U.S. patent application number 10/819775 was filed with the patent office on 2004-09-30 for laser with resonant reflector.
Invention is credited to Morgan, Robert A., Strzelecki, Eva M..
Application Number | 20040188695 10/819775 |
Document ID | / |
Family ID | 25021913 |
Filed Date | 2004-09-30 |
United States Patent
Application |
20040188695 |
Kind Code |
A1 |
Morgan, Robert A. ; et
al. |
September 30, 2004 |
Laser with resonant reflector
Abstract
A laser is provided that includes a top mirror layer upon which
a resonant reflector is situated. The resonant reflector includes a
first material layer having a thickness that is about an odd
multiple of a quarter of the wavelength to which the laser is
tuned. Patterned regions extend into the first material layer so
that the thickness of the first material layer in the patterned
regions is reduced. Some patterned regions are at least partially
filled with a second material having a refractive index that is
greater than the refractive index of the first material layer. The
second material also has a thickness of about an odd multiple of a
quarter of the wavelength to which the laser is tuned. Finally, a
third layer positioned immediately adjacent the first material
layer has a refractive index that is greater than the refractive
index of the first material layer.
Inventors: |
Morgan, Robert A.;
(Plymouth, MN) ; Strzelecki, Eva M.; (Plymouth,
MN) |
Correspondence
Address: |
WORKMAN NYDEGGER (F/K/A WORKMAN NYDEGGER & SEELEY)
60 EAST SOUTH TEMPLE
1000 EAGLE GATE TOWER
SALT LAKE CITY
UT
84111
US
|
Family ID: |
25021913 |
Appl. No.: |
10/819775 |
Filed: |
April 7, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10819775 |
Apr 7, 2004 |
|
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09751423 |
Dec 29, 2000 |
|
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6727520 |
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Current U.S.
Class: |
257/98 |
Current CPC
Class: |
H01S 2301/166 20130101;
H01S 5/18327 20130101; H01S 5/18369 20130101; H01S 5/18358
20130101; H01S 5/18391 20130101; H01S 5/18377 20130101; H01S
5/18308 20130101; H01S 5/18388 20130101 |
Class at
Publication: |
257/098 |
International
Class: |
H01L 033/00 |
Claims
What is claimed is:
1. A laser, comprising: a top layer; and a resonant reflector
disposed proximate the top layer and comprising: a first material
layer with a first refractive index and having a thickness of about
an odd multiple of a quarter of a wavelength to which the laser is
tuned, the first material layer having at least one patterned
region that extends at least partially into the first material
layer thus reducing the thickness of the first material layer in
the at least one patterned region; a second material at least
partially filling at least one selected patterned region and having
a second refractive index, one of the first and second refractive
indices being greater than the other of the first and second
refractive indices; and a third layer positioned immediately
adjacent the first material layer, the third layer having a
refractive index greater than the refractive index of the first
material layer.
2. The laser as recited in claim 1, wherein one of the at least one
patterned region extends completely through the first material
layer.
3. The laser as recited in claim 1, wherein the second refractive
index is greater than the first refractive index.
4. The laser as recited in claim 1, wherein the second material has
a thickness of about an odd multiple of a quarter of the wavelength
to which the laser is tuned.
5. The laser as recited in claim 1, wherein the second material
also extends above at least one non-patterned region of the first
material layer.
6. The laser as recited in claim 1, wherein the at least one
patterned region is configured such that reflectivity of the
resonant reflector is reduced in the at least one patterned
region.
7. The laser as recited in claim 1, wherein the at least one
patterned region is configured and arranged to facilitate mode
control for the laser.
8. The laser as recited in claim 1, wherein the top layer comprises
a mirror layer.
9. The laser as recited in claim 1, wherein the first material
substantially comprises SiO.sub.2, the second material
substantially comprises Si.sub.3N.sub.4 or TiO.sub.2, and the third
material substantially comprises AlGaAs.
10. The laser as recited in claim 1, wherein the first material
layer comprises a top mirror layer of a DBR mirror.
11. The laser as recited in claim 1, wherein the top layer
substantially comprises GaAs/AlGaAs.
12. A laser, comprising: a top layer; and a resonant reflector
disposed proximate the top layer and comprising: a first mirror
region having a top mirror layer, the top mirror having at least
one patterned region that extends at least partially into the top
mirror layer, and the top mirror layer further including at least
one non-patterned region; and a second mirror region formed on at
least one selected non-patterned region of the top mirror
layer.
