GB2283130A

GB2283130A - A semiconductor light emitting device

Info

Publication number: GB2283130A
Application number: GB9423843A
Authority: GB
Inventors: Tetsuya Yagi; Tsuneo Okada
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1991-02-12
Filing date: 1991-12-18
Publication date: 1995-04-26
Anticipated expiration: 2011-12-18
Also published as: GB9423843D0; GB2282263A; GB2283130B; GB2282263B; GB9423844D0

Description

1 2283130

A Semiconductor Light Emitting Device FIELD OF THE INVENTION

The present invention relates to an AlGaAs/GaAs system semiconductor light emitting device. BACKGROUND OF THE INVENTION

Figure 1 is a cross-sectional view of a prior art semiconductor light emitting device. In figure 1, reference numeral 1 designates an n type (hereinafter, referred to as 'n-') GaAs substrate. An n-Al0_35Ga0_65As first cladding layer 2 is disposed on the n-GaAs substrate 1. A p type(hereinafter, referred to as 'p-1) A10.06%.94As active layer 3 is disposed on the nA10.35%.65As first cladding layer 2. A p-Al0_35Ga0_65As second cladding layer 4 is disposed on the p-Al0_06Ga0.94As active layer 3. An n-Alo.loGao.90As contact layer 5 is disposed on the p-Alo.35Gao. 65As second cladding layer 4. A contact hole 7 is produced at the center of the contact layer 5. The contact layer 5 is doped to produce a Zn d1f f used p+ region 6, the dif fusion depth of which is controlled such that the diffusion front reaches the pAlO.35Ga0_65As second cladding layer 4 at a region under the contact hole 7 while it remains within the n-Al0. j0Ga0_q0As contact layer 5 at the other regions-

A description is given of the operation.

I A bias is applied between the Zn diffused p+ region 6 2 of the n-AlO. lOGaO. 9OAs contact layer 5 and the n-GaAs substrate 1 with the former as positive side. No current flows at regions where the contact hole 7 is not produced because a pnpn junction is produced viewed from above the device while a current flows at the region where the contact hole 7 is produced because the n-AlO.1OGaO.9OAs contact layer 5 is converted into the p+ region due to the Zn diffusion and only a pn junction is formed between the pA'0.00a.0.94As active layer 3 and the n- Alo.35Gao.65AS first cladding layer 2 which are biased by the bias voltage in a forward direction. Thus, the current flows along the current path 8 shown in figure 1. Holes and electrons injected into the p-Alo. 00ao.94As active layer 3 due to the current 8 are recombined and radiate a light. The light has an energy corresponding to the energy band gap of the material constituting the active layer. For example, when the active layer is constituted by A10.06%.94As, the peak wavelength of the light is approximately 830 nm.

In the prior art semiconductor light emitting device, a light generated in the p-AlO.06GaO.94As active layer 3 and spreading in a direction toward the substrate excites the nGaAs substrate 1 and electron-hole pairs are generated in the n-GaAs substrate 1- Since the n-GaAs substrate is generally produced by a horizontal Bridgman (HB) method, it cannot be doped at high concentration and it is usually f 3 doped at the concentration of around 1-3 A 1018cm-3. In this carrier concentration, an intensity of photoluminescence (hereinafter, referred to as PL) light generated due to the band to band transitions is sufficiently strong. Accordingly, the electron-hole pairs are excited in the n-GaAs substrate 1 by the light spreading from the p-Alo.00ao.94As active layer 3 in a direction toward the substrate and recombined in the substrate, so that the band to band transitions of GaAs occur and generate PL light of wavelength of 870 nm. As a result, a light generated in the active layer 3 with the PL light superposed thereon is emitted from the device. Figure 2 shows a spectrum of the light emitted from the device. As shown in figure 2, the spectrum has a main peak of 830 nm followed by a sub peak of 870 nm. The semiconductor light emitting device having such spectrum cannot be used for the optical communication including multiple wavelengths, such as three wavelengths of 830 m, 850 nm, and 870nm, because an interference arises therebetween.

Figure 3 is a cross-sectional view illustrating a construction of a prior art light emitting diode (hereinafter, referred to as LED) capable of suppressing the sub peak in the light emission intensity, disclosed in Japanese, Published Patent Application 63-240083. In figure 3, the same reference numerals as those of figure 1

4 designate the same or corresponding portions. Reference numeral 102 designates a non-doped GaAs layer of less than 1X 1016cm-3 dopant concentration, disposed between the n-GaAs substrate I and the n-AlGaAs cladding layer 2. Figure 4 a diagram showing the relation between the dopant concentration of the GaAs layer and the intensity of PL light.

