Nanoparticles embedded in thin-film solar cells show promise to improve cell efficiency and are expected to play a key role in the future of silicon-based photovoltaic cells.
A method has been devised for sputter depositing nanoscale superlattices of silicon nitrides and oxides to form silicon nanoparticles through post deposition annealing. The technique creates ultra thin layers of generic hydrogen-free SiOxNy with better stoichiometry control, which identifies the size, density and distribution of nanoparticles.
The benefits of employing magnetron sputtering for the accurate deposition of thin multi-layers was demonstrated through the use of the Oxford Instruments PlasmaPro400 system shown in Figure 1. The instrument’s rotation capability was utilized for cyclically passing the wafer beneath the active target at a rate of up to 12 rpm. This approach optimizes deposition uniformity and decreases layer thickness by means of an efficient deposition duty factor.
Figure 1. The Oxford Instruments PlasmaPro400 system
After setting up the magnetron for pulsed DC deposition running at a low discharge power, the chamber was fed with the reactive gas from a mass flow controller for producing the suitable dielectric material. A combination of discharge power and time was used to control the layer thickness, and ellipsometry was used to measure the deposited films.
Figure 2. A schematic of the PlasmaPro System 400.
The data for the process deposition rate, absorption coefficient and refractive index of reactively deposited silicon oxide as a function of oxygen flow is given in Figure 3. Two clear zones for reactive deposition of SiOx are shown in Figure 3a: the region ≤3.5 sccm oxygen flow rate, where the deposition process progresses from a metallic target state; and the region ≥ to 3.8 sccm, where the conversion of the target surface into oxide takes place. This results in a much lower sputtering rate.
Figure 3. The deposition rate (a), refractive index (b) and absorption coefficient (c) of SiOx as a function of oxygen flow.
The transition region is the area where the material deposited is completely stoichiometric SiO2 with an absorption coefficient of <0.01 and a refractive index of 1.48. The process is repeatable, stable and occurs at a higher deposition rate because the target is constantly maintained on the 'metallic' side of this transition. SiO is determined by the region where the absorption coefficient and refractive index are 0.13 and 2.45, respectively. It is possible to adjust the process to a range of Si:O ratios to analyze the formation of nanoparticles.
A comparable data set for thin film oxy-nitride deposition as a function of oxygen and nitrogen flows changed linearly from Si3N4 conditions to SiO and SiO2 is shown in Figure 4. This is actually a subset of SiOxN and it is possible to sputter any stoichiometry through the selection of the oxygen and nitrogen ratio. This data shows the flexibility of the deposition method, which allows deposition of a whole variety of materials for superlattice structures for subsequent analysis of nanoparticles. This data is helpful for compositional control utilizing the absorption coefficient and refractive index for selecting process conditions.
Figure 4. The deposition rate (a), refractive index (b) and absorption coefficient (c) of SiOxNy as a function of oxygen and nitrogen flow.
The characterization of individual thin films of SiO, SiO2 and SiOxNy has been performed by creating a cumulative layer of 40-80 nm thickness from depositions of individual thin layers having a thickness of 1-4 nm. The refractive index was utilized to determine the composition state of silicon as a function of oxygen and/ or nitrogen concentration. After identifying the appropriate oxygen/ nitrogen concentrations silicon based dielectric multi-layers can be created for annealing into nanoparticles through high temperature reduction of the SiOx to Si and SiO2.
The results demonstrate that achieving the high degree of stoichiometry control required for depositing elaborate superlattices is possible in 'dynamic' deposition through the use of the motion control capability of the PlasmaPro System 400 sputtering tool. Moreover, the selection of single layer material is possible over the whole stoichiometry range from pure silicon to SiO, SiO2 and Si3N4.
This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Plasma Technology.
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