CA1077607A

CA1077607A - Solid state light-emitting device and method of making the same

Info

Publication number: CA1077607A
Application number: CA255,114A
Authority: CA
Inventors: Kunihiko Asahi; Morio Inoue; Kunio Itoh
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1975-06-20
Filing date: 1976-06-17
Publication date: 1980-05-13
Also published as: DE2627355C3; GB1543220A; FR2316747B1; DE2627355A1; DE2627355B2; JPS5811111B2; JPS51151090A; FR2316747A1

Abstract

Abstract A narrow mesa region of ?tripe geometry is formed on a GaAs cyrstal substrate. A crystal layer of higher resistivity than the substrate is located around the mesa region, the top faces of these regions being flush with each other. On these top faces there is thus formed a light-emitting assembly consisting of several epitaxial growth regions of semiconductor crystal, including an active region for lasing. Subsequently there is formed a contact isolation region having an opening with the ?ame stripe geometry as the mean region. Finally, on the contact isolation region there is formed a metal electrode contacting the uppermost layer of the epitaxial growth regions through the stripe-geometry opening.
Current flows from the stripe-shaped portion of the upper electrode to the similar stripe-shaped narrow mesa region in a narrow concentrated path. As a result the effective lasing region is sufficiently concentrated to enable lasing with a low threshold current. Furthermore, since insulation films, such as SiO2 films, are not used for contact isolation, and the regions around the active region have sufficiently large area ail the way across the substrate, almost no strain is set up in the semiconductor crystal around the active region thereby assuring long life with stable ?unction.

Description

1(:1 77607 This invention relates to a solid-state light-emitting device and particularly concerns a semiconductor laser.: -.
In semiconductor laser devices, as a result of the employment of a double.heterostructure, lasing at room temperature becomes possible and many practical uses of such a semiconductor laser device have begun to..attract attention.
Double heterostructure semiconductor lasers were described by M.B. Panish et al. at pages 326 and 327 of Applied Physics Letters, Volume 16, Number 8, published 15th April, 1970. The device described in this publication has a structure in which .-an n-type Gal xAlxAs region,a p-type GaAs region and a p-type .:
Gal xAl As region are formed sequentially on a substrate of n-type GaAs crystal. Current is made to flow from the p-type Gal Al As region to the n-type GaAs substrate,.and carriers as well as the -~ light are confined in the GaAs active region, which is a.thin region ~ disposed perpendicular to the direction of thé curren.t.
`~ Subsequently, as an improved device capable of ;~ ~ confining the light in a limited narrow part of an active :~ . .
region~ the so-called stripe-type semiconductor laser has ~ 20 been developed, in which it is p.ossible to decrease considerably : the threshold current for lasing, thereby enabling low -: , current operation. This becomes possible by confining the `~ :.
carriers and light within a narrow stripe region. However, :.
.~ even in such a stripe-type laser, the dispersion of current in the active region cannot be ignored, and the threshold ourrent is not greatly reduced, even when the width of the I ~
stripe~region ~is sufficiently narrowed. Moreover, in such a strip~e-type laser, on account of the insulation film of, for Instance, SiO2 or Si3N4 disposed on the surface of the ~:

~:30~: semlconductor wafer, except on the surface of the stripe -~ -~

;sh~ped electrode contact, considerable strain caused by a : : . . , - , . , ~ . . - , ~077607 difference of thermal expansion coefficient is produced at the interface between the semiconductor and the insulatlon film. The effect of this strain reaches the active region, resulting in deterioration of the lasing characteristic and a shortening of the life of the device.
The present invention aims to provide an improved semiconductor laser wherein current is concentrated in a sufficiently small part of its active region, thereby providing a semiconductor laser having a small threshold current of lasing. A further object of the present invention is to provide a semiconductor laser wherein substantially no strain takes place in the crystal structure.
According to one aspect of the invention there is provided a semiconductor solid state light emitting device comprising, on a semiconductor crystal substrate, a light emitting layer which emits light upon carrier injection, wherein the substrate has high resistivity semiconductor isolation regions surrounding a mesa region, said light ; 20 emitting layer extends over said mesa region and over said high resistivity semiconductor isolation regions, an uppermost crystal layer of one conductivity type having a narrow opening is formed on an underlying layer of opposite conductivity type, forming a p-n junction therebetween, said uppermost cyrstal layer and said other layer being formed completely over said light emitting layer, and an upper electrode is formed completely over said uppermost crystal layer, contacting the surface of said underlying layer through said narrow opening, said -, 30 part of the -upper electrode which contacts the surface of i~
said underlying layer being substantially above said mesa :.

