Method for manufacturing photonic crystal gallium nitride light emitting element and lighting device

Abstract

To obtain a photonic crystal GaN light emitting element by efficiently forming photonic crystals on the GaN layer that constitutes the light emitting element.SOLUTION: The present invention comprises a step for forming an epitaxial structure by epitaxially growing n-type GaN layer 12, MQW layer 14, and p-type GaN layer 16 sequentially on a sapphire substrate 10, and a step for forming photonic crystals on the surface of the p-type GaN layer 16 using electron beam lithography and inductively coupled plasma reactive ion etching.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a photonic crystal gallium nitride light emitting device. [Background technology]

[0002] In order to narrow the beam angle of light emitted from a light-emitting diode (LED) chip that uses gallium nitride (GaN), consideration is being given to equipping the LED device with a lens to concentrate the light and narrow the beam angle.

[0003] However, in the case of lenses, even a slight change in directivity requires changing the mold, which increases costs.In addition, in the case of ultraviolet and deep ultraviolet rays, quartz lenses are used in consideration of the lens absorption, which makes the lenses expensive.

[0004] Therefore, as a method of concentrating light and narrowing the directivity without equipping an LED device with a lens, the formation of photonic crystals with holes on the order of several hundred nanometers at a pitch of several hundred nanometers on the surface of the LED chip is being considered.

[0005] Patent Document 1 describes an invention relating to a gallium nitride photonic crystal and a manufacturing method thereof. That is, the invention aims to provide a high-performance gallium nitride semiconductor element, a gallium nitride semiconductor substrate, and a gallium nitride photonic crystal with a high yield by obtaining a nitride semiconductor with reduced dislocation density in a wide region during epitaxial growth, and the invention includes a process of forming a gallium nitride semiconductor layer on a substrate, a process of forming an array of trenches in the gallium nitride semiconductor layer, a process of performing a heat treatment in an atmosphere containing nitrogen to deform at least two or more adjacent trenches among the plurality of trenches, forming continuous cavities in the gallium nitride semiconductor layer corresponding to the positions of the deformed trenches, and a process of forming a gallium nitride semiconductor element on the gallium nitride semiconductor layer having the cavities.

[0006] Furthermore, Patent Document 2 describes a photonic crystal light-emitting diode having an objective of providing a photonic crystal light-emitting diode that is resistant to deterioration over a long period of time and has high energy efficiency and light-emitting efficiency, in which three layers, a first semiconductor layer, an active layer, and a second semiconductor layer, are stacked in that order, with a first electrode electrically connected to the first semiconductor layer and a second electrode electrically connected to the second semiconductor layer, in which voids penetrating at least the first semiconductor layer and the active layer out of the three layers are arranged two-dimensionally periodically to form a photonic crystal structure, and the first electrode covers the first semiconductor layer except for the voids and a first non-current injection region surrounding the voids. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2004-111766 A [Patent Document 2] JP 2011-54828 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, when light emitting devices such as LEDs are made of gallium nitride (GaN), the technology for forming photonic crystals in a GaN layer has not yet been established.

[0009] An object of the present invention is to provide a method for obtaining a photonic crystal GaN light-emitting device by efficiently forming a photonic crystal in a GaN layer that constitutes a light-emitting device, and a lighting device. [Means for solving the problem]

[0010] The present invention is a method for manufacturing a photonic crystal gallium nitride light-emitting device, comprising the steps of epitaxially growing an n-type gallium nitride layer, a multiple quantum well layer, and a p-type gallium nitride layer, successively, on a sapphire substrate to form an epitaxial structure, and forming a photonic crystal on a surface of the p-type gallium nitride layer using electron beam lithography and inductively coupled plasma reactive ion etching.

[0011] In one embodiment of the present invention, the step of forming the photonic crystal comprises the steps of performing a hydrophobic treatment on the surface of the n-type gallium nitride layer, applying a resist to the surface of the n-type gallium nitride layer, irradiating the resist with an electron beam to perform drawing, and removing the resist with chlorine gas by inductively coupled plasma reactive ion etching.

[0012] In another embodiment of the present invention, the method includes the steps of growing a buffer layer and an undoped gallium nitride layer, in that order, on the sapphire substrate before growing the n-type gallium nitride layer on the sapphire substrate.

