Browsing by Subject "AlGaN"

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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
AlGaN quadruple-band photodetectors
(IEEE, 2009) Gökkavas, Mutlu; Bütün, Serkan; Caban, P.; Strupinski, W.; Özbay, Ekmel
Quadruple back-illuminated ultraviolet metal-semiconductor-metal photodetectors with four different spectral responsivity bands were demonstrated. The average of the full-width at half-maximum (FWHM) of the quantum efficiency peaks was 9.98 nm.
Open Access
Contributions of impurity band and electron-electron interactions to magnetoconductance in AlGaN
(Taylor & Francis, 2010-06-30) Tasli, P.; Yildiz, A.; Kasap, M.; Özbay, Ekmel; Lisesivdin, S. B.; Ozcelik, S.
Low temperature electrical measurements of conductivity, the Hall effect and magnetoconductance were performed on a degenerate AlGaN sample. The sample exhibited negative magnetoconductance at low magnetic fields and low temperatures, with the magnitude being systematically dependent on temperature. The measured magnetoconductance was compared with models proposed previously by Sondheimer and Wilson [Proc. R. Soc. Lond. Ser. A 190 (1947) p. 435] and Lee and Ramakrishan [Rev. Mod. Phys. 57 (1985) p. 287]. Data were analyzed as the sum of the contribution of a two-band and electron-electron interactions to the magnetoconductance, applying these models to describe the observed behavior. Least-squares fits to the data are presented. In the sample, magnetoconductance can be explained reasonably well by assuming these contributions to the measured magnetoconductance. It was found that theoretical and experimental data were in excellent agreement.
Open Access
Design, fabrication and characterization of high-performance solarblind AlGaN photodetectors
(SPIE, 2005) Özbay, Ekmel
Design, fabrication, and characterization of high-performance AlxGal-xN-based photodetectors for solar-blind applications are reported. AlxGal-xN heterostructures were designed for Schottky, p-i-n, and metal-semiconductor-metal (MSM) photodiodes. The resulting solar-blind AlGaN detectors exhibited low dark current, high detectivity, and high bandwidth.
Open Access
Determination of the in-plane effective mass and quantum lifetime of 2D electrons in AlGaN/GaN based HEMTs
(2011) Celik O.; Tiras, E.; Ardali, S.; Lisesivdin, S.B.; Özbay, Ekmel
Magnetoresistance and Hall resistance measurements have been used to investigate the electronic transport properties of AlGaN/GaN based HEMTs. The Shubnikov-de Haas (SdH) oscillations from magnetoresistance, is obtained by fitting the nonoscillatory component to a polynomial of second degree, and then subtracting it from the raw experimental data. It is shown that only first subband is occupied with electrons. The two-dimensional (2D) carrier density and the Fermi energy with respect to subband energy (EF-E1) have been determined from the periods of the SdH oscillations. The in-plane effective mass (m*) and the quantum lifetime (τq) of electrons have been obtained from the temperature and magnetic field dependencies of the SdH amplitude, respectively. The in-plane effective mass of 2D electrons is in the range between 0.19 m0 and 0.22 m0. Our results for in-plane effective mass are in good agreement with those reported in the literature © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Effect of gate structures on the DC and RF performance of AlGaN/GaN HEMTs
(Institute of Physics Publishing, 2018) Toprak, Ahmet; Osmanoǧlu, Sinan; Öztürk, Mustafa; Yılmaz, Doğan; Cengiz, Ö.; Şen, Ö.; Bütün, Bayram; Özcan, Ş.; Özbay, Ekmel
This work analyzes the effect of various gate structures on the DC and radio frequency (RF) performance of AlGaN/GaN high-electron mobility transistors (HEMTs). AlGaN/GaN HEMT devices with a 3 μm drain-to-source spacing, 125 μm gate width and 0.3 μm gate length in various gate structures were fabricated to achieve the desired frequency response with a robust, high yield, and repeatable process. The maximum drain current (IDS,max), maximum DC transconductance (gm), pinch-off voltage (Vth), current-gain cutoff frequency (fT), maximum oscillation frequency (fmax), and RF characteristics of the devices in terms of the small-signal gain and RF output power (Pout) at 8 GHz were investigated. The results showed that the output power is increased by 1 dB when the gate structure is changed from field plate to gamma gate. The Vth, gm, fT and fmax values are maximized when the thickness of the passivation layer between the gate foot and the gate head is minimized. It is shown that the IDS,max is decreased and Pout is increased when the gate recess etching process is performed.