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region of the substrate.
According to another apsect of the invention there is provided a method of making a solid state light-emitting device comprising the steps of: (a) forming a mesa region on a semiconductor substrate, (b) locating high resis-tivity regions in recesses around said mesa region with top faces of said mesa region and said high resistivity regions flush with each other, (c) epitaxially growing a light-emitting layer all the way across said top faces, 10 forming a layer of one conductivity type completely over -said light emitting layer, forming a crystal layer of opposite conductivity type having a narrow opening on -said layer of one conductivity type, said narrow opening - -being located substantially above said mesa region of said substrate, and forming an electrode over said layer of opposite conductivity type to contact the surface of the underlying layer through said narrow opening.
A preferred embodiment of the invention is illustrated in the accompanying drawings.

Fig. 1 is a sectional elevation view of a semi-: - .
conductor laser embodying the present invention, Fig. 2(a) to (f) are sectional elevation views of -,.
, '', , ., .

- 3a -'' ' ~ ~ : ' ;;' . . . ' ,' ' -various steps in tlle making of tlle device of Fig. 1, and - Fig. 3 is a diagram showing the lasing characteristics of the example of the present invention (curve I) and of the prior art (curve II).
Fig. 1 shows one example of semiconductor laser.
The substrate l of GaAs is mesa-etched so as to retain a central part as mesa region 11 of stripe geometry, crystal regions 2 of higher resistivity than the substrate l being provided in the mesa-etched recesses. The higher resistivity regions 2 are of Gal Al As crystal where O<x<l. The mesa region 11 is buried up to its top part in the higher resistivity regions

2 in such a manner that the top faces of the mesa region ll and the regions 2 are flush with each other.
An n-type GaO 7Alo 3As region 3, a p-type GaAs active region 4, a p-type GaO 7Alo 3As region 5 and a p -type GaAs region 6 are sequentially formed in this order on the flush top surfaces of the regions 2 and 11. On the region 6 an n-type Gal yAl As tO<y<l) region 7 having an opening 71 of similar or the same stripe geometry as the mesa region 11 is formed, and a metal electrode 8 contacting the p -type GaAs region 6 at a portion 72 in the opening 71 is formed on the region 7.
The n-type region 7 formed on the p -type region 6 forms a p-n isolation junction therebetween, and accordingly, the region 7 serves as an electrode contact isolation layer. The strlpe shaped opening 71 is so disposed as to align with the .
top face of the mesa region 11, the regions 3,4,5 and 6 being between. The lasing current is fed from the top metal ele~ctrode 8 to a bottom metal electrode 9.
In a laser of this structure, both the effective contacting area of the metal electrode portion 72 and the mesa region 11 o the substrate are of narrow stripe geometry.

