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Deposition and Characterisation of Crystalline Silicon

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RACHOW, Thomas, 2014. Deposition and Characterisation of Crystalline Silicon [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Rachow2014Depos-28821, title={Deposition and Characterisation of Crystalline Silicon}, year={2014}, author={Rachow, Thomas}, address={Konstanz}, school={Universität Konstanz} }

In this thesis, crystalline silicon thin films by atmospheric pressure chemical vapour deposition have been studied. These silicon films can be deposited on silicon wafers or transferred to various substrates for photovoltaic applications. One of the main advantages is the flexibility in thickness and doping concentration which allows the application of the silicon thin films in various solar cell concepts. The combination of these films with an industrial solar cell fabrication process has a high efficiency potential and offers a cost reduction as well as reduced material consumption. The key aspects of this publication were the deposition process, the characterisation of silicon thin films and the implementation of these layers into different solar cell concepts. In the following subchapters the main findings will be summarised and suggestions for further investigation will be given.<br /><br />Silicon Deposition by APCVD at Temperatures from 1150 °C to 850 °C<br /><br />The silicon deposition by atmospheric pressure chemical vapour deposition (APCVD) using chlorosilanes is described by a set of gas phase and surface reactions. A general and a simplified model of these chemical reactions for the decomposition and the deposition process using TCS as precursor have been discussed. The identified assumptions and limitations of the simplified model underline the importance of process characterisation.<br />Since the deposition process also depends on the reactor setup, a general description of the lab-type deposition tool RTCVD100 and RTCVD160 was given. The presented characterisation of the deposition homogeneity is crucial for the material characterisation as well as solar cell fabrication and shows the limitations and the influence of the deposition properties. Based on the deposition of cSiTF at 1150 °C an additional process at 1050 °C has been developed.<br />The correlations between precursor composition, deposition temperature, deposition rate substrates orientation and doping incorporation show the complexity of crystalline silicon thin film (cSiTF) deposition. Therefore, detailed investigations of the deposition process at temperatures between 1150 °C and 850 °C with different gas mixtures have been carried out. The change in deposition rate from 0.5 ± 0.2 µm/min to 2.4 ±0.4 µm/min increases the throughput or allows the reproducible formation of silicon films with a thickness below 500 nm. The basic mechanism for the doping incorporation and the resulting doping range of 1x1015 1/cm³ to 2x1020 1/cm³ for boron and 1x1017 1/cm³ to 3x1020 1/cm³ for phosphorous have been explained. Additionally, the increase in doping incorporation by almost one order of magnitude depending on the Cl/H ratio has been investigated.<br />Subsequently, the process development has been used to further optimise the deposition of the epitaxial emitters. The deposition of advanced emitter profiles with a thickness from 500 nm to 3.5 µm and the deposition on textured as well as planar wafers has been analysed. Furthermore, the reduction of contact peak thickness from 500 nm to 25-50 nm by diffusion or epitaxial deposition and the influence of the following process step have been discussed.<br />Apart from the epitaxial deposition on silicon substrates at a deposition temperature from 1150 °C to 950 °C the direct deposition of microcrystalline silicon thin film (µcSiTF) on substrates with intermediate layers between 1050 °C and 850 °C has been realised using APCVD. In addition, the successful deposition of µcSiTF on temperature sensitive substrates like borosilicate glass has been presented. These µcSiTF can be applied as single junction thin film solar cell, as active layer in a tandem solar cell, as diffusion barrier or contact layer and as seed layer for recrystallisation. The basic understanding of this deposition process has been complemented by an investigation about the nucleation process. The experimental results show that the random nucleation density of silicon on SiNx and SiOx by PECVD is increasing with temperature and results in a homogeneous seed layer. However, the etching of thermal SiO2 in hydrogen atmosphere at temperatures above 900 °C reduces the nucleation density on this type of intermediate layer from approximately 270 to 40 counts/mm2.<br />The presented experimental results and the identified correlations are essential for the optimisation and further development of the deposition processes by APCVD.<br /><br />Crystallographic Characterisation of Silicon Thin Films<br /><br />Different measurement methods to characterise the crystallographic properties of cSiTF have been presented. The formation of various defects like etch pits, stacking faults and spikes in the epitaxial layer depending on the deposition temperature, precursor composition as well as substrate orientation have been shown. The increasing number of stacking faults and the change in recombination intensity of those defects at temperatures below 1100 °C can result in a VOC loss of approximately 200 mV.<br /><br />The microcrystalline silicon films by direct deposition have been characterised with EBSD, XRD and µRaman measurements. The crystallographic properties of these layers including a grain size of 2-3 µm and the columnar growth have been determined. Various measurements to determine the electrical material quality of microcrystalline silicon films depending on deposition and post treatment parameters have also been done. These experiments lead to the conclusion that the RTA and RPHP treatments are necessary for a deposition temperature below 1000 °C because the stress and temperature sensitive defects can be reduced.<br /><br />The limitations of established measurement techniques for the characterisation of cSiTF and the benefit of approaches like the calibrated µRaman, µPL and µLBIC measurements have been shown. Based on a set of calibration samples it is possible to determine the doping concentration above 6x1016 1/cm3 by analysing the Fano resonance. The presented measurements of epitaxial silicon films have been used to investigate the overall material quality and the stress distribution in these layers. Furthermore, the investigation of the growth interface between epitaxial films and the silicon substrates shows no increased stress or recombination activity. However, the influence of the growth regime of epitaxial films on the reflection properties of textures samples has been identified as crucial parameter.