United States Patent [19]
`Nakamura et al.
`
`111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US005777350A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,777,350
`Jul. 7, 1998
`
`[54] NITRIDE SEMICONDUCTOR LIGHT(cid:173)
`EMITTING DEVICE
`
`5,652,434
`5,670,798
`
`7/1997 Nakamura et al ........................ 257n6
`9/1997 Schelzina .................................. 257/96
`
`[75]
`
`Inventors: Shoji Nakamura. Tokushima; Shinichi
`Nagahama. Komatsushima; Naruhito
`Iwasa, Tokushima; Hiroyuki Kiyoku.
`Tokushima-ken. all of Japan
`
`[73] Assignee: Nichia Chemical Industries, Ltd ..
`Japan
`
`[21] Appl. No.: 565,101
`
`[22] Filed:
`
`Nov. 30, 1995
`
`[30]
`
`Foreign Application Priority Data
`
`Dec. 2, 1994
`Dec. 2, 1994
`Dec. 22, 1994
`Feb. 23, 1995
`Mar. 16, 1995
`Mar. 16, 1995
`Apr. 14, 1995
`
`[JP]
`[JP]
`[JP]
`[JP]
`[JP]
`[JP]
`[JP]
`
`Japan .................................... 6-299446
`Japan .................................... 6-299447
`Japan .................................... 6-320100
`Japan .................................... 7-034924
`Japan .................................... 7-057050
`Japan .................................... 7-057051
`Japan .................................... 7-089102
`
`Int. Cl.6
`..................................................... BOlL 33/00
`[51]
`[52] U.S. Cl .................................. 257/96; 257n6; 257/97;
`257/103; 257/13; 372/45
`[58] Field of Search .................................. 257n6. 94. 96,
`257/97, 14. 13. 103; 372143. 45. 46
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`FOREIGN PATENT DOCUMENTS
`
`6-21511
`
`l/1994
`
`Japan .
`
`OTHER PUBUC.'ITIONS
`
`Jpn. J. Appl. Phys. vol. 34(1995) pp. L797-L799.
`Jpn. J. Appl. Phys. vol. 34(1995) pp. L1332-L1335.
`Appl. Phys. Lett. 67(13). 25 Sep. 1995.
`
`Primary EXaminer-Mahshid D. Saadat
`Assistant Examiner-John Guay
`Attorney, Agent, or Firm-Nixon & Vanderhye
`
`[57]
`
`ABSTRACT
`
`A nitride semiconductor light-emitting device has an active
`layer of a single-quantum well structure or multi-quantum
`well made of a nitride semiconductor containing indium and
`gallium. A first p-type clad layer made of a p-type nitride
`semiconductor containing aluminum and gallium is pro(cid:173)
`vided in contact with one surface of the active layer. A
`second p-type clad layer made of a p-type nitride semicon(cid:173)
`ductor containing aluminum and gallium is provided on the
`first p-type clad layer. The second p-type clad layer has a
`larger band gap than that of the first p-type clad layer. An
`n-type semiconductor layer is provided in contact with the
`other surface of the active layer.
`
`5,602.418
`
`2/1997 lmai et al.
`
`.. ............................ 257/627
`
`21 Claims, 6 Drawing Sheets
`
`tOO
`~
`
`I.IJ
`>t-
`-:::>
`~0.
`~5
`0::0
`
`50
`
`i2
`
`tt
`
`tOO
`
`~
`
`I.IJ
`
`:::: .....
`t-:::>
`<10.
`...JI-
`1.1.1:::>
`a::o
`
`50
`
`50
`THICKNESS OF WELL LAYER
`
`tOO
`!A)
`
`0
`
`Vizio EX1012 Page 0001
`
`

`
`U.S. Patent
`
`Jul. 7, 1998
`
`Sheet 1 of 6
`
`5,777,350
`
`12
`
`H
`
`3t
`
`F I G. 1
`
`F I G. 3
`
`F I G. 2
`
`Vizio EX1012 Page 0002
`
`

`
`U.S. Patent
`
`Jul. 7, 1998
`
`Sheet 2 of 6
`
`5,777,350
`
`it -
`
`F I G. 4
`
`O.OOi
`
`0.01
`
`1.0
`0. t
`THICKNESS
`
`( ~m l
`
`100
`
`~
`0
`
`uJ
`~ ......
