U.S. patent application number 09/952845 was filed with the patent office on 2002-05-16 for nitride semiconductor light emitting device and apparatus including the same.
Invention is credited to Ito, Shigetoshi, Taneya, Mototaka, Tsuda, Yuhzoh, Yuasa, Takayuki.
Application Number | 20020056846 09/952845 |
Document ID | / |
Family ID | 18818983 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020056846 |
Kind Code |
A1 |
Tsuda, Yuhzoh ; et
al. |
May 16, 2002 |
Nitride semiconductor light emitting device and apparatus including
the same
Abstract
A nitride semiconductor light emitting device includes a worked
substrate including grooves and lands formed on a main surface of a
nitride semiconductor substrate, a nitride semiconductor underlayer
covering the grooves and the lands of the worked substrate and a
nitride semiconductor multilayer emission structure including an
emission layer including a quantum well layer or both a quantum
well layer and a barrier layer in contact with the quantum well
layer between an n-type layer and a p-type layer over the nitride
semiconductor underlayer, while the width of the grooves is within
the range of 11 to 30 .mu.m and the width of the lands is within
the range of 1 to 20 .mu.m.
Inventors: |
Tsuda, Yuhzoh; (Nara,
JP) ; Yuasa, Takayuki; (Nara, JP) ; Ito,
Shigetoshi; (Nara, JP) ; Taneya, Mototaka;
(Nara, JP) |
Correspondence
Address: |
Thomas E. Ciotti
Morrison & Foester LLP
755 Page Mill Rd.
Palo Alto
CA
94301-1018
US
|
Family ID: |
18818983 |
Appl. No.: |
09/952845 |
Filed: |
September 11, 2001 |
Current U.S.
Class: |
257/86 ; 257/87;
257/E21.119; 257/E21.131 |
Current CPC
Class: |
H01L 21/0254 20130101;
H01L 21/02389 20130101; H01L 21/02505 20130101; H01S 5/0207
20130101; H01L 21/02647 20130101; H01S 5/0021 20130101; H01L
21/0262 20130101; H01L 33/32 20130101; H01L 33/20 20130101; H01L
21/02458 20130101; H01S 5/3203 20130101; H01S 5/32341 20130101 |
Class at
Publication: |
257/86 ;
257/87 |
International
Class: |
H01L 027/15; H01L
031/12; H01L 033/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 13, 2000 |
JP |
2000-344847 |
Claims
What is claimed is:
1. A nitride semiconductor light emitting device comprising: a
worked substrate including grooves and lands formed on a main
surface of a nitride semiconductor substrate; a nitride
semiconductor underlayer covering said grooves and said lands of
said worked substrate; and a nitride semiconductor multilayer
emission structure including an emission layer including a quantum
well layer or both a quantum well layer and a barrier layer in
contact with said quantum well layer between an n-type layer and a
p-type layer over said nitride semiconductor underlayer, wherein
width of said grooves is within the range of 11 to 30 .mu.m, and
width of said lands is within the range of 1 to 20 .mu.m.
2. The nitride semiconductor light emitting device according to
claim 1, wherein the width of said grooves is larger than the width
of said lands.
3. The nitride semiconductor light emitting device according to
claim 1, wherein depth of said grooves is within the range of 1 to
10 .mu.m.
4. The nitride semiconductor light emitting device according to
claim 1, wherein the longitudinal direction of said grooves or the
longitudinal direction of said lands is substantially parallel to a
<1-100>direction of a crystal of said substrate.
5. The nitride semiconductor light emitting device according to
claim 1, wherein the longitudinal direction of said grooves or the
longitudinal direction of said lands is substantially parallel to a
<11-20>direction of a crystal of said substrate.
6. The nitride semiconductor light emitting device according to
claim 1, wherein said nitride semiconductor underlayer contains
Al.
7. The nitride semiconductor light emitting device according to
claim 1, wherein said nitride semiconductor underlayer contains
In.sub.xGa.sub.1-xN (0.01.ltoreq.x.ltoreq.0.18).
8. The nitride semiconductor light emitting device according to
claim 1, wherein said quantum well layer contains at least any of
As, P and Sb.
9. The nitride semiconductor light emitting device according to
claim 1, wherein said nitride semiconductor light emitting device
is either a laser device or a diode device.
10. An optical apparatus comprising the nitride semiconductor light
emitting device according to claim 1.
11. A semiconductor light emitting apparatus comprising the nitride
semiconductor light emitting device according to claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a nitride semiconductor
light emitting device having an improved emission life and an
apparatus including the same.
[0003] 2. Description of the Prior Art
[0004] Japanese Patent Laying-Open No. 2000-124500 discloses a
technique of forming convex portions on a GaN layer stacked on a
sapphire substrate, flatly covering the convex portions with a GaN
underlayer and forming a gallium nitride semiconductor laser device
on the GaN covering layer for improving the emission
characteristics of a nitride semiconductor light emitting device.
According this gazette, the distance between adjacent ones of the
convex portions is preferably within the range of 1 to 10 .mu.m,
the width of the upper surfaces of the convex portions is
preferably at least 1 .mu.m, and the height of the convex portions
is preferably within the range of 0.1 to 2 .mu.m. This gazette also
describes that the sapphire substrate may be replaced with a GaN
substrate.
[0005] Even if the sapphire substrate is replaced with a GaN
substrate in the nitride semiconductor laser device according to
the prior art, however, the oscillation life is still
insufficient.
SUMMARY OF THE INVENTION
[0006] Accordingly, a principal object of the present invention is
to provide a nitride semiconductor light emitting device having a
long oscillation life.
[0007] According to the present invention, the nitride
semiconductor light emitting device includes a worked substrate
including grooves and lands formed on a main surface of a nitride
semiconductor substrate, a nitride semiconductor underlayer
covering the grooves and the lands of the worked substrate and a
nitride semiconductor multilayer emission structure including an
emission layer including a quantum well layer or both a quantum
well layer and a barrier layer in contact with the quantum well
layer between an n-type layer and a p-type layer over the nitride
semiconductor underlayer, while width of the grooves is within the
range of 11 to 30 .mu.m and width of the lands is within the range
of 1 to 20 .mu.m.
[0008] The width of the grooves is preferably larger than the width
of the lands, and the depth of the grooves is preferably within the
range of 1 to 10 .mu.m.
[0009] The longitudinal direction of the grooves or the
longitudinal direction of the lands is preferably substantially
parallel to the <1-100> direction or the <11-20>
direction of a crystal of the substrate.
[0010] The nitride semiconductor underlayer preferably contains Al.
Further, the nitride semiconductor underlayer preferably contains
In.sub.xGa.sub.1. .sub.xN(0.01.ltoreq.x.ltoreq.0.18).
[0011] The quantum well layer preferably contains at least any of
As, P and Sb.
[0012] The aforementioned nitride semiconductor light emitting
device can be either a laser device or a diode device. Further,
such a light emitting device can be preferably employed in an
optical apparatus or a semiconductor light emitting apparatus.
