Spin resonance in novel environments
Keywords: electrically detected magnetic resonance
silicon nanostructure
silicon
electron spin resonance
dopant activation
recombination
quantum transport
perovskite
transistor
current rectification
optoelectronic

In this thesis I experimentally investigate semiconductor defects in novel environments. I do this by exploiting the unpaired defect-bound electrons that can be measured through electron paramagnetic resonance (EPR) effects. I extend my analysis to electrically detected magnetic resonance (EDMR) as it characterises the influence of paramagnetic recombination centre on a sample's conductivity, which is important for typical applications like optoelectronics, complementary metal oxide semiconductor devices and donor spin quantum information processing.
My experimentation can be divided into three parts. I firstly investigated rectification effects in nanoscale tri-gate transistors setup for EDMR measurements. An understanding of rectification currents is a stepping stone to effective EDMR measurements of non-linear devices in the future. By using a two-dimensional Taylor series, I accurately model rectification currents through a tri-gate nanoscale transistor situated in a microwave cavity. My model requires minimal coupling to exist between the gate and bias terminals. I show that the phase difference between the AC voltage on the gate and bias terminals is critical in determining the magnitude and direction of the rectification current. I then demonstrate that rectification currents can be used to my advantage in broadband microwave detection. By operating my device in a non-linear mode under the influence of microwave fields it becomes sensitive to small changes in the cavity's field. Microwave absorption by foreign paramagnetic materials in the sample space can then be measured by a change in the rectification current. This I demonstrated through the detection of 2,2-diphenyl-1-picrylhydrazyl at X-band frequencies and oxygen at D-band frequencies. My experiment demonstrates a novel way with which to perform EPR experiments in a small sample space, without the need to isolate high frequency D-band detection circuitry from the static field.
I secondly investigated EDMR in naturally occurring silicon ion-implanted with 33S and 75As. My aim was to observe spin-dependent recombination (SDR) of electron-hole pairs mediated by the shallow donor 75As in a neutral charge state and the deep donor 33S in the charge state S+. Continuous wave-EDMR (cwEDMR) measurements revealed the spin signature of 75As, a plethora of Pb0, Pb1 and E' interface defects and residual 31P. In each sample, the EDMR spectra gave no definitive indication of electrically active 33S, 33S clusters or 33S complexes. I attribute this to the photoneutralisation of the paramagnetic S+ state. I also detected amorphisation of the silicon crystal from ion-implantation, which recrystalised for longer anneal durations as expected. My results gave an indication of the interface-like defects created during amorphisation and the effect these have on hindering charge transport. Pulsed-EDMR was used to investigate the charge carrier interactions leading to the cwEDMR spectra. The decrease in current upon microwave excitation indicated SDR of electron-holes pairs across all resonances. An unremarkable 2 microseconds coherence time was determined for 75As-bound electrons and a 4.5 microseconds electronic transition time, which is comparable to published data involving shallow donors and interface charge traps. Electrically detected electron double resonance experiments demonstrated the formation of spin-pairs between 75As and the amorphisation defects, which led to electron-hole recombination.
Finally, EPR was used to investigate UV-irradiated PbI2 and MAPbBr3/MAPbI3 crystals, which had previously shown paramagnetism. Intrinsic defects are believed to be the cause of their long electron-hole diffusion lengths in organic-inorganic hybrid perovskite. In my measurements, the materials all appeared EPR-silent, which is attributed to the high purity of the lead halide precursor.