Open Access
Fabrication and characterization of graphene/AlGaN/GaN ultraviolet Schottky photodetector
(Institute of Physics Publishing, 2016) Kumar, M.; Jeong, H.; Polat, K.; Okyay, Ali Kemal; Lee, D.
We report on the fabrication and characterization of a Schottky ultraviolet graphene/AlGaN/GaN photodetector (PD). The fabricated device clearly exhibits rectification behaviour, indicating that the Schottky barrier is formed between the AlGaN and the mechanically transferred graphene. The Schottky parameters are evaluated using an equivalent circuit with two diodes connected back-to-back in series. The PD shows a low dark current of 4.77 × 10-12 A at a bias voltage of -2.5 V. The room temperature current-voltage (I-V) measurements of the graphene/AlGaN/GaN Schottky PD exhibit a large photo-to-dark contrast ratio of more than four orders of magnitude. Furthermore, the device shows peak responsivity at a wavelength of 350 nm, corresponding to GaN band edge and a small hump at 300 nm associated to the AlGaN band edge. In addition, we examine the behaviour of Schottky PDs with responsivities of 0.56 and 0.079 A W-1 at 300 and 350 nm, respectively, at room temperature. © 2016 IOP Publishing Ltd.
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
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.
Open Access
High bandwidth-efficiency solar-blind AlGaN Schottky photodiodes with low dark current
(Pergamon Press, 2005-01) Tut, T.; Bıyıklı, Necmi; Kimukin, I.; Kartaloglu, T.; Aytur, O.; Unlu, M. S.; Özbay, Ekmel
Al0.38Ga0.62N/GaN heterojunction solar-blind Schottky photodetectors with low dark current, high responsivity, and fast pulse response were demonstrated. A five-step microwave compatible fabrication process was utilized to fabricate the devices. The solarblind detectors displayed extremely low dark current values: 30 μm diameter devices exhibited leakage current below 3fA under reverse bias up to 12V. True solar-blind operation was ensured with a sharp cut-off around 266nm. Peak responsivity of 147mA/W was measured at 256nm under 20V reverse bias. A visible rejection more than 4 orders of magnitude was achieved. The thermally-limited detectivity of the devices was calculated as 1.8 × 1013cm Hz1/2W-1. Temporal pulse response measurements of the solar-blind detectors resulted in fast pulses with high 3-dB bandwidths. The best devices had 53 ps pulse-width and 4.1 GHz bandwidth. A bandwidth-efficiency product of 2.9GHz was achieved with the AlGaN Schottky photodiodes. © 2004 Elsevier Ltd. All rights reserved.
Open Access
High performance AlxGa1-xN-based avalanche photodiodes
(Elsevier BV, 2007-10) Tut, T.; Butun, B.; Gokkavas, M.; Özbay, Ekmel
We report high performance solar-blind photodetectors with reproducible avalanche gain as high as 820 under ultraviolet illumination. The solar-blind photodetectors have a sharp cut-off around 276 nm. We improved the device performance by designing different epitaxial wafer structure with thinner active multiplication layer. We compare the resulting fabricated devices from these wafers in terms of dark current, photoresponse, avalanche gain performances.
Open Access
High-performance solar-blind AlGaN photodetectors
(SPIE, 2005) Özbay, Ekmel; Tut, Turgut; Bıyıklı, N.
Design, fabrication, and characterization of high-performance Al xGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky, p-i-n, and metal-semiconductor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA leakage currents at 6 V and 12 V reverse bias were measured on p-i-n and Schottky photodiode samples respectively. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9×1014 cmHz1/2W-1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.15 A/W and 0.11 A/W peak responsivity values at 267 nm and 261 nm respectively. All samples displayed true solar-blind response with cut-off wavelengths smaller than 280 nm. A visible rejection of 4×104 was achieved with Schottky detector samples. High speed measurements at 267 nm resulted in fast pulse responses with >GHz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.
Open Access
High-performance solar-blind photodetectors based on AlxGa 1_xN heterostructures
(IEEE, 2004) Özbay, Ekmel; Bıyıklı, Necmi; Kimukin, I.; Kartaloglu, T.; Tut, T.; Aytür, O.
