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Timestamp: 2020-04-01 14:49:30
Document Index: 944216

Matched Legal Cases: ['art.\n26', 'art 23', 'art 23', 'art 23', 'art 23', 'art 23', 'art 23', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10', 'art 10']

Fabricating a Set of Semiconducting Nanowires, and Electric Device Comprising a Set of Nanowires - Bakkers, Erik Petrus Antonius Maria
United States Patent Application 20080224115
The method of fabricating a set of semiconducting nanowires (10) having a desired wire diameter (d) comprises the steps of providing a set of pre-fabricated semiconducting nanowires (10′), at least one pre-fabricated semiconducting nanowire having a wire diameter (d′) larger than the desired wire diameter (d), and reducing the wire diameter of the at least one pre-fabricated nanowire (10′) by etching, the etching being induced by light which is absorbed by the at least one pre-fabricated nanowire (10′), a spectrum of the light being chosen such that the absorption of the at least one pre-fabricated nanowire being significantly reduced when the at least one pre-fabricated nanowire reaches the desired wire diameter (d). The electric device (100) may comprise a set of nanowires (10) having the desired wire diameter (d). The apparatus (29) may be used to execute the method according to the invention.
Bakkers, Erik Petrus Antonius Maria (Eindhoven, NL)
Feiner, Louis Felix (Eindhoven, NL)
Balkenende, Abraham Rudolf (Eindhoven, NL)
10/584037
156/345.5, 257/E21.214, 257/E29.001, 438/733
H01L29/00; C01B31/02; C23F1/00; G01N33/543; H01L21/302
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1. A method of fabricating a set of semiconducting nanowires having a desired wire diameter the method comprising the steps of: providing a set of pre-fabricated semiconducting nanowires at least one pre-fabricated semiconducting nanowire having a wire diameter larger than the desired wire diameter and reducing the wire diameter of the at least one pre-fabricated nanowire by etching, the etching being induced by electromagnetic radiation which is absorbed by the at least one pre-fabricated nanowire a minimum wavelength of the electromagnetic radiation being chosen such that the absorption of the at least one pre-fabricated nanowire being significantly reduced when the at least one pre-fabricated nanowire reaches the desired wire diameter.
2. A method as claimed in claim 1, wherein: a radiation source is used which emits the electromagnetic radiation inducing the etching and electromagnetic radiation having a wavelength shorter than the minimum wavelength, and the electromagnetic radiation emitted by the radiation source is spectrally filtered for substantially reducing electromagnetic radiation having a wavelength shorter than the minimum wavelength.
3. A method as claimed in claim 1, wherein prior to the step of reducing the wire diameter substantially all the pre-fabricated semiconducting nanowires have a diameter larger than or equal to the desired wire diameter
16. A method as claimed in claim 10, wherein the step of providing the pre-fabricated semiconducting nanowires comprises the following sub-steps: providing the substrate a surface of the substrate being etchable, and growing semiconducting nanowires on the surface of the substrate, the grown semiconducting nanowires being the pre-fabricated semiconducting nanowires, and after the step of providing the pre-fabricated semiconducting nanowires and prior to the step of reducing the wire diameter of the at least one pre-fabricated nanowire by etching the exposed surface of the substrate is covered by an etch resistant layer.
17. A method as claimed in claim 10, wherein the pre-fabricated semiconducting nanowires are distributed over the surface a first part of the surface being irradiated by light for inducing the etch treatment, pre-fabricated semiconducting nanowires in a second part of the surface being prevented from etching.
20. A method of manufacturing an electric device comprising a set of nanowires having a desired wire diameter each nanowire of the set being electrically connected to a first conductor and to a second conductor the method comprising the steps of: fabricating the set of semiconducting nanowires having the desired wire diameter according to claim 1, and electrically contacting the nanowires of the set to a first conductor and to a second conductor
21. An electric device comprising a set of semiconducting nanowires the set comprising a first subset of nanowires each having a first wire diameter and a second subset of nanowires each having a second wire diameter Different from the first wire diameter the nanowires of the first subset being attached to a first part of a substrate, the nanowires of the second subset being attached to a second part of the substrate free from the first part.
26. An apparatus for light induced etching of nanowires comprising: a light source for emitting light inducing the etching of the nanowires and a monitor unit for monitoring a light signal emitted by the nanowires during the etching, the light signal being indicative for the wire diameter of the nanowires
27. An apparatus as claimed in claim 26, further comprising a system control unit 36 for controlling the light source in dependence of the light signal monitored by the monitor unit.
