WO2017215770A1

WO2017215770A1 - A photodetector and a method for producing a photocurrent using the photodetector

Info

Publication number: WO2017215770A1
Application number: PCT/EP2016/066558
Authority: WO
Inventors: Heiner Linke; Steven LIMPERT
Original assignee: Heiner Linke; Limpert Steven
Priority date: 2016-06-13
Filing date: 2016-07-12
Publication date: 2017-12-21

Abstract

The present invention relates to a photodetector, comprising a semiconducting optical waveguide arranged to upon illumination produce photo-generated charge carriers, such that an electrical potential difference is formed between two electrodes that are connected to the waveguide. The waveguide comprises a charge carrier separator separating first and second portions of the waveguide, the charge carrier separator is configured to pass charge carriers of a first type and to block charge carriers of a second type, the second type being different from the first type. The waveguide is arranged to upon illumination form a waveguide charge generating region within the first or within the second portion of the waveguide, wherein the charge carrier separator comprises a separator material having a bandgap larger than a first material forming the first portion of the waveguide and a bandgap larger than a second material forming the second portion of the waveguide.

Description

A PHOTODETECTOR AND A METHOD FOR PRODUCING A PHOTOCURRENT USING THE PHOTODETECTOR

Technical field

The present invention relates a photodiode and a method for producing a photocurrent using a photodetector.

Background

Photodetectors convert light into an electrical signal such as a voltage or a current. For a photodetector comprising a semiconductor material the conversion is achieved by absorbed photons generating electron hole pairs within the semiconductor material. More specifically, a free electron hole pair, i.e. an electron in the conduction band and a hole in the valence band are created when a photon, having an energy greater than the bandgap of the semiconductor material, is absorbed.

To provide a photodetector with small foot print, good sensitivity and high speed the semiconductor material may further comprise a pn-junction, thereby taking advantage of the, at equilibrium, formed potential difference across a depletion region of the pn-junction. The photodetector is further configured such that the electron hole pairs are photogenerated in the depletion region, or within a diffusion length of the depletion region. The electrical field associated with the potential difference may thereby separate the electrons and holes and drifts them in opposite directions such that a photocurrent may be produces in an external circuit. Applying a reverse bias over the pn-junction further increases the width of the depletion junction, decreases the junction capacitance thereby providing an increased

responsivity. The reverse bias may, however, increase the dark current of the photodetector.

The efficiency of a pn-junction based photodetector is limited. First, the capacitance of the pn-junction generally reduces its applicability in high frequency modulation applications. Second, a portion of the photons, especially for long wavelengths, may be absorbed outside the depletion region. In other words, the absorption takes place outside the region within which the electric field may separate and drift the photogenerated electrons and holes which reduces the magnitude of the photocurrent achieved.

For a photodetector comprising a pin-structure the problems described above are reduced, but the detection of light is still limited to light having energies above the bandgap of the semiconductor material used in the photodetector. Photodetectors comprising sandwiched semiconductor materials with different bandgaps have been used to increase efficiency and to further offer color information by wavelength selected detection schemes. Color information may alternatively be achieved, for example, by a silicon photodetector comprising vertically stacked pn-junctions and the utilization of the wavelength-specific absorption depth in silicon. These multilayer semiconductor structures, however, add complexity to the fabrication of the photodetectors. The footprint for these photodetectors is also increased.

Hence, there is a need to improve the efficiency of photodetectors and to provide photodetectors which offer color information while maintaining small footprints.

Summary of the invention

In view of the above, it is an object of the present invention to provide an improved photodetector.

According to a first aspect a photodetector is provided. The

photodetector comprising: a photo-absorbing semiconducting optical waveguide arranged to upon illumination produce photo-generated charge carriers, the waveguide comprising: a charge carrier separator separating a first and a second portion of the waveguide, the charge carrier separator is configured to pass charge carriers of a first type and to block charge carriers of a second type, the second type being different from the first type; a first electrode electrically connected to the waveguide at the first portion thereof, and; a second electrode electrically connected to the waveguide at the second portion thereof, such that an electrical potential difference is formed between the first and the second electrodes upon the illumination, wherein the waveguide is arranged to upon illumination form a waveguide charge generating region within the first or within the second portion of the

waveguide, and wherein the charge carrier separator comprises a separator material having a bandgap larger than a first material forming the first portion of the waveguide and a bandgap larger than a second material forming the second portion of the waveguide.

The waveguide guides electromagnetic waves along its extension, i.e. the electromagnetic waves propagate with minimal loss of energy by restricting expansion to one dimension. An advantage being that a photodetector with improved light management is provided. An impedance mismatch within the photo diode may cause reflection of propagating electromagnetic waves, which added to the incoming waves may create local maximum in the form of for example a standing wave. The respective ends of the waveguide and/or the presence of the contacts may cause the impedance mismatch. A local maximum creates an electric field enhancement within at least a portion of the waveguide such that the waveguide charge generating region is formed. The waveguide charge generating region may be formed within the first or within the second portion of the waveguide. An increased amount of charge carriers may thereby be generated in the waveguide upon illumination. A larger photocurrent is, moreover, provided.

A further advantage being that the charge carrier separator allows for efficient separation of the photo-generated charge carriers of the first and the second type. Recombination of charge carriers of the first and second type is thereby reduced. An increased photocurrent is thereby provided.

The photodetector further may comprise a plasmonic structure arranged to upon the illumination generate surface plasmons thereby forming a plasmon charge generating region within the first or within the second portion of the waveguide.

The plasmonic structure increases the electric field within the waveguide as will be described further below. As a result an increased amount of charge carriers may be generated in the waveguide upon illumination and a larger photocurrent is provided. The direction of the photocurrent may, moreover, be tailored by further providing the plasmon charge generating region within the first or within the second portion of the waveguide. The location of the plasmon charge generating region in the waveguide may depend on the wavelength of the light illuminating the photodetector. To this end, the photodetector thereby allows for color detection by measuring the polarity of the resulting

photovoltage/-current, as will be described further below.

The plasmon charge generating region may overlap with the

waveguide charge generating region. The plasmon charge generating region and the waveguide charge generating region may thereby together form a region with enhanced electric field. A more efficient production of free charge carriers is thereby provided.

