CA1078949A

CA1078949A - Light emitting devices

Info

Publication number: CA1078949A
Application number: CA282,548A
Authority: CA
Inventors: Yuichi Ono; Mitsuhiro Mori; Makoto Morioka; Kazuhiro Ito; Masahiko Kawata; Kazuhiro Kurata
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1976-07-21
Filing date: 1977-07-12
Publication date: 1980-06-03
Also published as: JPS5312288A; DE2732808A1; GB1581768A; NL7708047A

Abstract

LIGHT EMITTING DEVICES

Abstract of the Disclosure A light emitting device has a III-V compound semi-conductor substrate whose bandgap is wider than the energy range corresponding to the radiation emitted by the device and which has a predetermined conductivity type. A second III-V compound semiconductor layer is deposited on an upper surface of the substrate and has the opposite conductivity type to that of the substrate. A current control layer covers an upper surface of the second layer and has a hole for current flow. A first electrode is provided on the current control layer and is in ohmic contact with the second III-V
compound semiconductor layer. An ohmic contact electrode is provided on a bottom surface of the substrate and has a light extracting window at its central part. The current control layer is made of a layer of high carrier concentration having the opposite conductivity type to that of the second III-V
compound semiconductor layer. The arrangement avoids a prior art difficulty in which the radiation region becomes extended by reason of the "current spreading phenomenon and becomes larger than the hole for current flow in the current control layer. Avoidance of this difficulty increases the efficiency of the device and especially enables efficient coupling with an optical fiber.

Description

9~9 Field of the Invention This invention relates to high radiance, light emitting devices for optical communications, and methods for manufacturing them. More particularly, it relates to light emitting devices that have a structure permitting highly efficient coupling with an optical fiber, and methods for manufacturing them.
Brief Description of_the Drawings Figure 1 is a vertical sectional view of the prior art light emitting device, Figure 2 is a vertical sectional view of an embodiment of a light emitting device according to an embodiment of this invention, Figure 3 is a vertical sectional view of another embodiment of light emitting device of this invention, Figure 4 is a view for explaining the operation of a device of this invention, Figures 5a - 5e are process diagrams showing an embodiment of a manufacturing method according to the invention, Figure 6a ~is an exploded view showing components of a device of this invention, Figure 6b shows the finished product, and Figure 7 is a view showing still another embodiment of the invention.
Description of the Prior Art A known light emitting device comprising 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 Figure 1, on a semiconductor substrate 11 having a bandgap wider than the energy range corresponding co the radiation emitted by the device, there is gr~wn an B-`
. . , ~078949 epitaxial layer 12 of opposite conductlvity type to that of the substrate 11. Thereafter, a glass film layer 13 for current confinement is provided with a hole in its central part. Ohmic contacts 14 and 15 are formed on the bottom of the substrate and on the layer 13, respectively. A p-n junction 16 is formed between the substratè 11 and the epitaxial layer 12. Radiated light produced in the p-n junction 16 is introduced into an optical fiber (not shown) through a window 17 as-indicated by the arrow L.
Another prior art device is disclosed in French patent application 7416054 of D. Diguet et al., published May 12, 1975 under No. 2270753. It includes a semiconductor substrate which is made of a material having a first bandgap, and an epitaxial layer which is made of a semic~nductor material having a second bandgap wider than the first one. 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 to penetrate the semiconductor substrate beyond the epitaxial layer.
~0 Of these`prior art devices, the first is disadvantageous in that the area defined by the glass film layer for current confinement and the area of the actual radiation region do not agree, the radiation region becoming extended on account of the "current spreading phenomenon". Since the p-n junction 16 extends to a side surface 18, it touches the external air and causes non-radiative recombination due to a surface recombination current. As a result the external efficiency is low. Further, the semiconductor substrate 11 that exhibits the wider bandgap has a low carrier concentration of the order of 10 cm for the reason that in the preparation of a crystal its ohmlc contact resistively with the electrode layers 14, 15 is comparatively high, so that the energy efficiency in the case ~ .,.

