United States Patent [19J
`Dutta et al.
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US005861636A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,861,636
`Jan. 19, 1999
`
`[54] SURFACE EMITTING VISIBLE LIGHT
`EMITING DIODE HAVING RING-SHAPED
`ELECTRODE
`
`[56]
`
`References Cited
`
`FOREIGN PATENT DOCUMENTS
`
`[75]
`
`Inventors: Achyut Kumar Dutta; Akira Suzuki,
`both of Tokyo, Japan
`
`[73] Assignee: NEC Corporation, Tokyo, Japan
`
`[21] Appl. No.: 629,470
`
`[22] Filed:
`
`Apr. 11, 1996
`
`[30]
`
`Foreign Application Priority Data
`
`Apr. 11, 1995
`
`[JP]
`
`Japan .................................... 7-085128
`
`[51]
`
`Int. Cl.6
`
`..................................................... H01L33/00
`
`[52] U.S. Cl. ................................. 257/91; 257/95; 257/98;
`257/99; 257/100
`
`3-190287
`4-174567
`4-259263
`
`8/1991
`6/1992
`9/1992
`
`Japan .
`Japan .
`Japan .
`
`Primary Examiner---Minh Loan Tran
`Attorney, Agent, or Firm---Sughrue, Mion, Zinn, Macpeak
`& Seas, PLLC
`
`[57]
`
`ABSTRACT
`
`Disclosed is a surface emitting visible light emitting diode,
`which has: a first conductivity-type substrate; and a first
`conductivity-type buffer layer, a first conductivity-type clad(cid:173)
`ding layer, an active layer, a second conductivity-type clad(cid:173)
`ding layer, a second conductivity-type current spreading
`layer and a second conductivity-type cap layer which are in
`turn grown on the substrate; wherein the second
`conductivity-type cap layer has a ring shaped electrode
`formed thereon.
`
`[58] Field of Search .................................. 257/91, 94, 95,
`257/98, 99, 100
`
`2 Claims, 16 Drawing Sheets
`
`14 PASSIVATION LAYER
`13 LIGHT OUTPUT
`?
`1J
`
`9 p-TYPE CONTACT ELECTRODE
`8 DIELECTRIC LAYER
`
`.._.._-~~~I.!:U.---=r-7 p-TYPE GaAs CAP LAYER
`
`19 LED
`
`12 LIGHT EMITTING SURFACE
`
`6 CURRENT SPREADING LAYER
`
`p-TYPE CLADDING LAYER
`
`5a SPACER LAYER
`
`4 ACTIVE LAYER
`
`3 n-TYPE CLADDING LAYER
`
`1 n-GaAs SUBSTRATE
`
`It:============:;jL 11 n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0001
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 1 of 16
`
`5,861,636
`
`25 ELECTRODE
`
`FIG. lA
`PRIOR ART
`
`27 LIGHT EMITTING SURFACE
`
`D 2!{ u 28 LIGHT OUTPUT
`
`7 CAP LAYER
`
`1
`
`•
`
`FIG. 18
`PRIOR ART
`
`~ ,.---27
`1
`~k-9 CURRENT SPREADING LAYER
`p-TYPE CLADDING LAYER
`~5
`--
`"'
`4 ACTIVE LAYER
`3 n-TYPE CLADDING LAYER
`1-- -
`2 n-TYPE BUFFER LAYER
`
`~~ 1 GaAs SUBSTRATE
`t::::=::=::==:=:::::j-.....__11 n-TYPE ELECTRODE
`
`FIG.2
`PRIOR ART
`LIGHT
`INTENSITY
`(a.u.) 1
`
`29
`
`29 LIGHT INTENSITY
`
`- - LIGHT EMITTING SURFACE
`t 29 r-=-1 t 29
`
`Vizio EX1014 Page 0002
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 2 of 16
`
`5,861,636
`
`FIG.3 PRIOR ART
`
`28 LIGHT OUTP
`
`UTtt~ 2f
`
`27 LIGHT EMITTING SURFACE
`lJ
`
`I
`
`7 CAP LAYER
`-
`-6 cu RRENT SPREADING LAYER
`-
`
`::
`
`,...
`
`30 BLOCKING LAYER
`
`-
`
`5
`
`p-TYPE CLADDING LAYER
`
`-A
`
`ACTIVE LAYER
`
`---3
`
`n-TYPE CLADDING LAYER
`
`n-TYPE BUFFER LAYER
`
`--2
`
`...
