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1. Extension Doping with Low‐Resistance Contacts for P‐Type Monolayer WSe ₂ Field‐Effect Transistors

   https://doi.org/10.1002/aelm.202400843  

   Chen, Sihan; Zhang, Yue; King, William_P; Bashir, Rashid; van_der_Zande, Arend_M (December 2024, Advanced Electronic Materials)

   Abstract Source/Drain extension doping is crucial for minimizing the series resistance of the ungated channel and reducing the contact resistance of field‐effect transistors (FETs) in complementary metal–oxide–semiconductor (CMOS) technology. 2D semiconductors, such as MoS₂and WSe₂, are promising channel materials for beyond‐silicon CMOS. A key challenge is to achieve extension doping for 2D monolayer FETs without damaging the atomically thin material. This work demonstrates extension doping with low‐resistance contacts for monolayer WSe₂p‐FETs. Self‐limiting oxidation transforms a bilayer WSe₂into a hetero‐bilayer of a high‐work‐function WO_xSe_yon a monolayer WSe₂. Then, damage‐free nanolithography defines an undoped nano‐channel, preserving the high on‐current of WO_xSe_y‐doped FETs while significantly improving their on/off ratio. The insertion of an amorphous WO_xSe_yinterlayer under the contacts achieves record‐low contact resistances for monolayer WSe₂over a hole density range of 10¹²to 10¹³cm⁻²(1.2 ± 0.3 kΩ µm at 10¹³cm⁻²). The WO_xSe_y‐doped extension exhibits a sheet resistance as low as 10 ± 1 kΩ □⁻¹. Monolayer WSe₂p‐FETs with sub‐50 nm channel lengths reach a maximum drain current of 154 µA µm⁻¹with an on/off ratio of 10⁷–10⁸. These results define strategies for nanometer‐scale selective‐area doping in 2D FETs and other 2D architectures. 

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2. 3,4,5‐Trimethoxy Substitution on an N‐DMBI Dopant with New N‐Type Polymers: Polymer‐Dopant Matching for Improved Conductivity‐Seebeck Coefficient Relationship

   https://doi.org/10.1002/anie.202110505  

   Han, Jinfeng; Chiu, Arlene; Ganley, Connor; McGuiggan, Patty; Thon, Susanna M.; Clancy, Paulette; Katz, Howard E. (November 2021, Angewandte Chemie International Edition)

   Abstract Achieving high electrical conductivity and thermoelectric power factor simultaneously for n‐type organic thermoelectrics is still challenging. By constructing two new acceptor‐acceptor n‐type conjugated polymers with different backbones and introducing the 3,4,5‐trimethoxyphenyl group to form the new n‐type dopant 1,3‐dimethyl‐2‐(3,4,5‐trimethoxyphenyl)‐2,3‐dihydro‐1H‐benzo[d]imidazole (TP‐DMBI), high electrical conductivity of 11 S cm⁻¹and power factor of 32 μW m⁻¹ K⁻²are achieved. Calculations using Density Functional Theory show that TP‐DMBI presents a higher singly occupied molecular orbital (SOMO) energy level of −1.94 eV than that of the common dopant 4‐(1, 3‐dimethyl‐2, 3‐dihydro‐1H‐benzoimidazol‐2‐yl) phenyl) dimethylamine (N‐DMBI) (−2.36 eV), which can result in a larger offset between the SOMO of dopant and lowest unoccupied molecular orbital (LUMO) of n‐type polymers, though that effect may not be dominant in the present work. The doped polymer films exhibit higher Seebeck coefficient and power factor than films using N‐DMBI at the same doping levels or similar electrical conductivity levels. Moreover, TP‐DMBI doped polymer films offer much higher electron mobility of up to 0.53 cm² V⁻¹ s⁻¹than films with N‐DMBI doping, demonstrating the potential of TP‐DMBI, and 3,4,5‐trialkoxy DMBIs more broadly, for high performance n‐type organic thermoelectrics. 

