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Inter-Valley Charge Transfer in Short-Wavelength InGaAs-AlAs
Quantum-Cascade Lasers
M.P. Semtsiv, M. Wienold, I. Bayrakli, S. Dressler, and W.T. Masselink
Humboldt-University, Berlin, Germany
2
2000 2200 2400 2600 2800 3000 3200 3400
5 4.5 4 3.5 3
HBr
H2O
CH2OHCl CH4
HCN
SO2
H2O
NON2O
COCO2
Abso
rban
ce
Wavenumber (cm-1)
Wavelength (µm)
“Short wavelength QCLs” emit in 1st atmospheric window, 3–5 µm.
Gas detection:EnvironmentalMedical diagnoses
Mid-IR imaging (medical)
Military countermeasures :Especially 3.8–4.2 µm
3
Short wavelength emission requires a large ∆Ec.
Injection Barrier Large ∆Ec for large hνHigh injection barrier to
prevent thermal escape over topMinimize thermal
backwash
For LIR QCLs,
2cEh ∆
≈ν
hν
LOE
4
5.6 5.7 5.8 5.9 6.0 6.1 6.2
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
AlSb
InAs
GaAs
AlAs
InP
(Ga,In)As
(Al,In)As
Ec( Γ
) (eV
)
Lattice Constant (A)
Al(As,Sb)
A large ∆Ec can be achieved in several direct-band-gap III-V heterosystems.
(Ga,In)As-(Al,In)As∆Ec≈0.53 eV
(Ga,In)As-Al(As,Sb)∆Ec≈1.6 eV
InAs-Al(As,Sb)∆Ec≈2.1 eV
(Ga,In)As-AlAs∆Ec≈1. 3 eV
5
Composite Barriers based on AlAs ⇒independent control of energy and strain
Addition of In0.55Al0.45As•shrinks miniband•partially compensates for thicker AlAs•independent control of confinement and strain
-4 -2 0 2
-6.0
-5.8
-5.6
-5.4
-5.2
-5.0
-4.8
-4.6
-4.4
InAs
GaAs
AlAs
In0.70Ga0.30As
In0.55Al0.45As
0.54 eV
1.33 eV
E C - E
vac (
eV)
Mismatch to InP (%)
InAlAs on InP InGaAs on InP
Replace the AlAs with In0.55Al0.45As + AlAs
Can also use AlAs withaddition of Al(As,Sb).
6
Primary barrier material is AlAs
In0.73Ga0.27As
In0.55Al0.45As
AlAsinjection barrier
ΓEC
LEC
I
II 2
III
F=76 kV/cm
1
Modified “bound-to-bound” design
7
The emission wavelength is limited by the indirect valleys in the well, not by ∆Ec.
-6.0
-5.5
-5.0
-4.5
In0.
55 A
l 0.45
As
AlAs
In0.
70 G
a 0.30
As
AlAs
In0.
55 A
l 0.45
As
E1
E2E c - E
vac (
eV)
Γ valley L valley X valley
Max E2-E1≅ 0.37 eV(3.3 µm)
E2 is limited by the indirect valleys in the InGaAs wells.
Question:How far above the indirect valleys can the upper laser state be located?
8
Strategy #1: InxGa1-xAs with x≈0.72 (large indirect-Γ energy separation with moderate strain)
0.5 0.6 0.7 0.8 0.9 1.0
0.2
0.4
0.6
0.8
1.0
0.53 eV
rel.
ener
gy (e
V)
x in InxGa1-xAs
0.61 eV0.68 eV
Γ
XL
0.53eV ⇒ λmin ≈ 3.8 µm0.61eV ⇒ λmin ≈ 3.3 µm0.68eV ⇒ λmin ≈ 2.9 µm
The indirect-Γ separation increases with In content.The maximum transition energy increases
correspondingly.
For relaxed InAs, λmin ≈ 2.7 µm.
9
Strategy #2: Rely on poor coupling between Γ and X.