The wording plasmonic structure may be construed as a structure in which plasmons may be excited. Plasmons may be understood as quanta of plasma oscillations associated with a collective oscillation of charge density. The charges may for instance be provided by electrons.

The plasmonic structure may support a surface plasmon resonance, SPR. The wording surface plasmon resonance, SPR, may be understood as a resonant oscillation of charges at the interface between a negative and positive permittivity material which is excited by for instance light. The resonance condition may be achieved when the frequency of incident photons matches the natural frequency of the charges, e.g. electrons, oscillate against the restoring force of for instance a positive nuclei.

To this end, when light comprising a p-polarized component is incident on the plasmonic structure in such a way that the propagation constant and energy of resultant evanescent wave is equal to that of an allowed surface plasmon wave at the interface, a strong absorption of light takes place as a result of transfer of energy and an optical spectrum reflected by the plasmonic structure may demonstrate a sharp dip at a particular wavelength known as the resonance wavelength. In other words, the generated propagating surface plasmon wave attenuates the reflection of light at the plasmonic structure.

The charge motion in a surface plasmon creates electromagnetic fields outside, as well as inside the plasmonic structure. The wording plasmon charge generating region is to be understood as a volume defined by the spatial extension of the electromagnetic field formed by the excited plasmons. The plasmons enhance the photoconversion in the semiconductor, for example by light trapping and/or, hot electron/hole transfer. Free charge carriers are thereby efficiently produced within the plasmon charge generating region.

According to a second aspect a photodetector is provided. The photodetector comprising: a photo-absorbing semiconducting optical waveguide arranged to upon illumination produce photo-generated charge carriers, the waveguide comprising: a charge carrier separator separating a first and a second portion of the waveguide, the charge carrier separator is configured to pass charge carriers of a first type and to block charge carriers of a second type, the second type being different from the first type; a first electrode electrically connected to the waveguide at the first portion thereof, and; a second electrode electrically connected to the waveguide at the second portion thereof, such that an electrical potential difference is formed between the first and the second electrodes upon the illumination, a plasmonic structure arranged to upon the illumination generate surface plasmons thereby forming a plasmon charge generating region within the first or within the second portion of the waveguide, wherein the charge carrier separator comprises a separator material having a bandgap larger than a first material forming the first portion of the waveguide and a bandgap larger than a second material forming the second portion of the waveguide.

The waveguide according to the second aspect may further be arranged to upon illumination form a waveguide charge generating region within the first or within the second portion of the waveguide.

In general, features of the second aspects of the invention provide similar advantages as discussed above in relation to the first aspect of the invention.

The plasmonic structure may be arranged to upon illumination with a first wavelength generate surface plasmons thereby forming a first plasmon charge generating region within the first portion of the waveguide, and wherein the plasmonic structure is arranged to upon illumination with a second wavelength, the second wavelength being different from the first wavelength, generate surface plasmons thereby forming a second plasmon charge generating region within the second portion of the waveguide.

The plasmonic structure allows for efficient tuning of the location of the plasmon charge generating regions within the waveguide. For light of a first wavelength a first plasmon charge generating region is formed within the first portion of the waveguide. A photocurrent having a first direction may thereby be provided allowing for detection of light of the first wavelength. For a light of a second wavelength a second plasmon charge generating region is in contrast formed within the second portion of the waveguide. A photocurrent having a second direction may thereby be provided, the second direction being different from the first direction. In other words, illumination of different wavelengths may be detected by the direction of the photocurrent generated. The photodetector thereby provides color detection.

The first electrode and/or the second electrode may form the plasmonic structure.

The first and/or the second electrode may induce plasmons in the waveguide. Fewer components are needed for forming the photodetector. A photodetector with smaller footprint may be provided. The first and the second electrode may be formed by a metal such as gold thereby forming a metal-semiconductor heterojunction. The charge generating region may thereby be formed in the vicinity of the electrodes.

The first electrode and the second electrode may form a dipole antenna.

The dipole antenna allows for efficient collection of light. A larger number of photons may contribute to the generation of free carriers in the waveguide. Thus, a larger photocurrent may be generated by the

photodetector.

The dipole antenna, also referred to as a doublet antenna may be understood as comprising two electrically conductive elements being bilaterally symmetrical. The first and the second electrodes form the electrically conductive elements. It should be noted that the additional conductive elements such as wires or rods of a metal may be used as antennas. The additional conductive elements may be formed by plasmonic elements as discussed below.

Alternatively, a monopole antenna formed by an elongated conductive element, such as a metal rod, may be arranged in the vicinity of the waveguide. Alternatively, a dipole antenna formed by a pair of elongated conductive elements, such as metal rods, may be arranged in the vicinity of the waveguide.

The first electrode and the second electrode may be arranged perpendicular to a long axis of the waveguide.

The first electrode and the second electrode may be arranged parallel to a long axis of the waveguide.

The propagation of light within the waveguide occurs along the long axis of the waveguide. The polarization dependence of the photodetector may be set by the arrangement of the electrodes in relation to the long axis of the waveguide, i.e. the orientation of the electrodes set the polarization at which the electrodes respond most strongly.

In the perpendicular configuration, the waveguide absorbs light most strongly when the electric field oscillates parallel to the waveguide and perpendicular to the electrodes. The electrons in the electrodes oscillate most strongly when the electric field oscillates parallel to the electrodes and perpendicular to the waveguide. Thus, by changing the angular orientation of the contact with respect to the waveguide, the polarization sensitivity of the photodetector may be tuned. For example, the electrodes may be parallel to the waveguide, with a small portion of respective electrodes being in contact with the waveguide so that the polarization sensitivities of the waveguide and the electrodes are aligned. Intermediate angles will combine polarization sensitivities to varying degrees.

The photodetector according may further comprise a gate electrode capacitively coupled to the waveguide.

The potential of the gate electrode influences the conductance in the waveguide. A reduction of dark current of the photodetector may thereby be achieved. Alternatively, the conductance of the waveguide may be increased by applying a voltage to the gate electrode.

