U.S. patent application number 16/423846 was filed with the patent office on 2020-12-03 for monolithically integrated gain element.
The applicant listed for this patent is Ciena Corporation. Invention is credited to Kelvin Prosyk, Ian Woods.
Application Number | 20200379174 16/423846 |
Document ID | / |
Family ID | 1000005218638 |
Filed Date | 2020-12-03 |
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United States Patent
Application |
20200379174 |
Kind Code |
A1 |
Prosyk; Kelvin ; et
al. |
December 3, 2020 |
Monolithically Integrated Gain Element
Abstract
A photonic integrated circuit including a photonic device and a
gain element, said gain element formed by a process including:
depositing by epitaxy a first doped layer onto a substrate;
depositing by epitaxy an active layer capable of optical gain onto
the first doped layer; depositing by epitaxy a second doped layer
onto the active layer; pattern etching at least the second doped
layer and the active layer to form a first ridge; and depositing by
epitaxy a current blocking layer laterally adjacent to the first
ridge at least partially filling the volume of active layer that
was removed by the pattern etching; wherein the current blocking
layer forms a portion of the photonic device.
Inventors: |
Prosyk; Kelvin; (Luskville,
CA) ; Woods; Ian; (Nepean, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ciena Corporation |
Hanover |
MD |
US |
|
|
Family ID: |
1000005218638 |
Appl. No.: |
16/423846 |
Filed: |
May 28, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/1228 20130101;
G02B 6/136 20130101 |
International
Class: |
G02B 6/122 20060101
G02B006/122; G02B 6/136 20060101 G02B006/136 |
Claims
1. A photonic integrated circuit comprising a photonic device and a
gain element, said gain element formed by a process comprising:
depositing by epitaxy a first doped layer onto a substrate;
depositing by epitaxy an active layer capable of optical gain onto
the first doped layer; depositing by epitaxy a second doped layer
onto the active layer; pattern etching at least the second doped
layer and the active layer to form a first ridge; depositing by
epitaxy a current blocking layer laterally adjacent to the first
ridge and at least partially filling the volume of active layer
that was removed by the pattern etching; providing a first
waveguide having a first overall length and a first overall width,
formed by a combination of the first ridge and an adjacent current
blocking layer* that is constant or tapering, and providing a
second waveguide having a second overall length and a second
overall width that is constant or tapering contiguous with the
first waveguide, formed by selectively etching the current blocking
layer; wherein the first overall width is lamer than the second
overall width, and wherein the first waveguide is weakly guided and
the second waveguide is strongly guided, and wherein the current
blocking layer forms a portion of the photonic device.
2. (canceled)
3. (canceled)
4. The photonic integrated circuit of claim 1, wherein the second
overall width tapers along the second length, providing a
transition from weakly guided to strongly guided in the second
waveguide.
5. The photonic integrated circuit of claim 1, wherein the gain
element is optically coupled to the photonic device.
6. The photonic integrated circuit of claim 1, further comprising a
strongly-guided third waveguide contiguous with the second
waveguide and having a third width, wherein the third width and
second width are selected to provide optimal coupling of light
between the second and third waveguides.
7. The photonic integrated circuit of claim 1, wherein the photonic
device is an optical modulator.
8. The photonic integrated circuit of claim 1, wherein the current
blocking layer comprises an undoped semiconductor.
9. The photonic integrated circuit of claim 1, wherein the
substrate comprises indium phosphide.
10. The photonic integrated circuit of claim 1, wherein any of the
first doped layer, the second doped layer, and the current blocking
layer is/are made substantially conducting or non-conducting via
implanting ions.
11. The photonic integrated circuit of claim 1, wherein any of the
first doped layer, the second doped layer, and the current blocking
layer is/are made substantially conducting or non-conducting via
diffusing dopants.
12. The photonic integrated circuit of claim 1, wherein the current
blocking layer comprises multiple sub-layers, each with different
dopants.
13. The photonic integrated circuit of claim 1, wherein the current
blocking layer functions as an overclad layer in the photonic
device.
