Semiconductor light-emitting device

A semiconductor light-emitting device of the present invention includes a nitride type alloy semiconductor layer. The nitride type alloy semiconductor layer is made of Al.sub.a Ga.sub.b In.sub.1-a-b N (0.ltoreq.a.ltoreq.1, 0.ltoreq.b .ltoreq.1, a+b.ltoreq.1) including at least one selected from the group consisting of Sc, Ti, V, Cr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a semiconductor light-emitting device made 
of a nitride type alloy semiconductor. More specifically, the present 
invention relates to a semiconductor light-emitting device capable of 
emitting light by band-to-band radiative recombination in a wavelength 
region of ultraviolet to blue or green with reduction of growth 
temperature difference. 
2. Description of the Related Art 
In the past, AlN, GaN, and InN have been studied as nitride type 
III-V-group alloy semiconductors for semiconductor light-emitting devices. 
Of ternary alloy semiconductors and quarternary alloy semiconductors, 
AlGaN and GaInN have been studied. In particular, Japanese Laid-Open 
Patent Publication No. 6-177423 discloses a light-emitting diode (LED) 
including the combination of an AlGaN barrier layer and a GaInN active 
layer as a device capable of emitting blue or green light. 
However, in the case of fabricating the above-mentioned LEDs, the 
difference in growth temperature between AlGaN and GaInN causes problems. 
For example, when using metal organic chemical vapor deposition (MOCVD), 
the typical growth temperature of AlGaN is about 1000.degree. C., while 
that of GaInN is about 800.degree. C. Thus, the difference in growth 
temperatures therebetween is about 200.degree. C. When using molecular 
beam epitaxy (MBE), the difference in growth temperature is about 
100.degree. C. or more. Conventionally, LEDs are produced as follows: an 
AlGaN barrier layer is grown at a high temperature, and a GaInN active 
layer is grown while the substrate temperature is lowered. Then, the 
substrate temperature is raised, and another AlGaN barrier layer is grown. 
In this process, problems arise in that the crystallinity of the GaInN 
active layer is degraded due to the increase in temperature after the 
growth of the GaInN active layer, and the flatness (steepness) of an 
interface is degraded. 
In the conventional semiconductor light-emitting devices made of 
III-V-group alloy semiconductors, blue or green light emission is realized 
by carrier recombination via impurity (trap) levels in GaInN. However, 
light emission by band-to-band radiative recombination is dominant in LEDs 
in a high carrier injection state, which makes it impossible to realize 
laser oscillation in light emission by recombination via impurity levels. 
Thus, in order to realize laser oscillation, it is required to realize 
blue or green light emission by band-to-band radiative recombination. 
However, when the GaInN active layer is used, it is difficult to obtain 
satisfactory crystallinity in a region having an indium (In) mole fraction 
of 0.2 or more. This makes it difficult to realize blue or green light 
emission by band-to-band radiative recombination. 
SUMMARY OF THE INVENTION 
In one embodiment, the semiconductor light-emitting device of this 
invention includes a nitride type alloy semiconductor layer, wherein the 
nitride type alloy semiconductor layer is made of Al.sub.a Ga.sub.b 
In.sub.1-a-b N (0.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.1, a+b.ltoreq.1) 
including at least one selected from the group consisting of Sc, Ti, V, 
Cr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. 
Alternatively, the semiconductor light-emitting device of this invention 
includes a barrier layer and an active layer made of a nitride type alloy 
semiconductor, wherein the barrier layer is made of AlGaInN, and the 
active layer is made of Al.sub.a Ga.sub.b In.sub.1-a-b N 
(0.ltoreq.a.ltoreq.1, 0.ltoreq.b.ltoreq.1, a+b.ltoreq.1) including at 
least one selected from the group consisting of Sc, Ti, V, Cr, Y, La, Ce, 
Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. 
Alternatively, the semiconductor light-emitting device of this invention 
includes a barrier layer and an active layer made of a nitride type alloy 
semiconductor, wherein the barrier layer is made of Al.sub.a Ga.sub.b 
In.sub.1-a-b N (0.ltoreq.a .ltoreq.1, 0.ltoreq.b.ltoreq.1, a+b.ltoreq.1) 
including at least one selected from the group consisting of Sc, Ti, V, 
Cr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and the 
active layer is made of Al.sub.c Ga.sub.d In.sub.1-c-d N 
(0.ltoreq.c.ltoreq.1, 0.ltoreq.d.ltoreq.1, c+d.ltoreq.1). 