[0013] In yet another embodiment of the present invention, the photonic crystal gallium nitride light emitting device has an emission wavelength of 380 nm to 390 nm.

[0014] The present invention also provides an illumination device comprising a photonic crystal gallium nitride light emitting element manufactured by the above manufacturing method, and an illumination optical system that irradiates an object with light from the photonic crystal gallium nitride light emitting element. Effect of the Invention

[0015] According to the present invention, it is possible to reliably form photonic crystals in a gallium nitride (GaN) layer constituting a GaN light-emitting device. This makes it possible to narrow the directivity of light emitted from the light-emitting device. In addition, it is possible to increase the luminous efficiency of the gallium nitride light-emitting device. [Brief description of the drawings]

[0016] [Figure 1] 1A to 1C are schematic cross-sectional views (part 1) illustrating a manufacturing method according to an embodiment. [Diagram 2] 1A and 1B are SEM images after etching by inductively coupled plasma reactive ion etching according to an embodiment, where (A) is an SEM image of the inner diameter of the hole and (B) is an SEM image of the outer diameter of the hole. [Diagram 3] 1A to 1C are explanatory diagrams of the creation of a flip chip according to an embodiment. [Figure 4] FIG. 4 is an IV characteristic diagram of the flip chip of the embodiment. [Diagram 5] FIG. 11 is an IL characteristic diagram of the flip chip of the embodiment. [Figure 6] FIG. 11 is a graph showing the emission intensity of the flip chip according to the embodiment. [Figure 7] FIG. 2 is an emission spectrum diagram of the flip chip of the embodiment. [Figure 8] FIG. 13 is a diagram showing the results of measuring the half-value width of the flip chip according to the embodiment. [Figure 9] FIG. 2 is a diagram showing the configuration of an illumination device and an exposure device according to the embodiment. [Figure 10] FIG. 2 is a diagram showing a configuration of a light source unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0018] The basic principle of this embodiment is to form photonic crystals in the GaN layer constituting a GaN light-emitting element by electron beam (EB) drawing and inductively coupled plasma reactive ion etching, taking into consideration that the etching rate of the GaN layer greatly differs depending on the structure and crystallinity of the substrate and the GaN layer, and to grow the GaN layer on a sapphire substrate rather than growing the GaN layer on a Si substrate or the like, and to form photonic crystals on the GaN layer thus grown by electron beam (EB) drawing and inductively coupled plasma reactive ion etching.

[0019] The present inventors have found that, compared with a GaN layer grown on a Si substrate, a GaN layer grown on a sapphire substrate has a significantly higher etching rate by electron beam (EB) lithography and inductively coupled plasma reactive ion etching, and that it is possible to very efficiently form holes at desired intervals and diameters on the surface of the GaN layer to form a photonic crystal.

[0020] FIG. 1 is a cross-sectional view that illustrates a method for producing a photonic crystal GaN light-emitting device (LED) in this embodiment.

[0021] As shown in FIG. 1, a low-temperature buffer layer (not shown) and an undoped GaN (u-GaN) layer (not shown) are laminated on a sapphire substrate 10 using a metal-organic chemical vapor deposition (MOCVD) apparatus, and an n-GaN layer (n-type GaN layer) 12, a multiple quantum well (MQW) layer 14, and a p-GaN layer (p-type GaN layer) 16 are sequentially laminated thereon to grow an epitaxial structure that emits light at 385 nm (375 to 390 nm).

[0022] More specifically, the n-GaN layer 12 is composed of an (AlInGaN) / (InGaN;Si) n-SLS (superlattice structure) layer and a (GaN;Si) contact layer. Here, for example, (GaN;Si) indicates that Si is doped GaN.

[0023] Moreover, the multiple quantum well (MQW) layer 14 is composed of an (InGaN / AlGaN) MQW layer.

[0024] More specifically, the p-GaN layer 16 is composed of a p-GaN (GaN;Mg) contact layer and an (AlGaN;Mg / GaN;Mg)p-SLS (superlattice structure) layer. Conventionally, the thickness of the p-GaN layer 16 is about 50 nm to 150 nm, but in this embodiment, in order to form a photonic crystal (PhC) in the p-GaN layer 16, the thickness is set to twice the conventional thickness, that is, about 100 nm to 300 nm, in order to reduce damage caused by the depth and dry etching during the formation of the PhC.

