High Speed Nanostructured Light Sources for Optical ChipLevel Interconnects

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High Speed Nanostructured Light Sources for
Optical Chip-Level Interconnects Tasks 4 and 5
Serge Oktyabrsky,
Collaborators Dr. Vadim Tokranov, Dr. Katharine
Dovidenko Mike Yakimov Alex Katsnelson, Matt
Lamberti Rene Todt
School of NanoSciences and NanoEngineering and
UAlbany Institute for Materials, University at
Albany - SUNY, Albany, NY
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Publications
S. Oktyabrsky, V. Tokranov, M. Yakimov, A.
Katsnelson, and K. Dovidenko. Vertical Stacks of
InAs Quantum Dots Embedded into Short-Period
AlAs/GaAs Superlattice. in Semiconductor Quantum
Dots, Mater. Res. Soc. Proc., 642 (2001)
P3.30.1-6. .M. Yakimov, V. Tokranov, and S.
Oktyabrsky. Dynamics of InAs Quantum Dots
Formation on AlAs and GaAs. in Growth, Evolution,
and Properties of Surfaces, Thin Films, and
Self-Organized Structures, Mater. Res. Soc.
Proc., 648 (2001) P2.6.1-6. .René Todt, K.
Dovidenko, A. Katsnelson, V. Tokranov, M.
Yakimov, and S. Oktyabrsky, Oxidation Kinetics
and Microstructure of Wet-Oxidized MBE-Grown
Short-Period AlGaAs. In Superlattices Progress
in Semiconductor Materials for Optoelectronic
Applications. Mater. Res. Soc. Proc., 692
(2002). V.Tokranov, M. Yakimov, A. Katsnelson, K.
Dovidenko, R. Todt, and S. Oktyabrsky, InAs
quantum dots in AlAs/GaAs short period
superlattices structure, optical characteristics
and laser diodes. In Progress in Semiconductor
Materials for Optoelectronic Applications. Mater.
Res. Soc. Proc., 692 (2002) .M.Yakimov,
K.Dovidenko, V.Tokranov, A. Katsnelson and S.
Oktyabrsky, InAs Quantum Dots Formation,
Evolution and Evaporation on GaAs and AlAs
Surfaces. In Current Issues in Heteroepitaxial
Growth - Stress Relaxation and Self-Assembly.
Mater. Res. Soc. Proc., 696 (2002) Serge
Oktyabrsky, James Castracane, and Alain
Kaloyeros, Emerging Technologies for Chip-Level
Optical Interconnects. Proc. SPIE, 4652
(2002). Vadim Tokranov, Michael Yakimov, Alex
Katsnelson, Katharine Dovidenko, Rene Todt, and
Serge Oktyabrsky, InAs Quantum Dot Laser Diodes
Structure, Characteristics and Temperature
Dependence. Proc. SPIE, 4656 (2002). .A.M.
Mintairov, P.A. Blaganov, O.V. Kovalenkov, C. Li,
J.L. Merz, S. Oktyabrsky, V. Tokranov, A.S.
Vlasov, D.A. Vinokurov, Mechanical interaction in
near-field spectroscopy of single semiconductor
quantum dots. in Materials and Devices for
Optoelectronics and Photonics. 722, Mater. Res.
Soc. Proc., (2002) K11.2. J. Zhu, M. Thaik, Y.
Yakimov, S. Oktyabrsky, A.E. Kaloyeros, and M.B.
Huang, Ion beam radiation effects on InAs
semiconductor quantum dots. In Progress in
Semiconductor Materials for Optoelectronic
Applications. Mater. Res. Soc. Proc., 692
(2002), H10.7
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Highlights

• Chip-level OIs Targets and Solutions
• Light Sources Challenges
• High Speed Semiconductor Lasers Status and
  Approaches (New Efforts)
• Nanoengineered QW and QD Structures for High
  Temperature VCSELs (Continuing Research)

