HK1237004A1 - Gas-phase synthesis of wires - Google Patents

Description

Gas phase synthesis of wire

The application is a divisional application of an invention patent application, and the application date of the parent application is 2011, 5 and 11, and the application number is 201180034310.X (PCT/SE2011/050599), and the invention name is 'gas phase synthesis of lines'.

Field of the invention

The present invention relates to the formation of wires (wires), and in particular to the gas phase synthesis of wires in the absence of a substrate. Gas phase synthesis is suitable for different materials, in particular semiconductor materials.

Background

Small elongated objects, commonly referred to as nanowires (nanowires), nanorods, nanowhiskers, etc., and typically comprising semiconductor materials, have so far been synthesized using one of the following routes:

liquid phase synthesis, for example by means of colloidal chemistry, as illustrated in US 2005/0054004 to Alivisatos et al,

epitaxially grown from a substrate, with or without catalytic particles, as exemplified in the studies presented by Samuelson et Al in WO 2004/004927A 2 and WO 2007/10781 Al, respectively, or

Gas phase synthesis by means of a laser assisted catalytic growth process, as exemplified in WO 2004/038767 a2 by Lieber et al.

The properties of the lines obtained using these routes are compared in the table below.

	Quality of material	Width/length and size control	Structural complexity	Scalability/production cost
					Liquid phase	Height of	Thin/short medium control	Is low in	High/high
Based on a substrate	Height of	All/all high control	Height of	Low/high
					Laser assisted	Medium and high grade	Thin/long medium control	Is low in	Moderate/moderate

Thus, the choice of synthetic route is a compromise between different linear properties and production costs. For example, substrate-based synthesis provides advantageous wire properties, but because the wires are formed in batches, the process scalability and therefore the production costs and throughput are limited.

Summary of The Invention

In view of the foregoing, it is an object of the present invention to provide a method and system for forming wires that enables large scale processes and with comparable structural complexity and material quality as wires formed using substrate-based synthesis.

The method comprises the following basic steps:

providing catalytic seed particles (catalytic seed particles) suspended in a gas,

providing a gaseous precursor comprising the components of the wire to be formed,

passing the gas-particle-precursor mixture through a reactor, typically a tube furnace, and

-growing wires from the catalytic seed particles in a gas phase synthesis comprising a gaseous precursor while suspending the catalytic seed particles in a gas.

In a first aspect of the invention, by varying the growth conditions during the growth of the individual wires, wires having different configurations may be provided, e.g. wires made of substantially the same material, unipolar wires or more complex wires, e.g. wires with axial pn-or pin-junctions, wires with radial pn-or pin-junctions, heterostructure wires or the like, such that wire segments (wire segments) are grown axially in their longitudinal direction on previously formed wire portions, or shells (shells) are grown radially in their radial direction on previously formed wire portions, or the material is added as a combination of axial and radial growth. The growth conditions may be varied between reaction zones by controlling one or more of the parameters associated with: precursor composition, precursor molar flow, carrier gas flow, temperature, pressure, or dopant. This variation is actually achieved by performing the wire growth in two or more zones, which may be maintained at different temperatures, and into which appropriate growth or dopant precursor molecules are injected by means of mass flow controllers or similar devices.

The growth conditions may also be varied over time by controlling one or more of the parameters relating to: the precursor composition, precursor molar flow, carrier gas flow, temperature, pressure, or dopant, or the size distribution of the catalytic seed particles, such that the strand properties may be varied from time to produce batches having a range of different strands, or to produce unique homogeneous batches.

The catalytic seed particles may be provided as an aerosol that is mixed with the gaseous precursor before or during initiation of wire growth. Alternatively, the catalytic seed particles are formed by being formed from gaseous reactants comprising at least one component of the catalytic particles, thereby enabling self-catalytic wire growth.

Preferably, the process of the present invention comprises providing a gas stream carrying catalytic seed particles and subsequently passing the partially or fully formed thread through one or more reactors, each reactor comprising one or more reaction zones. Such that the catalytic seed particles and any line sequences formed thereon flow through one or more reaction zones, wherein each reaction zone facilitates line growth by adding material to the line or etching the line. This enables to provide optimal conditions for each step during the growth process.

