Title: Long wavelength vertical cavity surface emitting laser
Abstract: Selectively oxidized vertical cavity lasers emitting at about 1290 nm using InGaAsN quantum wells that operate continuous wave below, at and above room temperature are reported. The lasers employ a semi-insulating GaAs substrate for reduced capacitance, high quality, low resistivity AlGaAs DBR mirror structures, and a strained active region based on InGaAsN. In addition, the design of the VCSEL reduces free carrier absorption of 1.3 μm light in the p-type materials by placing relatively higher p-type dopant concentrations near standing wave nulls.
Patent Number: 6,898,215 Issued on 05/24/2005 to Naone, et al.
| Inventors:
|
Naone; Ryan Likeke (Boulder, CO);
Jackson; Andrew W. (Boulder, CO);
Chirovsky; Leo M. F. (Superior, CO)
|
| Assignee:
|
Optical Communication Products, Inc. (Woodland Hills, CA)
|
| Appl. No.:
|
122707 |
| Filed:
|
April 11, 2002 |
| Current U.S. Class: |
372/4; 372/92; 372/96; 372/99 |
| Intern'l Class: |
H01S 003/30 |
| Field of Search: |
372/96,99,93,4,43-50,92
|
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| 5513204 | Apr., 1996 | Jayaraman.
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| 5557627 | Sep., 1996 | Schneider, Jr. et al.
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| 5606572 | Feb., 1997 | Swirhun et al.
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| 5719894 | Feb., 1998 | Jewell et al.
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| 5719895 | Feb., 1998 | Jewell et al.
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| 5805624 | Sep., 1998 | Yang et al.
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| 5825796 | Oct., 1998 | Jewell et al.
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| 5903589 | May., 1999 | Jewell.
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| 5912913 | Jun., 1999 | Kondow et al.
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| 5936266 | Aug., 1999 | Holonyak, Jr. et al.
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| 5960018 | Sep., 1999 | Jewell et al.
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| 6014395 | Jan., 2000 | Jewell.
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| 6052398 | Apr., 2000 | Brillouet et al.
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| 6570905 | May., 2003 | Ebeling.
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| 6720585 | Apr., 2004 | Wasserbauer et al.
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| 2001/0050934 | Dec., 2001 | Choquette et al.
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| Foreign Patent Documents |
| 0 822 630 | Feb., 1998 | EP.
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| HEI 7-154023 | Jun., 1995 | JP.
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| WO 9807218 | Feb., 1998 | WO.
| |
Other References
Lourdudoss, S, et al.; "Very rapid and selective epitaxy of InP around mesas
of height up to μm by hydride vapour phase epitaxy"; Mar. 27-30, 1994; InP
and related material, 1994 Conference Proceeding, Sixth International conference;
pp. 615-618.*
J. Boucart et al., 1-mW CW-RT Monolithic VCSEL at 1.55 μm, IEEE Photonics
Technology Letters, Jun. 1999, pp. 629-631, vol. II, No. 6.
Shiro Sakai et al., Band Gap Energy and Band Lineup of III-V Alloy Semiconductors
Incorporating Nitrogen and Boron, Jpn. J. Appl. Phys., Oct. 1993, pp. 4413-4416,
vol. 32 Part 1, No. 10.
Masahiko Kondow et al., Gas-Source molecular Beam Epitaxy of GaNxAs1-x
Using a N Radical as the N Source, Jpn. J. Appl. Phys., Aug. 1994, pp. 1056-1058,
vol. 33, Part 2, No. 8A.
Masahiko Kondow et al., Room-Temperature Pulsed Operation of GaInN As Laser Diodes
with Excellent High-Temperature Performance, Jpn. J. Appl. Phys., Nov. 1996, pp.
5711-5713, vol. 35, Part 1, No. 11.
Winston et al., Optoelectronic Device Simulation of Bragg Reflectors and Their
Influence on Surface Emitting Laser Characteristics, Department of Electrical
and Computer Engineering and the Optoelectronic Computing Systems Center University
of Colorado, Boulder, CO, 80309-0425, pp. 1-17 and 22-34.
Schneider, Jr. et al., Epitaxy of Vertical-Cavity Lasers, In Vertical-Cavity
Surface-Emitting Lasers, edited by C. Wilmsen, H. Tenkin, and L. Coldren, 1999,
pp. 137-139, Cambridge, UK: Cambridge University Press.
Lear K L et al. "High-frequency modulation of oxide-confined vertical cavity
surface emitting lasers" Electronics Letter, IEE Stevenage, GB, vol. 32, No. 5,
Feb. 29, 1996, pp. 457-458.
Ohnoki N et al. "Super-lattice AlAs/AlInAs for lateral-oxide current confinement
in InP-based lasers" Journal of Crystal Growth, North-Holland Publishing Co. Amsterdam,
NL, vol. 195, No. 1-4, Dec. 15, 1998, pp. 603-608.
|
Primary Examiner: Harvey; Min Sun
Assistant Examiner: Flores-Ruiz; Delma R.
Attorney, Agent or Firm: Barlow, Josephs & Holems, Ltd.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority of U.S. Provisional Patent application Ser.
No. 60/283,449, entitled "LONG WAVELENGTH VERTICAL CAVITY SURFACE EMITTING LASER"
filed on Apr. 11, 2001 and U.S. Provisional application Ser. No. 60/284,485, entitled
"LONG WAVELENGTH SURFACE EMITTING LASER ARRAY", filed on Apr. 17, 2001, and U.S.
Provisional Patent application Ser. No. 60/355,240, entitled "LONG WAVELENGTH VERTICAL
CAVITY SURFACE EMITTING LASER", filed Feb. 8, 2002 the contents of all of which
is incorporated herein by reference.