`r-::::>
`_. ......
`<lQ.
`u.J:::> a:o
`
`50
`
`F I G. 5
`
`0.001
`
`0.01
`
`1.0
`0.1
`THICKNESS
`
`( ~ml
`
`- tOO
`
`~ 0
`
`uJ
`>r-
`-::::>
`~Q_
`_J .....
`LLJ;:,
`0::0
`
`50
`
`F I G. 6
`
`50
`THICKNESS OF WELL LAYER
`
`100
`0
`(A)
`
`Vizio EX1012 Page 0003
`
`

`
`U.S. Patent
`
`Jul. 7, 1998
`
`Sheet 3 of 6
`
`5,777,350
`
`-::-!
`
`0
`
`100
`
`UJ ::1-
`
`t-::>
`c:tQ..
`-II-
`UJ::> o:o
`
`50
`
`F I G. 7
`
`tOO
`THICKNESS OF BARRIER LAYER
`
`200
`0
`{A l
`
`::-!
`0
`
`tOO
`
`l.LJ
`~~--
`t-::>
`<t Q..
`-II-
`UJ ::::>
`0:::0
`
`50
`
`F I G. 8
`
`PRIOR
`ART
`
`saw
`
`MOW
`
`POSITIVE
`ELECTRODE
`i29
`
`i26----~~~~~~
`t24
`
`NEGATIVE
`ELECTRODE
`23
`p:f;~:¢~~~~~'-422
`
`t2t
`
`F I G. 9
`
`Vizio EX1012 Page 0004
`
`

`
`U.S. Patent
`
`Jul. 7, 1998
`
`Sheet 4 of6
`
`5,777,350
`
`138
`
`436
`
`137 NEGATIVE
`ELECTRODE
`t33
`J:::s::j:::P=~~d::s:~C::S:~- t 32
`
`158
`
`t56 ~~~~~~
`
`F I G. 10
`
`i53
`
`F I G.
`
`t t
`
`158
`156
`
`F I G.
`
`t2
`
`POSITIVE
`ELECTRODE
`i59
`157
`
`NEGATIVE
`ELECTRODE
`
`i53
`•52
`
`t51
`
`Vizio EX1012 Page 0005
`
`

`
`U.S. Patent
`
`Jul. 7, 1998
`
`Sheet 5 of 6
`
`5,777,350
`
`ACTIVE
`LAYER
`
`SUBSTRATE
`
`F I G.
`
`t 3
`
`POSITIVE
`ELECTRODE
`
`248
`
`246..-.._..~~~~~~
`
`2 i 4 ....-~....__;:,.__;:...~-+--~
`
`42
`
`POSITIVE
`ELECTRODE
`
`F I G.
`
`t4
`
`F I G.
`
`'5
`
`Vizio EX1012 Page 0006
`
`

`
`U.S. Patent
`
`Jul. 7, 1998
`
`Sheet 6 of 6
`
`5,777,350
`
`NEGATIVE
`ELECTRODE
`313
`l;::o:±:s:~~¢~~~,..._ 312
`3H
`
`F I G.
`
`i6
`
`~ ......
`............
`
`..... .................
`p
`,,
`l
`--------- --------
`
`a
`
`300
`200
`tOO
`THICKNESS OF WELL LAYER
`F I G. 17
`
`0
`(A)
`
`(nm)
`560
`:I:
`540
`~
`<.!) z
`520
`uJ __.
`uJ >
`500
`<(
`~
`~ 480
`<l
`uJ 460
`a..
`z
`440
`0
`(f) 420
`
`(f) -~ 400
`
`IJ.J
`
`380
`
`Vizio EX1012 Page 0007
`
`

`
`5.777.350
`
`1
`NITRIDE SEMICONDUCTOR LIGHT(cid:173)
`EMITTING DEVICE
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`The present invention relates to a semiconductor light(cid:173)
`emitting device such as a light-emitting diode (LED) or a
`laser diode (LD), and in particular to a light-emitting device
`having a semiconductor structure formed of all nitride
`semiconductor materials.
