Semiconductor laser device

A semiconductor laser device includes a p type semiconductor substrate, an active layer having a smaller energy band gap than the p type semiconductor substrate and an n type semiconductor layer having a larger energy band gap than the active layer successively formed on the p type semiconductor substrate, a mesa formed by selectively etching the semiconductor substrate, active layer, and n type semiconductor layer, p-n-p layers having larger energy band gaps than the active layer and disposed at both sides of the mesa, a small energy band gap layer having a smaller energy band gap than the p type semiconductor substrate and disposed on the p-n-p layers, and an n type semiconductor layer disposed on the small energy band gap layer and on the n type semiconductor layer. The small energy band gap layers decrease the current flowing through the thyristor structure and are disposed close to the active region but in a different processing step from the formation of the active layer. A waveguide structure in which the active layer is surrounded by semiconductor layers having larger energy band gaps is realized.

FIELD OF THE INVENTION 
The present invention relates to a semiconductor laser device, and more 
particularly to a semiconductor laser device that realizes high speed 
response and high power output at the same time and that outputs light 
having a far-field pattern of Gaussian distribution. 
PRIOR ART 
FIG. 4 is a cross-sectional view of a prior art semiconductor laser device 
disclosed in Prescript of 29th Meeting of Japanese Society of Applied 
Physics (1982), p. 155. In FIG. 4, reference numeral 1 designates an n 
type InP substrate. InGaAsP layers 2 and 2' are disposed on the substrate 
1. P type InP layers 3 and 3' are disposed on the layers 2 and 2', 
respectively. Two stripe-shaped grooves penetrate the p type InP layers 3 
and 3' and the InGaAsP layers 2 and 2'. These grooves are formed by 
etching. A portion of the substrate 1, the InGaAsP layer 2, and the p type 
InP layer 3, which are between the two grooves, constitute a mesa 
structure. A p type InP layer 4 is disposed in the two stripe-shaped 
grooves and on the p type InP layer 3'. An n type InP layer 5 is disposed 
on the p type InP layer 4. A p type InP layer 6 is disposed on the n type 
InP layer 5 and on the p type InP layer 3. A p type InGaAsP contact layer 
7 is disposed on the p type InP layer 6. In addition, an n side electrode 
31 is disposed on the rear surface of the substrate 1 and a p side 
electrode 32 is disposed on the contact layer 7. 
Layers 3, 3', 4, 5, and 6 comprise InP similarly as the substrate 1 and 
have the same energy band gap as the substrate 1. The InGaAsP layers 2 and 
2' have smaller energy band gaps than the substrate 1. 
A description is given of the operation. When a bias voltage in forward 
direction to the pn junction is applied between the substrate 1 and the 
contact layer 7 from the n side electrode 31 and the p side electrode 32, 
respectively, the holes and electrons in the respective layers are 
injected into the active region 2, a semiconductor having a smaller energy 
band gap than the substrate 1, resulting in carrier recombinations that 
produce laser light. In such a semiconductor laser device, in order to 
inject the electrons and holes into the active region 2 with high 
efficiency, a current blocking structure is produced by the p-n-p-n 
thyristor comprising the p type semiconductor 6, n type semiconductor 5, p 
type semiconductors 3 and 4, and n type semiconductor 1. However, in this 
thyristor structure, when the voltage applied to the laser device is 
increased to increase the light output, the voltage applied to the 
thyristor structure increases and the current corresponding to the gate 
current increases, whereby the current flowing through the thyristor 
structure suddenly increases. As a result, the light output is not so 
large. On the other hand, in the structure shown in FIG. 4, since the 
semiconductor layer 2' having a small energy band gap is included in the 
thyristor structure, the gain of the n-p-n structure transistor 
constituting the thyristor decreases, so that less current flows through 
the thyristor structure as compared with a thyristor structure having no 
semiconductor layer 2'. Therefore, a large output of light can be 
obtained. 
