U.S. patent application number 17/106181 was filed with the patent office on 2022-06-02 for waveguide dual-depletion region (ddr) photodiodes.
The applicant listed for this patent is Infinera Corporation. Invention is credited to Peter W. Evans, Mingzhi Lu.
Application Number | 20220173258 17/106181 |
Document ID | / |
Family ID | 1000005341704 |
Filed Date | 2022-06-02 |
United States Patent
Application |
20220173258 |
Kind Code |
A1 |
Lu; Mingzhi ; et
al. |
June 2, 2022 |
WAVEGUIDE DUAL-DEPLETION REGION (DDR) PHOTODIODES
Abstract
Consistent with the present disclosure, a DDR photodiode is
provided on a substrate adjacent to a passive waveguide. In order
to efficiently capture light output from the waveguide, the
photodiode is coupled to the waveguide with a butt-joint. As a
result, the photodiode and the waveguide abut one another such that
the dominant mode of light propagating in the waveguide parallel to
the substrate is supplied directly to a side of the absorber layer
of the photodiode without, in one example, evanescent coupling, nor
is a resonant coupler required to supply light to the photodiode.
Thus, light is absorbed more efficiently in the photodiode such
that the photodiode may have a shorter length. In addition, since
substantially all light is input to the photodiode, nearly complete
absorption and nearly ideal quantum efficiency can be achieved in a
relatively short length. Further, the improved linearity associated
with DDR photodiodes is preserved with the exemplary butt joint
configurations disclosed herein.
Inventors: |
Lu; Mingzhi; (Fremont,
CA) ; Evans; Peter W.; (Tracy, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Infinera Corporation |
San Jose |
CA |
US |
|
|
Family ID: |
1000005341704 |
Appl. No.: |
17/106181 |
Filed: |
November 30, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 31/02327 20130101;
G02B 2006/12123 20130101; G02B 6/4296 20130101; G02B 6/12 20130101;
H01L 31/0203 20130101 |
International
Class: |
H01L 31/0232 20060101
H01L031/0232; G02B 6/42 20060101 G02B006/42; G02B 6/12 20060101
G02B006/12; H01L 31/0203 20060101 H01L031/0203 |
Claims
1. An optical receiver, comprising: a substrate; an optical
waveguide provided on a first region of the substrate; and a
photodiode provided on a second region of the substrate, such that
an interface between the optical waveguide and the photodiode
constitutes a butt joint, the photodiode includes: a first
semiconductor layer having a p-conductivity type, the first
semiconductor layer being a p-type cladding layer, a second
semiconductor layer having n-conductivity type, the second
semiconductor layer being an n-type cladding layer, an absorber
layer provided between the p-type cladding layer and the n-type
cladding layer, the absorber layer including a first undoped
semiconductor layer, such that the absorber layer is aligned with
the core layer of the optical waveguide to receive, via the
interface, an optical signal propagating in the optical waveguide,
and a second undoped semiconductor layer provided between the
absorber layer and the second semiconductor layer, such that, in an
absence of a reverse bias applied to the photodiode, a first
depletion region forms in the absorber layer and a second depletion
region forms in the second undoped semiconductor layer, wherein the
photodiode is configured to receive an optical signal supplied by
the optical waveguide, the optical signal having a propagation
direction in the optical waveguide, such that the interface between
the optical waveguide and the photodiode is provided at a
non-orthogonal angle relative to the direction of propagation of
the optical signal.
2. An optical receiver in accordance with claim 1, wherein the
absorber layer includes indium gallium arsenide.
3. An optical receiver in accordance with claim 2, wherein the
second undoped semiconductor layer includes indium phosphide.
4. An optical receiver in accordance with claim 1, wherein the
n-type cladding layer includes indium phosphide.
5. An optical receiver in accordance with claim 1, wherein the
p-type cladding layer includes indium phosphide.
6. An optical receiver in accordance with claim 1, further
including a band smoothing region provided between the p-type
cladding and the absorber layer, the band smoothing region
including a quaternary semiconductor alloy.
7. An optical receiver in accordance with claim 6, wherein a
composition of the quaternary semiconductor alloy changes along a
thickness of the smoothing region.
