Ellipsometric characterization of porous Silicon coated with atomic layer deposited ZnO Zsófia...

transcript

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

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,

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

The ellipsometry instrumentThe ellipsometry instrument

Woollam M-2000DI spectroscopic ellipsometer

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

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

ZnO ALD 80 cycles, tilt 7o

2.8 MeV 4He+ RBS

Simulated

Channels (Energy of backscattered He ions)

Comparing the results with RBS and EDS measurements –

short pulses

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

ZnO ALD 30 cycles, tilt 7o

2.8 MeV 4He+ RBS

Simulated

Channels (Energy of backscattered He ions)

Zs. Baji, J. Volk, Z. Lábadi, A L. Tóth, Zs. Zolnai, M. Fried,

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

Thank you for your kind attention!

Ellipsometric characterization of porous Silicon coated with atomic layer deposited ZnO Zsófia...

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