[0013] The foregoing and other objects, features, aspects and
advantages of the present invention will become more apparent from
the following detailed description of the present invention when
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic sectional view showing an exemplary
nitride semiconductor laser device formed on a covered worked
substrate according to the present invention;
[0015] FIG. 2A is a schematic sectional view showing an exemplary
nitride semiconductor worked substrate employable in the present
invention, and FIG. 2B is a top plan view thereof;
[0016] FIG. 3 is a schematic sectional view showing an exemplary
covered worked substrate employable in the present invention;
[0017] FIG. 4A illustrates grooves having two types of directions
perpendicular to each other in relation to modes of grooves
(concave portions) and lands (convex portions) formed on the worked
substrate employable in the present invention, FIG. 4B illustrates
grooves having two types of directions intersecting with each other
at an angle of 60.degree., and FIG. 4C illustrates grooves having
three types of directions intersecting with each other at an angle
of 60.degree.;
[0018] FIG. 5 is a graph showing a groove-land width range A
required in the worked substrate employed in the present
invention;
[0019] FIG. 6 is a graph showing the relation between a groove
depth in the worked substrate employed in the present invention and
the oscillation life of a laser device obtained through the worked
substrate;
[0020] FIG. 7 illustrates the relation between a position for
forming a ridge stripe portion of a nitride semiconductor laser
device formed on the covered worked substrate employable in the
present invention and the laser oscillation life;
[0021] FIG. 8 is a schematic sectional view showing preferable
areas for forming a light emitting device structure on the covered
worked substrate employable in the present invention;
[0022] FIG. 9 is a schematic sectional view showing another
exemplary covered worked substrate employable in the present
invention;
[0023] FIG. 10 is a schematic sectional view showing still another
exemplary covered worked substrate employable in the present
invention;
[0024] FIG. 11 is a schematic sectional view showing a further
exemplary covered worked substrate employable in the present
invention;
[0025] FIG. 12A is a schematic sectional view showing an exemplary
nitride semiconductor laser device having a ridge stripe structure,
and FIG. 12B is a schematic sectional view showing an exemplary
nitride semiconductor laser device having a current blocking layer
structure; and
[0026] FIG. 13 is a schematic block diagram showing an exemplary
optical apparatus including an optical pickup device utilizing the
nitride semiconductor laser device according to the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] Before describing various embodiments according to the
present invention, the meanings of some terms are now
clarified.
[0028] First, the term "grooves" stands for striped concave
portions formed on a main surface of a nitride semiconductor
substrate, and the term "lands" similarly stands for striped convex
portions (see FIGS. 2A and 2B, for example). The grooves and the
lands may not necessarily have rectangular sectional shapes as
shown in FIG. 2A, but may simply define steps of unevenness. In the
accompanying drawings, lengths, widths, thicknesses and depths are
arbitrarily changed for simplifying and clarifying the drawings,
and do not show actual dimensional relation.
[0029] The term "nitride semiconductor substrate" stands for a
substrate containing Al.sub.xGa.sub.yIn.sub.zN(0.ltoreq.x.ltoreq.1;
0.ltoreq.y.ltoreq.1; 0.ltoreq.z.ltoreq.1; and x+y+z=1). Not more
than about 10% of the nitrogen element contained in this nitride
semiconductor substrate may be replaced with at least one of As, P
and Sb (on the premise that the hexagonal crystal system of the
substrate is maintained). At least any of impurities Si, O, Cl, S,
C, Ge, Zn, Cd, Mg and Be may be added to the nitride semiconductor
substrate. In order to provide the nitride semiconductor substrate
with n-type conductivity, Si, O and Cl are particularly preferable
among these impurities.
[0030] The term "nitride semiconductor underlayer" stands for a
nitride semiconductor film covering a worked substrate, which
contains Al.sub.xGa.sub.yIn.sub.zN (1.ltoreq.x.ltoreq.1;
0.ltoreq.y.ltoreq.1; 0.ltoreq.z.ltoreq.1; and x+y+z=1). Similarly
to the case of the nitride semiconductor substrate, not more than
about 10% of the nitrogen element contained in this nitride
semiconductor underlayer may be replaced with at least one of As, P
and Sb, and at least any of impurities Si, O, Cl, S, C, Ge, Zn, Cd,
Mg and Be may be added to the nitride semiconductor underlayer.
[0031] The term "worked substrate" stands for a nitride
semiconductor substrate having a main surface formed with grooves
and lands. The widths of the grooves and the lands may have a
constant periodicity or may be different from each other. All
grooves may have a constant depth, or the grooves may have
different depths.
[0032] The term "emission layer" stands for a layer including at
least one quantum well layer or a plurality of barrier layers
alternately stacked with such a quantum well layer. An emission
layer of a single quantum well structure is formed by only a single
well layer or a barrier layer, a well layer and a barrier layer
stacked with one another. An emission layer of a multilayer quantum
well structure includes a plurality of well layers and a plurality
of barrier layers alternately stacked with one another, as a matter
of course.
[0033] The term "light emitting device structure" stands for a
structure including the emission layer as well as an n-type layer
and a p-type layer holding the same therebetween. The term "covered
worked substrate" stands for an improved substrate including the
worked substrate and the underlayer consisting of a nitride
semiconductor film covering the same (see FIG. 3).
[0034] [First Embodiment]
[0035] The worked substrate of a nitride semiconductor employed in
the present invention includes grooves and lands formed on a main
surface thereof. The width G of the grooves formed on the worked
substrate must be within the range of 11 to 30 .mu.m, and the width
L of the lands must be within the range of 1 to 20 .mu.m. The
restrictive range of such widths of the grooves and the lands is
hereinafter referred to as a groove-land width range A. According
to the present invention, the characteristics of the nitride
semiconductor light emitting device are improved by employing a
nitride worked substrate having the groove-land width range A, so
that the life of a nitride semiconductor laser device can be
increased or the emission intensity of a nitride semiconductor
light emitting diode device can be improved, for example.
[0036] The groove-land width range A is effective only in a worked
substrate consisting of a nitride semiconductor. This is because a
nitride semiconductor underlayer grown on a worked substrate
consisting of a material other than the nitride semiconductor
(hereinafter referred to as a hetero worked substrate) is subjected
to stress strain stronger than that applied to a nitride
semiconductor underlayer grown on the worked substrate of the
nitride semiconductor. Even if a hetero worked substrate having the
groove-land width range A is prepared, crystal strain in a nitride
semiconductor underlayer covering the hetero worked substrate is
not relaxed dissimilarly to the case of employing the worked
substrate of the nitride semiconductor.
[0037] (As to Groove-land Width Range A)
[0038] The inventors have investigated influences exerted by the
groove width G and the land width L on the laser oscillation life.
FIG. 5 shows the results. Referring to FIG. 5, the horizontal axis
shows the land width L (.mu.m) and the vertical axis shows the
groove width G (.mu.m). Black circles show laser oscillation lives
of at least 10000 hours, black squares show laser oscillation lives
of at least 5000 hours and less than 10000 hours, white circles
show laser oscillation lives of at least 1000 hours and less than
5000 hours, white squares show laser oscillation lives of at least
500 hours and less than 1000 hours, and crosses show laser
oscillation lives of less than 500 hours. The structure of nitride
semiconductor laser devices subjected to this measurement and a
method of manufacturing the same were similar to those of nitride
semiconductor laser devices according to a sixth embodiment of the
present invention described later.
[0039] As understood from FIG. 5, the groove width G and the land
width L substantially satisfying laser oscillation lives of at
least 5000 hours (black circles and black squares) of the nitride
semiconductor laser device were at least about 11 .mu.m and not
more than 30 .mu.m and at least about 1 .mu.m and not more than 20
.mu.m (within the groove-land width range A in FIG. 5)
respectively. More strictly, it was possible to prepare a nitride
semiconductor laser device having a laser oscillation life of at
least 5000 hours even when the groove width G was larger than 30
.mu.m. If the groove width G exceeds 30 .mu.m, however, the grooves
are hardly completely filled up with the nitride semiconductor
underlayer. When the grooves are hardly covered with the nitride
semiconductor underlayer, the area allowing formation of the
nitride semiconductor laser device is narrowed unpreferably in
consideration of an acquisition ratio of nitride semiconductor
laser devices per wafer.
[0040] When the widths of the grooves and the lands formed on the
worked substrate were within the groove-land width range A and
belonged to the upper left area of FIG. 5 divided by a straight
line A, the acquisition ratio of nitride semiconductor laser device
chips having laser oscillation lives of at least 5000 hours (black
circles and black squares) was increased. This proves that it is
important to increase the groove width G beyond the land width L
for improving the laser oscillation life.