This application contains subject matter that is related to co-pending patent
application Ser. No. 09/996,009, filed Nov. 28, 2001.
Claims
1. A long wavelength vertical cavity surface emitting laser, comprising:
a semi-insulating substrate;
an undoped first mirror adjacent said semi-insulating substrate;
an optical cavity, comprising an active region having one or more In
xGa
1-xAs
yN
1-y
quantum wells, adjacent said first mirror;
a p-type second mirror adjacent the optical cavity, said p-type second mirror
having a doping scheme that minimizes optical losses due to free carrier absorption
at long wavelength emissions in the range of between 1200 nm to 1600 nm;
a p-type ohmic contact above said active region; and
an n-type intra-cavity contact below said active region,
said optical cavity and said first and second mirrors being configured and arranged
to provide a long wavelength emission in the range of between 1200 nm to 1600 nm.
2. The vertical cavity surface emitting laser of claim 1 wherein said optical
cavity further comprises an n-type contact stack between the active region and
said undoped first mirror wherein the n-type intra-cavity contact is electrically
coupled to said n-type contact stack.
3. The vertical cavity surface emitting inset of claim 2 wherein said n-type
contact stack comprises GaAs.
4. The vertical cavity surface emitting laser of claim 2 wherein said n-type
contact stack comprises one or more n-type doping spikes located at or near nulls
in standing wave intensity pattern of said vertical cavity surface emitting laser.
5. The vertical cavity surface emitting laser of claim 1 wherein fractional concentration
of In, x, in said one or more quantum wells ranges from about 0.3-0.4.
6. The vertical cavity surface emitting laser of claim 1 wherein fractional concentration
of Nitrogen, 1-y, in said one or more quantum wells ranges from greater than 0.01
to less than about 0.02.
7. The vertical cavity surface emitting laser of claim 1 wherein said p-type
second mirror comprises alternating layers of AlGaAs/GaAs.
8. The vertical cavity surface emitting laser of claim 7 wherein Al content in
the alternating AlGsAs layers of said p-type second mirror ranges from about 0.8-0.96.
9. The vertical cavity surface emitting laser of claim 7 wherein said p-type
second mirror comprises compositional grading of Al at heterojunction interfaces
between said alternating layers.
10. The vertical cavity surface emitting laser of claim 9 wherein said compositional
Al grading comprises a biparabolic grading of the Al across upward interface of
a lower GaAs mirror layer and an AlGaAs layer.
11. The vertical cavity surface emitting laser of claim 10 wherein said p-type
second mirror further comprises an n-type doping spike and a p-type doping spike
at layer edges of the biparabolic upward interface between said alternating GaAs
and AlGaAs layers.
12. The vertical cavity surface emitting laser of claim 9 wherein said compositional
Al grading comprises a parabolic grading of Al across downward interface between
an AlGaAs mirror layer and a GaAs layer.
13. The vertical cavity surface emitting laser of claim 12 wherein said p-type
second mirror further comprises a p-type doping spike on the downward parabolic
interface of the Al concentration.
14. The vertical cavity surface emitting laser of claim 1 further comprising
an oxide aperture formed above the active region by steam oxidation of an Al-containing
semiconductor oxide layer.
15. The vertical cavity surface emitting laser of claim 14 wherein said oxide
layer comprises AlAs.
16. The vertical cavity surface omitting laser of claim 14 wherein said oxide
layer comprises AlGaAs and wherein aluminum composition in said oxide layer is
greater than aluminum composition in semiconductor layers in the p-type second mirror.
17. The vertical cavity surface emitting laser of claim 14 further comprising
a current spreading layer adjacent the oxide aperture.
18. The vertical cavity surface emitting laser of claim 17 wherein the oxide
aperture and the current spreading layer are positioned at or near a node in optical
standing wave intensity pattern of the vertical cavity surface emitting laser.
19. The vertical cavity surface emitting laser of claim 14 further comprising
a p-type transition layer adjacent said oxide aperture having an aluminum composition
that is less than aluminum composition of said oxide layer to ensure that oxidized
portion of the oxide aperture maintains a predetermined oxide thickness.
20. The vertical cavity surface emitting laser of claim 1 wherein said first
mirror comprises alternating layers of un-doped binary pairs of AlAs and GaAs.
21. A long wavelength vertical cavity surface emitting laser, comprising:
a semi-insulating substrate;
an undoped first minor stack adjacent said semi-insulating substrate;
an optical cavity, comprising an active region having one or more In
xGa
1-xAs
yN
1-y
quantum wells, adjacent said first mirror stack;
a p-type intra-cavity contact above the active region;
a patternable dielectric mirror adjacent the optical cavity and p-type intra-cavity
contact; and
an n-type intra-cavity contact below the active region,
said optical cavity, said undoped first mirror stack and said patternable dielectric
mirror being configured and arranged to provide a long wavelength emission in the
range of between 1200 nm to 1600 nm.
22. The vertical cavity surface emitting laser of claim 21 wherein fractional
concentration of In, x, in said one or more quantum wells ranges from about 0.3-0.4.
23. The vertical cavity laser of claim 21 wherein fractional concentration of
Nitrogen, 1-y, in said one or more quantum wells ranges from greater than 0.01
to less than about 0.02.
24. The vertical cavity surface emitting laser of claim 21 wherein said optical
cavity further comprises a p-type contact stack sandwiched between the active region
and dielectric mirror wherein the p-type intra-cavity contact is electrically coupled
to said p-type contact stack.
25. The vertical cavity surface emitting laser of claim 24 wherein said p-type
contact stack comprises one or more p-type doping spikes.
26. The vertical cavity surface emitting laser of claim wherein said one or more
p-type doping spikes are located at or near nulls in standing wave intensity pattern
of said vertical cavity surface emitting laser.