The diameter of the wire is determined in part by the size of the catalytic particles. Thus, by selecting the appropriate size or size distribution of the catalytic seed particles and by adapting the growth conditions to the size of the catalytic seed particles, the diameter of the line can be controlled.

In the case of the second reaction furnace or reaction zone, continuous wire growth takes place on the preformed semiconductor wire with attached catalytic particles formed in the first reactor. These lines act as flying substrates (growing) and therefore grow more readily than in the first zone, where line nucleation occurs on the seed particles. Thus, wire growth is more efficient and occurs at lower temperatures in the subsequent furnace. Depending on the growth conditions (reactor temperature and pressure, precursor type and concentration, seed particle/wire size and concentration, and reaction time), subsequent wire growth occurs in either the axial or radial direction or as a combination of both.

In one aspect of the invention, the method includes adding HC1 or other etching halide compounds to the aerosol stream to mimic the conditions in hydride vapor phase epitaxy, HVPE, to prevent growth on the hot walls of the reactor. HVPE sources are also useful in the present invention, wherein the metal group-III atoms are carried to the reaction zone as chlorides.

In another aspect of the invention, the seed particles/threads are heated by means of microwaves, infrared light or other electromagnetic radiation instead of or in addition to a hot-wall tube oven. This allows the gas to remain more or less cool, minimizing the amount of gas phase reactions, while allowing growth on hot particle/wire surfaces.

In yet another aspect of the invention, the method includes analyzing the wire or partially grown wire in situ to obtain a desired linear property. Means for controlling the line growth (means) include controlling the size of the catalytic seed particles, and controlling the growth conditions by controlling one or more of the parameters related to: precursor composition, precursor molar flow, carrier gas flow, temperature, pressure, or dopant. In situ analysis provides a means for getting feedback in the control loop (which is not available in e.g. substrate based synthesis). Any deviation from the desired property is detected quickly and growth conditions can be adjusted without significant delay or without having to discard a significant amount of the wire.

Means for in situ analysis include means for detecting the size of the catalytic seed particles and/or the formed lines, such as Differential Mobility Analyzers (DMA), illumination and detection of luminescence from the formed lines, absorption spectroscopy, raman spectroscopy, and in-flight X-ray powder diffraction (X-ray powder diffraction-the-fly), among others. In addition to possibly controlling line growth "in real time", in situ analysis may also be used to selectively classify lines having different properties (e.g., dimensions). Although described in terms of a line, it is to be understood that in situ analysis may also be performed on catalytic seed particles or partially formed lines.

In yet another aspect of the invention, the method includes a gas collection line that is self-carrying. The wires may be collected and stored for later use or they may be transferred to a different carrier or substrate to be incorporated in some structures to form devices.

To utilize a continuous stream of wires, the wires may be deposited and/or aligned on the substrate in a continuous process, such as a roll-to-roll process. Deposition and/or alignment may be assisted by an electric field applied over the substrate and further by charging the wires and optionally also the substrate. By locally charging the substrate in a predetermined pattern, the wires can be deposited in predetermined locations on the substrate. The present invention thus provides a continuous high-throughput method for producing aligned lines on a substrate, optionally with "real-time" feedback control, to achieve high quality lines.

The wires produced by the method of the invention can be used to realize wire-based semiconductor devices, such as solar cells, field effect transistors, light emitting diodes, thermoelectric elements, field emission devices, nano-electrodes for life sciences, etc., which are superior in many cases to conventional devices based on planar technology.

Although not limited to nanowires, semiconductor nanowires produced by the method of the present invention have several advantages over conventional planar processing. Although semiconductor devices fabricated using planar techniques suffer from certain limitations, such as lattice mismatch between successive layers, nanowires formed according to the present invention provide greater flexibility in selecting semiconductor materials in successive segments or shells, and thus greater possibilities to adapt the band structure of the nanowire. The nanowires also potentially have a lower defect density than the planar layer, and by replacing at least a portion of the planar layer in the semiconductor device with nanowires, defect-related limitations can be reduced. Furthermore, the nanowires provide a surface with a low defect density as a template for further epitaxial growth. In contrast to substrate-based synthesis, the lattice mismatch between the substrate and the wires does not have to be taken into account.

