Multiple wavelength light emitting device

Abstract: A multiple wavelength light emitting device having a single p-n junction and at least two light emitting layers within a diffusion region of a minority carrier comprises a p-type Ga 1-x1 Al x1 As layer, a p-type Ga 1-x2 Al x2 As layer, a p-­ type Ga 1-x3 Al x3 As layer, and an n-type Ga 1-y Al y As layer, where x₁, x₂, x₃ and Y represent aluminum arsenide mixed crystal ratios of the respective layers. The light emitting layers, i.e., the p-type Ga 1-X2 Al x2 As layer and the p-type Ga 1-x3 Al x3 As layer each have a different band gap to emit an infrared light and a visible light, respectively. Since the infrared and visible lights are simultaneously emitted, emission of the infrared light can be confirmed or monitored by the visible light. The light emitting device of the present invention utilizes an energy well for trapping electrons and an energy barrier for preventing electrons from diffusing to improve the whole light emitting efficiency.

Description: This invention relates to a multiple wavelength light emitting device having a single p-n junction, which is capable of simultaneously emitting lights having different light emitting wavelengths from one another. A multiple colour light emitting device can emit a plurality of colour lights simultaneously or each of them individually, and for such reasons it is used for photo-electric switches and the like. Recently, since infrared light is increasingly employed in many applications, taking advantage of its invisibility, demand for infrared light emitting devices made of compound semiconductor has also increased. Since the light emitted by such an infrared light emitting device belongs to an infrared region or an invisible region, it is difficult to determine by observation whether or not the device is malfunctioning, which gives rise to a problem in maintaining the performance of an electronic apparatus which employs such an infrared light emitting device. The compound semiconductor device which can emit an infrared light and a visible light is disclosed, for example, by Japanese Kokai No. JP-A- 57-28371. This prior art proposes a multiple colour light emitting device which comprises a first p-n junction formed of an n-type Ga X Al 1-X As crystal layer and a p-type Ga X Al 1-X As crystal layer epitaxially grown on a surface of an n-type GaAs crystal substrate, and a second p-n junction formed by diffusing impurities in the n-type Ga X Al 1-X As crystal layer exposed by removing at least a portion of the n-type GaAs crystal substrate, wherein these two p-n junctions are utilized to emit the above-mentioned infrared light and visible light. However, for manufacturing a device which can emit two kinds of light , i.e. a red light and an infrared light, on the basis of the multiple colour light emitting device constructed as mentioned above, it is necessary to form two p-n junctions, in a Ga X Al 1-X As crystal layer, oriented in opposite directions to one another by forming an epitaxially grown n-type Ga X Al 1-X As crystal layer, diffusing Zn in the surface of the grown layer to form a p-type layer, and removing the GaAs crystal substrate to form another p-type layer on the back surface of the grown layer. Thus, the manufacturing process for such a device having two p-n junctions is quite complicated. In addition, the prior art device requires adjustment of respective bias currents for red and infrared lights when they are simultaneously emitted, thereby making it difficult to employ a visible red light as a monitor for emission of an infrared light. Another prior art example of a single light emitting device made of compound semiconductor which can emit an infrared light and a visible light is disclosed by Japanese Kokai No. JP-A- 57-41979. This device makes use of the fact that the diffusion length of injected electrons from an n-region to a p-region in a light emitting diode made of GaAs doped with Si is sufficiently long. An n-type (doped with Si) GaAs mono-crystal layer, a p-type GaAs mono-crystal layer and a p-type GaAlAs mixed-crystal layer are grown on a GaAs substrate, wherein an infrared light is emitted from the GaAs mono-crystal layer, or a first p-type layer in a high current bias region, while a visible light determined by the forbidden band of the materials constituting the layer, for example a red light, is emitted from the GaAlAs mixed-crystal layer or a