Researchers from Taiwan’s National Yang Ming Chiao Tung University (NYCU) and TSMC Corporate Research have developed a new way to improve transistors made from atomically thin materials. They focuses on the tiny boundary where two materials meet, which allowed them to build transistors with an extremely thin insulating layer while maintaining strong electrical performance.According to Science Daily, atomically thin semiconductors have been studied for more than a decade because some can be just one atom thick while still showing useful electrical properties. Such materials could allow transistors to become smaller, faster and more energy efficient than conventional silicon-based devices.However, making these very thin transistors work well has been difficult. One major problem involves the gate dielectric, an extremely thin insulating layer that helps control the movement of electrons. Making it thinner can improve transistor control, but when it is placed on an atomically thin semiconductor, problems can develop at the boundary between the two materials. These can interfere with electron movement and reduce performance.The new study, published in Nature Electronics, addresses this problem by engineering the atomic interface between the semiconductor and the insulating layer.
Focus on Atomic Boundary
The NYCU team worked with TSMC Corporate Research to engineer the narrow region where the two materials meet. Instead of focusing only on finding a new semiconductor, the researchers changed the interface itself.The team used monolayer molybdenum disulfide, or MoS2, as the semiconductor. A monolayer is a material that is only one layer of atoms thick. The researchers first placed an ultrathin epitaxial aluminium layer directly on the MoS2.They then oxidised the aluminium to create an aluminium oxide layer about 0.42 nanometres thick. After this, they added a high-κ hafnium oxide gate dielectric, an insulating material that can provide strong electrical control while remaining very thin.The 0.42-nanometre aluminium oxide layer acted as a buffer between the MoS2 and hafnium oxide. It created a smoother surface, helping the hafnium oxide grow more uniformly, while also reducing unwanted electrical interactions between the two materials.“For many years, efforts to improve atomically thin transistors have largely focused on discovering better semiconductor materials,” said Professor Wen-Hao Chang, the study’s corresponding author from NYCU. “Our research shows that the atomic interface between materials can be just as important. By engineering that boundary, we were able to reduce one of the fundamental trade-offs that has limited two-dimensional transistors for many years,” he added.
Why interface matters
Atomically thin semiconductors create a particular challenge because their surfaces do not have dangling bonds. This makes it difficult to grow an extremely thin dielectric layer evenly across the surface.Standard deposition methods can leave gaps and defects at the interface. These can create electrical disorder and reduce carrier mobility, which refers to how easily electrons move through a material.
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Researchers have explored different dielectric materials, molecular seed layers and other oxide deposition methods. However, achieving a very thin equivalent oxide thickness, strong electrostatic control and high carrier mobility at the same time remains difficult.The challenge is especially important for wafer-scale CVD-grown monolayer MoS2. CVD, or chemical vapour deposition, is a method used to produce thin material layers over a larger area.
0.42-nanometre buffer layer
The researchers’ approach was to redesign the interface rather than replace the semiconductor or gate dielectric. They placed the ultrathin epitaxial aluminium layer on monolayer MoS2 and oxidised it to produce the 0.42-nanometre aluminium oxide layer. The high-κ hafnium oxide was then added on top.The team built short-channel top-gate transistors using CVD-grown monolayer MoS2. The devices had an equivalent oxide thickness of about one nanometre.Tests showed low leakage current and minimal hysteresis. Hysteresis refers to unwanted differences in a transistor’s electrical response depending on its previous operating condition.The transistors reached a maximum transconductance of 0.45 mS μm-1, with channel lengths of about 100 nanometres. Transconductance measures how strongly a transistor’s current responds when the gate voltage changes.The devices combined a very thin dielectric with strong electrical control and continued carrier transport, a combination that has been difficult to achieve in atomically thin transistors.
Relevance for future chips
The findings highlight the growing importance of interfaces as transistor components become smaller. When materials are only a few atomic layers thick, the region where two materials meet can strongly affect how the device performs.“When transistor components become only a few atomic layers thick, the interface is no longer simply the boundary between materials, it becomes an active part of the device,” said Professor Tsung-En Lee, the study’s corresponding author. “Learning to engineer these interfaces with atomic precision opens new opportunities for designing future semiconductor devices that would be difficult to achieve by changing individual materials alone,” professor Lee added.The researchers believe the approach could contribute to future two-dimensional electronics. Using CVD-grown monolayer MoS2 also brings the work closer to materials and processes that could eventually be considered for wafer-scale manufacturing.
