GB1581768A

GB1581768A - Device for light emission

Info

Publication number: GB1581768A
Application number: GB30533/77A
Authority: GB
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1976-07-21
Filing date: 1977-07-20
Publication date: 1980-12-17
Also published as: DE2732808A1; JPS5312288A; NL7708047A; CA1078949A

Description

(54) DEVICE FOR LIGHT EMISSION (71) We, HITACHI, LTD., a Japanese Company of 1-5-1 Marunouchi, Chiyodaku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a light-emitting device which can be capable of high output and suitable for use in optical communications. In a preferred form it relates to a device which has a structure permitting a highly efficient coupling with an optical fibre.

A light emitting diode for optical communication is described in "Material of the Society for Researches in Light Quantum Electronics, OQE 75-71" published by the Institute of Electrical Communication in 1975 in Japan. More specifically. as illustrated in the sectional view shown in Figure 1, of the accompanying drawings. on a semiconductor substrate ii having a band gap wider than that of a radiation region. an epitaxial layer 12 of the opposite conductivity type to that of the substrate l l is grown.

Thereafter. a glass film layer 13 for current confinement is provided in such a manner that a hole is located at the central part of the layer 12. Further, ohmic contacts 14 and 15 are provided on the bottom of the substrate and on the glass film laver 13.

respectively. A p-n junction 16 is formed by the substrate ii and the epitaxial layer 12.

In this case, radiated light produced adjacent the p-n junction 16 in introduced into an optical fiber (not shown) through a window for light emission 17 as indicated by arrow L.

Another previously proposed device is described in Japanese Published Patent Application No. 159688/1975. It includes a semiconductor substrate and an epitaxial layer which is made of a semiconductor material having a band gap wider than that of the substrate. The substrate and the epitaxial layer are of the same conductivity type. A p-n junction is formed by diffusing Zn from the outside surface of the epitaxial layer so that it enters the semiconductor substrate beyond the epitaxial layer.

Of these two devices, the first is disadvantageous in that the area which is determined by the glass film layer for current confinement and the area of the actual radiation region do not correspond the radiation region becoming extended, on account of the "current spreading phenome non". Since the edge of the p-n junction 16 extends to a side surface 18 of the device, it is in contact with external air and causes nonradiative recombination due to a surface recombination current, so that the external efficiency is low. Furthermore, the semiconductor substrate 11 exhibiting the wider band gap has a low carrier concentration of the order of 10'X cm-'. due to the manner in which the crystal was prepared, and its ohmic contact resistivity with the electrode layers 14,15 is comparatively high, so that the energy efficiency when coupling with an optical fiber is lowered.

In the second device, the surface recombination current is suppressed by the local- ized p-n junction produced by the Zn diffusion. In general, however. the quality of a crystal in a radiation region having a diffused junction is inferior to one having a junction formed by liquid phase epitaxial LPE growth. Consequently. the external efficiency of this device is low. Another disadvantage is that the life of this device is shorter than that of the device with a grown junction.

According to the present invention there is provided a light-emitting device having: a first layer of a Ill-V compound semiconductor which has a predetermined conductivity type; a second layer of Ill-V compound semiconductor which is provided on an upper surface of said first layer such that there is a p-n junction between said first layer and said second layer; a currentcontrol semiconductor layer which has the opposite conductivity type to that of said second layer and which overlies a substantial part of the upper surface of said second layer and which has an aperture therethrough; a third semiconductor layer positioned within said aperture and contacting said second layer or integral therewith, said third layer having the same conductivity type as said second layer; a first electrode which is provided directly on said current control layer, and which is in current flow communication with said second layer through said third layer; and a second electrode which is provided on a bottom surface of said first layer directly and/or with one or more intervening layers, said second electrode having a light-emission window in a central part for emitting light from said device; and wherein said current control layer comprises a layer of sufficiently high carrier concentration to provide a low contact resistivity between said current control layer and said first electrode and wherein light is producible normal to said p-n junction from a radiation region adjacent said junction so as to pass through said first layer and through said light-emission window.

said radiation region having a bandgap narrower than that of said first layer and said current-control layer acting to limit the extent of said radiation region.

Preferably the p-n junction responsible for radiation is not a diffused junction but it is formed by liquid or vapor phase epitaxial growth. Moreover the radiation region may be confined owing to the reverse-bias effect of the p-n junction.

