II/VI-compound semiconductor light emitting device

A lifetime of a II/VI-compound semiconductor light emitting device can be extended. The II/VI-compound semiconductor light emitting device includes an active layer (4) and a p-side cladding layer (6). An active-layer side portion (26) of the p-side cladding layer (6) is formed as a lightly impurity-doped region or a non-doped region.

BACKGROUND OF THE INVENTION 
The present invention relates to a II/VI-compound semiconductor light 
emitting device for use in light emitting devices, such as a light 
emitting diode and a semiconductor laser and, particularly to a green or 
blue semiconductor light emitting device. 
In a magneto-optical recording for recording or reproducing or recording 
and reproducing a magneto-optical signal by laser beams, a demand for 
using a short-wavelength laser, e.g., a blue semiconductor laser as a 
laser light source is progressively increased in order to improve a 
recording density. A II-VI ZnMgSSe semiconductor laser receives a 
remarkable attention as this kind of semiconductor laser. 
It is generally difficult to dope a p-type impurity in a II/VI-compound 
semiconductor, such as ZnSe, ZnSSe and ZnMgSSe. The doping of p-type 
impurity could be finally realized by using nitrogen N. When N is doped in 
ZnSe, an upper limit value of a hole concentration is 1.times.10.sup.18 
cm.sup.-3. When N is doped in ZnSSe, it is up to 10.sup.17 cm.sup.-3 which 
is a considerably low value. Since it is confirmed that nitrogen N itself 
is introduced into a crystal, it is considered that nitrogen N which does 
not increase hole density is inactivated. Therefore, it is considered that 
most of such nitrogen N become interlattice atoms. 
As shown in FIG. 1 of the accompanying drawings, the ZnMgSSe semiconductor 
laser made of the II/VI-compound semiconductor has a II/VI-compound 
semiconductor laser portion 7. The II/VI-compound semiconductor laser 
portion 7 has an n-GaAs substrate 1, a first cladding layer 2 made of 
n-ZnMgSSe, a first guide layer 3 made of n-ZNSE with or without S 
(hereinafter referred to as "ZN(S)Se"), an active layer 4 made of ZnCdSe, 
a second guide layer 5 made of p-Zn(S)Se, a second cladding layer 6 made 
of p-ZnMgSSe and an electrode 14 made of In, for example. The first 
cladding layer 2, the first guide layer 3, the active layer 4, the second 
guide layer 5 and the second cladding layer 6 are formed on the major 
surface of the n-GaAs substrate 1, in that order. The electrode 14 is 
formed on the other surface of the substrate 1 in ohmic contact. The 
ZnMgSSe semiconductor laser has a p-side electrode portion 12 formed on 
the II/VI-compound semiconductor laser portion 7. The p-side electrode 
portion 12 has a first semiconductor layer 8 made of p-Zn(S)Se as a 
capping layer, a second semiconductor layer 9 having a multiple quantum 
well (MQW) structure of ZnSE and ZnTe, a third semiconductor layer 10 made 
of p-ZnTe, an insulating layer 13 made of polyimide, for example, and a 
p-type metal electrode 11 having a multilayer structure in which Pd, Pt 
and Au are sequentially deposited. The first semiconductor layer 8, the 
second semiconductor layer 9, the third semiconductor layer 10 are 
sequentially formed by epitaxial growth. Then, the second and third 
semiconductor layers 9, 10 are selectively etched such that their center 
portions are left in a stripe fashion. The insulating layer 13 is buried 
in both side portions where the second and third semiconductor layers 9, 
10 are removed by etching. The metal electrode 11 is formed on the third 
semiconductor 10 in ohmic contact. 
Although the II/VI-compound semiconductor laser of this kind achieves 
continuous wave oscillation at room temperature, the II/VI-compound 
semiconductor laser can continuously oscillate in the order of seconds and 
is considerably short lifetime. Therefore, the II/VI-compound 
semiconductor laser could not be applied to an optical pickup device for 
high-density magneto-optical recording in actual practice. 
SUMMARY OF THE INVENTION 
In view of the aforesaid aspect, it is an object of the present invention 
to provide a long-life II/VI-compound semiconductor light emitting device. 
The same assignee of this application had found the following fact. As a 
doping concentration of a p-type impurity, e.g., nitrogen N is increased 
in a p-side region adjacent to an active layer, particularly in a cladding 
layer in order to reduce a resistivity, nitrogen (hereinafter referred to 
as interstitial nitrogen N.sub.int) located between lattices is increased. 
The interstitial nitrogen N.sub.int becomes a point defect in a crystal or 
induces another point defect, thereby contributing to the increase of 
dislocation. As a result, concentration of the interstitial nitrogen 
N.sub.int brings a harmful influence on lifetime of the II/VI-compound 
semiconductor light emitting device. The same assignee of this application 
obtained a long-life II/VI-compound semiconductor light emitting device by 
suppressing the p-type-impurity doping concentration. 
According to a first aspect of the present invention, a II/VI-compound 
semiconductor light emitting device includes an active layer and a p-side 
cladding layer. An active-layer side portion of the p-side cladding layer 
is a lightly impurity-doped region or a non-doped region. 
According to a second aspect of the present invention, in the 
II/VI-compound semiconductor light emitting device according to the first 
aspect of the present invention, the lightly impurity-doped region or 
non-doped region is provided at a portion in contact with the active 
layer. 
According to a third aspect of the present invention, in the II/VI-compound 
semiconductor light emitting device according to the first aspect of the 
present invention, the lightly impurity-doped region or non-doped region 
is provided at a portion in contact with the p-side guide layer. 
