Note: Descriptions are shown in the official language in which they were submitted.
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
CONTROL OF CURRENT SPREADING IN SEMICONDUCTOR
LASER DIODES
Background of the Invention
This application claims the benefit of U.S. Provisional Application No.
60/164,864, filed November 12, 1999. The present invention relates to
controlling the lateral extent of a region of a semiconductor laser diode that
is
exposed to a gain current, and specifically to controlling such lateral extent
in
a ridge waveguide semiconductor laser diode adapted to support selected
lateral modes of the emitted laser light.
Summary of the Related Art
Typical semiconductor lasers such as laser diodes are formed by a
body of semiconductor material having a thin, active region formed between
cladding layers and contact regions of opposite polarity. A waveguide is
formed in the structure by defining a stripe for light guiding and for current
injection. Light is generated in the active region when the stripe region is
subject to a current flow between the positive and negative contact regions.
Cladding and confinement regions, among others, are placed between the
contacts and the active region for guiding and confining the light along the
thickness of the layers. The various regions typically are formed as
substantially parallel thin layers grown epitaxially. When the current is
greater
than the threshold current for the active waveguide, amplified light is
generated. In general, the greater the current flowing into the active
waveguide, the more light is generated.
1
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
The active regions can be shaped like a thin layer having a specified
thickness, length and width. The oscillation of the electric and magnetic
fields
of the light waves are restricted to specific modes, depending on the
dimensions of the active layer. The longitudinal mode of the light, along the
direction of propagation, is determined by the longitudinal length of the
active
layer forming the laser cavity. Similarly, the thickness of the active layer
restricts oscillation of the light waves in the transverse direction,
perpendicular
to the plane of the layers along which light propagates. By appropriately
sizing
the thickness of the layer, oscillations can be restricted to the fundamental
mode or to other desired modes of the light.
However, in the lateral direction perpendicular to the length of the
cavity and in the same plane as the layers, the modes are not limited by the
size of the active layer, but rather by the width of the stripe and of the
current
flow region. More than one mode can thus co-exist simultaneously within the
active layer.
One problem encountered in this type of semiconductor laser diode is
that the light emitted may include more than one optical mode in the lateral
direction, as described above. The mufti mode light emitted from this type of
diode is thus of limited use, because it is formed by a complex pattern of
bright and dark areas. Many applications require laser light that has a far
field
pattern consisting of a single bright spot, made, for example, by light that
includes only the fundamental mode. For other uses, a different specific
pattern can be required, such as one that is achieved by generating light
having
various selected modes.
2
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
A conventional method used to control the lateral modes of the light
includes forming a positive conductor on top of the semiconductor layer,
having a lateral dimension selected to only support the desired modes. An
insulator can be placed between the active layer and the positive conductor
S layer outside of that lateral dimension, to prevent current flowing from the
positive conductor outside of the selected region.
This method works for low power applications, but when the gain
current flowing from the positive to the negative conductor and across the
active layer exceeds a certain value, the current tends to spread in the
lateral
direction as it travels perpendicular to the layers. The degree of lateral
current
spreading can also increase with increased gain or drive current levels. This
forms areas of high gain in the active layer that are larger than what is
necessary to support the selected modes. When this occurs, extraneous modes
can be supported by these enlarged gain areas of the active layer, and the
light
emitted is no longer of only the desired mode.
Accordingly, there is a need for a device and a method for controlling
the lateral spread of current through an active layer of a semiconductor laser
diode, so that the gain regions of the active layer can be limited in the
lateral
direction to only support desired lateral modes of the generated laser light,
and
in particular to support only the fundamental mode of the laser light.
Summary of the Invention
The present invention is directed to a semiconductor laser diode and a
related method that is adapted to control the lateral modes of the laser light
generated, so that only desired modes are supported. In particular, this
result is
3
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
achieved by controlling the lateral spread of the electric current that passes
through the active layer, so that only a selected portion of the active layer
has
a high gain, resulting in amplification of only the light crossing that
portion of
the layer. The other portions of the active layer that flank the selected
active
region inhibit the flow of current, and therefore have a lower gain which
results in less amplification, or no amplification of the light passing
through
those portions. The lateral dimensions of the high gain portion of the active
layer can be selected to support only desired modes of the laser light, such
as
the fundamental mode or a combination of the fundamental and other modes.
