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Title: Direct ion flux measurements at high-pressure-depletion conditions for microcrystalline silicon deposition

Abstract

The contribution of ions to the growth of microcrystalline silicon thin films has been investigated in the well-known high-pressure-depletion (HPD) regime by coupling thin-film analysis with plasma studies. The ion flux, measured by means of a capacitive probe, has been studied in two regimes, i.e., the amorphous-to-microcrystalline transition regime and a low-to-high power regime; the latter regime had been investigated to evaluate the impact of the plasma power on the ion flux in collisional plasmas. The ion flux was found not to change considerably under the conditions where the deposited material undergoes a transition from the amorphous to the microcrystalline silicon phase; for solar-grade material, an ion-to-Si deposition flux of ∼0.30 has been determined. As an upper-estimation of the ion energy, a mean ion energy of ∼19 eV has been measured under low-pressure conditions (<1 mbar) by means of a retarding field energy analyzer. Combining this upper-estimate with an ion per deposited Si atom ratio of ∼0.30, it is concluded that less than 6 eV is available per deposited Si atom. The addition of a small amount of SiH{sub 4} to an H{sub 2} plasma resulted in an increase of the ion flux by about 30% for higher power values,more » whereas the electron density, deduced from optical emission spectroscopy analysis, decreased. The electron temperature, also deduced from optical emission spectroscopy analysis, reveals a slight decrease with power. Although the dominant ion in the HPD regime is SiH{sub 3}{sup +}, i.e., a change from H{sub 3}{sup +} in pure hydrogen HPD conditions, the measured larger ion loss can be explained by assuming steeper electron density profiles. These results, therefore, confirm the results reported so far: the ion-to-Si deposition flux is relatively large but has neither influence on the microcrystalline silicon film properties nor on the phase transition. Possible explanations are the reported high atomic hydrogen to deposition flux ratio, mitigating the detrimental effects of an excessive ion flux.« less

Authors:
; ; ; ;  [1];  [1];  [2];  [1];  [3]
  1. Applied Physics Department, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven (Netherlands)
  2. (Netherlands)
  3. (DIFFER), P.O. Box 1207, 3430BE Nieuwegein (Netherlands)
Publication Date:
OSTI Identifier:
22218234
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 114; Journal Issue: 6; Other Information: (c) 2013 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; COLLISIONAL PLASMA; CRYSTALLIZATION; DEPOSITION; ELECTRON DENSITY; ELECTRON TEMPERATURE; EMISSION SPECTROSCOPY; HYDROGEN; HYDROGEN IONS 3 PLUS; SEMICONDUCTOR MATERIALS; SILICON; THIN FILMS