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Optical Chip-to-Chip Interconnects Concept and
Components
8x8 VCSEL Array
GaAs Resonant Cavity p-i-n photodetector
Micromirror Array

• Demonstration of chip-level optical
  interconnect
  system using
• High frequency high operation temperature VCSEL
  light sources
• Stress free, alignment tolerant, hybrid
  integration scheme
• Resonant Cavity GaAs-based Photodetectors
• 3D reconfigurable, free-space, MEMS-based
  interconnect medium

Goal
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Optical Interconnects Minimum Target Parameters
Derived from Projection of ITRS-99/01
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Target Specifications for VCSEL Arrays for
Chip-level Interconnects

• Major Challenges
• High temperature
• High frequency
• High density
• High efficiency
• High reliability

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Characteristics of High Speed Semiconductor Lasers
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Direct Current Modulation
Relaxation frequency

• Relaxation oscillations
• Gain switching

AN - differential gain (Medium property) F0 -
photon fluence in the cavity tp - photon lifetime
Gain switching
Light-output relaxation
160 mW, 30 GHz
18.5 mW, 11.5 GHz
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Q-Switching
Mode-Locking

• Modulation of cavity losses using saturable
  absorber (passive) or MQW Stark effect absorber
  (active)
• Pulse trains with low jitter demonstrated
• Highest frequency is obtained in monolithic
  colliding pulse lasers (1.5 THz)
• Short pulses for clock distribution !
• VCSEL needs external cavity

• Modulation of cavity losses using saturable
  absorber (passive) or MQW Stark effect absorber
  (active)
• Short pulses (lt2 ps) with low jitter demonstrated
  
• No active Q-switching in VCSELs demonstrated

Q-switched VCSEL
Colliding pulse mode-locked laser
Hudgings, 99
Chen, 92
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High Speed Semiconductor Lasers General
Considerations

• Low device resistance
• Intracavity contacts
• Low device capacitance
• Proton insulation
• Low integration (parasitic) capacitance and
  inductance
• In-plane contacts
• Semi-insulating substrate

High-speed VCSEL (Test Platform)
Proton-implant
Oxide aperture
PMGI (reflowable polymer)
n-contact
p-contact
n-DBR

• Platform will be used for development of
• High speed VCSELs
• Q-switched VCSELs
• VCSELs with external modulator

• Series resistance ( d-2 ) 100 W
• Device capacitance ( d2 ) 0.05 pF
• Pad capacitance 0.05 pF

For 10x10 mm2 RC 10 ps
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High Frequency VCSEL Process Flow
1. p-contact formation Pt-Ti-Pt-Au contact is
patterned by lift-off process Alignment marks
are formed 2. Ion implantation High energy H2
ions are used 3. First mesa etch (below the
active layer of the device) Wet chemical etch
(dry etch can be used) 3a. Wet oxidation Actual
electrical aperture is defined Highest
temperature in the process flow (400-450 oC) 4.
N-contact Au-Ge-Ni-Au contact is patterned by
lift-off process. Preliminary measurements of
device performance can be done
VCSEL after n-contact deposition
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High Frequency VCSEL Process Flow (Continued)
VCSEL after PMGI reflow
5. Second mesa etch Structure etched down to
semi-insulating substrate Wet chemical etch (dry
etch can be used) 6. Slope for metal deposition
is formed by PMGI reflow PMGI is a DUV
patternable, flowable dielectric 7. Metallization

• Maximum process temperature 400-450oC (wet
  oxidation)
• Temperature limitations after processing
• 400 oC (As evaporation from GaAs surfaces)
• 335 oC (PMGI decomposition)
• 250-270 oC - PMGI reflow temperature
• PMGI related limitations can be lifted by using
  different dielectric (e.g. reflowable glass) or
  reinforcing the airbridge and removing PMGI

Top view of processed VCSEL
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Completed Device
Capacitance-voltage characteristic of completed
device (30x30 mm2)
p-contact
n-contact
Forward diode bias
p-contact
PMGI
n-DBR

