Black phosphorus field-effect transistors (1401.4117v1)

Abstract: Two-dimensional crystals have emerged as a new class of materials with novel properties that may impact future technologies. Experimentally identifying and characterizing new functional two-dimensional materials in the vast material pool is a tremendous challenge, and at the same time potentially rewarding. In this work, we succeed in fabricating field-effect transistors based on few-layer black phosphorus crystals with thickness down to a few nanometers. Drain current modulation on the order of 10E5 is achieved in samples thinner than 7.5 nm at room temperature, with well-developed current saturation in the IV characteristics, both are important for reliable transistor performance of the device. Sample mobility is also found to be thickness dependent, with the highest value up to ~ 1000 cm2/Vs obtained at thickness ~ 10 nm. Our results demonstrate the potential of black phosphorus thin crystal as a new two-dimensional material for future applications in nano-electronic devices.

• The paper demonstrates the fabrication of few-layer black phosphorus FETs achieving a tenfold drain current modulation at room temperature.
• The paper employs mechanical exfoliation and ARPES to characterize a direct band gap of about 0.3 eV, aligning with ab initio calculations.
• The paper highlights that optimizing layer thickness is key, with peak mobility around 1000 cm²/V·s, underscoring its potential for high-speed devices.

Black Phosphorus Field-effect Transistors: A Technical Exploration

The paper of two-dimensional (2D) materials has seen significant advancements with the introduction of various compounds that promise to revolutionize electronics by enabling nanoscale device fabrication. This paper focuses on a prominent member of the 2D semiconductor family: black phosphorus. By fabricating few-layer black phosphorus field-effect transistors (FETs), the researchers explore this material's potential for future nano-electronic applications, delineating its structural and electronic properties that make it an attractive alternative to existing materials.

In their experimental workflow, the researchers successfully synthesized bulk black phosphorus crystals using high-temperature and high-pressure techniques. The band structure of bulk black phosphorus was characterized using angle-resolved photoemission spectroscopy (ARPES), consonant with ab initio calculations, showing a band gap of approximately 0.3 eV. The significance of these few-layer phosphorus transistors lies in their ambipolar behavior, substantial drain current modulation, and mobility that are either superior or on par with other existing 2D materials like MoS₂.

Key Experimental Findings and Numerical Results

The fabrication of few-layer black phosphorus transistors was achieved through mechanical exfoliation methods, depositing them on silicon substrates with thermally grown SiO₂ layers. These transistors demonstrated commendable electrical performance, evidenced by the following observations:

• The drain current modulation in transistors with a channel thickness of around 7.5 nm revealed a tenfold alteration at room temperature. This modulation is considerably higher than that observed in graphitic devices, stressing black phosphorus's potential as a digital electronic material.
• Mobility metrics are strikingly thickness-dependent, with a peak mobility of approximately 1000 cm²/V·s observed at a thickness of 10 nm. This surpasses typical values found in silicon-based transistors and is comparable with MoS₂ devices, marking black phosphorus as a formidable candidate for high-speed transistor applications.

The investigation of few-layer phosphorene FETs reveals that the electrical performance is reduced as the layers increase past 10 nm, primarily due to inter-layer screening and contact resistance effects. This behavior is consistent with that of other 2D materials, explaining the necessity to optimize layer thickness for specific applications.

Implications and Future Directions

The implications of these findings extend into both electronic and optoelectronic realms. The direct bandgap characteristic of black phosphorus that varies with the number of layers holds promise, particularly for infrared applications. Furthermore, the ambipolar behavior observed in few-layer phosphorene devices points towards potential applications in logic devices where the control of electron and hole conduction is necessary.

From a theoretical standpoint, the presence of a thickness-dependent direct bandgap offers a rich field for computational studies to further understand interlayer interactions and their effects on electronic properties. Practically, integrating black phosphorus with high-k materials as gate dielectrics or employing top-gate configurations could significantly improve device performance, advancing the catering of black phosphorus in commercial electronic components.

As black phosphorus continues to attract interest, future research may focus on addressing current limitations, such as reducing subthreshold swing and improving the on-state current. These steps are essential to fully harness the performance capabilities of black phosphorus-based devices. Furthermore, research into environmental stability and contact engineering will be crucial to overcoming challenges associated with device fabrication and scalability. Overall, this paper provides a comprehensive foundational paper that propels black phosphorus to the forefront of semiconductor research within the 2D materials framework.