# Patent

## Title
Electronic device containing nanowire(s), equipped with a transition metal buffer layer, process for growing at least one nanowire, and process for manufacturing a device

## Abstract

The electronic device comprises a substrate (1), at least one semiconductor nanowire (2) and a buffer layer (3) interposed between the substrate (1) and said nanowire (2). The buffer layer (3) is at least partly formed by a transition metal nitride layer (9) from which extends the nanowire (2), said transition metal nitride being chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride.

## Background
This application claims priority benefits to French Patent Application 12/60209 filed Oct. 26, 2012 and claims priority benefits to U.S. Provisional Patent Application Ser. No. 61/728,009 filed Nov. 19, 2012, the disclosures of both are incorporated herein by reference.


TECHNICAL FIELD OF THE INVENTION
The invention relates to the field of semiconductor materials and more precisely that of devices containing semiconductor nanowire(s).
The invention relates more particularly to a device comprising a substrate, at least one semiconductor nanowire and a buffer layer interposed between the substrate and said nanowire.
PRIOR ART
In the field of nanowire growth, it is known practice to use nucleation layers such as AlN (aluminum nitride) or TIN (titanium nitride). These layers may be deposited directly by LPCVD (low-pressure chemical vapor deposition) or by APCVD (atmospheric-pressure chemical vapor deposition) as described in document WO 2011/162715.
This document WO 2011/162715 states that semiconductor nanowires have a growth that may be promoted if the crystallographic orientation of a crystalline substrate enabling the growth is oriented in the direction [111] in a face-centered cubic structure of “NaCl” type, or in the direction [0001] or along the axis “c” in a “hexagonal” structure.
If the substrate is not correctly oriented. It is possible to deposit an AlN or TIN nucleation layer whose crystallographic structure will have a predominance of orientation in the direction [0001] for AlN which has a hexagonal structure and in the direction [111] for TIN which has an fcc structure.
It results from the foregoing that the crystallographic orientation of the growth support for nanowires is important. The predominance in a correct direction of a crystallographic structure should thus be optimized in order to promote the growth of the nanowires from this crystallographic structure.
Moreover, the nucleation layer should not be an obstacle to the polarization of a nanowire-based light-emitting nanodiode, for example performed by injection of electrons by the substrate to the nanowire via the nucleation layer, the nanowire constituting the N part of the PN junction of the nanodiode. A solution must therefore be found whereby this polarization may be optimized while at the same time conserving or providing new advantages during the growth of the nanowires.
OBJECT OF THE INVENTION
The aim of the present invention is to propose a device whose nanowire has a good crystallographic orientation and advantageously allows optimized polarization of the nanowire.
Steps toward this aim are taken by an electronic device comprising a substrate, at least one semiconductor nanowire and a buffer layer interposed between the substrate and said nanowire, the buffer layer being formed at least partly by a transition metal nitride layer from which extends the nanowire, said transition metal nitride being chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride.
Advantageously, the transition metal nitride layer comprises nitrogen, the concentration of which in the stable state of the transition metal nitride varies within a range of less than or equal to 10%.
According to one implementation, the nanowire is made of gallium nitride.
Preferably, the buffer layer is electrically conducting, so as to allow an electrical contact between at least one electrically conducting part of the substrate and the nanowire.
According to one embodiment, the end of the nanowire that is remote from the substrate is electrically doped according to a first type, and the device comprises a doped electrically conducting element of a second type placed at the end of the nanowire that is remote from the substrate so as to form an electrical junction, especially a light-emitting diode junction.
According to a perfection, the device comprises quantum wells placed at the interface between the nanowire and the doped electrically conducting element.
In addition, the device may comprise a nanowire polarization element so as to allow the generation of a light wave at said nanowire.
The invention also relates to a process for growing at least one semiconductor nanowire comprising a step of producing, on a substrate, a buffer layer at least pertly formed by a nucleation layer for the growth of the nanowire and a step of growth of the nanowire from the nucleation layer, the nucleation layer being formed by a layer of transition metal nitride chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride.
