Ge1-xSnx Epitaxial Growth on Ge Substrate by MOCVD
Ge1-xSnx is recognized as one of the post scaling materials, which can be used as a channel material and a stressor for Ge MOSFETs. For instance, it was reported that a hole mobility of pMOSFET with Ge1-xSnx channel was improved comparing to the pure Ge channel . Moreover, Ge1-xSnx can be used as an optical devices because the band structure changes from indirect to direct as Sn composition increasing more than 10 at.%. Ge1-xSnx is a very attractive material, and therefore often fabricated by epitaxial growth on the Ge substrate using CVD technique. However, the precursors used for CVD so far have a safety problem. GeH4 or Ge2H6 for Ge and SnH4 for Sn are very dangerous because they have a pyrophoric and explosive nature [2, 3]. In this paper, we succeeded in performing Ge1-xSnx epitaxial film growth by MOCVD (metal-organic chemical vapor deposition) using tertiary-butyl-german (t-C4H9GeH3) and tetra-ethyl-tin ((C2H5)4Sn) which have neither a pyrophoric nor explosive nature.
We synthesized and used t-C4H9GeH3 and (C2H5)4Sn as Ge and Sn precursors, respectively. Ge precursor has a high vapor pressure and can be decomposed at low temperature. Furthermore, it has few residual carbon impurities in the deposited film . Our selected Sn precursor also has high vapor pressure. Moreover, these precursors are sufficiently safe. In this experiment, we used Ge(100) wafer as a substrate. After the chemical cleaning of the substrates using NH4OH, HCl and HF, residual native oxide on the substrate was removed by thermal treatment at 450 ºC in the CVD process chamber. Then, the precursor vapor was injected into the chamber. The precursor supply was controlled by the flow rate of N2 carrier gas, and the temperatures of precursor bottles and pressure. In the experiment, the injected rates were estimated to be 1.4E-04 mol/min for both Ge and Sn precursors. The deposition temperature and chamber pressure were 360 ºC and 30 torr. The deposited films were investigated by transmission electron microscopy (TEM; JEM-2100, JEOL Ltd.) and X-ray diffractometry (XRD; Rigaku SmartLab).
Results and Discussion
Figure 1 (a) shows cross-sectional TEM (XTEM) image, here the film thickness is approximately 50 nm. Figure 1 (b) is the close-up image with enhanced brightness at the epitaxial interface indicated by the rectangle in Fig. 1 (a). No obvious defect was observed at the interface and in the film. The electron diffraction patterns taken from the Ge1-xSnx film (Fig. 2 (a)) and Ge substrate (Fig. 2 (b)) showed the same crystalline orientations, implying successful epitaxial growth.
Figure 3 shows the X-ray diffraction reciprocal space mapping (XRD-2DRSM) around -2-24 diffraction. The lattice constant was expanded approximately 0.6 % in the epitaxial film from the Ge substrate. This corresponds to approximately 2 at.% Sn incorporation in the film, which exceeds solid solubility limit of 1 at.% reported in the literature .
In summary, we have succeeded in the Ge0.98Sn0.02 heteroepitaxial film growth on the Ge substrate by MOCVD for the first time.
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