Ultra low temperature molecular beam epitaxy: A versatile tool for cutting edge group IV based electronics and optoelectronics

Semiconductors, in particular silicon (Si), form the indispensable basis of all modern data processing and communication technology. Facing a continuously limited scopefor further device miniaturization, today’s rapidly increasing demands in terms of performance and complexity while maintaining cost-effectiveness in fabrication pose major challenges for science and industrial research. This requires a technological and functional diversification retaining the present Si-based platform. In this context the properties of germanium (Ge) in form of strained epitaxial (Si)Ge films with high Ge content grown on Si(001) wake high prospects for a platform-compatible implementation of novel semiconductor devices and -functionalities for classical and emerging quantum technology. However, in the underlying Si/Si1−xGex heteroepitaxy the accessible parameter range for Ge fraction (x) and layer thickness (d) is fundamentally limited by the inherent lattice mismatch of about 4% among the Si and Ge crystal structures. For common epitaxial growth temperatures of TS ≥ 500◦C this quickly leads to plastic or elastic relaxation of the respective epilayer above a misfit-dependent critical thickness of just dc < 1 nm for pure Ge, which significantly restricts the flexibility of the material system with regard to the envisioned applications.
Within this PhD thesis it has been demonstrated that the TS-dependence of dc is a decisive leverage point to overcome the currently assumed limits for strained-layer Si/(Si)Ge heteroepitaxy. By applying ultra-low growth temperatures (ULTs) TS ≤ 350◦C in molecular beam epitaxy (MBE) the growth of strongly-supersaturated, pseudomorphic Si/Si1−xGex layers with excellent surface properties and no signs of elastic or plastic relaxation have been achieved within the x ≥ 0.4 composition range exceeding the theoretically predicted values for dc by far. In contrary to the general belief that ULTs inevitably entail a poor crystalline quality of the grown epilayers, further studies on the growth of pure Ge directly on Si in the range of 100 ≤ TS ≤ 300◦C and 1 ≤ d ≤ 16 nm show clear evidence that a high crystallinity can be maintained even at the lowest TS, enabled by an extraordinary pristine growth environment with active growth pressures close to or even within the extremely-high vacuum (XHV) range. Based on this convincing results three initial applications for ULT(-XHV) MBE epilayers in the fields of electronics, opto-electronics and optics have been pursued highlighting the excellent material quality and versatility of the approach: (i) Reconfigurable field-effect transistors (RFETs) which can reversibly switch between n- and p-type operation at runtime have been processed and tested by collaborators on the basis of (Si)Ge layers grown at ULTs on silicon-on-insulator (SOI) substrates. The respective joint efforts already led to the first demonstration of a top-down fabricated 4 nm Ge-channel RFET with highly-symmetric on-states. (ii) Prototypes of advanced group-IV light-emitting diodes hosting a Si/SiGe/Si double-heterostructure based on a 16 nm Si0.6Ge0.4 central layer grown at ULTs have been realized featuring a transition from a type-II to the more favorable type-I band alignment in forward operation with strong room-temperature electroluminescence within the telecom spectral range. (iii) By growing thin layers of carbon-doped Si:C at ULTs the all-epitaxial formation of C-based Si color centers with excellent optical properties has been achieved. Differently from the conventional fabrication via ion implantation the ULT(-XHV) MBE approach allows the control of the vertical emitter position within a structure at the nanoscale which is a crucial asset for efficient photonic- and device integration.