Patent Publication Number: US-9419183-B2

Title: Nanowire sized opto-electronic structure and method for manufacturing the same

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to nano sired opto-electronic structures, such as light emitting devices, e.g. diode structures, in particular arrays of nano sized based light emitting devices and in particular to contacting thereof. 
     BACKGROUND OF THE INVENTION 
     Light emitting diodes (LEDs) are increasingly used for lighting, but still there are some technological challenges to overcome, in particular with regard to large-scale processing, in order to reach the real breakthrough. 
     Over recent years the interest in nanowire technology has increased. In comparison with LEDs produced with conventional planar technology nanowire LEDs offer unique properties due to the one-dimensional nature of the nanowires, improved flexibility in materials combinations due to less lattice matching restrictions and opportunities for processing on larger substrates. Suitable methods for growing semiconductor nanowires are known in the art and one basic process is nanowire formation on semiconductor substrates by particle-assisted growth or the so-called VLS (vapor-liquid-solid) mechanism, which is disclosed in e.g. U.S. Pat. No. 7,335,908. Particle-assisted growth can be achieved by use of chemical beam epitaxy (CBE), metalorganic chemical vapour deposition (MOCVD), metalorganic vapour phase epitaxy (MOVPE), molecular beam epitaxy (MBE), laser ablation and thermal evaporation methods. However, nanowire growth is not limited to VLS processes, for example the WO 2007/102781 shows that semiconductor nanowires may be grown on semiconductor substrates without the use of a particle as a catalyst. One important breakthrough in this field was that methods for growing group III-V semiconductor nanowires, and others, on Si-substrates have been demonstrated, which is important since it provides a compatibility with existing Si processing and non-affordable III-V substrates can be replaced by cheaper Si substrates. 
     One example of a bottom emitting nanowire LED is shown in WO 2010/14032. This nanowire LED comprises an array of semiconductor nanowires grown on a buffer layer of a substrate, such as a GaN buffer layer on a Si substrate. Each nanowire comprises an n-type nanowire core enclosed in a p-type shell and a p-electrode with an active layer formed between the n-type and p-type regions that form a pn or pin junction. The buffer layer has the function of being a template for nanowire growth as well as serving as a current transport layer connecting to the n-type nanowire cores. Further the buffer layer is transparent since the light that is generated in the active area is emitted through the buffer layer. 
     Although having advantageous properties and performance the processing with regard to contacting of the nanowire LEDs requires new routes as compared to planar technology. Since nanowire LEDs comprise large arrays of nanowires, thereby forming a three-dimensional surface with high aspect ratio structures, deposition of contact material using line-of-sight processes is a challenging operation. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing one object of embodiments of the invention is to provide improved nanowire based structures, in particular opto-electronic structures such as LEDs and new routes for contacting thereof. 
     This object is achieved by a semiconductor device and a method for forming a semiconductor device in accordance with the independent claims. 
     A nanosized structure as disclosed herein comprises a plurality of nano elements arranged side by side. Each nano element comprises at least a first conductivity type (e.g., n-type) core. The core is preferably a nanowire core which forms a pn or pin junction with an enclosing second conductivity type (e.g., p-type) shell. The shell may be a part of the nano element or it may comprise a bulk semiconductor element. In operation, the junction provides an active region for light generation. While the first conductivity type of the core is described herein as an n-type semiconductor core and the second conductivity type shell is described herein as a p-type semiconductor shell, it should be understood that their conductivity types may be reversed. A p-electrode layer extends over a plurality of nano elements and is in electrical contact with at least A top portion of the nanoelements to connect to the p-type shell. The p-electrode layer can be at least partly bridged between the nano elements. “Bridged” for the purpose of this application means that the p-electrode layer extends across the distance between neighbouring nano elements thereby forming a continuous layer. The portions of the p-electrode extending between the wires can either rest on a support or be free-hanging (e.g., air-bridged). 
     Traditional, planar LEDs comprise functional layers in a sandwich structure. In their simplest form, the planar LEDs comprise at least three functional layers: a p-doped layer, an active region, and an n-doped layer. Functional layers may also include wells, barriers, intrinsic and graded layers (e.g., as part of the active region). The LED arrays described in embodiments of the invention distinguish themselves by at least one of the functional layers being electrically separated from the surrounding LEDs in the array. Another distinguishing feature is the utilization of more than one facet and non-planarity of functional layers as emission layers. 
     Although the fabrication method described herein preferably utilizes a nanowire core to grow semiconductor shell lavers on the cores to form a core-shell nanowire, as described for example in U.S. Pat. No. 7,829,443, to Seifert et al., incorporated herein by reference for the teaching of nanowire fabrication methods, it should be noted that the invention is not no limited. For example, as will be described below, in the alternative embodiments, only the core may constitute the nanostructure (e.g., nanowire) while the shell may optionally have dimensions which are larger than typical nanowire shells. Furthermore, the device can be shaped to include many facets, and the area ratio between different types of facets may be controlled. This is exemplified in figures by the “pyramid” facets and the vertical sidewall facets. The LEDs can be fabricated so that the emission layer formed on templates with dominant pyramid facets or sidewall facets. The same is true for the contact layer, independent of the shape of the emission layer. 
     The use of sequential (e.g., shell) layers may result in the final individual device (e.g., a pn or pin device) having a shape anywhere between a pyramid shape (i.e., narrower at the top or tip and wider at the base) and pillar shaped (e.g., about the same width at the tip and base) with circular or hexagonal or other polygonal cross section perpendicular to the long axis of the device. Thus, the individual devices with the completed shells may have various sizes. For example, the sizes may vary, with base widths ranging from 100 nm to several (e.g., 5) μm, such as 100 nm to below 1 micron, and heights ranging from a few 100 nm to several (e.g., 10) μm, 
     In prior art methods, arrays of nanowire LEDs are contacted by depositing a contact layer that covers essentially the whole surface of the nanowires and intermediate surfaces between the nanowires using sputtering or evaporation techniques. Due to the high aspect ratio, and often small spacing of the nanowires these line-of-sight processes results in a non-conformal coverage, In particular, there is a risk that the contact layer becomes discontinuous and that the contact layer on the intermediate surfaces (e.g., the horizontal surface exposed between vertical nanowires) becomes too thin. In operation, this will result in losing the effect of some nanowires and a poor current spreading in the device, respectively, With a bridged p-electrode in accordance with embodiments of the invention, the risk for discontinuities is reduced or eliminated, and the lateral current spreading is improved due to a uniform thickness of the p-electrode and optional additional layers deposited on the p-electrode. 
