Al compositional inhomogeneity of AlGaN epilayer with a high Al composition grown by metal-organic chemical vapour deposition

Nanoscale phase separation on an AlGaN surface characterized by scanning diffusion microscopy

AlGaN is an important material for deep ultraviolet optoelectronic devices and electronic devices. The phase separation on the AlGaN surface means small-scale compositional fluctuations of Al, which is prone to degrade the performance of devices. In order to study the mechanism of the surface phase separation, the Al_0.3Ga_0.7N wafer was investigated by the scanning diffusion microscopy method based on the photo-assisted Kelvin force probe microscope. The response of the surface photovoltage near the bandgap was quite different for the edge and the center of the island on the AlGaN surface. We utilize the theoretical model of scanning diffusion microscopy to fit the local absorption coefficients from the measured surface photovoltage spectrum. During the fitting process, we introduce as and ab parameters (bandgap shift and broadening) to describe the local variation of absorption coefficients α(as, ab, λ). The local bandgap and Al composition can be calculated quantitatively from the absorption coefficients. The results show that there is lower bandgap (about 305 nm) and lower Al composition (about 0.31) at the edge of the island, compared with those at the center of the island (about 300 nm for bandgap and 0.34 for Al composition). Similar to the edge of the island, there is a lower bandgap at the V-pit defect which is about 306 nm corresponding to the Al composition of about 0.30. These results mean Ga enrichment both at the edge of the island and the V-pit defect position. It proves that scanning diffusion microscopy is an effective method to review the micro-mechanism of AlGaN phase separation.

Quantum Dot-Like Behavior of Compositional Fluctuations in AlGaN Nanowires

We report on the structural and optical properties of AlxGa(1-x)N nanowire sections grown by plasma-assisted molecular beam epitaxy on GaN nanowire bases used as a template. Based on a combination of scanning electron microscopy, microphotoluminescence, time-resolved microphotoluminescence, and photon correlation experiments, it is shown that compositional fluctuations in AlxGa(1-x)N sections associated with carrier localization optically behave as quantum dots. Moreover, most of the micro-optical properties of such fluctuations are demonstrated to be very little dependent on kinetic growth parameters such as AlxGa(1-x)N growth temperature and AlN molar fraction in the alloy, which govern the macrostructural properties of AlxGa(1-x)N sections.