13. The laser as recited in claim 12, wherein the at least one
patterned region is configured to implement a phase shift, relative
to the at least one non-patterned region.
14. The laser as recited in claim 12, wherein the at least one
patterned region is configured and arranged to facilitate mode
control for the laser.
15. The laser as recited in claim 12, wherein the at least one
patterned region is configured such that reflectivity of the
resonant reflector is reduced in the at least one patterned
region.
16. A laser, comprising: a top layer; and a resonant reflector
disposed proximate the top layer and comprising: a top mirror with
a top mirror layer, the top mirror layer etched with a pattern
extending partially into the top mirror layer so that at least one
patterned region and at least one non-patterned region are formed,
the at least one patterned region serving to reduce the
reflectivity of the resonant reflector in the at least one
patterned region; and a cap mirror situated above at least one
selected non-patterned region of the top mirror layer.
17. The laser as recited in claim 16, wherein the at least one
patterned region is configured to implement a phase shift, relative
to the at least one non-patterned region.
18. The laser as recited in claim 16, wherein the at least one
patterned region is configured and arranged to facilitate mode
control for the laser.
19. The laser as recited in claim 16, wherein the top mirror layer
includes at least one of: at least one period of a semiconductor
DBR mirror; and, a narrow band dielectric reflection filter.
20. The laser as recited in claim 19, wherein a non-patterned
portion of the cap mirror substantially corresponds to a desired
optical cavity of the laser.
21. The laser as recited in claim 16, wherein the cap mirror
includes at least one of: at least one period of a semiconductor
DBR mirror; and, a narrow band dielectric reflection filter.
22. The laser as recited in claim 16, wherein the at least one
patterned region substantially circumscribes a desired optical
cavity of the laser.
23. The laser as recited in claim 16, further comprising a contact
layer proximate the top mirror layer.
24. The laser as recited in claim 16, further comprising a metal
layer disposed on at least one selected region of the top mirror
layer.
25. A vertical cavity surface-emitting laser (VCSEL), comprising: a
layer that at least partially defines an optical cavity having an
optical axis; and a resonant reflector layer extending across at
least part of the optical cavity, the resonant reflector layer
having a refractive index that does not abruptly change laterally
across the optical cavity, the refractive index of the resonant
reflector layer including contributions from a first material
having a first refractive index and a second material having a
second refractive index, at least one of the first material and the
second material comprising a polymer.
26. The VCSEL as recited in claim 25, wherein the first material is
substantially confined to a first region and the second material is
substantially confined to a second region, the first region and the
second region co-extending along an interface, at least part of the
interface being non-parallel with respect to the optical axis.
27. The VCSEL as recited in claim 25, wherein the first refractive
index is less than the second refractive index.
28. The VCSEL as recited in claim 25, wherein the first material
substantially comprises AlGaAs, and the second material
substantially comprises a polymer.
29. The VCSEL as recited in claim 28, wherein the polymer
substantially comprises one of: polyimide; or, benzocyclobuthene
(BCB).
30. The VCSEL as recited in claim 25, wherein a reflectivity of the
resonant reflector is at a maximum in a location proximate a center
of the optical cavity.
31. The VCSEL as recited in claim 25, wherein a reflectivity of the
resonant reflector is substantially symmetric about a center of the
optical cavity.
32. The VCSEL as recited in claim 25, further comprising an
additional layer disposed on top of the resonant reflector, the
additional layer comprising one of: at least one period of a
semiconductor DBR mirror; or, a narrow band dielectric reflection
filter.
33. The VCSEL as recited in claim 25, further comprising a contact
layer arranged proximate a periphery of the optical cavity.
34. The VCSEL as recited in claim 25, further comprising a mirror
having a top mirror layer positioned adjacent to the resonant
reflector layer.
35. The VCSEL as recited in claim 34, wherein the top mirror layer
has a refractive index that is greater than the first refractive
index and the second refractive index.
36. The VCSEL as recited in claim 34, wherein the top mirror layer
substantially comprises AlGaAs.
37. A vertical cavity surface-emitting laser (VCSEL), comprising: a
layer that at least partially defines an optical cavity having an
optical axis; and a resonant reflector defined by two substantially
planar opposing surfaces extending across at least a part of the
optical cavity of the laser, the resonant reflector layer having a
first region with a first refractive index and a second region with
a second refractive index, the first region and the second region
co-extending along an interface, at least part of the interface
being non-parallel with respect to the optical axis.
