U.S. patent application number 11/040246 was filed with the patent office on 2005-09-15 for method for making a high power semiconductor laser diode.
Invention is credited to Schmidt, Berthold, Sverdlov, Boris, Thies, Achim, Traut, Silke.
Application Number | 20050201438 11/040246 |
Document ID | / |
Family ID | 32028932 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050201438 |
Kind Code |
A1 |
Traut, Silke ; et
al. |
September 15, 2005 |
Method for making a high power semiconductor laser diode
Abstract
Semiconductor laser diodes, particularly high power ridge
waveguide laser diodes, are often used in opto-electronics as
so-called pump laser diodes for fiber amplifiers in optical
communication lines. To provide the desired high power output and
stability of such a laser diode and avoid degradation during use,
the present invention concerns an improved design of such a device,
the improvement concerns a method of suppressing the undesired
first and higher order modes of the laser which consume energy and
do not contribute to the optical output of the laser, thus reducing
it's efficiency. This novel effect is provided by a structure
comprising CIG--for Complex Index Guiding--elements on top of the
laser diode, said CIG being established by fabricating CIG elements
consisting of one or a plurality of layers and containing at least
one layer which provides the optical absorption of undesired modes
of the lasing wavelength. This CIG preferably contains an
insulating layer as a first contact layer to the semiconductor. The
CIG elements are manufactured by a selected sequence of processing
steps, in particular several masking steps, and are specifically
shaped, both in thickness and coverage of the lasers semiconductor
body, to provide desired suppression characteristics. Further, the
CIG elements may be combined with the contact layer usually
providing the electrical input power to the laser diode.
Inventors: |
Traut, Silke;
(Niederlenz/Zurich, CH) ; Schmidt, Berthold;
(Erlenbach/Zurich, CH) ; Sverdlov, Boris;
(Adliswil/Zurich, CH) ; Thies, Achim; (Zurich,
CH) |
Correspondence
Address: |
MARK D. SARALINO (GENERAL)
RENNER, OTTO, BOISELLE & SKLAR, LLP
1621 EUCLID AVENUE, NINETEENTH FLOOR
CLEVELAND
OH
44115-2191
US
|
Family ID: |
32028932 |
Appl. No.: |
11/040246 |
Filed: |
January 21, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11040246 |
Jan 21, 2005 |
|
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10245199 |
Sep 17, 2002 |
|
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6862300 |
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Current U.S.
Class: |
372/43.01 ;
438/29; 438/31 |
Current CPC
Class: |
H01S 2301/176 20130101;
H01S 5/2022 20130101; H01S 5/2219 20130101; H01S 5/22 20130101;
H01S 5/0655 20130101; H01S 5/04254 20190801; H01S 5/20 20130101;
H01S 2301/166 20130101 |
Class at
Publication: |
372/043.01 ;
438/029; 438/031 |
International
Class: |
H01S 005/00; H01L
021/00 |
Claims
1. A method for making a high power laser diode with a
semiconductor body and a ridge waveguide as active region,
comprising the following steps (a) providing said semiconductor
body with said ridge waveguide by a first mask, in particular a
photoresist mask, (b) depositing an insulator layer over at least
part of said semiconductor body including said first mask, (c)
depositing a photoresist on said insulator layer, (d) removing part
of said photoresist in a controlled way to provide a second mask,
(e) removing or thinning at least part of said insulator layer
where it is uncovered by said second mask, (f) removing both said
first and said second masks, and (g) depositing an absorption
layer, in particular as part of a complex index guiding (CIG)
element.
2. The method according to claim 1, wherein the second mask has a
predetermined size wider than the ridge--waveguide.
3. The method according to claim 1, wherein the absorption layer
serves as part of a complex index guiding (CIG) element and as
contact layer.
4. The method according to claim 1, wherein the insulator is
thinned to a predetermined thickness where it is uncovered by the
second mask to provide insulator areas of a first thickness under
said second mask and of a second thickness outside said second
mask.
5. The method according to claim 1, wherein a third mask is applied
as one of a plurality of process steps in generating multiple CIG
elements to both sides of the optical axis of the waveguide.
6. The method according to claim 5, wherein the third mask provides
for two or more longitudinally contiguous CIG sections, each said
section having a different lateral extension, in particular at
least one of said sections extending laterally to the border of the
semiconductor body.
7. The method according to claim 1, including (f1) removing
together with the first and the second masks at least part of a
first insulator layer, (f2) depositing a second, preferably thin,
insulator layer over at least part of the semiconductor body
including the ridge waveguide, and (f3) before depositing the
absorption layer, removing said second insulator layer at least
partly in a contact region of said ridge waveguide, (h) depositing
a contact layer, in particular a P-contact layer.
8. The method according to claim 7, including the steps of (f2')
after deposition of the second insulator layer, depositing an
absorption layer on said second insulator layer over at least part
of the semiconductor body including the ridge waveguide, and (f3')
at least partly removing said absorption layer and said second
insulator layer in a contact region of said ridge.
9. The method according to claim 7, including the steps of (f2")
after deposition of the second insulator layer, depositing a stack
of absorbing layers and insulator layers over at least part of the
semiconductor body including the ridge waveguide, and (f3")
removing said stack and said second insulator layer in a contact
region of said ridge waveguide, essentially leaving said stack as
CIG elements at both sides of said ridge waveguide.
