Multibandgap quantum well wafers by IR laser quantum well intermixing:

simulation of the lateral resolution of the process

O. Voznyy, R. Stanowski, J.J. DubowskiDepartment of Electrical and Computer Engineering

Research Center for Nanofabrication and Nanocharacterization Université de Sherbrooke, Sherbrooke, Québec J1K 2R1

Canada

2

Outline1. Motivation2. Modeling heat distribution and

photoluminescence (PL) in QW wafers3. Temperature profiles induced in

InGaAs/InGaAsP wafers by moving laser beam4. PL shift profiles5. Summary

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Multibandgap materials are needed for creation of photonic integrated circuits (lasers, modulators, waveguides, multi-color detectors etc. fabricated on same wafer)

Quantum well intermixing (QWI) – interdiffusion of wells and barriers resulting in the change of the well width, potential barrier height and energy of confined states.

Motivation >

E0 E1 E2 E3

Quantum well intermixing

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Current state of the problem

[1] A. McKee, et. al., IEEE J. Quantum Electron., vol. 33, pp. 45–55, Jan. 1997.[2] B.S.Ooi,et. al. IEEE J. Quantum Electron., vol. 40, pp.481–490, May 2004

Simulations [1] predict transition region ~300μm using CW Nd:YAG laser irradiation (photoabsorbtion induced disordering) with a shadow mask [1]. Also, pulsed laser IR disordering (2-step process) has been proposed (~2μm transtion region possible [2]).

Our aim is to investigate Laser-RTA (annealing with a moving CW laser beam) as a flexible (1-step process) and potentially cost-effective technique.

Motivation >

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Moving laser beamIn previous work [3] array of 12 lines of intermixed GaAs/AlGaAs QW material was successfully written with 5cm/s, 0.7mm CW Nd:YAG laser beam in a14 mm x6 mm sample.

This approach has the potential to write complex patterns of intermixed material.

[3] J.J. Dubowski, et. al., Proc. SPIE, 5339, (2004).Quantum well PL peak position measured across the sample irradiated with a fast scanning laser beam that was used to generate a 12-line pattern.

Motivation >

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Finite Element Method simulations

Heat transfer PDE:Subdomain equation: Q - (kT) = Cp(T/t)Boundary equation: kT=q0 + h(Tinf – T) + εσ(Tamb

4 – T4)

For correct results temperature dependent thermal conductivity k and optical absorption α should be taken into account.

To find heat distribution in a wafer we used FEMLAB commercial software.

Geometry is divided into small mesh elements with their own PDE parameters. Then the resulting system of PDEs is solved.

Computation details >

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1. Take diffusion coefficient as parameter

2. Find concentration profile for given D and time

3. Find energy profiles for electrons and holes (take into account bandgaps, band offsets, bandgap bowing)

4. Solve Schrödinger equation, find energy levels and PL

5. Approximate results as some function D(PL shift)

If T(t)=const (like with RTA):

LD = – diffusion length.Otherwise one needs to solve numericallyD assumed to be the same for different atomic species.

Finding PLshift(D)

DD L

Lxerf

L

Lxerf

CCCtxC

2

2/

2

1

2

2/

2

11

)(),( 212

Computation details >

Dt

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Finding D(T) and PLshift(T, t)

Computation details >

Compare simulations and experimental PLshift(Tanneal) data for the same annealing time, find D(Tanneal)

Build Arrhenius plot lnD(1/kT) and find parameters for D=D0exp(-EA/kT)

Now we can find PL shift for any T and time.

10,0 10,5 11,0 11,5 12,0 12,5 13,0-50

-49

-48

-47

-46

-45

-44

-43

InGaAsP / InGaAs / InGaAsP[McKee, et al IEEE J.Quant El, 33 (1997)]

ln(D

), m

2 s-1

1/kT, eV-1

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0,00 0,05 0,10 0,15 0,20900

950

1000

1050

1100

1150

1200No background heatingT

bg=523KT

bg=773K

d=12m

Tm

ax,

K

P, W

Laser power density and surface damage

To achieve T needed for intermixing, different power needed for different beam diameters.

For small diameters <0.5mm power densities become higher than surface damage threshold (>30W/mm2).

Needed power density can be reduced using background heating.

Computation details >

270 W/mm2 1500 W/mm2700 W/mm2

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Power density for moving beamWith laser fast scanning (Laser-RTA) we can heat samples to same temperatures, with smaller beam diameters and avoid surface damage.