8. An optical receiver in accordance with claim 6, wherein the
quaternary semiconductor alloy is indium gallium arsenic
phosphide.
9. An optical receiver in accordance with claim 8, wherein a
concentration of phosphorus in the smoothing region changes along a
thickness of the smoothing region.
10. An optical receiver in accordance with claim 1, further
including: a first band smoothing region provided between the
p-type cladding and the absorber layer, the first band smoothing
region including a first quaternary semiconductor alloy; and a
second band smoothing region provided between the n-type cladding
and the absorber layer, the second band smoothing region including
a second quaternary semiconductor alloy.
11. An optical receiver in accordance with claim 10, wherein the
first and second quaternary semiconductor alloys include indium
gallium arsenic phophide.
12. An optical receiver in accordance with claim 11, wherein a
concentration of phosphorus in the first smoothing region changes
along a thickness of the first smoothing region, and a
concentration of phosphorus in the second smoothing region changes
along a thickness of the second smoothing region.
13. An optical receiver in accordance with claim 1, wherein the
optical signal is amplitude modulated.
14. An optical receiver in accordance with claim 1, wherein the
optical signal is modulated in accordance with a m-quadrature
amplitude modulation (m-QAM) modulation format, wherein m is a
positive integer greater than 1.
15. An optical receiver in accordance with claim 1, wherein the
photodiode is a dual depletion region photodiode.
16. An optical receiver in accordance with claim 1, wherein the
optical signal travels in a propagation direction in the optical
waveguide, and the optical signal travels in the propagation
direction in the photodiode, a width of the optical waveguide
transverse to the propagation direction of the optical signal being
uniform along a length of the optical waveguide, and a width of the
photodiode transverse to the propagation direction of the optical
signal being uniform along a length of the photodiode.
17. An optical receiver in accordance with claim 1, wherein the
interface and the propagation direction define an angle, the angle
having a magnitude greater than or equal to 5 degrees and less than
or equal to 85 degrees.
18. An optical receiver, comprising: a substrate; an optical
waveguide provided on a first region of the substrate; a photodiode
provided on a second region of the substrate, such that an
interface between the optical waveguide and the photodiode
constitutes a butt joint, wherein the photodiode is configured to
receive an optical signal supplied by the optical waveguide, the
optical signal propagating in the optical waveguide in a
propagation direction, and the optical signal propagating in the
photodiode in the propagation direction, such that a width of the
optical waveguide increases in the propagation direction.
19. An optical receiver in accordance with claim 18, wherein a
width of the photodiode transverse to the propagation direction of
the optical signal is uniform along a length of the photodiode.
20. An optical receiver in accordance with claim 18, wherein a
width of the photodiode transverse to the propagation direction of
the optical signal decreases along a length of the photodiode.
21. An optical receiver in accordance with claim 18, wherein the
interface is oriented at a non-orthogonal angle relative to the
propagation direction of the optical signal.
22. An optical receiver in accordance with claim 21, wherein the
interface and the propagation direction define an angle, the angle
having a magnitude greater than or equal to 5 degrees and less than
or equal to 85 degrees.
23. An optical receiver in accordance with claim 18, wherein, in an
absence of a reverse bias applied to the photodiode, the photodiode
has a first depletion region and a second depletion region.
24. An optical receiver in accordance with claim 23, wherein the
first depletion region is in a first undoped semiconductor layer
and the second depletion is in a second undoped semiconductor
layer.
25. An optical receiver in accordance with claim 24, wherein the
first and second undoped semiconductor layers include first and
second semiconductors, respectively, the first and second
semiconductors being different from one another.
26. An optical receiver in accordance with claim 25, wherein the
photodiode includes a first cladding layer and a second cladding
layer, the first and second undoped semiconductor layers being
provided between the first and second cladding layers.
27. An optical receiver in accordance with claim 6, wherein the
absorber layer comprises a quaternary semiconductor alloy including
indium, gallium, arsenic, and phosphorus.
28. An optical receiver in accordance with claim 1, wherein the
absorber layer comprises a quaternary semiconductor alloy including
of indium, gallium, arsenic, and aluminum.
