Dr. Ümit Özgür Profile

Dr. Ümit Özgür

Associate Professor at Virginia Commonwealth Univ

SPIE Involvement:

Conference Program Committee | Author

Publications (71)

PROCEEDINGS ARTICLE | May 14, 2018

Finite size narrow-band transmission filters for real-time short wave IR spectroscopy and imaging

Proc. SPIE. 10656, Image Sensing Technologies: Materials, Devices, Systems, and Applications V

KEYWORDS: Infrared imaging, Short wave infrared radiation, Optical filters, Fabry–Perot interferometers, Spectroscopy, Dielectrics, Imaging spectroscopy, Infrared spectroscopy, Image filtering, Dielectric filters

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PROCEEDINGS ARTICLE | February 23, 2018

Recent progress in nonpolar and semi-polar GaN light emitters on patterned Si substrates

Proc. SPIE. 10532, Gallium Nitride Materials and Devices XIII

KEYWORDS: Light emitting diodes, Polarization, Optical properties, Crystals, Interfaces, Indium, Silicon, Scanning electron microscopy, Gallium nitride, Indium gallium nitride

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PROCEEDINGS ARTICLE | February 24, 2017

Quaternary BeMgZnO by plasma-enhanced molecular beam epitaxy for BeMgZnO/ZnO heterostructure devices

Proc. SPIE. 10105, Oxide-based Materials and Devices VIII

KEYWORDS: Magnesium, Polarization, Crystals, Zinc, Transmission electron microscopy, Gallium nitride, Field effect transistors, Zinc oxide, Beryllium, Heterojunctions, Molecular beam epitaxy

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We investigated the crystal structure, growth kinetics and electrical properties of BeMgZnO/ZnO heterostructures grown by Molecular Beam Epitaxy (MBE). Transmission Electron Microscopy (TEM) studies revealed that incorporation of Mg into the BeZnO solid solution eliminates the high angle grain boundaries that are the major structural defects in ternary BeZnO. The significant improvement of x-ray diffraction intensity from quaternary BeMgZnO alloy compared to ternary BeZnO was attributed to the reduction of lattice strain, which is present in the latter due to the large difference of covalent radii between Be and Zn (1.22 Å for Zn, 0.96 Å for Be). Incorporation of Mg, which has a larger covalent radius of 1.41Å, reduced the strain in BeMgZnO thin films and also enhanced Be incorporation on lattice sites in the wurtzite lattice. The Zn/(Be + Mg) ratio necessary to obtain single-crystal O-polar BeMgZnO on (0001) GaN/sapphire templates was found to increase with increasing substrate temperature:3.9, 6.2, and 8.3 at substrate temperatures of 450°C, 475°C, and 500°C, respectively. Based on analysis of photoluminescence spectra from Be_0.03Mg_yZn_0.97-yO and evolution of reflection high-energy electron diffraction patterns observed in situ during the MBE growth, it has been deduced that more negative formation enthalpy of MgO compared to ZnO and the increased surface mobility of Mg adatoms at elevated substrate temperatures give rise to the nucleation of a MgO-rich wurtzite phase at relatively low Zn/(Be + Mg) ratios. We have demonstrated both theoretically and experimentally that the incorporation of Be into the barrier in Zn-polar BeMgZnO/ZnO and O-polar ZnO/BeMgZnO polarization doped heterostructures allows the alignment of piezoelectric polarization vector with that of spontaneous polarization due to the change of strain sign, thus increasing the amount of net polarization. This made it possible to achieve Zn-polar BeMgZnO/ZnO heterostructures grown on GaN/sapphire templates with two-dimensional electron gas densities substantially exceeding those in Zn-polar MgZnO/ZnO and O-polar ZnO/MgZnO heterostructures with similar Mg content.

