CVD Growth of GeSnSiC Alloys Using Disilane, Digermane, Tin Tetrachloride and Methylsilane

Abstract

Ge-Sn-Si-C alloys offer a large range of strain engineering which gives the possibility to tailor the material properties for advanced photonic, electronic, photovoltaic and thermoelectric applications. The benefits of Ge-Sn-Si-C alloys are possibilities to obtain a direct-to-indirect transition (for Sn-based alloys) [1], high carrier mobility [2] and low cost process [3]. In this study, Ge1-x-y-zSnxSiyCz layers (0.01≤x≤ 0.24, 0≤y≤0.12 and 0≤z≤0.01) have successfully grown on Ge and Si by using RPCVD technique. It was demonstrated that the quality of epitaxial layers is dependent on the growth parameters, layer thickness and the quality of Ge virtual layer. It was found that a proper strain balance in the matrix during the epitaxy where the Si and Si-C is adjusted carefully compared to Sn flux improves the incorporation of Sn in Ge matrix. This is explained by the compensation of tensile strain induced by Si or Si-C with the compressive strain caused by Sn in order to obtain the minimum energy in Ge matrix. Fig. 1a and 1b show RBS spectra of two GeSnSi samples when the first one has a constant Si flux whereas the sample in Fig. 1b has a graded Si flux within GeSnSi layer. The Sn incorporation improved from 3% to 24% when Si flux was graded.Incorporation of P and B in GeSnSiC matrix has been studied and the effect of dopant concentrations on Sn content has been investigated. A low boron concentration in range of 1017 cm-3 could be incorporated in the epi-layers whereas P-doping in range of 1019 cm-3could be achieved. All Ge1-x-ySnxSiyCz layers were grown at 280-330 °C at 20 torr in a RPCVD reactor. The gas precursors were Si3H8, Ge2H6, 1% B2H6 in H2, 5% PH3 in H2, and SnCl4as Si, Ge, B, P and Sn sources, respectively. The strain in the epi-layers was measured by high-resolution x-ray reciprocal lattice maps (HRRLMs). Transmission electron microscopy (TEM) was applied to detect the defects in the epi-layers and to observe the interfacial quality. The Sn, Si and C content in the samples was measured by Rutherford backscattering spectrometry (RBS) and HRRLMs. The atomic dopant concentration in the epi-layers was measured by secondary ion mass spectrometry (SIMS). Four-probe measurements were done to determine the resistivity of the doped layers.The quality of carrier and reactant gases in terms of oxygen and water vapor was investigated by residual gas analyzer (RGA). The RGA instrument was connected to the exhaust gas output and was situated in 20 mm distance from the reactant gas chamber. A RGA spectrum reveals any absorption or desorption of molecules with accuracy of ppt range. The results showed that the level of oxygen during epitaxy was as low as 10 ppb and the contamination concetration was found as low as 1017 cm-3. This low oxygen level during epitxy is due to the high purity of trisilane, digermane and tetrachloride precursors as well as the hydrogen carrier gas. Acknowledgment The Swedish Research Council (VR) is greatly acknowledged for supporting this work.