Source: http://www.google.com/patents/US7354850?dq=6,064,942
Timestamp: 2015-07-01 20:09:48
Document Index: 642918623

Matched Legal Cases: ['Application No. 60', 'art 1', 'art 1', 'art 2', 'art 1', 'art 2']

Patent US7354850 - Directionally controlled growth of nanowhiskers - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsNanowhiskers are grown in a non-preferential growth direction by regulation of nucleation conditions to inhibit growth in a preferential direction. In a preferred implementation, <001> III-V semiconductor nanowhiskers are grown on an (001) III-V semiconductor substrate surface by effectively inhibiting...http://www.google.com/patents/US7354850?utm_source=gb-gplus-sharePatent US7354850 - Directionally controlled growth of nanowhiskersAdvanced Patent SearchPublication numberUS7354850 B2Publication typeGrantApplication numberUS 11/049,293Publication dateApr 8, 2008Filing dateFeb 3, 2005Priority dateFeb 6, 2004Fee statusPaidAlso published asCN1961100A, CN1961100B, EP1711649A2, US7911035, US20060019470, US20080142926, WO2005078167A2, WO2005078167A3Publication number049293, 11049293, US 7354850 B2, US 7354850B2, US-B2-7354850, US7354850 B2, US7354850B2InventorsWerner Seifert, Lars Ivar Samuelson, Bj�rn Jonas Ohlsson, Lars Magnus Borgstr�mOriginal AssigneeQunano AbExport CitationBiBTeX, EndNote, RefManPatent Citations (50), Non-Patent Citations (103), Referenced by (32), Classifications (43), Legal Events (3) External Links: USPTO, USPTO Assignment, EspacenetDirectionally controlled growth of nanowhiskers
US 7354850 B2Abstract
1. A method of growing a nanowhisker, comprising: disposing a catalytic particle on a substrate surface, the substrate surface providing a predetermined crystal plane; and growing at least a base portion of a nanowhisker from the catalytic particle in a <001> growth direction that is a non-preferential direction of growth for the nanowhisker, with nucleation conditions at the onset of growth being regulated to control an interface between the catalytic particle and the substrate such that said crystal plane is maintained as said substrate surface at the interface so as to define and stabilize said <001> growth; wherein:
prior to establishment of growth conditions, a mask of passivating material is formed on the substrate surface to inhibit bulk growth; and
the mask is a carbon-containing material that is evaporated on the substrate surface to provide a thin layer that contains carbon.
2. A method according to claim 1, wherein conditions are regulated such that nucleation of the nanowhisker is achieved with the catalytic particle having consumed substantially no material from the substrate.
3. A method according to claim 1, wherein conditions are regulated so as to avoid creation of a pit or depression at the interface between the catalytic particle and the substrate.
4. A method according to claim 1, wherein conditions are regulated such that there is not exposed at the interface between the catalytic particle and the substrate, a crystal facet that corresponds to a preferential direction for nanowhisker growth.
5. A method according to claim 1, wherein whisker nucleation is initiated without an initial annealing of the catalytic particle disposed on the substrate surface.
6. A method according to claim 1, wherein said nanowhisker comprises a III-V semiconductor compound.
7. A method according to claim 6, wherein indium is a group III constituent of the semiconductor compound.
8. A method according to claim 6, wherein the step of growing comprises growing by MOVPE.
9. A method according to claim 1, wherein said crystal plane is an (001) plane.
10. A method according to claim 1, wherein the substrate comprises a III-V semiconductor compound.
11. A method according to claim 10, wherein indium is a group III constituent of the semiconductor compound.
12. A method according to claim 1, wherein the mask is formed from an amino acid.
13. A method according to claim 1, wherein said catalytic particle is deposited from an aerosol onto the substrate surface.
14. A method according to claim 1, wherein the catalytic particle is preformed, at least in part, with a constituent element that will form part of the nanowhisker.
15. A method according to claim 1, wherein nanowhisker growth is carried out at a low temperature such as to inhibit substrate material contributing to growth of the nanowhisker.
16. A method of growing a nanowhisker, comprising: providing a catalytic particle on an (001) substrate surface; and establishing growth conditions wherein heat is applied and constituent materials are introduced in gaseous form so as to grow a nanowhisker having at least a base portion of a III-V semiconductor compound from the catalytic particle, wherein nucleation conditions at the onset of growth are regulated such that the base portion of the nanowhisker grows from the substrate surface in an <001> direction.