     With a bridged p-contact or electrode for top-emitting nano sized LEDs, a thick contact layer can directly contact the top portion of the nanowire LED. For top emitting LEDs, a transparent p-contact layer is used. Without the bridge, the p-electrode layer at the top portion must be made much thicker, which increases absorption. 
     Also, with the bridge p-contact or electrode for bottom-emitting nanosized LEDs, the reflective p-contact layer is only arranged on the top portion of the nano elements and not the whole circumferential nanowire area. A reflective layer extending down on the whole circumferential area would give significant losses due to total internal reflection. 
     Thus, embodiments of the invention make it possible to obtain an efficient nano sized device, such as a LED with regard to internal conductivity, light generation and coupling of light out from the nanowire LED. 
     Embodiments of the invention are defined in the dependent claims. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described with reference to the accompanying drawings, wherein: 
         FIG. 1  schematically illustrates a side cross sectional view of a basis of a nanowire LED in accordance with embodiments of the invention, 
         FIG. 2  schematically illustrates a side cross sectional view of a nanowire LED structure on a buffer layer in accordance with embodiments of the invention, 
         FIGS. 3 a - b    schematically illustrate side cross sectional views of bridged p-electrodes in accordance with embodiments of the invention, 
         FIGS. 4 a - h    schematically illustrate side cross sectional views of a first implementation of a method in accordance with one embodiment of the invention, 
         FIGS. 4 i - s    schematically illustrate side cross sectional views of a second implementation of a method in accordance with another embodiment of the invention, 
         FIG. 5  shows two scanning electron microscope images of a bridged p-electrode in accordance with embodiments of the invention, 
         FIGS. 6 a - i    schematically illustrate side cross sectional views of a third implementation of a method in accordance with another embodiment of the invention, 
         FIG. 7  shows a scanning electron microscope image of a nanowire LED structure manufactured according to the third implementation example, 
         FIG. 8  schematically illustrates a side cross sectional view of an array of nanowire LED structures comprising axial pn junctions and a bridged p-electrode in accordance with an alternative embodiment of the invention, 
         FIG. 9  illustrates a side cross sectional view of a device according to a further embodiment of the invention, 
         FIG. 10  shows the side view of the device of the embodiment of  FIG. 9  mounted on a submount with appropriate contacts, 
         FIG. 11  illustrates a side cross sectional view of a device according to a still further embodiment of the invention, and 
         FIG. 12  shows the side view of the device of the embodiment of  FIG. 11  mounted on a submount with appropriate contacts, 
         FIG. 13  illustrates a side cross sectional view of a still further embodiment, 
         FIG. 14  shows the embodiment of  FIG. 13  with appropriate contacts, 
         FIG. 15  illustrates a side cross sectional view of yet another embodiment, 
         FIG. 16  shows the embodiment of  FIG. 15  with appropriate contacts, 
         FIG. 17  shows a further embodiment of a nano sized structure, 
         FIG. 18  shows a still further embodiment of a nano sized structure, 
         FIG. 19  shows yet another embodiment of a nano sized structure, and 
         FIG. 20  shows still another embodiment of a nano sized structure. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     As used herein, the term “bridged electrode” is taken to mean an electrode structure that extends between adjacent individual devices over a filled spacer or to leave an empty space (e.g., air-bridge) between the adjacent devices. The empty space is preferably surrounded by the adjacent devices on the sides, the bridged electrode on the “top” and the support of the devices on the “bottom”, where the terms top and bottom are relative depending on which way the device is positioned. For example, in one embodiment in which each individual device is a radial core-shell nanowire, the bridged electrode covers the nanowire tips and the space between the nanowires, such that there is an empty space beneath the electrode between the nanowire support layer (e.g., substrate, buffer layer, a reflective or transparent conductive layer, insulating mask layer, etc.) and the electrode. 
     In the art of nanotechnology, nanowires are usually interpreted as nanostructures having a lateral size (e.g., diameter for cylindrical nanowires or width for pyramidal or hexagonal nanowires) of nano-scale or nanometer dimensions, whereas its longitudinal size is unconstrained. Such nanostructures are commonly also referred to as nanowhiskers, one-dimensional nano-elements, nanorods, nanotubes, etc. Generally, nanowires with a polygonal cross section are considered to have at least two dimensions each of which are not greater than 300 nm. However, the nanowires can have a diameter or width of up to about 1 μm. The one dimensional nature of the nanowires provides unique physical, optical and electronic properties. These properties can for example be used to form devices utilizing quantum mechanical effects (e.g., using quantum wires) or to form heterostructures of compositionally different materials that usually cannot be combined due to large lattice mismatch. As the term nanowire implies, the one dimensional nature is often associated with an elongated shape. In other words, “one dimensional” refers to a width or diameter less than 1 micron and a length greater than 1 micron. Since nanowires may have various cross-sectional shapes, the diameter is intended to refer to the effective diameter. By effective diameter, it is meant the average of the major and minor axis of the cross-section of the structure. 
     In the embodiments of the present invention, the finished structures are referred to as “nano elements”. Although in the figures the nano elements are shown to be pillar like and based on nanowire cores, i.e., more or less “one dimensional” cores, it should be noted that the cores can also have other geometries such as pyramids with various polygonal bases, such as square, hexagonal, octagonal, etc. Thus, as used herein, the core may comprise any suitable nano element having a width or diameter of less than 1 micron and a length greater than 1 micron and may comprise a single structure or a multi-component structure. For example, the core may comprise a semiconductor nanowire of one conductivity type or it may comprise the semiconductor nanowire of one conductivity type surrounded by one or more semiconductor shells of the same conductivity type and the core having a pillar or pyramid shape. For simplicity, a single component nanowire pillar core will be described below and illustrated in the figures. 