14. The photonic integrated circuit of claim 1, wherein the current
blocking layer comprises iron-doped indium phosphide.
15. (canceled)
16. (canceled)
17. (canceled)
18. (canceled)
19. A photonic integrated circuit comprising: a first doped layer
on a substrate; an active layer capable of optical gain on the
first doped layer; a second doped layer on the active layer; a
first ridge patterned on at least the second doped layer and the
active layer; a current blocking layer laterally adjacent to the
first ridge and at least partially filling the volume of active
layer of the first ridge, a first waveguide having a first overall
length and a first overall width, that includes a combination of
the first ridge and an adjacent current blocking laser, that is
constant or tapering; and a second waveguide having, a second
overall length and a second overall width that is constant or
tapering contiguous with the first waveguide, formed in the current
blocking layer. wherein the first overall width is larger than the
second overall width, and wherein the first waveguide is weakly
landed and the second waveguide is strongly guided.
20. (canceled)
21. The photonic integrated circuit of claim 19, wherein the second
overall width tapers along the second length, providing a
transition from weakly guided to strongly guided in the second
waveguide.
22. The photonic integrated circuit of claim 19, wherein the gain
element is optically coupled to the photonic device.
23. The photonic integrated circuit of claim 22, further comprising
a strongly-guided third waveguide contiguous with the second
waveguide and having a third width, wherein the third width and
second width are selected to provide optimal coupling of light
between the second and third waveguides.
24. The photonic integrated circuit of claim 19, wherein the
photonic device is an optical modulator.
25. The photonic integrated circuit of claim 19, wherein the
current blocking layer comprises an undoped semiconductor.
26. The photonic integrated circuit of claim 19, wherein the
substrate comprises indium phosphide.
Description
TECHNICAL FIELD
[0001] The present disclosure relates generally to photonic
components for use in optical devices and optical networks. More
specifically, the present disclosure relates to a modulator with a
monolithically integrated optical amplifier.
BACKGROUND
[0002] The various possible configurations of planar optical
waveguides used in photonic integrated circuits (PICs) are often
broadly sorted into two categories by their strength of lateral
guiding: strongly guided and weakly guided. The strength of lateral
guiding determines how well the optical mode is confined to the
central portion of the waveguide, and determines the minimum radius
of curvature that a curved optical waveguide can achieve before the
mode becomes unbound and radiates out of the waveguide. As
illustrated in FIG. 1A, a waveguide 3 may consist of a core guiding
region 5 with an optical index of refraction of Ng and a laterally
adjacent cladding region 7 with optical index of refraction of Nc.
Strong guiding refers to a large difference between Ng and Nc. For
example, in some indium phosphide (InP)-based PICs with
strongly-guided waveguides, the guiding core 5 may have an optical
index of refraction of Ng=3.54, while the cladding 7 consists of a
dielectric material, such as silicon dioxide (SiO.sub.2), with an
index of Nc=1.5. Conversely, weak guiding refers to designs where
the core index Ng is only slightly higher than the cladding index
Nc. For example, if an InP-based PIC lateral cladding 7 is
fabricated from semiconductor instead of silicon dioxide, the guide
section 5 would still have Ng=3.54, but the lateral cladding 7 may
have Nc=3.46, forming a weak guide. An alternative weak guiding
structure is shown in FIG. 1B. Here, there is no actual index
change in the material 7 adjacent to the waveguide core 5, but a
centrally located guiding ridge 9 above the core 5 creates an
effective index difference in the areas laterally adjacent the
guiding core 5. Such weakly guided waveguides 3 are also known as
shallow ridge waveguides, and may have an index contrast Ng-Nc of
<0.05.
[0003] Low-cost, small-size, and high-performance InP-based
Mach-Zehnder modulators (MZMs) are widely known in the photonic
components industry. In general, it is desirable that such
modulators use strongly-guided waveguides, like those in FIG. 1A,
with a dielectric cladding 7. Not only do strongly-guided
waveguides enable small size through the compact routing of
waveguides, they confine the mode to the central portion 5 of the
waveguide 3 that provides the optical modulation function, thereby
enabling high efficiency.