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3. Doping and compensation in heavily Mg doped Al-rich AlGaN films

   https://doi.org/10.1063/5.0082992  

   Bagheri, Pegah; Klump, Andrew; Washiyama, Shun; Hayden Breckenridge, M.; Kim, Ji Hyun; Guan, Yan; Khachariya, Dolar; Quiñones-García, Cristyan; Sarkar, Biplab; Rathkanthiwar, Shashwat; et al (February 2022, Applied Physics Letters)

   Record low resistivities of 10 and 30 Ω cm and room-temperature free hole concentrations as high as 3 × 10¹⁸ cm⁻³were achieved in bulk doping of Mg in Al_0.6Ga_0.4N films grown on AlN single crystalline wafer and sapphire. The highly conductive films exhibited a low ionization energy of 50 meV and impurity band conduction. Both high Mg concentration (>2 × 10¹⁹cm⁻³) and low compensation were required to achieve impurity band conduction and high p-type conductivity. The formation of V_N-related compensators was actively suppressed by chemical potential control during the deposition process. This work overcomes previous limitations in p-type aluminum gallium nitride (p-AlGaN) and offers a technologically viable solution to high p-conductivity in AlGaN and AlN. 

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4. Heterogeneous Integration of Atomically Thin Semiconductors for Non‐von Neumann CMOS

   https://doi.org/10.1002/smll.202202590  

   Pendurthi, Rahul; Jayachandran, Darsith; Kozhakhmetov, Azimkhan; Trainor, Nicholas; Robinson, Joshua A.; Redwing, Joan M.; Das, Saptarshi (July 2022, Small)

   Abstract Atomically thin, 2D, and semiconducting transition metal dichalcogenides (TMDs) are seen as potential candidates for complementary metal oxide semiconductor (CMOS) technology in future nodes. While high‐performance field effect transistors (FETs), logic gates, and integrated circuits (ICs) made from n‐type TMDs such as MoS₂and WS₂grown at wafer scale have been demonstrated, realizing CMOS electronics necessitates integration of large area p‐type semiconductors. Furthermore, the physical separation of memory and logic is a bottleneck of the existing CMOS technology and must be overcome to reduce the energy burden for computation. In this article, the existing limitations are overcome and for the first time, a heterogeneous integration of large area grown n‐type MoS₂and p‐type vanadium doped WSe₂FETs with non‐volatile and analog memory storage capabilities to achieve a non–von Neumann 2D CMOS platform is introduced. This manufacturing process flow allows for precise positioning of n‐type and p‐type FETs, which is critical for any IC development. Inverters and a simplified 2‐input‐1‐output multiplexers and neuromorphic computing primitives such as Gaussian, sigmoid, and tanh activation functions using this non–von Neumann 2D CMOS platform are also demonstrated. This demonstration shows the feasibility of heterogeneous integration of wafer scale 2D materials. 

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5. Intrinsic (Trap‐Free) Transistors Based on Epitaxial Single‐Crystal Perovskites

   https://doi.org/10.1002/adma.202205055  

   Bruevich, Vladimir; Kasaei, Leila; Rangan, Sylvie; Hijazi, Hussein; Zhang, Zhenyuan; Emge, Thomas; Andrei, Eva_Y; Bartynski, Robert_A; Feldman, Leonard_C; Podzorov, Vitaly (September 2022, Advanced Materials)

   Abstract The first experimental realization of the intrinsic (not dominated by defects) charge conduction regime in lead‐halide perovskite field‐effect transistors (FETs) is reported. The advance is enabled by: i) a new vapor‐phase epitaxy technique that results in large‐area single‐crystalline cesium lead bromide (CsPbBr₃) films with excellent structural and surface properties, including atomically flat surface morphology, essentially free from defects and traps at the level relevant to device operation; ii) an extensive materials analysis of these films using a variety of thin‐film and surface probes certifying the chemical and structural quality of the material; and iii) the fabrication of nearly ideal (trap‐free) FETs with characteristics superior to any reported to date. These devices allow the investigation of the intrinsic FET and (gated) Hall‐effect carrier mobilities as functions of temperature. The intrinsic mobility is found to increase on cooling from ≈30 cm²V⁻¹s⁻¹at room temperature to ≈250 cm²V⁻¹s⁻¹at 50 K, revealing a band transport limited by phonon scattering. Establishing the intrinsic (phonon‐limited) mobility provides a solid test for theoretical descriptions of carrier transport in perovskites, reveals basic limits to the technology, and points to a path for future high‐performance perovskite electronic devices. 

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