Ahmad I. Nusir

Self-Powered Near-Infrared Photodetector Based on Asymmetrical Schottky Interdigital Contacts

A self-powered metal-semiconductor Schottky diode with planar interdigital Au-Ti electrodes was fabricated and characterized for the detection of near-infrared light. The devices showed a significant increase in external quantum efficiency (EQE) and photoresponse at zero bias as electrode spacing reduces. A device with an electrode spacing of 5 μm exhibits EQE of 1% equivalent to responsivity of 6.45 mA/W at a wavelength of 800 nm and 0 V bias. Furthermore, the photocurrent to dark current ratio was 5×104 with a detectivity of 1.4×1011 cm.Hz0.5/W at 0 V bias. The recovery time constant was calculated from time-resolved photocurrent curve and found to be 0.19 s. The built-in electric field of the device was estimated to be on the order of 35 V/cm.

Modeling and optimization of Au-GaAs plasmonic nanoslit array structures for enhanced near-infrared photodetector applications

Abstract. This theoretical work explores how various geometries of Au plasmonic nanoslit array structures improve the total optical enhancement in GaAs photodetectors. Computational models studied these characteristics. Varying the electrode spacing, width, and thickness drastically affected the enhancement in the GaAs. Peaks in enhancement decayed as Au widths and thicknesses increased. These peaks are resonant with the incident near-infrared wavelength. The enhancement values were found to increase with decreasing electrode spacing. Additionally, a calculation was conducted for a model containing Ti between the Au and the GaAs to simulate the necessary adhesion layer. It was found that optical enhancement in the GaAs decreases for increasing Ti layer thickness. Optimal dimensions for the Au electrode include a width of 240 nm, thickness of 60 nm, electrode spacing of 5 nm, and a minimum Ti thickness. Optimal design has been shown to improve enhancement to values that are up to 25 times larger than for nonoptimized geometries and up to 300 times over structures with large electrode spacing. It was also found that the width of the metal in the array plays a more significant role in affecting the field enhancement than does the period of the array.

Hot Electrons in Microscale Thin-Film Schottky Barriers for Enhancing Near-Infrared Detection

Metallic microstructures composed from Au thin films were designed and fabricated to enhance the near-infrared detection of photodetectors based on GaAs. The devices showed significant increase in the photocurrent and the spectral response due to the generation of hot electrons in the Au thin films and their injection into the semiconductor. Enhancement in the order of 120% was achieved in the photocurrent after applying an array of Au thin films with a thickness of 10 nm. Furthermore, a photocurrent-sweep using red laser showed an increase in the photocurrent of the device, as the laser was swept over the Au thin film. The effect of adding Ti adhesive layer on damping the photocurrent enhancement was further studied by varying the thickness of Ti between 0 and 4 nm.

Computational electromagnetic analysis of plasmonic effects in interdigital photodetectors

Plasmonic nanostructures have been shown to act as optical antennas that enhance optical devices. This study focuses on computational electromagnetic (CEM) analysis of GaAs photodetectors with gold interdigital electrodes. Experiments have shown that the photoresponse of the devices depend greatly on the electrode spacing and the polarization of the incident light. Smaller electrode spacing and transverse polarization give rise to a larger photoresponse. This computational study will simulate the optical properties of these devices to determine what plasmonic properties and optical enhancement these devices may have. The models will be solving Maxwell’s equations with a finite element method (FEM) algorithm provided by the software COMSOL Multiphysics 4.4. The preliminary results gathered from the simulations follow the same trends that were seen in the experimental data collected, that the spectral response increases when the electrode spacing decreases. Also the simulations show that incident light with the electric field polarized transversely across the electrodes produced a larger photocurrent as compared with longitudinal polarization. This dependency is similar to other plasmonic devices. The simulation results compare well with the experimental data. This work also will model enhancement effects in nanostructure devices with dimensions that are smaller than the current samples to lead the way for future nanoscale devices. By seeing the potential effects that the decreased spacing could have, it opens the door to a new set of devices on a smaller scale, potentially ones with a higher level of enhancement for these devices. In addition, the precise modeling and understanding of the effects of the parameters provides avenues to optimize the enhancement of these structures making more efficient photodetectors. Similar structures could also potentially be used for enhanced photovoltaics as well.