However, in this structure, since the active layer 2 and the semiconductor 
layer 2' are formed at the same time, it is impossible to arbitrarily 
select the energy band gap of the semiconductor layer 2' to minimize the 
current flowing through the thyristor structure. In addition, when a 
semiconductor laser device performs high speed modulation, a structure 
shown in FIG. 5 is generally employed to decrease the parasitic 
capacitance of the element. In FIG. 5, stripe-shaped mesa grooves are 
formed at both sides of the active region from the surface of the contact 
layer 7 and reach into the substrate 1 and thus the whole laser element is 
formed into a mesa shape. In this case, if the mesa width is too narrow, 
the semiconductor layer 2' may be outside the mesa and the above-described 
current decreasing effect cannot be obtained. As a result, high speed 
response and high power output cannot be realized at the same time in this 
conventional structure. 
Meanwhile, FIG. 6 is a cross-sectional view of a prior art semiconductor 
laser device disclosed in Prescript of 33th Meeting of Japanese Society of 
Applied Physics (1986), p. 158. In FIG. 6, reference numeral 51 designates 
an n type InP substrate. An active layer 52 is disposed on the substrate 
1. A p type InP layer 53 is disposed on the active layer 52. A mesa 
structure is formed by etching the active layer 52, p type InP layer 53 
and the substrate 51. P type InGaAsP layers 58 are disposed at both sides 
of the mesa structure. The energy band gap of this p type InGaAsP layer 58 
is smaller than that of the substrate 1. P type InP layers 54 are disposed 
on the p type InGaAsP layers 58 and n type InP layers 55 are disposed on 
the p type InP layers 54. In addition, a p type InP layer 56 is disposed 
on the n type InP layers 55 and on the p type InP layer 53. A p type 
InGaAsP contact layer 57 is disposed on the p type InP layer 56. In 
addition, an n side electrode 21 is disposed on the rear surface of the 
substrate 51 and a p side electrode 22 is disposed on the contact layer 
57. 
In this semiconductor laser device, the semiconductor layer 58 has 
fundamentally the same effect as the semiconductor layer 2' of the laser 
device shown in FIG. 4, so that the current flowing through the thyristor 
structure can be significantly decreased. In addition, since this 
semiconductor layer 58 is formed in a different process step from that of 
the active layer 52, its composition can be chosen to have an optimum 
energy band gap. In addition, this layer 58 is adjacent to the active 
layer 52. Therefore, even when a mesa structure is formed by forming 
grooves 60 by etching thereby to decrease the parasitic capacitance, since 
the semiconductor layer 58 exists in the mesa, the effect of decreasing 
the current flowing through the thyristor structure can be maintained. 
FIG. 8 is a cross-sectional view of a prior art semiconductor laser device 
disclosed in Japanese Published Patent Application No. 2-143483. In FIG. 
8, reference numeral 101 designates a p type InP substrate. A p type InP 
first cladding layer 102 is disposed on the substrate 101. An InGaAsP 
active layer 103 is disposed on the first cladding layer 102. An n type 
InP second cladding layer 104 is disposed on the active layer 103. A mesa 
structure is formed by etching the second cladding layer 104, active layer 
103, and first cladding layer 102. P type InP first buried layers 105 are 
disposed at both sides of the mesa structure. N type InP second buried 
layers 106 are disposed on the first buried layers 105. P type InP third 
buried layers 107 having a high dopant impurity concentration are disposed 
on the second buried layers 106. P type InP fourth buried layers 108 are 
disposed on the third buried layers 107. An n type InP third cladding 
layer 109 is disposed on the second cladding layer 104 and on the fourth 
buried layers 108. An n type InGaAsP contact layer 110 is disposed on the 
third cladding layer 109. In addition, a p side electrode 111 is disposed 
on the rear surface of the substrate 101 and an n side electrode 112 is 
disposed on the contact layer 110. 
