Bahram Nabet

Picosecond response times in GaAs/AlGaAs core/shell nanowire-based photodetectors

High-speed metal-semiconductor-metal (MSM) photodetectors based on Schottky-contacted core/shell GaAs/AlGaAs and bare GaAs nanowires were fabricated and characterized. The measured core/shell temporal response has a ∼10 ps full-width at half-maximum and an estimated corrected value less than 5 ps. The bare GaAs devices exhibit a slower response (∼35 ps) along with a slow decaying persistent photocurrent (∼80 s). The core/shell devices exhibit significantly improved dc and high-speed performance over bare nanowires and comparable performance to planar MSM photodetectors. The picosecond temporal response, coupled with picoampere dark current, demonstrate the potential for core/shell nanowires in high-speed imaging arrays and on-chip optical interconnects.

Integrated plasmonic lens photodetector

Metal-semiconductor-metal (MSM) photodetectors may see increased responsivity when a plasmonic lens is integrated with the photodetector. The increased responsivity of the photodetector may be a result of effectively ‘guiding’ photons into the active area of the device in the form of a surface plasmon polariton. In one embodiment, the plasmonic lens may not substantially decrease the speed of the MSM photodetector. In another embodiment, the Shottkey contacts of the MSM photodetector may be corrugated to provide integrated plasmonic lens. For example, one or more of the cathodes and anodes can be modified to create a plurality of corrugations. These corrugations may be configured as a plasmonic lens on the surface of a photodetector. The corrugations may be configured as parallel linear corrugations, equally spaced curved corrugations, curved parallel corrugations, approximately equally spaced concentric circular corrugations, chirped corrugations or the like.

Analysis and analog implementation of directionally sensitive shunting inhibitory neural networks

The most significant property of these networks is their differential response to stimuli moving in opposite directions. A quantitative analysis shows that this directional response adapts to mean luminance levels and varies with size and speed of moving objects, as well as with coupling order among elements of a network. Both biophysical and analog hardware implementations of this class of networks are given here. Implementation of unidirectional coupling and the response to directional edges are demonstrated and shown to accord well with that of the neural network.

Effects of electron confinement on thermionic emission current in a modulation doped heterostructure

We discuss mechanisms responsible for the reduction of electron thermionic emission current from a Schottky contact to a modulation doped semiconductor compared to a bulk semiconductor. The effects discussed include metal to semiconductor barrier height enhancement due to proposed electron–electron cloud interaction, confined potential of the reduced dimensional systems, and the reduced dimensional nature of the density of states in the semiconductor. These effects describe the observed lowering of the dark current, and hence noise, of a modulation doped heterojunction based photodetector compared to a conventional bulk device.

A heterojunction metal-semiconductor-metal photodetector

A novel metal-semiconductor-metal photodetector is demonstrated in which a heterojunction is formed of doped Al/sub 0.24/Ga/sub 0.76/As layer on GaAs. The new device shows improved rectifying characteristics since the Schottky metal contacts a two-dimensional electron gas. The triangular quantum well that is formed at the hetero-interface due to doping of Al/sub 0.24/Ga/sub 0.76/As improves collection of optically generated electrons. The AlGaAs window also reduces reflection of light from air and eliminates GaAs surface recombination centers. The fabricated devices show up to a factor of four higher photocurrent and an order of magnitude less dark current than a conventional MSM. They also show smaller reach-through voltages consistent with the expected metal to two-dimensional gas contact properties.

On optical properties of GaAs and GaAs/AlGaAs core-shell periodic nanowire arrays

Optical reflection, transmission, and absorption in arrays of GaAs and GaAs/AlGaAs core-shell nanowires are studied using transfer matrix and photonic bandgap formalisms, analyzing the effects of size, geometry, height, packing density, and polarization. Energy dependence of the spectra demonstrates optical modes in the dielectric, similar to guided resonant modes, and also the air bands. Simulation of polarization dependence verifies higher absorption with the electric field along the wire axis. Higher absorption at much lower volume compared to thin film, combined with excellent charge transport, make core-shell nanowire arrays excellent candidates for optoelectronics applications.

