U.S. patent application number 16/660104 was filed with the patent office on 2020-04-30 for light emitting element.
This patent application is currently assigned to NICHIA CORPORATION. The applicant listed for this patent is NICHIA CORPORATION. Invention is credited to Makoto ABE.
Application Number | 20200135975 16/660104 |
Document ID | / |
Family ID | 70325524 |
Filed Date | 2020-04-30 |
United States Patent
Application |
20200135975 |
Kind Code |
A1 |
ABE; Makoto |
April 30, 2020 |
LIGHT EMITTING ELEMENT
Abstract
A light emitting element includes: an n-side semiconductor layer
made of a nitride semiconductor; a p-side semiconductor layer made
of a nitride semiconductor; and an active layer disposed between
the n-side semiconductor and the p-side semiconductor layer and
having a multi-quantum well structure in which a plurality of
nitride semiconductor well layers and a plurality of nitride
semiconductor barrier layers are alternately stacked, wherein the
light emitting element includes, between at least one of the
plurality of well layers and the barrier layer disposed adjacent
thereto on the p-side semiconductor side: a first layer and a
second layer disposed successively from the well layer side.
Inventors: |
ABE; Makoto; (Tokushima-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NICHIA CORPORATION |
Anan-shi |
|
JP |
|
|
Assignee: |
NICHIA CORPORATION
Anan-shi
JP
|
Family ID: |
70325524 |
Appl. No.: |
16/660104 |
Filed: |
October 22, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 33/32 20130101;
H01L 33/0008 20130101; H01L 33/06 20130101 |
International
Class: |
H01L 33/32 20060101
H01L033/32; H01L 33/06 20060101 H01L033/06; H01L 33/00 20060101
H01L033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 25, 2018 |
JP |
2018-200721 |
Claims
1. A light emitting element comprising: an n-side semiconductor
layer made of a nitride semiconductor; a p-side semiconductor layer
made of a nitride semiconductor; and an active layer disposed
between the n-side semiconductor and the p-side semiconductor layer
and having a multi-quantum well structure in which a plurality of
nitride semiconductor well layers and a plurality of nitride
semiconductor barrier layers are alternately stacked, wherein the
light emitting element comprises, between at least one of the
plurality of well layers and the barrier layer disposed adjacent
thereto on the p-side semiconductor side: a first layer and a
second layer disposed successively from the well layer side,
wherein a band gap of the first layer is larger than a band gap of
each of the well layers, wherein a thickness of the first layer is
lower than a thickness of each of the well layers, and wherein a
band gap of the second layer is smaller than the band gap of the
first layer and smaller than the band gap of each of the barrier
layers, wherein a thickness of the second layer is lower than the
thickness of each of the well layers.
2. The light emitting element according to claim 1, wherein the
well layers and the second layer are made of a nitride
semiconductor containing In, and wherein an In content of the
second layer is lower than an In content of each of the well
layers.
3. The light emitting element according to claim 1, wherein the
thickness of the first layer is in a range of 0.1 to 2.0 nm.
4. The light emitting element according to claim 1, wherein the
thickness of the second layer is in a range of 0.1 to 2.0 nm.
5. The light emitting element according to claim 1, wherein the
light emitting element comprises the first layer and the second
layer between each of the well layers except for a last well layer
closest to the p-side semiconductor layer, and each respective
barrier layer disposed adjacent thereto on the p-side semiconductor
layer side.
6. The light emitting element according to claim 2, wherein the
light emitting element comprises the first layer and the second
layer between each of the well layers except for a last well layer
closest to the p-side semiconductor layer, and each respective
barrier layer disposed adjacent thereto on the p-side semiconductor
layer side.
7. The light emitting element according to claim 1, wherein the
light emitting element comprises the first layer and the second
layer between each of the well layers except for a first well layer
closest to the n-side semiconductor layer, and each respective
barrier layer disposed adjacent thereto on the p-side semiconductor
layer side.
8. The light emitting element according to claim 2, wherein the
light emitting element comprises the first layer and the second
layer between every one of the well layers except a the first well
layer closest to the n-side semiconductor layer, and each
respective barrier layer disposed adjacent thereto on the p-side
semiconductor layer side.
9. The light emitting element according to claim 1, wherein the
light emitting element comprises the first layer and the second
layer between each of the well layers except for a first well layer
closest to the n-side semiconductor layer and the last well layer
closest to the p-side semiconductor layer, and each respective
barrier layer disposed adjacent thereto on the p-side semiconductor
layer side.
10. The light emitting element according to claim 2, wherein the
light emitting element comprises the first layer and the second
layer between every one of the well layers except for a first well
layer closest to the n-side semiconductor layer and the last well
layer closest to the p-side semiconductor layer, and each
respective barrier layer disposed adjacent thereto on the p-side
semiconductor layer side.