17. A method according to claim 16, wherein conditions are regulated so as not to create a pit or depression at an interface between the catalytic particle and the substrate.
18. A method according to claim 16, wherein conditions are regulated such that there is not exposed at an interface between the catalytic particle and the substrate, a crystal facet that corresponds to preferential direction for nanowhisker growth.
19. A method according to claim 16, wherein whisker nucleation is initiated without an initial annealing of the catalytic particle disposed on the substrate surface.
20. A method according to claim 16, wherein indium is a group III constituent of the semiconductor compound.
21. A method according to in claim 16, wherein the substrate comprises a III-V semiconductor compound.
22. A method according to claim 21, wherein indium is a group III constituent of the semiconductor compound.
23. A method according to claim 16, wherein the catalytic particle is deposited from an aerosol onto the substrate surface.
24. A method according to claim 16, wherein the catalytic particle is preformed, at least in part, with a constituent element that will form part of the nanowhisker.
25. A method according to claim 16, wherein nanowhisker growth is carried out at a low temperature such as to inhibit substrate material contributing to growth of the nanowhisker.
26. A method of growing a nanowhisker, comprising: providing a substrate having a major surface defined by a predetermined crystal plane; disposing a catalytic particle on the substrate surface; and establishing growth conditions wherein heat is applied and constituent materials are introduced in gaseous form so as to grow at least a base portion of a nanowhisker from the substrate surface via the catalytic particle in a <001> direction that is a non-preferential direction of growth for the nanowhisker, wherein, prior to establishment of growth conditions, a mask of passivating material is formed on the substrate surface to inhibit bulk growth that would compete with growth of the nanowhisker in said <001> direction, and the mask is a carbon containing material that is evaporated on the substrate surface to provide a thin layer that contains carbon.
27. A method according to claim 26, wherein said nanowhisker comprises a III-V semiconductor compound.
28. A method according to claim 27, wherein indium is a group III constituent of the semiconductor compound.
29. A method according to claim 26, wherein the substrate comprises a III-V semiconductor compound.
30. A method according to claim 29, wherein indium is a group III constituent of the semiconductor compound.
31. A method according to claim 26, wherein the mask is formed from an amino acid.
32. A method according to claim 26, wherein the mask is provided after the catalytic particle is disposed on the substrate surface.
33. A method according to claim 26, wherein growth of the nanowhisker is initiated without an immediately prior annealing step.
34. A method according to claim 26, wherein nucleation conditions at the onset of growth are regulated to control an interface between the catalytic particle and the substrate such that said crystal plane is maintained as said substrate surface at the interface so as to define and stabilize said growth direction.
35. A method of growing a nanowhisker, comprising: disposing a catalytic particle on an (001) substrate surface; and establishing growth conditions wherein heat is applied and constituent materials are introduced in gaseous form, so as to grow at least a base portion of a nanowhisker including a III-V semiconductor compound from the catalytic particle in an <001> direction relative to the substrate surface, wherein prior to establishment of growth conditions, a mask of passivating material is formed on the substrate surface to inhibit bulk growth of the semiconductor compound on the substrate surface.
36. A method according to claim 1, wherein the nanowhisker is grown in said predetermined direction over substantially its entire length.
37. A method according to claim 16, wherein said nanowhisker is grown in said <001> direction over substantially its entire length.
38. A method according to claim 26, wherein the nanowhisker is grown in said predetermined direction over substantially its entire length.
39. A method according to claim 35, wherein said nanowhisker is grown in said <001> direction over substantially its entire length.
40. A method comprising: providing a substrate having a major surface defined by a predetermined crystal plane; and growing a nanowhisker from the substrate surface by a MOVPE VLS process such that at least a base portion of the nanowhisker grows in a non-preferential growth direction other than a <111> B direction from the substrate surface, wherein the non-preferential growth direction comprises a direction normal to the substrate surface and the nanowhisker comprises a III-V semiconductor nanowhisker, and wherein the crystal plane comprises an (001) plane and the non-preferential growth direction comprises a <001> direction relative to the substrate surface.
41. The method of claim 1, wherein the <001> growth direction comprises a direction normal to the substrate surface.
42. The method of claim 9, wherein the <001> growth direction comprises a direction normal to the (001) plane.
43. The method of claim 26, wherein the <001> direction comprises a direction normal to the substrate surface.
44. The method of claim 43, wherein the crystal plane comprises an (001) plane.
This application claims the benefit of the priority of U.S. Provisional Patent Application No. 60/541,949 filed Feb. 6, 2004, the entirety of which is incorporated herein by reference.
