Germanium on SiC(0001): Surface Structure and Nanocrystals

Abstract

Germanium nanocrystals as potential candidates for a future optoelectronics in group-IV semiconductors have been grown on SiC(0001). A monoatomic wetting layer is formed in a Stranski-Krastanov growth mode. The surface structure of this wetting layer studied by STM and LEED depends on the SiC surface preparation. 3x3 and 4x4 Ge superstructures are observed by growing on the silicon-rich SiC(0001)-3x3 surface. Surface structures with mainly two-fold periodicity as well as 6x6 are observed after Ge deposition on silicon-deficient SiC(0001)-(?3x?3)R30° or (6?3x6?3) surfaces. Two-dimensional Ge islands of lateral dimensions between 2 and 4 nm and a density of 3 * 10 cm are initially formed on the wetting layer to reduce strain. Further deposition results in the growth of nanocrystals of lateral dimensions between 40nm and 150nm and heights between 5 and 30nm. A maximum density of 10 cm and minimum size of these nanocrystals has been obtained for low deposition temperature of 470°C and high rate of 1.5nm/min. The epitaxial orientation of the nanocrystals has been determined as (111) and (220) by TEM and XRD. Introduction Low-dimensional semiconductor structures are investigated for their interesting properties for electronic and optical applications [1]. Nanometer scale structures are expected to show electron confinement the stronger the smaller the size. Quantum dots confining the electrons in three dimensions can be grown by Stranski-Krastanov growth mode. Due to the strain induced by a different lattice constant of substrate and deposited material, an initially two-dimensional (2D) epitaxial growth changes to 3D island growth on a thin wetting layer. This formation of selfassembled islands reduces the strain and lowers the total energy of the system. Germanium nanocrystals embedded in a semiconductor matrix are potential candidates for a future optoelectronics in group-IV semiconductors. Although an indirect semiconductor, high intensity luminescence is expected for germanium quantum dots due to carrier confinement. Many investigations have dealt with Ge on Si(001) or Si(111) [e.g., 2-5]. We have grown Ge on SiC(0001) by solid-source MBE. The wide-band gap semiconductor SiC may give the possibility to arrange quantum dots at a p-n junction for electroluminescent devices. A Stranski-Krastanov-like growth mode is observed due to the huge lattice mismatch of some 30% between silicon carbide and germanium. We investigated the surface structure of Ge on differently prepared SiC(0001) by Scanning Tunneling Microscopy (STM) and Low-Energy Electron Diffraction (LEED). The formation of Ge islands by Stranski-Krastanov growth mode is measured in situ by STM and ex situ by Atomic Force Microscopy (AFM), High-Resolution Transmission Electron Microscopy (TEM), and X-ray diffraction (XRD). Materials Science Forum 353-356 (2001) 247-250.