Group-III nitride based light emitter

A group-III nitride based light emitter such as LED and LD, which has a double heterostructure and which comprises a diffusion suppressive layer between a p-type cladding layer and an active layer. The diode having a diffusion suppressive layer of the present invention has higher luminous intensity, greater forward voltage, and longer lifetime than the conventional diodes.

FIELD OF THE INVENTION 
The present invention relates to a group-III nitride based light emitter 
such as light emitting diodes and laser diodes. 
BACKGROUND OF THE INVENTION 
Of the light emitting diodes, a group-III nitride based light emitting 
diode emits short wavelength lights of, for example, from green to 
ultraviolet light. Some of them have a double hetero structure which 
basically comprises an n-type cladding layer, an active layer and a p-type 
cladding layer successively formed on a substrate via a buffer layer. 
Such n-type cladding layer, active layer and p-type cladding layer are 
doped with various dopants for the purpose of controlling the conduction 
type or for forming a light emitting center, wherein frequently used as 
said dopants are, for example, Si, Ge and the like for the n-type cladding 
layer; Zn, Cd, Si and the like for the active layer; and Zn, Mg, Cd, Be 
and the like for the p-type cladding layer. 
As exemplified in FIG. 2, a known group-III nitride based light emitting 
diode having a double heterostructure comprises an n-type cladding layer 
13, an active layer 14 and a p-type cladding layer 16 successively formed 
on a substrate 11 via a buffer layer 12. Specifically, a sapphire 
substrate is used for the substrate 11; AlGaN materials are used for the 
n-type cladding layer 13 and p-type cladding layer 16; InGaN material is 
used for the active layer 14; Si is used as a dopant for the n-type 
cladding layer 13; Mg is used as a dopant for the p-type cladding layer 
16; and Zn, Si and the like are used as dopants for the active layer 14. 
Of various dopants used for such n-type cladding layer, active layer and 
p-type cladding layer, Mg, Zn and the like which are doped into the active 
layer and p-type cladding layer easily diffuse into the adjacent 
semiconductor layers during growth of each layer mentioned above and light 
emission of the prepared diode. As a result, for example, the dopant which 
moved from the p-type cladding layer to the active layer due to diffusion 
decreases the amount of the dopant in the p-type cladding layer. In 
addition, the dopant diffused into the active layer may form a 
nonradiative recombination center. Consequently, the luminous intensity of 
the light emitting diode decreases to ultimately shorten the lifetime of 
the diode. For example, the use of the diode for 5,000 hours may reduce 
the luminous intensity to less than 50% of the initial luminous intensity. 
The diffusion is generally promoted with increasing temperatures, and the 
temperature during growth of semiconductor layers varies depending on the 
melting point of the materials used for each layer. Each layer of 
group-III nitride based light emitting diode composed of GaN, AlGaN 
materials, InGaN materials and the like needs to be grown at higher 
temperatures, thus promoting the diffusion of dopants. In addition, 
emission of the light accompanies generation of heat which inevitably 
causes diffusion, though gradual, of dopants. 
It is therefore an object of the present invention to suppress diffusion of 
dopants in a light emitter based on group-III nitride such as gallium 
nitride, whereby to prevent degradation of the property thereof. 
SUMMARY OF THE INVENTION 
The inventive group-III nitride based light emitter such as light emitting 
diodes and laser diodes has a double heterostructure wherein a diffusion 
suppressive layer is formed between a p-type cladding layer and an active 
layer. 
Such diffusion suppressive layer aims at suppression of a mutual diffusion 
of dopants which occurs between the p-type cladding layer and active layer 
and which degrades properties of the emitter. More particularly, the 
diffusion suppressive layer aims at suppression of diffusion of the dopant 
of the p-type cladding layer into the active layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The diode of the embodiment shown in FIG. 1 has a double heterostructure 
wherein a buffer layer 2 is formed on a substrate 1, and an n-type 
cladding layer 3, an active layer 4, a diffusion suppressive layer 5, and 
a p-type cladding layer 6 are successively formed thereon. 
The materials for the n-type cladding layer 3 and p-type cladding layer 6 
are determined according to the material used for the active layer 4, and 
are exemplified by GaN, AlGaN materials such as Al.sub.0.1 Ga.sub.0.9 N, 
InGaN materials such as In.sub.0.1 Ga.sub.0.9 N, and InGaAlN materials. 