This invention relates generally to structures produced by techniques of nanotechnology, and methods of producing such structures. More specifically, the invention relates to such structures and devices incorporating at least one element, essentially in one-dimensional form, which is of nanometer dimensions in its width or diameter, which is produced with the aid of a catalytic particle, and which is commonly termed a “nanowhisker.”
Although the growth of nanowhiskers catalyzed by the presence of a catalytic particle at the tip of the growing whisker has conventionally been referred to as the VLS (Vapor-Liquid-Solid process), it has come to be recognized that the catalytic particle may not have to be in the liquid state to function as an effective catalyst for whisker growth. At least some evidence suggests that material for forming the whisker can reach the particle-whisker interface and contribute to the growing whisker even if the catalytic particle is at a temperature below its melting point and presumably in the solid state. Under such conditions, the growth material, e.g., atoms that are added to the tip of the whisker as it grows, may be able to diffuse through a the body of a solid catalytic particle or may even diffuse along the surface of the solid catalytic particle to the growing tip of the whisker at the growing temperature. Persson et al., “Solid-phase diffusion mechanism for GaAs nanowires growth,” Nature Materials, Vol. 3, October 2004, pp 687-681, shows that, for semiconductor compound nanowhiskers there may occur solid-phase diffusion mechanism of a single component (Ga) of a compound (GaAs) through a catalytic particle. Evidently, the overall effect is the same, i.e., elongation of the whisker catalyzed by the catalytic particle, whatever the exact mechanism may be under particular circumstances of temperature, catalytic particle composition, intended composition of the whisker, or other conditions relevant to whisker growth. For purposes of this application, the term “VLS process,” or “VLS mechanism,” or equivalent terminology, is intended to include all such catalyzed procedures wherein nanowhisker growth is catalyzed by a particle, liquid or solid, in contact with the growing tip of the nanowhisker.
For the purposes of this specification the term “nanowhisker” is intended to mean a one-dimensional nanoelement with a width or diameter (or, generally, a cross-dimension) of nanometer size, the element preferably having been formed by the so-called VLS mechanism, as defined above. Nanowhiskers are also referred to in the art as “nanowires” or, in context, simply as “whiskers” or “wires.”
Freestanding nanowhiskers have drawn increasing attention for their potential use in applications in electronics and photonics.
As already shown in the early work of Wagner, referenced above, the preferential growth direction of such nanowhiskers is <111>. One drawback of <111> oriented nanowhiskers is the high density of stacking faults that commonly form perpendicular to the growth direction. These defects are expected to affect the physical properties of the nanowhiskers. Another drawback with this preferential growth direction is its non-compatibility with the (001) crystal face of the main surface of substrates commonly used in industrial applications. That is, the preferential growth direction is oblique rather than normal to the substrate main surface, the normal direction being <001>. For example, with III-V compounds, the commonly commercially available substrates have an (001) crystal face as the main surface. In contrast, nanowhiskers of III-V compounds preferentially grow in a <111>B direction from a (111)B crystal plane.
Hiruma et al., J. Appl. Phys., 77(2), 15 Jan. 1995, pages 447-462, reported the growth on InAs nanowhiskers on GaAs substrates having various surfaces including, in particular, the (001) surface. The InAs nanowires invariably grew in the <111> direction, resulting, for example, in pairs of wires ([1-11] and [-111]) tilted with an angle of 35� towards the (001) surface.
Other growth directions for nanowhiskers have been observed to occur sporadically during whisker growth. For instance, Wu et al, “Growth, branching, and kinking of molecular beam epitaxial <110> GaAs nanowires,” Applied Physics Letters, 20 Oct. 2003, Vol. 83, No. 16, pp 3368-3370, disclosed an <011> direction for GaAs nanowhiskers grown on GaAs (001) by molecular beam epitaxy (MBE).
Bj�rk et al, “One-Dimensional Heterostructures In Semiconductor Nanowhiskers,” Applied Physics Letters, Vol. 80, No. 6, 11 Feb. 2002, pages 1058-1060, described an <001> segment of an InAs/InP heterostructured nanowhisker grown from a (111)B GaAs surface in chemical beam epitaxy (CBE), the segment having deviated from the initial <111>B growth direction of the nanowhisker. More particularly, it had been observed that whereas most nanowhiskers grew in the <111>B direction, there were sporadically formed nanowhiskers in the form of a “hockey-stick” that initially grew in the <111>B direction, but “kinked” to the <001> direction. The nanowhisker disclosed had a base region of InAs and grew in the <001> direction as a result of the compressive strain at the InP/InAs interface. Growth of a nanowhisker in the <001> direction dramatically reduces the formation of defects, such as stacking faults.