29. An optical receiver in accordance with claim 1, wherein the
optical receiver includes: a local oscillator laser; and an optical
hybrid circuit that receives an optical output from the local
oscillator laser, wherein the optical signal is supplied by the
optical hybrid circuit.
Description
BACKGROUND
[0001] Optical communication systems are known whereby one or more
optical signals, each being modulated to carry information, are
transmitted from an optical transmitter to an optical receiver. The
optical receivers typically includes, among other things, one or
more photodiodes that convert the received optical signals into
corresponding electrical signals, which are then further processed.
In certain optical communication systems, various components or
devices in the receiver are integrated on a common substrate as a
photonic integrated circuit (PIC). Such components include optical
waveguides and photodiodes that, in some instances, receive light
supplied by the optical waveguides.
[0002] So-called dual-depletion region (DDR) photodiodes are known
in which an "absorber" layer is located above an undoped layer of
bandgap wider than the absorber, in such a manner that
photo-generated holes have a shorter distance to travel to the
p-type region anode above the absorber, while photo-generated
electrons have a longer distance to travel to the n-type region
cathode below the undoped layer. The depletion region extends from
above the absorber layer to below the lower undoped layer. Since
electron mobility is much higher than hole mobility in InP and
related materials, the total transit time of such holes and
electrons from the absorber to their respective contacts is
comparable or minimized. A similar photodiode without a undoped
layer below the absorber would have much higher capacitance, and a
similar photodiode with an absorber as thick as the two undoped
layers can absorb too much light at the input so that the high
speed photocurrent response is nonlinear. Accordingly, carrier
lifetime in the DDR photodiode is reduced, and the length and
capacitance of the photodiode may be optimized for both high
responsivity and high radio frequency (RF) bandwidth.
[0003] Conventional DDR photodiodes detect light that is incident
at a direction that is normal to the substrate upon which the DDR
photodiode is provided. PICs, however, include optical waveguides
that confine optical signals, whereby the optical signals propagate
in the optical waveguides in a direction parallel to the substrate.
Accordingly, conventional DDR photodiodes may not be suitable for
integration in a PIC, and the associated benefits of such
photodiodes may be difficult to obtain in optical receivers
including PICs.
SUMMARY
[0004] Consistent with an aspect of the present disclosure, an
optical receiver is provided that comprises a substrate and an
optical waveguide having a core layer provided on a first region of
the substrate. The receiver also includes a photodiode provided on
a second region of the substrate, such that an interface between
the optical waveguide and the photodiode constitutes a butt joint.
The photodiode includes a first semiconductor layer having a
p-conductivity type, the first semiconductor layer being a p-type
cladding layer. The photodiode also includes a second semiconductor
layer having n-conductivity type, the second semiconductor layer
being an n-type cladding layer. Further, the photodiode includes an
absorber layer provided between the p-type cladding layer and the
n-type cladding layer. The absorber layer has a first undoped
semiconductor layer, such that the absorber layer is aligned with
the core layer of the optical waveguide to receive, via the
interface, an optical signal propagating in the optical waveguide.
Moreover, the photodiode includes a second undoped semiconductor
layer provided between the absorber layer and the second
semiconductor layer, such that in an absence of a reverse bias
applied to the photodiode, a first depletion region forms in the
absorber layer and a second depletion region forms in the second
undoped semiconductor layer.
[0005] Consistent with a further aspect of the present disclosure,
an optical receiver is provided that comprises a substrate and an
optical waveguide provided on a first region of the substrate. In
addition, the optical receiver includes a photodiode provided on a
second region of the substrate, such that an interface between the
optical waveguide and the photodiode constitutes a butt joint.
Further, the photodiode is configured to receive an optical signal
supplied by the optical waveguide, wherein the optical signal has a
propagation direction in the optical waveguide. The interface
between the optical waveguide and the photodiode is provided at a
non-orthogonal angle relative to the direction of propagation of
the optical signal.
[0006] Consistent with an additional aspect of the present
disclosure, an optical receiver is provided that comprises a
substrate and an optical waveguide provided on a first region of
the substrate. Further, a photodiode provided on a second region of
the substrate, such that an interface between the optical waveguide
and the photodiode constitutes a butt joint. The photodiode is
configured to receive an optical signal supplied by the optical
waveguide, wherein the optical signal propagates in the optical
waveguide in a propagation direction, and the optical signal
propagates in the photodiode in the same propagation direction. A
width of the optical waveguide increases in the propagation
direction.