In a preferred embodiment. current flowcommunication between the first layer and the second electrode is effected via á region having a sufficiently high carrier concentration to lower its contact resistivity with the electrode. In this case the semiconductor in the region of the light emission window advantageously contains no doped impurities and thus has a low carrier concentration in order to reduce the internal absorption of light. Thus, a light emitting device which exhihits excellent characteristics especially as a light emitting diode for communications employing an optical fibre can be obtained.

Embodiments of the present invention will 100W be described by wav of example with reference to the accompanying drawings, wherein Figure 2 is a vertical sectional view of a first embodiment: Figure .3 is a vertical sectional view of a second embodiment: Figlrre 4 is a view illustrating the action of the current control layer of the embodiment shown in Figure 2; Figures 5a - Se are diagrams showing stages in the preparation of a third embodiment; Figures 6a and 6b are views showing respectively, the components necessary for assembling the device as a light emitting diode, and the structure of an assembled and finished product; and Figure 7 is a view showing a fourth embodiment.

Figure 2 shows a light emitting device having a crystal layer 21 for light transmission which is a p-conductivity type layer having a band gap wider than that of a radiation region in the adjacent layer 22.

N-type and n±type crystal layers 22 and 23, respectively. are successively and continuously grown on the crystal layer 21.

Layers 24 and 25 having high carrier concentrations are formed by selective diffusion of zinc into a) the p-type crystal layer 21 and b) the n-type crystal layer 22 and n -type crystal layer 23, respectively. The Zndiffused layer 24 exhibits a low contact resistivity to an electrode layer 26. The Zn diffused layer 25 exhibits a low contact resistivity to an electrode layer 27. and acts to confine the p-n junction current across a p-n junction 29. The ohmic contact electrodes 26 and 27 are made of a metal.

Shown at 28 is an exit window which allows the emission of radiation. indicated by arrow L, and to which an optical fibre (not shown) can be attached.

The p-n junction 29 is formed by liquid phase epitaxial growth. By controlling the diffusion depth of the Zn-diffused layer 25 formed from parts of the n-type crystal layer 22 and the nt-type crystal layer 23. it can be arranged that the radiation region of the device is of a size corresponding to the size of the exit window 28, and is confined to a small area of the p-n junction 29 as explained later, making it possible to attain light emission of very high radiance. The surface of the electrode layer 27 for the n-type ohmic contact is so formed as to be flat. without any unevenness over the n+type crystal layer 23 and the Zn diffused layer 25. in order that the electrode layer may efficiently give up heat when in close contact with a heat sink (not shown).

In the light emitting device shown in Figure 3. the electrode 26 is directly dis posed on the bottom of the p-type crystal layer 21 without providing an intermediate the Zn-diffused hwber 24 as shown in Figure 2.

It will now be explained why the current flow region is confined to a specific part only so that the radiation emanates from a small area of the p-n junction only. Figure 4 shows three possible current paths when a voltage is applied to the light emitting device shown in Figure 2. Electrons starting from the electrode layer 27 and following path B travel à long way through layer 22 which offers great resistance. Therefore, few follow path B. Since the p-n junction D between the n-type crystal layer 22 and the p-type Zn-diffused layer 25 is reversebiased, virtually no electrons flow along the path C. Therefore, almost all the electrons flow along a path like path A. At the p-n junction 2') the current is thus concentrated in a portion E indicated by a thick line, so that light is emitted from the portion E with high intensity.

It is possible to omit the n+ crystal layer 23 provided on the n-type crystal layer 22 as shown in Figure 2 and Figure 3 and to replace it with a further region of the n-type layer 22.

As is apparent from the above description, these light emitting devices embodying the present invention can simultaneously solve some of the problems of the prior art devices, i.e. the current spreading phenomenon of the radiation region, the lowering of the external efficiency ascribable to the surface recombination current, and other disadvantages such as short life and low reliability.

An example of a preparative process for a light emitting device embodying the present invention will now be described with reference to Figures 5a - Se.

