High-Performance P-Type Field-Effect Transistors: Substitutional Doping and Thickness Control in Two-Dimensional Materials

Academic Background

As semiconductor technology progresses, silicon-based field-effect transistors (FETs) are approaching fundamental physical limits in performance enhancement. To overcome these challenges, researchers have begun exploring two-dimensional (2D) materials as potential silicon replacements. Transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), and tungsten diselenide (WSe₂), have garnered significant attention due to their atomically smooth surfaces and excellent electronic properties. However, while n-type 2D FETs have shown significant progress, the development of p-type 2D FETs lags behind. This is primarily due to Fermi level pinning at the metal–2D material interface, which leads to inefficient p-type carrier injection and high contact resistance (Rc).

The aim of this study is to address performance bottlenecks in p-type 2D FETs through substitutional doping and thickness control. In particular, the authors achieved p-type doping in MoSe₂ and WSe₂ by introducing vanadium (V), niobium (Nb), and tantalum (Ta), significantly reducing Rc and improving device performance by optimizing material thickness.

Paper Information

This work was authored by Mayukh Das, Dipanjan Sen, Najam U Sakib, and others from institutions including Pennsylvania State University and the University of Chemistry and Technology Prague. It was published online on September 24, 2024, in the journal Nature Electronics.

Research Process and Results

1. Material Preparation and Characterization

The study began with the preparation of MoSe₂ and WSe₂ single crystals doped with V, Nb, and Ta using the chemical vapor transport (CVT) technique. The doping concentrations were determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy-dispersive X-ray spectroscopy (EDS) with scanning electron microscopy (SEM). Results indicated that the doping concentrations of V, Nb, and Ta in MoSe₂ and WSe₂ were below 0.8 at.% and 0.4 at.%, respectively.

To verify the electrical activation of dopants, Hall effect measurements were performed. Results showed that all doped samples exhibited p-type carrier characteristics, with doping concentrations consistent with their levels of electrical activation. For instance, the carrier concentrations in V-, Nb-, and Ta-doped MoSe₂ were 1.51×10¹⁹ cm⁻³, 1.57×10¹⁹ cm⁻³, and 5.36×10¹⁹ cm⁻³, respectively.

2. Device Fabrication and Performance Testing

The researchers mechanically exfoliated the doped crystals and transferred them onto silicon substrates coated with ALD-grown 50 nm thick aluminum oxide (Al₂O₃) as the back-gate dielectric. Electron beam lithography and electron beam evaporation were used to define the source and drain contacts, with palladium–gold (Pd/Au) employed as the contact metal. Pd’s high work function facilitates efficient hole injection.

Device testing of doped MoSe₂ and WSe₂ FETs with varying thicknesses revealed contrasting behavior: - Thick layers (4–6 layers): High on-state current (Ion) but poor electrostatic control, resulting in low on/off current ratios (Ion/Ioff). - Thin layers (1–3 layers): High Ion/Ioff ratios but reduced doping effectiveness due to quantum confinement effects (QCE), leading to high Rc and lower Ion.

3. Density Functional Theory (DFT) Calculations

To understand the relationship between doping effectiveness and material thickness, the researchers performed DFT calculations. Results indicated that as thickness decreased, doping effectiveness diminished, consistent with experimental observations. For instance, in 8-layer MoSe₂, Nb doping resulted in a Fermi level (Ef) shift of 300 meV relative to the valence band maximum (Ev), whereas for monolayer MoSe₂, the shift was only 30 meV. Additionally, DFT calculations revealed that doping caused bandgap narrowing (Eg) in MoSe₂, attributed to lattice strain from the atomic radius mismatch between dopants and molybdenum atoms.

4. Optimization of Contact Resistance

Through transfer length method (TLM) measurements, the study found that thick doped FETs exhibited significantly reduced Rc values. For example, Rc in Nb-doped MoSe₂ FETs reached as low as 95 Ω·µm. However, Rc increased substantially as thickness decreased. To balance low Rc with good electrostatic control, the researchers proposed a new FET structure: thick multilayer (>6 layers) regions under source/drain contacts and thin-layer (1–3 layers) regions in the channel.

5. High-Performance Dual-Gated FETs

To further improve device performance, a dual-gated structure was introduced. The channel length was scaled to 50 nm, and Al₂O₃ was used as both the top and bottom gate dielectric layers. This configuration achieved improved electrostatic control, yielding a high on-state current of 212 µA/µm and Ion/Ioff of 10⁴ in Nb-doped MoSe₂ FETs.

Conclusions and Implications

The study successfully demonstrated high-performance p-type 2D FETs using substitutional doping and thickness control. Key findings include: 1. Thick doped layers: Effective in reducing Rc, with Rc as low as 95 Ω·µm, but suffered from poor electrostatic control. 2. Thin layers: Achieved high Ion/Ioff ratios (>105) but had low Ion values due to high Rc. 3. Optimized device structure: Combining thin channel regions and thick contact regions resulted in devices with high Ion (85 µA/µm), low Rc (2 kΩ·µm), and high Ion/Ioff (10⁴). 4. Dual-gated architecture: Further improved on-state current to 212 µA/µm.

This research provides valuable insights into the design and optimization of p-type 2D FETs for potential applications in complementary metal–oxide–semiconductor (CMOS) technologies.

Research Highlights

1. High-Performance P-Type FETs: Achieved low Rc and high Ion/Ioff through combined substitutional doping and thickness control.
2. Novel Device Structure: Introduced a hybrid thick-contact/thin-channel design to optimize both Rc and electrostatic gate control.
3. Dual-Gate Design: Further enhanced device performance, achieving record high Ion values for p-type 2D FETs.
4. Theoretical Insights: Provided DFT-based evidence explaining the relationship between QCE, thickness, and doping effectiveness.

Future Directions

The study opens new pathways for the application of 2D materials in electronic devices. Future efforts will focus on: - Optimizing doping processes and exploring efficient dopants for broader 2D materials. - Advancing scalable synthesis methods (e.g., CVD) for large-area applications. - Integrating these devices into CMOS-compatible circuits to realize the next generation of electronic components.