Semiconductor laser device featuring group III and IV compounds doped with amphoteric impurity to vary electrical resistance according to direction of crystal plane

There is provided a high-quality semiconductor laser device having a current confinement feature along with a method of manufacturing the same in a simple manner. The upper clad layer 4 of a semiconductor laser device is a semiconductive layer made of a compound of elements of the III and V groups doped with an amphoteric impurity substance and the electric resistance of the lateral slopes is greater on the top of the mesa than on the upper clad layer 4 of the mesa. A method of manufacturing a semiconductor laser device comprises a step of repeating a cycle of crystal growth operation of sequentially forming a layer of an element of the III group, a layer of an amphoteric impurity substance and a layer of an element of the V group on said substrate by means of an MBE technique to produce said upper clad layer made of a compound of elements of the III and V groups.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to a semiconductor laser device using a solid and a 
method of manufacturing the same. 
2. Prior Art 
Compound semiconductor laser devices are popularly used as solid-state 
semiconductor light emitting devices in the field of optical 
telecommunication and other optical technologies. 
Semiconductor laser devices used for these applications are normally 
provided with a feature of confining the electric current running 
therethrough in order to reduce the threshold current level when starting 
laser oscillation. 
FIG. 9 of the accompanying drawings schematically illustrates a 
semiconductor laser device having such a current confinement feature. 
A semiconductor laser device as illustrated in FIG. 9 comprises an n-InP 
lower clad layer 12, an n-InGaAs active layer 13, a p-InP upper clad layer 
14, a p-InP layers 15, an n-InP layers 16, said p-InP layers 15 and n-InP 
layers 16 being provided as electric current blocking layers, a p-GaInAsP 
contact layer 17 and an insulation layer 18 arranged on an n-InP substrate 
11 in the above-mentioned order to form a multilayered structure, on the 
upper and lower surfaces of which a p-electrode 19 and an n-electrode 10 
are respectively mounted. 
A semiconductor laser device having a structure as shown in FIG. 9 is 
produced through a process as described below. 
In the first step of crystal growth operation, the n-InP lower clad layer 
12, the n-InGaAs active layer 13 and the p-InP upper clad layer 14 are 
sequentially formed on a substrate 11 by means of an MBE technique and 
thereafter the obtained intermediary multilayered structure is etched at a 
predetermined location to narrow the active layer 13. 
Then, in the second step of crystal growth operation, the p-InP layer 15 
and the n-InP layer 16 are sequentially formed as electric current 
blocking layers on the intermediary multilayered structure by means of an 
LPE technique. 
Finally, in the third step of crystal growth operation, the p-InGaAs 
contact layer 17 is formed atop by means of an MBE technique. 
A semiconductor laser device produced in a manner as described above can 
effectively confine the electric current running therethrough by utilizing 
the current blocking layers 15 and 16 of the pn reverse junction type 
formed on the opposite lateral sides of the active layer 13. 
Another semiconductor laser device also having a current confinement 
feature is illustrated in FIG. 10. 
A semiconductor laser device as illustrated in FIG. 10 comprises an n-InP 
lower clad layer 22, an n-InGaAs active layer 24 having its upper and 
lower surfaces covered by respective GaAs buffer layers 23 and 25, an 
Si-doped n-AlGaAs upper clad layer 26, a GaAs contact layer 27 and an 
SiO.sub.2 insulation layer 28 arranged on a p-GaAs substrate 21 in the 
above mentioned order to form a multilayered structure, on the upper and 
lower surfaces of which an n-electrode 29 and a p-electrode 20 are 
respectively mounted. 
The p-GaAs substrate 21 of the semiconductor laser device of FIG. 10 has 
different surface levels as it has a mesa between a pair of flat and low 
side areas. 
The flat surface of the highest central area of the mesa has a (100) plane, 
whereas the lateral slopes of the mesa connecting the higher level and the 
lower level have a (311) A plane. 