[0041] When the widths of the grooves and the lands formed on the
worked substrate were within the groove-land width range A and the
land width L was not more than 10 .mu.m, the acquisition ratio of
nitride semiconductor laser device chips having laser oscillation
lives of at least 10000 hours (black circles) was increased
much.
[0042] The inventors have further discovered that a cracking ratio
in the laser device chip can also be suppressed when the widths of
the grooves and the lands formed on the worked substrate belong to
the groove-land width range A. In general, it has been regarded
that a nitride semiconductor laser device formed on a GaN substrate
is hardly cracked. In practice, however, a conventional GaN
substrate is remarkably cracked after formation of a nitride
semiconductor laser device thereon. This is conceivably because the
nitride semiconductor laser device is formed by a multilayer
structure of various layers (for example, an AlGaN layer has a
smaller lattice constant than a GaN layer, and an InGaN layer has a
larger lattice constant than the GaN layer). It is also conceivable
that such cracking is influenced by latency of residual strain in
the GaN substrate obtained by the current technique itself.
[0043] When a nitride semiconductor laser device was formed on a
covered worked substrate (see FIG. 3) structured by the worked
substrate and the nitride semiconductor underlayer according to the
present invention, however, crack density was within the range of 0
to 3/cm.sup.2. When a nitride semiconductor laser device was formed
on a conventional GaN substrate, crack density was within the range
of about 5 to 8/cm.sup.2.
[0044] Thus, not only the laser oscillation life was improved but
also the crack density was suppressed according to the present
invention, and hence the long life of the nitride semiconductor
laser device has probably been obtained by an effect of relaxing
crystal strain in the nitride semiconductor crystal. Further, the
life of the nitride semiconductor laser device was remarkably
increased when the groove width G was increased beyond the land
width L, and hence the effect of relaxing crystal strain is
conceivably attained mainly by the grooves.
[0045] The aforementioned effects of increasing the life and
reducing the crack density according to the present invention are
not restricted to a nitride semiconductor laser device but
similarly attained also in a nitride semiconductor light emitting
diode device, as a matter of course.
[0046] (As to Groove Depth)
[0047] FIG. 6 shows the relation between groove depths H and laser
oscillation lives. The structure of nitride semiconductor laser
devices subjected to measurement in FIG. 6 and a method of
manufacturing the same were similar to those of the sixth
embodiment described later, and the groove width G and the land
width L were 18 .mu.m and 7 .mu.m respectively, i.e., within the
groove-land width range A.
[0048] As understood from FIG. 6, the laser oscillation life
started to increase as the groove depth H exceeded about 1 .mu.m.
When the groove depth H exceeded 2 .mu.m, the laser oscillation
life further increased and thereafter reached a substantially
constant saturation value. While the upper limit of the groove
depth H related to improvement of the laser oscillation life is not
particularly restricted, the grooves are hardly covered with the
nitride semiconductor underlayer if the groove depth H exceeds
about 10 .mu.m. If the grooves are hardly covered with the nitride
semiconductor underlayer, the area allowing formation of the
nitride semiconductor laser device is reduced unpreferably in
consideration of the acquisition ratio of nitride semiconductor
laser device chips per wafer. In the present invention, therefore,
the groove depth H is preferably at least 1 .mu.m and not more than
10 .mu.m, and more preferably at least 2 .mu.m and not more than 10
.mu.m.
[0049] The relation between the groove depth H and the laser
oscillation life exhibited a tendency similar to that in FIG. 6 so
far as the groove-land width range A was satisfied. The
aforementioned relation between the groove depth H and the emission
life is not restricted to the nitride semiconductor laser device
but is also applicable to a nitride semiconductor light emitting
diode device.
[0050] (Position for Forming Emission Part)
[0051] The inventors have made deep study to discover that the
laser oscillation life varies with the position of an emission part
(below a ridge stripe portion) of the nitride semiconductor laser
device formed on the covered worked substrate.
[0052] Referring to a graph shown in FIG. 7, the horizontal axis
shows the distance along the width direction between a ridge stripe
edge a and a groove center c of the covered worked substrate, and
the vertical axis shows the laser oscillation life. The distance
(hereinafter referred to as a c-to-a distance) between the groove
center c and the ridge stripe edge a is positive on the right side
of the groove center c and negative on the left side of the groove
center c along the width direction. The structure of nitride
semiconductor laser devices subjected to measurement in FIG. 7 and
a method of manufacturing the same were similar to those of the
sixth embodiment described later. The ridge stripe width, the
groove width G, the land width L and the groove depth H were 2
.mu.m, 18 .mu.m, 8 .mu.m and 2.5 .mu.m, respectively.
[0053] As shown in FIG. 7, the laser oscillation life of a nitride
semiconductor laser device having a ridge stripe portion formed
above the groove exhibited a tendency exceeding that of a nitride
semiconductor laser device having a ridge stripe portion formed
above the land. As a result of more detailed investigation, it has
been proven that the laser oscillation life is remarkably reduced
when the ridge stripe portion is formed in an area having a c-to-a
distance larger than -3 .mu.m and smaller than 1 .mu.m above the
groove. Considering that the width of the ridge stripe portion is 2
.mu.m, a c-to-a distance of -3 .mu.m corresponds to -1 .mu.m in
terms of the distance between the groove center c and another ridge
stripe edge b (hereinafter referred to as a c-to-b distance). In
other words, it has been proven that the laser oscillation life is
remarkably reduced when the ridge stripe portion of the nitride
semiconductor laser device is formed to be at least partially
included in an area of less than 1 .mu.m on the right and the left
of the groove center c along the width direction.
[0054] The area (in the range of less than 1 .mu.m on the right and
the left of the groove center c along the width direction) where
the laser oscillation life is remarkably reduced is referred to as
an area III. Therefore, the ridge stripe portion of the nitride
semiconductor laser device is preferably formed to be entirely (a-b
width) included in a range excluding the area III. In the range of
the groove width G, the range of at least 1 .mu.m on the right and
the left of the groove center c along the width direction is
referred to as an area I. This area I allows formation of a nitride
semiconductor laser device having a longer laser oscillation life
as compared with an area II described below.
[0055] Description similar to that for the areas above the groove
is applicable to areas above the land. When the ridge stripe
portion of the nitride semiconductor laser device was formed on an
area presenting a c-to-a distance larger than 10 .mu.m and smaller
than 14 .mu.m, the laser oscillation life was remarkably reduced.
Considering that the width of the ridge stripe portion is 2 .mu.m,
a c-to-a distance of -10 .mu.m corresponds to -1 .mu.m in terms of
the distance (hereinafter referred to as a d-to-b distance) between
the land center d and the ridge stripe edge b. Similarly, a d-to-a
distance of 14 .mu.m corresponds to -1 .mu.m in terms of the
distance (hereinafter referred to as a d-to-a distance) between the
land center d and the ridge stripe edge a. In other words, it has
been proven that the laser oscillation life is remarkably reduced
when the ridge stripe portion of the nitride semiconductor laser
device is formed to be at least partially included in an area of
less than 1 .mu.m on the right and the left of the land center d
along the width direction.
[0056] The area (in the range of less than 1 .mu.m on the right and
the left of the land center d along the width direction) where the
laser oscillation life is remarkably reduced is referred to as an
area IV. Therefore, the ridge stripe portion of the nitride
semiconductor laser device is preferably formed to be entirely (a-b
width) included in a range excluding the area IV. In the range of
the land width L, a range of at least 1 .mu.m on the right and the
left of the land center d along the width direction is referred to
as the area II. A nitride semiconductor laser device having a ridge
stripe portion formed on this area II exhibited a sufficient laser
oscillation life of several 1000 hours, although this life was
shorter than that in the aforementioned case of the area I.
[0057] FIG. 8 is a schematic diagram showing the aforementioned
results, i.e., the aforementioned areas I to IV on the covered
worked substrate according to the present invention. In the covered
worked substrate according to the present invention, the ridge
stripe portion of the nitride semiconductor laser device is
preferably formed at least on an area (the area I or II) excluding
the areas III and IV, most preferably formed on the area I and
next-preferably formed on the area II.
