Ziwu Ji

Influence of excitation power and temperature on photoluminescence in InGaN/GaN multiple quantum wells.

Excitation power and temperature dependences of the photoluminescence (PL) spectra are studied in InGaN/GaN multiple quantum wells (MQWs). The excitation power dependences of the PL peak energy and linewidth indicate that the emission process of the MQWs is dominated first by the Coulomb screening effect and then by the localized states filling at low temperature, and that the nonradiative centers are thermally activated in low excitation range at room temperature. The anomalous temperature dependences of the peak energy and linewidth are well explained by the localized carrier hopping and thermalization process, and by the exponentially increased density of states with energy in the band tail. Moreover, it is also found that internal quantum efficiency is related to the mechanism conversion from nonradiative to radiative mechanism, and up to the carriers escaping from localized states.

Transfer and recombination mechanism of carriers in phase-separated InGaN quantum wells

Photoluminescence (PL) properties of InGaN/GaN multiple quantum wells are studied. Two InGaN-related peaks are observed in the full PL spectrum and are assigned to the quasi-quantum dots (QDs) (2.42 eV) and the InGaN matrix (2.66 eV), due to a strong phase separation. As the carriers transfer from the matrix down to the QDs, an increase of the QDs-related PL intensity (ID) accompanied by the decrease of the matrix-related PL intensity (IM) results. A slight increase of the total PL intensity is also observed, and is attributed to the QDs providing deep potential levels to suppress the outflow of carriers toward surrounding nonradiative centers. A piezoelectric field resulting from the high indium content inside the QDs is observed, which is speculated from Coulomb screening effect. Additionally, we find that the sublinear dependence of the ID on excitation power (P) is due to the saturation of the QDs states, while the superlinear dependence of the IM on P is simultaneously attributed to the suppression o...

Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes

A hybrid ZnO micro-mesh and nanorod arrays (MMNR) was fabricated as a light output window for GaN-based light-emitting diodes (LEDs) to enhance the light extraction efficiency. The light output power of GaN-based LEDs with the ZnO MMNR is improved by 95% compared to the original planar LEDs. The ZnO MMNR is manufactured by photolithography techniques and a two-step wet chemical growth process. The incident angle-resolved light transmission of the ZnO MMNR beyond the critical angle of total internal reflection is greatly enhanced. The light diffraction pattern of the ZnO MMNR shows that it possesses both the two-dimensional diffraction grating effect of a ZnO micro-mesh and the light scattering effect of a ZnO nanorod array. LEDs with the ZnO MMNR have greater light extraction efficiency than those with only a ZnO micro-mesh or a ZnO nanorod array. The local optical field patterns of the ZnO micro-mesh and the ZnO MMNR are investigated using confocal scanning electroluminescence microscopy. The microscopic light extraction mechanism of the ZnO MMNR is analyzed in-depth.

Green and blue emissions in phase-separated InGaN quantum wells

We have investigated temperature-dependent photoluminescence (PL) of green and blue light-emitting InGaN/GaN multiple quantum wells at different excitation powers. Two InGaN-related PL peaks centered at around 2.4 and 2.7 eV are assigned to quasi-quantum dot (QD) emissions (PD) and the InGaN matrix emission (PM), respectively, due to a strong phase separation confirmed by high-resolution transmission electron microscopy. In contrast to the S-shaped temperature-dependent behavior of the PM peak energy, the PD peak energy initially decreased and then increased with increasing temperature up to 300 K, indicating that the carriers within QDs relax to stronger localized states first and then are thermalized to higher levels with increasing temperature. Interestingly, it was found that with increasing temperature both the emission intensities initially increased and then decreased. This behavior was attributed to an increased carrier localization effect and then enhanced non-radiative recombination with increas...

“W-shaped” injection current dependence of electroluminescence linewidth in green InGaN/GaN-based LED grown on silicon substrate

Injection current, and temperature, dependences of the electroluminescence (EL) spectrum from green InGaN/GaN multiple quantum well (MQW)-based light-emitting diodes (LED) grown on a Si substrate, are investigated over a wide range of injection currents (0.5 µA-350 mA) and temperatures (6-350 K). The results show that an increasing temperature can result in the change of injection current-dependent behavior of the EL spectrum in initial current range. That is, with increasing the injection current in the low current range, the emission process of the MQWs is dominated by filling effect of low-energetic localized states at the low temperature range of around 6 K, and by Coulomb screening of the quantum confinement Stark effect followed by a filling effect of the higher levels of the low-energetic localized states at the intermediate temperature range of around 160 K. However, when the temperature is further raised to the higher temperature range of around 350 K, the emission process of the MQWs in the low current range is dominated by carrier-scattering effect followed by non-radiative recombination process. The aforementioned current-dependent behaviors of the EL spectrum are mainly attributed to the strong localized effect of the green LED, as confirmed by the anomalous temperature dependence of the EL spectrum measured at the low injection current of 5 µA. In addition, the injection current dependence of external quantum efficiency at different temperatures shows that, with increasing temperature from 6 to 350 K, in addition to the enhanced non-radiative recombination, electron overflow becomes more significant, especially in the higher temperature range above 300 K.