`
`1t:::===============:::t~ 1 GaAs SUBSTRATE
`
`Vizio EX1014 Page 0003
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 3 of 16
`
`5,861,636
`
`F/G,4
`
`14 PASSIVATION LAYER
`13 LIGHT OUTPUT
`
`l
`]'
`
`9 p-TYPE CONTACT ELECTRODE
`8 DIELECTRIC LAYER
`1---~l::::::::li;:~L--------=:r---7 p-TYPE GaAs CAP LAYER
`12 LIGHT EMITTING SURF ACE
`
`19 LED
`
`6 CURRENT SPREADING LAYER
`
`5 p-TYPE CLADDING LAYER
`
`5a SPACER LAYER
`
`4 ACTIVE LAYER
`
`----------- 3 n-TYPE CLADDING LAYER
`1------------1
`1------------~~ 2 n-TYPE BUFFER LAYER
`
`1 n-GaAs SUBSTRATE
`
`lt:==================::;jL 11 n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0004
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 4 of 16
`
`5,861,636
`
`FIG.5A
`
`19
`
`12 LIGHT EMITTING SURFACE
`10 Au ELECTRODE
`
`12
`
`9(1) p-TYPE CONTACT
`ELECTRODE
`
`FIG. 58
`
`19-
`
`FIG.5C
`
`19
`
`Vizio EX1014 Page 0005
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 5 of 16
`
`5,861,636
`
`FIG.6
`
`14 PASSIVATION LAYER
`
`r.;::::::;~::;:::;:.
`
`Au ELECTRODE
`9 p-TYPE CONTACT ELECTRODE
`8 DIELECTRIC LAYER
`7 p-TYPE GaAs CAP LAYER
`----1-12 LIGHT EMITTING SURFACE
`6 CURRENT SPREADING LAYER
`
`\5 BLOCKING LAYER
`
`5 p-TYPE CLADDING LAYER
`
`4 ACTIVE LAYER
`
`3 n-TYPE CLADDING LAYER
`
`-.--.....___- 2 n-TYPE BUFFER LAYER
`
`n-GaAs SUBSTRATE
`
`19 LED---
`
`lt-------~L 11 n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0006
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 6 of 16
`
`5,861,636
`
`FIG.7
`
`14 PASSIVATION LAYER
`13 LIGHT OUTPUT
`
`I u 10 Au ELECTRODE
`
`9 p-TYPE CONTACT ELECTRODE
`8 DIELECTRIC LAYER
`7 p-TYPE GaAs CAP LAYER
`12 LIGHT EMITTING SURF ACE
`
`19 LED
`
`6 CURRENT SPREADING LAYER
`
`------ 5 p-TYPE CLADDING LAYER
`
`4 ACTIVE LAYER
`
`3 n-TYPE CLADDING LAYER
`
`16 DBR LAYER
`~~~~~~~-16(1)
`16(2)
`t-------~r--.. n-TYPE BUFFER LAYER
`
`lt---------IE=l n-GaAs SUBSTRATE
`
`11 n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0007
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 7 of 16
`
`5,861,636
`
`F /G. BA
`
`F
`16(1)-ft::------------=1=
`
`Fl6(1)
`
`16(2)
`
`---l.__.-16 DBR LAYER
`
`I
`I
`I
`I
`
`FIG.88
`
`tOO
`
`>-
`1--
`>
`i=
`(..) w
`....J u.. w a:=
`
`Vizio EX1014 Page 0008
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 8 of 16
`
`5,861,636
`
`FIG.9
`
`14 PASSIVA liON LAYER
`13 LIGHT OUTPUT
`)
`10 Au ELECTRODE
`ft
`9 p-TYPE CONTACT ELECTRODE
`I u
`8 DIELECTRIC LAYER
`-----r- 12 LIGHT EMITTING SURF ACE
`t----.~o::oL...-~:::.L..-J.:L...-~.-r---...- 7 p-TYPE GaAs CAP LAYER
`
`-------- 6 CURRENT SPREADING LAYER
`
`19 LED
`
`15 BLOCKING LAYER
`
`5 p-TYPE CLADDING LAYER
`
`ACTIVE LAYER
`
`3 n-TYPE CLADDING LAYER
`
`~~~~~~~ ........._ --16 DBR LAYER
`2 n-TYPE BUFFER LAYER
`
`n-GaAs SUBSTRATE
`
`It:=============~Ln n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0009
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 9 of 16
`
`5,861,636
`
`FIG.IO
`14 PASSIVATION LAYER
`13 LIGHT OUTPUT
`{?.
`10 Au ELECTRODE
`U
`r;:=~::::::::;-- 9 p-TYPE CONTACT ELECTRODE
`tt--~----1--- 8 DIELECTRIC LAYER
`7 p-TYPE GaAs CAP LAYER
`r--~o::~-~~:.~.-..~:~~..-__.