, :' :', ' ~:
. .
. . . - . - . . -Accordingly, the lines of electric force in the laser device are narrowly concentrated, on account of the narrow widths of the electrode portion 72 and the mesa region 11. The current in the active region 4 is thus well concentrated in a narrow stripe shaped area 4I, thereby improving lasing efficiency.
In this laser device, the thermal expansion -coefficients of the materials from the substrate 1, through the active region 4 to the isolation region 7 are almost the same, and the active region 4 is not subjected to any undesirable treatments such as mesa-etching or thermal oxidation.
Accordingly, there is little risk of any strain reaching the active region 4, and hence deterioration of the characteristics of the laser is avoided.
Fig. 2 illustrates the steps of making the laser device of Fig. 1. ;
The substrate 1 which is the starting material is (100)-oriented Te-doped n-type GaAs crystal of 2xlO cm concentration. As shown in Fig. 2(a), SiO2 films 20 of about 5000~ thick are formed on the GaAs substrate 1 by a known photochemical method so as to have a pattern of stripes of about 10 ~m disposed with a 250 ~m pitch and in the <110>
direction of the substrate crystal. Then, by employing the SiO2 films 20 as an etching mask, the n-type GaAs substrate 1 ;~ ~ 1s mesa-etched, the etchant consisting of sulfuric acid, hydrogen peroxide solution and water in a volume ra~io of 8:1:1. The GaAs substrate l is etched by this etchant at 60C~for 3 minutes, the etching being made to a depth of 6 ~m. The substrate 1 is thus mesa-etched as shown in Fig. 2(b).
30 ~ Then, as shown in Fig. 2(c), the higher resistivity GaAs crystal regions 2 are formed in the recesses 12 formed by the mesa-etching in such a manner that the top faces of the GaAs crystal regions 2 are flush with the top faces of the mesa-regions 11. The top faces of both the regions 2 and 11 are lapped to form a mirror-like flush face. The formation of these higher resistivity regions is car~ied out by a vapor-phase, epitaxial growth method, using ther~al decomposition of mixed gases of trimethygallium (Ga(CH3)3) and arsine(AsH3), employing the abovementioned SiO2 films as masks. Empirical -~
data show that a resistivity as high as 104Q cm can be obtainable with a temperature of thermal decomposition of 630C. As the higher resistivity regions 2, a mixed crystal of GaAlAs can also be used, and, in general, the higher resisti~ity regions 2 can be of Gal xAl As, where Ocx<l. In order to decrease strain around the higher resistivity regions 2, or more particularly strain in the active region 4 caused by strain in the higher resistivity regions 2, it is desirable to control the epitaxial growth process such that the x values have a gradient, e.g.
x=O at the bottom part (where the regions 2 contact the substrate 1) and x=0.3 at the top face (which contacts the region 3). The SiO2 films 20 are removed by a known method.
Next, as fihown by Fig. 2(d), region 3 of n-type GaO 7Alo 3As, region 4 of p-type GaAs, regio~ 5 of p-~ype GaO 7Alo 3As and region 6 of p -type GaAs are formed by sequential epitaxial growths on and across ~he mirror-lapped !:1 . :
flush top surface of ~he substrate 1 and the filled regions 2.
The reglon 7 of n-type Gal yAl As (O~y~l) is then formed on the region 6. The openings 71 of the stripe geometry are formed by known photoetching on the region 7, so as to expose the ~30 ~ underlying region 6. Since the regions 7 and 6 formed a heterostructure with each other, the seleoted parts of the r~ . -- 6 1,,,;: ~ : ~

107~7607 ~' . . .:
regions 7 can be etched away by hot phosphoric acid, retaining ~ -the underlying region 6 unharmed.
Einally, the top metal electrode 8 and the bottom metal electrode 9 are formed to cover all of the top and bottom surfaces, respectively, this being done by a known metal vapor deposition method, thereby making the wafer shown in Fig. 2(e). The wafer is then scribed and cut into individual units as shown in Fig. 2(f), the cutting lines being indicated by chain lines in Fig. 2(e).
Fig. 3 compares characteristic curves of a laser -embodying the present construction and according to the prior art, curve I showing the former while curve II relates to a -conventional stripe type laser using SiO2 films for contact isolation. The threshold current density is much influenced by the thicknesses of the four layers 1,3,4 and 5 which form a i double heterostructure. Accordingly, in the examples used for obtaining the curves of Fig. 3, the corresponding ones of these four layers were selected to be of equal thickness. The thicknesses oE the active region 4 were 0.2 1Im. The threshold current densitie8 are plotted as a function of stripe width, ~! ~ and Fig. 3 demonstrates that the laser embodying the present invention has a smaller threshold of lasing current in comparison with the conventional stripe type,laser, and especially a smaller threshold current for smaller stripe widths. This ,.~ : . .
result can be explained from the fact that, in the conventional structure, the current injected from the stripe electrode dlsp~erses widely when reaching the active region, the width of ~ ~
the current at the active region being generally 1.5 to 3 times ~ --the width of the stripe electrode with a stripe width of lO ~m, ,~
~30 ~ whllé in the present c,onstruction, on account of the stripe shqped contacting portion 72 of the 7 _ : . .
~ ':

electrode 8 and the stripe shaped mesa-part 11 disposed on both (the upper and lower) sides of the active region 4, the current is more concentrated in the active region 4.
~ further advantage of the present construction is long life with stable characteristic. As can be seen from Fig. 2(a) to (f), the lattice constants of the mesa part 11 and the higher resistivity regions 2 are almost equal to each other. Therefore, there is substantially no strain in the crystal structure in these regions. Also the lattice constants of the n-type Gal yAl As region 7 and the immediately under- `~
lying p -type GaAs region 6 are made to be almost equal.
Accordingly, there is little risk of introducing strain into the active region 4. This omission of strain assures stable characteristics for a long time. An example made in accordance with the foregoing description proved to have twice the life of conventional samples.
Although the above-described example is of a double ~- -heterostructure laser, the present invention is also applicable to lasers of single heterostructure, or homo-junction-structure.
It is also applicable to light emitting diodes.
Features of the apparatus disclosed are as follows:
(1) The solid state light-emitting device comprises a semiconductor substrate 1 bearing a light-emitting assembly