`2. Description of the Related Art
`As materials for a light-emitting device such as an LED
`or LD device which is expected of emitting a light ranging
`from ultraviolet to red, nitride semiconductor materials
`(In,.AlyGa 1 _ _._yN; O;§;x, O;§;y, x+y;§;l) are known, and blue
`and blue-green LEDs have been put to practical use in, for
`example, displays or signals.
`The light-emitting device such as a blue LED or a
`blue-green LED formed of nitride semiconductor materials
`and now actually used has a double-heterostructure. This
`light-emitting device is fundamentally constructed such that
`an n-type contact layer consisting of an n-type GaN. an
`n-type clad layer consisting of an n-type AlGaN. an n-type
`active layer consisting of an n-type loGaN. a p-type clad
`layer consisting of a p-type AlGaN and a p-type contact
`layer consisting of a p-type GaN are superimposed on a
`substrate made for example of sapphire in the order men(cid:173)
`tioned. The active layer is doped with a donor impurity such
`as Si or Ge and/or an acceptor impurity such as Mn or Mg. 30
`The light emission wavelength of the LED device can be
`changed from the ultraviolet region to the red region by
`varying the content of In in the composition of lnGaN of the
`active layer or by changing the kind of impurities to be
`doped into the active layer. The LED now put to practical 35
`use is an LED having an emission wavelength of 510 nm or
`less. with its active layer doped with both of donor and
`acceptor impurities. This LED is has a buffer layer formed
`of. for example, GaN or AlN between the substrate and the
`n-type contact layer.
`On the other hand. there have been many proposals on a
`structure of an LD device to date. For example, Unexamined
`Japanese Patent Application Publication (Kokai) 6-21511
`discloses a separation confinement type LD having a struc- 45
`ture wherein an active layer consisting of InGaN and having
`a thickness of not more than 100 angstroms is interposed
`between an n-type GaN layer and a p-type GaN layer, and
`the resultant composite is further sandwiched between an
`n-type AlGaN layer and a p-type AlGaN layer. Both of the 50
`AlGaN layers functioning as a light confinement layer.
`Through the realization of a double-heterostructure in an
`LED device as explained above. it has become possible to
`improve the light-emitting output. thus enabling the LED
`device to be actually used. However, since an lnGaN layer
`doped with impurities is utilized as an active layer in this
`LED device. this LED device is accompanied with a draw(cid:173)
`back that the half band width of emission spectrum can not
`avoid from becoming wide. For example. a full color display
`provided with an LED device having a luminescent spec- 60
`trum of wide half band width will exhibit a whitish lumi(cid:173)
`nescent light, thus narrowing the color-reproducing region
`thereof.
`As for LD device. it is theoretically possible to realize a
`laser oscillation in a double-heterostructure comprising an 65
`active layer formed of a non-doped InGaN as described in
`the above-mentioned Unexamined Japanese Patent Applica-
`
`2
`tion Publication 6-21511. but the laser oscillation is not yet
`realized with such a double-heterostructure. It is expected to
`greatly improve the emission output by making the active
`layer into a quantum well structure as described in this
`5 Publication. However, there are many problems to be
`solved, such as the preparation of an optical resonance
`surface or of an optical confinement layer, before the LD
`device can be actually realized.
`In order to realize an LD device. the active layer thereof
`10 is required to exhibit a sharp and strong band-to-band
`emission. Even with an LED device, it is possible te obtain
`an emission spectrum having a narrow half band width.
`provided a band-to-band emission can be realized. However.