       FIG. 1  schematically illustrates the basis for a nanowire LED structure in accordance with embodiments of the invention. In principle, one single nanowire is enough for forming a nanowire LED, but due to their small size, nanowires are preferably arranged in arrays comprising thousands of nanowires (i.e., nano-devices or devices) side by side to form the LED structure. For illustrative purposes the individual nanowire LED devices will be described herein as being made up from nanowires  1  having n-type nanowire core  2  and a p-type shell  3  at least partly enclosing the nanowire core  2  and an intermediate active layer  4 . However, for the purpose of embodiments of the invention, nanowire LEDs are not limited to this. For example the nanowire core  2 , the active layer  4  and the p-type shell  3  may be made up from a multitude of layers or segments. However, by controlling growth conditions the final geometry of a LED can range from elongated, narrow “pillar structures” to relatively wide based pyramid structures. As described above, in alternative embodiments, only the core  2  may comprise a nanostructure or nanowire by having a width or diameter below 1 micron, while the shell  3  may comprise a bulk p-type semiconductor element having a width or diameter above one micron. In order to function as a LED, the n-side and p-side of each nanowire  1  has to be contacted. 
     By growing the nanowires  1  on a growth substrate  5 , optionally using a growth mask  6  (e.g., a nitride layer, such as silicon nitride dielectric masking layer) to define the position and determine the bottom interface area of the nanowires  1 , the substrate  5  functions as a carrier for the nanowires  1  that protrude from the substrate  5 , at least during processing. The bottom interface area of the nanowires comprises the area of the core  2  inside each opening in the masking layer  6 . The substrate  5  may comprise different materials such as III-V or II-VI semiconductors, Si, Ge, Al 2 O 3 , SiC, Quartz, glass, etc., as discussed in Swedish patent application SE 1050700-2 (assigned to GLO AB), which is incorporated by reference herein in its entirety. In one embodiment, the nanowires  1  are grown directly on the growth substrate  5 . 
     Preferably, the substrate  5  is also adapted to function as a current transport layer connecting to the n-side of each nanowire  1 . This can be accomplished by having a substrate  5  that comprises a buffer layer  7  arranged on the surface of the substrate  5  facing the nanowires  1 , as shown in  FIG. 2 , by way of example a III-nitride layer, such as a GaN and/or AlGaN buffer layer  7  on a Si substrate  5 . The buffer layer  7  is usually matched to the desired nanowire material, and thus functions as a growth template in the fabrication process. For an n-type core  2 , the buffer layer  7  is preferably also doped n-type. The buffer layer  7  may comprise a single layer (e.g., GaN), several sublayers (e.g., GaN and AlGaN) or a graded layer which is graded from high Al content AlGaN to a lower Al content AlGaN or GaN. The nanowires can comprise any semiconductor material, but for nanowire LEDs III-V semiconductors such as a III-nitride semiconductor (e.g., GaN, AlInGaN, AlGaN and InGaN, etc.) or other semiconductors (e.g., InP, GaAs) are usually preferred. It should be noted that the nanowire  1  may comprise several different materials (e.g., GaN core, InGaN active layer and InGaN shell having a different In to Ga ratio than the active layer). In general the substrate  5  and/or the buffer layer  7  are referred to herein as a support or a support layer for the nanowires. As will be described in more detail with regard to  FIGS. 9-12 , a conductive layer (e.g., a mirror or transparent contact) may be used as a support instead of or in addition to the substrate  5  and/or the buffer layer  7 . Thus, the term “support layer” or “support” may include any one or more of these elements. 
     Thus, the buffer layer  7  provides means for contacting the n-side of the nanowires  1 . In prior art nanowire LEDs, the contacting of the p-side of each nanowire  1  is typically accomplished by depositing a p-electrode comprising a conductive layer that encloses the p-type shell  3  of each nanowire  1  and extends to an insulating layer on the substrate or buffer layer. The conductive layer extends on this insulating layer to adjacent nanowires. However, since the nanowires of a nanowire LED are closely spaced and being of high aspect ratio in order to obtain a high luminescence, the p-electrode deposition is a challenging operation. Typically line-of-sight processes, such as sputtering or evaporation are used for electrode deposition. Due to the line-of-side deposition, a preferential growth on the tips of the nanowires and a shadowing effect are observed that result in a tapering of the p-electrode with decreased thickness towards the base of the nanowires  1 . Hence, in order to obtain efficient lateral current spreading, the thickness of the p-electrode will become unnecessarily thick on the tips of the nanowires while being insufficiently thick in between the nanowires. The shadowing effect may also be so severe that there are discontinuities in the p-electrode. 
     A p-electrode  8  in accordance with embodiments of the invention may be least partly bridged between adjacent nanowires  1 .  FIG. 3 a    schematically illustrates a p-electrode  8  covering a group of nanowires  1 . As noted above, if the shell  3  of the nanowires  1  is n-type, then electrode  8  would be an n-electrode. However, electrode  8  is referred to herein as p-electrode for ease of description. In case of an air-bridged electrode, the p-electrode  8  is free-hanging between adjacent nanowires  1  and is only supported by the nanowires  1 . The p-electrode  8  encloses atop portion of each nanowire  1  and thereby contacts the p-side of the nanowire LED structure. The p-electrode may extend down along the sides of peripheral nanowires, e.g., in order to provide a connection to a pad arranged on the substrate  5  (as will be described in more detail below and as shown on the right and left edges of  FIG. 3 a   ). 
     Different additional layers may be deposited on the p-electrode. For example layers that improve electrical conductivity or coupling of light out from/into the nanowire may be deposited on the nanowire. 
     A nanowire LED structure of the embodiments of the present invention is either adapted for top emitting, i.e., light emission through the p-electrode, or bottom emitting, i.e., light emission through the support layer (i.e., through the conductive layer and/or buffer layer and/or substrate). The requirements on the p-electrode are different for these two cases. As used herein, the term light emission includes both visible light (e.g., blue or violet light) as well as UV or IR radiation. The embodiments of the present invention are suitable for bottom emitting devices. 
     For a bottom emitting LED, the p-electrode is preferably reflective. As shown in the following examples, the p-electrode may comprise one or more additional layers deposited on the p-electrode for improving the reflective and/or conductive properties. 