As shown in figure 4, the PL intensity of the GaAs y -3 layer of less than I '1016cm dopant concentration is less than a hundredth of that of the GaAs layer of approximately 1-3 X 1018cm-3 dopant concentration, which is generally used as a substrate. In this prior art LED, the non-doped GaAs layer 102 of less than 1 X 1016rm-3 dopant concentration is provided between the n GaAs substrate 1 and the n- AlGaAs cladding layer 2 and almost all of the light generated in the active layer 3 and spreading in a direction toward the substrate 1 is dissipated by the recombinations which do not contribute to the light emission in the nondoped GaAs layer 100, thereby suppressing the sub peak in the light emission intensity-

The prior art LED that suppresses the sub peak in the light emission intensity constituted as described above, arlses problems given as follows.

Firstly, at the time of growing the non-doped layer, a low dopant concentration layer can be produced in a growth is equipment having a clean inside. However, it cannot be produced when the inside of the growth equipment is contaminated by the impurities during repeating the growths. Therefore, the inside of the growth equipment must be always kept clean, thereby resulting in low productivity.

Secondly, even when the non-doped layer is produced in an epltaxial growth process, the Impurities are diffused from the n-GaAs substrate of 1-3 X 1018cm-3 dopant concentration and the n-AlGaAs cladding layer of 1- 5 X 1017cm-3 dopant concentration to the non-doped layer in the following annealing process such as Zn diffusion process and thus the dopant concentration' of the-non-doped layer is raised. Therefore, the PL intensity increases and the light emission intensity comes to have a sub peak.

Thirdly, the insertion of the non-doped layer increases the series resistance of the element and this increases the power consumption. Thus, the element deterioration is accelerated and the lifetime is shortened. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a semiconductor light emitting device that has no sub peak in the light emission spectrum and that has a good productivity and a lengthy lifetime.

Othev objects and advantages of the present invention will become apparent from the detailed description given

6 hereinafter; it should be understood, however that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.

According to an aspect of the present invention, a semiconductor light emitting device includes a buffer layer that has a smaller energy band gap than that of the light energy emitted by the light emitting device itself and has an indirect transition type energy band structure, which is inserted between a GaAs substrate and an AlGaAs cladding layer disposed at the substrate side. Therefore, light generated in an active layer and spreading in a direction toward the substrate is absorbed by the buffer layer before reaching the substrate. Furthermore, some of the electron-hole pairs produced in the buffer layer excited by the light from the active layer hardly cause band to band transition light emission, thereby preventing a sub peak to appear in the light emission spectrum of the semiconductor light emitting device.

7 BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

Figure 1 is a cross-sectional view illustrating a prior art semiconductor light emitting device;

Figure 2 is a diagram illustrating the light emission spectrum of the semiconductor light emitting device of figure 1; Figure 3 is a cross-sectional view illustrating another prior art semiconductor LED;

Figure 4 is a diagram showing a relation between the intensity of PL light generated by band to band transitions and the carrier concentration in n-GaAs.

Figure 5 is a perspective view illustrating a semiconductor light emitting device in accordance with a preferred embodiment of the present invention; Figure 6 is a cross-sectional view of the semiconductor light emitting device in a line II - II of figure 1; and Figure 7 is a diagram illustrating a light emission spectrum of a semiconductor light emitting device in accordance with the preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

An embodiment of the present invention will be described in detail with reference to the drawings.

Figure 5 is a perspective view illustrating a construction of a semiconductor light emitting device in 8 accordance with this embodiment of the present invention. In figure 5, reference numeral 11 designates an n-GaAs substrate on which an n-Ge buffer layer 12 is disposed. An n-A10.35GaO.65As first cladding layer 13 is disposed on the buffer layer 12. A p-A10.06GaO.94As active layer 14 is disposed thereon. A p-AlO.35GaO.65As second cladding layer 15 is disposed on the p-AlO.06GaO.qzAs active layer 14 and an n-AlO.jOGaO.qOAs contact layer 16 is disposed thereon. A contact hold 18 having a predetermined depth is produced at a central portion of the contact layer 16. An n side electrode 20 is produced on the rear surface of the substrate 11 and a p side electrode 21 is produced on the surface of the contact layer 16 except for the contact hole. Reference numeral 17 designates a p' region produced in the contact layer 16 by Zn diffusion.

As to typical thicknesses of respective layers, the contact layer 16 is about 3 jim, the second cladding layer 15 about 2 pm, the active layer 14 about 1 pm, the first cladding layer 13 about 2 pm, and the buffer layer 12 about 1. 0 pm. The contact hole 18 has about 35 pm diameter and about 2 pm depth.

Figure 6 is a cross-sectional view semiconductor light emitting device in a line of the II II of figure 1. As shown in figure 6, a Zn diffusion region 17 has a diffusion depth controlled such that the g i 1 9 diffusion front reaches the p-Alo.3_5Gao.65As second cladding layer 15 at a region under the contact hole 18 while it remains within the n-A10. 10Gao.90As contact layer 16 at the other regions.

A description will be given of the operation.