10. The method according to claim 7, wherein the contact layer
serves as part of the CIG element.
11. The method according to claim 1, wherein the insulator layer,
especially Si.sub.3N.sub.4, is deposited over essentially the whole
surface of the semiconductor body, the photoresist is deposited
over at least the center part of said semiconductor body, said
photoresist is removed, especially etched, to a desired distance
from said ridge waveguide, thus providing the second mask, said
insulator layer is thinned or removed, especially etched, in
particular etched down to the semiconductor surface so that only
insulator areas covered by said second mask remains, said masks are
removed by lifting off, at least one conductive layer is deposited
as absorption layer of a complex index guiding (CIG) element, said
conductive layer including at least one of Ti, Cr, Pt, Au, Si, or
Ge.
12. A method for making a laser diode with a semiconductor body
having an active region, a lower cladding layer, an upper cladding
layer with a ridge waveguide, and a top metallization for current
injection, said laser diode further including an optically
absorbing element for suppressing first and higher order modes of
said laser diode, said absorbing element being part of one or more
complex index guiding (CIG) elements, the method comprising:
fabricating an insulation layer and an absorption layer, the
insulation layer being provided on at least part of said upper
cladding layer, separating at least part of said absorption layer
from said laser semiconductor body, whereby said insulation layer
is fabricated with a predetermined thickness having a maximum close
to said ridge waveguide and a minimum, including zero, distant from
said ridge waveguide.
13. The method according to claim 12, wherein at least one CIG
element is fabricated to comprise or consist of two or more
sections located along the optical axis of the waveguide, each said
section having a predetermined extension along the optical axis of
said waveguide
14. The method according to claim 12, wherein two sections each of
substantially constant thickness, a first, greater thickness close
to the ridge waveguide and a second, smaller thickness distant from
said ridge waveguide are fabricated, said two sections forming the
insulation layer separating the absorption layer from the
semiconductor body.
15. The method according to claim 12, wherein the CIG element is
fabricated as a plurality, or stack of, insulating and absorption
layers.
16. The method according to claim 12, wherein at least two CIG
elements are fabricated as layered structures, preferably located
on both sides of the ridge waveguide and extending along part of or
the full length of the semiconductor body.
17. The method according to claim 12, wherein the CIG element is
shaped for maximizing the ratio of the suppression of first and
higher order modes to the suppression of the fundamental mode.
18. The method according to claim 12, wherein the semiconductor
body is made of a first material, including GaAs or InP based
materials, and the complex index guiding element is made of a
second material or a stack of second materials, in particular
either a conductor or a semiconductor, including at least one of
Ti, Cr, Pt, Au, Si, Ge, or an insulator, in particular at least one
of TiO.sub.2, Si.sub.3N.sub.4, AlN, SiO.sub.2.
19. The laser diode according to claim 12, wherein the insulation
layer is fabricated to separate the absorption layer from the laser
semiconductor body in the vicinity of the ridge waveguide only,
preferably covering at least part of the sides of said ridge and/or
part of said semiconductor body.
20. The method according to claim 12, wherein the first greater
thickness of the insulator close to the ridge waveguide is chosen
to minimize absorption of the fundamental mode by the absorption
layer, preferably to zero, and the second smaller thickness distant
from the ridge waveguide is chosen to maximize absorption of the
first and higher order mode, while keeping absorption of the
fundamental mode at a minimum.
21. The method according to claim 15, wherein materials and/or
thickness for at least one CIG element are selected to maximize the
ratio of the suppression of first and higher order modes to the
suppression of the fundamental mode, in particular maximizing
suppression of first and higher order modes while minimizing the
absorption of the fundamental mode.
22. The method according to claim 12, wherein the two sections of
the absorption layer are fabricated to abut against a common
shoulder which is self-aligned with the ridge waveguide.
23. A method for making a high power diode with a semiconductor
body and a ridge waveguide laser as active region, comprising the
following steps (a) providing a first mask over said ridge
waveguide, (b) depositing a first insulator layer over at least
part of said semiconductor body, (c) depositing a photoresist over
at least the center part of said semiconductor body, (d) thinning
or removing, especially etching, said photoresist to a desired
distance from said ridge waveguide, thus providing a second mask,
exposing part of said semiconductor body, (e) thinning or removing
at least part of said insulator layer where it is uncovered by said
second mask, (f) depositing at least one absorption layer as part
of a complex index guiding (CIG) structure over at least part of
said semiconductor body, and (g) lifting off both said masks, thus
exposing said ridge waveguide and said semiconductor body at least
partly, whereby parts of said first insulator layer and of said
absorption layer remain on said semiconductor body, and (h)
depositing a further layer as contact layer, especially a P-contact
layer.
24. The method according to claim 23, wherein step (f) is replaced
by step (f): depositing, over at least part of the semiconductor
body distant from the waveguide ridge, an insulating layer and an
absorption layer or a stack of alternating insulating and
absorption layers as part of a complex index guiding (CIG)
structure.