38. The VCSEL as recited in claim 37, wherein the first region is
positioned proximate a center of the optical cavity and includes
lateral edges that are non-parallel with respect to the optical
axis, and the second region includes lateral edges that co-extend
along the lateral edges of the first region.
39. A vertical cavity surface-emitting laser (VCSEL), comprising: a
top mirror layer having a thickness of an odd multiple of a quarter
of a wavelength to which the VCSEL is tuned, the top mirror layer
having at least one patterned region substantially circumscribing a
desired optical cavity of the VCSEL, the at least one patterned
region extending at least partly into the top mirror layer, and the
top mirror layer further including at least one non-patterned
region; and a second layer disposed on the top mirror layer so that
the second layer extends over at least a non-patterned region of
the top mirror layer, the second layer having a refractive index
less than a refractive index of the top mirror layer.
40. The VCSEL as recited in claim 39, wherein the second layer
extends over at least one patterned region of the top mirror
layer.
41. The VCSEL as recited in claim 39, further comprising an etch
stop layer positioned below the top mirror layer.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation, and claims the benefit,
of U.S. patent application Ser. No. 09/751,423, entitled SPATIALLY
MODULATED REFLECTOR FOR AN OPTOELECTRONIC DEVICE, filed Dec. 29,
2000, and incorporated herein in its entirety by this
reference.
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0002] This invention generally relates to optoelectronic
components such as lasers. More particularly, embodiments of the
invention are concerned with lasers that include a resonant
reflector for use in facilitating mode control in the laser.
BRIEF SUMMARY OF AN EXEMPLARY EMBODIMENT OF THE INVENTION
[0003] In one exemplary embodiment of the invention, a laser is
provided that includes a top mirror layer upon which a resonant
reflector is situated. The resonant reflector includes a first
material layer having a thickness that is about an odd multiple of
a quarter of the wavelength to which the laser is tuned. The first
layer also has one or more patterned regions that extend downward
into the first material layer a distance so that the thickness of
the first material layer in the patterned regions is thereby
reduced. Some of the patterned regions of the first material layer
are at least partially filled with a second material having a
refractive index that is greater than the refractive index of the
first material layer. Similar to the first material layer, the
second material has a thickness of about an odd multiple of a
quarter of the wavelength to which the laser is tuned. Finally, a
third layer is provided that is positioned immediately adjacent the
first material layer. The third layer has a refractive index that
is greater than the refractive index of the first material layer.
Among other things, this construction facilitates mode control in
the laser without necessitating significant additional processing
and manufacturing steps.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Other objects of the present invention and many of the
attendant advantages of the present invention will be readily
appreciated as the same becomes better understood by reference to
the following detailed description when considered in connection
with the accompanying drawings, in which like reference numerals
designate like parts throughout the figures thereof and
wherein:
[0005] FIG. 1 is a schematic diagram of a planar, current-guided,
GaAs/AlGaAs top surface emitting vertical cavity laser in
accordance with the prior art;
[0006] FIG. 2 is a schematic cross-sectional side view of a planar,
current-guided, GaAs/AlGaAs top surface emitting vertical cavity
laser with a first illustrative resonant reflector for increased
mode control in accordance with the present invention;
[0007] FIGS. 3A-3D are schematic cross-sectional side views showing
an illustrative method for making the resonant reflector of FIG.
2;
[0008] FIG. 4 is a schematic cross-sectional side view of a planar,
current-guided, GaAs/AlGaAs top surface emitting vertical cavity
laser with a second illustrative resonant reflector for increased
mode control in accordance with the present invention;
[0009] FIGS. 5A-5D are schematic cross-sectional side views showing
an illustrative method for making the resonant reflector of FIG.
