Best Student Poster Award
Stability of Tensile-strained Ge Studied by Transmission Electron Microscopy Meng Qia, William A. O'Briena, Chad A. Stephensona, Ning Caob, Brian J. Thibeaultb and Mark A. Wisteya a
Department of Electrical Engineering, University of Notre Dame, United States,
[email protected], bDepartment of Electrical Engineering, University of California, Santa Barbara , United States.
Tensile-strained germanium has been studied recently as a possible laser material due to its nearly-direct bandgap and the compatibility with CMOS technology. [1] Theoretically, 1.4% biaxial tensile strain could produce a direct bandgap in Ge. [2] But high strain can also lead to dislocations that reduce radiative efficiency and carrier mobilities. Also, the high circulating power in a laser can lead to damage in real time. This is observed as dark line defects, which are dislocation networks generated in the laser due to high thermal and electrical power. To reduce the defects generated by high biaxial strain, an alternative way is to use some out-of-plane strain instead of purely biaxial strain. Only 0.6% of hydrostatic strain is required for a direct bandgap in Ge. Even with the smaller strain, Ge may be susceptible to relaxation. In this study, we examined the sensitivity of strained Ge to damage by irradiating it with 300kV electrons and comparing which strain conditions led to dislocations. In this work, three-dimensional strain in Ge waveguides was introduced by SiNx stress liners, which are widely used for CMOS. Ge waveguides, with widths from 0.5 micron to 80 micron, were patterned by contact lithography and a 2 micron deep etch by reactive ion etching (RIE). Then 1 micron thick strained SiNx with stress of 1 GPa or 2 GPa was deposited by dual frequency plasma enhanced chemical vapor deposition (DF-PECVD). Because the ion bombardment in DF-PECVD can itself cause surface damage, some samples also had a thin, intermediate layer of strain neutral SiNx by PECVD to protect the Ge surface. The resulting strain was characterized by Raman spectroscopy. We observed a Ge peak shift of up to 11 cm-1. This shift corresponds to a sufficiently high strain that we expected direct bandgap emission. However, photoluminescence (PL) showed weak signals in highly strained waveguides and little wavelength shift in weakly strained waveguides. This suggested a relaxed (unstrained) interface or damage or dislocations at the interface. To study the strained Ge interface, we performed a time-dependent damage study under TEMirradiation on two 0.5 µm-width waveguides, one with 1 GPa stress and a 20 nm protection layer, the other with 2 GPa stress but no protection layer. A weakly strained Ge waveguide with the interface polished away was used as a control, assuming no strain. Two TEM specimens were made by focused ion beam (FIB) under identical conditions. We observed that, for highly-strained waveguides, the interface is severely damaged by e-beam within mere seconds, and the damage propagates into the deeper region with time. For the weakly strained sample, weak damage is confined within 2 nm of the interface and does not propagate. No obvious damage was observed in the unstrained control. Raman and PL were consistent with our observations from TEM. In Raman on highly strained waveguide, the width of the Ge peak and wide background signal indicate partially amorphous and damaged material. The highly strained structure has weak PL intensity, even with an additional GeOx passivation layer, while weakly strained waveguides showed much higher PL intensity and a shift in Γ and L emission wavelength compared with the control. In summary, we found that highly strained Ge interfaces were susceptible to damage under ebeam, and the dislocations propagated deep into the waveguide. This suggests possible limits to achievable strain and laser performance. We will report strain/stability tradeoffs. References [1] Jifeng Liu, X. Sun, Rodolfo Camacho-Aguilera, Lionel C. Kimerling, and Jurgen Michel. Optics Letters, 35(5), 2010. [2] M. V. Fischetti, S. E. Laux. Journal of Applied Physics, 80(4), 1996. 978-1-4577-1865-6/12/$26.00 ©2012 IEEE
Best Student Poster Award
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Fig. 1. a) SEM cross-section of a 0.8 micron width waveguide covered by 1 GPa compressive SiNx film. b) A series of TEM images taken at cross-section of 0.5 micron-width waveguide with 2 GPa strained SiNx for different e-beam exposure time. A degradation effect can be observed. The triangle shape cross-section of waveguide is due to imperfect lithography and isotropic etching in the narrowest waveguides.
b) a) Fig. 2. a) Strain profile simulated by COMSOL for 1 micron-width waveguide and 0.5 micron SiNx film. Colors in the Ge from warm to cold represent strain from tensile to compressive. A highest tensile strain of 0.25% is predicted at the top corner region. b) Strain measured by Raman spectroscopy on 0.5 micron-width waveguides with different strains. Highly strained waveguide is achieved by 2 GPa SiNx compressive-strained film, while weakly strained is achieved by 1 GPa strained waveguides with a 20 nm strain-neutral SiNx protect layer. Highest shift of 11 cm-1 is found in the highly strained waveguide, corresponding to 3.55% tensile strain assuming hydrostatic strain.
b) a) Fig. 3 a) Time dependent damage study by HRTEM for 0.5 micron-width waveguides with different strains: 1) Ge/SiNx interface has been polished away, with strain assumed relaxed. No obvious damage. 2) SiNx/Ge interface at weakly strained waveguide. Weak damage is only found within 2-3 nm from interface. 3) SiNx/Ge interface at highly strained waveguide, severe damage can be found propagating into deep area along with exposure time. b) Photoluminescence (PL) from a control sample and 0.5 micron-width waveguides with different strain profiles. Little wavelength shift for weakly strained waveguide can be observed. Highly strained waveguide has weak PL signal, which is consistent with the severe damage by TEM for high strains.