The plasmonic structure may comprise or be formed by a plasmonic element, the plasmonic element exhibiting a localized surface plasmon resonance condition upon resonant excitation, thereby forming the plasmon charge generating region.

An advantage being that an improved tailoring of the field

enhancement and the location of the charge generating region within the waveguide is achieved.

The wording localized surface plasmon resonance, LSPR, is to be understood as an excited state of the charge carriers within the plasmonic element, which can be excited by photons, i.e., by the electromagnetic field of light, incident on the plasmonic element. The resonant excitation of the plasmonic element thereby induces the LSPR.

Charge carriers such as electrons or holes may also be used to excite the plasmonic element inducing an LSPR condition. An electrical activation of the LSPR may thereby be achieved.

The LSPR condition is to be understood as a resonance condition associated to the collective oscillation of charge density and to the boundary conditions resulting from the finite size of the plasmonic element. As a result, a charge density wave is formed with a frequency/wavelength/energy that is set by the electronic properties of the material of the plasmonic element, its geometry, size and the material properties of the environment surrounding the plasmonic element. As an example, the LSPR typically occurs in the visible part of the electromagnetic wavelength spectrum if the plasmonic element is a gold nanoparticle having a diameter in the range of 50-100 nm. The LSPR may alternatively occur in the infrared or near-infrared visible part of the electromagnetic wavelength spectrum. The LSPR may alternatively occur in the ultraviolet part of the electromagnetic wavelength spectrum.

It should further be understood that the LSPR condition may occur when the electromagnetic radiation interacts with the plasmonic element. As a result an enhanced local electromagnetic field is created in the close vicinity of the plasmonic element. The enhanced electric field forms the plasmon charge generating region as will be described below. The strength of the enhancement and the spatial extent of the enhanced field depend on a number of parameters such as the material, size, shape, and environment of the plasmonic element. The enhanced electric field is beneficial as it improves the generation of charges charge generating region within the waveguide.

Resonant excitation may therefore be understood as resonant excitation by light or charge carriers such that the plasmonic element exhibits.

The spatial extension of the electromagnetic field from the excited localized surface plasmon resonance may form at least a part of the charge generating region. It is to be understood that the plasmonic element is arranged in relation to the waveguide such that the electromagnetic radiation generated by the plasmonic element at the resonance condition creates at least a part of the charge generating region within the waveguide. The charge generating region is characterized by an enhanced electric field. Since the spatial extension of this electromagnetic field depends both on the details of the plasmonic element, on the properties of the materials surrounding the plasmonic element and on the direction of the incident electromagnetic field in relation to the geometry of the plasmonic element, the volume of the charge generating region depends on all these parameters. To this end, the electromagnetic field related to the excited LSPR falls of gradually, often approximately exponentially, away from the plasmonic element such that the charge generating region typically has an extension on the length scale of 10- 500 nm. The extension of the exponential electromagnetic field depends sensitively on the material properties of the plasmonic element, the dielectric constants of its surrounding, as well as the properties of the light exciting the plasmonic elements.

The plasmonic structure may be formed by the plasmonic element. The plasmonic structure may comprise a plurality of plasmonic elements.

The first and the second material may be formed by the same material.

A simpler structure is provided. The manufacturing of the photodetector is further less complex. The waveguide charge generating region and/or the plasmon charge generating region may be arranged within a diffusion length of the first type of charge carries from the charge carrier separator.

An advantage being that charge carriers originating from the charge generating regions may efficiently be transported to the change carrier separator. The probability for photo-generated charge carriers of the first type from recombining with charge carriers of the second type is thereby reduced. An increased photo generated current is achieved. A more efficient

photodetector is thereby provided.

More specifically, when the charge generating region is/are located within a diffusion length for hot or thermalized charge carriers from the charge carrier separator, hot or thermalized charge carriers of the first type generated within the charge generating regions may diffuse to and pass through the charge carrier separator before cooling. The charge carriers of the second type may reach also reach the charge carrier separator but are blocked. The charge carrier separator thereby separated the charge carriers of the first type from the charge carriers of the second type. The cooling of the charge carriers of the first type reduces back flow of charges and a more efficient charge separation is thereby provided. As an example, a hot electron generated may pass over or through the charge carrier separator instead of recombining with a hole. To this end, the charge carrier separator may be construed as an energy filter in that the charge separator preferentially transmit charge carriers having higher energies than thermalized charge carries at lower energies, i.e. being closer to the corresponding band edges of the conduction/valence band. Further, the carrier's non equilibrium energy, for example, hot electrons may be used for higher voltage generation. A charge carrier separator comprising a potential barrier as disclosed below may be used to achieve the energy filtering.

The charge transport results in a measurable photocurrent through the photodetector, as well as in a corresponding measurable photovoltage.

According to a third aspect a method for producing a photocurrent is provided. The method comprising: illuminating a photodetector according to the above description, and measuring a current and a current direction of the photocurrent generated by photodetector.

The features of the third aspect of the invention provide when applicable the similar advantages as discussed above in relation to the first and the second aspect of the invention.

A further scope of applicability of the present invention will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

Hence, it is to be understood that this invention is not limited to the particular component parts of the device described or steps of the methods described as such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claim, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings do not exclude other elements or steps. Brief Description of the Drawings

The above and other aspects of the present invention will now be described in more detail, with reference to appended drawings showing embodiments of the invention. The figures should not be considered as limiting the invention to the specific embodiment; instead they are used for explaining and understanding the invention.

As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention. Like reference numerals refer to like elements throughout.

Figure 1 illustrates a schematic top view of a photodetector.

Figure 2 illustrates a schematic top view of a photodetector.

Figure 3 illustrates a schematic top view of a photodetector.

Figure 4 illustrates a schematic top view of a photodetector upon illumination of a first wavelength and a band diagram for the waveguide of the photodetector.

Figure 5 illustrates a schematic top view of a photodetector upon illumination of a second wavelength and a band diagram for the waveguide of the photodetector.

Figure 6 illustrates a flow chart illustrating a method for producing a photocurrent.

Figure 7 illustrates a schematic top view of a photodetector upon illumination of a third wavelength and a band diagram for the waveguide of the photodetector.