29. An apparatus as claimed in claim 26, further comprising an optical element for rotating a polarization of the light inducing the etching.
In order to reduce the wire diameter of the at least one pre-fabricated semiconducting nanowire having a wire diameter larger than the desired wire diameter, the set of pre-fabricated semiconducting nanowires is subjected to an etch treatment induced by electromagnetic radiation. The etch treatment induced by electromagnetic radiation, which is known, e.g., from U.S. Pat. No. 4,518,456, is a method in which a semiconducting object to be etched is placed in an, e.g. aqueous, solution of, e.g., H3PO4 or HCl. While the object is in contact with the solution, the parts of the object to be etched are illuminated by electromagnetic radiation. The electromagnetic radiation may be visible or invisible to the human eye and is referred to in the remainder of this application simply as “light”. The light is at least partly absorbed by the object to be etched, thereby generating electrons and holes. These light generated charge carriers, i.e. the electrons and/or the holes, then diffuse and induce chemical reactions at the interface between the object and the solution. In the course of these chemical reactions, which are in the art also referred to as photo etching, atoms of the nanowire may be ionized and dissolved in the solution. The ionization of these atoms may be induced by the light generated charge carriers such as e.g. the holes. The process of dissolving the ions thus generated may involve the combination of these ions with ions in the solution. These latter ions may be induced by the light generated charge carriers such as e.g. the electrons. For InP in a Fluorine comprising solution six holes may form In3+ and P3+ ions out of InP. These positive ions may combine with negative Fluorine ions F− that may be formed by a reaction F2+2 electrons resulting in 2 F−. Similar processes known in the art may be used for other nanowire compositions.
In this application the term “semiconducting” describes the class of materials in which electron hole pairs may be generated by light to induce etching, e.g. in the way described above. If not stated differently, in the remainder of the application the term “nanowire” implies a semiconducting nanowire.
From the article “Etching of colloidal InP nanocrystals with fluorides: photochemical nature of the process resulting in high photoluminescence efficiency” by D. Talapin et al., Journal of Physical Chemistry B, 2002, volume 106, page 12659-12663, it is known that nanodots having a size of 5.2 nm or less can be etched. According to this article the etching is induced by light that is absorbed by the nanodots. The spectrum of the light is chosen such that the absorption of the nanodots is significantly reduced when the nanodots reach the desired size.
In this application the term “nanowire” describes both nanowires with a solid core and nanowires with a hollow core. The latter are also referred to in the art as nanotubes. Also in the latter type of nanowires charge carriers such as electrons and holes are confined perpendicular to the longitudinal axis, i.e. in a radial direction, due to the relatively small dimensions perpendicular to the longitudinal axis. As a consequence the charge carriers have discrete quantum mechanical energy levels, which are determined mainly by the thickness of the core defining this type of nanowire. Due to the relatively large dimension along the longitudinal axis, the charge carriers are not confined in discrete quantum mechanical energy levels as function of the wire length, analogous to the nanowires having a solid core. When the nanowire has a hollow core, the wire diameter refers to the thickness of the core. The thickness of the core is the difference between the outer wire diameter and the inner wire diameter, i.e. the diameter of the hollow part.
In an embodiment a radiation source is used which emits the electromagnetic radiation inducing the etching and in addition to this also electromagnetic radiation having a wavelength shorter than the minimum wavelength. The electromagnetic radiation emitted by the radiation source is spectrally filtered for substantially reducing electromagnetic radiation having a wavelength shorter than the minimum wavelength. This latter electromagnetic radiation having a wavelength shorter than the minimum wavelength is able to induce etching of the pre-fabricated semiconducting nanowires having the desired wire diameter, i.e. it has a wavelength which is shorter than the wavelength at which the etching process terminates at the desired wire diameter. Prior to directing the electromagnetic radiation onto the pre-fabricated nanowires, the electromagnetic radiation emitted by the radiation source is spectrally filtered for substantially reducing electromagnetic radiation having a wavelength shorter than the minimum wavelength. In this way etching of pre-fabricated semiconducting nanowires having the desired wire diameter is substantially reduced and preferably effectively prevented. In this application the term “light source” is used as a synonym for the term “radiation source”. The term “light source” is not limited to radiation sources which emit visible electromagnetic radiation but may include radiation sources which emit electromagnetic radiation invisible to the human eye.