       FIG. 3 b    schematically illustrates one embodiment of a nanowire LED structure in accordance with embodiments of the invention. In principle it is the same structure as shown in  FIG. 3 a   , but the p-electrode comprises a comparatively thin conductive layer  8 ′ enclosing the p-type shell  3  and a comparatively thick bridged conductive layer  8 ″ arranged on the thin conductive layer  8 ′, The thin conductive layer  8 ′ extends down towards the base of the nanowire  1 , longer than the thick conductive layer  8 ″. The thin conductive layer  8 ′ can for example be deposited using atomic layer deposition or grown as an epitaxial layer on the p-type shell. Layer  8 ′ may be discontinuous between adjacent nanowires and may cover only the nanowires but not the masking layer  6  or buffer layer  7  between the nanowires. With this arrangement the thin conductive layer  8 ′ can be used to obtain an optimal interface to the p-type shell  3  and the thick conductive layer  8 ″ can be optimised for current spreading and/or light coupling and/or reflection. Thus, only layer  8 ″ may be used to form the bridge. The thick conductive layer  8 ″ is deposited as described below. 
     In an alternative embodiment, in addition to the mask layer  6 , the space between the nanowires can also be filled fully or partially with a dielectric (i.e., insulating) material, such as silicon oxide, For partially filled space, the gap size below the bridge is reduced. For fully filled space, there is no longer an air-bridge. Thus, for the embodiments described below with regard to the contact schemes for the nanowires, it should be understood than the nanowires may be contacted either in an air-bridged, non-air-bridged or non-bridged configurations. 
     In the following first implementation of a method for forming atop emitting nanowire LED structure is described with reference to  FIGS. 4 a - h   . In this embodiment, the same conductive layer is patterned to form both the p undo electrode layers. In this implementation, pads for connecting to the n-side and p-side of the device are formed in pad areas adjacent to the nanowires forming the nanowire LED. However, the invention is not limited to this configuration. 
       FIG. 4 a    shows an array of nanowires  1  grown from a buffer layer  7  through a growth masking layer  6 . The nanowires preferably comprise an n-type nanowire core  2  enclosed in a p-type shell layer  3  with an intermediate active layer  4  for light generation, as shown in  FIG. 1 . The growth mask  6  may be patterned by photolithography to define openings for the nanowire growth, as described for example in U.S. Pat. No. 7,829,443. In this implementation, the nanowires are grouped in an n-pad area, a non-active area, a LED area (i,e, the area which emits light) and a p-pad area. However, embodiments of the invention are not limited to this. For example the p-pad area may be arranged on top of the nanowires forming the light emitting part of the nanowire LED structure, whereby the p-pad area and the LED area coincide, as described in PCT International Application Publication Number WO 2010/014032 A1 to Konsek, et al., published Feb. 4, 2010 and incorporated herein by reference in its entirety. 
     Referring to  FIG. 4 b   , in the next step a protection layer  9  is deposited, at least in the LED-area where the nanowires will form LEDs, to protect nanowires from the subsequent processing. Otherwise residuals from photoresist and reactive ions from sputtering and reactive ion etching (RIE) may cause defects and/or contamination. ZnO deposited with atomic layer deposition (ALD) can be used as a protection layer. One advantage with ALD as a deposition technique is its perfect step coverage. Other materials such as other metal or silicon oxides, e.g., Al 2 O 3  or SiO 2 , deposited with ALD or other deposition techniques can also be used. This layer may fill an additional role as insulator on areas where it will be left. 
     Protection layer  9  deposition is followed by opening up, through lithography and etching, to the buffer layer  7  through the protection layer and the growth mask in the n-pad area  11 . In other words, as shown in  FIG. 4 b   , a photoresist or another masking layer (shown as dashed lines  12 ) is formed over the entire device and then removed in the n-pad area  11  by photolithography. The exposed protection layer  9  and the exposed masking layer  6  between the nanowires  1  are etched by any suitable etching method which can stop on the buffer layer (e.g., any wet or dry etching method which can etch a metal oxide or silicon oxide selectively with respect to a III-nitride semiconductor buffer layer). The purpose is to access the buffer layer  7  for arranging an electrode thereon, in order to provide an electrical connection through the buffer layer  7  to the n-side of the nanowires  1  (i.e., an electrical connection through n-type layer  7  to n-type nanowire cores  2 ). 
     Referring to  FIG. 4 c   , the next step is to form a sacrificial layer  10 , such as a photoresist or another suitable sacrificial material layer, with two different thicknesses extending over the non-active area and the p-pad area. The photoresist layer should completely cover the nanowires in the non-active area  13 , whereas it should partly cover the nanowires  1  in the LED area  14 , leaving a top portion of each nanowire  1  in the LED area  14  exposed. If the same contact materials are to be used in the n- and p-electrodes, areas which should be accessed as contact pads, i.e., the n-pad area  11  and the p-pad area  15 , the n-pad area is  11  preferably not covered by photoresist. This is clearly seen in the left part of  FIG. 4 c   . As appreciated by one skilled in the art, the photoresist layer can be formed, e.g., by depositing photoresist, and using two masks and two exposures and then development, or larger amplitude exposure in area  14  than in area  13  for a positive photoresist (or vice versa for negative photoresist). Also, the photoresist can comprise multiple layers (e.g., forming, exposing and developing a first resist in areas  13  and  14  and then forming, exposing and developing a second resist only in area  13  over the first resist). If desired, the photoresist  10  may comprise a portion of the photoresist layer  12  used in  FIG. 4 b    to pattern layers  9  and  6 . In this case, photoresist layer  12  is exposed a second time using the methods described above in areas  14  and  15  hut not in area  13  (or vice versa for a negative photoresist), and then developed (i.e., removed) fully in area  15  and partially in area  14 . 
     Referring to  FIG. 4 d   , the next step is to remove the protection layer  9  on at least the exposed top portions of the nanowires in the LED area  14  that are exposed outside of photoresist pattern  10 . This may be done by selective etching which selectively etches the oxide protective layer  9  but not the masking layer  6  (e.g., silicon nitride) or the semiconductor buffer layer  7  or semiconductor nanowires  1 . If desired, layer  9  may be left in areas where it does not interfere in a contact between the semiconductor material and a respective electrode to provide additional electrical insulation on top of the masking layer  6 . For example, an aluminium oxide layer may be used as such a permanent protective layer  9  in combination with silicon nitride masking layer  6 . 
