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This work utilizes an open bench time-domain spectroscopy system to measure the THz pulse and spectrum of photoconductive antennas (PCAs) with two different active layer materials: low temperature gallium arsenide (LT-GaAs) and 2D black phosphorous (BP). COMSOL Multiphysics modeling of the PCAs has shown that using BP as an active layer greatly increases the optical to THz conversion efficiency compared to LT-GaAs. However, the fabrication and measurement of both devices has demonstrated the opposite. 

In this project, Hall bar devices with black phosphorus (BP) as the semiconductor layer were fabricated to measure the Hall mobility and carrier density of exfoliated BP flakes obtained from bulk crystals acquired from various commercial sources. Black phosphorus is proposed as an alternative material for terahertz photoconductive antennas (PCAs) from the standard GaAs or InGaAs PCAs that are currently available commercially. Black phosphorus is an anisotropic material with a reported Hall mobility over three times greater than GaAs, but our preliminary testing of BP PCAs has shown dramatic differences of electrical properties between black phosphorus sourced from three different vendors. To determine the best quality black phosphorus source, Hall bar devices containing 40 nm BP flakes were used to measure the carrier mobility of the semiconductor. A Hall bar device is created by layering a 40nm BP flake underneath a hexagonal boron-nitride (hBN) flake, all on top of gold contacts in a Hall bar arrangement fabricated on a high-resistivity silicon substrate. The hBN acts as a passivation layer for the BP so that it may be safely removed from the glove box without damage. The Hall mobility of the material from different sources ranges from around 100 cm2/Vs to 1600 cm2/Vs, with only one source showing promising, high mobility results. This study allows BP with optimized electrical properties to be incorporated into THz PCAs for characterization via THz time domain spectroscopy.