Method of making distributed feedback laser having spatial variation of grating coupling along laser cavity length

A distributed feedback (DFB) laser is made with a spatially graded optical coupling (.kappa.) between its diffraction grating and its active layer by means of selective area epitaxial growth of the epitaxial layer from which the grating is formed. More specifically, the epitaxial layer is formed on a major surface of a semiconductor substrate on which a mask, such as a silicon dioxide mask, has been formed. The mask has a pair of segments spaced apart by a fixed distance, the segments having spatially variable widths. Advantageously the epitaxial layer has a refractive index that is different from that of the substrate at the operating wavelength of the laser. The epitaxial layer is then etched into stripes. In this way the heights of the resulting grating stripes will be spatially variable, and so will the coupling .kappa. between the grating and the active layer. In this way, the properties of the optical output of the laswer can be adjusted. Advantageously also, the spacer layer and the substrate have the same refractive indices at the operating wavelength of the laser, whereby accurate control over the depth of the etching is not required.

FIELD OF INVENTION 
This invention relates to semiconductor lasers and more particularly to 
methods of making such lasers that have distributed feedback. 
BACKGROUND OF INVENTION 
A semiconductor laser is ordinarily made of Group III-V semiconductor 
materials. One particularly useful form of such a laser has distributed 
feedback ("DFB"). That is to say, optical feedback is built into the laser 
along its cavity length. For example, such feedback is supplied by means 
of a DFB diffraction grating whose stripes ("teeth") run perpendicular to 
the length (longitudinal direction) of the laser cavity. Such lasers, 
however, tend to suffer from spatial holeburning (spatial variation in 
optical gain saturation along the longitudinal direction) and from 
adiabatic chirping. That is to say, they suffer from relatively low gain 
near the highly reflecting mirror of the laser owing to spatial variation 
in gain saturation, and from non-symmetrical spectral intensity 
distribution around the spectral maximum. In turn, such holeburning and 
chirping cause, among other things, an undesired lack of single mode 
operation as well as an undesired lack of linearity of laser response to 
applied signals. 
U.S. Pat. No. 5,329,542 teaches a semiconductor DFB laser in which improved 
single-mode behavior of such a laser is improved by reducing the feedback 
at or near one or both ends of the DFB grating. In particular, the 
feedback is reduced by reducing the coupling coefficient .kappa. between 
the DFB grating and the optical cavity. This reducing in the coupling 
coefficient .kappa. is achieved by spatially varying the depth of the 
grating's teeth and/or by spatially varying the spacing between adjacent 
teeth (while maintaining a fixed periodicity). 
The aforementioned patent further teaches methods to achieve this spatially 
varying coupling .kappa.. More specifically, the patent teaches a method 
of spatially varying either the depths or the spacing of the teeth. The 
method uses a crystallographically dependent (angular; non-vertical) 
chemical etching in combination with a patterned photoresist masking layer 
having a spatial duty cycle (i.e., variable tooth density) that varies in 
the longitudinal laser direction. This spatially varying duty cycle cannot 
be achieved by a relatively quick method--for example, a holographic 
interference exposure method--that produces a constant duty cycle over the 
entire surface in a single exposure; but it requires other, more time 
consuming methods. The resulting tooth depth is a critical parameter that 
determines .kappa.. Such a method thus critically relies on the chemistry 
of the crystallographic etching required to produce the teeth. 
Consequently the angle of etching, and hence the depth of the teeth, is 
very sensitive to the chemistry of the crystallographic etching. Therefore 
it is relatively difficult to control the average coupling resulting in 
the gratings made by such methods. It would therefore be desirable to have 
a method of making a DFB laser with a coupling .kappa. that varies in the 
longitudinal laser direction but which does not rely on crystallographic 
etching and which can be achieved with a constant spatial duty cycle. 