According to a fourth aspect of the present invention, in the 
II/VI-compound semiconductor light emitting device according to the third 
aspect of the present invention, the lightly impurity-doped region is 
provided in the p-side guide layer. 
According to a fifth aspect of the present invention, in the II/VI-compound 
semiconductor light emitting device according to the fourth aspect of the 
present invention, a portion, which is in contact with the active layer, 
of the p-side guide layer is a non-doped region. 
According to a sixth aspect of the present invention, in the II/VI-compound 
semiconductor light emitting device according to the fourth aspect of the 
present invention, an impurity doping concentration of the lightly 
impurity-doped region of the p-side cladding layer is small as compared 
with that of the lightly impurity-doped region of the second guide layer. 
According to a seventh aspect of the present invention, in the 
II/VI-compound semiconductor light emitting device according to the first 
aspect of the present invention, an impurity doping concentration of the 
p-side cladding layer is progressively decreased toward the active layer. 
According to an eighth aspect of the present invention, the II/VI-compound 
semiconductor light emitting device includes the active layer and the 
p-side guide layer. An active-layer side portion of the p-side guide layer 
is a lightly impurity-doped region or a non-doped region. 
According to a ninth aspect of the present invention, in the II/VI-compound 
semiconductor light emitting device according to the first aspect of the 
present invention, the lightly impurity-doped region has an interstitial 
impurity concentration of 2.times.10.sup.17 cm.sup.-3 or smaller. 
According to a tenth aspect of the present invention, in the II/VI-compound 
semiconductor light emitting device according to the first aspect of the 
present invention, when the lightly impurity-doped region is made of 
ZnMgSSe, an impurity doping concentration of the lightly impurity-doped 
region is 7.times.10.sup.17 cm.sup.-3 or smaller. 
According to an eleventh aspect of the present invention, in the 
II/VI-compound semiconductor light emitting device according to the tenth 
aspect of the present invention, when the lightly impurity-doped region is 
made of ZnMgSSe with a band gap energy of 2.9 eV or larger, an impurity 
doping concentration of the lightly impurity-doped region is 
3.times.10.sup.17 cm.sup.-3 or smaller. 
According to a twelfth aspect of the present invention, in the 
II/VI-compound semiconductor light emitting device according to the first 
aspect of the present invention, when the lightly impurity-doped region is 
made of Zn (S) Se, an impurity doping concentration of the lightly 
impurity-doped region is 8.times.10.sup.17 cm.sup.-3 or smaller. 
According to a thirteenth aspect of the present invention, the 
II/VI-compound semiconductor light emitting device includes the active 
layer and the p-side guide layer. A portion, which is in contact with the 
active layer, of the p-side cladding layer is a non-doped region and a 
region thereof other than the non-doped region is a doped region. 
According to a fourteenth aspect of the present invention, in the 
II/VI-compound semiconductor light emitting device according to the 
thirteenth aspect of the present invention, an active-layer side portion 
of the guide layer is a non-doped region.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will hereinafter be described with 
reference to the drawings. 
In the following embodiments, the present invention is applied to a 
II/VI-compound semiconductor laser. 
FIG. 2 shows a first embodiment according to the present invention. 
As shown in FIG. 2, a II/VI-compound semiconductor laser 25 has a substrate 
1 made of n-GaAs single crystal doped with Si, for example, a first 
cladding layer 2 made of n-ZnMgSSe doped with C1 with a thickness of 0.8 
.mu.m, for example, a first guide layer 3 made of n-Zn(S)Se doped with Cl 
with a thickness of 120 nm, for example, an active layer 4 made of ZnCdSe 
with a thickness of 7 nm, for example, a second guide layer 5 made of 
Zn(S)Se with a thickness of 120 nm, for example, and a second cladding 
layer 6 made of p-ZnMgSSe doped with nitrogen N with a thickness of 0.6 
.mu.m, for example. The first cladding layer 2, the first guide layer 3, 
the active layer 4, the second guide layer 5 and the second cladding layer 
6 are sequentially formed on the major surface of the substrate 1 by 
epitaxial growth to form a semiconductor laser portion 7. 
In the first embodiment, nitrogen doping concentration in a p-side region 
from the second cladding layer 6 to the second guide layer 5 is reduced 
gradually toward the active layer 4 to present a multistage doping 
concentration distribution of nitrogen N which is a p-type impurity. 
Specifically, as shown in FIG. 2, the second guide layer 5 has a non-doped 
region 21 made of Zn(S)Se formed at its portion in contact with the active 
layer 4 and a lightly impurity-doped region 22 made of p-Zn(S)Se, which 
will be described later on, formed on the second cladding layer 5 side 
thereof. The second cladding layer 6 has a lightly impurity-doped region 
23, which will be described later on, formed at its portion in contact 
with the second guide layer 5 and a heavily impurity-doped region 24 
formed at the rest portion. 
At least the second cladding layer 6 has the multistage doping 
concentration distribution of p-type impurity having two stages or more. 