of coupling with the optical fiber is lowered.
In the second prior art device, the surface recombination current is suppressed by the locali~ed p-n junction owing to the Zn diffusion. In general, however, a diffused junction is inferior to a grown junction by LPE in the degree of perfection of the crystal at the radiation region. Consequently, the external efficiency of this device is low. Another disadvantage is that the life of this device is shorter than that of a device with a grown junction.
Summary of the Invention This invention has for its primary object to provide a light emitting device having a structure for eliminating or reducing the phenomenon by which the area of a radiation region becomes larger than the area determined by a glass film for current confinement, and also to provide a method of manufacturing such device.
To this end the invention provides in a light emitting device having a III-V compound semiconductor substrate whose bandgap is wider than the energy range corresponding to the radiation emitted by the device and which has a predetermined conductivity type, a second III-V compound semiconductor layer--deposited on an upper surface of said III-V compound semiconduc-tor substrate and having the opposite conductivity type to that of said substrate, a current control layer that covers an upper surface of said second III-V compound semiconductor layer and has a hole for current flow, a first electrode provided on said current control layer and being in ohmic contact with said second III-V compound semiconductor layer, and an ohmic contact elec-trode provided on a bottom surface of said III-V compound semi-conductor substrate and having a light extracting window at itscentral part, wherein said current control layer is made of a layer of high carrier .- :
' . ~ .' :,, ' ' . , ' .

1078~49 concentration having the opposite conductivity type to that of said second III-V compound semiconductor layer.
In the preferred embodiment, the junction for radiation is not a diffused junction but is formed by liquid or vapor phase epitaxial growth. Also the regional confinement for the junction is achieved by the reverse-bias effect of the p-n junction. The ohmic contact region of the III-V compound semiconductor having the wider bandgap is given a sufficiently high carrier concentration as to lower its contact resistivity with the ohmic contact. The portion of a window for radiation extraction is doped with no impurity and is left at a low carrier concentration, thereby to reduce the internal absorption of light.
Embodiments of this invention will now be described.
Description of the Preferred Embodiments Embodiment 1:
Figure 2 shows an embodiment of light emitting device according to this invention. Numeral 21 designates a crystal layer for light transmission that is formed of a p-conductivity type layer having a bandgap wider than the energy range corresponding to the radiation emitted by the device. Numerals 22 and 23 designate n-type and n -type-crystal layers, respectively, which are successively and continuously grown on the crystal layer 21. Numerals 24 and 25 indicate layers formed in the p-type crystal layer 21 and in the n-type crystal layer 22 and n -type crystal layer 23, respectively, said layers being formed by diffusing Zn thereinto. The Zn diffused 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 on a p-n junction 29 to confine the p-n junction current. The electrode layers 26 and 27 are made of metal.

B

.. .

.
Shown at 28 is a window for extracting radiation, indicated by the arrow L, and an optical fiber (not shown) is attached to this portion. The p-n junction 29 is formed by the liquid phase epitaxial growth. By controlling the diffusion depth of the Zn diffused, p -type layer 25 in the n-type crystal layer 22 and the n -type crystal layer 23, the radiation region can be formed in a size corresponding to the size of the extracting window 28.
In this manner the Zn layer 25 is diffused in the ;
0 n-type crystal layer 22 and the n -type crystal layer 23, and the diffusion depth is controlled, whereby the radiation region 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 for the n-type ohmic contact 27 is so formed as to be flat without any unevenness over the layer 23 and the layer 25 in order that the electrode layer may efficiently radiate heat in close contact with a heat sink (not shown).
The electrode 26 may be disposed directly on the bottom of the p-type crystal layer 21, without providing the Zn diffused layer 24, as shown in Figure 3.
With this structure the current flow reg-ion is confined to a specific part only, the radiation being emitted ~-from the small area of the p-n junction. Figure 4 shows current paths when applying a voltage to the device shown in Figure 2. Three cases can be considered; when electrons starting from the electrode layer 27 travel along arrows A, B and C. When electrons flow over a long distance in the n-type crystal layer 22, as indicated by the arrow B, the resistance is much higher than in the case where th~y flow along the arrow A. Therefore, the number of electrons flowing - : . . : :. . - , ~078949 as indicated by the arrow B i5 almost zero. Since the p-n junction D between the n-type crystal layer 22 and the p-type Zn diffused layer 25 is reverse-biased, the electrons cannot flow as indicated by the arrow C. Therefore, the electrons essentially flow as indicated by the arrow A without fail, and in the p-n junction 29 currents are crowded into a portion E indicated by a thick line, so that light of high radiance is emitted from the portion E (arrow L).
It is also possible to omit the n crystal layer 23 provided on the n-type crystal layer 22 shown in Figure 2 and Figure 3 and to make the corresponding portion n-type.
As apparent from this description~ this device simultaneously solves the problems of the prior art, i.e., the current spreading phenomenon of the radiation region, the lowering of the external efficiency ascribable to the surface recombination current, and the disadvantage of short life, low reliability, etc.
Embodiment 2:
An examp~e of a manufacturing process will now be described with referenee to ~igures 5a - Se.
As shown in Fig~re 5a, on a III-V eompo~nd semieonduetor substrate doped with an impurity bestowing a predetermined conductivity type, for example~ an n-type orp-type (1 0 0) GaAs substrate 30 whose carrier concentration is in the order of 10 em , a p-type Gal xAlxAs (0 < x < 1) layer 31 about 200 ~m thiek is grown by the liquid phase epitaxial growth.
By way of example, the value x may deerease eonti.nuously from 0.4 to 0.1 upwards from the substrate surfaee. Subsequently, the grown layer is polished until the AlAs eomposition of its upper surfaee beeomes above 15 % (above x = 0.15), and it has aehieved a mirror surfaee. Aeeording to a eapaeitanee -voltage measurement, .~

