page_title: Surface photovoltage
section_title: Experimental methods
hierarchical_section_title: Surface photovoltage / Experimental methods
caption_attribution_description: Surface photovoltage measurements are an electro-optical method for the characterization of semiconductors. The figure schematically illustrates a surface photovoltage apparatus, including a chopped light source whose wavelength is scanned and a Kelvin probe used to measure the modulation of the surface potential. Drawn by Alison Chaiken using xfig.
context_page_description: Surface photovoltage measurements are a widely used method to determine the minority carrier diffusion length of semiconductors. Since the transport of minority carriers determines the behavior of the p-n junctions that are ubiquitous in semiconductor devices, surface photovoltage data can be very helpful in understanding their performance. As a contactless method, SPV is a popular technique for characterizing poorly understood compound semiconductors where the fabrication of ohmic contacts or special device structures may be difficult.
context_section_description: Surface photovoltage measurements are performed by placing a wafer or sheet film of a semiconducting material on a ground electrode and positioning a kelvin probe a small distance above the sample.   The surface is illuminated with light whose wavelength is scanned using a monochromator so as to vary the absorption depth of the photons.   The deeper in the semiconductor that carrier generation occurs, the fewer the number of minority carriers that will reach the surface and the smaller the photovoltage.   On a semiconductor whose spectral absorption coefficient is known, the minority carrier diffusion length can in principle be extracted from a measurement of photovoltage versus wavelength.   The optical properties of a novel semiconductor may not be well known or may not be homogeneous across the sample.   The temperature of the semiconductor must be carefully controlled during an SPV measurement test thermal drift complicate the comparison of different samples.  Typically SPV measurements are done in an AC-coupled fashion using a chopped light source rather than a vibrating Kelvin probe.
caption: Schematic of a typical SPV apparatus
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