[0058] It is inferred from the above results that, when the worked
substrate having the groove width G and the land width L according
to the present invention is covered with the underlayer of the
nitride semiconductor film (i.e., on the covered worked substrate),
areas of the nitride semiconductor underlayer located on the
grooves of the worked substrate have a larger effect of relaxing
crystal strain as compared with areas of the underlayer located on
the lands.
[0059] A nitride semiconductor laser device formed on a covered
worked substrate having a groove width G and a land width L
belonging to the groove-land width range A according to the present
invention can obtain the aforementioned relation between the
position for forming the ridge stripe portion and the laser
oscillation life. Also when the width of the ridge stripe portion
is other than 2 .mu.m, a tendency similar to the relation shown in
FIG. 7 is attained.
[0060] The aforementioned relation between the position for forming
the ridge stripe portion and the laser oscillation life is not
restricted to a nitride semiconductor laser device having a ridge
stripe structure shown in a schematics sectional view of FIG. 12A,
for example. In a nitride semiconductor laser device having a
current blocking structure shown in a schematic sectional view of
FIG. 12B, for example, the aforementioned ridge stripe portion
corresponds to a current narrowing part of the laser device, and
the ridge stripe width corresponds to the width of the current
narrowing part. In more general expression, the effects according
to the present invention are attained when an emission part
(substantial current injection part of an emission layer)
contributing to laser oscillation of the nitride semiconductor
laser device is present on the area I and/or the area II shown in
FIG. 8.
[0061] In practice, however, the laser oscillation life of the
nitride semiconductor laser device having a current blocking
structure was lower by about 20 to 30% as compared with the nitride
semiconductor laser device having the aforementioned ridge stripe
structure. In the nitride semiconductor laser device having a
current blocking structure, further, the yield was remarkably
reduced by cracking as compared with the nitride semiconductor
laser device having a ridge stripe structure. While the causes
therefor are uncertain, a step of growing a nitride semiconductor
crystal on a current blocking layer formed with a current narrowing
part is conceivably problematic. In the step of growing the nitride
semiconductor crystal on the current blocking layer, the wafer is
temporarily taken out from a crystal growth apparatus (to normal
temperature) to prepare the current narrowing part thereon in the
process of preparing a light emitting device structure and
thereafter the wafer is introduced into the crystal growth
apparatus again to grow the remaining emission structure at about
1000.degree. C., for example. When a heat history including abrupt
temperature change is applied in an intermediate stage of forming
the light emitting device structure, it is conceivable that crystal
strain in the light emitting device structure is not sufficiently
relaxed but cracking takes place even in the nitride semiconductor
laser device according to the present invention.
[0062] (As to Longitudinal Direction of Groove)
[0063] The longitudinal direction of grooves formed on a nitride
semiconductor substrate having a main surface of a {0001} C-plane
is most preferably parallel to a <1-100>direction, and
next-preferably parallel to a <11-20>direction. The
longitudinal direction of the grooves related to such specific
directions is not substantially influenced by a deviation angle of
about .+-.5.degree. in the {0001} C-plane.
[0064] The preference of formation of the grooves along the
<1-100>direction of the nitride semiconductor substrate
resides in extremely large effects of relaxing crystal strain and
suppressing cracking. When a nitride semiconductor film is grown in
grooves formed along this direction, a {11-20} plane mainly grows
on the side wall surfaces of the grooves, which in turn are covered
with the nitride semiconductor film. The {11-20} plane is
perpendicular to the main surface of the substrate, and hence the
grooves are covered with the nitride semiconductor film while
presenting substantially rectangular cross sections. In other
words, the nitride semiconductor film hardly grows on the bottom
surfaces of the grooves, which in turn are covered from the side
walls thereof. The nitride semiconductor film attains sufficient
growth in a direction parallel to the main surface of the substrate
(hereinafter referred to as lateral growth), to conceivably bring
the very large effects of relaxing crystal strain and suppressing
cracking. When the nitride semiconductor film hardly grows on the
bottom surfaces of the grooves, the lateral growth is promoted
keeping a large depth of the grooves (close to the depth of grooves
formed on a worked substrate), while the grooves being covered with
the lateral growth, to preferably increase the crystal area having
the effects of relaxing crystal strain and suppressing
cracking.
[0065] Further, the aforementioned specific longitudinal direction
of the grooves can increase the lateral growth in combination with
the groove width G within the groove-land width range A, thereby
more efficiently attaining the effects of relaxing crystal strain
and suppressing cracking.
[0066] On the other hand, the preference of formation of grooves
along the <11-20>direction of the nitride semiconductor
device resides in that the nitride semiconductor film filling up
the grooves exhibits good surface morphology on areas located on
the grooves. When the nitride semiconductor film grows in the
grooves formed along this direction, a {1-101} plane mainly grows
on the side wall surfaces of the grooves, which in turn are covered
with the nitride semiconductor film. The {1-101} side wall surfaces
are extremely flat, and edge portions where the side wall surfaces
are in contact with upper surfaces of lands are very sharp.
Therefore, the grooves formed along the <11-20>direction are
covered with the nitride semiconductor film in a hardly meandering
state as viewed from above, as shown in FIG. 2B. The nitride
semiconductor film exhibits excellent surface morphology on the
areas located on the grooves covered in such a manner. When an
underlayer consisting of the nitride semiconductor film has
excellent surface morphology, the defective ratio of nitride
semiconductor light emitting devices formed on the underlayer is
preferably reduced.
[0067] While all of the aforementioned grooves or lands are
striped, the striped shape is preferable in the following point.
When a part (below a ridge stripe portion) contributing to
oscillation of a nitride semiconductor laser device is striped and
the aforementioned preferable area (the area I or II) for forming
the ridge stripe portion is also striped, the part contributing to
oscillation can be readily formed on the preferable area I or II.
Alternatively, cross-striped grooves may be formed as shown in
FIGS. 4A to 4C, in place of the striped grooves or lands.
[0068] FIG. 4A is a top plan view of a worked substrate, formed
with two different types of grooves perpendicular to each other,
having concave and convex portions. FIG. 4B is a top plan view of a
worked substrate, formed with two different types of grooves
intersecting with each other at an angle of 60.degree., having
concave and convex portions. FIG. 4C is a top plan view of a worked
substrate, formed with three different types of grooves
intersecting with each other at an angle of 60.degree., having
concave and convex portions.
[0069] (As to Nitride Semiconductor Underlayer)
[0070] The underlayer consisting of a nitride semiconductor film
for covering the worked substrate can be formed by a GaN film, an
AlGaN film or an InGaN film, for example.
[0071] A nitride semiconductor underlayer of a GaN film is
preferable in the following points: The GaN film of a binary mixed
crystal has excellent controllability for crystal growth. Further,
the surface migration length of the GaN film is larger than that of
an AlGaN film and smaller than that of an InGaN, and hence proper
lateral growth can be attained to completely and flatly cover
grooves and lands.
[0072] A nitride semiconductor underlayer of an AlGaN film is
preferable in the following points: The AlGaN film containing Al
has a smaller surface migration length as compared with a GaN film
and an InGaN film. The nitride semiconductor film having a small
surface migration length is hardly deposited on the bottom portions
of the grooves while laterally covering the grooves. In other
words, crystal growth of the AlGaN film is promoted from the side
walls of the grooves to remarkably present lateral growth, to be
capable of further relaxing crystal strain. The composition ratio x
of Al contained in an Al.sub.xGa.sub.1-xN film is preferably at
least 0.01 and not more than 0.15, and more preferably at least
0.01 and not more than 0.07. If the composition ratio x of Al is
smaller than 0.01, the aforementioned surface migration length may
be undesirably increased. If the composition ratio x of Al exceeds
0.15, the surface migration length may be so excessively reduced
that the grooves are hardly flatly filled up with the underlayer.