27. The vertical cavity surface emitting laser of claim 21 wherein said optical
cavity further comprises an n-type contact stack sandwiched between said active
region and said undoped first mirror stack, and wherein said n-type intra-cavity
contact is electrically coupled to said n-type contact stack.
28. The vertical cavity surface emitting laser of claim 27 wherein said n-type
contact stack comprises one or more n-type doping spikes and wherein said one or
more n-type doping spikes are located at or near nulls in standing wave intensity
pattern of said vertical cavity surface emitting laser.
29. An array of long wavelength vertical cavity surface emitting lasers, comprising:
two or more long wavelength vertical cavity lasers monolithically formed at discrete
locations on a semi-insulating substrate,
wherein each of said two or more long wavelength vertical cavity surface emitting
lasers is optically arid electrically independent,
wherein each of said two or more long wavelength vertical cavity surface emitting
lasers comprise,
a semi-insulating substrate;
an undoped first minor adjacent said semi-insulating substrate;
an optical cavity, comprising an active region having one or more In
xGa
1-xAs
yN
1-y
quantum wells, adjacent said undoped first mirror stack;
a second mirror adjacent the optical cavity;
a p-type ohmic contact above said active region; and
an n-type intra-cavity contact below said active region.
30. The array of vertical cavity surface emitting lasers of claim 29 wherein
said optical cavity further comprises an n-type contact stack between the active
region and said undoped first mirror wherein the n-type intra-cavity contact is
electrically coupled to said n-type contact stack.
31. The array of vertical cavity surface emitting lasers of claim 30 wherein
said n-type contact stack comprises one or more n-type doping spikes located at
or near nulls in standing wave intensity pattern.
32. The array of vertical cavity surface emitting lasers of claim 29 wherein
fractional concentration of In, x, in said one or more quantum wells ranges from
about 0.3-0.4.
33. The array of vertical cavity surface emitting lasers of claim 29 wherein
fractional concentration of Nitrogen, 1-y, in said one or more quantum wells ranges
from greater than 0.01 to less than about 0.02.
34. The array of vertical cavity surface emitting lasers of claim 29 wherein
said second mirrors arc doped p-type end have a doping scheme that minimizes optical
losses due to free carrier absorption.
35. The array of vertical cavity surface emitting lasers of claim 34 wherein
said p-type second mirror comprises alternating layers of AlGaAs/GaAs.
36. The array of vertical cavity surface emitting lasers of claim 35 wherein
Al content in the alternating AlGaAs layers of said p-type second mirror ranges
from about 0.8-0.96.
37. The array of vertical cavity surface emitting laser of claim 35 wherein said
p-type second mirror comprises compositional grading of Al at heterojunction interfaces
between said alternating layers.
38. The array of vertical cavity surface emitting lasers of claim 37 wherein
said compositional Al grading comprises a biparabolic grading of the Al across
upward interface of a lower GaAs mirror layer and an AlGaAs layer.
39. The array of vertical cavity surface emitting lasers of claim 38 wherein
said p-type second minor further comprises an n-type doping spike and a p-type
doping spike at layer edges of the biparabolic upward interface between said alternating
GaAs and AlGaAs layers.
40. The array of vertical cavity surface emitting lasers of claim 37 wherein
said compositional Al grading comprises a parabolic grading of Al across downward
interface between an AlGaAs mirror layer and a GaAs layer.
41. The array of vertical cavity surface emitting lasers of claim 29 wherein
said second mirror comprises a dielectric mirror having alternating layers of a
first dielectric material having a first index of refraction and a second dielectric
material having a second index of refraction greater than, said first index of refraction.
42. The array of vertical cavity surface emitting lasers of claim 41 wherein
said p-type ohmic contact comprises a p-type intra-cavity contact above said active region.
43. The array of vertical cavity surface emitting lasers of claim wherein said
optical cavity further comprises a delta doped cladding layer and wherein said
p-type intra-cavity contact is adjacent said delta doped cladding layer.
44. The array of vertical cavity surface emitting lasers of claim 43 wherein
said optical cavity further comprises a highly doped p-type layer adjacent said
delta doped cladding layer and wherein said p-type intra-cavity contact is adjacent
said highly doped p-type layer.
45. The array of vertical cavity surface emitting lasers of claim 29 wherein
each of said two or more vertical cavity surface emitting laser further comprise
an oxide aperture formed above the active region by steam oxidation of an Al-containing
semiconductor oxide layer.
46. The array of said vertical cavity surface emitting laser of claim 45 wherein
said oxide layer comprises AlAs.
47. The vertical cavity surface emitting laser of claim 21 wherein comprising
an oxide aperture formed above the active region by steam oxidation of an Al-containing
semiconductor oxide layer.
48. The vertical cavity surface emitting laser of claim 47 wherein said oxide
layer comprises AlAs.
49. The vertical cavity surface emitting laser of claim 47 wherein said oxide
layer comprises AlGaAs and wherein aluminum composition in said oxide layer is
greater than aluminum composition in semiconductor layers in the p-type second mirror.
50. The vertical cavity surface emitting laser of claim 47 further comprising
a current spreading layer adjacent the oxide aperture.
51. The vertical cavity surface emitting laser of claim 50 wherein the oxide
aperture and the current spreading layer are positioned at or near a node in optical
standing wave intensity pattern of the vertical cavity surface emitting laser.
52. The vertical cavity surface emitting laser of claim 47 further comprising
a p-type transition layer adjacent said oxide aperture having an aluminum composition
that is less than aluminum composition of said oxide layer to ensure that oxidized
portion of the oxide aperture maintains a predetermined oxide thickness.
53. The vertical cavity surface emitting laser of claim 47 wherein said first
mirror comprises alternating layers of un-doped binary pairs of AlAs and GaAs.