4;
[0010] FIG. 6 is a schematic cross-sectional side view of a planar,
current-guided, GaAs/AlGaAs top surface emitting vertical cavity
laser with yet another illustrative resonant reflector for
increased mode control in accordance with the present
invention;
[0011] FIGS. 7A-7D are schematic cross-sectional side views showing
a first illustrative method for making the resonant reflector of
FIG. 6;
[0012] FIGS. 8A-8E are schematic cross-sectional side views showing
another illustrative method for making the resonant reflector of
FIG. 6;
[0013] FIGS. 9A-9E are schematic cross-sectional side views showing
yet another illustrative method for making the resonant reflector
of FIG. 6;
[0014] FIG. 10 is a schematic cross-sectional side view showing
another illustrative embodiment of the present invention similar to
that shown in FIGS. 5A-5D;
[0015] FIG. 11 is a schematic cross-sectional side view showing
another illustrative embodiment of the present invention similar to
that shown in FIGS. 5A-5D; and
[0016] FIG. 12 is a schematic cross-sectional side view showing
another illustrative embodiment of the present invention similar to
that shown in FIGS. 8A-8E.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0017] FIG. 1 is a schematic illustration of a planar,
current-guided, GaAs/AlGaAs top surface emitting vertical cavity
laser 10 in accordance with the prior art. Formed on an n-doped
gallium arsenide (GaAs) substrate 14 is an n-contact 12. Substrate
14 is doped with impurities of a first type (i.e., n type). An
n-type mirror stack 16 is formed on substrate 14. Formed on stack
16 is a spacer 18. Spacer 18 has a bottom confinement layer 20 and
a top confinement layer 24 surrounding active region 22. A p-type
mirror stack 26 is formed on top confinement layer 24. A p-metal
layer 28 is formed on stack 26. The emission region may have a
passivation layer 30.
[0018] Isolation region 29 restricts the area of the current flow
27 through the active region. Region 29 may be formed by deep H+
ion implantation. The diameter "g" may be set to provide the
desired active area, and thus the gain aperture of the VCSEL 10.
Further, the diameter "g" may be set by the desired resistance of
the p-type mirror stack 26, particularly through the non-conductive
region 29. Thus, non-conductive region 29 performs the gain guiding
function. The diameter "g" is typically limited by fabrication
limitations, such as lateral straggle during the implantation
step.
[0019] Spacer 18 may contain a bulk or quantum-well active region
disposed between mirror stacks 16 and 26. Quantum-well active
region 22 may have alternating layers of aluminum gallium arsenide
(AlGaAs) barrier layers and GaAs quantum-well layers. InGaAs
quantum wells may also be used in the active region, particularly
where an emission wavelength (e.g. .lambda.=980 nm) is desired
where GaAs is transparent. Stacks 16 and 26 are distributed Bragg
reflector (DBR) stacks, and may include periodic layers of doped
AlGaAs and aluminum arsenide (AlAs). The AlGaAs of stack 16 is
doped with the same type of impurity as substrate 14 (e.g., n
type), and the AlGaAs of stack 26 is doped with the other kind of
impurity (e.g., p type).
[0020] Metal contact layers 12 and 28 are ohmic contacts that allow
appropriate electrical biasing of laser diode 10. When laser diode
10 is forward biased with a more positive voltage on contact 28
than on contact 12, active region 22 emits light 31 which passes
through stack 26.
[0021] Most VCSELs of practical dimensions are inherently multi
(transverse) mode. Single lowest-order mode VCSELs are favored for
coupling into single-mode fibers, and are advantageous for
free-space and/or wavelength sensitive systems, and may even be
beneficial for use in extending the bandwidth-length product of
standard 50 .mu.m and 62.5.mu.m GRIN multi-mode fiber. However, it
has long been known that, although the short optical cavity
(2.lambda.) of the VCSEL favors single longitudinal mode emission,
the multi-wavelength (10.lambda.) lateral dimensions facilitate
multi-transverse mode operation.
[0022] As indicated above, higher order modes typically have a
greater lateral concentration of energy away from the center of the
optical or lasing cavity. Thus, the most obvious way to force the
laser to oscillate in only a lowest order circularly symmetric mode
is to make the lateral dimension "g" of the active area small
enough to prevent higher-order modes from reaching threshold.
However, this necessitates lateral dimensions of less than about 5
.mu.m for typical VCSELs. Such small areas may result in excessive
resistance, and push the limits obtainable from conventional
fabrication methodologies. This is particularly true for
implantation depths of greater than about 1 .mu.m, where lateral
straggle may become a limiting factor. Thus, control of transverse
modes remains difficult for VCSEL's of practical dimensions.
[0023] One illustrative approach for controlling transverse modes
of an optoelectronic device is shown in FIG. 2. FIG. 2 is a
schematic cross-sectional side view of a planar, current-guided,
GaAs/AlGaAs top surface emitting vertical cavity laser, as in FIG.
