Introduction and motivation

In photovoltaic as well as in microelectronic industry the goal is to drive costs down by bringing yields up at the same time. To reach this goal it is important to analyze the defects existing in the used semiconductor materials and their impact on the latter device performance. Therefore the two contactless electrical characterization methods MDP and MD-PICTS for defect investigation are presented and results at different semiconductor materials are reviewed.

Due to the advanced microwave detection technique both methods have an advantage of sensitivity. As a consequence a high measurement speed is enabled. Mapping of 156 mm mc-Si wafer takes under a second [1]. The wide measurable injection range from 1010 to 1017 cm-3 is another benefit. Extraction of several defect parameters like the activation energy of the main recombination center from injection dependent investigations is possible. The high sensitivity is also used for measurements on lower qualitative materials or thin epitaxial layers on various substrates [2]. Using the varying penetration depth of light from different wavelength reveals even information about interface defects.

As one example the influence of metal contaminations as iron and chromium in silicon, diffusing into the material during the melting process and reducing the efficiency of solar cells and causing breakdowns of devices, is widely discussed in literature [3], as well as lifetime degradation caused by BO2 [4]. MDP is a suitable tool for high resolution mappings of the density of iron as well as chromium and boron-oxygen-complexes. Furthermore due to high sensitivity and steady state measurements iron mapping in multicrystalline silicon bricks is even possible as inline measurement, revealing the concentration of all electrically active Fe.

MD-PICTS is an advancement of conventional photo induced current transient spectroscopy (PICTS) without the necessity of contacting the samples and with a higher sensitivity, opening new fields of applications on a variety of semiconductors revealing so far not accessible defect information. The technique is sensitive to defects acting as carrier traps while the DLTS method gives more information about the dominating recombination center in the material.

To achieve a better understanding of measured results a versatile numerical simulation tool was developed. It strictly starts from first principles rather than relying e.g. on SRH formalism and similar approximations. Application of this tool makes it possible to determine the impact of certain defect properties on important material parameters as minority carrier lifetime, photoconductivity or diffusion length. Thus it is used to simulate MD-PICTS and MDP measurements by taking different defects into account.

Experimental details

Microwave detected photoconductivity

The novel method MDP is well suited for both, defect investigation by e.g. injection dependent minority carrier lifetime measurements, as well as mapping of wafers or even bricks for inline metrology. Its major advantage is the combination of sensitivity, resolution and speed, giving MDP the flexibility for a wide variety of different applications.