characterizing polarity-controlled nucleation layers on Si (111) substrates. An initial AlN layer, which is demonstrated to adopt Al-(N-)polarity for N-(Al-)rich growth conditions, is utilized to configure the polarity of subsequently grown GaN layers as determined by piezoresponse force microscopy (PFM), polarity-dependent surface reconstructions, and polarity-sensitive etching. Polarity-dependent surface reconstructions observed in reflection high-energy electron diffraction (RHEED) patterns were found to be particularly useful for in situ verification of the nucleation layer polarity, prior to mask deposition, patterning, and selective-area regrowth of the GaN NW arrays. N-polar templates produced fast-growing nanowires with vertical m-plane side walls and flat c-plane tips, while Ga-polar templates produced slow-growing pyramidal structures bounded by (1 (1) over bar 02) r-planes. The selective-area nanowire growth process window, bounded by nonselective and no-growth conditions, was found to be substantially more relaxed for NW arrays grown on N-polar templates, allowing for long-range selectivity where the NW pitch far exceeds the Ga diffusion length.
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  • Hybrid Glass Structures for Telecommunication Applications [4640-10]

    Hayden, Joseph S.   Conzone, Samuel D.   Callicoatt, Berton   Roshko, Alexana  

    "Hybrid glass parts composed of dissimilar glass sections are an attractive route to integrate multiple functions onto a single substrate and offer the potential to fabricate advanced laser sources; amplifiers; lossless splitters and other photonic devices such as Fabry-Perot etalons. We review the most promising bonding technologies; placing particular emphasis on techniques that do not require the use of high processing temperatures. In particular; we discuss in detail a recently developed low temperature bonding technology that relies on inorganic adhesives. Characterization of interfacial joints prepared with this inorganic technology indicate low insertion loss; high mechanical strength and chemical resistance to attack during the conventional lithographic and ion exchange steps employed to fabricate waveguide structures."
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