(Invited) UV Excimer Laser Assisted Heteroepitaxy of (Si)GeSn on Si(100)

The perspective to achieve Photonic Integrated Circuits (PICs) using CMOS compatible direct bandgap Group-IV heterostructures is highly attractive from scientific, technological, and economic point of view. Research activities on modeling, growing and processing adequate Group-IV materials and heterostructures for developing new emitters, waveguides and detectors have therefore been intensified over the last decade.

Several approaches directed to material and strain engineering of Silicon and Germanium micro- or nanostructures were studied and already led to significant progress in this field. However, to ensure further prospects, more versatile material systems would be of interest and theoretical as well as experimental studies in this direction identified the Silicon-Germanium-Tin (SiGeSn) ternary as an attractive but also challenging system. These studies converged finally to the intensive exploration on how to produce and process Germanium-Tin (GeSn) binary and Silicon containing Germanium-Tin (Si)GeSn alloys, that are heteroepitaxially grown on Silicon wafers or virtual Germanium (v-Ge) substrates. As a result, the possibility to tune independently band gap and lattice constant over a wide range and to achieve, by adjusting strain and composition, the predicted direct bandgap material within this system, is currently explored. As a consequence, (Si)GeSn is now expected to boost the development of new high speed micro- and nanoelectronic components, as well as of CMOS compatible opto-electronic devices and of new GeSn and SiGeSn strain platforms on conventional Si wafers, as successfully employed in Si(Ge) technology [1-4].

Major issues in growing and processing these binary and ternary (Si)GeSn alloys are the large mismatch between the lattices of the pure elements (0.54nm for Si, 57nm for Ge, and 0.69nm for a-Sn), the very low solid phase solubility of Sn in Ge of less than 1% and the tendency of  Sn to cluster and to form surface precipitates, when subjected to thermal treatment [5-7].

Due to these interconnected issues, increased attention has been devoted to, Si-technology compatible, non-equilibrium growth techniques that allow such an independent adjustment of lattice parameters and band gaps in a precise and cost efficient way. Among several well established techniques, like low temperature Molecular Beam Epitaxy (MBE) and Chemical Vapor Deposition (CVD), also Pulsed Laser Induced Epitaxy (PLIE) using Excimer lasers has been identified as a promising candidate.

This contribution will give an overview on recent achievements obtained through 193 nm ArF-Excimer Laser PLIE of (Si)GeSn alloys on v-Ge substrates and on Si wafers. The technique itself is based on local rapid melting and solidification processes, that are induced by 25 ns short ArF-Excimer laser pulses and are completed within an ultra-short time scale (<500 ns). Applying these heat treatments to Si, Ge and Sn heterostructures that are grown on Si(100), leads to epitaxial alloys with gradients in composition and strain that depend on the intermixing of these elements in the molten phase and ultimately on the thermal gradients produced by adjusting “precursor-heterostructure”, laser energy density and number of laser pulses [8-11].

This contribution will give an overview on the following PLIE approaches that have been used to obtain the binary and ternary alloys with different crystallinity, strain and stoichiometry, including Sn content of up to 20% and their respective advantages or drawbacks:

i)      PLIE treatment of low cost precursor heterostructures, like thermally evaporated Sn on Si(100) or on v-Ge.

ii)     PLIE of v-Ge wafers coated with Sn films through Molecular Beam Epitaxy (MBE).

iii)   PLIE of Boron doped or intrinsic a-Si:H or a-Ge:H films, produced by Laser Induced Chemical Vapour Deposition (LCVD) and MBE, on Sn/v-Ge.

iv)   Scanning of the substrate through the laser beam to achieve large area treatment.

v)    Production of micro-patterns by Mask Projection Assisted PLIE.

Acknowledgment

This work was partially financed by Spanish (MAT2008-02350, MAT2011-24077), Galician (2010/83) and local P.P.00VI131H64102 (UVigo) grants, co-financed by FEDER funds.

References

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[10] S. Stefanov et al., TSF 520 (2012) 3282-3285

[11] S.Stefanov et al., MEE (2014) DOI: 10.1016/j.mee.2014.03.017