We investigated the liquid-phase growth of silicon-germanium (SiGe) epitaxial layers formed on Si substrates by a simple and rapid process of screen-printing of Al-Ge paste followed by high temperature annealing. Aluminum (Al) and germanium (Ge) (7:3) mixed paste was prepared, screen-printed on a Si substrate and annealed in an infra-red (IR) furnace at peak temperatures between 800°C to 1000°C. After reaching the Al melting point at 660°C, the melted Al dissolves the Si surface and Ge powder in the paste forming a thick liquid phase of melted Al-Ge-Si layer on the dissolved surface of the Si substrate. During the cooling process, a SiGe layer starts to grow epitaxially at the Si interface. The formed SiGe was observed by scanning electron microscope (SEM), and energy dispersive X-ray spectrometry (EDX) and characterized by X-ray diffraction reciprocal space mapping (XRD-RSM). The thickness of the SiGe layer depended on the peak temperature reaching about 25mm at 1000oC. The results suggest the epitaxially grown SiGe is strain-relaxed with a graded Ge concentration.
A mixed paste of aluminum (Al) and germanium (Ge) (7:3) was prepared and screen-printed on silicon (Si) substrates, followed by annealing at a peak temperature of 1000 °C in an infrared rapid thermal annealing furnace to investigate the liquid-phase growth of silicon-germanium (SiGe) epitaxial layers. The gas ambient during annealing was changed to investigate the effect on SiGe layer quality and physical properties. The SiGe-formed samples were observed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). Oxygen-containing atmosphere suppressed the SiGe layer formation by oxidizing the Al particle surface limiting the reaction of the particle to the Si surface. On the other hand, annealing in an argon atmosphere without oxygen resulted in the formation of SiGe layers with a thickness of over 30 μm.
Silicon-germanium (SiGe) has been considered as an important alternative material to silicon due to its high carrier mobility, low power consumption and excellent performance [1]. However, due to both material unique phase diagram with melting points separated by 376 °C, it is too difficult to realize a good bulk quality of polycrystalline-free SiGe single crystals with controlled Ge contents. In this work, crystalline thick epitaxial layers of SiGe with Ge contents exceeding 30% are fabricated by screen-printing technology on Si wafers. As illustrated in Figure 1, Aluminum-germanium (AlGe) paste with a mole ratio of 7:3 was prepared and screen-printed on p-type Cz-Si <111> substrate, dried at 100 °C for 10 minutes and then annealed in Ar ambient using IR image furnace at 900 °C for 5 minutes. The Al begins to melt at 660 °C, dissolving the silicon wafer surface and the Ge in the paste. The melted Al, Si and Ge form the liquid phase of the Al-Si-Ge, which when cooling process start, regrown epitaxially at the melt/Si interface to form the Al-doped SiGe crystalline layer. The formed SiGe was observed by scanning electron microscope (SEM), and energy dispersive X-ray spectrometry (EDX). As can be seen in the cross-sectional SEM image in Figure 2, SiGe layer of about 20mm was formed on the Si substrate surface. It is worth mentioning that the SiGe/Si interface is formed straight as the reaction stopped perfectly on façade <111> forming SiGe layer. EDX mapping of elements suggests that the Al in the Al-Si-Ge liquid phase formed at high temperature is segregated from the liquid to the Al-Ge paste layer during the cooling process and remains only as dopant in the SiGe layer due to its low solid solubility in Si and Ge. The Al-Ge paste residue layer will be etched chemically and further step of chemical mechanical polishing will be performed to improve the surface flatness of SiGe layer to be an epi-ready SiGe/Si substrate for different semiconductor devices applications. Reference [1] M. L. Lee and E. A. Fitzgerald, Strained Si/strained Ge dual-channel heterostructures on relaxed Si0.5Ge0.5 for symmetric mobility p-type and n-type metal-oxide-semiconductor field-effect transistors, Appl. Phys. Lett. 83, 4202 (2003). Figure 1
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