Yueh-Chien Lee

Two distinct carrier localization in green light-emitting diodes with InGaN/GaN multiple quantum wells

The effect of carrier localization in InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes is investigated by photoluminescence (PL) and time-resolved PL (TRPL) measurements. PL results show that two peaks obtained by Gaussian fitting both relate to the emission from localized states. By fitting the TRPL lifetimes at various emission energies, two localization depths corresponding to the In-rich regions and quasi-MQWs regions are obtained. Using a model we proposed, we suggest that compositional fluctuations of In content and variation of well width are responsible for carrier localization in In-rich regions and quasi-MQWs regions, respectively.

Investigation of the light emission properties and carrier dynamics in dual-wavelength InGaN/GaN multiple-quantum well light emitting diodes

Three dual-wavelength InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) with increasing indium content are grown by metal-organic chemical vapor deposition, which contain six periods of low-In-content MQWs and two periods of high-In-content MQWs. For the low-In-content MQWs of three studied samples, their internal quantum efficiency (IQE) shows a rising trend as the emission wavelength increases from 406 nm to 430 nm due to the suppression of carriers escape from the wells to the barriers. However, for the high-In-content MQWs, the sample IQE falls rapidly with a further increase of emission wavelength from 496 nm to 575 nm. Theoretical calculation reveals that the electron-hole wave function overlap in the high-In-content MQWs is reduced because of an increase in the internal polarization field as indium content is increased. In addition, time-resolved photoluminescence decay curves show that the carriers generated in the low-In-content MQWs can be effectively transferred to the high-In-...

Observation of persistent photoconductivity in 2H‐MoSe2 layered semiconductors

We report the observation of persistent photoconductivity (PPC) effect in 2H‐MoSe2 layered semiconductors. The decay behavior of PPC can be well described by a stretch-exponential function. Experimental results indicate that the lattice relaxation of DX-like impurity is responsible for PPC in MoSe2. In addition, the small capture barrier created by lattice relaxation and the temperature-dependent resistance measurement exhibit that the magnitude of PPC effect in MoSe2 is small, consistent with the PPC behavior in indirect gap materials.

Raman spectra investigation of InAlGaN quaternary alloys grown by metalorganic chemical vapor deposition

Raman analysis of the A1 (LO) mode of AlGaN-like phonons of the InAlGaN/GaN heterostructures in the composition range 1.38% ≦ In ≦ 2.73% and 8.01% ≦ Al ≦ 13.97% is presented. The line shape of A1 (LO) mode of AlGaN-like phonons was observed to exhibit a significant asymmetry and Raman linewidth toward the lower energy side. The spatial correlation model is discussed and is shown to account the line shape. The spatial correlation model calculations also indicate the lack of a long-range order in the higher Al to In ratio of InAlGaN/GaN alloys. These results were confirmed by x-ray diffraction and the correlation length L decreases as the increasing of Al to In ratio corresponding to the absence of the long-range order in the alloy. The Raman linewidth of the AlGaN-like A1 (LO) mode was found to exhibit a maximum at the higher Al to In ratio indicative of a random disordered alloy system.

Temperature-Dependent Excitonic Luminescence in ZnO Thin Film Grown by Metal Organic Chemical Vapor Deposition

In this study, the excitonic luminescence behaviors of ZnO thin films in the temperature range of 10–300 K were investigated. The photoluminescence (PL) spectrum exhibits the bound-exciton emission and the donor-acceptor recombination accompanying its multiphonon replicas at low temperatures. The observed features exhibit redshift with an increase in temperature, and the temperature dependence of the peak position was analyzed by the Varshni empirical expression. The study showed the dominant presence of bound excitonic transition below 160 K or free excitonic transition at higher temperatures for the observed PL spectrum. The free exciton emission can be observed up to room temperature. The Debye temperature of ZnO was evaluated by taking into account the elastic constants of ZnO and utilized as a parameter in the Varshni empirical expression giving an accurate description of the free exciton emission behavior up to room temperature.