`6 CURRENT SPREADING LAYER
`12 LIGHT EMITTING SURF ACE
`17 PROTON IMPLANTATION REGION
`
`19 LED~
`
`4 ACTIVE LAYER
`
`3 n-TYPE CLADDING LAYER
`
`1-----------1....___-2 n-TYPE BUFFER LAYER
`
`1 n-GaAs SUBSTRATE
`
`n-TYPE ELECTRODE
`
`FIG. II
`
`21 MOLDING
`
`19 LED
`
`20 LEAD-FRAME
`
`Vizio EX1014 Page 0010
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 10 of 16
`
`5,861,636
`
`FIG. 12A
`
`FIG. 128
`
`~. ~ [i]
`~ ~ [iJ
`
`Vizio EX1014 Page 0011
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 11 of 16
`
`5,861,636
`
`FIG.13
`
`LIGHT
`
`l0(3)} Au ELECTRODE
`10(2)
`9 p-TYPE CONTACT
`8 DIELECTRIC LAYER
`7 CAP LAYER
`
`6 CURRENT SPREADING LAYER
`
`14
`
`5 p-TYPE CLADDING LAYER
`~---------t.....-- 5a SPACER LAYER
`4 ACTIVE LAYER
`
`3 n-TYPE CLADDING LAYER
`
`2 n-TYPE BUFFER LAYER
`
`J__ 1 GaAs SUBSTRATE
`1
`t::===============::jj_ 11 n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0012
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 12 of 16
`
`5,861,636
`
`FIG.14
`
`LIGHT
`
`12
`
`10(3)} Au ELECTRODE
`10(2)
`E~~- 9 p-TYPE CONTACT
`8 DIELECTRIC LAYER
`7 CAP LAYER
`
`6 CURRENT SPREADING LAYER
`
`5 p ... TYPE CLADDING LAYER
`~--------:L,..--50 ~PACER LAYER
`4 ACTIVE LAYER
`3 n-TYPE CLADDING LAYER
`15 DBR LAYER
`
`2 n-TYPE BUFFER LAYER
`
`t GaAs SUBSTRATE
`
`l
`ll::=:::::=============::i
`
`11 n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0013
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 13 of 16
`
`5,861,636
`
`FIG.15
`
`LIGHT
`
`10(3)'\
`10(2)) Au ELECTRODE
`9 p-TYPE CONTACT
`kf--8 DIELECTRIC LAYER
`~~~==~==~r-7 CAP LAYER
`6 CURRENT SPREADING LAYER
`16 PROTON IMPL.ANT ATION REGION
`
`_._--5 P-TYPE CLADDING LAYER
`5a SPACER LAYER
`4 ACTIVE LAYER
`3 n-TYPE CLADDING LAYER
`
`2 n-TYPE BUFFER LAYER
`
`------1 GaAs SUBSTRATE
`
`1l:::======::::::::j..L 11 n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0014
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 14 of 16
`
`5,861,636
`
`FIG.16
`
`13 LIGHT EMITTING
`SURFACE
`
`10(2) Au ELECTRODE
`9 p-TYPE CONTACT
`
`18 PASSIV A TlON
`LAYER
`
`8 DIELECTRIC LAYER
`l====:f:a::~tii::lt2i:===~-7 CAP LAYER
`17 GalnP LAYER
`
`14
`
`6 CURRENT SPREADING LAYER
`
`5 p-TYPE CLADDING LAYER
`
`5a SPACER LAYER
`~~~~~~~~-4 ACTIVE LAYER
`
`3 n-TYPE CLADDING LAYER
`
`2 n-TYPE BUFFER LAYER
`
`1 GaAs SUBSTRATE
`
`1t:=:=:=:=:=:=::::::::Jl11 n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0015
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 15 of 16
`
`5,861,636
`
`FIG.17
`
`13 LIGHT EMITTING
`SURFACE
`
`18 PASSIVATION
`LAYER
`
`10(2) Au ELECTRODE
`- 9 p-TYPE CONTACT
`......,....__ 8 DIELECTRIC LAYER
`b===:lri:~~-12~!:::: ~=="'""=~
`7 CAP LAYER
`14 ---.. .
`17 GalnP LAYER
`
`.;::;
`
`~
`
`I - -
`
`6 CURRENT SPREADING LAYER
`
`- -
`
`5 p-TYPE CLADDING LAYER
`~
`
`v
`
`5a SPACER LAYER
`4 ACTIVE LAYER
`
`-
`
`1- 3 n-TYPE CLADDING LAYER
`
`1--15 DBR LAYER
`
`1- 2 n-TYPE BUFFER LAYER
`
`L.