Excitation of Local Field Enhancement on Silicon Nanowires

The interaction between light and reduced-dimensionality silicon attracts significant interest due to the possibilities of designing nanoscaled optical devices, highly cost-efficient solar cells, and ultracompact optoelectronic systems that are integrated with standard microelectronic technology. We demonstrate that Si nanowires (SiNWs) possessing metal-nanocluster coatings support a multiplicatively enhanced near-field light-matter interaction. Raman scattering from chemisorbed probing molecules provides a quantitative measure of the strength of this enhanced coupling. An enhancement factor of 2 orders of magnitude larger than that for the surface plasmon resonance alone (without the SiNWs) along with the attractive properties of SiNWs, including synthetic controllability of shape, indicates that these nanostructures may be an attractive and versatile material platform for the design of nanoscaled optical and optoelectronic circuits.

Simple analytical model of bias dependence of the photocurrent of metal-semiconductor-metal photodetectors

The current-voltage (I-V) characteristics of metal-semiconductor-metal (MSM) photodetectors under various light intensities are examined. The current shows an initial increase followed by saturation and a subsequent sharp increase as bias increases. We propose a theoretical model for bias dependence in all regions of operation except for breakdown, based on drift collection of carriers in the depleted regions under the contacts and diffusion and recombination in the undepleted region. This is based on the solution of the diffusion equation in the undepleted area between the two contacts of the MSM structure. The solution is subject to boundary conditions on excess minority carriers at the cathode end and continuity of current at the anode end. The latter is written in terms of a parameter, denoted as effective diffusion length, which describes the collection efficiency of carriers at the anode. The closed-form solution thus derived corroborates with physical expectations in several limiting cases. To compare theory with experiment, we propose methods to extract parameters that are used to normalize the I-V curves and calculate depletion widths under different light intensities, from current- and capacitance-voltage measurements. A close match between experimental and theoretical results is observed, and possible breakdown mechanisms are discussed.

Polarization anisotropy of individual core/shell GaAs/AlGaAs nanowires by photocurrent spectroscopy

We investigate the photodetection properties of individual core/shell GaAs/AlGaAs nanowires (NWs) and, in particular, their behavior under linearly polarized light. The NWs are grown by Au-assisted metalorganic vapor phase epitaxy and electrical contacts are defined on NWs by electron beam induced deposition. The spectral photocurrent of the single NW is measured and the dependence of the polarization anisotropy ρ (varying from ∼0.1 to ∼0.55) on the absorption wavelength is found to be clearly affected by the core/shell structure. High quantum efficiency values (10% at 600 nm) are obtained which are attractive for a wide range of optoelectronic devices.

Resonant-cavity-enhanced heterostructure metal–semiconductor–metal photodetector

We report a GaAs-based high-speed, resonant-cavity-enhanced, heterostructure metal–semiconductor–metal photodetector with Al0.24Ga0.76As/Al0.9Ga0.1As distributed Bragg reflector operating around 850 nm. The photocurrent spectrum shows a clear peak at this wavelength with full width at half maximum (FWHM) of around 30 nm. At resonance wavelength, a seven-fold increase can be achieved in quantum efficiency compared to a detector of the same absorption depth. The top reflector is a delta modulation doped Al0.24Ga0.76As that also acts as the barrier enhancement layer thus providing very low dark current values. The breakdown voltage is above 20 V. Time response measurements show rise time, fall time, and FWHM of 8.8 ps, 9 ps, and 8.1 ps, respectively, giving a 3-dB bandwidth of about 33 GHz. Combination of low dark current, fast response, wavelength selectivity, and compatibility with high electron mobility transistors makes this device especially suitable for short haul communications purposes.