Abstract:
Nano-engineered structures are disclosed, incorporating nanowhiskers of high mobility conductivity and incorporating pn junctions. In one embodiment, a nanowhisker of a first semiconducting material has a first band gap, and an enclosure comprising at least one second material with a second band gap encloses said nanoelement along at least part of its length, the second material being doped to provide opposite conductivity type charge carriers in respective first and second regions along the length of the of the nanowhisker, whereby to create in the nanowhisker by transfer of charge carriers into the nanowhisker, corresponding first and second regions of opposite conductivity type charge carriers with a region depleted of free carriers therebetween. The doping of the enclosure material may be degenerate so as to create within the nanowhisker adjacent segments having very heavy modulation doping of opposite conductivity type analogous to the heavily doped regions of an Esaki diode. In another embodiment, a nanowhisker is surrounded by polymer material containing dopant material. A step of rapid thermal annealing causes the dopant material to diffuse into the nanowhisker. In a further embodiment, a nanowhisker has a heterojunction between two different intrinsic materials, and Fermi level pinning creates a pn junction at the interface without doping.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the priority of U.S. Provisional Patent Application No. 60/459,990 filed Apr. 4, 2003, the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to structures and devices produced by techniques of nanotechnology. 
     More specifically, the invention relates to such structures and devices incorporating at least one element, essentially in one-dimensional form, and that is of nanometer dimensions in its width or diameter, and that preferably is produced by the so-called Vapor-Liquid-Solid (VLS) mechanism. For the purposes of this specification, such element will be termed a “nanowhisker”. 
     2. Brief Description of the Prior Art 
     Nanotechnology covers various fields, including that of nanoengineering, which may be regarded as the practice of engineering on the nanoscale. This may result in structures ranging in size from small devices of atomic dimensions, to much larger scale structures for example on the microscopic scale. Commonly, such structures include nanostructures. In certain contexts nanostructures are considered to be those having at least two dimensions not greater than about 100 nm, with some authors using the term to identify structures having at least two dimensions not greater than about 200 nm. Nevertheless, some procedures for fabricating such small structures are useful for structures having at least two dimensions somewhat greater, e.g., structures having at least two dimensions not greater than about 1 micrometer (μm). Ordinarily, layered structures or stock materials having one or more layers with a thickness less than 1 μm are not considered to be nanostructures. Thus, although the term “nanostructure” is more classically considered to refer to structures having at least two dimensions not greater than about 100 nm, in the following discussion, the term “nanostructure”, “nanowhisker”, or “nanoelement” is intended to include a structure having at least two dimensions not greater than about 1 μm. 
     Nanostructures include so-called one-dimensional nanoelements, essentially in one-dimensional form, that are of nanometer dimensions in their width or diameter, and that are commonly known as nanowhiskers, nanorods, nanowires, nanotubes, etc. 
     As regards nanowhiskers, the basic process of whisker formation on substrates, by the so-called VLS (vapor-liquid-solid) mechanism, is well known. A particle of a catalytic material, usually gold, for example, on a substrate is heated in the presence of certain gases to form a melt. A pillar forms under the melt, and the melt rises up on top of the pillar. The result is a whisker of a desired material with the solidified particle melt positioned on top. (See E.I Givargizov,  Current Topics in Materials Science , Vol. 1, pages 79-145, North Holland Publishing Company, 1978.) The dimensions of such whiskers were in the micrometer range. 
     Although the growth of nanowhiskers catalyzed by the presence of a catalytic particle at the tip of the growing whisker has conventionally been referred to as the VLS (Vapor-Liquid-Solid) process, it has come to be recognized that the catalytic particle may not have to be in the liquid state to function as an effective catalyst for whisker growth. At least some evidence suggests that material for forming the whisker can reach the particle-whisker interface and contribute to the growing whisker even if the catalytic particle is at a temperature below its melting point and presumably in the solid state. Under such conditions, the growth material, e.g., atoms that are added to the tip of the whisker as it grows, may be able to diffuse through a the body of a solid catalytic particle or may even diffuse along the surface of the solid catalytic particle to the growing tip of the whisker at the growing temperature. Evidently, the overall effect is the same, i.e., elongation of the whisker catalyzed by the catalytic particle, whatever the exact mechanism may be under particular circumstances of temperature, catalytic particle composition, intended composition of the whisker, or other conditions relevant to whisker growth. For purposes of this application, the term “VLS process”, “VLS method”, or “VLS mechanism”, or equivalent terminology, is intended to include all such catalyzed procedures wherein nanowhisker growth is catalyzed by a particle, liquid or solid, in contact with the growing tip of the nanowhisker. 
     International Application Publication No. WO 01/84238 discloses in  FIGS. 15 and 16  a method of forming nanowhiskers wherein nanometer sized particles from an aerosol are deposited on a substrate and these particles are used as seeds to create filaments or nanowhiskers. 