[0007] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory only and are not restrictive of the invention, as
claimed.
[0008] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate several
embodiments and together with the description, serve to explain the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 shows an optical receiver employing direct detection
consistent with an aspect of the present disclosure;
[0010] FIG. 2a shows an optical receiver employing coherent
detection consistent with an additional aspect of the present
disclosure;
[0011] FIG. 2b shows an example of a balanced (differential)
detector consistent with the present disclosure;
[0012] FIGS. 3a-3c show plan view of waveguide-photodiode
combinations consistent with aspects of the present disclosure.
[0013] FIG. 4 shows a perspective view of a waveguide-photodiode
configuration shown in FIG. 3a;
[0014] FIG. 5a shows a cross-sectional view of photodiode and
waveguide consistent with a further aspect of the present
disclosure;
[0015] FIG. 5b shows a cross-sectional view of photodiode and
waveguide consistent with an additional aspect of the present
disclosure;
[0016] FIG. 5c shows a cross-sectional view of photodiode and
waveguide consistent with a further aspect of the present
disclosure;
[0017] FIG. 6 shows an example of an energy band diagram consistent
with an additional aspect of the present disclosure;
[0018] FIG. 7 is a plot of mode overlap ratio with a passive
waveguide vs. absorber thickness consistent with an aspect of the
present disclosure;
[0019] FIG. 8 shows plots of total harmonic distortion vs. bias
voltage consistent with an additional aspect of the present
disclosure.
DESCRIPTION OF THE EMBODIMENTS
[0020] Consistent with the present disclosure, a DDR photodiode is
provided on a substrate adjacent to a passive waveguide. In order
to efficiently capture light output from the waveguide, the
photodiode is coupled to the waveguide with a butt-joint. As a
result, the photodiode and the waveguide abut one another such that
the dominant mode of light propagating in the waveguide parallel to
the substrate is supplied directly to a side of the absorber layer
of the photodiode without, in one example, evanescent coupling, nor
is a resonant coupler required to supply light to the photodiode.
Thus, light is absorbed more efficiently in the photodiode such
that the photodiode may have a shorter length. In addition, since
substantially all light is input to the photodiode, nearly complete
absorption and nearly ideal quantum efficiency can be achieved in a
relatively short length. Further, the improved linearity associated
with DDR photodiodes is preserved with the exemplary butt joint
configurations disclosed herein.
[0021] Reference will now be made in detail to the present
exemplary embodiments of the disclosure, which are illustrated in
the accompanying drawings. Wherever possible, the same reference
numbers will be used throughout the drawings to refer to the same
or like parts.
[0022] FIG. 1 shows a high level circuit diagram of receiver 104
consistent with an aspect of the present disclosure. Receiver 104
includes a waveguide 194 that carries optical signals which may be
amplitude modulated. The optical signals are supplied to photodiode
192, which may be appropriately biased. In one example, the
photodiode is reversed biased such that a positive reference or
bias voltage +Vdd is supplied to cathode 192-2 and a negative
reference or bias voltage -Vdd is supplied to anode 192-1.
[0023] FIG. 2a shows coherent optical receiver 104c, which is
another example of an optical receiver consistent with the present
disclosure. In one example, a polarization multiplexed optical
signal is supplied to receiver 104C. In that case, receiver 104 may
include a polarization beam splitter (PBS) 202 operable to receive
the input optical signal and to separate the signal into orthogonal
polarizations, i.e., vector components of the optical E-field of
the incoming optical signal transmitted on optical fiber medium
108, into signals X and Y. One of the polarizations is parallel to
the local oscillator (LO) polarization, and the other is rotated to
be parallel to the LO (201). The LO output is split by an optical
coupler (203). X and Y light are then each mixed with portions of
the LO output in their own 90 degree optical hybrid circuits
("hybrid") 204-1 and 204-2. Hybrid 204-1 outputs four optical
signals O1a, O1b, O2a, and O2b, and hybrid 204-2 outputs four
optical signals O3a, O3b, O4a, and O4b, each representing the
in-phase and quadrature components of the optical E-field of X and
Y signals, and each including light from local oscillator 201 and
light from polarization beam splitter 202 or mixing products.