An effect similar to that of the AlGaN film can be attained so far
as the nitride semiconductor underlayer contains Al.
[0073] A nitride semiconductor underlayer of an InGaN film is
preferable in the following points. The InGaN film containing In is
more elastic as compared with a GaN film and an AlGaN film.
Therefore, the InGaN film fills up the grooves of the worked
substrate to diffuse crystal strain from the nitride semiconductor
substrate over the nitride semiconductor film and relax difference
in strain between the areas located on the grooves and the lands.
The In composition ratio x in an In.sub.xGa.sub.1-xN film is
preferably at least 0.01 and not more than 0.18, and more
preferably at least 0.01 and not more than 0.1. If the In
composition ratio x is smaller than 0.01, the effect of elasticity
due to In may be hardly attained. If the In composition ratio x
exceeds 0.18, crystallinity of the InGaN film may be reduced. An
effect similar to that of the InGaN film can be attained so far as
the nitride semiconductor underlayer contains In.
[0074] (As to Thickness of Nitride Semiconductor Underlayer)
[0075] In order to completely cover the worked substrate, the
nitride semiconductor film forming the underlayer must have a
sufficient thickness. In order not to completely cover the worked
substrate, the nitride semiconductor film forming the underlayer
must have a small thickness. In order to solve problems related to
the present invention, the worked substrate need not necessarily be
covered completely with the nitride semiconductor film. In
consideration of the acquisition ratio of a light emitting device
chip, however, the worked substrate is preferably covered
completely with the nitride semiconductor underlayer. Therefore,
the thickness of the nitride semiconductor film is preferably about
at least 2 .mu.m and not more than 20 .mu.m. If the thickness of
the nitride semiconductor film is smaller than 2 .mu.m, it starts
to be difficult to completely and flatly fill up the grooves with
the nitride semiconductor film, depending on the width and the
depth of the grooves formed on the worked substrate. If the
thickness of the nitride semiconductor film is larger than 20
.mu.m, vertical growth (perpendicular to the main surface of the
substrate) on the worked substrate may gradually become remarkable
as compared with lateral growth, leading to a possibility of
insufficient effects of relaxing crystal strain and suppressing
cracking.
[0076] (As to Method of Verifying Worked substrate)
[0077] In order to confirm whether or not a covered worked
substrate includes a worked substrate having the groove width G and
the land width L according to the present invention in a nitride
semiconductor light emitting device including a light emitting
device structure grown on the covered worked substrate, the light
emitting device structure may be partially or entirely ground with
an apparatus such as a grinder and the device may be observed with
a cathode luminescence (CL) device. According to a result of CL
measurement made by the inventors, grooves formed on a nitride
semiconductor substrate (worked substrate) were observed as a
pattern of bright and dark stripes. The bright and dark stripes
corresponded to grooves and lands formed on the worked substrate,
and it was possible to measure the widths of the grooves and the
lands formed on the worked substrate by measuring the widths of the
stripes. According to a result of deep study made by the inventors,
the bright stripes corresponded to the grooves, and the dark
stripes corresponded to the lands.
[0078] In place of partially or entirely grinding the light
emitting device structure with an apparatus such as a grinder, the
substrate of the nitride semiconductor light emitting device may be
partially ground with an apparatus such as a grinder. When the
ground surface of the device is observed with a CL device, a result
of observation similar to the above can be obtained.
[0079] [Second Embodiment]
[0080] A method of preparing a covered worked substrate according
to a second embodiment of the present invention is described with
reference to FIG. 3. Items not particularly mentioned in relation
to this embodiment are similar to those of the first
embodiment.
[0081] FIG. 3 is a schematic sectional view showing a covered
worked substrate covered with an underlayer of a nitride
semiconductor film, which can be prepared as follows. A dielectric
film of SiO.sub.2 or SiN.sub.x is first deposited on the main
surface, oriented along the (0001) plane, of an n-type GaN
substrate. A general resist material is applied onto this
dielectric film for forming a striped mask pattern by lithography.
Along this mask pattern, grooves are formed on the n-type GaN
substrate through the dielectric film by dry etching. Thereafter
the resist material and the dielectric film are removed for
preparing a worked substrate. The grooves and lands formed in the
aforementioned manner along the <1-100>direction of the
n-type GaN substrate present a groove width of 17 .mu.m, a groove
depth of 3 .mu.m and a land width of 8 .mu.m. Alternatively, a
low-temperature GaN buffer layer may be formed on the n-type GaN
substrate having the main surface oriented along the (0001) plane
at a relatively low temperature of about 450 to 600.degree. C., in
order to form an n-type GaN layer on the low-temperature GaN buffer
layer and thereafter prepare a worked substrate by the
aforementioned method.
[0082] The prepared worked substrate is subjected to sufficient
organic cleaning and thereafter introduced into an MOCVD
(metal-organic chemical vapor deposition) apparatus for stacking an
underlayer consisting of a GaN film having a thickness of 6 .mu.m
thereon. In order to form the GaN underlayer, NH.sub.3 (ammonia) as
a source for a group V element and TMGa (trimethyl gallium) or TEGa
(triethyl gallium) as a source for a group III element are supplied
onto the worked substrate set in the MOCVD apparatus, and SiH.sub.4
(Si impurity concentration: .times.10.sup.18/cm.sup.3) is added to
the source materials at crystal growth temperature of 1050.degree.
C. Under such growth conditions, portions located on the grooves
and the lands are flatly covered with the underlayer of the GaN
film, as shown in FIG. 3.
[0083] In order to form the grooves and the lands on the nitride
semiconductor substrate, a general resist material may be directly
applied to the surface of the nitride semiconductor substrate
without through the aforementioned dielectric film, followed by a
process similar to the above. According to an experiment made by
the inventors, however, damage (particularly on the surfaces of the
lands) on the substrate was preferably reduced during formation of
the grooves in the case that the resist material was applied
through the dielectric film.
[0084] In this embodiment, the low-temperature GaN buffer layer may
be a low-temperature Al.sub.xGa.sub.1-xN buffer layer
(0.ltoreq.x.ltoreq.1), or the low-temperature buffer layer may be
omitted. However, a currently supplied GaN substrate is not
sufficiently preferable in surface morphology, and hence the
low-temperature Al.sub.xGa.sub.1-xN buffer layer is preferably
inserted in consideration of improvement of the surface morphology.
The term "low-temperature buffer layer" stands for a buffer layer
formed at a growth temperature of about 450 to 600.degree. C., as
hereinabove described. A buffer layer formed in such a relatively
low growth temperature range is polycrystalline or amorphous.
[0085] The grooves, formed by dry etching in this embodiment, may
alternatively be formed by another method. For example, wet
etching, scribing, wire sawing, electric discharge machining,
sputtering, laser beam machining, sandblasting, focus ion beam
machining or the like is employable.
[0086] The grooves, formed along the <1-100>direction of the
n-type GaN substrate in this embodiment, may alternatively be
formed along the <11-20>direction.
[0087] While the GaN substrate has the main surface along the
(0001) plane in this embodiment, another surface orientation or
another nitride semiconductor substrate may alternatively be
employed. As to the surface orientation of the nitride
semiconductor substrate, the C-plane {0001}, the A-plane {11-20},
the R-plane {1-102}, the M-plane {1-100} or the {1-101} plane is
preferably employable. A substrate having a main surface at an off
angle within 2.degree. from such surface orientation has good
surface morphology.
[0088] As to the width and the depth of the grooves formed on the
worked substrate and the width of the lands in this embodiment,
other numerical values may be employed so far as the same satisfy
the conditions for the numerical ranges described above with
reference to the first embodiment. This also applies to the
remaining embodiments.
[0089] [Third Embodiment]
[0090] A third embodiment of the present invention is similar to
the first and second embodiments except that the widths of lands
formed on a worked substrate are set not to a constant value but to
various different values.