According to a first embodiment, the buffer layer is deposited as a vapor phase from a gas mixture comprising nitrogen and a transition metal chosen from vanadium, chromium, zirconium, niobium, molybdenum, hafnium or tantalum.
According to a second embodiment, the buffer layer is produced via the following steps: deposition on the substrate of a layer of a transition metal chosen from vanadium, chromium, zirconium, niobium, molybdenum, hafnium or tantalum; nitridation of at least part of the deposited transition metal layer so as to form the layer of transition metal nitride having a surface intended for the growth of the nanowire.
Advantageously, and in a manner that is applicable to the process in general, the transition metal nitride layer is made such that, once formed, it comprises nitrogen, the concentration of which in the stable state of the transition metal nitride varies within a range of less than or equal to 10%.
According to an implementation that is applicable to the second embodiment, the nitridation step of said at least part of the transition metal layer is performed so as to at least partly modify the crystallographic structure of the transition metal layer toward a face centered cubic or hexagonal, associated with the transition metal nitride layer.
Advantageously, the nitridation step comprises: a first nitridation substep at least partly performed at a first temperature by imposing an injection of a nitridation gas at a first flow rate; a second nitridation substep at least partly performed at a second temperature less than or equal to the first temperature by imposing an injection of the nitridation gas at a second flow rate different from the first flow rate.
According to a particular example, the injected nitridation gas is ammonia, and the first temperature is between 1000° C. and 1050° C., especially equal to 1050° C.; the first flow rate is between 500*V/8 sccm and 2500*V/8 sccm, especially equal to 1600*V/8 sccm; the second temperature is between 950° C. and 1050° C., especially equal to 1000° C.; the second flow rate is between 500*V/8 sccm and 2500*V/8 sccm, especially equal to 500*V/8 sccm; in which V is the total capacity in liters of a corresponding nitridation chamber.
Preferably, the nitridation step is performed in a nitridation chamber placed at a pressure of between 50 mbar and 800 mbar, especially 100 mbar.
For example, the nanowire growth step is performed after the second nitridation substep, or is initiated during the second nitridation substep.
Advantageously, the nanowire growth step comprises a step of injecting Ga so as to form the gallium nitride nanowire, said nanowire extending from a growth surface of the nucleation layer.
According to one implementation, with the substrate being silicon, the step of depositing the transition metal layer is configured such that the interdiffusion of silicon into the deposited transition metal layer is less than 10 nm and/or so as to conserve a non-silicided slice of the transition metal layer of at least 2 nm.
If the deposited transition metal is chosen from Cr, V or Ti, said transition metal is preferentially deposited at a temperature below 100° C.
In the case where the substrate 1 is based on silicon, the step of depositing the transition metal layer 6 preferentially comprises a preliminary step of determining the thickness of the transition metal layer to be deposited, comprising: a step of determining a first diffusion length of the silicon into the transition metal layer during the future deposition of the transition metal layer as a function of the transition metal used and of the deposition temperature; a step of determining a second diffusion length of the silicon into the transition metal layer during the future nitridation step of the transition metal layer; said thickness of the transition metal layer to be deposited being dependent on the desired thickness of the transition metal nitride layer and on a thickness of a silicided slice of transition metal obtained in the future transition metal layer from the first and second determined diffusion lengths.
According to one implementation, the process may comprise a step in which the substrate is provided such that it has a resistivity of between 1 mΩ·cm and 100 mΩ·cm.
Preferably, the process comprises, before deposition of the transition metal layer, a step of deoxidation of a surface of the substrate intended to receive the transition metal layer.
The invention also relates to a process for manufacturing a device as described, comprising a step of performing the growth process as described.
The manufacturing process may advantageously comprise the following steps: the electrical doping of a first type of at least one end of the nanowire which is opposite the substrate; the formation of an electrically doped element of a second type opposite the first type at the end of the nanowire opposite the substrate.
In addition, the manufacturing process may comprise a step of forming quantum wells placed at the interface between the nanowire and the electrically doped element of the second type.