The lateral control of the electric current is achieved by implanting
high energy ions, such as protons, in the portions of the active layer that
require a reduced conductivity, while shielding from the ion implant the
portion of the active layer where high conductivity, therefore high gain is
desired. This shielding can be obtained, for example, by placing a photoresist
layer between the source of ions and the active layer. The photoresist layer
can
be shaped with openings that correspond to the size of the desired conductive
portion of the active layer.
To achieve these and other advantages and in accordance with the
purpose of the invention as embodied and broadly described, in one aspect the
invention is a ridge waveguide semiconductor laser diode adapted to support
desired lateral modes of generated light, comprising a first conductor layer
for
application of a current, a second conductor layer facing the first conductor
layer, an active layer disposed between the first and second conductor layers,
a
conduction region of the active layer adapted for conducting the current, and
4
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
reduced conductivity regions of the active layer, flanking the conduction
region, adapted to impede passage of the current.
Brief Description of the Drawings
The accompanying drawings are included to provide a further
understanding of the invention, are incorporated in and constitute a part of
the
specification, illustrate embodiments of the invention and, together with the
description, serve to explain the objects, advantages and principles of the
invention.
In the drawings:
Figure 1 is a schematic perspective view of one embodiment of a
semiconductor laser diode incorporating the present invention;
Figure 2 is a schematic perspective view of the semiconductor laser
diode shown in Figure 1, also including a barrier layer;
Figure 3 is a schematic front elevation view showing a detail of the
embodiment of Figure 1; and
Figure 4 is a schematic view showing individual semiconductor laser
elements disposed in an array.
Detailed Description of the Preferred Embodiments of the Invention
Semiconductor laser diodes are used in a variety of devices such as
optical data storage and compact disc drives, for printing processes such as
those used in laser printers, and also for displays. For certain applications,
a
plurality of laser diodes can be assembled in an array, so that the light from
all
5
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
of the arrayed laser diodes has the same mode and in some cases also the same
phase.
For high brightness applications, a gain current up to 20 or 30 times the
threshold current of the active layer in the laser diodes can be used to
obtain a
high brightness spot or beam from the semiconductor material.
Figure 1 shows one embodiment of a semiconductor laser diode
according to the present invention. Semiconductor lasers consist of epitaxial
layers grown on a single-crystal substrate 15. Typically, the substrate 15 is
n-
type. The exemplary semiconductor laser diode 1 has an active layer 10 that
can include a quantum well structure. Active layer 10 can be formed, for
example, of un-doped InGaAs or InGaAsP. A positive conductor 12 can be
applied facing one surface of the active layer 10, and several p-type clad
region layers and confinement layers can also be deposited in region 18,
between the active layer 10 and the positive conductor 12, according to a
conventional manner of construction of semiconductor laser devices. A
negative conductor layer 14 can be formed on the substrate 15, facing the
opposite surface of active layer 10. A conventional arrangement of
confinement layers and n-type clad layers can also be disposed in region 20,
between the active layer 10 and the substrate 1 S.
Positive conductor layer 12 can be a strip as shown in Figure 1, or can
extend the entire width of semiconductor laser diode I. The strip shaped
positive conductor layer 12 can preferably have dimensions corresponding to a
region of active layer 10 that supports the desired optical modes. A
dielectric
insulator layer 16 can be used to prevent the flow of current from entering
the
6
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
top layers of region 18, outside of a selected lateral area. Insulator layer
16
thus defines an opening 17 in the insulator layer, that corresponds to the
area
where positive conductor 12 is in contact with region 18. In this manner, the
current flowing from positive conductor 12 to negative conductor 14 is
allowed to enter the portion of region 18 under opening 17, but is prevented
from entering the remainder of region 18 by insulator layer 16. In a different
embodiment, insulator 16 can be omitted from the device, particularly if
positive conductor 12 is shaped as a strip of desired width, and does not
extend the entire width of semiconductor laser diode 1.
