Browsing by Subject "Photodiodes."

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    A1GaN UV photodetectors : from micro to nano
    (2011) Bütün, Serkan
    The absorption edge of AlGaN based alloys can be tuned from deep UV to near UV by changing the composition. This enables the use of the material in various technological applications such as military, environmental monitoring and biological imaging. In this thesis, we proposed and demonstrated various UV photodetectors for different purposes. The multi-band photodetectors have the unique ability to sense the UV spectrum in different portions at the same time. We demonstrated monolithically integrated dual and four-band photodetectors with multi layer structures grown on sapphire. This was achieved through epitaxial growth of multi AlGaN layers with decreasing Al content. We suggested two different device architectures. First one has separate filter and active layers, whereas the second one has all active layers which are used as filter layers as well. The full width at half maximum (FWHM) values for the dual band photodetector was 11 and 22 nm with more than three orders of magnitude inter-band rejection ratio. The self-filtering four band photodetector has FWHMs of 18, 17, 22 and 9 nm from longer to shorter bands. Whereas photodetector with separate filter layers has FWHMs of 8, 12, 11 and 8 nm, from longer to shorter bands. The overall inter-band rejection ration was increased from about one to two of magnitude after incorporating the passive filter layers. The plasmonic enhancement of photonic devices has attracted much attention for the past decade. However, there is not much research that has been conducted in UV region. In the second part of this thesis, we fabricated nanostructures on GaN based photodetectors and improved the responsivity of the device. We have fabricated Al nano-particles on sapphire with e-beam lithography. We characterized their response via spectral extinction measurements. We integrated these particles with GaN photodetectors and had enhancement of %50 at the plasmonic resonance of the nano-particles. Secondly, we have fabricated sub-wavelength photodetectors on GaN coupled with linear gratings. We had 8 fold enhancement in the responsivity at the plasmonic resonance frequency of the grating at normal incidence. Numerical simulations revealed that both surface plasmons and the unbound leaky surface waves played a role in the enhancement. We, finally, conducted basic research on the current transport mechanisms in Schottky barriers of AlGaN based materials. Experiments revealed that the tunneling current plays a major role in current transport. In addition incorporation, of a thin insulator between metalsemiconductor interface reduces the undesired surface states thereby improving the device performance.
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    The growth, fabrication and characterization of high performance AI(formula)Ga(formula)N metal-semiconductor-metal photodiodes
    (2006) Bütün, Serkan
    High performance UV photodetectors have attracted unwarranted attention for various applications, such as in military, telecommunication, and biological imaging, as an AlxGa1-xN material system is also rather suitable for such applications. Its direct band gap covers the spectrum from 200 nm to 360 nm by way of changing the Al concentration in the compound. In this present thesis, the design and growth of an Al0.75Ga0.25N template on sapphire substrate and a deep-UV photodiode with a cut off wavelength of 229 nm that was fabricated on the Al0.75Ga0.25N template is presented. A responsivity of 0.53 A/W was attained corresponding to a detectivity of 1.64 × 1012 cmHz 1/2/W at a 50 V bias and 222 nm UV light illumination. The UV/VIS rejection ratio of seven orders of magnitude was achieved from the fabricated devices. The second work that was conducted in this thesis was the growth of a semiinsulating (SI-) GaN template. We also fabricated visible-blind photodetectors on this semi-insulating (SI-) GaN template. Furthermore, we fabricated identical samples on a regular GaN template in order to investigate any possible i improvement. The improvement found was obvious in terms of dark current. A dark current density of 1.96 × 10-10 A/cm2 at a 50 V bias voltage for an SI-GaN photodetector was obtained, which is four orders of magnitude lower than devices on a regular GaN template. Devices on an SI-GaN had very high detectivity, and therefore, SI-GaN was used for low level power detection. The photogenerated current was well above the dark current that was under the illumination of just a few picowatts of UV light.
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    Ultraviolet-visible nanophotonic devices
    (2010) Bütün, Bayram
    Recently in semiconductor market, III-Nitride materials and devices are of much interest due to their mechanical strength, radiation resistance, working in the spectrum from visible down to the deep ultraviolet region and solar-blind device applications. These properties made them strongest candidates for space telecommunication, white light generation, high power lasers and laser pumping light emitting diodes. Since, like other semiconductors, there have been material quality related issues, ongoing research efforts are concentrated on growing high quality crystals and making low p-type ohmic contact. Also, in light emitting device applications, similar to the visible and infrared spectrum components, there are challenging issues like high extraction efficiency and controlled radiation. In this thesis, we worked on growth and characterizations of high quality (In,Al)GaN based semiconductors, fabricating high performance photodiodes and light emitting diodes. We studied different surface modifications and possibilities of obtaining light emitting diode pumped organic/inorganic hybrid laser sources