University of Cambridge
Structure-Property Relationships in Nitride Electronic Devices
Abstract
dc:description.abstractGallium nitride (GaN) based semiconductors can be used to fabricate high electron mobility transistors (HEMTs) which are typically used for power and radio frequency applications. To reach the full potential of GaN-based HEMT structures, which often have numerous material defects, nanoscale electrical characterisation techniques are needed to provide insights into their structure-property relationships. Scanning capacitance microscopy (SCM) is an AFM-based technique that provides access to information about the electrical properties of semiconductors. This thesis describes the development of a series of SCM-based techniques for plan-view and cross-sectional GaN-based HEMT structure characterisations, that help to meet the need for greater access to nanoscale structure-property relationships. Optimised workflows have been developed for plan-view conventional SCM (C-SCM) and DataCube SCM (DCUBE-SCM). In both cases, this requires a diamond coated tip and a cantilever with a high spring constant. The tip is positioned near the Ohmic contact and the sample edge. After tip selection and positioning, the tip should be milled blunt enough by continuously scanning it across the sample surface in contact mode, with a higher deflection setpoint and a greater scan rate than those used in C-SCM scans, in order to achieve strong electrical signals. A multi-microscopy approach is employed to achieve both a high-resolution topographical image and strong electrical signals of the same scan area so that a direct correlation between nanoscale features and local electrical properties can be established. In plan-view C-SCM, a dc bias should be applied to the tip in addition to an ac bias. Two optimised dc bias values provide strong but opposite SCM contrast, highlighting the impact of surface defects and subsurface AlxGa1-xN barrier layer inhomogeneities on the local electrical properties. In plan-view DCUBE-SCM, a map of threshold voltage (Vth) distribution across the surface at the nanoscale is extracted from the dataset. Most surface defects reduce the magnitude of the local Vth. Vth variations within non-defect areas are related to subsurface AlxGa1-xN barrier layer inhomogeneities which are also quantified. Two samples having similar two-dimensional electron gas (2DEG) sheet carrier density but different AlxGa1-xN barrier layers are compared using the two developed techniques. The thicker AlxGa1-xN barrier layer with a lower Al content leads to a more homogeneous Vth distribution. The cross-section of a sample is also characterised using C-SCM and DCUBE-SCM. The contrast of two well-defined conductive layers is observed in cross-sectional C-SCM, showing opposite carrier types: one is n-type while another is p-type. Their local dC/dV-V curves, collected through DCUBE-SCM, match those of the p-type and n-type regions in a GaN-based p-n junction sample. The evolution of the local dC/dV-V curves between the two conductive layers also match with the nature of a 2DEG and two-dimensional hole gas (2DHG). It is proposed that the n-type carrier-containing layer is a 2DEG layer, while the p-type carrier-containing layer is most likely a 2DHG layer. Overall, the developed methodologies enabled a comprehensive understanding that could be crucial for future optimisation of GaN-based HEMT structures and other semiconductor materials and devices.
Degree
thesis:*- Name dc:type.qualificationname
- Doctor of Philosophy (PhD)
- Level dc:type.qualificationlevel
- Doctoral
- Grantor dc:publisher.institution
- University of Cambridge
- Year dc:date.issued
- 2025
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Chen, Chen
- Advisor dc:contributor.advisor
-
- Oliver, Rachel
Subjects
dc:subject × 6Rights
dc:rightsIdentifiers
dc:identifier.*- DOI dc:identifier.doi
- https://doi.org/10.17863/CAM.120198
- OAI identifier oai:identifier
- oai:www.repository.cam.ac.uk:1810/387403