     For the purposes of this specification the term nanowhiskers is intended to mean “one-dimensional” nanoelements with a width or diameter (or, generally, a cross-dimension) of nanometer size, the elements having been formed by the so-called VLS mechanism. Nanowhiskers are also referred to in the art as “nanowires” or, in context, simply as “wires”, and such terminology, as used in this application, is equivalent to the term “nanowhiskers”. 
     Several experimental studies on the growth of nanowhiskers have been made, including those reported by Hiruma et al. They grew III-V nano-whiskers on III-V substrates in a metal organic chemical vapor deposition (MOCVD) growth system. (See K. Hiruma, et al.,  J. Appl. Phys.  74, page 3162 (1993); K. Hiruma, et al.,  J. Appl. Phys.  77, page 447 (1995); K. Hiruma, et al.,  IEICE Trans. Electron . E77C, page 1420 (1994); K. Hiruma, et al.,  J. Crystal Growth  163, pages 226-231 (1996)). 
     Hiruma et al. have made pn junctions within nanowhiskers by doping GaAs whiskers with Si using Si 2 H 6 , during the growth process, and switching the dopant to an opposite conductivity type (carbon) during growth: K. Hiruma et al.,  J. Appl. Phys.  77(2), 15 Jan. 1995 p.447, see pages 459-461; and K. Hiruma et al  J. Appl. Phys.  75(8) 4220 (1994). In general, there are problems in that the definition of the junction within the nanowire is not good enough for electrical components, and in that the presence of dopant ions within the crystal creates crystal imperfections and reduces carrier mobility. 
     In another approach by Lieber et al, WO-A-03/005450, nanowires were produced, different wires being doped with opposite conductivity type dopants, and two wires of opposite conductivity type were physically crossed, one on top of the other, so that a pn junction was formed at their point of contact. A difficulty with this approach is the extra step required of physically positioning the nanowires. 
     In planar semiconductor processing, various doping techniques are known. One technique that is valuable with heterojunctions is known as modulation doping. In this technique, carriers from a doped layer of, e.g., AlGaAs, diffuse across an interface with an undoped material, e.g., GaAs, and form a very thin layer of carriers of very high mobility, within a potential well, next to the interface—see for example FIG. 1 of WO 02/19436. 
     U.S. Pat. No. 5,362,972 discloses an FET wherein the current flowpath between source and drain is composed of GaAs nanowhiskers. The nanowhiskers are surrounded by n-doped AlGaAs, to create by modulation doping a one-dimensional electronic gas within each nanowhisker. 
     WO 02/020820 discloses a modulation doping technique in Coaxial Heterostructure Nanowires, wherein dopants in an outer coaxial layer donate free carriers to an inner nanowire. 
     In other techniques, doping of a semiconductor region in a planar semiconductor device occurs by diffusion of ions from an adjacent region of polymer; see Guk et al.,  Semiconductors  Vol. 33(3), pp. 265-275, March 1999. 
     In co-pending U.S. patent application Ser. No. 10/613,071, filed on Jul. 7, 2003, in the names of Samuelson and Ohlsson, the contents of which are incorporated herein by reference, a process was disclosed for producing nanowhiskers, and structures were disclosed incorporating nanowhiskers. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide new and improved nano-engineered structures incorporating nanowhiskers and other one-dimensional nanoelements, the nanoelements having improved conductivity characteristics. 
     It is a further object of the invention to provide new and improved nano-engineered structures incorporating nanowhiskers and other one-dimensional nanoelements, wherein the nanoelements contain improved pn junctions. 
     In at least a preferred embodiment of the invention, nanowhiskers or other one-dimensional nanoelements are grown as pure crystals without doping. The nanowhisker is then enclosed in an enclosure comprising a surrounding layer or matrix of a further different material that will usually be a semiconductor material. Dopant ions are incorporated into this further material, by an appropriate process during or after its deposition. Carriers liberated in the further material transfer into the nanowhisker. The band structures of the nanoelement and the further material ensure that it is energetically favorable for the carriers to diffuse into the nanoelement; this is effectively by the process known as modulation doping wherein a potential well is defined within the nanowhisker. Thus effectively the nanowhisker is doped with carriers, but that these are of high mobility, since the absence of dopant ions within the nanowhisker ensures that the crystalline structure is not deformed. 
     The preferred embodiment provides a method of producing a one-dimensional nanoelement of desired conductivity, the method comprising the steps of (1) forming a one-dimensional nanoelement of a first material, (2) surrounding the nanoelement with a second material, different from that of the nanoelement, the second material containing dopant material whereby charge carriers from the dopant material diffuse into the nanowhisker to create said desired conductivity. More specifically, the method comprises the steps of (1) forming by the VLS method a nanowhisker on a substrate, the nanowhisker including a first semiconducting material, and (2) forming a coaxial layer around the nanowhisker of a second semiconducting material, and (3) incorporating dopant material into the coaxial layer whereby charge carriers from the dopant material diffuse into the nanowhisker to create said desired conductivity. 
