Abstract

We propose a robust fabrication process for growing Ge and Ge-based heterostructures in growth windows with Si sidewalls which can be applied to growth in thick Si optical waveguides. Sidewall growth is eliminated by the presence of a dielectric spacer layer which covers the sidewalls. We demonstrate the effectiveness of this process by selective-area growth of Ge and Ge/SiGe quantum wells, and show an improved performance and increased process reliability over previous work.

© 2012 OSA

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References

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  1. J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics4(8), 527–534 (2010).
    [CrossRef]
  2. M. Kim, O. O. Olubuyide, J. U. Yoon, and J. L. Hoyt, “Selective epitaxial growth of Ge-on-Si for photodiode applications,” ECS Trans.16, 837–847 (2008).
    [CrossRef]
  3. S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
    [CrossRef]
  4. N. N. Feng, S. Liao, D. Feng, X. Wang, P. Dong, H. Liang, C. C. Kung, W. Qian, Y. Liu, J. Fong, R. Shafiiha, Y. Luo, J. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “Design and fabrication of 3μm silicon-on-insulator waveguide integrated Ge electro-absorption modulator,” Opt. Express19(9), 8715–8720 (2011).
    [CrossRef] [PubMed]
  5. H. Ishii, Y. Takahashi, and J. Murota, “Selective Ge deposition on Si using thermal decomposition of GeH4,” Appl. Phys. Lett.47(8), 863–865 (1985).
    [CrossRef]
  6. S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett.98(15), 151108 (2011).
    [CrossRef]
  7. W. Franz, “Influence of an electric field on an optical absorption edge,” Z. Naturforsch. B13a, 484–489 (1958).
  8. L. V. Keldysh, “The effect of a strong electric field on the optical properties of insulating crystals,” Sov. Phys. JETP7, 788–790 (1958).
  9. Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
    [CrossRef] [PubMed]
  10. D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE97(7), 1166–1185 (2009).
    [CrossRef]
  11. G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4(8), 518–526 (2010).
    [CrossRef]
  12. N. N. Feng, P. Dong, D. Zheng, S. Liao, H. Liang, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “Vertical p-i-n germanium photodetector with high external responsivity integrated with large core Si waveguides,” Opt. Express18(1), 96–101 (2010).
    [CrossRef] [PubMed]
  13. S. Ren, T. I. Kamins, and D. A. B. Miller, “Thin dielectric spacer for the monolithic integration of bulk germanium or germanium quantum wells with silicon-on-insulator waveguides,” IEEE Photon. J.3(4), 739–747 (2011).
    [CrossRef]
  14. A. Nayfeh, C. O. Chui, K. C. Saraswat, and T. Yonehara, “Effects of hydrogen annealing on heteroepitaxial-Ge layers on Si: Surface roughness and electrical quality,” Appl. Phys. Lett.85(14), 2815–2817 (2004).
    [CrossRef]

2012

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

2011

N. N. Feng, S. Liao, D. Feng, X. Wang, P. Dong, H. Liang, C. C. Kung, W. Qian, Y. Liu, J. Fong, R. Shafiiha, Y. Luo, J. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “Design and fabrication of 3μm silicon-on-insulator waveguide integrated Ge electro-absorption modulator,” Opt. Express19(9), 8715–8720 (2011).
[CrossRef] [PubMed]

S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett.98(15), 151108 (2011).
[CrossRef]

S. Ren, T. I. Kamins, and D. A. B. Miller, “Thin dielectric spacer for the monolithic integration of bulk germanium or germanium quantum wells with silicon-on-insulator waveguides,” IEEE Photon. J.3(4), 739–747 (2011).
[CrossRef]

2010

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4(8), 518–526 (2010).
[CrossRef]

N. N. Feng, P. Dong, D. Zheng, S. Liao, H. Liang, R. Shafiiha, D. Feng, G. Li, J. E. Cunningham, A. V. Krishnamoorthy, and M. Asghari, “Vertical p-i-n germanium photodetector with high external responsivity integrated with large core Si waveguides,” Opt. Express18(1), 96–101 (2010).
[CrossRef] [PubMed]

J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics4(8), 527–534 (2010).
[CrossRef]

2009

D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE97(7), 1166–1185 (2009).
[CrossRef]

2008

M. Kim, O. O. Olubuyide, J. U. Yoon, and J. L. Hoyt, “Selective epitaxial growth of Ge-on-Si for photodiode applications,” ECS Trans.16, 837–847 (2008).
[CrossRef]

2005

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

2004

A. Nayfeh, C. O. Chui, K. C. Saraswat, and T. Yonehara, “Effects of hydrogen annealing on heteroepitaxial-Ge layers on Si: Surface roughness and electrical quality,” Appl. Phys. Lett.85(14), 2815–2817 (2004).
[CrossRef]

1985

H. Ishii, Y. Takahashi, and J. Murota, “Selective Ge deposition on Si using thermal decomposition of GeH4,” Appl. Phys. Lett.47(8), 863–865 (1985).
[CrossRef]

1958

W. Franz, “Influence of an electric field on an optical absorption edge,” Z. Naturforsch. B13a, 484–489 (1958).

L. V. Keldysh, “The effect of a strong electric field on the optical properties of insulating crystals,” Sov. Phys. JETP7, 788–790 (1958).