Design, fabrication, and characterization of high-performance AI xGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky. p-i-n, and metal-semicondnctor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA, leakage currents at 6-V reverse bias were measured on p-i-n samples. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9×1014 cmHz1/2W -1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.09 and 0.11 A/W peak responsivity values at 267 and 261 nm, respectively. A visible rejection of 2×104 was achieved with Schottky samples. High-speed measurements at 267 nm resulted in fast pulse responses with greater than gigahertz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.
Open Access
Improved Tmax estimation in GaN HEMTs using an equivalent hot point approximation
(IEEE, 2020) Odabaşı, Oğuz; Akar, Mehmet Ömer; Bütün, Bayram; Özbay, Ekmel
In this article, heat generation distribution and maximum device temperature of gallium-nitride (GaN) high-electron-mobility transistors (HEMTs) are investigated by using the 2-D electrothermal and finite-element method (FEM) simulations. Devices with different gate lengths and source-to-drain spacing are investigated. It is observed that the maximum device temperature (TMAX) depends on the drain-to-source spacing and is almost independent of the gate length and that the assumption of a uniform heat generation region, under the gate, is not accurate; this is contrary to conventional calculation methods. Moreover, based on the results, a new approximation is proposed to use in the FEM simulations that can estimate TMAX more accurately. This method does not require physics-based technology computer-aided design (TCAD) simulations and can work with a low mesh density. The performance is compared with prior methods.
Open Access
Improvement of breakdown characteristics in AlGaN/GaN/AlxGa 1-xN HEMT based on a grading Al xGa 1-xN buffer layer
(Wiley, 2010-08-03) Yu, H.; Lisesivdin, S. B.; Ozturk, M.; Bolukbas, B.; Kelekci, O.; Ozturk, M. K.; Ozcelik, S.; Caliskan, D.; Cakmak, H.; Demirel, P.; Özbay, Ekmel
To improve the breakdown characteristics of an AlGaN/GaN based high electron mobility transistor (HEMT) for high voltage applications, AlGaN/GaN/Al xGa 1-xN double heterostructure (DH-HEMTs) were designed and fabricated by replacing the semi-insulating GaN buffer with content graded Al xGa 1-xN (x=x 1 → x 2, x 1 > x 2), in turn linearly lowering the Al content x from x 1=90% to x 2=5% toward the front side GaN channel on a high temperature AlN buffer layer. The use of a highly resistive Al xGa 1-xN epilayer suppresses the parasitic conduction in the GaN buffer, and the band edge discontinuity limits the channel electrons spillover, thereby reducing leakage current and drain current collapse. In comparison with the conventional HEMT that use a semi-insulating GaN buffer, the fabricated DH-HEMT device with the same size presents a remarkable enhancement of the breakdown voltage.
Open Access
Integrated AlGaN quadruple-band ultraviolet photodetectors
(IOP Publishing, 2012-04-27) Gökkavas, M.; Butun, S.; Caban, P.; Strupinski, W.; Özbay, Ekmel
Monolithically integrated quadruple back-illuminated ultraviolet metalsemiconductormetal photodetectors with four different spectral responsivity bands were demonstrated on each of two different Al xGa 1-xN heterostructures. The average of the full-width at half-maximum (FWHM) of the quantum efficiency peaks was 18.15nm for sample A, which incorporated five 1000nm thick epitaxial layers. In comparison, the average FWHM for sample B was 9.98 nm, which incorporated nine 500nm thick epitaxial layers.
Open Access
Investigation of a hybrid approach for normally-off GaN HEMTs using fluorine treatment and recess etch techniques
(Institute of Electrical and Electronics Engineers Inc., 2019) Kurt, Gökhan; Gülseren, Melisa Ekin; Salkım, Gurur; Ural, Sertaç; Kayal, Ömer Ahmet; Öztürk, Mustafa; Bütün, Bayram; Kabak, M.; Özbay, Ekmel
A hybrid approach for obtaining normally off high electron mobility transistors (HEMTs) combining fluorine treatment, recess etch techniques, and AlGaN buffer was studied. The effects of process variations (recess etch depth and fluorine treatment duration) and epitaxial differences (AlGaN and carbon doped GaN buffers) on the DC characteristics of the normally off HEMTs were investigated. Two different epitaxial structures and three different process variations were compared. Epitaxial structures prepared with an AlGaN buffer showed a higher threshold voltage (V th = +3.59 V) than those prepared with a GaN buffer (V th = +1.85 V).
