14 results on '"Collazo, Ramón"'
Search Results
2. Thermal conductivity of GaN single crystals: Influence of impurities incorporated in different growth processes.
- Author
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Rounds, Robert, Sarkar, Biplab, Sochacki, Tomasz, Bockowski, Michal, Imanishi, Masayuki, Mori, Yusuke, Kirste, Ronny, Collazo, Ramón, and Sitar, Zlatko
- Subjects
THERMAL conductivity ,GALLIUM nitride ,GALLIUM compounds ,VAPOR phase epitaxial growth ,CRYSTAL growth from vapor ,MASS spectrometry - Abstract
The thermal conductivity of GaN crystals grown by different techniques is analyzed using the 3ω method in the temperature range of 30 K to 295 K. GaN wafers grown by the ammonothermal method show a significant variation in thermal conductivity at room temperature with values ranging between 164 W m
−1 K−1 and 196 W m−1 K−1 . GaN crystals produced with the sodium flux and hydride vapor phase epitaxy methods show results of 211 W m−1 K−1 and 224 W m−1 K−1 , respectively, at room temperature. Analysis using secondary ion mass spectrometry indicates varying amounts of impurities between the respective crystals and explains the behavior of thermal conductivity trends in the samples. The observed difference between thermal conductivity curves suggests that scattering of phonons at point defects dominates the thermal conductivity of GaN within the investigated temperature range. Deviations of model curves from thermal conductivity measurements and disparities between modelled characteristic lengths and actual sample thicknesses indicate that phonon resonances are active in GaN. [ABSTRACT FROM AUTHOR]- Published
- 2018
- Full Text
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3. Schottky contacts to N-polar GaN with SiN interlayer for elevated temperature operation.
- Author
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Khachariya, Dolar, Szymanski, Dennis, Reddy, Pramod, Kohn, Erhard, Sitar, Zlatko, Collazo, Ramón, and Pavlidis, Spyridon
- Subjects
HIGH temperatures ,SCHOTTKY barrier diodes ,GALLIUM nitride ,STRAY currents - Abstract
In this Letter, we unveil the high-temperature limits of N-polar GaN Schottky contacts enhanced by a low-pressure chemical vapor deposited (LPCVD) SiN interlayer. Compared to conventional Schottky diodes, the insertion of a 5 nm SiN lossy dielectric interlayer in-between Ni and N-polar GaN increases the turn-on voltage (V
ON ) from 0.4 to 0.9 V and the barrier height (ϕB ) from 0.4 to 0.8 eV. This modification also reduces the leakage current at zero bias significantly: at room temperature, the leakage current in the conventional Schottky diode is >103 larger than that observed in the device with the SiN interlayer, while at 200 °C, this ratio increases to 105 . Thus, the rectification ratio (ION /IOFF ) at ±1.5 V reduces to less than one at 250 °C for the conventional Schottky diode, whereas for SiN-coated diodes, rectification continues until 500 °C. The I–V characteristics of the diode with an SiN interlayer can be recovered after exposure to 400 °C or lower. Contact degradation occurs at 500 °C, although devices are not destroyed yet. Here, we report N-polar GaN Schottky contact operation up to 500 °C using an LPCVD SiN interlayer. [ABSTRACT FROM AUTHOR]- Published
- 2022
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4. The role of Ga supersaturation on facet formation in the epitaxial lateral overgrowth of GaN.
- Author
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Wang, Ke, Kirste, Ronny, Mita, Seiji, Washiyama, Shun, Mecouch, Will, Reddy, Pramod, Collazo, Ramón, and Sitar, Zlatko
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SUPERSATURATION ,GALLIUM nitride ,CHEMICAL vapor deposition ,DENSITY functional theory ,SCANNING electron microscopy - Abstract
In this paper, facet formation of (0001) { 11 2 ¯ 0 } { 11 2 ¯ 2 } facets during epitaxial lateral overgrowth (ELO) of GaN is investigated for different Ga vapor supersaturations. The ELO was conducted via metalorganic chemical vapor deposition on patterned GaN/sapphire templates with SiO
2 masks aligned along the ⟨ 1 1 ¯ 00 ⟩ direction of GaN. Scanning electron microscopy was used to characterize the cross section shapes of the ELO GaN islands. A correlation of supersaturation, facet formation, and the shape of the ELO GaN islands is found. It is shown that { 11 2 ¯ 2 } facets are favored under high Ga vapor supersaturation, while { 11 2 ¯ 0 } facets are favored under low Ga vapor supersaturation. A qualitative model based on Wulff construction and density functional theory calculation is proposed to illustrate the mechanism of the facet formation of the ELO GaN islands. [ABSTRACT FROM AUTHOR]- Published
- 2022
- Full Text
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5. On the characteristics of N-polar GaN Schottky barrier contacts with LPCVD SiN interlayers.