second p-type layer in the same manner. However, the above light emitting device has a drawback in that electrons injected from the n-type layer almost recombine in the first p-type layer and therefore do not diffuse and recombine in the second p-type layer. Even if electrons recombine in the second p-type GaAlAs mixed-crystal layer to achieve light emission, the light is self-absorbed. Thus, its internal light emitting efficiency is quite low. In view of the problems mentioned above, it is an aim of the present invention to provide an economical multiple wavelength light emitting device having a single p-n junction which is capable of simultaneously emitting multiple colour lights, specifically a red light and an infrared light, with a single forward direction bias current, wherein the red light is utilized as a monitor for emission of the infrared light. It is another aim of the present invention to provide a multiple wavelength emitting light emitting device which has a high light emitting efficiency with respect to both infrared and visible lights. It is a further aim of the present invention to provide a multiple wavelength light emitting device which is capable of arbitrarily changing a light emitting ratio of infrared and visible lights. The present invention in one aspect provides a multiple wavelength light emitting device having a single p-n junction as set out in Claim 1. The multiple wavelength light emitting device according to the invention may suitably comprise: a p-type Ga 1-X1 A1 x1 As layer; a p-type Ga 1-x2 Al x2 As layer; a p-type Ga 1-x3 Al x3 As layer; and an n-type Ga 1-y Al y As layer; wherein x₁, x₂, x₃ and Y represent aluminium arsenide mixed crystal ratios of the respective layers, and wherein said p-type Ga 1-x2 Al x2 As layer and said p-type Ga 1-x3 Al x3 As layer are light emitting layers respectively having a different band gap. The invention in another aspect provides a method of manufacturing a multiple wavelength light emitting device having a single p-n junction, comprising the steps of: preparing a GaAs mono-crystal substrate; epitaxially growing a p-type Ga 1-x1 Al x1 AS layer, a p-type Ga 1-x2 Al x2 As layer, a p-type Ga 1-x3 Al x3 As layer and an n-type Ga 1-y Al y As layer sequentially on said GaAs mono-crystal substrate; and removing said GaAs mono-crystal substrate after said epitaxial growth step has been completed; and wherein said p-type Ga 1-x2 Al x2 As layer and said p-type Ga 1-x3 Al x3 As layer are light emitting layers respectively having a different band gap. The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view showing a structure of a multiple wavelength light emitting device according to the present invention; Figures 2 and 3 are diagrams respectively showing an energy band structure before and after applying a forward direction bias current to a first embodiment of the light emitting device of the present invention; Figure 4 is a diagram showing an emission spectrum of the light emitting device of the present invention; and Figures 5 and 6 are diagrams respectively showing an energy band structure before and after applying a forward direction bias current to a second embodiment of the present invention. A first embodiment of the present invention will be hereinafter explained with reference to Figures 1 through 4. Figure 1 shows a structure of a multiple wavelength light emitting device 10 which has three p-type layer 1, 2 and 3 and an n-type layer 4 as epitaxially grown layers deposited on a GaAs mono-crystal substrate 11. The multiple wavelength light emitting device 10 of Figure 1 is formed by epitaxially growing a p-type Ga 0.3 Al 0.7 As layer 1, a double-hetero type light emitting layer, i.e. a p-type Ga 0.95 Al 0.05 As layer 2, a single-hetero type light emitting layer, i.e. a p-type Ga 0.62 Al 0.38 As layer 3, and an n-type Ga 0.3 Al 0.7 As layer 4 sequentially on the GaAs mono-crystal substrate 11. It should be noted that the substrate 11 is removed by etching after the layers are completely grown. As can be seen, the multiple wavelength light emitting device 10 solely has a single p-n junction, and the p-type light emitting layer 2 and 3 are formed in a minority carrier diffused region. The p-type Ga 0.3 Al 0.7 As layer 1 is a layer having a thickness of 150 µm in which Zn is doped in a concentration of 2 x 10¹⁷/cm³, the p-type Ga 0.95 Al 0.05 As layer 2 a layer having a thickness of 1 µm in which Zn is