     The invention provides a means of creating a pn junction within a one-dimensional nanoelement by modulation doping. 
     Specifically the invention provides a nanoengineered structure including a one-dimensional nanoelement of a first semiconducting material having a first bandgap, an enclosure comprising at least one second material having a second bandgap enclosing said nanoelement along at least part of its length, and said second material being doped to provide opposite conductivity type charge carriers in respective first and second regions along the length of the of the nanowhisker, whereby to create by transfer of charge carriers into said nanoelement, corresponding first and second regions of opposite conductivity type charge carriers with a pn junction therebetween in said nanoelement, and wherein the bandgaps are such that it is energetically favorable for the charge carriers to remain in said nanoelement. 
     The enclosure for the nanoelement may be a coaxial jacket. In one preferred form a thin nanowhisker of GaAs is grown, then the growth conditions are changed from those appropriate for catalytic growth to those appropriate for bulk growth, so that a coaxial jacket is formed around the sides of the nanowhisker. The material may be AlGaAs. It is necessary to dope the lower part of the AlGaAs jacket with one conductivity type dopant material, and the upper parts of the coaxial jacket with opposite conductivity dopant ions. One exemplary technique for achieving this is to embed the coaxial jacket within a polymer matrix comprising upper and lower layers as, for example, spin on glass or polymer substances. The lower layer has one conductivity type dopant material, and the upper layer has the opposite conductivity type dopant material. Rapid thermal annealing causes diffusion of the dopant material into the coaxial jacket. The thermal annealing step is stopped before appreciable diffusion into the one-dimensional nanoelement. The presence of the dopant ions within the coaxial jacket creates modulation doping within the nanowhisker, and a pn junction between the two regions of opposite conductivity type material. The space charge within each region is maintained within the nanoelement, and the depletion region of the pn junction may be sharp or as diffuse as desired (typically within the range 50 nm to 1 μm). The diameter of the nanowhisker is preferably small, about 20 nm. The coaxial jacket may be as small as 10 nm thick, but it may in other cases be preferable to have a jacket that is 200 nm thick or one that even fills the volume between the nanowires completely in an array of nanowires. 
     The materials of the nanowire and jacket may be GaAs and AlGaAs, for example. Other material combinations could be InAs in the core section and AlSb in the surrounding material or a germanium core and a silicon jacket. 
     In a modification, the coaxial jacket is doped in one conductivity type during its formation. A layer of spin-on glass is then formed partway along the length of the nanowire, containing opposite type conductivity ions, that are sufficiently concentrated to reverse the conductivity type in the lower part of the coaxial jacket. In a further modification, the coaxial jacket is grown in an undoped condition, and a layer of spin-on glass is then formed partway along the length of the nanowire, containing one type conductivity ions. The structure is then exposed to a gas containing opposite type conductivity ions that diffuse into the upper part of the coaxial jacket, where they create a region of opposite conductivity type. The ions in the gas also diffuse into the layer of spin on glass, but not in a sufficient concentration to overcome the existing concentration of the one type conductivity. A pn junction is thereby formed in the nanowire. 
     In an alternative form, the one-dimensional nanoelement is encapsulated within an enclosure formed by first and second layers of polymer material or spin on glass, each layer having opposite conductivity dopant material. Direct charge transfer of the carriers from the polymer matrix creates modulation doping within the nanowhisker, and two separate regions of oppositely signed charge carriers with a pn junction between them. 
     In a further alternative form of the invention the doping is so heavy as to create degenerate doping within the nanoelement, that is to say the Fermi level exists, in one region, in the conduction band, and in the other region, in the valence band. In this state, the nanoelement comprises a tunnel diode or Esaki diode wherein in known manner, forward biasing of the junction creates a negative resistance caused by tunnelling between the valence and the conduction bands. 
     In a further aspect, a nanowhisker or other one-dimensional nanoelement is surrounded by a material containing dopant ions. For example the surrounding material may be a polymer material. By a process, for example a subsequent step of rapid thermal annealing, the dopant ions in the matrix material are permitted themselves to diffuse into the nanowhisker, to create a desired conductivity. This provides advantages over a direct doping into the nanoelement, by providing an extra degree of control over the doping process, and permitting the diffusion of certain dopants into the nanoelement that would not be possible by a more direct process. Although a polymer material is preferred, which is evaporated or spun onto a substrate so as to surround the nanowhisker, other materials may be employed, such as for example semiconductor material or dielectric material grown onto the substrate. The dopant material may be incorporated in the surrounding material before application to the substrate, during the application to the substrate, or as a subsequent step after the surrounding material is formed on the substrate. 