- Author
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Khachariya, Dolar, Szymanski, Dennis, Breckenridge, M. Hayden, Reddy, Pramod, Kohn, Erhard, Sitar, Zlatko, Collazo, Ramón, and Pavlidis, Spyridon
- Subjects
SCHOTTKY barrier ,GALLIUM nitride ,CAPACITANCE measurement ,X-ray photoelectron spectroscopy ,SCHOTTKY barrier diodes ,ELECTRIC capacity - Abstract
We study the behavior of N-polar GaN Schottky diodes with low-pressure chemical vapor deposited (LPCVD) SiN interlayers and unveil the important role of an amphoteric miniband formed in this interlayer due to a previously identified and dominating Si dangling bond defect. Through analysis of temperature-dependent current–voltage (I–V–T), capacitance–voltage (C–V), and x-ray photoelectron spectroscopy measurements, we observe that when nickel is deposited on LPCVD SiN pretreated with hydrofluoric acid, the SiN/GaN interface is responsible for determining the overall system's barrier height. By contrast, contact formation on oxidized LPCVD SiN leads to a metal/SiN-dominant barrier. We, consequently, propose band diagrams that account for an amphoteric miniband in LPCVD SiN, leading to a new understanding of LPCVD SiN as a lossy dielectric with surface barrier-dependent behavior. [ABSTRACT FROM AUTHOR]
- Published
- 2021
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6. Smooth cubic commensurate oxides on gallium nitride.
- Author
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Paisley, Elizabeth A., Gaddy, Benjamin E., LeBeau, James M., Shelton, Christopher T., Biegalski, Michael D., Christen, Hans M., Losego, Mark D., Mita, Seiji, Collazo, Ramón, Sitar, Zlatko, Irving, Douglas L., and Maria, Jon-Paul
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OXIDES ,OXYGEN compounds ,GALLIUM nitride ,GALLIUM compounds ,NITRIDES - Abstract
Smooth, commensurate alloys of 〈111〉-oriented Mg
0.52 Ca0.48 O (MCO) thin films are demonstrated on Ga-polar, c+ [0001]-oriented GaN by surfactant-assisted molecular beam epitaxy and pulsed laser deposition. These are unique examples of coherent cubic oxide|nitride interfaces with structural and morphological perfection. Metal-insulator-semiconductor capacitor structures were fabricated on n-type GaN. A comparison of leakage current density for conventional and surfactant-assisted growth reveals a nearly 100x reduction in leakage current density for the surfactant-assisted samples. HAADF-STEM images of the MCO|GaN interface show commensurate alignment of atomic planes with minimal defects due to lattice mismatch. STEM and DFT calculations show that GaN c/2 steps create incoherent boundaries in MCO over layers which manifest as two in-plane rotations and determine consequently the density of structural defects in otherwise coherent MCO. This new understanding of interfacial steps between HCP and FCC crystals identifies the steps needed to create globally defect-free heterostructures. [ABSTRACT FROM AUTHOR]- Published
- 2014
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7. Polarity characterization by anomalous x-ray dispersion of ZnO films and GaN lateral polar structures.
- Author
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Shelton, Christopher T., Sachet, Edward, Paisley, Elizabeth A., Hoffmann, Marc P., Rajan, Joseph, Collazo, Ramón, Sitar, Zlatko, and Maria, Jon-Paul
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POLARITY (Physics) ,DIFFRACTOMETERS ,GALLIUM nitride ,GALLIUM compounds ,ZINC oxide films - Abstract
We demonstrate the use of anomalous x-ray scattering of constituent cations at their absorption edge, in a conventional Bragg-Brentano diffractometer, to measure absolutely and quantitatively the polar orientation and polarity fraction of unipolar and mixed polar wurtzitic crystals. In one set of experiments, the gradual transition between c+ and c- polarity of epitaxial ZnO films on sapphire as a function of MgO buffer layer thickness is monitored quantitatively, while in a second experiment, we map the polarity of a lateral polar homojunction in GaN. The dispersion measurements are compared with piezoforce microscopy images, and we demonstrate how x-ray dispersion and scanning probe methods can provide complementary information that can discriminate between polarity fractions at a material surface and polarity fractions averaged over the film bulk. [ABSTRACT FROM AUTHOR]
- Published
- 2014
- Full Text
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8. Complexes and compensation in degenerately donor doped GaN.
- Author
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Baker, Jonathon N., Bowes, Preston C., Harris, Joshua S., Collazo, Ramón, Sitar, Zlatko, and Irving, Douglas L.