25. A method for making a high power diode with a semiconductor
body and a ridge waveguide laser as active region, comprising the
following steps (a) providing a first mask over said ridge
waveguide, (b) depositing a photoresist over at least part of said
semiconductor body, (c) removing, especially etching, said
photoresist to a desired, variable distance from said ridge
waveguide, thus providing a second mask, exposing part of said
semiconductor body, (d) depositing an absorption layer as part of a
complex index guiding (CIG) structure over at least part of said
semiconductor body, said first mask and said second mask, (e)
lifting off both said first and said second masks, thus exposing at
least part of said ridge waveguide and of said semiconductor body,
whereby parts of said absorption layer remain on said semiconductor
body, (f) depositing an insulator layer over at least part of said
semiconductor body, (g) opening a contact area, especially on top
of said ridge waveguide, and (h) depositing a further layer as
contact layer, especially a P-contact layer.
26. The method according to claim 25, wherein the second mask has a
predetermined size wider than said ridge waveguide.
27. The method according to claim 25, wherein step (d) is replaced
by step (d'): depositing an absorption layer and an insulator layer
or a stack of absorption and insulator layers over at least part of
the semiconductor body as part of a complex index guiding (CIG)
structure.
28. The method according to claim 25, wherein a third mask is
applied as one of a plurality of process steps in generating
multiple CIG elements to both sides of the optical axis of the
waveguide.
29. A high power laser diode fabricated according to a method
defined in any of the claims 1, 4, 7, 10, 11, 22, or 24, said laser
diode comprising a semiconductor body, a ridge waveguide, an active
region, and a structure of optically absorbing elements for
suppressing selected modes of said laser diode, said structure
constituting part of one or more complex index guiding (CIG)
elements, said laser diode preferably having front and back facets
and, extending between said facets, said ridge waveguide having: a
center segment with a substantially constant first cross section,
preferably having a length of 40-70% of the diode length, two
tapered segments extending and widening from the center segment
towards said facets in opposite direction, and two end segments
between said tapered segments and said facets, each said end
segment having a substantially constant cross section larger than
said first cross section, in particular a first one of said tapered
segments having a length of about 30-60% of the diode length and a
second one of said tapered segments having a length of up to 10% of
the diode length.
30. A high power laser diode fabricated according to a method
defined in claim 25, said laser diode comprising: a semiconductor
body, an active region, a lower cladding layer, an upper cladding
layer with a ridge waveguide, a top metallization for current
injection, said laser diode further including a structure of
optically absorbing elements constituting part of one or more
complex index guiding (CIG) elements for suppressing selected,
especially first and higher, order modes of said laser diode, said
structure extending along the length of said semiconductor body
with a predetermined width across said semiconductor body,
preferably having a variable width or having sections with at least
two widths, preferably one wider width extending across a first
part of the semiconductor body and one narrower width extending
across only a fraction or a second part of said semiconductor body.
Description
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 10/245,199, filed Sep. 17, 2002. The entire
disclosure of this application is hereby incorporated herein by
reference.
DESCRIPTION
[0002] 1. Field of the invention
[0003] This invention relates to semiconductor laser diodes, in
particular to ridge waveguide (RWG) diodes, and a method for making
such diodes. RWG laser diodes are especially used as pump lasers in
fiber optic networks and similar applications since they provide
the desired narrow-bandwidth optical radiation with a stable light
output power in a given frequency band. Naturally, output power and
stability of such laser diodes are of crucial interest. The present
invention relates to an improved method for making such a laser
diode, i.e. an improved manufacturing process, the improvement in
particular concerning the structure and design of the laser diode;
it also relates to laser diodes manufactured by such an improved
process.
[0004] 2. Background of the Invention
[0005] Coupling light of a semiconductor laser diode into an
optical fiber is a central problem within the field of optical
networks, in particular when high power transmission/coupling is
desired. Due to increasing channel density in DWDM (Dense
Wavelength Division Multiplexing) long haul networks, and the power
requirements at elevated temperatures in metro networks, maximizing
the laser diode's operating light output power is a primary design
criterion. The useful operating power is mainly limited by a "kink"
in the L-I curves, i.e. the light output over current curves,
indicating a beam steering in lateral direction. The occurrence of
such a kink is influenced by the real refractive index step, the
gain profile as well as spatial hole burning and local heating in
the laser diode. Depending on the device structure, the laser diode
suffers at a certain power level from the resonance between the
fundamental mode and higher order modes in lateral direction. This
has been shown by J. Guthrie et al in "Beam instability in 980 nm
power lasers: Experiment and Analysis" in IEEE Pot. Tech. Lett.
6(12), 1994, pp. 1409-1411. Generation of higher order modes is
highly undesirable since efficient laser to fiber coupling is only
possible with the fundamental mode.
[0006] Since weakly guided semiconductor devices like ridge
waveguide (RWG) laser diodes are preferred for high power
applications, as shown by B. E. Schmidt et al in "Pump laser
diodes", Optical Filter Telecommunications IVA, Editors: Kaminov
and Li, Academic Press, 2002, ISBN 0-12-395172-0, pp. 563-586, an
improvement in RWG designs appears highly desirable.
[0007] Bowler U.S. Pat. No. 6,141,365 describes a semiconductor
laser with a kink suppression layer. Reportedly, the latter limits
the establishment of higher order lateral modes and thus increases
the kink power of the device. Bowler also discloses disposing an
optical layer along the optical axis of an RWG laser on both sides
of the laser's ridge. However, shape and size of this kink
suppression layer is essentially determined by the photoresist mask
used to form the ridge. Bowler does not address utilizing the kink
suppression layer's shape, thickness, and/or material for any
particular purpose apart from general kink suppression. Also, the
lasers described by Bowler have output powers of no more than 200
to 300 mW which is insufficient for many of today's technical
applications.