Description
FIELD OF THE INVENTION
The present invention relates generally to vertical cavity surface emitting lasers
("VCSELs"), and more particularly, to VCSELs that emit light at a nominal wavelength
of 1.3 μm or higher.
BACKGROUND
Vertical cavity surface emitting lasers (VCSELs) emitting at 850 nm have
been widely and rapidly adopted into Gigabit Ethernet and other applications. Short
wavelength VCSELs are particularly suitable for multi-mode optical fiber local
area networks due to their reliability, reduced threshold current, circular output
beam, and inexpensive and high volume manufacture. However, there is strong interest
in developing VCSELs that emit at long wavelengths, such as in the 1240 nm to 1600
nm regime. VCSELs that emit at 1.3 μm, for example, may be used to leverage
high bandwidth single mode fiber that is often already installed as well as to
operate at the dispersion minimum of silica optical fiber.
SUMMARY OF THE INVENTION
An exemplary embodiment of the present invention provides a VCSEL that is suitable
for high speed optical communications at a nominal wavelength of 1.3 μm (and
longer) and transmission speeds up to 10 Gb/s over distances of up to 20 km or
more. The VCSEL is fabricated on a GaAs substrate, utilizes high quality AlGaAs
DBR mirror structures, and a strained active region based on InGaAsN.
In addition, the design of the VCSEL reduces free carrier absorption of 1.3 μm
light in the p-type materials by placing relatively higher p-type dopant concentrations
near standing wave nulls. Furthermore, in certain variations of the VCSEL, improvements
may be used to reduce the device resistance that is inherent in current approaches
to 1.3 μm VCSELs. In addition, a linear array of VCSEL devices may be formed
that provides substantially uniform light output at a given current and temperature.
An exemplary VCSEL array may therefore be integrated into parallel optical links
using standard MTP parallel connectors.
In one aspect of the present invention a vertical cavity surface emitting laser
includes an undoped first mirror adjacent a semi-insulating substrate, an optical
cavity, comprising an active region having one or more In
xGa
1-xAs
yN
1-y
quantum wells, adjacent the first mirror, a p-type second mirror adjacent the optical
cavity, a p-type ohmic contact above the optical cavity and an n-type intra-cavity
contact below the optical cavity.
In another aspect of the present invention a vertical cavity surface emitting
laser includes an n-type mirror comprising alternating layers of a first semiconductor
material having a first index of refraction and a second semiconductor material
having a second index of refraction greater than the first index of refraction
and a step graded interfacial transition layer there between, an optical cavity
comprising an active region having one or more In
xGa
1-xAs
yN
1-y
quantum wells adjacent the n-type mirror and a second mirror adjacent the optical cavity.
In a further aspect of the present invention a vertical cavity surface emitting
laser includes an undoped first mirror stack adjacent a semi-insulating substrate,
an optical cavity, comprising an active region having one or more In
xGa
1-xAs
yN
1-y
quantum wells, adjacent the first mirror stack, a p-type intra-cavity contact adjacent
the optical cavity, a dielectric mirror adjacent the optical cavity and p-type
intra-cavity contact and an n-type intra-cavity contact below the optical cavity.
In another aspect of the present invention an array of vertical cavity surface
emitting lasers includes two or more vertical cavity lasers monolithically formed
at discrete locations on a semi-insulating substrate, wherein each of the two or
more vertical cavity surface emitting lasers comprise an undoped first mirror adjacent
the semi-insulating substrate, an optical cavity, comprising an active region having
one or more In
xGa
1-xAs
yN
1-y quantum
wells, adjacent the undoped first mirror stack, a second mirror adjacent the optical
cavity, a p-type ohmic contact above the optical cavity and an n-type intra-cavity
contact below the optical cavity.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present invention
will become better understood with regard to the following description, appended
claims, and accompanying drawings, in which:
FIG. 1 is a cross-sectional view of an exemplary VCSEL structure in accordance
with an exemplary embodiment of the present invention;
FIG. 2 graphically illustrates the alloy compositions and doping levels of the
VCSEL of FIG. 1 including one period of the lower mirror stack adjacent the active
region, through one period of the upper mirror stack in accordance with an exemplary
embodiment of the present invention;
FIG. 3 graphically illustrates the alloy composition of that portion of the
VCSEL structure shown in FIG. 2, overlayed with the standing wave intensity profile
of the optical field as a function of vertical position within the VCSEL in accordance
with an exemplary embodiment of the present invention;
FIG. 4 graphically illustrates the alloy composition and doping levels of one
pair of an exemplary p-type upper mirror stack of the VCSEL of FIG. 1 in accordance
with an exemplary embodiment of the present invention;
FIG. 5 is a scanning electron micrograph of the VCSEL of FIG. 1 in accordance
with an exemplary embodiment of the present invention;
FIG. 6 is a mask diagram for forming the VCSEL of FIG. 1 in accordance with
an exemplary embodiment of the present invention;
FIGS. 7A-7J are cross-sectional views illustrating an exemplary process for
forming the VCSEL illustrated in FIG. 6 in accordance with an exemplary embodiment
of the present invention;
FIG. 8 is a light intensity diagram graphically illustrating the performance
of the VCSEL of FIG. 1 over temperature in accordance with an exemplary embodiment
of the present invention;
FIG. 9 graphically illustrates the lasing spectra of the VCSEL of FIG. 1 at
1289 nm with over 30 dB side mode suppression for both a 10 Gb/sec modulation using
a pseudo-random bit sequence and unmodulated in accordance with an exemplary embodiment
of the present invention;
FIG. 10 is a 10 GBit/sec Eye diagram of the VCSEL of FIG. 1 in accordance with
an exemplary embodiment of the present invention;
FIG. 11 is a cross sectional view of an optical cavity including a cavity extension
layer in accordance with an exemplary embodiment of the present invention;
FIGS. 12