1, with a top mounted mode control resonant reflector 50. The
resonant reflector 50 includes a patterned first material layer 56
that is positioned on top of the top mirror layer 52 of VCSEL 54. A
second material layer 58 is provided over the patterned first
material layer 56, as shown.
[0024] The first material layer 56 preferably has a refractive
index that is greater than the refractive index of the second
material layer 58, and the first and second material layers 56 and
58 preferably have a refractive index that is less than the
refractive index of the top mirror layer 52 of the optoelectronic
device 54. In one example, the first material layer 56 is
SiO.sub.2, the second material layer 58 is Si.sub.3N.sub.4 or
TiO.sub.2, and the top mirror layer 52 is AlGaAs, although other
suitable material systems are contemplated. Each layer is
preferably an even multiple of one-quarter wavelength (.lambda./4)
thick. This causes a reduction in reflectivity of the resonant
reflector 50 in those regions that correspond to the etched regions
60 (see FIG. 3B) in the first material layer 56, that is, those
regions that are filled with the second material layer 58. By
designing the etched regions to circumscribe the desired optical
cavity, this difference in reflectivity can be used to help provide
mode control for VCSEL 54.
[0025] In forming the resonant reflector 50, and referring now to
FIG. 3A, the first material layer 56 is provided over the top
mirror layer 52. As shown in FIG. 3B, the first material layer 56
is patterned, preferably by etching away the first material layer
56 in the region or regions circumscribing the desired optical
cavity of the VCSEL 54. As shown in FIG. 3C, a second material
layer 58 is provided over the first material layer 56. The second
material layer 58 is preferably provided over both the etched 60
and non-etched regions of the first material layer 56, but may be
confined to the non-etched regions if desired. Selected regions,
such as regions 62a and 62b of the second material layer 58 may
then be removed to provide access to the top mirror layer 52. Then,
and as shown in FIG. 3D, a contact layer 64 may be provided on the
exposed regions of the top mirror layer 52. The contact layer 64
may provide electrical contact to the top mirror layer 52.
[0026] In a related embodiment, a top mirror layer of the
optoelectronic device may function as the first material layer 56
discussed above. Thus, the top mirror layer may be patterned,
preferably by etching at least partially into the top mirror layer
in the region or regions circumscribing the desired optical cavity
of the optoelectronic device. In one embodiment, the layer 52 below
the top mirror layer may function as an etch stop layer. Then, a
second material layer 58 is provided over the top mirror layer. The
second material layer is preferably provided over both the etched
and non-etched regions of the top mirror layer, but may only be
provided over the non-etched regions, if desired. In this
embodiment, the regions labeled 56 in FIGS. 2-3 should have the
same cross-hatch pattern as layer 53, and the refractive index of
these regions should be less than the refractive index of layer
52.
[0027] Another illustrative approach for controlling transverse
modes of an optoelectronic device is shown in FIG. 4. FIG. 4 is a
schematic cross-sectional side view of a planar, current-guided,
GaAs/AlGaAs top surface emitting vertical cavity laser, as in FIG.
1, with another illustrative top mounted mode control resonant
reflector 70. In this embodiment, the resonant reflector 70 is
formed by etching down into but not all the way through one or more
of the top mirror layers 72 of the optoelectronic device. The
etched region, generally shown at 74, preferably circumscribes the
desired optical cavity of the optoelectronic device, and has a
depth that causes a phase shift that reduces the reflectivity of
the resonant reflector 70 at the desired operating wavelength, such
as a depth that corresponds to an odd multiple of .lambda./4. To
provide further differentiation, a cap mirror 76 having one or more
additional layers may be provided on selected non-patterned regions
78 of the top mirror layer 72, such as over the desired optical
cavity of the optoelectronic device. The cap mirror 70 may include
one or more periods of a conventional semiconductor DBR mirror, or
more preferably, a narrow band dielectric reflection filter. A
metal layer may be provided on selected regions of the top mirror
layer 72. The metal layer may function as a top contact layer.
[0028] In forming the resonant reflector 70, and referring now to
FIGS. 5A-5B, a top mirror layer 72 (or some other top layer) is
patterned and etched to form one or more etched regions 74. The
etched regions 74 are preferably formed to circumscribe the desired
optical cavity of the optoelectronic device. Also, the etched
regions 74 are preferably etched down to a depth that causes a
phase shift in the incident light, thereby reducing the
reflectivity of the resonant reflector 70 in those regions.
