InAs Quantum Dot Laser Diodes: Structure, Characteristics, Temperature Dependence and Future Directi

1
InAs Quantum Dot Laser Diodes Structure,
Characteristics, Temperature Dependence and
Future Directions.
V. Tokranov, A. Katsnelson, K. Dovidenko, M.
Yakimov, R. Todt, and S. Oktyabrsky School of
Nanoscience and Materials UAlbany Institute for
Materials University of Albany - SUNY

• Highlights
• Introduction
• Experimental details of growth structures
• Results of AFM, TEM, PL EL measurements
• Conclusions and future directions

2
Abstract
One of the goals of the study self-assembled
quantum dots (QDs) is the development of the
active medium for laser diodes operating at
elevated temperatures. We have investigated the
influence of 2 monolayers (ML) AlAs under- and
overlayers on the formation of InAs QDs using
transmission electron microscopy (TEM), atomic
force microscopy (AFM), photoluminescence (PL)
and electroluminescence (EL) to achieve high
internal quantum efficiency and temperature
stability of the active medium. Single sheets of
InAs QDs with 1.9-2.4ML average coverage were
grown on GaAs (001) substrate by molecular beam
epitaxy using 4750C growth temperature inside a
2ML/8ML - AlAs/GaAs short-period superlattice
with various combinations of under- and
overlayers. We have found from TEM and AFM
measurements, that InAs QDs with GaAs underlayer
and 2ML AlAs overlayer exhibit the lowest QD
surface density of 4.21010 cm-2 and the largest
QD lateral size of about 19 nm as compared to the
other combinations of cladding layers. This 2.4ML
InAs QD ensemble has also shown the highest room
temperature PL intensity with a peak at 1.21µm
and the narrowest linewidth, 34meV. The optical
properties of the QD PL structures and EL
characteristics of the QD laser structures were
also compared with those of InGaAs quantum well
(QW) structures generally used as a laser active
medium. Thermal quenching of the PL intensity
from 77 K to 420 K was found to be 30 for QDs vs.
5000 for QW. Successfully fabricated 1.9 ML InAs
QDs with AlAs overlayer edge-emitting broad-area
lasers (length 1000µm and width 50 µm) with
cleaved mirrors demonstrated 210 A/cm2 threshold
current density and 1010 nm emission wavelength,
that were similar to the characteristics of 90 ?
In0.21Ga0.79As QW lasers (170A/cm2 and 1009 nm).
QDs lasers have also shown a higher thermal
stability of CW threshold current density than QW
lasers for the temperatures up to 100oC. Next
step will be the realization of this QDs
advantages in VCSEL structures.
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Quantum Dots (QDs) Motivation
3D islands
15nm
5nm

• 3D quantum confinement of carriers
• discrete atomic-like electronic spectrum

Substrate
FWHM lt0.15 meV up to 60 K

• Atomic Force Microscopy (AFM)
• DI Nanoscope IIIa
• Room temperature, ex-situ, contact mode

Each peak corresponds to a single QD
M.Grundmann et al, Phys. Rev. Lett., 1996 V74
N20 pp.4043-4046

• almost no thermal broadening of electronic
  spectrum
• ?
• superior active medium for semiconductor lasers
  and other devices

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Physical Advantages of QD Lasers
Potentially lower threshold current and higher
efficiency
Excellent thermal stability
Excellent modulation characteristics
(Bimberg et al. 1998)
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Experimental Details- Growth

• EPI GEN II Molecular Beam Epitaxy (MBE) system
• In flux calibrated for InGaAs (5-20 In) using
  Reflection High Energy Electron Diffraction
  (RHEED) oscillations, adjusted using Beam Flux
  Monitor (BFM). InAs growth rate 0.05 ML/s. As2
  flux As2/In 10
• Temperature readouts of thermocouple and
  pyrometer were calibrated using 2x2?2x4
  transition in RHEED pattern (480oC). Growth
  temperature 450-5000C
• in situ process monitoring by 10 keV RHEED system
• QD layer(s) imbedded into 2ML-AlAs/8ML-GaAs short
  period superlattice (SPSL)

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Band-gap structure of QDs QW samples

• AFM samples with d1,d2,d3 0. No any doping.
• TEM PL samples d120nm, d210nm, d35nm. No
  any doping.
• Edge-emitting laser structures d10.25?m,
  d21.0?m, d30.4?m. n?51017 cm-3ltSigt, n?31018
  cm-3ltSigt, p?51017 cm-3 ltBegt, p?31019 cm-3
  ltBegt.