[0025] In this embodiment, the term "GaN layer" does not necessarily mean a single layer having a GaN composition, but means a single layer or multiple layers having a composition containing GaN, such as AlInGaN, InGaN, AlGaN, etc. In short, it means a single layer or multiple layers having GaN as the main component.

[0026] In this embodiment, the emission wavelength of the epitaxial layer is 385 nm. When a wavelength longer than this is used, the basic epitaxial structure is the same, but in terms of band gap energy, the Al content of the (InGaN / AlInGaN) MQW emission layer 14 is made larger and the In content is made smaller for 385 nm than for longer wavelengths.

[0027] Next, a photonic crystal (PhC) is formed on the surface of the p-GaN layer 16, as shown by dashed-dotted region 18 in Fig. 1. Although multiple dashed-dotted regions 18 are formed on the surface of the p-GaN layer 16 in the figure, they may be formed over the entire surface of the p-GaN layer 16.

[0028] After that, ITO is evaporated onto the p-GaN layer 16 and etched down to the nGaN layer 12 to form a mesa shape. Furthermore, an n-pad is formed in the etched area and its height is adjusted to match the height of the mesa shape. Then, SiO2 is evaporated onto the entire surface to form a protective film, SiO2 is partially etched on the n-pad and ITO, and Au is evaporated onto the etched portion by sputtering UBM (under bump metal), to produce a GaN light emitting element (LED chip).

[0029] Here, we will explain in more detail the method for producing the photonic crystal shown in Fig. 1. The photonic crystal (PhC) is formed by developing a pattern in a resist by electron beam (EB) lithography, and then etching by inductively coupled plasma reactive ion etching. Therefore, prior to forming the photonic crystal, it is necessary to set the EB lithography conditions and the etching conditions by inductively coupled plasma reactive ion etching. EXAMPLES

[0030] A GaN wafer with an emission wavelength of 385 nm grown on a sapphire substrate was rinsed and then dried for 5 minutes at 180°C. After that, a treatment (HMDS treatment) was performed to make the GaN wafer surface hydrophobic, and 500 nm of UV resist was applied using a spin coater, followed by pre-baking for 3 minutes at 180°C.

[0031] Next, a PhC pattern of 2 mm x 2 mm size (total of 5 locations) was formed by changing the exposure dose using a high-speed maskless exposure device.

[0032] In parallel with this, in order to confirm the etching rate, holes of 100 μm x 100 μm size (total of three places) were formed with a constant exposure dose. The exposure dose for each size is as follows: <2mm x 2mm (total of 5 places)> Exposure dose = 140, 150, 160, 170, 180 (μC / cm 2 ) <100μm×100μm> Exposure dose = 200 μC / cm 2

[0033] A GaN wafer with a UV resist was etched for 10 minutes with chlorine gas (30sccm) in an inductively coupled plasma reactive ion etching system. The etching depth was measured with a UV stylus step gauge, and the etching rate was 55.2nm / min.

[0034] Figure 2 shows an SEM image after etching with an inductively coupled plasma reactive ion etching system. The holes in the PhC are slightly tapered, with an inner diameter of 200 nm to 230 nm and an outer diameter of 270 nm to 300 nm. From the viewpoint of taper, the exposure dose (dose) was 180 μC / cm 2 ~200μC / cm 2 Good results (i.e., less taper) were obtained at an exposure dose of 170 μC / cm. 2Figure 5(B) is an SEM image of the inner diameter of a PhC pattern formed under the same conditions, with the size being 226.67 nm. Figure 5(C) is an SEM image of the outer diameter of a PhC pattern formed under the same conditions, with the size being 285.33 nm.

[0035] In this way, it was confirmed that holes on the order of several hundred nanometers were formed at a pitch of several hundred nanometers on the surface of the GaN wafer, and a photonic crystal (PhC) was formed.

[0036] It was also confirmed that when an epitaxial layer was grown in a similar manner using a patterned sapphire substrate (PSS) instead of a sapphire substrate and a PhC was formed, holes of the same shape as those formed on the sapphire substrate were formed and the etching rate was increased by approximately 1.2 times.

