Electrically active defects in novel Group IV semiconductors.

This thesis presents the electrical characterisation of defects in novel group IV semiconducting materials: semiconducting diamond and silicon germanium (SiGe) virtual substrates. Several methods to clean diamond surfaces are introduced, which lead to the fabrication of a diamond Schottky diode with acceptable characteristics. Current-Voltage (I-V) and Capacitance-Voltage (C-V) measurements were carried out to study the electrical properties of both the diamond and SiGe Schottky diodes. Deep level transient spectroscopy (DLTS) and Laplace DLTS were then carried out to investigate the deep electronic states in these devices. Scanning Electron Microscopy (SEM) was also used to investigate defects in the diamond samples. For the diamond Schottky diodes, I-V and C-V measurements confirmed the quality of the fabricated Schottky diode; the measured phase angle between capacitance and voltage was close to 90° for temperatures greater than 300K and frequencies above 200 kHz and the device clearly exhibited rectification. DLTS and LDLTS measurements of the diamond did not show any signatures that could be attributed to isolated point defects. This could be due to the fact that it was necessary to take the samples to higher temperatures in order to fully ionize the boron in the sample. The boron acceptor is at 0.37 eV above the valence band and therefore only about 5% is ionised at room temperature. During the major part of the study at Manchester, there was no access to a high temperature cryostat. However, a clear capacitance transient was observed at lower temperatures and it is proposed that this is due to emission of holes from boron. Deep traps will be located deeper in the band gap than the boron. An additional problem was that the sample was of polycrystalline structure and is full of grain boundaries, which appear to be implicated in the leakage currents present in our devices. I-V, C-V, DLTS and LDLTS were also used to investigate the deep states in the SiGe virtual substrate. I-V and C-V measurements showed that the SiGe Schottky diode showed some leakage (reported by the suppliers) but nevertheless the diode exhibited rectification. Analysis of the DLTS data showed the presence of a defect in the SiGe samples which could be a structural defect, probably dislocation-related. However, the low background doping meant that a considerable depth below the surface was being measured in DLTS and depth profiling was not possible.