     Thereafter the p-electrode layer  16  is deposited. Since the p-electrode becomes elevated and does not have to extend down deeply into the narrow space between the nanowires  1 , line-of-sight processes such as sputtering or evaporation can be used. Of course the n-electrode layer is formed at the same time since the n-pad area  11  is exposed. It should be noted that p-electrode  16  does not contact then-type buffer layer  7  in the p-pad area  15  because the buffer layer  7  is covered by the masking layer  6  in the p-pad area. Thus, a short circuit between the p-electrode and the n-buffer layer/n-nanowire cores is avoided. However, if the left side portion of layer  16  is used to form the n-electrode, then it this portion of layer  16  contacts the exposed buffer layer  7  between the nanowires in the n-pad area  11 . It should be noted that layer  16  does not contact the nanowires  1  in the non-active area  13  which is covered by the photoresist  13 . 
     Referring to  FIG. 4 e   , next step is to do another lithography step leaving another photoresist pattern  17  in the p-pad area  15 , the LED area  14  and the n-pad area  11 . This may be done by forming another photoresist layer over the device shown in  FIG. 4 d    (including over the metal electrode  16  covered resist pattern  10  in non-active area  13 ) and then exposing and developing the photoresist to leave the photoresist pattern  17  on both sides of the metal electrode  16  covered resist pattern  10 . 
     Referring to  FIG. 4 f   , the next step is to remove the electrode material  16  on the areas where electrode material is not covered by resist pattern  17  from the previous step, i.e., in the non-active area  13 , which can be done by selective dry or wet etching which does not remove the photoresist patterns  10  and  17 . This causes the electrode layer  16  to become discontinuous such that it is removed in the non-active area  13  between the n-pad area  11  and the active and p-pad areas  14 ,  15 . 
     Referring to  FIG. 4 g   , next step is removal of all remaining photoresist  10 ,  17 , which can be done by dissolving and/or plasma etching. This leaves the p-electrode layer  16  free-hanging between the nanowires  1  in the LED area  14 . This forms the air-bridge with empty space  18  between the electrode  16 , nanowires  1  and the masking layer  6 . 
     Optionally, the photoresist layer could be left underneath the bridge layer and then other material choices can be made. 
     Thus, in the case where it is desired to leave material underneath the bridged p-electrode, the process should be modified. Instead of applying photoresist to the entire device, another material such as spin-on glass, polymer, oxide (e.g., silicon oxide), nitride (e.g., silicon nitride)is deposited where the bridged p-electrode is to be located. These materials will not be affected by the etch that removes the photoresist. The layers could have purposes to guide light, change extraction properties, add isolation between p-contact and n-side or increase electrical conductivity to the p-side. 
     Referring to  FIG. 4 h   , finally residues of the protective layer  9  on the non-active area  13  that still may be present is removed. Thus, layer  16  forms the p-electrode  16   a  which contacts the tips of the nanowire  1  p-shells  3  and contacts the masking layer  6  in the p-pad area, as well as the n-electrode  16   b  which contacts the n-buffer layer  7  in the n-pad area  11 .  FIG. 5  shows two scanning electron microscope images of a bridged p-electrode in accordance with this embodiment of the invention. The intersection between the non-active area and LED area with the air-bridged p-electrode is visible on the left. 
     Since layer  16  was removed in non-active area  13 , the same layer  16  may be used to form both p- and n-electrodes. Thus, in the above process sequence illustrated by  FIGS. 4 a  to 4 h   , the p-electrode and n-electrode are deposited in the same step. The n-electrode layer  16   b  comprises an n-pad area  11  on a first part of the buffer layer  7 . The p-electrode layer  16   a  comprises a p-pad area  15  on the nanowires in a LED active area  14  or on a dielectric masking layer  6  on the buffer layer  7  adjacent to the nanowires in the LED active area. The n-pad area and the p-pad area are separated by a non active area  13  comprising dummy nanowires  1  which do not contact the p-electrode (i.e., these nanowires do not emit light). 
     However, in an alternative second embodiment, the p-electrode is provided in a first step and the n-electrode is formed from a different material at a later stage. Such a process is discloses in  FIGS. 4 i  to 4 s    and will be briefly described below. The description of the same elements and steps from  FIGS. 4 a -4 h    will not be repeated below for brevity. 
     The first two steps in the second embodiment method are identical to the first embodiment method, i.e.  FIGS. 4 a  and  b    represent the same steps as  FIGS. 4 i  and  j   . However, the protective layer  9  and masking layer  6  are not removed in the n-pad area  11   FIG. 4 j    as in  FIG. 4   b.    
     In the next step, a sacrificial (e.g., resist) layer  10   a  is deposited in two different thicknesses such that no nanowires are left uncovered in the n-pad area  11  as in the first embodiment. Thus, in the left hand side of  FIG. 4 k    it can be seen that the nanowires in area  11  are entirely covered just as the central nanowires in the non-active region  13 , as opposed to in  FIG. 4 c    where the leftmost nanowires in n-pad area  11  are completely uncovered. The nanowires in the LED area  14  are partially exposed on the top in the photoresist  10   a . The p-pad area  15  is completely exposed in photoresist pattern  10   a.    
       FIG. 41  shows that the protective layer  9  is at least partially removed from the exposed nanowire tips in LED area  13  in order to provide for contact between the p-shell  3  of the nanowires in area  13  and the p-electrode. 
     The p-electrode layer  16  is then deposited as shown in  FIG. 4 m   . Layer  16  covers the entire structure. The inactive  13  and n-contact  11  areas are now covered by the photoresist  10   a  and layer  16  is formed on top of the photoresist  10   a . Layer  16  contacts the exposed p-shells  3  of the nanowires in LED area  14  and the masking layer  6  in the p-pad area  15 . 
     As shown in  FIG. 4 n   , a second photoresist pattern  17   a  is now provided over the p-electrode layer  16  in the LED area  14  and the p-pad area  15 . Photoresist pattern  17   a  is removed in areas  13  and  11 . Thus, layer  16  is exposed in areas  11  and  13 . 
     The exposed p-electrode layer  16  is then removed from areas  11  and  13  by selective etching, as shown in  FIG. 4 o   . 
     As shown in  FIG. 4 p   , all photoresist  10   a ,  17   a  is removed such that the p-electrode layer  16  forms an air-bridge with underlying empty spaces  18  between the nanowires in LED area  14 , and forms a p-contact pad in area  15 . 