Optical signals O1a, O1b; O2a, O2b; O3a, O3b; and O4a, O4b are
supplied to respective one of photodetector circuits 209, 211, 213,
and 215. Each photodetector circuit includes a pair of photodiodes
("2 PDs") configured as single ended or else as balanced detector,
as shown in the example FIG. 2b.
[0024] FIG. 2b shows photodetector circuit 209 in greater detail.
It is understood that remaining photodetector circuits 211, 213,
and 215 have a similar construction and operate in a similar manner
as photodetector circuit 209. As shown in FIG. 2b, optical
waveguides 194-1 and 194-2 supply optical signals O1a and O1b,
respectively, from optical hybrid 204-1 to a corresponding one of
photodiodes 292 and 294. Photodiodes 292 and 294 are connected in a
balanced detector configuration, and the output, E1, of
photodetector circuit 209 is supplied to a transimpedance amplifier
(TIA) circuit, whose output then supplies an analog-to-digital
conversion (ADC) circuit for further processing.
[0025] Each of remaining photodetector circuits 211, 213, and 215
generates a corresponding one of electrical signals E2 to E4 in a
similar manner as that described above with respect to
photodetector circuit 209. Signals E2 to E4 are also supplied to
respective TIA/ADC circuits. Electrical signals E1 to E4 are
indicative of data carried by optical signals input to PBS 202.
[0026] FIGS. 3a to 3c show plan views or layouts of examples of
passive waveguide-photodiode combinations consistent with the
present disclosure. As shown in FIG. 3a an optical signal
represented by optical mode 302 propagates in direction 304 in
waveguide 194. Waveguide 194 has a width transverse to direction
304 that increases in direction 304. For example, at location L1,
waveguide 194 has a width w1, and, at location L2, waveguide 194
has a width w2 that is greater than width w1. Location L1 is
farther away from interface 306 than location L2.
[0027] As further shown in FIG. 3a, interface 306 is present
between optical waveguide 194 and photodiode 192. In one example,
interface 306 constitutes a butt joint, whereby photodiode 194 is
configured to receive optical mode 302 directly from optical
waveguide 194. Moreover, in the example shown in FIG. 3a, interface
306 is oriented or provided at a non-orthogonal angle a relative to
direction 304. Angle a may have a magnitude that is greater than or
equal to 5.degree. and less than or equal to 85.degree..
[0028] Optical mode or signal 304 next propagates into photodiode
192 and is absorbed along a length of photodiode 192. In one
example, a width of photodiode 192 narrows in a direction
corresponding to propagation direction 304, such that at location
L3 photodiode 192 has a width w3, which is greater than a width w4
of photodiode 192 at location L4. Location L3 is nearer interface
306 than location L4.
[0029] In another example, as shown in FIG. 3b, the width w5 of
photodiode 192 is uniform along a length extending in a propagation
direction 304. However, the width of waveguide 194 increases in a
direction corresponding to propagation direction 304, as shown in
FIG. 3a.
[0030] In a further example, as shown in FIG. 3c, both waveguide
194 and photodiode 192 have the same width w6, and such width is
uniform in propagation direction 304.
[0031] FIG. 4 shows a perspective view of waveguide 194 and
photodiode 192 corresponding to the configuration shown in FIG.
3a.
[0032] FIG. 5a shows a view of waveguide 194 and photodiode 192
taken along cross-section 5 (see FIG. 3a) adjacent interface 306.
It is understood that, in one example, waveguide 194 and photodiode
192 shown in FIGS. 3b and 3c will have a similar construction as
that shown FIG. 5a, and, in further examples, waveguide 194 and
photodiode 192 have the structures shown in FIGS. 5b and 5c.
[0033] As shown in FIG. 5a, waveguide 194 is provided on region 594
of substrate 404, and photodiode 192 is provided on region 592.
