Semiconductor laser with active layer having a radiation emitting active region therein which extends through and is bounded by a current limiting blocking layer

A semiconductor laser having a layer structure including a first and a second passive layer of opposite conductivity types, an active layer therebetween which forms a pn junction with one of the passive layers, and a current-limiting blocking layer which forms a reverse-biased pn junction bounding a radiation emitting active region of the active layer. The active region has a thickness which exceeds that of the remainder of the active layer, and extends through the blocking layer at least as far as the other passive layer. This achieves effective electrical and optical confinement of the active region, enabling a sufficiently low threshold current for laser operation at room temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a semiconductor laser comprising a semiconductor 
body having a substrate region of a first conductivity type and a layer 
structure disposed thereon which includes at least one first passive layer 
of the first conductivity type, a second passive layer of the second 
opposite conductivity type and an active layer interposed between the 
first and second passive layers. The active layer forms a pn junction 
which at a sufficiently high current strength in the forward direction can 
produce coherent electromagnetic radiation in a direction at right angles 
to that of the active layer in an active region of the active layer 
located within a resonant cavity situated between two reflectors. The 
layer structure also includes a current-limiting blocking layer of the 
second conductivity type, is interrupted at the area of the active region. 
The first and second passive layers and the blocking layer all have a 
larger band gap and a smaller refractive index for the radiation produced 
than the active layer, the first and second passive layers being 
electrically connected to connection conductors and the active region 
being laterally bounded by the blocking layer. 
2. Description of the Related Art 
Such a semiconductor laser is known from U.S. Pat. No. 4,309,670. This 
discloses a semiconductor laser in which an active layer having a 
homogeneous thickness produces radiation which emanates in a direction at 
right angles to the active layer. Current limitation is obtained by means 
of a buried blocking layer, which forms a reverse-biased pn junction with 
the adjoining semiconductor material. In one of the embodiments of this 
U.S. Patent, the active region is laterally bounded by an epitaxial 
semiconductor region having a larger band gap and a smaller refractive 
index for the emitted radiation than the active region. 
Semiconductor lasers of the relevant type, in which the radiation produced 
is emitted in a direction at right angles to the active layer, have the 
advantage that they can be readily coupled to an optical fibre, the 
emanating beam is not very divergent and no accurate positioning of the 
crystal is required. 
Since the length of the active region within which amplification occurs 
corresponds in known lasers to the thickness of the active layer and is 
consequently very small, a high current density is required to obtain the 
amplification required for the laser effect. In order to keep the 
threshold current at an acceptable value, fairly complicated epitaxial 
structures are required, while, as in the aforementioned U.S. Pat. No. 
4,309,670, additional epitaxial passive layers and/or locally diffused 
blocking layers are used besides the active layer. For this reason inter 
alia, the said known laser structures can generally be realized only with 
great difficulty. 
SUMMARY OF THE INVENTION 
The present invention has for its object to avoid the said disadvantages 
inherent in known semiconductor lasers of this type or to reduce them at 
least to a considerable extent. The invention provides a new laser 
structure, in which both electrical and optical confinement of the active 
region are realized in a satisfactory manner, which has a comparatively 
small wavelength and a comparatively low threshold current, and which can 
be manufactured in a comparatively simple manner by means of modern 
techniques. 
The invention is based inter alia on the recognition of the fact that this 
can be achieved by the use of an active layer which has a increased 
thickness in the active region. 
According to the invention, a semiconductor laser of the kind described in 
the opening paragraph is characterised in that the active region of the 
active layer has a larger thickness than the remaining part of the active 
layer, and extends at the area of the interruption through the blocking 
layer at least as far as the first passive layer. The active layer is of 
the first conductivity type, and the blocking layer is disposed between 
the active layer and the substrate region. 
In contrast with the mentioned known lasers, in the semiconductor laser 
according to the invention the thickness of the active region exceeds that 
of the blocking layer, which ensures the electrical and optical 
confinement. As a result, the active layer itself forms part of the 
current-limiting layer structure, which thus achieves a much simpler 
construction and can be realized in a simpler manner. Further, due to the 
very effective electrical and optical confinement, the threshold current 
can be low so that operation at room temperature becomes possible. 
