Gas Source Techniques for Molecular Beam Epitaxy of Highly Mismatched Ge Alloys

Abstract

Ge and its alloys are attractive candidates for a laser compatible with silicon integrated circuits. Dilute germanium carbide (Ge 1−x C x ) offers a particularly interesting prospect. By using a precursor gas with a Ge 4 C core, C can be preferentially incorporated in substitutional sites, suppressing interstitial and C cluster defects. We present a method of reproducible and upscalable gas synthesis of tetrakis(germyl)methane, or (H 3 Ge) 4 C, followed by the design of a hybrid gas/ solid-source molecular beam epitaxy system and subsequent growth of defect-free Ge 1−x C x by molecular beam epitaxy (MBE). Secondary ion mass spectroscopy, transmission electron microscopy and contactless electroreflectance confirm the presence of carbon with very high crystal quality resulting in a decrease in the direct bandgap energy. This technique has broad applicability to growth of highly mismatched alloys by MBE.

Nature of the band gap of Ge:C alloys: insights from hybrid functional density functional theory calculations

“…Recently Stephenson et al [3] presented a route to overcome this problem, using a hybrid gas/solid-source MBE approach with tetrakis(germyl)methane (4GeMe) as the C source, a molecule that has one C atom bonded to four Ge atoms. This allowed growth of high quality Ge:C alloys in the composition range of 0.2% C [11].Their measurements were accompanied by density functional theory (DFT) calculations on Ge 127 C 1 supercells, indicating that Ge:C at 0.78% C is a direct gap material. These recent experimental and theoretical investigations suggest that Ge:C could therefore be a promising candidate for future silicon-based technologies.…”

Electronic structure evolution in dilute carbide Ge1−xCx alloys and implications for device applications

“…36 Consistent with this analysis, Park et al 37 showed that it would be virtually impossible to achieve fully substitutional growth of Ge 1−x C x alloys by conventional molecular beam epitaxy (MBE) techniques. Recently, Stephenson et al 38 presented a route to overcome this problem, using a hybrid gas/solid-source MBE approach with tetrakis(germyl)methane (4GeMe) as the C source, a molecule that has one C atom bonded to four Ge atoms. This enabled growth of high quality Ge 1−x C x alloys having C compositions x ≈ 0.2% C. 24 These samples have been characterised using structural techniques and photo-modulated reflectance spectroscopy, 24 but there has to date been no reports of optical emission.…”