[0029] Next, and as shown in FIG. 5C, a cap mirror 76 is formed on
the patterned top mirror layer 72. As indicated above, the top
mirror layer 72 may include one or more semiconductor DBR mirror
periods, and/or a narrow band dielectric reflection filter. In
either case, and to provide further differentiation in
reflectivity, the cap mirror 76 may be etched away except on those
areas that correspond to the desired optical cavity of the
optoelectronic device. This is shown in FIG. 5D. Alternatively, the
patterned cap mirror 76 may be formed using well known lift-off
techniques. Thereafter, a contact layer 80 may be provided on the
selected regions of the top mirror layer 72. The contact layer 80
may provide electrical contact to the top mirror layer 72.
[0030] Another illustrative approach for controlling transverse
modes of an optoelectronic device is shown in FIG. 6. FIG. 6 is a
schematic cross-sectional side view of a planar, current-guided,
GaAs/AlGaAs top surface emitting vertical cavity laser, as in FIG.
1, with yet another illustrative top mounted mode control resonant
reflector 90. In this illustrative embodiment, the resonant
reflector 90 has a refractive index that does not abruptly change
across the optical cavity of the optoelectronic device.
[0031] In a preferred embodiment, the resonant reflector 90 has at
least one resonant reflector layer 92 that has a refractive index.
The refractive index may include, for example, contributions from
both a first material 94 having a first refractive index and a
second material 96 having a second refractive index. In a preferred
embodiment, the first material 94 is confined to a first region and
the second material is confined to a second region, wherein the
first region and the second region co-extend along an interface 98.
By making the interface 98 non-parallel with the optical axis 100
of the optoelectronic device, the refractive index of the resonant
reflector layer, at least when viewed laterally along the optical
cavity of the optoelectronic device, does not change abruptly
across the optical cavity. Rather, there is a smooth transition
from one refractive index to another. This reduces the diffraction
effects caused by abrupt changes in the refraction index. It is
contemplated that one or more periods of a conventional
semiconductor DBR mirror, or more preferably, a narrow band
dielectric reflection filter 106, may be provided on top of the
resonant reflector 90, as shown. Finally, a contact layer 102 may
be provided around the periphery of the optical cavity. In the
embodiment shown, the contact layer 102 is in direct contact with
the top mirror layer 104 and provides electrical contact to the top
mirror layer 104.
[0032] The smooth transition from one refractive index to another
is further illustrated in chart 110. The X axis of chart 110
represents the lateral position along the optical cavity of the
device shown above the chart. The Y axis of chart 110 corresponds
to the reflectivity "R" of the top mirror, including the resonant
reflector 90 and conventional semiconductor DBR mirror or
dielectric reflection filter 106. The reflectivity "R" of the top
mirror, at least in the region of the resonant reflector 90, is
dependent on the refractive index of the resonant reflector layer
90.
[0033] Traveling from left to right laterally along the optical
cavity of the optoelectronic device, the reflectivity starts at a
first value 112. The first value 112 is shown relatively low
because the resonant reflector 90 and the conventional
semiconductor DBR mirror or dielectric reflection filter 106 do not
extend out into this region. The contact layer 102 may also
decrease the reflectivity in this region.
[0034] At the edge of the resonant reflector 90, the reflectivity
increases to a value 114, which includes contributions from the
second material 96 of the resonant reflector layer 90 and the
conventional semiconductor DBR mirror or dielectric reflection
filter 106. Moving further to the right, the refractive index of
the resonant reflector 90 begins to change due to the relative
contributions of the first material 94 and the second material 96.
This causes the reflectivity of the resonant reflector 90 to
smoothly increase toward the center of the desired optical cavity,
as shown. Preferably, the reflectivity of the resonant reflector 90
reaches a maximum 116 in or around the center of the desired
optical cavity. The reflectivity of the resonant reflector 90 then
decreases to the right of the center of the desired optical cavity,
in an opposite manner to that described above. As can be seen, the
refractive index, and thus the reflectivity, of the resonant
reflector 90 does not change abruptly across the optical cavity.
Rather, there is a smooth transition from one refractive index to
another. This reduces the diffraction effects often caused by
abrupt changes in the refraction index of a resonant reflector.
[0035] FIGS. 7A-7D are schematic cross-sectional side views showing
a first illustrative method for making the resonant reflector of
FIG. 6. In this illustrative embodiment, a first substantially
planar layer of material 94 is provided on, for example, a top
mirror layer 104 of a conventional DBR mirror. The top mirror layer
104 preferably has a refractive index that is higher than the
refractive index of the first layer of material 94. The top mirror
layer 104 may be, for example, AlGaAs, and the first layer of
material 94 may be, for example, TiO2, Si3N4, or a polymer such as
polyamide or Benzocyclobuthene (BCB).