The active layer can be directly provided on the blocking layer. However, 
in given circumstances, it may be preferable that a further semiconductor 
layer of the first conductivity type having a larger band gap and a 
smaller refractive index for the emitted radiation than the active layer 
be provided between the active layer and the blocking layer. During 
manufacture, leakage currents, which could adversely affect the electrical 
confinement, are then less liable to occur. 
The resonant cavity can be formed in different ways. For example, one or 
both reflectors may be formed by a periodical variation of the effective 
refractive index in the direction of the emitted radiation, according to 
the principle of devices known as DBR or DFB lasers. According to another 
preferred embodiment, the reflectors are constituted by optically flat end 
surfaces of the layer structure, the substrate region being provided with 
a cavity having a flat bottom, through which the radiation emanates, this 
cavity extending throughout the thickness of the substrate to the first 
passive layer. 
The invention further relates to a particularly suitable method of 
manufacturing the semiconductor laser. This method is characterized in 
that in order of succession a first passive semiconductor layer of the 
first conductivity type and a blocking layer of the second opposite 
conductivity type are grown epitaxially on a monocrystalline substrate 
region of the first conductivity type. Then a cavity is formed by etching, 
which extends through the blocking layer to the first passive layer, and 
subsequently an active layer of the first conductivity type is formed by 
epitaxial growth from the liquid phase, an active region of this active 
layer filling the cavity. Finally, a second passive layer of the second 
conductivity type is formed by epitaxial growth on the active layer and a 
contact layer of the second conductivity type is formed on this passive 
layer, whereupon an opening extending as far as the second passive layer 
is etched into the contact layer opposite to the active region and a 
dielectric layer having a thickness equal to an optical path length of an 
integral number of times a quarter wavelength of the emitted radiation is 
provided in this opening. The substrate region and the contact layer are 
then provided with connection conductors.

The Figures are schematic and are not drawn to scale, while more 
particularly the dimensions in the direction of thickness are exaggerated 
for the sake of clarity. 
Corresponding parts are generally provided with the same reference numerals 
in the various embodiments. Semiconductor regions of the same conductivity 
type are generally cross-hatched in the same direction. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows diagrammatically in cross-section a semiconductor laser 
according to the invention. For the sake of simplicity, the semiconductor 
laser is assumed to be rotation-symmetrical about the line M M, although 
this is not absolutely necessary. 
The semiconductor laser comrises a semiconductor body having a substrate 
region 1, which in this embodiment consists of a monocrystalline region of 
gallium arsenide of a first conductivity type, in this case the p 
conductivity type. On this substrate region 1 is disposed a layer 
structure comprising a first passive layer 2, which is also of the first 
(p) conductivity type, a second passive layer 3 of the second opposite (so 
in this case n) conductivity type and an active layer 4 situated between 
the first passive layer 2 and the second passive layer 3 and having a pn 
junction 5, which at a sufficiently high current strength in the forward 
direction can produce coherent electromagnetic radiation in the direction 
of the arrow 6, so at right angles to that of the active layer 4. The 
radiation is produced in an active region 4A of the active layer 4 and 
this active region is situated in a resonant cavity between two 
reflectors, which in this embodiment are constituted by reflecting end 
surfaces 7 and 8 of the layer structure. Further, the layer structure 
comprises a current-limiting blocking layer 9 of the second (in this case 
n) conductivity type, which has an interruption at the area of the active 
region 4A. The active layer 4, the first and second passive layers 2 and 3 
and the blocking layer 9 in this embodiment all consist of gallium 
aluminium arsenide, the contents of aluminium being chosen so that the 
layers 2, 3 and 9 all have a larger band gap and consequently a lower 
refractive index for the radiation produced than the active layer 4. The 
first and second passive layers (2 and 3) are electrically connected to 
connection conductors constituted by electrode layers 10 and 11, 
respectively; the layer 2 is connected thereto through the highly doped 
substrate 1 and the layer 3 through an also highly doped contact layer 12 
of n-type gallium arsenide. The active region 4A is laterally bounded by 
the blocking layer 9. 
According to the invention, the active region 4A has a larger thickness 
than the remaining part of the active layer 4 and it extends through the 
area of interruption of blocking layer 9 at least as far as the first 
passive layer 2. Further, according to the invention, the passive layer 2 
is of the first (so in this case of the p) conductivity type so that the 
pn junction 5 is situated between the layers 3 and 4, while the blocking 
layer 9 is situated between the active layer 4 and the substrate region 1. 
