Physical understanding and technological control of carrier lifetimes in semiconductor materials and devices: A critique of conceptual development, state of the art and applications

A method based on two phase shift measurements at two different modulation frequencies is proposed to determine simultaneously the actual bulk lifetime τb and the surface recombination velocity S in silicon wafers Such a determination works, irrespectively, of the physical state of the surface or the passivation level, and is based on a microwave contactless technique, which allows mapping of τb and S with a spatial resolution of 50 μm

Contactless measurement of bulk lifetime and surface recombination velocity in silicon wafers

The influence of intentional iron bulk contamination on the performances of boron doped p-type multicrystalline silicon solar cells was investigated. Solar cells were made from iron contaminated wafers, with an initial dissolved iron concentration 100 times higher than that of standard wafers. Nevertheless, the conversion efficiency of these cells was not impacted by this intentional contamination. We showed that this tolerance toward iron was due to the efficiency of the gettering and hydrogenation effects, complementary in this material. While phosphorus diffusion (extracting more than 99% of the iron from the bulk) is slightly limited in regions of high dislocation density, hydrogen diffuses through the whole thickness of the wafer and passivates defects and remaining impurities, with its diffusion being faster along extended defects

Effect of intentional bulk contamination with iron on multicrystalline silicon solar cell properties

Highly sensitive lifetime mapping equipment using the UV laser excited differential microwave photoconductivity decay technique was developed to observe the defect and contamination distribution in ultrathin silicon-on-insulator (SOI) layers. The technique was successively applied to the evaluation of state-of-the-art SOI wafers. Through theoretical analysis, it was quantitatively confirmed that the lifetime is shortened as the SOI film becomes thin, due to an increase in interface recombination. The large improvement in sensitivity attained by the differential detection enabled us to evaluate nonuniformities in the active layer quality by overcoming the difficulty of detection normally experienced by the decrease in the thickness of the SOI layer. Using light illumination with an extremely short penetration depth and the barrier effect of the buried oxide layer, it was found that the generated carriers were confined to the active layer, and lifetime data reflecting the quality of the SOI layer could be obtained. In the lifetime map for an SOI layer with a thickness of 52 nm, numerous islandlike spots with long lifetime, which were believed to be caused by HF defects or thin silicon areas, were detected.

Lifetime Mapping Technique for Ultrathin Silicon-on-Insulator Wafers

http://pv.cecs.anu.edu.au/files/KelvinSEMSC2014aspublished.pdf

An accurate method for calibrating photoluminescence-based lifetime images on multi-crystalline silicon wafers

The microwave phase-shift (μW-PS) technique is used to determine the bulk lifetime ( τ b ) of minority carriers. In this contactless technique, the phase-shift between a microwave beam (10 GHz) and a sine-modulated infrared excitation is related to τ b and to the surface recombination velocity ( S ). Surfaces are passivated by means of an aqueous iodine solution or by short phosphorus diffusion (850 °C for 20 min). By varying the excitation, the modulation frequency ( S ) can be deduced. The technique works at a quasi-constant excitation level, using either a large excitation beam or a focused beam (50 μm) to establish a lifetime scan map. The phosphorus-diffused samples are transformed into n + p diodes and light beam-induced current (LBIC) maps lead to a mapping of minority carrier diffusion length ( L ). From the τ b and L maps, we can get a map of the minority carrier diffusion coefficient (or mobility). Comparison of the measured values of τ b in surface-passivated samples leads to an evaluation of S , which is in agreement with that obtained directly by changing the excitation frequency. The technique proposed works for Si wafers containing oxygen precipitates and for multicrystalline Si wafers. Features of extended defect recombination strength, at the defects and far from them, as well as the local variation of S , can be deduced. In conclusion, using in association the lifetime maps obtained by μW-PS and the diffusion length maps obtained by LBIC leads to an enhanced knowledge of the properties of silicon wafer minority carriers.

Mapping of minority carrier lifetime and mobility in imperfect silicon wafers

Improved resolution lifetime scan map of minority carriers in excess in monocrystalline and multicrystalline silicon wafers are obtained by means of the phase-shift technique at a practically constant injection level. This technique is contactless, based on reflected microwave power variations which occur when the sample is illuminated by a focused near infrared light. Surfaces are passivated by means of an aqueous iodine solution, whose passivating efficiency is remarkably constant. Scan maps are obtained with a lateral resolution of 50 μm thanks to a coaxial cable which directs 9,4 GHz microwaves onto the wafer and a fiber coupled laser diode which generates minority carriers in excess. Lifetime scan maps are found to be in acceptable agreement with minority carrier diffusion length scan maps obtained by the light beam induced current technique.

Minority carrier lifetime scan map in crystalline silicon wafers

High resolution lifetime mappings of excess minority carriers in silicon wafers are obtained by means of the phase-shift technique at practically constant injection level. This technique is contactless, based on reflected microwave power variations which occur when the sample is illuminated by a focused near infrared light. It measures the phase shift φ between a sinusoidal modulation of the excitation and the reflection power of microwaves. Surfaces are passivated by means of an aqueous iodine solution, whose passivating efficiency is remarkably constant. Scan maps are obtained with a lateral resolution of 50 μm by means of a coaxial cable which directs 9.4-GHz microwaves onto the wafer and a fiber coupled laser diode which generates minority carriers in excess. Lifetime mappings are in agreement with minority carrier diffusion length mappings obtained by the light beam induced current technique, especially for multicrystalline wafers.

High resolution lifetime scan maps of silicon wafers

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Mapping of minority carrier lifetime and mobility in imperfect silicon wafers

Minority carrier lifetime scan map in crystalline silicon wafers

High resolution lifetime scan maps of silicon wafers