According to the present invention, a nitride type compound semiconductor 
layer is made of Al.sub.a Ga.sub.b In.sub.1-a-b N (0.ltoreq.a.ltoreq.1, 
0.ltoreq.b.ltoreq.1, a+b.ltoreq.1) including at least one selected from 
the group consisting of Sc, Ti, V, Cr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, 
Dy, Ho, Er, Tm, Yb, and Lu. 
Thus, the invention described herein makes possible the advantage of 
providing a semiconductor light-emitting device in which semiconductor 
layers with satisfactory good crystallinity are formed by reducing the 
difference in growth temperature of the semiconductor layers and which is 
capable of emitting light by band-to-band radiative recombination in a 
wavelength region of blue or green by reducing a bandgap. 
These and other advantages of the present invention will become apparent to 
those skilled in the art upon reading and understanding the following 
detailed description with reference to the accompanying figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 12 shows the relationship between the ion radiuses of elements (Al, 
Ga, and In) forming a nitride type III-V type compound semiconductor and 
the lattice constant in an a-axis direction of the nitride type III-V type 
compound semiconductor (AlN, GaN, and InN). As is understood from this 
figure, the ion radius is in a one-to-one correspondence with the lattice 
constant. This figure also shows the ion radiuses of trivalent ions of Sc, 
Ti, V, Cr, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. 
Of these elements, the ion radiuses of Sc, Ti, V, and Cr are larger than 
those of Al and Ga. Thus, by adding these elements to, for example, AlN of 
the AlGaInN layer to obtain AlXN (X is made of at least one of Sc, Ti, V, 
and Cr, and the added amount of X to Al is appropriate), the lattice 
constant of AlN is made close to or identical with that of GaN, and 
consequently, bandgap can be decreased. These elements are heavier than 
Al, so that the growth temperature of these elements is lowered so as to 
be closer to that of GaN. Alternatively, by adding these elements to GaN 
to obtain GaXN (X is made of at least one of Sc, Ti, V, and Cr, and the 
added amount of X to Ga is appropriate), the lattice constant can be made 
closer to that of InN or the bandgap can be decreased. 
Similarly, the ion radiuses of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, 
Er, Tm, Yb, and Lu are larger than that of In, and these elements are 
heavier than Al and Ga. Thus, by adding these elements to, for example, 
AlN and GaN of the AlGaInN layer, the growth temperature of AlN and GaN 
can be lowered so as to be equal to or lower than that of a layer 
containing indium (In), the bandgap can be decreased, and the lattice 
constant can be increased. 
Hereinafter, the present invention will be described by way of illustrative 
examples with reference to the drawings. 
EXAMPLE 1 
In the present example, a semiconductor light-emitting device emitting 
green light comprises barrier layers made of Ga.sub.0.9 In.sub.0.1 N and 
an active layer made of Ga.sub.0.93 Gd.sub.0.07 N. 
FIG. 1 is a cross-sectional view showing a structure of a semiconductor 
light-emitting device of Example 1. The semiconductor light-emitting 
device includes a low temperature growth GaN buffer layer 2, an n-type GaN 
contact layer 3, an n-type Ga.sub.0.9 In.sub.0.1 N barrier layer 4, an 
i-type Ga.sub.0.93 Gd.sub.0.07 N active layer 5, a p-type Ga.sub.0.9 
In.sub.0.1 N barrier layer 6, and a p-type GaN contact layer 7 formed in 
this order on a sapphire substrate 1. The n-type barrier layer 4, the 
i-type Ga.sub.0.93 Gd.sub.0.07 N active layer 5, the p-type Ga.sub.0.9 
In.sub.0.1 N barrier layer 6, and the p-type GaN contact layer 7 are 
partially removed so that a portion of the n-type contact layer 3 is 
exposed. An n-type electrode 8 is formed on the exposed portion of the 
n-type contact layer 3 and a p-type electrode 9 is formed on the p-type 
contact layer 7. The low temperature growth GaN buffer layer 2 has a 
thickness of 300 .ANG.; the n-type GaN contact layer 3 has a thickness of 
0.1 .mu.m and a carrier concentration of 1.times.10.sup.18 cm.sup.-3 ; the 
n-type Ga.sub.0.9 In.sub.0.1 N barrier layer 4 has a thickness of 3.0 
.mu.m and a carrier concentration of 5.times.10.sup.17 cm.sup.-3 ; the 
i-type Ga.sub.0.93 Gd.sub.0.07 N active layer 5 has a thickness of 500 
.ANG.; the p-type Ga.sub.0.9 In.sub.0.1 N barrier layer 6 has a thickness 
of 0.5 .mu.m and a carrier concentration of 5.times.10.sup.17 cm.sup.-3 ; 
and the p-type GaN contact layer 7 has a thickness of 0.2 .mu.m and a 
carrier concentration of 2.times.10.sup.18 cm.sup.-3. 