As shown in Figure Sa, a base 30 of a III-V compound semiconductor doped with an impurity bestowing a predetermined conductivity type, for example. an n-type or p-type (1 0 0) GaAs substrate 30 whose carrier concentration is of the order of 10l7 cm-. has a one surface a layer 31 of a Ga1-xAlxAs 31 (O < xcl) substrate which is about 2001m thick and which was grown by liquid phase epitaxial growth so that. for example. the value x may continuously decrease from 0.4 to O. l upwards from the surface of the base 3(). Subsequently. the grown layer 31 is polished until its upper surface has an AlAs composition, greater than 15% (above x = 0.1S) and a mirror-like appearance. According to a capacitance voltage measurement, the carrier concentration of the crystal layer 31 was 5 x 1017 cm-'. As the next step, using the crystal layer 31 as a substrate and by a sliding method emploving a graphite jig. a first layer 32 (p-tvpe Gal xAlxAs layer. () < xs 1)* a second layer 33 (n-tvpe Ga, xAlxAs layer.

O < xl). and a third laver 34 (n±tvpe Gal xAIxAs layer. () < xsl)are successively and continuously crystal grown from a Ga solution [in which GaAs or Al is used as a solute, and Zn or Si (p-type impurities) or Te (n-type) is used as dopant].

At this time, the thicknesses of the first layer 32, the second layer 33 and the third layer 34 were, for example, about 30 clam, 2 Fm and 1 ,um, respectively. The carrier concentrations df the respective layers are controlled by the added quantities of the dopants Zn, Si and Te, and were for example 2 to 3 x 1018 cm-3, 1 x 1018 cm3 and 5 x 101 cm-3 respectively.

Subsequently the substrate 30 and parts of the crystal layer 31 are polished away. The total thickness may be reduced to 150 ttm.

The exposed surface of the crystal layer 31 is given a mirror-like finish. Thereafter. an Al,OX film 35 and a PSG (Phospho-Silicate Glass) film 36 which are 1000 A and 2000 A thick respectively are deposited on the top and bottom surfaces of the resultant structure. Next, the outer peripheral parts of the films 35 and 36 are removed to form a diffusion mask of diameter of 40 ,um on the side of the layer 34 and a diffusion mask of a diameter of 150 um on the surface of the crystal layer 31 (Figure Sb). (When the device of Figure 3 is to be produced, a diffusion mask is formed only on the top surface, layer 34).

Thereafter, the resultant structure is vacuum-sealed into a quartz ampoule together with a ZnAs2 source, and Zndiffused layers 37 and 38 about 2.5 tim thick, as shown in Figure Sc, are formed by a heat treatment at 65() C for 120 minutes (when the device of Figure 3 is to be fabricated, the glass layer except the portion corresponding to the light emission window at the bottom of the substrate is removed in advance). At this time, the spacing between the surface A of the Zn diffused layer 37 and the layer 32, that is, the remaining thickness of the second layer 33 at this part is about 0.5 ,um.

Subsequently. as shown in Figure Sd, using the films 35 and 36 as an evaporation mask, AuZn or AuSbZn, to act as an ohmic contact electrode layer 39 on the bottom, p-side is evaporated to a thickness of about 2 M m.

Thereafter. as shown in Figure Se, part of the ohmic contact electrode layer 39 to and all of the films 35 and 36 underlying it which were employed as the diffusion mask are removed by photo-lithography to form a light-emission window 42. During the photo-lithographic treatment the diffusion mask (films 35 and 36) on the top, n-side is covered with apiezon. On completion of the photo-lithographic treatment. the apiezon is removed with trichloroethylene, and the films 35 and 36 are successively removed.

Subsequently. AuGe-Ni-Au 4d is evaporated onto the upper surface of the resultant structure to form an n-type ohmic contact electrode layer 40 with a thickness of about 1 m. Next. an Au layer 41, about 9 ptm thick. is deposited on the electrode layer 40 by electrolytic plating.

Thereafter, the resultant structure which is in the form of a wafer, is cut by inscribing and is formed into a chip of about 600 Ftm x 6()0 Rm. Thus, a light emitting diode chip (hereinafter abbreviated to 'LED chip") incorporating a light emitting device embodying this invention can be obtained.

In a modification of the above embodiment, GaAs is used as the base 30 and a layer of a mixed crystal having a wider band gap is grown with it. This step of providing an n -type mixed crystal layer is optional and need not be carried out in some device structures.

