Fabrication and Characterisation of Buffer Layers in AlGaN/GaN on Si Power Devices

While silicon (Si) is reaching its performance limits, gallium nitride (GaN) has become the preferred choice for power electronics due to its superior properties. Carbon (C) doping is widely used to achieve high buffer resistivity, yet its intrinsic charge transport behaviour remains insufficiently understood. This thesis systematically investigates the role of carbon doping in GaN buffer structures.
A detailed analysis of carbon-doped GaN (CGaN) layers revealed a monotonic relationship between positive charge storage and carbon concentration. For the first time, dynamic RON was found to decrease with increasing carbon, contradicting prior assumptions. This was attributed to the evolution of prominent vertical leakage paths along the dislocation. Additionally, carbon incorporation was observed to enhance crystal quality.
The impact of CGaN thickness was further examined. While thicker buffers are conventionally favoured for high breakdown voltage, this study found that thinner CGaN layers improve crystal quality and exhibit lower dynamic RON. High-resolution X-ray diffraction (HRXRD) analysis indicated dislocation segregation as a key factor.
The effect of varying carbon concentration in the strain relief layer (SRL) was also investigated. Wafers with lower SRL carbon exhibited gap dependency due to enhanced lateral leakage, whereas those with higher carbon showed dominant vertical leakage, indicating weakly gap-dependent behaviour. The highest SRL carbon concentration resulted in the highest positive charge storage and the lowest dynamic RON. At high substrate bias, electron injection was more prominent in wafers with lower SRL carbon, reinforcing the need for high SRL resistivity.
Finally, a C-Si co-doped buffer layer was introduced. While the Si incorporation improved the crystal quality, an increased substrate leakage and dynamic RON degradation can be observed with increasing Si. These findings offer critical insights into GaN buffer design, reinforcing the need for highly resistive buffer structures.