Since the dopant Si of the n-AlGaAs upper clad layer 26 of the 
semiconductor laser device of FIG. 10 is an amphoteric impurity substance, 
the (100) plane of the layer is of n-type, whereas the (311) A plane of 
the layer is of p-type. 
With such an arrangement, a pn junction is formed within the Si-doped 
n-AlGaAs upper clad layer 26 to block any transversal electric currents 
that may flow therein. 
Besides, the electric current confinement structure of the semiconductor 
laser device illustrated in FIG. 10 can be formed by a single epitaxial 
growth operation. 
3. Problems to be Solved by the Invention] 
Semiconductor laser devices such as those illustrated in FIGS. 9 and 10 are 
accompanied by the following problems that remain unsolved. 
(1) A mesa etching step is inevitably required in the process of crystal 
growth for forming the layers of a semiconductor laser device as 
illustrated in FIG. 9. This means that the process of crystal growth needs 
to be interrupted, consequently making the operation of controlling the 
overall process of manufacturing such a semiconductor laser device rather 
complicated. 
(2) The use of a different material for a semiconductor laser device as 
illustrated in FIG. 19 inevitably results in the failure of providing it 
with a current confinement feature. 
For instance, if an InAlAs clad layer and an InGaAs active layer are formed 
on an InP substrate having different surface levels, while the (100) plane 
of the substrate is turned to n-type by doping the InAlAs clad layer with 
Si, the (111) A plane that needs to become p-type is also turned to n-type 
having a degree of carrier concentration similar to that of the (100) 
plane. 
Thus, a current confinement operation cannot be successfully carried out by 
means of pn junction if a laser structure is formed on the (100) plane of 
the InP structure that also has a (111) A plane or a (311) A plane. 
In view of the above described technological problems, it is therefore an 
object of the present to provide an advanced high-quality semiconductor 
laser device having a current confinement feature as well as a method of 
manufacturing the same in a simple manner. 
SUMMARY OF THE INVENTION 
According to the present invention, the above object is achieved by 
providing a semiconductor laser device comprising a lower clad layer, an 
active layer and an upper clad layer formed on a normal mesa-shaped 
substrate, said upper clad layer being a semiconductive layer made of a 
compound of elements of the III and V groups doped with an amphoteric 
impurity substance, the electric resistance of the lateral slopes of the 
mesa being greater on the top of the mesa than on said upper clad layer of 
the mesa by more than 10.sup.2 times. 
According to the present invention, the above object is also achieved by 
providing a method of manufacturing a semiconductor laser device having a 
lower clad layer, an active layer and an upper clad layer formed on a 
normal mesa-shaped substrate comprising a step of repeating a cycle of 
crystal growth operation of sequentially forming a layer of an element of 
the III group, a layer of an amphoteric impurity substance and a layer of 
an element of the V group on said substrate by means of an MBE technique 
to produce said upper clad layer made of a compound of elements of the III 
and V groups. 
The present invention is based on a new scientific discovery that when a 
semiconductor made of a compound of elements of the III and V groups 
having differently directed crystal planes is doped with an amphoteric 
impurity substance, the electric resistance of the semiconductor varies 
depending on the direction of crystal plane. 
The phenomenon may be explained by the fact that when the sites of atoms of 
the element of the III group are doped with an amphoteric impurity 
substance, the latter becomes of n-type, that when the sites of atoms of 
the element of the V group are doped with the same amphoteric impurity 
substance, the latter becomes of p-type and that, when the two sites are 
doped with a same amphoteric impurity substance to a same extent, the 
impurities on the two sites compensate each other to show a high-electric 
resistance. 
Although atoms of an amphoteric impurity substance normally enters the 
sites of atoms of an element of the III group and not the sites of atoms 
of an element of the V group if sites of two different types exist, the 
amphoteric impurity may enter the sites of atoms the element of the V 
group if a stabilizing plane of the element of the III group is 
artificially formed and doped with the amphoteric impurity. 
The above phenomenon is utilized in a semiconductor laser device according 
to the invention in a manner as described below. 
