Short Wavelength Quantum Cascade Lasers
Dmitry G. Revin
University of Sheffield, United Kingdom
IQCLSW2012
People involved
The University of Sheffield.
The results obtained in the following
groups will be cited:
Physics and Astronomy Department
Group of Prof. John Cockburn:
J.P.Commin,
C.N.Atkins
and former members
Electronic and Electrical Engineering
Department
S.Y.Zhang,
A.B.Krysa,
K.Kennedy,
and former members
University of Montpellier
(A.N.Baranov, R.Teissier)
Humboldt-University, Berlin
(W.T. Masselink, M.Semtsiv)
Northwestern University
(M.Razeghi )
Institute for Quantum Electronics, Zurich
(J.Faist )
Texas A&M University
(A.Belyanin)
Institute of Ion Beam Physics and Materials
Research, Dresden
Corning, Inc., USA
Glasgow University
IQCLSW2012
Outline
1. Short wavelength QCLs, applications and materials
choice.
2. InGaAs/AlAs(Sb)/InP QCLs.
3. Indirect valleys.
4. Single mode QCLs.
5. State-of-the art short wavelength QCLs.
IQCLSW2012
Short wavelength lasers and their main
applications in 3 - 4 µm wavelength range
Gas spectroscopy
Strong absorption
of hydrocarbons
Medicine (breath
analysis)
Environmental
monitoring
Security (explosives)
Monitoring of
industrial processes
Exploration (oil, gas)
λ ~ 3–4 µm ⇒ E ~ 310–410 meV
(λ ~ 4–6 µm ⇒ E ~ 210–310 meV )
IQCLSW2012
QCL materials choice: QW depth
Fundamental requirement for high performance short wavelengths QCLs:
large conduction band offset (∆Ec) for good confinement of electrons in upper
laser level (especially at high temperatures and electric fields)
barrier
Thermal activation
well
∆Ec
Tunnelling
E
IQCLSW2012
Material systems for QCLs
Al0.5As0.5Sb
1. GaAs-based:
GaAs/AlGaAs
2. InP-based:
InGaAs/(In)AlAs(Sb)
In0.5Ga0.5As
3.
InAs-based:
InAs/AlSb.
IQCLSW2012
Material choice for λ ~ 3-4 µm QCLs
Primary requirement - large ∆Ec!
λ ~ 3–4 µm ⇒ E ~ 310–410 meV
GaAs/
Al0.45Ga0.55As
> 8µm
∆Ec <~ 0.38eV,
cannot go
below ~8µm
In0.7Ga0.3As/
ln0.4Al0.6As
3.5µm
Strain
compensated,
∆Ec ~ 0.80.9eV,
InP waveguide/
substrate
In0.7Ga0.3As/
AlAs
3µm
AlAs in the
active regions,
∆Ec ~ 1.4eV,
high strain,
InP waveguide/
substrate
InAs/AlSb
InGaAs/
AlAsSb
3µm
AlAs(Sb) barriers,
∆Ec ~ 1.6eV, very
flexible for strain
compensation,
InP waveguide/
substrate
3µm
∆Ec ~ 2.1eV,
low strain,
InAs
substrates
IQCLSW2012
First short wavelength (λ
(λ ~ 3.5 µm)
µm) QCLs
Strain compensated In0.7Ga0.3As/In0.4Al0.6As/InP
λ ~ 3.5 µm at 10 K;
operated in pulsed regime only up to 280 K
J.Faist, et al, APL 72, 680 (1998).
IQCLSW2012
First short wavelength InAs/
InAs/AlSb QCLs
The problems with appropriate
waveguide had to be overcome.
First successful waveguide:
n-InAs – n+-InAs
• λ ~ 4.5 µm, up to 300 K.
R. Teissier et al, APL 85, 167 (2004).
• λ ~ 3.55 µm, up to 140 K
J. Devenson et al, El. Lett. 42 (2006).
n+-InAs cladding layers with
InAs/AlSb superlattice spacers
surrounding the active zone was
introduced later.
IQCLSW2012
1.0
M3253
M3255
20
Jth (kA/cm 2)
Normalized intensity (a.u.)
First short wavelength InGaAs/
InGaAs/AlAsSb QCLs
10
8
6
0.8
T0=130K
M3253
M3255
4
0.6
2
0.4
100
200
300
Temperature (K)
Lattice matched to InP
In0.53Ga0.47As/AlAs0.56Sb0.44
Laser emission :
at λ ~ 3.55 µm only up to 300 K
at λ ~ 3.05 µm only up to 120 K
0.2
20K
0.0
3.0
80K
300K
3.5
4.0
4.5
Wavelength (µm)
D.Revin, et al, APL 90, 021108 (2007).