Figure 8 illustrates a schematic top view of a photodetector.

Figure 9 illustrates an experimental realization of a photodetector.

Figure 10 illustrates current reversal for a photodetector.

Figure 1 1 illustrates theoretical modeling of a photodetector.

Figure 12 illustrates schematically the results of the theoretical modeling shown in figure 1 1 .

Figure 13 illustrates a schematic side view of a photodetector comprising an array of waveguides.

Detailed description

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and to fully convey the scope of the invention to the skilled person. Figure 1 is a schematic top view of a photodetector 100. The photodetector comprises a photo-absorbing semiconducting optical waveguide 102, a first electrode 104 electrically connected to the waveguide 102 at the first portion thereof 106, and a second electrode 108 electrically connected to the waveguide 102 at a second portion 1 10 thereof.

The waveguide 102 comprises a charge carrier separator 1 12 separating the first 106 and the second 1 10 portion of the waveguide 102. The charge carrier separator 1 12 is a single barrier structure 1 14 as will be discussed below. The charge carrier separator 1 12 is configured to pass charge carriers of a first type and to block charge carriers of a second type, the second type being different from the first type as will be discussed further below.

The waveguide 102 is arranged to upon illumination produce photo- generated charge carriers. The waveguide 102 is further arranged to upon illumination form a waveguide charge generating region 1 16 within the first portion 106 of the waveguide 102. An increased amount of charge carriers may thereby be generated in the waveguide 102 upon illumination.

Alternatively, the waveguide 102 may be arranged to upon illumination form a waveguide charge generating region 1 16 within the second 1 10 portion of the waveguide, not shown.

The waveguide may be arranged to upon illumination with a first wavelength create an electric field enhancement within at least a portion of the first portion of the waveguide such that the waveguide charge generating region is formed and wherein the waveguide is arranged to upon illumination with a second wavelength, the second wavelength being different from the first wavelength, create an electric field enhancement within at least a portion of the second portion of the waveguide such that the waveguide charge generating region is formed. The waveguide allows for efficient tuning of the location of the waveguide charge generating regions within the waveguide. For light of a first wavelength, a first waveguide charge generating region is formed within the first portion of the waveguide. A photocurrent having a first direction may thereby be provided allowing for detection of light of the first wavelength. For a light of a second wavelength, a second waveguide charge generating region is in contrast formed within the second portion of the waveguide. A photocurrent having a second direction may thereby be provided, the second direction being different from the first direction.

The electrodes 104 and 108 are arranged to conduct a current. The electrodes 104 and 108 may be a charge conducting material such as a metal, a doped semiconductor or a conductive polymer.

The photodetector 100 may comprise a plasmonic structure 1 18 as illustrated in figure 2, arranged to upon illumination generate surface plasmons thereby forming a plasmon charge generating region 120 within a first portion 106 of the waveguide 102. The first electrode 104 and the second electrode 108 form the plasmonic structure 1 18. According to other

embodiments the first or the second electrode may form a plasmonic structure. The plasmonic structure 1 18 increases the electric field within the waveguide 102. As a result an increased amount of charge carriers may be generated in the waveguide 102 upon illumination and a larger photocurrent may be provided.

The photodetector 100 may thereby take advantage of combined waveguiding and plasmonic effect in order to provide enhanced generation of charge carriers. The plasmon charge generating region 120 may further overlap with the waveguide charge generating region 1 16. A more efficient production of free charge carriers is thereby provided.

It should be noted that a plasmonic structure may alternatively be arranged to upon illumination generate surface plasmons thereby forming a plasmon charge generating region within the second portion of a waveguide as shown below.

The first 104 and the second 108 electrode may form a dipole antenna 122. The dipole antenna allows for efficient collection of light. A larger number of photons may contribute to the generation of free carriers in the waveguide 102.

The first electrode 104 and the second electrode 108 are arranged perpendicular to a long axis of the waveguide 102. In the perpendicular configuration, the waveguide 102 absorbs light most strongly when the electric field oscillates parallel to the waveguide 102 and perpendicular to the electrodes 104, 108.

Figure 3 illustrates a photodetector 200 similar to the photodetector 100 figures 1 and 2, but the electrodes 104, 108 are instead arranged parallel to a long axis of the waveguide 102. By changing the angular orientation of the electrodes 104, 108 with respect to the waveguide, the polarization sensitivity of the photodetectors 100, 200 may be tuned.

Next the function of the photodetector 100 will be described with reference to figures 4 - 6. The color detection capability of the photodetector is further discussed. Figures 4 and 5 illustrate the photodetector 100 under illumination with a first and a second wavelength, respectively. Band diagrams illustrating the band structure for the waveguide 100 are also illustrated in the figures 4 and 5. Figure 6 illustrates a flow chart of a method 200 for producing a photocurrent, using any of the photodetectors disclosed.

Figure 4 illustrates the photodetector 100 illuminated with light of a first wavelength λι . Upon the act of illuminating 202, see figure 6, the

photodetector 100 the photo-absorbing semiconducting optical waveguide 102 produce photo-generated charge carriers 124, as illustrated in the bad diagram of figure 4.

The plasmonic structure 1 18 is arranged to upon illumination with light of the first wavelength λι generate surface plasmons thereby forming a first plasmon charge generating region 126 within the first portion 106 of the waveguide 102.

The plasmons enhance the photoconversion in the plasmon charge generating region 126 of the waveguide 102. Free charge carriers 124 are thereby efficiently produced within the plasmon charge generating region 126. The free carriers 124 generated may diffuse to the charge carrier separator 1 12. The charge carrier separator 1 12 is arranged between the first 106 and the second 1 10 portion of the waveguide 102. The charge carrier separator 1 14 is configured to pass charge carriers of a first type 126, for example electrons, and to block charge carriers of a second type 128, for example holes. This is achieved by the charge carrier separator 1 12 comprising a separator material 130 having a bandgap larger than a first material 131 forming the first portion 106 of the waveguide 102 and a bandgap larger than a second material 133 forming the second portion 1 10 of the waveguide 102, see figure 4. The larger bandgap of the separator material 130 forms potential barriers for the charge carriers 124. Charge carriers of the first type 126, i.e. electrons, having kinetic energies larger than a threshold value may, however, pass over the potential barrier structure 1 14 such that a

photocurrent is produced. Charge carriers of the second type 128, i.e. the holes, are in contrast blocked by the potential barrier 1 14. The charge carrier separator 1 12 thereby separated the charge carriers of the first type 126 from the charge carriers of the second type 128.

