Light emitter device based on a photonic crystal with pillar- or wall-shaped semiconductor elements, and methods for the operation and production thereof

Abstract

A light emitter device (100) comprises a substrate (10) and a photonic crystal (20), which is arranged on the substrate (10) and comprises pillar- and/or wall-shaped semiconductor elements (21), which are arranged periodically standing out from the substrate (10), wherein the photonic crystal (20) forms a resonator, in which the semiconductor elements (21) are arranged in a first resonator section (22) with a first period (d.sub.1), in a second resonator section (23) with a second period (d.sub.2) and in a third resonator section (24) with a third period (d.sub.3), wherein on the substrate (10) the second resonator section (23) and the third resonator section (24) are arranged on two mutually opposing sides of the first resonator section (22) and the second period (d.sub.2) and the third period (d.sub.3) differ from the first period (d1), the first resonator section (22) forms a light-emitting medium and the third resonator section (24) forms a coupling-out region, through which a part of the light field in the first resonator section (22) can be coupled out of the resonator in a light outcoupling direction parallel to a substrate surface (11) of the substrate (10). Methods for operating and producing the light emitter device (100) are also described.

Claims

1. A light emitter device, comprising: a substrate, and a photonic crystal, which is arranged on the substrate and comprises a plurality of at least one of pillar-shaped and wall-shaped semiconductor elements, which are arranged in a periodic manner protruding from the substrate, wherein the photonic crystal forms a resonator, in which the semiconductor elements are arranged in a first resonator segment at a first period (d.sub.1), in a second resonator segment at a second period (d.sub.2), and in a third resonator segment at a third period (d.sub.3), wherein the second period (d.sub.2) and the third period (d.sub.3) differ from the first period (d.sub.1), wherein the second resonator segment surrounds the first resonator segment on multiple sides, the third resonator segment is arranged on a single side of the first resonator segment, and the photonic crystal forms a circular resonator, the first resonator segment forms a light emitting medium, the third resonator segment forms an output coupling region, through which some of a light field in the first resonator segment can be coupled out of the resonator in a light output coupling direction parallel to a substrate surface of the substrate, and at least one waveguide segment is provided, which adjoins the third resonator segment.

2. The light emitter device according to claim 1, wherein the second resonator segment surrounds the first resonator segment on the substrate over an angular range of at least 300°.

3. The light emitter device according to claim 1, wherein the third resonator segment has a smaller dimension in the light output coupling direction than the second resonator segment.

4. The light emitter device according to claim 1, wherein the second period (d.sub.2) and the third period (d.sub.3) are equal.

5. The light emitter device according to claim 1, wherein the at least one waveguide segment is formed by additional semiconductor elements or integrally with the substrate.

6. The light emitter device according to claim 1, wherein the semiconductor elements are arranged adjoining the second resonator segment or the waveguide segment in a deflection segment at a fourth period (d.sub.4), which differs from the first period, second period and third period (d.sub.1, d.sub.2, d.sub.3), and form a vertical coupler.

7. The light emitter device according to claim 6, wherein the fourth period (d.sub.4) is selected to satisfy a second-order Bragg condition.

8. The light emitter device according to claim 1, wherein the substrate comprises a contact layer, on which the photonic crystal is formed and which is arranged on a support material having a bandgap that is larger than ae light energy of emitted light, and having a refractive index that is smaller than the refractive index of the first resonator segment.

9. The light emitter device according to claim 1, wherein the substrate comprises silicon.

10. The light emitter device according to claim 9, wherein the substrate comprises a silicon dioxide layer having a silicon contact layer.

11. The light emitter device according to claim 1, wherein the semiconductor elements are arranged in a dielectric embedding layer.

12. The light emitter device according to claim 11, wherein the embedding layer carries an electrically conductive contact layer.

13. A method for operating a light emitter device according to claim 1, comprising the steps: coupling the light emitter device to a voltage source device or a pump device; electrically or optically exciting the first resonator segment; and emitting light through the third resonator segment.

14. The method for producing a light emitter device according to claim 1, comprising the steps: providing the substrate; and growing the semiconductor elements of the photonic crystal on the substrate.