The target material is vaporized and transported over the substrate 20. This results in the growth of nanowires 10 under the catalysis of the nanoparticles formed out of the metal film. InP nanowires are grown when the substrate temperature is in the range 450-500° C. The higher the temperature, the larger is the wire diameter of the nanowires grown. At a temperature above 500° C. InP nanotubes, i.e. a nanowire with a hollow core, may be formed. The pressure during growth is in the range 100-200 mbar and an argon flow between 100-300 sccm is applied. The length of the nanowires may be, e.g., 2-10 micron when 15000 laser pulses are applied. Shorter and longer nanowires may be obtained with less and more laser pulses, respectively. The resulting wire diameter is determined by the thickness of the metal film and by the substrate temperature during growth. Dopants may be added at a concentration of, e.g. 0.001-1.0 mol % to obtain n-type and/or p-type InP nanowires. The n-type dopants may comprise e.g. S, Se and Te, the p-type dopants may comprise e.g. Zn. The dopants may be added to the target illuminated by the excimer laser or they may be provided as a gas to the oven, independent from the illumination of the target. The resulting level of active dopants in the nanowire is 1017-1020 atoms/cm3. E.g. by shifting the laser beam to another target, e.g. selected from one of the targets described above, during the growth process, a junction may be built in the wire, i.e. a p-n junction and/or hetero-junction.
In an embodiment substantially all the pre-fabricated nanowires 10′ have a diameter d′ larger than or equal to the desired wire diameter d prior to the step of reducing the wire diameter. To this end the metal film used for forming the catalyst nanoparticles may be relatively thick such that substantially all pre-fabricated nanowires have a wire diameter larger than the desired wire diameter. After performing the etching treatment substantially all nanowires have the desired wire diameter d. The terms “substantially all the pre-fabricated nanowires” and “substantially all the nanowires” imply that the pre-fabrication of the nanowires 10′ is designed to produce nanowires having a diameter d′ larger than the desired diameter d. Due to incidental unwanted formation of one or a few small nanoparticles out of the metal film one or a few of the nanowires may have a wire diameter d′ which is accidentally smaller than the desired wire diameter d.
The light of the light source 30 may be focused on the substrate 20 having the pre-fabricated nanowires 10 by an objective 33. The power density of the light inducing the etching depends on the magnification of the objective used. The magnification may be e.g. between 50 and 1000×. The power density may be between 0.5 and 10 kW/cm2 at a wavelength of 457 nm. The polarization vector may be rotated by, e.g., a polarization rhomb. The maximum excitation polarization ratio obtained with an InP nanowire is 0.95. A blue shift and/or an intensity increase in the photoluminescence is typically observed after photo-etching 3-120 minutes. The maximum increase in emission intensity obtained is a factor 1300.
After the light induced etch treatment those nanowires 10a whose longitudinal axis is parallel to the axis 40 have the desired wire diameter d, whereas those nanowires 10b whose longitudinal axis is perpendicular to the axis 40 are etched substantially less efficient, i.e. after the etch treatment they have a wire diameter db which is substantially the same as prior to the etch treatment, see FIG. 4B. For nanowires whose longitudinal axis is neither parallel nor perpendicular to the axis 40, exemplary indicated by reference numerals 10c and 10d, the absorption efficiency is between these two extremes. In general the absorption efficiency scales with a trigonometric function of the angle between the longitudinal axis of the nanowire 10 and the axis 40. As a result the wire diameter of these nanowires with the intermediate position is reduced during the etching, cf. the initial wire diameters dc′ and dd′ versus the wire diameters dc and dd in FIG. 4B. The reduction in the wire diameter depends on the orientation of the longitudinal axis with respect to the axis 40. The light induced etching may be stopped when the nanowires 10a parallel to the axis 40 have the desired wire diameter d. The time instant when to stop the etch treatment may be determined by monitoring a light signal indicative for the wire diameter. When this light signal comprises a component indicative for the desired wire diameter the etch treatment may be stopped.
