Single Crystalline GeSn On Silicon By Solid Phase Crystallization

Single crystalline GexSn1-x (GeSn) alloys are promising for electronic and optical applications [1]. GeSn has been predicted to exhibit carrier mobilities exceeding that of Ge [2], which makes this material suitable as channel material for high-speed complementary metal-oxide semiconductor (CMOS) technology [3]. In addition to increased carrier mobility, the GeSn material system is interesting for its optical properties. GeSn exhibits a direct band gap between 0.61 and 0.35 eV for Sn concentrations between 6 and 15 % [4]. This makes it much better suited for optoelectronic applications than Si and Ge, especially at long wavelengths. Alloying Ge with Sn has several challenges, which hamper the formation of high quality layers. First the limited solubility of Sn in Ge of 0.5 % prohibits the formation of alloys with high Sn content [5,6,7]. Another problem with GeSn epitaxy is the large lattice mismatch of tin with germanium and silicon of 18 % and 23 %, respectively. The mismatch between layer and substrate complicates the formation of high quality layers by introducing lattice defects, stress, roughness, compositional fluctuations etc. Suppressing these effects is critical to obtain high quality layers and high performance devices. Several growth methods have been used to obtain GeSn layers [8]. Chemical vapor deposition [9,10], molecular beam epitaxy [6], sputtering [7] and pulsed laser deposition [11] have been demonstrated. Solid phase epitaxy of GeSn on Si(001) with a Ge buffer has been tried, however with limited structural quality [12]. We demonstrate single crystalline GeSn layers on Si(111) substrates by solid phase epitaxy (SPE) of amorphous GeSn layers with excellent structural quality. We do not use surfactants, capping layer nor buffer layer. Amorphous GeSn layers are obtained by evaporation of Ge and Sn atoms while exposing the substrate to a N2 gas beam of 1.2 standard cubic centimeters per minute (SCCM) and using a deposition temperature around room temperature. Previously we have demonstrated the influence of an inert gas beam during deposition of amorphous Ge on subsequent solid phase epitaxy [13]. The surface adatom mobility is reduced by the presence of inert molecules, which consequently help to decrease the order of the deposited layer. In-situ reflection high electron diffraction (RHEED) shows a diffuse image without the presence of diffraction spots, indicating that the GeSn layer is amorphous and not poly or single crystalline. XRD ω/2θ scan of the as-deposited layer does not show diffraction of GeSn, confirming that the GeSn does not grow epitaxially with the growth conditions that were applied, see Figure 1 (black, solid line with squares). Subsequent annealing transforms the amorphous GeSn into a single crystalline layer by solid phase epitaxy. An intense and narrow GeSn(111) diffraction peak is observed, indicating high crystal quality, as observed from ω/2θ XRD scan, see Figure 1 (red, solid line). The (111) crystal planes of the GeSn layer are parallel to the (111) Si surface. The presence of Pendellösung fringes indicates a smooth GeSn surface and interface between GeSn and Si. The crystalline quality has been investigated by XRD, Rutherford backscattering and channeling spectrometry (RBS/C) and transmission electron microscopy (TEM). The Sn content and strain has been determined by XRD reciprocal space mapping (RSM). RBS has been used to confirm the Sn content obtained by XRD RSM analysis. Excellent structural quality is demonstrated for layers with 3.8 % of Sn. The XRD rocking curve FWHM of the (111) GeSn reflection shows a value of 162 arc sec. Strain relaxation of 106.7 % has been measured for 38 nm layers, which makes this method promising for strain engineering. Furthermore the surface and interface are smooth, especially when considering the relaxation of the layers. The surface RMS roughness is smaller than 1 nm as measured by atomic force microscopy (AFM) and X-ray reflectivity (XRR). XRR shows a sharp interface between the silicon substrate and GeSn with a RMS roughness smaller than 1 nm. Finally, we show an increased optical absorption for the GeSn layers in respect with Ge, which is promising for integration of optical devices on silicon substrates. Fig. 1: Comparison of XRD ω/2θ scans of as-deposited (black, solid line with squares) and crystallized (red, solid line) GeSn on Si(111). An intense and narrow GeSn(111) diffraction peak is observed after annealing, indicating high crystal quality. The presence of fringes indicates a smooth GeSn surface and interface between GeSn and Si. 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