a-12
d graphically illustrate asymmetric mirror
designs for extending the cavity in accordance with an exemplary embodiment of
the present invention;
FIG. 13 is a cross-sectional view of an alternate long wavelength VCSEL structure
in accordance with an exemplary embodiment of the present invention;
FIG. 14 graphically illustrates the alloy compositions and doping levels of
the VCSEL of FIG. 13 including a lower mirror period adjacent the active region,
through one period of the upper mirror stack in accordance with an exemplary embodiment
of the present invention;
FIG. 15 graphically illustrate the alloy composition and doping levels of an
exemplary n-type mirror stack of the VCSEL of FIG. 13 in accordance with an exemplary
embodiment of the present invention;
FIG. 16 graphically illustrates the alloy composition of that portion of the
VCSEL structure shown in FIG. 14, overlayed with the standing wave intensity profile
of the optical field as a function of vertical position within the VCSEL in accordance
with an exemplary embodiment of the present invention;
FIG. 17 is a cross-sectional view of a VCSEL having a p-type intra-cavity contact
and an n-type intra-cavity contact in accordance with an exemplary embodiment of
the present invention;
FIG. 18 is a cross-sectional view illustrating a method for contacting the VCSEL
of FIG. 17 in accordance with an exemplary embodiment of the present invention;
FIG. 19 is a cross-sectional view of a VCSEL utilizing an etch stop layer to
selectively remove a highly doped p-type contact stack from within the ohmic aperture
in accordance with an exemplary embodiment of the present invention;
FIG. 20 is a cross-sectional view of the VCSEL of FIG. 19 following exemplary
processing steps in accordance with an exemplary embodiment of the present invention;
FIG. 21 is a cross-sectional view of linear array of VCSEL devices in accordance
with an exemplary embodiment of the present invention;
FIG. 22 is a light intensity diagram graphically illustrating the performance
of a linear array of twelve VCSELs over temperature in accordance with an exemplary
embodiment of the present invention;
FIG. 23 is a cross-sectional view of a bottom emitting long wavelength VCSEL
having a low electrical resistance n-type lower mirror formed on a GaAs substrate; and
FIG. 24 is a side view, partly in cross-section, of an optical subassembly incorporating
the long wavelength VCSEL in accordance with an exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
An exemplary embodiment of the present invention provides a high speed, continuous
wave optoelectronic device that emits at a wavelength of 1200 nm and longer. An
exemplary light emitting diode comprises a layered structure formed from Group
III-V or II-VI compound semiconductor materials. In an exemplary embodiment the
light emitting diode comprises a vertical cavity surface emitting laser (VCSEL)
in which an optical cavity is normal to the p-n junction of the semiconductor wafer
from which it was fabricated.
One of skill in the art will appreciate that the design of an efficient VCSEL
for operation at a particular wavelength involves the balancing of a plurality
of constraints. For example, conventional VCSEL designs rely on a relatively small
active region volume within the optical cavity, to achieve a low threshold current.
Small active regions however, have relatively low optical gain so that VCSELs typically
require highly reflective optical mirrors above and below the optical cavity to
achieve lasing. The upper and lower mirrors, often distributed Bragg reflectors
(DBRs), may have their peak reflectivity at the emission wavelength of choice,
e.g. 1310 nm or 1550 nm.
The production of VCSELs emitting at 1240-1600 nm wavelengths also requires laser
quality active layer material that has strong light emission at a wavelength that
is appropriate for device emission at the desired wavelength. Active layer materials
for VCSELs that emit in the 1240 to 1600 nm region are presently the subject of
intense research and development. For example, InGaAsP active layers have demonstrated
excellent emission strength in the 1240-1600 nm region. However, InGaAsP has a
lattice constant that is more closely matched to InP than to other binary III-V
semiconductor substrates, for example, GaAs. Thus, many of the lasers emitting
at 1240-1600 nm to date have been grown on InP substrates to minimize defects that
may degrade the light emission performance or reliability of the device.
However, DBRs formed from the InP based material systems typically have
a relatively low index contrast. Therefore, a very large number of layer pairs,
up to sixty periods, may be required to construct InP based DBRs with sufficient
reflectivity. Disadvantageously, if a large number of pairs is used in the DBR
it becomes difficult to manufacture the mirror and, moreover, the yield of the
device deteriorates. In addition, device resistance and power consumption also
increase with increasing number of mirror periods. In addition, the thermal conductivity
of InP based DBR mirrors is relatively poor, resulting in devices with poor thermal properties.
Alternatively, high quality DBR mirrors that have a peak reflectivity
at long wavelengths such as, for example, 1310 nm and 1550 nm may be constructed
using an AlGaAs system grown on a GaAs substrate. However, until recently a suitable
high quality active layer material for use with the well-developed AlGaAs DBR technology
that is lattice matched to a GaAs substrate has not been produced. As a result,
it has been necessary to perform complicated processes such as wafer bonding to
take advantage of both the high quality InGaAsP active layer and the high quality
AlGaAs DBRS. However, the manufacture of such devices requires multiple complex
steps, including as many as three epitaxial growths, so that such devices are not
expected to be highly manufacturable.
More recently it has been shown that adding nitrogen to InGaAs, decreases the
peak transition energy and thereby increases the peak transition wavelength as
described by Kondow et al., in an article entitled "GaInNAs: A Novel Material for
Long-Wavelength-Range Laser Diodes with Excellent High-Temperature Performance,"
Jpn. J. Appl. Phys., vol. 35, pp. 1273-1275, February 1996. In addition, Kondow
et. al. have also shown that active layers formed from InGaAsN can be substantially
lattice-matched to a GaAs substrate.
