Abstract
We introduce a robust low-budget Kelvin probe design that is optimized for the long-term acquisition of surface photovoltage (SPV) data, especially developed for highly resistive systems, which exhibit—in contrast to conventional semiconductors—very slow photoinduced charge relaxation processes in the range of hours and days. The device provides convenient optical access to the sample, as well as high mechanical and electrical stability due to off-resonance operation, showing a noise band as narrow as 1 mV. Furthermore, the acquisition of temperature-dependent SPV transients necessary for SPV-based deep-level transient spectroscopy becomes easily possible. The performance of the instrument is demonstrated by recording long-term SPV transients of the ultra-slowly relaxing model oxide strontium titanate (SrTiO) over 20 h.
Highlights
The electrical characterization of wide-bandgap materials, to which a variety of current topical material systems such as functional complex oxide structures [1] belongs, has been a methodological challenge for decades [2]
The electrical transport across these structures is crucially influenced by electronic defect states, especially at surfaces and interfaces [3]; we need reliable experimental techniques for analyzing the defect state distribution across a given bulk, film-substrate, hetero, or device structure
The insets of Diagrams (c) and (d) show the noise bands in the dark and under illumination to be smaller than 1 mV, which is the state-of-the-art for surface photovoltage (SPV) measurements using unmodulated illumination schemes
Summary
The electrical characterization of wide-bandgap materials, to which a variety of current topical material systems such as functional complex oxide structures [1] belongs, has been a methodological challenge for decades [2]. Optical methods are one way to overcome this problem; see, e.g., [2,4] In this context, a promising and very versatile contactless technique to characterize the (surface- and interface-related) distribution of defect states in wide-bandgap materials and to extract their parameters is provided by the analysis of the surface photovoltage [5] as a function of:. The SPV is defined to be the light-induced shift of the band bending of a semiconductor surface or interface In principle, this shift can be detected by any method that is sensitive to the work function. The goal of the Kelvin probe is to find exactly that external bias voltage Uext (in the following referred to as the Kelvin signal) that discharges the capacitor.
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