[0008] Thus, it is a general object of this invention to devise a
reliable design for a high power RWG laser diode which in
particular provides a stable light output under all operating
conditions and a sufficiently long life of such laser diodes.
Hereinbelow, the term "high power" is used for an optical output
power approximating 1 W. Laser diodes with 918 mW linear kink-free
power have been realized with a design according to the present
invention.
[0009] It is a further primary object of this invention to provide
an advantageous and economical manufacturing method for a novel
high power RWG laser diode, allowing reliable mass production of
such laser diodes.
[0010] It is a more specific object of this invention to provide a
RWG laser diode design optimally suited for realizing laser diodes
with kink-free output powers in the 1 W region, and an increase of
about 25% in median linear power (taken over about 700 devices)
compared to a standard design.
DISCLOSURE OF THE INVENTION
[0011] The principal design idea of the invention is to develop a
structure of a high power RWG laser diode which controllably
introduces additional optical losses for first and higher order
modes, whereas the fundamental (or 0th order) mode experiences only
minor influences.
[0012] It is known that high order lateral modes, e.g. the first
order mode, exhibit a broader extension of the optical field in
lateral direction than the fundamental mode. In other words, the
lateral extension of the desired fundamental mode is smaller than
that of the undesirable first order and higher order modes. These
undesired modes can be suppressed by introducing optically
absorbing regions parallel to the ridge waveguide.
[0013] Hence, depending on the location, an absorbing layer can
function as a suppression layer for the first and higher order
modes, without introducing significant absorption of the
fundamental mode.
[0014] Due to the increased loss in the first order mode, resonant
coupling occurs at much higher power levels and hence the linear
power, i.e. the kink-free power, of the laser diode is
significantly increased. Since attenuation of first and higher
order modes is stronger than the same for the fundamental mode,
this layer acts as a mode-discrimination element.
[0015] The absorption layer can be made of any material in which
the imaginary part of the complex index of refraction is not zero
for the wavelength in question, i.e. the lasing wavelength. The
element that discriminates first and higher order modes can be a
single layer or contain multiple layers, where at least one layer
must have the desired absorption properties. Number and location of
these mode-discrimination elements (or Complex Index Guiding, CIG,
elements) within the laser diode structure as well as shape and
number of layers contained within the element depend on the laser
design and have to be individually optimized.
[0016] The improvement achieved by adding CIG elements to a
standard RWG structure can be demonstrated. The linear power for a
laser diode with CIG elements as described is significantly higher
than for a similar standard laser diode. In one trial embodiment of
a laser diode according to the invention, about 900 mW kink-free
light output power was reached at an operating current of around
1.1 A. The median linear, i.e. kink-free, power taken over about
700 laser diodes increased by about 25% for laser diode structures
containing CIG elements compared to standard diodes.
[0017] In a first series of experiments, the photoresist etching
mask already used for ridge etching was employed as mask for RIE
etching the insulating layer, similar to the method described by
Bowler in U.S. Pat. No. 6,141,365, cited above. The insulating
layer at both sides of the ridge was etched down to the
semiconductor. Subsequently, the p-contact metallisation (Ti/Pt/Au)
was deposited. The Ti layer of the metallisation functioned as the
optically absorbing layer, i.e. the CIG element, in this case.
Depending on the laser design, the linear power was increased
anywhere from 10% to 20%. At the same time, the efficiency
decreased by 10% to 20%, indicating significant absorption of the
fundamental mode.
[0018] In further experiments, the design was improved by laterally
varying the distance of the CIG elements relative to ridge and
herewith the extension of the modes. The purpose of this variation
is to optimize absorption of higher order modes relative to the
fundamental mode and thus optimize linear power and minimize
efficiency losses. Furthermore, a thin insulating layer was added
to the CIG element. This layer is electrically insulating and does
not absorb light of the lasing wavelength. It is located between
the semiconductor body and the absorption layer. The overall
absorption now not only depends on the material of the absorption
layer and the location of the CIG element, but also on the
thickness of this insulating layer, i.e. the vertical distance of
the modes from the absorption layer. Additionally, the insulator
electrically separates the absorption layer, which is a conductor
in the present case, from the semiconductor and thus eliminates the
possibility of leaking currents.
[0019] These variations rendered very interesting results and thus
form an essential part of this invention. They will be described in
detail later. In three variations, the CIG elements were located at
0, 300, and 600 nm distance relative to both sides of the ridge,
i.e. measured from ridge etching mask. The thin insulating layer,
here Si.sub.3N.sub.4, was part of the CIG elements for all
experiments and had a thickness of about 25 nm. On average, the
linear power of these laser diodes increased by about 25% relative
to laser diodes without CIG elements. Relative to standard laser
diodes, the average efficiency was reduced by about 10% for lasers,
where the CIG elements were located right next to the ridge, i.e.
at 0 nm from ridge etching mask. For the two designs where the CIG
elements were taken further from the ridge, i.e. 300 nm and 600 nm
relative to the ridge etching mask, the efficiency was reduced by
only about 5%.
[0020] In one embodiment, the lateral and vertical far-field show
stable single mode outputs above 900 mW and no lateral beam
steering was observed in the whole power range.