However, a unique challenge for long wavelength VCSELs relative to 850 nm
VCSELs is that the optical absorption of the p-type doping required for a p-type
DBR mirror may be as much as ten times higher in the 1240-1600 nm range. The magnitude
of optical loss is further exaggerated for long wave devices by the fact that mirror
layers are inherently thicker at longer wavelengths than shorter wavelengths. Thicker
layers may also complicate the processing of such devices. For example, etch back
processes for longwave devices may be more difficult to accurately control due
to the increased depth of the etch.
Furthermore, the operating performance of a VCSEL (slope efficiency
and threshold) typically varies as a function of temperature. However, certain
variations of long wavelength VCSELs may suffer from excessive device resistance
and self heating that decrease the efficiency of the device and limit the long
term reliability of the device. Device resistance may result from voltage drops
across the lower mirror, upper mirror or both.
Therefore, an exemplary embodiment of the present invention may include
features to mitigate the impact of free carrier absorption in the p-type materials
incorporated in a typical VCSEL. Furthermore, an exemplary embodiment of the present
invention may further include additional features that reduce the device resistance
which is inherent in current longwave VCSEL designs and to provide high speed,
high power single mode operation.
Referring to FIG. 1, an exemplary light emitting device
10 is a
layered structure epitaxially-grown on a semiconductor substrate. In the described
exemplary embodiment, a lower mirror stack
14 is formed above a semi-insulating
substrate
12, such as for example GaAs, and an n-type contact stack
16
may be formed above the lower mirror stack
14 and below an active region
18 in an optical cavity
19. Further, an upper mirror stack
20
may be formed above the optical cavity
19. In the described exemplary embodiment
an oxide aperture
22 is formed between the active region
18 and the
upper mirror stack
20, completing the optical cavity
19 (which also
includes the n-type contact stack
16). The oxide aperture
22 may
comprise, for example, a low index layer of AlGaAs that is selectively oxidized
in part to provide electrical and optical confinement. In an exemplary embodiment
of the present invention the VCSEL layers are etched downward to an upper surface
of the n-type contact stack
16 forming a mesa to provide access to the oxidation
layers used to form the oxide aperture
22 and the n-type contact stack
16.
In the described exemplary embodiment the VCSEL may be contacted with a p-type
ohmic contact
26 formed above the upper mirror stack
20 and an n-type
intracavity contact
24 formed below the active region
18. In one
embodiment the p-type ohmic contact is deposited before the formation of the mesa
and patterned by an etching or lift off step. In this embodiment the p-type ohmic
contact
26 may either be protected by an overlying layer deposited and patterned
for use as an etch mask, or the upper electrode may form at least a part of the
etch mask. The p-type ohmic contact may be formed, for example, by depositing a
p-type metalization, such as gold with 2% beryllium added or a layered structure
of titanium/platinum above the upper mirror stack defining an annular opening therein
by a lithographic masking and lift-off process. The p-type ohmic contact may be
deposited for example, by electron beam evaporation. In one embodiment the n-type
intra-cavity contact
24 may be formed, for example, by depositing an n-type
metalization such as AuGe/Ni/Au on the upper surface of the n-type contact stack
16.
In an exemplary embodiment, the annular opening formed through the p-type ohmic
contact
26 is generally sized to be at least as large in diameter as the
oxide-free portion
28 of the oxide aperture
22, but smaller in diameter
than the top surface of the mesa. In this way, light may be efficiently coupled
out from the light-emitting device
10 through the central opening while
allowing the electrical current to be efficiently coupled from the p-type ohmic
contact
26 into the upper mirror stack
20, and therefrom to the active
region
18.
FIG. 2 graphically illustrates the alloy compositions and doping levels of the
described exemplary light emitting device including the first period of the lower
mirror stack adjacent to the active region, through the first period of the upper
mirror stack adjacent to the oxide aperture. In an exemplary embodiment, the upper
mirror stack may be formed from quarter-wavelength-thick alternating layers of
AlGaAs/GaAs for operation at a wavelength near 1.3 μm. One of skill in the
art will appreciate that the Al content in the AlGaAs upper mirror stack may vary
in the range of about 0.8-0.96. Further, the upper limit of the Al fraction in
the upper mirror stack may be determined by the Al composition of the alloy used
to form the oxide aperture.
In the described exemplary embodiment the lower mirror stack may comprise alternating
layers of un-doped binary pairs of AlAs
100 and GaAs
102 with abrupt
interfaces at the layer edges. Advantageously, the utilization of binary pairs
having a 100% concentration of Al reduces the overall thermal impedance of the
lower mirror stack. In addition, the utilization of un-doped mirror pairs reduces
the optical loss of the lower mirror stack as compared to conductive or doped mirror pairs.
In the described exemplary embodiment, the upper mirror stack is doped p-type.
In an exemplary embodiment, the upper mirror stack may comprise on the order of
twenty four mirror periods and the lower mirror stack may comprise on the order
of thirty mirror periods. The upper and lower mirror stacks are highly reflective,
having greater than 99% reflectivity. Conventionally, highly reflective DBRs have
the disadvantage of being highly resistive with significant levels of self heating
that may impair the performance of the device.
For example, the operating performance of a VCSEL (slope efficiency and threshold)
typically varies as a function of temperature. In addition, long term laser reliability
may also be compromised in high resistivity devices. Therefore, in the described
exemplary embodiment, the upper and lower mirror stacks are designed to reduce
the voltage drop as well as the loss or absorption associated with conducting current
into the active region of the device.
For example, an exemplary embodiment of the present invention includes an n-type
intra-cavity contact (See FIG. 1) for injecting electrons into the optical cavity.