In the present example, the active layer 5 and the barrier layers 4 and 6 
are grown by MBE at 600.degree. C. The active layer 5 is doped with Gd, so 
that its growth temperature can be set as low as the barrier layers 4 and 
6. Therefore, no decrease in crystallinity and flatness of an interface 
are observed. Furthermore, the lattice constant of the active layer 5 is 
almost equal to those of the barrier layers 4 and 6 so as to obtain 
lattice matching. In this manner, satisfactory crystallinity is obtained. 
FIG. 2 shows a current-voltage characteristic of the semiconductor 
light-emitting device of the present example. The current-voltage 
characteristic of the semiconductor light-emitting device is almost equal 
to that of the conventional semiconductor light-emitting device using an 
AlGaN barrier layer/GaInN active layer. FIG. 3 shows an EL spectrum. In 
the conventional semiconductor light-emitting device using the AlGaN 
barrier layer/GaInN active layer, a full width at half maximum (FWHM) of 
light emission is large (i.e., 80 nm) because light is emitted by 
recombination via impurity levels. In contrast, in the semiconductor 
light-emitting device of the present example, crystallinity is 
satisfactory and light can be emitted by band-to-band radiative 
recombination, so that a full width at half maximum (FWHM) of light 
emission is remarkably reduced (i.e., 16 nm). Furthermore, the bandgap can 
be reduced by doping the active layer 5 with Gd, so that the 
light-emission wavelength can be enlarged and the color of light thus 
obtained can be made close to purer green, compared with the conventional 
semiconductor light-emitting device including the AlGaN barrier 
layer/GaInN active layer. 
EXAMPLE 2 
In the present example, a semiconductor light-emitting device emitting blue 
light is produced by using barrier layers made of GaN and an active layer 
made of Al.sub.0.88 La.sub.0.12 N. 
FIG. 4 is a cross-sectional view showing a structure of a semiconductor 
light-emitting device of Example 2. The semiconductor light-emitting 
device includes a low temperature growth GaN buffer layer 2, an n-type GaN 
contact layer 3, an n-type GaN barrier layer 10, an i-type Al.sub.0.88 
La.sub.0.12 N active layer 11, a p-type GaN barrier layer 12, and a p-type 
GaN contact layer 7 in this order on a sapphire substrate 1. The n-type 
GaN barrier layer 10, the i-type Al.sub.0.88 La.sub.0.12 N active layer 
11, the p-type GaN barrier layer 12, and the p-type GaN contact layer 7 
are partially removed so that a portion of the n-type contact layer 3 is 
exposed. An n-type electrode 8 is formed on the exposed portion of the 
n-type contact layer 3 and a p-type electrode 9 is formed on the p-type 
contact layer 7. The low temperature growth GaN buffer layer 2 has a 
thickness of 300 .ANG.; the n-type GaN contact layer 3 has a thickness of 
0.1 .mu.m and a carrier concentration of 1.times.10.sup.18 cm.sup.-3 ; the 
n-type GaN barrier layer 10 has a thickness of 3.0 .mu.m and a carrier 
concentration of 5.times.10.sup.17 cm.sup.-3 ; the i-type Al.sub.0.88 
La.sub.0.12 N active layer 11 has a thickness of 500 .ANG.; the p-type GaN 
barrier layer 12 has a thickness of 0.5 .mu.m and a carrier concentration 
of 5.times.10.sup.17 cm.sup.-3 ; and the p-type GaN contact layer 7 has a 
thickness of 0.2 .mu.m and a carrier concentration of 2.times.10.sup.18 
cm.sup.-3. 