The first or the second portions of the waveguide may be formed by a semiconductor material having an effective mass for the charge carriers of the first type that is smaller than the effective mass of the charge carries of the second type. Efficient passing of the charge carries of the first type and efficient blocking of the charge carries of the second type are thereby provided.

The skilled person in the art realizes that the threshold value depends on several factors such as the effective mass of the charge carries, the distance from the charge carrier separator at which the charge carriers are photogenerated etc.

The first and the second portions of the waveguide may be formed by the same material having the same bandgap.

The first and second material may alternatively be formed by different materials.

The first and the second portions of the waveguide may have the same doping. The doping may be n-type, p-type, or intrinsic.

The first and second portions of the waveguide are formed by different materials having different bandgaps. The different materials may have different bandgap allowing for increased wavelength tuning of the absorption of the photons.

The plasmon charge generating region 126 may be arranged within a diffusion length L_diff of the first type of charge carries 126 from the charge carrier separator 1 12. The probability for photo-generated charge carriers of the first type 126 from recombining with charge carriers of the second type128 is thereby reduced.

More specifically, when the plasmon charge generating region 126 is located within a hot-carrier diffusion length, L_diff, from the charge carrier separator 1 12 , hot charge carries of the first type 126 generated within the plasmon charge generating region 126 may diffuse to and pass over the barrier structure 1 14 of the charge carrier separator 1 12 before cooling. The cooling of the charge carriers of the first type 126 further reduces back flow of charges and a more efficient charge separation is thereby provided. As an example, a hot electron generated may pass over the barrier structure or through the charge carrier separator instead of recombining with a hole. The charge transport results in a measurable photocurrent through the

photodetector.

The wording hot-charge carriers may be construed as non-equilibrium electrons or holes in a semiconductor created when a photon having an energy larger than the bandgap of the semiconductor strikes a

semiconductor.

The hot-carrier diffusion length may be in the order of a few hundred nanometers.

The method 200 further comprises the act of measuring 204 a current direction, , of the photocurrent generated by photodetector 100. The measuring 204 may be made by an amperemeter 132. The current direction pertains to the direction of the charge carrier of the first type 126 passing the charge carrier separator 1 12.

The plasmonic structure 1 18 is further arranged to upon illumination with a second wavelength λ₂, the second wavelength being different from the first wavelength λι, generate surface plasmons thereby forming a second plasmon charge generating region 134 within the second portion 1 10 of the waveguide 102, see figure 5.

The function of the charge carrier separator 1 12 is the same for the charge carriers generated in the second portion 1 10 as for the charge generators generated in the first portion 106, disclosed in relation to figure 4, and will for brevity not be discussed again. It is, however, noted that for a light of the second wavelength X₂ a photocurrent having a second direction i₂ is provided, the second direction i₂ being different from the first direction . In other words, illumination of different wavelengths may be detected by measuring 204 the direction of the photocurrent generated. The photodetector 100 may thereby provide information on the wavelength of the illuminating light, i.e. the photodetector 100 allows for color detection.

The magnitude of the photocurrent may further be measured.

The method may further comprise cooling the photodetector.

The waveguide 102 may be a semiconductor nanowire. The first portion 106, the charge carrier separator 1 12 and the second portion 1 10 of the waveguide102 may thereby be formed within a single nanowire, i.e. as an axial heterostructure. The axial heterostructure may be construed waveguide formed by that the wire material is varied along the growth direction during normal nanowire growth techniques, including, for example, the particle- assisted growth mechanism. The atomically sharp interfaces formed between the materials in the axial heterostructure allows for efficient charge transport within the waveguide. Efficient strain relaxation in the nanowires further allows for the combination of non-lattice-matched materials along the nanowire. The bandgap along the nanowire may thus be tuned locally along the length of a nanowire, e.g. allowing for efficient formation of barrier structures, double barrier heterostructures and quantum-confined structures.

The semiconductor nanowire may have a length in the range of 100 - 2 000 nm, and a diameter in the range of 10 - 500 nm.

The charge carrier separator 1 14 may comprise a double barrier structure 136, as illustrated in figure 7. Illumination with a third wavelength λ₃ generates free charge carries 124 in the first portion 106 of the waveguide 102, see the charge generating region 126. The double barrier 136 may form a resonant-tunneling diode structure. More specifically, the resonant-tunneling diode comprises a resonant-tunneling structure through which charge carriers of the first type 126 may pass and charge carriers of the second type 128 are blocked. In other words, the charge carriers of the first type may tunnel through via resonant states at certain energy levels within the tunneling barrier 136, not shown. The double barrier structure 136 provides efficient separation of charges of the first 126 and the second type 128 such that a photocurrent having a first direction i₃ is provided.

The double barriers are formed by a material having a bandgap larger than the bandgap of the material forming the first and the second portion of the waveguide. The double barrier structure 136 may be formed in a semiconductor nanowire as will be discussed further below.

The charge carrier separator may alternatively comprise a multi-barrier structure. The potential landscape of the charge carrier separator may be tailored. The potential barriers for the first type of charge carriers may thereby be made lower than for the second type of charge carriers.

The semiconductor nanowire may for example be formed be a semiconductor material such as InAs and the barriers may be formed by InP. The skilled person in the art realizes that other semiconductor materials may be used as long as the band structure of the charge carrier separator allows for efficient separation of charge carries of the first and the second type.

The photodetector 100 of figure 7 further comprises a gate electrode 138 capacitively coupled to the waveguide 102. The gate electrode 138 may be formed by a substrate onto which the waveguide is arranged.

