In vapor-liquid-solid (VLS) epitaxy, precursor atoms contribute to the growth process via an interaction with the liquid-vapor interface of the metal catalyst. Experiments showed that different sizes of liquid-vapor interface result in different VLS growth rates due to different doses of precursor atoms that reach the metal catalyst, even with the same CVD growth conditions. By tuning the size of the liquid-vapor interface, a solidified eutectic microstructure composed of metal catalyst and semiconductor(s) with unique (semiconductor) nanostructures can be achieved, due to different eutectic compositions. The ability to control and isolate these semiconductor nanostructures are attractive as building blocks for novel opto- and nano-electronic device applications.

An epitaxial semiconductor nanostructure obtained by tuning liquid-vapor interface of VLS epitaxy

2. Integration of Dissimilar Semiconductors for Si Photonics

The vapor-liquid-solid (VLS) mechanism revealed the role of metal as a catalyst that can be used to control the first nucleation sites. For decades, this has been seen as an attractive way to realize small, one-dimensional wires to confine carriers that had led to the birth of many nanometer-scale wire (nanowire) based devices. Recently, larger-scale VLS growth is gaining “back” some attention for their potential applications in advanced heteroepitaxy to fabricate high-quality semiconductor heterostructures, highly desirable for Si photonics applications. For these purposes, larger-sized films are often preferred for industrial applications. 

Metal catalysts can be used to realize lateral crystal growth. In this way, the first nucleated dislocation can be engineered to propagate laterally along with the crystal growth front without nucleating new dislocations along the way resulting in films with low threading dislocation densities. The metal catalyst-assisted VLS heteroepitaxy promises across-wafer films, bufferless films, and a direct electrical connection of films to the substrate. Also, It promises low temperature and high growth rates which are advantageous for VLSI applications. Utilizing the VLS mechanism, we have successfully grown high-quality Ge and InP films on Si substrates for integrable mid-infrared photodetector and lasers, respectively.

Controls on the Ge epilayer size by tuning the Au size with an agreement between experiments and theory (red curve)

3. Novel Semiconductor Heterostructures for Quantum Technologies

Recently, GeSiSn, which is an alloy of the group-IV semiconductors, Si, Ge, and semimetal Sn, has been an attractive material for group IV-based photonics in the mid-infrared (MIR) regime, which offers various potential applications from molecular spectroscopy until MIR imaging. This is possible due to its tunable narrow bandgap.

However, thermal noise due to its narrow bandgap is still one of critical issues. To suppress the thermal noise, it is important to develop devices with a functionality of negative differential resistance such as resonant tunneling diodes (RTDs) based on GeSn utilizing quantum resonant tunneling transport. In this study, we demonstrated the formation of GeSn/GeSiSn quantum heterostructures with an ultra-thin thickness that shows resonant tunneling properties.

First observation of quantum resonant tunneling in GeSn-based quantum heterostructures (sample 1)

Cambridge Centre for Gallium NitrideDept. Materials Science and Metallurgy, University of Cambridge27 Charles Babbage Road, Cambridge CB3 0FSUnited Kingdom