When light inducing the etch treatment has a minimum wavelength λ chosen such that the light induced etching treatment is self terminated at the desired wire diameter d, the light induced etch treatment may be continued when reaching the state schematically depicted in FIG. 4B. Since the nanowires 10a parallel to the axis 40 have the desired wire diameter d, they do not absorb the light inducing the etching relatively effectively anymore. As a result they are etched substantially less efficient. Effectively they may be not etched at all. Since the nanowires 10b perpendicular to the axis 40 do not absorb the light inducing the etching relatively effectively either, they are etched substantially less efficient as well. Effectively they may be not etched at all. The nanowires 10c, 10d that have an intermediate orientation are etched relatively efficiently until they reach the desired wire diameter d at which the absorption of the light inducing the etching and thus the efficiency of the etching are largely reduced. The set of nanowires thus obtained is depicted schematically in FIG. 4C.
In addition to the linearly polarized light described above and also referred to as the first component, the set of randomly oriented pre-fabricated nanowires may be illuminated by a second component of light inducing an etch treatment. The second component may be linearly polarized along a second axis perpendicular to the first axis, e.g. parallel to the longitudinal axis of nanowire 10b shown in FIGS. 4A-4C. This second component may induce relatively effectively etching of nanowires 10b which were etched relatively ineffectively with the first component. The first component may have a first spectrum with a first minimum wavelength λ1 and the second component may have a second spectrum with a second minimum wavelength 2 different from the first minimum wavelength λ1. The first minimum wavelength λ1 and the second minimum wavelength λ2 may correspond to an energy of, e.g. 1.6 and 2.0 eV, respectively. The nanowires parallel to the second axis having a band gap smaller than, in this example, 2.0 eV absorb the second component and are thus etched until they have a band gap of, in this example, 2.0 eV. In this way nanowires perpendicular to the axis 40 may be etched effectively as well to a desired wire diameter which may be different from the desired wire diameter d determined by the first minimum wavelength λ1.
The set of pre-fabricated nanowires provided prior to the etch treatment shown in FIG. 5A comprises nanowires 10h which are substantially horizontal, nanowires 10v which are substantially vertical, and nanowires 10i which are intermediate, i.e. neither substantially horizontal nor substantially vertical. When such a set is illuminated by light with a relatively short wavelength such that the light is absorbed by the nanowires until they are falling apart, nanowires may be removed from the set.
In the example of FIGS. 5A and 5B the light is linearly polarized along the axis 40, i.e. it is vertically polarized. In this case the nanowires 10v substantially parallel to the axis 40 absorb the light relatively effectively and may be removed from the set whereas the nanowires 10h substantially perpendicular to the axis 40 absorb the light relatively ineffectively. As a consequence they are not removed from the set. Whether or not nanowires 10i which are intermediate, i.e. neither substantially horizontal nor substantially vertical, are removed from the set depends on the time duration of the illumination. When the illumination is terminated directly after removing the last substantially vertical nanowire 10v, the nanowires 10i may remain. When the illumination is continued, they may be removed as well. The longer the illumination is continued in this case, the better defined is the orientation of the remaining nanowires 10h.
In the example of FIGS. 5A and 5C the light is linearly polarized along the axis 41 which is perpendicular to axis 40, i.e. it is horizontally polarized. In this case the nanowires 10h substantially parallel to the axis 41 absorb the light relatively effectively and may be removed from the set whereas the nanowires 10v substantially perpendicular to the axis 41 absorb the light relatively ineffectively. As a consequence they are not removed from the set. Whether or not nanowires 10i which are intermediate, i.e. neither substantially horizontal nor substantially vertical, are removed from the set depends on the time duration of the illumination. When the illumination is terminated directly after removing the last substantially vertical nanowire 10h, the nanowires 10i may remain. When the illumination is continued, they may be removed as well. The longer the illumination is continued in this case, the better defined is the orientation of the remaining nanowires 10v.
When the pre-fabricated nanowires 10 are supported by a substrate 20, during the etch treatment the substrate 20 may have a surface 23 constituted by a part 23a supporting the pre-fabricated semiconducting nanowires 10 and another part 23b being free from the part 23a, at least the other part 23b. The substrate 20 may be homogeneous and entirely consist of a material which is etch resistant, such as e.g. Teflon. The substrate 20 may comprise a first layer 24 which is not etch resistant such as an e.g. native oxide layer on a silicon wafer, and a second layer 25 which is etch resistant, the second layer 25, shown in FIG. 6, constituting the other part 23b of the surface 23. The second layer 25 may be connected to the first layer 24 by chemical bonds which results in a relatively strong interconnection between these two layers and consequently in a relatively efficient protection of the first layer 24. The second layer 25 may be composed of one or more materials selected from alkyltriethoxysiloxane and alkyltrimethoxysiloxane such as e.g. aminopropyltrietoxysiloxane (APTES). The alkyl may be propyl (C3), butyl (C4), pentyl (C5) up to C12. The amino-group may be replaced by a mercapto- or carboxyl-group.