[0004] Modulators of the sort described herein typically mix a
continuous wave (CW) optical carrier input, having no data content,
with a broadband electrical signal that carries data. The frequency
of the CW carrier may be, for example, approximately 193 THz and is
typically desired to be as narrowband as possible, for example 100
kHz line width. The bandwidth of the electrical data signal may
span, for example, from 500 MHz to 10 GHz, or from 500 MHz to 70
GHz in modern high-capacity telecommunications systems. Depending
on the modulation format being used, the bandwidth can provide a
data rate of 10 Gbit/s to 400 Gbit/s or more. The data is
transmitted as an optical carrier wave at the frequency of the
original CW optical carrier input, with a modulation envelope
determined by the electrical data signal. The modulators,
therefore, perform an up-conversion function from original radio
frequency (RF) data baseband to optical frequencies, in order to
enable transmission through optical fiber. The modulators often
further combine several RF data tributaries that are in an
amplitude-modulated format into a more complex combined
phase-and-amplitude-modulated format that may give advantages, for
example, in the signal to noise ratio (SNR) of the data at the
receiving end of the fiber.
[0005] Critical to the function of such modulators is the CW
optical carrier input. In some applications, the CW optical carrier
is provided by an external laser that is coupled through a short
length of optical fiber to the modulator input if they are packaged
separately, or through a micro-lens or optical guiding system if
they are co-packaged. In these applications, the light lost during
the coupling and modulation processes is an important performance
parameter. If too much of the CW optical carrier light is lost, the
up-converted data signal emitted from the modulator will have low
power, and is subject to a poor SNR at the receiver. One obvious
solution is to use a high-power laser to compensate the losses.
However, there are technology limitations to how high the laser
power can be, and engineering consequences in terms of power
dissipation, performance non-idealities, and cost. Monolithically
integrating a semiconductor optical amplifier, or SOA, with the
modulator can overcome these problems.
[0006] In other applications, it is preferable to monolithically
integrate the laser itself with the modulator, thereby avoiding
optical coupling losses, packaging complexity, and cost associated
with an external solution. Of course, laser integration could also
be combined with an integrated SOA to further boost the optical
output power. Lasers and SOAs can be referred to generally as
active or gain elements. Whether it is a laser or SOA, the state of
the art in lateral optical guiding means is essentially similar.
Often with such integration comes the need to integrate photonic
components other than a modulator, laser, or SOA: detectors,
optical monitors, phase tuning elements, variable optical
attenuators, and so on. The extension of this invention to the
integration of a gain element (laser or SOA) with photonic
components other than a modulator is thus fundamentally
important.
[0007] Known shallow ridge, or stripe, lasers and SOAs have a
configuration similar to FIG. 1B. In addition to being weakly
guided, they lack lateral current confinement capability, such that
current spreads non-uniformly and inefficiently over a large area,
thereby reducing gain.
[0008] Known plain deep ridge lasers and SOAs, similar to FIG. 1A,
with a lateral cladding 7 of dielectric, as in most modulators, are
almost never used, as the etched sidewall of the guiding core 5
leaves dangling chemical bonds that act as mid-level traps. These
mid-level traps add a significant non-radiative component to the
associated carrier recombination, making the current-gain curve
highly unfavorable. Properly preparing the sidewall with chemical
treatment and overgrowing it with semiconductor can remove these
dangling chemical bonds.
[0009] Known buried heterostructure (BH) laser and SOA structures
come in numerous variants and represent the current industry
standard, utilizing a more sophisticated arrangement of
current-blocking layers to achieve better current confinement to
the multi-quantum well (MQW) core. One shortcoming is the complex
regrowth recipe that is difficult to monolithically integrate with
a modulator in a manufacturable manner. Further, BH lasers and SOAs
with Al-containing cores are notorious for their questionable
reliability. As illustrated in FIG. 2A, the BH structure 15a is
manufactured by blanketing the N-InP substrate 10 with a first
growth of MQW material 12. A selective etch is then performed to
form a MQW ridge. Subsequently, a multi-layer stack 16 is
selectively grown around the MQW ridge on the N-InP substrate 10,
with sensitive critical dimensions. Finally, an overgrowth of
P-type semiconductor 18 is blanketed over the MQW ridge and
multi-layer stack 16. Although the current is much better confined,
the waveguide is still weakly guided.