     Next, a new photoresist pattern  19  is applied to cover areas  13 ,  14  and  15  but not the n-pad area  11 , as can be seen in  FIG. 4 q   . The protective layer  9  and masking layer  6  are removed from exposed area  11 . 
     N-electrode layer  20  is then deposited over the entire structure, as shown in  FIG. 4 r   . Layer  20  may comprise Ti and Al sublayers or any other suitable metal. Layer  20  contacts the exposed buffer layer  7  and “dummy” shorted nanowires in area  11 . Layer  20  rests on photoresist  19  in areas  13 ,  14  and  15 . 
       FIG. 4 s    shows a lift-off step in which the photoresist pattern  19  is removed to lift off layer  20  in areas  13 ,  14  and  15 , such that the remaining layer  20  in area  11  forms the n-electrode, There is no electrode layers  16 ,  20  in the non-active area  13 . This prevents shorting of layer  16  and  20 . Dummy nanowires are located in the non-active area  13 . 
       FIGS. 4 h  and 4 s    show in process devices prior to formation of contacts (e.g., lead wires or bump electrodes) to the p-electrode  16   a ,  16  and n-electrode  16   b ,  20 , respectively. However, it should be understood that the contacts described with respect to  FIGS. 6, 10 or 12  are made to the p-electrode in p-pad area  15  and the n-pad area  11 , respectively. Furthermore, as noted above, the p-pad area  15  may be on top of nanowires (e.g., areas  14  and  15  are combined) rather than between nanowires as shown in  FIGS. 4 h    and  4   s.    
     The following third implementation of a method for forming a bottom emitting nanowire LED structure is described with reference to  FIGS. 6 a - h   . In this implementation, pads for connecting to The n-side and p-side are again formed in n-pad areas and p-pad areas, respectively, adjacent to the nanowires forming the nanowire LED. However, the invention is not limited to this. The same elements that were described above will not be described again below for brevity. 
       FIG. 6 a    shows the structure which is similar to  FIG. 4 a   . As in the previously described with respect to  FIG. 4 a   , a photoresist layer or pattern  10   b  with two thicknesses completely covers nanowires in the non-active area  13  and partially encloses the nanowires in the LED area  14 , leaving the top nanowire portions exposed. The n-pad area  11  and the p-pad area  15  are open and not covered by the photoresist pattern  10   b.    
     Referring to  FIG. 6 b   , in a next step, the protective layer  9  on the exposed top portion of the nanowires in the LED area  14  is selectively removed. Then, a p-electrode layer  16   c , a current spreading layer  16   d  and one or more reflector layers  16   e  are deposited over the entire device by for instance sputtering or evaporation. One or more of these layers may be omitted (e.g., the reflector layer  16   d  may be omitted if a separate mirror will be used), as long as at least one conductive layer is formed. 
     Referring to  FIG. 6 c   , in a next step the photoresist  10   b  is removed to lift off layers  16   c ,  16   d  and  16   e , and is optionally followed by a heat treatment to tune the properties of the layers. This leaves layers  16   c - 16   e  in areas  11 ,  14  and  15 . An air bridge is formed in area  14  with empty spaces  18  described above. This separates layer  16   c - e  into p-electrode  22  and n-electrodes  23 , as shown in  FIG. 6   d.    
     Referring to  FIG. 6 d   , in next step, the residues of the protective layer  9 , on the non-active area  13  is removed if desired. 
     Referring to  FIG. 6 e   , in a next step solder ball bumps (SBB) (e.g., p-bump  21   a  and n-bump  21   b ) are attached to the p-pad  15  and n-pad  11  areas, respectively. In the p-pad area  15 , the p-electrode  22  is isolated from the n-buffer layer  7  by the masking layer  6 . The p-electrode  22  provides electrical contact between the p-bump  21   a  and the p-shells  3  in area  14 . The n-electrode  23  provides contact between n-bump  21   b  and the n-buffer layer  7  and n-cores  2 . Thus, the buffer layer is accessed by the n-electrode/n-bump and the shells are accessed by the p-electrode/p-bump to provide an external electrical connection to the LEDs. 
     Referring to FIG. Of, in a next step the chip, i.e., the LED structure, is flipped over and dipped in a conductive adhesive  23  which remains on the bumps  21   a ,  21   b , in addition to providing electrical conductivity, the conductive adhesive may improve the heat dissipation properties, 
     Referring to  FIG. 6 g   , in a next step the chip is mounted on a carrier  24  pre-processed with p-and n-electrodes  25  and  26 . Although described in terms of a SBB arrangement it is appreciated by a person skilled in the art that there are other contact alternatives, such as lead wire or lead frame connections. 
     Referring to FIG. Oh, in a next step, the space between the chip and the carrier is underfilled, for example by an epoxy material  27  The underfill provides structural rigidity and may also contribute to improved heat dissipation. 
     Referring to  FIG. 6 i   , in a next step the Si substrate  5  is removed completely or partially by for example wet or dry etching to form an opening  28  exposing the buffer layer  7 . If desired, the buffer layer  7  may also be removed through opening  28  to expose the nanowire  1  bases, 
       FIG. 7  shows a nanowire structure obtained by this implementation of the method with a solder bump arranged on the nanowires. The p- and n-electrodes are accessed via the carrier wafer using a p-contact  29  and an n-contact  30 . This forms a bottom emitting LED device which emits light from LED areas  14  through the buffer layer  7 . 
     As mentioned above, nanowires may comprise heterostructures of compositionally different materials, conductivity type and/or doping such as the above exemplified radial heterostructures forming the pn or pin junction. In addition, axial heterostructures within the nanowire core may also be formed. These axial heterostructures can form pn- or p-i-n-junctions that can be used for light generation in a nanowire LED.  FIG. 8  schematically illustrates a plurality of nanowires with axial pn-junctions (e.g., the p-portion  3  located above the n-portion  2  in the axial direction contacted on the p-side  3  with a bridge electrode  8  arrangement. 
     Although the present invention is described in terms of contacting of nanowire LEDs, it should be appreciated that other nanowire based semiconductor devices, such as field-effect transistors, diodes and, in particular, devices involving light absorption or light generation, such as, photodetectors, solar cells, lasers, etc. can be contacted in the same way, and in particular the bridge arrangement can be implemented on any nanowire structures. 