In International Patent Application Publication No. WO 2004/004927 (the entirety of which is incorporated herein by reference), there is disclosed in FIG. 24( b) a technique for controlling the growth direction of whiskers wherein, by applying strain to the whisker during formation, by change of growth conditions, the direction of growth of the whisker can be changed to the <100> direction from the usual <111> direction. Alternatively, a short bandgap segment of a wide bandgap material may be grown at the base of the nanowhisker.
Still further improvements in the control of the growth direction of nanowhiskers are desirable. For example, a method that would provide for an initial whisker growth direction normal to an incompatible substrate surface—that is, where the preferential growth direction of the whisker is oblique to the surface—would be highly desirable, as would structures produced by such method. Such a method would allow for the growth of whiskers that are normal to the surface over their entire length (or, more generally, at least the initial portion of their length) as opposed to the kinked nanowhiskers having an initial growth direction oblique to the surface as previously observed.
The present invention relates to directionally controlled growth of nanowhiskers and to structures including such nanowhiskers, and provides, among other things, methods and structures having the highly desirable characteristics just described. The invention thus provides methods and structures in which a nanowhisker has at least a base portion grown in a non-preferential growth direction from a substrate surface.
The invention, from one perspective, recognizes that nanowhiskers of particular semiconductor materials have preferential directions for growth, and that commonly available substrates of particular semiconductor materials have particular crystal facets defining a major surface that does not correspond to a preferential growth direction. The present invention therefore provides a mechanism for growth of nanowhiskers that will permit growth of nanowhiskers in a non-preferential growth direction from a substrate major surface defined by a crystal facet that does not correspond to a preferential growth direction.
The invention also provides a nano-engineered structure that comprises a nanowhisker upstanding from (or at least having an initial or base portion upstanding from) a substrate major surface in a non-preferential growth direction, wherein a crystal facet defining the surface corresponds to the non-preferential direction of growth of the nanowhisker. The growth direction of the nanowhisker is preferably maintained over its entire length. However, it is within the broader scope of the invention that the growth direction can be changed from the initial, non-preferential growth direction by changing growth conditions (e.g., constituent materials) of the whisker after growth of the whisker base portion.
More generally, the invention provides nanostructures incorporating nanowhiskers grown on substrates, having improved structural form.
For the purposes of this specification, it will be understood that where a surface or crystal facet is defined by Miller indices (hkl), where h, k and l are numerical values, then this “corresponds” to a nanowhisker growth direction <hkl>.
The present invention, from another perspective, recognizes that where it is desired deliberately to grow a nanowhisker from a substrate surface in a non-preferential growth direction, nucleation conditions at the onset of growth can be regulated such that there is not created at an interface between a catalytic particle and the substrate a condition that would cause growth of the nanowhisker in a preferential growth direction.
Thus, in accordance with another of its aspects, the invention provides a method of growing nanowhiskers on a substrate surface providing a predetermined crystal plane, the method comprising providing at least one catalytic particle on the substrate surface, and growing a nanowhisker from each said catalytic particle in a predetermined growth direction that is a non-preferential growth direction for the nanowhisker, with nucleation conditions at the onset of growth being regulated to control the interface between each said catalytic particle and the substrate such that said crystal plane is maintained as said substrate surface at the interface so as to define and stabilize said predetermined growth direction. The nanowhisker can be of the same material or of a different material from that of the substrate.
In accordance with the present invention, the nanowhisker growth direction can be defined and stabilized by controlling the surface conditions at the onset of the nucleation event. This nucleation can be strongly affected by pre-treatment of the catalytic particle at the substrate surface. Conventionally, an annealing step at high temperature is performed subsequent to providing the catalytic particles on the substrate surface and prior to initiation of nanowhisker growth. In such an annealing step, substrate material is consumed by or dissolved into the catalytic particles, and this creates depressions or recesses in the substrate surface in which the catalytic particles sit. Such depressions may expose crystal facets such as (111) that may bring about nanowhisker growth perpendicular to such facets in a preferential direction such as <111>. Thus, in practice of the present invention, such an annealing step is preferably omitted. More generally, the nucleation stage of initial nanowhisker growth preferably comprises absorbing constituent materials from the gaseous phase to create supersaturation conditions within the catalytic particle; substrate material does not contribute to a significant extent.
