Bismuth Ohmic Contacts for 2D Transistors With Low Contact Resistance
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Solution Overview
Problem
Two-dimensional semiconductor devices face challenges due to high contact resistance at metal/semiconductor interfaces, hindering their performance and scalability beyond Moore's law, as conventional metals like gold, titanium, and platinum form Schottky barriers that impede current injection at low temperatures.
Innovation Solution
Depositing bismuth on two-dimensional materials forms a new phase with a lower work function, inducing a high density of carriers and creating energy-barrier free, ohmic contacts with ultra-low contact resistance, achieving degenerate surfaces and enabling high ON-currents even at low temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional metals (gold, titanium, platinum) are used for contact interfaces, then the device structure is simple and manufacturing is easy, but high Schottky barriers form that impede current injection and increase contact resistance
Solution Approach 1:
The patent uses a composite contact structure consisting of bismuth deposited on top of conventional metal contacts (such as titanium, platinum, or gold). This composite structure combines the beneficial properties of both materials: the conventional metal provides structural stability and ease of manufacturing, while the bismuth layer reduces the Schottky barrier height and enables efficient current injection into the two-dimensional semiconductor channel, achieving ohmic contact behavior
Solution Approach 2:
The patent changes the work function parameter of the contact interface by introducing bismuth, which has a lower work function than conventional metals. This parameter change reduces the Schottky barrier height at the metal/semiconductor interface, enabling thermionic emission and ohmic contact behavior. The bismuth layer modifies the electronic properties of the contact interface without requiring changes to the underlying metal structure
2Reliability
If bismuth is deposited to form new phase with lower work function, then contact resistance is reduced to ultra-low levels, but the deposition process and phase control become more complex
Solution Approach 1:
The patent employs preliminary action by first depositing a layer of conventional metal (such as titanium, platinum, or gold) before depositing bismuth. This preliminary metal layer serves as a foundation that controls the nucleation and growth of bismuth, guiding it to form the desired low-work-function phase. The preliminary metal layer acts as a template that ensures reproducible phase formation during subsequent bismuth deposition, reducing the complexity of phase control
Solution Approach 2:
The conventional metal layer serves as an intermediary between the substrate and the bismuth layer. It mediates the interaction between the deposition process and the underlying substrate, controlling the phase formation of bismuth. This intermediary layer simplifies the manufacturing process by providing a stable platform for bismuth deposition and ensuring consistent phase formation without requiring complex in-situ phase control mechanisms
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The use of bismuth on two-dimensional materials results in ohmic contact interfaces with contact resistance approaching the quantum limit, exceeding the performance of conventional metals, enabling high-speed, low-energy devices for advanced technologies like AI and 5G communication.
Implementation Method 1
The deposition of bismuth on two-dimensional materials allows the formation of a new phase of bismuth
Implementation Method 2
The work function of such a new phase of bismuth can depend, at least in part, on the morphology of the underlying 2D material substrate, and can be different than that of other phases of bismuth deposited without the underlying 2D materials
Data Source
AI summary
Devices, such as transistors, that use bismuth to create ohmic contacts are provided, as are methods of manufacturing the same. The transistors, such as field-effect transistors, can include one or more two-dimensional materials, and electrical contact areas can be created on the two-dimensional material(s) using bismuth. The bismuth can help to provide energy-barrier free, ohmic contacts, and the resulting devices can have performance levels that rival or exceed state-of-the-art devices that utilize three-dimensional materials, like silicon. The two-dimensional materials can include transition metal dichalcogenides, such as molybdenum disulfide.