Subsequently, the II/VI-compound semiconductor laser 25 has a p-side 
electrode portion 12 formed on the semiconductor laser portion 7. The 
p-side electrode portion 12 has a first semiconductor layer 8 serving as a 
capping layer made of n-Zn(S)Se doped with N, for example, with a 
thickness of 0.6 .mu.m, for example, a second semiconductor layer 9 having 
a multiple quantum well (MQW) structure of ZnSe and ZnTe, a third 
semiconductor layer 10 made of n-ZnTe doped with N, an insulating layer, 
e.g., polyimide layer 13 and a metal electrode 11 having a multilayer 
structure in which Pd, Pt and Au are sequentially laminated. The first 
semiconductor layer 8, the second semiconductor layer 9 and the third 
semiconductor layer 10 are continuously formed on the second cladding 
layer 6 by epitaxial growth. The second and third semiconductor layers 9, 
10 are selectively removed by etching such that their center portions are 
left in a stripe fashion. The polyimide layer 13 is buried in both side 
portions where the second and third semiconductor layers 9, 10 are removed 
by etching. The metal electrode 11 is formed on the striped third 
semiconductor layer 10 so as to have ohmic contact to the latter. The 
semiconductor laser portion 7 has the other electrode 14 made of In formed 
on the other surface of the substrate 1 so as to have ohmic contact to the 
substrate 1. 
FIG. 3 shows an activated acceptor concentration (NA-ND) and concentration 
of an interstitial nitrogen N.sub.int relative to nitrogen doping 
concentration measured when nitrogen, a p-type impurity, was doped in 
ZnMgSSe (with a band gap energy of 2.74 eV at room temperature) forming 
the cladding layers 2, 6. In FIG. 3, a solid curve I represents the 
acceptor concentration and a broken curve II represents the interstitial 
nitrogen concentration. 
The acceptor concentration starts being saturated from the nitrogen doping 
concentration of 10.sup.17 cm.sup.-3. A saturation start point S at this 
time is defined by an intersection point of both tangents a and b of the 
solid curve I. As shown in FIG. 3, an region A is a saturated region 
([N]&gt;7.times.10.sup.17 cm.sup.-3) and a region B is a non-saturated region 
([N].ltoreq.7.times.10.sup.17 cm.sup.-3). 
The interstitial nitrogen concentration is increased as the nitrogen doping 
concentration [N]is increased. 
In order to suppress the point defects introduced by the interstitial 
nitrogen N.sub.int, it is desirable to suppress the interstitial nitrogen 
concentration to 2.times.10.sup.17 cm.sup.-3 or smaller, though it is a 
rough standard, preferably 1.times.10.sup.17 cm.sup.-3 or smaller. If the 
interstitial nitrogen concentration exceeds 2.times.10.sup.17 cm.sup.-3, 
then the interstitial nitrogen is diffused intensively. Study of FIG. 3 
reveals that it is possible to set the interstitial nitrogen concentration 
to 2.times.10.sup.17 cm.sup.-3 or smaller by setting the nitrogen doping 
concentration [N]to 7.times.10.sup.17 cm.sup.-3 or smaller and to set the 
interstitial nitrogen concentration to 1.times.10.sup.17 cm.sup.-3 or 
smaller by setting the nitrogen doping concentration [N]to 
5.times.10.sup.17 cm.sup.-3 or smaller. 
ZnMgSSe becomes saturated at lower acceptor concentration as its band gap 
energy becomes large. FIG. 4 shows an acceptor concentration (N.sub.A 
-N.sub.D) and an interstitial nitrogen concentration relative to nitrogen 
doping concentration measured when nitrogen was doped in ZnMgSSe with a 
band gap energy of 2.90 eV at room temperature. In FIG. 4, a solid curve 
III represents the acceptor concentration and a broken curve IV represent 
the interstitial nitrogen concentration. 
Study of FIG. 4 shows that it is possible to set the interstitial nitrogen 
concentration to 1.4.times.10.sup.17 cm.sup.-3 or smaller by setting the 
nitrogen doping concentration [N]to 3.times.10.sup.17 cm.sup.-3 or 
smaller. 
FIG. 5 shows an activated-acceptor concentration (N.sub.A -N.sub.D) and an 
interstitial nitrogen concentration relative to nitrogen doping 
concentration measured when nitrogen N, the p-type impurity, was doped in 
ZnSe forming the guide layers. In FIG. 5, a solid curve V represents the 
acceptor concentration and a broken curve VI represent the interstitial 
nitrogen concentration. Study of FIG. 5 shows that it is possible to set 
the interstitial nitrogen concentration to 2.times.10.sup.17 cm.sup.-3 or 
smaller by setting the nitrogen doping concentration [N]to 
8.times.10.sup.17 cm.sup.-3 or smaller and to set the interstitial 
nitrogen concentration to 1.times.10.sup.17 cm.sup.-3 or smaller by 
setting the nitrogen doping concentration [N]to 6.times.10.sup.17 
cm.sup.-3 or smaller. In case of ZnSSe obtained by including S of about 6% 
in ZnSe, its concentration distribution is substantially similar to that 
of ZnSe. 
However, in the first embodiment shown in FIG. 2, the second cladding layer 
6 has a portion 23 in contact with the second guide layer 5 and a main 
portion 24 other than the portion 23. The main portion 24 is formed as the 
high impurity-doped region with a nitrogen doping concentration [N]of 
[N]&gt;7.times.10.sup.17 cm.sup.-3 such that the acceptor concentration is 
set in the saturated region A on the basis of the concentration 
distribution graph shown in FIG. 3 for suppressing the increase of a total 
resistance. The second cladding layer 6 has the portion 23 in contact with 
the second guide layer 5 is formed as the lightly impurity-doped region 
with the nitrogen doping concentration of [N].ltoreq.5.times.10.sup.17 
cm.sup.-3, preferably [N].ltoreq.1.times.10.sup.17 cm.sup.-3. 