`the thickness of the active layer in the conventional LED
`15 device is relatively thick. i.e., 0.1 to 0.2 j.lm. thus the
`thickness of lnGaN layer hetero-epitaxially grown on an
`AlGaN layer already exceeds over the critical thickness, so
`that it was impossible with the conventional LED device to
`realize a strong band-to-band emission, thus failing to real-
`20 ize a laser oscillation. Meanwhile, if an LED device is
`formed into a quantum well structure by greatly thinning the
`active layer as shown in the above-mentioned Unexamined
`Japanese Patent Application Publication 6-21511. it may be
`possible to obtain a strong band-to-band emission. However,
`25 if the thickness of the active layer is thinned. the light
`confinement may then become insufficient thus making it
`impossible to realize a laser oscillation.
`
`40
`
`SUMMARY OF THE INVENTION
`Therefore, a first object of the present invention is to
`provide a nitride semiconductor light-emitting device which
`is capable of generating an excellent laser oscillation.
`A second object of the present invention is to provide a
`nitride semiconductor light-emitting device which is capable
`of exhibiting an improved emission output.
`These and other objects which will become apparent from
`the following detailed description have been achieved
`according to the present invention by a nitride semiconduc(cid:173)
`tor light-emitting device comprising:
`an active layer of a quantum well structure comprising a
`nitride semiconductor containing indium and gallium,
`and having first and second main surfaces;
`a first p-type clad layer comprising a p-type nitride
`semiconductor containing aluminum and gallium, and
`provided in contact with the second main surface of the
`active layer;
`a second p-type clad layer comprising a p-type nitride
`semiconductor containing aluminum and gallium, hav(cid:173)
`ing a larger band gap than that of the first p-type clad
`layer, and provided on the first p-type clad layer; and
`an n-type semiconductor layer provided in contact with
`the first main surface of the active layer.
`According to the present invention. there is further pro-
`55 vided a nitride semiconductor light-emitting device com(cid:173)
`prising:
`an active layer comprising of a quantum well structure
`comprising a nitride semiconductor containing indium
`and gallium. and having first and second main surfaces;
`a first n-type clad layer made of an n-type nitride semi(cid:173)
`conductor containing aluminum and gallium or of an
`n-type GaN. and provided in contact with the first main
`surface of the active layer, the first n-type clad layer
`having a thickness within a range of 10 angstroms to
`1.0 j.!ID;
`a second n-type clad layer comprising an n-type nitride
`semiconductor having a larger band gap than that of the
`
`Vizio EX1012 Page 0008
`
`

`
`5.777.350
`
`5
`
`3
`first n-type clad layer. and provided on the first n-type
`clad layer; and
`a p-type semiconductor layer provided in contact with the
`second main surface of the active layer.
`According to the present invention. there is further pro-
`vided a nitride semiconductor light-emitting device com(cid:173)
`prising:
`an active layer of a quantum well structure comprising a
`nitride semiconductor containing indium and gallium.
`and having first and second main surfaces;
`a first n-type clad layer made of an n-type nitride semi(cid:173)
`conductor containing aluminum and gallium. or of an
`n-type GaN. and provided in contact with the first main
`surface of the active layer;
`a second n-type clad layer comprising an n-type nitride
`semiconductor having a larger band gap than that of the
`first n-type clad layer. and provided on the first n-type
`clad layer;
`a first p-type clad layer comprising a p-type nitride
`semiconductor containing aluminum and gallium. and
`provided in contact with the second main surface of the
`active layer: and
`a second p-type clad layer comprising a p-type nitride
`semiconductor containing aluminum and gallium. hav(cid:173)
`ing a larger band gap than that of the first p-type clad
`layer. and provided on the first p-type clad layer.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com(cid:173)
`prising an active layer of a quantum well structure compris(cid:173)
`ing a nitride semiconductor containing indium and gallium
`and interposed between an n-type nitride semiconductor
`layer and a p-type semiconductor layer. the p-type semicon(cid:173)
`ductor layer including a p-type clad layer provided in
`contact with the active layer. the p-type clad layer compris(cid:173)
`ing a p-type nitride semiconductor containing aluminum and
`gallium and having a thickness within a range of 10 ang- 35
`stroms to 1.0 Jllll·
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com(cid:173)
`prising an active layer of quantum well structure interposed
`between an n-type nitride semiconductor layer and a p-type 40
`semiconductor layer. the active layer comprising a nitride
`semiconductor containing indium and gallium. and provided
