GeSn heterobipolar phototransistors

• Applicant: Dr.-Ing. Michael Oehme
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 546089873

The aim of this project proposal is to develop and build an integratable Group IV HPT. Under vertical light irradiation, this phototransistor should have an optical responsivity of significantly greater than 10 A/W at the telecommunications wavelength of 1550 nm. To achieve this, scientific and material engineering challenges must be overcome. This includes in particular the development of a device-relevant database of the physical properties of the heterostructures used for a precise simulation of optoelectronic components, material optimization and stabilization of quantum well structures made of the material GeSn, which is highly unstable as a bulk material, and the gradual further development of an integrable transistor type, the one increase in signal-to-noise ratio allowed by internal amplification. The simulation of quantum devices requires information on the band gap, but also on its division into energy jumps at the valence and conduction band edges (band offsets). These quantities are strongly dependent on the Sn content and the elastic strain of the thin layers. At high concentrations, the necessary doping of the layers leads to a merging of the doping levels with the near band edge, a phenomenon that is known in semiconductor physics under the keyword “bandgap narrowing”, but is often neglected in idealized simulations. The influence of this effect is important in real simulations, since the connections of the actual devices are highly doped to achieve a well-conductive metal-semiconductor transition. The phase diagram of GeSn in volume equilibrium shows a decomposition into a mixture of two phases for more than 1% Sn. Thin GeSn layers on Ge/Si could be stabilized to over 25% Sn content in the diamond lattice. However, these layers exhibit two types of defects with energy levels in the band gap that result in a dark current several orders of magnitude larger than the ideal dark current of a p/n junction. In this project, we will try to achieve further stabilization by balancing the elastic stresses (minimizing the stress energy of multiple layers through alternating tensile and compressive stresses) with the focus on dark currents that are as low as possible. Internal gain in a transistor structure improves the signal-to-background ratio. We choose the hetero-bipolar phototransistor as the device because preliminary work on this has shown encouraging results. Technologically, the realization can take place in two stages, first with a free-floating base with a multi-quantum well absorber and then with the application of an optimal base voltage. The latter version is associated with increased technological effort for the individual transistor, but is much more integration-friendly. The hetero-bipolar phototransistors must be designed in such a way that they can be integrated on the silicon platform. This is the only way they can be used in a CMOS-integrated camera system.

• DFG Programme: Research Grants