SUMMARY DESCRIPTION OF THE DRAWINGS
Other advantages and characteristics will emerge more clearly from the description which follows of particular embodiments of the invention, which are given as nonlimiting examples and represented on the attached drawings, in which:
FIG. 1 illustrates a particular embodiment of an electronic device according to the invention,
FIG. 2 is a view in cross section of a step of formation of a nucleation layer,
FIG. 3 is a view in cross section of a step of nucleation of at least one nanowire from the nucleation layer,
FIG. 4 illustrates a representation of the X-ray diffraction spectrum for identifying the types of crystallographic structures present in an Nb-based transition metal layer before nitridation and after nitridation,
FIG. 5 illustrates a representation of the X-ray diffraction spectrum for identifying the types of crystallographic structures present in an Hf-based transition metal layer before nitridation and after nitridation,
FIG. 6 represents in detail an implementation of a nitridation step according to one embodiment of the invention,
FIGS. 7 and 8 illustrate different steps for preparing a nucleation layer.



DESCRIPTION OF PREFERENTIAL MODES OF THE INVENTION
The device described below differs from the prior art especially in the materials used and intercalated between the nanowire and the substrate.
The term “microwire” or “nanowire” in the rest of the description preferentially means a three-dimensional structure of elongated shape whose longitudinal dimension is at least equal to once the transverse dimension(s), preferably at least five times and even more preferentially at least ten times. The transverse dimension(s) are between 5 nm and 2.5 μm. In certain embodiments, the transverse dimensions may be less than or equal to about 1 μm, preferably between 100 nm and 300 nm. In certain embodiments, the height of each nanowire may be greater than or equal to 500 nm, preferably between 1 μm and 50 μm.
As illustrated in FIG. 1, the electronic device 4 comprises a substrate 1, at least one semiconductor nanowire 2 and a buffer layer 3 interposed between the substrate 1 and said nanowire 2. The buffer layer 3 is formed at least partly by a transition metal nitride layer from which extends the nanowire 2, said transition metal nitride being chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride. The term “extends from” means that the nanowire 2 has a defined length between two longitudinal ends 2a, 2b, a first longitudinal end 2a being in contact with the transition metal nitride layer and a second longitudinal end 2b being remote from the transition metal nitride layer. Preferably, the second longitudinal end 2b is the most distant from the transition metal nitride layer.
The choice of this transition metal nitride layer makes it possible to give said transition metal nitride layer electrical conduction nature of metallic type and refractory nature of ceramic type. These properties (metallic/refractory), which are in principle antagonist, may be achieved by the transition metal nitrides mentioned above. In point of fact, the refractory nature may be achieved by a material whose melting point is associated with a temperature above about 1800° C., which is the case for the transition metal nitrides targeted above. In addition, such a transition metal nitride layer was advantageously able to serve as a nucleation layer during growth of the nanowire so as to obtain a nanowire oriented along the axis C substantially perpendicular to the substrate 1. The term “substantially perpendicular to” means exactly perpendicular or perpendicular to more or less 10°. This slight disorientation of more or less 10° nevertheless permits the performance of subsequent technological steps for preparing more complete devices.
Tungsten, although present in the same column of the Periodic Table as chromium and molybdenum, is set aside from the list since it has insufficient stability properties at high temperatures, which does not allow efficient growth of the nanowires. In other words, such a device 4 cannot be obtained, or will be greatly defective, if the transition metal nitride layer is based on tungsten.
Moreover, titanium nitride is also set aside since, during its deposition, it gives rise to defects in the structure of the layer not allowing optimum growth of nanowire(s). Thus, the device 4 would have greater probabilities of being defective.
In point of fact, the transition metal nitride layer advantageously comprises nitrogen, the concentration of which in the stable state of the transition metal nitride varies within a range of less than or equal to 10%. The term “stable state” means that the transition metal nitride remains thermally and chemically stable irrespective of the annealing conditions. The transition metal nitride may be noted MxNy, with M representing the transition metal used, N the nitrogen compound, x and y representing the stoichiometric conditions of the transition metal nitride. By setting x as 1, y may vary within a certain measure while at the same time conserving a stable state of the transition metal nitride, this variation also being known as the stoichiometric deviation and being noted ΔN (corresponding to the range of less than or equal to 10% targeted above). If ΔN is too large, the associated structure may be considered as defective. This is due to the fact that, in general, a mononitride structure consists of two subnetworks: the metal atoms occupy a first subnetwork of face-centered cubic orientation and the nitrogen atoms occupy all or part (especially as a function of the composition) of the octahedral sites of the metal network to form a second but offset face-centered cubic subnetwork, especially offset by a/2, “a” representing the lattice parameter of the fcc structure. In the particular case of titanium nitride, the composition in the stable state may be between Ti1N0.6 and Ti1N1.1 (or an N concentration from 37.5% to 52.4%, i.e. a ΔN of 15%). This size of ΔN means that for Ti1N0.6 the nitrogen subnetwork is lacunar and that for Ti1N1.1 the metal subnetwork is lacunar. For comparative purposes: for vanadium nitride, the concentration of N in the stable state ranges between 41.9% and 50%, i.e. a ΔN of 8.1%; for niobium nitride, the concentration of N in the stable state ranges between 48% and 51.4%, i.e. a ΔN of 3.4%, for tantalum nitride, the concentration of N in the stable state ranges between 48% and 50%, i.e. a ΔN of 2%; for chromium nitride, the concentration of N in the stable state ranges between 49.5% and 50%, i.e. a ΔN of 0.5%; for molybdenum nitride, the concentration of N in the stable state ranges between 29% and 35.4%, i.e. a ΔN of 6.4%; for hafnium nitride, the concentration in the stable state ranges between 42.8% and 52.8%, i.e. a ΔN of 10%; for zirconium nitride, the concentration in the stable state ranges between 40% and 50%, i.e. a ΔN of 10%.
