JP2002111058A - Light emitting diode and exposure system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the quantity of light that a light emitting diode stops down into a very small spot with a very small light emission diameter and a narrower radiation angle. SOLUTION: On a sapphire substrate 11, an n-GaN low-temperature buffer layer 12, an n-GaN buffer layer 13, an n-In0.05Ga0.95N buffer layer 14, an n-Al0.15 Ga0.85N clad layer 15, an n-GaN light guide layer 16, an undoped active layer 17, a p-GaN light guide layer 18, a p-Al0.15Ga0.85N clad layer 19, and a p-GaN gap layer 20 are grown. The total film thickness of the active layer 17 and light guide layer 16 is >=400 nm. Then an epitaxial layer not in a light emission area is etched away until the n-GaN buffer layer 13 is exposed. Then a similar process is used to etch the p-Al0.15Ga0.85N clad layer 19 away to its halfway point except a ridge-shaped stripe area of 4 μm in width, thereby forming a ridge type waveguide structure.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a light emitting diode having an optical waveguide structure provided with a cladding layer of AlGaN and an exposure system using the same.

[0002]

2. Description of the Related Art As a conventional high-quality photographic print system, the present applicant has used a semiconductor laser as a red light source and SHG (Second Harmonic G) as a blue and green light source.
A digital printing apparatus “Frontier” equipped with an exposure system for exposing a light-sensitive material for ordinary visible light exposure using a semiconductor laser-excited solid-state laser utilizing eneration) has been commercialized. In addition, CMY (Cyan, Magenta, Yel) was used by using semiconductor laser light of two wavelengths of infrared light and one wavelength of red light.
low) A relatively inexpensive system called "Pictrography" for exposing photosensitive materials that develop color has also been commercialized. Since these exposure systems use laser light as a light source, they can perform high-speed light beam scanning exposure using a polygon or the like, and can output prints at high speed. Further, it is not necessary to use high-speed paper feed such as a drum for scanning, and simple paper feed such as continuous paper supply is possible.

In the former, the characteristics are most stable,
Ordinary color paper that is sensitive to the three primary colors, which is inexpensive, can be used. However, although semiconductor lasers can be used for red, green and blue semiconductor lasers have not been put to practical use. Therefore, as a laser element, a gas laser or a semiconductor laser pumped solid-state laser second harmonic (SHG: Second Harmonic Generation)
By using the laser light source according to (1), it is possible to provide small and high-quality laser light at relatively low cost.
In addition, since the first stage of the electro-optical conversion is a semiconductor laser, the reliability is also improved.

On the other hand, in the latter case, it is possible to use commercially available semiconductor lasers (for example, oscillation wavelengths of 810 nm, 750 nm, and 680 nm), but it is necessary to use a special photosensitive material that is sensitive to these wavelengths. Low versatility and high printing cost. In addition, the sensitivity is shifted to a longer wavelength side than usual in order to match a commercially available semiconductor laser. For this reason, special care is required in handling, for example, the durability is generally lower than that of a light-sensitive material for a shorter wavelength. In addition, in such a method, the printing apparatus can be manufactured at a lower cost than when the solid-state laser is used, but it is necessary to prepare a special photosensitive material according to the wavelength of the semiconductor laser. For this reason, the material cost increases, and the operation cost and the printing cost increase.

To further reduce the cost of these devices, it is essential to reduce the cost of the laser light source, which is a key device. Although there is no problem with red semiconductor lasers as the light source for high-density magneto-optical disks and DVDs has been reduced in price, there is no prospect of obtaining semiconductor lasers for blue and green light. Therefore, it is difficult to reduce the cost as in a semiconductor laser due to the configuration of the apparatus.

It is ideal to use a laser light source that emits light in three primary colors of RGB as a light source of a conventional digital printing apparatus for silver halide photography. When a material having high sensitivity such as a general-purpose silver salt photographic material is used, the light intensity on the photographic material may be a low light output of about 1 μW. Therefore, instead of an expensive laser, an LED having the advantages of a laser beam having a small emission spot diameter and a narrow beam emission angle may be used.

