Ge-on-Si: Single-Crystal Selective Epitaxial Growth in a CVD reactor (abstract)

Ge-on-Si: Single-Crystal Selective Epitaxial Growth in a CVD reactor

A. Sammak, W. B. de Boer and L.K. Nanver Delft Institute of Microsystems and Nanoelectronics

(DIMES), Delft University of Technology, Feldmannweg 17, 2628 CT Delft, The Netherlands, Phone: +3115 27 82 506, Fax: +31 (0)15 26 22 163,

This article presents a Chemical Vapor Deposition (CVD) technique for selective growth of epitaxial Ge on Si substrate. A standard Si/SiGe ASM CVD reactor that was recently modified for merging GaAs and Si epitaxial growth in one system [1], is utilized to achieve intrinsic and doped epitaxial Ge-on-Si with low threading dislocation and defect density. For this purpose, the system is equipped with 2% diluted GeH4 as the main precursor gas for Ge deposition; and 0.7% diluted AsH3 and B2H6 precursor gases as well as a TriMethylGallium (TMGa) bubbler system for As, B and Ga doping of epitaxial Ge, respectively. The quality of Ge epitaxy on Si is investigated by cross-sectional transmission electron-microscopy (TEM) and atomic-force electron-microscopy (AFM) analysis.

The starting material is 4-inch n- or p-type Si wafers. Experiments were performed with less than 1 µm SiO2 first being deposited on the surface of the wafers and the areas where Ge deposition is desired are opened by plasma etching to the Si with soft landing. Just before loading in the CVD reactor a HF dip-etch and Marangoni cleaning are performed, while in the reactor a 3-min baking step at 850°C is used in order to ensure that the surface is free of native oxide. Then diluted GeH4 gas is introduced into the reactor chamber at a pressure of 20 Torr. Growth temperatures from 550˚C to 700˚C were found to render good selectivity to the exposed Si surface with only very few Ge nuclei formation on oxide masking regions.

It is shown that at the deposition temperature of 700˚C, most of the lattice mismatch-defects are trapped within first 300 nm of Ge growth. This is basically due to inter-diffusion of Ge and Si that occurs at this deposition temperature at the interface and hence, good quality single crystal Ge can be achieved within a layer thickness of approximately 1 µm on window sizes up to hundreds of μm2_.

The very smooth surface of the Ge grown at 700˚C is underlined by the AFM imaging as shown in Fig. 1 for the selective epitaxial growth of Ge on Si in windows-size of 30×30 µm2_{. The Ge thickness is around} 1 µm.

Fig. 1. AFM surface scan of a Ge island grown on Si with the size of 30×30 µm2_{. The surface roughness is 4.3 Å RMS and 2 nm peak}

to peak.

A surface roughness of 4.3 Å RMS by AFM indicates a smooth surface of Ge and the high quality of the growth.

Also lateral overgrowth of Ge over SiO2 has been studied by Ge growing in the patterns containing narrow 1-µm-wide lines with 1-µm spacing’s between them. Fig. 2 is a cross-sectional TEM image of selective epitaxial growth of Ge with overgrowth on SiO2. It is evident from Fig. 2a that Ge lateral overgrowth from both sides joins perfectly on top of the oxide without any defect formation at the interface. This is specifically promising for fabrication of Ge-on-Insulator wafers with Si as substrate. In Fig. 2b a small sample of the Ge crystallography structure is shown.

(a)

(b)

Fig. 2. a) Cross-sectional TEM image of selective epitaxial Ge-on-Si with lateral overgrowth on SiO2 and b) a high resolution TEM

image of the Ge crystal at a thickness of 500 nm.

While previous work has shown that good quality diodes can be fabricated in this type of Ge-on-Si [2], this paper will give details of an improved process. Moreover, various analysis and electrical measurement results of the intrinsic and doped Ge epitaxial layers will be discussed in the full manuscript.

[1] A. Sammak, W. de Boer, A. van den Bogaard and L.K. Nanver, “Merging standard CVD techniques for GaAs and Si epitaxial growth”, ECS Trans., Vancouver, Canada, April 2010. [2] A. Sammak, W.B. de Boer, L. Qi and L.K. Nanver, “High-quality p+_{n Ge diodes selectively grown on Si with a sub-300nm} transition region”, ESSDERC 2011, 12-16 September 2011, Helsinki, Finland, ISBN 978-1-4577-0707-0, pp. 359-362.