[0010] Recently, an attempt has been made to simplify the complex
arrangement of current-blocking layers of the BH laser by using a
self-aligned single-growth technique. The structure exhibits
beneficial current confinement. As illustrated in FIG. 2B, the
simplified BH structure 15b is manufactured by blanketing the N-InP
substrate 10 with a first growth of MQW 12 and P-type semiconductor
18 material. A selective etch is performed to form an MQW ridge.
Subsequently, a single, blanket layer of undoped InP 17 is grown
over the structure 15b. The undoped InP 17 is removed from the
ridge top using a self-aligned etch technique. Like the more
complex BH structure 15a (FIG. 2A), this simplified BH structure
15b (FIG. 2B) has good current confinement but is still weakly
guided.
[0011] Neither the shallow ridge structure (FIG. 1B) nor the BH
structure (FIG. 2B) can be directly optically coupled to a
modulator because the waveguides are incompatible. Some
interconnection means is necessary to bridge the strongly guided
and weakly guided waveguides, such as that provided in U.S. Pat.
No. 7,184,207, for example.
[0012] Thus, what are still needed in the art are structures and
processes that enable reliable, manufacturable modulators
monolithically integrated with lasers and optical amplifiers having
well confined current injection.
SUMMARY
[0013] In various exemplary embodiments, the present disclosure
provides a design whereby an optical amplifier is efficiently
monolithically integrated with a deeply-etched ridge waveguide
modulator, and, in particular, a multi-growth modulator formed on
an InP substrate, such as that provided in U.S. Pat. No. 9,182,546,
for example. The design enables the re-use of existing undoped
overgrowth in the TWE modulator for the purpose of current
blocking. Subsequent deep etching of the current-blocked buried
ridge provides for independent control of the confinement factor
and enables efficient coupling to a deeply-etched modulator.
[0014] Thus, the present disclosure provides a means to re-use an
overgrowth that already exists in the standard modulator process
sequence, thereby reducing cost, complexity, and problems
associated with many epitaxial growths, such as reliability issues.
The present disclosure provides better current confinement, and
therefore better electrical efficiency, than alternative shallow
ridge solutions. The present disclosure decouples current
confinement (provided by u-InP blocks described in greater detail
herein below) from optical confinement (provided by etched areas
described in greater detail herein below). Accordingly, the present
disclosure provides an efficient alternative means to couple light
from the modulator to the gain section without introducing an
additional or new optical element into the design, such as that
provided in U.S. Pat. No. 7,184,207, for example.
[0015] The present disclosure provides a modulator with an optical
amplifier, including: an N-type layer; a multi-quantum well
material disposed on the N-type layer; a P-type layer disposed on
the multi-quantum well material opposite the N-type layer; wherein
a portion of the N-type layer, the multi-quantum well material, and
a portion of the P-type layer collectively form a ridge structure;
and a material that is not intentionally doped (undoped, or u-type)
disposed on the N-type layer and about side portions of the ridge
structure using selective area epitaxy. Optionally, the u-type
material is further deeply etched to form a strongly guided
structure. The N-type layer includes N-InP. The P-type layer
includes one of P-InGaAs and P-InP. The u-type material includes
u-InP, but may alternatively be any type of suitable
current-blocking material that impedes current flow, such as
semi-insulating iron-doped InP. Optionally, over all or some
portion of the length, a width of the strongly guided structure is
selected to couple efficiently to a strongly guided modulator
waveguide.