In case of [N]5.times.10.sup.17 cm.sup.-3, the acceptor concentration 
becomes 3.times.10.sup.17 cm.sup.-3 or smaller and the concentration of 
the interstitial nitrogen N.sub.int becomes 1.times.10.sup.17 cm.sup.-3 or 
smaller. Therefore, the interstitial nitrogen N.sub.int is reduced and the 
diffusion of the interstitial nitrogen N.sub.int is suppressed. If the 
interstitial nitrogen concentration has the above value, it is possible to 
suppress a harmful influence resulting from the point defects. 
In case of [N].ltoreq.1.times.10.sup.17 cm.sup.-3, the acceptor 
concentration becomes 8.times.10.sup.16 cm.sup.-3 or smaller and the 
concentration of the interstitial nitrogen N.sub.int becomes 
1.times.10.sup.16 cm.sup.-3 or smaller. If the interstitial nitrogen 
concentration has the above value, the point defects is substantially 
negligible. 
Moreover, the second guide layer 5 shown in FIG. 2 has a portion 22 in 
contact with the p-type second cladding layer 6 and a portion 21 in 
contact with the active layer 4. 0n the basis of the concentration 
distribution graph shown in FIG. 5, the portion 22 is formed as the 
lightly impurity-doped region with a nitrogen doping concentration [N] of 
[N].ltoreq.2.times.10.sup.17 cm.sup.-3 and the portion 21 is formed as the 
non-doped region. When the nitrogen doping concentration [N] of the second 
guide layer 5 made of Zn(S)Se is set as [N].ltoreq.2.times.10.sup.17 
cm.sup.-3, the interstitial nitrogen concentration is 1.times.10.sup.16 
cm.sup.-3 or smaller. Specifically, the II/VI-compound semiconductor light 
emitting device 25 according to the first embodiment shown in FIG. 2 has a 
multistage doping concentration distribution in which the nitrogen doping 
concentration in the region of the second cladding layer 6 and the second 
guide layer 5 becomes lowered stepwise toward the active layer 4. 
According to the II/VI-compound semiconductor laser 25 having the 
above-mentioned arrangement shown in FIG. 2, since the II/VI-compound 
semiconductor laser 25 has the multistage impurity doping concentration 
distribution in the region from the p-type second cladding layer 6 to the 
p-type second guide layer 5 and particularly the portion 23, which is in 
contact with the second guide layer 5, of the second cladding layer 6 is 
formed as the lightly impurity-doped region with 
[N].ltoreq.5.times.10.sup.17 cm.sup.-3, it is possible to reduce the 
interstitial nitrogen concentration at the portion adjacent to the active 
layer 4 and to suppress the point defects (non-radiative recombination 
factor) introduced by interstitial nitrogen near the active layer 4. 
Specifically, as described above, if the interstitial nitrogen 
concentration in the lightly impurity-doped region 23 of the second 
cladding layer 6 is 1.times.10.sup.17 cm.sup.-3 or smaller, then it is 
possible to suppress the harmful influence of the point defects resulting 
from the interstitial nitrogen N.sub.int. If the interstitial nitrogen 
concentration in the lightly impurity-doped region 23 of the second 
cladding layer 6 is 1.times.10.sup.16 cm.sup.-3 or smaller, then it is 
possible to substantially disregard the point defects resulting from the 
interstitial nitrogen N.sub.int, Since the second cladding layer 6 side 
portion 22 of the second guide layer 5 has the nitrogen doping 
concentration [N] of 2.times.10.sup.17 cm.sup.-3 or smaller, its 
interstitial nitrogen concentration becomes 1.times.10.sup.16 cm.sup.-3 or 
smaller. Since the active layer 4 side portion 21 of the second guide 
layer 5 is the non-doped region, the portion 21 has no interstitial 
nitrogen. Accordingly, it is possible to disregard the harmful influence 
of the point defects in the second guide layer 5. 
On the other hand, since the main portion 24 of the second cladding layer 6 
is the heavily impurity-doped region with a nitrogen doping concentration 
[N] of [N]&gt;7.times.10.sup.17 cm.sup.-3, it is possible to lower the 
resistance thereof. 
Accordingly, it is possible to extend a lifetime of the semiconductor laser 
25 by providing the lightly impurity-doped regions 22, 23 and the 
non-doped region 21 while the increase of a total voltage, i.e., an 
operating voltage of the semiconductor laser is suppressed by providing 
the heavily impurity-doped region 21. Thus, it becomes more practical to 
use the II/VI-compound semiconductor laser, i.e., the green or blue 
semiconductor laser. 
FIG. 6 shows a second embodiment according to the present invention, 
In the second embodiment, particularly a second guide layer 5 is formed as 
a p-type guide layer. The second guide layer 5 has a second cladding layer 
6 side portion 28 doped with nitrogen at a concentration at which its 
interstitial nitrogen concentration is 5.times.10.sup.16 cm.sup.-3 or 
smaller. The second guide layer 5 has a portion 28 formed in contact with 
the active layer 4 such that a nitrogen doping amount of the portion 28 is 
lower than that of the portion 27. 
Other elements and parts are similar to those of the first embodiment shown 
in FIG. 2. Therefore, they are marked with the same references and need 
not be described in detail. 
In a II/VI-compound semiconductor laser 29 having such arrangement shown in 
FIG. 6, since the interstitial nitrogen N.sub.int at a portion adjacent to 
an active layer 4 is small in number, it is possible similarly to the 
first embodiment to suppress the point defects introduced by the 
interstitial nitrogen while the increase of the operating voltage is 
suppressed and to thereby extend the lifetime of the semiconductor laser. 