The thickness of each layer is 2-6 .mu.m for n-type cladding layer 3 and 
about 0.3-1.5 .mu.m for p-type cladding layer 6. While the material of the 
active layer 4 vary depending on the wavelength region of the light 
emitted from the diode to be prepared, InGaN materials such as In.sub.0.1 
Ga.sub.0.9 N, In.sub.0.2 Ga.sub.0.8 N and In.sub.0.4 Ga.sub.0.6 N, GaN and 
the like can be used, and the thickness is about 0.001-0.1 .mu.m. The 
dopants for n-type cladding layer 3 are preferably elements of Si, Ge, Sn 
and the like in view of n-type conduction. Si is particularly preferable, 
since it affords high doping efficiency and fine surface of the doped 
layer. Doping is performed so that the carrier concentration becomes 
1.times.10.sup.17 -1.times.10.sup.19 cm.sup.-3. Similarly, the elements of 
Zn, Mg, Cd, Be and the like are preferable for p-type cladding layer 6 in 
view of p-type conduction. Among them, Mg is particularly preferable, 
since it shows smaller acceptor level and is easily activated. In this 
case, the doping is performed so that the carrier concentration becomes 
3.times.10.sup.17 -8.times.10.sup.18 cm.sup.-3. Various dopants are used 
for the active layer 4 depending on the wavelength of the light emitted 
from the diode. Si is preferably used for emitting ultraviolet light, and 
Zn, Cd, Si and the like are preferably used for emitting blue to blue 
green light. Of the dopants used for emitting blue to blue green light, Zn 
and Si are particularly preferable in terms of handling of the precursor 
thereof. 
Of the various dopants to be doped into the above-mentioned n-type cladding 
layer 3, p-type cladding layer 6 and active layer 4, however, Zn and Mg 
are easily diffused into the adjacent layers. Thus, a diffusion 
suppressive layer 5 is formed between the p-type cladding layer 6 into 
which Mg and the like are doped, and the active layer 4, in the group-III 
nitride based light emitting diode of the present invention having a 
double heterostructure, to suppress or prevent diffusion of such 
easily-movable dopants. Such easily-movable dopants may move due to 
diffusion or migration. The diffusion suppressive layer of the present 
invention suppresses or prevents move of the dopants due to diffusion 
and/or migration. Accordingly, diffusion of dopants in the present 
invention also includes move due to migration. 
The diffusion suppressive layer 5 suffices for use as long as it can 
suppresses at least most of the diffusion of the dopants between the 
p-type cladding layer 6 and the active layer 4. It is more preferable that 
it substantially and certainly suppress or prevent the diffusion. For the 
improvement of the suppressive or preventive performance of the diffusion 
suppressive layer 5, the thickness of the diffusion suppressive layer 5 is 
generally increased, in which case, the forward voltage required for light 
emission needs to be increased, thus causing problems such as low light 
emission efficiency. On the other hand, when the thickness of the 
diffusion suppressive layer 5 is insufficient, the suppressive effect 
thereof becomes poor. Thus, the thickness of the diffusion suppressive 
layer 5 is 0.001-2 .mu.m, particularly 0.005-0.5 .mu.m, and more 
particularly 0.01-0.2 .mu.m. 
Smaller concentrations of the impurities in the diffusion suppressive layer 
5 are preferable in that the diffusion of the dopants between the 
above-mentioned layers can be absorbed in the diffusion suppressive layer 
5, while the degree of diffusion of impurities in the diffusion 
suppressive layer 5 can be decreased to more effectively suppress the 
diffusion. For this end, the diffusion suppressive layer 5 is particularly 
preferably an undoped layer. 
When the group-III nitride based light emitting diode is prepared by 
epitaxial growth, the material of the diffusion suppressive layer 5 is 
subject to no particular limitation as long as it can lattice match with 
either the p-type cladding layer 6 or the active layer 4, with preference 
given to In.sub.x Ga.sub.y Al.sub.1-x-y N wherein 
0.ltoreq..times..ltoreq.1, 0.ltoreq.y.ltoreq.1, and x+y.ltoreq.1, from the 
aspect of the aforementioned lattice match, since the materials of the 
both layers sandwiching the diffusion suppressive layer 5 are mainly GaN, 
AlGaN materials, InGaN materials and InGaAlN materials. Specifically, GaN, 
InN, AIN, InGaN, AlGaN, InAlN and InGaAlN are exemplified as the material 
of the diffusion suppressive layer 5. 