     Specifically, the invention provides a method of forming a one-dimensional nanoelement of a desired conductivity, comprising:
         (a) forming a one-dimensional nanoelement on a substrate, the nanoelement being formed of a first material;   (b) forming at least a first layer of a further material on the substrate and surrounding, at least partially, the nanoelement, the further material having a first conductivity type dopant material therein, and   (c) processing the further material so that said dopant material diffuses into the nanoelement, whereby to create a desired conductivity therein.       

     Additionally, the invention provides a nanoengineered structure, comprising a one-dimensional nanoelement, of a first material, disposed on a substrate, and at least a first layer of material formed on the substrate and surrounding, at least partially, the nanoelement, the first layer having a first conductivity type dopant material therein, said first conductivity type dopant material having diffused into the nanoelement, whereby to create a desired conductivity within the nanoelement. 
     In a further aspect, a pn junction is created within a one-dimensional nanoelement, preferably a nanowhisker. A nanowhisker is grown on a substrate, and embedded in a surrounding material. The material consists of first and second layers, formed on the substrate, one on top of the other, for example as polymer layers evaporated or spun onto the substrate. Alternatively the layers may be of some other material, for example dielectric material or semiconducting material grown on the substrate. The first layer extends partway up the nanowhisker, and has a first dopant material incorporated within it or subsequently injected, providing charge carriers of a first type. The second layer extends towards the top of the nanowhisker, and has a second dopant material contained within it or subsequently injected into it, providing charge carriers of an opposite conductivity type. The surrounding material is treated, as for example by rapid thermal annealing, so that the dopant ions themselves diffuse into the respective first and second regions of the nanoelement, to create an effective pn junction within the nanowhisker. In this case, the surrounding layers may be commercially available polymer layers evaporated or spun onto the substrate. The dopant materials are incorporated into the polymer materials before, during, or after the application of the polymer materials to the substrate. 
     In either case, the effective pn junction can be made as sharp as desired, approaching that of a few nanometers. More than one pn junction may be created by employing multiple layers, each layer having appropriate dopant material. 
     Specifically the invention provides a nanoengineered structure including a one-dimensional nanoelement with at least one pn junction therein, comprising a nanowhisker upstanding from a substrate, and a first layer of a material formed on the substrate and surrounding and extending partway up the nanowhisker, the first layer having a first conductivity type dopant material therein, and a second layer of material formed on top of the first layer and surrounding and extending towards the top of the nanowhisker, and having a second conductivity type dopant material therein, whereby to create by diffusion from said first and second layers into respective first and second regions of the nanowhisker, a pn junction within the nanowhisker between the first and second regions. 
     The invention also provides a method of forming a one-dimensional nanoelement with a pn junction therein, comprising:
         (a) forming a nanowhisker upstanding from a substrate,   (b) forming a first layer of material on the substrate and surrounding and extending partway up the nanowhisker, the first layer having a first conductivity type dopant material therein, and   (c) forming a second layer of material on top of the first layer and surrounding and extending towards the top of the nanowhisker, and having a second conductivity type dopant material therein, so that diffusion from the first and second layers into respective first and second regions of the nanowhisker creates a pn junction within the nanowhisker between the first and second regions.       

     In a fifth aspect, the invention recognises that there are problems in chemical doping of dopant ions in a nanowhisker of III-V semiconductor material, since for most III-V semiconductors, solid solubility at room temperature is limited, and during cooling out-diffusion is fast with these nanodimensions. Thus the amount of doping within the nanowhisker may be difficult to accurately predetermine. The invention recognises that the interface between a nanoelement and a surrounding medium, or a heterojunction interface within a nanoelement, may have a stronger and more significant role than hitherto realised in determining the electrical characteristics of the nanoelement. 
     It is known that localised surface “trap” states exist at the surfaces of bulk semiconductors; this is exhibited for example in Schottky diodes. This creates what is known as Fermi Level Pinning, where the surface trap states determine the relative levels of the conduction and valence bands in the junction materials. See, e.g., “Defective Heterojunction Models”, Freeouf J L, Woodall J M,: IBM Corp,:  Surface Science,  1986, V 168, N1-3, P 518-530. 
     The invention recognises that for a one-dimensional nanoelement, where there may be a wide range of possibilities to combine III-V semiconductors in spite of lattice mismatch, Fermi Level Pinning is a constructive way to make pn-junctions by choosing semiconductor alloy composition to determine carrier type. In such devices the band gap can be engineered and the carrier type can be controlled to make new types of semiconductor devices. When a semiconductor crystal ends abruptly at an interface, and “band bending” tends to occur to equalize the Fermi Levels on the two sides of the interface, Fermi Level Pinning, arising from the existence of surface trap states, counteracts this effect to reduce the amount of charge transfer across the interface. 
     The invention further provides a one-dimensional nanoelement including a first segment of a first semiconductor crystalline material, and a second segment of a second semiconductor crystalline material different from that of the first, and with a heterojunction therebetween, whereby the first and second materials are selected such that charge carriers of opposite conductivity type are provided at the opposite sides of the heterojunction interface so as to create a pn junction with predetermined characteristics, which characteristics are at least partially determined by Fermi level pinning. 
