First principles modelling of tunnel field-effect transistors based on heterojunctions of strained Germanium/InGaAs alloys

  • João Carlos Correia de Abreu

Student thesis: Doctoral ThesisDoctor of Philosophy


The limitations of the down-scaling of the silicon metal–oxide–semiconductor (MOS) field-effect transistor (FET) and the demand for energy-efficient appliances motivate the study of alternative devices. The device under consideration is the biaxial tensile-strained Germanium on InGaAs alloy tunnel FET (TFET). The type of device depends on the band alignments at the heterojunction. For example, it determines if it can work as a TFET or a MOSFET. In turn, the band alignments depend on the atomistic details of the interface structure.

First-principles calculations were performed to obtain the band alignments of the simpler Ge/GaAs system. The difference of the Ge and GaAs band gaps, calculated at Many-Body Perturbation Theory (MBPT) level, is added to the potential line-up of a supercell structure containing the Ge/GaAs interface, calculated using Density Functional Theory (DFT). It was studied how the band alignment is affected by the disorder, the stoichiometry composition and the diffusion of atoms at the interface. Calculations on the relative interface formation energy were performed to determine the relative stability of the various models for the interface. Results show how the heterojunction type changes when going from As-rich to Ga-rich conditions and with the diffusion of atoms over a few layers. These results explain the interval range of the experimental measurements and highlight the challenge of obtaining a Ge/GaAs TFET as this implies the control of atomic diffusion at the interface. Results also show that As-rich interface conditions are more favourable than Ga-rich conditions and that the contribution of disorder at the interface to the potential line-up is negligible. Further, preliminary studies were performed for band gaps of strained Ge and InGaAs alloys. Besides, an alternative approach to the expensive MBPT was developed based on enforcing Koopman’s theorem to DFT to obtain accurate band gaps at a low computational cost.
Date of AwardJul 2020
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorMyrta Grüning (Supervisor) & Jorge Kohanoff (Supervisor)

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