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We report on the dual-character of an InGaN/GaN multiple quantum well (MQW) device structure epitaxially grown by MOCVD on a 150 mm Si-on-insulator (SOI) wafer. The fabricated mesa device exhibits photovoltaic (PV) response mainly for λ < 500 nm and strong electroluminescence (EL) for 470 nm < λ < 750 nm with a major amber component about 580 nm < λ < 620 nm. Multiple, high amplitude oscillations are observed in the luminescence spectra and verified by theoretical modelling as strong Fabry-Perot interference effects. HR-XRD ω -2θ (0002) scan estimates a mean In composition of ∼22% in the MQWs, consistent with the PV absorption edge and weak photoluminescence (PL) emission when excited by a 518 nm laser at 20K. Under 405 nm laser excitation at low temperatures, PL emission for 600 nm < λ < 740 nm is greatly enhanced. Asymmetric broadening of the spectral peak to 570 nm < λ < 740 nm occurs when the excitation energy is increased with a 325 nm laser, and is attributed to radiative transitions associated with MQW interface defects. This is supported by the (105) reciprocal space map which shows slight relaxation of the MQWs. The strong but broad long-wavelength emission, is ascribed to the MQW interface defects and highly recombinative In-rich clusters. These hardly contribute to PV response and are responsible for the protracted Stokes shift that constitutes the apparent near-mutual exclusion of EL and PV absorption wavelengths. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth based perovskite NdGaO3 (NGO) and the systematic quench of the NGO photoluminescence (PL) by Ce doping. Study of room temperature PL was performed in both single-crystal and polycrystalline NGO (substrates and pellets) respectively. Several NGO pellets were prepared with varying Ce concentration and their room temperature PL was studied using 325 nm laser. It was found that the PL intensity shows a systematic quench with increasing Ce concentration. XPS measurements indicated that nearly 50% of Ce atoms are in the 4+ state. The PL quench was attributed to the novel concept of super hydrogenic dopant (SHD)”, where each Ce4+ ion contributes an electron which forms a super hydrogenic atom with an enhanced Bohr radius, due to the large dielectric constant of the host. Based on the critical Ce concentration for complete quenching this SHD radius was estimated to be within a range of 0.85 nm and 1.15 nm whereas the predicted theoretical value of SHD radius for NdGaO3 is ~1.01 nm.

In this paper, we identified how the growth environment chemistry can critically influence the type and nature of the incorporated K defect in ZnO films grown using the aqueous solution route, which explains the switching between p- and n-type conductivities under different doping or thermal annealing conditions. This was achieved by relating the growth environment to the structural, optical, and electrical characteristics of the films. The thermal behavior of these defects up to 700 °C confirms the proposed doping mechanism. It is found that the best route to realizing p-type conductivity is through minimizing the amount of Ki and KZn–Ki complexes because films with high concentrations of Ki have a slow p-type recovery caused by the slow out-diffusion of Ki. The highest hole concentrations for as-grown films and those that were annealed at 700 °C for 30 min were 2.6 × 1016 and 3.2 × 1017 cm–3, respectively. The upper limit for p-type doping using this route appears to be about mid-1017 cm–3. Our results ...

Zinc oxide (ZnO) is a wide band-gap material with excellent optical properties for optoelectronics applications. However, device fabrication has been hampered by difficulties in obtaining a stable p-type doping. Here, we present the first report on the growth and doping of ZnO film through the incorporation of potassium (K) from group I in aqueous solution at 90 °C to yield a stable p-type doping. The contribution of potassium toward p-type conductivity is confirmed using Hall effect measurements and SIMS. A new growth strategy was introduced to obtain a good film coverage with a lower native defect density without the use of surfactants. Photoluminescence measurements confirmed the reduction of defect-related emissions and enhancement of UV band-edge emissions. Variation of carrier concentrations with temperature points to the presence of unstable hydrogen donors that can be removed by annealing at temperatures above 400 °C for extended durations. The instability of these hydrogen defects is attributed t...

In this study, we report on the enhancement in the light extraction efficiency of GaN blue LEDs topped with ZnO nanorods. The ZnO nanorods were grown by a two-step hydrothermal synthesis with pre-coated ZnO nanoparticles under optimized condition to give the appropriate size and quality, giving an increase in the light output efficiency of 66%. This improvement is attributed to the optimal rod size and spacing with improved thermal dissipation as compared to light extraction from plain GaN surface. During the ZnO growth on the LEDs, 0.55 M of NH 3 was added and the ZnO sample was later annealed at 475 °C in N 2 ambient, to drive out interstitial oxygen atoms from the tetrahedral unstable site. As a result, a high ratio of UV to orange defect band emission was achieved. The two-step growth of ZnO nanorods on GaN LEDs was effective in generating array of ZnO nanorods which serve as reflector to enhance light extraction from LEDs.

ZnO nanorods have been hydrothermally grown on substrates pre-coated with ZnO nanoparticles. This work presents the detailed chemistry to control the growth morphology and defects incorporation during the hydrothermal synthesis based on fine tuning the interfacial energy, the zinc solubility co and surface charges. When co 1.56 mmol/l, large clustered rods are obtained. Finally, when 0.88

Vertically aligned ZnO nanorods were grown at 90 °C by both microwave synthesis and traditional heated water bath method on Si (100) substrate with a precoated ZnO nanoparticle seed layer. A detailed comparison in the morphology, defects, and optical properties of the ZnO nanorods grown by the two methods across the pH range of 10.07-10.9 for microwave synthesis and conventional heated water bath method was performed using scanning electron microscopy, photoluminescence, and X-ray photoelectron spectroscopy. The results show that the microwave route leads to more uniformly distributed nanorods with a lower density of native defects of oxygen interstitials and zinc vacancies. The microwave synthesis presents a promising new approach of fabricating metal oxide nanostructures and devices toward green applications.