11. The light emitting element according to claim 1, wherein an In
content of a last second layer closest to the p-side semiconductor
layer is lower than an In content of any other second layer.
12. The light emitting element according to claim 2, wherein an In
content of a last second layer closest to the p-side semiconductor
layer is lower than an In content of any other second layer.
13. The light emitting element according to claim 1, wherein an In
content of an initial second layer closest to the n-side
semiconductor layer is lower than an In contents of any other
second layer.
14. The light emitting element according to claim 2, wherein an In
content of an initial second layer closest to the n-side
semiconductor layer is lower than an In contents of the other
second layer.
15. The light emitting element according to claim 1, wherein an In
content of an initial second layer closest to the n-side
semiconductor layer and an In content of a last second layer
closest to the p-side semiconductor layer are lower than an In
contents of any other second layer.
16. The light emitting element according to claim 2, wherein an In
content of an initial second layer closest to the n-side
semiconductor layer and an In content of a last second layer
closest to the p-side semiconductor layer are lower than an In
contents of any other second layer.
17. The light emitting element according to claim 1, wherein the
barrier layers and the first layers are made of a nitride
semiconductor containing Ga.
18. The light emitting element according to claim 1, wherein a
thickness of each barrier layer is in a range of 3.0 to 5.0 nm.
19. The light emitting element according to claim 1, wherein a
thickness of each well layer is lower than a thickness of each
barrier layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims priority to
Japanese Patent Application No. 2018-200721, filed on Oct. 25,
2018, the entire contents of which are incorporated herein by
reference.
BACKGROUND
[0002] The present invention relates to a light emitting
element.
[0003] Semiconductor light emitting elements such as light emitting
diodes are utilized in various applications. Depending on the
application, more stringent brightness requirements are being
requested for a light emitting element. Although raising the drive
current is the simplest way to increase the brightness, driving a
light emitting element at higher currents can induce a phenomenon
known as efficiency droop, which in turn reduces the emission
efficiency.
[0004] Japanese Patent Publication No. 2017-045798 discloses a
background technology that can reduce the droop phenomenon. This
patent publication discloses a technique to improve the
semiconductor layer structure of a semiconductor light emitting
element to thereby increase the emission efficiency of the light
emitting element.
SUMMARY
[0005] In recent years, however, there has been a need for a light
emitting element having even higher emission efficiency when driven
at higher currents. One object of the present invention is to
provide a light emitting element that demonstrates high emission
efficiency when driven at higher currents.
[0006] According to one embodiment, a light emitting element
includes an n-side semiconductor layer made of a nitride
semiconductor, a p-side semiconductor layer made of a nitride
semiconductor, and an active layer disposed between the n-side
semiconductor layer and the p-side semiconductor layer and having a
multi-quantum well structure alternately stacking a plurality of
nitride semiconductor well layers and a plurality of nitride
semiconductor barrier layers, wherein the light emitting element
has, between any one of the plurality of well layers and the
barrier layer disposed adjacent thereto on the p-side semiconductor
layer side, a first layer having a larger band gap and a lower
thickness than that of any well layer, and a second layer having a
smaller band gap than the first layer and any barrier layer and a
lower thickness than any well layer, which are disposed
successively from the well layer.
[0007] The light emitting element according to the embodiment of
the present invention can increase the emission efficiency when
driven at higher currents.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic cross-sectional view of a light
emitting element according to one embodiment.
[0009] FIG. 2 is an enlarged cross-sectional view of region A in
FIG. 1.
[0010] FIG. 3 is a schematic energy diagram showing the size of the
band gap of each semiconductor layer in range C in FIG. 2.
[0011] FIG. 4 is an enlarged cross-sectional view of region B in
FIG. 1.
[0012] FIG. 5 is a schematic cross-sectional view of a light
emitting element according to another embodiment.
[0013] FIG. 6 is a schematic cross-sectional view of a light
emitting element according to another embodiment.
[0014] FIG. 7 shows the results of measuring the output of the
light emitting element of Example 1 when a drive current of 32.5 mA
was applied.
[0015] FIG. 8 shows the results of measuring the output of the
light emitting element of Example 1 when a drive current of 120 mA
was applied.
[0016] FIG. 9 shows the results of measuring the output of the
light emitting element of Example 2 when a drive current of 32.5 mA
was applied.
[0017] FIG. 10 shows the results of measuring the output of the
light emitting element of Example 2 when a drive current of 120 mA
was applied.
[0018] FIG. 11 shows the results of measuring the effect of the
positions of the first layer and the second layer on the output of
a light emitting element.