When nucleation conditions are initiated, it is preferred to maintain temperature as low as possible, consistent with ensuring proper nucleation and growth. Further, it has been found that the catalytic particle, when heated under such conditions, may perform an “ironing” effect on the underlying substrate surface to “iron-out” irregularities, atomic steps, etc., and this further contributes to maintaining a well-defined surface. The surface may amount to an atomically flat surface, with no atomic steps, etc., so that a nanowhisker has no other possibility than to grow in the desired non-preferential direction. However, perfect atomic flatness may not be necessary in order to force growth in the desired direction.
Further, the catalytic particle, while usually of a non-reactive material such as Au, may be formed of, or include, a constituent element (e.g., a group III material such as In) of the nanowhisker. Nucleation and supersaturation conditions may thus be achieved more quickly, with a reduced amount of the constituent material being absorbed into the catalytic particle from its surroundings.
In a further aspect, the invention provides a structure comprising a substrate having a surface providing an (001) crystal plane, and at least one nanowhisker extending, at least initially, from the surface in an <001> direction relative to the surface, wherein the <001> direction corresponds to a non-preferential direction of growth for the nanowhisker, in that one or more other directions are more conducive to growth.
In a more general aspect, the invention provides a structure comprising a substrate having a major surface defined by a predetermined crystal plane, and at least one nanowhisker extending, at least initially, from the surface in a direction corresponding to the crystal plane, wherein the direction of the nanowhisker is a non-preferential direction of growth for the nanowhisker, in that one or more other directions are more conducive to growth.
It has been found that the (001) surface of III-V materials is particularly conducive to bulk epitaxial growth, when exposed to appropriate materials in gaseous form. This may compete with, inhibit or obstruct nanowhisker growth.
In accordance with a further aspect of the invention, a method of growing nanowhiskers on a substrate surface comprises disposing at least one catalytic particle on the substrate surface, the substrate surface providing a predetermined crystal plane, and growing a nanowhisker from each said catalytic particle in a predetermined direction from the surface that does not correspond to a preferential direction of growth for the nanowhisker, and wherein prior to establishment of growth conditions, a mask of passivating material is formed on the substrate surface to inhibit bulk growth of material used to form the nanowhisker.
In one preferred implementation, the invention provides a method of growing nanowhiskers on an (001) surface of a substrate of III-V semiconductor material, wherein at least one catalytic particle is disposed on the (001) substrate surface, and growth conditions are established wherein heat is applied, and constituent materials are introduced in gaseous form, from which to grow a nanowhisker from each said catalytic particle in an <001> direction relative to the surface, and wherein prior to establishment of growth conditions, a mask of passivating material is formed on the substrate surface to inhibit bulk growth.
In a further aspect, the invention provides a structure comprising a substrate having a surface defined by a predetermined crystal facet, and at least one nanowhisker extending from said surface in a direction corresponding to said crystal facet, but not corresponding to a preferential direction of growth for the nanowhisker, and a layer of passivating material disposed on the substrate surface.
In one preferred form, the structure comprises a substrate of III-V semiconductor material having an (001) surface, and at least one nanowhisker extending from said surface in an <001> direction relative to the surface, and a layer of passivating material disposed on the substrate surface.
The mask of passivating material may be silicon oxide or silicon nitride, as described in copending U.S. patent application Ser. No. 10/751,944, filed Jan. 7, 2004 (the entirety of which is incorporated herein by reference). Each catalytic particle may be disposed in a respective aperture within the mask. Alternatively, and as preferred, a layer of carbon containing material can be employed that is deposited over the substrate and the catalytic particles. A particularly preferred material is Lysine (an amino acid). When heated, the material decomposes to leave a thin layer, which may be as thin as a monolayer, of a material that contains carbon and serves to inhibit bulk growth on the substrate. Although the layer coats in addition the catalytic particle, it is so thin that it does not disturb or inhibit nanowhisker growth.
It is preferred to use, as a method of positioning the catalytic particle on the substrate surface, deposition of the particle from an aerosol. A method as described in International Patent Application Publication No. WO 01/84238 (the entirety of which is incorporated herein by reference) may be employed. In the situation where it is desired to use a lithographic process for positioning catalytic particles such as gold on the substrate, it is appropriate to ensure that none of the process steps, such as etching a mask to define positions for the catalytic particles, creates undesirable depressions.