Advantageously, the nanowire(s) 2 are made of gallium nitride Gallium nitride is a good candidate for forming an electrooptic device. Specifically, such a nanowire 2 made of gallium nitride makes it possible to form a light nanoemitter. GaN-based quantum wells may be added either in shell form around the nanowire, or in the continuity of the axis of the nanowire (axial structure). Depending on the composition of these GaN-based quantum wells, the spectral domain of the light emission may cover a wide wavelength range extending from ultraviolet to infrared.
Preferably, the buffer layer 3 is electrically conductive so as to allow an electrical contact between at least an electrically conductive part of the substrate 1 and the nanowire 2. The substrate 1 in its entirety may also be electrically conductive, and may then be made, for example, of n-doped silicon.
According to a particular embodiment, the end 2b of the nanowire 2 that is remote from the substrate 1 is electrically doped according to a first type (for example of n type), and the device comprises a doped electrically conductive element 5 of a second type (for example of p type) placed at the end 2b of the nanowire 2 that is remote from the substrate 1 so as to form an electrical junction. This electrical function is preferentially a light-emitting diode junction. The element 5 doped so as to form a junction with the end 2b of the nanowire 2 may at least partly cover the nanowire 2 at said end 2b. Preferably, the doped element 5 forms a sheath around the end 2b of the nanowire 2.
Preferentially, the device 4 comprises quantum wells (not shown) placed at the interface between the nanowire 2 and the doped electrically conductive element 5.
The device may comprise an element 100 for polarization of the nanowire 2 so as to allow the generation of a light wave at said nanowire. It is thus understood that the device may be an electrooptic device. A polarization element comprises the means necessary for generating a current crossing the device. In the particular example of FIG. 1, the polarization element 100 may comprise a first electrical contact 100a in direct contact with the doped electrically conductive element 5, a second electrical contact 100b in direct contact with the substrate 1 and an energy source 100c. This makes it possible, for example, to inject electrons and holes so as to bring about their recombinations in a zone Z1 of the nanowire 2.
Moreover, in the context of an electrooptic device, the quantum wells may form, where appropriate, confinement zones so as to increase the emission yield of the nanowire 2.
An example based on a nanowire 2 has been given: needless to say, the device 4 may comprise a plurality of nanowires extending from the metal nitride layer so as to form, for example, a matrix of pixels. The elements 5 may then all be electrically linked to each other.
The device described above may be advantageously made at least partly via the process described below.
In general, in FIGS. 2 and 3, the process for growing at least one semiconductor nanowire 2 comprises a step of producing, on the substrate 1, the buffer layer 3 (FIG. 2) formed at least partly by a nucleation layer for the growth of the nanowire 2 and a step of growth of the nanowire 2 (FIG. 3) from the nucleation layer. On the example of FIGS. 2 and 3, the buffer layer 3 consists of the nucleation layer. In particular, the nucleation layer is formed by a layer of a transition metal nitride chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride. In other words, it may also be considered that the nucleation layer consists of a transition metal nitride layer as described. In order to optimize the growth, this nucleation/transition metal nitride layer may have a minimum thickness of 2 nm, and preferably between 2 nm and 50 nm.
This buffer layer 3 may be made via any type of deposition technique. The transition metal nitride layer (i.e. the nucleation layer) thus obtained also makes it possible, by virtue of the transition metal used, to have a smaller gap than the AlN-based nucleation layers that have been very commonly used to date as nucleation layer. Thus, when the substrate 1 is based on silicon, the buffer layer 3 according to the invention has at its interface with the substrate 1 a potential barrier that is easier to cross than in the case where AlN is used, this giving an advantage when it is desired to polarize one or more nanowires from the substrate 1.
According to a first embodiment, the buffer layer 3 is deposited as a vapor phase from a gas mixture comprising nitrogen and a transition metal chosen from vanadium, chromium, zirconium, niobium, molybdenum, hafnium or tantalum. The nucleation layer is thus obtained directly, after this deposition. According to this first embodiment, the buffer layer 3 may consist of the nucleation layer, which itself consists of the transition metal nitride layer.
According to a second embodiment, the buffer layer 3 is produced via the following steps: the deposition onto the substrate 1 of a layer of a transition metal chosen from vanadium, chromium, zirconium, niobium, molybdenum, hafnium or tantalum; and the nitridation of at least part of the deposited transition metal layer so as to form the transition metal nitride layer having a surface intended for growing the nanowire(s). According to one implementation applicable to the different transition metals, especially for Hf, Nb, Ta, the deposited transition metal layer may have a thickness of between 20 nm and a few hundred nanometers (for example 200 nm). For the other transition metals, a thickness of 20 nm will be preferred. The deposition of the transition metal layer may be performed by PVD (physical vapor deposition) from a metal target, for example a continuous-current spray passing over the target. During this step of deposition of the transition metal, the substrate may be maintained at room temperature. In a general manner applicable throughout the description, the term “room temperature” means a temperature preferably between 20° C. and 50° C. The pressure in the PVD chamber during the deposition of the transition metal may be between 3×10−3 mbar and 6×10−3 mbar.
After various tests, it was possible to observe that the growth of the nanowire(s) was promoted by this nucleation layer formed in two steps, it is thus understood that this second mode is preferred.
