Wavelength-sensitive detector with elongate nanostructures

This application claims the benefit under 35 U.S.C. § 119(e) of U.S. provisional application Ser. No. 60/704,108, filed Jul. 29, 2005, and foreign priority benefit under 35 U.S.C. § 119(a)-(d) of European Patent Application No. 05077989.1, filed on Dec. 27, 2005, the disclosures of which are hereby incorporated by reference in their entirety and are hereby made a portion of this application.

Wavelength-sensitive detectors using elongate nanostructures are provided, more particularly, detection systems such as photodetection systems, including photodetectors, monochromators and optics, techniques which can be used to obtain spectral information about an incident photon beam, and the use of elongate nanostructures such as Nanowires (NW) or nanotubes, e.g., carbon nanotubes (CNT).

A photodetector is a device, which measures the intensity of an incident photon beam by outputting an electrical signal proportional to the number of incident photons. A photodetector is sensitive to incident photons, provided that the photon energy is larger than the bandgap of the photodetector material. In a conventional setup, wavelength-sensitivity is achieved for example by using spectral filters in front of the photodetector, such that only photons in a limited spectral range are incident onto the photodetector. If these filters are produced on the photodetector substrate, the processing costs will be very high. It is also a research topic on its own to develop materials, which have a sharp pass/block transition around the desired wavelength. In addition, the minimum photon energy, which can be detected, is still determined by the photodetector material. Another way to achieve wavelength-sensitivity is to use a monochromator in front of the detector. The monochromator diffracts the incident photon beam and a grating then selects a particular (narrow) spectral range of the incident photon beam, which then is incident on the detector. By scanning the grating of the monochromator, the whole spectral density of the incident photon beam can be determined. At the same time, a monochromator often takes a lot of space and requires additional equipment to control the grating of the monochromator.

Nanocrystals (NC) and nanowires (NW) have attracted considerable attention nowadays due to the interesting fundamental properties present in such low-dimensional systems and the exciting prospects for utilizing these materials in nanotechnology-enabled electronic and photonic applications.

The photonic applications of nanowires is described in Gudiksen et al. (J. Phys. Chem. B 106, 4036, 2002), specifically, size-dependent photoluminescence from single indium phosphide nanowires. This publication presents the change in peak frequency of the photoluminescent spectrum of a nanowire, as the diameter of the nanowire varies. This effect is explained by radial quantum confinement of electrons and holes in the narrow nanowires. This publication mentions the possibility to use several materials and wires with different diameters simultaneously, but this is a conventional setup, which includes for example the use of a monochromator to determine the frequency of the incident light.

Wang et al. describes highly polarized photoluminescence from and polarization-sensitive photodetection with single indium phosphide nanowires (Science 293, 1455, 2001). This publication presents a horizontal nanowire used as a photoconductor, and demonstrates the sensitivity of the nanowire to the polarization of the incident light. Suggestions are made about the use of other materials for the nanowire.

The preferred embodiments relate to a wavelength-sensitive detector. Said wavelength-sensitive detector can comprise at least a first and a second photoconductor unit. Said photoconductor units are situated on a substrate lying in a plane. Each of the at least first and second photoconductor units can comprise a first electrode having a first longitudinal direction and a first sidewall lying in a first plane parallel with the first longitudinal direction and a second electrode having a second longitudinal direction and a second sidewall lying in a second plane parallel with the second longitudinal direction, said first and second plane being substantially parallel to each other and being substantially perpendicular to the plane of the substrate.

Each individual photoconductor unit can further comprise a plurality of elongate nanostructures, each elongate nanostructure having a longitudinal axis, the axis of the elongate nanostructures being substantially parallel to each other and being substantially perpendicular to the first and second plane of the first and second electrode. Said plurality of elongate nanostructures can be positioned in between the first electrode and the second electrode.

The first photoconductor unit can further comprise a plurality of first type elongate nanostructures, and the second photoconductor unit comprises a plurality of second type elongate nanostructures, the first type elongate nanostructures being different from the second type elongate nanostructures.

A wavelength-sensitive detector is provided comprising at least a first and a second photoconductor unit on a substrate lying in a plane, each of the at least first and second photoconductor units comprising a first electrode having a first longitudinal direction and a first sidewall lying in a first plane parallel with said first longitudinal direction and a second electrode having a second longitudinal direction and a second sidewall lying in a second plane parallel with said second longitudinal direction, the first and second plane of a photoconductor unit being substantially parallel to each other and being substantially perpendicular to the plane of the substrate, wherein the first photoconductor unit comprises a plurality of first type elongate nanostructures being positioned in between the first electrode and the second electrode of the first photoconductor unit, the elongate nanostructures each having a longitudinal axis, the longitudinal axes of the first type elongate nanostructures being substantially parallel to each other and being substantially perpendicular to the first and second plane of the first and second electrodes of the first photoconductor unit, and wherein the second photoconductor unit comprises a plurality of second type elongate nanostructures being positioned in between the first electrode and the second electrode of the second photoconductor unit, the elongate nanostructures each having a longitudinal axis, the longitudinal axes of the second type elongate nanostructures being substantially parallel to each other and being substantially perpendicular to the first and second plane of the first and second electrodes of the second photoconductor unit, the first type elongate nanostructures being different from the second type elongate nanostructures.

The term “elongate nanostructures” as used herein is a broad term, and is to be given its ordinary and customary meaning to a person of ordinary skill in the art (and is not to be limited to a special or customized meaning), and refers without limitation to any two-dimensionally confined pieces of solid material in the form of wires (nanowires), tubes (nanotubes), rods (nanorods) and similar elongated substantially cylindrical or polygonal nanostructures having a longitudinal axis. A cross-dimension of the elongate nanostructures preferably lies in the region of 1 to 500 nanometers. According to the preferred embodiments, organic elongate nanostructures, such as e.g. carbon nanotubes, or inorganic elongate nanostructures, such as e.g. semiconducting nanowires (e.g. silicon nanowires) can be used.

The elongate nanostructures in each photoconductor are preferably formed of a semiconducting material. The plurality of elongate nanostructures can for example be single walled carbon nanotubes.

The plurality of elongate nanostructures in each of the first and second photoconductor units preferably have substantially a same diameter and can be formed of a same material. According to the preferred embodiments, the plurality of elongate nanostructures in said first photoconductor unit (also referred to as first type elongate nanostructures) and the plurality of elongate nanostructures in said second photoconductor unit (also referred to as second type elongate nanostructures) are different from each other. For example, they can be different from each other in material and/or diameter.

As an alternative the wavelength-sensitive detector can comprise at least one photoconductor unit wherein the plurality of elongate nanostructures in at least one of the first and second photoconductor units are partly n-type doped and partly p-type doped such that a pn diode is created within said at least one photoconductor unit.

In a preferred set-up the plurality of elongate nanostructures in a photoconductor unit can be stacked next to and/or on top of each other. The plurality of elongate nanostructures can be positioned in an array comprising rows and columns of elongate nanostructures. According to some embodiments, the array can be a periodic array.

The first and second electrodes of a photoconductor unit are preferably made of conductive materials, such as e.g. metals, alloys, poly-Si, metal-silicides, etc.

The plurality of elongate nanostructures used in the at least first and second photoconductor units preferably have a diameter from 0.3 nm up to 300 nm and more preferred a diameter smaller than 100 nm, i.e. a diameter from 0.3 nm to 100 nm.