[0019] FIG. 12 shows the results of measuring the effect of the
positions of the first layer and the second layer on the drive
voltage for a light emitting element.
DETAILED DESCRIPTION
[0020] Certain embodiments of the present invention will be
explained below based on the drawings.
First Embodiment
[0021] FIG. 1 is a schematic cross-sectional view of a light
emitting element according to one embodiment of the present
invention. FIG. 2 is an enlarged cross-sectional view of region A
in FIG. 1. FIG. 3 is a schematic energy diagram showing the size of
the band gap of each semiconductor layer in range C in FIG. 2. FIG.
4 is an enlarged cross-sectional view of region B in FIG. 1.
[0022] As shown in FIG. 1, the light emitting element 100 includes
a substrate 102, an n-side semiconductor layer 104, an active layer
106, and a p-side semiconductor layer 108. For the substrate 102,
for example, a sapphire (Al.sub.2O.sub.3) substrate, SiC substrate,
GaN substrate or the like can be used.
[0023] The n-side semiconductor layer 104 is disposed on the
substrate 102. The n-side semiconductor layer 104 is a nitride
semiconductor. For the n-side semiconductor layer 104, for example,
an n-type nitride semiconductor can be used. As one example, an
Si-added GaN layer can be formed. A buffer layer may further be
formed between the n-side semiconductor layer 104 and the substrate
102. For the buffer layer, a nitride semiconductor, such as GaN,
AlGaN or the like, can be used. The active layer 106 is disposed on
the n-side semiconductor layer 104. The p-side semiconductor layer
108 is disposed on the active layer 106. The p-side semiconductor
layer 108 is a nitride semiconductor. For the p-side semiconductor
layer 108, for example, a p-type nitride semiconductor can be used.
As one example, an Mg-added GaN layer can be formed.
[0024] As shown in FIG. 2 and FIG. 4, the active layer 106 has a
multi-quantum well structure in which a plurality of well layers
204 and a plurality of barrier layers 202 are alternately stacked.
The well layers 204 and the barrier layers 202 are nitride
semiconductors. The barrier layers 202 may be a nitride
semiconductor containing Ga. For example, the barrier layers 202
may be GaN. The thickness of a barrier layer 202 may be in a range
of 3.0 to 5.0 nm. The well layers 204 may be a nitride
semiconductor containing In. For example, the well layers 204 may
be InGaN. The thickness of a well layer 204 is preferably lower
than the thickness of a barrier layer 202.
[0025] In this embodiment, a first layer 206 and a second layer 208
are disposed between each of the well layers 204 and the barrier
layer 202 disposed adjacent thereto on the p-side semiconductor
layer 108 side. That is, as shown in FIG. 2 and FIG. 4, the first
layer 206 and the second layer 208 are disposed between each well
layer 204 and the barrier layer 202 disposed thereon (on the p-side
semiconductor layer 108 side). The first layer 206 and the second
layer 208 are disposed successively from the well layer 204
side.
[0026] As shown in FIG. 2, for the sake of convenience, the well
layer 204 closest to the n-side semiconductor layer 104 is denoted
as "the first well layer 204a," and the first layer 206 and the
second layer 208 disposed between the first well layer 204a and the
barrier layer 202 disposed thereon are denoted as "the initial
first layer 206a" and "the initial second layer 208a,"
respectively. As shown in FIG. 4, the well layer 204 closest to the
p-side semiconductor layer 108 is denoted as "the last well layer
204b," and the first layer 206 and the second layer 208 disposed
between the last well layer 204b and the barrier layer 202 disposed
thereon are denoted as "the last first layer 206b" and "the last
second layer 208b," respectively.
[0027] As shown in FIG. 3, the band gap of a first layer 206 is
larger than that of any well layer 204. The band gap of a second
layer 208 is smaller than that of any first layer 206 and any
barrier layer 202. In this embodiment, the first layers 206 and the
barrier layers 202 have the same band gap. Moreover, the second
layers 208 and the well layers 204 have the same band gap.
[0028] The thickness of a first layer 206 is lower than the
thickness of any well layer 204. The first layers 206 may be
nitride semiconductors containing Ga. For example, the first layers
206 may be GaN. The thickness of a second layer 208 is lower than
the thickness of any well layer 204. The second layers 208 are
nitride semiconductors containing In. The second layers 208 may be
InGaN. In the case of using InGaN for the second layers 208, the In
mixed crystal composition ratio can be set to a range of from 3% to
50%, preferably from 5% to 30%, more preferably 10% to 20%.
[0029] On one part of the surface of the p-side semiconductor layer
108, a p-electrode 114 is disposed to be electrically connected to
the p-side semiconductor layer 108. An n-electrode 112 is disposed
on the surface of the n-side semiconductor layer 104 exposed by
partially removing the p-side semiconductor layer 108 and the
active layer 106 to be electrically connected to the n-side
semiconductor layer 104.