     All references to top, bottom, base, lateral, etc are introduced for the easy of understanding only, and should not be considered as limiting to specific orientation. Furthermore, the dimensions of the structures in the drawings are not necessarily to scale. 
     In a further aspect of the invention, processes are provided for contacting arrays of nanostructures as described above. Such processes and the resulting devices will be described below with reference to  FIGS. 9-16 . The methods described render the LED devices bottom emitting. 
     In general the contacting entails providing reflective means, such as a mirror, at or near i.e. adjacent the top portions of each individual light emitting nanoelement so as to direct the emitted light backwards through the buffer layer of the device. 
     Thus, in  FIG. 9  there is shown a first embodiment in which the p-contact  90 , provided as described above, is transparent and suitably made of a conductive oxide, such as indium tin oxide (ITO). As can be seen, this p-contact layer will exhibit some topology. In some embodiments, this topology may be planarized when providing a mirror  92 . This can be done by applying, for example, a transparent glass layer  91  (e.g., spin-on glass, SOG) by a spinning technique to a thickness such that a planarization can be performed without causing damage. 
     The glass layer may be planarized by suitable methods such as polishing, reflow, and/or etching. 
     In order to make contact from the outside to the p-contact, a hole  93  is provided in the glass layer. This hole may be made by etching, such as dry etching, the glass layer  91  using a photoresist mask. 
     When a sufficient degree of planarity has been achieved, a reflective material  92 , such as Ag, is deposited on the glass layer  91 . In principle, any other reflective conductive material would be usable. Methods of deposition can be selected from sputtering, metal evaporation, electroplating and electroless plating. Suitably, the reflective layer may be provided in a thickness of about 500-1000 nm. As can be seen, the Ag layer  92  will be deposited also in the hole  93  in the glass layer and thus establish an electrical contact. 
     For bonding to a carrier substrate, a eutectic bonding method may be used. For example, a bonding medium such as a AuSn layer  95  can be used. However, a diffusion barrier  96  may first be suitably provided on the reflective layer  92 . The diffusion barrier may be in the form of a layer of a suitable metal selected from e.g. Ti, Ni, Pd, etc. 
     Eutectic bonding is well known to he skilled man per se and will not be described in detail. It is sufficient to mention that the bonding material may be provided on either the carrier  100 A shown in  FIG. 10  or on the LED device surface. The carrier and the LED device are brought in contact under slight pressure and optionally heating. 
     When bonding is finished, the original substrate (e.g., substrate  5  in  FIG. 2 ) is removed so as to expose the buffer layer  7  (e.g. AlGaN layer  7  in  FIG. 2 ) which will form the emitting surface of the LED device. The surface of the buffer layer is optionally etched in order to remove AIN (which may be part of the buffer layer) so as to allow contact to GaN and/or roughened to increase light outcoupling. 
     Finally, an n-contact layer  94 , suitably made of Ti/Al or other suitable metal material(s), is deposited on selected areas of the buffer layer  7  to provide a base for wire bonding. The entire assembly thus produced is now “flip-chipped” to a mount structure  100 A using a conductive material  103  and the eutectic bonding layer  95 , as shown in  FIG. 10 . A protective “bulb”  101  suitably made of silicone or similar material may be provided over the entire structure. A conductive wire  102  may electrically connect the contact layer with a second part of the mount structure  100 B. 
     Turning now to  FIGS. 11 and 12  a further embodiment is described. Here, instead of providing the p-contact  90  as a “bridged” contact, the p-contact  90  covers the entire light emitting nano element, i.e. the material extends all the way down to the insulation  6  (mask layer) between the nano elements and along. the sides of the nano elements  1 . This is referred to as a “semi-conformal” contact which includes the continuous contact layer  90  containing openings through which the nano elements  1  protrude. As can be seen in  FIG. 11 , not all nano elements  1  are coated with this se conformal contact layer  90 . 
     To form the contact layer  90  over a first set but not a second set of the nano elements  1 , a mask photoresist) may be formed over the second set of the nano elements and the contact layer is deposited over the exposed first set of nano elements and over the mask. The mask is then lifted off to remove the contact layer  90  from over the second set of nano elements  1  while leaving the contact layer  90  over and between the first set of nano elements. Alternatively, the contact layer  90  may be deposited over the entire device and then patterned through photolithography and etching (e.g., by forming a photoresist mask on the layer  90  over the first set of nano elements and etching away the portion of layer  90  over the second set of nano elements), The contact layer  90  may be deposited by any suitable method, such as sputtering. 
     When this semi-conformal p-contact has been deposited, an electrically insulating material  97 , such as silicon oxide, etc., is deposited so as to fill the spaces over the contact layer  97  located between the nano elements  1 . However, this material is only deposited up to a portion of the height of the nano elements  1  to expose the upper portion of the contact layer  90  located on top of some of the nano elements in the hole  93  in the spin on glass layer  91  to allow the mirror layer  92  to electrically contact the contact layer  90 , as can be seen in  FIGS. 11 and 12 . Thereafter the process is the same as in the embodiment described with reference to  FIGS. 9 and 10 , and will not be repeated here. 
     A still further variant of the device described above is disclosed in  FIGS. 13 and 14 . Here, the p-contact  90  is deposited so as to cover the surface between the nano elements and also the circumference of individual nano elements up to a certain height, but not more than about 90% of the height. Preferably about 30-90%, such as about 80% of the nano element height is coated. The contact includes the continuous contact layer  90  containing openings through which the nano elements  1  protrude. In this embodiment, the hole  93  through the insulating layer  97  and the spin on glass layer  91  extends down to the lower part of the contact layer  90  (e.g., the horizontal portion of the layer  90  on the mask layer  6  between the nano elements  1 ) to allow the mirror layer  92  to electrically contact the contact layer  90 . Again, in this embodiment the same procedure is used as describe with reference to  FIGS. 9 and 10  to finish the device, and will not be repeated here. 