[0016] In one exemplary embodiment, the present disclosure provides
a photonic integrated circuit including a photonic device and a
gain element, said gain element formed by a process including:
depositing by epitaxy a first doped layer onto a substrate;
depositing by epitaxy an active layer capable of optical gain onto
the first doped layer; depositing by epitaxy a second doped layer
onto the active layer; pattern etching at least the second doped
layer and the active layer to form a first ridge; and depositing by
epitaxy a current blocking layer laterally adjacent to the first
ridge at least partially filling the volume of active layer that
was removed by the pattern etching; wherein the current blocking
layer forms a portion of the photonic device. Optionally, the
photonic integrated circuit further includes a first waveguide
having a first length and a first width, formed by a combination of
the first ridge and the adjacent second doped layer; and a second
waveguide having a second length and a second width contiguous with
the first waveguide, formed by selectively etching the current
blocking layer. Optionally, the first width is larger than the
second width, and wherein the first waveguide is weakly guided and
the second waveguide is strongly guided. Optionally, the second
width tapers along the second length, providing a transition from
weakly guided to strongly guided in the second waveguide. The gain
element is optically coupled to the photonic device. Optionally,
the photonic integrated circuit further includes a strongly-guided
third waveguide contiguous with the second waveguide and having a
third width, wherein the third width and second width are selected
to provide optimal coupling of light between the second and third
waveguides. Optionally, the photonic device is an optical
modulator. Optionally, the current blocking layer includes an
undoped semiconductor. Optionally, the substrate includes InP.
Optionally, any of the first doped layer, the second doped layer,
and the current blocking layer is/are made substantially conducting
or non-conducting using ion implantation means. Optionally, any of
the first doped layer, the second doped layer, and the current
blocking layer is/are made substantially conducting or
non-conducting using dopant diffusion means. Optionally, the
current blocking layer includes multiple sub-layers, each with
different dopants. Optionally, the current blocking layer functions
as an overclad layer in the photonic device. Optionally, an area of
deposition of the current blocking layer is shaped in a zig-zag
pattern along one or more edges thereof. Optionally, the current
blocking layer includes iron-doped InP.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The present disclosure is illustrated and described herein
with reference to the various drawings, in which like reference
numbers are used to denote like assembly components/method steps,
as appropriate, and in which:
[0018] FIGS. 1A and 1B are a series of schematic diagrams
illustrating conventional strongly-guided and weakly-guided
(shallow ridge) waveguides, respectively;
[0019] FIGS. 2A and 2B are a series of schematic diagrams
illustrating the fabrication of conventional BH lasers or SOA
structures;
[0020] FIG. 3 is a series of schematic diagrams illustrating the
fabrication of a conventional gain element;
[0021] FIG. 4 is a series of schematic diagrams illustrating the
fabrication of one exemplary embodiment of the gain element of the
present disclosure;
[0022] FIG. 5 is a series of schematic diagrams illustrating the
fabrication of another exemplary embodiment of the gain element of
the present disclosure, as well as the coupling of the associated
SOA and modulator;
[0023] FIG. 6 is a schematic diagram illustrating the flaring of a
waveguide SOA or modulator width to match an associated modulator
SOA optical mode in accordance with the methods of the present
disclosure;
[0024] FIG. 7 is another schematic diagram illustrating the flaring
of a waveguide modulator width to match an SOA optical mode in
accordance with the methods of the present disclosure; and
[0025] FIG. 8 is a schematic diagram illustrating a novel geometry
for the edge of a mask used to overgrow an epitaxial layer in
accordance with the methods of the present disclosure, suppressing
undesirable growth enhancement.
DESCRIPTION OF EMBODIMENTS
[0026] Again, in various exemplary embodiments, the present
disclosure provides a design whereby an optical amplifier is
efficiently monolithically integrated with a deeply-etched ridge
waveguide modulator, and, in particular, a multi-growth modulator
formed on an InP substrate, such as that provided in U.S. Pat. No.
9,182,546, for example. The design enables the re-use of existing
undoped overgrowth in the TWE modulator for the purpose of current
blocking. Subsequent deep etching of the current-blocked buried
ridge provides for independent control of the confinement factor
and enables efficient coupling to a deeply-etched modulator.