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Quantum Well - Based VCSEL Arrays current state
of art

• VCSEL Emission (room temperature)
• l 847, 985
• Threshold current 0.8 mA (110 A/cm2)

• 8x8 array of VCSELs
• Pitch 100 mm
• Aperture 15 mm

Intensity , a.u.
VCSEL cross-section
830
840
850
860
870
Wavelength, nm
(Oktyabrsky et al., Workshop on Interconnections
Within High Speed Digital Systems 2000)
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VCSEL Array Tests Two Wavelength Integration
l850 nm, l980 nm
Bond pad
Aperture
Packaged VCSEL Arrays
Two, 1 x 16 Linear Arrays

• Sub-mA threshold current
• 100 micron spacing,
• 6-15 micron aperture
• 1 mW output power

Expanded View
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Transmission electron microscopy (TEM) QDs
Imbedded into Short Period SL

• 200 keV FEG TEM (JEOL 2010 FEG)
• Plan-view and cross-sectional samples
• Preparation polishing, dimpling, ion-milling, Ar
  at 5 keV

Cross-Sectional high-resolution TEM Image of a
single QD
Cross-Sectional TEM Image of QD array (2.4ML
InAs)
(Oktyabrsky et al., MRS Proc. 2000)

• QD layers imbedded into short-period GaAs/AlAs
  superlattice
• Effective wide-bandgap material grown at low
  temperature
• Effective smoothening of the structure (low
  roughness of top DBR for VCSEL)
• Another variable to control emission wavelength

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TEM - Plan-view measurements Single QD Layer
Imbedded into GaAs/AlAs SPSL
QD sizes

• TEM of QDs grown at 4750C on AlAs
• small QDs 14 nm

QD density

• Vertical error bars correspond to QD size
  dispersion

(Oktyabrsky et al., MRS Proc. 2000)

• All results are for 2.4 ML of InAs
• Single sheets of InAs QDs were grown using 4750C
  growth temperature with various combinations of
  under- and overlayers.
• QDs with GaAs underlayer and 2ML AlAs overlayer
  exhibit the lowest QD surface density of 4.21010
  cm-2 and the largest QD lateral size of about 19
  nm as compared to the other combinations of
  cladding layers.

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Photoluminescence (PL). Single QD Layer Imbedded
into SPSL Optical Properties

• Excitation Ar-ion laser, ?514nm, 0.05-200W/cm2
• Temperature 77-430K
• Liquid-nitrogen-cooled Ge-detector with the
  standard lock-in technique

PL of 2.4ML InAs QDs Temperature 300K
Excitation intensity 10 W/cm2
PL peak energy and FWHM of QD (2.4 ML InAs in
SPSL) at RT with different combinations of under-
and overlayers

• PL peak of 2.4ML InAs shifts to lower energies
  with increasing of QD sizes.
• QD ensemble with 2ML AlAs overlayer has shown 10
  times higher room temperature PL intensity and 2
  times narrower FWHM than QDs with 2ML AlAs
  underlayer

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PL-Efficiency QDs QW vs. Excitation Intensity
T300K

• High PL-efficiency up to a very low excitation
  intensity (lt0.1W/cm2) for QD - faster radiative
  rate in QDs, suppression of excitation transport.
  
• Fast drop of PL-efficiency with decreasing of
  excitation intensity (lt5W/cm2) for QW.
• Lower (2-3 times) efficiency at high excitation
  level in case of QDs in comparison with QW - QD
  saturation at high excitation intensity.

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Thermal Quenching of Photoluminescence
Normalized PL Integral Intensity vs. Temperature
for 2.4ML InAs QDs and 90A In 0.21Ga0.79As
QW PL-Excitation 10W/cm2 Ar-laser, 514nm
Thermal quenching is reduced for large QDs capped
with 2 ML AlAs (19 nm, 4.2.1010 cm-2) PL
intensity drops (from 77 K to 420 K) 30 times
for QDs 5000 for QW
QD
QW
(Tokranov et al., MRS Proc. 2001)
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Electroluminescence (EL) of QDs QW
Edge-Emitting Laser

• Stripe lasers with stripes width from 5?m to
  200?m
• Stripe up laser crystal mounting on heatsink by
  In
• Laser measurements at Continuous Wave (CW) mode
  of operation

• Si-detector for laser power measurements
• Temperature 77-430K
• Liquid-nitrogen-cooled Ge-detector with the
  standard lock-in technique for spectral
  measurements

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Thermal Quenching of the Threshold Current
Threshold currents of edge-emitting lasers
(stripe 30 x 1000 ?m, CW, stripe up)
QDs
QW
(Tokranov et al., SPIE Proc 2001)
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Summary

• InAs QDs with GaAs underlayer and 2ML AlAs
  overlayer exhibit the lowest QD surface density
  of 4.21010 cm-2 and the largest QD lateral size
  about 19 nm
• Single sheet of QDs with 2ML AlAs overlayer
  imbedded into GaAs/AlAs SPSL allowed to achieve
  high internal quantum efficiency at room
  temperature at 1010 - 1210 nm
• Edge-emitting lasers with QDs in active medium
  have shown a higher thermal stability of CW
  threshold current density than QW lasers for the
  temperatures up to 100oC

Future Directions

• Evaluation of optical performance of 3D QD layers
  with high radiative efficiency and narrow size
  distribution (PL FWHM lt 30 meV) embedded into
  GaAs/AlAs SPSL to increase the effective bandgap
  of barrier material and suppress excitation
  transfer
• Demonstration of QD VCSELs with superior
  performance at elevated temperatures and higher
  modulation cut-off frequencies