In the present example, the active layer 11 and the barrier layers 10 and 
12 are grown by MOCVD at 1000.degree. C. For the source of Al and Ga 
elements, trimethylaluminum (TMA) and trimethylgallium (TMG) are used, and 
for the source of La, a DPM((CH.sub.3).sub.3.C.CO.CH.sub.2 
CO.C(CH.sub.3).sub.3) type .beta.-diketone complex is used. The active 
layer 11 is doped with La, so that its growth temperature can be set as 
low as the barrier layers 10 and 12. Therefore, no decrease in 
crystallinity and flatness of an interface are found. Furthermore, the 
lattice constant of the active layer 11 is almost equal to those of the 
barrier layers 10 and 12 so as to obtain lattice matching. In this manner, 
satisfactory crystallinity is obtained. 
FIG. 5 shows a current-voltage characteristic of the semiconductor 
light-emitting device of the present example. The current-voltage 
characteristic of the semiconductor light-emitting device is almost equal 
to that of the conventional semiconductor light-emitting device using an 
AlGaN barrier layer/GaInN active layer. FIG. 6 shows an EL spectrum. In 
the conventional semiconductor light-emitting device using the AlGaN 
barrier layer/GaInN active layer, FWHM of light emission is large (i.e., 
80 nm) because light is emitted by recombination via impurity levels. In 
contrast, in the semiconductor light-emitting device of the present 
example, crystallinity is satisfactory and light can be emitted by 
band-to-band radiative recombination in the same way as in Example 1, so 
that FWHM of light emission is remarkably reduced (i.e., 15 nm). 
Furthermore, the bandgap can be reduced by doping the active layer 11 with 
La, so that the light-emission wavelength can be enlarged and the color of 
light thus obtained can be made close to purer blue, compared with the 
conventional semiconductor light-emitting device including the AlGaN 
barrier layer/GaInN active layer. 
EXAMPLE 3 
In the present example, a semiconductor light-emitting device emitting 
ultraviolet light is produced by using an active layer made of GaN and 
barrier layers made of Al.sub.0.9 Cr.sub.0.1 N. 
FIG. 7 is a cross-sectional view showing a structure of a semiconductor 
light-emitting device of Example 3. The semiconductor light-emitting 
device includes a low temperature growth GaN buffer layer 2, an n-type GaN 
contact layer 3, an n-type Al.sub.0.9 Cr.sub.0.1 N barrier layer 13, an 
i-type GaN active layer 14, a p-type Al.sub.0.9 Cr.sub.0.1 N barrier layer 
15, and a p-type GaN contact layer 7 in this order on a sapphire substrate 
1. The n-type Al.sub.0.9 Cr.sub.0.1 N barrier layer 13, the i-type GaN 
active layer 14, the p-type Al.sub.0.9 Cr.sub.0.1 N barrier layer 15, and 
the p-type GaN contact layer 7 are partially removed so that a portion of 
the n-type GaN contact layer 3 is exposed. An n-type electrode 8 is formed 
on the exposed portion of the n-type GaN contact layer 3 and a p-type 
electrode 9 is formed on the p-type GaN contact layer 7. The low 
temperature growth GaN buffer layer 2 has a thickness of 300 .ANG.; the 
n-type GaN contact layer 3 has a thickness of 0.1 .mu.m and a carrier 
concentration of 1.times.10.sup.18 cm.sup.-3 ; the n-type Al.sub.0.9 
Cr.sub.0.1 N barrier layer 13 has a thickness of 3.0 .mu.m and a carrier 
concentration of 5.times.10.sup.17 cm.sup.-3 ; the i-type GaN active layer 
14 has a thickness of 500 .ANG.; the p-type Al.sub.0.9 Cr.sub.0.1 N 
barrier layer 15 has a thickness of 0.5 .mu.m and a carrier concentration 
of 5.times.10.sup.17 cm.sup.-3 ; and the p-type GaN contact layer 7 has a 
thickness of 0.2 .mu.m and a carrier concentration of 2.times.10.sup.18 
cm.sup.-3. 
In the present example, the active layer 14 and the barrier layers 13 and 
15 are grown by MBE at 600.degree. C. The barrier layers 13 and 15 are 
doped with Cr, so that its growth temperature can be set as low as the 
active layer 14. Therefore, no decrease in crystallinity and flatness of 
an interface are observed. Furthermore, the lattice constant of the active 
layer 14 is almost equal to those of the barrier layers 13 and 15 so as to 
obtain lattice matching. In this manner, satisfactory crystallinity is 
obtained. 
