Test & Measurement

X-Ray Diffraction Reveals 3D Mechanics of Germanium-Crystal Arrays

Scientists at the European Synchrotron Radiation Facility (ESRF) are using X-ray diffraction to thoroughly understand a new type of nanostructure — square arrays of germanium (Ge) pillars that have very complex geometries and physical attributes.

Germanium grown epitaxially on micrometre-sized silicon pillars forms perfect crystals free of defects. The resulting Ge/Si “virtual substrate” is lighter and tougher than a free-standing Ge substrate and clears a path towards high-resolution CMOS X-ray detectors and high-efficiency multi-junction solar cells for both space and terrestrial applications.

Micrometre-scale layers of pure Ge are attractive for infra-red photodetectors, the final layer of triple-junction solar cells, or as “virtual substrates” for the deposition of GaAs-based layers. Increasing the Ge thickness still further (to several tens of micrometres) would enable the realisation of X-ray and particle detectors which are more efficient than their existing Si-based counterparts. The increased efficiency coupled with the good compatibility between Ge and Si leads to the final goal of a pixel detector in which the Ge absorbs the photons or particles and the resulting signals are handled by Si CMOS readout electronics.

The growth technique of low-energy plasma-enhanced chemical vapour deposition (LEPECVD) [3] has led to the efficient and fast deposition of such layers, but, once the Ge layer is more than a few micrometres thick, the thermal contraction during the cooling of the substrate back to room temperature after growth causes bending of the substrate and cracking of the Ge layer.

Learn more from ESRF.