o. voznyy , r. stanowski, j.j. dubowski department of electrical and computer engineering
DESCRIPTION
Multibandgap quantum well wafers by IR laser quantum well intermixing: simulation of the lateral resolution of the process. O. Voznyy , R. Stanowski, J.J. Dubowski Department of Electrical and Computer Engineering Research Center for Nanofabrication and Nanocharacterization - PowerPoint PPT PresentationTRANSCRIPT
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
3
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
4
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 >
5
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 >
6
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 >
7
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
8
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 >
12
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
13
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
14
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
15
• 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