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GALLIUM nitride ,DENSITY functional theory - Abstract
Gallium nitride is an increasingly technologically relevant material system. While donor doping GaN to low and intermediate dopant concentrations using silicon and germanium has become routine, compensation mechanisms activate under very high donor doping, limiting the maximum electron concentration achievable with either dopant in the degenerate doping regime. This effect, and how it differs between the two dopants, is investigated by hybrid functional density functional theory calculations and grand canonical thermodynamics models and is found to be due to the onset of multi-member Ga vacancy-donor substitutional complexes under degenerate doping conditions. The differing energetics of Ge- and Si-related complexes leads to different responses, ultimately making Ge the more effective donor in degenerate conditions. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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9. Impact of impurity-based phonon resonant scattering on thermal conductivity of single crystalline GaN.
- Author
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Bagheri, Pegah, Reddy, Pramod, Kim, Ji Hyun, Rounds, Robert, Sochacki, Tomasz, Kirste, Ronny, Bockowski, Michał, Collazo, Ramón, and Sitar, Zlatko
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THERMAL conductivity ,DELOCALIZATION energy ,GALLIUM nitride ,MASS spectrometry ,RAMAN spectroscopy ,PHONON scattering ,PHONONS - Abstract
The impact of impurities on the thermal conductivity of halide vapor phase epitaxy gallium nitride (GaN) was studied. Phonon resonances with impurities, modeled as Lorentz oscillators, were used to explain the much lower thermal conductivity than predicted by the Debye–Callaway model. The resonance energies for the oscillators were determined by Raman spectroscopy for Mn and by mass difference approximation for C and Fe. Employing the obtained resonance energies and proportionality factors extracted as fitting parameters, the modified model showed a good agreement with the experimental data. While the doping decreased thermal conductivity for all temperatures, the room temperature values started decreasing significantly once the doping levels approached ∼10
19 cm−3 . Consequently, required doping levels to achieve certain GaN-based devices may reduce the thermal conductivity of GaN by as much as 1/3. [ABSTRACT FROM AUTHOR]- Published
- 2020
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10. Temperature dependent photoluminescence of lateral polarity junctions of metal organic chemical vapor deposition grown GaN.
- Author
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Kirste, Ronny, Collazo, Ramón, Callsen, Gordon, Wagner, Markus R., Kure, Thomas, Sebastian Reparaz, Juan, Mita, Seji, Xie, Jinqiao, Rice, Anthony, Tweedie, James, Sitar, Zlatko, and Hoffmann, Axel
- Subjects
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PHOTOLUMINESCENCE , *GALLIUM nitride , *POLARITY (Physics) , *METAL organic chemical vapor deposition , *SCANNING electron microscopy , *THERMAL neutrons - Abstract
We report on fundamental structural and optical properties of lateral polarity junctions in GaN. GaN with Ga- to N-polar junctions was grown on sapphire using an AlN buffer layer. Results from scanning electron microscopy and Raman spectroscopy measurements indicate a superior quality of the Ga-polar GaN. An extremely strong luminescence signal is observed at the inversion domain boundary (IDB). Temperature dependent micro photoluminescence measurements are used to reveal the recombination processes underlying this strong emission. At 5 K the emission mainly arises from a stripe along the inversion domain boundary with a thickness of 4-5 μm. An increase of the temperature initially leads to a narrowing to below 2 μm emission area width followed by a broadening at temperatures above 70 K. The relatively broad emission area at low temperatures is explained by a diagonal IDB. It is shown that all further changes in the emission area width are related to thermalization effects of carriers and defects attracted to the IDB. The results are successfully used to confirm a theoretical model for GaN based lateral polarity junctions. Due to the strong and pronounced emission of IDBs even at elevated temperatures, it is demonstrated that lateral polarity junctions exhibit a strong potential for future high efficiency devices. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
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11. Improvement in detection limit for time-of-flight SIMS analysis of dopants in GaN structures.
- Author
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Klump, Andrew, Zhou, Chuanzhen, Stevie, Frederick A., Collazo, Ramón, and Sitar, Zlatko
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GALLIUM nitride ,DOPING agents (Chemistry) ,SECONDARY ion mass spectrometry ,TIME-of-flight spectrometry ,BISMUTH - Abstract
Secondary ion mass spectrometry (SIMS) has been used extensively to monitor dopant levels in semiconductor materials. The preponderance of these measurements has been made with magnetic sector or quadrupole analyzers. Use of time-of-flight (ToF) analyzers has been limited because of an inability to match the detection limit of the other analyzers. Optimization of the ToF-SIMS analysis beam pulse width and analysis frames per cycle is shown to provide as much as an order of magnitude improvement in detection limit. The magnesium dopant in GaN structures was used for the study and analysis was made with Cs
+ sputtering source and Bi3 + . The count rate for CsMg+ increased by a factor of 11.3 with both improvements applied. This was evidenced by a detection limit improvement for magnesium from 7.5 × 1017 atoms/cm3 to low 1017 atoms/cm3 . Increasing the number of analysis frames from one to ten causes cycle time to increase by a factor of five. Hence, there is a tradeoff between improved detection limit and analysis time. [ABSTRACT FROM AUTHOR]- Published
- 2018
- Full Text
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12. Step-free GaN surfaces grown by confined-area metal-organic vapor phase epitaxy.