The efficiency of the described exemplary VCSEL is increased by having the n-type
intra-cavity contact above the lower mirror stack, which reduces the series voltage
across the VCSEL by avoiding conduction through the lower mirror stack. In one
embodiment the n-type intra-cavity contact is coupled to the n-type contact stack
16. In an exemplary embodiment, the n-type contact stack
16 may comprise
a GaAs layer doped with a suitable n-type dopant such as, for example, silicon.
The n-type contact stack may include a constant doped region
104 with a
concentration in the range of about 5×10
17 cm
-3. In
addition, in an exemplary embodiment the n-type contact stack may further include
one or more n-type doping spikes
106(
a-c) having a concentration
in the range of about 5×10
18 cm
-3 to reduce the lateral
resistance created by the electrical connection.
FIG. 3 displays the alloy composition of that portion of the VCSEL structure
shown in FIG. 2, overlayed with the standing wave intensity profile of the optical
field as a function of vertical position within the VCSEL. The standing wave intensity
profile is related to the intensity of the light in the VCSEL. Hence, the standing
wave maxima are where the circulating light in the cavity is most intense, and
the standing wave minima are where the light is least intense. Light is more readily
absorbed by high doped semiconductor materials and less absorbed by low doped materials.
Therefore, in the described exemplary embodiment, the heavily doped n-type spikes
106(
a-c) in the n-type contact stack (see FIG. 2) may be positioned
at a node or minimum
150(
a-c) in the standing wave intensity pattern
of the VCSEL structure to reduce the loss associated with these regions.
Referring back to FIG. 2, the optical cavity may include an active region
108 having one or more undoped In
yGa
1-yAs
1-xN
x
quantum wells
110 (also known as active layers) separated by barrier layers
and sandwiched between two carrier confinement layers or separate confinement heterostructures
(SCHs). There are a variety of material compositions that may be utilized as the
active layer within the active region. However, the proper selection of an active
layer material preferably balances the gain requirements of the material with the
mechanical stability of the device.
For example, increasing the indium concentration in the quantum wells tends to
increase the emission wavelength but also increases the strain in the quantum well
layers, thereby necessitating a reduction in quantum well thickness to avoid stress
induced dislocations. However, reducing the well thickness also leads to a reduction
in the emission wavelength due to increased quantum confinement. Furthermore, increasing
the nitrogen concentration in the quantum well layers tends to increase the operating
wavelength, and further provides strain compensation for the indium. Adding too
much nitrogen, however, may reduce the optical quality of the active layer material.
Therefore, in an exemplary embodiment, the semiconductor alloy composition
of the InGaAsN quantum wells may be optimized to achieve emission at a nominal
wavelength of about 1.3 μm without exceeding the critical thickness of the
quantum wells. In the described exemplary embodiment concentration of nitrogen
within the quantum well layers is minimized to improve the optical quality of the
material. It has been found that a nitrogen concentration of greater than 1% and
less than about 2% provides substantially defect free, high optical quality material.
In an exemplary embodiment of the present invention the active region may comprise
three In
0.34Ga
0.66As
0.988N
0.012 quantum
wells. In the described exemplary embodiment each quantum well
110 may be
in the range of about 3-10 nm thick. The quantum wells may be separated by barrier
layers of undoped GaAs that are approximately eighty angstroms thick when separating
a pair of adjacent quantum wells. In accordance with an exemplary embodiment the
confinement layers may also be formed from GaAs and may be on the order of about
one hundred fifty to five hundred angstroms thick. In the described exemplary embodiment
the confinement layers are undoped to improve device reliability and to reduce
optical loss and current spreading. However, one of skill in the art will appreciate
that a portion of the confinement layers may be doped the same conductivity type
as the adjacent semiconductor layers i.e. the lower confinement layer may be n-type
and the upper confinement layer may be p-type in the described exemplary embodiment.
In the described exemplary embodiment, the barrier layers have an energy bandgap
intermediate between the energy bandgaps of the quantum-wells and the oxidation
aperture and lower mirror stack. The confinement layers may have an energy bandgap
equal to that of the barrier layers or intermediate between the energy bandgap
of the barrier layers and the oxidation aperture and lower mirror stack.
The quantum-wells provide quantum confinement of electrons and holes therein
to enhance recombination for the generation of light. The number and location of
quantum-wells may further provide means for increasing the optical gain. In the
described exemplary embodiment, the quantum-well layers
110 may be positioned
near an antinode or peak (i.e. maximum)
152 of the electric field of the
light in the optical cavity to increase the efficiency for light generation therein
(see FIG.
3). In the described exemplary embodiment the confinement layers
may have a semiconductor alloy composition that is uniform in the growth direction,
forming a separate confinement heterostructure (SCH) active region
108.
In an exemplary VCSEL structure, the oxide aperture may be formed above the optical
cavity by the steam oxidation of an Al-containing semiconductor layer. The oxidized
outer portion of the oxide aperture has increased resistivity providing lateral
current constriction to reduce the volume of the gain region. In the described
exemplary embodiment the current constriction formed by the oxidized portion of
the oxide aperture reduces the diameter of the current aperture below the diameter
formed by the VCSEL p-type ohmic contact. In addition, an oxide free central portion
of the oxide aperture remains substantially transmissive to light allowing for
the injection of current into the active region.
Conventionally, oxide apertures create a large index step that provides
a significant waveguiding effect in the transverse dimension. Therefore, conventional
oxide confined VCSELs typically have relatively small diameters, generally on the
order of about five microns or less, to ensure single-mode operation at wavelengths
above 1200 nm. In an exemplary embodiment the thickness of the oxide aperture may
be reduced and the aperture may be placed near a node in the standing wave intensity
pattern of the optical field to reduce the index step seen by the optical mode.
The reduced index step allows for the utilization of a larger diameter oxide aperture
while maintaining single-mode operation. The larger diameter oxide aperture further
lowers the resistance of the device.