We show that the rod-like morphology of hydrothermally-grown ZnO nanorods can be obtained on any substrate, pre-coated with ZnO nanoparticles, when pH10.1. When pH>10.1, three types of morphology, from uniform nanorods to large clustered rods, can be distinguished based on the solubility of zinc. In line with classical thermodynamics, the density of nanorods increases when the solubility of zinc decreases. We also report that the orange defect peak photoluminescence can be reduced, while the UV peak increased by decreasing the pH of the growth solution.

The fabrication of a p-shell/n-core coaxial nanorod ZnO homojunction light-emitting diode by inexpensive solution method is demonstrated. The p-type conductivity of the ZnO shell arises from the incorporation of potassium while the n-type conductivity of the core is due to unintentional doping.

ZnO nanorods are grown by hydrothermal synthesis on a GaN substrate. The nanorods are charcterised by X-ray diffraction, photoluminescence and electron diffraction. A phenomenological model is proposed to predict the areal density and rod length.

We demonstrated a novel and widely accessible method for determining the electron effective mass and scattering time of ZnO films with different carrier concentrations by combining terahertz time-domain spectroscopy with Hall measurement. The terahertz time domain spectroscopy (THz-TDS) transmission spectra (0.1–2THz) were well described by Drude model. It is found that electron effective mass varied from 0.23m0 to 0.26m0 as the electron concentration changes from 5.9 × 1017 cm−3 to 4.0 × 1019 cm−3. The carrier concentration dependent characteristic is ascribed to the non-parabolicity of conduction band. Free carrier localization mechanism explained the discrepancy in mobilities obtained from THz-TDS and Hall measurements.

GaN grown on sapphire is electrochemically etched in HF and in KOH. Etching in HF results in a network of nanopillars while that etched in KOH results in a network of pores. The higher density of voids from the network of pores shows the highest strain relaxation for a 1.2 µm thick GaN overgrown on the porous templates. In general, a light-emitting diode (LED) on the porous templates gives about 1.5 times higher intensity and a spectral envelop shift towards the red due to a higher In incorporation. The higher intensity is attributed to enhanced light extraction due to light scattering at the voids formed from the pores and improved material quality with dislocation reduction. The formation of larger overgrowth GaN islands which merges to give a continuous GaN film over the porous template reduced the dislocation density and also accounted for higher strain relaxation for the growth of the quantum dots (QDs) and quantum well layers. This reduced the extent of peak shift of LEDs grown on porous GaN template and improved its performance.

Bright and stable structured green luminescence (GL) is achieved from solution-grown Cu-doped ZnO nanorods. Dependence of photoluminescence on the annealing parameters reveals that GL is correlated with creation of Zn vacancies (VZn) and then formation of Cu dopants at Zn sites (CuZn). High internal quantum efficiency (43%) of the GL can be sustained up to 240 K due to negative thermal quenching. In contrast to the poor stability of defects-related visible emission, the structured GL shows good stability with respect to sample heating. Cu-doped ZnO nanorods with strong and stable GL have potential applications in visible light display and lighting.

Phosphor-free amber light emitting quantum dots with bimodal size distribution have been effectively incorporated in InGaN/GaN light emitting diodes (LEDs). With overgrowth of the LEDs structure on electrochemically etched nanoporous GaN templates, a reduction in density of threading dislocations and lower biaxial stress are achieved. A higher density of smaller quantum dots ∼4.5×109 cm−2 is incorporated in the multiple quantum well of LEDs on the nanoporous GaN template, generating higher energy emission, and enhanced light output by 1.45 times over LEDs grown on conventional GaN. The carrier capture kinetic by the bimodal distributed quantum dots is discussed with its energy band profile.

Poor near-band-edge emission (NBE) prohibits the application of solution-grown ZnO nanorods in optoelectronics, thus their photoluminescence (PL) was studied with respect to post-annealing temperature and duration. A universal behavior was revealed: NBE was enhanced by one order (or two) of magnitude after annealing in air (or H2) at about 425 °C for 30 min, while the enhancement factor starts to decrease after annealing at higher temperatures. The evolution of PL was mainly ascribed to annealing-induced activation and dissociation of hydrogen donor, which was identified as HO by both PL and Raman analyses. Results from nanorods with different diameters and annealing gases further support this assignment. The results provide new insights to understand and optimize the properties of solution-grown ZnO.

We present an empirical model based on the solubility of zinc and temperature to explain the observed variations in density and length of hydrothermally grown zinc oxide (ZnO) nanorods on GaN substrate as reported by Le et al. [J. Cryst. Growth, 293, 36 (2006)]. The ionic equilibrium of the growth solution is solved numerically. By using the experimental data and calculated equilibrium concentrations of species in the solution, expressions were formulated to represent the dependence of the ZnO nanorod density and length on the solubility of zinc and temperature. Activation energies of 0.77 and -2.11 eV were found for the length and density, respectively. The proposed model can be used to predict the ZnO nanorod density and length on GaN substrate within the growth temperature range of 333 to 423 K, precursor solution pH between 9.7 and 10.6, and a degree of saturation 20 < S < 60.