- Author
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Shelton, Christopher T., Bryan, Isaac, Paisley, Elizabeth A., Sachet, Edward, Ihlefeld, Jon F., Lavrik, Nick, Collazo, Ramón, Sitar, Zlatko, and Maria, Jon-Paul
- Subjects
GALLIUM nitride ,METAL organic chemical vapor deposition ,SEMICONDUCTORS - Abstract
A two-step homoepitaxial growth process producing step-free surfaces on low dislocation density, Ga-polar ammonothermal GaN single crystals is described. Growth is conducted under very low supersaturation conditions where adatom incorporation occurs predominantly at step edges, and lateral growth is strongly preferred. The achievable step-free area is limited by the substrate dislocation density. For ammonothermal crystals with an average dislocation density of ∼1 × 10
4 cm−2 , step-free mesas up to 200 × 200μ m2 in size are achieved. These remarkable surfaces create a unique opportunity to study the effect of steps on the properties and performance of semiconductor heterostructures. [ABSTRACT FROM AUTHOR]- Published
- 2017
- Full Text
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13. Fermi Level Control of Point Defects During Growth of Mg-Doped GaN.
- Author
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Bryan, Zachary, Hoffmann, Marc, Tweedie, James, Kirste, Ronny, Callsen, Gordon, Bryan, Isaac, Rice, Anthony, Bobea, Milena, Mita, Seiji, Xie, Jinqiao, Sitar, Zlatko, and Collazo, Ramón
- Subjects
FERMI surfaces ,GALLIUM nitride ,GALLIUM compounds ,METAL organic chemical vapor deposition ,CHEMICAL vapor deposition - Abstract
In this study, Fermi level control of point defects during metalorganic chemical vapor deposition (MOCVD) of Mg-doped GaN has been demonstrated by above-bandgap illumination. Resistivity and photoluminescence (PL) measurements are used to investigate the Mg dopant activation of samples with Mg concentration of 2 × 10 cm grown with and without exposure to ultraviolet (UV) illumination. Samples grown under UV illumination have five orders of magnitude lower resistivity values compared with typical unannealed GaN:Mg samples. The PL spectra of samples grown with UV exposure are similar to the spectra of those grown without UV exposure that were subsequently annealed, indicating a different incorporation of compensating defects during growth. Based on PL and resistivity measurements we show that Fermi level control of point defects during growth of III-nitrides is feasible. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
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14. Lateral epitaxial overgrowth of nitrogen polar GaN on smooth nitrogen polar GaN templates by metalorganic chemical vapor deposition.
- Author
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Hussey, Lindsay, Mita, Seiji, Xie, Jinqiao, Guo, Wei, Akouala, Christer-Rajiv, Rajan, Joseph, Bryan, Isaac, Collazo, Ramón, and Sitar, Zlatko
- Subjects
EPITAXY ,CHEMICAL vapor deposition ,GALLIUM nitride ,TRANSMISSION electron microscopes ,DISLOCATIONS in crystals - Abstract
Nitrogen (N)-polar GaN lateral epitaxial overgrowth (LEO), with a reduced dislocation density, is grown by metalorganic chemical vapor deposition on patterned, smooth N-polar templates. For LEO growth conditions identical to that of smooth N-polar GaN on sapphire, mask orientation dependent morphologies and faceting of the overgrowth regions are observed. N-polar LEO oriented along the <1-100> direction exhibit flat sidewall morphologies while LEO oriented along the <11-20> direction exhibit inclined sidewalls. A lateral to vertical growth ratio for the <1-100> and <11-20> oriented LEO was found to be ∼0.3 and 0.2, respectively. Transmission electron microscope observations reveal a reduction of dislocations of up to three orders of magnitude in the overgrowth regions from ∼1010 cm-2 in the template. Additionally, dislocation bending likely initiated from interfacial tension between the N-polar GaN and SiO2 mask is observed, resulting in a reduction of dislocation density to ∼109 cm-2 in the window region of re-growth. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
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