N-type cladding layer 512 , in one example, may be provided on
substrate 402 that extends over both regions 594 and 592. The
n-type cladding layer includes, in a further example, indium
phosphide (InP). An additional n-type or n-doped layer 511 may be
provided on a first cladding layer 512. Layer 511 may also extend
over both region 594 and region 592. Layer 511 may also include
InP.
[0034] As further shown in FIG. 5a, waveguide 194 provided over
region 594 includes, in one example, a core layer 610, which
includes a quaternary semiconductor material, such as, indium
gallium arsenide phosphide (InGaAsP) or other suitable quaternary
semiconductor material, such as AlGalnAs. Layer 610 is typically
undoped or else lightly doped (.ltoreq.1 E17) n-type. As used
herein, undoped means unintentionally doped or a doping
concentration that is less than or equal to 1.times.1016. Waveguide
194 may also include undoped or n-layer 508, which is further
provided on region 594, and a layer 506 provided on layer 508.
Layer 508 may be undoped or lightly doped n-. Waveguide 194
provided over region 594 further includes a second cladding layer
504, which, in one example, is doped p-type or implanted with H or
He to be semi-insulating, and, in further example includes InP.
[0035] Photodiode 192, as noted above, is formed over region 592 of
substrate 404. Photodiode 192 may be a DDR photodiode including an
absorber layer. In the example shown in FIG. 5a, photodiode 192
includes absorber layer 522, which includes, for example, indium
gallium arsenide (InGaAs). In a further example, absorber layer 522
is undoped or undoped. Photodiode 192 further includes cladding
layer 520, which may be p-type and includes InP. Cladding layer 520
is provided on absorber layer 522. In addition, contact layer 518,
which is also p-type and includes InGaAs, may be provided on p-type
cladding 520. As further shown in FIG. 5a, an additional contact
layer 516 including a conductor or metal may be provided on contact
layer 518.
[0036] As noted above, the absorber layer, such as layer 522 of
photodiode 192 in configured with undoped layer 508 below or has a
thickness or electron/hole mobility combination such that
photo-generated holes have a shorter distance to travel to the
p-type anode (520) while photo-generated electrons have a longer
distance to travel to the n-type cathode (511) or photodiode 192.
As further noted above, since the electron mobility is higher than
the hole mobility for InP and related materials, the transit time
of such holes and electrons is substantially the same. Accordingly,
carrier lifetime in the photodiode 192 is reduced, and radio
frequency (RF) bandwidth is increased.
[0037] Moreover, absorber layer 522 is provided in a manner to be
aligned with and abuts core layer 610 of waveguide 192, such that
light is efficiently input to absorber layer 522 via interface 306
with minimal loss.
[0038] FIG. 5b shows an example similar to that shown in FIG. 5a.
In FIG. 5b, however, optical core layer 510 extends over region 592
and constitutes part of photodiode 592. In the example shown in
FIG. 5b an optical mode or optical signal 503 propagates in a
direction indicated by arrow 514 in core 510. In photodiode 592, a
tail portion 503-1 or mode 503 extends into absorber layer 522, and
this generates electron-hole pairs in a manner similar to that
described above. In FIG. 5b, mode 503 is evanescently coupled to
absorber 522.
[0039] FIG. 5c shows an example similar to that shown in FIG. 5b.
In FIG. 5c, however, photodiode 192 includes an undoped layer 524
and an undoped or n-doped layer 526 provided between layer 508 and
absorber layer 522. Layer 524 is provided on quaternary layer 526,
in this example. In this example, layer 524 includes, for example,
InP that is undoped and layer 526 includes n-type InP.
[0040] In the examples shown in FIG. 5c, optical mode or optical
signal 503 propagates in waveguide 194 in the direction indicated
by arrow 514. Layers 524 and 526 are configured to facilitate a
quaternary mode transfer from core 510 to absorber 522 by way of
resonate coupling. As a result, mode 503 generates electron-hole
pairs in absorber layer 522 in a manner similar to that described
above.
[0041] In each of the examples shown in FIGS. 5a-5c, appropriate
biases are applied to contact 516 and a contact, for example, to
substrate 402 so that the above-described electron-hole pairs
generate a photocurrent indicative of the optical signal 503.