-4 -2 0 2 4 6-0.20.00.20.40.60.81.01.21.41.6
Ener
gy (e
V)
X2X1Γ1
Γ2
Position (nm)
3 nm
0 2 4 6 810-2
10-1
100
τfast=1.5 psτslow=6.2 ps
∆T /
T 0 (%
)X
1
2 τ2X
τX1
τ21
experiment biexponential fit
Delay time (ps)
Relaxation from Γ2 directly to Γ1 is 4 times faster than via the X valleys.
(H. Schneider’s Thursday Poster)
10
Strategy #3: Move the upper laser state away from the indirect valleys
injector
extractor76 kV/cm
23
1
#1597, z2,1=0.36nm, z3,1=0.09nm
3
#1596, z2,1=0.47nm, z3,1=0.19nm
injector1
extractor
2
Thin injection barrier ⇒lasing is 2→1 (vertical).
Thick injection barrier ⇒lasing is 3→1 (diagonal).
Diagonal upper laser state less well coupled to indirect valleys in QW.
“Diagonal” transitions can increase the emission energy through the Stark effect.
11
Strong coupling and vertical transitions makes the best lasers for moderate wavelength.
excited states
injector 32
1
76 kV/cmextractor
z2,1=0.24nmz3,1=0.13nm
short wavelength:77K: 3.6–3.8 µm300K: 3.8–3.9 µm
low thresholds:77K: 0.6 kA/cm2
300K: 3.8 kA/cm2
The structure was designed for 3.6 µm based on the injection miniband lasing into state 1.
60 800.30
0.352–1
Ener
gy (e
V)
Field (kV/cm)
3–1
12
Strategy #4: Change well material for upper laser state
3 2
extractor miniband
100 kV/cm
1
injector miniband
EΓ
c , EX
c , EL
c
Upper laser states mostly located in AlInAs.
Upper laser states are high in energy and not well coupled to indirect valleys in lower QW.
13
Strategy #5: InAs insert in QW with lower laser state
3 2
extractor miniband
100 kV/cm
1
injector miniband
EΓ
c , EX
c , EL
c
This design emits at 3.05 µm.
QW with lower laser state contains InAs insert.
InAs insert lowers lower laser state, increasing the transition energy.
14
The 3.05-µm QCL emits >100 mW power at 80K, but operates only up to 150K.
0 2 4 6 80
10
20
0
50
100
150
Bia
s (V
)
Jth (kA/cm2)
80 K
Peak
pow
er (m
W)
300 400 500
4 3.5 3 2.5
72 K
80 K
Inte
nsity
(arb
. u.)
2.7 kA/cm2
1.25% d.c.
Energy (meV)
300 K(x10)
Wavelength (µm)
A similar design for 3.3 µm emits up to 600 mWand operates to 200K.
15
Short wavelength designs have difficulties at elevated temperatures.
100 2002
T'0= 15 K
0.2 % d.c.
J th (k
A/cm
2 )
Temperature (K)
T0= 200 K
λ~ 3.3µmSteep increase in Jthbeginning at 150K is probably due to scattering into the indirect valleys.
The indirect valleys do not prevent lasing, but still have adverse effects.
16
These design strategies allow QCL emission over the entire 1st atmospheric window.
3.0 3.2 3.4 3.6 3.8 4.0 4.2
150 K 350K300K
Wavelength (µm)
200 K 300K
QCL spectra taken at Tmax for different designs.
17
Both the maximum temperature of operation and the onset of low T0 decrease with emission wavelength.
2.5 3.0 3.5 4.0 4.50
100
200
300
400
Inte
nsity
(arb
. uni
ts)
Wavelength (µm)
77 K
Max
. ope
r. te
mp.
(K)
Extrapolating these results implies an ultimate limit of about 2.7 µm.
The trend of steadily decreasing temperature for the onset of low T0, despite many changes to the active region, implies that the culprit is the indirect valleys.
18
Summary
•For large enough ∆Ec , it is the indirect valleys of the well material that limit emission wave length.
•New design features to avoid emptying upper laser state into InGaAs indirect valleys – the upper laser state can be higher.
•QCLs to 3.6 µm: RT lasing, high efficiencies and T0
•Record of 3.05 µm recently achieved•QCLs limited by indirect valleys to about <3.0 µm
(1st atmospheric window covered)