The Walter Schottky Institute

1
MIOMD-VII Lancaster University, UK, 2005
Recent Progress on MIR Laser Diodes
M.- C. Amann, M. Grau, and A. Friedrich
Walter Schottky Institute Technical University
Munich Germany

• now with
• VERTILAS GmbH, Garching, Germany

2
Contents

• Introduction
• GaSb-based type-I laser diodes for wavelengths
  3µm
• Staircase-type quantum cascade lasersfor
  wavelength of 8-10µm
• Conclusion

3
Contents

• Introduction
• GaSb-based type-I laser diodes for wavelengths
  3µm
• Staircase-type quantum cascade lasersfor
  wavelength of 8-10µm
• Conclusion

4
Laser types and covered wavelength range
1. Introduction
5
Wavelength ranges for cw operation
1. Introduction
Room-temperature cw
6
Wavelength ranges for cw operation
1. Introduction
Room-temperature cw
no RT cw !
7
Some gas lines in 3-6µm range
1. Introduction
cw-laser diodes
InP-based lasers
Quantum cascade lasers
GaSb-based lasers
wavelength (µm)
8
Contents

• Introduction
• GaSb-based type-I laser diodes for wavelengths
  3µm
• Staircase-type quantum cascade lasersfor
  wavelength of 8-10µm
• Conclusion

9
Long wavelength GaSb-based lasers
GaSb-Lasers
sources for MIR cw operation _at_ RT GaSb-based
lasers or QCLs

• status of GaSb-based lasers
• high power lasers realized for wavelengths up to
  2.8 µm
• RT cw emission up to 3.1 µm
• pulsed RT operation at 3.16 µm

10
Bandstructure of a 3 µm MIR-Laser
11
Challenges for GaSb-based lasers gt 3 µm
GaSb-Lasers

• increase of Indium in GaInAsSb active material
  results in type-II band-alignment
• small valence band offset results in strong
  temperature dependece of performance (low T0, cw
  difficult for wavelengths gt 3 µm)
• strain in QWs helps, but critical thickness sets
  limits

12
Strategies for lasers beyond 3 µm
GaSb-Lasers

• improve heat managment of previously shown lasers
  with pulsed RT operation? gold heat spreader on
  top of ridge waveguide lasers
• improve band alignment for better hole
  confinement ? new barrier material quinternary
  AlGaInAsSb

13
Strategies for lasers beyond 3 µm
GaSb-Lasers

• improve heat managment of previously shown lasers
  with pulsed RT operation? gold heat spreader on
  top of ridge waveguide lasers
• improve band alignment for better hole
  confinement ? new barrier material quinternary
  AlGaInAsSb

14
Lasers with gold heat spreader
GaSb-Lasers
top view
cross section

• ridge waveguide lasers with electroplated gold
  heat spreader

15
Lasers with gold heat spreader
GaSb-Lasers
simulation of heat transfer with Quickfield
16
Lasers with gold heat spreader
GaSb-Lasers

• cw operation at RT
• wavelength 3.18 µm

17
Strategies for lasers beyond 3 µm
GaSb-Lasers

• improve heat managment of previously shown lasers
  with pulsed RT operation? gold heat spreader on
  top of ridge waveguide lasers
• improve band alignment for better hole
  confinement ? new barrier material quinternary
  AlGaInAsSb

18
New quinternary barrier material
GaSb-Lasers

• increased valence band offset with AlGaInAsSb
  barriers
• improved hole confinement in QWs
• reduction of conduction band offset, improves
  homogenity of electron injection in multiple
  quantum well structures

19
Structural quality of quinternary material
GaSb-Lasers

• good structural quality of MBE grown bulk
  AlGaInAsSb
• laser structure comprising compressivley strained
  GaInAsSb-QWs shows sharp satellite peaks in HRXRD
  measurements

20
Lasers with quinternary AlGaInAsSb barriers
GaSb-Lasers
21
3,26 µm - Laser with quinternary barriers

• Tmax pulsed 50C !

• Potential for cw _at_ RT

22
Contents

• Introduction
• GaSb-based type-I laser diodes for wavelengths
  3µm
• Staircase-type quantum cascade lasersfor
  wavelength of 8-10µm
• Conclusion

23
Concept of usual QC lasers
Staircase-Lasers
Quantum-cascade laser with injection miniband

• transfer of electrons
• working in a wide electric field range
• doping stable current flow
• suppression of thermal backfilling

Periodic repetition of two sections ( 30)
24
QC lasers without injector regions
Staircase-Lasers
Quantum-cascade laser without injection miniband

• no optically passive sections
• compact gain regions
• Problems to solve
• electron injection?
• limited electric field range?
• scattering from donor impurities?
• thermal backfilling?