The apparatus of the invention comprises at least one reactor for growing the wire, said reactor comprising one or more reaction zones, means (means) for supplying the reactor with catalytic seed particles suspended in a gas, means for supplying the reactor with a gaseous precursor comprising the components of the wire to be formed, and means for collecting the wire grown from the catalytic seed particles in a gas-phase synthesis comprising the gaseous precursor while the catalytic seed particles are suspended in the gas.

Multiple reactors, each providing one reaction zone, or reactors divided into different reaction zones, or combinations thereof, may be used to enable the growth conditions to be varied during the growth of the individual wires. During processing of the catalytic particles, partially grown and fully grown strands are sequentially carried through the reactor by the gas stream.

Preferably the apparatus further comprises means for analysing the formed wires in situ. In one embodiment of the invention, the means for in situ analysis is arranged after one of the reaction zones for detecting linear properties and the signal from the means for in situ analysis is fed back to the means for controlling upstream growth conditions.

One advantage of the method and apparatus of the present invention is that the wire can be grown at an unexpectedly high rate. The growth rate may be higher than 1 μm/s, which means that for a typical line of size 0.4 x 3 μm, the growth time is a few seconds. This means that the throughput is enormous in the continuous process of the present invention.

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 drawings and the claims.

Brief Description of Drawings

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which

Figure 1 schematically illustrates the axial growth of nanowires of the invention,

FIG. 2 schematically illustrates a system for forming wires in accordance with the present invention in (a) a system having a single reactor, and in (b) a system extending to a module having multiple reactors, in the example of (c-h) different sub-modules,

figure 3 schematically illustrates the axial growth of a wire comprising a pn-junction according to the invention,

figure 4 schematically illustrates the core-shell growth of a wire comprising a pn-junction according to the invention,

figure 5 schematically illustrates a system for forming a wire including an in situ analysis module according to the present invention,

figure 6 schematically illustrates a first embodiment of a system for forming nitride based LED structures having different emission wavelengths according to the present invention,

figure 7 schematically illustrates a second embodiment of a system for forming nitride based LED structures having different emission wavelengths according to the present invention,

figure 8 schematically illustrates an arrangement of in-situ luminescence measurements in a system for forming lines according to the present invention,

FIG. 9 schematically illustrates an arrangement of in-situ absorption measurements in a system for forming lines, in accordance with the present invention, an

Fig. 10 and 11 show wires having different configurations formed in a system according to the present invention.

Detailed description of the embodiments

For the purposes of this application, the term "thread" refers to an elongated object. As mentioned above, the width or diameter of these wires may be substantially of nanometer size, commonly referred to as nanowires, nanowhiskers, nanorods, and the like, but is not limited thereto.

Referring to FIG. 1, fundamentally, a method for forming the inventive wire comprises

Providing catalytic seed particles 2 suspended in a gas,

providing gaseous precursors 3, 4 comprising the components of the thread 1 to be formed, and

growing the wire 1 from the catalytic seed particles 2 in a gas phase synthesis comprising gaseous precursors 3, 4 while suspending the catalytic seed particles in a gas.

The growth, or at least a part thereof, is carried out at high temperature, usually in a furnace or in some other kind of reactor, and is initiated by initiating growth and nucleation by catalytically decomposing the gaseous precursors 3, 4 on the surface of the catalytic seed particles 2. After nucleation, the wire 1 grows directionally and forms an elongated object, i.e., a wire. Preferably, the gas flows through the reactor, thereby carrying at least the catalytic seed particles and the threads thus formed on the catalytic seed particles through the reactor.

The method is described herein in terms of semiconductor materials, particularly but not limited to III/V-materials. By way of example, FIG. 1 schematically illustrates the formation of a catalyst composed of catalytic seed particles 2 (e.g., gold) and the gaseous precursors TMGa 3 and AsH₃4 form GaAs line 1. As shown, the catalytic particles are carried forward by the gas into the reactor where the gaseous precursors 3, 4 are present and react. The precursor gas may be added to the gas stream prior to entering the reactor or may be added directly to the reactor.

