Non-Contact Metrology for Electrical Characterization of Photo-Voltaic Materials
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Non-Contact Metrology for Electrical Characterization of Photo-Voltaic Materials M.I. Current, V.N. Faifer, T.M.H. Wong, W. Walecki and T. Nguyen Frontier Semiconductor, San Jose, CA 95112 ABSTRACT A non-contact metrology for electrical characterization of p-n junctions has been developed for use on photo-voltaic materials. By analysis of junction photo-voltage (JPV) measurements at multiple light beam modulation frequencies and multiple light penetration depths, a comprehensive electrical analysis of photo-voltaic materials is provided for junction sheet resistance, leakage current, capacitance and bulk carrier diffusion length. The JPV analysis can be made with screen and native oxides on the surface, so no pre-measurement chemical etching is required. Since the probe does not contact the p-n junction surface, measurements can be made in a continuous fashion with the junction moving under the probe, providing a method for efficient high-resolution mapping of local electrical properties. The non-contact probing also allows for nondamaging measurements in both “hard” (Si) and “soft” (organic) photo-voltaic materials. Examples of p-n junction properties in polished, crystalline-Si and cast poly-Si will be discussed. PROCESS PARAMETERS FOR SOLAR PHOTOVOLTAIC JUNCTIONS The success of Si photo-voltaic devices depends on monitoring and controlling a wide range of fundamental properties of p-n junctions in semiconductor materials. The basics of conversion of light to electricity in semiconductor materials are sketched in Fig. 1. If a photon with energy larger than the bandgap of a semiconductor material (1.12 eV for Si), is absorbed in the semiconductor, electron-hole pairs are created. If the semiconductor has a p-n junction on its surface, these carriers can be sweep out of the depletion layer into majority carrier regions (electrons into n-type and holes into p-type) and removed from the device through surface metal contacts. Photon in Photo-current out
Junction Photo-voltage
Junction Sheet Resistance
Surface junction
Junction Leakage Current
Carrier generation
Substrate contacts
Carrier recombination
Junction Capacitance (substrate doping)
Photo-current out
Figure 1. Sketch of photo-current generation by generation of electrical carriers, diffusion and collection at surface contacts. Also shown is a carrier recombination event and, along the right side, are RsL measurement parameters for photo-voltaic structures.
If the bulk of the semiconductor contains a high density of carrier recombination centers (lattice defects, metal centers, oxygen precipitates, etc.) the carriers can recombine before they have a chance to contribute to the photo-current output of the solar cell device. Another key quantity is the conductivity of the surface junction and substrate materials. For high conductivity junctions, with low sheet resistance values, efficient carrier spreading in the junction increases the photo-current and provides low contact resistance to the metal grid lines. Also shown in Fig. 1, along t
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