[0021] The three experimentally evaluated locations of the CIG
elements show clearly that optimization reduces the detrimental
effect on the fundamental mode and thus increases the efficiency
and kink power even further.
[0022] The laser diodes with the improved CIG design were tested
under accelerated conditions for stability, failures and
degradation. The CIG-improved lasers showed stable performance,
indicating highly reliable operation. No distinctive features were
observed compared to standard laser diodes. The operating
conditions were 900 mA constant current at 85.degree. C. heat sink
temperature, 3000 hrs.
[0023] To summarize, the invention concerns a process for making a
novel high power ridge waveguide semiconductor laser design
containing one or more CIG elements (Complex Index Guiding
elements). These CIG elements consist of at least one layer that
absorbs light of the lasing wavelength, but may contain a plurality
of absorbing and non-absorbing layers. The novel laser exhibits
high stability with increased kink power. The CIG elements are
preferably located to both sides of the ridge along the optical
axis. Precise location and shape of the CIG element as well as
number and location of layers in the CIG element depend on the
laser design and are chosen to achieve maximum efficiency and/or
maximum kink power.
[0024] The novel manufacturing process according to the invention
allows control of the distance relative to the extension of
fundamental and first order modes and hence optimization of
increased kink power vs. optical losses. Experimental results show
an increased kink power of about 25% (median) and very good
life-time results.
[0025] As already addressed, the position of the absorbing layer
relative to the fundamental mode is rather critical. This is due to
the fact that absorption of the first order mode is desired, but
absorption of the fundamental mode is undesirable since it results
in reduced efficiency. The described novel manufacturing method
allows control of the distance of the absorbing layer relative to
the ridge by a self-aligning process. This optimizes the kink power
increase by absorption of the first order mode without
significantly loosing efficiency by absorption of the fundamental
mode. Since the location of the CIG elements can be defined
independently of the ridge and its etching mask, any epitaxial
design and any ridge design can be used.
[0026] The fabrication method according to the invention has the
further advantage that it does not put limitations on the CIG
elements in terms of position, thickness, material and deposition
method. Also, the novel method facilitates the introduction of a
thin insulating layer underneath the absorption layer to
electrically separate the semiconductor from the metal and thus
avoid leaking currents and to modify the overall absorption.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0027] In the following, various embodiments of the invention,
including some basic considerations and both the laser structure
and the manufacturing process, shall be described by reference to
the drawings, in which:
[0028] FIG. 1 shows the influence of an absorbing layer on various
modes of an RWG laser;
[0029] FIG. 2 shows the measured P-I curve of a standard device
compared to a CIG-improved device according to the invention;
[0030] FIGS. 3a-3g illustrate the preferred manufacturing process
of an RWG laser diode according to the invention;
[0031] FIG. 3h depicts the structure of a first example of an RWG
laser diode according to the invention;
[0032] FIG. 3i shows the optical energy distribution of an RWG
laser diode according to FIG. 3h;
[0033] FIGS. 4a-4c illustrate the first alternative manufacturing
process of an RWG laser diode according to the invention; and
[0034] FIG. 4d shows the design of a second example of an RWG laser
diode according to the invention;
[0035] FIGS. 5a-5c illustrate a second alternative manufacturing
process of an RWG laser diode according to the invention; and
[0036] FIG. 5d shows the design of a third example of an RWG laser
diode according to the invention.
[0037] FIGS. 6a-6c illustrate a third alternative manufacturing
process of an RWG laser diode according to the invention; and
[0038] FIG. 6d shows the design of a fourth example of an RWG laser
diode according to the invention.
[0039] FIGS. 7a-7b illustrate a fourth alternative manufacturing
process of an RWG laser diode according to the invention;
[0040] FIG. 7c shows the design of a fifth example of an RWG laser
diode according to the invention;
[0041] FIGS. 8a-8d illustrate a sixth alternative manufacturing
process of an RWG laser diode according to the invention;
[0042] FIG. 8e shows the design of a sixth example of an RWG laser
diode according to the invention; and
[0043] FIG. 9 shows the design of a seventh example of an RWG laser
diode according to the invention.
[0044] FIG. 1 is a schematic representation of the principle
influence of an absorbing layer on various modes of an RWG laser
diode. The upper part of FIG. 1 shows the distribution of the
"gain" extending over the lateral extension "x" of an RWG laser
diode without and with an absorbing layer, the latter according to
the invention (dotted lines). As explained above, it is clearly
visible that the addition of an absorbing layer reduces the gain in
lateral regions, but not in the center region of the diode.
[0045] The lower part of FIG. 1 now shows the calculated lateral
distribution of the optical energy of the fundamental mode (dotted
line) and the first order mode, again over the lateral extension
"x" and the vertical extension "y". It is obvious that the first
order mode shows a significantly different lateral distribution of
its optical energy, in particular shows it a much higher level than
the fundamental mode (dotted lines) in the laterally more distant
regions and; a minimum at the center region. Here, the invention
sets in by providing lateral absorbing layers, appropriately
positioned parallel to the waveguide, which significantly suppress
the first order mode. This in turn leads to an increase in the
linear power of the pump laser device since resonant coupling of
the first order mode now occurs at higher power levels.