Figure 6a and Figure 6b are sectional views, showing respectively the components which may be required for assembling a light emitting diode with the use of the LED chip described above. and the finished light emitting diode. In these figures. numeral 61 designates a stem having an insulating part 61a, numeral 62 a submount, numeral 63 the LED chip, numeral 64 a fibre connector, and numeral 65 an optical fibre.

The sequence of assembly is as follows.

First, the submount 62 and the LED chip 63 are bonded together. Subsequently. the integral unit comprising the submount 62 and the LED chip 63 are bonded onto the lower surface of the fibre connector 64. The resultant structure is bonded onto the stem 61 by means of a layer 66 of a soft metal such as indium, and the stem 61 and the fibre connector 64 are hermetically fixed together with an epoxy resin 67. Thereafter, the optical fibre 65 is caused to pass into the fibre connector 64. The optical fibre 65 has its lower end face. brought into close contact with the light emission window of the LED chip 63. and is fixed to the fibre connector 64 with an epoxy resin 68.

Measurements were made on tulle assembled diode giving the results mentioned below. The optical fiber 65 had a numerical aperture of 0.16. a core diameter of 85 m, and a length of 5() cm. When a d.c. current of l0() mA was passed the optical fibre output was 350 aW on average. the centre wavelength of light emission was 830() A.

and the spectral half-width was 27() A.

Before the fibre was attached the chip had the remarkably large light output of 4 - 7 mW. The thermal resistance was as low as 30 - 50 deg./W.

lio the present example. the thermal resistance was low as noentioned above. and the heat radiation was favourable, so that the saturation of light output versus the increase of the bias current was small. When the bias current was 1()() mA"~lz and the modulation depth was 4()C/f . the modulation distortion of the light output was is low as of -50 dB. The current - voltage characteiistics were also measured. the following results being obtained: no leakage current a good forward voltage of 1.65 V (IF = 100 mA, d.c.); and a breakdown voltage of about 10 V.

The radiation region was measured. It was found that the radiation diameter was as small as about 45 llm, and it was verified that the radiation region hardly spread from the area confined by the selective Zndiffusion layer 25 in Figure 2. In this manner, light emission of a very high radiance can be obtained from a very small area.

Figure 7 shows the sectional structure of a further embodiment having a light emission window 51 formed in such a way that a portion of layer 43 underlying the light emission window 28 in Figure 2 is removed by mask etching with the etchant H2SOA H2O - H9O. In this structure, a p + region 47 in a p-type portion need not be formed by selective diffusion using a central mask. but can be formed by diffusion over the entire area of a wafer surface, followed by removal by mask etching of a central portion to a depth slightly greater than the diffusion depth. i.e. the mask-etched portion is slightly deeper than the diffused layer 47. The other preparative steps may be executed similarly to those illustrated in Figures Sa Se.

An advantage of the present embodiment. is that. by suitably selecting the diameter of the light emission window 51 to be etched and removed, a good coupling of the device with an optical fibre can be achieved and the troublesome operation of mask registration can be omitted. Needless to say, there is added the advantage that, by such deep etching and removal. the light output is enhanced by the amount of light which would have been absorbed by the removed portion.

In Figure 7. numeral 43 indicates a p-conductivity type layer. numeral 44 an n-conductivity type layer. numeral 45 an n±conductivity type layer. numerals 46 and 47 Zn diffused layers formed simultaneous- ly. numeral 48 a p-n junction, numeral 49 an electrode layer for n-type ohmic contact.

and numeral 5() an electrode layer for p-type ohmic contact.

Although. in the foregoing embodiments, only the employment of Ga1.',AlAs (() < xSl) as the semiconductor material has been mentioned. it hardlv needs to be said that similar effects may be achieved with mixed crystals of other II1-V compound semiconductors such as G a A s P N (() < xl). In,Ga1-xAs was (0 < x#1). GaAs l xSbx (O < xl) and Ga1-xInxP (() < xsl) or with hetero-junctions employing lll-V compound semiconductor materials different from cach other. The process of crystal growth is not restricted to liquid phase growth. but a similar method of manufac ture can be applied and similar effects achieved by vapor phase growth.

Furthermore, although the above description contains, for the sake of brevity, descriptions concerning individual light emitting devices, this invention the is also applicable to functional elements in which a large number of light emitting diodes are integrated on a single semiconductor substrate.