doped in a concentration of 1 x 10¹⁷/cm³, the p-type Ga 0.62 Al 0.38 As layer 3 a layer having a thickness of 5 µm in which Zn is doped in a concentration of 1 x 10¹⁸/cm³, and the n-type Ga 0.3 Al 0.7 As layer 4 a layer having a thickness of 50 µm in which Te is doped in a concentration of 1 x 10¹⁷/cm³. In the p-type grown layers 1, 2 and 3, a diffusion length over which electrons diffuse beyond the p-n junction is 30 µm. With this value the Ga 0.62 Al 0.38 AS layer 3 requires the above-mentioned thickness of approximately 5 µm and a hole concentration of approximately 1 x 10¹⁸/cm³ to emit a light with a practical intensity. Generally, the the p-type Ga 0.95 Al 0.05 As layer 2 has the highest emission efficiency when its length is approximately 1 µm, taking account of self-absorption and the like. It is preferable that the thickness of the Ga 0.3 Al 0.7 As layer 1 ranges from 100 to 150 µm in order to facilitate handling when the GaAs crystal substrate 11, which undesirably performs as an emitted light absorbing layer if it remains, is removed. Further, the thickness of the n-type Ga 0.3 Al 0.7 As layer 4 preferably has a thickness of more than 40 µm, taking account of its emission efficiency. Assuming that an Al mixed crystal ratio of the layers 1, 2, 3 and 4 are x₁ (=0.7), x₂ (=0.05), x₃ (=0.38) and Y (=0.7), the following relationships are satisfied among x₁, x₂, x₃ and Y: Y > x₃ > x₂ x₁ > x₃ Therefore, the energy band structure of the multiple wavelength light emitting device 10 is as shown in Figure 2, in which a thermal equilibrium condition is maintained where electrons 12 in an n-region and holes 13 in a p-region do not diffuse. Incidentally, reference letters E F in Figure 2 designate the Fermi level. Thus, when a forward direction bias current is applied to the light emitting device 10, electrons 12 are injected from the n-region to the p-region and recombine with holes 13 in the p-region, with the result that the light emitting layers 2 and 3 emit infrared and red lights, respectively. Since the relationship of the AlAs mixed crystal ratios x₁, x₂ , x₃ and Y of the layers 1, 2, 3 and 4 are determined by the above expressions (1) and (2), band gaps e₁, e₂, e₃ and e₄ of the respective layers 1 - 4 are e₁ = 2.1 eV, e₂ = 1.5 eV, e₃ = 1.8 eV and e₄ = 2.1 eV, respectively. The p-type Ga 0.95 Al 0.05 As layer 2 has its aluminium arsenide mixed crystal ratio smaller than that of the Ga 0.62 Al 0.38 As layer 3, so that an energy well a is formed as shown in Figure 3, whereby the injected electrons 12 are trapped in the well. The injected electrons 12 which have passed through the p-type Ga 0.95 Al 0.05 As layer 2 reach the p-type Ga 0.62 Al 0.38 As layer 3. However, as mentioned above, the band gap of this layer is chosen to be larger than that of the layer 2, whereby an energy barrier b is formed, as shown in Figure 3, which prevents the electrons 12 from passing beyond same. As the result, an electron density in the p-type Ga 0.95 Al 0.05 As layer 2 becomes higher, making it possible to emit a strong infrared light, corresponding to the band gap of the layer 2. Thus, the light emitting layer 2 (e₂ = 1.5 eV) emits an infrared light which has a longer wavelength, while the light emitting layer 3 (e₃ = 1.8 eV), having a larger band gap, emits a red light which a shorter wavelength. The emission spectrum of these red and infrared lights is shown in Figure 4 from which it can be seen that peak values of the wavelength of the respective lights are λ₁ = 650 nm and λ₂= 840 nm. The p-type Ga 0.95 Al 0.05 As layer 2 together with the two compound semiconductor layers 1, 3 on both sides of the layer 2, respectively having a large band gap, constitute a double hetero structure by which the infrared light emitted from the p-type Ga 0.95 Al 0.05 As layer 2 is never absorbed, with the result that the external emitting efficiency of the light emitting device 10 is largely improved. The red light emitted from the p-type Ga 0.62 Al 0.38 As layer 3 may be absorbed by the p-type Ga 0.95 Al 0.05 As layer 2. However, a single hetero structure constituted by the n-type Ga 0.3 Al 0.7 As layer 4 and the compound semiconductor layer having a larger band gap than the layer 3, i.e. the layer 1, prevents the red light from being absorbed, whereby a highly efficient red light emission can be achieved. Next, a second embodiment of the present invention will be explained with reference to Figures 5 through 