At present, as the LED (Light Emitting Diode), "The Blue Laser Diode" published in 1997, S. Nakamura
and G. Fasol, Springer, Berlin, have realized high-brightness blue and green LEDs using InGaN-based materials. This LED has a light emitting surface of about several hundred μm square, like other red and other LEDs, and the light is divergent light. Therefore, in order to form a spot with a diameter of several tens of μm using this LED as a light source in order to construct a high-definition printer,
The amount of light that couples the divergent light into the finite aperture of the optical system is extremely small, and since the image is formed by a reduction optical system of about 1/2 to 1/10, the distance between the lens and the light source is relatively large. There is a problem that the coupling efficiency is reduced.

[0008]

In order to solve the above-mentioned problems, in Japanese Patent Application Laid-Open No. 11-074559, the present inventors have proposed a semiconductor light emitting device and an exposure apparatus having an optical waveguide structure having a minute light emitting region. ing. It has been found that the semiconductor light emitting device having the stripe structure disclosed in the above publication can condense a minute spot completely in a spontaneous emission light region without stimulated emission. However, Al
In the case of an element using GaN, AlGaN has a disadvantage that a crystal is cracked and cracks when grown thickly because it has a large built-in strain, and high-quality emission light having a minute spot diameter is obtained. It has become difficult.

In view of the above circumstances, the present invention is directed to a light emitting diode having a cladding layer made of AlGaN and a light guide structure of a stripe shape, and by performing more effective light guide, the amount of light that can be narrowed down to a minute spot is reduced. It is an object of the present invention to provide a low-cost and highly reliable exposure system using the increased number of light emitting diodes and the light emitting diodes as a light source.

[0010]

A light emitting diode according to the present invention comprises a light guide layer and at least one cladding layer made of a semiconductor containing AlGaN in this order from the side of the active layer above and below the active layer. In the upper and lower light guide layers and the cladding layer, the refractive index of the light guide layer is higher than the refractive index of the cladding layer. The total film thickness is 400 nm or more.

The total thickness of the active layer and the two light guide layers is more preferably 1 μm or more.

Each clad layer has a thickness of 500 nm.
The following is preferred.

The light guide layer is preferably made of GaN.

An exposure system according to the present invention is an exposure system having a scanning exposure system including a light source and a deflecting unit for scanning light emitted from the light source onto an object to be exposed. A light emitting diode according to the present invention is included.

[0015]

According to the light emitting diode of the present invention, with the above-described structure, the amount of light seeping from the active layer to the cladding layer can be suppressed, the waveguide efficiency is improved, and the Gaussian distribution is reduced. Emitted light having a light emission diameter and a narrow emission angle can be obtained. Therefore, when the emitted light is narrowed down, the amount of light at a minute spot can be increased.

When the total thickness of the active layer and the two light guide layers is 1 μm or more, the amount of light in the minute spot can be further increased.

In addition, when the present invention is applied to a light emitting diode having a cladding layer having a thickness of 500 nm or less, which has a problem of light seepage due to its small thickness, it is more effective to prevent light seepage. is there.

When the light guide layer is made of GaN, GaN
Since it is composed of two elements, it is easy to manufacture, and because of its high refractive index, the waveguide efficiency can be improved.

According to the exposure system of the present invention, a light source,
In an exposure system having a scanning exposure system including a deflecting unit for scanning light emitted from the light source onto an object to be exposed, the light source includes the light emitting diode of the present invention having the above configuration, A low-cost and highly reliable exposure system can be realized. In particular, there is an advantage that an expensive semiconductor laser does not need to be used as a light source in a blue, green or ultraviolet wavelength range. Therefore, in an exposure system using three primary color light sources, for example, a light emitting diode according to the present invention is used as a blue light source and / or a green light source, and other light sources are provided by using light emitting diodes or semiconductor lasers which are currently provided at low cost. In addition, an exposure system using a low-cost and highly reliable three-primary-color light source using a conventional ordinary photosensitive material having sensitivity to the three primary colors of visible light can be realized.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings.