The diffusion suppressive layer 5 may be a single layer composed of a 
single material or of a multi-layer structure. The interface between 
layers in the multi-layer structure tends to trap the dopants which have 
entered the diffusion suppressive layer 5, whereby the suppressive effect 
on the diffusion of the dopant of the diffusion suppressive layer 5 can be 
advantageously improved. Such multi-layer structure can be formed by 
alternatively laminating at least two kinds of materials as mentioned 
above. For example, a first layer is prepared from GaN and a second layer 
is formed from AlGaN. This pair of layers is laminated by alternatively 
forming each layer to give a desired multi-layer structure. 
While any number of layers and materials can be used for forming the 
multi-layer structure, the total number of the layers is preferably 2-50, 
more preferably 2-10, and the number of materials is 2-4, preferably 2, 
from the reasons of easiness of manufacture. 
Be it a single layer or of a multi-layer structure, the materials of the 
upper and lower layers which come into contact with the p-type cladding 
layer or active layer preferably have superior lattice matching property 
to facilitate manufacture. When the diffusion suppressive layer 5 is a 
single layer, this layer and the p-type cladding layer are formed from the 
same semiconductor material and only the p-type cladding layer is 
subjected to necessary doping, whereby the both layers are extremely 
easily formed. 
In the embodiment of FIG. 1, the n-type cladding layer formed right above 
the buffer layer 2 may be exchanged with the p-type cladding layer, in 
which case a buffer layer, a p-type cladding layer, a diffusion 
suppressive layer, an active layer and an n-type cladding layer are 
successively formed on the substrate layer. 
The present invention is described in more detail in the following by 
illustrative examples. 
Example 1 
An LED having the structure shown in FIG. 1 was manufactured. A sapphire 
(0001) substrate was used as the substrate 1, an AlN layer was formed as 
the buffer layer 2, an Si-doped GaN layer was formed as the n-type 
cladding layer 3, a Zn-doped InGaN layer was formed as the active layer 4, 
an undoped GaN layer was formed as the diffusion suppressive layer 5 and 
an Mg-doped GaN layer was formed as the p-type cladding layer 6. 
The LED was prepared by the following method. First, the substrate 1 was 
placed in a growth chamber wherein the substrate temperature was set to 
1050.degree. C. in a hydrogen atmosphere and the substrate was subjected 
to heat treatment for 10 min. Then, the substrate temperature was lowered 
to 500.degree. C., and the AlN buffer layer 2 (0.03 .mu.m) was formed by 
supplying trimethylaluminum (TMA) at a flow rate of 30 cc/min and NH.sub.3 
at a flow rate of 4 l/min into the growth chamber. The substrate 
temperature was raised to 1020.degree. C., and the Si-doped GaN n-type 
cladding layer 3 (ca. 3 .mu.m) was formed by supplying trimethylgallium 
(TMG) at a flow rate of 50 cc/min, NH.sub.3 at a flow rate of 4 l/min, and 
SiH.sub.4 at a flow rate of 30 cc/min into the growth chamber. Then, the 
substrate temperature was lowered to 700.degree. C., and the Zn-doped 
In.sub.0.2 Ga.sub.0.8 N active layer 4 (ca. 0.01 .mu.m) was formed by 
supplying trimethylindium (TMI) at a flow rate of 200 cc/min, TMG at a 
flow rate of 40 cc/min, NH.sub.3 at a flow rate of 4 l/min and 
dimethylzinc (DMZ) at a flow rate of 10 cc/min into the growth chamber. 
The substrate temperature was raised to 1020.degree. C., and the undoped 
GaN diffusion suppressive layer 5 (ca. 0.03 .mu.m) was formed by supplying 
TMG at a flow rate of 50 cc/min and NH.sub.3 at a flow rate of 4 l/min 
into the growth chamber. Finally, the Mg-doped GaN p-type cladding layer 6 
(ca. 0.8 .mu.m) was formed by supplying TMG at a flow rate of 50 cc/min, 
NH.sub.3 at a flow rate of 4 l/min and biscyclopentadienylmagnesium 
(Cp.sub.2 Mg) at a flow rate of 20 cc/min into the growth chamber at a 
substrate temperature of 1020.degree. C. In this way, a group-III nitride 
based light emitting diode was prepared. Then, a heat treatment was 
applied at 700.degree. C. for 15 min in a nitrogen atmosphere to activate 
Mg. The carrier concentration of the n-type cladding layer 3 and p-type 
cladding layer 6 was 1.times.10.sup.18 cm.sup.-3 and 7.times.10.sup.17 
cm.sup.-3, respectively. 
