Naser M. Ahmed

High sensitivity and fast response and recovery times in a ZnO nanorod array/p-Si self-powered ultraviolet detector

High quality, vertically aligned ZnO nanorods were grown on a silicon substrate, using microwave-assisted chemical bath deposition with poly (vinyl alcohol)-Zn(OH)2 nanocomposites as seed layer. The structure and surface morphology of the prepared ZnO nanorod arrays were characterized using X-ray diffraction and scanning electron microscopy. The optical properties were assessed using photoluminescence measurements; the results showed a high-intensity UV peak, and a lower intensity, broader visible peak. Upon exposure to 395 nm light at a zero-bias voltage, the UV detector showed a high sensitivity of 8000% and fast response and recovery times of 25 and 22 ms, respectively.

Comparative study of ultraviolet detectors based on ZnO nanostructures grown on different substrates

Pd/ZnO/Pd metal-semiconductor-metal photodetectors have been successfully fabricated using a variety of high-quality ZnO nanostructures. The nanostructures used included well-aligned nanorods, tetrapod-like nanorods, and hair-like nanowires and were synthesized on Si (100), porous silicon (PS/Si), and quartz substrates, respectively, using a catalyst-free vapor–solid mechanism for comparison. The morphological, structural, and optical properties of these nanostructures were investigated. Upon illumination with ultraviolet light (365 nm), the responsivity values of the fabricated photodetectors on PS/Si, Si, and quartz substrates were 0.22, 0.073, and 0.053 A/W, which correspond to quantum efficiencies of 85%, 28%, and 20%, respectively, at an applied bias of 5 V. The present study demonstrated that ZnO nanowires/PS exhibited a relatively fast photoresponse, with a rise time of 0.089 s and fall time of 0.085 s. The ZnO nanorods/Si and ZnO nanotetrapods/quartz exhibited a slow response, with rise times of 0...

High Sensitivity pH Sensor Based on Porous Silicon (PSi) Extended Gate Field-Effect Transistor

In this study, porous silicon (PSi) was prepared and tested as an extended gate field-effect transistor (EGFET) for pH sensing. The prepared PSi has pore sizes in the range of 500 to 750 nm with a depth of approximately 42 µm. The results of testing PSi for hydrogen ion sensing in different pH buffer solutions reveal that the PSi has a sensitivity value of 66 mV/pH that is considered a super Nernstian value. The sensor considers stability to be in the pH range of 2 to 12. The hysteresis values of the prepared PSi sensor were approximately 8.2 and 10.5 mV in the low and high pH loop, respectively. The result of this study reveals a promising application of PSi in the field for detecting hydrogen ions in different solutions.

A highly sensitive flexible SnS thin film photodetector in the ultraviolet to near infrared prepared by chemical bath deposition

This study involves a novel fabrication of a high performance, low cost and flexible (SnS nanoflake based) photodetector on a polyethylene terephthalate (PET) substrate by using chemical bath deposition. The photoresponse properties of the fabricated SnS nanostructure in the ultraviolet (UV) to near infrared (NIR) region were studied. The photodetector was subjected to illumination from four light emitting diodes (LEDs) characterized by spectra peaks of 380, 530, 750 and 850 nm, respectively. The photodetector exhibited a good photoresponse, fast response and high reproducibility with time. The sensitivity values of the photodetector were determined to be approximately 2990, 1604, 2591, and 446 for wavelengths of 380, 530, 750, and 850 nm at a bias voltage of 3 V. At the bias voltage of 5 V, the sensitivity values of 2575, 1262, 1635 and 295 were recorded for 380, 530, 750 and 850 nm respectively. The photodetector also manifests fast photoresponse for the illumination wavelengths. Based on the abovementioned results, in addition to its low cost, flexibility, and non-toxic nature, the SnS photodetector is a promising photoelectronic device that is effectively applicable over the UV-vis-NIR range.

Fabrication of lateral polysilicon gap of less than 50 nm using conventional lithography

We report a thermal oxidation process for the fabrication of nanogaps of less than 50 nmin dimension. Nanogaps of this dimension are necessary to eliminate contributions from double-layer capacitance in the dielectric detection of protein or nucleic acid. The method combines conventional photolithography and pattern-size reduction techniques. The gaps are fabricated on polysilicon-coated silicon substrate with gold electrodes. The dimensions of the structure are determined by scanning electron microscopy (SEM). An electrical characterization of the structures by dielectric analyzer (DA) shows an improved conductivity as well as enhanced permittivity and capacity with the reduction of gap size, suggesting its potential applications in the detection of biomolecule with very low level of power supply. Two chrome Masks are used to complete the work: the first Mask is for the nanogap pattern and the second one is for the electrodes. An improved resolution of pattern size is obtained by controlling the oxidation time. The method expected to enable fabrication of nanogaps with a wide ranging designs and dimensions on different substrates. It is a simple and cost-effective method and does not require complicated nanolithography process for fabricating desired nanogaps in a reproducible fashion.