     The invention also provides a method of forming a pn junction comprising:
         a. forming a one dimensional nanoelement having a first segment of a first crystalline material, and a second segment of a second crystalline material different from that of the first, with a heterojunction therebetween,   b. the first and second materials being selected so as to provide charge carriers of opposite conductivity type at the heterojunction so as to create a pn junction with predetermined characteristics, which characteristics are at least partially determined by Fermi level pinning.       

     In accordance with the invention, the charge carriers can be provided by the intrinsic nature of the first and second materials. For III-V materials, stoichiometric compositions of ternary or quaternary materials can be chosen for desired conductivity characteristics. 
     It has been found that the present invention is particularly applicable to III-V compounds epitaxially grown (CBE or MOCVD, or MOVPE), under group III rich conditions. Under these conditions, the outermost atomic surface layers may have excess Ga or In ions and these create defect states, as further described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic of the CBE apparatus used for making the described embodiments. 
         FIG. 2  is a cross-sectional schematic view showing a step in the formation of the first embodiment of the invention, with an accompanying energy band diagram. 
         FIG. 3A  shows a cross-section of a nanowhisker prepared by the VLS method. 
         FIG. 3B  shows a cross-section of a nanowhisker having an enclosure or jacket according to the invention. 
         FIG. 3C  shows an array of such clad nanowhiskers extending from a (111) surface. 
         FIG. 3D  shows an enlarged view of a nanowhisker having been separated from the surface. 
         FIG. 3E  is a view of the cross-section of the clad nanowhisker showing a hexagonal structure that is characteristic of nanowhiskers growing in a &lt;111&gt; direction. 
         FIG. 3F  is a luminescence curve showing characteristic peaks at approximately 1.5 and 1.8 eV, which represent GaAs and AlGaAs materials respectively. 
         FIG. 4  is a cross-sectional view of a second embodiment of the invention. 
         FIG. 5  is a cross-sectional schematic view of a third embodiment of the invention. 
         FIG. 6  is a cross-sectional schematic view of a fourth embodiment of the invention. 
         FIG. 7  is a cross-sectional schematic view of a fifth embodiment of the invention. 
         FIG. 8  is a cross-sectional schematic view of a sixth embodiment of the invention. 
         FIG. 9  is a graph showing the energy levels of the bands and surface states of a number of semiconductor materials. 
         FIG. 10  is a schematic energy level diagram for the doped nanowhiskers of  FIG. 7  and  FIG. 8 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments to be described are all formed with nanowhiskers, preferably according to the Chemical Beam Epitaxy method (CBE) described in copending U.S. patent application Ser. No. 10/613,071 filed Jul. 7, 2003, the contents of which are herein incorporated by reference. 
     As indicated above, in the following detailed description of the invention, the term “nanoengineered structures” signifies a structure that includes structures, e.g., elements, parts, or the like, having dimensions as defined above, i.e., structures having at least two dimensions less than about 1 micrometer. Such structures are referred to herein as “nanoelements” or nanostructures, and/or, because of their generally elongated shape, as “nanowhiskers” or “nanowires”. 
     Chemical Beam Epitaxy (CBE) combines a beam epitaxial technique like Molecular Beam Epitaxy (MBE) and the use of chemical sources similar to Metal Organic Chemical Vapor Deposition (MOCVD). In MOCVD or related laser ablation techniques, the pressure inside the reactor is usually greater than 10 mbar and the gaseous reactants are viscous, which means that they have a relatively high resistance to flow. The chemicals reach the substrate surface by diffusion. CBE reduces the pressure to less than 10 −4  mbar and the mean free path of the diffusants then becomes longer than the distance between the source inlet and the substrate. The transport becomes collision free and occurs in the form of a molecular beam. The exclusion of the gas diffusion in the CBE system means a fast response in the flow at the substrate surface and this makes it possible to grow atomically abrupt interfaces. 
     The CBE apparatus shown in  FIG. 1  consists of a UHV growth chamber  100  where the sample  102  is mounted on a metal sample holder  104  connected to a heater  106 . Around the chamber there is a ring  108  filled with liquid nitrogen that is called the cryoshroud. The cryoshroud pumps away species that don&#39;t impinge or that desorb from the substrate surface. It prevents contamination of the growing surface layer and reduces the memory effect. Vacuum pumps  110  are provided. 