IQCLSW2012
Strain--compensated InGaAs
Strain
InGaAs//AlAsSb QCLs
IQCLSW2012
Intervalley scattering
InGaAs
IQCLSW2012
Satellite conduction band minima
AlAs0.56Sb0.44
Γ
∆EC~1.6eV
X
X
X, L
In0.53Ga0.47 As
λ ~ 3.7 µm places the upper
laser level ABOVE the
predicted satellite valley in
lattice matched InGaAs/AlAsSb
system
Does (and if) intervalley
scattering influence laser
operation below λ ~ 3.7 µm ?
∆EΓ-L~520meV
IQCLSW2012
Strain compensated InGaAs/AlAsSb QCLs
1.2
X valley
L valley
1.0
0.8
Γ valley
0.6
0.4
0.4
0.5
0.6
0.7
0.8
Indium fraction in InGaAs
0.9
650
Γ-X separation
Γ-L separation
600
550
500
0.4
st
ra
in
ed
Energy (eV)
1.4
700
Energy separation (meV)
1.6
Calculations based on Vurgaftman et al,
JAP. 89, (2001)
lattice matched
0.5
0.6
0.7
0.8
Indium fraction in InGaAs
0.9
1. Larger separation between Γ-(X,L) valleys (effectively deeper QWs) reduction of possible electron intervalley scattering;
2. To compensate the strain the fraction of Sb in AlAsSb should be smaller –
better layer quality is expected (away from miscibilty gap for AlAsSb).
IQCLSW2012
λ~3.1 µm strain
strain--compensated
In0.67Ga0.33As/AlAs0.75Sb0.25/InP QCL
0.4
eL3
Energy (eV)
0.2
e1
eL2
eL1
e2
0.0
L-valley
-0.2
e1
-0.4
-0.6
λ ~ 3µm
0
10
20
30
40
50
60
Distance (nm)
Upper laser level e2 is predicted to be just 20meV above level eL3.
IQCLSW2012
λ~3.1 µm strain
strain--compensated
In0.67Ga0.33As/AlAs0.75Sb0.25/InP QCL
Peak optical power (W)
1.5
10µm 3mm
as-cleaved
80K
120K
1.0
200K 250K
80K
160K
200K
0.5
240K
280K 295K
3.00
0.0
0
5
10
15
20
3.05
3.10
3.15
Wavelength(µm)
Current density (kA/cm2)
λ ~ 3.15 µm - the shortest RT QCL on InP
S.Zhang, et al., APL, 031106 (2009).
IQCLSW2012
High performance of λ ~ 3.7 µm strain
strain-compensated QCLs with AlAs barriers
IQCLSW2012
InGaAs//AlAsSb interfaces: complications
InGaAs
InGaAs AlAsSb InGaAs
• Interdiffusion of Sb is difficult to control
Sb
• High quality interfaces hard to achieve
• Can lead to reduced laser performance
As
As
InGaAlAsSb
InGaAs AlAsSb InGaAs
IQCLSW2012
X-STM image for a InGaAs/
InGaAs/AlAsSb QCL
The brightest
spots – Sb atoms
(The data were
obtained in 2004
on our one of the
earliest
structures. )
Collaboration with
P. Offermans et al,
Eindhoven
10 nm
AlAsSb barriers
IQCLSW2012
InGaAs/AlAs(Sb) strain compensated lasers with
AlAs barriers
Active region
1.5
AlAs
barriers
Injection region
Active region
AlAsSb
barriers
AlAs
barriers
Energy (eV)
1.0
0.5
- Two identical (layer thicknesses)
λ~3.7 µm slightly diagonal boundto-continuum designs grown for
comparison.
0.0
-0.5
-1.0
0.0
1.5
λ = 3.7µm
0.1 0.2
AlAsSb
barriers
0.3
0.4
0.5
AlAsSb
barriers
Energy (eV)
1.0
0.6
0.7
AlAsSb
barriers
- AlAs barriers in the active regions,
comparable (?) ∆Ec ~ 1.4 eV
0.5
- Higher quality interfaces are
expected in the regions of optical
transitions.