A bias is applied between the Zn diffusion region 17 at the surface of the n-A10.10Gao.90As contact layer 16 and the n-GaAs substrate 11 with the former as positive side. No current flows at regions where the contact hole 18 is produced because a pnpn junction is produced viewed from above the device, while a current flows at the region where the contact hole 18 is produced because the n-A10.10Gao.90As J contact layer 16 Is converted into the p"' region due to Zn diffusion and only a pn junction is formed between the pA10_06GaO.94As active layer 14 and the n-AlO_35GaO.65As first cladding layer 13 which are biased by the bias voltage in a forward direction. Thus, the current flows along the current path 19 shown in figure 5. Holes and electrons injected into the p-Al GaO.94As active layer 14 due to 0.06 -the current 19 are recombined and radiate a light. The light has an energy corresponding to the energy band gap of the material constituting the active layer- For example, when the active layer is constituted by A10.06Gao.94AS, the peak wavelength of the light is approximately 830 run.

In this embodiment, the n-Ge buffer layer 12 is disposed on the n-GaAs substrate 11. Ge has an indirect transition type energy band structure in which the extreme value of conduction band is not opposite to the extreme value of valence band, and the energy band gap thereof is smaller than that of A10.06Gao.94As constituting the active layer. Since the lattice constant of Ge is almost the same as that of GaAs, it can easily be grown on the GaAs substrate. The relaxation time of the radiating recombination in the indirect transition type semiconductor such as Ge is about one second even in the simplest band to band trAnsition while the mean lifetime of the minority carr3.er Is about 1 millisecond at the longest. Accordingly, i 1 1 1 the probability that the radiating recombination occurs is quite small as compared with that of the non-radiative recombination in the indirect transition type semiconductor. Therefore, in this embodiment, although the n-Ge buffer layer 12 is excited by the light spreading from the pA10. 06Gao.94As active layer 14 in a direction toward the substrate and electron-hole pairs are generated in the n-Ge buffer layer 12, S.:Lnce the buffer layer 12 is an ndirect transition type semiconductor, almost all the electron-hole pairs are subjected to non-radiative recombination and the band to band transition light emission hardly occurs. As a result, the light is emitted only from the active layer 14 and the light emission spectrum has no sub peak as shown in figure 7. in the above- described bmbodiment, the n-Ge buffer layer that has a smaller energy band gap than that of the light emitted from the active layer and an indirect transition type band structure is disposed on the n-GaAs substrate. Therefore, the light generated in the active layer and spreading in a direction toward the substrate is absorbed by the buffer layer before reaching the substrate and the electron-hole pairs produced in the buffer layer excited by the light from the active layer do not cause band to bcnd transition light emission, thereby preventing a sub peak in the light emission spectrum of the semiconductor 12 light emitting device. In addition, as compared with the semiconductor LED provided with a non-doped buffer layer, the series resistance of the element can be reduced, thereby reducing the power consumption and preventing the element deterioration due to heat generation. This makes it possible to obtain an element having a lengthy lifetime.

In the above-described embodiment, Ge is used as a material of the buffer layer, but other indirect transition type semiconductor materials having a smaller energy band gap than that of the material constituting the active layer may be used.

According to the invention a buffer layer that has a smaller energy band gap than that of the light emitted from the light emitting device and has an indirect transition type energy band structure is disposed between the GaAs substrate and the AlGaAs cladding layer disposed at the substrate side. Therefore, a lengthy lifetime semiconductor light emitting device having no sub peak in the spectrum of the emission light can be obtained easily at high productivity.

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Claims

WHAT IS CLAIMED IS:

An AlGaAs/GaAs system light emitting device comprising:

a GaAs substrate doped with a first conductivity type impurity; a first conductivity type AlyGaj_YAs first cladding layer, an Al.Gal-,As (0 < x < y) active layer, and an AlYGa,_YAs second cladding layer having a conductivity type opposite to the first conductivity type, which are successively disposed on said substrate; a light being taken out from the upper surface of said second cladding layer; and a buffer layer disposed between said GaAs substrate and said first cladding layer, having an indirect transition type energy band structure, the energy band gap of which is smaller than that of the light emitted - from this light emitting device itself.
2. A semiconductor light emitting device of claim 1 wherein the carrier concentration of said GaAs substrate is 1 X 1018CM-3 - 3x 1018CM-3.
3. A semiconductor light emitting device of claims 1 or 2 wherein said buffer layer comprises Ge doped with firt conductivity type impurity.
4. A semiconductor light emitting device of claim 1 14 wherein said first conductivity type is n-type.
5. A semiconductor light emitting device of claim 4 wherein said buffer layer is an n type Ge layer.
6. A semiconductor light emitting device of claim 5 wherein the thickness of said buffer layer is approximately 1 gm.
7. A semiconductor light emitting device constructed adapted and arranged to operate substantially as described hereinbefore with reference to and as shown in figures 5 to 7 of the accompanying drawings.

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