Terahertz Conductivity Analysis for Highly Doped Thin-Film Semiconductors

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 THz-TDS 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 102Ω− 1cm− 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 104Ω− 1cm− 1 and provides a very high precision for calculating photoconductivity up to 104Ω− 1cm− 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%.

Phase segregation in mixed-halide perovskites affects charge-carrier dynamics while preserving mobility

“…1 ). The presence of such phonon modes is known to result in a series of distortions in the THz photoconductivity spectra, including artifacts 30 , 31 . The distinctive derivative shape on the real part of the spectrum evident in Fig.…”

Revealing Ultrafast Charge-Carrier Thermalization in Tin-Iodide Perovskites through Novel Pump–Push–Probe Terahertz Spectroscopy

“…Upon addition of tin fluoride to the perovskite during deposition, a similar transmission to that of the bare quartz substrate is recovered, indicating negligible conductivity of the thin film. Using an analysis of THz transmission through these thin films, 19 , 51 we determine a background doping density of 5 × 10 19 cm –3 for the FA 0.83 Cs 0.17 SnI 3 film without tin-fluoride additive. Existing studies evaluating tin vacancy suppression methods in tin-halide perovskites have found that the minimal doping levels achievable through tin-fluoride addition in these materials are >10 17 cm –3 .…”

“…We note that the conductivity transients shown in Figures 2 B and 3 represent spectral averages of THz conductivity over the probe’s bandwidth. 51 …”

A Review of the Terahertz Conductivity and Photoconductivity of Carbon Nanotubes and Heteronanotubes

“…Moreover, in Equation ( 8) the assumption of a small change in the transmission is used. Therefore, in the case of doped and highly conductive samples, the full expression has to be used: [139,140] σ…”

“…It was found that Equation ( 3) is suitable only for materials with conductivity up to 10 4 Ω −1 cm −1 , above which the large complex refractive index invalidates the assumption of negligible optical thickness. [140] In highly doped thin films with high dark conductivity, the approximations (Equation ( 8)) result in a significant underestimate of the photoconductivity, and Equation ( 9) must be used.…”