When an upper clad layer is formed on a normal mesa-shaped substrate 
(having a trapezoidal cross section), the direction of crystal plane of 
the upper clad layer on the top of the mesa is different from that of the 
upper clad layer on the lateral slopes of the mesa. Moreover, if such an 
upper clad layer is appropriately doped with an amphoteric impurity 
substance, the sites through which atoms of the amphoteric impurity 
substance enter and the electric resistance of the upper clad layer on the 
top of the mesa respectively differ from those on the lateral slopes of 
the mesa. 
Thus, a semiconductor laser device according to the invention and having an 
electric resistance which is greater on the lateral slopes than on the 
upper layer of the mesa can effectively confine the injected electric 
current on the top of the mesa. 
A method of manufacturing a semiconductor laser device having a lower clad 
layer, an active layer and an upper clad layer formed on a substrate 
according to the present invention comprises a step of repeating a cycle 
of crystal growth operation of sequentially forming a layer of an element 
of the III group, a layer of an amphoteric impurity substance and a layer 
of an element of the V group on said substrate by means of an MBE 
technique to produce said upper clad layer made of a compound of elements 
of the III and V groups. 
With such an arrangement, since the probability with which atoms of the 
amphoteric impurity substance are combined with atoms of the element of 
the III group varies depending on the direction of the crystal plane, the 
electric resistance of the clad layer also varies depending on the 
direction of the crystal layer to provide the semiconductor laser device 
with desired properties. 
Now, the present invention will be described by way of the accompanying 
drawings that illustrate a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A first preferred embodiment of the semiconductor laser device of the 
present invention is schematically illustrated in FIG. 1 in cross section. 
Referring to FIG. 1, the first preferred embodiment of the invention 
comprises a p-InP lower clad layer 2, a non-doped InGaAs active layer 3, 
an n-InAlAs upper clad layer 4, an n-InAlAs contact layer 5 and an 
insulation layer 6 arranged on a p-InP substrate 1 in the above-mentioned 
order to form a multilayered structure and said upper clad layer 4 is 
provided with current blocking sections 9. 
An n-electrode 7 and a p-electrode 8 are mounted respectively on the 
insulation layer 6 and on the lower surface of the p-InP substrate 1. 
According to the invention, a semiconductor laser device as illustrated in 
FIG. 1 is prepared in a manner as described below. 
The principal crystal plane of the p-InP substrate 1 is a (100) plane. 
The p-InP substrate 1 has a mesa which is formed by etching and trapezoidal 
in cross section and the lateral slopes of the mesa are (111) A planes. 
When a substrate 1 having a configuration as described above is set in an 
MBE apparatus in order to grow crystals on it, the oxide film existing on 
the surface of the substrate 1 is removed by a preliminary ordinary heat 
treatment. 
The substrate 1 is held to a predetermined position by a holder in the 
vacuum chamber of the MBE apparatus and molten materials of elements of 
the III and V groups are held to high temperature in respective effusion 
cells (molecular beam cells) in the apparatus and Si of the IV group is 
solidified at 900.degree. C. to 1,200.degree. C. 
Among the conditions to be met for crystal growth by a method of 
manufacturing a semiconductor laser device according to the invention, the 
growth temperature needs to be kept between 300.degree. and 650.degree. C. 
and the intensity of molecular beam of the V group should be 2 to 10.sup.2 
times as high as that of the III group. 
To begin with, the shutters of the effusion cells of the elements of a 
lower clad layer 2 are made open to allow atoms of the elements to burst 
out, collide and eventually get to the substrate 1 to form semiconductive 
crystals, which then grow to form a complete Be-doped p-InAlAs lower clad 
layer 2. 
Then, atoms of the elements of an active layer are effused from the 
respective effusion cells of the elements to form crystals of a non-doped 
InGaAs active layer 4 on the lower clad layer 2. The crystals are then 
made to grow to become a complete active layer 3 there. 