`
`1
`
`~L..
`
`t 1 GaAs SUBSTRATE
`
`c=:==========~ 1t n-TYPE ELECTRODE
`
`Vizio EX1014 Page 0016
`
`

`

`U.S. Patent
`
`Jan. 19, 1999
`
`Sheet 16 of 16
`
`5,861,636
`
`FIG.18
`
`14
`
`7 CAP LAYER
`t5::::=::t:Zl:='Z:zZJI::fZI:=~L 17 GalnP LAYER
`f.£>0<~-16 PROTON IMPLANTATION REGION
`
`---.<~
`
`5 p-TYPE CLADDING LAYER
`
`5a SPACER LAYER
`~~~~~~§~-4 ACTIVE LAYER
`
`~~~~~~~~~15 DBR LAYER
`
`2 n-TYPE BUFFER LAYER
`
`3 n-TYPE CLADDING LAYER
`
`~-1 GaAs SUBSTRATE
`
`11::============::=:1-l-11 n-TYPE ELECTRODE .
`
`Vizio EX1014 Page 0017
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`

`

`5,861,636
`
`1
`SURFACE EMITTING VISIBLE LIGHT
`EMITING DIODE HAVING RING-SHAPED
`ELECTRODE
`
`FIELD OF THE INVENTION
`
`This invention relates to a surface emitting visible light
`emitting diode, and more particularly to, a surface emitting
`visible light emitting diode which is used as an optical
`source in a POF(plastic optical fiber) based optical data link
`system and also as an outdoor display or automobile indi(cid:173)
`cator.
`
`BACKGROUND OF THE INVENTION
`
`A conventional surface emitting visible light emitting
`diode(hereinafter also referred to as 'surface emitting visible
`LED') generally has a top electrode with a square or circular
`shape which is located at the center of the light emitting
`surface of LED. In the conventional surface emitting visible
`LED, the external quantum efficiency is dependent on the
`structure of the LED and the shape of an electrode for
`spreading current into the junction. To enhance the light
`output, the electrode area needs to be as small as possible
`and the light emitting surface needs to be as wide as
`possible.
`Though the conventional surface emitting visible LED
`allows easiness in fabrication, it causes the disruption of a
`gaussian type beam output, broadening the beam with light
`emission concentrated around the electrode. The emitted
`light intensity gets decreased as the distance from the
`electrode gets increased. This is because the current is
`almost centrally crowded and is not distributed far away
`from the electrode. The light intensity achieved is directly
`proportional to the current density obtained at the point. This
`type of LED with broad light beam cannot be implemented
`in the optical data-link system, because the coupling effi(cid:173)
`ciency even with a larger core fiber is low due to its beam
`divergence.
`For example, known is a conventional surface emitting
`visible LED, which comprises an-type buffer layer, an-type
`cladding layer, an active layer, a p-type cladding layer, a
`current spreading layer and a p-type cap layer which are in
`turn grown on an-type GaAs substrate. On the cap layer, an
`electrode composed of square or circular shaped metal is
`formed.
`Observing an approximated near field pattern of the light
`emitted from the light emission surface of the conventional
`surface emitting visible LED, it is proved that the light
`intensity is maximum around the electrode and gets lowered
`at the position far away from the electrode, which indicates
`that the spreading of current is mainly crowded under the
`electrode contact. Thus, the light output obtained could not
`be enhanced as expected. To avoid this current crowding
`around the electrode, a blocking layer is conventionally
`employed which is formed prior to the growth of the current
`spreading layer.
`However, this type of LED needs the two step layer
`growth, which needs further time and increases the produc(cid:173)
`tion cost. In addition, the darkness at the center portion due
`to the circular shaped electrode reduces the coupling effi(cid:173)
`ciency even with a larger core fiber.
`Many papers and Japanese patent applications disclose
`concerning III-V semiconductors based visible LED which
`has a wavelength range from 580 to 670 nm. In every case,
`the electrodes for current spreading have a square or circular
`shape and are located at the center of LED. One typical
`
`5
`
`10
`
`2
`example is disclosed in Sugawara et al., Japan Journal of
`Applied Physics, part 1, Vol.l, No.8, pp.2446-2451. In this
`report, a cross shaped electrode is used in the top emitting
`LED for spreading the current. A blocking layer is also used
`to prevent the current from crowding under the contact. The
`light output achieved by the cross shaped electrode is still
`below the level necessary for the optical data link system. To
`increase the current spreading outside the contact, a thick
`window layer with a low resistivity is needed. However,
`even with this thick window layer, the current spreading
`outside the contact is limited up to a certain level and the
`light intensity is not enough for practical application.