     In another embodiment, the p-contact layer and the mirror layer are combined or integrated into a single layer  92 , as shown in  FIGS. 15 and 16 . The mirror layer  92  may be bridged between the nano elements. For example, the reflective mirror layer  92  may be located on the insulating material  97  located between the nano elements and in contact with the top portions of some of the nano elements  1  exposed in the insulating material  92  such that the mirror layer acts as a p-type contact to the nano elements. In this embodiment, the spin on glass layer  91  and the hole may be omitted. The integrated mirror/p-contact layer  92  may be formed by the procedures described with reference to  FIGS. 4 a - f   , Thus, after the reflective p-contact layer  92  is formed, a diffusion barrier  96  is provided over the reflective contact layer  92  to enable the subsequent eutectic bonding using bonding layer  95  formed over the barrier  96  without contamination of the reflective p-contact  92 . The resulting assembly is then “flip-chipped” to a mount structure, by the eutectic bonding as described above and as shown in  FIG. 16 . 
     It should be noted that nano elements or nanostructures  1  different from the ones illustrated in  FIG. 1-8  may be used in the embodiments of  FIGS. 9-16 .  FIGS. 17-20  illustrate several alternative embodiments of the nano elements or nanostructures  1  useable in the embodiments of  FIGS. 9-16 . Specifically,  FIGS. 17-20  illustrate a nano element  1  having the same general constitution as the ones disclosed in  FIGS. 1 and 2 , i.e., the nano element  1  has an n-type nanowire core  2  and optionally a p-type shell  3  at least partly enclosing the nanowire core  2  and an intermediate active layer  4 . The nano element is provided on a substrate  5 . 
     In all  FIGS. 17-20 , any of the layers provided on the base structure can be highly reflective or highly transparent. However, if all layers shown in  FIGS. 17-20  are transparent, then a further reflective layer is required on top, so as to provide a bottom emitting device. Furthermore, there is also shown in broken lines how adjacent nano elements are provided in a contiguous relationship. 
       FIG. 17  illustrates a p-contact layer  170  on the nano element  1 . This layer is suitably made from metal, TCO or conductive polymer and covers the sidewalk of the nano element and optionally on the substrate surface (e.g., on the mask layer  6  over the buffer layer  7  on the substrate  5 ) and/or on the tips of the nano elements in addition to the sidewalk of the nano elements. This layer constitutes the p-contact layer for the nano structure. The material forming this layer could be highly reflective or highly transparent. 
     There may also be provided another conductive layer  172 , such as metal, TCO or conductive polymer covering (only) the surface of the substrate  5 , i.e. the conductive layer  172  does not extend up onto the side walls of the nano elements. Rather, the conductive layer  172  forms a conductive connection between adjacent nano elements, and is thus intended to increase current conduction capacity. The material for this layer  172  should be chosen to give high heat dissipation properties. The material could be highly reflective or highly transparent. 
     Finally, the nano element may be provided with an electrically insulating passivating layer  174  (numbered  97  in  FIG. 9 ) such as polymer, oxide, nitride or similar insulating materials, to decrease leakage, influence from ambient and also to modify light extraction properties. The material for the passivating layer should be chosen to give high heat dissipation properties, and could he highly reflective or highly transparent, 
     The embodiment illustrated in  FIG. 18  has the same general constitution as the embodiment in  FIG. 17 . However, here the structure is covered with a further conductive layer  176  such as metal, TCO or conductive polymer. This layer  176  covers the nano element tips and extends further in the lateral direction so as to create a bridge  178  between adjacent nano elements (for illustration purposes, a neighboring element is shown in broken lines only). The purpose of this layer  176  is to increase current conduction capacity, as does the layer  172  in  FIG. 17 . In a preferred embodiment, this layer  176  forms the actual p-electrode layer for the structure. Preferably, this layer is reflective so as to render the device bottom emitting in operation, In such case, the p-contact layer  170  is optional. The material for the further conductive layer  176  should be chosen to give high heat dissipation properties, and could be highly reflective or highly transparent. In this embodiment the passivating layer  174  does not extend over the entire structure as in  FIG. 17 , but extends just partway along the sidewalls from the bottom and upwards (e.g., along 50-90% of the nano element height from the bottom of the nano element). By virtue of this, it forms a support for the bridged electrode layer  176 . 
       FIG. 19  shows yet another embodiment with a similar base structure. However, in this embodiment, the p-contact layer  170  extends only on the vertical sidewalls of the nano element. A further, thicker conductive layer  180  is provided on the p-contact layer  170 . This layer  180  is suitably made from a material such as metal, TCO or conductive polymer. The layer  180  covers the sidewalls and optionally on the substrate surface (e.g., on the mask layer  6  over the buffer layer  7  on the substrate  5 ) and/or on the tips of the nano elements, and is intended to increase current conduction capacity of the device. The material for this layer is chosen so as to give high heat dissipation properties, and it can be highly reflective or highly transparent. 
     The entire structure shown in  FIG. 19  (similar to that shown in  FIGS. 17 and 18 ) is covered with a passivating layer  174  such as polymer, oxide, nitride or similar to decrease leakage, influence from ambient and also to modify light extraction properties. The material for the passivating layer should be chosen to give high heat dissipation properties, and can be highly reflective or highly transparent. The passivating layer  174  can cover the top and sides of the conductive layer  180 , such that the electrical contact to layer  180  is made through a hole in layer  174 . 
     In  FIG. 20 , a structure is shown that differs from the structure illustrated in  FIG. 17  in that there is provided a conductive layer  182  that extends over the entire structure, i.e. it covers the substrate surface (e.g., mask layer  6  surface) and nano element side walls and the tip of the nano element (this configuration is referred to as a “turtle-neck”). The purpose here is also to increase current conduction capacity. The material is chosen to give high heat dissipation properties, and can be highly reflective or highly transparent. In this embodiment, layer  170  is optional. 
     Again, a passivating layer  174  is provided, such as polymer, oxide, nitride or similar to decrease leakage, influence from ambient and also to modify light extraction properties. The passivating layer  174  can cover the top and sides of the conductive layer  182 , such that the electrical contact to layer  182  is made through a hole in layer  174 . 
     In particular, it should be emphasized that although the figures illustrate embodiments having a pillar like geometry and are based on nano wire core, i.e. “one dimensional” cores, it should be understood that the cores can have other geometries such as pyramidal shapes by changing growth conditions. Also, by changing growth conditions, the final nano element can have a pyramidal shape, or any shape between a pillar like and a pyramid shape. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, on the contrary, it is intended to cover various modifications and equivalent arrangements within the scope of the appended claims.