Energy-Loss Filtered Imaging of Segregation-Induced Interface Broadening in SiGe/Si P-Channel MOSFET Device Structures
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ABSTRACT SiGe/Si p-channel heteroepitaxial MOSFET test structures have been fabricated using solid-source molecular beam epitaxy. High-resolution transmission electron microscopy and energy-loss filtered imaging have been used to quantitatively determine the nanoscale Ge distributions across the SiGe alloy channel. The Ge profile at the edges of the alloy channel were found to be asymmetrical due to the effect of Ge segregation, with an exponential-like distribution directed toward the surface. The results agree well with the predictions of segregation theory and indicate that the concentration of Ge in the extended distribution lay in the range 10%-1% over a distance of several nanometers from the body of the channel. Secondary ion mass spectrometry measurements upon the same samples were insensitive to this short range extended Ge distribution. INTRODUCTION At present, there is a considerable effort directed towards the development of an advanced SiGe-based device technology for the fabrication of high-speed analogue and digital electronics. One particular device structure that is currently under investigation is the strained p-channel
field-effect transistor (FET), a component of which consists of a pseudomorphic layer of SiGe grown by molecular beam epitaxy (MBE) on Si. A major requirement for good electrical performance is the need for interfaces to be sharp on either side of the SiGe layer since small scale interface roughness leads to unwanted hole scattering.' Interface sharpness can be difficult to control and, indeed, under certain conditions the surface of the strained layer can exhibit wave-like undulations caused by thermodynamically driven growth instability. 2 It is possible to produce more uniform alloy layers by performing growth at a relatively low-temperature, capping with Si, and then annealing at high-temperature to restore the required electrical properties. However, even after implemention, interfaces may still not be optimally sharp due, for example, to the effect of Ge segregation.4 Hence, it is crucial that the composition profile across such interfaces can be characterised with nanometer-scale resolution. Interfaces of this type have been analysed using secondary ion mass spectroscopy 3,5 (SIMS); however, this method tends to sample large areas, to give averaged results, and consequently it is difficult to obtain nm-scale resolution in the high-concentration regime. Small electron-probe methods 6,7 involving electron energy-loss spectroscopy (EELS) and energy dispersive-X-ray spectroscopy (EDS) have been used to profile the Ge distribution. Moreover, imaging techniques include energy-filtered TEM (EFTEM) or high-angle annular dark-field (HAADF): previous EFTEM work on SiGe layer structure includes, Si-K deficit profiling of 167 Mat. Res. Soc. Symp. Proc. Vol. 5890 2001 Materials Research Society
relatively thin multiple channel layers. 8 Whereas, HAADF imaging was performed on morphologically distorted SiGe layers 9 to analyse the effects of inter-diffusion and segregation. In this wor
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