Papers by Author: Peter R. Wilshaw

Abstract: In this work we use Saw Damage Gettering (SDG) in combination with emitter formation to improve the minority carrier lifetime of highly contaminated multi-crystalline silicon wafers. This process is applied to wafers from the bottom of ingots, commonly referred to as the “red zone”, which are currently discarded since their high concentration of impurities limits the efficiency of solar cells produced therefrom. SDG is a potentially simple technique designed to upgrade these wafers. In this technique, bulk impurities are dissolved via annealing. The wafers are then cooled which generates a super-saturation of impurities in solution. The system then relaxes through the formation of precipitates in the saw damaged region. SDG is shown to be enhanced when using a temperature dependent cooling rate which maximizes the flux of impurities to the saw damaged regions. In addition, these benefits were observed even after an additional gettering process occurring during an emitter formation procedure. The SDG annealing conditions required to achieve the maximum lifetime were altered by the introduction of the emitter formation process. The enhancement generated by the SDG process may be sufficient to enable red-zone wafers to be processed is the same manner as higher quality no-red zone wafer wafers without adversely affecting the resultant cell efficiency. Due to its simplicity, it is expected that SDG can easily be incorporated into current production methods.

Abstract: A new technique is described by which ionic species can be rapidly transported into oxide films, and once there provide effective and stable field effect passivation to silicon surfaces. Field effect passivation in thermally grown oxide films has been achieved by embedding potassium ions using a combined drift and diffusion mechanism at high temperature. This process has been shown to be over 10 times faster than a pure diffusion process. The resulting passivation stable for periods exceeding 600 days, with lifetimes reaching 1.4 ms, equivalent to a surface recombination velocity (SRV) ≤ 5.7 cm/s, on 1 Ωcm, n-type, FZ-Si.

Abstract: An investigation of the parasitic surface recombination mechanisms in high-lifetime oxidized n-Si is presented. An approximate analytical expression describing recombination at the edge of square shaped specimens is derived. This shows that edge recombination can have a significant effect on the effective lifetime as measured using the transient photo-conductance technique and that for well passivated high quality material edge recombination can be the dominant mechanism in reducing the effective lifetime below the intrinsic level. For 3 x 3 cm² pieces of silicon measured using a Sinton photo-conductance lifetime instrument, it is shown that recombination at the edge of the sample results in an additional component to the measured lifetime of around 16 ms at an injection level of 10¹⁵ cm^-3. When this effect is taken into account measurements of 1 Ωcm FZ-Si show that a SRV as low as 1.5 cm/s is possible when the surface is passivated using a corona charge concentration of +2.2 x 10¹² q/cm² deposited on a 100 nm oxide layer.

Abstract: Effective reduction of front surface carrier recombination is essential for high efficiency silicon solar cells. Dielectric films are normally used to achieve such reduction. They provide a) an efficient passivation of surface recombination and b) an effective anti-reflection layer. The conditions that produce an effective anti-reflection coating are not necessarily the same for efficient passivation, hence both functions are difficult to achieve simultaneously and expensive processing steps are normally required. This can be overcome by enhancing the passivation properties of an anti-reflective film using the electric field effect. Here, we demonstrate that thermally grown silicon dioxide is an efficient passivation layer when chemically treated and electrically charged, and it is stable over a period of ten months. Double layers of SiO₂ and SiN also provided stable and efficient passivation for a period of a year when the sample is submitted to a post-charge anneal. Surface recombination velocity upper limits of 9 cm/s and 19 cm/s were inferred for single and double layers respectively on n-type, 5 Ωcm, Cz-Si.

Abstract: Silicon nanocrystals (Si NCs) are a promising candidate for the top cell of an all-Si tandem solar cell with a band gap from 1.3-1.7 eV, tuneable by adjusting NC size. They are readily produced within a Si-based dielectric matrix by precipitation from the Si excess in multilayers of alternating stoichiometric and silicon-rich layers. Here we examined the luminescence and transport of Si NCs embedded in SiC. We observed luminescence that redshifts from 2.0 to 1.5 eV with increasing nominal NC size. Upon further investigation, we found that this redshift is to a large extent due to Fabry-Pérot interference. Correction for this effect allows an analysis of the spectrum emitted from within the sample. We also produced p-i-n solar cells and found that the observed I-V curves under illumination could be well-fitted by typical thin-film solar cell models including finite series and parallel resistances, and a voltage-dependent current collection function. A minority carrier mobility-lifetime product on the order of 10^-10 cm²/V was deduced, and a maximum open-circuit voltage of 370 mV achieved.

Abstract: The concept of fully encapsulated, semi-insulating silicon (SI-Si), Czochralski-silicon-on-insulator (CZ-SOI) substrates for silicon microwave devices is presented. Experimental results show that, using gold as a compensating impurity, a Si resistivity of order 400 kΩcm can be achieved at room temperature using lightly phosphorus doped substrates. This compares favourably with the maximum of ~180kΩcm previously achieved using lightly boron doped wafers and is due to a small asymmetry of the position of the two gold energy levels introduced into the band gap. Measurements of the temperature dependence of the resistivity of the semi-insulating material show that a resistivity ~5kΩcm can be achieved at 100°C. Thus the substrates are suitable for microwave devices working at normal operating temperatures and should allow Si to be used for much higher frequency microwave applications than currently possible.

Abstract: As the thickness of multi-crystalline silicon solar cells continues to reduce, understanding the mechanical properties of the material is of increasing importance. In this study, a variety of techniques are used to study multi-crystalline silicon. Fracture tests are performed using four- and three-point bending. The fracture stress of as-sawn material reduces with increasing beam width and is increased in beams with a polished front surface. This indicates that fracture initiates from surface flaws. Modifications to standard fracture testing, including testing under liquid, are made so that beams fracture into just two pieces. By determining the crystallography either side of the location of fracture, multi-crystalline silicon was found to fail by transgranular fracture in the samples studied. Further evidence for this is gained from indentation experiments at grain boundaries. In order to understand the relative strength of grain boundaries, new approaches need to be considered. Therefore, a novel micromechanical technique, which enables individual grain boundaries to be studied, has started to be applied to multi-crystalline silicon. A focused ion beam is used to mill micron-scale cantilevers across notched grain boundaries, which are then loaded to fracture using the tip of a nanoindenter. The technique is shown to reproduce the known fracture toughness of {110} planes in single-crystal silicon, giving a value of 0.7 ± 0.3MPam1/2. Preliminary results are presented for fracture of multi-crystalline silicon.

Abstract: Dislocation locking by nitrogen impurities has been investigated in float-zone silicon with nitrogen concentrations of 2.2 x 1015cm-3 and 3 x 1014cm-3. The stress required to unlock dislocations pinned by nitrogen impurities was measured as a function of annealing time (0 to 2500 hours) and temperature (550 to 830°C). For all conditions investigated the locking effect was found to increase linearly with annealing time before saturating. It is assumed that the rate of increase of unlocking stress with annealing time is a measure of transport of nitrogen to the dislocation core. This rate of increase was found to depend linearly on nitrogen concentration, which is consistent with transport by a dimeric species, whose activation energy for diffusion is approximately 1.4eV. The saturation unlocking stress has been found to be dependent on the nitrogen concentration. Additionally, the temperature dependence of the stress required to move dislocations immobilised by nitrogen impurities has been studied. By assuming a value for the binding energy of the nitrogen to the dislocation, the density of the locking species at the dislocation core has been calculated.