As indicated previously in the particular description of the device, and in a manner applicable to the various embodiments of the growth process, the transition metal nitride layer is made such that, once formed, it comprises nitrogen, the concentration of which in the stable state of the transition metal nitride varies within a range of less than or equal to 10%.
An example has been given based on the growth of a nanowire, but the growth process is not limited to this sole example and makes it possible, during the growth step, to grow a plurality of nanowires side by side from the nucleation layer.
It is understood from the foregoing that the function of the nucleation layer is to promote the nucleation of the nanowire(s). In addition, this nucleation layer is preferably chosen so as to protect the substrate 1 from any degradation during the growth (which may be the case if the substrate is made of silicon and the nanowire made of gallium nitride), and/or to conserve good stability at high temperatures (in the case of temperatures above 500° C.), and/or to give good electrical conduction especially when it is desired to polarize each nanowire and to inject current via the substrate 1. The materials of the nucleation layer targeted above make it possible to satisfy these problems.
As regards the substrate 1, the process may, in a nonlimiting manner comprise a step in which the substrate is provided such that it has a resistivity of between 1 mΩ·cm and 100 mΩ·cm. This resistivity is advantageous when it is desired to polarize the nanowires as indicated above across the nucleation layer via the substrate 1.
In order to optimize the growth of the nanowire(s), it will thus be sought to have, at the level of the nucleation layer, a crystallographic orientation adapted to the growth of the nanowires 2. The denser, i.e. the more predominant, this crystallographic orientation, the more the density of these nanowires 2 may be magnified.
In general, the step of growth of said at least one nanowire 2 may comprise a step of injecting a material intended at least partly to form the nanowire 2. In particular, this will be an injection of Ga so as to form the gallium nitride nanowire, said nanowire extending from the growth surface of the nucleation layer. To form the gallium nitride nanowire 2, the injection of Ga may be performed concomitantly with an injection of NH3 or N2. In general, for the synthesis of GaN, it is the reaction of Ga with NH3 and not with N2 that is used. The injection of Ga may be performed in a chamber adapted to the growth of nanowires.
The description below applies to the second embodiment of the growth process, i.e. to the metal nitride layer obtained in two steps.
In the case of the second embodiment of the growth process, it turns out that the metal nitride layer thus formed has growth sites whose crystallographic orientation is more favorable to the growth of nanowires than in the first mode. These growth sites are in greater number and have a better distribution than in the first mode.
It results from the foregoing that the crystallographic orientation of the nucleation layer, especially on a surface intended for growing the nanowire(s), is of importance in the context of promoting the growth of nanowires.
Thus, the step of deposition of the transition metal layer is preferentially performed such that said transition metal layer has, before the nitridation step, at least partly a crystallographic structure of centered cubic (CC) type for a layer of a transition metal chosen from Cr, Mo, V, Nb or Ta, or of hexagonal type for a layer of transition metal chosen from Zr and Hf.
The specific orientations targeted previously make it possible to perform the step of nitridation of said at least part of the transition metal layer so as to at least partly modify the crystallographic structure of the transition metal layer toward a face-centered cubic structure, which is especially oriented [111], or hexagonal, which is especially oriented [0001] or along the direction of the axis “C”, associated with the transition metal nitride layer. The transition metals of columns 4 and 6 preferentially form nitrides having an fee crystallographic structure, whereas the transition metals of column 5 may form nitrides having air fcc or hexagonal crystallographic structure.
FIGS. 4 and 5 each illustrate an X-ray diffraction spectrum for identifying the crystallographic phases or structures present. FIG. 4 shows that for the curve C1 representing the crystallographic structure of the layer of transition metal of Nb type before nitridation, there is indeed a predominance of the orientation [110] of the centered cubic (cc) structure of Nb, and that for the curve C2 representative of the hexagonal crystallographic structure of the transition metal nitride layer NbN, there is indeed a predominance of the orientation [0004] of the hexagonal structure and of its orientation harmonic [0008], i.e. of similar orientation to [0001]. FIG. 5 shows that for the curve C3 representative of the crystallographic structure of the transition metal layer of Hf type before nitridation, there is indeed a predominance of the orientation [0002] of the hexagonal structure, and that for the curve C4 representative of the face centered cubic (fcc) crystallographic structure of the transition metal nitride layer HfN, there is indeed a predominance of the orientation [111] of the face-centered cubic structure. On FIGS. 3 and 4, only the peaks are important for visualizing the predominance, the rest of the curve representing a continuous background due to the experimental device and the sample. Similar curves may be produced by a person skilled in the art for the other transition metals and the conclusions would be substantially identical, for example for tantalum nitride, there would be a predominance of the orientation [111] of the face-centered cubic structure of tantalum nitride.
According to a particular implementation of the second mode, especially illustrated in FIG. 6, the nitridation step may comprise a first substep of nitridation En1 performed at least partly at a first temperature by imposing an injection of a nitridation gas at a first flow rate, and a second substep of nitridation En2 performed at least partly at a second temperature less than or equal to the first temperature by imposing an injection of the nitridation gas at a second flow rate different from the first flow rate. This makes it possible to optimize the crystallographic orientation of the nucleation layer (transition metal nitride layer), it goes without saying that these two nitridation substeps are performed one after the other. In particular, the first substep En1 makes it possible to perform a rapid nitridation and the second substep En2 makes it possible to perform annealing which stabilizes the nitride phase of the transition metal. Following these two substeps En1 and En2, the transition metal nitride layer is chemically and thermally stable and can act as protection for the substrate (in particular if this substrate is made of silicon) during the growth of the nanowire(s).
