Abstract
Efficient and reliable electronic devices beyond silicon technology are necessary to handle the ever-growing demand for electric power and advanced electronic technologies. Wide band gap semiconductors entailing high breakdown field characteristics are the ideal candidates to meet the demand for revolutionary power and communication electronics. Particularly gallium nitride (GaN) is a suitable material given its ability to form a 2D electron gas (2DEG) combined with the high electron mobility resulting in high device performance. Despite the great material properties and past research progress, many challenges remain for GaN technology, particularly when silicon substrates are used. The device buffer is one key component to ensure a high vertical breakdown voltage, however, it is prone to different liabilities compromising the device performance. Defects and impurities are inherently or deliberately incorporated during buffer growth defining the electronic properties during device operation. Understanding their behavior upon external stress imposed by the device operation or its environment is crucial to ensure stable and reliable device performance.Dislocations are an inevitable consequence of GaN growth on silicon substrates given the different lattice properties. Threading dislocations have been found to act as localized vertical conductive paths jeopardizing the breakdown characteristics of the buffer. A novel measurement methodology combining Kelvin Probe Force Microscopy (KPFM) and substrate bias ramps was developed in order to probe the role of vertical leakage paths during off-state stress. KPFM measurements under high negative back bias show local drops in surface potential related to local 2DEG depletion regions at the location of conductive dislocations. A model is presented correlating the observed surface potential effect to large deformations of the off-state buffer potential around threading dislocations causing high localized electric field stress in the channel. The measurement suggests that breakdown is more of a localized phenomenon instead of a bulk buffer effect.
Dopants such as carbon are used to make the buffer more insulating. The doping profile with depth is specifically engineered to maintain ideal channel properties while ensuring good blocking and dynamic switching performance of the buffer. However, the resulting electronic band structure and charge concentrations are sensitive to illumination changing the charge distribution across the buffer. Absorption of ultraviolet (UV) light with an energy greater than the GaN band gap increases the channel conductivity which recovers only over long periods of time, known as persistent photoconductivity (PPC). The effect was measured and simulated for a wide range of device configurations. It is shown how light absorption and charge generation affect the electronic band diagram and charge distribution in the buffer. The magnitude of the effect is found to be directly related to the net doping density (difference of acceptor concentration and donor concentration), especially in the channel layer. This proposes that the UV-induced PPC effect can be used to determine the net doping density, a quantity that is extremely difficult to measure by any other method.
| Date of Award | 6 Dec 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Martin H H Kuball (Supervisor) & Michael J Uren (Supervisor) |