Method for fabricating a nanostructure

Abstract

A method for fabricating a nanostructure comprises the steps of growing a first nanowire on a substrate, forming a dielectric layer on the substrate, the dielectric layer surrounding the first nanowire, wherein a thickness of the dielectric layer is smaller than a length of the first nanowire, and removing the first nanowire from the dielectric layer, thereby exposing an aperture in the dielectric layer.

Claims

1. A method for fabricating a nanostructure, comprising: growing a first nanowire on a substrate; forming a dielectric layer on said substrate, said dielectric layer surrounding said first nanowire, wherein a thickness of said dielectric layer is smaller than a length of said first nanowire; and removing said first nanowire from said dielectric layer, thereby exposing an aperture in said dielectric layer; and growing a second nanowire in said aperture on said substrate wherein growing said second nanowire comprises growing a support element in said aperture, and extending said support element above said dielectric layer, and growing a body element around at least a portion of said support element that extends above said dielectric layer.

2. The method according to claim 1, wherein growing said first nanowire on said substrate comprises: forming a mask layer on said substrate; forming an opening in said mask layer, wherein said opening extends to said substrate; and growing said first nanowire on said substrate in said opening.

3. The method according to claim 2, wherein said mask layer is formed at a thickness no larger than 80 nm.

4. The method according to claim 1, wherein said dielectric layer is formed on said substrate at a thickness of at least 100 nm.

5. The method according to claim 1, wherein a diameter of said body element is at least two times larger than a diameter of said support element.

6. The method according to claim 1, wherein said nanostructure is adapted to emit a laser signal at a wavelength , and a diameter of said support element is smaller than /(2n), wherein n denotes an index of refraction of said support element.

7. The method according to claim 1, wherein said nanostructure is adapted to emit a laser signal at a wavelength , and a diameter of said body element is no smaller than /n, wherein n denotes an index of refraction of said body element.

8. The method according to claim 1, wherein growing said first nanowire comprises molecular beam epitaxy or metal organic chemical vapor deposition.

9. The method according to claim 1, wherein said first nanowire is grown in a direction perpendicular to an upper surface of said substrate.

10. The method according claim 1, wherein said first nanowire is grown at an angle inclined to a surface normal of said substrate.

11. The method according to claim 4, wherein said thickness of said dielectric layer is at least 150 nm.

12. The method according to claim 10, wherein said diameter of said body element is at least three times larger than said diameter of said support element.

13. The method according to claim 10, wherein said angle is at least 20 degrees.

14. A method for fabricating a nanostructure, comprising: growing a first nanowire on a substrate; forming a dielectric layer on said substrate, said dielectric layer surrounding said first nanowire, wherein a thickness of said dielectric layer is smaller than a length of said first nanowire; and removing said first nanowire from said dielectric layer, thereby exposing an aperture in said dielectric layer; wherein said nanostructure is adapted to emit a laser signal at a wavelength , wherein a thickness of said dielectric layer is an integer multiple of /(2n), wherein n denotes an index of refraction of said dielectric layer.

15. The method according to claim 14, wherein said dielectric layer is formed on said substrate at a thickness of at least 100 nm.

16. The method according to claim 15, wherein said thickness of said dielectric layer is at least 150 nm.

17. The method according to claim 14, further comprising a step of growing a second nanowire in said aperture on said substrate; wherein growing said second nanowire comprises growing a support element in said aperture, and extending said support element above said dielectric layer, and growing a body element around at least a portion of said support element that extends above said dielectric layer.

18. The method according to claim 17, wherein a diameter of said body element is at least two times larger than a diameter of said support element.

19. The method according to claim 17, wherein said nanostructure is adapted to emit a laser signal at a wavelength , and a diameter of said support element is smaller than /(2n), wherein n denotes an index of refraction of said support element.

20. The method according to claim 17, wherein said nanostructure is adapted to emit a laser signal at a wavelength , and a diameter of said body element is no smaller than /n, wherein n denotes an index of refraction of said body element.

Description

DESCRIPTION OF EMBODIMENTS

(1) The features and advantages of the method for fabricating a nanostructure according to the present invention will become best apparent from a detailed description of exemplary embodiments in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a flow diagram illustrating a method for fabricating a nanostructure according to an example;

(3) FIGS. 2a to 2f are schematic cross-sectional views illustrating the fabrication of apertures in the dielectric structure according to an example of the invention;

(4) FIG. 3 illustrates the subsequent growth of a nanowire array in the aperture pattern formed according to the example method of FIGS. 2a to 2f; and

(5) FIG. 4 is a schematic cross-sectional view that shows one of the nanowires of FIG. 3 in additional detail.