FIG. 7 shows a third embodiment according to the present invention. 
In the third embodiment, a second guide layer 5 is formed as a non-doped 
region or a p-type region with an interstitial nitrogen concentration of 
5.times.10.sup.16 cm.sup.-3 or smaller. A p-type second cladding layer 6 
has a portion 23 in contact with the second guide layer 5 formed as a 
lightly impurity-doped region with a nitrogen doping concentration [N] of 
[N].ltoreq.5.times.10.sup.17 cm.sup.-3, preferably 
[N].ltoreq.1.times.10.sup.17 cm.sup.-3. The second cladding layer 6 has a 
rest portion 24 formed as a heavily impurity-doped region with a nitrogen 
doping concentration of 7.times.10.sup.17 cm.sup.-3 or larger similarly to 
the first embodiment. Other elements and parts are similar to those of the 
first embodiment shown in FIG. 2. Therefore, they are marked with the same 
references and need not be described in detail. In a II/VI-compound 
semiconductor laser 31 having such arrangement shown in FIG. 7, it is 
possible to suppress the point defects introduced by the interstitial 
nitrogen while the increase of an operating voltage is suppressed and to 
thereby extend a lifetime of the semiconductor laser. 
FIG. 8 shows a fourth embodiment according to the present invention. 
In the fourth embodiment, a second guide layer 5 is formed as a non-doped 
region or a p-type region with an interstitial nitrogen concentration of 
5.times.10.sup.16 cm.sup.-3 or smaller. A p-type second cladding layer 6 
has a portion 32 in contact with the second guide layer 5 formed as a 
non-doped region. 
The second cladding layer 6 has a main portion 24 other than the portion 32 
in contact with the second guide layer 5, the main portion 24 being formed 
as a heavily impurity-doped region with an interstitial nitrogen 
concentration of 7.times.10.sup.17 cm.sup.-3 or larger similarly to the 
first embodiment. 
Other elements and parts are similar to those of the first embodiment shown 
in FIG. 2. Therefore, they are marked with the same references and need 
not be described in detail. 
In a II/VI-compound semiconductor laser 33 having such arrangement shown in 
FIG. 8, it is possible to suppress the interstitial nitrogen introduced 
near the active layer by forming the portion 32, which is in contact with 
the second guide layer 5, of the second cladding layer 6 as the non-doped 
region. It is possible to suppress the point defects introduced by the 
interstitial nitrogen while the increase of an operating voltage is 
suppressed and to extend the lifetime of the semiconductor laser. 
FIG. 9 shows a fifth embodiment according to the present invention. 
In the fifth embodiment, a second cladding layer 6 is formed as a p-type 
region with a normal impurity concentration. A second guide layer 5 has a 
second cladding layer 6 side portion 43 formed as a p-type region with a 
normal impurity concentration. The second guide layer 5 has a portion 44 
in contact with an active layer 4 formed as a non-doped region or a 
lightly impurity-doped region as described above. 
Other elements and parts are similar to those of the first embodiment shown 
in FIG. 2. Therefore, they are marked with the same references and need 
not be described in detail. 
In a II/VI-compound semiconductor laser 45 having such arrangement shown in 
FIG. 9, it is possible to suppress the interstitial nitrogen introduced 
near the active layer by forming the portion 44, which is in contact with 
the active layer 4, of the second guide layer 5 as the non-doped region or 
the lightly impurity-doped region. It is possible to suppress the point 
defects introduced by the interstitial nitrogen while increase of an 
operating voltage is suppressed and to extend the lifetime of the 
semiconductor laser. 
In each of the first to ninth embodiments respectively shown in FIGS. 2 and 
6 through 9, the present invention is applied to a semiconductor laser 
having a so-called separate confinement heterostructure (SCH) in which an 
active layer is sandwiched by guide layers and cladding layers are 
disposed outside the guide layers, particularly the SCH having a guide 
layer with small thickness of about 900 .ANG. to 1200 .ANG.. FIG. 10 shows 
a sixth embodiment in which the present invention is applied to an SCH 
having a guide layer with large thickness of 1500.ANG. or 2000.ANG. or 
larger, for example. 
In the sixth embodiment shown in FIG. 10, a second guide layer 5 has a 
portion 35 in contact with an active layer 4 and a portion 36 which is a 
region except the portion 35. The portion 35 is formed as a non-doped 
region or a lightly impurity-doped region with a nitrogen doping 
concentration [N] of 8.times.10.sup.17 cm.sup.-3 or smaller, for example, 
preferably 6.times.10.sup.17 cm.sup.-3 or smaller which is smaller than a 
nitrogen doping concentration of the portion 36. A p-type second cladding 
layer 6 is formed as a heavily impurity-doped region in which an acceptor 
concentration is set in a saturated region A shown in FIG. 3. 
Other elements and parts are similar to those of the first embodiment shown 
in FIG. 2. Therefore, they are marked with the same references and need 
not be described in detail. 
It is possible to set a nitrogen doping concentration in a region from the 
portion 36 to the portion 35 to 8.times.10.sup.17 cm.sup.-3 or smaller to 
reduce the nitrogen doping concentration stepwise toward the active layer 
4. 
In a II/VI-compound semiconductor laser 37 having such arrangement shown in 
FIG. 10, since the second guide layer 5 has a stepwise distribution of the 
nitrogen doping concentration in which the second guide layer 5 has low 
nitrogen doping concentration on its active layer 4 side, it is possible 
to suppress the point defects introduced by the interstitial nitrogen and 
to thereby extend the lifetime of the semiconductor laser. 