An InGaN-based blue light emitting diode according to a first embodiment of the present invention will be described along its manufacturing process. FIG. 1 shows a cross-sectional view of the light emitting diode perpendicular to the waveguide direction. On a sapphire C-plane substrate 11, an n-GaN low-temperature buffer layer 12, an n-GaN buffer layer 13 (Si-doped, 5 μm thick), an n-In _0.05 Ga _0.95 N buffer layer 14 (Si
Doping, thickness 0.1 μm), n-Al _0.15 Ga _0.85 N cladding layer 15
(Si-doped, 0.4 μm thick), n-GaN optical guide layer 16 (Si-doped, 0.7 μm thick), undoped active layer 17, p-GaN optical guide layer 18 (Mg-doped, 0.7 μm thick), p -Al _{0.15 Ga0.85} N cladding layer 19 (Mg doped, 0.4 μm thick), p-GaN cap layer 20
(Mg doped, 0.3 μm thick). Thereafter, the p-type impurity is activated by heat treatment in a nitrogen gas atmosphere. The active layer 17 is made of undoped In _0.05 Ga _0.95 N (thickness 10 nm), undoped In _0.28 Ga _0.72 N quantum well layer (thickness 3 nm), undoped In _0.05 Ga _0.95 N (thickness 5 nm), undoped In _0.28
Ga _0.72 N quantum well layer (thickness 3 nm), undoped In _0.05 Ga
_0.95 N (thickness 10 nm), undoped Al _0.1 Ga _0.9 N (thickness 10
nm) double quantum well structure. Next, unnecessary portions other than the light emitting region are removed by photolithography and etching. After that, RIBE (reactive i
epi-layer except for the light-emitting region by n-GaN
The etching is removed until the buffer layer 13 is exposed. At this time, the light emitting surface of the present LED is formed. Next, by using the same process, the p-Al _0.15 Ga _0.85 N clad layer 19 is partially removed by etching, leaving a ridge-shaped stripe region having a width of 4 μm. After the SiN film 21 is formed on the entire surface by plasma CVD, a stripe-shaped window (4 μm in width) for current injection into the SiN film 21
m) is opened, and the SiN film 21 other than the n-electrode formation region is removed by etching. The light emitting diode 100 is manufactured by depositing Ti / Al / Ti / Au as the n-electrode 23 and Ni / Au as the p-electrode 22 and performing annealing in nitrogen to form an ohmic electrode.

The light emitting diode 100 of this embodiment has an edge emitting stripe structure and emits light in the blue 470 nm band. The chip had a width of 300 μm and a length of 300 μm, and a non-reflective coating film was formed on one light emitting end face. The light emitting diode manufactured under the above conditions has a light emitting spot diameter of 4
Micro emission of about μm × 1 μm can be obtained. This is a normal
It is extremely small compared to LEDs and has sufficient characteristics as an exposure light source for printers.

Next, the light condensing ability of the light emitting diode according to the present invention and the light emitting diode according to the prior art to a minute spot was evaluated using the optical system shown in FIG. Edge emitting LED100
The light emitted from the lens 101 is condensed on a pinhole 109 having a diameter of 100 μm by a lens 101 (NA = 0.5, magnification 20 ×, focal length 3.2 mm). The output power was compared.

When the total light quantity of the light emitting diode 100 was 70 μW, a pinhole transmission power of 5 μW was obtained. As a light emitting diode according to the prior art, it has the same structure as the light emitting diode 100, but the n-GaN light guide layer 16 and the p-GaN
The thickness of the light guide layer 18 is the same as that of a conventional semiconductor laser, etc.
When the device with 0.1 μm was measured, the pinhole transmission power was 1.7 μW when the total light amount was 70 μW.

In the light emitting diode according to the above embodiment, by increasing the total film thickness of the active layer and the light guide layer, light radiated outside the cladding layer could be reduced, so that the light collection efficiency could be improved. . In the conventional device, the amount of light radiated from the substrate or the cladding layer is large to the total amount of light. However, in the present invention, the amount of light emitted from the light guide region (the region including the active layer and the two light guide layers) increases, The light collection efficiency was improved because the amount of light emitted from the layer was reduced.

