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Epitaxial Growth of Direct-gap Group IV Materials

There are several routes for bandgap engineering a fundamental direct bandgap semiconductor solely from Group IV elements. Germanium is considered to be a promising candidate, since a small difference in energy between the direct Γ-valley and the indirect L-valley of only ~140 meV is inherited. Starting from bulk Ge, the incorporation of Sn atoms, the employment of tensile strain or a combination of both have been known to further reduce and even reverse the valley separation. Theoretical calculations, obtained with colleagues from the University of Leeds, exemplarily show this behavior for tensile strained Ge and cubic GeSn (Fig. 2 b). Since the position of the indirect-to-direct transition in GeSn strongly depends on the residual compressive strain, optimizing the epitaxial growth conditions is of great importance.

bandstructure calculations for biaxial tensile strained Ge (bottom, from [6])Figure 1.: Schematic crystal structure of cubic GeSn alloys.

Perhaps the most promising approach so far is the incorporation of high, non-equilibrium concentrations of Sn into the Ge lattice. Together with colleagues from the Paul Scherrer Institut (PSI) in Switzerland, we have recently shown the first unambiguous proof of a fundamental direct bandgap group IV semiconductor. We employed a joint density of states (JDOS) model to associate the temperature dependent photoluminescence (PL) signal with the band offset ΔE between Γ- and L-valleys. The direct gap behavior of a partially relaxed GeSn alloy with 13 at.% Sn is definitely demonstrated in the upper part of Figure 2. From our measurements we were able determine the required Sn concentration for the indirect-to-direct transition of cubic GeSn alloys to approx. 9 at.%, as shown in the lower part of Figure 2.
Nevertheless, our efforts are not limited to GeSn alloys. PL measurements performed on SiGeSn are suggesting that engineering a direct gap is feasible also in ternary group IV alloys.

Temperature-dependent PL signal and JDOS model for obtaining band offsets between ?- and L-valleys.Figure 2. (a): Temperature-dependent PL signal and JDOS model for obtaining band offsets between ?- and L-valleys (from [7]).

Indirect-to-direct transition for cubic GeSn alloys (both from [7]).Figure 2. (b): Indirect-to-direct transition for cubic GeSn alloys (from [7]).

The epitaxial growth of (Si)GeSn alloys on Si and Ge is hampered by the large lattice mismatch of 15% between Ge and alpha-Sn. Moreover, the tendency of Sn to segregate to the surface and form precipitations in the volume makes the optimization of the non-equilibrium growth conditions very challenging. Our growth studies of GeSn and SiGeSn alloys on 200 mm Si(100) and Ge virtual substrates (Ge-VS) are performed in a cold-wall reduced-pressure chemical vapor deposition (RPCVD) tool (CVD cluster) with showerhead design, as sketched in Fig. 3 a).

Figure 3. (a): Schematic of a CVD reactor with showerhead design.Figure 3. (a): Schematic of a CVD reactor with showerhead design.

Figure 3. (b): Growth schematics of GeSn epitaxy on Ge-VS using Ge2H6 and SnCl4.Figure 3. (b): Growth schematics of GeSn epitaxy on Ge-VS using Ge2H6 and SnCl4.

We employ Si2H6, Ge2H6 (10% diluted in H2) and SnCl4 precursors and growth temperatures between 325°C-475°C. Our reactor allows the use of PH3 and B2H6 for in-situ n- and p-type doping of grown layers, beneficial for fabricating readily doped heterostructures for opto- and nanoelectronic devices. An artistic view of the reactor chamber is depicted in Fig. 3 b).
X-ray diffraction (XRD) (Bruker-XRD tool) θ-2θ (004) scans of compressively strained GeSn layers with Sn contents up to 13 at.% are shown in Fig. 4 a). Well-defined Pendellösung fringes are visible, evidencing high crystalline quality layers with smooth surfaces and abrupt interfaces. Rutherford Backscattering Spectroscopy (Tandetron) in both random and channeling alignment can be performed on GeSn alloys. Both spectra for a pseudomorphic GeSn sample are shown in Fig. 4 b). The small χmin value, defined as ratio between random and channeling spectra, of about 7 % indicate a high substitutionality of Sn atoms in the Ge lattice.

Figure 5 (a): XRD scans of several pseudomorphic GeSn & SiGeSn alloys (from [6]).Figure 4. (a): XRD scans of several pseudomorphic GeSn & SiGeSn alloys. © IOP Publishing. Reproduced with permission. All rights reserved [9].

Figure 4. (b): RBS spectrum of pseudomorphic GeSn layer.Figure 4. (b): RBS spectrum of pseudomorphic GeSn layer.