[0030] In this embodiment, "a first layer 206 and a second layer
208 are disposed between each of the well layers 20 and the barrier
layer 202 disposed adjacent thereto on the p-side semiconductor
layer 108 side," but the present invention is not limited to such
an embodiment. In another embodiment of the present invention, a
first layer 206 and a second layer 208 may be disposed only between
some of the well layers 204 and the barrier layers 202 respectively
disposed adjacent thereto on the p-side semiconductor layer 108
side. For example, a first layer 206 and a second layer 208 may be
disposed between only one of the well layers 204 and the barrier
layer 202 disposed adjacent thereto on the p-side semiconductor
layer 108 side.
[0031] More carriers are present in the region of a well layer 204
on the p-side semiconductor 108 side than on the n-side
semiconductor 104 side. Accordingly, by disposing at this location
a first layer 206 having a larger band gap and lower thickness than
those of a well layer 204, and a second layer 208 having a smaller
band gap than that of a barrier layer and a lower thickness than
that of a well layer 204, electrons can be efficiently accumulated.
This is believed to facilitate efficient recombination to increase
the internal quantum efficiency to thereby increase the emission
efficiency of the light emitting element 100 even when driven at
higher currents that readily induce the droop phenomenon.
[0032] The thickness of a first layer 206 is preferably in a range
of 0.3 to 2.0 nm. This is because an excessively thick first layer
206 reduces the internal quantum efficiency, which reduces the
emission efficiency. The thickness of a second layer 208 is
preferably in a range of 0.3 to 2.0 nm. This is because an
excessively thick second layer 208 causes the layer to emit light,
generating light of a wavelength other than that intended, which
reduces the internal quantum efficiency. The In content of a second
layer 208 is preferably lower than the In content of a well layer
204. This can make the band gap of the second layer 208 larger than
that of the well layer 204 thereby restraining the emission in the
second layer 208.
[0033] Each semiconductor layer in this embodiment can be formed by
any known technique, such as metalorganic chemical vapor deposition
(MOCVD), hydride vapor phase epitaxy (HVPE), molecular beam epitaxy
(MBE), or the like. In the case of forming each semiconductor layer
by MOCVD, trimethylgallium (TMG) or triethylgallium (TEG) can be
used as the raw material for gallium, trimethylaluminum (TMA) can
be used as the raw material for aluminum, trimethylindium (TMI) can
be used as the raw material for indium, and NH.sub.3 can be used as
the raw material for nitrogen. In the case of adding Si as an
n-type impurity, silane gas can be used as the raw material, and in
the case of adding Mg as a p-type impurity, Cp.sub.2Mg (bis
cyclopentadienyl magnesium), for example, can be used as the raw
material.
[0034] The n-electrode 112 and the p-electrode 114 can be formed by
a method such as vapor deposition or sputtering. Specifically, a
resist mask having openings at the locations where the n-electrode
112 or/and the p-electrode 114 are to be formed is applied, and an
electrode material layer that can become the n-electrode 112 or/and
the p-electrode 114 is formed by vapor deposition, sputtering, or
the like. Subsequently, by removing the resist mask and the
electrode material layer formed on the resist mask by a lift-off
technique, the n-electrode 112 or/and p-electrode 114 can be
formed. For the material for the n-electrode 112 and p-electrode
114, a single metal, such as Ag, Al, Ni, Rh, Au, Cu, Ti, Pt, Pd,
Mo, Cr, W or the like, or an alloy having these metals as main
components can be used. For example, Ti and Au can be successively
stacked to form an electrode material layer.
[0035] In the description herein, expressions such as "on/thereon"
and "under/thereunder" that are used to indicate the directions or
positions of constituent elements describe the relative directions
or positions of the constituent elements in cross-sectional
diagrams, and are not intended to indicate their absolute positions
unless otherwise specifically noted.
Second Embodiment
[0036] FIG. 5 is a cross-sectional view schematically showing a
light emitting element according to a second embodiment. This
embodiment is a variation of the first embodiment. In this
embodiment, the components, members, parts, or elements having the
same functions as those in the first embodiment are denoted by the
same reference numerals, and repeated explanations may be
omitted.
[0037] This embodiment is configured such that the light emitting
element has a first layer 206 and a second layer 208 disposed
between each of the well layers 204, except for the last well layer
204b closest to the p-side semiconductor layer 108, and the barrier
layer 202 disposed adjacent thereto on the p-side semiconductor
layer 108 side. That is, as shown in FIG. 5, there is no last first
layer 206b or last second layer 208b shown in FIG. 4 disposed
between the last well layer 204b and the barrier layer 202 disposed
thereon, but there are first layers 206 and second layers 208
disposed between the other well layers 204 and the barrier layers
202 respectively disposed thereon.
