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 electrolyte PTFE current generator metal backcontact Si wafer substrate: p +, (100), Si (  =0.005  cm) temperature: 20 °C sample area: 4-24 cm 2 electolyte: HF/water/ethanol (c HF =20 wt%) etching time/layer: 60 s etching current density: j=25 mA/cm 2 porosity from gravimetry: 67% pore diameter: 5-50 nm 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 ALD process for ZnO deposition

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 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; 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 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 models Surface roughness c-Si EMA: Void f v ZnO f ZnO substrate Surface roughness c-Si EMA : Void fv ZnO f ZnO substrate Surface roughness substrate c-Si EMA :Void f v ZnO f ZnO 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 f v ZnO f ZnO Surface roughness c-Si EMA : Void fv ZnO f ZnO substrate c-Si EMA :Void f v ZnO f ZnO

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 ± 6 Thick2 = 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 fit Supposing a double layer did not improve the fit at all, but introducing graded layers improved the fit somewhat Model 1 Model 1g Thick = 1121 ± 0.3 nm, void % = 24 ± 1, ZnO % = 36 ± 1

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 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