[0036] The first layer of material is then patterned, as shown in
FIG. 7A. This is typically done using a conventional etch process.
As shown in FIG. 7B, the patterned first layer of material 104 is
then heated, which causes it to reflow. This results in a
non-planar top surface 98. Then, and as shown in FIG. 7C, a second
layer of material 96 is provided over the first layer of material
94. The top surface 105 of the second layer of material 96 is
preferably substantially planar, but it may be non-planar if
desired. The second layer of material 96 preferably has a
refractive index that is lower than the refractive index of the
first layer of material 94. The second layer of material 96 may be,
for example, SiO2, a polymer, or any other suitable material. When
desired, the top surface 105 of the second layer of material 96 may
be planarized using any suitable method including, for example,
reflowing the second layer of material 96, mechanical, chemical or
chemical-mechanical polishing (CMP) the second layer of material
96, etc. In some embodiments, the top surface 105 is left
non-planar.
[0037] The second layer of material 96 is preferably provided over
the entire top surface of the resonant reflector, and etched away
in those regions where a top contact 102 is desired. Once the
second layer of material 96 is etched, a contact layer 102 is
provided on the exposed regions of the top mirror layer 104. The
contact layer 102 provides electrical contact to the top mirror
layer 104. As shown in FIG. 7D, a cap mirror 106 may then be
provided above the second layer of material 96. The cap mirror 106
may include one or more periods of a conventional semiconductor DBR
mirror, or more preferably, a narrow band dielectric reflection
filter.
[0038] FIGS. 8A-8E are schematic cross-sectional side views showing
another illustrative method for making the resonant reflector of
FIG. 6. In this illustrative embodiment, and as shown in FIG. 8A, a
first substantially planar layer of material 94 is provided on, for
example, a top mirror layer 104 of a conventional DBR mirror. The
top mirror layer 104 preferably has a refractive index that is
higher than the refractive index of the first layer of material 94.
The top mirror layer 104 may be, for example, AlGaAs, and the first
layer of material 94 may be, for example, TiO2, Si3N4, or any other
suitable material. Next, a photoresist layer 110 is provided and
patterned on the first layer of material 94, preferably forming an
island of photoresist above the desired optical cavity of the
optoelectronic device.
[0039] As shown in FIG. 8B, the photoresist layer 110 is then
heated, causing it to reflow. This results in a non-planar top
surface on the photoresist layer 110. That is, the top surface of
the photoresist layer 110 may have portions that taper down toward
the first layer of material 94. Next, and as shown in FIG. 8C, the
photoresist layer 110 and the first layer of material 94 are etched
for a specified period of time. The etchant preferably selectively
etches both the photoresist layer 110 and the first layer of
material 94. This transfers the shape of the non-planar top surface
of the photoresist layer 110 to the first layer of material 94.
[0040] As shown in FIG. 8D, a second layer of material 96 is then
provided over the first layer of material 94. The second layer of
material 96 preferably has a refractive index that is less than the
refractive index of the first layer of material 94. The second
layer of material 96 is preferably provided over the entire top
surface of the resonant reflector, and etched away in those regions
where a top contact 102 is desired. Once the second layer of
material 96 is etched, a contact layer 102 is provided on the
exposed regions of the top mirror layer 104. The contact layer 102
provides electrical contact to the top mirror layer 104.
Preferably, the top surface of the second layer of material 96 is
substantially planar. As shown in FIG. 8E, a cap mirror 106 may be
provided above the second layer of material 96, if desired. The cap
mirror 106 may include one or more periods of a conventional
semiconductor DBR mirror, or more preferably, a narrow band
dielectric reflection filter.
[0041] FIGS. 9A-9E are schematic cross-sectional side views showing
yet another illustrative method for making the resonant reflector
of FIG. 6. In this illustrative embodiment, and as shown in FIG.
9A, a first substantially planar layer of material 94 is provided
on, for example, a top mirror layer 104 of a conventional DBR
mirror. Like above, the top mirror layer 104 preferably has a
refractive index that is higher than the refractive index of the
first layer of material 94.
