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Ellipsometric characterization of
porous Silicon coated with atomic layer deposited ZnO
Zsófia Baji, János Volk, Attila Lajos Tóth, Zoltán Lábadi, Zsolt Zolnai, Miklós Fried
Research Centre for Natural Sciences, Institute for Technical Physics and Materials Science
Hungarian Academy of Science
Outline
Fabrication of hybrid solar cells based on:• Porous Si• Pores covered with n-ZnO • Filled with polymer
Problems:• How to fill the pores
with ZnO unifomly ?• How to controll the
process?
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried
Research Centre for Natural Science, Institute for Technical Physics and Materials Science
Preparation of mesoporous Si layers
HF - os Pt electrode
HF electrolytePTFE
current generator
metal backcontactSi wafer
• substrate: p+, (100), Si (=0.005 cm) • temperature: 20 °C• sample area: 4-24 cm2
• electolyte: HF/water/ethanol (cHF=20 wt%)
• etching time/layer: 60 s• etching current density: j=25 mA/cm2
• porosity from gravimetry: 67%• pore diameter: 5-50 nm
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. FriedResearch Centre for Natural Science, Institute for Technical Physics and Materials Science
The ALD process for ZnO The ALD process for ZnO depositiondeposition
The ALD process for ZnO The ALD process for ZnO depositiondeposition
The ALD process for ZnO The ALD process for ZnO depositiondeposition
The ALD process for ZnO The ALD process for ZnO depositiondeposition
The ALD process for ZnO The ALD process for ZnO depositiondeposition
The ALD process for ZnO The ALD process for ZnO depositiondeposition
Atomic layer deposition
• Sequential gas-surface interactions• Self limiting monolayer by monolayer growth
through chemisorption• Reaction only occurs on the substrate surface• Uniform and conformal coverage• Thickness control with monolayer accuracy• High aspect ratio structures can also be covered• Highly reproducible• Doping is easy and precise
Zs. Baji, J. Volk, Z. Lábadi, M. Fried, A L. Tóth, Zs. Zolnai,
Research Centre for Natural Science, Institute for Technical Physics and Materials Science
Details of the ALD processDetails of the ALD process
• Approach I: Traditional deposition parameters with varying the number of deposition cycles:
Zn pulse:0.2 s, purge:4s, Oxygen pulse:0.2s, purge:5 s; 30-50-80 cycles
• Approach II: extra long pulses (5x) and purging:Zn pulse 1s, purge: 40s, Oxygen pulse: 1s, purge: 50s; 80 cycle
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried
Research Centre for Natural Science, Institute for Technical Physics and Materials Science
The ellipsometry instrumentThe ellipsometry instrument
Woollam M-2000DI spectroscopic ellipsometer
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried
Research Centre for Natural Science, Institute for Technical Physics and Materials Science
Ellipsometric modelsEllipsometric models
Surface roughness
c-SiEMA: Void fv
ZnO fZnO
substrate
Surface roughness
c-SiEMA : Void fv
ZnO fZnO
substrate
Surface roughness
substrate
c-SiEMA :Void fv
ZnO fZnO
Model 1:
Effective Medium Approximation (EMA) of the
porous Si with a surface
roughness layer
Model 1/g:
A grading of the porous layer is also taken into
account
Model 2/g:
A grading of the porous layer is also taken into
account
Model 2:
Effective medium approximation of
two porous Si layers with a
surface roughness layer
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried, Research Centre for Natural Science, Institute for Technical Physics and Materials Science
graded c-Si
EMA: Void fv ZnO fZnO
Surface roughness
c-SiEMA : Void fv
ZnO fZnO
substrate
c-SiEMA :Void fv
ZnO fZnO
Results of the model fitting- short ALD pulses
Zs. Baji, Z. Lábadi, Zs. E. Horváth, A. L. Tóth, Zs. Zolnai,M. Fried Research Institute for Technical Physics and Materials Science MFA
A single layer did not describe the SE result perfectly
Supposing a double layer improved the fitting considerably, but introducing graded layers did not result in further improvement
Model 1 Model 2/g
Thick1 = 452 ± 5 nm, void % = 3 ± 4, ZnO % = 64 ± 6Thick2 = 544 ± 5 nm, void % = 46 ± 3, ZnO % = 0 ± 4
Results of the model fitting- long ALD pulses
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried
Research Centre for Natural Science, Institute for Technical Physics and Materials Science
The single-layer model resulted in a good fitSupposing a double layer did not improve the fit at all, but introducing graded layers improved the fit somewhat
Model 1
Model 1gThick = 1121 ± 0.3 nm, void % = 24 ± 1, ZnO % = 36 ± 1
50 100 150 200 250 300 350 4000
500
1000
1500
2000
2500
3000
3500
ZnO ALD 80 cycles, tilt 7o
2.8 MeV 4He+ RBS
Simulated
Yie
ld (
Co
un
ts)
Channels (Energy of backscattered He ions)
ZnO
Si
Comparing the results with RBS and EDS measurements –
short pulses
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried
Research Centre for Natural Science, Institute for Technical Physics and Materials Science
Both methods showed a decrease of the Zn concentration as a function of the depth
Comparing the results with RBS and EDS measurements – long
pulses
50 100 150 200 250 300 350 4000
500
1000
1500
2000
2500
ZnO ALD 30 cycles, tilt 7o
2.8 MeV 4He+ RBS
Simulated
Yie
ld (
Co
un
ts)
Channels (Energy of backscattered He ions)
ZnO
Si
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried,
Research Centre for Natural Science, Institute for Technical Physics and Materials Science
Both methods showed a uniform Zn concentration throughout the whole depth of the layer
40% ZnO (SE 36%)
Conclusions
• ALD has been proved to be an excellent method for a controllable coating of porous surfaces
• Short pulses fill the pores on the top (first 400 nm), and block them
• Long pulse length allows full in-depth coating (1000 nm), leaving voids behind for the polymer
• Ellipsometric models have been eveloped• The results of the ellipsometric fittings are also in
good agreement with two independent characterization techniques
• Possible in-line nondestructive characterization to check the remaining porosity
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried
Research Centre for Natural Science, Institute for Technical Physics and Materials Science
Thank you for your kind attention!
Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried
Research Centre for Natural Science, Institute for Technical Physics and Materials Science