Nanodot Formation in Thermally Annealed UHV-RTCVD Grown Si 1 - X Ge X Epitaxial Layers on Silicon for Photovoltaics
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Nanodot Formation in Thermally Annealed UHV-RTCVD Grown Si1-XGeX Epitaxial Layers on Silicon for Photovoltaics Abdennaceur Karoui1, and Anita S. Ethiraj Photovoltaic Nanotechnology and Nanosensors Lab., Shaw University, Raleigh, NC 27601, USA 1 : [email protected], (919) 719 1998 ABSTRACT The surface and interface of SiGe layers on Si were found to incur drastic changes during layer rapid growth and post-growth rapid annealing. As deposited and thermal annealed samples were characterized using Energy dispersive X-ray Analysis (EDX) enhanced by Monte Carlo simulation for precise evaluation of Ge concentration. X-ray Diffraction (XRD) data exhibited a small shift of the SiGe (400) peak towards low 2 T values, which was attributed, primarily, to change in the Ge concentration. Confocal Raman Spectroscopy of samples showed regions of high and low strain that resulted from fluctuations in Ge concentrations. Nano- and submicronpyramidal features at the surface of Si1-xGex layers (x=17% and 28%) were revealed by Atomic Force Microscopy (AFM) and SEM. Additionally, pyramidal nanodots were revealed for [Ge]=17% samples and high density nanostructure for 28% appeared along the crosshatch strain pattern induced by misfit dislocations, when annealed at 700°C and 900°C, respectively. The observed Ge-rich nano-features, which were obtained with low thermal budget low cost techniques, are expected to be useful for bandgap engineering and third generation solar cells. INTRODUCTION It is well known that the material system Si1-xGex on Si is of great technological importance for it offers band gap engineering [1] and carrier mobility enhancement [2]. The introduction of SiGe heteroepitaxial growth in silicon based microelectronics has advanced electronic devices. Application of SiGe is even more prominent for Photo-detectors [3] and solar cells [4, 5, 6] in both crystalline or amorphous [7] forms. Incorporation of lower band gap material like Ge (Eg=0.67 eV) into the base material or using SiGe layers and stacks improves the near-infrared (NIR) absorption (0.67 eV Eg 1.12 eV) for thin Si Solar cells [4]. Additionally, SiGe layers offer a wide range of options for designing PV devices capable of high energy conversion efficiency. It is required that high temperature processing be done with low thermal budgets for PV process viability. Therefore, it becomes essential to study the effect of thermal processing on the Si and Ge material system. Although superior techniques like MBE have been employed to deposit Si1-xGex on Si [8], we use RTCVD as low cost growth technique for photovoltaic cells. During SiGe processing, the surface as well as the SiGe/Si interface are strongly altered, which in turn drastically effects the interface properties to the point it may degrade device performances [9]. Therefore, it is necessary to analyze SiGe/Si surface structure of samples annealed under different conditions. Few reports exist on the effect of annealing on SiGe layers. Lee et al. [10] investigated the thermal annealing of a strained relaxed
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