6 in which parts corresponding to those of the first embodiment are designated by the same reference numerals and detailed explanation thereof will be omitted. In the second embodiment, layers epitaxially grown on the GaAs mono-crystal substrate 11 (see Figure 1) are a p-type Ga 0.3 Al 0.7 As layer 1, a p-type Ga 0.62 Al 0.38 As layer 2, a p-type Ga 0.95 Al 0.05 As layer 3 and an n-type Ga 0.3 Al 0.7 As layer 4. The layers 1, 2 and 4 have the same thickness and dopant density as those of the first embodiment, while the p-type Ga 0.95 Al 0.05 As layer 3 is a layer having a thickness of 1 µm in which Zn is doped in a density of 1 x 10¹⁷/cm³. It should be noted that the thickness of the layers 2 and 3 preferably ranges from 1 to 2 µm in consideration of self-absorption of the lights emitted therefrom. In the second embodiment, the AlAs mixed crystal ratios x₁, x₂, x₃ and Y of the layers 1, 2, 3 and 4 are chosen to be 0.7, 0.38, 0.05 and 0.7, respectively. Therefore, the following relationships are satisfied among x₁, x₂, x₃ and Y: Y > x₂ > x₃ x₁ > x₂ Thus, the energy band structure of the multiple wavelength light emitting device of the second embodiment is as shown in Figure 6 where a thermal equilibrium condition is maintained in the same manner as the first embodiment. When a forward direction bias current is applied to the light emitting device 10, electrons 12 are injected from an n-region to a p₃-region and recombine with holes 13 existing in the p-region, whereby the light emitting layer 3 emits an infrared light. When the applied bias voltage is further increased, the electrons 12 from the p₃-region are injected into a p₂-region and recombine with the hole 13, whereby a red light is emitted from the light emitting layer 2. In this event, the electrons 12 injected into the p₂-region are prevented from diffusing in the p₁-region by the band gap of the same. Since the relationship among the AlAs mixed crystal ratios x₁, x₂ , x₃ and Y of the layers 1, 2, 3 and 4 are determined by the above expressions (3) and (4), band gaps e₁, e₂, e₃ and e₄ of the respective layers 1 - 4 are e₁ = 2.1 eV, e₂ = 1.8 eV, e₃ = 1.5 eV and e₄ = 2.1 eV, respectively. As a result, high potential barriers a and b for blocking the electrons 12 injected in the p₂ and p₃ regions are formed as shown in Figure 6. The light emitting layer 2 which has a larger band gap (e₂ = 1.8 eV) emits a red light having a shorter wavelength, while the light emitting layer 3 which has a smaller band gap (e₃ = 1.5 eV) emits an infrared light having a longer wavelength. The emission spectrum of these lights is the same as the first embodiment as shown in Figure 4. The potential barrier a which blocks the electrons 12 injected in the p₃-region, together with a double-hetero structure formed by the n-type Ga 0.3 Al 0.7 As layer 4 effectively improve the light emitting efficiency. As described above, the electrons 12 overflowed by a higher bias voltage are injected into the layer 2. The high potential barrier b is formed by the p₁-region within the diffusion length of the thus injected electrons 12, whereby the diffusion length of the electrons 12 injected into the p₂-region is substantially shortened. The diffusion length thus shortened and an additional window effect can further improve the red light emitting efficiency of the light emitting device 10. As explained above in detail, the present invention provides a quite simple structure of the multiple wavelength light emitting device 10 which solely has a single p-n junction, and accordingly the epitaxial and device processes in a manufacturing procedure can be quite simple. Further, since the light emitting device of the present invention can simultaneously emit an infrared light and a visible light, emission of the infrared light can be confirmed or monitored by the visible light, so that the light emitting device 10, if applied to a security apparatus, can improve the reliability of the same. Also, the light emitting device of the present invention, which utilizes an energy well for trapping electrons and an energy barrier for preventing electrons from diffusing as described above, can improve the whole light emitting efficiency in comparison with conventional devices which do not have such an energy band structure. Furthermore, the light emitting device of the present invention can also improve the external light emitting efficiency by virtue of the window effect.