Epitaxy of SiGeSn layers allows variation of Si and Sn concentrations up to 20 at.% and 14 at.%, respectively. Theoretical studies have shown that in fully compressively strained (Si)GeSn layers, occurring naturally from low temperature growth on Ge-VS, rather huge Sn concentrations are required to achieve the indirect-to-direct bandgap transition. Therefore strain relaxation, e.g. by growing layers beyond the critical thickness for strain relaxation, has to be achieved maintaining high crystalline quality at the same time. A gradual strain relaxation in GeSn layers at a fixed Sn concentration of about 12.5 at.% and thicknesses between 50 nm and 1 µm is shown in Figure 5 a). The thickest layer of about 970 nm contains no more than -0.4% residual compressive strain, as determined by XRD reciprocal space mapping technique. Despite the huge thickness, a high crystalline quality, as required for optoelectronic and laser applications, is obtained, proved by XTEM micrographs in Fig. 5 b).

Figure 6. (a): Continuous strain relaxation in thick GeSn alloys with fixed composition. Figure 5. (a): Continuous strain relaxation in thick GeSn alloys with fixed composition. Reprinted with permission from Chem. Mater. 27, 4693-4702. Copyright 2015 American Chemical Society [8].

Figure 6. (b): High crystalline quality in thick GeSn layer, with defects confined at the GeSn/Ge-VS interface (all from [8]).Figure 5. (b): High crystalline quality in thick GeSn layer, with defects confined at the GeSn/Ge-VS interface. Reprinted with permission from Chem. Mater. 27, 4693-4702. Copyright 2015 American Chemical Society [8].

[1] S. Wirths, D. Buca, G. Mussler, A. T. Tiedemann, B. Holländer, P. Bernardy, T. Stoica, D. Grützmacher, and S. Mantl,
ECS J. Solid State Sci. Technol., vol. 2, no. 5, pp. N99–N102, Mar. 2013.
Reduced Pressure CVD Growth of Ge and Ge1-xSnx Alloys

[2] S. Wirths, A. T. Tiedemann, Z. Ikonic, P. Harrison, B. Holländer, T. Stoica, G. Mussler, M. Myronov, J. M. Hartmann, D. Grützmacher, D. Buca, and S. Mantl,
Appl. Phys. Lett., vol. 102, no. 19, p. 192103, May 2013.
Band engineering and growth of tensile strained Ge/(Si)GeSn heterostructures for tunnel field effect transistors

[3] S. Wirths, Z. Ikonic, A. T. Tiedemann, B. Holländer, T. Stoica, G. Mussler, U. Breuer, J. M. Hartmann, A. Benedetti, S. Chiussi, D. Grützmacher, S. Mantl, and D. Buca,
Appl. Phys. Lett., vol. 103, no. 19, p. 192110, 2013.
Tensely strained GeSn alloys as optical gain media

[4] S. Wirths, D. Buca, Z. Ikonic, P. Harrison, A. T. Tiedemann, B. Holländer, T. Stoica, G. Mussler, U. Breuer, J. M. Hartmann, D. Grützmacher, and S. Mantl,
Thin Solid Films, vol. 557, pp. 183–187, Apr. 2014.
SiGeSn growth studies using reduced pressure chemical vapor deposition towards optoelectronic applications

[5] S. Wirths, Z. Ikonic, N. von den Driesch, G. Mussler, U. Breuer, A.T. Tiedemann, P. Bernardy, B. Holländer, T. Stoica, J.M. Hartmann, D. Grützmacher, S. Mantl and D. Buca,
ECS Transactions, vol. 64, no.6, pp. 689-696, 2014.
Growth Studies Of Doped SiGeSn/Strained Ge(Sn) Heterostructures

[6] S. Wirths, D. Stange, M.-A. Pampillón, A. T. Tiedemann, G. Mussler, A. Fox, U. Breuer, B. Baert, E. San Andrés, N.D. Nguyen, J.M. Hartmann, Z. Ikonic, S. Mantl, and D. Buca,
ACS Appl. Mater. Interfaces, vol. 7, pp. 62–67, 2015.
High-k Gate Stacks on Low Bandgap Tensile Strained Ge and GeSn Alloys for Field-Effect Transistors

[7] S. Wirths, R. Geiger, N. von den Driesch, G. Mussler, T. Stoica, S. Mantl, Z. Ikonic, M. Luysberg, S. Chiussi, J. M. Hartmann, H. Sigg, J. Faist, D. Buca, and D. Grützmacher,
Nat. Photonics, vol. 9, pp. 88–92, 2015.
Lasing in direct-bandgap GeSn alloy grown on Si

[8] N. von den Driesch, D. Stange, S. Wirths, G. Mussler, B. Holländer, Z. Ikonic, J. M. Hartmann, T. Stoica, S. Mantl, D. Grützmacher, D. Buca,
Chemistry of Materials, vol. 27, no. 13, p. 4693-4702, 2015.
Direct Bandgap Group IV Epitaxy on Si for Laser Applications

[9] S. Wirths, R. Troitsch, G. Mussler, J.M. Hartmann, P. Zaumseil, T. Schroeder, S. Mantl, D. Buca,
Semicond. Sci. Technol., vol. 30, no. 5, pp. 055003, 2015.
Ternary and quaternary Ni(Si)Ge(Sn) contact formation for highly strained Ge p- and n-MOSFETs


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