15. The method according to claim 14, comprising the step: contacting the photonic crystal with contact electrodes.

16. The method according to claim 14, wherein growing the semiconductor elements comprises at least one member selected from the group consisting of direct epitaxial growth of the semiconductor elements on the substrate, vapor-liquid-solid based growth of the semiconductor elements on the substrate, and mask-based deposition of the semiconductor elements on the substrate.

17. The method according to claim 14, wherein the semiconductor elements are embedded in a transparent embedding layer.

18. The light emitter device according to claim 9, wherein a substrate surface of the substrate comprises silicon.

19. The light emitter device according to claim 12, wherein the electrically conductive contact layer is a reflective contact layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further details and advantages of the invention are described below with reference to the accompanying drawings, in which:

(2) FIG. 1 shows in a schematic side view a photonic crystal of a claimed light emitter device according to the first embodiment of the invention;

(3) FIG. 2 shows in a sectional view the first embodiment of the light emitter device according to the invention;

(4) FIG. 3 shows in a schematic plan view the first embodiment of the light emitter device according to the invention;

(5) FIG. 4 shows in a schematic plan view the second embodiment of the light emitter device according to the invention; and

(6) FIG. 5 shows a graph for characterizing the resonator of a light emitter device according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(7) Features of preferred embodiments of the invention are described below by way of example with reference to a light emitter device which is provided with a deflection segment and is configured as a surface emitter device having directional emission into the half-space above the substrate surface. The surface emitter device has particular advantages when used as a vertical emitter. The practical implementation of the invention is not limited to this embodiment, however. Indeed the light emitter device can be realized without the deflection segment and designed for emission parallel to the substrate surface.

(8) Forms of the semiconductor elements of the light emitter device are, for example, the form of a pillar (or pin, wire or rod) or the form of a wall (or plate). The thickness of the semiconductor elements preferably equals at least 5 nm and/or at most 500 nm, and the height of the semiconductor elements preferably equals at least 300 nm and/or at most 10 μm. The semiconductor elements are preferably produced from a GaN or GaAs compound semiconductor, and are optionally doped, for instance with Si, Ge or Mg (GaN) or with Si, Be, C or Te (GaAs).

(9) Reference is made by way of example to a light emitter device having a photonic crystal that is produced from nanowires. The invention is not limited to the use of nanowires, but can be realized in a corresponding manner by a photonic crystal that is produced from nanowalls or from nanowires and nanowalls. The light emitter device can be designed in particular for electrical excitation or optical excitation of the photonic crystal. In addition, a plurality of photonic crystals can be arranged jointly on one substrate.

(10) FIG. 1 shows a schematic sectional view of a photonic crystal 20 having a plurality of nanowires 21 arranged in a periodic manner on a substrate (not shown in FIG. 1), which form a first resonator segment 22, a second resonator segment 23, a third resonator segment 24 and a deflection segment 25. The sectional view shows the photonic crystal 20 along the plane perpendicular to the substrate surface (x-z plane), wherein the segments 22 to 25 are arranged adjacent to one another in a cavity main direction (light output coupling direction, optical axis OA, x-direction in FIG. 1), and the photonic crystal 20 forms a linear resonator (first embodiment of the invention).

(11) In the plane parallel to the substrate surface (x-y plane, perpendicular to the drawing plane in FIG. 1), all of the segments 22 to 25 have a rectangular basic shape, and therefore the overall photonic crystal 20 has a rectangular basic shape and is not covered in the y-direction (see below).

(12) The nanowires 21 form, for instance, (In,Ga)N-based quantum-well heterostructures, which are produced at a diameter of 50 nm and length of 500 nm. In said segments 22 to 25, the nanowires 21 have different lattice spacings (periods). The first period d.sub.1 of the nanowires 21 equals e.g. 110 nm in the first resonator segment 22.

(13) In the central first resonator segment 22, light is emitted by the nanowires 21 in response to optical or electrical excitation (see FIG. 2). The dimension of the first resonator segment 22 along the optical axis OA (distance between the second and third resonator segments 23, 24) is selected for resonant superposition of the emitted light according to the wavelength λ of said light (see double-ended arrow), and equals, for example, 2.2 μm.