3,4,5 ~ 6 which emits light by the injection of carriers, the substrate 1 comprising a narrow part 11 located between or among higher resistivity semiconductor isolation regions 2.
(2) In the device of this item (1), the light-emitting assembly 3,4,5 & 6 and the higher resistivity isolation regions 2 are formed of semiconductor crystals of a ;30 III-V compound, especially of GaAs or GaAlAs.
(3) In the device of item (1), the light-emitting . . .. . .. .. .. . . . .. . . . .. ..

assembly comprises a heterostruc~ure, more particularly a heterostructure comprising a GaAs-GaAlAs junction.

(4) In the device of item (1), the light-emitting assembly comprises a double heterostructure and on both sides (upper and lower) of the light~emitting assembly there are formed higher resistivity semiconductor isolation regions 2 and semiconductor isolation regions 7, respectively, for limiting the carriers to narrowed paths.

(5) The method of making such a device comprises the following steps:
(a) forming a mesa region 11 on a semiconductor substrate 1 which is the starting material, (b) placing higher resistivity regions 2 in recesses .
12 a.round the mesa region 11, the top faces of the regions :
11 and 2 being flush with each other, and (c) epitaxially growing the light-emitting regions : 3,4,5 & 6 all the way across and on the flush top faces of the regions 11 and 2. . ..

(6) The method of item (5) further comprising the step of forming semiconductor isolation regions 7 which form a contact isolation junction with the underlying region 6, ~ .
the contact isolation junction being defined by the stripe shaped contacting portion 72 of the contact electrode 8 in the .`
opening 71 formed in the electrode 7. .

(7) The method of item (5) wherein the higher resistivity regions are formed by vapor phase epitaxial growth method.

,.: .

(8) The method of items (5) to (7) wherein the semi-conductor regions comprise GaAs and GaAlAs crystal regions.

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Claims

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

1. A semiconductor solid state light emitting device comprising, on a semiconductor crystal substrate, a light emitting layer which emits light upon carrier injection, wherein the substrate has high resistivity semiconductor isolation regions surrounding a mesa region, said light emitting layer extends over said mesa region and over said high resistivity semiconductor isolation regions, an uppermost crystal layer of one conductivity type having a narrow opening is formed on an underlying layer of opposite conductivity type, forming a p-n junction therebetween, said uppermost cyrstal layer and said other layer being formed completely over said light emitting layer, and an upper electrode is formed completely over said uppermost crystal layer, contacting the surface of said underlying layer through said narrow opening, said part of the upper electrode which contacts the surface of said underlying layer being substantially above said mesa region of the substrate.

2. A device of claim 1 wherein said high resistivity semiconductor isolation regions are of a III-V compound.

3. A device of claim 1 wherein said higher resistivity semiconductor regions are of GaAs or GaAlAs.

4. A device of claim 1, wherein said p-n junction comprises a GaAs-GaAlAs junction.

5. A device of claim 1, wherein said light-emitting layer comprises a double heterostructure of GaAlAs-GaAs, semiconductor isolation regions of GaAlAs being formed on said light-emitting assembly with isolation regions disposed above and with corresponding geometry to said high resistivity semiconductor isolation regions.

6. A method of making a solid state light-emitting device comprising the steps of:
(a) forming a mesa region on a semiconductor substrate, (b) locating high resistivity regions in recesses around said mesa region with top faces of said mesa region and said high resistivity regions flush with each other, (c) epitaxially growing a light-emitting layer all the way across said top faces, forming a layer of one conductivity type completely over said light emitting layer, forming a crystal layer of opposite conductivity type having a narrow opening on said layer of one conductivity type, said narrow opening being located substantially above said mesa region of said substrate, and forming an electrode over said layer of opposite conductivity type to contact the surface of the underlying layer through said narrow opening.

7. A method of claim 6, wherein said higher resistivity regions are formed by a vapor phase epitaxial growth method.

8. A method of claim 6, wherein said semiconductor regions are formed by a combination of epitaxial growths of GaAs and GaAlAs crystals.