As regards the material of the catalytic particle, this may comprise a non-reactive material such as Au. Alternatively, the Au may be alloyed with a group III material, for example, that forms part of the nanowhisker compound. Alternatively, the particle may be heterogeneous—for example, with one region of Au and another region of group III material. Alternatively, the catalytic particle may be formed wholly of a group III material, such as In.
As regards the material of the nanowhiskers, this need not be the same as that of the substrate and may be of any desired material. When forming semiconductor nanowhiskers on a semiconductor substrate, the material of the whiskers can be of the same semiconductor group as that of the substrate material, or of a different semiconductor group. A specific implementation described hereinafter, and having achieved excellent results, involves InP nanowhiskers grown in an <001> direction on an (001) InP substrate surface.
In addition to III-V materials, substrates of groups IV and II-VI semiconductor materials, and other substrate materials, may be used for nanowhisker growth. Substrates of III-V materials are commonly provided in commerce with (001) surfaces; substrates with (111) surfaces are much more expensive. While specific implementation of the invention is described hereinafter with reference to InP, the invention may also be practiced with GaP and InAs substrates, for example. Substrates of material that contain Ga may have thicker and more resistant oxide formations that should preferably be removed prior to nucleation, without forming a depression in the substrate surface.
Among group IV semiconductor substrate materials, Si is widely used as a substrate material in the manufacture of electronic components. Stable silicon surfaces include (001), (111), and (113). Most electronic components are fabricated on (001) surfaces. Heretofore, nanowhiskers have been grown in the preferential (and oblique) <111> direction from (001) surfaces. In accordance with the techniques of the present invention, however, nanowhiskers can be grown in a direction normal to such surfaces not corresponding to a preferential growth direction—for example, in an <001> direction from an (001) surface.
The present invention is, in principle, applicable to any of the materials that may be used in the manufacture of nanowhiskers and substrates therefor. Such materials are commonly semiconductors formed of Group II through Group VI elements. Such elements include, without limitation, the following:
Group VI: O, S, Se, Te.
Semiconductor compounds are commonly formed of two elements to make III-V compounds or II-VI compounds. However, ternary or quaternary compounds are also employed involving, e.g., two elements from Group II or from Group III. Stoichiometric or non-stoichiometric mixtures of elements can be employed.
AIN, GaN, SiC, BP, InN, GaP, AlP, AlAs, GaAs, InP, PbS, PbSe, InAs, ZnSe, ZnTe, CdS, CdSe, AlSb, GaSb, SnTe, InSb, HgTe, CdTe, ZnTe, ZnO.
There are many other semiconductor materials to which the invention is applicable: see, for example, “Handbook of Chemistry and Physics”—CRC—Properties of Semiconductors—for a more complete treatment.
In accordance with the invention, a semiconductor substrate may be selected from one of the above group IV, III-V or II-VI materials, and the nanowhiskers may be selected from the same or another of the group IV, III-V or II-VI materials.
FIGS. 1( a) to 1(d) are SEM-images of InP nanowhiskers on a substrate of InP (001), in accordance with the invention: (a) top-view, (b) enlarged top-view, (c) view on a substrate tilted by 30�, (d) magnification of a single whisker after a clockwise rotation of the substrate by 40�.
FIGS. 2( a) and 2(b) are schematic diagrams for explaining the invention in terms of different growth directions by different start (nucleation) conditions.
FIGS. 3( a) to 3(e) are TEM-images of InP nanowhiskers to illustrate comparative characteristics of nanowhiskers grown in a non-preferential <001> direction in accordance with the invention relative to nanowhiskers grown in a preferential <111>B direction.
FIG. 4( a) shows photoluminescence spectra from an <001> nanowhisker in accordance with the invention (thick line) and a typical <111>B whisker (thin line), and FIG. 4( b) is an SEM image of the <001> whisker having the photoluminescence spectrum in FIG. 4( a).
In accordance with the present invention, it is has been found that growth of a nanowhisker in a non-preferential direction (e.g., in an <001> direction from an (001) crystal plane) is, once established, stable. The present invention more particularly recognizes the possibility to define and to stabilize the growth direction by controlling conditions at the onset of the nucleation event.
The following discussion describes an exemplary application of the invention to [001] InP nanowhiskers grown by metal-organic vapor phase epitaxy directly on (001) InP substrates. The nanowhiskers were characterized by scanning electron microscopy and transmission electron microscopy and found to have structural characteristics substantially superior to those of comparative whiskers grown in the preferential <111>B direction, as will be discussed in detail below.
