Chen-Ke Shu

Isoelectronic In-doping effect in GaN films grown by metalorganic chemical vapor deposition

The isoelectronic In-doping effect in GaN films grown by metalorganic chemical vapor deposition was investigated by using Raman scattering, scanning electron microscopy (SEM), and x-ray and photoluminescence (PL) measurements. In our study, the phonon spectra of films remain sharp without alloy formation after incorporation of small amounts of In atoms. The SEM pictures of the sample surface reveal greatly reduced nanopits indicating better surface flatness that is also supported by the multiple interference effect in the PL signals. More importantly, isoelectronic doping has caused the linewidth at 15 K of the near-band-edge emission of GaN to decrease sharply to 10 meV or less, reflecting improved optical property.

Raman and x-ray studies of InN films grown by metalorganic vapor phase epitaxy

Thin InN films were deposited on the (0001) sapphire substrate at various temperatures from 325 to 600 °C by metalorganic vapor phase epitaxy. We used Raman scattering and x-ray diffraction to investigate the film properties and crystalline structures. Significant line broadening, softening and intensity evolution were observed at the growth temperatures between 375 and 450 °C. This can be attributed to the formation of the mixed hexagonal and cubic structures and the related dislocation defects. As the growth temperature is further increased, the hexagonal phase is found to be dominant in the deposited InN film.

Isoelectronic As doping effects on the optical characteristics of GaN films grown by metalorganic chemical-vapor deposition

We have studied the As doping effects on the optical characteristics of GaN films by time-integrated photoluminescence and time-resolved photoluminescence. When As is incorporated into the film, the localized defect levels and donor–acceptor pair transition become less resolved. The recombination lifetime of neutral-donor-bound exciton (I2) transition in undoped GaN increases with temperature as T1.5. However, the I2 recombination lifetime in As-doped GaN first decreases exponentially from 98 to 41 ps between 12 and 75 K, then increases gradually to 72 ps at 250 K. Such a difference is related to the isoelectronic As impurities in GaN, which generate nearby shallow levels that dominate the recombination process.

Photoluminescence and photoluminescence excitation studies of as-grown and P-implanted GaN: On the nature of yellow luminescence

We have studied optical and electronic properties of isoelectronic P-implanted GaN films grown by metalorganic chemical vapor phase epitaxy. After rapid thermal annealing, a strong emission band around 430 nm was observed, which is attributed to the recombination of exciton bound to isoelectronic P-hole traps. From the Arrhenius plot, the hole binding energy of ∼180 meV and the exciton localization energy of 28 meV were obtained. According to first-principle total-energy calculations, the implantation process likely introduced NI and P-related defects. By using photoluminescence excitation technique, we found that the P-implantation-induced localized states not only increase the yellow luminescence but also suppress the transitions from the free carriers to deep levels.

Growth temperature effects on InxGa1−xN films studied by X-ray and photoluminescence

The InGaN films were grown between 850 C and 600 C by the metalorganic chemical vapor deposition method and characterized by X-ray diffraction and photoluminescence (PL). The incorporation of In into the ternary films was found to increase from x = 0.01 to 0.28 as the temperature decreases. In films grown at 750°C and higher. both the X-ray and PL results show gradual changes and indicate 5% In molar fraction difference that may be due to the alloy composition fluctuation. However, in films grown at 700 C and lower, the near band edge emission disappears and the impurity transitions (IT) become dominant in the PL spectra. in contrast to X-ray diffraction where the line width broadens sharply from less than 300 arcsec to larger than 500 arcsec. We also found that IT is relatively insensitive to the sample temperature, Besides, the correlation between enhancing PL intensity and patterned micro-structure is observed.

Optical and electrical investigations of isoelectronic In-doped GaN films

Abstract The optical and electrical properties of isoelectronic In-doped GaN films grown by metalorganic vapor phase epitaxy (MOVPE) were investigated by X-ray, photoluminescence (PL), Hall and Raman measurements. As a result, adequate In-doping quantity causes not only a reduction of yellow luminescence and unintentional background concentration, but an enhanced mobility and decrease in the widths. The improved crystalline and optical qualities of GaN films may be attributed to the decrease in defects.

Time-resolved photoluminescence study of isoelectronic In-doped GaN films grown by metalorganic vapor-phase epitaxy

Time-resolved photoluminescence spectra were used to characterize isoelectronically doped GaN:In films. Our results indicate that the recombination lifetime of the donor-bound-exciton transition of undoped GaN exhibits a strong dependence on temperature. When In is doped into the film, the recombination lifetime decreases sharply from 68 to 30 ps, regardless of the measured temperature and In source flow rate. These observations might be related to the isoelectronic In impurity itself in GaN, which creates shallow energy levels that predominate the recombination process.

Raman and X-Ray Studies of InN Films Grown at Different Temperatures by Metalorganic Vapor Phase Epitaxy

We used Raman scattering and X-ray diffraction (XRD) methods to investigate the properties of InN films deposited at temperatures ranging from 325 to 600°C by metalorganic vapor phase epitaxy (MOVPE). Significant line broadening, softening and intensity evolution were observed from films at growth temperatures between 375 and 450°C. This can be attributed to the formation of mixed hexagonal and cubic structures and related dislocation defects. As the growth temperature was further increased, the hexagonal phase was found to be dominant in the deposited InN film.

Luminescence Intensity Reduction in Mg-Doped GaN Grown by Metalorganic Chemical Vapor Phase Epitaxy

Photoluminescence dynamic responses of a heavily Mg-doped GaN grown by metalorganic chemical vapor phase epitaxy have been investigated. As the probe power density increased from 2.5 to 925 W/cm2, a Mg-related emission band shifted from 2.82 to 3.16 eV revealing characteristics of donor-acceptor pair recombination. The intensity evolution also showed slow decay that suggests the metastable emission behavior. A simple potential barrier model was proposed to occur between the Mg related deep levels and common Mg acceptor level, which impedes electron relaxation and thus slows down the luminescence decay. From the variation of decay time constant with temperature, a barrier energy was deduced to be about 69 meV which is very close to the electrical result of 68 meV.

Metastable photoluminescence in heavily Mg-doped GaN grown by metalorganic chemical vapor phase epitaxy

The long-term transient spectra of heavily Mg-doped GaN have been investigated. As the excitation power density increased, the broad Mg-induced emission band showed blue- shift revealing characteristic of donor-acceptable pair (DAP) recombination. We also observed an unusually slow intensity decay. The characteristic time constants range from several tenths to a few hundred seconds for emission between 360 and 460 nm. Our results are interpreted in terms of metastability due to compound effects of differential DAP population and recombination rates and uneven acceptor distribution.