L-band automatic-gain-controlled erbium-doped fiber amplifier utilizing C-band backward-amplified spontaneous emission and electrical feedback monitor

A new L-band automatic-gain-controlled (AGC) erbium-doped fiber amplifier (EDFA) for dense wavelength-division-multiplexing transmission systems is presented, in which a single 1480 nm laser with an internal thermoelectric cooler is used as a primary pump for stable amplification. All C-band amplified spontaneous emission (ASE) is recycled by the secondary pump to enhance the gain efficiency. A fraction of the output signal is used as an electrical feedback monitor for the AGC to improve the gain-clamped (GC) flatness. Experimental results prove that the AGC EDFA has a gain flatness of better than 0.46 dB/40 nm, i.e., below 1.5%, and a higher gain of approximately 36.5 dB compared to that of approximately 35.3 dB for the conventional GC EDFA at -30 dBm input signal power. The best gain flatness of +/-0.25 dB can be achieved over the dynamic range greater than 20 dB. The dynamic range of noise figure is between 6.7 and 7.1. The 3 dB down bandwidth is more than 40 nm. Overall dynamics measurements for the AGC EDFA feedback stabilization have been carried out. The recorded corresponding rise time of 1.565 ms indicates that the system does not exhibit any overshoot of gain or ASE variation due to the signal at the beginning of the pulse.

Optical studies in distributed Bragg reflectors built from ZnO/MgO multilayer films

We present the distributed Bragg reflectors (DBRs) with 10- and 15-period zinc oxide (ZnO)/MgO multilayer films deposited on silicon by sputtering technology. The reflectivity for the 10-period ZnO/MgO stacks can reach to 91.4% and for the 15-period ZnO/MgO stacks can be increased to 98.7%. Furthermore, the transfer matrix method takes account of the Sellmeier equation and the random thickness model, plus it can well describe the measured reflectivity spectra. The investigation indicates that a refined control of the individual layer thickness and the number of layer periods are significant subjects to improve the DBRs performance.

Optical, structural, and nuclear scientific studies of AlGaN with high Al composition

AlGaN epilayers with higher Al-compositions were grown by Metalorganic Chemical Vapor Deposition (MOCVD) on (0001) sapphire. Trimethylgallium (TMGa), trimethylaluminium (TMAl) and NH3 were used as the source precursors for Ga, Al, and N, respectively. A 25 nm AlN nucleation layer was first grown at low-temperature of 590 °C at 300 Torr. Followed, AlxGa1-xN layers were grown at 1080 °C on low-temperature AlN nucleation layers. The heterostructures were characterized by a series of techniques, including x-ray diffraction (XRD), Rutherford backscattering (RBS), photoluminescence (PL), scanning electron microscopy (SEM) and Raman scattering. Precise Al compositions were determined through XRD, RBS, and SEM combined measurements. Room Temperature Raman Scattering spectra shows three major bands from AlGaN alloys, which are AlN-like, A1 longitudinal optical (LO) phonon modes, and E2 transverse optical (TO) band, respectively, plus several peak comes from the substrate. Raman spectral line shape analysis lead to an optical determination of the electrical property free carrier concentration of AlGaN. The optical properties of AlGaN with high Al composition were presented here.

Carrier dynamics in ZnxCd1-xO films grown by molecular beam epitaxy

In this work, the carrier dynamics in Zn1-xCdxO thin films with different Cd contents grown by molecular beam epitaxy system have been investigated using photoluminescence and time-resolved photoluminescence measurements. The carrier lifetime can be estimated from the PL decay curve fitted by triple exponential function. The emission energy dependence and temperature dependence of the PL decay time indicate that carrier localization dominate the luminescence mechanism of the ZnCdO alloy semiconductor.

Influence of Rapid Thermal Annealing on Raman Scattering of InN Epilayers

We studied the Raman scattering of the InN epilayers with rapid thermal annealing (RTA). The longitudianl optical (LO) phonon in Raman spectrum shifts toward lower frequency and increases asymmetric broadening as the RTA temperature is increased. We suggest that the formation of indium-related defects, such as metallic indium clusters or indium vacancies, are responsible for the change in the asymmetric ratio in the LO mode. The E2 (high) mode in the Raman spectrum does not exhibit significant change after RTA since the indium atom does not involve the E2 (high) mode.