Terahertz Conductivity Analysis for Highly Doped Thin-Film Semiconductors
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Terahertz Conductivity Analysis for Highly Doped Thin-Film Semiconductors Aleksander M. Ulatowski1 · Laura M. Herz1 · Michael B. Johnston1 Received: 17 July 2020 / Accepted: 19 August 2020 / © The Author(s) 2020
Abstract The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique can be used to gain an insight into the transient photoconductivity of the semiconductor, revealing the dynamics and the mobility of photoexcited charge carriers. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from THzTDS data in the regime of optically thin samples. Here, we assess the accuracy of these approximated analytical formulas in thin films of highly doped semiconductors, finding significant deviations of the calculated photoconductivity from its actual value in materials with background conductivity comparable to 102 −1 cm−1 . We propose an alternative analytical expression, which greatly improves the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally. Our approximation remains valid in thin films with high dark conductivity of up to 104 −1 cm−1 and provides a very high precision for calculating photoconductivity up to 104 −1 cm−1 , and therefore is highly relevant for studies of photoexcited charge-carrier dynamics in electrically doped semiconductors. Using the example of heavily doped thin films of tin-iodide perovskites, we show a simple experimental method of implementing our correction and find that the commonly used expression for photoconductivity could result in an underestimate of charge-carrier mobility by over 50%.
Michael B. Johnston
[email protected] 1
Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, UK
International Journal of Infrared and Millimeter Waves
Keywords THz-TDS · OPTP · Thin films · Doped semiconductors · THz conductivity analysis
1 Introduction The progress of thin-film technology in photovoltaic (PV) applications has led to highly efficient solar cells employing a variety of semiconductors being developed in recent years [1]. The benefits of devices based on thin films include their ease of fabrication, wide choice of substrate materials and low cost of production [2]. One of the promising groups of materials used as sub-micron thin films are hybrid metal-halide perovskites, which exhibit excellent optoelectronic properties and are relatively easy to fabricate through solution processing using widely available precursors [3–5]. Teraher
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