Abstract
Ion implantation has been investigated as an alternative technique to epitaxial deposition for the synthesis and doping of strained Si1-xGex alloy layers. Use of an all-implanted process has the potential to overcome many issues and difficulties of doping, reproducibility and yield in the commercial production of SiGe heterostructure devices. Samples with different germanium peak concentrations, x (0 ≤ x ≤ 15) were synthesised by implantation of Ge+ into bulk Si, and were heavily doped with boron or with arsenic (10 20 cm-3). Some layers were also post-amorphised by Si+ implantation. Room temperature sheet resistance and Hall mobility were measured using the Van der Pauw method. Depth profiles were obtained using Secondary Ion Mass Spectroscopy (SIMS) and Spreading Resistance Profiling (SRP), to determine dopant distribution and sheet carrier concentration, respectively. The re-grown material quality of selected samples was investigated using cross-sectional Transmission Electron Microscopy (XTEM). As a general trend, an increase in sheet resistance and a decrease in Hall mobility was observed with increasing germanium peak concentration in the ion beam synthesised layers. However, a speculative approach used to propose values and trends in drift mobility (muD) suggested some improvements compared to bulk Si. Some confidence in the values of muD proposed is given by corresponding values of the Hall scattering factor, rH, which are consistent with values reported in the literature for similar germanium content and layer thickness. Electrical junction depths were reduced in the synthesised layers compared to bulk Si. Evidence from transmission electron microscopy (TEM) and electrical measurements shows that the highest germanium peak concentrations used exceed the critical threshold for production of strained SiGe layers after solid phase epitaxial growth (SPEG) in accordance with Paine's experimental model of strain relaxation in ion beam synthesised SiGe layers. It is proposed that estimated trends in rH may provide a useful source of evidence for strain relaxation. Furthermore, use of ion beam synthesised SiGe leads to a reduction in diffusion of both boron and arsenic, compared to bulk-Si. This is attributed to the formation of BGe pairs and/or boron-interstitial (BI) clusters, and GeAs precipitates and/or AS4V- clusters, which are also responsible for reduced carrier activation in the synthesised layers. In the As-doped samples, this trend differs from findings reported in the literature for deposited strained SiGe layers. Silicon post-amorphisation improves the carrier activation in As-doped samples; hence it is postulated that end-of-range (EOR) defects act as a barrier to arsenic enhanced diffusion. It is concluded that an all-implanted route to the production of strained, doped SiGe layers is worth incor-porating into commercial device manufacture.