Space-Filling Arrays of Three-Dimensional Epitaxial Ge and Si1-xGex Crystals

In this paper, a Si substrate is patterned into high-aspect ratio pillars or ridges by conventional photolithography and deep reactive ion etching, to expose limited areas on which to grow Ge and Si_1-xGe_x layers. Subsequent epitaxial growth by low-energy plasma-enhanced chemical vapor deposition (LEPECVD) takes advantage both of geometric shielding of the growth species arriving at the patterned substrate surface, as well as of growth parameters (e.g. low temperature, high growth rate) designed to limit the surface diffusion length, thus favoring vertical over lateral growth. This results in a uniform space-filling array of three-dimensional epitaxial crystals. The crystalline quality, tilt and strain of the Ge and Si_1-xGe_x crystals are investigated by high resolution X-ray diffraction (XRD) with reciprocal space mapping (RSM) around the Si(004) and Si(224) reflections. The results provided evidence for the nearly perfect crystal structure of the epitaxial material, and showed the crystals grown on the Si pillars to be strain-free. Synchrotron submicron diffraction experiments performed with a focused (~300×500 nm) X-ray beam revealed tilted small tilt of epitaxial Ge crystals with respect to the Si pillars. Faceted crystals with height, size and shape tunable over a wide range by growth and substrate parameters, are shown to be defect-free by transmission electron microscopy and defect etching. The electrical properties of p-i-n heterojunctions between the epitaxial crystals and the Si-substrate and the interplay between surface and volume effects were investigated by in-situ SEM conductivity experiments. The measured I-V characteristics showed clear diode behavior with dark currents of the order of 10^-4 A/cm².