[0042] FIG. 6 shows an example of an energy band diagram 600
consistent with a further aspect of the present disclosure. As
shown in FIG. 6, energy band diagram 600 includes regions 712, 711,
722, and 720 corresponding to the energy bands present in layers
511, 508, 522 and 520 of FIG. 5a. Energy band diagram 600 further
includes "band smoothing" regions 602 and 604, which, in one
example, includes a layer or layers comprising a quaternary
semiconductor alloy, such as InxGa1-xAsyP1-y, with a bandgap
intermediate between that of the InGaAs absorber and InP that
facilitates carrier transit. Region 602 may be provided between InP
layer 508 and absorber layer 522, and region 604 may be provided
between absorber layer 522 and InP layer 520 in FIG. 5a. In one
example, the concentration of phosphorus in the quaternary alloy
InxGa1-xAsyP1-y varies continuously along a thickness of region 602
and varies along a thickness of region 604, e.g., in a direction
away from the substrate (along the x-axis in FIG. 6).
Alternatively, the smoothing region 602 and 604 includes one or
more layers, including a ternary semiconductor alloy or
composition, and each layer within the band smoothing region may
have a discrete composition and corresponding bandgap. Thus, band
smoothing region 602 may include either a quaternary or ternary
semiconductor alloy and band smoothing region 604 may include
either a quaternary or ternary semiconductor alloy.
[0043] As further shown in FIG. 6, band smoothing region 604 is
provided between region 720 (corresponding to layer 520) and region
722 (corresponding to absorber layer 522). A valence band edge Ev
within region 604 has an associated first energy that is between a
second energy associated with a valence band edge Ev within region
722 and a third energy associated with a valance band edge Ev
within region 720. In addition, band smoothing region 602 is
provided between region 722 and region 711 (corresponding to layer
508). A conduction band edge Ec within region 602 has an associated
fourth energy that is between a fifth energy associated with a
conduction band edge Ec within region 722 and a sixth energy
associated with a conduction band edge Ec within region 711.
[0044] In each of the above example, the concentration of
phosphorus in the band smoothing regions 602 and 604 may vary along
a thickness of such regions, e.g., in a direction along the x-axis
in FIG. 6.
[0045] In a further example, each of band smoothing regions 602 and
604 includes AlGalnAs.
[0046] FIG. 7 is a plot 700 of the overlap ratio of the optical
mode in photodiode 192 to the optical mode in waveguide 194 vs
absorber thickness for FIG. 5a. FIG. 7 shows an example in which an
absorber thickness of 0.1 microns results in a maximum amount of
overlap of the optical modes propagating in waveguide 192 and
photodiode 194, nearly 100%.
[0047] FIG. 8 illustrates plots 800 of total harmonic distortion
(THD) for two DDR photodiodes consistent with aspects of the
present disclosure. These plots were generated in connection with
an example in which light output from two lasers was mixed to
obtain a beating signal having a 1 GHz beating tone. The beating
signal was received by a photodiode consistent with the present
disclosure, such as photodiode 192. The total harmonic distortion
may be defined as the ratio between the total power of the
harmonics (2 GHz, 3 GHz, etc.) over the RF power at 1 GHz. Optical
signals incident from the two lasers provide photocurrents of 4.6
mA and 0.4 mA, respectively. The dashed curved in FIG. 8 represents
the THD as a function of bias voltage of a photodiode having band
smoothing layers similar to those described above. The solid curve
represent the THD as a function of bias voltage of a photodiode
without smoothing layers. The dashed curve indicates that for a
given bias voltage, the THD, and thus the linearity is greater for
the photodiode having smoothing layers, such as a band smooth layer
between the p-type core layer 520 and absorber layer 522, and
between 522 and 508 below, than that of the photodiode without such
smoothing layers. The measurements depicted in FIG. 8 were obtained
from photodiodes with a 3 micron input width.
[0048] In a further example, a responsivity of 1.1. NW was measured
in connection with a 25 micron long photodiode.
[0049] Other embodiments will be apparent to those skilled in the
art from consideration of the specification. It is intended that
the specification and examples be considered as exemplary only,
with a true scope and spirit of the invention being indicated by
the following claims.
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