In the described exemplary embodiment, the alloy composition and thickness of
the layers forming the oxide aperture are different from the composition and layer
thickness of any of the other compound semiconductor layers. As an example, the
oxide aperture layers may be formed from AlAs or from AlGaAs with an aluminum composition
higher than the aluminum composition of AlGaAs high-bandgap semiconductor layers
in the upper mirror stack. In the described exemplary embodiment, the semiconductor
layers forming the oxide aperture contain an Al concentration of about 98%. In
an exemplary embodiment the oxide aperture
120 may be doped with a dopant
type that is the same as the mirror layer immediately adjacent to the oxide aperture.
Thus, in the described exemplary embodiment the oxide aperture which is located
between the active region and the p-type upper mirror stack may be p-type with
a carbon dopant at a concentration of about 1×10
17 cm
-3 to
1×10
19 cm
-3.
At high currents oxide confined VCSELs tend to develop non-uniform current distribution
with "current crowding" at the edges of the high resistivity or oxidized portion
of the aperture. Therefore, an exemplary VCSEL structure may further comprise a
current spreading layer
122 above the oxide aperture. In the described exemplary
embodiment the current spreading layer
122 is highly conductive, comprising
a carbon doping spike at a concentration of about 1×10
20 cm
-3.
The current spreading layer provides a more uniform current distribution across
the oxide aperture improving current injection into the optical cavity and further
reducing the device resistance. Referring to FIG. 3, in the described exemplary
embodiment, the oxide aperture and the heavily doped p-type current spreading layer
are positioned at a node
154 in the standing wave intensity pattern of the
VCSEL structure to reduce the loss associated with the heavily doped regions and
to reduce the effective index step seen by the optical mode.
In an exemplary embodiment of the present invention, the oxidized outer portion
of the oxide aperture generally has an annular shape with the oxidation extending
inward from one or more etched sidewalls of the mesa. The lateral shape of the
annular oxidized portion of the oxide aperture is influenced by the semiconductor
alloy composition of the layers surrounding the oxide aperture. The selective oxidation
is due to a strong compositional dependence in the lateral oxidation of Al
xGa
1-xAs
layers, for x in the range of about 0.8 to 1.0.
Therefore, the described exemplary embodiment may include a p-type, AlGaAs
transition layer
124 having an aluminum composition of approximately 75%
to ensure that the oxidized portion of the aperture maintains the desired oxide
thickness which is determined by the thickness of the layers forming the oxide
aperture
120. The higher the aluminum content in layer
124, the more
non-uniform and thicker the lateral shape of the oxide becomes. A low aluminum
content poses a high energy barrier to hole flow from the transition layer (
124)
to the oxide aperture layer
120. Thus, there is a trade-off between control
of the oxide shape and electrical resistance.
FIG. 2 also illustrates the material composition of the first mirror pair of
an exemplary upper mirror, having a high index
130 GaAs layer adjacent to
the oxide aperture and current spreading layer. The highly doped p-type region
206 serves a similar purpose as the carbon doping spike shown in FIG.
4.
FIG. 4 graphically illustrates the structural details of an exemplary p-type upper
mirror from the center of one GaAs layer to the center of the next GaAs layer.
In the described exemplary embodiment, the mesa etched into the upper mirror stack
reduces the current carrying volume of the upper mirror stack, increasing the resistance
thereof with a consequent increase in self heating. VCSEL heating may be further
exaggerated in the upper mirror stack due to the current constriction. The oxide
aperture further confines the current flowing in the upper mirror stack so that
the current density in the constricted region is orders of magnitude higher than
it otherwise would be in an un-constricted mirror.
However, the series resistance of a DBR may be decreased by decreasing the
valence band discontinuity at the interface between the alternating layers of the
mirror stack. Therefore, an exemplary p-type upper mirror stack may utilize techniques,
such as compositional grading at the heterojunction interface between alternating
layer pairs to reduce the band-discontinuity and the resistance of the p-type upper
mirror stack.
For example, an exemplary p-type upper mirror may include compositional grading
of the Al concentration and doping across the heterojunction interface between
alternating layer pairs. An exemplary p-type upper mirror stack includes biparabolic
grading of the Al concentration
200 across the interface of a lower GaAs
mirror layer
210 and an AlGaAs layer
220. The biparabolic grading
of the Al concentration flattens the valence band by increasing the energy at the
bottom of the band and decreasing the energy at the top of the band.
Thus the biparabolic grading may decrease the valence band discontinuity at
the hetero-interface between a high index layer and a low index layer. The series
resistance is exponentially dependent on the magnitude of the band-discontinuity
of the valence band at the heterojunction and may also be significantly reduced.
In addition, the described exemplary p-type upper mirror stack may include a parabolic
grading
230 of the Al concentration across the downward interface between
an AlGaAs mirror layer and a GaAs layer. The parabolic grading improves the lateral
conductivity of the mirror stack reducing the overall resistivity of the device.
In an exemplary embodiment of the present invention the GaAs mirror layers
210
are p-type with a carbon dopant at a concentration in the range of about 2×10
17-2×10
18
cm
-3. However, as shown in FIG. 4 an exemplary p-type upper mirror
stack may include an n-type doping spike
240 and a p-type doping spike
250
at the layer edges of the biparabolic upward interface
200 between alternating
GaAs and AlGaAs layers. The n-type
240 and p-type
250 doping spikes
flatten the valence band, and further reduce the bandgap discontinuity across the
layer interface and therefore further improve the vertical conductivity of the
mirror stack.
In an exemplary embodiment of the present invention the n-type doping spike
240
may comprise a silicon dopant at a concentration in the range of about 2×10
17-2×10
18
cm
-3 with a preferred concentration of about 5×10
17cm
-3.