FIG. 9 is a cross-sectional view of a prior art semiconductor laser device 
disclosed in Japanese Published Patent Application No. 2-143483. This 
laser device is similar to the device shown in FIG. 8 except that high 
impurity concentration p type InP fifth buried layers 113 are disposed on 
the p type fourth buried layers 108. 
In the laser devices shown in FIGS. 8 and 9, since the high impurity 
concentration buried layers 107 and 113 are included, those buried layers 
produce a p-n-p.sup.+ -p-n thyristor structure (FIG. 8) or a p-n-p.sup.+ 
-p-p.sup.+ -n thyristor structure (FIG. 9). Therefore, during the laser 
operation, electrons injected into the thyristor structure are prevented 
from being transferred by barriers formed by the high impurity 
concentration buried layers and the thyristor is hard to turn on. Also in 
these structures, since the high impurity concentration buried layers are 
arranged close to the active layer, a mesa structure can be formed to 
reduce the parasitic capacitance. 
In the prior art semiconductor laser device shown in FIG. 6 that can 
realize high speed response and high power output at the same time, the 
semiconductor layer 58 has a refractive index different from those of the 
semiconductor layers 51, 53, 54, 55 and 56, and current confinement in the 
width direction of the active region is not sufficient in the waveguide 
structure constituted by the active layer 52, the semiconductor layer 58 
and the peripheral semiconductor layers, so that the far-field pattern of 
the output light does not have an ideal Gaussian distribution. 
In the prior art semiconductor laser devices shown in FIGS. 8 and 9, high 
concentration impurity layers 107 and 113 are provided to make the 
thyristor hard to turn on. However, it is technically difficult to form a 
high carrier concentration film of good quality. In addition, in order to 
make the laser structures shown in FIGS. 8 and 9 effective, the high 
impurity concentration layers 107 and 113 are required to be as thick as 
about 0.5 micron or more, restricting design freedom. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a semiconductor laser 
device that can realize high power output without losing the effect of 
decreasing current flowing through the thyristor structure even when a 
mesa structure is employed for reducing parasitic capacitance in which the 
far-field pattern of the output light has a Gaussian distribution. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given hereinafter; it should be understood, 
however, that the detailed description and specific embodiment are given 
by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description. 
In accordance with the present invention, a semiconductor laser device 
includes a p type semiconductor substrate, an active layer comprising a 
semiconductor having smaller energy band gap than that of the p type 
semiconductor substrate and an n type semiconductor layer having larger 
energy band gap than that of the active layer, which are successively 
formed on the p type semiconductor substrate, a mesa formed by selectively 
etching the semiconductor substrate, active layer and n type semiconductor 
layer so as to leave the active layer and n type semiconductor layer in a 
stripe shape, p-n-p buried layers having larger energy band gaps than the 
active layer and disposed at opposite sides of the mesa, small energy band 
gap layers having smaller energy band gaps than the p type semiconductor 
substrate and disposed on the buried layers, and an n type semiconductor 
layer disposed on the small energy band gap layers and on the n type 
semiconductor layer. Therefore, the small energy band gap layers for 
decreasing the current flowing through the thyristor structure is disposed 
close to the active region in a different process step from that in which 
the active layer is formed and, furthermore, a waveguide structure in 
which the active layer is surrounded by semiconductor layers having large 
energy band gaps is realized.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will be described in detail with 
reference to the drawings. 