[0042] Next, and as shown in FIG. 9B, the first layer of material
94 is patterned, preferably forming an island above the desired
optical cavity of the optoelectronic device. This results in the
first layer of material 94 having lateral surfaces 118 that extend
up to a top surface 116 that is defined by top peripheral edges
120. A photoresist layer 114 is then provided over the patterned
first layer of material 94, including over the lateral surfaces
118, the top peripheral edges 120 and the top surface 116. Because
of the step between the top surface 116 and the lateral surfaces
118, the photoresist layer 114 is thinner near the top peripheral
edges 120 than along either the lateral surfaces 118 or top surface
116 of the patterned first layer of material 94, as shown.
[0043] As shown in FIG. 9C, the photoresist layer 114 and the first
layer of material 94 are then etched for a specified period of
time. During this etch step, those regions of the first layer of
material 94 that are adjacent the thinner regions of the
photoresist layer 114 are subject to the etchant for a longer
period of time than those regions that are adjacent thicker regions
of the photoresist layer 114. Thus, and as shown in FIG. 9C, the
top peripheral edges 120 of the first layer of material 94 are
etched more than those regions away from the top peripheral edges
120, resulting in tapered edges 122.
[0044] After the etching step, and as shown in FIG. 9D, a second
layer of material 96 may be provided over the first layer of
material 94. Like above, the second layer of material 96 preferably
has a refractive index that is less than the refractive index of
the first layer of material 94. The second layer of material 96 is
preferably provided over the entire top surface of the resonant
reflector, and etched away in those regions where a top contact 102
is desired. Once the second layer of material 96 is etched, a
contact layer 102 is provided on the exposed regions of the top
mirror layer 104. The contact layer 102 provides electrical contact
to the top mirror layer 104. Preferably, the top surface of the
second layer of material 96 is substantially planar.
[0045] As shown in FIG. 9E, a cap mirror 106 may be provided above
the second layer of material 96, if desired. The cap mirror 106 may
include one or more periods of a conventional semiconductor DBR
mirror, or more preferably, a narrow band dielectric reflection
filter.
[0046] FIG. 10 is a schematic cross-sectional side view showing
another illustrative embodiment of the present invention similar to
that shown in FIGS. 5A-5D. In this embodiment, a top layer 110,
which may be the top mirror layer of the top DBR mirror stack of
the optoelectronic device or an additional layer on top of the top
mirror layer, may be etched down--but not all the way through. The
etched region preferably circumscribes the desired optical cavity
of the optoelectronic device, and has a depth that causes a phase
shift that reduces the reflectivity of the resonant reflector at
the desired operating wavelength, such as a depth that corresponds
to an odd multiple of .lambda./4. Like in FIGS. 5A-5D, a cap mirror
112 having one or more additional layers may be provided on
selected non-patterned regions of layer 110, such as over the
desired optical cavity of the optoelectronic device, to provide
further differentiation in reflectivity. A metal layer 114 may then
be provided on the etched region of layer 110. The metal layer may
function as the top contact. By extending the metal layer 114 all
the way or near the cap mirror 112, better current spreading can be
achieved for the optoelectronic device.
[0047] FIG. 11 is a schematic cross-sectional side view showing
another illustrative embodiment of the present invention similar to
that shown in FIGS. 5A-5D. This embodiment is similar to that of
FIG. 10, but the metal layer, now labeled 116, extends over the
etched region of layer 110 and over the cap mirror 112. For back
illumination devices, this may provide even better current
spreading for the optoelectronic device.
[0048] FIG. 12 is a schematic cross-sectional side view showing
another illustrative embodiment of the present invention similar to
that shown in FIGS. 8A-8E. A resonant reflector is provided that
has a refractive index that does not change abruptly across the
optical cavity of the optoelectronic device. The illustrative
resonant reflector includes at least one resonant reflector layer
that has a refractive index that includes contributions from, for
example, both a first material 120 having a first refractive index
and a second material 122 having a second refractive index. In the
embodiment shown, the first material 120 is confined to a first
region and the second material 122 is confined to a second region,
wherein the first region and the second region co-extend along an
interface. A metal layer 124 is then provided over the entire
structure. For back illumination devices, the metal layer 124 may
provide enhanced current spreading when compared to the device
shown in FIGS. 8A-8E.
[0049] Having thus described the preferred embodiments of the
present invention, those of skill in the art will readily
appreciate that the teachings found herein may be applied to yet
other embodiments within the scope of the claims hereto
attached.
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