Tomorrow’s electronics are getting smaller so researchers are looking for tiny components that function reliably in increasingly narrow configurations. Promising elements include the chemical compounds indium selenide (InSe) and gallium selenide (GaSe). In the form of ultra-thin layers, they form two-dimensional (2D) semiconductors. But they degrade when they get in contact with air during manufacturing.
A new technique allows the sensitive material to be integrated in electronic components without losing its desired properties. Encapsulated transistors were made that are based on indium selenide and gallium selenide. The encapsulation technique protects the sensitive layers from external impacts and preserves its performance. For encapsulation, the scientists use hexagonal boron nitride (hBN), which can be formed into a thin layer and is also inert, so it does not respond to its environment.
Indium and gallium selenide are seen as promising candidates for various applications in areas such as high-frequency electronics, optoelectronics, and sensor technology. These materials can be made into flake-like films only 5 to 10 atomic layers thick that can be used to produce electronic components of extremely small dimensions. During encapsulation, the 2D flakes are arranged between two layers of hBN and thus completely enclosed. The upper hBN layer is responsible for outward insulation and the lower one for maintaining distance to the substrate.
One of the particularly big challenges posed by the encapsulation technique was to apply external contacts to the semiconductors. The usual method of evaporation deposition using a photomask is unsuitable because during this process, the sensitive materials come into contact both with chemicals and with air and thus degrade. Researchers employed a lithography-free contacting technique involving metal electrodes made of palladium and gold embedded in hBN foil. This means the encapsulation and the electric contact with the 2D layer underneath can be achieved concurrently.
In order to produce the contacts, the desired electrode pattern is etched onto the hBN layer so that the holes created can be filled with palladium and gold by means of electron beam evaporation. The hBN foil is then laminated with the electrodes onto the 2D flake. When there are several contacts on an hBN wafer, contact with several circuits can be made and measured. For later application, the components will be stacked in layers.
Experiments showed complete encapsulation with hexagonal boron nitride protects the 2D layers from decomposition and degradation and ensures long-term quality and stability. The encapsulation technique is robust and easy to apply to other complex 2D materials. The new 2D semiconductors are inexpensive to produce and can be used for various applications such as detectors that measure light wavelengths. Another example of use would be as couplers between light and electronic current by generating light or switching transistors using light.