<br />These experiments have been necessary to explain and improve the material quality of cSiTF in general. Furthermore, these measurements prove the applicability of epitaxial emitters for wafer based solar cells.<br /><br />Lifetime measurements of cSiTF and wafer based solar cells<br /><br />The challenges and solutions for reliable effective minority carrier lifetime measurements on various cSiTF have been discussed. Based on the upgraded setup and an improved measurement routine the MWPCD signal intensity has been increased from approximately 4 mV to values above 35 mV. In combination with a new analysis algorithm, which incorporates the effect of carrier trapping on the decay transient, it is possible to determine reliable lifetimes below 1 µs of various cSiTF. Additional experiments to determine the influence of the excitation laser wavelength, the excess carrier density, the substrates and the passivation have been evaluated. The measurements of cSiTF deposited in the RTCVD160 show an effective diffusion length of about 90-120 µm and epitaxial cSiTF on recrystallised templates of 50-80 µm. Subsequently, MDP, LBIC and IQE measurements of cSiTF solar cells have been conducted to validate the MWPCD results and to determine the material quality of cSiTF.<br />Furthermore, QSSPC measurements of silicon wafers with epitaxial emitters and experiments to investigate the material degradation at high temperatures have been presented. The effective lifetime and emitter saturation current measurements underline the successful optimisation of the epitaxial emitter deposition. At 1050 °C the effective minority carrier lifetimes of 200 µs for p-type and 250 µs for n-type show an increase of more than one order of magnitude compared to the standard epitaxial process at 1150 °C. The emitter saturation current of J0e < 45 fA/cm2 for p-type and J0e < 30 fA/cm2 for n-type FZ wafers show the potential for an application of epitaxial emitters into industrial and high efficiency solar cell concepts.<br />In addition, the lifetime degradation of silicon substrates during the standard deposition process above 1100 °C has been analysed. The experiments show a minor lifetime degradation due to the formation of thermal defects by the high thermal budget and the cooling ramp. However, the diffusion of impurities depending on duration, ambient gas and temperature in the reactor has been identified as the dominating effect. Iron concentrations measurements on reference wafers of 1x108 1/cm3 increases to values of 4x1011 1/cm3 after a 4 min annealing process at 1100 °C. Alternative effects like the formation of mono vacancies and the migration of defects from the silicon surface have also been discussed.<br /><br />Solar cells using cSiTF and µcSiTF<br /><br />The properties and possible adjustments to cSiTF solar cells have been investigated using the epitaxial wafer equivalent (EpiWE) concept. An alternative formation of the BSF by diffusion from a gaseous source with similar absorber material quality EPD < 2x104 1/cm² and the integration of a plasma texture have been carried out. An optimisation in absorber thickness to 30 µm and the decrease of open circuit voltages at deposition temperatures below 1100 °C has been presented. Furthermore, the development and a proof of concept of a solar cell process for the n-type cSiTF with p-type epitaxial emitter by APCVD with a record VOC of 655 mV and an efficiency of 13.9 % have been realised.<br />Additional experiments have been conducted to investigate the different applications for the microcrystalline silicon layers by direct deposition using APCVD below 1000 °C. The presented experiments evaluate the solar cell properties of these layers including the parameters contact resistance, series resistance and material lifetime. The solar cell batches proof the feasibility of these concepts and show a VOC of 466 mV and a current of 14 mA without light trapping and texturing for a 10 µm thick absorber. Additional results and simulation by PC1D lead to the conclusion that the absorber layer has to be reduced to a thickness of 6 µm. Using microcrystalline silicon layers as seed layer on glass in combination with e-beam crystallisation shows voltages of 539 mV. A more industrial feasible approach is the crystallisation by ZMR resulting in voltages of 614 mV [8].<br />These results and the characterisation of cSiTF solar cells will be used to increase the efficiency and the overall understanding of various cSiTF concepts.<br /><br />Wafer based solar cells with epitaxial emitter<br /><br /><br />In the last Chapter the development of n- and p-type solar cell concepts with epitaxial emitters has been presented. Solar cell simulations by PC1D and process simulations by Sentaurus Process have been conducted to determine the potential and limitations of epitaxial emitters by APCVD and to optimise the fabrication process. Based on these simulations and the improvement in material quality, the solar cell batches show record efficiencies of eta = 16.4 % for n-type solar cells and eta = 16.1 % for p-type cells on mc wafers. Optimised deposition process for mc wafers at 1000 °C and for mono crystalline wafers at 1025 °C have been established.<br />The adjustments in metallisation and passivation lead to optimised concepts and new solar cell processes for epitaxial emitters and for cSiTF concepts as well. The record solar cells with a conversion efficiency of eta = 20.0 % on n-type, FZ and eta = 18.4 % on p-type, FZ proves that epitaxial emitters are able to compete with the state of the art diffusion process. These results are an increase in efficiency of over 2 % absolute over the last 3 years und show the successful process optimisation. The diffusion length Leff > 1500 µm and the similar open circuit voltages compared to the diffused emitter of 658 mV for n-type solar cells also underline their potential for high efficiency and industrial application.<br />The improvement and the transfer of the epitaxial emitter from a simplified development concept to the high efficiency concepts includes the development of the plasma texture with in-situ selective emitter formation (JSC = + 2.85 mA/cm²) and an epitaxial BSF.<br />In summary it can be stated that the advancements in process development presented in this thesis have already been used to optimise the current deposition process of cSiTF and will contribute to the ongoing development. Furthermore, the thorough characterisation and the application of alternative characterisation methods will contribute to a better understanding of cSiTF. Abscheidung und Charakterisierung von kristallinen Silizium 2014 2014-08-19T09:16:55Z Rachow, Thomas eng Rachow, Thomas 2014-08-19T09:16:55Z Deposition and Characterisation of Crystalline Silicon terms-of-use

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