Sinthiptharakoon, K; (2015) Investigation of individual donors in silicon at cryogenic temperature with atomic-scale resolution for atomic electronic devices. Doctoral thesis , UCL (University College London).

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Abstract

There is an urgent need for characterisation of group V donors in silicon necessary for the development of quantum information processing (QIP) devices, and my PhD work has been contributed towards this objective. In this thesis, three different group V donors were individually studied with scanning tunnelling microscopy (STM) and spectroscopy (STS), combined with DFT calculation and simulated STM images where necessary. Si(001) dosed with phosphine (PH3) at room temperature prior and imaged at 77 K a few minutes later was investigated. Novel phosphine-related features were observed and compared to the room-temperature results [4]. Some features were the same as the room-temperature dissociative products but a new dissociative mechanism was proposed and novel bonding configurations were assigned to the features. A transformation between adsorbates was seen to occur on the surface at 77 K and was attributed to a tip-induced effect. The appearance of the Si-P heterodimer was found to be different at 77 K and room temperature [5] and the reasons for this difference was discussed. Subsurface As and Bi donors below the Si(001):H surface were separately investigated at 77 K. Special sample annealing (flashing) procedures were created for both studies. There were two classes of As features commonly observed while there were three types for Bi. The appearance of the As features related to the As wavefunction informed that the subsurface As donors were electrically neutral but could be reversibly switched to being ionised (positively-charged) by changing the sample bias. Some subsurface As donors can also be negatively-charged, depending on their distances from the surface. DFT calculations were performed and simulated STM images were generated to compare with the experimental data, allowing us to assign the features to As donors at their exact lattice positions. The appearance of the Bi features indicated that the Bi donors were negatively-charged but could be changed to being positively-charged. With the unequal rate of tunnelling in and off the donor energy level, the Bi donors could be switched back to being negatively-charged again. The Bi wavefunction projection was speculated, based on the Bi features.

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