.

1078~9 the carrier concentration of the crystal layer 31 was 5 x 10 cm 3. In the next step, using the crystal layer 31 as a substrate and a sliding method employing a graphite jig, a first layer 32 (p-type Gal xAl As layer, 0 ~ x _ 1), a second layer 33 (n-type Gal Al As layer, 0 < x _ 1) and a third layer 34 (n -type Gal Al As layer, 0 < x _ 1) are successively and continuously crystal-grown from a Ga solution (in which GaAs or Al is used as a solute, Zn or Si representing a p-type bestowing impurity or Te representing an n-type bestowing impurity being used as dopant).
The thickness of the layers 32, 33 and 34 were, for example, about 30 ~m, 2 ~m and 1 ~m, respectively. The carrier concentrations of the respective layers were controlled by the quantities of dopants Zn, Si and Te, and were, for exa~ple, 2 3 x 10 cm , 1 x 10 8 cm 3 and 5 x 1018 cm 3.
Subsequently, as shown in Figure 5b, parts of the substrate 30 and the crystal layer 31 are polished and removed so that the total thickness may become 150 rum, and the exposed surface of the crystal layer 31 is finished into a mirror surface. Thereafter, an A1203 film 35 and a PSG (Phospho-Silicate-Glass) film 36, which are 1000 R and 2000 ~ thick respectively, are deposited on each of the front and rear surfaces of the resultant structure. The outer peripheral parts of the films 35 and 36 are then removed (when the device of Figure 3 is to be produced, the films 35 and 36 are deposited entirely on the bottom surface~, to form a diffusion mask of a diameter of 40 ~m on the side of the third layer 34 and a diffusion mask of a diameter of 150 ~m on the side of the crystal layer 31.
3Q Thereafter, the resultant structure is vacuum-sealed into a quartz ampoule together with a ZnAs2 source, and Zn ; 10789~9 diffused layers 37 and 38 about 2.5 ~m thick, as shown in Figure 5c, are formed by a heat treatment at 650 C for 120 minutes (when the device of Figure 3 is to be fabricated, the glass layer except the light extracting portion at the bottom of the substrate is removed in advance). At this time, the spacing between the diffusion surface A of the Zn diffused layer 37 and the first layer 32, that is, the thickness of the second layer 33 is about 0.5 ~m.
Subsequently, as shown in Figure 5d, using the films 35 and 36 as an evaporation mask, AuZn or AuSbZn forming an ohmic contact electrode layer 39 on the p-side is evaporated to a thickness of about 2 ~m.
Further, as shown in Figure 5e, that part of the ohmic contact electrode layer 39 which corresponds to a light extracting window 42 and the films 35 and 36 which have been employed as the diffusion mask are respectively removed by photo-lithography. At this time, the diffusion mask (films 35 and 36) on the n-side or on the upper side in the illustration is covered with apiezon in advance. After completion of the photo-lithographic treatment, the apiezon is removed with trichloro-ethylene, and the films 35 and 36 that have been employed as the diffusion mask on the n-side (upper side) are successively removed. Subsequently, AuGe-Ni-Au 40 is evaporated on the upper surface of the resultant structure as an n-type ohmic contact electrode layer to a thickness of about 1 ~m.
Further, Au 41 being about 9 ~m thick is deposited on the electrode layer 40 by the electrolytic plating.
Thereafter, the resultant structure in the form of a wafer is cut by scribing into a chip of about 600 ~m x 600 ~m. Thus, a light emitting diode chip (abbreviated to "LED chip") according to this invention is obtained.

