Chinese researchers have unveiled a novel material that could revolutionize the development of two-dimensional, low-power computer chips. The team from the Shanghai Institute of Microsystem and Information Technology at the Chinese Academy of Sciences created an ultra-thin layer of aluminum oxide, just 1.25 nm thick, using a unique oxidation method at ambient temperature on single-crystalline aluminum. This material meets the stringent requirements set by the International Roadmap for Devices and Systems, offering low gate leakage, low interface state density, and high dielectric strength.
Advancing 2D Field-Effect Transistors (FETs)
As traditional silicon field-effect transistors (FETs) approach their miniaturization limits, new materials are needed to address challenges like short-channel effects. Two-dimensional (2D) materials, such as molybdenum disulfide (MoS2), have emerged as promising candidates due to their atomic thinness and high carrier mobility. However, the lack of high-quality dielectric materials has hindered the full potential of 2D FETs.
Common amorphous oxide dielectrics, like SiO2, Al2O3, and HfO2, struggle to form smooth interfaces with 2D materials, resulting in high gate leakage and low dielectric strength. While crystalline dielectrics such as hexagonal boron nitride (hBN) and calcium fluoride (CaF2) offer smoother interfaces, they still face issues like high leakage currents and challenges in scaling down to atomic thickness.
Recently, atomically thin metal oxides have shown promise for overcoming these limitations. These materials can be easily formed on metal surfaces, providing adequate dielectric properties and flat surfaces, making them ideal for 2D FETs.
Breakthrough in 2D FET Fabrication
To explore the electronic properties of a 2D FET based on single-crystalline aluminum oxide (c-Al2O3), researchers developed a self-aligned MoS2 FET using a 2 nm thick layer of c-Al2O3. Each FET featured an aluminum gate with a width of 100 µm and a length of 250 nm, with a tiny air gap between the gold and aluminum gates ensuring complete insulation. The channel length measured 300 nm, and cross-sectional transmission electron microscopy (TEM) images highlighted the MoS2 FET’s structure.
The fabrication process used a van der Waals (vdW) transfer method to align the source, drain, dielectric, and gate materials on a graphene/Ge donor wafer before transferring the entire FET stack onto the channel material in a single step. This approach resulted in a 2D FET with excellent contact and dielectric interfaces. The MoS2 FET’s output and transfer characteristics demonstrated promising current control and saturation, with a sharp increase in drain current in the subthreshold region.
For long-channel MoS2 FETs, the researchers achieved a subthreshold swing (SS) of just 61 mV/dec, very close to the theoretical limit of 60 mV/dec at room temperature (300 K). These devices also exhibited very low hysteresis, indicating fewer issues with trapped charges in the c-Al2O3 layer.
Superior Performance and Scalability
When comparing FETs made with single-crystalline aluminum oxide (c-Al2O3) to those made with amorphous aluminum oxide (a-Al2O3), the former performed significantly better, with lower SS and less hysteresis, suggesting more efficient operation and reliability.
To demonstrate scalability, the researchers fabricated FET arrays on a 4-inch chemical vapor deposition (CVD) MoS2 wafer. The high-quality dielectrics ensured excellent electrostatic control, and the arrays exhibited uniform n-type characteristics across 100 MoS2 FETs. Approximately 70% of the devices displayed SS values between 75 and 175 mV/dec and an on/off current ratio greater than 10^6, indicating that this technique could enable the fabrication of high-performance transistors and complex circuits in the future.
“This breakthrough will serve as the foundation for further advancements in the diversity, scalability, and manufacturability of single-crystalline oxides, facilitating the seamless transition of 2D semiconductors from laboratories to industrial environments,” the research team stated.
By Impact Lab
