In this thesis the electrical transport properties o f high resolution doping structures grown by Si:MBE have been studied. The high degree o f profile control, can be exploited in the production of new devices such as the delta doped field effect transistor studied in this work. A periodic sequence of high resolution doping spikes, known as a doping superlattice, in which enhanced carrier mobilities were found by Nakagawa et al (1987) was further investigated. Thin layer doping (or delta doping) allowed the study o f 2-Dimensional phenomena in a silicon system without the presence o f an oxide barrier. This was investigated by tunnelling spectroscopy. Increasing the width o f the doping layer allowed a 2D -3D transition to be observed in silicon, by magnetoresistance studies.
Temperature dependent Hall coefficient measurements were used to characterise Si:B and Si:Sb doping superlattices. T h e anomalous high mobility observed by Nakagawa et a l (1986) in a boron doping superlattice with a period o f 60nm, was not observered by the author in a similar structure. Si:Sb doping superlattices designed to have a high mobility showed apparent mobility enhancement with increasing space/mark ratio. This effect was interpreted as being due to increasing carrier spillage into the low doped regions o f the superlattice.
Further work on doping superlattices should involve the use delta doping to achieve the Esaki criterion [Esaki 1970) o f a large potential modulation with a short superlattice period. Delta doping with opposite type spikes should produce a saw-tooth band-edge profile and allow the formation o f minibands. This may then be observable
via transport measurement parallel and perpendicular to the superlattice. Such a structure would be very challenging to produce by Si:MBE, as two different growth processes would be used to produce n and p type delta layers.
The 2D subband structure associated with delta doping was investigated via tunnelling spectroscopy. Reported for th e first time in this work, are results of tunnelling studies o f boron delta layers in silicon. The observed structure was compared to theoretical calculations which gave broad agreement with the experimental results.
A future tunnelling spectroscopy study could investigate the variation o f the measured subband energy with delta layer concentration and layer width. This would be easier to achieve with Sb delta layers, du e to the fewer number o f occupied subbands, giving more easily resolvable spectra. It may then be possible to improve theoretical calculations o f subbands energies. Further investigations o f the subband spectrum of boron delta layers in silicon is required.
Transport measurements were then continued on a series o f Si:Sb doped thin layers, with widths o f 10, 20 and 80nm. A metal-insulator transition was observed as the width of the conducting channel was decreased. Also reported for the first time, is a 2D to 3D transition in silicon. Current theories o f weak localisation and electron- electron interaction effects have been used to describe the corrections to the conductivity. The layers studied have a large amount of disorder, where the values of k f l a 1. 2D transport formula have been used to extract values o f the screening parameter F* and the phase relaxation tim e is found to vary as T 1. Magnetoresistance studies on a lOnm width Sb doped layer, has shown a large anisotropy between sample orientations parallel and perpendicular to the magnetic field. Values of the phase relaxation length (L*) have been extracted from the negative magnetoresistance found for perpendicular sample orientations. T he value o f L ,-3 4 n m justifies the use o f 2D transport formula. A 20nm width sample was described using formula for a quasi 2D system parallel to the magnetic field. Again, sensible values o f the phase relaxation
length were found. In contrast a 80nm width sample could be satisfactorily described using 3D transport formula and the magnetoresistance was found to be temperature independent and isotropic.
The study o f weak localisation and electron-electron interaction phenomena effects should be extended to a Si:Sb delta layer. In such a system, the 2D transport formula should apply for magnetic fields up to 40T. Further theoretical work is required to explain the Zeeman spin spitting effects seen in these layers, as the form of the magnetoresistance for high magnetic field cannot be adequately described. A metal- insulator transition may possibly be observed for a suitable sample on the application of a magnetic field.
Lastly, the device applications o f delta doping were investigated by the fabrication o f the first p-channel delta doped FET using a boron delta layer. A low temperature processing schedule, combined with a plasma enhanced oxide growth process was used to minimise diffusion of the delta layer. A model is presented describing the operation o f a long channel delta doped FET. This was used to extract device parameters such as the channel mobility.
The delta doped FET produced in this work has shown that a working device can be fabricated using Si:MBE delta layer. Optimisation o f the RTA stages is needed to reduce further dopant diffusion. Adoption of an n + polysilicon gate would allow an investigation o f submicron devices using a self aligned process to reduce the parasitic capacitances. This processing would then be more typical o f that used to produce CMOS devices. The p-channel F E T needs to be demonstrated with an n-type punchthrough stopper. Use o f two opposite type delta layers, raises the possibility of the production o f a CMOS devices. A mask set with diagnostic test structures, such as Hall Bar and an MOS capacitor to determine carrier concentrations, oxide quality and dopant profiles is needed to correlate the extracted transistor parameters.