0.0
-0.5
-1.0
0.0
λ = 3.7µm
0.1
0.2
0.3 0.4 0.5
Distance (nm )
0.6
0.7
IQCLSW2012
λ~3.7 µm In0.7Ga0.3As/
As/AlAs
AlAs((Sb
Sb)) strain compensated
lasers
AlAsSb barriers everywhere
AlAs barriers in the active regions
AlAsSb barriers in the injectors
3
2
300K
300K
10
320K
T0~115K
340K
5
1
360K
0
380K
0
2
4
6
8
10
12
14
16
18
0
15
260K
280K 3
300K
320K
340K 2
360K
380K
400K 1
300K
10
5
T0~108K
0
0
2
2
4
6
8
10
12
14
2
16
18
0
Peak optical power (W)
Voltage (V)
280k
15
20
Voltage (V)
260k
Peak optical power (W)
10µm 3mm
as-cleaved ridge
20
4
10µm 3mm
as-cleaved ridge
Current density (kA/cm )
Current density (kA/cm )
Reduced threshold current (at 300K) from ~8.2 to 3.7 kA/cm2.
Higher maximum operating temperature.
J.Commin, et al., APL, (2009).
IQCLSW2012
λ~3.5 µm In0.7Ga0.3As/AlAs(Sb) strain compensated
lasers with AlAs barriers
Peak optical power (W)
with AlAs barriers, slightly diagonal transition design.
4 mm long, high reflectivity (HR) coated laser
240K
260K
280K
300K
320K
10µm 4mm
HR coating
5
T0~115K
4
3
340K
360K
2
380K
400K
1
0
0
2
4
6
8
10
12
300K
3.2
3.4
3.6
3.8
Wavelength (µm)
14
Current density (kA/cm2)
At 300K: threshold current – 2.5 kA/cm2,
peak optical power – 4 W (8.8 W for 20µm wide lasers).
IQCLSW2012
λ~3.3 µm In0.7Ga0.3As/AlAs(Sb) strain compensated
lasers with AlAs barriers
with AlAs barriers, slightly diagonal transition design.
4 mm long, HR coated laser
Peak optical power (W)
5.0
10µm 4mm
HR coating
240K
260K
280K
T0~105K
300K
4.0
3.0
80K 300K
320K
340K
2.0
360K
3.0
1.0
0.0
380K
3.2
3.4
Wavelength (µm)
400K
0
2
4
6
8
10
12
14
2
Current density (kA/cm )
At 300K: threshold current – 3.5 kA/cm2,
peak optical power – 3.5 W (7 W for 20µm wide lasers).
The highest optical peak power reported at this wavelength.
J.Commin, et al,, APL, 031108 (2010).
IQCLSW2012
λ~3.6 µm strain compensated laser
with AlAs barriers, the design with “vertical” laser transitions
0.4
0.4
Energy (eV)
Energy (eV)
0.2
0.0
-0.2
F=120kV/cm
λ=3.45µm
-0.4
0
10
0.2
0.0
-0.2
F=125kV/cm
λ=3.3µm
-0.4
20
30
40
50
60
70
Distance (nm)
our “standard” design: slightly
diagonal transitions
0
10
20
30
40
50
60
70
Distance (nm)
the design with vertical transitions
Increased matrix element for laser transition.
Higher optical power is predicted.
IQCLSW2012
λ~3.6 µm strain compensated laser
with AlAs barriers, vertical transition design
4 mm long HR coated laser
260K
7
280K
6
5
300K
4
320K
340K
3
360K
2
380K
1
400K
0
2
8
10µm 3mm, no HR, T0=133K
240K
10µm 4mm
HR coating
Jth (kA/cm )
Peak optical power (W)
9
10
9
8
7
6
5
4
2
4
6
8
10
Current density (kA/cm2)
240K
300K
400K
3
2
3.4 3.5 3.6 3.7 3.8
Wavelenght (µm)
1
0
10µm 4mm, HR, T0=122K
240 260 280 300 320 340 360 380 400
Temperature (K)
At 300K:
- threshold current density 3.5 kA/cm2;
- peak power per facet ~ 5.0 W;
- average power at 2% duty cycle ~ 50 mW.
IQCLSW2012
λ~3.6 µm strain compensated laser
HR coated 30 µm wide laser
30
30µm 4mm
HR coating
240K
8
260K
300K
25
280K 20
6
300K
320K 15
4
340K
10
360K
2
380K 5
400K
0
0
2
4
6
8
10
Peak optical power (W)
Wall plug efficiency (%)
10
0
Current density (kA/cm2)
Nearly linear increase in the output power for wider (20 and 30 µm) lasers.
More than 25 W peak power from a single facet is achievable at 240K.
D.Revin, et al., Phot.Tech.Lett. (2010).
