Novel method to introduce uniaxial tensile strain in Ge by microfabrication of Ge/Si1−xGex structures on Si(0 0 1) substrates
Introduction
A Ge channel metal–oxide-semiconductor field effect transistor (MOSFET) is one of the most promising candidates in which to realize the drive current improvement which should accompany the mobility enhancement. Moreover, strained-Ge technologies should become of even more-consequence in the future considering the importance of strained Si technology at present. Recently, it has been proposed that strained Ge channel MOSFETs provide higher carrier mobility than bulk Ge [1]. It is also known that the hole mobility is enhanced in Ge p-MOSFET by compressive strain in the direction along the channel or tensile strain in the direction perpendicular to the channel [2]. Additionally, a theoretical calculation suggested that in-plane tensile strain in the Ge layer allows us to obtain much higher mobility not only for holes but also for electrons [3]. On the other hand, from the viewpoint of optoelectronics, Ge is also an attractive waveguide photodetector material as it has the detectable wavelength ranging from 1.3 to 1.55 μm, which is a requirement in telecommunication. Moreover, the detectable wavelength range can be extended to a longer wave length by applying uniaxial or biaxial tensile strain to Ge, which enables a wider range in a multiple transmission.
Therefore, tensile strained Ge is very interesting for MOSFET and optoelectronic devices. Previously, some fabrication methods were proposed to realize tensile strained Ge [4], [5], [6], [7]. However, the strain control technology of Ge has not been established. We focused on the elastic strain relaxation of microfabricated Si1−xGex layers [8] in order to introduce the tensile strain into the Ge layer. In this study, we propose a novel method to realize by microfabrication a uniaxial tensile strained Ge layer with the elastic strain relaxation of a Si1−xGex buffer layer and investigate the strain and dislocation structure of microfabricated Ge/Si1−xGex stacked layers on Si(0 0 1) substrates.
Section snippets
Concept and method of sample preparation
Fig. 1 is a schematic diagram showing how to realize a tensile strained Ge layer by the microfabrication of Ge/Si1−xGex stacked layers. First, a pseudomorphic Si1−xGex layer is epitaxially grown on a Si(0 0 1) substrate, following the epitaxial growth of a strain-relaxed Ge layer on that as shown in Fig. 1a. Note that there is an in-plane compressive strain in the Si1−xGex layer just after growth. After patterning of the strain-relaxed-Ge/pseudomorphic-Si1−xGex/Si(0 0 1) structure to form a
Results and discussion
Fig. 3a shows the X-ray diffraction two-dimensional reciprocal space map (XRD 2DRSM) around Si and Ge 224 reciprocal lattice points after the growth of Si0.60Ge0.40 and Ge layers. Fig. 3b shows the cross-sectional TEM image of the same sample. The value of the reciprocal lattice, Qx for Si0.60Ge0.40 224 corresponds to that for Si 224 as shown in Fig. 3a and no dislocation at the Si0.60Ge0.40/Si interface is found as shown in Fig. 3b. These results indicate the pseudomorphic growth of the Si0.60
Conclusions
We have proposed and demonstrated a novel method to realize by microfabrication a uniaxial tensile strained Ge layer due to the elastic strain relaxation of a Si1−xGex buffer layer. A fully strain-relaxed Ge layer can be formed on a compressive strained Si0.60Ge0.40 layer after PDA and the strain of the Si0.60Ge0.40 layer is elastically and anisotropically relaxed by microfabrication which produces stripe lines with a width of 250 nm. As a result, a uniaxial tensile strained Ge layer can be
Acknowledgement
This work was partly supported by the Ministry of Education, Culture, Sports, Science and Technology, through a Grant-in-Aid for Scientific Research for Priority Areas, No. 18063012, in Japan.
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