The construction of semiconductor laser diode 1 can also include a
ridged waveguide 22 extending parallel to a longitudinal axis of
semiconductor laser device 1, and extending along the entire length L of the
device. For example, ridge waveguide 22 can have a width of about 3 - 5
microns. Ridge waveguide 22 channels the light being emitted and amplified
in active layer 10, so that the light is directed for the most part along the
longitudinal dimension of the semiconductor laser device 1.
In the lateral direction, along the active layer 10, the extent of the laser
light is governed by the width of the waveguide, the lateral index step
between
the waveguide and the region external to it, and the amount of lateral current
spread. In the case of a ridge waveguide, the lateral index step is the index
difference between the region under the ridge 22, and the regions under the
channels 23 on each side of the ridge 22.
If the index step is not present, the waveguide is "gain guided". In this
case, the light is guided along the current path by virtue of the absorption
7
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
difference, or gain vs. loss ratio, between the current flow region and its
lateral
regions. When the index step is present, the waveguide is "index guided", so
that guiding of the light is achieved by the index step. Index guiding can be
more advantageous than gain guiding, because it results in single-spatial mode
operation and reduced astigmatism.
At low power settings, only a small portion of the active layer 10,
approximately corresponding to the width of the non-insulated region of
positive conductor layer 12, is crossed by the current. At high power
settings,
when the current flowing from positive conductor 12 to negative conductor 14
is relatively large, the combination of a stripe-like positive conductor 12
and
an insulator 16 is insufficient to maintain the flow of current only within
the
general dimension of opening 17. Instead, the current tends to spread
laterally
outward to a region of active layer 10 having greater width than the opening
17. The current thus no longer follows a straight line from positive conductor
12 to negative conductor 14, but instead flares laterally outwards, as
schematically shown by the dashed line 31 in Figure 1. This broadens the
effective width of the gain region generating light, and allows the waveguide
to support additional lateral modes.
To prevent the occurrence of unwanted modes, active layer 10 is
divided in a defined gain region 24 of high conductivity, through which the
current flowing from positive conductor element 12 to negative conductor
element 14 can easily pass, and flanking regions 26 of reduced conductivity,
through which the current cannot easily pass. In this manner, the current
flowing from positive conductor element 12 to negative element 14 is
8
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
prevented from spreading laterally away from defined gain region 24, and
follows a path shown by the solid line 32. The lateral dimension of the high
gain portion of active layer 10 is thus limited to the size of defined gain
region
24. By properly sizing the defined gain region 24, only the desired lateral
modes of the laser light can be sustained, while other modes are not amplified
and will decay.
For example, defined gain region 24 could be sized to only support the
fundamental mode of the laser light in active layer 10, so that a sharp,
single
spot output laser beam can be generated by the device. Generation of laser
light of the fundamental mode is useful for lasers used in telecommunications.
For other applications, different modes can be useful. For example, a non-
gaussian beam formed by the fundamental and the second transverse mode can
be useful in laser printing applications to improve printing sharpness.
The reduced conductivity regions 26 formed in the active layer 10 can
be obtained, for example, by implanting ions such as protons in the material
of
active layer 10. The proton implant damages the structure of the active layer
10, and causes the affected region of active layer 10 to become non-
conducting. The extent of the transformation incurred by active layer 10 is
dependent on the strength and the duration of the proton implant. For example,
successful results can be obtained by implanting protons having an energy of
between approximately 130 KeV and 170 KeV. The implant can preferably
have a duration of between about 1 and 5 minutes, and can be repeated more
than once.
9
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
As shown in Figure 1, the laser light is amplified within a light
amplification portion 30 of defined gain region 24. The actual shape of light
amplification portion 30 depends on the desired mode being sustained. For
example, in the case shown, the fundamental mode is supported, which results
in a light amplification portion 30 shaped like a single elliptical spot
through
active layer 10, where the light amplification takes place.
The location within active layer 10 of reduced conductivity regions 26
must be selected carefully. The width of the defined gain region 24 of active
layer 10 determines which lateral modes of the laser light will be supported,
therefore reduced conductivity regions 26 must be placed sufficiently close to
light amplification portion 30 of active layer 10, so that additional modes
will
not be sustained by the active layer 10.
To satisfy these requirements, interface 40 between defined gain region
24 and reduced conductivity regions 26 must be selected to be just outside
light amplification portion 30 of active layer 10. At the same time, interface
40
must not be so far from light amplification portion 30 that the defined gain
region 24 can support modes other than the desired mode of the laser light.