[0038] The light emitting element of this embodiment is different
from the light emitting element 100 of the first embodiment (see
FIG. 4) only with respect to the absence of the last first layer
206b and the last second layer 208b, and the remaining parts are
the same as those of the light emitting element 100 of the first
embodiment. If the last first layer 206b and the last second layer
208b are disposed between the last well layer 204b closest to the
p-side semiconductor layer 108 and the barrier layer 202 disposed
thereon, the presence of the last first layer 206b and the last
second layer 208b might hinder the movement of electrons and holes.
Accordingly, not disposing the last first layer 206b and the last
second layer 208b can reduce the risk of hindering the movement of
electrons and holes, thereby further improving the emission
efficiency of the light emitting element.
Third Embodiment
[0039] FIG. 6 is a cross-sectional view schematically showing a
light emitting element according to a third embodiment. This
embodiment is a variation of the first embodiment. In this
embodiment, the components, members, parts, or elements having the
same functions as those in the first embodiment are denoted by the
same reference numerals, and repeated explanations may be
omitted.
[0040] This embodiment is configured such that the light emitting
element has a first layer 206 and a second layer 208 disposed
between each of the well layers 204, except for the first well
layer 204a closest to the n-side semiconductor layer 104, and the
barrier layer 202 disposed adjacent thereto on the p-side
semiconductor layer 108 side. That is, as shown in FIG. 6, there is
no initial first layer 206a or initial second layer 208a shown in
FIG. 2 disposed between the first well layer 204a and the barrier
layer 202 disposed thereon, but there are first layers 206 and
second layers 208 disposed between the other well layers 204 and
the barrier layers 202 respectively disposed thereon.
[0041] The light emitting element of this embodiment is different
from the light emitting element 100 of the first embodiment only
with respect to the absence of the initial first layer 206a and the
initial second layer 208a, and the remaining parts are the same as
those of the light emitting element 100 of the first embodiment. If
the initial first layer 206a and the initial second layer 208a are
disposed between the first well layer 204a closest to the n-side
semiconductor layer 104 and the barrier layer 202 disposed thereon,
the presence of the initial first layer 206a and the initial second
layer 208a might hinder the movement of electrons and holes.
Accordingly, not disposing the initial first layer 206a and the
initial second layer 208a can reduce the risk of hindering the
movement of electrons and holes, thereby further improving the
emission efficiency of the light emitting element.
Fourth Embodiment
[0042] This embodiment is a variation of the first embodiment. In
this embodiment, the components, members, parts, or elements having
the same functions as those in the first embodiment are denoted by
the same reference numerals, and repeated explanations may be
omitted.
[0043] This embodiment is configured such that the light emitting
element has a first layer 206 and a second layer 208 between each
of the well layers 204, except for the first well layer 204a
closest to the n-side semiconductor layer 104 and the last well
layer 204b closest to the p-side semiconductor layer 108, and the
barrier layer 202 disposed adjacent thereto on the p-side
semiconductor layer 108 side.
[0044] To explain using FIG. 2 and FIG. 4 that show the first
embodiment, this embodiment does not have the initial first layer
206a or the initial second layer 208a disposed between the first
well layer 204a and the barrier layer 202 disposed thereon.
Furthermore, it does not have the last first layer 206b or the last
second layer 208b disposed between the last well layer 206b and the
barrier layer 202 disposed thereon. First layers 206 and second
layers 208 are disposed between the well layers 204, excluding the
first well layer 204a and the last well layer 204b, and the barrier
layers 202 respectively disposed thereon.
[0045] The light emitting element according to this embodiment is
different from the light emitting element 100 of the first
embodiment only with respect to the absence of the initial first
layer 206a, the initial second layer 208a, the last first layer
206b, and the last second layer 208b, and the remaining parts are
the same as those of the light emitting element 100 of the first
embodiment. Disposing the initial first layer 206a, the initial
second layer 208a, the last first layer 206b and the last second
layer 208b might hinder the movement of electrons and holes.
Accordingly, not disposing the initial first layer 206a, the
initial second layer 208a, the last first layer 206b, and the last
second layer 208b can reduce the risk of hindering the movement of
electrons and holes, thereby further improving the emission
efficiency of the light emitting element.
Fifth Embodiment
[0046] This embodiment is a variation of the first embodiment. In
this embodiment, the components, members, parts, or elements having
the same functions as those in the first embodiment are denoted by
the same reference numerals, and repeated explanations may be
omitted.