Thereafter, atoms of the elements of an upper clad layer 3 are also sprayed 
out from the respective effusion cells of the elements to form crystals of 
an n-InAlAs upper clad layer 4 on the non-doped active layer 3. The 
crystals are then made to grow to become a complete n-InAlAs upper clad 
layer 4 there. 
It should be noted, however, that the crystal growth of the n-InAlAs upper 
clad layer 4 is controlled by frequently turning on and off the shutters 
of the effusion cells. 
FIG. 2 shows a timing chart of opening and closing the shutters. 
In each line of FIG. 2, the higher level indicates a condition where the 
shutter of the effusion cell is open, whereas the lower level indicates a 
condition where the shutter of the effusion cell is closed. 
Of the three lines of FIG. 2, the uppermost line 41 represents a first 
effusion cell that contains In and Al, both of which belong to the III 
group, the middle line 42 represents a second effusion cell containing Si 
and the lowermost line 43 represents a third effusion cell containing As. 
The first effusion cell 41 is, however, divided into two compartments, one 
for In and the other for Al. 
Referring to FIG. 2, first of all, the first effusion cell 41 is made open 
to let out atoms of the elements of the III group to cover the surface for 
crystal growth. When the surface for crystal growth is completely covered 
by atoms of the elements of the III group approximately by a monolayer, 
the first effusion cell 41 is closed. Thereafter, the second effusion cell 
containing Si which is a dopant is made open instantaneously and quickly 
closed again. Then, the first effusion cell 41 is opened again in order to 
cover the dopant Si with atoms of the elements of the III group. The first 
effusion cell 41 is closed again when the dopant Si is covered by the 
elements of the III group. 
Now, the Si is sandwiched by the elements of the III group. 
With such an arrangement, since each of the Si atoms can be easily combined 
with one or more of the atoms of the elements of the III group, the Si 
atoms are highly probably taken into the sites of As atoms. 
Finally, the third effusion cell 43 is made open for a predetermined period 
of time to let out As atoms in order to accelerate the growth of non-doped 
InAlAs. 
The above described steps of opening and closing the effusion cells 
constitute a crystal growth cycle, which is then repeated for several to 
several thousand times to complete the operation of producing a complete 
n-InAlAs upper clad layer 4. 
An n-InAlAs upper clad layer 4 prepared in this manner has an InAlAs layer 
having a high electric resistance on each of the lateral slopes of the 
mesa. Then, a current blocking section 9 is formed in part of each of the 
highly resistive InAlAs layers. 
It should be noted here that the electric resistance of each of the current 
blocking sections 9 formed on the (111) A planes or the lateral slopes of 
the mesa is greater than that of the (100) plane or the top of the mesa 
approximately by 10.sup.2 to 10.sup.6 times. 
After forming an n-InAlAs upper clad layer 4, an n-InAlAs contact layer 5 
is made to grow on it by a known technique. Thereafter, an insulation 
layer 6 is formed thereon by using, for instance, a plasma CVD technique. 
Then an oblong n-electrode 7 is arranged on the insulation layer by any 
known photolithography technique and a p-electrode 8 is arranged on the 
lower surface of the p-InP substrate 1 by known means. 
Now, the relationship between the electric resistance and the direction of 
crystal plane of an Si-doped InAlAs layer will be described by way of an 
example and a comparative example obtained as a result of an experiment. 
In the example, where an Si-doped InAlAs layer was formed on each Fe-doped 
semiinsulated InP substrate having a normal mesa by using the above 
described technique of opening and closing effusion cells (means for 
doping a surface selectively with elements of the III group), a number or 
substrates were prepared in advance, including those having a principal 
plane agreeing with the (100) plane, those having a principal plane 
misoriented by 1.degree. and 2.degree. respectively from the (100) plane, 
those having a principal plane agreeing with the (111) A plane and those 
having a principal plane misoriented by respectively 1.degree. and 
2.degree. from the (111) A plane, on each of which the above described 
layer was formed. 
The temperature of the substrate was kept to 400.degree. C. during the 
experiment. 