[0091] FIG. 9 is a schematic sectional view showing a covered
worked substrate according to this embodiment, which has a groove
width G1 of 15 .mu.m, a groove depth H1 of 2.5 .mu.m and land
widths L1 and L2 of 5 .mu.m and 10 .mu.m respectively. An AlGaN
film having a thickness of 5 .mu.m is stacked on this worked
substrate, to prepare the covered worked substrate according to the
third embodiment.
[0092] While the worked substrate according to this embodiment has
two types of different land widths, the worked substrate may
alternatively have more different land widths.
[0093] [Fourth Embodiment]
[0094] A fourth embodiment of the present invention is similar to
the first and second embodiments except that the widths of grooves
formed on a worked substrate are set not to a constant value but to
various different values.
[0095] FIG. 10 is a schematic sectional view showing a covered
worked substrate according to this embodiment, which has a land
width L1 of 5 .mu.m, a groove depth Hi of 1 .mu.m and groove widths
G1 and G2 of 11 .mu.m and 20 .mu.m respectively. An InGaN film
having a thickness of 3.5 .mu.m is stacked on this worked
substrate, to prepare the covered worked substrate according to the
fourth embodiment.
[0096] While the worked substrate according to this embodiment has
two types of different groove widths, the worked substrate may
alternatively have more different groove widths. Further, the
fourth embodiment may be combined with the third embodiment.
[0097] [Fifth Embodiment]
[0098] A fifth embodiment of the present invention is similar to
the first and second embodiments except that the depths of grooves
formed on a worked substrate are set not to a constant value but to
various different values.
[0099] FIG. 11 is a schematic sectional view showing a covered
worked substrate according to this embodiment, which has a groove
width G1 of 18 .mu.m, a land width L1 of 7 .mu.m and groove depths
H1 and H2 of 1.5 .mu.m and 5 .mu.m respectively. A GaN film having
a thickness of 6 .mu.m is stacked on this worked substrate, to
prepare the covered worked substrate according to the fifth
embodiment.
[0100] While the worked substrate according to this embodiment has
two types of different groove depths, the worked substrate may
alternatively have more different groove depths. Further, the fifth
embodiment may be combined with the third or fourth embodiment.
[0101] [Sixth Embodiment]
[0102] According to a sixth embodiment of the present invention, a
nitride semiconductor laser device is formed on the covered worked
substrate of any of the first to fifth embodiments.
[0103] (Crystal Growth)
[0104] FIG. 1 illustrates a nitride semiconductor laser device
grown on a covered worked substrate. The nitride semiconductor
laser device shown in FIG. 1 includes a covered worked substrate
100 consisting of a worked substrate (n-type GaN substrate) 101 and
an n-type Al.sub.0.05Ga.sub.0.95N underlayer 102, an n-type
In.sub.0.07Ga.sub.0.93N anti-cracking layer 103, an n-type
Al.sub.0.1Ga.sub.0.9N cladding layer 104, an n-type GaN light guide
layer 105, an emission layer 106, a p-type Al.sub.0.2Ga.sub.0.8N
carrier blocking layer 107, a p-type GaN light guide layer 108, a
p-type Al.sub.0.1Ga.sub.0.9N cladding layer 109, a p-type GaN
contact layer 110, an n electrode 111, a p electrode 112 and an
SiO.sub.2 dielectric film 113.
[0105] In order to prepare this nitride semiconductor laser device,
the covered worked substrate 100 according to any of the first to
fifth embodiments is first formed. In the sixth embodiment, grooves
are formed along the <1-100>direction of the GaN
substrate.
[0106] Then, TMIn (trimethyl indium) as a source for a group III
element and SiH.sub.4 (silane) as an impurity are added to NH.sub.3
(ammonia) as a source for a group V element and TMGa (trimethyl
gallium) or TEGa (triethyl gallium) as a source for a group III
element over the covered worked substrate 100 in an MOCVD
apparatus, and the n-type In.sub.0.07Ga.sub.0.93N anti-cracking
layer 103 is grown in a thickness of 40 nm at a crystal growth
temperature of 800.degree. C. Then, the substrate temperature is
increased to 1050.degree. C., for growing the n-type
Al.sub.0.1Ga.sub.0.9N cladding layer 104 (Si impurity
concentration: 1.times.10.sup.18/cm.sup.3) of 0.8 .mu.m thickness
by using TMAl (trimethyl aluminum) or TEAl (triethyl aluminum) as a
source for a group III element and then growing the n-type GaN
light guide layer 105 (Si impurity concentration:
1.times.10.sup.18/cm.sup.3) of 0.1 .mu.m thickness.
[0107] Thereafter the substrate temperature is reduced to
800.degree. C. for forming the emission layer (multiple quantum
well structure) 106 by alternately stacking In.sub.0.01Ga.sub.0.99N
barrier layers of 8 nm thickness and In.sub.0.15Ga.sub.0.85N well
layers of 4 nm thickness. According to this embodiment, the
emission layer 106 has a multiple quantum well structure starting
and ending with barrier layers, and includes three (three cycles
of) quantum well layers. An Si impurity is added to both of the
barrier layers and the well layers in a concentration of
1.times.10.sup.18/cm.sup.3. A crystal growth interruption interval
of at least 1 second and not more than 180 seconds may be inserted
between any barrier layer growth and the next well layer growth or
between any well layer growth and the next barrier layer growth. In
this case, the layers are preferably improved in flatness to reduce
the half-width of an emission spectrum.
[0108] AsH.sub.3 or TBAs (tertiary butyl arsine) may be employed
when As is added to the emission layer 106, PH.sub.3 or TBP
(tertiary butyl phosphine) may be employed when P is added to the
emission layer 106, and TMSb (trimethyl antimony) or TESb (triethyl
antimony) may be employed when Sb is added to the emission layer
106. Alternatively, N.sub.2H.sub.4 (dimethyl hydrazine) may be
employed in place of NH.sub.3 as a source for N when the emission
layer 106 is formed.
[0109] Then, the substrate temperature is increased to 1050.degree.
C. again, for successively growing the p-type Al.sub.0.2Ga.sub.0.8N
carrier blocking layer 107 of 20 nm thickness, the p-type GaN light
guide layer 108 of 0.1 .mu.m thickness, the p-type
Al.sub.0.1Ga.sub.0.9N cladding layer 109 of 0.5 .mu.m thickness and
the p-type GaN contact layer 110 of 0.1 .mu.m thickness. As a
p-type impurity, Mg (EtCP.sub.2Mg: bisethyl cyclopentadienyl
magnesium) is added in a concentration of
5.times.10.sup.19/cm.sup.3 to 2.times.10.sup.20/cm.sup.3. The
p-type impurity concentration in the p-type GaN contact layer 110
is preferably increased toward the interface between the same and
the p electrode 112. Thus, contact resistance is reduced at the
interface between the p-type GaN contact layer 110 and the p
electrode 112. Further, a small amount of oxygen may be introduced
during growth of the p-type layers, in order to remove residual
hydrogen that inactivates Mg not to serve as the p-type
impurity.
[0110] After the aforementioned growth of the p-type GaN contact
layer 110, gas in a reactor of the MOCVD apparatus is entirely
replaced with nitrogen carrier gas and NH.sub.3, and the substrate
is cooled at a rate of 60.degree. C./min. When the substrate
temperature reaches 800.degree. C., supply of NH.sub.3 is stopped
and the substrate is held at this temperature for five minutes and
thereafter cooled to the room temperature. The substrate is
preferably held at a temperature between 650.degree. C. and
900.degree. C. for at least 3 minutes and not more than 10 minutes.
The rate of cooling the substrate to the room temperature is
preferably at least 30.degree. C./min. In actual evaluation by
Raman measurement, a crystal-grown film formed in the
aforementioned manner already exhibited p-type characteristics
(i.e., Mg was activated) in a state not subjected to conventional
annealing for attaining p-type conductivity. Further, contact
resistance was also reduced in formation of the p electrode 112.