IQCLSW2012
Questions still remain:
Where are the indirect valleys?
How they influence the laser
performance?
IQCLSW2012
Femtosecond pumppump-probe spectroscopy
InGaAs/AlAsSb MQWs lattice matched to InP
Relaxation from Γ2 to Γ1 is
4 times faster than via the
X valleys.
The coupling between Γ
and X is poor.
So, there should not be a
big problem to put Γ2
above X?
C. V.-B. Tribuzy et al, APL 89, 171104 (2006).
IQCLSW2012
Using high magnetic field
λ ~ 3.1 µm strain-compensated In0.73Ga0.27As/(In)AlAs
It was suggested that when the N=0 Landau
level is nearly resonant with the indirect
valleys lasing stops.
This would make the following values for
In0.73Ga0.27As/AlAs :
the lowest indirect valley is 70 meV above the
upper laser level, and about 640 meV above
the minimum of Γ point.
It is ~ 40 meV higher than expected...
Semtsiv et al, APL 93, 071109 (2008).
IQCLSW2012
Applying high hydrostatic pressure
The energy of Γ valley increases, the
energy of L valley decreases.
They move towards each other.
Courtesy S. Sweeney
InAs/AlSb QCLs
Surrey/Montpellier
IQCLSW2012
Calculating the energies of the satellite valleys
The band structure of lattice matched
Ga0.47In0.53As calculated with the 30band k·p method.
The calculated energy value for Г-L
separation is ~60 meV higher than
usually assumed.
Y.-H. Cho and A. Belyanin, JAP 107, 053116 (2010).
IQCLSW2012
Single mode λ ~ 3.33.3-3.5 µm
room temperature QCLs
IQCLSW2012
Single mode short wavelength QCLs
First order surface DFB grating,
InAs/AlSb QCLs, holography
O. Cathabard, et al. El. Lett. 45, (2009).
IQCLSW2012
Single mode short wavelength QCLs
First order high aspect ratio lateral DFB grating,
InGaAs/AlAsSb/InP QCLs, electron beam lithography
T.Slight, et al, Phot.Tech.Lett. 23, 420 (2011).
IQCLSW2012
Single mode short wavelength QCLs
Buried third order grating
Low losses
High overlap
Possibility of using the
standard photolithography
Easier targeting of the
desired emission wavelength
IQCLSW2012
Single mode QCLs with third order DFB gratings
Pitch period
Λ (µm)
λ (µm)
Jth(kA/cm2)
1.58
3.36
8.9
1.60
3.40
6.8
1.62
3.45
5.8
1.64
3.49
5.8
Laser size: 3 mm 10 µm, no HR
Normalised intensity (a.u.)
Four InGaAs/InP buried gratings
fabricated on λ ~ 3.5 µm
InGaAs/AlAs(Sb) wafer by
photolithography
1
FP spectrum
envelope
T=300K
0
3.3
3.4
3.5
3.6
3.7
Wavelength (µm)
The wavelengths correspond to the grating periods reduced
by factor of 3 (third order gratings).
J.Commin, et al., APL (2010).
IQCLSW2012
Single mode QCLs with third order DFB gratings :
temperature tuning
3.35
270K
360K
3.380
Wavelength (µm)
Normalised intensity (a.u)
3.385
3.375
3.370
3.365
3.360
3.355
3.350
3.36
3.37
3.38
Wavelength (µm)
3.39
260
280
300
320
340
360
Temperature (K)
Continuous tuning from 3.358 – 3.380 µm for 270 – 360 K
Tuning rate ~ 0.24 nmK-1
IQCLSW2012
Single mode Sb
Sb--free QCLs
External cavity configuration, T=288K ,
strain-compenstaed with combined
barriers In0.72Ga0.28As/In0.52Al0.48As–AlAs
Tuning range:
λ ~ 3.15 - 3.4 µm,
HR/AR coated
J.Faist group, Semi. Sci. Tech. 27, 045013 (2012).
IQCLSW2012
State-of
Stateof--the art short wavelength
QCLs based on other materials
IQCLSW2012
InAs//AlSb QCLs
InAs
18
1200
D385-22
12 µm x 4 mm
100 ns / 1 kHz
800
10
320 K
600
8
340 K
6
400
4
360 K
2
380 K
0
400 K
T0=185K
2
Voltage (V)
300 K
12
L= 3.97 mm
d = 12 µm
1000
Jth (kA/cm )
14
D385
280 K
Peak Power (mW)
16
10
1
200
0
1
2
3
4
0
80K
3.2 3.4
λ (µm)
0.1
100
Current (A)
Tmax > 400 K
At 300 K: Jth=3.0 kA/cm2
400K
300K
3.2 3.4
λ (µm)
200
300
3.2 3.4
λ (µm)
400
Temperature (K)
P~1W
IQCLSW2012
The shortest λ~2.63 µm wavelength QCLs
InAs/AlSb on InAs
pulsed regime
APL 96, 141110 (2010).