Next, the dependency of the condensing power in the pinhole of the light emitting diode on the total thickness of the active layer and the light guide layer will be described. FIG. 3 shows a graph showing this. Total light 70
It was measured at the time of μW.

As shown in FIG. 3, when the total film thickness of the active layer and the light guide layer is 400 nm or more, the condensing power in the pinhole becomes 4 μW, which is more than twice as large as that when the thickness is smaller than 400 nm. Understand. Further, when the total film thickness is 1000 nm or more, the condensing power is further increased.

In a semiconductor laser, when the total film thickness of the active layer and the light guide layer becomes large, a higher-order transverse mode oscillates, so there is an upper limit. However, since the light emitting diode of the present invention does not oscillate, it has It is more efficient to increase the total thickness of the active layer and the optical guide layer to achieve multimode waveguide that allows many higher-order modes, because the spontaneous emission light entering the interface between the guide layer and the cladding layer at different angles is more efficient. Light can be well guided to the end face. Therefore, in practice, it is effective to make the light guide layer thicker than the stripe width or more.

Next, the InGa according to the second embodiment of the present invention will be described.
The N-based blue light emitting diode will be described. FIG. 4 is a cross-sectional view of the light emitting diode perpendicular to the waveguide direction. The light emitting diode according to the present embodiment has the same basic layer structure and manufacturing method as those of the first embodiment, and the same components are denoted by the same reference numerals and description thereof is omitted.

As shown in FIG. 4, in the present embodiment, the n-GaN optical guide layer 16 and the p-Ga
The thickness of each of the N light guide layers 18 is set as thick as 1.5 μm,
The ridge processing is performed until the n-GaN optical guide layer 16 is removed at portions other than the stripe portion. As described above, by forming the entire light guide layer into a stripe shape, the spontaneous emission light is effectively guided to the end face, and the light collection efficiency is further improved.

Next, In according to the third embodiment of the present invention will be described.
A GaN-based blue light emitting diode will be described, and a cross-sectional view of the light emitting diode is shown in FIG. The basic structure and manufacturing method are the same as those of the first embodiment except that a 6H-SiC substrate 31 having n-type conductivity is used for the substrate, and each element is denoted by the same reference numeral. And the description is omitted.

As shown in FIG. 5, in the light emitting diode according to the present embodiment, the substrate has n-type conductivity.
An H-SiC substrate 31 is used. Therefore, the configuration is such that the n-type electrode 23 is formed on the back surface of the substrate.

In the above three embodiments, a light emitting diode having an emission wavelength in the blue region has been described. However, the present invention is not limited to this wavelength region, and is not limited to this wavelength region, but is a green or longer wavelength region, or a violet or shorter wavelength of 450 nm or less. It goes without saying that the present invention can be applied to a light emitting diode having an emission wavelength in the ultraviolet region. For example, an ultraviolet light emitting diode uses GaN or AlGaN as an active layer, and uses an AlGaN
It is desirable to have a layer configuration including an optical guide layer and an AlGaN cladding layer.

In the above embodiment, a stripe structure having a ridge shape for forming an insulating film is described as an element structure. However, the present invention is applied to a semiconductor laser having a stripe structure proposed so far, such as a buried structure made of AlGaN or the like. The various structures described above can be applied.

Next, a description will be given of a high-speed and simple exposure system using a light-emitting diode of the present invention as described in the above-mentioned embodiment, which can efficiently condense light into a minute spot with a small light emission diameter. FIG. 6 shows a schematic configuration diagram thereof.

As shown in FIG. 6, a light beam emitted from the light emitting diode 200 of the present invention according to the first embodiment is converted into a first lens 201 for further reducing the spread angle.
Then, the light is polarized by a movable mirror 202 such as a galvanometer mirror and condensed on a medium through a condenser lens 203. Unlike a conventional light emitting diode that emits divergent light, it has a very small spot diameter, so an exposure system can be manufactured at low cost using a simple optical system similar to that using a laser as a light source. It can be configured, and can be deflected at a high speed like the laser beam.