Citation Formats

Bronneberg, A. C., Kang, X., Palmans, J., Janssen, P. H. J., Lorne, T., Creatore, M., Solliance Solar Research, High Tech Campus 5, 5656AE Eindhoven, Sanden, M. C. M. van de, and Dutch Institute for Fundamental Energy Research. Direct ion flux measurements at high-pressure-depletion conditions for microcrystalline silicon deposition. United States: N. p., 2013. Web. doi:10.1063/1.4817859.
Bronneberg, A. C., Kang, X., Palmans, J., Janssen, P. H. J., Lorne, T., Creatore, M., Solliance Solar Research, High Tech Campus 5, 5656AE Eindhoven, Sanden, M. C. M. van de, & Dutch Institute for Fundamental Energy Research. Direct ion flux measurements at high-pressure-depletion conditions for microcrystalline silicon deposition. United States. doi:10.1063/1.4817859.
Bronneberg, A. C., Kang, X., Palmans, J., Janssen, P. H. J., Lorne, T., Creatore, M., Solliance Solar Research, High Tech Campus 5, 5656AE Eindhoven, Sanden, M. C. M. van de, and Dutch Institute for Fundamental Energy Research. Wed . "Direct ion flux measurements at high-pressure-depletion conditions for microcrystalline silicon deposition". United States. doi:10.1063/1.4817859.
@article{osti_22218234,
title = {Direct ion flux measurements at high-pressure-depletion conditions for microcrystalline silicon deposition},
author = {Bronneberg, A. C. and Kang, X. and Palmans, J. and Janssen, P. H. J. and Lorne, T. and Creatore, M. and Solliance Solar Research, High Tech Campus 5, 5656AE Eindhoven and Sanden, M. C. M. van de and Dutch Institute for Fundamental Energy Research},
abstractNote = {The contribution of ions to the growth of microcrystalline silicon thin films has been investigated in the well-known high-pressure-depletion (HPD) regime by coupling thin-film analysis with plasma studies. The ion flux, measured by means of a capacitive probe, has been studied in two regimes, i.e., the amorphous-to-microcrystalline transition regime and a low-to-high power regime; the latter regime had been investigated to evaluate the impact of the plasma power on the ion flux in collisional plasmas. The ion flux was found not to change considerably under the conditions where the deposited material undergoes a transition from the amorphous to the microcrystalline silicon phase; for solar-grade material, an ion-to-Si deposition flux of ∼0.30 has been determined. As an upper-estimation of the ion energy, a mean ion energy of ∼19 eV has been measured under low-pressure conditions (<1 mbar) by means of a retarding field energy analyzer. Combining this upper-estimate with an ion per deposited Si atom ratio of ∼0.30, it is concluded that less than 6 eV is available per deposited Si atom. The addition of a small amount of SiH{sub 4} to an H{sub 2} plasma resulted in an increase of the ion flux by about 30% for higher power values, whereas the electron density, deduced from optical emission spectroscopy analysis, decreased. The electron temperature, also deduced from optical emission spectroscopy analysis, reveals a slight decrease with power. Although the dominant ion in the HPD regime is SiH{sub 3}{sup +}, i.e., a change from H{sub 3}{sup +} in pure hydrogen HPD conditions, the measured larger ion loss can be explained by assuming steeper electron density profiles. These results, therefore, confirm the results reported so far: the ion-to-Si deposition flux is relatively large but has neither influence on the microcrystalline silicon film properties nor on the phase transition. Possible explanations are the reported high atomic hydrogen to deposition flux ratio, mitigating the detrimental effects of an excessive ion flux.},
doi = {10.1063/1.4817859},
journal = {Journal of Applied Physics},
number = 6,
volume = 114,
place = {United States},
year = {Wed Aug 14 00:00:00 EDT 2013},
month = {Wed Aug 14 00:00:00 EDT 2013}
}
  • An investigation of the effect of the total gas pressure on the deposition of microcrystalline thin films form highly diluted silane in hydrogen discharges was carried out at two different frequencies. The study was performed in conditions of constant power dissipation and constant silane partial pressure in the discharge while using a series of plasma diagnostics as electrical, optical, mass spectrometric, and in situ deposition rate measurements together with a simulator of the gas phase and the surface chemistry of SiH{sub 4}/H{sub 2} discharges. The results show that both the electron density and energy are affected by the change ofmore » the total pressure and the frequency. This in turn influences the rate of high energy electron-SiH{sub 4} dissociative processes and the total SiH{sub 4} consumption, which are favored by the frequency increase for most of the pressures. Furthermore, frequency was found to have the weakest effect on the deposition rate that was enhanced at 27.12 MHz only for the lowest pressure of 1 Torr. On the other hand, the increase of pressure from 1 to 10 Torr has led to an optimum of the deposition rate recorded at 2.5 Torr for both frequencies. This maximum is achieved when the rate of SiH{sub 4} dissociation to free radical is rather high; the flux of species is not significantly hindered by the increase of pressure and the secondary gas phase reactions of SiH{sub 4} act mainly as an additional source of film precursors.« less
  • Plasma conditions for microcrystalline silicon deposition generally require a high flux of atomic hydrogen, relative to SiH{sub {alpha}=0{yields}3} radicals, on the growing film. The necessary dominant partial pressure of hydrogen in the plasma is conventionally obtained by hydrogen dilution of silane in the inlet flow. However, a hydrogen-dominated plasma environment can also be obtained due to plasma depletion of the silane in the gas mixture, even up to the limit of pure silane inlet flow, provided that the silane depletion is strong enough. At first sight, it may seem surprising that the composition of a strongly depleted pure silane plasmamore » consists principally of molecular hydrogen, without significant contribution from the partial pressure of silane radicals. The aim here is to bring some physical insight by means of a zero-dimensional, analytical plasma chemistry model. The model is appropriate for uniform large-area showerhead reactors, as shown by comparison with a three-dimensional numerical simulations. The SiH{sub {alpha}} densities remain very low because of their rapid diffusion and surface reactivity, contributing to film growth which is the desired scenario for efficient silane utilization. Significant SiH{sub {alpha}} densities due to poor design of reactor and gas flow, on the other hand, would result in powder formation wasting silane. Conversely, hydrogen atoms are not deposited, but recombine on the film surface and reappear as molecular hydrogen in the plasma. Therefore, in the limit of extremely high silane depletion fraction (>99.9%), the silane density falls below the low SiH{sub {alpha}} densities, but only the H radical can eventually reach significant concentrations in the hydrogen-dominated plasma.« less
  • Hydrogenated microcrystalline silicon growth by very high frequency plasma-enhanced chemical vapor deposition is investigated in an industrial-type parallel plate R and D KAI reactor to study the influence of pressure and silane depletion on material quality. Single junction solar cells with intrinsic layers prepared at high pressures and in high silane depletion conditions exhibit remarkable improvements, reaching 8.2% efficiency. Further analyses show that better cell performances are linked to a significant reduction of the bulk defect density in intrinsic layers. These results can be partly attributed to lower ion bombardment energies due to higher pressures and silane depletion conditions, improvingmore » the microcrystalline material quality. Layer amorphization with increasing power density is observed at low pressure and in low silane depletion conditions. A simple model for the average ion energy shows that ion energy estimates are consistent with the amorphization process observed experimentally. Finally, the material quality of a novel regime for high rate deposition is reviewed on the basis of these findings.« less
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