In this embodiment the step of providing the pre-fabricated semiconducting nanowires comprises the sub-steps of providing the substrate 20 which may have a first layer 24. At least a part of the substrate 20 is not etch resistant. The semiconducting nanowires 10 are grown on a surface 23a of the substrate 20. The semiconducting nanowires thus grown are the pre-fabricated semiconducting nanowires 10. After the step of providing the pre-fabricated semiconducting nanowires 10 and prior to the step of reducing the wire diameter of the at least one pre-fabricated nanowire 10 by etching as described e.g. above, the part 23b of surface 23 of the substrate 20 is covered by an etch resistant layer 25.
The electric device 100 may comprise a set of nanowires 10, the set comprising a first subset of nanowires 10a each having a first wire diameter da and a second subset of nanowires 10b each having a second wire diameter db different from the first wire diameter da. The nanowires 10a of the first subset may be attached to a first part of the substrate 20, which in the example of FIGS. 8A-12B is constituted by the first conductor 110a. The nanowires 10b of the second subset may be attached to a second part of the substrate 20, which in the example of FIGS. 8A-12B is constituted by the first conductor 110b and which is free from the first part.
The nanowires 10a of the first subset may be electrically connected to a conductor, which in the example of FIGS. 8A-12B is constituted by the first conductor 110a. The nanowires 10b of the second subset may be electrically connected to a further conductor, which in the example of FIGS. 8A-12B is constituted by the first conductor 110b and which may be electrically insulated from the further conductor.
In a first step a substrate 20, which may be a silicon wafer, is provided with isolation zones 102 which may be shallow trench insulation (STI) regions shown in FIGS. 8A and 8B, and with a first conductor 110 for electrically contacting the nanowires 10 to be formed later on. The first electrical conductor 110 may be formed by doping regions of the substrate outside the STI regions. Alternatively or in addition, a conductor may be deposited for forming the first conductor 110. The substrate 20 may be an insulator such as a quartz substrate. In this case the isolation zones 102 are not required. In the embodiment of FIGS. 8A-12B three parallel, mutually insulated first conductors 110 are provided. However, the invention is not limited to three mutually insulated first conductors 110. Alternatively, the first conductor 110 may be electrically conductively be connected to all nanowires 10 of the set, or it may comprise N mutually insulated electrically conductors where N is an integer larger than one. Here and in the remainder of the application the term “mutually electrically insulated” implies that the conductors are not directly electrically connected. It does not exclude that the conductors are electrically connected indirectly, i.e. via one or more additional elements such as e.g. the nanowires 10 and/or the second conductors 120. The substrate 20 may be transparent to visible light.
For inducing the etch treatment of the nanowires 10a attached to the first conductor 110a shown in FIGS. 10A and 10B light having a first minimum wavelength may be used resulting in a desired wire diameter da. During this etch treatment of the nanowires 10a attached to the first conductor 110a, etching of the nanowires 10b and 10c attached to the first conductor 110b and 110c, respectively, may be prevented, e.g. by using a mask. Subsequently, the nanowires 10c attached to the first conductor 110c shown in FIGS. 10A and 10B may be etched by light induced etching using light having a second minimum wavelength resulting in a desired wire diameter dc. During this etch treatment of the nanowires 10c attached to the first conductor 110c, etching of the nanowires 10a and 10b attached to the first conductor 110a and 110b, respectively, may be prevented, e.g. by using a mask. If required, the nanowires 10b attached to the first conductor 110b may be etched as well to obtain a desired wire diameter db. The etching of the nanowires 10a, 10b, if relevant, and 10c may be self terminating or may be terminated in dependence of a light signal indicative of the wire diameter.
In this method a set of nanowires 10a, 10b, 10c is obtained which consists of a three subsets of nanowires, each subset having a wire diameter which is different from the wire diameter of the nanowires of the other two subsets. Each subset is connected to a particular first conductor 110a, 110b, 110c.