SUMMARY OF INVENTION 
In accordance with the invention, a DFB laser is made by a sequence of 
steps comprising 
(a) forming, on a horizontal (xy) major surface of a semiconductor 
substrate, a masking layer having at least a pair of segments with 
opposing first and second surfaces oriented perpendicular to the major 
surface, whereby a space is formed between the segments whereat the major 
surface of the semiconductor substrate is exposed, the widths of the 
segments measured along a first (y) direction varying, advantageously 
monotonically, along a second (x) direction between two planes parallel to 
the first and second directions (for example, FIG. 1); 
(b) growing, preferably by organo-metalic vapor phase epitaxy ("OMVPE"), an 
epitaxial semiconductive layer, advantageously having a different 
refractive index from that of the substrate, on the exposed major surface 
of the semiconductor substrate in the space between the segments of the 
masking layer, whereby the epitaxial layer has a thickness, measured in 
the vertical (z) direction, that varies--advantageously 
monotonically--with distance along the second (x) direction (for example, 
FIG. 2); and 
(c) patterning the epitaxial semiconductive layer into a multiplicity of 
stripes, such as by means of holographic interference photolithography and 
dry etching, each of the stripes having a geometrical component running 
along the major surface in the first (y) direction parallel to the first 
direction (for example, FIG. 3). 
Step (c) is then advantageously followed by removing the phototlithographic 
mask and then epitaxially growing in succession a (bottom) spacer layer, 
an active layer, and a (top) cladding layer for the laser (illustratively, 
FIG. 3). Advantageously further, the laser is completed by subsequent 
further processing and epitaxial growth, as known in the art of making 
single-mode DFB lasers. This further processing advantageously includes 
vertical etching from the top of the laser down into the substrate ("mesa 
etching"). Advantageously this etching is performed such that the length 
of the entire portion of the laser located above the substrate has a 
constant length in the y direction, in order to define an optical 
waveguide that will support a single mode operation in the xy plane. In 
this way a grating layer is formed having stripes, composed of a material 
having a refractive index that is different from that of the substrate, 
whose heights measured along the vertical (z) direction spatially vary, 
advantageously monotonically, along the second (x) direction. As used 
herein the term "monotonically" includes the case in which the thickness 
does not vary in some regions along the second (x) direction. 
Advantageously, the spacer layer has the same refractive index as that of 
the substrate. In this way precise control over the patterning (e.g., over 
the depth of the dry etching) of the epitaxial semiconductive layer is not 
required. 
The coupling .kappa. is dependent on the varying sizes (e.g., heights) of 
the stripes as well as on the refractive index of the grating layer 
relative to that of the spacer layer contiguous with the stripes. Because 
of the spatial variation in heights of the stripes of the grating layer, 
the coupling .kappa. of the grating layer to the active layer will also 
spatially vary, as is desired. 
The inventive method is adaptable to mass making of many DFB lasers, by 
means of suitable dicing (cleaving into pieces) of the substrate together 
with its superstructure that has been formed according to the 
above-described steps. Advantageously, this dicing is followed by 
providing respective optical coatings of suitable reflectivities on the 
resulting yz facets.

DETAILED DESCRIPTION 
As shown in FIG. 1, a single crystal semiconductive substrate 10 has a top 
major surface oriented parallel to the xy plane. Advantageously for mass 
making of many DFB lasers at the same time, the pattern of the structure 
shown in FIG. 1 has spatial periodicities in both the x and y 
directions--i.e., along the plane of the major surface of the substrate 
10. Typically the substrate 10 is made of a Group III-V compound 
semiconductor, as known in the art. Silicon dioxide layer segments 31 and 
32 are formed on the top major surface of the substrate 10, such as by a 
standard chemical vapor deposition technique. These layer segments 31 and 
32 are patterned by standard masking and (wet or dry) etching, such that 
they have surfaces 41 and 42, respectively parallel to the xz plane. In 
addition, these layer segments 31 and 32 are patterned such that they have 
continuous (in the x direction), curvilinear vertical surfaces 51 and 52, 
respectively, opposing these planar vertical surfaces 41 and 42. The 
curvilinear vertical surfaces 51 and 52 are made such that the widths of 
the silicon dioxide layer segments 31 and 32 monotonically decrease in the 
x direction going from (imaginary) plane .beta. to (imaginary) planes 
.alpha. and .gamma.. As used herein, the term "monotonically decrease" 
includes the case in which some portions monotonically non-increase. The 
heights (in the z direction) of the silicon dioxide layer segments 31 and 
32 typically are made to be uniform and equal. 