(6) With reference to the flow diagram of FIG. 1, in a first step S10, a first nanowire is grown on a substrate. In a subsequent step S12, a dielectric layer is formed on said substrate, said dielectric layer surrounding said first nanowire, wherein the thickness of said dielectric layer is smaller than the length of said first nanowire. In a third step S14, said first nanowire is removed from said dielectric layer, thereby exposing a aperture in said dielectric layer.

(7) A method for fabricating a nanostructure will now be described with reference to FIGS. 2 to 3 for the specific example of fabricating a nanowire laser array. However, the invention is not so limited, and is generally applicable whenever an aperture with a diameter in the nanometer range shall be formed in a dielectric layer, in particular in a dielectric layer that is much thicker than the diameter of the apertures to be formed.

(8) FIGS. 2a to 2f are schematic cross-sectional drawings that illustrate a sequence of steps for fainting an aperture pattern 10 with a plurality of apertures in a dielectric layer 14. FIG. 2 shows a method for forming four apertures 12.sub.1 to 12.sub.4 in the dielectric layer 14. However, the invention is not so limited, and may just as well be employed for the formation of a single aperture or a large number of apertures in the dielectric layer 14. In fact, it is a particular advantage of the method according to the present invention that it allows the formation of arrays comprising thousands or even millions of apertures at carefully selected positions and with accurately controlled dimensions by employing semiconductor fabrication techniques.

(9) As illustrated in FIG. 2a, a thin dielectric mask layer 16 may be formed on a substrate layer, such as a silicon substrate 18, by means of e-beam lithography or nano imprint lithography. For instance, the dielectric mask layer 16 may comprise SiO.sub.2 at a thickness in the range of 5 to 30 nm. As further illustrated in FIG. 2a, a patterned resist layer 20 with apertures 22 may be formed on the dielectric mask layer 16. For instance, the resist layer 20 may comprise PMMA.

(10) Subsequent reactive ion etching (RIE) and/or hydrofluoric acid (HF) etching may then be employed to extend the apertures 22 down through the dielectric mask layer 16 and to the upper surface of the silicon substrate 18. The resulting structure (after removal of the resist layer 20) is shown in FIG. 2b.

(11) As illustrated in FIG. 2c, sacrificial nanowire structures 24 are then grown in the apertures 22 in the vertical direction. These sacrificial nanowire structures 24 only need to be as high as a few hundred nanometers, and in particular just as high as the desired thickness of the dielectric layer 14. The sacrificial nanowire structures 24 may be grown to a maximum diameter of approximately 200 nm, and are preferably composed of a material that can be easily etched by wet chemistry or decomposed thermally at temperatures below the respective decomposition/dissolution temperature of the desired dielectric layer 14. For instance, the sacrificial nanowire structures 24 may be formed of a combination of group III/group V semiconductor material, such as InAs, InGaAs, or GaAs.

(12) These nanowires can be thermally evaporated under both ambient and vacuum conditions at temperatures of 400 to 800 C., and hence well below the onset for subliming SiO.sub.2 at 1000 C. The resulting structure comprising the sacrificial nanowires 24 is show in FIG. 2c.

(13) Once the sacrificial nanowire structures 24 have been grown, in a subsequent step a thick dielectric layer deposit 26 may be grown on the dielectric mask layer 16 and the sacrificial nanowire structures 24, as schematically illustrated in FIG. 2d. Growth of the dielectric layer deposit 26 may involve techniques such as sputtering, plasma-enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD) or epitaxial techniques such as molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). For instance, the dielectric layer deposit 24 may comprise SiO.sub.2 or SiN at a thickness of 200 nm or higher. The use of epitaxial techniques such as MBE and/or MOCVD also allows to form a dielectric layer 26 comprising a distributed Bragg reflector and/or a high contrast grating, which may be beneficial for some laser applications.

(14) As schematically shown in FIG. 2d, the sacrificial nanowire structures 24 may be parasitically covered by the dielectric layer deposit 26 on all their facets. In order to minimize the parasitic deposits on the side walls of the sacrificial nanowire structure 24 that protrude above the desired dielectric layer 14, the inventors found it advantageous to employ highly directional sputtering techniques for forming the dielectric layer deposit 26, rather than more homogeneous techniques such as atomic layer deposition. This is because sputtering from a target under near normal incidence will usually result in a more anisotropic growth of the parasitic dielectric, with substantial dielectric layer material 26 on the top facet of the sacrificial nanowire structures 24, but only relatively thin layers of dielectric material on the side facets, as schematically shown in FIG. 2d. In contrast, atomic layer deposition usually results in a more isotropic growth with parasitic dielectric layer deposit 26 uniformly distributed all around the sacrificial nanowire structures 24.