FIG. 11 shows a seventh embodiment according to the present invention. 
In the seventh embodiment, the present invention is applied to a 
II/VI-compound semiconductor laser having a so-called double 
heterostructure (DH) in which an active layer is directly sandwiched by 
cladding layers. 
In the seventh embodiment, as shown in FIG. 10, a II/VI-compound 
semiconductor laser 41 has a substrate 1 made of an n-type single crystal 
GaAs doped with Si, for example, a first cladding layer 2 made of 
n-ZnMgSSe doped with Cl, for example, an active layer 4 made of ZnCdSe, 
and a second cladding layer 6 made of p-ZnMgSSe doped with N. The first 
cladding layer 2, the active layer 4 and the second cladding layer 6 are 
formed on the surface of the substrate 1 by epitaxial growth to form a 
semiconductor laser portion 7. In the seventh embodiment, particularly the 
second cladding layer 6 has a portion 39 in contact with the active layer 
4 formed as the lightly impurity-doped region or the non-doped region as 
described with reference to FIG. 2 and a rest portion 40 other than the 
portion 39 formed as the heavily impurity-doped region as described with 
reference to FIG. 2. It is possible to form the portion 39, which is in 
contact with the active layer 4, of the second cladding layer 6 as the 
non-doped region. 
Subsequently, similarly to the first embodiment shown in FIG. 2, the 
II/VI-compound semiconductor laser 41 has a p-side electrode portion 12 
formed on the second cladding layer 6 and an electrode 14 formed on the 
other surface of the substrate 1. 
In the II/VI-compound semiconductor laser 41 having such arrangement shown 
in FIG. 11, since the portion 39, which is in contact with the active 
layer 4, of the p-type second cladding layer 6 is formed as the lightly 
impurity-doped region or the non-doped region, it is possible to suppress 
the point defects introduced near the active region and to thereby extend 
the lifetime of the semiconductor laser. 
FIG. 12 shows an eighth embodiment according to the present invention. 
In the eighth embodiment, a second guide layer 5 has a region in contact 
with an active layer 4 formed as a non-doped region 42 and a rest region 
other than the non-doped region 42 formed as a heavily impurity-doped 
region 43 (a so-called p-type guide region) in which an acceptor 
concentration is set in the saturated region A shown in FIG. 3. A second 
cladding layer 6 has a region in contact with the second guide layer 5 
formed as a non-doped region 44 and a rest region other than the non-doped 
region 44 formed as a heavily impurity-doped region 45 in which an 
acceptor concentration is set in the saturated region A shown in FIG. 3. 
A thickness t.sub.1 of the non-doped region 42 of the second guide layer 5 
can be set in a range of from 300 .ANG. to 900 .ANG. and a thickness 
t.sub.2 of the heavily impurity-doped region 43 can be set in a range of 
from 900 .ANG. to 300 .ANG.. A thickness t.sub.3 of the non-doped region 
44 of the second cladding layer 6 can be set in a range of from 50 .ANG. 
to 2000 .ANG. and a thickness t.sub.4 of the heavily impurity-doped region 
45 can be set in a range of from 7500.ANG. to 5500.ANG.. 
In the eighth embodiment shown in FIG. 12, the thickness t.sub.1 of the 
non-doped region 42 of the second guide layer 5 is set to 700 .ANG. and 
the thickness t.sub.2 of the heavily impurity-doped region 43 thereof is 
set to 500 .ANG.. The thickness t.sub.3 of the non-doped region 44 of the 
second cladding layer 6 is set to 500 .ANG. and the thickness t.sub.4 of 
the heavily impurity-doped region 45 is set to 7100 .ANG.. A thickness of 
a first cladding layer 2 is set to 7600 .ANG. and a thickness of a fist 
guide layer 3 is set to 1200.ANG.. A thickness of the active layer 4 is 
set to 70.ANG. and a thickness of a first semiconductor layer 8 serving as 
a capping layer was set to 4000 .ANG.. 
Other elements and parts are similar to those of the first embodiment shown 
in FIG. 2. Therefore, they are marked with the same references and need 
not be described in detail. 
According to a II/VI-compound semiconductor laser 46 having such 
arrangement shown in FIG. 12, since the region, which is in contact with 
the active layer 4, of the second guide layer 5 is formed as the non-doped 
region 42 and the region, which is in contact with the second guide layer 
5, of the second cladding layer 6 is formed as the non-doped region 44, it 
is possible to control the point defects introduced by the interstitial 
nitrogen and further to reduce an amount of electrons overflowing into the 
second cladding layer 6. Thus, it is possible to extend the lifetime of 
the semiconductor laser. Moreover, it is possible to lower the threshold 
current value Ith. 
If the thickness t.sub.1 of the non-doped region 42 of the second guide 
layer 5 is smaller than 300 .ANG., then it is impossible to achieve an 
effect of controlling the point defects. If the thickness t.sub.1 exceeds 
900 .ANG., then the amount of electrons overflowing into the second 
cladding layer 6 and the threshold current value Ith are both increased. 
If the thickness t.sub.3 of the non-doped region 44 of the second cladding 
layer 6 is smaller than 50 .ANG., then it is impossible to achieve an 
effect of suppressing the point defects. If the thickness t.sub.3 exceeds 
2000 .ANG., then an influence occurs which is caused by the increase of a 
total resistance. 
FIG. 13 shows a ninth embodiment according to the present invention. 