`with a well layer having a thickness of not more than 70
`angstroms.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com- 45
`prising an active layer of a quantum well structure having
`first and second main surfaces. and comprising a nitride
`semiconductor containing indium and gallium; and a first
`n-type clad layer comprising an n-type nitride semiconduc(cid:173)
`tor containing indium and gallium.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com(cid:173)
`prising an active layer of a quantum well structure having
`first and second main surfaces. and comprising a nitride
`semiconductor containing indium and gallium; and a first 55
`p-type clad layer comprising a p-type nitride semiconductor
`containing indium and gallium.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com(cid:173)
`prising:
`an active layer comprising a nitride semiconductor con(cid:173)
`taining indium and gallium. and having first and second
`main surfaces;
`a first n-type clad layer comprising an n-type nitride
`semiconductor not containing aluminum. and provided 65
`in contact with the first main surface of the active layer;
`and
`
`4
`a p-type clad layer comprising a p-type nitride semicon(cid:173)
`ductor and having a surface region. at least the surface
`region comprising a p-type nitride semiconductor con(cid:173)
`taining aluminum and gallium. the p-type clad layer
`being provided in contact with the second main surface
`of the active layer.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com(cid:173)
`prising an active layer of a quantum well structure compris-
`10 ing a nitride semiconductor; an negative electrode; a positive
`electrode; an n-type GaN contact layer provided in contact
`with the negative electrode; and a p-GaN contact layer
`provided in contact with the positive electrode.
`According to the present invention. there is further pro-
`15 vided a nitride semiconductor light-emitting device com(cid:173)
`prising an active layer having first and second main surfaces.
`and comprising a nitride semiconductor containing indium
`and gallium; and a first n-type clad layer comprising an
`n-type nitride semiconductor containing indium and
`20 gallium. having a larger band gap than the active layer. and
`being provided in contact with the first main surface of the
`active layer.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com-
`25 prising an active layer having first and second main surfaces.
`and comprising a nitride semiconductor containing indium
`and gallium; and a first p-type clad layer comprising a p-type
`nitride semiconductor containing indium and gallium. hav(cid:173)
`ing a larger band gap than the active layer. and being
`30 provided in contact with the second main surface of the
`active layer.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com-
`prising:
`an active layer comprising a nitride semiconductor con(cid:173)
`taining indium and gallium. and having first and second
`main surfaces;
`a first n-type clad layer comprising an n-type nitride
`semiconductor containing indium and gallium. having
`a larger band gap than that of the active layer. and
`provided in contact with the first main surface of the
`active layer;
`a first p-type clad layer comprising a p-type nitride
`semiconductor containing indium and gallium. having
`a larger band gap than that of the active layer. and
`provided in contact with the second main surface of the
`active layer;
`a second n-type clad layer comprising an n-type nitride
`semiconductor containing aluminum and gallium. hav(cid:173)
`ing a larger band gap than that of the first n-type clad
`layer. and provided in contact with the first n-type clad
`layer; and
`a second p-type clad layer comprising a p-type nitride
`semiconductor containing aluminum and gallium. hav(cid:173)
`ing a larger band gap than that of the first p-type clad
`layer. and provided on the first p-type clad layer.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com-
`60 prising an active layer comprising a nitride semiconductor
`containing at least indium and interposed between a first
`n-type clad layer comprising an n-type nitride semiconduc(cid:173)
`tor having a smaller thermal expansion coefficient than that
`of the active layer and a first p-type clad layer comprising a
`p-type nitride semiconductor having a smaller thermal
`expansion coefficient than that of the active layer. wherein
`the active layer is of a single-quantum well structure or of a
`
`50
`
`Vizio EX1012 Page 0009
`
`

`
`5.777.350
`
`6
`FIG. 15 is a cross-sectional view schematically illustrat(cid:173)
`ing a structure of a light-emitting device according to a still
`further embodiment of the present invention;
`FIG. 16 is a cross-sectional view schematically illustrat-
`ing a structure of a light-emitting device according to a ninth
`embodiment of the present invention; and
`FIG. 17 is a graph showing a relationship between the
`thickness of a well layer and the emission peak wavelength.