Open Access
Investigation of AlGaN buffer layers on sapphire grown by MOVPE
(SPIE, 2004) Van Gemmern, P.; Dikme, Y.; Bıyıklı, Necmi; Kalisch, H.; Özbay, Ekmel; Jansen, R. H.; Heuken, M.
In this work, AlGaN layers were grown on sapphire by metal-organic vapor phase epitaxy (MOVPE) on (0001)-oriented sapphire substrates, with the intention to investigate the effect of varying Al/MO and V/III ratios on the Al incorporation into the AlGaN layers. The parameters Al/MO and V/III describe the proportions of source material inside the reactor. With the help of optical transmission measurements, characteristic cut-off wavelengths of the Al xGa(1-x)N layers were determined. These wavelengths were used to calculate the Al content x of the layers, leading to values between 26.6% and 52.1%. Using the two process parameters Al/MO and V/III as input and the Al content of the AlGaN layers as a response variable, the experimental results were further investigated with the help of the software STATGRAPHICS. An estimated response surface for the variable x was generated. It was found that the Al incorporation is only tunable within a wide range for high V/III ratios of about 900. For constant Al/MO ratios and varying V/III ratios, two different growth characteristics were observed at high and low Al/MO values. This behavior is ascribed to the superposition of two oppositional effects.
Open Access
Investigation of angstrom-thick aluminium oxide passivation layers to improve the gate lag performance of GaN HEMTs
(IOP, 2019-07) Gülseren, Melisa Ekin; Kurt, Gökhan; Ulusoy Ghobadi, Türkan Gamze; Ghobadi, Amir; Salkım, Gurur; Öztürk, Mustafa; Bütün, Bayram; Özbay, Ekmel
In this paper, we report an angstrom-thick atomic layer deposited (ALD) aluminum oxide (Al2O3) dielectric passivation layer for an AlGaN/GaN high electron mobility transistor (HEMT). Our results show a 55% improvement in the gate lag performance of the design and a decrease by half in interface state density upon coating with two cycles of ALD Al2O3. DC characteristics such as current density, threshold voltage, and leakage currents were maintained. ALD Al2O3 passivation layers with thicknesses up to 10 nm were investigated. XPS analyses reveal that the first ALD cycles are sufficient to passivate GaN surface traps. This study demonstrates that efficient passivation can be achieved in atomic-scale with dimensions much thinner than commonly used bulk layers.
Open Access
Mobility limiting scattering mechanisms in nitride-based two-dimensional heterostructures with the InGaN channel
(IOP Publishing, 2010-03-16) Gökden, S.; Tülek, R.; Teke, A.; Leach, J. H.; Fan, Q.; Xie, J.; Özgür, Ü.; Morkoç, H.; Lisesivdin, S. B.; Özbay, Ekmel
The scattering mechanisms limiting the carrier mobility in AlInN/AlN/InGaN/GaN two-dimensional electron gas (2DEG) heterostructures were investigated and compared with devices without InGaN channel. Although it is expected that InGaN will lead to relatively higher electron mobilities than GaN, Hall mobilities were measured to be much lower for samples with InGaN channels as compared to GaN. To investigate these observations the major scattering processes including acoustic and optical phonons, ionized impurity, interface roughness, dislocation and alloy disorder were applied to the temperature-dependent mobility data. It was found that scattering due mainly to interface roughness limits the electron mobility at low and intermediate temperatures for samples having InGaN channels. The room temperature electron mobilities which were determined by a combination of both optical phonon and interface roughness scattering were measured between 630 and 910 cm2 (V s)-1 with corresponding sheet carrier densities of 2.3-1.3 × 1013 cm-2. On the other hand, electron mobilities were mainly limited by intrinsic scattering processes such as acoustic and optical phonons over the whole temperature range for Al0.82In 0.18N/AlN/GaN and Al0.3Ga0.7N/AlN/GaN heterostructures where the room temperature electron mobilities were found to be 1630 and 1573 cm2 (V s)-1 with corresponding sheet carrier densities of 1.3 and 1.1 × 1013 cm-2, respectively. By these analyses, it could be concluded that the interfaces of HEMT structures with the InGaN channel layer are not as good as that of a conventional GaN channel where either AlGaN or AlInN barriers are used. It could also be pointed out that as the In content in the AlInN barrier layer increases the interface becomes smoother resulted in higher electron mobility.