Asghari, M.

Chui, C. O.

A. Nayfeh, C. O. Chui, K. C. Saraswat, and T. Yonehara, “Effects of hydrogen annealing on heteroepitaxial-Ge layers on Si: Surface roughness and electrical quality,” Appl. Phys. Lett.85(14), 2815–2817 (2004).
[CrossRef]

Claussen, S. A.

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

Cunningham, J.

Cunningham, J. E.

Dong, P.

Feng, D.

Feng, N. N.

Fong, J.

Franz, W.

W. Franz, “Influence of an electric field on an optical absorption edge,” Z. Naturforsch. B13a, 484–489 (1958).

Gardes, F. Y.

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4(8), 518–526 (2010).
[CrossRef]

Ge, Y.

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Harris, J. S.

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett.98(15), 151108 (2011).
[CrossRef]

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Hoyt, J. L.

M. Kim, O. O. Olubuyide, J. U. Yoon, and J. L. Hoyt, “Selective epitaxial growth of Ge-on-Si for photodiode applications,” ECS Trans.16, 837–847 (2008).
[CrossRef]

Ishii, H.

H. Ishii, Y. Takahashi, and J. Murota, “Selective Ge deposition on Si using thermal decomposition of GeH4,” Appl. Phys. Lett.47(8), 863–865 (1985).
[CrossRef]

Kamins, T. I.

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett.98(15), 151108 (2011).
[CrossRef]

S. Ren, T. I. Kamins, and D. A. B. Miller, “Thin dielectric spacer for the monolithic integration of bulk germanium or germanium quantum wells with silicon-on-insulator waveguides,” IEEE Photon. J.3(4), 739–747 (2011).
[CrossRef]

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Keldysh, L. V.

L. V. Keldysh, “The effect of a strong electric field on the optical properties of insulating crystals,” Sov. Phys. JETP7, 788–790 (1958).

Kim, M.

M. Kim, O. O. Olubuyide, J. U. Yoon, and J. L. Hoyt, “Selective epitaxial growth of Ge-on-Si for photodiode applications,” ECS Trans.16, 837–847 (2008).
[CrossRef]

Kimerling, L. C.

J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics4(8), 527–534 (2010).
[CrossRef]

Krishnamoorthy, A. V.

Kung, C. C.

Kuo, Y. H.

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Lee, Y. K.

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Li, G.

Liang, H.

Liao, S.

Liu, J.

J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics4(8), 527–534 (2010).
[CrossRef]

Liu, Y.

Luo, Y.

Mashanovich, G.

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4(8), 518–526 (2010).
[CrossRef]

Michel, J.

J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics4(8), 527–534 (2010).
[CrossRef]

Miller, D. A. B.

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett.98(15), 151108 (2011).
[CrossRef]

S. Ren, T. I. Kamins, and D. A. B. Miller, “Thin dielectric spacer for the monolithic integration of bulk germanium or germanium quantum wells with silicon-on-insulator waveguides,” IEEE Photon. J.3(4), 739–747 (2011).
[CrossRef]

D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE97(7), 1166–1185 (2009).
[CrossRef]

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Murota, J.

H. Ishii, Y. Takahashi, and J. Murota, “Selective Ge deposition on Si using thermal decomposition of GeH4,” Appl. Phys. Lett.47(8), 863–865 (1985).
[CrossRef]

Nayfeh, A.

A. Nayfeh, C. O. Chui, K. C. Saraswat, and T. Yonehara, “Effects of hydrogen annealing on heteroepitaxial-Ge layers on Si: Surface roughness and electrical quality,” Appl. Phys. Lett.85(14), 2815–2817 (2004).
[CrossRef]

Olubuyide, O. O.

M. Kim, O. O. Olubuyide, J. U. Yoon, and J. L. Hoyt, “Selective epitaxial growth of Ge-on-Si for photodiode applications,” ECS Trans.16, 837–847 (2008).
[CrossRef]

Qian, W.

Reed, G. T.

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4(8), 518–526 (2010).
[CrossRef]

Ren, S.

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett.98(15), 151108 (2011).
[CrossRef]

S. Ren, T. I. Kamins, and D. A. B. Miller, “Thin dielectric spacer for the monolithic integration of bulk germanium or germanium quantum wells with silicon-on-insulator waveguides,” IEEE Photon. J.3(4), 739–747 (2011).
[CrossRef]

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Rong, Y.

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett.98(15), 151108 (2011).
[CrossRef]

Roth, J. E.

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Saraswat, K. C.

A. Nayfeh, C. O. Chui, K. C. Saraswat, and T. Yonehara, “Effects of hydrogen annealing on heteroepitaxial-Ge layers on Si: Surface roughness and electrical quality,” Appl. Phys. Lett.85(14), 2815–2817 (2004).
[CrossRef]

Schaevitz, R. K.

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

Shafiiha, R.

Takahashi, Y.