     The sources  112  for CBE are in liquid phase and they are contained in bottles which have an overpressure compared to the chamber. The sources are usually as follows: TMGa, TEGa, TMIn, TBAs, and TBP. The bottles are stored in constant-temperature baths and by controlling the temperature of the liquid source, the partial pressure of the vapor above the liquid is regulated. The vapor is then fed into the chamber through a pipe complex  114  to, in the end of the pipe just before the growth chamber, a source injector  116 . The source injector is responsible for injection of the gas sources into the growth chamber  100 , and for generation of a molecular beam with stable and uniform intensity. The III-material, from the metal organic compounds TMIn (trimethylindium), TMGa (trimethylgallium) or TEGa (triethylgallium), will be injected by low temperature injectors to avoid condensation of the growth species. They will decompose at the substrate surface. The V-material is provided by the metal-organic compounds, TBAs (tertiarybutylarsine) or TBP (tertiarybutylphosphine). As opposed to the decomposition of the III-material, the V-material will be decomposed before injection into the growth chamber  100 , at high temperatures, in the injectors  116 . Those injectors  116  are called cracking cells and the temperatures are kept around 900° C. The source beam impinges directly on the heated substrate surface. Either the molecule gets enough thermal energy from the surface substrate to dissociate in all its three alkyl radicals, leaving the elemental group III atom on the surface, or the molecule get desorbed in an undissociated or partially dissociated shape. Which of these processes dominates depends on the temperature of the substrate and the arrival rate of the molecules to the surface. At higher temperatures, the growth rate will be limited by the supply and at lower temperatures it will be limited by the alkyl desorption that will block sites. 
     This Chemical Beam Epitaxy method permits formation of heterojunctions within a nanowhisker, which are abrupt, in the sense there is a rapid transition from one material to another over a few atomic layers. 
     Referring now to  FIG. 2 , a first embodiment of the invention is formed by positioning a gold aerosol particle  2  on a III-V substrate  4 , e.g., a gallium arsenide substrate. With appropriate conditions of temperature and pressure a nanowhisker of indium arsenide is grown by injecting organic materials TMIn and TBAs in a conventional VLS procedure, e.g., in a chemical beam epitaxial method, using the apparatus described above, or by a metal organic vapor phase epitaxy (MOVPE), or the like. Indium and arsenide ions are absorbed in the gold particle  2  and supersaturation conditions create a solid pillar  6  of indium arsenide. 
     Once the indium arsenide whisker has been grown, different materials TEGa and TBP are used to create a coaxial jacket or surrounding layer  8  of GaP around the nanowhisker  6 . Layer  8  may be created by CBE; using the apparatus of  FIG. 1 , wherein the conditions of temperature ( 106 ) and/or pressure ( 112 ) are changed to inhibit growth by the VLS mechanism, and instead to support bulk growth. Alternatively the gold melt particle  2  can be removed mechanically, so that subsequent growth of GaP will occur in bulk form 
     The resulting energy level bandgap diagram is shown with an energy gap of 2.3EV separating the conduction bands for gallium phosphide, whereas there is a bandgap of 0.3EV for the central indium arsenide whisker. 
     The jacket or shell material (GaP in this case) may then be doped, e.g., via the vapor phase, resulting in a sheath at the periphery of the GaP jacket which will contain donor dopants such as tellurium. 
     As an alternative to tellurium, any donor dopant materials that are commonly used for GaP may be used, see for example CRC The Handbook of Chemistry and Physics, Semiconductor Properties, e.g., Si, Sn, Te, Se, S, or the like. Alternatively, if an acceptor-doped jacket or shell is desired, appropriate acceptor materials, e.g., Zn, Fe, Mg, Be, Cd, or the like, can be incorporated. 
     As an alternative to InAs/GaP, any other combination of materials may be used, subject to the bandgaps providing energetically favorable conditions—the band gap of the surrounding layer should be wider than that of the nanowhisker; thus for a whisker of InAs, covering materials of GaAs, GaP or InP may be employed—see for example CRC The Handbook of Chemistry and Physics, Semiconductor Properties. 
     The effect of doping with tellurium ions is to liberate charge carrier electrons within gallium phosphide layer  8 . These electrons preferentially transfer into the central nanowhisker, where the energy states (conduction band/valence band levels) determine that the electrons are in energetically favorable condition. The theory is essentially that of modulation doping that is a technique employed in planar technology as described in WO 02/1438. 
     This therefore creates a nanowhisker with a desired electrical conductivity. The nanowhisker also has a high mobility because there are no dopant ions within the crystal lattice deforming the lattice structure. 
     Referring now to  FIGS. 3A to 3F  there is shown a specific example of the first embodiment of the invention. Nanowhiskers  6  of gallium arsenide were grown from gold catalytic particles by an epitaxial process from a GaAs substrate having a (111) surface. The growth conditions were then changed by altering the temperature, and modifying the gaseous pressure of the As-containing gas, so as to grow epitaxially, by bulk growth rather than catalytic growth, material of AlGaAs along the side of the GaAs nanowhiskers. The result as shown in  FIGS. 3B and 3C  are cylinders, in the form of a candle, with an inner core  6  of a GaAs 20 nanometers in diameter, and an outer cladding  8  of AlGaAs between 100 and 5000 nanometers in diameter. 