Subsequently, the pre-fabricated electric device 100 shown in FIGS. 10A and 10B may be provided with a, preferably transparent, dielectric layer 130 such as e.g. a spin on glass (SOG), shown in FIGS. 11A and 11B. The upper surface of the pre-fabricated electric device 100 thus obtained may be provided with a second conductor 120 for electrically contacting the upper end portion of the nanowires 10.
The upper end portion of the nanowires 10a, 10b, 10c may be electrically connected to second conductors 120a, 120b, 120c shown in FIGS. 12A and 12B, which are mutually electrically insulated. The first conductors 110a, 110b and 110c and the second conductors 120a, 120b and 120c are mutually perpendicular and form, in this example, a three by three array. In the embodiment of FIGS. 8A-12B one nanoparticle 111 and thus one nanowire 10 is provided at each intersection area defined by the first conductors 110 and the second conductors 120 which define an, in this example rectangular three by three, array. The invention is not limited to an array of this shape or size. The invention is not limited to just one nanoparticle 111 and one nanowire 10 per intersection area. Instead some or all intersection areas may have more than one nanoparticle 111 and one nanowire 10.
The second conductor 120a, 120b, 120c may be at least partly transparent to visible light. They may be composed e.g. of indium tin oxide (ITO). The first conductor 110 and/or the second conductor 120 may be composed of zinc or a zinc alloy.
The nanowires 10a, 10b, 10c may each comprise a p-doped part 10p and a n-doped part 10p forming a p-n-junction, shown in FIG. 13. When sending an electrical current from the first conductor 110 through the nanowire 10 to the second conductor 120, electrons and holes are injected from the respective n-doped part 10 n and p-doped part 10p. When these charge carriers recombine, light is emitted. The light is emitted mainly in the p-doped part 10p close to the p-n junction due to the higher mobility of the electrons as compared to the holes.
In the electric device 100 shown in FIGS. 12A and 12B, the nanowires 10a, 10b and 10c may each comprise a p-n junction. The wavelength of the light emitted by the above described recombination of the holes and electrons depends on the bandgap and hence on the wire diameter at the location of the recombination. In the embodiment of FIGS. 12A and 12B the nanowires 10a, 10b and 10c may have different wire diameters da, db and dc. as a consequence they may emit light of different wavelength. The nanowires may be composed of InP with the n-doped part 10n being doped by e.g. S, Se and/or Te, and the p-doped part 10p being doped by e.g. Zn or Cd. The concentration of the dopants may be e.g. between 1017-1020 cm−3.
The p-n junction may serve as a selection device, i.e. a pixel of the array formed by the first conductors 110 and the second conductors 120 may be selected by biasing the respective first conductor 110 and the second conductor 120. The nanowire 10bb located at the intersection of first conductor 110b and the second conductor 120b may be selected by biasing these two conductors. At the intersection more than one nanowire 10bb may be located and selected.
In the embodiment shown in FIG. 13 the n-doped part 10n is electrically connected to the first conductor 110 having a first distance ln to the p-n junction. The p-doped part 10p is electrically connected to the second conductor 120 having a second distance 1p to the p-n junction which is smaller than the first distance ln. The n-doped part 10n has a wire diameter dn which is larger than a wire diameter dp of the p-doped part 10p. Due to the presence of the p-n junction the electron-hole pairs generated by the absorption of the light inducing the etching are separated such that the electrons flow to the n-doped part 10n and the holes flow to the p-doped part 10p. The holes are mainly responsible for the light induced etching. The higher hole concentration in the p-doped part 10p results in a more efficient etching and thus in a relatively small wire diameter dp. As a result, the nanowire may have two regions, the n-doped part 10n and the p-doped part 10p, having different diameters dn and dp, respectively. The n-doped region may have a diameter which may be similar to the wire diameter prior to etching. The wire diameter dp of the p-doped part 10p may be predetermined by the minimum wavelength of the light used for inducing the etching. The light signal indicative for the wire diameter may be observed when etching nanowires 10 having a n-doped part 10n and a p-doped part 10p. The light emitted due to recombination of electrons and holes in the p-doped part 10p is indicative for the wire diameter dp of this part. Once the light signal indicates that the desired wire diameter dp is reached, the light inducing the etching may be blocked to prevent any further etching of the n-doped part which may result in an unwanted further reduction of the wire diameter dn of the n-doped part 10n.
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