Measuring dopant concentrations in compensated p-type crystalline silicon via iron-acceptor pairing

We present a method for measuring the concentrations of ionized acceptors and donors in compensated p-type silicon at room temperature.Carrier lifetimemeasurements on silicon wafers that contain minute traces of iron allow the iron-acceptor pair formation rate to be determined, which in turn allows the acceptor concentration to be calculated. Coupled with an independent measurement of the resistivity and a mobility model that accounts for majority and minority impurity scatterings of charge carriers, it is then possible to also estimate the total concentration of ionized donors. The method is valid for combinations of different acceptor and donor species.D.M. is supported by an Australian Research Council fellowship. L.J.G. would like to acknowledge SenterNovem for support

Recombination activity of interstitial iron and other transition metal point defects in p- and n-type crystalline silicon

Interstitial iron in crystalline silicon has a much larger capture cross section for electrons than holes. According to the Shockley–Read–Hall model, the low-injection carrier lifetime in p-type silicon should therefore be much lower that in n-type silicon, while in high injection they should be equal. In this work we confirm this modeling using purposely iron-contaminated samples. A survey of other transition metal impurities in silicon reveals that those which tend to occupy interstitial sites at room temperature also have significantly larger capture cross sections for electrons. Since these are also the most probable metal point defects to occur during high temperature processing, using n-type wafers for devices such as solar cells may offer greater immunity to the effects of metal contaminants.This work has been supported by the Australian Research Council and The Netherlands Agency for Energy and the Environment

Dynamics of light-induced FeB pair dissociation in crystalline silicon

The dynamics of light-induced dissociation of iron–boron (FeB) pairs in p-type crystalline silicon is investigated. The dissociation is observed to be a single-exponential process which is balanced with thermal repairing. The dissociation rate is proportional to the square of the carrier generation rate and the inverse square of the FeB concentration. This suggests that the dissociation process involves two recombination or electron capture events. A proportionality constant of 5×10⁻¹⁵s describes the dissociation rate well in the absence of other significant recombination channels. The dissociation rate decreases in the presence of other recombination channels. These results can be used for reliable detection of iron in silicon devices and materials, and for further elucidation of the electronically driven FeB dissociation reaction.This work was supported by NOVEM (The Netherlands Agency for Energy and the Environment) and the Australian Research Council

Iron detection in crystalline silicon by carrier lifetime measurements for arbitrary injection and doping

An existing technique for accurate measurement of iron in silicon, which was previously restricted to low injection and a narrow doping range, has been extended to arbitrary injection and doping levels. This allows contactless lifetime measurement techniques to be used for very sensitive and rapid iron detection under a wide range of conditions. In addition, an easily measured and unambiguous “fingerprint” of iron in silicon has been identified. It is based on the invariant nature of the excess carrier density at which the injection-dependent lifetime curves, measured before and after iron–boron pair dissociation, cross over. This characteristic crossover point lies in the narrow range of 1.4 to 2.0×10¹⁴ cm⁻³, provided only that the boron concentration is below 5×10¹⁶ cm⁻³. To demonstrate the value of these techniques, they have been applied to photovoltaic-grade cast multicrystalline silicon wafers.This work has been supported by NOVEM (The Netherlands Agency for Energy and the Environment) under contract no. 2020.01.13.11.2002

Dermal contributions to human interfollicular epidermal architecture and self-renewal

© 2015 by the authors; licensee MDPI, Basel, Switzerland. The human interfollicular epidermis is renewed throughout life by populations of proliferating basal keratinocytes. Though interfollicular keratinocyte stem cells have been identified, it is not known how self-renewal in this compartment is spatially organized. At the epidermal-dermal junction, keratinocytes sit atop a heterogeneous mix of dermal cells that may regulate keratinocyte self-renewal by influencing local tissue architecture and signalling microenvironments. Focusing on the rete ridges and complementary dermal papillae in human skin, we review the identity and organisation of abundant dermal cells types and present evidence for interactions between the dermal microenvironment and the interfollicular keratinocytes