       FIG. 3C  shows an array of such clad nanowhiskers extending from a (111) surface. 
       FIG. 3D  shows an enlarged view of a nanowhisker having been separated from the surface. 
       FIG. 3E  is a view of the cross-section of the clad nanowhisker showing a hexagonal structure that is characteristic of nanowhiskers growing in a &lt;111&gt; direction. 
       FIG. 3F  is a luminescence curve showing characteristic peaks at approximately 1.5 and 1.8 eV, which represent GaAs and AlGaAs materials respectively. An intermediate hump is thought to be caused by spatially indirect transitions. 
     Referring now to  FIG. 4  there is shown a second embodiment of the invention. Similar parts to those of  FIG. 2  are identified by the same reference numeral. A structure is produced comprising an inner nanowhisker of GaAs material  6 , produced from a catalytic particle  2 . The whisker is surrounded by a coaxial jacket  8  of AlGaAs. Then, first and second layers  20 ,  22  are provided, being of polymer or glass material spun on to the surface of substrate  4 . Layer  20  contains n type dopant ions  24 , and layer  22  contains p type dopant ions  26 . A rapid thermal annealing step ensures that dopant ions  24 ,  26  migrate into corresponding regions  28 ,  30  of coaxial jackets  8 . The annealing step is controlled such that there is no appreciable diffusion into the nanowhisker  6 . 
     The result is that the dopant ions within regions  28 ,  30  create corresponding regions  32 ,  34  within nanowhisker  6  by modulation doping of opposite conductivity type. These regions that are stable space charge regions create a region  36  depleted of free carriers resembling the depletion region of a pn junction between semiconductor materials of opposite conductivity type. 
     The level of dopant concentration within regions  28 ,  30  may be such that highly degenerative doping is produced, with correspondingly heavy modulation doping of the segments  32 ,  34  of the nanowhisker. Such heavy modulation doping may create a condition analogous to that existing in an Esaki or tunnel diode, with corresponding tunnelling between the regions and an associated negative resistance effect. 
     Referring now to  FIG. 5 , there is shown a third embodiment of the invention, wherein similar parts to those of  FIG. 2  are denoted by the same reference numeral. Thus an indium arsenide nanowhisker  6  is grown on a gallium arsenide substrate  4  by chemical beam epitaxy employing a gold catalytic particle  2 . 
     After formation of the nanowhisker, a first layer  50  of polymer material is evaporated (preferred) or spun onto the substrate  4 . There is commercially available a wide range of dielectric materials formed of carbon or silicon based polymers, some of which are doped and have defined electrical conductivity characteristics. The polymer material has contained within it a desired concentration of dopant ions of a desired type. As may be seen, layer  50  extends towards the top of the nanowhisker. The depth of layer  50  can be determined very accurately with evaporation of polymer. 
     The entire structure is then subject to rapid thermal annealing. This permits the dopant ions in the polymer material layer  50  to diffuse into the nanowhisker regions  54 , to provide a controlled doping of the regions  54 . The temperature of the annealing step depends on the materials employed. 
     There is thus provided a nanowhisker with a desired degree of conductivity, the method of doping providing a high degree of control over the conductivity. 
     Referring now to  FIG. 6 , there is shown a fourth embodiment of the invention, wherein similar parts to those of  FIG. 2  are denoted by the same reference numeral. Thus an indium arsenide nanowhisker  6  is grown on a gallium arsenide substrate  4  by chemical beam epitaxy employing a gold catalytic particle  2 . 
     After formation of the nanowhisker, a first layer  60  of polymer material is evaporated (preferred) or spun onto the substrate  4 . There is commercially available a wide range of dielectric materials formed of carbon or silicon based polymers, some of which are doped and have defined electrical conductivity characteristics. The polymer material has contained within it a desired concentration of dopant ions of a desired type. As may be seen, layer  60  extends roughly halfway along the length of the nanowhisker. Thus, for a nanowhisker that is 2 micrometers long, the depth of layer  60  is 1 micrometer. The depth can be determined very accurately with evaporation of the polymer. 
     A second layer  62  of polymer material of the same type as the first but having a dopant material of opposite conductivity type is evaporated on to layer  60  and extends up to the top of the nanowhisker, to a height approximately the same as the gold particle  2 . 
     The entire structure is then subject to rapid thermal annealing. This permits the dopant ions in the polymer material layer  60  to diffuse into the adjacent nanowhisker region  64 , to provide a controlled doping of the region  64 . Further, the dopant ions in the polymer material layer  62  diffuse into the adjacent nanowhisker region  66 , to provide a controlled doping of the region  66 . The temperature of the annealing step depends on the materials employed. 
     Thus, region  64  of whisker  6  may contain for example negative charge carriers, whereas positive charge carriers from layer  62  are contained in region  66  of whisker  6 . This effectively creates a pn junction  68  between the two regions  64 ,  66 . 
