Intragrain Defect Characterization Of Solar Grade Silicon Using Near-Field Scanning Optical Microscopy

Abstract

Multicrystalline silicon (mc-Si) is a material used in the photovolatic (PV) industry because of its lower production cost in comparison to its single crystal or thin film silicon counterparts. Multicrystalline silicon grown by the block casting technique, in which molten silicon is cooled in a growth crucible, generates thermally induced stress, creating intragrain dislocation clusters. These dislocation clusters act as minority carrier recombination centers, reducing the overall cell efficiency. To gain understanding of the recombination behavior of these defects a characterization tool with submicron resolution is needed.

Materials scientists have a number of microscopy options at their disposal to characterize the structural, chemical, and electrical properties of semiconductors. Optical microscopy, e.g., differential interference contrast (DIC) techniques such as Nomarski microscopy, is used to observe surface defects delineated by chemical etching. However, far-field optical spatial resolution is limited by the Abbe limit, which states that the minimum resolvable distance between two objects is limited by the wavelength of the incident radiation. Electron microscopy, including scanning electron microscopy (SEM) and transmission electron microscopy (TEM), is employed to provide electrical and structural information about bulk defect interactions. Although capable of sub-angstrom resolution, electron microscopy requires sample preparation that destroys the sample surface. Advances in scanning probe microscopy (SPM) have allowed scientists to break the far-field limit to produce images with nanometer and subnanometer resolution.

Near-field scanning optical microscopy (NSOM) is a form of scanning probe microscopy. NSOM has the ability to image both surface and bulk properties of a material in a non-evasive manner with greater spatial resolution than far-field optical microscopy and electron microscopy. I intend to demonstrate NSOM as a characterization tool in photovoltaic (PV) silicon wafers, using carrier lifetime and photoinduced current variation as contrast mechanisms.

The motivation for and development of NSOM as an characterization tool to map defect recombination behavior is first described. Then, an account of the carrier dynamics associated with the NSOM contrast modes is given. Next, the design challenges associated with the construction of the NSOM system are explained. An analysis of the intragrain defect lifetime and recombination behavior follows, using results from both existing characterization techniques and NSOM imaging. Finally, a summary of findings and description of areas for future study is given.