A potential 140 applied to the gate electrode 138 influences the conductance in the waveguide 102. As an example, increased voltage, either positively or negatively, depending on whether the waveguide is of a p-type or n-type semiconductor material, may decrease the channel conductivity, similarly to the depletion mode operation of a MOSFET. An advantage is that the charge transport within the waveguide 102 may be tuned. A reduction of dark current of the photodetector may thereby be achieved.

The localized surface plasmon resonance condition may occur when the electromagnetic radiation interacts with the plasmonic element. As a result an enhanced local electromagnetic field is created in the close vicinity of the plasmonic element. The enhanced electric field forms the plasmon charge generating region as will be described below. The strength of the enhancement and the spatial extent of the enhanced field depend on a number of parameters such as the material, size, shape, and environment of the plasmonic element. The enhanced electric field is beneficial as it improves the generation of charges charge generating region within the waveguide.

In figure 8 a photodetector 100 comprising plasmonic elements is illustrated.

The plasmonic structure 1 18 comprise a pair of plasmonic elements 142 arranged to collectively create the plasmon charge generating region 144 within the waveguide 102 upon resonant excitation of the plasmonic structure 1 18. The resonant excitation is illustrated as illumination of light having a resonant wavelength _R. An improved field enhancement in the plasmon charge generating region 144 is thereby obtained. The plasmonic elements 142 are arranged at opposite sides of the waveguide 102. A favourable formation of the plasmon charge generating region 144 within the

waveguide102 is thereby provided.

The extension of the plasmon charge generating region 144 within the waveguide 102 may, moreover, be tailored by the arrangement of the plasmonic elements 142, e.g. by the separation of the plasmonic elements 142. A closer separation between the plasmonic elements 142 may increase the field enhancement in the plasmon charge generating region 144. In figure 8 the plasmonic elements are illustrated as discs. The skilled person in the art realizes that other types of plasmonic elements such as triangular, rods, stars etc may be used. The resonance frequency, i.e. the wavelength for which the localized plasmon resonance condition occurs may be tuned by for example changing the material, inter distance, size or shape of the plasmonic elements. An advantage being that an improved tailoring of the field enhancement and the location of the charge generating region within the waveguide is achieved.

The photodetector may comprise additional structures such as lenses, anti-reflection windows for improving the capture of light. The photodetector may comprise a current amplifier. I the above given examples, the charge carriers of the first type are electrons and the charge carriers of the second type are holes. Alternatively, the charge carriers of the first type may be holes and the charge carriers of the second type may be electrons.

Examples

The following descriptions of experiments are intended to be examples of the embodiments disclosed herein, and are not intended to be limiting. In the following an experimental realization of a photodetector according to the invention is described.

Materials and Methods

Next, a photodetector comprising an InAs nanowire as waveguide is described. The InAs nanowire has a bandgap, E_G = 0.39 eV, corresponding to light with a bandgap wavelength _G = 3180 nm, and electron-hole effective mass and mobility asymmetries. A charge carrier separator is formed by an InP segment or InP segments integrated in the InAs nanowire, see figure 9.

The InP material form a barrier for free charge carries generated within the nanowire as InP has a bandgap E_G = 1 .34 eV corresponding to light with a bandgap wavelength _G = 925 nm. Two types of charge carrier separators comprising InAs/lnP nanowire heterostructures are disclosed: (i) double- barriers forming charge separation by resonant tunneling, and (ii) single thermionic barriers. These material structures provide photogeneration of high-energy, fast-diffusing electrons and low-energy, slow-diffusing holes, thereby assisting in electron-hole separation.

Results

Figure 9 summarizes a non-limiting example of a photodetector produced. Figure 9a shows a scanning transmission electron high angle annular dark field micrograph of InAs/lnP heterostructure nanowire in which for illustration both a single and a double-barrier with InP segments are shown. Figure 9b illustrates a band diagram of a single-barrier detector under short-circuit current conditions. A charge generating region formed upon illumination is indicated by the dashed oval on the right side of the potential barrier forming the charge carrier separator discussed above. The

photodetector provides a photogeneration rate gradient across the InP barrier. High-energy photogenerated electrons, see upper right corner of figure 9b, diffuse over the InP barrier, while low-energy photogenerated holes, see lower right corner of figure 9b are blocked by the InP barrier. Figure 9c and d illustrates band diagrams under short-circuit conditions of the single- barrier InP heterostructure and of the double-barrier heterostructure, respectively, used in the experiments. Bandgaps, band offsets and

heterostructure layer thickness are indicated.

The nanowires were grown by chemical beam epitaxy and transferred onto a 200 nm thick silicon dioxide layer atop a degenerately doped n-type silicon substrate. Sulphur-passivated layered Ni/Au Ohmic contacts were fabricated around targeted heterostructures by metal evaporation after electron beam lithography exposure of polymethynnethacrylate, PMMA, resist. The photodetectors were characterized in a variable-temperature, T = 6 K - 300 K probe station with optical fiber access. DC electrical measurements where made using a Yokagawa 7651 DC source, a Stanford Research Systems SR570 current preamplifier, a Hewlett Packard 34401A multimeter and a Keithley 2636B SourceMeter.

For photodetector characterization, a Fianium Femtopower 1060 Supercontinuum Source with emission from 500 nm to 1850 nm, a maximum power output of 8 W and a repetition rate of 82.5 MHz coupled into a

Princeton Instruments SP2150 Double Monochromator was used. All presented measurements were performed using the monochromator's 150 gratings per millimeter, 800 nm blaze grating. The narrowband spectra used for illuminating the photodetectors were measured with an Avantes Avaspec- 3648-usb2 silicon CCD spectrometer and the narrowband power was measured with a Thorlabs power meter and silicon and germanium

photodetectors. A typical measured illumination spectrum was found to have a Gaussian line profile with a standard deviation of 15 nm.