Due to the construction of the layer structure according to the invention, 
in which the active region 4A is thicker than the remaining part of the 
active layer 4 and extends through the blocking layer 9 at least as far as 
the passive layer 2, a very efficient electrical and optical confinement 
of the generated radiation is obtained in a comparatively simple manner. 
As a result, a comparatively low threshold current can be attained. 
In the semiconductor laser according to this embodiment, there is moreover 
provided between the active layer 4 and the blocking layer 9 a 
semiconductor layer 13 of the first (so in this case p) conductivity type 
consisting of gallium aluminium arsenide having a larger band gap and a 
smaller refractive index for the radiation produced than the active layer 
4. This layer serves to prevent that in the case of possible leakage 
currents through the active layer 4 the current-limiting properties of the 
reverse-biased pn junction between the n-type blocking layer 9 and the 
adjoining p-type material would be aversely affected. 
Further, in this embodiment, the substrate region 1 is provided with a 
cavity 14 having a flat bottom, through which the radiation emanates. This 
cavity extends throughout the thickness of the substrate region 1 to the 
first passive layer 2 and is adapted to receive a glass fibre coupled to 
the laser, as a result of which the coupling becomes very simple. 
In this embodiment, the following compositions, dopings and thicknesses of 
the various layers are used. 
__________________________________________________________________________ 
Refractive 
Doping Thickness 
index 
Layer 
Composition 
Type 
concentration 
(.mu.m) 
(for .lambda. = 750 nm) 
__________________________________________________________________________ 
1 GaAs P 2 .times. 10.sup.19 at/cm 
90 .mu.m 
2 Ga.sub.0.50 Al.sub.0.50 As 
P 10.sup.18 
3 3.26 
3 Ga.sub.0.50 Al.sub.0.50 As 
N 10.sup.18 
3 3.26 
4 Ga.sub.0.86 Al.sub.0.14 As 
P 10.sup.18 
0.5 3.49 
9 Ga.sub.0.80 Al.sub.0.20 As 
N 2 .times. 10.sup.17 
1 3.45 
12 GaAs N 3 .times. 10.sup.18 
1 
13 Ga.sub.0.80 Al.sub.0.20 As 
P 10.sup.18 
0.5 3.45 
__________________________________________________________________________ 
The radiation emitted by this laser has a wavelength of 750 nm. The 
diameter (a) of the active region is about 3 .mu.m; the diameter (b) of 
the substrate opening 14 is about 20 .mu.m at the area of the bottom 8. 
The electrode layer 10 on the substrate 1 is, for example, a platinum 
molybdenum-gold layer or a platinum-tantalum-gold layer. The electrode 
layer 11 on the contact layer 12 of highly doped gallium arsenide 
consists, for example, of a gold-germanium-nickel layer, which is located 
opposite to the opening 14 within an opening in the contact layer 12 on a 
silicon oxide layer 15 having a thickness of 0.15 .mu.m. This thickness 
corresponds to an optical path length of an integral number of times a 
quarter wavelength of the emitted radiation so that the radiation on this 
side of the laser structure is reflected substantially completely. The 
threshold current of the laser was 10 mA at 30.degree. C. 
The semiconductor laser described can be manufactured according to the 
invention in the following manner. The starting material is a substrate 1 
of monocrystalline p-type gallium arsenide having a doping concentration 
of 2.10.sup.19 atoms per cm.sup.3 and a thickness of, for example, 350 
.mu.m. After the surface thereof, which preferably has a (100) 
orientation, has been polished and etched, there are successively grown on 
this surface, for example from the liquid phase (designated as LPE=Liquid 
Phase Epitaxy) a 3 .mu.m thick layer 2 of p-type Ga.sub.0.50 Al.sub.0.50 
As having a doping concentration of 10.sup.18 atoms per cm.sup.3, a 1 
.mu.m thick layer 9 of n-type Ga.sub.0.8 Al.sub.0.2 As having a doping 
concentration of 2.10.sup.17 atoms per cm and a 0.5 .mu.m thick layer 13 
of p-type Ga.sub.0.8 Al.sub.0.2 As having a doping concentration of 
10.sup.18 atoms per cm.sup.3. This growth may also be effected by means of 
metal-organic epitaxy from the vapour phase, known under the designations 
MOCVD or OMVPE (Organic Metallic Vapour Phase Epitaxy) technique, by 
chemical decomposition of organic metal compounds. For details about the 
LPE technique, reference may be made to the book by D. Elwell and H. J. 