• Devices were fabricated using technology
  described above
• Preliminary tests show technology fulfills the
  requirements for a test VCSEL structure

Oxide aperture
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Integration on Si (Details in the Poster )
p-contact
FIB image of Si/BCB/n-DBR interface
PMGI
n-contact
DBR
Ø75 ?m x 10 ?m device bonded on Si by BCB
BCB
BCB
Si
Si

• BCB for GaAs wafer bonding
• GaAs substrate removal by wet etching
• VCSEL processing on Si wafer
• PMGI reflow/planarization

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High Temperature VCSEL Development MQW Structures
3xMQWs in SPSL AlAs/GaAs 53x(2ML/6ML)

• Bandgap engineered MQW structures
• Wide bandgap barrier material
• to reduce thermal evaporation of carriers
• to increase separation between the ground and
  first excited levels

Threshold current and Efficiency and Lasing
spectra of 12 mm MQW VCSEL (l0.99 mm)
Bottom DBR
Top DBR
Tokranov, Oktyabrsky et. al. 2002
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Physical Advantages of QD Lasers
Excellent thermal stability
Evolution of threshold current density in
in-plane lasers
Excellent modulation characteristics
(From Zia Laser )
(Bimberg et al. 1998)
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Optical Properties of Single QD Layer Imbedded
into SPSL
Small QDs and wetting layer are dissolving and
substituted by AlAs
AlAs under layer
GaAs under layer
GaAs over layer
AlAs over layer
Tokranov, Oktyabrsky et. al. 2002

• PL peak of 2.4ML InAs shifts to lower energies
  with increasing of QD sizes
• QD ensemble with 2ML AlAs overlayer has shown
  highest room temperature PL intensity and lowest
  FWHM (38meV) in comparison with other designs

Yakimov, Tokranov, Oktyabrsky 2000
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Quantum Dot Active Layer
Multiple QD stacks to increase saturation gain
Comparison of PL of QW , QD and 3xQD structures
(room temperature) Top 1.5 W/cm2 (0.6 A/cm2
) Bottom 1 kW/cm2 (400 A/cm2 )

• Major Challenges
• High gain
• Low size distribution
• High density
• High radiative efficiency

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Quantum Dot Laser Medium

• Ground state 3 stack QD edge-emitting laser
• Jth110A/cm2 , l 1.23 mm
• Ground state maximum modal gain 6.6 cm-1 (vs. 3.5
  cm-1 )
• Small optical confinement factor (dw0.8?m)
• Max. lasing temperature 130 0C vs. 80 0C)

Evolution of threshold current density in
in-plane lasers
How to increase saturation gain?
Funded by NSF
Increase number of QD layers to 10
Control nucleation of QDs in-situ (using
interference of optical surface waves)
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Summary

• Status
• Developed test bench for high frequency VCSELs
• Demonstrated 200 0C operation in
  bandgap-engineered MQW VCSEL
• Enhanced high-temperature performance of QD gain
  medium (edge-emitting laser operates at 120 0C on
  the ground state).
• Bonding and processing protocol of III-V
  optoelectronic components was proposed and tested

• Challenge
• High frequency VCSEL light sources
• High operation temperature VCSEL light sources
• Hybrid integration

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Future Directions

• Continue development of nanoscale InAs quantum
  dot active medium for VCSELs with superior
  performance characteristics (threshold current,
  efficiency) at high temperatures (gt100 0C).
• Design, development and demonstration of VCSELs
  for direct high-frequency (gt10 GHz) modulation at
  high temperatures (gt100 0C)
• Development of GaAs-based monolithically
  integrated VCSEL - MQW Modulator (external or
  intracavity) component for high speed (gt40 GHz)
  performance with a design amenable to integration
  with Si platform.

Integrated VCSEL-Modulator
p-contact Modulator
MQW Modulator
PMGI
n-contact VCSEL
oxy-aperture
VCSEL MQW