PROCEEDINGS ARTICLE | February 27, 2016

Exciton localization and large Stokes shift in quaternary BeMgZnO grown by molecular beam epitaxy

Proc. SPIE. 9749, Oxide-based Materials and Devices VII

KEYWORDS: Thin films, Magnesium, Ultraviolet radiation, Luminescence, Zinc, Gallium nitride, Optoelectronics, Optoelectronic devices, Sapphire, Solids, Excitons, Carrier dynamics, Zinc oxide, Beryllium, Molecular beam epitaxy, Absorption

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Owing to wide range bandgap tunability to more than 5 eV, the quaternary (Be,Mg)ZnO solid solutions are attractive for a variety of UV optoelectronic applications, inclusive of solar blind photodetectors, and intersubband transition devices. The mutual compensation effects of Be and Mg on the formation energy and strain allows a wide range of compositions and bandgaps beyond those achievable by MgZnO and BeZnO ternaries. Localization effects are well pronounced in such wide-bandgap semiconductor alloys due to large differences in metal covalent radii and the lattice constants of the binaries, resulting in strain-driven compositional variations within the film and consequently large potential fluctuations, in addition to that possibly caused by defects. However, carrier localization may suppress recombination through nonradiative channels, and thus, facilitate high-efficiency optoelectronic devices. To investigate potential fluctuations and localization in Be_xMg_yZn(_1-x-y)O films grown by plasma-assisted molecular beam epitaxy, optical absorption and steady-state and time-resolved photoluminescence (PL) measurements were performed. O-polar Be_xMg_yZn(_1-x-y)O samples grown on GaN templates with compositions up to x = 0.04 and y = 0.18 were used for timeresolved studies, and O-polar Be_xMg_yZn(_1-x-y)O samples grown on sapphire with compositions up to x = 0.19 and y = 0.52 were used for absorption measurements. From spectrally resolved PL transients, BeMgZnO samples with higher Mg/Be content ratio were found to exhibit smaller localization depth, Δ₀=98 meV for Be_0.04Mg_0.17Zn_0.79O and Δ₀=173 meV for Be_0.10Mg_0.25Zn_0.65O, compared to samples with smaller Mg/Be ratio, Δ₀=268 meV for Be_0.11Mg_0.15Zn_0.74O. Similar correlation is observed in temporal redshift of the PL peak position of 8 meV, 42 meV and 55 meV for Be_0.04Mg_0.17Zn_0.79O, Be_0.10Mg_0.25Zn_0.65O and Be_0.11Mg_0.15Zn_0.74O, respectively, that originates from potential fluctuations and removal of band filling effect in the localized states. PL transients indicate that emission at low temperature is dominated by recombination of localized excitons, which exhibit decay times as long as τ = 0.36 ns at the peak position. The Sshaped behavior of PL peak with change in temperature was observed for the quaternary alloy Be_0.04Mg_0.17Zn_0.79O. The degree of localization σ was determined to be 22 meV. Relatively high potential fluctuations and localization energy lead to a strong Stokes shift, which increased with bandgap reaching ~0.5 eV for O-polar BeMgZnO on sapphire with 4.6 eV absorption edge.

PROCEEDINGS ARTICLE | February 26, 2016

Wurtzite/zinc-blende electronic-band alignment in basal-plane stacking faults in semi-polar GaN

Proc. SPIE. 9748, Gallium Nitride Materials and Devices XI

KEYWORDS: Ultrafast phenomena, Quantum wells, Femtosecond phenomena, Polarization, Spectroscopy, Electrons, Interfaces, Gallium nitride, Sapphire, Optical spectroscopy, Excitons, Picosecond phenomena, Laser beam diagnostics, Scanning transmission electron microscopy

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PROCEEDINGS ARTICLE | February 26, 2016

Optical investigation of microscopic defect distribution in semi-polar (1-101 and 11-22) InGaN light-emitting diodes

Proc. SPIE. 9748, Gallium Nitride Materials and Devices XI

KEYWORDS: Nanostructures, Light emitting diodes, Luminescence, Indium, Quantum efficiency, Scanning electron microscopy, Gallium nitride, Near field scanning optical microscopy, Indium gallium nitride, Epitaxial lateral overgrowth, Near field optics

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PROCEEDINGS ARTICLE | February 26, 2016

Exciton localization in (11-22)-oriented semi-polar InGaN multiple quantum wells

Proc. SPIE. 9748, Gallium Nitride Materials and Devices XI

KEYWORDS: Quantum wells, Light emitting diodes, Luminescence, Indium, Quantum efficiency, Gallium nitride, Lab on a chip, Excitons, Picosecond phenomena, Indium gallium nitride, Gallium, Heterojunctions, Epitaxial lateral overgrowth, Temperature metrology

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PROCEEDINGS ARTICLE | February 26, 2016

