Group IV heterostructures are interesting for two reasons: Application in microelectronics, photonics, photovoltaics, and as basic material system for strained heterostructure studies.

-   Condensation and crystallization of group IV elements on Si

Driving force of the crystalline growth is minimizing of energy because the perfect crystal is the energetically favorable solid. A requirement for epitaxy is a clean substrate surface, otherwise the information about orientation of the substrate is disordered or even destroyed and the consequences are crystalline defects up to polycrystalline or amorphous growth. Modern epitaxy techniques like molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) allow the growth of very thin crystalline silicon, silicon germanium or germanium layers with well-defined dopants by very low growth temperatures. In this temperature regime, volume diffusion is negligible. Furthermore, the critical thickness of SiGe heterostructures is increasing. However, in this low temperature growth regime surface segregation of dopant or alloy atoms has turned out to be a dominant mechanism for profile smearing.

-   MBE and CVD techniques

An MBE system involves the generation of molecular beams of matrix material such as silicon, germanium or tin and doping species and their interaction with the substrate surface to form a single crystal deposit under ultrahigh vacuum (UHV) conditions. Atomic or molecular beams of the necessary species are directed toward the heated substrate and grow into epitaxial layer. The atomic or molecular fluxes of elemental constituents are evaporated or sublimated in special electron beam evaporators or in heated effusion cells. For a precise control of the beam fluxes over the substrate, all sources exhibit rapidly acting mechanical shutters and the flux of the deposition materials can be measured directly with a flux monitor, e.g. a quadrupol mass spectrometer, or indirectly over the effusion cell temperature. The formation of layers is called chemical vapor deposition (CVD) if the vapor contains chemical complexes - the precursors - which have to undergo chemical reactions to form the layer materials. CVD is a very common process in microelectronics manufacturing. The basics of silicon CVD are described in many textbooks; the reader is referred to (Chang, 1996). Rapid thermal chemical vapor deposition (RTCVD) and reduced pressure chemical vapor deposition (RPCVD) have rapidly emerged as the main production technologies for group IV alloys.  Group IV based alloys, namely SiGe, SiGeC, SiC, SiSn, SiGeSn, and GeSn, are well known to be key materials for extending the capabilities of the silicon based technologies that dominate the micro-electronics industry.   These alloys are fully compatible with Si technology and display advantageous electrical, optical, chemical, and mechanical properties.

-   Strain and misfit dislocations: Critical thickness and metastable growth

SiGe (silicon germanium) is lattice mismatched against the Si substrate commonly used in microelectronics. The lattice mismatch f is nearly linear with Ge content x (Vegard’s law: f = 0.0417x). Exactly, a slight parabolic bow (Dismukes law) is proven for both bulk SiGe and epitaxial SiGe.

The lattice mismatch causes strained layer SiGe growth (pseudomorphic SiGe) up to a critical thickness t_c. Thicker layers start to relax the strain by the introduction of misfit dislocation networks at the interface. By kinetic reasons (nucleation of dislocations at surface sites) the interface dislocation network is connected to the surface by threading dislocations. The onset of strain relaxation in the metastable regime is retarded by dislocation nucleation barriers and by slow motion of misfit dislocations. For clean surfaces as used in microelectronics processing, we proposed (Kasper, 2012) a metastable critical thickness band with a lower and an upper bound.

-   Accommodation of lattice mismatched heterostructures: Thin virtual substrates

Structures able to adjust the strain of heterostructure layers on top of a Si substrate are called strain platforms. A universal solution to strain platforms is given by the so called virtual substrates which are composed of a thin Si substrate and an ultrathin strain relaxed buffer layer of a mismatched material system (e.g. SiGe or GeSn) offering a surface lattice constant from the underlying Si substrate. Our group (Lyutovich, 1999) managed to adjust the lattice constant in ultrathin SiGe buffers by nucleation of misfit dislocations from supersaturated point defect concentrations. Supersaturation of point defects was controlled by Si ion impact (100 eV to 1000 eV energy) or very low growth temperature intervals.

-   Dopant segregation and methods to overcome it

Surface segregation during growth which is well-known from studies on dopant incorporation in Si has attracted attention for Ge/Si and Sn/Ge. Quantitatively, surface segregation is described by the segregation coefficient r_s  that expresses the ratio of the impurity surface concentration n_s to the bulk concentration n. In general, r_s may not solely depend on growth parameters (temperature T and growth rate R) but also on surface coverage itself (Berbezier, 2011).

-   GeSi in microelectronics, photonics and photovoltaics: Status and prospects

Heterostructure SiGe is extensively used in microelectronics circuits as stressor for PMOS channels in processors and as high frequency booster in microwave circuits (e.g. SiGe HBT for automotive radar). In future, high mobility Ge channels in CMOS will allow continuation of the anticipated performance progress. In Si photonics, integration of SOI-based waveguides with Ge detectors, modulators, and lasers is rapidly progressing. GeSn devices on Si will expand the wavelengths window from 1.2µm to beyond 2µm into the mid infrared. For photovoltaics, thick Ge buffer layers on Si are used in high efficiency tandem solar cells.

References:

C.Y. Chang and S.M. Sze, ULSI technology, McGraw-Hill (1996)

E. Kasper, N. Burle, S. Escoubas, J. Werner, M. Oehme and K. Lyutovich, "Strain relaxation of metastable SiGe/Si: Investigation with two complementary X-Ray Techniques", J. App. Phys., 111, 063507 (2012)

 Lyutovich, K., Ernst, F., Kasper, E., Bauer, M. Oehme, M. Interaction between Point Defects and Dislocations in SiGe, Solid State Phenomena  69-70, p. 179-184 (1999)

I. Berbezier, J. P. Ayoub, A. Ronda, M. Oehme, K. Lyutovich, E. Kasper, M. Di Marino, G. Bisognin, E. Napolitani, M. Berti, "Strain engineered segregation regimes for the fabrication of thin Si1-xGex layers with abrupt n-type doping", J. Appl. Phys. 107, 034309 (2010)

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