     The junction  68  may be sharply defined within the nanowhisker. For types of dopant materials, any of the commonly used materials may be used. See, e.g., CRC The Handbook of Chemistry and Physics, Semiconductor Properties. 
     Three or more layers of polymer may be deposited, each with appropriate dopant materials. This permits the formation of multiple pn junctions within the whisker. 
     Referring now to  FIGS. 7 to 10 , there is shown fifth and sixth embodiments of the invention. In  FIG. 7  a nanowhisker is shown upstanding from a substrate  70 , having a gold catalytic particle  72  at its top, and being composed of a material  74 , preferably a III-V compound such as GaAs, InAs, InP. The nanowhisker has its sides defined by (110) surfaces. The whisker is formed by the CBE method as described above. The nanowhisker is embedded in a surrounding layer  76  of a second material different from that of the first, but preferably also a III-V compound such as GaAs, InAs, InP. The material of region  74  may be gallium arsenide, whereas material region  76  may be indium arsenide. Material region  76  is also grown by CBE, with conditions of temperature and/or pressure adjusted to support bulk growth, rather than VLS growth. 
     Preferably, nanowhiskers of III-V compounds are grown under group-III rich growth conditions (In, Ga, Al, B) that is for example an excess of TEGa is used for CBE growth of whiskers containing Ga. This ensures that the outermost surface of the nanowhisker has a slight excess of the group III compound Ga, and is therefore intrinsically p-type. The embedding layer  76  is InP, which embedding layer also grown under group III rich conditions to ensure a slight excess of In. The outermost surfaces of the nanowhisker are (110) surfaces. 
     Thus, a pn-junction results by combining GaAs (p-type intrinsically) with InP (n-type intrinsically). Another example would be InAs, which is almost degenerately n-type intrinsically. 
     By way of explanation, it is well understood that, at the free surface of a semiconductor, surface relaxation and surface reconstruction may take place, to minimise free energy, in particular from charge imbalance. Surface reconstruction may involve rearrangement of the crystal lattice; this is particularly so for GaAs (111) surfaces. Further, surface trap states are created in the bulk band gap, and this strongly modifies the charge balance at the surface. This creates, in known manner, a deformation of the band structure near the surface. The band edges bend upwards so that the surface states cross the Fermi level and start to empty, decreasing the surface charge density. The region over which the bands are bent is termed the depletion region because it has been depleted of mobile carriers. If the surface state density at a semiconductor surface has a high value, the band bending will saturate. At this point the Fermi level is said to be pinned by the surface states. 
     Since in this embodiment, the nanowhisker is grown under group III rich conditions, the surface reconstruction creates, from these excess group III atoms, deep-level like defects, the energy position of which are related to the vacuum level, not to the band edges of the semiconductors (this corresponds to the situation for other deep level impurities in bulk III-V semiconductors). 
     Referring to  FIG. 9 , this shows the band gaps for a range of III-V compounds grown under group III rich conditions, with surface trap states indicated by crosses occurring in the band gaps. It will be noted that for all the compounds, the energy levels for the trap states are roughly equal, relative to vacuum level. This implies that pn junctions can simply be created by Fermi Level pinning at an interface between two such materials. 
     Thus, the situation arises that the surface of a GaAs whisker is p-type, whereas the surface of an InP whisker is n-type. Further the surface of layer  76  surrounding and embedding the whisker will have a conductivity governed by similar considerations. Thus Fermi Level pinning will ensure that the surface of a surrounding InP layer is n-type; hence if the whisker is GaAs, a pn junction is created by the Fermi Level Pinning effects. The situation is shown in  FIG. 10 , where the relative levels of the band gaps of GaAs and InP are determined by Fermi Level Pinning, arising from the surface trap states. 
     In an alternative, where the whisker and surrounding layer are grown by MOVPE, then the MOVPE process has to be tuned to give Group III rich conditions of growth. 
     In a further embodiment as shown in  FIG. 8 , a heterojunction  88  within a nanowhisker  82  between an indium phosphide segment  84  and a gallium arsenide segment  86  assumes the character of a pn junction along a (001) or (100) crystal plane. This is because GaAs is intrinsically p-type whereas indium phosphide is intrinsically n-type. The side facets of the whisker are (111) planes that have many surface states which establish a surface Fermi level (pinned Fermi level) which is characteristic of p-type or n-type semiconductor material, respectively. For nanowhiskers of a diameter of about 100 nm or less, there is insufficient diametral distance to permit band bending in the interior of the whisker to a level characteristic of the bulk semiconductor. Consequently, the conductivity type of each of the segments  82 ,  84  is determined by the Fermi-level pinning produced by the surface states on the side facets of each segment. Accordingly, the heterojunction  88  becomes a pn junction between the indium phosphide segment  84  and the gallium arsenide segment  86  of the nanowhisker. 
     The skilled practitioner will, of course, recognize that the above-described embodiments are illustrative of the present invention and not limiting.