The present invention relates to a light emitting diode (LED) including a photonic crystal on the active layer or another layer.
Researches have been made to obtain an LED having a high light emitting efficiency, among which one including a two-dimensional photonic layer is a major candidate.
A photonic crystal is a body provided with an artificial periodic structure formed within a dielectric (matrix). The periodic structure is generally made by forming areas having a refractive index different from that of the matrix appearing periodically in the matrix. Each of the areas can be made by embedding a small body having a refractive index different from that of the matrix in the matrix, whereas a hole in the matrix is preferable as the area because it renders a larger difference in the refractive index from the matrix and is easier to manufacture.
In a photonic crystal, owing to the periodic structure, a band structure of photon energy is formed including an energy region or regions within which light cannot propagate, that is called “photonic bandgap”. The bandgap depends on the refractive index of the dielectric matrix and the periodic structure formed in it. In a two-dimensional photonic crystal, light having the wavelength falling within the bandgap cannot propagate along the slab body in which the periodic structure is formed, and can propagate perpendicular to the slab body. Thus, by providing a two-dimensional photonic crystal within an LED device, the light emitted from the active layer of the LED does not propagate along the two-dimensional photonic crystal but goes out solely perpendicular to it, which increases the luminous efficiency of the LED.
In Patent Document 1 shown below, an LED including a two-dimensional photonic crystal is disclosed, where the LED is composed of a p-type semiconductor cladding layer (“p-type doped layer” in Patent Document 1), an active layer and an n-type semiconductor cladding layer (“n-type doped layer”) are provided between a pair of electrodes, and air holes penetrating through the three layers are periodically arranged in the plane of the layers. In the LED, holes injected from the p-type semiconductor cladding layer and electrons injected from the n-type semiconductor cladding layer recombine in the active layer to generate light, wherein the generated light cannot propagate along the plane of the layers, and can be taken out in the direction perpendicular to the plane. This confers the high light emitting efficiency on the LED.
[Patent Document 1] Unexamined Japanese Patent Publication No. 2004-289096 ([0009]-[0010], [0015], [0020]-[0023], [0025],
In Patent Document 1, the surface recombination is suppressed by using materials comprising group III elements such as Gallium, Indium, Aluminum, etc. and Nitrogen which have slow surface recombination speed in the active layer.
Thus an object of the present invention is to provide a two-dimensional photonic crystal LED less vulnerable to the surface recombination than conventional two-dimensional photonic crystal LEDs, and has a high light emitting efficiency and high energy efficiency.
According to the present invention, in an LED including a p-type semiconductor cladding layer, an active layer of light-emitting material and an n-type semiconductor cladding layer placed between a pair of electrodes, air holes penetrating through the layers are provided periodically, and at least a part of the inner wall of the air holes is oxidized in either one or both of the p-type semiconductor cladding layer and the n-type semiconductor cladding layer.
In the above structure of the LED, there may be other layers, such as a spacer layer, between the p-type semiconductor cladding layer and the active layer, or between the active layer and the n-type semiconductor cladding layer. The p-type semiconductor cladding layer, the active layer and the n-type semiconductor cladding layer can be made of the same material as conventional ones.
The p-type semiconductor cladding layer may be made of a single layer of the same material, or a stack of multiple layers of different materials. The same applies for the active layer and the n-type semiconductor cladding layer.
In one feature of the present invention, such a material that is more easily oxidized than the active layer is used for either one or both of the p-type semiconductor cladding layer and the n-type semiconductor cladding layer.
The p-type semiconductor cladding layer, the active layer of light-emitting material and the n-type semiconductor cladding layer in which air holes are provided periodically act as a two-dimensional photonic crystal. The air holes may penetrate through all the three layers, or they may be restricted within the p-type semiconductor cladding layer alone, or/and within the n-type semiconductor cladding layer alone. The air holes may be arranged in the rectangular lattice or in the triangular lattice as conventional two-dimensional photonic crystals, and the shape of each air hole may be cylindrical or other shape also as conventional ones.
The later-described effect of the present invention can be obtained by oxidizing the whole inner wall of the air holes of the p-type semiconductor cladding layer or of the n-type semiconductor cladding layer, and some effect can be obtained by oxidizing only a part of the inner wall of the air holes of the p-type semiconductor cladding layer or/and of the n-type semiconductor cladding layer. But, ultimately it is most preferable to oxidize the entire inner wall of the air holes of the p-type semiconductor cladding layer and the n-type semiconductor cladding layer.
The function and effect of the two-dimensional photonic crystal of the present invention is explained with reference to
It is preferred that the inner wall of the air hole in the active layer is not oxidized. When the inner wall of the air hole in the active layer is oxidized, a defect energy level or levels are generated, and recombination of holes and electrons producing heat rather than light are apt to occur. But, even when the inner wall of the air hole in the active layer is oxidized, the possibility of holes and electrons entering the oxidized region of the active layer decreases and the deleterious effect of recombination is minimized by providing oxidized regions in the p-type semiconductor cladding layer and in the n-type semiconductor cladding layer.