`This type of LED may have no problem for use in the
`outdoor application. However, in an application such as a
`short distance data link system, especially based on POF, the
`15 use of this conventional LED with the square, circular or
`cross shaped electrode usually exhibits low coupling effi(cid:173)
`ciency to be completely impractical in the POF based
`communication system. For the POF based data link system,
`it would be highly desirable to design a LED which exhibits
`20 not only high brightness but also high coupling efficiency.
`Japanese patent application laid-open No.2-174272
`(inventor: Kim) discloses a high brightness LED. In this
`LED, the brightness is enhanced by employing a n-p-n-p
`structure under a contact, since the current crowded under
`25 the contact can be spread to the light emitting surface. The
`main drawback of this LED is that prior to making a
`p-contact a mesa structure up to an active region is formed
`in the light emitting surface to make a current path and also
`dopant-like zinc(Zn) is diffused at a high ambient tempera-
`30 ture. The mesa formation up to the active region and also the
`post growth high temperature for Zn diffusion may have an
`influence on its performance characteristics. As the emitting
`surface has a high p-doping a large fraction of the light
`(depending on the light energy) might be absorbed in the
`35 emitting surface. In fabrication of this type of LED, it needs
`to go through several processes to make the LED fabrication
`cost higher. Further, it is difficult to make this type of LED
`high speed because of the large capacitance induced due to
`the contact area which is proportional to the LED size.
`Japanese patent application laid-open No.3-190287
`(inventor:Tajiri) also discloses a high brightness LED array.
`In this LED, to facilitate the wire bonding, a contact is made
`on the upper layer following the mesa formation. In this
`case, the shape of the contact is changed to achieve a high
`45 light output. Since this device is closely related to a LED
`array and the mesa structure is needed prior to the formation
`of electrode, there will be difficulties to form the electrode
`unless a thick contact region is provided. Therefore, the cost
`of LED fabrication increases and it is completely impractical
`50 for a single LED to be made.
`Japanese patent application laid-open No. 4-174567
`(inventors: Kato et al.) discloses a high brightness LED
`array for the use of a printer. This LED is provided with a
`bottom distributed Bragg reflector(DBR) for reflecting the
`55 light returned from a substrate. In this case, the same idea as
`in the fabricating of a surface emitting laser is implemented.
`In this DBR, pairs of GaAs/AlAs were used for reflecting
`880 nm wavelength light emitted towards a substrate. The
`type and number of the pairs in this DBR are dependent on
`60 the wavelength of output light and this type of DBR cannot
`be used in a visible LED where 600 to 650 nm wavelengths
`are to be considered. This type of LED array could be used
`for a printer, but it cannot be implementable in another
`application such as in a data link system. This is because the
`65 shape of a top electrode is the same as that of the previously
`mentioned conventional LED therefore resulting in provid(cid:173)
`ing a very low coupling efficiency to a fiber.
`
`40
`
`Vizio EX1014 Page 0018
`
`

`

`5,861,636
`
`3
`Japanese patent application laid-open No. 4-259263
`(inventors: Nitta et al.) discloses a InAlGaP based visible
`LED. In this LED, to avoid the Zn diffusion into an active
`region, an additional layer of InGaP is employed which has
`a bandgap greater than that of the active region layer. No 5
`additional layer for current spreading is used. However, as
`described previously, this type of visible LED with a circular
`shaped electrode centrally located is not useful in a short
`distance data link system.
`As explained above, the conventional LEDs have draw(cid:173)
`backs that a sufficient light output and coupling efficiency
`cannot be obtained in the short distance data link system,
`especially in the POP based data system.
`
`SUMMARY OF THE INVENTION
`
`15
`
`4
`FIGS. SA and 8B are explanatory diagrams showing a
`structure of the DBR layer in the surface emitting visible
`LED in the third and a fourth preferred embodiment and its
`approximated reflecting characteristics,
`FIG. 9 is a cross sectional view showing a surface
`emitting visible LED in a fourth preferred embodiment
`according to the invention,
`FIG. 10 is a cross sectional view showing a surface
`emitting visible LED in a fifth preferred embodiment
`10 according to the invention,
`FIG. 11 is a cross sectional view showing a mold package
`structure for a surface emitting visible LED in a sixth
`preferred embodiment according to the invention,
`FIGS. 12A and 12B are top views showing surface
`emitting visible LED arrays in a seventh preferred embodi(cid:173)
`ment according to the invention,
`FIG. 13 is a cross sectional view showing a surface
`emitting visible LED in an eighth preferred embodiment
`according to the invention,
`FIG. 14 is a cross sectional view showing a surface
`emitting visible LED in a ninth preferred embodiment
`according to the invention,
`FIG. 15 is a cross sectional view showing a surface
`emitting visible LED in a tenth preferred embodiment
`according to the invention,
`FIG. 16 is a cross sectional view showing a surface
`emitting visible LED in an eleventh preferred embodiment
`30 according to the invention,
`FIG. 17 is a cross sectional view showing a surface
`emitting visible LED in a twelfth preferred embodiment
`according to the invention, and
`FIG. 18 is a cross sectional view showing a surface
`emitting visible LED in a thirteenth preferred embodiment
`according to the invention.