`
`5
`
`5
`multi-quantum well structure. thereby to emit a light of
`lower energy than the inherent band gap energy of the nitride
`semiconductor forming the active layer.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com(cid:173)
`prising a first n-type clad layer made of an n-type nitride
`semiconductor containing indium or of an n-type GaN; and
`an active layer comprising a nitride semiconductor contain(cid:173)
`ing indium. having a larger thermal expansion coefficient
`than that of the first n-type clad layer and provided in contact 10
`with the first n-type clad layer. wherein the active layer is of
`a single-quantum well structure or of a multi-quantum well
`structure. thereby to emit a light of lower energy than the
`inherent band gap energy of the nitride semiconductor
`forming the active layer.
`According to the present invention. there is further pro(cid:173)
`vided a nitride semiconductor light-emitting device com(cid:173)
`prising an active layer comprising a nitride semiconductor
`containing indium; and a first p-type clad layer comprising
`a p-type nitride semiconductor containing aluminum. having 20
`a smaller thermal expansion coefficient than that of the
`active layer and provided in contact with the active layer
`wherein the active layer is of a single-quantum well struc(cid:173)
`ture or of a multi-quantum well structure. thereby to emit a
`light of lower energy than the inherent band gap energy of 25
`the nitride semiconductor forming the active layer.
`
`15
`
`DErAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`In the following description. In,.Ga 1_.fi (O<X<l) is some(cid:173)
`times referred to simply as InGaN. Likewise. (O<y<l) is
`sometimes referred to simply as AlyGai->N.
`According to a first embodiment of the present invention.
`there is provided a nitride semiconductor light-emitting
`device provided with an active layer interposed between an
`n-type nitride semiconductor layer and a p-type nitride
`semiconductor layer. the active layer being formed of a
`nitride semiconductor containing indium and gallium. and
`constructed into a quantum well structure (single-quantum
`well or multi-quantum well structure). The above mentioned
`p-type nitride semiconductor layer comprises. as mentioning
`from the active layer side. a first p-type clad layer formed of
`a p-type nitride semiconductor containing aluminum and
`gallium. and a second p-type clad layer having a larger
`band-gap than that of the first p-type clad layer and formed
`of a p-type nitride semiconductor containing aluminum and
`30 gallium.
`FIG. 1 shows a cross-sectional view schematically illus(cid:173)
`trating a structure of a light-emitting device (LD structure)
`according to the first embodiment of the present invention.
`Referring to FIG. 1. the light-emitting device comprises a
`35 substrate 11 on which an n-type contact layer 12. an n-type
`clad layer 13. an active layer 14. a first p-type clad layer 61.
`a second p-type clad layer 62 and a p-type contact layer 15
`are superimposed in the mentioned order. On the surface of
`the p-type contact layer 15. there is formed a current-
`40 contracting layer 16 formed of an insulating material and
`having an opening 16a formed therein. On the surface of this
`current-contracting layer 16 is formed a positive electrode
`(p-electrode) 17 connected to the p-type contact layer 15
`through the opening 16a. On the other hand. a negative
`45 electrode (n-electrode) 18 is formed on the surface of the
`n-type contact layer 12. In the case of an LED device. the
`positive electrode 17 is directly formed on the p-type contact
`layer 15. without forming the current-contracting layer 16.
`The substrate 11 may be made of sapphire (including the
`5o C-plane, R-plane and A-plane thereof). SiC (including
`6H-SiC and 4H-SiC). Si. ZnO. GaAs. spinel (MgA120 4 •
`particularly (111) plane). and a monocrystalline oxide hav(cid:173)
`ing a lattice constant which is close to that of the nitride
`semiconductor may be employed. Among them. sapphire
`55 and SiC are generally employed. Although a buffer layer is
`not specifically shown in FIG. 1. a buffer layer formed of
`GaN or AlN several hundred angstroms in thickness is often
`formed between the substrate and the nitride semiconductor
`for the purpose of relieving the mismatching of lattice
`60 constants of these materials. Since. however. this buffer
`layer can be omitted if the substrate is formed of SiC or ZnO
`whose lattice constant is very close to that of the nitride
`semiconductor. the buffer layer is not shown in FIG. 1.