The manufacturing process for semiconductor laser diode 1 will be
described with reference to Figure 2. Positive and negative conductor layers
12 and 14, insulating layer 16, active layer 10 and the remaining layers
forming semiconductor laser diode 1 are grown in a conventional manner. In a
preferred embodiment of the device according to the present invention,
reduced conductivity areas 26 are formed by implanting protons in the layers
of the device. A source of protons 44 can be, for example, hydrogen.
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
A barner layer such as photoresist layer 42 is used to shield from the
protons areas of active layer 10 that are to remain conductive. For example,
photoresist layer 42 can be placed above defined gain region 24, so that
protons generated by source 44 are stopped by photoresist layer 42, and do not
affect defined gain region 24. The remaining portions of the device, above and
including the reduced conductivity regions 26, are implanted with protons,
resulting in a loss of conductivity for those regions. In an embodiment
according to the invention, only sections lying below the channels 23 on the
sides of the ridge 22 are implanted. The depth of the implant can correspond
approximately to the thickness of clad region 18 and can reach through active
layer 10.
Figure 3 shows a detail of a region of the semiconductor laser depicted
in Figure 1. This exemplary embodiment includes an implant region of
constant depth 'd' obtained, for example, with an implant of given energy and
duration. Because of the presence of channels 23 on the sides of ridge 22, the
implant depth 'd' reaches active layer 10 in regions 26, below channels 23.
Regions 26', located beyond channels 23, also receive an implant of depth 'd'.
However, the layers of the laser diode above the active layer 10 have greater
thickness at that point, and thus the implant in region 26' may not reach the
active layer 10.
Figure 3 also shows lines 32 that indicate the path of current traveling
through active layer 10 from positive conductor 12, and lines 33 indicating
the
path of spreading current blocked by reduced conductivity areas 26.
11
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
Protons from source 44 can travel across metallic layers and the
various layers that form semiconductor laser diode l, so that the proton
implant can take place in a single step process, after all the layers of the
wafer
have been grown in a conventional manner. Photoresist layer 42 can be made,
for example, of a long chain polymer such as SHIPLEY Photoresist Type AZ
4620, having a thickness of about 5 p.m to 7 p.m. However, other materials
that
absorb protons from source 44 can be used in photoresist layer 42.
Since the energy of the implant determines the amount of loss
occurring in the implanted layer, the loss in selected regions of the active
layer
10 can be controlled with the location and energy of the implant. In this
manner, the waveguide loss in the lateral direction can be selected by
modifying the location of the implant. This loss introduced by the implant is
another mechanism that can be used to control the laser light mode in the
waveguide, in addition to controlling the current spread in reduced
conductivity areas 26.
In a preferred embodiment, photoresist layer 42 can be placed above
the outer surface of conductor layer 12, and can be removed after the implant
has been performed. However, other configurations of photoresist layer 42 can
be utilized, as long as photoresist layer 42 is placed between the source of
protons 44 and the areas that are to remain conductive after the implant.
The semiconductor laser diode according to the invention is well suited
for use in an array of laser diodes. For example, Figure 4 shows an exemplary
array of individually-addressable laser elements 50. Each element 50 includes
a ridge single-mode waveguide element 52, shown upside down in the figure.
12
CA 02388858 2002-05-10
WO 01/35506 PCT/US00/31048
In an exemplary embodiment, a dielectric layer 54 and a p-side metallization
layer 56 are formed with an appropriate shape and thickness to define the
ridges 58. The elements 50 are thermally and electrically separated by V-
grooves 64, etched through the active region 60. In an exemplary embodiment,
the element 50 are separated by approximately SOpm, resulting in an array that
contains more than 200 laser elements, with a density of 50 elements per cm.
All the laser elements in the exemplary embodiment produce a spot of laser
light 62 in active layer 60 having the same mode characteristics.
It will be apparent to those skilled in the art that various modifications
and variations can be made in the structure and the methodology of the present
invention, without departing from the spirit or scope of the invention. Thus,
the present invention is intended to encompass the modifications and
variations that come within the scope of the appended claims and their
equivalents.
13