10789~9 In a concrete example of this embodiment, a GaAs substrate is used as the starting substrate, and the grown substance is obtained by growing a layer of a mixed crystal with the substrate material that has a bandgap wider than that of the substrate material. The step of providing the n+-type mixed crystal layer need not be carried out in some devices.
Figure 6a and Figure 6b are sectional, exploded and assembled views respectively, showing components for assembling a light emitting diode using the LED chip described above.
In these figures, numeral 61 designates a stem having an insulating part 61a, numeral 62 is a submount, numeral 63 is the LED chip, numeral 64 is a fiber connector, and numeral 65 is an optical fiber.
The sequence of assembly is as follows. The submount 62 and the LED chip 63 are first bonded together into an integral form. The submount 62 and the LED chip 63 in this integral form are then bonded onto the lower surface of the fiber connector 64. The resultant structure is then bonded into the stem 61 by means of a layer 66 of a low fusing metal such as indium, and the stem 61 and the fiber connector 64 are hermetically secured together with an epoxy resin 67.
Thereafter, the optical fiber 65 is passed through the fiber connector 64 so that its lower end face is brought into close contact with the light extracting window of the LED -' chip 63. It is then fixed to the fiber connector 64 by epoxy resin 68.
Measurements were taken and the characteristics mentioned below were observed. The optical fiber 65 had a numerical aperture of 0.16, a core diameter of 85 ~m, and a length of 50 cm. When a d.c. current of 100 mA was passed, the optical fiber output was 350 ~W on the average, the center g 1078g49 wavelength of light emission was 8300 A, and the spectral half width was 270 A. When the fiber was not attached, a much larger value of 4 - 7 mW light output was obtained. The thermal resistance was as low as 30 - 50 deg./W.
Since the thermal resistance was low, as mentioned, and the heat radiation was favorable, saturation of the light output versus increase of bias current was slight. When the bias current was an average of 100 mA and the modulation depth was 40 %, the modulation distortion of the light output was as low as -50 dB. The current - voltage characteristics were also inspected. No leakage current was found and such good characteristics as a forward voltage of 1.65 V (IF = 100 mA, d.c.) and a breakdown voltage of about 10 V were exhibited.
The radiation region was also measured. As a result, the radiation diameter was extremely small, i.e. about 45 ~m, and it was verified that the radiation region hardly spread from the area confined by the selective Zn diffusion layer 25 in Figure 2. In this manner, a light emission of extraordinarily high radiance can be obtained from a very small areaO
Embodiment 3:
Figure 7 shows a section through a light emitting device according to another embodiment of this invention. A
light extracting window 51 is formed in such a way that a portion corresponding to the light extracting window 28 in Figure 2 is removed by mask etching with an etchant of H2S04 -H202 - H20. In this case a p region 47 in a p-type portion need not be formed by selective diffusion, but it may be formed in such a way that, after diffusion over the entire area of a wafer surface, removal by mask etching is carried out to a depth slightly greater than the diffusion depth, i.e., the mask-etched portion becomes slightly deeper than the .

.

~078~49 diffused layer 47. The other steps of manufacture may be similar to those illustrated in Figures Sa - 5e.
An advantage in this case is that, by suitably selecting the diameter of the light extracting window 51 to be etched and removed, the coupling of the device with an optical fiber is very effective and the difficult operation of mask registration can be omitted. There is added the advantage that, by such deep etching and removal, the light output is enhanced to the amount of the light absorption by the removed portion.
In Figure 7, numeral 43 indicates a p-conductivity type layer, numeral 44 and n-conductivity type layer, numeral 45 an n -conductivity type layer, numerals 46 and 47 Zn diffused layers formed simultaneously, numeral 48 a p-n junction, numeral 49 an electrode layer for n-type ohmic contact, and numeral 50 an electrode layer for p-type ohmic contact.
Although, in the embodiments described, only the use of Gal_xAlxAs (0 < x _ l) as the semiconductor material has been stated, it is needless to say that similar effects can be achieved with mixed crystals of other III-V compound semiconductors, such as GaAsl xPX ( ' x _ 1), InxGal_xAs (0 < x _ 1), GaAsl xSbx (0 _ x ' 1~ and Gal_xInxP (0 _ x < 1) or with hetero-junctions employing III-V compound semiconductor materials different from éach other. The process of crystal growth is not restricted to liquid phase growth; a similar method of manufacture is applicable and similar effects can be achieved with vapor phase growth.
Further, although for simplicity the above description relates to the fabrication of individual light emitting devices, the invention is applicable to the fabrication of a function ,,~. : ,.

