Papers by Keyword: Time-Resolved Photoluminescence

Abstract: Silver (Ag) nanoparticles (NPs) were deposited on the surface of bulk Nitrogen-Boron co-doped 6H silicon carbide (SiC), and the Ag NPs were observed to induce localized surface plasmons (LSP) resonances on the SiC substrate, which was expected to improve the internal quantum efficiency (IQE) of the emissions of the donor-acceptor pairs of the SiC substrate. Room-temperature measurements of photoluminescence (PL), transmittance and time-resolved photoluminescence (TRPL) were applied to characterize the LSP resonances. Through the finite-difference time-domain (FDTD) simulation of the LSP resonance of an Ag nanoparticle on the SiC substrate, it is predicted that when the diameter of the cross section on the xy plane of the Ag nanoparticle is greater than 225 nm, the LSP starts to enhance the PL intensity. With implementation of a 3rd order exponential decay fitting model to the TRPL results, it is found that the average minority carrier lifetime of the SiC substrate decreased.

Abstract: Thick and low-doped epilayers with a low Z_1/2 center concentration were grown on 8^o off-cut 4H-SiC(0001)Si-face substrate. Two post-growth processes, namely, the C⁺-implantation/annealing process or the thermal oxidation/Ar annealing process, were applied to the thick epilayers. The dependence of the Z_1/2 center concentration and the carrier lifetime on process conditions was investigated. Under proper conditions, both processes could eliminate the Z_1/2 center to a depth of 100 μm or more, and considerably improved the carrier lifetime while maintaining the surface roughness comparable to that of the as-grown sample. The effect of the post-growth processes applied on C-face is also presented.

Abstract: Luminescence properties of blue emission InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs), grown on sapphire substrates by metal organic chemical vapor deposition (MOCVD), were studied by time-resolved photoluminescence (TRPL) spectroscopic technique. Samples involved have similar basic structures of three QWs but different well-composition and barrier/well dimensions. TRPL results show that PL intensity and decay time increase with the number of QWs and the indium composition. Correlation of physical properties with crystalline perfection open the way for optimized designs of InGaN MQW LED, with controlled the indium composition and QW numbers.

Abstract: We investigate the carrier lifetimes in very thick 4H-SiC epilayers (~250 μm) by means of time-resolved photoluminescence and microwave photoconductive decay. Both the minority carrier lifetime and the high injection lifetime are found to reach 18.5 μs by applying the carbon implantation/annealing method to the as-grown epilayers. We also study the epilayer thickness dependence of the carrier lifetime by successive experiments involving lifetime measurement and polishing. Based on the relationships between epilayer thickness and carrier lifetime, the bulk carrier lifetime and the hole diffusion constant are discussed.

Abstract: Luminescence properties of blue emission InGaN/GaN multiple quantum well (MQW) have been studied by temperature dependent photoluminescence (PL), photoluminescence excitation (PLE) and time-resolved photoluminescence (TRPL) spectroscopic techniques. Two typical samples are studied, both consisting of five periods of InGaN wells with different indium compositions of 21% and 24%, respectively. According to the PL and PLE measurement results, large values of activation energy and Stokes’ shift are obtained. This indicates that higher Indium composition results in an increase of composition fluctuation in the InGaN MQW region, showing the stronger carrier localization effect. The lifetime at the low-energy side of the InGaN peaks is longer for higher indium composition, as expected from the larger Stokes shift.

Abstract: We report on the luminescence spectra related to Ti impurity in both 4H- and 6H-SiC polytypes. The spectrum depends strongly on the polarization. They are two families of lines in 4H and three in 6H. The main no-phonon line of each family is shown as a triplet and its phonon structure contains both sharp and broad replicas. The higher energy family has also extra lines at high energy appearing when the temperature increases. The spectra can be detected with excitation energy below the excitonic bandgap and even with excitation energy below the spectrum itself. Time-resolved photoluminescence reveals 0.1 ms long lifetime at low temperature.

Abstract: Improved structural quality and radiative efficiency were observed in GaN thin films grown by metalorganic chemical vapor deposition on in situ-formed SiN and TiN porous network templates. The room temperature carrier decay time of 1.86 ns measured for a TiN network sample is slightly longer than that for a 200 μm-thick high quality freestanding GaN (1.73 ns). The linewidth of the asymmetric X-Ray diffraction (XRD) (1012) peak decreases considerably with the use of SiN and TiN layers, indicating the reduction in threading dislocation density. However, no direct correlation is yet found between the decay times and the XRD linewidths, suggesting that point defect and impurity related nonradiative centers are the main parameters affecting the lifetime.

Abstract: Minority carrier (hole) lifetime investigations are conducted on identical 6H-SiC p+-n structures by electrical (reverse recovery, open circuit voltage decay) and optical (time-resolved photoluminescence) techniques. The p+-n diodes are fabricated by Al implantation. Depending on the particular analysis technique, the lifetime is determined either electrically in different regions of the p+-n diode or optically in the n-type 6H-SiC epilayer and results, therefore, in different values ranging from ≈10 ns to 2.5 µs.