Browsing by Subject "Photon detectors."

Now showing 1 - 7 of 7

Open Access
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.
Open Access
GaN/AlGaN-based UV photodetectors with performances exceeding the PMTS
(2008) Tut, Turgut
The recent developments in high Al-content AlxGa1−xN material growth technology made it possible to fabricate high performance solar-blind photodetectors operating in the ultraviolet (UV) spectral region with improved receiver sensitivity, low noise, low dark current density, and high speed. AlGaN-based Schottky, p-i-n, and metal-semiconductor-metal photodetectors (MSM) with very high performances have already been demonstrated. The UVfiltering nature of the atmospheric ozone molecules blocks the solar radiation to reach the earth’s surface for wavelengths shorter than 280 nm. In this case, UV photodetectors with cutoff wavelengths around 280 nm, which are also called solarblind detectors, can detect very weak UV signals under intense background radiation. These devices have important applications including missile plume detection, chemical/biological agent sensing, flame alarms, covert space-tospace and submarine communications, ozone-layer monitoring, and gas detection. Due to their high responsivity (600 A/W), high speed, high cathode gain (on the order of a million), and low dark current properties, photomultiplier tubes (PMTs) are frequently used in such applications. However, PMTs are very expensive and bulky. Besides, they require a cooling system, and they have high operation voltages in excess of 1000 V. To achieve solar-blind detection, PMTs should also be integrated with complex and expensive filters. In order to avoid these disadvantages, high performance solid-state UV photodetectors with high internal gain are needed. Wide band-gap semiconductor photodetectors, such as AlxGa1−xN with x=0.4, are ideal candidates for this purpose. These devices are intrinsically solar blind, in which no additional filters are needed, they have low noise, and fast response times. The lack of high internal gain has been the major limitation for the usage of AlGaN photodetectors for applications that require high sensitivity detectors. There have been several theoretical research works that examined the avalanche effect in GaN and AlGaN-based structures. However, reproducible high gain in AlGaN-based APDs is still a major limitation. We have designed, fabricated, GaN/AlGaN based photodetectors, and according to characterization measurements, the Schottky, p-i-n, and avalanche detectors have high performance in terms of quantum efficiency, dark current, detectivity, high speed response, and high reproducible avalanche gain.
Open Access
High speed and high efficiency infrared photodetectors
(2004) Kimukin, İbrahim
The increasing demand for telecommunication systems resulted in production of high performance components. Photodetectors are essential components of optoelectronic integrated circuits and fiber optic communication systems. We successfully used resonant cavity enhancement technique to improve InGaAs based p-i-n photodetectors. The detectors had 66% peak quantum efficiency at 1572 nm which showed 3 fold increases with respect to similar photodetector without resonant cavity. The detectors had 28 GHz 3-dB bandwidth at the same time. The bandwidth efficiency product for these detectors was 18.5 GHz, which is one of the best results for InGaAs based vertical photodetector. The interest in high speed photodetectors is not limited to fiber optic networks. In the recent years, data communication through the air has become popular due to ease of installation and flexibility of these systems. Although the current systems still operate at 840 nm or 1550 nm wavelengths, the advantage of mid-infrared wavelengths will result in the production of high speed lasers and photodetectors. InSb based p-i-n type photodetectors were fabricated and tested for the operation in the mid-infrared (3 to 5 µm) wavelength range. The epitaxial layers were grown on semi-insulating GaAs substrate by molecular beam epitaxy method. The detectors had low dark noise and high differential resistance around zero bias. Also the responsivity measurements showed 49% quantum efficiency. The detectivity was measured as 7.98×109 cm Hz1/2/W for 60 µm diameter detectors. Finally the high speed measurements showed 8.5 and 6.0 GHz bandwidth for 30 µm and 60 µm diameter detectors, respectively.
Open Access
Passivation of InSb infrared photodetectors
(2010) Yumrukçu, Samed
Infrared detectors have wide range applications in both military and civilian life. One of the most commonly used infrared detectors is InSb detectors. InSb detector technology has been developing since 1950s. Fabricating p-n diodes to detect infrared radiation is a common way of constructing InSb detectors. Due to high free carrier concentration at room temperature, InSb detectors need to be cooled down to operate properly and usually liquid nitrogen is preferred for cooling. However, even at 77 K, tunneling and generation-recombination and surface leakage are not negligible and these effects result in dark current. Improving the photo current-to-dark current ratio is the main goal in design and fabrication of InSb photo detectors. One way of decreasing the dark current is passivating the exposed edges of the detector to reduce surface leakage current. Passivating the edges can result in decreasing in the surface leakage by eliminating the surface states (dangling bonds). Dielectric thin films like SiO2 and SiNx are commonly used for passivation. In this work, different sized detectors are fabricated and characterized by measuring I-V curves and spectral response. Different approaches are tested for passivation and a detailed comparison between detectors with different treatments is presented.
Open Access
Plasmonically enhanced silicon infrared Schottky detector
(2011) Polat, Kazım Gürkan
Open Access
Silicon Germanium multi quantum wells for high efficiency optoelectronic devices
(2011) Yeşilyurt, Alper
Open Access
ZnO based photo-thin-film-transistors with actively tunable photoresponse in the visible spectrum
(2013) Aygün, Levent Erdal
Zinc oxide, ZnO, is an important material for wide range of optoelectronic device applications. Especially, ZnO is famous with its large exciton binding energy of 60 meV which makes it a good candidate for ultraviolet light emitting diodes and lasers. Moreover, its high carrier mobility and wide band gap of 3.37 eV (368 nm) makes it a promising material for transparent electronics and UV photodetectors. However, ZnO has crystallographic defect states (e.g. oxygen vacancies, zinc interstitials) which degrade the performance of ZnO based LEDs, lasers and UV photodiodes. In this thesis, ZnO based photo-thin-film-transistors (photo-TFTs) with visible light response by using their defect states to absorb subbandgap photons are investigated. The design, fabrication and characterization of ZnO based photoTFTs are presented. A photo-TFT is a three-terminal optoelectronic device that is a photoconductor structure with an additional gate terminal which actively tunes electrical and optical properties of photoconductive material. In a clean room environment, ZnO based photo-TFTs with various device sizes are fabricated at different ZnO channel layer deposition temperatures (ranging from 80 to 250 °C). Initially, TFT characteristics of fabricated devices are characterized to show that the gate terminal dynamically modulates ZnO’s channel conductivity. Moreover, the effects of the device size and the deposition temperature on device performance are investigated. Then, the optical characterization of ZnO film deposited at 250° C is conducted via absorption and photoluminescence measurements in order to investigate its visible light absorption characteristics and the energy levels of its defect states in the forbidden band gap of ZnO. After that, the responsivity measurements are reported from ZnO based photo-TFTs fabricated at 250 °C and the active tuning mechanism of visible light photoresponse is discussed. Finally, the effects of the deposition temperature and the device size on the visible light responsivity are presented.