[0027] Thus, the present disclosure provides a means to re-use an
overgrowth that already exists in the standard modulator process
sequence, thereby reducing cost, complexity, and problems
associated with many epitaxial growths, such as reliability issues.
The present disclosure provides better current confinement, and
therefore better electrical efficiency, than alternative shallow
ridge solutions. The present disclosure decouples current
confinement (provided by u-InP blocks described in greater detail
herein below) from optical confinement (provided by etched areas
described in greater detail herein below). Accordingly, the present
disclosure provides an efficient alternative means to couple light
from the modulator to the gain section without introducing an
additional or new optical element into the design, such as that
provided in U.S. Pat. No. 7,184,207, for example.
[0028] In general, the present disclosure provides a modulator with
an optical amplifier, including: an N-type layer; a multi-quantum
well material disposed on the N-type layer; a P-type layer disposed
on the multi-quantum well material opposite the N-type layer;
wherein a portion of the N-type layer, the multi-quantum well
material, and a portion of the P-type layer collectively form a
ridge structure; and a material that is not intentionally doped
(undoped, or u-type) disposed on the N-type layer and about side
portions of the ridge structure using selective area epitaxy.
Optionally, the u-type material is further deeply etched to form a
strongly guided structure. The N-type layer includes N-InP. The
P-type layer includes one of P-InGaAs and P-InP. The u-type
material includes u-InP, but may alternatively be any type of
suitable current-blocking material that impedes current flow, such
as semi-insulating iron-doped InP. Optionally, over all or some
portion of the length, a width of the strongly guided structure is
selected to couple efficiently to a strongly guided modulator
waveguide.
[0029] FIG. 3 illustrates the fabrication of a conventional
modulator structure 25, such as by the method provided in U.S. Pat.
No. 9,182,546, for example. The modulator structure 25 is
fabricated by blanketing the N-type substrate 10 with a first
growth of MQW material 12 and a P-type layer 18. In a u-type ridge
embodiment, a selective etch of the P-type layer 18 is then
performed, and a u-InP layer 20 is selectively grown in the etched
area. The P-type layer 18 and MQW material 12 are then selectively
etched, leaving a u-InP-capped ridge in the u-type ridge SOA
embodiment and a P-type-capped ridge in a P-type ridge modulator
embodiment. Thus, a selective u-type growth step is typically
utilized. It is still desirable to monolithically integrate a SOA
with a modulator and provide performance that approximates that of
a BH structure 15 (FIGS. 2A and 2B), without the introduction of a
critically-aligned growth step. It is also still desirable to, for
the monolithically integrated SOA, create a BH-like structure that
has a lateral optical mode that is compatible for optical coupling
to a deeply etched modulator ridge.
[0030] Referring now specifically to FIG. 4, in one exemplary
embodiment, the gain element structure 35 of the present disclosure
is fabricated by blanketing an N-type layer 10 with a first growth
of optical gain material 12 and a P-type layer 18. The P-type layer
18, optical gain material 12, and N-type layer 10 are then
selectively etched, leaving a P-capped ridge that is covered with a
mask 19. A u-type growth step is then utilized to fill the lateral
areas of the ridge with u-InP material 20. Here, the lateral u-InP
material 20 provides superior current blocking, without the extra
growth steps and critical alignments associated with BH
fabrication.
[0031] FIG. 5 is a series of schematic diagrams illustrating the
fabrication of another exemplary embodiment of the gain element
structure 35 of the present disclosure, as well as the means for
coupling of the associated SOA 37 and modulator 39. The u-InP
material 20 and N-type material 10 are etched to form u-InP
material walls on either side of the ridge, thereby providing
strong guiding on either side of the ridge. Further, the deeply
etched modulator waveguide can be widened to provide lateral
optical matching. It will be appreciated that, in FIG. 5, the
bottom schematic represents the modulator waveguide that has been
width-matched to the SOA input/output waveguide, shown in the
middle schematic. For the SOA 37, W1 (current confinement) and W2
(optical mode overlap) can be varied independently, allowing the
gain to be varied along the length of the SOA 37, for example to
mitigate the effects of spatial hole burning.