[0047] This embodiment is configured such that, with reference to
FIG. 4, the In content of the last second layer 208b closest to the
p-side semiconductor layer 108 is lower than the In contents of the
other second layers 208. For example, in the case of composing the
second layers 208 with InGaN and setting the mixed crystal
composition ratio of In in the other second layers 208 as 15%, the
mixed crystal composition ratio of In in the last second layer 208b
is set to from 1% to 10%. As described earlier, if the last first
layer 206b and the last second layer 208b are disposed between the
last well layer 204b closest to the p-side semiconductor layer 108
and the barrier layer 202 disposed thereon, the presence of the
last first layer 206b and the last second layer 208b might hinder
the movement of electrons and holes. Reducing the In content of the
last second layer 208b can make the band gap of the last second
layer 208b larger than those of the other second layers 208. This
reduces the rise of the last second layer 208b hindering the
movement of electrons and holes, thereby increasing the emission
efficiency of the light emitting element.
Sixth Embodiment
[0048] This embodiment is a variation of the first embodiment. In
this embodiment, the components, members, parts, or elements having
the same functions as those in the first embodiment are denoted by
the same reference numerals, and repeated explanations may be
omitted.
[0049] This embodiment is configured such that, with reference to
FIG. 2, the In content of the initial second layer 208a closest to
the n-side semiconductor layer 104 is lower than the In contents of
the other second layers 208. For example, in the case of composing
the second layers 208 with InGaN and setting the mixed crystal
composition ratio of In in the other second layers 208 as 15%, the
mixed crystal composition ratio of In in the initial second layer
208a is set to from 1% to 10%. As described earlier, if the initial
first layer 206a and the initial second layer 208a are disposed
between the first well layer 204a closest to the n-side
semiconductor layer 104 and the barrier layer 202 disposed thereon,
the presence of the initial first layer 206a and the initial second
layer 208a might hinder the movement of electrons and holes.
Reducing the In content of the initial second layer 208a can make
the band gap of the initial second layer 208a larger than those of
the other second layers 208. This reduces the risk of the initial
second layer 208a hindering the movement of electrons and holes,
thereby increasing the emission efficiency of the light emitting
element.
Seventh Embodiment
[0050] This embodiment is a variation of the first embodiment. In
this embodiment, the components, members, parts, or elements having
the same functions as those in the first embodiment are denoted by
the same reference numerals, and repeated explanations may be
omitted.
[0051] This embodiment is configured such that, with reference to
FIGS. 2 and 4, the In contents of the initial second layer 208a
closest to the n-side semiconductor layer 104 and the last second
layer 208b closest to the p-side semiconductor layer 108 are lower
than the In contents of the other second layers 208. For example,
in the case of composing the second layers 208 with InGaN and
setting the mixed crystal composition ratio of In in the other
second layers 208 as 15%, the mixed crystal composition ratio of In
in the initial second layer 208a and the last second layer 208b is
set to from 1% to 10%. As described earlier, disposing the initial
first layer 206a, the initial second layer 208a, the last first
layer 206b and the last second layer 208b might hinder movement of
electrons and holes. Reducing the In contents of the initial second
layer 208a and the last second layer 208b can make the band gaps of
the initial second layer 208a and the last second layer 208b larger
than those of the other second layers 208. This reduces the risk of
the initial second layer 208a and the last second layer 208b
hindering the movement of electrons and holes, thereby increasing
the emission efficiency of the light emitting element.
EXAMPLE 1
[0052] A light emitting element 100 was produced as described
below.
[0053] For the substrate 102, a sapphire (C-plane) was used. In a
MOCVD reactor, the surface of the substrate 102 was cleaned in a
hydrogen ambiance at 1050.degree. C. The temperature was then
reduced to 550.degree. C. and an AlGaN buffer layer was grown on
the substrate to a thickness of about 12 nm by using TMA, TMG, and
NH.sub.3 as source gases. Then an n-side semiconductor layer 104
made of n-type GaN doped with Si at 1.times.10.sup.19/cm.sup.3 was
grown at 1150.degree. C. to a thickness of 6 .mu.m by using TMG,
TMA, ammonia, and monosilane.
[0054] The temperature was then reduced to 840.degree. C. to allow
a total of nine sets of layers, each having a GaN barrier layer 202
having a thickness of Tb (3.0 to 5.0 nm) formed by using TEG, TMI,
and ammonia as source gasses, an In.sub.0.15Ga.sub.0.85N well layer
204 having a thickness of Tw, a GaN first layer 206 having a
thickness of T1, and an In.sub.0.10Ga.sub.0.90N second layer 208
having a thickness of T2, stacked in that order, to grow. Then by
growing a GaN barrier layer 202 to a thickness of 4 nm, the active
layer 106 was formed.