(15) In a subsequent step, the parasitic deposits of the dielectric layer deposit 26 on the sacrificial nanowire structures 24 may be etched, such as by employing buffered hydrofluoric acid (HF). The inventors found that dipping the sample very shortly into the buffered HF solution is sufficient to remove the thin parasitic layer of the dielectric layer deposit 26 from the sacrificial nanowire structures 24. The resulting sample is shown in FIG. 2e. The dielectric layer that remains on the substrate 18 corresponds to the dielectric layer 14 at the desired thickness.

(16) The sample may now be transferred to an annealing device capable of providing temperatures in the range of approximately 800 C. At these temperatures, the sacrificial nanowire structures 24 dissolve thermally. For instance, thermal decomposition of GaAs sacrificial nanowires 80 in high vacuum of 10.sup.6 to 10.sup.9 Torr requires annealing temperatures in the range of 650 to 750 C. The inventors found it helpful to monitor the evaporating species during the annealing procedure, such as by mass spectrometry in situ. The annealing process can be considered completed once no more measurable traces of the evaporating species are detected in the mass spectrometer. This allows the determination of the time duration of the annealing procedure.

(17) The annealing step leaves behind the configuration shown in FIG. 2f, with thin nano-apertures 12.sub.1 to 12.sub.4 whose shapes and diameters corresponds to the diameters of the (removed) sacrificial nanowire structures 24 and that extend through the entire dielectric layer 14 down to the substrate 18.

(18) By means of the technique described above with reference to FIGS. 2a to 2f, thin nano-apertures 12.sub.1 to 12.sub.4 with diameters in the range of 100 nm or less may be formed in a dielectric layer 14 at a thickness of 200 nm or even more.

(19) The aperture pattern 10 illustrated in FIG. 2f can be used in a large variety of nanostructure fabrication techniques that require thin apertures 12.sub.1 to 12.sub.4 with carefully defined positions and sizes. For instance, FIG. 3 illustrates the growth of a nanowire laser array 28 in the aperture pattern 10.

(20) In the sequel, a growth process for a nanowire layer array 28 comprising a plurality of nanowire laser structures 30.sub.1 to 30.sub.4 made from group III/group V semiconductors is described, but one skilled in the art will understand that similar techniques may be employed to fabricate nanowire laser structures from other semiconductor materials. In a self-catalyzed growth process, the group-III element may form a metallic droplet seed inside the apertures 12.sub.1 to 12.sub.4 during the nucleation step. This seed droplet can comprise Ga or In, depending on whether GaAs or InGaAs-based nanowire lasers are desired. The growth temperature can be chosen so as to promote formation of the metal droplet only inside the apertures 12.sub.1 to 12.sub.4, and not on the dielectric SiO.sub.2 layer 14. Good growth selectivity can be achieved by using high substrate temperatures in the range of 500 C. and beyond.

(21) A support element of the nanowire may first be grown to a length of 1 m or larger, such as 5 to 20 m, to define the length of the nanowire laser cavity. In this case, a typical diameter of the support element may be in the range of approximately 20 to 150 nm, depending on the growth conditions, such as III-V flux/pressure and temperature. This diameter range is typically insufficient to allow for an effective mode confinement. However, after growth of the inner nanowire support element, the growth process may be changed from axial to lateral growth, so as to widen the nanowire structure and thereby form a body element surrounding the support element above the dielectric layer. This may be achieved by lowering the growth temperature to exploit the facet-dependent growth rate differences and induce radial growth along the {110} or {112} stable planes of compound semiconductor nanowires. Depending on the desired optical confinement of the fundamental optical modes, the radial growth may result in a diameter of the body element in the range of 200 nm or larger, for instance 300 to 600 nm for GaAs and InGaAs-based nanowires. During this radial growth step, the group-V pressure/flux may be increased so that growth takes place under group-V rich conditions. This has the additional benefit that the V-rich growth environment consumes the self-catalyzed metal droplet at the nanowire growth front, and therefore leads to a specular mirror-like end facet.

(22) The result is a nanowire laser array 28 with a plurality of nanowire laser structures 30.sub.1 to 30.sub.4 in which the respective body elements extend laterally only above the SiO.sub.2 dielectric layer 14, with a funnel-like support element at the base that connects the respective body element to the underlying substrate 18.

(23) An example for a nanowire laser structure 30.sub.1 to 30.sub.4 (henceforth denoted by reference numeral 30) as shown in FIG. 3 will now be described in greater detail with reference to FIG. 4.

(24) The nanowire laser structure 30 comprises the substrate 18, the elongated body element 32 (shown in black) resulting from the axial growth and extending vertically from the silicone substrate 18, and the elongated body element 34 (shown hatched in FIG. 4) that results from the subsequent lateral growth and is formed on and around the support element 32.

