Encapsulation of Si: P devices fabricated by scanning tunnelling microscopy

This thesis demonstrates the effective use of low temperature molecular beam epitaxyto encapsulate planar Si:P (phosphorus-in-silicon) devices lithographically patternedby scanning tunnelling microscopy (STM) without significant redistribution ofthe dopants. To achieve this goal, low temperature magnetotransport is used in combinationwith STM, Auger electron spectroscopy and secondary ion-mass spectrometry toanalyse Si:P δ-doped samples fabricated under different doping and growth conditions.An important aspect of this project is the use of large 1 × 1 cm2 Si(001) sampleswhich are about five times larger than standard STM samples. The larger samplesize is necessary for post-STM fabrication lithography processes in a cleanroom butpresents problems for preparing atomically clean surfaces. The ability to prepare cleanand atomically flat Si(001) surfaces for STM lithography on such 1 × 1 cm2 samplesis demonstrated, and it is shown that Si:P δ-doped layers fabricated on these surfacesexhibit complete electrical activation.Two dopant sources (gaseous PH3 and solid GaP source) were investigated to assesstheir compatibility with STM-lithography on the H:Si(001) surface. The findings showthat while the PH3 and GaP sources result in near identical electrical qualities, onlyPH3 molecules are compatible with H-resist based lithography for controlled nano-scaledoping.For achieving complete activation of the P dopants, it is shown that an anneal to∼ 350 ◦C to incorporate P atoms into the Si surface prior to encapsulation is critical.While it is known that the presence of H during growth degrades the quality of Siepitaxy, investigations in this thesis indicate that it has no significant effect on dopantactivation. Systematic studies performed to assess the impact of growth temperaturerecommend an encapsulation temperature of 250 ◦C for achieving optimal electricalqualities with minimal dopant segregation. In addition, it is shown that rapid thermalanneals (RTAs) at temperatures &lt 700 ◦C provide only marginal improvement in theelectrical quality of Si:P δ-doped samples encapsulated at 250 ◦C, while RTA temperatures&gt 700 ◦C should be avoided due to the high probability of dopant redistribution.To elucidate the nature of 2D transport in Si:P δ-doped devices, a detailed analysisof the low temperature magnetotransport for Si:P δ-doped layers with doping densitiesin the range ∼ 0.2 − 2 × 1014 cm−2 was carried out. Using conventional 2D theoriesfor disordered systems, both weak localisation (WL) and electron-electron interactions(EEI) are shown to contribute almost equal corrections to the 2D conductivity. In particular,it is found that EEI can introduce a significant correction in the Hall coefficientRH (hence Hall density) especially in the low density/temperature regime and the needto correct for this when using the Hall density to estimate the activated electron densityis highlighted. While the electronic mean free path in such highly doped δ-layersis typically &lt 10 nm making ballistic transport in these devices difficult to observe, thephase coherence length can extend to almost 200 nm at about 0.30.5 K for dopingdensities of ∼ 1 − 2 × 1014 cm−2.Finally, the optimised encapsulation strategy developed in this thesis is appliedto a 2D square device fabricated by STM. The device exhibits Ohmic conductivitywith complete dopant activation. An analysis of its low temperature magnetotransportshows that the device behaves similarly to a Si:P δ-doped layer encapsulated undersimilar conditions, thus highlighting that the STM patterning process had no adverseeffect on device quality