H. Ishii, Y. Takahashi, and J. Murota, “Selective Ge deposition on Si using thermal decomposition of GeH4,” Appl. Phys. Lett.47(8), 863–865 (1985).
[CrossRef]

Thomson, D. J.

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4(8), 518–526 (2010).
[CrossRef]

Wang, X.

Yonehara, T.

A. Nayfeh, C. O. Chui, K. C. Saraswat, and T. Yonehara, “Effects of hydrogen annealing on heteroepitaxial-Ge layers on Si: Surface roughness and electrical quality,” Appl. Phys. Lett.85(14), 2815–2817 (2004).
[CrossRef]

Yoon, J. U.

M. Kim, O. O. Olubuyide, J. U. Yoon, and J. L. Hoyt, “Selective epitaxial growth of Ge-on-Si for photodiode applications,” ECS Trans.16, 837–847 (2008).
[CrossRef]

Zheng, D.

Appl. Phys. Lett.

H. Ishii, Y. Takahashi, and J. Murota, “Selective Ge deposition on Si using thermal decomposition of GeH4,” Appl. Phys. Lett.47(8), 863–865 (1985).
[CrossRef]

S. Ren, Y. Rong, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Selective epitaxial growth of Ge/Si0.15Ge0.85 quantum wells on Si substrate using reduced pressure chemical vapor deposition,” Appl. Phys. Lett.98(15), 151108 (2011).
[CrossRef]

A. Nayfeh, C. O. Chui, K. C. Saraswat, and T. Yonehara, “Effects of hydrogen annealing on heteroepitaxial-Ge layers on Si: Surface roughness and electrical quality,” Appl. Phys. Lett.85(14), 2815–2817 (2004).
[CrossRef]

ECS Trans.

M. Kim, O. O. Olubuyide, J. U. Yoon, and J. L. Hoyt, “Selective epitaxial growth of Ge-on-Si for photodiode applications,” ECS Trans.16, 837–847 (2008).
[CrossRef]

IEEE Photon. J.

S. Ren, T. I. Kamins, and D. A. B. Miller, “Thin dielectric spacer for the monolithic integration of bulk germanium or germanium quantum wells with silicon-on-insulator waveguides,” IEEE Photon. J.3(4), 739–747 (2011).
[CrossRef]

IEEE Photon. Technol. Lett.

S. Ren, Y. Rong, S. A. Claussen, R. K. Schaevitz, T. I. Kamins, J. S. Harris, and D. A. B. Miller, “Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides,” IEEE Photon. Technol. Lett.24(6), 461–463 (2012).
[CrossRef]

Nat. Photonics

J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics4(8), 527–534 (2010).
[CrossRef]

G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics4(8), 518–526 (2010).
[CrossRef]

Nature

Y. H. Kuo, Y. K. Lee, Y. Ge, S. Ren, J. E. Roth, T. I. Kamins, D. A. B. Miller, and J. S. Harris, “Strong quantum-confined Stark effect in germanium quantum-well structures on silicon,” Nature437(7063), 1334–1336 (2005).
[CrossRef] [PubMed]

Opt. Express

Proc. IEEE

D. A. B. Miller, “Device requirements for optical interconnects to silicon chips,” Proc. IEEE97(7), 1166–1185 (2009).
[CrossRef]

Sov. Phys. JETP

L. V. Keldysh, “The effect of a strong electric field on the optical properties of insulating crystals,” Sov. Phys. JETP7, 788–790 (1958).

Z. Naturforsch. B

W. Franz, “Influence of an electric field on an optical absorption edge,” Z. Naturforsch. B13a, 484–489 (1958).

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Figures (4)

Fig. 1
Fig. 1

(a) Desired growth results using the substrate preparation process suggested by Ren et al. [13]. (b) Scanning electron microscope (SEM) image of selective area growth of Ge/SiGe QWs. Implementation of the spacer fabrication process proved to be not very robust, and often resulted in undesired sidewall growth originating from the top corner of the Si sidewall, directly under the SiO2 mask edge (circled region). The dielectric spacer was likely removed from this corner during a dry etching step in the substrate fabrication, due to insufficient coverage of the spacer by the top SiO2 mask.

Fig. 2
Fig. 2

Proposed new dielectric spacer fabrication process. The SiO2 spacer on the sidewalls is protected by the overhanging SiO2 mask layer, preserving it during subsequent fabrication steps and ensuring that the sidewall remains completely covered.

Fig. 3
Fig. 3

Longer wet etch times of the Si sidewalls lead to greater amounts of undercut, as shown by these cross section SEM images of samples that were etched for (a) 2 minutes, (b) 5 minutes, and (c) 10 minutes. The wet etch used in this work, tetramethylammonium hydroxide, etches different crystal planes of Si at different rates, resulting in the faceting evident at longer etch times.

Fig. 4
Fig. 4

SEM images of selective-area Ge growth using the dielectric spacer fabrication process in Fig. 2. (a) Angled image of a sample etched in TMAH for 5 minutes. Ge nucleation is evident on the top surface of the SiO2 mask, indicating imperfect selectivity. (b) Cross section SEM showing Ge crystal facets at the edge of the growth window.

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