`
`20
`
`Accordingly, it is an object of the invention to provide a
`surface emitting visible LED by which a high light output
`and a high coupling efficiency to an optical fiber can be
`obtained.
`According to the invention, a surface emitting visible
`light emitting diode, comprises:
`a first conductivity-type substrate; and a first
`conductivity-type buffer layer, a first conductivity-type
`cladding layer, an active layer, a second conductivity- 25
`type cladding layer, a second conductivity-type current
`spreading layer and a second conductivity-type cap
`layer which are in turn grown on the substrate;
`wherein the second conductivity-type cap layer has a ring
`shaped electrode formed thereon.
`In accordance with the invention, the shape of the elec(cid:173)
`trode is properly designed to distribute the current on the
`emitting surface. When the ring shaped electrode with an
`optimized diameter on the emitting surface is used, the
`current can be uniformly distributed over the emitting sur- 35
`face. In the use of a large emitting surface, two or more rings
`may be employed to keep the uniform current spreading.
`Both the speed and the light output of LED are dependent on
`the number of rings and the distance between the rings.
`Thus, by optimizing these factors, the light output can be 40
`maximized and the speed can be enhanced. Also, the use of
`the ring shaped electrode can allow the light to be collected
`at the central portion to enhance the coupling efficiency to
`the POF and the speed of the LED.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The invention will be explained in more detail in con(cid:173)
`junction with the appended drawings, wherein:
`FIGS. 1A and 1B are top and cross sectional views
`showing a conventional surface emitting visible LED,
`FIG. 2 is an explanatory diagram showing an approxi(cid:173)
`mated near field pattern of the light emitted from the light
`emission surface of the LED in FIGS. 1A and 1B,
`FIG. 3 is a cross sectional view showing another conven-
`tional surface emitting visible LED,
`FIG. 4 is a cross sectional view showing a surface
`emitting visible LED in a first preferred embodiment accord(cid:173)
`ing to the invention,
`FIGS. SA to 5C are top views showing ring shaped
`electrodes employed in the first embodiment,
`FIG. 6 is a cross sectional view showing a surface
`emitting visible LED in a second preferred embodiment
`according to the invention,
`FIG. 7 is a cross sectional view showing a surface 65
`emitting visible LED in a third preferred embodiment
`according to the invention,
`
`55
`
`Before explaining a surface emitting visible LED in the
`preferred embodiment, the aforementioned conventional
`LED will be explained in FIGS. 1 to 3.
`FIGS. 1A and 1B are top and cross sectional views
`45 showing an example of the conventional surface emitting
`visible LED. As shown in FIGS. 1A and lB, n-type GaAs
`buffer layer 2, n-type (AlxGa1 _x))n1 _xp layer 3, active layer
`4, p-type(AlxGa1_J 0 .5In05P layer 5, current spreading layer
`6 and p-type GaAs cap layer 7 are in turn grown on an-type
`50 GaAs substrate 1. On the p-type GaAs cap layer 7, an
`electrode 25 composed of square or circular shaped metal26
`is formed. As the current spreading layer 6, thick
`A10 .8 Ga0 .9As is usually used.
`FIG. 2 shows an approximated near field pattern of the
`light emitted from the light emission surface 27 of the above
`LED. In FIG. 2, as described above, it is proved that the light
`intensity 29 is maximum around the electrode 25 and gets
`lowered at the position far away from the electrode 25,
`which indicates that the spreading of current is mainly
`60 crowded under the electrode contact. Thus, the light output
`28 could not be enhanced as expected.
`In order to avoid this current crowding around the
`electrode, a blocking layer is conventionally employed
`which is formed prior to the growth of the AlGaAs
`(spreading) layer 6. FIG. 3 shows a conventional surface
`emitting visible LED with the blocking layer 30. This type
`of LED needs the two step layer growth.
`
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`
`

`

`5,861,636
`
`5
`Next, a surface emitting visible light emitting diode in the
`first preferred embodiment will be explained in FIG. 4,
`wherein like parts are indicated by like reference numerals
`as used in FIGS. 1B and 3.