The basic system for forming a wire according to the invention is schematically illustrated in fig. 2 a. The system comprises at least one reactor 8 for growing the wire 1, means 9 for providing the reactor 8 with catalytic seed particles suspended in a gas, and means 10 for providing gaseous precursors 3, 4 comprising the components of the wire 1, the wire 1 being grown from the catalytic seed particles in a gas phase synthesis comprising the gaseous precursors while suspending the catalytic seed particles in the gas. Optionally the system further comprises means 11 for collecting the line 1. The system may also include means for analyzing 12 the particles and the line formed in the reactor 8 in situ, such as a Differential Mobility Analyzer (DMA) or other analytical tool to monitor the size or other properties of the line.

In one embodiment of the process of the invention, the wire growth is carried out in one or more reactors arranged in series and/or in parallel, wherein a continuous flow of catalytic seed particles is supplied as an aerosol, mixed with the gaseous precursors 3, 4, and the gas mixture is then passed into the first reactor of said one or more reactors in which the wire growth is initiated. The catalytic seed particles 2 may also be formed by gaseous reactants within the first reactor, thereby enabling self-catalytic wire growth. When wire growth is performed in multiple reactors, each reactor increases the complexity of the wire, e.g., creating pn-junctions or heterostructures in the axial or radial direction.

The reactor of the system, the means for providing the catalytic seed particles, the means for in situ analysis, etc. do not have to be a separate chamber or arrangement. Preferably the system is a modular system incorporated in an on-line production facility. In particular, each reactor may comprise one or more reaction zones arranged in series and/or parallel, as described above for the reactors. Thus, these terms are used interchangeably hereinafter as the reaction zone has the same function as the reactor. Fig. 2b schematically illustrates a modular system with a particle delivery system 9, several growth modules arranged in series and in parallel and means for collecting particles and threads carried out by the growth modules by the gas flow. FIG. 2 shows other examples of modules that may be incorporated into the system: (c) a wire growth module, (d) a shell growth module, (e) a passivation layer growth module, (f) an in situ analysis tool 12 (where the arrows illustrate possible feedback controls), such as DMA, (g) an evaporation module with an evaporation source 13, and (h) a plasma-enhanced chemical vapor deposition module with a plasma source 14, but is not limited thereto.

Figure 3 schematically illustrates how the method of the invention may be used to form a GaAs wire comprising an axial pn-junction between a p-doped GaAs segment and an n-doped GaAs segment. Precursors 3, 4 comprising group III and group V materials, respectively, and a p-dopant are provided to the reactor, and after nucleation, the p-doped GaAs grows axially from the catalytic seed particles, forming a first axial segment of the GaAs wire. Subsequently, the growth conditions are changed by exchanging the p-dopant for the n-dopant, while substantially maintaining the other parameters related to the growth conditions, such that the second axial line segment is axially grown in its longitudinal direction on the previously formed first segment. This illustrates that during axial growth it is possible to change the growth conditions to obtain axial segments with different properties.

Figure 4 schematically illustrates the formation of a GaAs wire comprising a radial pn-junction between a p-doped GaAs core and an n-doped GaAs shell. Precursors 3, 4 comprising group III and group V materials, respectively, and a p-dopant are provided to the reactor, and after nucleation, the p-doped GaAs grows axially from the catalytic seed particles, forming nuclei of GaAs wires. Subsequently, the growth conditions are changed by increasing the temperature and/or the V/III ratio to promote radial growth and by changing the p-dopant to an n-dopant. Thereby radially growing the shell in a radial direction thereof on the previously formed core. This illustrates that it is possible to change the growth conditions to switch between axial growth and radial growth.

Although illustrated with GaAs, it is understood that other III/V semiconductor materials, as well as semiconductor materials comprising group II and group VI materials, can be processed in the same manner. Gaseous precursors such as the above example may be exchanged for TMIn and PH₃To form InP lines. As will be appreciated by those skilled in the art, the reactor configuration need not be altered to form lines from different gaseous precursors, but simply to convert the gaseous precursors. In addition, for example, in FIG. 3 and the drawings4 may be carried out with or without a dopant. Insulators may also be grown. A single or multiple reactors or reaction zones within a reactor may be used to improve the formation of fragments, cores or shells having different compositions, doping or conductivity types. Furthermore, the axial and radial growth need not be completely separated, but the wire may grow radially and axially at the same time. By selecting the appropriate gaseous precursors, flow rates, temperatures, pressures and particle sizes, the wire material can be grown in either the axial or radial direction, or in a combination of the two growth modes.

