Deniz Caliskan

The behavior of the I‐V‐T characteristics of inhomogeneous (Ni∕Au)–Al0.3Ga0.7N∕AlN∕GaN heterostructures at high temperatures

We investigated the behavior of the forward bias current-voltage-temperature I-V-T characteristics of inhomogeneous Ni/ Au –A l0.3Ga0.7N / AlN / GaN heterostructures in the temperature range of 295– 415 K. The experimental results show that all forward bias semilogarithmic I-V curves for the different temperatures have a nearly common cross point at a certain bias voltage, even with finite series resistance. At this cross point, the sample current is temperature independent. We also found that the values of series resistance Rs that were obtained from Cheung’s method are strongly dependent on temperature and the values abnormally increased with increasing temperature. Moreover, the ideality factor n, zero-bias barrier height B0 obtained from I-V curves, and Rs were found to be strongly temperature dependent and while B0 increases, n decreases with increasing temperature. Such behavior of B0 and n is attributed to Schottky barrier inhomogeneities by assuming a Gaussian distribution GD of the barrier heights BHs at the metal/semiconductor interface. We attempted to draw a B0 versus q /2 kT plot in order to obtain � +

Growth of high crystalline quality semi-insulating GaN layers for high electron mobility transistor applications

Semi-insulating character (sheet resistivity of 3.26×1011Ω∕sq) of thick GaN layers was developed for AlGaN∕GaN high electron mobility transistor (HEMT) applications on an AlN buffer layer. Electrical and structural properties were characterized by a dark current-voltage transmission line model, x-ray diffraction, and atomic force microscope measurements. The experimental results showed that compared to semi-insulating GaN grown on low temperature GaN nucleation, the crystal quality as well as surface morphology were remarkably improved. It was ascribed to the utilization of a high quality insulating AlN buffer layer and the GaN initial coalescence growth mode. Moreover, the significant increase of electron mobility in a HEMT structure suggests that this is a very promising method to obtain high performance AlGaN∕GaN HEMT structures on sapphire substrates.

Low dark current and high speed ZnO metal–semiconductor–metal photodetector on SiO2/Si substrate

ZnO thin films are deposited by radio-frequency magnetron sputtering on thermally grown SiO2 on Si substrates. Pt/Au contacts are fabricated by standard photolithography and lift-off in order to form a metal-semiconductor-metal (MSM) photodetector. The dark current of the photodetector is measured as 1 pA at 100 V bias, corresponding to 100 pA/cm2 current density. Spectral photoresponse measurement showed the usual spectral behavior and 0.35 A/W responsivity at a 100 V bias. The rise and fall times for the photocurrent are measured as 22 ps and 8 ns, respectively, which are the lowest values to date. Scanning electron microscope image shows high aspect ratio and dense grains indicating high surface area. Low dark current density and high speed response are attributed to high number of recombination centers due to film morphology, deducing from photoluminescence measurements. These results show that as deposited ZnO thin film MSM photodetectors can be used for the applications needed for low light level de...

Metal–semiconductor–metal photodetector on as-deposited TiO2 thin films on sapphire substrate

TiO2 thin films are prepared on c-plane sapphire substrates by the RF magnetron sputtering method. The performance of the Pt contact metal–semiconductor–metal (MSM) photodetector fabricated on as-deposited films is studied. The dark current density and the responsivity obtained were 1.57 × 10−9 A/cm2 at 5 V bias and 1.73 A/W at 50 V bias, respectively. Breakdown is not observed up to 50 V bias. Rise and fall times for the photocurrent were 7 and 3 s, respectively. Our results show that high quality MSM photodetectors can be fabricated without high temperature and complicated fabrication steps.

Metalorganic chemical vapor deposition growth and thermal stability of the AlInN/GaN high electron mobility transistor structure

The AlxIn1−xN barrier high electron mobility transistor (HEMT) structure has been optimized with varied barrier composition and thickness grown by metalorganic chemical vapor deposition. After optimization, a transistor structure comprising a 7 nm thick nearly lattice-matched Al0.83In0.17 N barrier exhibits a sheet electron density of 2.0 × 10 13 cm −2 with a high electron mobility of 1540 cm 2 V −1 s −1 .A n Al0.83In0.17N barrier HEMT device with 1 μm gate length provides a current density of 1.0 A mm −1 at VGS = 0 V and an extrinsic transconductance of 242 mS mm −1 , which are remarkably improved compared to that of a conventional Al0.3Ga0.7N barrier HEMT. To investigate the thermal stability of the HEMT epi-structures, post-growth annealing experiments up to 800 ◦ C have been applied to Al0.83In0.17N and Al0.3Ga0.7N barrier heterostructures. As expected, the electrical properties of an Al0.83In0.17N barrier HEMT structure showed less stability than that of an Al0.3Ga0.7N barrier HEMT to the thermal annealing. The structural properties of Al0.83In0.17N/GaN also showed more evidence for decomposition than that of the Al0.3Ga0.7N/GaN structure after 800 ◦ C post-annealing. (Some figures in this article are in colour only in the electronic version)

