Encapsulation technique enables super-thin semiconductors
A research group at the HZDR Institute for Ion Beam Physics and Materials Research in Dresden has succeeded in producing encapsulated transistors based on indium selenide and gallium selenide. The encapsulation technique protects the sensitive layers from external influences and preserves their performance. The scientists use hexagonal boron nitride (hBN) for encapsulation. It is ideally suited for this purpose because it can be formed into a thin layer and is inert, i.e. it does not react with the environment
Indium and gallium selenide are considered promising candidates for advanced high-performance applications, for example in high-frequency electronics, optoelectronics or sensor technology. The materials can be used to form flake-like layers with a thickness of only 5 to 10 atomic layers, which in turn enable the production of electronic components with extremely small dimensions. The electrical conduction in these layers takes place only in a 2-dimensional plane.
In the new encapsulation method the two-dimensional flakes are arranged between two platelets of hexagonal boron nitride and thus completely enclosed. The upper hBN layer provides the insulation to the outside, the lower layer serves as a spacer to the carrier material. This technique was originally developed by a research group at Columbia University in New York, where scientist Himani Arora learned it during a research stay. Now, as a doctoral student at the International Helmholtz Research School (IHRS) NanoNet, she is further developing the technique at the HZDR.
A particular challenge in the encapsulation technology is to establish the contacts of the semiconductor to the outside. The conventional method of vapor deposition using a photomask is not suitable for this purpose because the sensitive materials would come into contact with both chemicals and air during the process and degrade. The researchers at the HZDR therefore use a lithography-free contacting technique. This involves metal electrodes made of palladium and gold embedded in hexagonal boron nitride. In this way, an encapsulation and an electrical connection with the underlying two-dimensional layer can be achieved simultaneously.
To produce the contacts, the desired electrode pattern is etched into the hBN layer to fill the resulting holes with palladium and gold by electron beam evaporation. Then the hexagonal boron nitride with the electrodes is laminated to the 2D flake. With several contacts in an hBN flake, several circuits can be contacted and measured on a single semiconductor flake. In the later application the components are stacked in layers on top of each other.
As experiments have shown, the complete encapsulation with hexagonal boron nitride protects the two-dimensional layers from decomposition and degradation and gives them high quality and stability for a long time. This encapsulation technique is said to be robust and easily transferable to other complex two-dimensional materials. The new two-dimensional semiconductors can be produced cost-effectively and used for various applications, for example in detectors that measure the wavelengths of light. They could also be used as couplers between light and electricity, for example by generating light or switching transistors with light.
More information: www.hzdr.de
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