[0037] Next, the PL wavelength and intensity were measured using GaN wafers with PhC formed. As a result, when comparing the intensity of the PhC-formed area with that of the surrounding areas without PhC formed, it was confirmed that the PhC-formed area had an intensity about 1.5 to 1.6 times higher than that of the areas without PhC formed. This is thought to be because the light extraction efficiency is improved by the PhC formation. It was also confirmed that the PL wavelength in the areas with PhC formed tended to shift to the longer wavelength side by 0.5 to 1.0 nm compared to the areas without PhC formed.

[0038] Next, a photonic crystal (PhC) is formed on the surface of the p-GaN layer 16, ITO is evaporated onto the p-GaN layer 16, and the layer is etched down to the nGaN layer 12 to form a mesa shape. An n-pad is formed in the etched area and its height is adjusted to match that of the mesa shape. SiO2 is evaporated onto the entire surface to form a protective film. SiO2 is partially etched onto the n-pad and ITO, and Au is evaporated onto the etched portion by sputtering UBM (under bump metal). A GaN light emitting element (LED chip) is manufactured and mounted on a mounting substrate by flip chip. The IV characteristics, IL characteristics, emission spectrum, and emission intensity are measured.

[0039] FIG. 3 shows a plan view of forming photonic crystals on the order of several hundreds of nm in a total of five regions 22a, 22b, 22c, 22d, and 22e on the surface of a 4-inch wafer of a sapphire substrate 20 to create a chip with a light-emitting area of ​​79×79 μm. FIG. 3(A) is a plan view of the wafer, and FIG. 3(B) shows a total of five photonic crystal formation regions 22a-22e formed at predetermined positions on the wafer at intervals. FIG. 3(C) is a partial enlarged view of one region 22c in FIG. 3(B). An LED chip was manufactured in the region where the photonic crystals were formed and flip-chip mounted, and an LED chip was manufactured in the region where the photonic crystals were not formed and mounted on a mounting substrate by flip-chip mounting.

[0040] Figure 4 shows the measurement results of the IV characteristics of a chip with a photonic crystal (Sapphire-PhC) and a chip without a photonic crystal (Sapphire). Both show good characteristics, and there is almost no difference between the presence and absence of a photonic crystal.

[0041] Figure 5 shows the measurement results of the IL characteristics of a chip with a photonic crystal (Sapphire-PhC) and a chip without a photonic crystal (Sapphire). Both show good characteristics, and there is almost no difference between the presence and absence of a photonic crystal.

[0042] Figure 6 shows the results of measuring the intensity of a chip with a photonic crystal (Sapphire-PhC) and a chip without a photonic crystal (Sapphire). Figures 7 and 8 show the emission spectra and full width at half maximum of a chip with a photonic crystal (Sapphire-PhC) and a chip without a photonic crystal (Sapphire), respectively. In Figure 8, WH indicates the half width (full width at half maximum) and PO indicates the emission intensity.

[0043] As can be seen from FIGS. 6 and 8, it was confirmed that the emission intensity of the chip having the photonic crystal was improved by about 1.74 times compared to the chip not having the photonic crystal.

[0044] In this embodiment, as described above, photonic crystals are formed on the surface of the p-GaN layer 16, ITO is deposited on the p-GaN layer 16, and then etching is performed down to the n-GaN layer 12 to form a mesa shape, so it is presumed that no photonic crystals are present in the mesa-etched portion. However, the applicant of the present application has confirmed that the color of the entire surface of the chip having the mesa shape has changed, and it is possible that the photonic crystals on the surface of the p-GaN layer 16 are also formed in the n-GaN layer 12 during mesa etching, which may contribute to improving the light extraction efficiency.

[0045] In this manner, a photonic crystal (PhC) can be formed on the surface of the p-GaN layer 16. Then, an illumination device including the GaN light emitting element thus manufactured and an exposure device including the illumination device can be obtained.

[0046] Fig. 9 is a diagram showing a schematic configuration of an exposure apparatus equipped with an illumination device IU using the GaN light-emitting element manufactured by the manufacturing method according to the embodiment described above as a light source. Fig. 10(A) is a plan view showing a schematic configuration of a light source unit 120 that supplies exposure light to the illumination device IU, and Fig. 10(B) is a diagram showing a schematic internal configuration of the light source unit 120 and the output optical system 130.