As set forth above, the radiation region of a p-n junction is confined to a very small area thereby to attain light emission of high radiance and high efficiency. A diffused layer of high carrier concentration provided at a region of contact with an electrode layer can lower the contact resistivity. Moreover leaving a light passage corridor of low carrier concentration can reduce the absorp tion of light. Coupling with an optical fibre can be easily conducted, all of which can make devices of this invention highly effec tive as light emitting devices.

WHAT WE CLAIM IS: 1. A light-emitting device having: a first layer of a III-V compound semiconductor which has a predetermined conductivity type; a second layer of III-V compound semiconductor which is provided on an upper surface of said first layer such that there is a p-n junction between said first layer and said second layer; a current control semiconductor layer which has the opposite conductivity type to that of said second layer and which overlies a substan tial part of the upper surface of said second layer and which has an aperture there through: a third semiconductor layer posi tioned within said aperture and contacting said second layer or integral therewith. said third layer having the same conductivity type as said second layer; a first electrode which is provided directly on said current control layer and which is in current flow communication with said second layer through said third layer: and a second electrode which is provided on a bottom surface of said first layer directly and/or with one or more intervening layers. said second electrode having a light-emission window in a central part for emitting light from said device: and wherein said current control layer comprises a layer of sufficiently high carrier concentration to provide a low con tact resistivitv between said current control layer and said first electrode and wherein light is producible normal to said p-n junc tion from a radiation region adjacent said junction so as to pass through said first laver and through said light-emission window.

said radiation region having a bandgap narrower than that of said first layer. and said current-control layer acting to licit the extent of said radiation region.

2. A device according to claim I having a fourth semiconductor layer interposed between said first layer and said second electrode, said fourth layer having an aperture for passage of light in register with said window in said second electrode, the fourth layer having a carrier concentration sufficiently high to provide a low resistance connection with said second electrode.

3. A device according to either of the preceding claims wherein in said aperture in said current control layer there is a semiconductor region of the same conductivity type as said second layer and containing more dopant that said second layer.

4. A device according to the preceding claims wherein said current control layer is formed by diffusion of metal atoms into a semiconductor region at least part of which, before such diffusion, was part of a single semiconductor layer which also provides said second layer.

5. A device according to any one of the preceding claims wherein one of said first and second layers has been formed by epitaxial growth of semiconductor material on the other.

6. A light emitting diode chip including a device according to any one of claims 1 to 5.

7. An element having a plurality of devices, each according to any one of claims 1 to 5. on a single substrate.

8. A device according to any one of claims 1 to 5 in combination with an optical fibre.

9. A device for light emission substantially as herein described with reference to any of Figures 2 to 7.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. ture can be applied and similar effects achieved by vapor phase growth. Furthermore, although the above description contains, for the sake of brevity, descriptions concerning individual light emitting devices, this invention the is also applicable to functional elements in which a large number of light emitting diodes are integrated on a single semiconductor substrate. As set forth above, the radiation region of a p-n junction is confined to a very small area thereby to attain light emission of high radiance and high efficiency. A diffused layer of high carrier concentration provided at a region of contact with an electrode layer can lower the contact resistivity. Moreover leaving a light passage corridor of low carrier concentration can reduce the absorp tion of light. Coupling with an optical fibre can be easily conducted, all of which can make devices of this invention highly effec tive as light emitting devices. WHAT WE CLAIM IS:

1. A light-emitting device having: a first layer of a III-V compound semiconductor which has a predetermined conductivity type; a second layer of III-V compound semiconductor which is provided on an upper surface of said first layer such that there is a p-n junction between said first layer and said second layer; a current control semiconductor layer which has the opposite conductivity type to that of said second layer and which overlies a substan tial part of the upper surface of said second layer and which has an aperture there through: a third semiconductor layer posi tioned within said aperture and contacting said second layer or integral therewith. said third layer having the same conductivity type as said second layer; a first electrode which is provided directly on said current control layer and which is in current flow communication with said second layer through said third layer: and a second electrode which is provided on a bottom surface of said first layer directly and/or with one or more intervening layers. said second electrode having a light-emission window in a central part for emitting light from said device: and wherein said current control layer comprises a layer of sufficiently high carrier concentration to provide a low con tact resistivitv between said current control layer and said first electrode and wherein light is producible normal to said p-n junc tion from a radiation region adjacent said junction so as to pass through said first laver and through said light-emission window.