Enhancement of indium incorporation to InGaN MQWs on AlN/GaN periodic multilayers

Proc. SPIE. 9748, Gallium Nitride Materials and Devices XI

KEYWORDS: Nanostructures, Quantum wells, Satellites, Luminescence, X-ray diffraction, Indium, Quantum efficiency, Gallium nitride, Aluminum nitride, Indium gallium nitride, Gallium, Heterojunctions, Indium nitride

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PROCEEDINGS ARTICLE | March 13, 2015

Strong exciton-photon coupling in hybrid InGaN-based microcavities on GaN substrates

Proc. SPIE. 9363, Gallium Nitride Materials and Devices X

KEYWORDS: Quantum wells, Luminescence, Photons, Reflectivity, Gallium nitride, Excitons, Polaritons, Indium gallium nitride, Optical microcavities, Temperature metrology

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PROCEEDINGS ARTICLE | March 13, 2015

Indium-incorporation efficiency in semipolar (11-22) oriented InGaN-based light emitting diodes

Proc. SPIE. 9363, Gallium Nitride Materials and Devices X

KEYWORDS: Light emitting diodes, Indium, Quantum efficiency, Chemical vapor deposition, Gallium nitride, Indium gallium nitride, Metalorganic chemical vapor deposition, Heterojunctions, Indium nitride, Epitaxial lateral overgrowth

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PROCEEDINGS ARTICLE | March 13, 2015

Enhancement of optical and structural quality of semipolar (11-22) GaN by introducing nanoporous SiNx interlayers

Proc. SPIE. 9363, Gallium Nitride Materials and Devices X

KEYWORDS: X-ray optics, Luminescence, X-rays, X-ray diffraction, Chemical vapor deposition, Scanning electron microscopy, Gallium nitride, Sapphire, Excitons, Epitaxial lateral overgrowth

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PROCEEDINGS ARTICLE | March 13, 2015

Active region dimensionality and quantum efficiencies of InGaN LEDs from temperature dependent photoluminescence transients

Proc. SPIE. 9363, Gallium Nitride Materials and Devices X

KEYWORDS: Quantum wells, Light emitting diodes, External quantum efficiency, Polarization, Luminescence, Quantum efficiency, Excitons, Indium gallium nitride, Internal quantum efficiency, Temperature metrology

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PROCEEDINGS ARTICLE | March 13, 2015

Enhancement of coherent acoustic phonons in InGaN multiple quantum wells

Proc. SPIE. 9363, Gallium Nitride Materials and Devices X

KEYWORDS: Mirrors, Quantum wells, Indium, Gallium nitride, Terahertz radiation, Phonons, Indium gallium nitride, Acoustics, Laser beam diagnostics, Superlattices

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PROCEEDINGS ARTICLE | March 13, 2015

Strong carrier localization in stacking faults in semipolar (11-22) GaN

Proc. SPIE. 9363, Gallium Nitride Materials and Devices X

KEYWORDS: Point spread functions, Quantum wells, Polarization, Luminescence, Crystals, Gallium nitride, Aluminum nitride, Excitons, Applied physics, Temperature metrology

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PROCEEDINGS ARTICLE | March 13, 2015

Temperature-dependent time-resolved photoluminescence measurements of (1-101)-oriented semi-polar AlGaN/GaN MQWs

Proc. SPIE. 9363, Gallium Nitride Materials and Devices X

KEYWORDS: Quantum wells, Oscillators, Polarization, Luminescence, Silicon, Gallium nitride, Lab on a chip, Excitons, Picosecond phenomena, Anisotropy

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PROCEEDINGS ARTICLE | March 8, 2014

Polarized time-resolved photoluminescence measurements of m-plane AlGaN/GaN MQWs

Proc. SPIE. 8986, Gallium Nitride Materials and Devices IX

KEYWORDS: Quantum wells, Polarization, Luminescence, 3D modeling, Gallium nitride, Excitons, Picosecond phenomena, Tantalum, Temperature metrology, Anisotropy

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PROCEEDINGS ARTICLE | March 8, 2014

Optical properties of m-plane GaN grown on patterned Si(112) substrates by MOCVD using a two-step approach

Proc. SPIE. 8986, Gallium Nitride Materials and Devices IX

KEYWORDS: Point spread functions, Silicon, Chemical vapor deposition, Scanning electron microscopy, Gallium nitride, Near field scanning optical microscopy, Image quality, Excitons, Metalorganic chemical vapor deposition, Near field optics