The catalytic seed particles may be composed of a single element or a combination of two or more elements to facilitate wire growth or doping of the wire. Gaseous precursors may also be used to dope the wires.

In case of a preformed catalytic seed particle, the means for providing a catalytic seed particle 9 may comprise a particle generator. By a range of prior art methods, particle generators produce aerosols with more or less size-selected particles. Particle generation can be performed by evaporation/condensation, spray or vapor pyrolysis, spark discharge, laser ablation, electrospray of colloidal particles, and the like. Size selection can be made by gas mobility classification, for example, by using DMA, virtual collision or only by well-controlled particle formation. For many applications, it is desirable for the aerosol particles to be electrically charged, which can be accomplished by radioactive sources, corona discharge, thermal or optical emission of electrons, and the like. Typical systems for particle generation are described in Magnusson et al, gold nanoparticles: production, reshaping, and thermal charting (gold nanoparticles: production, modification and charging), J nanoparticie Res, 1, 243-.

As mentioned above, the system may comprise one or more reactors or reaction zones, wherein each reactor or reaction zone adds a new functional layer to the line. Such a modular system is shown in fig. 5 and described further below. Depending on growth parameters such as precursor molecules, temperature, pressure, flow, particle density and particle size, a new functional layer may be added as an axial extension of the previously formed wires, as a radial shell, or as a combination of both axial and radial growth. The layers formed may be of similar or different materials, i.e., homo-or heteroepitaxy, and of similar or different conductivity types, e.g., pn-junctions. The functional layer is not limited to a crystalline layer formed by epitaxy, but may also be an amorphous layer, such as an oxide, providing passivation and/or insulation functions. Chemical reactions using surfactant or polymer shell coating lines, or condensing sacrificial layers (sacrificial layers) for later redispersion are other possibilities.

For some growth conditions, additional modules may be added to the reactor or reaction zone. For example, a plasma generator may be added to alter the chemical reaction so that the reaction rate can be higher. This is important, especially if the lines or layers formed on the wires are grown at low temperatures by stable precursors, which usually require high temperatures to decompose. A typical example where this may be useful is the growth of nitrides from ammonia.

Before or between the reactors or reaction zones, other components may be placed, such as devices for charging the particles or wires. By utilizing a relatively low diffusion coefficient of the wire, a tube-type absorption filter can be used to remove precursor molecules and small particles from the gas stream. The precursors and reactants can thus be exchanged between growth reactors, not just added. Size classification tools (e.g., DMA or virtual collider) may also be used to refine the gas stream (i.e., aerosol), or as an in situ analysis, as explained below.

With reference to fig. 5, in the following, an implementation of the inventive method is described in terms of growing GaAs lines containing pn-junctions. The system comprises a particle delivery system, which may consist of any of the aforementioned particle generators. Producing particles, which are subsequently flowed from a particle delivery system by a gas (e.g., H)₂Or N₂) Is carried. Hereinafter, the gas stream containing the particles or threads is referred to as an aerosol. The GaAs (n-type) wire growth module consists of a reaction furnace and a gas delivery system for the precursor molecules. In this case, the precursor molecules are TMGa, AsH₃And SiH₄. TMGa and AsH₃Forming a GaAs material, and SiH₄The wire is doped with Si, resulting in an n-type material. The precursor molecules are mixed with the aerosol and then enter the reaction furnace. After entering the reaction furnace, the precursor reacts with the particles in the aerosol to form n-type GaAs wires. The growth parameters (temperature, flow, pressure, etc.) are varied to obtain the desired properties (length, crystal structure, shape, etc.). After the GaAs (n-type) wire growth module, the aerosol, now consisting of carrier gas and n-type GaAs wire, exits the GaAs wire growth module and is split into a small flow and a large flow. Small flows enter the DMA, which analyzes the line size distribution. A larger flow enters the next wire growth module. The GaAs (p-type) line growth module is designed to grow an axial extension of p-type GaAs on top of a previously grown n-type GaAs line. The growth module has essentially the same design as a GaAs (n-type) wire growth module, except that the precursors are now made of TMGa, AsH₃And DEZn. TMGa and AsH₃An axial extension of GaAs material is formed, while DEZn dopes the wire with Zn, resulting in a p-type material. The growth parameters in the furnace do not have to be the same as those of the previous growth module, but are optimized to obtain an axial extension of the wires with high quality p-type GaAs material. After leaving the GaAs (p-type) wire growth module, the aerosol separates into a small flow and a large flow. Small flows enter the DMA, which analyzes the line size distribution. The large flow enters a line collection module that can collect the line by any of the aforementioned methods.