In the ninth embodiment, a second guide layer has a region in contact with 
an active layer 4 formed as a non-doped region 42 and a rest region other 
than the region 42 formed as a heavily impurity-doped region 43 in which 
an acceptor concentration is set in a saturated region A shown in FIG. 3. 
A second cladding layer 6 is formed as a heavily impurity-doped region in 
which an acceptor concentration is set in the saturated region A shown in 
FIG. 3. 
A thickness t.sub.1 of the non-doped region 42 of the second guide layer 5 
can be set in a range of from 300 .ANG. to 900 .ANG. and a thickness 
t.sub.2 of the heavily impurity-doped region 43 can be set in a range of 
from 900 .ANG. to 300 .ANG.. In the ninth embodiment shown in FIG. 13, the 
thickness t.sub.1 is set to 700 .ANG. and the thickness t.sub.2 is set to 
500 .ANG.. A thickness of each of first and second cladding layers 2, 6 is 
set to 7600 .ANG. and a thickness of a fist guide layer 3 is set to 1200 
.ANG.. A thickness of the active layer 4 is set to 70 .ANG. and a 
thickness of a first semiconductor layer 8 serving as a capping layer is 
set to 4000 .ANG.. Other elements and parts are similar to those of the 
first embodiment shown in FIG. 2. Therefore, they are marked with the same 
references and need not be described in detail. 
In a II/VI-compound semiconductor laser 47 having such arrangement shown in 
FIG. 13, since the region with the thickness t.sub.1, which is in contact 
with the active layer 4, of the second guide layer 5 is formed as the 
non-doped region 42, it is possible to control the point defects 
introduced by the interstitial nitrogen and to lower an amount of 
electrons overflowing into the second cladding layer 6. Thus, it is 
possible to extend the lifetime of the semiconductor. Moreover, it is 
possible to lower the threshold current value Ith. 
Since the semiconductor laser 46 shown in FIG. 12 has the non-doped region 
44 formed also in the second cladding layer 6, the semiconductor laser 46 
achieves a greater effect of suppressing the point defects as compared 
with the semiconductor laser 47. 
FIG. 14 is a graph showing simulated results of the threshold current value 
Ith and an amount (Pn) of electron overflowing into the second cladding 
layer 6 obtained when there were compared the semiconductor laser 46 
having the arrangement shown in FIG. 12, the semiconductor laser 47 having 
the arrangement shown in FIG. 13 and the semiconductor laser 33 having the 
arrangement shown in FIG. 8 in which the second guide layer 5 was formed 
as the non-doped region and the portion 32, that was in contact with the 
second guide layer 5, of the second cladding layer 6 was formed as the 
non-doped region. In FIG. 14, a solid curve VII represents the threshold 
current value Ith and a solid curve VIII represents the electron overflow 
amount (Pn). 
The semiconductor laser 33 shown in FIG. 8 used in the above comparison had 
the first cladding layer 2 with a thickness of 7600 .ANG., the first guide 
layer 3 with a thickness of 1200 .ANG., the active layer 4 with a 
thickness of 70 .ANG., the second guide layer 5 with a thickness of 1200 
.ANG. and the non-doped region 32 of the second cladding layer 6 with a 
thickness of 500 .ANG., and the heavily impurity-doped region 24 thereof 
with a thickness of 7100 .ANG.. 
Study of the graph shown in FIG. 14 reveals that the threshold current 
values Ith and electron overflow amounts (Pn) of the semiconductor lasers 
46, 47 respectively shown in FIGS. 12 and 13 were lowered as compared with 
the semiconductor laser 33 shown in FIG. 8. Specifically, since an 
effective barrier to electron is lowered as thicknesses of the non-doped 
regions in the second cladding layer 6 and the guide layer 5 are 
increased, the amount of electrons overflowing into the p side of the 
semiconductor laser from the n side thereof is increased. Accordingly, by 
employing the arrangements of the II/VI-compound semiconductor lasers 46, 
47 respectively shown in FIGS. 12 and 13, it is possible to further 
suppress the amount of electrons overflowing into the p-side cladding 
layer and to thereby extend the lifetime of the semiconductor laser. 
Moreover, it is possible to suppress the increase of the threshold current 
Ith. Thus, the blue semiconductor laser becomes more practical. 
While N is used as the. p-type impurity as described above, the present 
invention is not limited thereto and other group I or IV elements, such as 
indium In can be used with similar effects being achieved. 
While ZnMgSSe is used to form the first and second cladding layers 3, 6 as 
described above, the present invention is not limited thereto and a 
superlattice structure of ZnMgSSe and ZnSe may be used. In this case, an 
impurity doping concentration of an active-layer side lightly 
impurity-doped region is selected in response to a composition of the 
superlattice structure such that an interstitial impurity concentration 
should be 2.times.10.sup.17 cm.sup.-3 or smaller. 
According to a method of controlling the above doping concentration of the 
p-type impurity, i.e., N stepwise, when active nitrogen excited by plasma 
is used, it is possible to control the nitrogen doping concentration 
stepwise by changing a plasma power. 
Moreover, while the second cladding layer 6 and/or the active-layer side 
portion of the second guide layer 5 in the p-side region which is 
intensively influenced by the interstitial nitrogen are formed as the 
lightly impurity-doped region or the non-doped region as described above, 
it is possible that the first cladding layer 2 and/or the first guide 
layer 3 in the n-side region have the same arrangement. 