FIG. 1(a) is a cross-sectional view of a semiconductor laser device in 
accordance with a first embodiment of the present invention. In FIG. 1(a), 
reference numeral 10 designates a p type InP substrate. An active layer 11 
comprising In.sub.0.71 Ga.sub.0.29 As.sub.0.62 P.sub.0.38 which has a 
smaller energy band gap than the substrate 10 is disposed on the substrate 
10. An n type InP layer 12 is disposed on the active layer 11. The n type 
InP layer 12, active layer 11 and substrate 10 are formed into mesa shape 
by etching and p type InP layer 13, n type InP layer 14, p type InP layer 
15 and In.sub.0.82 Ga.sub.0.18 As.sub.0.4 P.sub.0.6 layer 16 are 
successively deposited at both sides of the mesa. An n type InP layer 17 
is disposed on the mesa and on the In.sub.0.82 Ga.sub.0.18 As.sub.0.4 
P.sub.0.6 layer 16. An n type In.sub.0.82 Ga.sub.0.18 As.sub.0.4 P.sub.0.6 
contact layer 18 is disposed on the n type InP layer 17. A p side 
electrode 19 is disposed on the rear surface of the substrate 10 and an n 
side electrode 20 is disposed on the contact layer 18. 
FIG. 2 is a perspective view of the semiconductor laser device of FIG. 
1(a), in which reference numerals 21 and 22 designate cleavage facets. 
A description is given of the production method. FIGS. 3(a) to 3(c) 
illustrate a method for producing the semiconductor laser device shown in 
FIGS. 1(a) and 2. In FIGS. 3(a) to 3(c), the same reference numerals as 
those in FIGS. 1(a) and 2 designate the same or corresponding parts. 
As shown in FIG. 3(a), an In.sub.0.71 Ga.sub.0.29 As.sub.0.62 P.sub.0.38 
active layer 11 of 0.1 micron thickness and an n type InP layer 12 of 1.0 
micron thickness are successively grown on the p type InP layer 10 by 
liquid phase epitaxy (LPE), metal organic chemical vapor deposition 
(MOCVD) or the like. Then, as shown in FIG. 3(b), a mesa is formed over 
the wafer using photolithography and chemical etching. The etching depth 
is about 4 microns. Thereafter, as shown in FIG. 3(c), p type InP layer 13 
of 1 micron thickness, n type InP layer 14 of 1.5 microns thickness, p 
type InP layer 15 of 1.5 microns thickness and In.sub.0.82 Ga.sub.0.18 
As.sub.0.4 P.sub.0.6 layer 16 of 0.1 micron thickness are successively 
grown by LPE so as to bury the mesa. Then, on the mesa and the In.sub.0.82 
Ga.sub.0.18 As.sub.0.4 P.sub.0.6 layer 16, an n type InP layer 17 of 2 
microns thickness and an n type In.sub.0.82 Ga.sub.0.18 As.sub.0.4 
P.sub.0.6 contact layer 18 of 1.0 micron thickness are successively grown, 
completing the growth process. Thereafter, a p side electrode 19 is formed 
on the rear surface of the substrate 10 and an n side electrode 20 is 
formed on the contact layer 18 by sputtering. Then, resonator facets 21 
and 22 are formed by cleavage, resulting in the laser device shown in 
FIGS. 1(a) and 2. 
This laser device operates as follows. When voltage is applied to the p 
type semiconductor substrate 10 and the n type contact layer 18, electrons 
and holes are injected into the active region 11, resulting in carrier 
recombinations that produce laser light. 
The semiconductor layers 13, 14, 15, 16 and 17 constitute a p-n-p-n 
structure and when a low voltage is applied, this p-n-p-n structure serves 
as a current blocking layer and current flows into the active region 
efficiently. When a high voltage is applied to increase the light output, 
the p-n-p-n structure is usually turned on and the current blocking effect 
is lost, so that the light output is not increased. In this first 
embodiment of the present invention, however, since the small energy band 
gap layer 16 has a smaller energy band gap than the substrate 10, the 
p-n-p-n thyristor structure is not turned on until a relatively high 
voltage is applied, so that high power output can be obtained. 