Power needed to heat the wafer to TQWI increases a little, but fluence drops down significantly (shorter dwell time).

0,00 0,05 0,10 0,15 0,20 0,25 0,30900

950

1000

1050

1100

1150

1200

v=0v=5cm/sv=50cm/s

No background heatingTbg

=523KTbg

=773K

d=12mT

ma

x, K

P, W

Computation details >

0,0 0,5 1,0 1,5 2,0900

950

1000

1050

1100

1150

1200

d=100m

No background heatingTbg

=523KTbg

=773K

v=0v=5cm/sv=50cm/s

Tm

ax,

K

P, W

TQWI TQWI

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d=12μm

Depth, μm100

50

0 0 50 100 Lateral, μm

d=100μm

Depth, μm100

50

0 0 50 100 Lateral, μm

Depth dependenceFor small beam diameters T drops down with depth very fast.

InP is transparent to Nd:YAG wavelength at RT, butEg(InP)=1.165eV at 500°C, and α=104-106cm-1 at higher T.

Thus, all the energy is absorbed on the surface and goes inside only by heat conduction.

Computation details >

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Scanning speed and bg heating• For small samples slower speed results in raise of background temperature.• For big wafers heat dissipates faster and temperature profiles don’t depend

on scanning speed (laser power is adjusted to achieve same Tsurface).

• Background heating helps to achieve wanted T.

Temperature profiles >

0 20 40 60 80 100300

400

500

600

700

800

900

1000

1100

Surface

2m depth

2m depth

Surface

Tbg

=773K

No bg heating

d=100m

T,

K

Lateral position, m

0 2 4 6 8 10 12300

400

500

600

700

800

900

1000

1100

2m depth

Surface

2m depth

Surface

Tbg

=773K

No bg heating

d=12m

T,

K

Lateral position, m

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Temporal T behavior during scan

To calculate PL shift profile for moving beam we need:

• calculate concentration and energy profiles using given T(t) and D(T) at different distances from line center,

• solve Schrödinger equation and find PL shift.

PL shift profiles >

19 20 21 22300

400

500

600

700

800

900

1000

1100

1200

center2m5m10m15m

InP5cm/s100m

dwell time

T,

K

t, ms

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PL shift profile for moving beamPL shift profile shape doesn’t depend on Tmax.

PL shift profiles >

Due to varying T(t), PL shift profile for moving beam differs from that of stationary beam, although temperature profiles are the same.

Higher temperatures reduce processing time significantly.

0 1 2 3 4 5800

900

1000

1100

1200

90 s RTAintermixingthreshold

P=0.04W

P=0.05W

d=12mT

bg=773K

T,

K

Lateral position, m

0 1 2 3 4 50

20

40

60

80

100

Transition region width

InPd=12mT

bg=773K

v=0t=90s

v=50cm/st=500 h with T

max=1073K

t=80 h with Tmax

=1173K

PL

sh

ift,

nm

Lateral position, m

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• Processing time for 100nm PL shift along one 2-inch line assuming Tmax=1073K (which requires 90s to get the same PL shift with RTA).• Practical applications will require shifts < 50nm.

300 400 500 600 700 800

0

5

10

15

20

25

500s

2m depthSurface

2m depth

Surface

Processing time forv=0 (one point)v>0 (2-inch line)

90s, 1250 h

90s, 50 h

200s, 110h

2000s, 3000 h

750s, 4170h

3000 h, 5x1011h5 h

90s, 140 h

90s, 500 h

d=12m

d=100m

Tra

nsi

tion

re

gio

n,

m

Tbg

, K

PL shift resolution and processing time

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Summary1. Irradiation with the moving CW Nd:YAG laser been has been investigated

for selective area writing of the QWI material.2. For large size wafers (2 inch) temperature profiles don’t depend on scanning

speed (assuming that beam power is adjusted to achieve the same Tmax).3. Processing time to achieve targeted PL (badgap) shifts depends on beam

diameter and Tmax. 4. To achieve reasonable processing time without loss in resolution

a) QWs should be very close to surface, b) Tmax should be as high as allowed by material decomposition temperature

4. Background heating can be used to further decrease processing time (especially for deep QWs) but decreasing also resolution.

5. Lateral PL shift resolution of 5μm is feasible (InGaAs/InGaAsP QW material system) with the 12μm beam Laser-RTA.

SupportNatural Sciences and Engineering Research Council of Canada (NSERC)Canada Research Chair (CRC) Program