(25) In the configuration of FIG. 4, the support element 32 extends through the body element 34, and hence serves as a core of the nanowire laser structure 30. The body element 34 extends around the support element 32, and hence can be considered a shell of the nanowire laser structure 30.

(26) Contrary to the support element 32, the body element 34 does not extend all the way down to the substrate 18, but is separated from the substrate 18 by means of the dielectric layer 14 that surrounds the lower part of the support element 32 and extends between an upper surface of the substrate 18 and a lower surface of the body element 34.

(27) The dimensions of the nanowire laser structure 30 are chosen in accordance with the laser wavelength of the laser signal that the nanowire laser structure 30 is supposed to emit. For instance, a diameter d.sub.1 of the support element 32 may be chosen smaller than /(2.Math.n), when n denotes an index of refraction of the support element 32. A diameter d.sub.1</(2.Math.n) prevents laser wavelengths from forming and resonating within the support element 32, and hence prevents direct contact or coupling of these laser wavelengths with the underlying substrate 18 on which the support element 32 is formed.

(28) An outer diameter d.sub.2 of the body element 34 may be at least two times larger than the diameter d.sub.1 of the support element 32, and in particular may amount to at least /n, wherein n denotes an index of refraction of the body element 32. Hence, laser wavelengths may form and resonate in the body element 34. The body element 34 thus serves as a laser cavity.

(29) Depending on the laser wavelength, the diameter d.sub.1 of the support element 32 may be in the range between 80 nm and 300 nm. Correspondingly, the diameter d.sub.2 of the body element 34 may amount to at least 160 nm, in particular at least 400 nm or at least 600 nm.

(30) The dielectric layer 14 serves to reflect the laser modes at the lower end of the body element 34, and therefore provides resonant recirculation of the optical modes for lasing of the nanowire structure 30. The inventors found that the dielectric layer 14 enhances the modal reflectivity at the nanowire interface to 0.9 or even beyond, two to three orders of magnitude higher than what could be obtained by reflecting the laser modes directly off the silicon substrate 18. Even though the nanowire structure 30 is anchored directly to the silicon substrate 14 via the support element 32, it hence maintains excellent wave-guiding properties with confinement factors larger than 90% and spontaneous emission factors of =0.2 due to the low order mode laser cavity formed by the body element 34 being separated from the substrate 18 by means of the dielectric layer 14.

(31) Preferably, a thickness t of the dielectric layer 14 may be chosen as an integer multiple of /(2.Math.n), wherein n denotes an index of refraction of the dielectric layer 14. This thickness allows reflection both on a upper surface side of the dielectric layer 14 at the interface with the body element 34 and on a lower surface side of the dielectric layer 14 at the interface with the underlying substrate 18, and thereby further enhances the reflective properties by exploiting constructive interference of light reflections from said interfaces 34/14 and 14/18 inside said body element 34.

(32) Depending on the desired laser wavelength, the thickness t of the dielectric layer 14 may be chosen in the range between 100 nm and 800 nm.

(33) A length 1 of the body element 34 may amount to at least 1 m. With a cavity length of at least 1 m, the inventors found that losses may be efficiently suppressed. If the reflectivity of the dielectric layer 14 is particularly good, even shorter body elements may be used.

(34) For some applications, the support element 32 and/or the body element 34 may be doped individually. The doping may be homogeneous. Alternatively, graded dopings may be employed.

(35) With reference to FIGS. 2 and 3, a configuration has been described in which thin holes 12.sub.1 to 12.sub.4 are formed in the dielectric layer 14, wherein the size and dimension of each of the holes 12.sub.1 to 12.sub.4 correspond to the dimensions of the corresponding nanowire 24. However, this is simply an example, and apertures of other sizes and shapes may be formed. For instance, in an example the sacrificial nanowire structures 24 may be grown on the substrate 18 in close spatial proximity, so that they merge and together form a nano wall. Upon removal of the nano wall from the substrate 18, the aperture corresponds to a deep and narrow trench formed in the dielectric layer 14.

(36) The description and the drawings merely serve to illustrate the invention, but should not be understood to imply a limitation. The scope of the invention is determined by the appended claims.

REFERENCE SIGNS

(37) 10 hole pattern 12.sub.1-12.sub.4 holes of hole pattern 10 14 dielectric layer 16 dielectric mask layer 18 substrate 20 patterned resist layer 22 holes of patterned resist layer 20 24 sacrificial nanowire structures 26 dielectric layer deposit 28 nanowire laser array 30, 30.sub.1-30.sub.4 nanowire laser structures of nanowire laser array 28 32 support element of nanowire laser structure 30, core 34 body element of nanowire laser structure 30, shell