The injected gas may be ammonia (NH3) or dinitrogen (N2). NH3 is preferred since it makes it possible to nitride the transition metal layer more rapidly. In point of fact, in the NH3 form, the nitriding power is greater than for N2. This rapid nitridation may be important especially it the transition metal is capable of being transformed into silicide: this point is addressed hereinbelow.
According to a particular example, the injected nitridation gas being ammonia, the first temperature is between 1000° C. and 1850° C., especially equal to 1850° C., the first flow rate is between 500 sccm and 2500 sccm (sccm means “standard cubic centimeters per minute”), especially equal to 1600 sccm, the second temperature is between 950° C. and 1050° C., especially equal to 1000° C., the second flow rate is between 500 sccm and 2500 sccm, especially equal to 500 sccm.
The flow rates mentioned above correspond to, the volume capacity of the nitridation chamber used, i.e. a total volume of gas (for example N2+NH3) of 8 liters in the example mentioned. For a chamber of a different volume, the flow rates must be adapted (for example: for an 18-liter chamber, the first flow rate will especially have to be equal to 4000 sccm and the second flow rate especially equal to 1280 sccm). In other words, the first flow rate is between 500*V/8 sccm and 2500*V/8 sccm, especially equal to 1600*V/8 sccm, and the second flow rate is between 500*V/8 sccm and 2500*V/8 sccm, especially equal to 500*V/8 sccm. V is the total capacity in liters of a corresponding nitridation chamber. The term “corresponding nitridation chamber” means herein the chamber in which the nitridation of the transition metal layer is performed.
In general, the nitridation step is performed in a nitridation chamber placed at a pressure of between 50 mbar and 800 mbar, especially 100 mbar.
FIG. 6 illustrates in a detailed manner the nitridation step by representing the change in temperature as a function of the time in a nitridation chamber. In a first time T1, the temperature in the nitridation chamber rises gradually, for example at 2° C./s up to a 1050° C. stage. The first nitridation substep under NH3 En1 targeted above begins when the temperature reaches 200° C. During this first substep, the NH3 flow rate remains constant at 1600 sccm. In a second time T2, concomitant with at least part of the first substep, the temperature is maintained at 1050° C. for a time of between 5 minutes and 15 minutes. In a third time T3, the first substep En1 is continued while the temperature passes from 1050° C. to 1000° C. in 60 s. In a fourth time T4, the temperature in the nitridation chamber is maintained at 1000° C. for a time of between 5 minutes and 15 minutes and the second substep En2 is started. In a fifth time T5, the introduction of heat into the nitridation chamber is stopped so that the temperature of the nitridation chamber falls until it returns to room temperature. The duration of T5 may correspond to the inertia of the nitridation chamber. The second nitridation substep may be continued for a given time during the fifth time T5. The fifth time T5 may correspond to stoppage of the heating of the chamber and then to its decrease in temperature or may also correspond to a step of growth of the nanowires if the chamber used for the nitridation is also the MOCVD chamber dedicated to the synthesis of the nanowires.
According to a particular implementation, the step of growth of the nanowire 2 is performed after the second nitridation substep En2, or is initiated during the second nitridation substep En2.
The use of Ga to form said at least one nanowire 2 is advantageous in the sense that the face-centered cubic or hexagonal structures of the transition metal nitride layer (and thus of the nucleation layer) are favorable to epitaxy of GaN. Nanowires made of gallium nitride, the crystallographic structure of which is a hexagonal structure (wurtzite structure) oriented along the axis [0001] or the axis C of FIG. 3, may be readily nucleated from the nucleation layer as described. Alternatively, the nanowires may also be made of ZnO, InN or SiC.
In order to achieve optimized nanowire growth, it is preferable for the transition metal layer to be sparingly silicide-treated (silicided). Silicidation of the transition metal layer may arise, if the substrate is based on silicon, according to two cases: during the step of deposition of the transition metal, and/or when it is desired to nitride the transition metal layer to delimit the nucleation layer. The first case may be explained in the following manner. In point of fact, at high temperature (about 1000° C.), the formation of silicide compounds MSi2 is promoted (M being the transition metal used). Among these silicides, only silicides of transition metals from column V (VSi2, NbSi2, TaSi2), plus chromium silicide (CrSi2) form crystallographic phases of hexagonal structure, which are potentially advantageous (if textured along the axis c) for the growth of gallium nitride nanowire(s). However, the disagreement in lattice parameter “a” between these hexagonal phases and gallium nitride (3.19 Å) is so large, respectively −30%, −36%, −33% and −25% for VSi2, NbSi2, TaSi2 and CrSi2, that epitaxy of gallium nitride is highly improbable. Typically, the lattice parameter “a” for the hexagonal compounds VSi2, NbSi2, TaSi2, CrSi2 is, respectively: 4.57 Å, 4.97 Å, 4.78 Å and 4.28 Å. Thus, a subfamily may be formed from the following materials: Ti, V, Cr, Nb, Ta, Mo, i.e. metals for which the interdiffusion coefficient with Si is high, which implies rapid growth kinetics of the new MSi2 phase. By way of example, Cr has a coefficient of interdiffusion with Si at 850° C. of 1.5×10−7 cm2/s, i.e. a diffusion length of about 11.6 μm in 15 minutes, whereas at about 100° C., this diffusion length falls to about 80 nm in 15 minutes. For the reasons mentioned above, if the deposited transition metal is chosen from Cr, V or Ti, it is preferentially deposited at a temperature below 100° C. so as to limit the diffusion of the silicon derived from the substrate 1. For Nb, the Nb—Si interdiffusion length over 15 minutes is 12 nm and 2 nm, for 800° C. and 700° C., respectively. Nb may thus be deposited at high temperature up to 700-750° C. without—or with very little silicidation. The other materials: Zr, Hf and Ta having smaller coefficients of interdiffusion with Si than Nb, may thus be readily deposited from room temperature up to 750° C.