`FIG. 4 shows a surface emitting visible light emitting
`diode in the first embodiment. As shown in FIG. 4, n-type
`GaAs buffer layer 2, n-type (A10 .7 Ga0 .3 ) 05In0 .5P cladding
`layer 3, active layer 4, p-type (A10 .7 Ga0 .3 ) 05In05P cladding
`layer S, current spreading layer 6 and high doped p-type
`GaAs cap layer 7 are subsequently grown on an-type GaAs 10
`substrate 1. As the active layer 4, bulk or multiquantum well
`of (AlxGa1_J 05In0 .6P based material with a desired wave(cid:173)
`length can be used. As the current spreading layer 6, high
`doped p-type III-V based semiconductors having a band gap
`higher than that of used as the active layer 4 and also having
`a lattice constant similar to that of its underlying p-type 15
`cladding layer S can be used.
`It should be noted here that since the high doping is
`necessary in the thick current spreading layer 6 and cap layer
`7 during the epitaxial growth, the dopant diffusion into the
`active layer 4 may deteriorate the optical characteristics of 20
`LED. In order to prevent the dopant from diffusing into the
`active layer 4, the low doped spacer layer Sa with a thickness
`dependent on the thickness of the current spreading layer 6
`and cap layer 7 is formed following the formation of the
`active layer 4. As the spacer layer Sa, the same type of low 25
`doped materials as the p-type cladding layer S can be used.
`After the epitaxial growth, a dielectric layer 8 with over
`150 nm thickness of silicon nitride, silicon dioxide or the
`like is deposited at a temperature below 300° C.
`This is followed by opening the window through the
`dielectric layer 8 to provide a light emitting surface 12.
`Then, a p-type contact electrode 9 is formed using the lift-off
`process. The top view of the electrode 9 is as shown in FIGS.
`SA to SC.
`In LED's, since the current density is higher than that of
`optical devices such as a laser diode, the contact resistance
`mainly induced due to the p-type contact electrode 9 should
`be as low as 1 ohm for its reliable long time operation. For
`this reason, in the formation of the p-type contact electrode 40
`9, the more reliable metallization is to be selected in
`avoiding any spike formation in the GaAs cap layer 7. For
`the p-type contact electrode 9, Au:Zn or Ti/Pt/Au can be
`used to provide the reliability higher than the conventional
`type metallization such as Cr:Au.
`When an LED is used in the data link system, the optical
`coupling efficiency of LED to the fiber is of more concern.
`Therefore, the light emitting surface should be designed
`such that after the coupling more light can emit into the fiber.
`The coupling efficiency is mainly dependent on several 50
`factors such as the type of a fiber used and the emitting
`surface of LED used. In surface emitting visible LED, the
`emitting surface needs to be designed such that more light is
`concentrated on the center region rather than the outer
`region of the surface. This is more dependent on the top 55
`shape of an electrode. Namely, the coupling efficiency can
`be maximized by optimizing the top shape of an electrode.
`As explained previously, in the conventional surface
`emitting visible LED in which electrodes mainly are located
`at the center portion of the surface, the output light is 60
`broadened to the outer region of the surface to lower the
`coupling efficiency.
`When ring shaped electrodes as shown in FIGS. SA to SC
`according to the invention are used, the light can be made to
`concentrate on the center of LED, which can provide the 65
`coupling efficiency higher than that of the conventional
`LED.
`
`6
`The design of the ring is also another factor for enhancing
`the coupling efficiency. In general, to achieve the maximum
`coupling efficiency(50% or more) to the fiber, the ring
`diameter should be designed such that the ratio of the fiber
`5 core diameter to the emitting surface is greater than 5. For
`example, when the step index(SI) type POF fiber with core
`size of 0.98 mm and numerical aperture(NA) of 0.5 is used,
`the ring diameter is to be less than about 100 ,urn to achieve
`the coupling efficiency over 70%.
`Also, in order to enhance the output power, the width of
`the ring should be as low as possible. Since the contact
`resistance is dependent on its contact area, two or three rings
`as shown in FIGS. SB to SC may be used to improve the
`contact resistance.
`After making the p-type contact electrode 9, to facilitate
`the wire bonding, an Au electrode 10 is plated on the p-type
`contact electrode 9 or partially on the portion where the wire
`bonding is laid.
`Then, the light emitting surface 12 is covered with a
`passivation layer 14. The passivation layer 14, which can be
`made of alumina(Al2 0 3 ), silicon dioxide(Si0 2 ) or silicon
`nitride(SiNx), is deposited at a room temperature or a
`temperature less than 200° C. The passivation layer 14 can
`prevent the light emitting surface 12 from absorbing water
`molecules or oxygen from the environment and enhance the
`device reliability in long time operation.