IQCLSW2012
Low temperature Sb
Sb--free λ~3.05 µm QCLs
Strain-compensated with combined barriers
In0.73Ga0.27As/In0.55Al0.45As–AlAs/InP
Operates only up to 150K
Very diagonal laser transitions to
avoid intervalley scattering;
Lower laser level is located in
InGaAs
Upper laser level – in InAlAs
M.Semtsiv et.al, APL 90, 051111 (2007).
IQCLSW2012
High temperature Sb
Sb--free λ~3.39µm QCLs
Strain-compensated with combined barriers
In0.72Ga0.28As/In0.52Al0.48As–AlAs
3.8 mm long laser, HR coated
J.Faist group, APL 98, 191104 (2011).
IQCLSW2012
Commercial CW short wavelength CW QCLs
Corning, Inc.
Strain-compensated
InGaAs/InAlAs/InP material
λ ~ 4.6 µm , max CW power
λ ~ 4.0 µm , max CW power
λ ~ 3.8 µm , max CW power
λ ~ 3.5 µm , max CW power
– 1200 mW
– 600 mW
– 200 mW
– 60 mW
Feng Xie et al, IEEE J. of Sel.Topics in Q.El. 17, 1445 (2011).
IQCLSW2012
High temperature CW λ~3.39µm QCLs
Highly strain-compensated (more than 1.8% mismatch for InGaAs)
In0.8Ga0.2As/In0.18Al0.82As/InP
5 mm long laser,
HR coated,
epi-down on
diamond
M.Razeghi group. APL 100, 212104 (2012).
IQCLSW2012
What are the immediate challenges
for the short wavelength QCLs?
IQCLSW2012
Laser core region design and the growth
It is possible to make a theoretical design
for a QCL at wavelength below 3 µm.
BUT:
For such a design the thickness of
individual layers become less than
2-4 monolayers.
This thickness should be
controlled better than an integer
number of Ǻ.
Is the growth technology capable
to grow such layers (with strain)?
How sharp the interfaces can be?
How they would affect the barrier
heights and well depths?
What about upper minibands?
Should we remember or forget
about intervalley scattering?
IQCLSW2012
InP based waveguide
TM mode profile
λ=3µm
3.0
n-InP
InGaAs
InGaAs
n-InP
refractive index, n
λ=5µm
n+-InP
2.5
λ=10µm
2.0
1.5
1.0
0.5
0.0
1E17
1E18
1E19
Electron concentration (cm-3)
Low free carrier absorption at λ = 3 µm - low waveguide losses ~ (0.5-1 cm-1),
however, the plasmon enhanced waveguide is less efficient than at λ = 10 µm.
Intersubband losses become very important!
IQCLSW2012
The most promising material system
Al(In)As(Sb)
InP substrate and waveguide
Highly strained InGaAs
(approaching InAs)
InGaAs
Al(In)As(Sb) (single or combined)
barriers with various
compositions to compensate the
strain (preferably the local one
not just average!)
IQCLSW2012
Short wavelength second harmonic
generation in deep QW QCLs
An electrically pumped intersubband
laser for the wavelengths of ~ 1.5–2.5 µm
is proposed.
GaInAs/AlAsSb/InP
The fundamental laser transition
2 to 1 is effectively limited by
the lowest L(X) valley.
Y.-H. Cho and A. Belyanin, JAP 107, 053116 (2010).
IQCLSW2012
Wide bandgap nitrides
•GaN/AlGaN provides very deep quantum wells (~1.7eV), potential for ISB
device operation down to telecoms wavelengths
•ISB absorption, optically pumped emission observed
•Problems with stable resonant tunneling for electrically pumped devices
Driscoll et al,
APL 94,
081120, (2009)
Nevou et al, APL 90,
223511, (2007)
IQCLSW2012
Concluding remarks
1. “Ultimate” short wavelength limit at ~2.7 µm is due to technological
limitations and intervalley scattering.
2. InGaAs/Al(In)As(Sb)/InP is the best suited material.
3. Further progress on laser performance is hugely dependent on the
future growth and processing advances.
4. Some ideas for new laser designs?
IQCLSW2012