When the conventional annealing for attaining p-type conductivity
was additionally introduced, the activation ratio of Mg was further
preferably improved.
[0111] In this embodiment, the layers needed to be formed from the
worked substrate 101 up to the nitride semiconductor laser device
may be continuously crystal-grown, or growth process from the
worked substrate 101 to the covered worked substrate 100 may be
carried out in advance so that re-growth is thereafter performed to
form the nitride semiconductor laser device.
[0112] In this embodiment, the In.sub.0.07Ga.sub.0.93N
anti-cracking layer 103 may alternatively have an In composition
ratio other than 0.07, or may be omitted. When lattice mismatch
between the cladding layer 104 and the GaN substrate 101 is
increased, the InGaN anti-cracking layer 103 is preferably
inserted.
[0113] The emission layer 106, starting and ending with the barrier
layers in this embodiment, may alternatively start and end with
well layers. The number of the well layers included in the emission
layer 106 is not restricted to three. So far as the number of the
well layers is not more than 10, the value of the threshold current
is low allowing continuous oscillation at the room temperature. The
value of the threshold current is preferably reduced particularly
when the number of the well layers is at least 2 and not more than
6.
[0114] Si, added to both of the well layers and the barrier layers
in the concentration of 1.times.10.sup.18/cm.sup.3 in the emission
layer 106 according to this embodiment, is not necessarily to be
added. However, emission intensity is increased when Si is added to
the emission layer 106. The impurity added to the emission layer
106 is not restricted to Si but at least any of O, C, Ge, Zn and Mg
may alternatively be employed. The total amount of the impurity is
preferably about 1.times.10.sup.17/cm.sup.3 to
1.times.10.sup.19/cm.sup.3. Further, the impurity may not be added
to both of the well layers and the barrier layers but may
alternatively be added to only either the well layers or the
barrier layers.
[0115] According to this embodiment, the p-type
Al.sub.0.2Ga.sub.0.8N carrier blocking layer 107 may alternatively
have an Al composition ratio other than 0.2, or may be omitted.
However, the value of the threshold current is reduced when the
carrier blocking layer 107 is provided. This is because the carrier
blocking layer 107 confines carriers in the emission layer 106. The
Al composition ratio of the carrier blocking layer 107 may
preferably be increased in order to strengthen confinement of the
carriers. Further, the Al composition ratio may preferably be
reduced within a range for maintaining confinement of the carriers,
in order to increase carrier mobility in the carrier blocking layer
107 and reduce electrical resistance.
[0116] The Al composition ratio of Al.sub.0.1Ga.sub.0.9N employed
for the p-type cladding layer 109 and the n-type cladding layer 104
in this embodiment may be other than 0.1. If the Al ratio in the
mixed crystal is increased, energy gap difference and refractive
index difference between the cladding layers 109 and 104 and the
emission layer 106 are increased so that the carriers and light are
efficiently confined in the emission layer 106 and the value of the
laser oscillation threshold current can be reduced. If the Al
composition ratio is reduced in a range for maintaining confinement
of the carriers and light, carrier mobility in the cladding layers
109 and 104 is so increased that an operating voltage of the device
can be reduced.
[0117] The thickness of each of the AlGaN cladding layers 109 and
104 is preferably within the range of 0.7 .mu.m to 1.0 .mu.m, in
order to attain a unimodal vertical lateral mode, increase light
confinement efficiency, improve optical characteristics of the
laser device and reduce the value of the laser threshold
current.
[0118] The cladding layers 109 and 104 are not restricted to
ternary mixed crystals of AlGaN but may be quaternary mixed
crystals of AlInGaN, AlGaNP or AlGaNAs. Further, the p-type
cladding layer 109 may have a superlattice structure including a
p-type AlGaN layer and a p-type GaN layer or including a p-type
AlGaN layer and a p-type InGaN layer, in order to reduce electrical
resistance.
[0119] While an MOCVD apparatus is employed for crystal growth in
this embodiment, it may alternatively be performed by molecular
beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE) or the
like.
[0120] (Chip Step)
[0121] An epi-wafer (a wafer including nitride semiconductor layers
epitaxially grown on a covered worked substrate) formed by the
aforementioned crystal growth is taken out from the MOCVD apparatus
and processed into a laser device. The epi-wafer formed with a
nitride semiconductor laser layer has a flat surface, i.e., grooves
and lands formed on the worked substrate 101 are completely filled
up with a nitride semiconductor underlayer and a light emitting
device structure layer.
[0122] Hf and Al are successively stacked to form the n electrode
111 on the rear surface the covered worked substrate 100 consisting
of an n-type nitride semiconductor (see FIG. 1). Alternatively, Ti
and Al, Ti and Mo, or Hf and Au may be stacked to form the n
electrode 111. Hf is preferably employed for the n electrode 111,
to reduce the contact resistance thereof The p electrode is etched
in a striped manner along the grooves of the worked substrate 101,
thereby forming a ridge stripe portion (see FIG. 1). When
cross-striped grooves are formed on the worked substrate 101, the
<1-100>and <11-20>directions of the nitride
semiconductor substrate may be selected as the longitudinal
directions of these grooves. The ridge stripe portion, having a
stripe width W of 2.0 .mu.m, is formed to be included in the
aforementioned area I. Thereafter the SiO.sub.2 dielectric film 113
is deposited and the upper surface of the p-type GaN contact layer
110 is exposed from the SiO.sub.2 dielectric film 113 so that Pd,
Mo and Au are deposited thereon for forming the p electrode 112.
Alternatively, Pd, Pt and Au, Pd and Au, or Ni and Au may be
stacked to form the p electrode 112.
[0123] Finally, the epi-wafer is cloven perpendicularly to the
longitudinal direction of the ridge stripe portion, to prepare a
Fabry-Prot resonator having a resonator length of 500 .mu.m. The
resonator length is preferably within the range of 300 .mu.m to
1000 .mu.m in general. The resonator formed along the grooves in
the <1-100>direction has a mirror end surface defined by the
M plane {1-100} of a nitride semiconductor crystal. In order to
form this mirror end surface, cleavage and division for a laser
chip are carried out from the rear surface of the covered worked
substrate 100 with a scriber. However, cleavage is performed not
after scribing the wafer across the overall rear surface thereof
but after partially scribing the wafer only on both ends thereof.
Thus, no shavings resulting from the scribing and the sharp edge of
the end surface adhere to the surface of the epi-wafer, whereby the
yield of the device is improved.
[0124] The laser resonator may also adopt a feed-back system
generally known as DFB (distribution feedback) or DBR (distribution
Bragg reflection) or the like.
[0125] Dielectric films of SiO.sub.2 and TiO.sub.2 are alternately
deposited on the mirror end surface of the Fabry-Prot resonator to
form a dielectric multilayer reflection film having reflectance of
70%. Alternatively, a multilayer film of SiO.sub.2/Al.sub.2O.sub.3
or the like may be employed for the dielectric multilayer
reflection film.
[0126] While the n electrode 111 is formed on the rear surface of
the covered worked substrate 100, the n-type
Al.sub.0.05Ga.sub.0.95N film 102 may be partially exposed from the
front side of the epi-wafer so that the n electrode is formed on
the exposed area.
[0127] (Packaging)
[0128] The semiconductor laser device chip obtained in the
aforementioned manner is packaged. When a nitride semiconductor
laser device having a high output (at least 30 mW) is employed,
attention must be drawn to measures for heat radiation. While the
high-output nitride semiconductor laser device can be connected to
the body of a package by an In solder material with its
semiconductor junction being upward or downward, it is preferably
connected to the body of the package with its semiconductor
junction being downward. While the high-output nitride
semiconductor laser device can be directly mounted on the body of
the package or a heat sink part, it may be connected through a
submount of Si, AlN, diamond, Mo, CuW, BN, Fe, Cu, SiC or Au.
