Low Temperature RPCVD Epitaxial Growth of Si1-xGex and Ge Using Si2H6 and Ge2H6

Abstract

The need to improve the electronic, thermo-electric or optic device performance as well as an all-Si based integration has significantly increased the requirements for Si/SiGe material. The introduction of strain in Si(Ge), which induces strong energy band modification and through this enhance carrier mobility and absorption/emission properties, has been the dominant technique to enhance the Si(Ge) device performance. However, the epitaxial growth of highly strained layers is very challenging, since growth at very low temperatures and with appropriate precursors is required. We will present epitaxial growth studies of pseudomorphic Si1-xGex and relaxed Ge layers on 200 mm Si(100) wafers using an AIXTRON Tricent® RPCVD tool. This is the first report of epitaxial Si(Ge) layer growth using an AIXTRON cold wall reactor with a showerhead technology. Compared to conventional CVD tools the consumption of precursors is strongly reduced. Using Si2H6 and Ge2H6 precursors with a flow of carrier gas (H2) of a maximum of 5 standard liters per minute epitaxial growth is obtained at temperatures as low as 425 °C at a constant growth pressure of 60 mbar. Prior to the epitaxial growth the wafers were cleaned using only O3/ vapor HF or O3/diluted HF/HCl which allows an in-situ pre-bake step at about 850°C (surface temperature). The layer thicknesses, composition and morphology were analyzed by RBS/C, XRD and TEM. Moreover Si/SiGe multi quantum well structures (MQW) with highly in-situ B-doped Si bottom and top layers (contacts) will be presented. B2H6 precursor was employed for in-situ doping. Photoluminescence (PL) and ToF-SIMS measurements were carried out to prove the high quality of the heterostructure. Fig. 1a shows the growth rate versus the Ge concentration up to 45at. % and the low channeling min. yield of the grown layers. The SiGe-layers were deposited at constant partial pressure of Si2H6 of 15 Pa at 600 °C. Up to 45at. % Ge the growth rate is linearly dependent on the Ge concentration and also the alloying increases linearly with the Ge2H6 partial pressure. RBS/Channeling measurements indicate single crystal quality with min. yield of 5% for strained Si1-xGex (0.15<x<0.4) with thicknesses up to 100 nm (Fig 1a). Hartmann et al. [1] showed that the critical thickness for plastic relaxation of SiGe can be increased compared to previous theories [2] by decreasing the deposition temperature. Their results can be seen in figure 1b. In our growth conditions we expect to further increase the critical thickness for plastic relaxation by using Si2H6 /Ge2H6 instead of SiH2Cl2 /GeH4 for deposition temperatures below 600 °C. The blue marked point in this plot represents a Si0.61Ge0.39 layer grown by our group at 600 °C. In Fig. 2a an XRD rocking curve of Si/SiGe MQW grown on SOI at 600 °C is shown. The MQW consists of a 4 times Si/Si0.67Ge0.33 (30 nm/10 nm) heterostructure. On top and below that heterostructure Si:B layers (100 nm) were deposited. The clear XRD peaks and the perfect matching with the simulated curve (red) indicate sharp interfaces. The inset of Fig. 2a showing the PL spectra at 5 K underline the high quality of the structure. B concentrations in the Si:B layers of about 1-5x1019cm-3 were extracted from sheet resistance measurements. In addition, ToF-SIMS analysis (Fig. 2b) indicate that the B concentration increases and decreases abruptly at the Si:B interfaces, respectively. The results for the low temperature Ge growth using Ge2H6 are shown in Fig. 3. Two different Ge2H6 flows are employed at temperatures between 375°C and 425°C. Growth rates up to 9 nm/min and root mean square (rms) values for the roughness of 1.5 nm are obtained for a growth temperature of 425°C.