Then (FIGS. 1 and 2), using the silicon dioxide layer segments 31 and 32 as 
masks, an epitaxial Group III-V semiconductor layer 20 (hereinafter, 
simply called "the epitaxial layer 20"), having a continuous (in the x 
direction) curvilinear top surface 210, is grown on the top surface of the 
substrate 10 by means of selective area growth. As used herein, the term 
"selective area growth" refers to a process in which the growth conditions 
are adjusted so that growth of any semiconductor material occurs only on 
surfaces of exposed semiconductor, as known in the art. The epitaxial 
layer 20 illustratively is an epitaxial quaternary Group III-V 
semiconductor layer or an epitaxial multiple-quantum-well Group III-V 
semiconductor layer. The epitaxial layer 20 advantageously has a 
refractive index or an effective refractive index at the DFB laser 
operating wavelength that differs from the of the substrate 10. As 
indicated in FIG. 2, this epitaxial layer 20 will have greater thicknesses 
(greater heights) measured in the z direction in the neighborhood of the 
plane .beta. than in the neighborhoods of the planes .alpha. and .gamma.. 
Moreover, it will have intermediate, monotonically increasing heights 
between the planes .alpha. and .beta.. These spatial variations in heights 
are attributable to the influence of the above-described monotonically 
decreasing widths of the silicon dioxide layer segments 31 and 32 between 
the planes .alpha. and .beta.--i.e., monotonically increasing widths going 
from the plane .alpha. to the plane .beta.. Therefore the epitaxial layer 
20 has a monotonically increasing height going from the plane .alpha. to 
the plane .beta.. As used herein, the term "monotonically increasing" thus 
includes the case in which some portions are monotonically non-decreasing. 
Next, as indicated in FIG. 3, the epitaxial layer 20 is masked and 
vertically etched, preferably dry etched, using a standard patterned 
photoresist masking and vertical etching technique, whereby exemplary 
epitaxial stripes 21, 22, 23, 24, and 25 are formed. Only for the purpose 
of clarity, in FIG. 3 the number of stripes shown is reduced. 
Advantageously, in order to form a pattern in the photoresist mask, the 
photoresist is holographically exposed. Typically, the photoresist has a 
thickness of approximately 0.05 .mu.m. 
In a preferred embodiment, the layer 11 and the substrate have 
substantially equal refractive indices at the operating wavelength of the 
DFB laser 200 (FIG. 3) that is being made. In such an embodiment, the 
etching need not be material selective, that is, need not be dependent on 
the materials encountered in the etching, whereby the resulting etched 
grooves located between any of the adjacent stripes 21, 22, 23, 24, and 25 
may penetrate into the substrate 10 by an amount that need not be 
controlled. 
In any event, the epitaxial stripes 21, 22, 23, 24, and 25 are made to run 
parallel to the y direction, advantageously with mutually equal widths 
parallel to the x direction. However, the heights of these epitaxial 
stripes 21, 22, 23, 24, and 25 will monotonically increase with increasing 
x between the planes .alpha. and .beta., because these stripes derive from 
the epitaxial layer 20 that has a monotonically increasing height in the z 
direction going from the plane .alpha. to the plane .beta.. 
Then a spacer layer 11 is epitaxially grown to at least such a thickness 
that it fills in the grooves located between the stripes 21, 22, 23, 24, 
and 25. Next, an active layer 12 and a cladding layer 13 are epitaxially 
grown. These layers 11, 12, and 13 typically are made of Group III-V 
semiconductor material suitable for making a DFB laser 200 (FIG. 3). 
Advantageously the spacer layer 11 has the same refractive index as that 
of the substrate 10, so that precise control over the depth and 
selectivity of the etching of the stripes 21, 22, 23, 24, and 25 is not 
required. 