[0046] FIG. 2 illustrates the improvement achieved by adding CIG
elements according to the invention to the RWG laser diode
structure. The figure shows the P-I curve, i.e. power versus
injected current in arbitrary units (a.u.), for a standard diode
compared to a CIG-improved diode. Indicated is the first occurring
kink, i.e. instability of the optical output power of a standard
diode vs. a CIG-improved diode. The first kink clearly occurs at a
much lower power level for the standard diode than for the same
diode comprising CIG elements. As stated before, stable output
powers of more than 900 mW were achieved with the improved CIG
design with good life test results. Heretofore, it was difficult or
impossible to reach an output power of more than 900 mW with both
stability and long life of the laser diodes.
[0047] Initially, a manufacturing method of RWG laser diodes
according to the invention shall be described since many details
will become clear from the preferred manufacturing process.
Different stages and variations of this method are illustrated in
FIGS. 3a to 9. A person skilled in the art may of course vary this
process, e.g. by modifying and/or deleting certain steps and/or by
adding further steps, without departing from the invention.
[0048] Please note that the figures showing the RWG laser diode are
not to scale, in particular are the thicknesses of the various
layers greatly exaggerated to make them visible. Please note also
that the manufacturing process is only explained with regard to the
present invention and is insofar incomplete as those steps and
measures known to the person skilled in the art are not mentioned
or described.
[0049] FIG. 3a starts with the ridge formation by a wet etching
process. The part of a semiconductor body 2 which is supposed to
form the ridge of the final RWG laser diode is covered by a
photoresist mask 1, the ridge etching mask. GaAs or AlGaAs are the
preferred materials for the body 2. However, the process is not
limited to these materials, but can be applied also to InP or any
other optical semiconductor material. The etching step results in a
semiconductor body 2 having the shape shown in FIG. 3a, i.e. the
ridge is formed. Here, the shape results from a wet etching
process, but the CIG element formation process will work as well on
other ridge shapes having, e.g. straight side walls or sidewalls of
other shapes. Important for the later described CIG layer
self-aligned masking process is only the presence of some kind of a
mesa structure.
[0050] In the next step, shown in FIG. 3b, a thin insulator layer
3, preferably Si.sub.3N.sub.4, is deposited across the entire
structure. The deposition can be achieved by a PECVD process, i.e.
by Physically Enhanced Chemical Vapor Deposition. The thickness of
this insulator layer 3 is in the region of 200 to 300 nm,
preferably about 220 nm. The insulator layer 3 can also be made of
alternative materials, such as SiO.sub.2, AlN, or TiO.sub.2, and be
deposited by alternative deposition methods, such as PVD, i.e.
Physical Vapor Deposition, or CVD, i.e. Chemical Vapour
Deposition.
[0051] Whereas the steps themselves above are more or less state of
the art, they form the basis for subsequent steps focusing on the
invention.
[0052] The steps illustrated in FIGS. 3c to 3g produce the mask
that defines the location of the absorbing CIG elements. It is
effectively the thick Si.sub.3N.sub.4 insulator layer 3 that acts
as a mask for the desired absorbing layer. This insulator layer
optically separates the light generated in the waveguide from the
absorbing layer. In the regions with thick Si.sub.3N.sub.4, any
absorbing layer deposited on top will not (or only marginally)
contribute to the absorption.
[0053] In the step shown in FIG. 3c, a photoresist layer 4 is
deposited over the whole semiconductor body 2, including insulator
layer 3 and mask 1. Preferably, the photoresist is spun over the
semiconductor body 2, resulting in a thicker photoresist layer near
the ridge and a thinner photoresist layer in the body region. The
thickness of the resulting photoresist layer 4 is preferably about
2.5 .mu.m in the region of the ridge and about 1 .mu.m in the body
region. The thickness gradient of the photoresist layer 4 is
important for the variability of the absorbing layer (or CIG
element) location and shape, as will be shown later. The
photoresist deposition of FIG. 3c prepares the device for the
subsequent Si.sub.3N.sub.4 masking.
[0054] To provide the masking necessary for the fabrication the CIG
element(s), the photoresist is etched to a desired shape, here
specifically a variable width or distance, measured from the ridge
center. A preferred method for this shaping step is RIE, i.e.
Reactive Ion Etching. This results in the shaping masks 5
illustrated in FIG. 3d. The control of the width of this shaping
mask may be facilitated by a rather directional etching process
and/or the choice of an appropriate etch time. A person skilled in
the art will know how to modify the etching process in order to
achieve the desired result.
[0055] More precisely, FIG. 3d shows three different masks: a
narrow one, essentially of the same width as the ridge etching
photoresist mask 1; a middle one, somewhat wider than said ridge
etching mask 1; and a wide one, identified by the outermost line
shown in FIG. 3d. All three widths are shown to clearly demonstrate
the variability of the mask.
[0056] In a subsequent step, shown in FIG. 3e, the insulator layer
3, established earlier as described above with FIG. 3b, is etched
down to the semiconductor body 2. After this etching, the insulator
layer 3 remains only at the flanks of the ridge and underneath the
photoresist mask which was established in the previous step (FIG.
3d) and forms the insulator strips 6a and 6b on both sides of the
ridge. They extend preferably along the whole length of the
semiconductor body 2, but may be shorter than the latter if
desired. The total width of the insulator strips 6a and 6b varies
with the width of the shaping mask 5. The shape of the insulator
strips also determines the effective location of the CIG element,
i.e. the location where absorption of light mainly occurs.