Results on the reversal of photocurrent for different wavelengths illuminating the photodetector is illustrated in figure 10 for a separator structure comprising an InP double-barrier, see figures 7 and 9 for the design of the photodetector. At a temperature of T= 6 K and a back-gate voltage V_g = -20 V of the gate electrode, the double-barrier detectors behaved as insulators in the dark, i.e. the darkcurrent of the photodetector is suppressed, see figure 10a. Under illumination, the photodetector of figure 10 became photoconductive and produced electrical power and V_oc up to -80 mV, see figures 10 a-c. It was further experimentally verified that for illumination with light of different wavelengths, achieved by narrowband illumination, the direction of the photocurrent depended on wavelength in the double-barrier detectors. The photodetector exhibited at least three reversals of the photocurrent direction between for illumination of light having wavelengths ranging from 500 nm to 1500 nm, see figure 10b. Measurements from additional detectors displayed similar photocurrent reversals with wavelength, not shown. The observed sign reversal of the photocurrent and photovoltage achieved by the photodetector are caused by the non-uniform light absorption achieved in the nanowire waveguides. The formation of local charge generating regions within the nanowires was confirmed by COMSOL wave optic modelling of the photodetectors. The theoretical modelling took the materials, the dimensions and relative arrangement of features of the photodetector into account. The theoretical modelling shows that the location of the charge generating regions within the nanowires changes with wavelength in the visible spectrum of the illumination light, λ<700 nm, see figures 1 1 a-d.

Figure 1 1 illustrates the COMSOL wave optic modelling of the photodetectors, i.e. of a contacted, single nanowire double-barrier

photodetector. For increased clarity, figure 12 illustrates schematically the results of the theoretical modeling shown in figure 1 1 .

Figure 1 1 a illustrated the nanowire portion of 3D wave optics model to scale. Air, S1O2 substrate and Au contacts are included in the model.

White/black sections of the NW are InAs/lnP. Position and width of Au contacts, striped regions, are based on the actual photodetector for which data are shown in figure 10, whereas wire segment lengths are averages from transmission electron microscopy images of other nanowires from the same batch. Indicated on figure 1 1 are the volumes to the left and to the right of the double-barrier in the electrically active region of the photodetector over which the absorption rate density, G, is integrated for comparison in Fig. 10d. Figures 1 1 b,c show modeled, normalized absorption rate density for normally incident, unpolarized light at a few wavelengths: The sequence illustrates how the charge generation regions, indicated by dashed ovals, in the electrically active region of the detector shifts relative to the double-barrier, offering verification of the observed dependence of photocurrent direction on wavelength.

In contrast, in the infrared regime, λ>700 nm, plasmonic absorption develops at the nanowire/contact interfaces, shifting the position of the charge generation for this particular photodetector to the metal contacts, see figure 1 1 d . The modelling results show that the location of the charge generating regions, providing enhanced photo-absorption and generation of free carries in the nanowire, may shift to either side of the charge separating double- barrier structure, depending on the wavelength of the illumination used. Thus verifying the observed reversal of the sign of the photocurrent as a function of λ as discussed above.

It should be noted that according to the above experiments it is the position of the charge carrier separator relative to the charge generating portion that determines whether or not a reversal of photocurrent occurs as a function of wavelength. It should further be noted that the result is

independent of whether the charge carrier separator comprises a single, double barrier or multibarrier structure, but instead depends on the

waveguiding properties and the plasmonic properties of the contacts.

As non-limiting examples, the nanowires may have a total length from approximately 1 μιτι to 1 .5 μιτι.

As non-limiting examples, the nanowires may have a diameter from approximately 45 nm to 65 nm.

As non-limiting examples, the single barrier segment length may be in the range of 10 nm to 100 nm, preferably within 20 nm to 60 nm

As non-limiting examples, each of the double barrier segments may have a length in the range of 1 nm to 10 nm, preferably 2 nm to 6 nm. As non-limiting examples, the distance between the two double barrier segments may be in the range of 10 nm to 50 nm, preferably 10 nm to 30 nm.

The inner edges of the electrode may be separated from each other by a distance in the range of 100 nm to 1 000 nm.

The electrodes may be formed by a bottom layer of Ni on top of which a layer of gold is formed. The nickel layer may, for example, have a thickness of 25nm and the gold top layer may have a thickness of 75 nm. The skilled person in the art realizes that other materials and thicknesses of the electrodes may be used.

The electrodes may be formed by gold.

The width of the electrodes may vary in the range of 200 nm to 2 μιτι.

The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

For example, the plasmonic element may be formed as a disk, rod, wire, ellipse, polygon, triangle, sphere, cube, star, hole in thin metal film, nanoshell, core-shell particle, nanorice or nanoring.

The plasmonic structure may comprise a semiconductor and/or a metal. The metal may be selected from a group consisting of Ag, Au, Cu, Al, Mg, Ni, Pd and Pt, or alloys comprising at least one metal selected from the group.

The nanowire may be formed by a ll-VI, a IV or a lll-V semiconductor material, or combinations thereof.

The waveguides discussed above may be arranged vertically or horizontally in relation to a surface such as a substrate.

The long axis of the waveguide may be arranged at an angle to a surface such as the substrate. The angle may, for example, be in the range of 5 to 45 degrees.

The waveguides may be etched out mesa structures.

A plurality of waveguides may further be arranged to form an array of waveguides. A waveguide array, such as an array of nanowires, constitutes a two dimensional photonic crystal where multiple optical modes of the array contribute simultaneously to the optical response, including the absorption. In other words, the collective electromagnetic response to the illumination of the nanowires generates waveguide charge generating regions within the wires. The absorption in a nanowire array further show strong spatial dependence, both in the axial and radial direction within the nanowires. For example, the absorption in the top part of the nanowires tends to be much stronger than in the bottom part. Thus, a waveguide charge generating region may be formed in an upper part of a waveguide. The location of the waveguide charge generating region dependes further on the wavelength of the light illuminating the nanowire array. The skilled person in the art realizes that the absorption in the nanowire array depends critically on the spacing between the nanowires, the diameter of the nanowires etc.

Theoretical modeling, not shown, illustrate that it is possible to design nanowire diameter and array pitch such that a waveguide charge generating region is present within the top 300 - 3000 nm of the nanowires, where a charge carrier separator may be placed. Alternatively or in combination plasmonic structures may be used to provide plasmonic charge generating regions within waveguides of the array.

The array offers parallel operation of many waveguides and thus a photodetector having higher sensitivity may be achieved. Moreover, the array may offer an additional way of controlling the position of the waveguide charge generating region relative to the charge carrier separator through the collective photonic behavior of the array.