Fast UV detection and hydrogen sensing by ZnO nanorod arrays grown on a flexible Kapton tape

ZnO nanorod arrays were grown on a flexible Kapton tape using microwave-assisted chemical bath deposition. High crystalline properties of the produced nanorods were proven by X-ray diffraction patterns and field emission scanning electron microscopy. Additionally, the photoluminescence spectrum showed higher UV peaks compared with visible peaks, which indicates that the ZnO nanorods had high quality and low number of defects. The metal-semiconductor-metal (MSM) configuration was used to fabricate UV and hydrogen gas detectors based on the ZnO nanorods grown on a flexible Kapton tape. Upon exposure to 395 nm UV light, the UV device exhibited fast response and decay times of 37 ms and 44 ms, respectively, at a bias voltage of 30 V. The relative sensitivities of the gas sensor made of the ZnO nanorod arrays, at hydrogen concentration of 2 %, at room temperature, 150 °C and 200 °C, are 0.42, 1.4 and 1.75 respectively.

A Study on the UV Photoresponse of Hydrothermally Grown Zinc Oxide Nanorods With Different Aspect Ratios

We report on the effect of the aspect ratio of zinc oxide (ZnO) nanorods (NRs) prepared by the hydrothermal process. It was found that increasing the precursor molar amount resulted in the decrease of the aspect ratio of ZnO NRs. Furthermore, the aspect ratios showed a significant effect on the structural and optical properties of the prepared ZnO NRs. The ZnO NRs were fabricated into a metal-semiconductor-metal (MSM) UV photodetector. The performances of the prepared MSM ZnO NRs were also studied, and the high aspect ratio showed that the highest responsivity had a value of 33 A/W at a bias voltage of 5 V and a wavelength of 380 nm. The responsivity, rise time, and full time of the prepared ZnO NRs showed a trend of behavior as the molar amount of the precursor was varied.

Fabrication of 6 nm gap on silicon substrate for power- saving appliances

We document a thermal oxidation process for the reproducible fabrication of 6-nm gaps on silicon-oninsulator (SOI) substrate. Nanogaps sizes of this dimension are implicated to eliminate contributions from double-layer capacitance in the dielectric sensing of proteins or nucleic acids. The method combines conventional photolithography and pattern-size reduction technique to create a desired-size gap. The gaps are physically characterized with a field emission scanning electron microscopy (FESEM). Preliminary results show that gap-size reduction provides an improvement in conductivity, permittivity and capacitance parameters, reflecting the potential applications of the fabricated structures in low-power consuming electrical devices. The task is completed with two chrome masks: the first mask is for the nanogap pattern and the second one is for the electrodes. An improved resolution of pattern size is obtained by controlling the oxidation time of the final cycle. The reproducibility of the method is proven in triplicate experiments. We believe the method can be used in the industrial production of desired-size nanogaps on a variety of low-cost substrates.

A comparative study of InN growth on quartz, silicon, C-sapphire and bulk GaN substrates by RF magnetron sputtering

In this work, we investigate the growth of indium nitride (InN) films on quartz, bulk GaN, sapphire (001) and Si (111) substrates. An InN buffer layer was first deposited on all the substrates, then an InN film was grown on bare substrate and InN buffered substrates. The films were polycrystalline in nature with preferred orientation along (002) plane. Best structural quality was observed on InN buffered Si substrate. The structural properties were explained by calculating the full width at half maximum, crystallite size, micro-strain, and dislocation density. The morphology of the films revealed similar granular features except for bare sapphire substrate which showed cracks and more oxygen percentage. The application of buffer layer increased the surface roughness for quartz and reduced in other cases. The band gap of InN films was determined using UV–visible reflectance spectroscopy. The lowest band gap value was observed for InN buffered quartz substrate.

Responsivity Dependent Anodization Current Density of Nanoporous Silicon Based MSM Photodetector

Achieving a cheap and ultrafast metal-semiconductor-metal MSM photodetector PD for very high-speed communications is ever-demanding. We report the influence of anodization current density variation on the response of nanoporous silicon NPSi based MSM PD with platinum Pt contact electrodes. Such NPSi samples are grown from n-type Si 100 wafer using photoelectrochemical etching with three different anodization current densities. FESEM images of as-prepared samples revealed the existence of discrete pores with spherical and square-like shapes. XRD pattern displayed the growth of nanocrystals with 311 lattice orientation. The nanocrystallite sizes obtained using Scherrer formula are found to be between 20.8 nm and 28.6 nm. The observed rectifying behavior in the I-V characteristics is ascribed to the Pt/PSi/n-Si Schottky barrier formation, where the barrier height at the Pt/PSi interface is estimated to be 0.69 eV. Furthermore, this Pt/PSi/Pt MSM PD achieved maximum responsivity of 0.17 A/W and quantum efficiency as much as 39.3%. The photoresponse of this NPSi based MSM PD demonstrated excellent repeatability, fast response, and enhanced saturation current with increasing anodization current density.