Example 2 
A diode was prepared in the same manner as in Example 1 using the same 
materials, method and conditions, except that the thickness of the 
diffusion suppressive layer 5 was set for 0.1 .mu.m. 
Example 3 
A diode was prepared in the same manner as in Example 1, except that the 
thickness of the diffusion suppressive layer 5 was set for 0.5 .mu.m. 
Example 4 
A diode was prepared in the same manner as in Example 1, except that a 0.03 
.mu.m thick undoped In.sub.0.2 Ga.sub.0.8 N diffusion suppressive layer 5 
was grown at a substrate temperature of 700.degree. C., TMI flow rate of 
200 cc/min, TMG flow rate of 40 cc/min and NH.sub.3 flow rate of 4 l/min. 
Example 5 
A diode was prepared in the same manner as in Example 1, except that a 
0.005 .mu.m thick GaN was grown at a substrate temperature of 1050.degree. 
C., TMG flow rate of 50 cc/min and NH.sub.3 flow rate of 4 l/min, and a 
0.005 .mu.m thick Al.sub.0.05 Ga.sub.0.95 N was grown at TMG flow rate of 
50 cc/min, TMA flow rate of 10 cc/min and NH.sub.3 flow rate of 4 l/min, 
whereby to give a layer of GaN/Al.sub.0.05 Ga.sub.0.95 N, followed by two 
repeats thereof to give a 0.03 .mu.m thick diffusion suppressive layer 5 
composed of three layers of GaN/Al.sub.0.05 Ga.sub.0.95 N as a single 
layer. 
Comparative Example 
A diode was prepared in the same manner as in Example 1, except that a 
diffusion suppressive layer 5 was not formed. 
With respect to the diodes of Examples 1-5 and Comparative Example, 
luminous intensity, forward voltage and lifetime of light emission were 
determined, and the results are shown in Table 1. The forward voltage was 
that at the forward current of 20 mA, and lifetime of light emission was 
the time lapsed until the luminous intensity became 50% of the initial 
value when the forward current set for 50 mA. 
TABLE 1 
______________________________________ 
Com. 
Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 
______________________________________ 
n-type cladding 
Si--GaN (carrier concentration 1 .times. 10.sup.18 cm.sup.-3) 
layer 
active layer 
Zn--In.sub.0.2 Ga.sub.0.8 N 
p-type cladding 
Mg--GaN (carrier concentration 7 .times. 10.sup.17 cm.sup.-3) 
layer 
material of 
GaN GaN GaN InGaN GaN/ -- 
diffusion sup- AlGaN 
pressive layer 
thickness (.mu.m) of 
0.03 0.1 0.5 0.03 0.03 -- 
diffusion sup- 
pressive layer 
luminous 1070 900 850 1000 1000 700 
intensity (mcd) 
forward voltage 
4.0 4.5 5.5 4.0 4.0 4.0 
(V) 
lifetime (h) 
.gtoreq.10000 
.gtoreq.10000 
.gtoreq.10000 
.gtoreq.10000 
.gtoreq.10000 
5000 
______________________________________ 
As shown in Table 1, the luminous intensity of the diodes of Examples 1-5 
were about 1.2-1.5 times higher than that of the diode of Comparative 
Example, and the forward voltage was greater in Example 3 than in 
Comparative Example, and the diode of Example 2 showed slightly greater 
voltage. The lifetime of the diode was more than twice longer in Examples 
1-5 than in Comparative Example. 
The diodes of the Examples with a diffusion suppressive layer showed about 
1.2-1.5 times higher luminous intensity of light emission than without the 
diffusion suppressive layer, and the lifetime was not less than 10,000 
hours which was more than twice longer than the conventional diodes. It 
was also confirmed that thicker diffusion suppressive layer had higher 
forward voltages.