The reason why high power output operation can be attained in this 
embodiment will be described with reference to an equivalent circuit 
diagram. FIG. 1(b) shows an equivalent circuit of the semiconductor laser 
device of FIG. 1(a). In FIG. 1(b), a diode D1 is constituted by the p type 
substrate 10, active layer 11 and n type semiconductor layer 12. A diode 
D2 is constituted by the p type semiconductor layer 15, small energy band 
gap layer 16 and n type semiconductor layer 17. A PNP transistor Q1 is 
constituted by the p type semiconductor layer 13, n type semiconductor 
layer 14 and p type semiconductor layer 15. An NPN transistor Q2 is 
constituted by the n type semiconductor layer 14, p type semiconductor 
layer 15, small energy band gap layer 16 and n type semiconductor layer 
17. Resistors R1 to R5 are connected to the diodes D1, D2 and transistors 
Q1, Q2. In this embodiment, since the small energy band gap layer 16 has a 
smaller energy band gap than that of the substrate 10, the characteristics 
of the diode D2 and the transistor Q1 change, and the current flowing 
through the diode D1 increases when the current flowing through the other 
paths decreases, resulting in high power output operation. 
In this laser structure, since the small energy band gap layer 16 is formed 
in a different process step from the process forming the active region 12, 
it is possible to select the energy band gap of the layer 16 to obtain the 
highest output power. In addition, the small energy band gap layer 16 is 
formed so that the ends of the layer 16 at the active region side reach 
the upper edges of the mesa, i.e., the ends of the layer 16 are aligned 
with respective edges of the active layer 11. Therefore, even when grooves 
25 are formed by etching to form a mesa structure that reduces parasitic 
capacitance as shown in FIG. 10, the small energy band gap layers 16 are 
reliably inside the mesa, whereby high speed response can be realized with 
high power output. In addition, the small energy band gap layers 16 are 
spaced apart from the active region 11 and the active layer 11 is 
surrounded by semiconductor layers having a large energy band gap. 
Therefore, light is sufficiently confined in the active region 11 and a 
far-field pattern having an ideal Gaussian distribution can be obtained. 
In addition, the small energy band gap layer 16 can be easily formed as 
compared with the conventional high carrier concentration layers 107 and 
113 shown in FIGS. 8 and 9 and a thickness of only 0.1 micron or less is 
enough to exhibit the effect, resulting in great merits in production and 
design. 
While in the above-described embodiment the semiconductor layer 11 having a 
small energy band gap, which serves as an active layer, is formed directly 
on the p type semiconductor substrate 10, a buffer layer having an energy 
band gap the same as or different from that of the substrate 10 may be 
inserted between the layer 11 and the substrate 10. In this case, the 
energy band gaps of the respective semiconductor layers are chosen on the 
basis of the energy band gap of the buffer layer. 
In addition, like a distributed feedback type semiconductor laser, a layer 
having a different energy band gap from that of the substrate 11 may be 
formed on or beneath the active layer 12. 
As is evident from the foregoing description, according to the present 
invention, a semiconductor laser device includes a p type semiconductor 
substrate, an active layer comprising a semiconductor having a smaller 
energy band gap than the p type semiconductor substrate and an n type 
semiconductor layer having larger energy band gap than that of the active 
layer, which are successively formed on the p type semiconductor 
substrate, a mesa formed by selectively etching the semiconductor 
substrate active layer and n type semiconductor layer leaving the active 
layer and n type semiconductor layer in a stripe-shape, p-n-p buried 
layers having larger energy band gap than the active layer and disposed at 
both sides of the mesa, a small energy band gap layer having smaller 
energy band gap than the p type semiconductor substrate and disposed on 
the buried layers, and an n type semiconductor layer disposed on the small 
energy band gap layer and on the n type semiconductor layer. Therefore, 
the small energy band gap layer for decreasing the current flowing through 
the thyristor structure is disposed close to the active region in a 
different process step from the formation of the active layer and, 
furthermore, a waveguide structure in which the active layer is surrounded 
by semiconductor layers having large energy band gaps is realized. As a 
result, a semiconductor laser device that provides high speed response and 
high power output at the same time and that outputs light having a 
far-field pattern in a Gaussian distribution is realized.