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.

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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)