J.D. Zhang

Micro-Raman and photoluminescence studies of neutron-irradiated gallium nitride epilayers

GaN epilayers grown on sapphire substrate were irradiated with various dosages of neutrons and were characterized using Micro-Raman and photoluminescence. It was found that the A1(LO) peak in the Raman spectra clearly shifted with neutron irradiation dosage. Careful curve fitting of the Raman data was carried out to obtain the carrier concentration which was found to vary with the neutron irradiation dosage. The variation of the full width at half maximum height of the photoluminescence was consistent with the Raman results. The neutron irradiation-induced structural defects (likely to be GeGa) give rise to carrier trap centers which are responsible for the observed reduction in carrier concentration of the irradiated GaN.

Large lattice relaxation deep levels in neutron-irradiated GaN

Deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) measurements have been carried out in neutron-irradiated n-type hydride-vapor-phase-epitaxy-grown GaN. A defect center characterized by a DLTS line, labeled as N1, is observed at EC−ET=0.17eV. Another line, labeled as N2, at EC−ET=0.23eV, seems to be induced at the same rate as N1 under irradiation and may be identified with E1. Other defects native to wurtzite GaN such as the C and E2 lines appear to enhance under neutron irradiation. The DLOS results show that the defects N1 and N2 have large Frank-Condon shifts of 0.64 and 0.67eV, respectively, and hence large lattice relaxations. The as-grown and neutron-irradiated samples all exhibit the persistent photoconductivity effect commonly seen in GaN that may be attributed to DX centers. The concentration of the DX centers increases significantly with neutron dosage and is helpful in sustaining sample conductivity at low temperatures, thus making possible DLTS measurements o...

Electronic stability and noise reduction in Doppler broadening spectroscopy systems

Abstract At the University of Hong Kong’s positron beam two related positron defect spectroscopies are being developed, namely, positron deep level transient spectroscopy (PDLTS), and coincidence Doppler broadening spectroscopy. Both of these spectroscopies require the use of high purity Ge detectors and the standard high gain nuclear instrumentation amplifiers. In this paper we review some of the difficulties that we have encountered in making such measurements satisfactorily in one of the busiest cities of the world. Attempts, such as using battery power supply, opto-isolating from local mains, connecting to an independent ground and adding a spectrum stabilizer, have been made to overcome the noise problem. However, the results are not particularly promising. The reasons why are discussed.

Raman Scattering and X-Ray Diffraction Study of Neutron Irradiated GaN Epilayers

Neutron irradiation induced defects and their effects on the carrier concentration of GaN epilayers are investigated with Raman scattering and X-ray diffraction techniques. Relative to the as-grown sample, the neutron-irradiated samples exhibit a clear variation in the position and lineshape of the A1(LO)-mode Raman peak as well as in the full-width at half-maximum height (FWHM) of the XRD rocking curves. Careful curve fitting and adequate calculations give the carrier concentrations of the irradiated GaN. It is found that the defects induced by neutron irradiation act as carrier trap centres which capture the electron carriers so that the carrier concentration of the irradiated GaN is reduced