It is advantageous in manufacturing a two-dimensional photonic crystal device to use a material having larger tendency of oxidization than that of the active layer in at least a part of the p-type semiconductor cladding layer or of the n-type semiconductor cladding layer. For example, when materials easier to be oxidized are used for the p-type semiconductor cladding layer and the n-type semiconductor cladding layer, and a material harder to be oxidized is used for the active layer, the two cladding layers can be selectively oxidized by exposing the whole device to an oxidizing atmosphere. Another example is that a material having a certain degree of oxidization is used for the active layer, and materials having a larger oxidizing rate are used for the two cladding layers. In this case, by oxidizing the whole device in a very short time for the active layer to be oxidized, the inner wall of the air holes in the two cladding layers can be selectively oxidized.
For the well-oxidizable materials of the p-type semiconductor cladding layer and the n-type semiconductor cladding layer, semiconductor materials containing Al can be used, where AlGaAs, AlGaP, AlGaInP, AlGaN, etc. are examples. In this case, for the active layer, a material devoid of Al, such as GaAs, GaP, GaInP, GaN, etc. can be used. When these materials are used for the p-type semiconductor cladding layer, n-type semiconductor cladding layer and active layer, and the inner wall of the air holes are exposed to water vapor, the materials containing Al are easily oxidized, while the material of the active layer is not oxidized. Thus, the selective oxidization of the inner wall of the two cladding layers is facilitated.
Alternatively, for the active layer, a material having substantially the same composition as the well-oxidizable materials of the cladding layers but containing less Al compared to Ga. In this case, by controlling the length of time for which the inner wall of the air holes are exposed to water vapor, the inner wall in the cladding layers can be adequately oxidized while that in the active layer remains scarcely oxidized.
A two-dimensional photonic crystal LED embodying the present invention is explained referring to
Through the p-type semiconductor cladding layer 12, the active layer 11 and the n-type semiconductor cladding layer 13, air holes 16 are provided substantially perpendicular to these layers. The air holes are formed so that they penetrate through the p-type semiconductor cladding layer 12 and the active layer 11, and stop within the n-type semiconductor cladding layer 13. As shown by the A-A′ cross-sectional view of
The air holes 16 can be formed so that they penetrate through the n-type semiconductor cladding layer 13 and the active layer 11, and stop within the p-type semiconductor cladding layer 12. Otherwise they may penetrate through all the three layers 12, 11, and 13.
Then, oxidized regions 17 are formed on the inner wall of the p-type semiconductor cladding layer 12 and the n-type semiconductor cladding layer 13. The oxidized regions 17 are made by oxidizing the material AlGaAs of the two layers 12 and 13. The thickness of the oxidized layers 17 is 0.05 μm in the present embodiment.
The operation of the two-dimensional photonic crystal LED of the present embodiment is as follows. When a voltage is applied between the upper electrode 14 and the lower electrode 15 with the upper electrode 14 positive, holes are ejected from the upper electrode 14 to the p-type semiconductor cladding layer 12, and electrons are ejected from the lower electrode 15 to the n-type semiconductor cladding layer 13. Since, in the semiconductor cladding layers 12 and 13, the conductivity of the oxidized regions 17 is lower than that of the other regions, the holes and electrons flow through the other regions and avoid the oxidized regions 17, i.e. the surface of the inner wall of the air holes 16. Then the holes and electrons are injected from the semiconductor cladding layers 12 and 13 to the active layer 11 and are combined to generate light in the place remote from the inner wall of the air holes 16. Thus, recombination near the surface of the inner wall of the air holes 16, i.e. the energy loss, is minimized, and the light emitting efficiency is maximized.
As described before, in the present embodiment, the light generated in the active layer 11 cannot propagate in parallel to the plane of the layers because the p-type semiconductor cladding layer 12 and the n-type semiconductor cladding layer 13 act as a two-dimensional photonic crystal with a band gap prohibiting the light, which forces the light only to propagate perpendicularly to the plane. The light emitted toward the lower electrode 15 is reflected by the lower electrode 15, so that the light generated in the active layer 11 is taken out from the upper electrode 14 at a high light emitting efficiency.
A method of manufacturing the two-dimensional photonic crystal LED of the present embodiment is described referring to
The inner wall of the air holes 16 is then exposed to a vapor of 400° C. for 60 seconds, whereby the surface of the inner wall of the air holes 16 in the p-type semiconductor cladding layer 12 and the n-type semiconductor cladding layer 13 is oxidized to a certain depth, forming the oxidized region 17 (
Number | Date | Country | Kind |
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2005-366117 | Dec 2005 | JP | national |