Electroluminescence properties of InGaN/GaN multiple quantum well-based LEDs with different indium contents and different well widths

Two InGaN/GaN multiple quantum well (MQW)-based blue light emitting diodes (LEDs) emitting photons at approximately the same wavelength, with different indium contents and well widths, are prepared, and the temperature-dependences of their electroluminescence (EL) spectra at different fixed injection currents are investigated. The results show that, compared with sample B with its lower indium content and larger well width, sample A with its higher indium content and smaller well width, has a stronger carrier localization effect and higher external quantum efficiency (EQE) at the lower fixed currents; however, upon increasing the injection current, both the localization effect and EQE for sample A decrease at a faster rate. The former is mainly attributed to the deeper potential levels due to the larger indium fluctuations originating from the higher indium content, and to the smaller well width-induced stronger carrier quantum-confine effect (QCE); the latter is mainly attributed to the more significant growing in the electron leakage and/or electron overflow originating from the smaller well width and larger lattice mismatch-induced stronger piezoelectric field, and to the more significant reduction in carrier localization effect originating from the smaller well width-induced smaller density of high-energy localized states.

Fabrication of p-ZnO:Na/n-ZnO:Na homojunction by surface pulsed laser irradiation

An ingenious method of preparation of ZnO homojunctions for on-chip integration purposes is proposed, by local multiple pulse laser irradiating (MPLI) ZnO:Na film (NZO). The importance of this method lies in realization of p- and n-ZnO using only one kind of dopant (Na element) with a single layer of NZO film. The p-NZO as prepared by pulsed laser deposition (PLD) easily changes to n type after a couple of hours. However, more than ∼150 times MPLI with laser fluence of 60 mJ cm−2 can be used to efficiently control the Na dopants to occupy the substitutional (NaZn) sites to realize a more stable p-NZO. The low temperature (6 K) PL spectra of the p-NZO at excitation power of 6 mW and the first principle calculation show that the acceptor energy level is ∼161 meV. The current–voltage (I–V) curve of the p–n NZO homojunction fabricated by local MPLI shows good rectifying behavior, with turn on voltage at ∼2.47 V under forward bias voltage and reverse breakdown voltage bias ∼11.22 V. This convenient p–n junction technique could be used to simplify and improve controllability and accuracy of fabrication of microchip electronic devices.

Structural properties of Alq3 nanocrystals prepared by physical vapor deposition and facile solution method

Tris(8-hydroxyquinoline) aluminum (Alq3) nanostructures are promising materials for nanooptoelectronic devices and molecular spintronics. In this paper, we report Alq3 nanocrystals prepared by both physical vapor deposition (PVD) and facile solution method. The transmission electron microscopy (TEM) and high resolution scanning electron microscope (SEM) measurements show that the Alq3 nanomaterials prepared by PVD technique are e-Alq3 nanoflowers, while the Alq3 nanostructures prepared by solution method are α-Alq3 nanorods. Our experiments indicate that the α-Alq3 nanomaterials prepared by using solution method are more suitable for the fabrication of molecular spintronic devices than that of PVD method.

Type-I interband transition in undoped ZnSe/BeTe type-II quantum wells under high excitation density

A spatially direct photoluminescence (PL) spectrum associated with type-I interband transition of a ZnSe layer in undoped ZnSe/BeTe type-II quantum structures was investigated by varying the photo-excitation density. For a sample with a narrower ZnSe layer both PL intensity and linewidth of the trion show a superlinear increase with increasing excitation density in comparison to that with a wider ZnSe layer. The results are explained by the effective increase of the electron concentration and an enhanced dephasing rate of the trion resulting from the electron–trion scattering. The effective increase of the electron concentration in the ZnSe layer is considered to be originating from both the greater transfer rate of the hole into the BeTe layer due to the narrower ZnSe layer and the efficient spatially indirect transition PL of the complex states through the interface. With decreasing excitation density, no indication of any shift in peak energy density was observed indicating that the studied structures are of excellent quality.

EFFECT OF THE VOIGT MAGNETIC FIELD ON THE EXCITON COMPLEXES IN ZnSe/BeTe TYPE-II SINGLE QUANTUM WELLS

We have performed the magneto-photoluminescence (PL) measurements on ZnSe/BeTe quantum well (QW) structures with a type-II band alignment in a Voigt configuration. The PL spectra were composed of two components. The lower energy PL peak energy shows red-shifts applying magnetic field. A model of the energy dispersion shift of the spatially indirect exciton complexes probably explains these results.