-800° C. at most. Excessive silicidation would have the consequence of not making it possible later to obtain a transition metal nitride layer of sufficient thickness. In other words, to generalize, when the substrate is silicon, the step of deposition of the transition metal layer is configured such that the interdiffusion of silicon into the deposited transition metal layer is less than 10 nm and/or so as to conserve a non-silicided slice of the transition metal layer of at least 2 nm. In point of fact, this non-silicided slice is opposite the substrate 1 and is intended to form the transition metal nitride layer. In FIG. 7, reference 6 indicates the transition metal layer initially deposited on the substrate 1, during the deposition of the layer, a slice 7 of this layer 6 may be silicided such that only part 8 of the layer 6 is composed of the pure transition metal that can serve to form, by nitridation, the nucleation layer. In the second case, the nitridation step may make it necessary to work at 1050° C. for a few minutes. To do this, use will preferably be made as nitriding gas of NH3, since, by virtue of its high nitriding power, the nitridation reaction rate is higher than the silicidation reaction rate. In point of fact, in the ideal case, it is sought to form during the nitridation step at least one transition metal nitride layer 9 (FIG. 8) in the deposited transition metal layer 6, the thickness of said metal nitride layer 9 advantageously being between 2 nm and 50 nm. In order to limit the production of a new silicide compound, the nitridation step will be optimized, in point of fact, after the nitridation step, it is understood, as illustrated in FIG. 8, that the layer that it was sought to produce by depositing a transition metal 6 may comprise a first transition metal silicide layer 7 obtained during the deposition of said transition metal, a second transition metal silicide layer 10 deposited in the continuity of the first transition metal silicide layer 7 and the nucleation layer 9 derived from the nitridation of the layer 8 of FIG. 7. Optionally, it is also possible that a residual layer 11 of pure transition metal remains intercalated between layer 9 and layer 10, this depending at least partly on the deposited thickness of the transition metal layer.
It results from the explanation of the first and second cases that if the substrate 1 is made of silicon, a person skilled in the art will be capable of determining the thickness of the transition metal layer to be deposited as a function of the type of transition metal to be deposited, of the temperature of deposition of the transition metal, of the duration of the transition metal deposition step, and also of the duration of the nitridation step so that it is possible to obtain a layer of transition metal nitride of a predetermined thickness. In other words, with the substrate 1 being based on silicon, the step of deposition of the transition metal layer 6 may comprise a preliminary step of determining the thickness of the transition metal layer 6 to be deposited, said step of determining the thickness comprising: a step of determining a first diffusion length of silicon into the transition metal layer 6 during the future deposition of the transition metal layer as a function of the transition metal used and of the deposition temperature; a step of determining a second diffusion length of silicon into the transition metal layer 6 during the future step of nitridation of the transition metal layer 6. Said thickness of the transition metal layer 6 to be deposited being a function of the desired thickness of the transition metal nitride layer and of a thickness of a slice of transition metal silicide obtained in the future transition metal layer 6 from the first and second determined diffusion lengths.
Preferably, the predominant crystallographic structure of the substrate 1 is of orientation [100] at least at the interface between the substrate 1 and the transition metal layer 6. This makes it possible especially to reduce the manufacturing costs.
In general, the substrate 1 will advantageously be prepared before the deposition of the transition metal layer 6. To do this, the process may comprise, before the step of deposition of the transition metal layer 6, a step of deoxidation of a surface of the substrate 1 intended to receive the deposit of the transition metal layer 6. More particularly, this step of deoxidation of the surface of the silicon may be performed either chemically (HF bath) or physically (etching of the surface by applying a bias tension to the substrate 1). This makes it possible especially to remove the layer of native silicon oxide (SiO2) which is an “insulating” barrier to the injection of electrons into the nucleation layer and into the gallium nitride nanowire.
Preferentially, the growth process described above in its various embodiments may be used in the context of forming a device as described previously.
Thus, the invention may also relate to a process for manufacturing a device, for example as described above, comprising a step of implementing the growth process as has been described above, especially in its various implementations or embodiments.
Moreover, the manufacturing process may also comprise a step of electrical doping of a first type of at least one end 2b of the nanowire 2 opposite the substrate 1. This first type is, preferably, doping of n type. In addition, the process also comprises a step of forming an electrically doped element 5 of a second type opposite the first type at the end 2b of the nanowire 2 opposite the substrate 1. This second type of doping is preferentially of p type. Thus, the end 2b of the nanowire 2 and the doped element 5 associated with this end 2b may form a junction of a diode intended to emit light. This junction is preferably a homojunction, i.e. the nanowire 2 and the associated doped element 5 are based on the same materials, for instance gallium nitride. The preparation of a heterojunction is also possible: for example, it is possible to use ZnO in the form of an n-doped nanowire, and then to add quantum wells based on ZnO and to use the element 9 made of electrically P-doped GaN. In point of fact, it is currently difficult to p-dope ZnO.
The manufacturing process may also comprise a step of forming quantum wells placed at the interface between the nanowire 2 and the electrically doped element 5 of the second type.

