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1. Challenges in Measurement of Broadband THz Photoconductive Antennas

   Zach Uttley; Jose Santos Batista; Mahmudul Doha; Katie Welch; Hugh Churchill; and Magda El-Shenawee (January 2023, ARFTG Microwave Measurement Conference)

   ARFTG (Ed.)

   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. 

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2. Realization of Quantum Hall Effect in Chemically Synthesized InSe

   https://doi.org/10.1002/adfm.201904032  

   Yuan, Kai; Yin, Ruoyu; Li, Xinqi; Han, Yimo; Wu, Meng; Chen, Shulin; Liu, Shuai; Xu, Xiaolong; Watanabe, Kenji; Taniguchi, Takashi; et al (August 2019, Advanced Functional Materials)

   Abstract Recently, 2D electron gases have been observed in atomically thin semiconducting crystals, enabling the observation of rich physical phenomena at the quantum level within the ultimate thickness limit. However, the observation of 2D electron gases and subsequent quantum Hall effect require exceptionally high crystalline quality, rendering mechanical exfoliation as the only method to produce high‐quality 2D semiconductors of black phosphorus and indium selenide (InSe), which hinder large‐scale device applications. Here, the controlled one‐step synthesis of high‐quality 2D InSe thin films via chemical vapor transport method is reported. The carrier Hall mobility of hexagonal boron nitride (hBN) encapsulated InSe flakes can be up to 5000 cm²V⁻¹s⁻¹at 1.5 K, enabling to observe the quantum Hall effect in a synthesized van der Waals semiconductor. The existence of the quantum Hall effect in directly synthesized 2D semiconductors indicates a high quality of the chemically synthesized 2D semiconductors, which hold promise in quantum devices and applications with high mobility. 

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3. Black phosphorus photoconductive terahertz antenna: 3D modeling and experimental reference comparison

   https://doi.org/10.1364/JOSAB.419996  

   Batista, Jose_Santos; Churchill, Hugh_O_H; El-Shenawee, Magda (March 2021, Journal of the Optical Society of America B)

   This paper presents a 3D model of a terahertz photoconductive antenna (PCA) using black phosphorus, an emerging 2D anisotropic material, as the semiconductor layer. This work aims at understanding the potential of black phosphorus (BP) to advance the signal generation and bandwidth of conventional terahertz (THz) PCAs. The COMSOL Multiphysics package, based on the finite element method, is utilized to model the 3D BP PCA emitter using four modules: the frequency domain RF module to solve Maxwell’s equations, the semiconductor module to calculate the photocurrent, the heat transfer in solids module to calculate the temperature variations, and the transient RF module to calculate the THz radiated electric field pulse. The proposed 3D model is computationally intensive where the PCA device includes thin layers of thicknesses ranging from nano- to microscale. The symmetry of the configuration was exploited by applying the perfect electric and magnetic boundary conditions to reduce the computational domain to only one quarter of the device in the RF module. The results showed that the temperature variation due to the conduction of current induced by the bias voltage increased by only 0.162 K. In addition, the electromagnetic power dissipation in the semiconductor due to the femtosecond laser source showed an increase in temperature by 0.441 K. The results show that the temperature variations caused the peak of the photocurrent to increase by $\sim <\#comment/>3.4\mathrm{\%<\#comment/>}$and $\sim <\#comment/>10\mathrm{\%<\#comment/>}$, respectively, under a maximum bias voltage of 1 V and average laser power of 1 mW. While simulating the active area of the antenna provided accurate results for the optical and semiconductor responses, simulating the thermal effect on the photocurrent requires a larger computational domain to avoid false rise in temperature. Finally, the simulated THz signal generation electric field pulse exhibits a trend in increasing the bandwidth of the proposed BP PCA compared with the measured pulse of a reference commercial LT-GaAs PCA. Enhancing signal generation and bandwidth will improve THz imaging and spectroscopy for biomedical and material characterization applications. 

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4. Resonant Tunneling and Negative Differential Resistance in Black Phosphorus Vertical Heterostructures

   https://doi.org/10.1002/aelm.202000318  

   Xu, Kai; Wynne, Eric; Zhu, Wenjuan (July 2020, Advanced Electronic Materials)

   Abstract Resonant tunneling diodes with negative differential resistance (NDR) have attracted significant attention due to their unique quantum resonant tunneling phenomena and potential applications in terahertz emission/detection and high‐density logic/memory. In this paper, resonant tunneling devices, where the carriers tunnel through a hexagonal boron nitride (hBN) barrier sandwiched by two black phosphorus (BP) layers, are explored. The resonance occurs when the energy bands of the two black phosphorus layers are aligned. The conductive atomic force microscopy (CAFM) measurements reveal prominent NDR peaks with large peak‐to‐valley ratios at room temperature. It is found that the positions of the NDR peaks are very sensitive to the amplitude and the shape of the voltage waveform used in CAFM, which can be explained by the charge trapping effect. Furthermore, resonant tunneling transistors are demonstrated based on BP/hBN/BP stacks in which the locations of the NDR peaks are tunable by the electrostatic gating. As compared to the traditional tunneling diodes based on bulk materials, the tunneling devices based on thin boron nitride tunneling barrier and high mobility black phosphorus offer ultra‐high‐speed response. This feature, together with the NDR characteristics, provides the potential for applications in THz oscillators and multi‐value logic devices. 

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5. Solution-Processed Organic and ZnO Field-Effect Transistors in Complementary Circuits

   https://doi.org/10.3390/electronicmat2020006  

   Barron, John; Pickett, Alec; Glaser, James; Guha, Suchismita (June 2021, Electronic Materials)

   null (Ed.)

   The use of high κ dielectrics lowers the operating voltage in organic field-effect transistors (FETs). Polymer ferroelectrics open the path not just for high κ values but allow processing of the dielectric films via electrical poling. Poled ferroelectric dielectrics in p-type organic FETs was seen to improve carrier mobility and reduce leakage current when compared to unpoled devices using the same dielectric. For n-type FETs, solution-processed ZnO films provide a viable low-cost option. UV–ozone-treated ZnO films was seen to improve the FET performance due to the filling of oxygen vacancies. P-type FETs were fabricated using the ferroelectric polymer poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) as the dielectric along with a donor–acceptor polymer based on diketopyrrolopyrrole (DPP-DTT) as the semiconductor layer. The DPP-DTT FETs yield carrier mobilities upwards of 0.4 cm2/Vs and high on/off ratios when the PVDF-TrFE layer is electrically poled. For n-type FETs, UV–ozone-treated sol–gel ZnO films on SiO2 yield carrier mobilities of 10−2 cm2/Vs. DPP-DTT-based p- and ZnO-based n-type FETs were used in a complementary voltage inverter circuit, showing promising characteristic gain. A basic inverter model was used to simulate the inverter characteristics, using parameters from the individual FET characteristics. 

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