Kai Qiu

The effect of low-temperature annealing on ferromagnetic Ga1−xMnxAs thin films studied by photoemission spectroscopy

A systematic photoemission and variable temperature resistivity measurements are carried out on Ga0.946Mn0.054As ferromagnetic semiconductors. It is showed that the Mn 3d valence band and the As 3d core-level spectrum are modified by the annealing treatment. Correlating these modifications with the observed changes in the resistivity and Curie temperatures, we have identified that the low-temperature annealing most likely induces three changes: interstitial Mn out-diffusion, slight increase of the substitution Mn components, and reduction of excess As.

Threading dislocations related persistent photoconductivity effect in hydride vapor phase epitaxy grown GaN epilayers

Deep-level defect-related properties of as-grown and annealed GaN epilayers grown by hydride vapor phase epitaxy (HVPE) are investigated using persistent photoconductivity (PPC) and photoluminescence (PL) measurements. PPC phenomena are seen in these HVPE-GaN epilayers which do not contain the yellow-luminescence (YL) band, suggesting that PPC and YL phenomena are not related. The PPC decay kinetics indicates that the characteristic of the defects which are responsible for the PPC phenomenon would be different from the point defects. The structural qualities of these HVPE-GaN epilayers were characterized by X-ray diffraction. As the densities of threading dislocations decrease, a decrease of photocurrent was clearly observed. The results suggest that the negatively charged threading dislocations are the candidate for PPC effect in our HVPE-GaN epilayers.

Persistent photo‐conductivities from extended defects in GaN films with high resistivity

Excitation intensity and spectral dependence of persistent photoconductivity (PPC) in undoped GaN has been measured. The observed current decay behavior can be attributed to extend defects which served as negatively charged centers. According to this model, the recapture barrier would increased gradually when the electrons were recaptured by the extend defects. The recapture barrier Ec measurements indicate that large lattice relaxation took place during trapping and detrapping process.

Study of the transport properties of annealed (Ga,Mn)As by x-ray absorption spectroscopy

A systematic Mn L-edge x-ray absorption is carried out on carefully prepared Ga0.946Mn0.054As ferromagnetic semiconductors with varying As2∕Ga flux ratio. It is found that the L3 peak of the absorption spectroscopy is enhanced after low temperature (LT) annealing. Furthermore it is shown that a more localized electronic structure nearly like the d5 high-spin state is obtained. It can be attributed to breaking the MnS–MnI pairs during the annealing process. Furthermore the authors present a direct evidence for a slightly increase of the Mn substitutional concentration due to LT annealing.