By using multiple means for in situ analysis, such as the two in situ DMAs of fig. 5, the intermediate state of the in-line growth can monitor the line growth process and, if necessary, can adjust the growth parameters to obtain consistent high quality lines with the desired properties.

As mentioned above, the method and system of the present invention can be used to form complex line structures. By way of example, fig. 6 schematically illustrates a system for growing nitride based Light Emitting Diodes (LEDs) adapted to provide emission at different wavelengths. The system comprises a particle delivery system, a GaN (n-type) wire growth module arranged in series, followed by an InGaN shell growth module arranged in parallel, followed by AlGaNN (p-type) shell module, and finally a device for particle/wire collection. Thus, the gas flow is divided into InGaN shell growth modules In parallel, which are adapted to form InGaN shells having different compositions, i.e., In_xGa_1-xN、In_yGa_1-yN and In_yGa_1-yN, wherein x ≠ y ≠ z. The lines will get different emission characteristics due to different growth conditions in each branch. For example, lines suitable for emission in the red, green and blue wavelength regions may be implemented. By collecting at least partially formed wires from an InGaN shell growth module into a common gas flow, different wires can be grown and collected simultaneously for an assembly of white LEDs.

Fig. 7 schematically illustrates a similar system as shown in fig. 6, but with possibly more control during growth, since different InGaN quantum wells result in different shells individually adapted to the quantum well structure. Each InGaN shell growth module is followed by a p-AlGaN growth module, in addition to the parallel InGaN shell growth modules of the system of fig. 6. However, the n-GaN line growth module and the AlO passivation layer growth module following the p-AlGaN shell growth module may be the same for different lines to reduce the complexity of the system.

The flexibility of the system allows several in situ analysis tools 12 to measure and monitor properties that are not available using other wire growth techniques. This allows for instantaneous feedback to adjust the system so that it is possible to continuously fine-tune the material parameters in a manner not possible in other methods.

By way of example, line size measurement and classification may be achieved through the use of DMA. The DMA or any other device for in situ analysis may be coupled in series or in parallel depending on whether the measurement is invasive or non-invasive with respect to the gas flow. Serially coupled DMAs can be classified according to the size of the aerosol centerline. The classified sizes and size distributions depend on the nature and setting of the DMA. When coupled in parallel, a small aerosol stream can be extracted to the DMA for nearly noninvasive measurements. In this case, the DMA may be scanned over its size detection range to obtain the size distribution of the aerosol. This can be achieved by consuming only a small portion of the gas stream, thus maintaining a high productivity of the line.

By illuminating the gas stream, the optical properties of the thread can be studied in a non-invasive manner. The light source should preferably be a laser with a higher light energy than the band gap of the material or materials of which the line consists. By using a photodetector, the luminescence from the line can be studied. This enables monitoring of the optical properties of the wire, which can be used to adjust the growth parameters to obtain the desired linear properties. This is different from other growth methods in that the wire can be cooled down rapidly after each successive growth reactor or reaction zone, and in wire growth, between each step, temperature sensitive luminescence techniques can be used.

Other possible in situ optical methods include absorption spectroscopy, where the absorption path ideally flows along a line; raman spectroscopy (especially coherent anti-stokes raman spectroscopy, CARS), which can also be used inside the reactor to study molecular decomposition and temperature gradients; and in-flight X-ray powder diffraction.

Depending on the type of thread to be produced, different collection methods are possible. For charged wires, they are easily collected on any substrate by means of an electric field. The aerosol may be bubbled through a liquid to remove strands from the gas stream, with or without surfactant molecules, to prevent agglomeration of the strands. The easily redispersible strands can be collected in the filter as a dry powder.