[0047] In FIG. 9, the illumination device IU includes a first focusing optical system 140 that focuses the exposure light from the light source unit 120 via the output optical system 130, a fly's eye lens FEL onto which the exposure light from the first focusing optical system 140 is incident, and a second focusing optical system 160 that focuses the exposure light from the fly's eye lens FEL to illuminate a reticle REI as an irradiated surface.

[0048] The exposure apparatus also includes an illumination device IU, and a projection optical system PO that forms an image of a pattern on a reticle illuminated by the illumination device IU onto a wafer serving as an exposed substrate.

[0049] Next, the light source unit 120 and the output optical system 130 will be described with reference to FIG.

[0050] In FIG. 10(A), the light source unit 120 includes a plurality of LED (Light Emitting Diode) chips 123 (5×5 in FIG. 10(A)) arranged on a substrate 121, for example. These LED chips are GaN light emitting elements manufactured by the manufacturing method of the above-mentioned embodiment. The number of LED chips 123 may be appropriately changed as necessary. Each of the plurality of LED chips 123 has a light emitting portion 231, and the peak wavelength of the light emitted from the light emitting portion 231 is within a range of 380 to 390 nm. That is, the light emitting portion 231 is an ultraviolet LED (UV LED). It is more preferable that the peak wavelength of the light emitted from the light emitting portion 231 is 385 nm. The light emitting surface of the light emitting portion 231 may be a square, or may be a polygon such as a rectangle or a hexagon. The LED chips 123 may be arranged on a heat sink, for example, instead of on a substrate.

[0051] The configuration of the light source unit 120 and the output optical system 130 will be described with reference to Fig. 10(B). In Fig. 10(B), the two directions in which the LED chips 123 are arranged are the X direction and the Y direction. The X direction and the Y direction are perpendicular to each other. The direction perpendicular to the X direction and the Y direction is the Z direction. The Z direction approximately coincides with the traveling direction of the light emitted from the light emitting section 231. Note that Fig. 10(B) shows only four LED chips 123 aligned in a row along the Y direction for clarity of the drawing.

[0052] As shown in FIG. 10B, the output optical system 130 includes a plurality of magnifying optical systems for forming magnified images of the light emitting parts 231 of the LED chips 123. Each of the magnifying optical systems is arranged to correspond to the arrangement of the LED chips 123, and is a double-telecentric optical system that magnifies and projects the light emitting parts 231 under a magnification ratio. In FIG. 10B, each of the magnifying optical systems of the output optical system 130 is illustrated as having two positive lenses, but the number of lenses constituting the magnifying optical system is not limited to two. The magnifying optical system may be configured as a reflective system or a catadioptric system. The output optical system 130 may include a unity magnification optical system or a reduction optical system instead of or in addition to the magnifying optical system.

[0053] 9 and 10, each magnifying optical system of the output optical system 130 forms a magnified image of the light emitting unit 231 near the final optical member of the output optical system 130. As a result, a plurality of light source images that are densely arranged are located near the final optical member of the output optical system 130.

[0054] Returning to FIG. 9, each component of the illumination device IU will be described.

[0055] The first light-collection optical system 140 is disposed so that its front focal point is located at or near the position of the light source image formed by the light source unit 120 and the output optical system 130, and provides Koehler illumination to the entrance surface of the fly's eye lens FEL.

[0056] The fly-eye lens FEL is formed by, for example, arranging a plurality of lens elements having positive refractive power vertically and horizontally and densely so that their optical axes are parallel to the reference optical axis AX. Each lens element constituting the fly-eye lens FEL has a rectangular cross section similar to the shape of the illumination field to be formed on the reticle REI (and thus the shape of the exposure area to be formed on the wafer W).

[0057] Therefore, the light beam incident on the fly-eye lens FEL is wavefront-split by the multiple lens elements, and one light source image is formed on or near the rear focal plane (exit surface) of each lens element. In other words, a substantial surface light source consisting of multiple light source images, that is, a secondary light source, is formed on or near the rear focal plane (exit surface) of the fly-eye lens FEL.