Claims: A multiple wavelength light emitting device (10) having a single p-n junction, comprising: a first p-type layer (1), an n-type layer (4), and at least two p-type light emitting layers (2, 3) respectively having a different band gap, said light emitting layers being positioned between said p-type and n-type layers, within a diffusion region of a minority carrier; one of said light emitting layers (2) being adjacent to said first p-type layer (1); and another of said light emitting layers (3) being adjacent to said n-type layer (4). A multiple wavelength light emitting device as claimed in claim 1, characterized in that said p-type layer (1) has a band gap to form a high potential barrier for preventing electrons injected in said light emitting layers (2, 3) from diffusing. A multiple wavelength light emitting device as claimed in claim 2, characterized in that it is operable by a single forward direction bias current. A multiple wavelength light emitting device as claimed in claim 3, characterized in that said light emitting layers (2, 3) simultaneously emit a visible light and an infrared light, respectively. A multiple wavelength light emitting device as claimed in any of claims 1 to 4, characterized by comprising: a p-type Ga 1-x1 Al x1 As layer (1); a p-type Ga 1-x2 Al x2 As layer (2); a p-type Ga 1-x3 Al x3 As layer (3); and an n-type Ga 1-y Al y As layer (4); wherein x₁, x₂, x₃ and Y represent aluminium arsenide mixed crystal ratios of the respective layers, and wherein said p-type Ga 1-x2 Al x2 As layer (2) and said p-type Ga 1-x3 Al x3 As layer (3) are light emitting layers respectively having a different band gap. A multiple wavelength light emitting device as claimed in claim 5, characterized in that said aluminium arsenide mixed crystal ratios of the respective layers satisfy the following expressions: Y > x₃ > x₂ and x₁ > x₃. A multiple wavelength light emitting device as claimed in claim 5, characterized in that said aluminium arsenide mixed crystal ratios of the respective layers satisfy the following expressions: Y > x₂ > x₃ and x₁ > x₂. A multiple wavelength light emitting device as claimed in any of claims 5 to 7, characterized in that said p-type Ga 1-x1 Al x1 As layer (1) has a higher band gap than both said p-type Ga 1-x2 Al x2 As layer (2) and said p-type Ga 1-x3 Al x3 As layer (3) to form a high potential barrier. A method of manufacturing a multiple wavelength light emitting device (10) having a single p-n junction, comprising the steps of: preparing a GaAs mono-crystal substrate (11); epitaxially growing a p-type Ga 1-x1 Al x1 As layer (1), a p-type Ga 1-x2 Al x2 As layer (2), a p-type Ga 1-x3 Al x3 As layer (3) and an n-type Ga 1-y Al y As layer (4) sequentially on said GaAs mono-crystal substrate; and removing said GaAs mono-crystal substrate (11) after said epitaxial growth step has been completed; and wherein said p-type Ga 1-x2 Al x2 As layer (2) and said p-type Ga 1-x3 Al x3 As layer (3) are light emitting layers respectively having a different band gap.

Inventors: TAKENAKA TAKUO, YAMADA MASATO, YAMAGUCHI YUKIHIRO, TAKENAKA, TAKUO, YAMADA, MASATO, YAMAGUCHI, YUKIHIRO
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1990
Main classification: H01L33
IPC codes: H01L33