The substrate 10 together with its superstructure including the epitaxial 
stripes 21, 22, 23, 24, and 25 is cleaved along the planes .alpha., 
.beta., and .gamma., as well as along planes parallel to the xz plane, to 
form many individual DFB lasers such as DFB laser 200 (FIG. 3). An 
optically highly reflecting coating 201 is formed on the plane .alpha. and 
an anti-reflection coating 202 is formed on the plane .beta.. Finally, 
electrical contacts (not shown) are attached to the top and bottom 
surfaces of the DFB laser 200, as known in the art. 
The epitaxial stripes 21, 22, 23, 24, and 25 form a DFB grating, as known 
in the art. The optical coupling .kappa. between the active layer 12 and 
this DFB grating layer will monotonically increase going from the plane 
.alpha. to the plane .beta., because of the increasing cross-section (yz) 
area of these stripes 21, 22, 23, 24, and 25, respectively, along the x 
direction. In addition, part of the variation of .kappa. among the stripes 
21, 22, 23, 24, and 25 may be attributable to a variation of refractive 
index or of effective refractive index among these stripes caused by 
variation of composition of, or of effective bandgap of, the semiconductor 
material of these stripes. 
EXAMPLE 
By way of an illustrative example, the thickness (height in the z 
direction) of the silicon dioxide masking layer segments 31 and 32 
typically are equal to approximately 0.3 .mu.m. The length of the DFB 
laser 200 along the x direction typically is equal to approximately 0.4 
mm. The epitaxial stripes 21, 22, 23, 24, 25, etc., all have the same 
width typically equal to approximately 0.10 or 0.12 .mu.m in the x 
direction. More generally, the width of the stripes 21, 22, 23, 24, 25, 
etc., in the x direction typically is equal to approximately 30 
percent-to-70 percent of the spatial periodicity (in the x direction) of 
the grating formed by these stripes. In addition, the stripes 21, 22, 23, 
24, 25, etc., have a spatial periodicity typically equal to approximately 
0.20 .mu.m or 0.24 .mu.m, respectively, for a DFB laser 200 operating at a 
wavelength of 1.31 .mu.m or 1.55 .mu.m, respectively. Thus there typically 
are approximately two thousand epitaxial stripes 21, 22, 23, 24, 25, etc., 
in the DFB laser 200. The width of the substrate 10 of the DFB laser 200 
in the y direction is typically equal to approximately 0.25 mm (=250 
.mu.m). However, by means of vertical etching not only the stripes 21, 22, 
23, 24, 25, etc., but also the layers 11, 12, and 13 have a width in the y 
direction that typically is approximately equal to only 1 .mu.m. 
Although the invention has been described in detail in terms of a specific 
embodiment, various modifications can be made without departing from the 
scope of the invention. For example, instead of, or in addition to, the 
silicon dioxide layer segments 31 and 32 having planar vertical opposing 
surfaces 41 and 42, they can have curvilinear vertical opposing surfaces 
like 51 and 52. 
Instead of silicon dioxide, other materials can be used for the masking 
layer segments 31 and 32. Such other materials include, for example, 
silicon nitride or amorphous silicon. In addition, such other materials 
include other dielectric layers that inhibit the growth of the epitaxial 
semiconductor layer 20 on their surfaces but that enable the diffusion on 
their surfaces of the reacting chemical species into the exposed 
semiconductor regions where the epitaxial layer 20 is growing. 
Also, the surfaces 51 and 52 can have stepwise y-discontinuities along the 
x direction. In such cases, the contour of the surface 210 will not be as 
discontinuous as the surfaces 51 or 52 because of the diffusion, on the 
surfaces of the masking layer segments 31 and 32, of the semiconductor 
material being deposited. 
Finally, as an alternative, immediately after the epitaxial growth of the 
epitaxial layer 20, a thin epitaxial layer (not shown), advantageously 
having the same refractive index as that of the substrate 10, is grown on 
the top surface 210. In this way, as known in the art, the quality of the 
stripes 21, 22, 23, 24, 25, etc., is maintained.