[0057] After the etching process described in FIG. 3e, the
photoresist shaping mask 5 used for the Si.sub.3N.sub.4 or any
similar etching as well as the ridge etching photoresist mask 1 are
removed, e.g. by lift-off. The result is illustrated in FIG.
3f.
[0058] As shown in FIG. 3g, a layer of optically absorbing
material, resulting in an uninterrupted layer covering the whole
semiconductor body 2 is deposited. This layer has two
functions:
[0059] It provides the contact layer for the usual P-contact
metallisation on top of the ridge.
[0060] It provides the absorption necessary for suppressing the
undesired first and higher order modes of the laser by forming
absorption layers (or CIG elements) 8a and 8b at both sides of the
ridge. As stated earlier, the location where absorption takes
place, i.e. where the CIG element is effective, is confined to
those areas left and right of the ridge where the semiconductor
body 2 is not covered by the thick insulator strips 6a and 6b. If
desired, the absorption layer may extend over only part of the
semiconductor body's length. A person skilled in the art will know
how to achieve this.
[0061] Consequently, this absorption layer must have two important
material properties:
[0062] It must be a material in which the imaginary part of the
complex refractive index is non-zero for the wavelength in
question, i.e. the lasing wavelength.
[0063] For the process described, it must also be suitable as a
first contact layer for the p-contact metallisation. Conductors
such as Ti and Cr are suitable in this case.
[0064] FIG. 3h shows the nearly complete RWG laser diode structure
having the additional P-contact layers 9 deposited necessary for
electrical powering of the diode.
[0065] Any other steps in the manufacturing process to complete the
RWG laser diode remain essentially standard and are well known to a
person skilled in the art. These steps thus need not be described
here.
[0066] FIG. 3i finally shows, somewhat similar to FIG. 1, the
optical power distribution of the RWG laser diode shown in FIG. 3h
approximately in relative dimensions to the structure in FIG. 3h.
It is clearly visible that the fundamental mode has its usual peak
in the center of the laser diode, whereas the first order mode--as
any higher order modes--extend further into the areas where the CIG
elements are located. Thus, the first and higher modes are strongly
attenuated, which is what the invention intends to achieve.
[0067] Depending on the laser design (e.g. ridge shape, epitaxial
design) the lateral extension of the modes within the laser diode
varies. Accordingly, changes must be made with regard to the
optimal location of the CIG elements to achieve the desired maximum
absorption of first and higher order modes and minimum absorption
of the fundamental mode. It is therefore important to have a
process that allows variable placement and shape of the CIG
elements independent of, but adapted to, the laser's ridge shape
and design. The present invention provides this flexibility and
adaptability.
[0068] Some alternatives for the deposition and the arrangement of
the absorption layer(s) or complex index guiding (CIG) element(s)
will be addressed in the following.
[0069] FIGS. 4a to 4d show a first alternative starting after the
formation step of mask 5 in FIG. 3d. In this case, the thick
insulator layer 3, established earlier as explained above with FIG.
3b, is not etched down to the semiconductor body 2, but to a
predetermined thickness on the body. This etching results in
relatively thin insulating layers 7a and 7b, as shown in FIG. 4a,
extending over the whole of or part of the semiconductor body.
Their thickness may be selected in the region of 15 to 40 nm,
preferably about 25. The choice depends on the desired overall
absorption of the CIG element. Again, the insulator strips 6a and
6b and/or the thin outer insulator layers 7a and 7b extend
preferably along the whole length and width of the semiconductor
body, but may also be shorter and/or narrower than the latter if
desired. The thin outer insulator layers 7a and 7b electrically
separate the absorbing material from the semiconductor and thus
avoid any undesired leak currents and/or undesired material
interactions at the interface. Furthermore, they may be utilized to
modify the overall absorption of the CIG element.
[0070] FIG. 4b shows the structure after lift-off of the
photoresist masks 1 and 5, as previously described for FIG. 3f. The
next step is the deposition of the absorbing layer 8a and 8b as
part of the p-metallisation. The result is demonstrated in FIG. 4c
and was previously described for FIG. 3g.
[0071] FIG. 4d shows the nearly complete RWG laser diode structure
having the additional P-contact layer 9 deposited necessary for
electrical powering of the diode. The CIG elements located left and
right of the ridge now consist of two layers: the thin insulating
layers 7a and 7b and the optically absorbing layers 8a and 8b.
[0072] FIGS. 5a to 5d show a second alternative for fabricating a
CIG element with an insulation layer underneath the absorbing
layer. As described for the first process and shown in FIGS. 3e and
3f, the thick insulating layer is etched down to the semiconductor
body 2. The photoresist masks are subsequently removed by lift-off
to result in a structure containing the semiconductor body 2 with
the ridge and the two thick insulating layers 6a and 6b to both
sides of the ridge. This is shown in FIG. 5a.
[0073] In a next step a thin insulating layer, again preferably 25
nm, is deposited covering the entire semiconductor body 2, thus
forming the first layer of the CIG elements 7a and 7b as shown in
FIG. 5b. The material can now be chosen and deposited independent
of the thick insulating material. Standard materials and deposition
methods for this purpose are insulators such as Si.sub.3N.sub.4,
TiO.sub.2, SiO.sub.2, AlN deposited by PVD, CVD or MOCVD.
