(Keynote) Epitaxy-Based Strain-Engineering Methods for Advanced Devices

In Si microtechnology with the economy of scale slackening its pace, energy efficiency is the technology driver of the future. The replacement of metallic by optical interconnects and the rigorous reduction of the supply voltage (V_DD) of logic circuitry are suitable options to drastically reduce power consumption. Thus the integration of optoelectronic components and steep slope devices, like the T-FET, will meet this demands. However, both elements will either need or benefit from a direct band gap material, which is not supplied by Si. Possible solutions are the given by III/V compound or by Ge based semiconductors, such as GeSn alloys grown on Si and combinations thereof. These are challenging tasks and many obstacles have to be overcome, like large lattice mismatch, alloys with compositions far beyond the solid solubility limit, antiphase domains and interface and surface states. In this paper we will discuss several epitaxial methods to address these challenges. Furthermore, basing on profound Si technology, possible fabrication routes for electronic and optoelectronic devices will be discussed.  

Nanotechnology in combination with advanced selective area epitaxy offer unique paths to fabricate high quality III/V nanostructures on Si. In particular templated selfassembly of vertical core/ shell and lateral nanowires on nanopatterned Si substrates will be discussed. The structural properties have been investigated using high resolution transmission microscopy. Fig. 1 depicts a lateral InAs nanowires grown on Si (110) substrates revealing (111) facets at the InAs/Si interface and rotational twins in the InAs nanowire.

Regarding photonic devices the most promising group IV material is Ge, since the conduction band minimum at the G-point of the Brillouin-zone (referred to as G-valley) is placed only ca. 140 meV above the fourfold degenerate indirect L-valley. To band engineer Ge towards a direct bandgap semiconductor, two approaches are very promising, Ge with a large tensile strain (> 1.5%) and relaxed GeSn alloys with Sn concentrations above 10% [1,2]. Here, both systems are discussed, however, special emphasis is put on the growth of GeSn and SiGeSn alloys. These are predicted to have a direct band gap energy of less than 0.5 eV. Thus GeSn provides advantages for optoelectronics as well as for ultralow power T-FETs, since the small direct band gap is beneficial for the realization of large tunnel currents. However, due to the large lattice mismatch to Si, the small solubility of Sn in Ge and the necessity to strain engineer the layers, epitaxy is extremely demanding. Here we introduce an epitaxial growth method using enhanced surface kinetical reactions to provide high quality SiGeSn/GeSn heterostructures on Ge virtual substrates on Si with Sn concentrations above 10%.

Fig. 2 shows a TEM cross section of a strained Ge layer embedded in relaxed SiGeSn cladding layers grown on a relaxed GeSn buffer layer. The double heterostructure provides a type I band alignment and includes doping levels for active optoelectronic devices.  Structural characterization using X-ray diffraction and RBS to determine strain and crystallographic perfection have been performed. Photoluminescence and absorption spectroscopy give insights into the indirect to direct band gap transition in these materials.

Finally, novel concepts for Tunnel-FET´s using III/V and group IV heterostructures will be discussed. The recently achieved strained-Si nanowire TFET exhibiting a subthreshold swing of less than 60 mV and large I_on/I_off ratios are base for the integration of these heterostructure concepts. SiGeSn alloys potentially promise an energy efficient platform by the integration of ultralow power and optoelectronic devices.

 [1] Camacho-Aguilera, R. E. et al. An electrically pum-ped germanium laser. Opt. Express 20, 11316–20 (2012).

[2] Bauer, M. et al. Ge–Sn semiconductors for band-gap and lattice engineering. Appl. Phys. Lett. 81, 2992 (2002).