Ümit Özgür, Ph.D.
Engineering Foundation Professor, Department of Electrical and Computer Engineering
- Engineering West Hall, Room 340, Richmond VA UNITED STATES
Professor Özgür specializes in optical spectroscopy and photonic devices
Media
Links
Social
Biography
Industry Expertise
Areas of Expertise
Education
Duke University
Ph.D.
Physics
2003
Duke University
M.A.
Physics
1999
Bogazici University
B.S.
Physics
1996
Bogazici University
B.S.
Electrical Engineering
1996
Research Focus
Optical Spectroscopy
Current research focuses on synthesis of novel electronic materials, high performance UV/visible light emitters, dielectric and plasmonic optical narrow bandpass filters, and nanostructured biosensors. We use time-resolved and time-integrated optical characterization spectroscopy methods to evaluate the quality of materials and performance of devices.
Polarity control and residual strain in ZnO epilayers grown by molecular beam epitaxy
Cover picture - Phys. Stat. Sol. RRL
2016
We reported on the polarity control of ZnO grown by plasma assisted molecular beam epitaxy on (0001) GaN/sapphire template simply via the oxygen-to-Zn (VI/II) ratio during nucleation growth at 300 °C. The authors have demonstrated that ZnO polarity can be inverted, thus O-polar ZnO can be grown even if the (0001)GaN surface is kept under continuous Zn beam exposure prior to the nucleation growth. Following the Zn pre-exposure, ZnO nucleated with VI/II ratio below 1.5 showed Zn-polarity and those nucleated with VI/II ratios above 1.5 exhibited O-polarity. Scanning tunneling electron microscopy (STEM) imaging reveals that polarity inversion takes place without formation of any vertical inversion domain and within one monolayer of presumably non-stoichiometric GaOx at the interface. A direct correlation between polarity and residual strain sign was found. Zn-polar ZnO were under tensile biaxial strain whereas the O-polar material showed compressive strain. Moreover, the amount of strain varied linearly with the VI/II ratio used during low-temperature nucleation growth.
Energy Gap Tuning and Carrier Dynamics in Colloidal Ge1−xSnx Quantum Dots
2016
We studied the tunability of energy gap and carrier dynamics in colloidally synthesized 2.0±0.8 nm Ge1−xSnx QDs (x = 0.055 − 0.236) and developed a model for their radiative recombination pathways. Energy gaps at 15 K, as deduced from steady-state PL measurements, were confirmed to reach the visible spectral range, varying from 1.88 eV to 1.61 eV by changing Sn content from x = 0.055 to x = 0.236, respectively. Taking the size and compositional variation of these QDs into account, experimental energy gap values were fairly consistent with theoretically calculated ones. PL decay times were found to be 3 − 27 μs at 15 K due to the slow recombination of spin-forbidden dark excitons and recombination of carriers trapped at surface states. They dramatically decreased to 9 − 28 ns at room temperature owing to the thermal activation of spin-allowed bright excitons and carrier de-trapping from surface states.
Research Grants
SusChEM: Synthesis and Structure-Property Elucidation of Direct-Bandgap Group IV Alloy Nanocrystals for Optoelectronic Applications
National Science Foundation
The production of high-efficiency optoelectronic materials based solely on low-cost, non-toxic, and abundant Group IV elements such as silicon is challenging as Group IV elements are less efficient in electron-photon conversion process compared to the widely used optoelectronic materials. This project utilizes the unique nanoscale size confinement effects and alloying with tin to produce silicon-tin, germanium-tin, and silicon-germanium-tin nanocrystals that exhibit superior light absorption and emission properties. The collaborative team supports the synthetic efforts along with advanced optical characterization and theoretical calculations to garner a deep understanding of the emerging materials properties and enhance the optoelectronic performance. The interdisciplinary research provides valuable training to the graduate and undergraduate students in the areas from smart material design to device testing. Other education and outreach activities include K-12 nanoscience outreach efforts and involving the under-represented high school students and women in summer research activities.
Courses
Electromagnetic Fields
Fundamentals of engineering electromagnetics, including electrostatics, magnetostatics, electrodynamics, analysis and understanding of the phenomena associated with electric and magnetic fields, wave dynamical solutions of Maxwell's equations, reflection and transmission of electromagnetic waves in dielectric materials, waveguides and transmission line structures, and radiation from antennas.
Microwave and Photonics Engineering
Wireless and optical communications applications of electromagnetic fields. Theory of microwave transmission line and waveguiding structures including impedance transformation and matching. Essential concepts from geometrical and physical optics and the interaction of photons with materials will be studied. Operating principles and design considerations of fiber optics, photodetectors and receivers are considered.
Semiconductor Optoelectronics
Discussions of optical processes in semiconductors and semiconductor heterostructures in terms of radiative and nonradiative processes, as well as absorption. Also covers in depth the theory and practice of light-emitting diodes, including those intended for solid-state lighting, lasers and detectors.
Nonlinear Optical Materials and Devices
Principles of nonlinear optics and operation of photonic devices and systems that utilize various second and third order nonlinear optical effects. The topics include electromagnetic wave propagation in anisotropic media, nonlinear optical susceptibility tensor, linear and quadratic electro-optic effects, second harmonic, sum-, and difference-frequency generation, phase-matching, parametric amplification, optical switching, multi-photon absorption, self-focusing and self-phase modulation.
Selected Articles
Polarity Control and Residual Strain in ZnO Epilayers Grown by Molecular Beam Epitaxy on (0001)-GaN/Sapphire
Physica Status Solidi - Rapid Research Letters - Wiley2016
Ultra-Small Ge1-xSnx quantum dots with visible photoluminescence
Chemical Communications - The Royal Society of Chemistry2016
Optical properties of organic-inorganic hybrid perovskite CH3NH3PbI3: theory and experiment
Physical Review B - Americal Physical Society2016
Energy Gap Tuning and Carrier Dynamics in Colloidal Ge1-xSnx Quantum Dots
The Journal of Physical Chemistry Letters - American Chemical Society2016
Lattice Parameters and Electronic Structure of BeMgZnO Quaternary Solid Solutions: Experiment and Theory
Journal of Applied Physics - American Institute of Physics2016
Metal-Semiconductor Hybrid Aerogels: Evolution of Optoelectronic Properties in a Low-Dimensional CdSe/Ag Nanoparticle Assembly
ACS Nano - American Chemical Society2015
Enhancement of Be and Mg incorporation in wurtzite quaternary BeMgZnO alloys with up to 5.1 eV optical bandgap
Journal of Crystal Growth - Elsevier2014
Ballistic transport in InGaN based LEDs: impact on efficiency
Semiconductor Science and Technology - Institute of Physics2011, Topical issue in honor of the Nobel Laureate Zh. I. Alferov
GaN-based Light-Emitting Diodes: Efficiency at High Injection Levels
Proceedings of the IEEE2010
ZnO Devices and Applications: A review of current status and future prospects
Proceedings of the IEEE2010
A comprehensive review of ZnO materials and devices
Journal of Applied Physics - American Institute of Physics2005