Scheel, "Crystal Growth from High-Temperature Solutions", Academic Press 
1975, p. 433-467. For details about the OMVPE technique, reference may be 
made to the article by M. J. Ludowise "Metal-organic Chemical Vapour 
Deposition of III-IV Semiconductors" in Journal of Applied Physics, Vol. 
58, Oct. 15, 1985, p. R31-R55. Subsequently, in the layer structure thus 
obtained a hole is etched having in this example a diameter of about 3 
.mu.m just through the blocking layer 9 and preferably having a flat 
bottom so that the situation of FIG. 2 is obtained. The etching technique 
used may be, for example, "reactive ion etching" (RIE). 
Now by means of OMVPE a very thin (5 nm) layer of gallium arsenide is grown 
(not indicated in the Figure). This is necessary to permit of carrying out 
the next epitaxial growth because epitaxial growth on gallium aluminium 
arsenide is very difficult. 
The etched hole is now filled by epitaxial growth of gallium aluminium 
arsenide from the liquid phase by means of the LPE technique. In this 
case, the hole is closed very rapidly by growth, after which the further 
layers are formed on the practically flat surface obtained. Thus, in order 
of succession the active layer 4, the passive layer 3 and the highly doped 
contact layer 12 (of GaAs) are grown having thicknesses of 0.5 .mu.m, 3 
.mu.m and 1 .mu.m, respectively, and having the composition described 
above (cf. FIG. 3). 
A hole having a diameter of 15-20 .mu.m is now etched into the contact 
layer 12 opposite to the surface 4A by means of a selective etching 
liquid, which attacks practically only a GaAs, but does not attack 
Ga.sub.x Al.sub.1-x As, for example a mixture of hydrogen peroxide and 
ammonia. The etching process then terminates at the optically flat 
interface of GaAs and Ga.sub.x Al.sub.1-x As. Subsequently, the substrate 
1 is reduced to a thickness of about 90 .mu.m by etching in, for example, 
a mixture of concentrated sulphuric acid, hydrogen peroxide (30%) and 
water (volume ratio 3:1:1), after which an opening having a diameter of 
about 20 .mu.m is etched also selectively into the substrate opposite to 
the active region 4A by means of an aqueous solution of H.sub.2 O.sub.2 
and NH.sub.4 OH down to the optically flat interface with the layer 2, 
which serves as the second reflector. After a 0.15 .mu.m thick silicon 
oxide layer 15 have been provided in the opening in the contact layer 12 
and the electrode layers 10 and 11 have been formed, the structure of FIG. 
1 is obtained. 
Another embodiment of a semiconductor laser according to the invention will 
now be described with reference to the FIGS. 4 to 6. Also in this case, 
the laser is assumed to be rotated-symmetrical about the line M--M, 
although this is not essential to the invention. This semiconductor laser 
is suitable to produce radiation having greater wavelengths (1.3 .mu.m), 
such as frequently used for optical telecommunication purposes. 
The starting material is a substrate of indium phosphide, which is p-type 
conducting, has a thickness of 350 .mu.m and has a doping of 2.10.sup.18 
atoms per cm.sup.3. Since in this case the substrate region and the first 
passive layer consist of the same material, an intermediate layer 20 
should first be formed on the substrate. This layer has in this embodiment 
a thickness of 0.3 .mu.m and consists of p-type In.sub.0.72 Ga.sub.0.28 
As.sub.0.60 P.sub.0.40. Subsequently on the layer 20, which can be etched 
selectively with respect to indium phosphide, there are grown from the 
liquid phase a first passive layer 2 of p-type indium phosphide having a 
thickness of 6 .mu.m and a doping concentration of 10.sup.18 atoms per 
cm.sup.3 and then also from the liquid phase an about 1 .mu.m thick 
blocking layer of n-type indium phosphide having a doping concentration of 
10.sup.18 atoms per cm.sup.3. Then, a hole having a diameter of, for 
example, 2 .mu.m is etched into the surface and this hole extends 
throughout the thickness of the layer 9 into the layer 2, as a result of 
which the structure shown in FIG. 4 is obtained. 