Figure 13 illustrates a schematic side view of a photodetector 300 comprising an array 302 of waveguides 102. The waveguides 102 are arranged vertically with respect to a surface 304 such as surface of a substrate.

The waveguides 102 may be nanowires grown epitaxially on top of a substrate or transferred to the substrate.

The array of waveguides 302 constitutes a two dimensional photonic crystal where multiple optical modes of the array may contribute

simultaneously to the optical response of the array 302. Thus, a collective electromagnetic response to illumination, e.g. of light having a wavelength λ in the infrared, visible or ultraviolet part of the light spectrum, generates waveguide charge generating regions 306 within the waveguides 102. The absorption in the top part, i.e. the first portion 104 of the waveguide 102 may be calculated, not shown, to be much stronger than in the bottom part, i.e. the second portion 106, of the waveguides 102. Thus, the waveguide charge generating regions 306 may thereby be formed efficiently in the upper parts, i.e. the first portions 104, of the waveguides 102. By arranging the charge carrier separator 1 12 in the upper part of the waveguide 102 charge carries photogenerated in the waveguide charge generating region 306 may efficiently reach the charge carrier separator 102 such that charge carriers of the first and second type are efficiently separated. An improved photocurrent is thereby achieved.

The plurality of waveguides 102 further increases the absorption of light and a larger photocurrent may be produced. A photodetector with increased sensitivity may thereby be provided.

The photodetector 300 may further comprise a plasmonic structure 308. The plasmonic structure 308 is arranged to upon the illumination generate surface plasmons thereby forming a plasmon charge generating region, not shown, within the first portions 104 of the waveguides 102. The plasmonic structure 308 increases the electric field within the waveguide 102 as described above. As a result an increased amount of charge carriers may be generated in the waveguide upon illumination and a larger photocurrent is provided.

The plasmon charge generating region may overlap with the waveguide charge generating region 306. The plasmon charge generating region and the waveguide charge generating region 306 may thereby together form a region with enhanced electric field. A more efficient production of free charge carriers is thereby provided.

The plasmonic structure may be formed by an electrode, such as the first electrode described above. The substrate may form the second electrode.

The plasmonic structure may comprise a plasmonic element or plasmonic elements as describe above. In the above description of the array 302 the plasmon charge generating region and the waveguide charge generating 306 region have been described to be formed in the first portion of the waveguide 102. It should, however, be noted that in accordance with the above description relating to single waveguides, the plasmon charge generating region and the waveguide charge generating region may be formed in the second portion of the waveguide.

To this end, the location of the plasmon charge generating region and the waveguide charge generating region may depend on the wavelength of the light illuminating the waveguide.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

Claims

1 . A photodetector comprising:

a photo-absorbing semiconducting optical waveguide arranged to upon illumination produce photo-generated charge carriers, the waveguide comprising:

a charge carrier separator separating a first and a second portion of the waveguide, the charge carrier separator is configured to pass charge carriers of a first type and to block charge carriers of a second type, the second type being different from the first type;

a first electrode electrically connected to the waveguide at the first portion thereof, and;

a second electrode electrically connected to the waveguide at the second portion thereof, such that an electrical potential difference is formed between the first and the second electrodes upon the illumination,

wherein the waveguide is arranged to upon illumination form a waveguide charge generating region within the first or within the second portion of the waveguide, and

wherein the charge carrier separator comprises a separator material having a bandgap larger than a first material forming the first portion of the waveguide and a bandgap larger than a second material forming the second portion of the waveguide.

2. The photodetector according to claim 1 , wherein the photodetector further comprises a plasmonic structure arranged to upon the illumination generate surface plasmons thereby forming a plasmon charge generating region within the first or within the second portion of the waveguide.

3. A photodetector comprising:

a photo-absorbing semiconducting optical waveguide arranged to upon illumination produce photo-generated charge carriers, the waveguide comprising: a charge carrier separator separating a first and a second portion of the waveguide, the charge carrier separator is configured to pass charge carriers of a first type and to block charge carriers of a second type, the second type being different from the first type;

a plasmonic structure arranged to upon the illumination generate surface plasmons thereby forming a plasmon charge generating region within the first or within the second portion of the waveguide, and

4. The photodetector according to claim 3, wherein the waveguide is further arranged to upon illumination form a waveguide charge generating region within the first or within the second portion of the waveguide.

5. The photodiode according to claim 2 or 3, wherein the plasmonic structure is arranged to upon illumination with a first wavelength generate surface plasmons thereby forming a first plasmon charge generating region within the first portion of the waveguide, and

wherein the plasmonic structure is arranged to upon illumination with a second wavelength, the second wavelength being different from the first wavelength, generate surface plasmons thereby forming a second plasmon charge generating region within the second portion of the waveguide.

6. The photodetector according to claim 2 or 3, wherein the first electrode and/or the second electrode form the plasmonic structure.

7. The photodetector according to any one of the claims 1 - 6, wherein the first electrode and the second electrode forms a dipole antenna.

8. The photodetector according to claim 7, wherein the first electrode and the second electrode are arranged perpendicular to a long axis of the waveguide.

9. The photodetector according to claim 7, wherein the first electrode and the second electrode are arranged parallel to a long axis of the waveguide.

10. The photodetector according to any one of the claims 1 - 9, further comprising a gate electrode capacitively coupled to the waveguide.

1 1 . The photodetector according to any one of the claims 2 - 10, wherein the plasmonic structure comprises or is formed by a plasmonic element, the plasmonic element exhibiting a localized surface plasmon resonance condition upon resonant excitation, thereby forming the plasmon charge generating region.

12. The photodetector according to any one of the claims 1 - 1 1 , wherein the first and the second material are formed by the same material.

13. The photodetector according to any one of the claims 1 or 3, wherein the waveguide charge generating region and/or the plasmon charge generating region is arranged within a diffusion length of the first type of charge carries from the charge carrier separator.

14. A method for producing a photocurrent, the method comprising:

illuminating a photodetector according to any one of the claims

1- 13,

measuring a current direction of the photocurrent generated by the photodetector.