Critical assessment of contact resistance and mobility in tin perovskite semiconductors

TL;DR

This study critically evaluates contact resistance effects on mobility measurements in tin perovskite semiconductors, demonstrating that four-point probe methods provide more accurate mobility values than traditional two-probe techniques, especially at low temperatures.

Contribution

The paper introduces gated four-point probe measurements for tin perovskite FETs, revealing more accurate mobility values and highlighting the importance of device geometry to minimize contact resistance effects.

Findings

Gated 4PP mobility is consistent and independent of bias conditions.

2P measurements underestimate or overestimate mobility due to contact resistance.

Contact resistance effects are more significant at lower temperatures.

Abstract

Recent reports highlight the potential of tin-based perovskite semiconductors for high-performance p-type field-effect transistors (FETs) with mobilities exceeding 20 cm2V-1s-1. However, these high mobilities--often obtained via two-probe (2P) methods on devices with small channel length-to-width ratios (L/W < 0.5) operating in the saturation regime at high drain-source currents--raise concerns about overestimation due to contact resistance and non-ideal FET characteristics. Here, we performed gated four-point probe (4PP) FET measurements on Hall bar devices (L/W = 6) of Cs0.15FA0.85SnI3, obtaining a consistent mobility of 3.3 cm2V-1s-1. Upon comparing these with gated 2P measurements of narrow-channel FETs (L/W = 0.1) on the same chip, we resolved the contact resistance (R_C). The 2P linear mobility is underestimated due to voltage drops across R_C, while the 2P saturation mobility is overestimated because of high (dR_C)/(dV_G) near the threshold. Contact resistance effects become more pronounced at lower temperatures. Contact-corrected four-point-probe (4PP) mobilities are independent of bias conditions and are observed to flatten at temperatures lower than 180 K. Future reports of perovskite FET mobilities should include gated 4PP measurements and use devices with larger L/W ratios to minimize nonidealities arising from contact resistance effects.