[0074] Since this thin insulator covers the entire surface of the
semiconductor body, it also covers the contact area on top of the
ridge. In this latter area, the thin insulator must be removed to
provide electrical contact of the semiconductor with the p-metal.
This can be done by any common method with photoresist masks and
subsequent etching, preferably RIE etching. A person skilled in the
art will know how to realize this. The result is shown in FIG.
5c.
[0075] Finally, the p-metal layer 9, which also provides and
functions as the absorption layer of the CIG elements 8a and 8b, is
deposited resulting in a structure shown in FIG. 5d.
[0076] The third alternative process is similar to the previous
one, but allows the utilization of different materials for the CIG
element independent of the thick insulating layer(s) and the
p-metal layer. FIG. 6a shows the structure with the thick
insulating layers 6a and 6b at both sides of the ridge and the thin
insulating layer 7a/b deposited across the entire semiconductor
surface. This structure is generated in the same manner as
described earlier for FIG. 5b.
[0077] In a next step, an absorption layer is deposited, also
covering the entire body and forming the necessary absorption
layers for the CIG elements. This is shown in FIG. 6b. As in the
process described previously with FIGS. 5a to 5d, the two layers
forming the CIG element 7a/b and 8a/b must be removed from the
p-contact area. This is again done by any common masking and
subsequent etching step and results in the structure shown in FIG.
6c.
[0078] FIG. 6d finally shows the RWG structure after deposition of
the p-metallisation, i.e. the p-contact layer 9. The advantage of
the process described last is the ability to choose any stack of
materials for the CIG element composition independent of
p-metallisation. The only requirement for the absorption layer
remains now the absorption property at the lasing wavelength. In
the previously described processes, the choice of materials was
limited to materials providing good contact to the semiconductor,
preferably a conductor of the type Ti, Cr, Pt. For this last
process however, any material and thickness can be used as long as
the material provides absorption at the lasing wavelength.
Additionally the CIG element can be modified to any shape to cover
only part of the semiconductor body.
[0079] A fourth alternative is described in FIGS. 7a to 7c.
Starting from a structure as in FIG. 3e or 4a, an absorption layer
8 is deposited over the semiconductor body 2, including both the
ridge etching mask 1 and the photoresist shaping masks 5. This is
shown in FIG. 7a. When the photoresist masks are now lifted off,
CIG elements 8a and 8b remain, extending over the semiconductor
body except the ridge and its vicinity, i.e. the insulator strips
6a, 6b and 7a, 7b. The result, shown in FIG. 7b, are two separate
CIG elements 8a and 8b. Again, if desired, the absorption layer may
extend over only part of the semiconductor body. A person skilled
in the art will know how to achieve this.
[0080] FIG. 7c shows the structure after lifting-off the masks and
depositing the usual P-contact metallisation layer 9. The advantage
of this alternative is that material and thickness for the
P-contact metallization and the CIG element can be chosen
independently. In contrast to the previously described process, cf.
FIGS. 6a-6c, the contact areas do not have to be opened in a
separate step. This is facilitated by the lift-off step also used
for structuring the CIG elements.
[0081] FIGS. 8a to 8c show a fifth alternative process for
fabricating a high power laser with CIG elements. Here, as shown in
FIG. 8a, the mask providing the variable distance from the ridge is
fabricated directly after ridge etching. Subsequently, the
absorption layer 8 is deposited as part of the CIG element, as
shown in FIG. 8b. Following this stage, further layers may be
deposited, being absorbing and/or non-absorbing layers. Both masks
1 and 5 are lifted off resulting in the CIG elements 8a and 8b.
Then the insulating layer 7 is deposited, shown in FIG. 8c. Finally
the P-contact area is opened on top of the ridge by any
state-of-the-art method, such as photoresist masking and RIE
etching and the P-metallization deposited, as shown in FIGS. 8d and
8e. FIG. 8e shows the almost finished device, including the
P-contact 9. A person skilled in the art knows how to execute these
steps, so they need not be described here in detail. Similar to the
methods describes above, a stack of alternating absorption and
insulation layers may be deposited, resulting in a layered
structure of the CIG element(s).
[0082] FIG. 9 shows a laser device having multiple CIG elements 10a
to 10f. The limitation of the CIG area perpendicular to the optical
axis of the laser is facilitated by any of the above processes, as
described in connection with the FIGS. 3 to 8. The limitation of
the CIG area along the optical axis of the device can be achieved
by any state of the art masking and etching or masking and
deposition process. Such limitation of the CIG area provides a
further degree of control in governing the level of absorption
experienced by different optical modes. The limitation of the CIG
areas is a technique that is applicable to all of the different
designs illustrated in the present application. Again, a person
skilled in the art will easily see how to fabricate such a
device.
[0083] Any of the above described embodiments my be applied to a
laser diode of the so-called "straight-flared-straight" structure
as disclosed in Pawlik et al. U.S. Pat. No. 6,798,815, assigned to
the assignee of the present invention and incorporated herein by
reference.
[0084] Further modifications will readily occur to a person skilled
in the art and the invention is therefore not limited to the
specific embodiments, details, and steps shown and described
hereinbefore. Modifications may be made without departing from the
spirit and scope of the general inventive concepts as defined in
the appended claims.
* * * * *