[0032] FIG. 6 is a schematic diagram illustrating the flaring of a
waveguide SOA or modulator width to match an associated modulator
SOA optical mode in accordance with the methods of the present
disclosure. Any arbitrary combination of central width and exterior
width, with tapers in between, is possible. For example, only the
central portion could be tapered, omitting the leading and trailing
constant sections. This provides practical advantages for spatial
hole burning by having high optical confinement at the beginning,
and increasing injection, but lowering optical overlap at the
end.
[0033] FIG. 7 is another schematic diagram illustrating the flaring
of the waveguide modulator width to match the SOA optical mode in
accordance with the methods of the present disclosure. Here, the
SOA waveguide 37 is coupled to the modulator waveguide 39 via a
tapering passive waveguide 38 or a tapering portion of the SOA
waveguide 37 or modulator waveguide 39.
[0034] In the conventional modulator structure 25 of FIG. 3, there
is already a selective growth of u-InP layer 20, the thickness of
which may be determined by the optimization of the modulator
design. This same growth is used herein to provide better current
blocking for a SOA. As shown in FIG. 4, the u-InP layer 20 from the
conventional modulator may also be deposited on either side of the
SOA ridge 19. The design requirements of the modulator 25, however,
may not provide a thickness of u-InP layer 20 sufficient to match
the height of the SOA ridge 19. To improve the manufacturability of
subsequent processing steps, enhanced growth may be used to make a
more planar top surface. It should be noted that, for regions not
requiring the increased lateral confinement of the current-blocking
mesa, width can be increased so as to improve the thermal impedance
of the SOA. For increased lateral optical confinement (to match the
modulator mode and/or improve SOA efficiency), the deep etch ridge
can be used.
[0035] By way of an enabling technology, FIG. 8 illustrates a novel
geometry 50 for the edge of a mask used to overgrow an epitaxial
layer. There is often an undesirable degree of enhanced crystal
growth that occurs at the mask edge where the pattern is in the
[011] or [0-11] directions, for example. The use of a non-linear
geometry 50 for the mask edge suppresses this undesirable growth
enhancement. The use of a zigzag pattern on the mask edge, rather
than a straight edge, is a novel solution. Unintentional growth
enhancement at a mask edge is an ongoing problem in selective area
growth. Solutions to date have focused on altering the growth
conditions for the epitaxial growth. Selective area epitaxy is a
technique that is used in the production of many InP optical
modulators. This technique and the use of mask patterns to reduce
unintentional enhancement are related to multi-growth modulators on
InP substrate as described in U.S. Pat. No. 9,182,546. Suppressing
unintentional enhancement by changing growth conditions has the
disadvantage of also suppressing the intentional growth
enhancement, which is often the purpose of pursuing selective area
growth in the first place. The conventional use of straight-sided
selective area masks (SAMs) leads to linear structures of enhanced
growth material along the mask edge that are fragile and break off
to cause contamination of the device surface, which has an adverse
effect on manufacturability and reliability. The use of a geometric
solution 50 suppresses the intentional growth enhancement at the
edge of the mask without compromising the intended growth
enhancement. The effect of unintentional enhancement at a mask edge
is anisotropic on the wafer surface. Along one directional axis,
the effect is strong, but along the perpendicular axis it is weak.
By placing a zigzag pattern 50 along the edge susceptible to
enhancement, almost none of that boundary is parallel to the line
along which the unintentional enhancement occurs.
[0036] Although the present disclosure is illustrated and described
herein with reference to preferred embodiments and specific
examples thereof, it will be readily apparent to those of ordinary
skill in the art that other embodiments and examples may perform
similar functions and/or achieve like results. All such equivalent
embodiments and examples are within the spirit and scope of the
present disclosure, are contemplated thereby, and are intended to
be covered by the following non-limiting claims for all
purposes.
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