Next, an exposure system according to another embodiment of the present invention will be described. FIG. 7 shows a schematic configuration diagram thereof.

As shown in FIG. 7, in the present embodiment, a light beam emitted from a light emitting diode 210 passes through a first lens 211 for further reducing the spread angle, and a polygon mirror is used as a light scanning deflection device. The light is deflected by the light 212 and condensed on the medium through the condenser lens 213. Also in the present embodiment, a highly reliable exposure system can be realized at low cost.

The configuration of the exposure system is not limited to the above two embodiments, and various configurations are possible as a device using a deflector for light scanning and a condenser lens.

In the above two embodiments,
Although only the case of a single light source has been described, it is also possible to combine a plurality of light sources such as three types of light sources of red, green and blue.

When a plurality of light sources are used, it is not necessary to use the light emitting diodes of the present invention for all the light sources. For example, the light emitting diodes of the present invention are used for the green and blue light sources, and the red semiconductor laser is used for the red light source. , Can be combined with other light sources.

According to the present invention, a high-speed and high-quality printer can be realized by realizing a light emitting diode capable of efficiently narrowing down a minute spot in a blue, green or ultraviolet wavelength region where a semiconductor laser is difficult to realize. Can be. In addition, by configuring a scanning exposure system using the light emitting diodes, an exposure system can be constructed extremely easily.

[Brief description of the drawings]

FIG. 1 is a sectional view of a light emitting diode according to a first embodiment of the present invention.

FIG. 2 is a light condensing optical system using a light emitting diode according to the first embodiment of the present invention.

FIG. 3 is a graph showing the dependence of the condensing power in a pinhole on the total thickness of an active layer and an optical guide layer.

FIG. 4 is a sectional view of a light emitting diode according to a second embodiment of the present invention.

FIG. 5 is a sectional view of a light emitting diode according to a third embodiment of the present invention.

FIG. 6 is a schematic configuration diagram of an exposure system according to an embodiment of the present invention.

FIG. 7 is a schematic configuration diagram of an exposure system according to another embodiment of the present invention.

[Explanation of symbols]

11 Sapphire C-plane substrate 12 n-GaN low temperature buffer layer 13 n-GaN buffer layer 14 n-In _0.05 Ga _0.95 N buffer layer 15 n-Al _0.15 Ga _0.85 N cladding layer 16 n-GaN optical guide layer 17 undoped active layer 18 p-GaN optical guide layer 19 p-Al _{0.15 Ga0.85} N cladding layer 20 p-GaN cap layer 21 SiN film 22 p electrode 23 n electrode

 ──────────────────────────────────────────────────続 き Continued on front page F term (reference) 5F041 AA06 AA31 AA43 CA05 CA14 CA33 CA34 CA40 CA46 CA65 CA73 CA74 CA82 CA92 CB05 CB11 EE11 EE25 FF13 5F073 AA13 AA43 AA74 AA83 AB27 AB29 BA07 CA07 CB05 EA07 EA18

Claims (5)

[Claims] An optical guide layer and at least one cladding layer made of a semiconductor containing AlGaN are provided in this order above and below the active layer in the stacking direction of the active layer. In the clad layer, the refractive index of the light guide layer is higher than the refractive index of the clad layer, and in a light emitting diode having a stripe-shaped optical waveguide structure, the total film thickness of the active layer and the two light guide layers is 400 nm.
Light emitting diode characterized by the above. 2. The total thickness of the active layer and the two light guide layers is 1 μm or more.
A light-emitting diode as described. 3. The thickness of each clad layer is 500 nm.
The light emitting diode according to claim 1, wherein: 4. The light emitting diode according to claim 1, wherein said light guide layer is made of GaN. 5. An exposure system having a scanning exposure system including a light source and a deflecting unit for scanning light emitted from the light source onto an object to be exposed, wherein the light source is any one of claims 1 to 4. An exposure system comprising the light emitting diode according to claim 1.