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PhD RESEARCH AREAS
His research focuses on III-Nitride photonic devices which are made of direct bandgap AlGaInN compounds covering a wide spectra from deep ultraviolet (UV) to near infrared. Robustness of this material system makes them promising candidates for applications in harsh environments such as everyday life, battlegrounds, outdoors and space.
Ultraviolet region is very important as many biological agents (such as anthrax and plague) are luminescent in UV. Scattering of short-wavelengths in atmosphere enables non-line-of-sight secure communications in rugged terrains whereas strong reflection/absorption of UV at ionosphere promises secure space-to-space communications. UV detectors also find applications in astronomy for cosmic events analysis and in space exploration for extraterrestrial object investigations. Where photomultiplier tubes are found to be bulky and fragile, and Si(C)-based photodiodes require external filter elements, his world’s highest performing UV avalanche photodiodes (APDs) (gains of 51000, and external quantum efficiency of 57%) can be employed. Via Geiger-mode operation, he has realized (world’s first) UV single photon detectors (SPDs) with single photon detection efficiencies as high as 32%, that could detect (identify) even a single photon (chemical). The use of III-Nitride APDs presents key advantages such as lower operation voltages, much reduced sizes, and no need for cooling, which enable the fabrication of more compact, lower power, and all-solid-state APD/CMOS integrated arrays, suitable for integration into space shuttles/stations, airplanes, and military vehicles for secure communication and aerial countermeasures.
Solid state lighting (SSL) holds the promise of a more energy-efficient, longer-lasting, more compact, and lower maintenance substitute for today's incandescent and fluorescent light sources. The total annual energy consumption in the United States for lighting is approximately 800 Terawatt-hours and costs $80 billion to the public. The energy consumed for lighting throughout the world entails to greenhouse gas emission equivalent to 70% of the emissions from all the cars in the world. A novel solution to lighting with higher efficiency will drastically reduce the energy consumption and help greenhouse gas emissions to be lowered. Novel green light emitting diodes are the key components of an affordable, durable and environmentally benign lighting solution that can perform at superior energy conversion efficiency.
Terahertz (THz) emitters enable identification of pharmaceutical ingredients (for example, in a drug). Penetration through nonconductors (fabrics, wood, plastic) enables a more efficient way of performing security checks (for example at airports) with THz emission, as illegal drugs and explosives could be detected. Being a non-ionizing radiation, THz radiation is environment-friendly enabling a safer analysis environment than conventional X-ray based techniques. Due to deep penetration depth through body and tissue selectivity, THz waves are employed in medicine for the cancer cell detection as well as for bone analysis (such as tooth cavity detection). Thanks to the large longitudinal optical phonon energy, III-Nitrides is a promising candidate for room temperature operation of terahertz emitters. His on-going research on III-Nitride intersubband devices will lead to a continuous monitoring of an environment ensuring a better security than conventional security check-points in airports without effecting privacy as well as better diagnostic tools for cancer detection and medical imaging.
RESEARCH INTERESTS
Dr. Bayram's Ph.D. research area is wide bandgap semiconductor devices including III-N materials (AlGaInN) and II-VI materials (ZnO). His research interests include semiconductor device design/simulation, material growth/characterization, device processing/packaging/measurement.
He has performed more than 3000 MOCVD growths up-to-date. He has improved AlxGayIn(1-x-y)N layers (where [0,0]< [x, y] < [1,1]), and integrated them into self-designed nitride optoelectronic devices. By using state of the art material characterization tools such as atomic force microscopy, scanning electron microscopy, photoluminsecence measurements, X-ray diffraction equipments, and Hall measurements, he has correlated the material growth, characterization and (structural (surface, crystallographic), optical, electrical) material quality that leaded to world's first and highest performance nitride optoelectronic devices.
By using conventional and state-of-the-art semiconductor fabrication techniques and equipments (such as rapid thermal annealing, electron cyclotron resonance reactive ion etching, electron beam metal evaporator, plasma-enhanced chemical vapor deposition, photo- and e-beam-lithography systems), he has fabricated more than 500 wide bandgap semiconductor devices ranging from UV APDs to blue and green LEDs, near- to mid-IR intersubband absorption devices to resonant tunneling diodes. Combining the device performance with the material growth, a unique blend of semiconductor knowledge is gathered in-house, and being implemented.
His PhD research interests include avalanche and single photon detection in UV spectral region and high performance novel blue-green-white light emitting diodes. He is currently developing high quality Al(Ga)N/GaN-based intersubband devices operating from near- and mid-infrared towardsTHz wavelength regime.
RECENT RESEARCH INTERESTS
With the recent revision of the bandgap of InN at ~0.65 eV, the bandgap of the InGaN material system now ranges from the infrared (~0.7 eV) to the ultraviolet (~3.4 eV) region. This direct and wide bandgap range makes the InGaN material system useful for photovoltaic applications due to the possibility of fabricating not only high-efficiency multijunction solar cells but also third-generation devices such as intermediate-band solar cells based solely on the nitride material system.
While the maximum reported efficiency for a solar cell is 41% under concentrated suns, achieved by a triple-junction GaInP–GaInAs–Ge tandem, such devices are approaching maturity in terms of efficiency limits. Detailed balance modeling indicates that in order to achieve practical terrestrial photovoltaic efficiencies of greater than 50%, materials with bandgaps greater than 2.4 eV are required. In addition to the wide bandgap range, the nitrides also demonstrate favorable photovoltaic properties such as low effective carrier mass, high mobility, high peak and saturation velocities, high absorption coefficient, and radiation tolerance. The III-V nitride technology has demonstrated the ability to grow high-quality crystalline structures and fabricate optoelectronic devices, which confirms its potential in high-efficiency photovoltaics.
Micro/Nano-electromechanical (MEMs/NEMs) systems based on wide bandgap materials find diverse applications including chemical, biological and gas sensors, microfluidic sensors and other fluid devices, microactuators, RF-MEMS (filters, resonators, switches), micro-opto-electromechanical systems. The high Young’s modulus of wide bandgap semiconductors enables AlGaN to achieve higher frequencies and quality factors in resonant devices at the same geometrical dimensions in comparison with silicon. Besides, AlGaN/GaN-heterostructures form a highly conductive two-dimensional electron gas at the interface, which is sensitive to mechanical load, as well as to chemical modification of the surface, and can be used for novel sensing principles. With the development of the III-nitride material technologies, these miracle wide bandgap materials systems will play a major role in future MEMs/NEMs.
In conclusion, understanding this material system will also lead to many other novel engineered devices such as water splitting for hydrogen generation, terahertz emitters (i.e. quantum cascade lasers and plasmonic resonance transistors), and piezoelectric energy harvesters. Exploring innovative design approaches and novel fabrication methods in III-Nitrides will advance the current devices used in photonics and nanotechnology. III-nitrides will gain more interest in nanotechnology and energy efficient ultraviolet-to-terahertz photonic devices due to their unique material characteristics.