`After forming the passivation layer 14, the backside of the
`substrate 1 is polished to around 100 ,urn to form a n-type
`contact region 11 thereon. The n-type contact region 11 can
`be made of Au:Ge/Ni/Au. After forming the n-type contact
`region 11 on the backside of the substrate 1 polished, each
`LED is scribed for packaging. The ring shaped electrode in
`LED according to invention not only improves the light
`output power and coupling efficiency but also reduces the
`35 fabrication cost of the LED.
`FIG. 6 shows a surface emitting visible light emitting
`diode in the second preferred embodiment according to the
`invention, wherein like parts are indicated by like reference
`numerals as used in FIG. 4, so that repeated explanation is
`omitted here.
`In the second embodiment, for fabricating LED, two step
`epitaxial growth is necessary. After growing the p-type
`cladding layer S, a blocking layer 1S is grown, which is
`45 followed by the formation of a pattern thereof. The blocking
`layer 1S helps to prevent the current from crowding under
`the contact. As the blocking layer 1S, the same n-type
`materials as the cladding layer 3 can be used. After pattern-
`ing the blocking layer 1S, the current spreading layer 6 and
`high doped Ga bs cap layer 7 for p-type contact region are
`grown.
`The other process following this step is carried out as
`described in the first embodiment, so that explanation
`thereof is omitted here. Here, it should be noted that the
`p-type contact electrode 9 be made at the position just below
`the pattern of the blocking layer 1S. The use of the blocking
`layer 1S with the ring shape top electrode in the p-type
`contact region helps to enhance the light output.
`FIG. 7 shows a surface emitting visible light emitting
`diode in the third preferred embodiment according to the
`invention, wherein like parts are indicated by like reference
`numerals as used in FIGS. 4 and 6, so that repeated expla(cid:173)
`nation is omitted here.
`In the third embodiment, n-type distributed Bragg
`refiector(DBR) mirror 16 is grown on then-buffer layer prior
`to growing the cladding layer 3, active layer 4 and current
`spreading layer 6. DBR to be used as a mirror consists of
`
`30
`
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`

`5,861,636
`
`7
`semiconductor pairs 16(1) and 16(2) having high refractive
`index differences. The number and type of the pairs also
`determine the amount of the light reflected back from the
`DBR mirror 16. The semiconductor layers to be used as
`DBR should also have low absorbency with respect to the 5
`light emitting from the active region.
`FIGS. SA and 8B show a structure of the DBR layer 16
`and its approximated reflecting characteristics. As explained
`previously, the number and type of the semiconductor to be
`used in the pairs are dependent on the output light wave- 10
`length. In optical devices such as a surface emitting visible
`LED, since the emission spectra covers a broad wavelength,
`the type of a semiconductor to be used in the pairs should be
`selected such that maximum reflectivity can be obtained in
`the broad wavelength. As shown in FIG. 8B, a wide wave- 15
`length range from A.1 , to A.2 is necessary to achieve the
`maximum light reflectivity. For example, in designing an
`LED of 650 nm, the use of 15 pairs of A1As/(Al05Ga05)
`05In05P can reflect 100% light within the wavelength range
`from 590 to 710 nm. Table 1 summarizes the types and 20
`reflection characteristics of DBR pairs useful to design a
`surface emitting visible LED.
`
`pair type
`
`A1As/Al5 Ga5As
`Al5 ln5 P/
`(Al5 Ga5 ).5 ln5 P
`Al 6ln5 P/
`Ga5 ln5 P
`AlAs/
`(A1 6Ga5 ).5 ln5 P
`AlAs/GaAs
`
`TABLE 1
`
`number of
`pairs
`
`reflectivity
`(%)
`
`wavelength range
`A.,-A-2 (nm)
`
`'::';20
`'::';15
`
`'::';25
`
`'::';12
`
`'::';15
`
`99.9
`99.9
`
`99.9
`
`99.9
`
`90.0
`
`620-685
`605-700
`
`625-680
`
`590-710
`
`620-660
`
`25
`
`30
`
`35
`
`In designing DBR, the formation of the parabolic or
`graded junction to minimize the resistance is as important as
`the reflection characteristics. For making the graded DBR
`junction, superlattice or graded junction to match with the
`pairs as shown in Table 1 can be used. Following the 40
`formation of DBR 16, the other layers such as a n-type
`cladding layer 3, active layer 4, p-type cladding layer 5,
`current spreading layer 6 and p-type GaAs cap layer 7 are
`grown in a single chamber. The other process following this
`step are the same as describe