`The n-type contact layer 12 may be formed of a n-type
`65 nitride semiconductor. If it is formed of a binary or ternary
`mixed crystal such as GaN or AlGaN. a contact layer of
`excellent crystallinity can be obtained. If the n-type contact
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a cross-sectional view schematically illustrating
`a structure of a light-emitting device according to a first
`embodiment of the present invention;
`FIG. 2 is a cross-sectional view schematically illustrating
`a structure of a light-emitting device according to a second
`embodiment of the present invention;
`FIG. 3 is a cross-sectional view schematically illustrating
`a structure of a light-emitting device according to a third
`embodiment of the present invention;
`FIG. 4 is a graph showing a relationship between the
`thickness of a first p-type clad layer and the emission output
`according to a first embodiment of the present invention;
`FIG. 5 is a graph showing a relationship between the
`thickness of a first n-type clad layer and the emission output
`according to a second embodiment of the present invention;
`FIG. 6 is a graph showing a relationship between the
`thickness of a well layer and the emission output;
`FIG. 7 is a graph showing a relationship between the
`thickness of a barrier layer and the emission output;
`FIG. 8 is a graph showing the output of a light-emitting
`device of the present invention in comparison with that of
`the conventional light-emitting device;
`FIG. 9 is a cross-sectional view schematically illustrating
`a structure of a light-emitting device according to a fifth
`embodiment of the present invention;
`FIG. 10 is a cross-sectional view schematically illustrat(cid:173)
`ing a structure of a light-emitting device according to a sixth
`embodiment of the present invention;
`FlG. 11 is a cross-sectional view schematically illustrat(cid:173)
`ing a structure of a light-emitting device according to a
`seventh embodiment of the present invention;
`FlG. 12 is a cross-sectional view schematically illustrat(cid:173)
`ing a structure of a light-emitting device according to a still
`further embodiment of the present invention;
`FIG. 13 is a oblique view of the structure of FlG. 12;
`FIG. 14 is a cross-sectional view schematically illustrat(cid:173)
`ing a structure of a light-emitting device according to a
`eighth embodiment of the present invention;
`
`Vizio EX1012 Page 0010
`
`

`
`5.777.350
`
`7
`layer 12 is formed of GaN in particular. an excellent ohmic
`contact with a negative electrode material can be achieved.
`A preferable n-conductivity can be obtained by doping the
`contact layer with a donor impurity such as Si. Ge or S. As
`for the materials for the negative electrode 18, the use of a 5
`metallic material containing both Ti and Au. or both Ti and
`AI is preferable.
`The n-type clad layer 13 may be formed of a p-type nitride
`semiconductor. If it is formed of a binary or ternary mixed
`crystal such as GaN. AIGaN or InGaN. a clad layer of 10
`excellent crystallinity can be obtained. If the n-type clad
`layer 13 is formed of InGaN or GaN in particular. the
`formation of a relatively thin active layer 14 can be made
`possible. so that an active layer of a single-quantum well
`(SQW) or a multi-quantum well (MQW) structure will be 15
`realized, thus greatly improving the output of the light(cid:173)
`emitting device. It is possible to omit the n-type clad layer
`13 in an LED device. The n-type clad layer 13 should
`desirably be formed in the case of an LED device to a
`thickness ranging from 10 angstroms to 1.0 J.lm. preferably 20
`from 30 angstroms to 0.1 J.lffi. while in the case of an LD
`device to a thickness ranging from 100 angstroms to 1.0 J.liD.