[0129] The nitride semiconductor laser device according to this
embodiment is prepared in the aforementioned manner.
[0130] While the worked substrate 100 of GaN is employed in this
embodiment, a worked substrate of another nitride semiconductor may
alternatively be employed. In the case of a nitride semiconductor
laser device, for example, a layer having a lower refractive index
than a cladding layer must be in contact with the outer side of the
cladding layer in order to attain a unimodal vertical lateral mode,
and thus an AlGaN substrate can be preferably employed.
[0131] According to this embodiment, the nitride semiconductor
laser device is formed on the covered worked substrate 100, thereby
to relax crystal strain, suppress cracking, obtain a laser
oscillation life of about 15500 hours and improve a device yield
due to the effect of suppressing cracking.
[0132] [Seventh Embodiment]
[0133] In a seventh embodiment of the present invention, a nitride
semiconductor light emitting diode device is formed on any covered
worked substrate obtained in the first to fifth embodiments. At
this time, a nitride semiconductor light emitting diode device
layer is formed by a method similar to the prior art.
[0134] In the nitride semiconductor light emitting diode device
according to this embodiment, emission intensity is improved as
compared with the prior art. In particular, a light emitting diode
device made of nitride semiconductor material to have a short
emission wavelength (not more than 440 nm) or a long emission
wavelength (at least 600 nm) can attain emission intensity of at
least about 1.6 times as compared with the prior art by forming the
same on any covered worked substrate obtained in the first to fifth
embodiment.
[0135] [Eighth Embodiment]
[0136] An eighth embodiment of the present invention is similar to
the sixth and seventh embodiments, except that an emission layer
contains a substitutional element of at least one of As, P and Sb
for substituting for part of N. More specifically, the
substitutional element of at least one of As, P and Sb is contained
in the emission layer of a nitride semiconductor light emitting
device in substitution for part of N contained at least in well
layers. Assuming that x represents the total composition ratio of
As, P and/or Sb contained in the well layers and y represents the
composition ratio of N, the composition ratio x is smaller than the
composition ratio y, and x/(x+y) is not more than 0.3 (30%),
preferably not more than 0.2 (20%). The lower limit of the
preferable total concentration of As, P and/or Sb is at least
1.times.10.sup.18/cm.sup.3.
[0137] This is because concentration separation causing certain
areas of different composition ratios of the substitutional
element(s) in the well layers starts to take place when the
composition ratio x of the substitutional element(s) exceeds 20 %,
and then the concentration separation starts to change to crystal
system separation causing a hexagonal system and a cubic system in
a mixed state when the composition ratio x exceeds 30%, thereby
increasing the possibility of reducing crystalline quality of the
well layers. When the total concentration x of the substitutional
element(s) is reduced below 1.times.10.sup.18/cm.sup.3 to the
contrary, the effect of introducing the substitutional element(s)
into the well layers can be hardly attained.
[0138] According to this embodiment, the effective mass of
electrons and holes in the well layers is reduced and mobility
thereof is increased due to the substitutional element of at least
one of As, P and Sb contained in the well layers. In the case of a
semiconductor laser device, small effective mass means that carrier
inversion distribution for laser oscillation is obtained with a
small current injection ratio while large mobility means that new
electrons and holes can be injected at a high speed due to
diffusion even if electrons and holes disappear in the emission
layer due to emission recombination. In other words, a
semiconductor laser having lower threshold current density and good
self-oscillation characteristic (good low-noise characteristic) can
be obtained in this embodiment as compared with an InGaN nitride
semiconductor laser device containing none of As, P and Sb in an
emission layer. Further, the laser oscillation life generally tends
to increase when the threshold current density is reduced, and
hence a nitride semiconductor laser device having a longer laser
oscillation life can be obtained according to this embodiment.
[0139] When this embodiment is applied to a nitride semiconductor
light emitting diode, an In composition ratio in well layers can be
reduced as compared with a conventional nitride semiconductor light
emitting diode device including InGaN well layers by introducing a
substitutional element of As, P and/or Sb into well layers. This
means that reduction of crystalline quality caused by concentration
separation of In can be suppressed. Therefore, the effect attained
by adding the substitutional element(s) is multiplied with the
effect related to the nitride semiconductor diode device according
to the seventh embodiment, so that emission intensity can be
further improved and color irregularity can be reduced in addition.
Particularly when the light emitting diode device is prepared of
nitride semiconductor material to have a short emission wavelength
(not more than 440 nm) or a long emission wavelength (at least 600
nm), well layers can be formed with a low In composition ratio or
without containing In, whereby smaller color irregularity and
larger emission intensity can be attained as compared with the
conventional InGaN nitride semiconductor light emitting diode
device.
[0140] [Ninth Embodiment]
[0141] According to a ninth embodiment of the present invention,
the nitride semiconductor laser device of the sixth or eighth
embodiment is applied to an optical apparatus. A blue-purple
nitride semiconductor laser device (oscillation wavelength: 380 to
420 nm) of the sixth or eighth embodiment can be preferably applied
to various optical apparatuses, and is applicable to an optical
pickup apparatus, for example, preferably in the following point.
Such a nitride semiconductor laser device stably operates with a
high output in a high-temperature atmosphere and has a long laser
oscillation life, and is hence optimum for a highly reliable
high-density recording/reproducing optical disk apparatus
(recording/reproduction in higher density is enabled as the
oscillation wavelength is reduced).
[0142] FIG. 13 is a schematic block diagram showing an exemplary
optical disk device such as a DVD including an optical pickup to
which a nitride semiconductor laser device of the sixth or eighth
embodiment is applied. In this optical information
recording/reproducing apparatus, a laser beam 3 outgoing from the
nitride semiconductor laser device 1 is modulated by an optical
modulator 4 in response to input information. Then the modulated
light information is recorded on a disk 7 through a scan mirror 5
and a lens 6. A motor 8 rotates the disk 7. In reproduction, a
photodetector 10 detects a reflected laser beam optically modulated
by pit arrangement on the disk 7 through a beam splitter 9, thereby
obtaining a reproduced signal. A control circuit 11 controls
operations of these elements. The laser device 1 generally has an
output of 30 mW in recording and about 5 mW in reproduction.
[0143] The laser device according to the present invention is
applicable not only to the aforementioned optical disk
recording/reproducing apparatus but also to a laser printer, a bar
code reader, a projector with a laser beam of the three primary
colors (blue, green and red).
[0144] [Tenth Embodiment]
[0145] According to a tenth embodiment of the present invention,
the nitride semiconductor light emitting diode device of the
seventh or eighth embodiment is applied to a semiconductor light
emitting apparatus. The nitride semiconductor light emitting diode
device is applicable to a display (exemplary semiconductor light
emitting apparatus) as a device for at least one of the three
primary colors (red, green and blue). A display having less color
irregularity and high emission intensity can be prepared by
utilizing such a nitride semiconductor light emitting diode
device.
[0146] The nitride semiconductor light emitting diode device
capable of emitting light of the three primary colors is also
applicable to a white light source apparatus. A nitride
semiconductor light emitting diode device according to the present
invention, having an emission wavelength in the ultraviolet to
violet regions (about 360 to 440 nm), is also employable as a white
light source device when a fluorescent paint is applied
thereto.
[0147] When such a white light source is employed, a backlight
having low power consumption and high brightness can be implemented
in place of a halogen light source employed for a conventional
liquid crystal display. This white light source can also be applied
as a backlight for a liquid crystal display of a man-machine
interface of a portable notebook-sized personal computer or a
portable telephone, so that a miniature liquid crystal display
having sharp picture quality can be provided.
[0148] According to the present invention, as hereinabove
described, the emission life and emission intensity can be improved
in a nitride semiconductor light emitting device.
[0149] Although the present invention has been described and
illustrated in detail, it is clearly understood that the same is by
way of illustration and example only and is not to be taken by way
of limitation, the spirit and scope of the present invention being
limited only by the terms of the appended claims.
* * * * *