25
Former injectorless QC lasers
Staircase-Lasers

• Low performance
  
  ? High threshold current
  densities (1Jth ? 5.9 kA/cm2,
• 2Jth ? 4 kA/cm2, 3Jth ? 3.6 kA/cm2,
  77 K)
  ? Limited maximum
  operating temperature (Tmax ? 200 K)
• Reasons for our previous results1,3
  
  
  ? Design not
  optimized
  
  ? Highly doped GaInAs-cladding
  ? high waveguide-losses
  
• (?w ? 60 cm-1, ? ? 10 µm)
  
  

1N. Ulbrich et al., Appl. Phys. Lett., 2002, 80,
pp. 4312-4314 2M. Wanke et al., Appl. Phys.
Lett., 2001, 78, pp. 3950-3952 3G. Scarpa et al.,
IEEE Proceedings IPRM, 2002, pp. 735-738
26
Design
Staircase-Lasers
Improved design Active section
Al0.56In0.44As/Ga0.4In0.6As (in nm)
3.4/4.0/1.3/5.2/0.9/2.6/1.9/3.2
Doping nAR 6.4?1010 cm-2 AR 60 periods

• Four-level staircase
• LO-Phonon resonant tunneling injection
• Layers where radiativ transition takes place are
  left undoped to avoid scattering on donor
  impurities

27
Design
Staircase-Lasers
Increased electric field

• Double LO-Phonon resonant condition
• wide electric field range
• reduces thermal backfilling
• Diagonal transition strong wavelength shift due
  to voltage induced Stark-effect
  ?80kV/cm ? 10 µm ?110kV/cm ? 8.4 µm

28
Design
Staircase-Lasers
Concept 4-level staircase

• no optically passive sections
• compact gain regions
• Problems solved
• electron injection
• limited electric field range
• scattering from donor impurities
• thermal backfilling

29
Results
Staircase-Lasers

• Pulsed operation 250 ns, 250 Hz
• L 4 mm, W 30 mm
• Jth 0.9 kA/cm2 (77 K),
  Jth 3.1 kA/cm2 (300 K)
• ?77 K 10 mm l300 K 8.4 mm
• Jth reduced by a factor of 3
  
• Tmax 350 K

30
Improved active region
Staircase-Lasers
Improved design Active section
Al0.56In0.44As/Ga0.4In0.6As (in nm)
3.4/4.4/1.1/6.0/0.8/2.2/1.9/3.0

• New structure
• Same concept (4-level staircase)
• Vertical transition (increased probability
  density, small wavelength shift)
• Shorter wavelength, l7.8 mm (larger confinement
  GAR, smaller losses aw)

Doping nAR 8.6?1010 cm-2 AR 65 periods
31
Results
Staircase-Lasers

• L 3.2 mm, W 12 mm
• Jth 0.21 kA/cm2 (77 K),
  Jth 2.4 kA/cm2 (300 K)
• ?300 K 7.9 mm
• Tmax 400 K

32
Comparison to usual QCLs
Staircase-Lasers
R. Green et al., Appl. Phys. Lett., 2004, 85, pp.
5529-5531

• Comparison to usual QWCLs in same wavelength
  region
• Low temperatures Exceedingly small threshold
  current densities
• Room-temperature values comparable

33
Contents

• Introduction
• GaSb-based type-I laser diodes for wavelengths
  3µm
• Staircase-type quantum cascade lasersfor
  wavelength of 8-10µm
• Conclusion

34
Conclusion

• GaSb-based type-I lasers up to 3.26 µm at 300 K
• CW room-temperature operation up to 3.18 µm
• Obstacle of lacking hole confinement removed by
  quinternary AlGaInAsSb barriers
• Injectorless quantum cascade lasers (i. e.
  Staircase lasers) at 8-10 µm realized
• Room-temperature performance comparable to
  stanndard quantum cascade lasers
• Significantly smaller ( factor 3-4) threshold
  current densities at low temperatures

35
Acknowledgement

• G. Böhm
• C. Lin
• R. Meyer
• G. Scarpa
• G. Xu
• L. Mora
• R. Heilmann
• now with State Key Laboratory of Functional
  Materials for Informatics, Chinese Academy of
  Sciences, Shanghai, China