[0055] Then using TMG, ammonia, and Cp.sub.2Mg, a p-side
semiconductor layer 108 made of GaN doped with Mg at
5.times.10.sup.20/cm.sup.3 was grown to a thickness of 23 nm. After
the layer was grown, the wafer was placed in a reactor and annealed
in a hydrogen ambiance at 700.degree. C. to reduce the resistance
of the p-side semiconductor layer 108.
[0056] After annealing, the surface (the electrode forming face)
for forming an n-electrode 112 was exposed by removing the p-side
semiconductor layer 108 and the active layer 106 in a region.
Finally, a p-electrode 114 and an n-electrode 112 are formed on the
surface of the p-side semiconductor layer 108 and the electrode
forming face, respectively.
[0057] The output of the light emitting element 100 of Example 1
was measured by driving it at forward currents I.sub.f of 32.5 mA
and 120 mA. The output was measured by placing the light emitting
element in an integrating sphere and driving it at a prescribed
current. The output in the central region of the substrate was
measured.
[0058] FIG. 7 shows the results of measuring the outputs of the
light emitting element of Example 1 and the reference sample of the
light emitting element described later when driven at a forward
current I.sub.f of 32.5 mA. The vertical axis of the graph in FIG.
7 represents the outputs Po of the light emitting elements.
[0059] In FIG. 7, the sample labeled "T1=0.7 nm, T2=0.6 nm" was
produced by the method described above, and in this light emitting
element 100, the thickness T1 of a first layer 206 was 0.7 nm and
the thickness T2 of a second layer 208 was 0.6 nm. In the light
emitting element 100, moreover, the thickness Tb of a barrier layer
202 was 4.3 nm and the thickness Tw of a well layer 204 was 3.2
nm.
[0060] In FIG. 7, the sample labeled "Ref" is a reference sample
that has a similar structure to that of the light emitting element
100 of Example 1 except for not including first layers 206 and the
second layers 208.
[0061] As is understood from the results shown in FIG. 7, in the
case where the forward current If was 32.5 mA, the light emitting
element 100 of Example 1 had a similar output to that of the
reference sample.
[0062] FIG. 8 shows the results of measuring the outputs of the
light emitting element 100 of Example 1 and the reference sample
when driven at a forward current I.sub.f of 120 mA. FIG. 8 shows
the results obtained by changing the forward current I.sub.f to 120
mA applied to the same light emitting elements used in the
measurements shown in FIG. 7. As is understood from the results
shown in FIG. 8, when driven at a higher current, the output of the
light emitting element 100 of Example 1 was higher than the output
of the reference sample. This is believed to be because the first
layers 206 and second layers 208 provided on the well layers 204 on
the p-side semiconductor layer 108 side in the light emitting
element 100 can efficiently accumulate electrons, facilitating
efficient recombination to increase the internal quantum efficiency
even when the light emitting element 100 is driven at a higher
current, thereby increasing the emission efficiency of the light
emitting element.
EXAMPLE 2
[0063] A light emitting element 100 of Example 2 in which the
thickness Tb of a barrier layer 202 was 4.0 nm, the thickness Tw of
a well layer 204 was 3.0 nm, and the thickness T1 of a first layer
206 and the thickness T2 of a second layer 208 were 0.5 nm, was
produced using the same method as that describe with reference to
Example 1.
[0064] FIG. 9 shows the results of measuring the output of the
light emitting element 100 of Example 2 when driven at a forward
current of 32.5 mA. In FIG. 9, "T1=T2=0.5 nm" indicates that the
thickness T1 of a first layer 206 and the thickness T2 of a second
layer 208 are 0.5 nm. The other labels in FIG. 9 have the same
meaning as those in FIG. 7. The reference sample used for this
example had the same structure as the reference sample used in
Example 1 except for having different thicknesses of the barrier
layers 202 and well layers 204. The thickness of a barrier layer
202 and the thickness of a well layer 204 of the reference sample
used for this example were 4.0 nm and 3.0 nm, respectively.
[0065] According to the results shown in FIG. 9, the output of the
light emitting element 100 of Example 2 in which the thickness Tb
of a barrier wall 202 is 4.0 nm was similar to the output of the
reference sample when the forward current I.sub.f was 32.5 mA.
[0066] FIG. 10 shows the results of measuring the output of the
light emitting element 100 of Example 2 when driven at a forward
current of 120 mA. FIG. 10 shows the results obtained by changing
the forward current I.sub.f to 120 mA applied to the same light
emitting elements used in the measurements shown in FIG. 9. As is
understood from the results shown in FIG. 10, when driven at a
higher current, the output of the light emitting element 100 in
which Tb is 4.0 nm was higher than that of the reference sample.