Ultrafast and sensitive bioassay using split ring resonator structures and microwave heating

In this paper, we have reported that split ring resonators (SRRs) structures can be used for bioassay applications in order to further improve the assay time and sensitivity. The proof-of-principle demonstration of the ultrafast bioassays was accomplished by using a model biotin-avidin bioassay. While the identical room temperature bioassay (without microwave heating) took 70 min to complete, the identical bioassay took less than 2 min to complete by using SRR structures (with microwave heating). A lower detection limit of 0.01 nM for biotinylated-bovine serum albumin (100-fold lower than the room temperature bioassay) was observed by using SRR structures.

Effects of rapid thermal annealing on the structural and local atomic properties of ZnO: Ge nanocomposite thin films

We have investigated the structural and local atomic properties of Ge nanocrystals (Ge-ncs) embedded ZnO (ZnO: Ge) thin films. The films were deposited by sequential sputtering of ZnO and Ge thin film layers on z-cut quartz substrates followed by an ex-situ rapid thermal annealing (RTA) at 600 °C for 30, 60, and 90 s under forming gas atmosphere. Effects of RTA time on the evolution of Ge-ncs were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), hard x-ray photoelectron spectroscopy (HAXPES), and extended x-ray absorption fine structure (EXAFS). XRD patterns have clearly shown that fcc diamond phase Ge-ncs of sizes ranging between 18 and 27 nm are formed upon RTA and no Ge-oxide peak has been detected. However, cross-section SEM images have clearly revealed that after RTA process, Ge layers form varying size nanoclusters composed of Ge-ncs regions. EXAFS performed at the Ge K-edge to probe the local atomic structure of the Ge-ncs has revealed that as prepared ZnO:Ge possesses G...

Chemical Visualization of a GaN p-n junction by XPS

We report on an operando XPS investigation of a GaN diode, by recording the Ga2p3/2 peak position under both forward and reverse bias. Areal maps of the peak positions under reverse bias are completely decoupled with respect to doped regions and allow a novel chemical visualization of the p-n junction in a 2-D fashion. Other electrical properties of the device, such as leakage current, resistivity of the domains are also tapped via recording line-scan spectra. Application of a triangular voltage excitation enables probing photoresponse of the device.

Planar Indium Tin Oxide Heater for Improved Thermal Distribution for Metal Oxide Micromachined Gas Sensors.

Metal oxide gas sensors with integrated micro-hotplate structures are widely used in the industry and they are still being investigated and developed. Metal oxide gas sensors have the advantage of being sensitive to a wide range of organic and inorganic volatile compounds, although they lack selectivity. To introduce selectivity, the operating temperature of a single sensor is swept, and the measurements are fed to a discriminating algorithm. The efficiency of those data processing methods strongly depends on temperature uniformity across the active area of the sensor. To achieve this, hot plate structures with complex resistor geometries have been designed and additional heat-spreading structures have been introduced. In this work we designed and fabricated a metal oxide gas sensor integrated with a simple square planar indium tin oxide (ITO) heating element, by using conventional micromachining and thin-film deposition techniques. Power consumption–dependent surface temperature measurements were performed. A 420 °C working temperature was achieved at 120 mW power consumption. Temperature distribution uniformity was measured and a 17 °C difference between the hottest and the coldest points of the sensor at an operating temperature of 290 °C was achieved. Transient heat-up and cool-down cycle durations are measured as 40 ms and 20 ms, respectively.

Spectral response modification of TiO 2 MSM photodetector with an LSPR filter

We fabricated UVB filtered TiO₂ MSM photodetectors by the localized surface plasmon resonance effect. A plasmonic filter structure was designed using FDTD simulations. Final filter structure was fabricated with Al nano-cylinders with a 70 nm radius 180 nm period on 360 nm SiO₂film. The spectral response of the TiO₂ MSM photodetector was modified and the UVB response was reduced by approx. 60% with an LSPR structure, resulting in a peak responsivity shift of more than 40 nm. To our knowledge, this is the first published result for the spectral response modification of TiO₂ photodetectors with LSPR technique.