[0058] Here, when focusing on one lens element of the fly-eye lens FEL, a plurality of light source images formed by the light source unit 120 and the output optical system 130 are formed on the rear focal plane (exit surface) of this one lens element or in its vicinity, and these plurality of light source images are distributed in an area having a similar shape to the area where the plurality of LED chips 123 are arranged. For this reason, the area where the plurality of LED chips 123 are arranged and the cross section of each lens element of the fly-eye lens FEL may have a similar shape to each other.

[0059] Now, the light beam from the secondary light source formed on or near the rear focal plane (exit surface) of the fly-eye lens FEL is incident on the aperture stop 150 arranged nearby. In this embodiment, the rear focal plane (exit surface) of the fly-eye lens FEL and the LED chip array surface of the light source unit 120 are optically conjugate.

[0060] The aperture stop 150 is disposed at a position that is nearly optically conjugate with the entrance pupil plane of the projection optical system PO, and has a variable aperture for defining the range that contributes to the illumination of the secondary light source. The exposure light from the secondary light source that passes through the aperture stop 150 is subjected to the focusing action of the second focusing optical system 160, and then illuminates a reticle REI on which a predetermined pattern is formed in a superimposed manner.

[0061] Then, the exposure light from the reticle REI illuminated by the illumination device IU (exposure light passing through a pattern located within the illumination field on the reticle REI) is incident on the projection optical system PO and forms an image of the pattern of the reticle REI on the wafer W as the exposed substrate. As a result, the pattern of the reticle REI is exposed onto the exposure area of ​​the wafer W. The projection magnification of the projection optical system PO may be a reduction magnification, an enlargement magnification, or an equal magnification. The projection optical system PO may also be a refractive optical system, a reflective optical system, or a catadioptric optical system.

[0062] As described above, according to this embodiment, photonic crystals can be manufactured by forming holes on the order of several hundreds of nanometers at a pitch of several hundreds of nanometers on the surface of a p-GaN layer grown on a sapphire substrate. This makes it possible to provide directivity to the LED chip alone without using a lens, and eliminates the need to create a mold, reducing costs. In addition, the light emission efficiency of the LED chip can be increased by approximately 1.5 times or more compared to when a photonic crystal is not formed.

[0063] In addition, it is difficult to control the beam angle of micro UV-LED flip chips smaller than 100 μm square because it is difficult to micro-machine the quartz lens. Furthermore, since lift-off is performed after mounting the GaN substrate, it is effective for controlling the beam angle of the microchip and increasing efficiency in situations where the effect of the high refractive index of the PSS (Patterned Sapphire Substrate) substrate cannot be applied. [Explanation of symbols]

[0064] 10 sapphire substrate, 12 n-GaN layer, 14 MQW layer, 16 p-GaN layer.

Claims

1. forming an epitaxial structure by epitaxially growing an n-type gallium nitride layer, a multiple quantum well layer, and a p-type gallium nitride layer on a sapphire substrate in this order; forming a photonic crystal on a surface of the p-type gallium nitride layer using electron beam lithography and inductively coupled plasma reactive ion etching; A method for manufacturing a photonic crystal gallium nitride light-emitting device comprising the steps of:

2. The step of forming the photonic crystal includes: a step of subjecting a surface of the p-type gallium nitride layer to a hydrophobic treatment; applying a resist to a surface of the p-type gallium nitride layer; irradiating the resist with an electron beam to perform writing; removing the resist with chlorine gas by inductively coupled plasma reactive ion etching; The method for manufacturing a photonic crystal gallium nitride light emitting device according to claim 1 , comprising:

3. Prior to growing the n-type gallium nitride layer on the sapphire substrate, a buffer layer and an undoped gallium nitride layer are sequentially grown on the sapphire substrate. The method for manufacturing a photonic crystal gallium nitride light emitting device according to claim 1 , comprising:

4. The method for producing a photonic crystal gallium nitride light-emitting device according to any one of claims 1 to 3, wherein the emission wavelength of the photonic crystal gallium nitride light-emitting device is 375 nm to 390 nm.

5. A photonic crystal gallium nitride light emitting device manufactured by the manufacturing method according to any one of claims 1 to 3; an illumination optical system that illuminates an object with light from the photonic crystal gallium nitride light emitting device; A lighting device comprising:

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