`The active layer 14 should most preferably be formed of
`a non-doped InGaN (no impurity is doped). thereby allow(cid:173)
`ing an emission of 660 nm to 365 nm to be obtained through 25
`a band-to-band emission. In order to prepare an InGaN
`active layer of excellent crystallinity having a thickness
`which is sufficiently thin enough to provide a single(cid:173)
`quantum well or multi-quantum well structure, it is very
`preferable to form in advance, as an n-type clad layer 13. an 30
`InGaN layer having a larger band gap than the active layer
`14 that will be subsequently grown on this InGaN layer.
`When the active layer 14 is of an SQW or MQW structure,
`a device of very high emission output can be obtained. By
`the expressions of SQW and MQW structures. it is meant a 35
`structure of active layer from which a light emission of
`inter-quantum level through a non-doped In_.Ga1 _ _.N com(cid:173)
`position can be obtained. For example. an active layer of the
`SQW structure is constituted by a single layer (well layer)
`formed of a single composition of In_.Ga1 _ _.N (O<x<l). and 4D
`the clad layers 13 and 61 sandwiching the active layer
`constitute barrier layers. When the thickness of this In .. Ga 1_
`..N layer is 100 angstroms or less. the active layer undergoes
`a plastic deformation. thus making it possible to obtain a
`strong light emission of inter-quantum level. On the other 45
`hand. an active layer of the MQW structure is formed of a
`laminate of well and barrier layers. alternately stacked. The
`well and barrier layers may consist of In .. Ga1_fl layer (in
`this case, x may be from 0 to 1) different in composition
`from each other. In an active layer of the MQW structure. the so
`two outermost thin films of the active layer constitute well
`layers. When the active layer is of an SQW or MQW
`structure, it is possible to obtain a visible LED of high output
`of inter-quantum level light emission having a wavelength
`ranging from about 365 nm to 660 nm. When the thickness 55
`of the well layer of the quantum well structure is not more
`than 70 angstroms, preferably not more than 50 angstroms,
`a light-emitting device of high emission output will be
`obtained. This in turn indicates that this degree of thickness
`is not more than the critical thickness of the InGaN active 60
`layer. Since the Bohr radius of electron in the loGaN is about
`30 angstroms, the quantum effect of the InGaN is obtained
`when the thickness is 70 angstroms or less.
`Likewise. it is preferable in the case of multi-quantum
`well structure to adjust the thickness of the well layer to 70 65
`angstroms or less. and to adjust the thickness of the barrier
`layer to 150 angstroms or less.
`
`8
`Next. the first p-type clad layer 61 also featuring the first
`embodiment of the present invention will be explained
`below.
`This first p-type clad layer 61 is formed of a p-type nitride
`semiconductor containing AI and Ga. and most preferably is
`formed of a ternary mixed crystal of AIGaN. This first p-type
`clad layer 61 functions as a light-guiding layer in an LD
`device. Namely. in the case of an LD device according to the
`first embodiment of the present invention. if the thickness of
`the active layer 14 is made sufficiently thin to provide a
`quantum well structure, the confinement of light within the
`active layer 14 may become insufficient. Therefore. this first
`p-type clad layer 61 is provided for functioning it as a
`light-guiding layer for confining light. Moreover. the AIGaN
`layer is most preferable because it can be easily made into
`a p-type layer of high carrier concentration. and at the same
`time is suited for enlarging a difference in band offset or
`refractive index thereof relative to the InGaN active layer 14
`as compared with other kinds of nitride semiconductors.
`Additionally. as compared with other kinds of nitride
`semiconductors. a p-type AIGaN is less susceptible to
`decomposition during the growth thereof. so that it has an
`effect of inhibiting the decomposition of loGaN of the active
`layer as it is grown by way of the MOVPE method for
`example. Because of this. an active layer of excellent
`crystallinity can be obtained. thus improving the output
`thereof. Under the circumstances. the first p-type clad layer
`should most preferably be made of AIGaN. On the other
`hand. if the first p-type clad layer is formed of a p-type GaN.
`the emission output will be decreased to '/' of the case where
`the p-type AIGaN is employed. The reason of this phenom(cid:173)
`enon may be ascribed to the facts that GaN is less likely to
`be turned into p-type as compared with AIGaN and that GaN
`is more likely to be deco