The results show the same tendency as that in the light emitting
element 100 of Example 1 shown in FIG. 8. That is, the emission
efficiency of the light emitting element 100 can be increased even
when driven at a higher current by disposing the first layers 206
and the second layers 208 that promote efficient recombination.
Moreover, the thickness of a well layer 204 is preferably lower
than that of a barrier layer 202.
EXAMPLE 3
[0067] A light emitting element 100 of Example 3 in which the
thickness T1 of a first layer 206 and the thickness T2 of a second
layer 208 were 0.3 nm was produced using the same method as that
described with reference to Example 1. The thickness Tb of a
barrier layer 202 in the light emitting element 100 of Example 3
was 4.3 nm, and the thickness Tw of a well layer 204 was 3.2
nm.
[0068] In order to investigate the effect of the positions of the
first layers 206 and the second layers 208, a light emitting
element was prepared in which a first layer 206 and a second layer
208 were also disposed between each of the well layers 204 and the
barrier layer 202 disposed adjacent thereto on the n-side
semiconductor layer 104 side. That is, to explain using FIG. 2,
this sample had a first layer 206 and a second layer 208 disposed
between each well layer 204 and the barrier layer 202 disposed
thereunder (on the n-side semiconductor layer 104 side). The first
layer 206 and the second layer 208 are disposed successively from
the well layer 204 side. This sample is hereinafter referred to as
the "comparative sample."
[0069] FIG. 11 shows the results of measuring the effect of the
positions of the first layers 206 and the second layers 208 on the
output of the light emitting elements. In FIG. 11, "T1=0.3 nm,
T2=0.6 nm" indicates that the thickness T1 of a first layer 206 is
0.3 nm, and the thickness T2 of a second layer 208 is 0.6 nm. In
FIG. 11, "p-Side" represents the light emitting element 100 of
Example 3, and "n-Side" represents the comparative sample. The
remaining labels in FIG. 11 mean the same as those in FIG. 7. The
reference sample used for this example has the same structure as
that of the reference sample used for Example 1.
[0070] FIG. 11 shows the output measurements obtained when driving
the light emitting element of Example 3, the comparative sample,
and the reference sample at a forward current of 20 mA. The results
show that the light emitting element 100 of Example 3 (the p-Side)
had a similar output to that of the reference sample, and the
output of the comparative sample (the n-Side) was lower than the
other two.
[0071] FIG. 12 shows the results of measuring the effect of the
positions of the first layers 206 and the second layers 208 on the
drive voltage for the light emitting elements. The vertical axis in
FIG. 12 represents drive voltage V.sub.f. That is, FIG. 12 shows
the results of measuring the drive voltage V.sub.f when driving the
light emitting element of Example 3, the comparative sample, and
the reference sample at a forward current of 20 mA. The other
labels in FIG. 12 mean the same as those in FIG. 11.
[0072] According to the results shown in FIG. 12, the light
emitting element 100 of Example 3 and the reference sample showed a
similar drive voltage, but the comparative sample showed a higher
drive voltage than the other two. This indicates that a higher
drive voltage is required to drive the comparative sample than that
to drive the light emitting element 100 and the reference sample.
Considering this in combination with the results shown in FIG. 11,
the comparative sample has a higher drive voltage requirement and a
lower output than the light emitting element 100 of Example 3 and
the reference sample. That is, the comparative light emitting
element has extremely low emission efficiency. This is believed to
be attributable to scarce carriers on the face of a well layer 204
on the n-side semiconductor layer 104 side, and even if the first
layer 206 and the second layer 208 are disposed there, they do not
function as layers for accumulating electrons, but rather generate
an unnecessary internal electric field. This is believed to
consequently hinder recombination to thereby reduce the internal
quantum efficiency. Moreover, because the light emitting element of
Example 3 has a similar structure to that of Example 1, it can
increase the emission efficiency even when driven at a higher
current.
[0073] The measurement results of the examples described above
revealed that disposing a first layer 206 and a second layer 208
between a well layer 204 and the barrier layer 202 disposed
adjacent thereto on the p-side semiconductor layer 108 side can
increase the emission efficiency of the light emitting element 100
when driven at a higher current.
[0074] Although the present invention has been explained in the
foregoing with reference to certain embodiments and examples, the
technical scope of the present invention is not limited to the
scope of the embodiments and examples. A person having ordinary
skill in the art can evidently make various modifications and
improvements to the embodiments described above. It is clear from
the scope of the claims that such modifications and improvements
will also be encompassed by the technical scope of the present
invention. For example, although the embodiments have been
explained in detail above for the purpose of making the present
invention easily understood, the present invention is not
necessarily limited to one having all of the elements described.
Moreover, some of the elements in each embodiment described can be
replaced with other elements or removed.
* * * * *