Jason Tan

Imaging interstitial iron concentrations in boron-doped crystalline silicon using photoluminescence

D.M. is supported by an Australian Research Council QEII Fellowship. The Centre of Excellence for Advanced Silicon Photovoltaics and Photonics at UNSW is funded by the Australian Research Council.

Accurate measurement of the formation rate of iron-boron pairs in silicon

This paper presents new data regarding the formation rate of iron–boron (Fei–B) pairs in p-type crystalline silicon. Improvements in the temperature control of the sample, a reduction in measurement error of the effective lifetime of the sample after all Fei–B pairs have reformed, and improved statistical analysis have led to a revision of the value of the pre-factor in the equation relating the association time constant of iron–acceptor pairs to the acceptor concentration. The new equation predicts a 14% slower repairing time than a previous commonly used equation, and reduces the uncertainty in determining the acceptor concentration from the repairing time from ±24% to ±7%.

Dissolution of metal precipitates in multicrystalline silicon during annealing and the protective effect of phosphorus emitters

The degradation of the carrier lifetime in multicrystalline silicon due to the dissolution of metal precipitates during high temperature annealing is well known. This letter presents evidence indicating that the presence of phosphorus emitters during annealing can help reduce this recontamination. Part of the degradation observed is due to increased interstitial iron concentrations caused by the dissolution of iron precipitates during annealing. However, dissolution of other metals also seems to contribute to the reduced carrier lifetimes observed.

A Contactless Method for Determining the Carrier Mobility Sum in Silicon Wafers

In this paper, we present a new method to determine the simultaneous injection and temperature dependence of the sum of the majority and minority carrier mobilities in silicon wafers. The technique is based on combining transient and quasi-steady-state photoconductance measurements. It does not require a full device structure or contacting but only adequate surface passivation. The mobility dependence on both carrier injection level and temperature, as measured on several test samples, is discussed and compared with well-known mobility models. The potential of this method to measure the impact of dopant concentration, compensation ratio, injection level, and temperature on the mobility is demonstrated.

Impurities in solar-grade silicon

Unintentional impurities can play a significant role in reducing the efficiency of crystalline silicon solar cells. With the advent of low-cost solar-grade silicon feedstocks, this is likely to remain the case well into the future. The purpose of this paper is to review the most important impurities in directionally-solidified ingot-grown multicrystalline silicon, their chemical states, where they come from, and the most commonly used techniques to detect them.

FTIR Analysis of Microwave-Excited PECVD Silicon Nitride Layers

This paper presents infrared absorption (FTIR) measurements of SiN layers and correlates them to their ability to passivate silicon wafer surfaces. The best passivation was obtained for films having a nitrogen to silicon atomic composition in the proximity of N/Si=1.2, together with a high concentration of Si-N bonds (approximately 1times1023 cm-3) and a refractive index in the vicinity of n=2. The total hydrogen concentration in these films remained practically unchanged after a high temperature firing cycle, which indicates a good thermal stability. In contrast, silicon rich layers (higher refractive index and lower Si-N bond density) suffered a large reduction in the total hydrogen content. These results support the suggestion by ECN researchers that the Si-N bond concentration can be a good indicator of the ultimate electronic impact of the SiN layers

Removal of hydrogen and deposition of surface charge during rapid thermal annealing

The submission of a hydrogenated oxide-passivated silicon wafer to a rapid thermal anneal (RTA) leads to a complicated change in effective lifetime τeff. Within seconds, τeff decreases rapidly before increasing to near its initial level, and then decreasing more slowly. The initial decline is shown to be due to hydrogen loss from the SiO2—Si interface, as is consistent with the literature. The recovery in τeff is due to the deposition of surface charge, somehow related to the placement of the sample on an as-cut silicon baseplate within the RTA chamber; this surface charge can be removed by washing the sample in isopropanol. The mechanism behind the final decrease in lifetime is unknown.

Iron imaging in multicrystalline silicon wafers via photoluminescence

We have extended the development of a recent interstitial iron imaging technique based on photoluminescence (PL) imaging and iron-boron pair dissociation. The method is best applied below the lifetime crossover point, in order to avoid FeB pair breaking during the PL measurements. We have applied this high resolution iron imaging technique to a range of multicrystalline silicon wafers from different parts of an ingot, both before and after phosphorus gettering. The high spatial resolution mega-pixel images of the dissolved iron concentration generated in this way help to better understand the behavior of iron in this material, and its response to cell processing steps.

Passivation of highly boron doped silicon surfaces by sputtered AlO x and PECVD SiN, a comparison

We show that boron-diffused emitters can be passivated with AlO<inf>x</inf> deposited using RF sputtering of an Al target. The surface passivation achieved so far is inferior to that obtained using an optimised PECDV SiN process that includes a chemically grown SiO<inf>2</inf> interfacial layer. Nevertheless, the levels of passivation obtained, expressed by emitter recombination current densities of J<inf>oE</inf>=228–349 fA/cm² for sheet resistances of 88–210 Ω/□, are already consistent with solar cell with efficiencies in the 20% range.