The InP nanowhiskers were grown using low-pressure metal-organic vapor phase epitaxy (MOVPE). Aerosol-produced, 50 nm Au-particles were deposited on (001) InP substrates, which were then placed within a horizontal reactor cell on an RF-heated graphite susceptor. A hydrogen carrier gas flow of 6 l/min at a pressure of 100 mBar (10 kPa) was used. A constant phosphine flow at a molar fraction of 1.5�10−2 was supplied to the reactor cell and the samples were heated up to 420� C. over 5 minutes. After this temperature ramp step, growth of nanowhiskers was immediately commenced by introducing trimethylindium (TMI) into the reactor cell. The TMI molar fraction was 3�10−6, and the typical growth time was 8 minutes. It should be noted that this method of producing whiskers differs from the often-used procedure of whisker growth, where Au particles are annealed at higher temperature prior to whisker growth in order to de-oxidize the surface and alloy the Au catalyst with the semiconductor material. In addition, in order to improve the growth of [001] nanowhiskers in relation to competing bulk growth at the (001) surface, the substrate with the deposited Au particles was dipped into a solution of poly-L-Lysine before inserting it into the growth chamber. L-Lysine (2,6 diaminocaproic acid) is known to be an adhesion-active substance with low vapor pressure. The monohydrate melts under decomposition between 212-214� C., leaving a thin passivation layer at the surface. This layer prevents InP-growth on the bare (001) InP surface.
Sample characterization was carried out using a JSM 6400 F field emission scanning electron microscope (SEM), operated at 15 kV. FIGS. 1( a) to 1(d) show SEM-images of [001] InP nanowhiskers grown by the procedures described above. FIG. 1( a) is a top view. FIG. 1( b) is an enlarged top view. FIG. 1( c) is view on a substrate tilted by 30�, and FIG. 1( d) shows magnification of a single whisker after a clockwise rotation of the substrate by 40�. In FIG. 1( b), a rectangular whisker shape formed by stepped {110} side-facets of the [001] oriented whiskers is clearly evident.
A most remarkable effect of the whisker growth in [001] is the high crystalline perfection observed. FIGS. 3( a) to 3(e) show high-resolution transmission electron microscopy (TEM) images of InP wires grown in [001] and <111>B in comparison. The [001] wires appear to be defect-free, whereas <111>B grown whiskers contain a high concentration of stacking faults. The energetic differences for hexagonal or cubic stacking sequences in <111>B are small, and the stacking faults, as planar defects vertical to the growth direction, can freely end at the nanowhisker side facets. The formation of similar defects during growth in [001] would need to overcome an activation barrier for the creation of Frank partial dislocations. FIG. 3( a) is a side view showing a defect-free [001]-grown nanowhisker. FIG. 3( b) is an enlargement of the boxed area in FIG. 3( a), showing the atomic lattice of the defect-free zincblende structure in a [110] projection. FIG. 3( c) is a Fourier transform of the [110] projection. FIG. 3( d) is a side view showing a conventionally grown <111>B-directed nanowhisker with stacking faults all along the wire. FIG. 3( e) is a close-up of the nanowhisker of FIG. 3( d), showing mirror plane stacking faults resulting in wurtzite-structure segments.
The TEM images of FIGS. 3( a) to 3(e) were taken from nanowhiskers broken off from the substrate by touching a TEM grid to the nanowhisker substrate.
The higher materials perfection for nanowhiskers grown in [001] was also evident in photoluminescence studies. For photoluminescence (PL) studies, nanowhiskers were transferred to a thermally oxidized Si wafer on which a gold pattern was created to facilitate localization and identification of the whiskers studied by PL. The measurements were performed at liquid-He temperatures. A frequency-doubled Nd-YAG laser emitting at 532 nm was used for excitation. The luminescence was collected through an optical microscope, dispersed through a spectrometer, and detected by a liquid-N2 cooled CCD.
Photoluminescence measurements of single [001] InP nanowhiskers grown in accordance with the invention exhibited a narrow and intense emission peak at approximately 1.4 eV, whereas <111>B conventionally grown reference whiskers showed additional broad luminescence peaks at lower energy. FIG. 4( a) shows photoluminescence spectra from an <001> nanowhisker of the invention, with strong bandgap-related luminescence associated with the whisker (thick line) and a typical <111>B whisker with weaker luminescence and an additional broad peak at lower energies (thin line; small peaks superimposed on top of the broad main feature are artifacts resulting from interference within the CCD). FIG. 4( b) shows an SEM image of the <001> whisker, showing the strong PL in FIG. 4( a).