In the comparative example, where, an Si-doped InAlAs layer was grown on 
each Fe-doped semiinsulated InP substrate by means of a known technique of 
using effusion cells respectively containing In, Al, As and Si which were 
simultaneously made open for doping (bulk doping means), a number or 
substrates were prepared in advance as in the case of the example 
described above, including those having a principal plane agreeing with 
the (100) plane, those having a principal plane misoriented by 1.degree. 
and 2.degree. respectively from the (100) plane, those having a principal 
plane agreeing with the (111) A plane and those having a principal plane 
misoriented by respectively 1.degree. or 2.degree. from the (111) A plane, 
on each of which the above described layer was formed. 
The electric resistance of each of the samples obtained in the example and 
the comparative example was measured. FIG. 3 shows the result of the 
measurement. 
As clearly seen from FIG. 3, the Si-doped InAlAs formed on the (111) A 
plane (that corresponds to the side slopes of the mesa) of the samples of 
the example showed a specific electric resistance greater than that of the 
top of the mesa by 10.sup.5, whereas the difference of specific electric 
resistance between the side slopes and the top of the mesa of the samples 
of the comparative example was by far lower than that of the sample of the 
example. 
For the purpose of the present invention, it should be noted that the 
following technological particulars are found within the scope of the 
invention. 
The direction of the principal plane of the substrate may be misoriented 
approximately by 5.degree. from that of the (100) plane. 
The plane of each of the side slopes of the mesa may be a (111) A plane, a 
(211) A plane, a (311) A plane, a (411) A plane or a (511) A plane. The 
direction of the plane of each of the side slopes of the mesa may be 
misoriented approximately by 5.degree. from that of any of these planes. 
The side slopes of the mesa need not be restricted to a crystal A plane and 
may well comprise a crystal B plane. 
The substrate may alternatively be a GaAs, GaSb, Si or GaAsP substrate and 
the active layer may alternatively be a quantum well layer or a quaternary 
layer of InAlGaAs. The upper and/or lower clad layers may alternatively 
have a SCH, GRIN or GRIN-SCH structure. 
AsGaAs and C may be used respectively for the upper clad layer and the 
amphoteric impurity. Alternatively, AlGaInAs and Si may be used 
respectively for the upper clad layer and the amphoteric impurity. 
Timing of opening and closing the effusion cells for forming an upper clad 
layer is not limited to the one shown in FIG. 2 and may be replaced by any 
of those shown in FIGS. 4, 5, 6, 7 and 8. 
Since the timing charts of FIGS. 4 through 8 are similar to that of FIG. 2 
and therefore may be easily understood, they will not be described any 
further here. 
As described in detail above, since a semiconductor laser device according 
to the present invention comprises a lower clad layer, an active layer and 
an upper clad layer formed on a normal mesa-shaped substrate is 
characterized in that said upper clad layer is a semiconductive layer made 
of a compound of elements of the III and V groups doped with an amphoteric 
impurity substance and that the electric resistance of said lateral slopes 
is greater on the top of the mesa than on the upper clad layer of the 
mesa, the injected electric current can be confined on the top of the mesa 
to consequently reduce the threshold current level when starting laser 
oscillation. 
As described above, a method of manufacturing a semiconductor laser device 
according to the invention comprises a step of repeating a cycle of 
crystal growth operation of sequentially forming a layer of an element of 
the III group, a layer of an amphoteric impurity substance and a layer of 
an element of the V group on said substrate by means of an MBE technique 
to produce said upper clad layer made of a compound of elements of the III 
and V groups. 
Thus, with a method according to the invention, a semiconductor laser 
device can be prepared by a simple and continuous operation of crystal 
growth by means of an MBE technique. Since a semiconductor laser device 
prepared by means of the method of the invention is not subjected to 
degradation of the crystal quality, unlike the case of a device prepared 
by a conventional method of crystal growth that require interruptions of 
the crystal growth operation, the method can effectively provide 
high-quality semiconductor laser devices.