Hilal Cansizoglu

Enhanced Photocurrent and Dynamic Response in Vertically Aligned In2S3/Ag Core/Shell Nanorod Array Photoconductive Devices

Enhanced photocurrent values were achieved through a semiconductor-core/metal-shell nanorod array photoconductive device geometry. Vertically aligned indium sulfide (In2S3) nanorods were formed as the core by using glancing angle deposition technique (GLAD). A thin silver (Ag) layer is conformally coated around nanorods as the metallic shell through a high pressure sputter deposition method. This was followed by capping the nanorods with a metallic blanket layer of Ag film by utilizing a new small angle deposition technique combined with GLAD. Radial interface that was formed by the core/shell geometry provided an efficient charge carrier collection by shortening carrier transit times, which led to a superior photocurrent and gain. Thin metal shells around nanorods acted as a passivation layer to decrease surface states that cause prolonged carrier lifetimes and slow recovery of the photocurrent in nanorods. A combination of efficient carrier collection with surface passivation resulted in enhanced photocurrent and dynamic response at the same time in one device structure. In2S3 nanorod devices without the metal shell and with relatively thicker metal shell were also fabricated and characterized for comparison. In2S3 nanorods with thin metal shell showed the highest photosensitivity (photocurrent/dark current) response compared to two other designs. Microstructural, morphological, and electronic properties of the core/shell nanorods were used to explain the results observed.

Enhanced photoresponse of conformal TiO2/Ag nanorod array-based Schottky photodiodes fabricated via successive glancing angle and atomic layer deposition

In this study, the authors demonstrate a proof of concept nanostructured photodiode fabrication method via successive glancing angle deposition (GLAD) and atomic layer deposition (ALD). The fabricated metal-semiconductor nanorod (NR) arrays offer enhanced photoresponse compared to conventional planar thin-film counterparts. Silver (Ag) metallic NR arrays were deposited on Ag-film/Si templates by utilizing GLAD. Subsequently, titanium dioxide (TiO2) was deposited conformally on Ag NRs via ALD. Scanning electron microscopy studies confirmed the successful formation of vertically aligned Ag NRs deposited via GLAD and conformal deposition of TiO2 on Ag NRs via ALD. Following the growth of TiO2 on Ag NRs, aluminum metallic top contacts were formed to complete the fabrication of NR-based Schottky photodiodes. Nanostructured devices exhibited a photo response enhancement factor of 1.49 × 102 under a reverse bias of 3 V.

Effect of grain size and strain on the bandgap of glancing angle deposited AZO nanostructures

Vertical and tilted nanorod arrays of Al-doped ZnO were deposited by glancing angle deposition method at room temperature. The morphologies of the samples were varied by adjusting the rotation speed of the substrate. The applicability of various analytical methods was considered in order to understand the structural results. Analysis of X-ray diffraction data showed that the most suitable evaluation of some structural parameters was performed with a ‘strain profile’ model. The values of the grain size and strain obtained from the best fits of this model to the experimental data were reasonable. The bandgap increased with decreasing grain size and increasing strain for the samples. The change in bandgap of the samples was explained by our detailed structural analysis.

Enhanced light trapping and plasmonic properties of aluminum nanorods fabricated by glancing angle deposition

Vertically aligned arrays of aluminum (Al) nanorods were fabricated by glancing angle deposition (GLAD) method. Nanorods with maximum lengths of 200 and 350 nm were grown on 100 nm flat Al thin film. Total and diffuse reflectance profiles were measured using an ultraviolet–visible–near infrared (UV-Vis-NIR) spectrophotometer utilizing an integrating sphere to study detailed optical properties of Al nanorods in comparison to conventional planar Al thin film samples. Finite-difference-time-domain (FDTD) optical modeling method was utilized to simulate the optical response of Al nanorod array and thin film structures. FDTD simulations were carried out for periodic and random arrays of Al nanorods as well as for an isolated single nanorod in order to investigate effects of geometrical structure on plasmonic and light trapping effects. UV-Vis-NIR spectrum results reveal that total reflectance is inversely proportional with nanorod length, and decreases down to as low as ∼25%–30% in the visible spectrum at wave...

HIPS-GLAD core shell nanorod array photodetectors with enhanced photocurrent and reduced dark current

Vertically aligned core/shell nanorod array photodetectors were fabricated by high pressure sputter (HIPS) deposition of copper indium sulfide (CIS) films on glancing angle deposited (GLAD) indium sulfide (In2S3) nanorods. For comparison, we also studied nanorod photodetectors with conventional low pressure sputtered (LPS) CIS film coatings and counterpart thin film devices incorporating HIPS or LPS-CIS on In2S3 films. HIPS-GLAD core/shell photodetectors have shown a superior photocurrent density response along with lowest dark current density. Photoresponsivity defined with the photocurrent density/dark current density ratio γ = |J ph/J dark| was about ~1820 for HIPS-GLAD nanorod devices, which is several orders of magnitude higher compared to those of LPS-CIS thin film (γ ~ 2) and HIPS-CIS thin film (γ ~ 9) devices, and also about four-fold higher than LPS-CIS nanorod devices (γ ~ 490). Enhanced photoresponsivity is attributed to the porous microstructure and improved conformality of HIPS-CIS film around the In2S3 nanorods confirmed by SEM and EDS measurements. Due to randomization of the sputtered flux at higher working gas pressures, HIPS can provide a more conformal while at the same time a voidy low-density film around nanostructured surfaces. Reduced interelectrode distance and improved p–n junction interface due to the more uniform conformality of HIPS-CIS result in a higher photocurrent in our HIPS-GLAD devices. In addition, the voids in HIPS-CIS film as a result of its porous nature can behave as highly resistive spots that lower the dark current. Therefore, we have demonstrated that by utilizing a simple and low-temperature HIPS-GLAD method, high-photocurrent and low-dark-current photodetectors can be achieved by controlling the conformality and microstructure of a shell layer around nanorod arrays. HIPS shell coating method can be extended to almost any type of nanostructured substrate.