The differences between the situation with and without annealing may be explained by the schematics in FIGS. 2( a) and 2(b). FIG. 2 (a) shows growth from an Au-droplet at the (001) surface after annealing. InP will be locally dissolved to form an Au/In alloy, resulting in the formation of a pit. Two side facets within the pit are of {111}B-character. At high temperature (>500� C.), InP will be locally dissolved in a reaction with the Au. Typical Au/semiconductor interfaces, which develop under such conditions within the pit, are the low-energy facets {111}B and {011}, rather than the (001) facet which defines the substrate major surface. Nucleation on such low-energy facets could be the starting point for the commonly observed whisker growth in [1-11] and [-111], as well as the more seldom observed <011> direction reported for GaAs-MBE.
FIG. 2( b) shows growth, in accordance with the invention, from an Au-droplet without annealing. The Au/In-alloy forms by reaction of the Au with TMI, such that the (001) surface underneath the Au-droplet remains essentially intact. Without annealing at higher temperature, the reaction between InP and Au will be suppressed. In and P, dissolved within the Au-droplet, will be mostly from the supply of TMI and PH3 from the vapor phase, and not at the cost of the substrate material. Upon reaching a critical supersaturation, nucleation starts at the InP (001)/Au interface, and, consequently, wire growth can be controlled to occur in the [001] direction. In all samples, areas were found where [001] wires were dominant, but also areas with dominantly <111>B wires. Since slightly misoriented substrates (0.2�) were used, this different behavior may be due to lateral differences in the step structure at the (001) substrate surface.
It will thus be appreciated that the invention can achieve, among other advantages, (1) nano-wires which are highly perfect zincblende crystals that are free of stacking faults, exhibiting intense single-wire luminescence, and (2) the capability of vertical growth on the industrially viable (001) substrate orientation.
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2013Qunano AbMethod of producing precision vertical and horizontal layers in a vertical semiconductor structureUS8563395Nov 30, 2010Oct 22, 2013The Royal Institute For The Advancement Of Learning/Mcgill UniversityMethod of growing uniform semiconductor nanowires without foreign metal catalyst and devices thereofUS8664094Oct 18, 2012Mar 4, 2014Qunano AbMethod of producing nitride nanowires with different core and shell V/III flow ratiosUS8691011Mar 7, 2007Apr 8, 2014Qunano AbMethod for metal-free synthesis of epitaxial semiconductor nanowires on siUS8772626Dec 31, 2007Jul 8, 2014Qunano AbNanostructures and methods for manufacturing the sameUS8796119May 3, 2013Aug 5, 2014Qunano AbNanoelectronic structure and method of producing such* Cited by examinerClassifications U.S. Classification438/604, 257/E21.463, 257/E21.131, 257/E21.132, 977/825, 257/E21.126International ClassificationC30B29/62, B01J23/52, C30B11/12, C30B25/00, C30B29/60, H01L21/28Cooperative ClassificationY10S977/825, H01L21/02387, H01L21/02392, H01L21/02488, H01L21/02639, B82Y30/00, H01L21/0262, H01L21/02551, C30B25/005, C30B29/62, H01L21/02543, H01L21/02538, H01L21/02653, B01J23/52, C30B29/605, C30B11/12European ClassificationH01L21/02K4A1B, H01L21/02K4E3C, H01L21/02K4C1B, H01L21/02K4A1B2, H01L21/02K4C1B2, H01L21/02K4E3S3, H01L21/02K4B1J, H01L21/02K4C1C, H01L21/02K4E3S8, C30B29/62, B01J23/52, C30B11/12, B82Y30/00, C30B25/00F, C30B29/60DLegal EventsDateCodeEventDescriptionNov 10, 2006ASAssignmentOwner name: QUNANO AB,SWEDENFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:BTG INTERNATIONAL LIMITED;US-ASSIGNMENT DATABASE UPDATED:20100216;REEL/FRAME:18506/577Owner name: QUNANO AB,SWEDENFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:BTG INTERNATIONAL LIMITED;REEL/FRAME:018506/0577Effective date: 20050922Owner name: QUNANO AB,SWEDENFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:BTG INTERNATIONAL LIMITED;US-ASSIGNMENT DATABASE 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