## Claims

1. An electronic device comprising a substrate, at least one semiconductor nanowire and a buffer layer interposed between the substrate and said nanowire,
wherein the buffer layer is at least partly formed by a transition metal nitride layer from which extends the nanowire, said nanowire being in direct contact with the transition metal nitride layer, said transition metal nitride being chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride,
wherein the transition metal nitride layer comprises nitrogen, the concentration of which in the stable state of the transition metal nitride varies within a range of less than or equal to 10%.

2. The device as claimed in claim 1, wherein the nanowire is made of gallium nitride.

3. The device as claimed in claim 1, wherein the buffer layer is electrically conductive so as to allow an electrical contact between at least one electrically conductive part of the substrate and the nanowire.

4. The device as claimed in claim 1, wherein the end of the nanowire which is remote from the substrate is electrically doped according to a first type, and wherein the device comprises a doped electrically conductive element of a second type placed at the end of the nanowire which is remote from the substrate so as to form an electric junction.

5. The device as claimed in claim 4, comprising quantum wells placed at the interface between the nanowire and the doped electrically conductive element.

6. The device as claimed in claim 1, comprising an element for polarizing the nanowire so as to allow the generation of a light wave at said nanowire.

7. A process for growing at least one semiconductor nanowire comprising producing, on a substrate, a buffer layer formed at least partly by a nucleation layer for the growth of the nanowire and growing the nanowire from the nucleation layer, wherein the nucleation layer is formed by a layer of a transition metal nitride in direct contact with the nanowire, said transition metal nitride being chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride so as to obtain the device as claimed in claim 1.

8. The process as claimed in claim 7, wherein the buffer layer is deposited as a vapor phase from a gas mixture comprising nitrogen and a transition metal chosen from vanadium, chromium, zirconium, niobium, molybdenum, hafnium or tantalum.

9. The process as claimed in claim 7, wherein the buffer layer is produced via the following:
deposition onto the substrate of a layer of a transition metal chosen from vanadium, chromium, zirconium, niobium, molybdenum, hafnium or tantalum,
nitridation of at least part of the deposited transition metal layer so as to form the transition metal nitride layer having a surface intended for the growth of the nanowire.

10. The process as claimed in claim 7, wherein the transition metal nitride layer is made so that, once formed, the transition metal nitride layer comprises nitrogen, the concentration of which in the stable state of the transition metal nitride varies within a range of less than or equal to 10%.

11. The process as claimed in claim 9, wherein the nitridation of said at least part of the transition metal layer is performed so as to at least partly modify the crystallographic structure of the transition metal layer toward a face-centered cubic or hexagonal, associated with the transition metal nitride layer.

12. The process as claimed in claim 9 wherein the nitridation comprises:
a first nitridation at least partly performed at a first temperature by imposing an injection of a nitridation gas at a first flow rate,
a second nitridation at least partly performed at a second temperature less than or equal to the first temperature by imposing an injection of the nitridation gas at a second flow rate different from the first flow rate.

13. The process as claimed in claim 12, wherein the injected nitridation gas is ammonia, and wherein:
the first temperature is in a range of from 1000° C. to 1050° C.,
the first flow rate is in a range of from 500*V/8 sccm to 2500*V/8 sccm,
the second temperature is in a range of from 950° C. to 1050° C.,
the second flow rate is in a range of from 500*V/8 sccm to 2500*V/8 sccm,
in which V is the total capacity in liters of a corresponding nitridation chamber.

14. The process as claimed in claim 12, wherein the nitridation is performed in a nitridation chamber placed at a pressure in a range of from 50 mbar to 800 mbar.

15. The process as claimed in claim 12, wherein the growth of the nanowire is at least one of (i) performed after the second nitridation and (ii) initiated during the second nitridation.

16. The process as claimed in claim 7, wherein the growth of the nanowire comprises injecting Ga so as to form the gallium nitride nanowire, said nanowire extending from a growth surface of the nucleation layer.

17. The process as claimed in claim 9, wherein the substrate is silicon, and wherein the depositing of the transition metal layer is configured so that at least one of (i) the interdiffusion of silicon into the deposited transition metal layer is less than 10 nm and (ii) a non-silicided slice of the transition metal layer of at least 2 nm is conserved.

18. The process as claimed in claim 9, wherein the deposited transition metal is chosen from Cr, V or Ti, and wherein said transition metal is deposited at a temperature below 100° C.

19. The process as claimed in claim 9 wherein the substrate is based on silicon, and wherein the depositing the transition metal layer comprises a preliminary determining of the thickness of the transition metal layer to be deposited, comprising:
determining a first diffusion length of silicon into the transition metal layer during the future deposition of the transition metal layer as a function of the transition metal used and of the deposition temperature,
determining a second diffusion length of silicon into the transition metal layer during the future nitridation of the transition metal layer,
wherein the thickness of the transition metal layer to be deposited is dependent on the desired thickness of the transition metal nitride layer and on a thickness of a slice of transition metal silicide obtained in the future transition metal layer from the first and second determined diffusion lengths.

20. The process as claimed in claim 7, comprising providing the substrate so that the substrate has a resistivity in a range of from 1 mΩ·cm to 100 mΩ·cm.

21. The process as claimed in claim 7, comprising, before deposition of the transition metal layer, deoxidizing a surface of the substrate intended to receive the transition metal layer.

22. A process for manufacturing a device as claimed in claim 1, comprising performing the growth process of at least one semiconductor nanowire, wherein the growth process comprises:
producing, on a substrate, a buffer layer formed at least partly by a nucleation layer for the growth of the nanowire, and
growing the nanowire from the nucleation layer,
wherein the nucleation layer is formed by a layer of a transition metal nitride chosen from: vanadium nitride, chromium nitride, zirconium nitride, niobium nitride, molybdenum nitride, hafnium nitride or tantalum nitride.

23. The process as claimed in claim 22, comprising performing:
electrical doping of a first type of at least one end of the nanowire opposite the substrate,
formation of an electrically doped element of a second type opposite the first type at the end of the nanowire opposite the substrate.

24. The process as claimed in claim 23, which comprises forming quantum wells placed at the interface between the nanowire and the electrically doped element of the second type.

25. The device as claimed in claim 1, wherein the transition metal nitride layer comprises nitrogen, the concentration of nitrogen in the stable state of the transition metal nitride is within a range of 0.5% to 8.1%.