The carrier distributions and defect states in GaN-based semiconductor devices are included in crucial parameters for the overall performance and reliability. However, experimental investigations of carrier distributions and defect states in GaN-Based semiconductors have been limited, while a number of researches and efforts theoretical simulations with assumptions have been the main source of the research.
In this study, we experimentally investigated the carrier distributions and defects in GaN-Based semiconductor devices by electrical and optical spectroscopy through the modified capacitance-voltage (C-V) techniques.
Firstly, C-V depth profile with additional laser illumination were performed at various temperatures (80K to 300K) to determined the carrier distributions in active region of InGaN/GaN blue light-emitting diodes and compared those results with the simulations. The higher density of accumulated carriers in n-side QWs indicates slower recombination rate than carriers in p-side QWs. The stronger piezoelectric field in a quantum well would separate farther electron and hole envelop wavefunction resulting in the smaller overlap integral and the slower radiative recombination rate. This non-uniformity of the piezoelectric field through the QWs and the stronger field in the n-side QWs are possibly due to the partial relaxation of the strain through the MQWs, which is consistent with our previous report. In the MQW structure this partial relaxation is expected to occur subsequently through the MQWs, so that the p-side QWs that are more weakly strained resulting in possessing lower piezoelectric fields.
Secondly, the distributions, densities and energies of defects near InGaN/GaN quantum wells in blue LEDs were simultaneously determined by utilizing C-V measurements. By combining the modulation frequency , temperature dependency and C-V depth profiling with additional laser illumination, the densities and the locations of the defective layers could be determined. The relative defect densities of devices were directly compared by monitoring the magnitude of the frequency dependence. This frequency dependency varies distinctly as the sample temperature changes. The activation energies of defects are then determined by analyzing the frequency dependency of C-V with temperature. We found that three different defects states were formed in a low-temperature-grown un-doped GaN (LT-GaN) layer inserted under the active layer. The activation energies of those defects were determined to be 3.96, 12.1 and 45.9 meV. The formation of additional defects states in the active layers induced by the insertion of LT-GaN layer was also observed. Our proposed technique to analyze the distributions, densities and energies of defects near/in the active region by combining frequency dependency, temperature dependency and C-V depth profiling with laser illumination can be readily applicable to not only GaN-related deep UV and green LEDs, but also to other sophisticated optoelectronic devices such as laser diode and solar cell structures.
Additionally, high defect density in AlGaN deep ultra-violet (DUV) LED were investigated by frequency dependant C-V and current-voltage characteristic. In particularly, huge defect density in active region of AlGaN DUV result in significant leakage current.
Finally, We performed frequency dependence C-V measurements with resonant optical excitation in order to investigate the defects distributions and their effects on MOS and SB AlGaN/GaN HEMT devices. Strong frequency dependency caused by large densities of defect states in the SiO2 layerofthe MOS HEMT was observed. The capacitance in the GaN capsulation layer was increased by photogenerated carriers at the SiO2/GaN interface states. And the Vth was strongly influenced by the properties of the interface states. This negative change in Vth is attributed to changes in the Fermi level pinning point by the detrapping of neutral deep states at the SiO2/GaN interface, which leads to changes in the electric field of the device.