FIG. 8 schematically illustrates an arrangement of in-situ luminescence (PL) measurements in a system for forming lines according to the present invention. The PL arrangement comprises a light source and a light detector arranged at e.g. a transparent quartz tube. For proper luminescence measurements, the light source should be a laser with higher light energy than the band gap of the semiconductor material of the wire flowing through the transparent quartz tube.

FIG. 9 schematically illustrates an arrangement of in-situ absorption measurements in a system for forming lines according to the present invention. The in-situ absorption measurement arrangement comprises a light source and an absorption detector arranged at, for example, a transparent quartz tube. For absorption measurements, light should be emitted by a white light source with collimated light. The absorption detector is preferably placed in alignment with the light source so as to maximize the absorption volume of the aerosol.

As further examples of wires formed by the methods and systems of the present invention, fig. 10 and 11 show Scanning Electron Microscope (SEM) images of GaAs nanowires grown under two different growth conditions (hereinafter referred to as (i) and (ii), respectively). Au agglomerates were generated from the molten Au in a high temperature furnace at a set temperature of (i)1775 ℃ or (ii)1825 ℃. Au agglomerates were 1680 sccm N between the different modules of the growth system₂Carried by a carrier gas (hereinafter the carrier gas containing Au agglomerates/particles is referred to as aerosol). After the high temperature furnace, the Au agglomerates each carry a single electron. By using this single electron charge, the Au agglomerate was size-selected by a differential mobility analyzer, in this case, set to 50 nm. The aerosol was passed through a sintering furnace at 450 ℃ which compacted the Au agglomerates into spherical Au particles. After the sintering furnace, the aerosol is mixed with the precursor gases TMGa and AsH₃Mixing, setting molar flow rate at 2.4 x 10^-2mmol/min and 2.2 x 10^-2mmol/min. The aerosol (including precursor gas) enters the reactor set at a temperature of (i) 450 ℃ or (ii) 625 ℃. In the reaction furnace, the precursor decomposes to form the material components Ga and As. The material composition is supplied to the Au particles in the gas phase, and the GaAs seed crystals nucleate on the Au particles. The continuous growth of the lines continues via two different growth modes: (i) an axial growth mode, in which material is incorporated at the interface between the Au grains and the GaAs seed crystal forming the wire, (ii) a combination of axial and radial growth modes, in which the material composition is incorporated both at the Au grain-GaAs interface and on the side of the formed wire, forming a wire with a conical shape. After the reaction furnace, the wire was carried by a carrier gas to a deposition chamber where a voltage of 6kv was applied to the Si substrate to deposit a charged wire. As shown in fig. 10, the Au particles are visible and have a bright contrast compared to the darker nanowires. As shown in fig. 11, the Au particles are visible with a bright contrast at the tip of the cone shaped nanowires.

The formation of GaAs nanowires typically occurs at temperature regimes of 380-700 ℃, depending on the desired shape and properties of the nanowires formed. Higher temperatures generally result in higher growth rates, i.e., longer nanowires for a set growth time, and also have an effect on the crystal structure and impurity incorporation in the conical shape. In addition to temperature, the ratio of group V material precursors to group III material precursors, i.e., the V/III ratio, is important. If the V/III ratio is too low, typically below 0.2, nanowire growth continues in a group III rich environment, which can reduce growth rate and material quality. If the V/III ratio is too high, typically above 5, the nanowires are difficult to nucleate because the group III material is insoluble in the Au particles. The formation of GaAs nanowires typically occurs at a total reactor internal pressure of 50-1100 mbar. The lower pressure reduces the degree of supersaturation in the gas phase, which can reduce parasitic gas phase reactions. Higher pressure increases the degree of supersaturation in the gas phase, which can increase the degree of supersaturation in the Au particles and increase the growth rate. Pressure may also be used to control the residence time in the growth reactor.

It should be noted that the parameters (e.g. temperature, precursor flow, V/III ratio and pressure) depend on the precursor molecules used, since only the material that actually reaches the growth interface is incorporated. The nanowire-forming reaction is most likely to occur at higher temperatures if the precursor can withstand higher temperatures without reacting.

The above discussion of growth parameters is primarily valid for single-stage growth, where nucleation and wire growth occur in a single reaction zone. For multi-stage growth, the first nucleation stage should generally be carried out at a higher temperature, lower precursor flow rate and lower V/III ratio than the subsequent growth step.