(14) In the second and third resonator segments 23, 24, the respective second and third periods d.sub.2=d.sub.3 are selected such that the Bragg condition according to equation (1) is satisfied for the resonant wavelength λ of the light emitted in the first resonator segment 22 having an effective refractive index n, and photons emitted in the first resonator segment 22 are confined in this segment:
d.sub.B=λ/2n  (1)

(15) In a practical example, the second and third periods d.sub.2, d.sub.3 are set to 150 nm for an emission wavelength λ of 500 nm in (In,Ga)N-based nanowires 21.

(16) The second and third resonator segments 23, 24 are not identical. Along the optical axis OA, the dimension of the third resonator segment 24 is smaller than the dimension of the second resonator segment 23. In the radiation output coupling direction, the second resonator segment 23, which forms a fully reflecting mirror, comprises 15 nanowires, for instance, whereas the third resonator segment 24, which forms an output coupler, comprises only 5 nanowires, with the result that the dimensions of the second resonator segment 23 and of the third resonator segment 24 equal e.g. 2.1 μm and 0.75 μm along the optical axis OA.

(17) In the deflection segment 25, the nanowires 21 are arranged at a larger spacing. The fourth period (deflection period) d.sub.4 is selected, for example, such that in the deflection segment 25 having the effective refractive index n, the second-order Bragg condition according to equation (2)
d.sub.0=λ/n  (2)
is satisfied. Vertical extraction in the z-direction of the light coupled out of the first resonator segment 21 is achieved by this fourth period.

(18) The fourth period d.sub.4 need not be tuned exactly to the second-order Bragg condition according to equation (2). Instead it is sufficient in the example mentioned for the fourth period d.sub.4 to vary about a value of 300 nm. By means of test experiments or numerical simulations, the fourth period d.sub.4 can be selected so as to maximize the efficiency of the deflection in the vertical direction. The dimension of the deflection segment 25 equals e.g. 3 μm along the optical axis OA.

(19) FIG. 2 shows a schematic sectional view of a preferred embodiment of the light emitter device 100 according to the invention comprising a substrate 10 and a photonic crystal 20. The substrate 10 has a Si-based layered structure, which provides a substrate surface 11 made of silicon. In detail, the layered structure comprises a support layer 12, which is made of silicon or another semiconductor material or ceramic material, a silicon dioxide layer 13, and a silicon contact layer 14, which forms the substrate surface 11. Depending on the method for producing the nanowires 21 (see below), a buffer layer 15, for instance made of AlN, and a masking layer 16 made of silicon dioxide can be provided on the silicon contact layer 14. The masking layer 16 includes apertures at which the nanowires 21 are grown onto the buffer layer 15, and in which the nanowires 21 are in electrical contact with the silicon contact layer. The substrate 10 has a thickness of 0.5 mm for example.

(20) Adjacent to the photonic crystal 20 is provided a contact electrode 31 on the silicon contact layer 14. The contact electrode 31 preferably comprises a metal layer, for instance made of gold having a thickness of 100 nm.

(21) The nanowires 21 of the photonic crystal 20 are embedded in an embedding layer 26, for instance made of HSQ, which electrically isolates the nanowires 21 from one another and provides mechanical stability to said nanowires. On the top face of the embedding layer 26, the ends of the nanowires 21 are connected to an electrically conductive contact layer 32. The contact layer 32 for instance consists of gold having a thickness of 100 nm.

(22) The contact electrode 31 and the contact layer 32 are provided for connecting to a voltage source and for supplying an electrical excitation current to the semiconductor elements (nanowires 21). Since the electrical excitation of the nanowires 21 is provided solely in the first resonator segment 22, the contact layer 32 can be restricted to the extent of the first resonator segment 22 (see FIG. 3).

(23) FIG. 3 shows a schematic plan view of the first embodiment of a light emitter device 100 according to the invention comprising the substrate 10 and the photonic crystal 20. The photonic crystal 20 comprises the central first resonator segment 22, which is enclosed in the x-direction by the second resonator segment 23 and the third resonator segment 24. FIG. 3 also shows the contact electrode 31 on the silicon contact layer 11 on the top face of the substrate 10, and the contact layer 32, which covers (shown dashed) at least the first resonator segment 22 of the photonic crystal 20.