After that, like in the preceding embodiment, the etched cavity is filled 
very rapidly by epitaxial growth from the liquid phase, after which on a 
practically flat surface thus obtained there is provided the remaining 
about 0.2 .mu.m thick part of the active layer 4, which in this case 
consists of p-type In.sub.0.72 Ga.sub.0.28 As.sub.0.60 P.sub.0.40. 
Subsequently, in order of succession a second passive layer 3 of n-tye 
indium phosphide having a thickness of 2 .mu.m and a doping concentration 
of 10.sup.18 atoms per cm.sup.3 and an about 0.5 .mu.m thick contact layer 
12 of n-type In.sub.0.72 Ga.sub.0.28 As.sub.0.60 P.sub.0.40 having a 
doping concentration of 5.10.sup.18 atoms per cm are grown, also from the 
liquid phase. 
Then, like the preceding embodiment, an opening having a diameter of about 
20 .mu.m is etched into the contact layer 12 opposite to the active region 
4A by means of a selective etchant, which does not attack the layer 3, so 
that an optically flat surface 7 is obtained. In this opening, a 
dielectric layer 15 of, for example, silicon oxide is provided, which has 
an optical thickness of an integral number of quarter wavelengths of the 
emitted radiation. After the substrate region 1 has been etched down to a 
thickness of about 50 .mu.m, an opening 14 is etched into the substrate 
opposite to the active region 4A by means of a selective etchant, for 
example hydrochloric acid, which does not or substantially does not attack 
the intermediate layer 20. Subsequently, by means of another selective 
etchant, for example a solution of 5 g of potassium permanganate in 1 
cm.sup.3 of concentrated sulphuric acid and 50 cm.sup.3 of water, which 
attacks only the intermediate layer 20, but which does not attack the 
indium phosphide layer 2, the layer 20 is etched down to the optically 
flat interface 8 between the layers 2 and 20, which serves as the second 
reflector. After the electrode layers 10 and 11 have been formed, the 
semiconductor laser shown in FIG. 6 is obtained, which can emit in the 
direction of the arrow 6 coherent electromagnetic radiation having a 
wavelength of about 1.3 .mu.m in, for example, a fibre mounted in the 
cavity 14. 
The reflectors need not consist of reflecting end surfaces of the laser. 
Other solutions are also possible, such as, for exmple, shown with 
reference to FIG. 7, in which instead of the reflecting end surface 8 of 
the preceding embodiments, a so-called DBR (Distributed Bragg Reflection) 
reflector is used. This reflector consists of a number of thin layers 30 
alternately consisting of Ga.sub.1-w Al.sub.w As and Ga.sub.1-y Al.sub.y 
As, where 0.ltoreq.w.ltoreq.1.0.ltoreq.y.ltoreq.1 and w&gt;y. As a result, a 
periodical variation of the effective refractive index is obtained in the 
direction of the emitted radiation 6, as also described in the 
aforementioned U.S. Pat. No. 4,309,670. The layers 30 are grown on a 
p-type surface 1 of GaAs; the semiconductor laser may otherwise be 
constructed in the same manner as that according to the embodiment of FIG. 
1. The layer structure 30 may be obtained, for example, by epitaxial 
growth according to the MBE (Molecular Beam Epitaxy) or OMVPE 
(Organo-Metallic Vapour Phase Epitaxy) techniques. The layers consist, for 
example, alternately of GaAs and AlAs and all have a thickness of about 80 
nm. The total number of layers of the structure 30 is, for example, 
twenty. 
The invention is not limited to the embodiments described because many 
variations are possible for those skilled in the art without departing 
from the scope of the invention. For example, semiconductor materials 
other than those mentioned in the embodiments may be used. Further, the 
conductivity types may be all (simultaneously) replaced by the opposite 
conductivity types. Other layer thicknesses may be used according to the 
application desired. Moreover, the laser structure need not be 
rotation-symmetrical at all. For example, rectangular structures may also 
be used. Further, instead of one reflector, both reflectors may also be 
constructed as "Distributed Bragg Reflectors" (of the type as indicated by 
(30) in FIG. 7).