As described above, according to the present invention, it is possible to 
obtain the room-temperature continuous-wave-oscillation and long-life 
II/VI-compound semiconductor laser, i.e., the green or blue semiconductor 
laser. Accordingly, it becomes much more practical to use the 
II/VI-compound laser in the optical pickup apparatus for high-density 
magneto-optical recording. 
While the present invention is applied to the II/VI-compound semiconductor 
laser as described above, the present invention is not limited thereto and 
can be applied to a light emitting diode of the II/VI-compound 
semiconductor. 
As described above, according to the present invention, since the 
active-layer side portion of the second cladding layer 6 is formed as the 
lightly impurity-doped region or the non-doped region, it is possible to 
suppress the point defects introduced by the interstitial nitrogen near 
the active layer 4 and to extend the lifetime of the II/VI-compound 
semiconductor light emitting device. 
According to the present invention, it is possible to extend the lifetime 
of the II/VI-compound semiconductor light emitting devices having DH and 
SCH structures. 
In addition, according to the present invention, since the II/VI-compound 
semiconductor light emitting device has the lightly impurity-doped region 
or the non-doped region in its portion from the second cladding layer 6 to 
the second guide layer 5, it is possible to further extend the lifetime of 
the SCH II/VI-compound semiconductor light emitting device. 
Since the above SCH II/VI-compound semiconductor light emitting device 
having the lightly impurity-doped region further has the portion, which is 
in contact with the active layer 4, of the second guide layer 3 formed as 
the non-doped region, it is possible to suppress the point defects 
introduced by the interstitial nitrogen near the active layer 4 and to 
further extend the lifetime of the SCH II/VI-compound semiconductor light 
emitting device. 
Moreover, since the SCH II/VI-compound semiconductor light emitting device 
has the lightly impurity-doped region from the second cladding layer 6 to 
the second guide layer 5 where the impurity doping concentration at the 
active-layer side portion thereof is selected to be smaller than the other 
portion thereof, it is possible to further suppress the influence of the 
point defects resulting from the interstitial impurity and to increase the 
lifetime of the SCH II/VI-compound semiconductor light emitting device. 
Since the impurity doping concentration of the second cladding layer 6 is 
decreased stepwise toward the active layer 4, it is possible to suppress 
the influence of the point defects resulting from the interstitial 
impurity in the second cladding layer 6 and to increase the lifetime of 
the II/VI-compound semiconductor light emitting device. 
Since the SCH II/VI-compound semiconductor light emitting device has the 
second guide layer 5 whose active-layer 4 side portion is formed as the 
lightly impurity-doped region or the non-doped region, it is similarly 
possible to increase the lifetime of the SCH II/VI-compound semiconductor 
light emitting device. This arrangement is also effective in the SCH 
II/VI-compound semiconductor light emitting device having the thick guide 
layers. 
Since the impurity doping concentration of the lightly impurity-doped 
region is selected such that the interstitial impurity concentration 
becomes 2.times.10.sup.17 cm.sup.-3 or smaller, it is possible to suppress 
the harmful influence of the point defects and to reliably increase the 
lifetime of the II/VI-compound semiconductor light emitting device. 
Since the interstitial impurity concentration of the lightly impurity-doped 
region made of ZnMgSSe can be selected to be 2.times.10.sup.17 cm.sup.-3 
or smaller by setting the impurity doping concentration thereof to 
7.times.10.sup.17 cm.sup.-3 or smaller, it is possible to extend the 
lifetime of the II/VI-compound semiconductor light emitting device. 
Since the interstitial impurity concentration of the lightly impurity-doped 
region made of ZnMgSSe with the band gap energy of 2.9 eV or larger can be 
selected to be 1.4.times.10.sup.17 cm.sup.-3 or smaller by setting the 
impurity doping concentration thereof to 3.times.10.sup.17 cm.sup.-3 or 
smaller, it is possible to extend the lifetime of the II/VI-compound 
semiconductor light emitting device. 
Since the interstitial impurity concentration of the lightly impurity-doped 
region made of Zn(S)Se can be selected to be 2.times.10.sup.17 cm.sup.-3 
or smaller by setting the impurity doping concentration thereof to 
8.times.10.sup.17 cm.sup.-3 or smaller, it is possible to extend the 
lifetime of the II/VI-compound semiconductor light emitting device. 
Since the second guide layer 5 has the portion in contact with the active 
layer 4 formed as the non-doped region and has the rest portion other than 
the no-doped region formed as the impurity-doped region, it is possible to 
remove the harmful influence of the point defects and to reduce the amount 
of electrons overflowing into the second cladding layer 6. Thus, it is 
possible to extend the lifetime of the II/VI-compound semiconductor light 
emitting device. Moreover, it is possible to suppress the increase of the 
threshold current value Ith. 
Since the second guide layer 5 has the region in contact with the active 
layer 4 formed as the non-doped region and the second cladding layer 6 has 
the active-layer 4 side portion formed as the non-doped region, it is 
possible to further extend the lifetime of the II/VI-compound 
semiconductor light emitting device and to suppress the increase of the 
threshold current value Ith. 
Thus, when the present invention is applied to the II/VI-compound 
semiconductor light emitting device, it becomes possible to use the 
II/VI-compound semiconductor light emitting device in the optical pickup 
device suitable for the high-density magneto-optical recording. 
Having described preferred embodiments of the invention with reference to 
the accompanying drawings, it is to be understood that the present 
invention is not limited to those precise embodiments and that various 
changes and modifications can be effected therein by one skilled in the 
art without departing from the spirit or scope of the invention as defined 
in the appended claims.