element 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 ~unction 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 is provided at a portion of contact with an electrode layer thereby to lower the contact resistivity, a portion of a light passage is left at a low carrier concentration thereby to reduce the absorption of light, and the coupling with an optical fiber can be easily conducted, so that the device is very effective as a light emitting device.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a light emitting device having a III-V compound semiconductor substrate whose bandgap is wider than the energy range corresponding to the radiation emitted by the device and which has a predetermined conductivity type, a second III-V
compound semiconductor layer deposited on an upper surface of said III-V compound semiconductor substrate and having the opposite conductivity type to that of said substrate, a current control layer that covers an upper surface of said second III-V
compound semiconductor layer and has a hole for current flow, a first electrode provided on said current control layer and being in ohmic contact with said second III-V compound semiconductor layer, and an ohmic contact electrode provided on a bottom surface of said III-V compound semiconductor substrate and having a light extracting window at its central part, wherein said current control layer is made of a layer of high carrier concentration having the opposite conductivity type to that of said second III-V compound semiconductor layer.

2. A light emitting device according to claim 1, wherein a layer of high carrier concentration is provided between the bottom surface of said III-V compound semiconductor substrate and said ohmic contact electrode.

3. A light emitting device according to claim 2, wherein the upper part of said second III-V compound semiconductor layer that lies within said hole of said layer of high carrier concentration provided on the upper surface of said III-V
compound semiconductor substrate is made of a layer which is doped with a conductivity type bestowing impurity in excess.

4. A light emitting device according to claim 3, wherein on the bottom surface of said III-V compound semiconductor substrate, a region corresponding to said light extracting window of said ohmic contact electrode is formed by providing a groove which extends to a position of said layer of high carrier concentration.

5. A method for manufacturing a light emitting device comprising the steps of (a) preparing a III-V compound semiconductor substrate and epitaxially growing thereon a first III-V compound semicon-ductor layer having a bandgap wider than that of the substrate and being of a predetermined conductivity type in such a manner that a composition ratio of the III-V compound varies from an upper surface of said substrate, (b) polishing and removing the said epitaxially grown III-V compound semiconductor layer from its upper sur-face down to a portion of specified composition range, and epitaxially growing further second, third and fourth layers successively onto said polished and removed surface of the first epitaxially grown layer, the second layer having the same conductivity type as that of the first layer, while the third and fourth layers have the opposite conductivity type, the bandgaps of said second, third and fourth layers being wider than the energy range corresponding to the radiation emitted by the device, (c) polishing and removing a part of said substrate at a bottom of the resultant III-V compound semiconductor and a part of the epitaxially grown layer on an upper side thereof, (d) coating an upper surface and a bottom surface of the resultant III-V compound semiconductor with glass films, and removing a part of the glass film on said upper surface that is other than a portion for introducing a current into said III-V compound semiconductor substrate, (e) diffusing an impurity that bestows the same conductivity type as that of the second epitaxially grown layer into the resultant III-V compound semiconductor, (f) removing the glass film layers on the upper and bottom surfaces of tile resultant III-v compound semiconductor, and (g) bringing metal electrodes into ohmic contact with those parts of the upper and bottom surfaces of the resultant III-V compound semiconductor that are other than a light extracting portion.

6. A method according to claim 5, wherein at step (d) the part of the glass film on said bottom surface that is other than the light extracting portion is removed.

7. A method according to claim 5, wherein after completion of step (b) an epitaxial layer doped in excess with an impurity bestowing the same conductivity type as that of the final epitaxial layer is provided on said final epitaxial layer, and wherein after completion of step (d) the part of said epitaxial layer doped with an impurity in excess that is other than a portion with the glass film installed thereon is removed.