(24) The regions of the second resonator segment 23 and of the third resonator segment 24, which regions are opposite one another, form the end and output coupler mirrors of a linear resonator, the main cavity direction of which (optical axis OA, x-direction) is shown by a dashed line. Linear modes are generated along the optical axis. In accordance with the orientation of the resonator, during operation of the light emitter device 100, light that is excited by optical pumping or electrical excitation and amplified by resonance in the first resonator segment 22, can be coupled out through the third resonator segment 24 (see dotted arrow), and deflected in the deflection segment 25 into the vertical z-direction (perpendicular to the drawing plane).

(25) FIG. 4 shows by way of example a schematic plan view of the second embodiment of the light emitter device 100 comprising the substrate 10 and the photonic crystal 20, wherein in this case the first resonator segment 22 is designed by a hexagonal shape to generate circular modes (see dotted double-ended arrow). The light generated in the first resonator segment 22 is coupled out through the third resonator segment 24 into a waveguide arrangement 27. Onward transmission takes place from the waveguide arrangement 27 into the adjoining space, into an additional waveguide integrated in the substrate, or into a deflection segment (see FIG. 3).

(26) The dimensions of the photonic crystal 20 parallel to the surface of the substrate 10 are selected such that circular modes are generated resonantly at the emission wavelength λ of the nanowires in the first resonator segment 22, and are confined in the radial direction by the second resonator segment 23 except for the region of the third resonator segment 24, and such that output coupling into the waveguide segment 27 takes place at the third resonator segment 24. The dimensions of the second resonator segment 23 and of the third resonator segment 24 in the radial direction can be determined specifically by numerical simulations of the confinement at the second resonator segment 23 and of the output coupling at the third resonator segment 24. In a practical example, the diameter of the first resonator segment 22 equals 350 nm, the width of the second resonator segment 23 in the radial direction equals 2.1 μm, and the width of the third resonator segment 24 in the radial direction equals 100 nm.

(27) A light emitter device 100 according to the invention is preferably produced using a “bottom-up” technique. The nanowires 21 grow selectively in the holes of a mask, which has advantages for precise control of the position of the nanowires 21. For the GaN-based nanowires 21, an AlN buffer layer 15 is applied to the silicon contact layer 14, followed by the deposition of a silicon oxide masking layer 16. The masking layer 16 has openings, which are arranged in accordance with the lattice pattern in which the nanowires 21 are meant to be grown on the substrate 10. The pn heterostructure containing quantum wells for the charge-carrier recombination, which heterostructure is desired for electrical excitation, is achieved with the epitaxial deposition of the semiconductor material in the axial or radial direction along the nanowires 21.

(28) After the growth of the nanowires 21, the embedding layer 26 is formed by applying an HSQ precursor solution to the arrangement of the nanowires 21, for instance by a spin-on process. Once the HSQ has been cured, surface treatment and planarization are carried out until the top ends of the nanowires 21 are exposed.

(29) Then for a light emitter device that is designed for electrical excitation, a gold layer is applied as the electrically conductive contact layer 32 by sputtering, for instance, in order to produce the electrical connections.

(30) Using silicon as the substrate material has a number of advantages in terms of costs and the capability for integration with other optical and electrical components. However, since silicon, as an absorbing material at short wavelengths, can drastically impair the resonator quality (Q factor), the layered structure shown in FIG. 2 having the silicon dioxide support layer 13 is preferably used. The effect of the absorption can be reduced by means of the support layer 13, while the thin silicon contact layer 14 still allows an electrical connection to be made between the nanowires 21 and the contact electrode 31. For long wavelengths, the silicon dioxide support layer 13 allows confinement of the light field from below.

(31) FIG. 5 shows the result of a numerical simulation of the resonator quality (Q factor) as a function of the thickness of the silicon contact layer 14 on the silicon dioxide support layer 13. As a result of the light absorption in the silicon, the resonator quality falls exponentially with increasing thickness of the silicon contact layer 14. While the resonator quality without the silicon contact layer 14 would equal over 15,000, the quality drops to 5000 for a thickness of the silicon contact layer of just 50 nm and as far as 1000 for a thickness greater than 150 nm. Advantageously, however, a silicon contact layer 14 having a thickness of less than 40 nm is sufficient to ensure electrical contact is made to the nanowires 21.

(32) The features of the invention disclosed in the above description, in the drawings and in the claims may be relevant, both individually and in combination or in sub-combination, to realizing the invention in its various embodiments.