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PROCEEDINGS ARTICLE | March 8, 2014

Impact of extended defects on optical properties of (1-101)GaN grown on patterned Si

Proc. SPIE. 8986, Gallium Nitride Materials and Devices IX

KEYWORDS: Point spread functions, Light emitting diodes, Polarization, Luminescence, Silicon, Gallium nitride, Near field scanning optical microscopy, Excitons, Indium gallium nitride, Near field optics

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PROCEEDINGS ARTICLE | March 8, 2014

Determination of carrier diffusion length in p- and n-type GaN

Proc. SPIE. 8986, Gallium Nitride Materials and Devices IX

KEYWORDS: Magnesium, Luminescence, Diffusion, Doping, Gallium nitride, Sapphire, Integration, Indium gallium nitride, Line scan image sensors, Absorption

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PROCEEDINGS ARTICLE | February 27, 2014

Estimation of carrier leakage in InGaN light emitting diodes from photocurrent measurements

Proc. SPIE. 9003, Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting XVIII

KEYWORDS: Continuous wave operation, Light emitting diodes, Electrodes, Plasma etching, Indium gallium nitride, Metalorganic chemical vapor deposition, Helium cadmium lasers, Electron transport, Heterojunctions, Band structure simulations

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PROCEEDINGS ARTICLE | March 27, 2013

Microwave performance of AlGaN/AlN/GaN-based single and coupled channels HFETs

Proc. SPIE. 8625, Gallium Nitride Materials and Devices VIII

KEYWORDS: Plasmons, Lithium, Reliability, Gallium nitride, Aluminum, Aluminum nitride, Phonons, Transistors, Microwave radiation, Heterojunctions

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PROCEEDINGS ARTICLE | March 4, 2013

InGaN based multi-double heterostructure light-emitting diodes with electron injector layers

Proc. SPIE. 8625, Gallium Nitride Materials and Devices VIII

KEYWORDS: Quantum wells, Light emitting diodes, External quantum efficiency, Luminescence, Quantum efficiency, Electroluminescence, Indium gallium nitride, Heterojunctions, Internal quantum efficiency, Temperature metrology

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PROCEEDINGS ARTICLE | March 4, 2013

Depth distribution of carrier lifetimes in semipolar (1101) GaN grown by MOCVD on patterned Si substrates

Proc. SPIE. 8625, Gallium Nitride Materials and Devices VIII

KEYWORDS: Light emitting diodes, Optical properties, Luminescence, Silicon, Gallium nitride, Near field scanning optical microscopy, Carrier dynamics, Metalorganic chemical vapor deposition, Temperature metrology, Near field optics

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PROCEEDINGS ARTICLE | March 4, 2013

GaN-based vertical cavity lasers with semiconductor/dielectric and all dielectric reflectors

Proc. SPIE. 8625, Gallium Nitride Materials and Devices VIII

KEYWORDS: Reflectors, Quantum wells, Modulation, Etching, Dielectrics, Reflectivity, Semiconductor lasers, Gallium nitride, Indium gallium nitride, Epitaxial lateral overgrowth

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PROCEEDINGS ARTICLE | March 4, 2013

Investigation of microwave and noise properties of InAlN/GaN HFETs after electrical stress: role of surface effects

Proc. SPIE. 8625, Gallium Nitride Materials and Devices VIII

KEYWORDS: Interfaces, Reliability, Fourier transforms, Gallium nitride, Capacitance, Phonons, Field effect transistors, Phase measurement, Microwave radiation, Heterojunctions

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PROCEEDINGS ARTICLE | March 4, 2013

Recombination dynamics in non-polar m-plane GaN investigated by time- and polarization-resolved photoluminescence

Proc. SPIE. 8625, Gallium Nitride Materials and Devices VIII

KEYWORDS: Continuous wave operation, Polarization, Spectroscopy, Luminescence, Polarizers, Gallium nitride, Excitons, Time resolved spectroscopy, Temperature metrology, Dual energy x-ray absorptiometry

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PROCEEDINGS ARTICLE | February 28, 2012

Recombination and diffusion processes in polar and nonpolar bulk GaN investigated by time-resolved photoluminescence and nonlinear optical techniques