Nanowire formation in the described process typically occurs at a lower V/III ratio but similar temperature compared to MOVPE. Since the parameters (e.g., temperature, pressure, flow rate, and V/III ratio) depend on the exact chemistry (exact chemistry) used to form the nanowires, it is understood that different materials may be formed under different parameters. For example,due to NH₃The higher stability of the precursor, the III-nitride can be formed at higher temperatures, while InAs growth is performed at lower temperatures.

Suitable materials for forming the wires of the present methods and systems include, but are not limited to:

InAs, InP, GaAs, GaP and alloys thereof (In)_xGa_1-xAs_yP_1-y)

InSb, GaSb and their alloys (In)_xGa_1-xSb)

AlP, AlAs, AlSb and alloys thereof, e.g. AlP_1-xAs_x

InGaAsP alloyed with Al, e.g. Al_xGa_1-xAs

InGaAsP alloyed with Sb, e.g. GaAs_ySb_1-y

InN, GaN, AlN and alloys thereof (In)_xGa_1-xN)

Si, Ge and alloys thereof, i.e. (Si)_xGe_1-x)

-CdSe, CdS, CdTe, ZnO, ZnS, ZnSe, ZnTe, MgSe, MgTe and alloys thereof

- SiO_xC (diamond), C (carbon nanotube), SiC, BN

Suitable materials for catalyzing the seed particles include, but are not limited to:

- Au、Cu、Ag

- In、Ga、Al

- Fe、Ni、Pd、Pt

- Sn、Si、Ge、Zn、Cd

alloys of the above, e.g. Au-In, Au-Ga, Au-Si

Suitable gases for carrying the catalytic seed particles and threads in the process includeBut are not limited to: h₂、N₂Or mixtures thereof; or He, Ar.

Suitable dopants include, but are not limited to

-InGaAl-AsPSb system: n-dopant: s, Se, Si, C and Sn; p-dopant: zn, Si, C, Be

-AlInGaN system: n-dopant: si; p-dopant: mg (magnesium)

-Si: n-dopant: p, As, Sb; p-dopant: B. al, Ga, In

CdZn-OSSeTe system: p-dopant: li, Na, K, N, P, As; n-dopant: al, Ga, In, CI, I

According to the usual nomenclature for chemical formulae, a compound consisting of an element a and an element B is generally denoted AB, which should be interpreted as a_xB_1-x。

It is understood that the wire growth may comprise one or more etching steps, wherein material is removed rather than grown on the wire. Etching may also be used to separate radial and axial growth, which for example enables a reduction of the tapering (taping) of the wire or a simple shape control of the wire.

The size of the wire depends on many factors, such as the material from which the wire is formed, the intended application of the wire, and the requirements on the quality of the formed wire. Preferably the diameter of the wires is less than 10 μm, more preferably the wire diameter is less than 300nm, especially for wires forming layers or segments comprising a lattice mismatch.

Since the wire of the present invention can have various cross-sectional shapes, the diameter (interchangeably referred to as width) is intended to mean the effective diameter.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims (6)

1. A system for forming wires, the system comprising at least one reactor (8) for growing nanowires (1), wherein the system comprises:

-means (9) for supplying catalytic seed particles (2) suspended in a gas to a reactor (8),

-means for providing gaseous precursors (3, 4) comprising components of the nanowires (1) to be formed to a reactor (8), at least one of said gaseous precursors (3, 4) being SiH4, DEZn, a group V material precursor or a group III material precursor,

-means for producing seeds at the surface of the catalytic seed particles, and

-means for collecting semiconductor nanowires (1) epitaxially grown from the formed seeds in a gas phase synthesis comprising gaseous precursors (3, 4) while suspending catalytic seed particles (2) in a gas.

2. The system of claim 1, wherein the group III material precursor is TMIn or TMGa.

3. The system of claim 1, wherein the group V material precursor is PH3, AsH3, or NH 3.

4. The system of claim 1, wherein the system comprises a plurality of reactors (8) arranged in series and/or in parallel.

5. The system of any one of claims 1 to 4, further comprising means for in situ analysis (12) of the formed nanowires (1).

6. The system of claim 5, further comprising means for feedback in situ analysis and control of growth conditions.