Ga2O3 as both gate dielectric and surface passivation via sol-gel method at room ambient

We report the use of sol-gel method at room ambient to grow nanoscale thin film of Ga2O3 on Si surface for both surface passivation and gate dielectric. The admittance measurements were carried out in the frequency range of 20 kHz-1 MHz at room temperature. Voltage dependent profile of interfacial trap density (Dit) was obtained by using low and high frequency capacitance method. The capacitance (C)-voltage (V) analyses show that the structures have a low interfacial trap density (Dit) of 1x1012 cm-2eV-1. The Ga2O3 thin film synthesized via sol-gel method directly on devices to function as a gate dielectric film is found to be very effective. We also present our experimental results for a number of gate dielectric and device passivation applications.

Comparison of Heterojunction Device Parameters for Pure and Doped ZnO thin films with IIIA (Al or In) Elements Grown on Silicon at Room Ambient

In this work, pure and IIIA element doped ZnO thin films were grown on p type silicon (Si) with (100) orientated surface by sol-gel method, and were characterized for comparing their electrical characteristics. The heterojunction parameters were obtained from the current-voltage (I-V) and capacitance-voltage (C-V) characteristics at room temperature. The ideality factor (n), saturation current (Io) and junction resistance of ZnO/p-Si heterojunction for both pure and doped (with Al or In) cases were determined by using different methods at room ambient. Other electrical parameters such as Fermi energy level (EF), barrier height (ΦB), acceptor concentration (Na), built-in potential (Φi) and voltage dependence of surface states (Nss) profile were obtained from the C-V measurements. The results reveal that doping ZnO with IIIA (Al or In) elements to fabricate n-ZnO/p-Si heterojunction can result in high performance diode characteristics.

Enhanced light trapping and carrier collection in glancing angle deposited nanostructures

Nanostructured materials have become an attractive alternative to their thin film and bulk counterparts in photovoltaic (PV) research. They owe this attention mainly to their superior optical and electrical properties. Light trapping in vertically aligned nanostructures results in high optical absorption and core/shell type of nanostructured devices provide enhanced carrier collection by utilizing a radial junction. Combination of these two features can potentially lead to the development of high efficiency nanostructured solar cells. Here, results from optical absorption properties of indium sulfide (In2S3; n-type semiconductor) nanostructures as a model material system in different geometries and their photoconductive properties in an In2S3-nanorods-core/metal-shell device design are presented and discussed. Glancing angle deposition (GLAD) technique was used to grow In2S3 nanostructures in different shapes (i.e., zigzags, springs, screws, tilted rods, and vertical rods). Optical absorption was found to strongly depend on the shapes of semiconducting nanostructures through ultraviolet-visible (UV-Vis) spectroscopy measurements. Numerical solutions of finite difference time domain (FDTD) optical modelling show that diffracted light is distributed uniformly within the 3D nanostructure geometries, indicating an enhanced diffuse light scattering and light trapping. A high pressure sputter deposition method was used to get a conformal silver (Ag) layer around GLAD In2S3 nanorods and produce the nanostructured core/shell photoconductive devices. Core/shell geometry was observed to enhance radial interface and shorten charge carrier transit times. This provides efficient carrier collection and results in superior photocurrent and gain. Slow recovery of photocurrent arisen from prolonged carrier lifetimes due to high surface states in nanorods is also eliminated by the metal shell, which provides surface passivation and decreases surface states. Overall, we demonstrate that GLAD nanostructures provide both efficient charge carrier collection and enhanced light trapping, and therefore can lead to the utilization of low quality (i.e. low cost) materials for high efficiency solar cells.

SAD-GLAD core-shell nanorod arrays for fuel cell, photodetector, and solar cell electrode applications

The glancing angle deposition (GLAD) technique, unlike a conventional physical vapor deposition (PVD) process, incorporates a flux of atoms that are obliquely incident on a tilted and rotating substrate. Instead of a continuous thin film coating, these atoms can form arrays of three-dimensional nanostructures due to a shadowing effect. By simply controlling the deposition angle and substrate rotation speed, nanostructures of a large variety of materials in the shapes of rods, screws, or springs can be obtained easily that are otherwise difficult to produce by conventional lithographical techniques. In this study, a brief overview of the growth mechanisms of GLAD nanostructures is presented. In addition, a new small angle deposition (SAD) technique as a simple means of conformally coating nanorod or nanowire arrays is described. SAD utilizes a small tilt angle during PVD on nanostructured substrates, which allows the effective exposure of nanorod sidewalls to the incoming flux and leads to enhanced thin film conformality. In this work, some recent results on core-shell nanorod arrays obtained by coating GLAD nanorods with a SAD shell will be presented. It will be shown that core-shell nanostructured geometries obtained by the simple SAD-GLAD method can significantly enhance catalyst activity for fuel cell electrodes, and charge carrier collection efficiency in photoconductive/semiconductor nanostructured materials.

Quantum Efficiency Enhancement of Mid Infrared Photodetectors with Photon Trapping Micro-Structures

The study proposes to use the photon trapping micro-structures to enhance quantum efficiency of the mid infrared photodetectors. The nanostructure that is consist of micro holes reduces reflection and bends the near normally incident light into the lateral modes in the absorbing layer.