FIG. 8 shows a current-voltage characteristic of the semiconductor 
light-emitting device of the present example. The current-voltage 
characteristic of the semiconductor light-emitting device is almost equal 
to that of the conventional semiconductor light-emitting device using an 
AlGaN barrier layer/GaInN active layer. FIG. 9 shows an EL spectrum. In 
the conventional semiconductor light-emitting device using the AlGaN 
barrier layer/GaInN active layer, FWHM of light emission is large (i.e., 
80 nm) because light is emitted by recombination via impurity levels. In 
contrast, in the semiconductor light-emitting device of the present 
example, crystallinity is satisfactory and light can be emitted by 
band-to-band radiative recombination in the same way as in Example 1, so 
that FWHM of light emission is remarkably improved (i.e., 15 nm). 
EXAMPLE 4 
In the present example, optical pumping is conducted with a nitrogen laser 
(wavelength: 337.1 nm; pulse width: 10 nsec; duty: 10.sup.-7) as shown in 
FIG. 10 in a structure described with respect to Examples 1 through 3. 
FIGS. 11A through 11C are graphs showing the relationships between the pump 
light power density and the optical output of the semiconductor 
light-emitting devices of Examples 1 through 3 according to the present 
invention, respectively. As is apparent from these figures, laser 
oscillation can be realized by optical pumping in any of the samples. 
In Examples 1 through 4, Gd, La, and Cr are used as an element X to be 
added to an AlGaInN layer. However, any of Sc, Ti, V, Cr, Y, La, Ce, Pr, 
Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu can be used in place of Gd, 
La, and Cr. Two kinds or more thereof can also be used. Stable crystal 
structures of nitrides of these elements (i.e., Sc, Ti, V, Cr, Y, La, Ce, 
Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) are rock-salt 
structures. In the case where any of these elements is added in a small 
amount to AlGaInN which is stable in a wurtzite structure, the AlGaInN 
with any of these elements added has the same crystal structure as that of 
AlGaInN. This is similar to the case where MgS and MgSe which are stable 
in a rock-salt structure is added to ZnS and ZnSe which are stable in a 
zinc-blend structure. Although the added amount (y in the case of 
(AlGaIn).sub.1-y X.sub.y N of the element X depends upon the ion radius of 
the element to be added, it is preferably 20% or less, more preferably 10% 
or less in terms of stability of the crystal structure. 
The mole fractions a and b of an Al.sub.a Ga.sub.b In.sub.1-a-b N 
semiconductor layer are not limited to those shown in the above examples. 
AlN, GaN, InN, and combinations thereof such as ternary and quarternary 
semiconductors can be used. 
In Examples 1 through 4, LEDs with a double hetero structure are used. In 
place of these, LEDs with a homojunction structure or a single hetero 
structure; and semiconductor laser devices with a separate confinement 
heterostructure (SCH), graded index separate confinement heterostructure 
(GRIN-SCH), or the like can be used. 
The barrier layer or the active layer is doped with the above-mentioned 
elements. Both layers can be doped. Other AlGaInN layers can be doped. In 
these cases, effects such as the decrease in growth temperature 
difference, matching of lattice constants, and the decrease in bandgap can 
be obtained. 
As is apparent from the above description, according to the present 
invention, the difference in growth temperature of semiconductor layers 
can be decreased and the decrease in crystallinity and flatness of an 
interface can be prevented, which have been difficult in the conventional 
semiconductor light-emitting device using AlGaN/GaInN ternary compounds. 
Furthermore, the lattice constants of the semiconductor layers can be made 
closer to each other, so that satisfactory crystallinity can be obtained. 
Furthermore, a semiconductor light-emitting device can be obtained, in 
which light emission wavelength is enlarged by decreasing bandgap, light 
is emitted by band-to-band radiative recombination in a wavelength region 
from ultraviolet to blue or green (which has been difficult to achieve in 
the conventional semiconductor light-emitting device), and laser 
oscillation is realized. 
Various other modifications will be apparent to and can be readily made by 
those skilled in the art without departing from the scope and spirit of 
this invention. Accordingly, it is not intended that the scope of the 
claims appended hereto be limited to the description as set forth herein, 
but rather that the claims be broadly construed.