Performance enhancement of InAs quantum dots solar cells by using nanostructured antireflection coating with hydrophobic properties

Abstract. Enhancement of the performance of an InAs quantum dots (QDs) solar cell was investigated by using a nanostructured antireflection coating with hydrophobic properties. The surface modification was performed by growing ZnO nanoneedles on top of an InAs QDs solar cell’s surface, then the nanoneedles’ hydrophobicity treatment was achieved by using stearic acid. The QDs solar cell’s performance remarkably improved by 50% as noticed in the power conversion efficiency, external quantum efficiency, and spectral response after the surface modification. Additionally, the contact angle of the hydrophobic surface was found to be 153 deg.

Structural characteristics of Au-GaAs nanostructures for increased plasmonic optical enhancement

This research has been performed to improve upon optical qualities exhibited by metallic-semiconductor nanostructures in terms of their ability to excite electrons and generate current through the fabricated device. Plasmonic interactions become very influential at this scale, and can play an important role in the generation of photocurrent throughout the semiconductor. When the device is fabricated to promote the coupling of these radiated electromagnetic fields, a very substantial optical enhancement becomes evident. A GaAs substrate with an array of Au nanowires attached to the surface is studied to determine structural qualities that promote this enhancement. Using computational electromagnetic modeling and analysis, the effect of the Ti adhesion layer and various structural qualities are analyzed to promote photocurrent generation. Emphasis is placed on the amount of enhancement occurring in the semiconductor layer of the model. The photocurrent is then calculated mathematically and generalized for optimization of the device.

Silicon nanowires to enhance the performance of self-powered near-infrared photodetectors with asymmetrical Schottky contacts

Silicon nanowires were etched vertically in the channel between asymmetrical interdigital electrodes. The self-powered near-infrared photodetector consists of a planar structure of Au-vertically aligned Si nanowire-Ti. The devices were characterized by measuring the current-voltage characteristics, the external quantum efficiency (EQE), and the spectral response. An enhancement of 32% in the short-circuit current was achieved after applying the Si nanowires. The EQE of the device with Si nanowires consists of a strong peak covering the near-infrared spectral range with a maximum EQE of 10.3% at 965 nm and 0 V. Furthermore, the spectral response measurements showed enhancement and broadening in the spectrum of devices with Si nanowires.

Effect of ligand exchange on the photoresponsivity of near-infrared sensors based on PbSe nanocrystals

The effect of ligand exchange was investigated on the optical and electrical characteristics of near-infrared sensors fabricated from PbSe nanocrystals. Different ligands such as oleic acid, ethanedithiol, and mercaptoacetic acid were used to cap the nanocrystals. The bandgap of the PbSe nanocrystals was measured using the optical absorption and tuned into the near-infrared region by controlling the growth time. The electrical properties of the sensors fabricated from the PbSe nanocrystals treated with different ligands were extracted by measuring the current-voltage characteristics. Furthermore, the optical properties of the different sensors were found by measuring the spectral response of each sensor.

Semi-insulating GaAs and Au Schottky barrier photodetectors for near-infrared detection (1280 nm)

Schottky barriers formed between metal (Au) and semiconductor (GaAs) can be used to detect photons with energy lower than the bandgap of the semiconductor. In this study, photodetectors based on Schottky barriers were fabricated and characterized for the detection of light at wavelength of 1280 nm. The device structure consists of three gold fingers with 1.75 mm long and separated by 0.95 mm, creating an E shape while the middle finger is disconnected from the outer frame. When the device is biased, electric field is stretched between the middle finger and the two outermost electrodes. The device was characterized by measuring the current-voltage (I-V) curve at room temperature. This showed low dark current on the order of 10-10 A, while the photocurrent was higher than the dark current by four orders of magnitude. The detectivity of the device at room temperature was extracted from the I-V curve and estimated to be on the order of 5.3x1010 cm.Hz0.5/W at 5 V. The step response of the device was measured from time-resolved photocurrent curve at 5 V bias with multiple on/off cycles. From which the average recovery time was estimated to be 0.63 second when the photocurrent decreases by four orders of magnitude, and the average rise time was measured to be 0.897 second. Furthermore, the spectral response spectrum of the device exhibits a strong peak close to the optical communication wavelength (~1.3 μm), which is attributed to the internal photoemission of electrons above the Schottky barrier formed between Au and GaAs.