R. M. Smith

Hybrid III-Nitride/Organic Semiconductor Nanostructure with High Efficiency Nonradiative Energy Transfer for White Light Emitters

A novel hybrid inorganic/organic semiconductor nanostructure has been developed, leading to very efficient nonradiative resonant-energy-transfer (RET) between blue emitting InGaN/GaN multiple quantum wells (MQWs) and a yellow light emitting polymer. The utilization of InGaN/GaN nanorod arrays allows for both higher optical performance of InGaN blue emission and a minimized separation between the InGaN/GaN MQWs and the emitting polymer as a color conversion medium. A significant reduction in decay lifetime of the excitons in the InGaN/GaN MQWs of the hybrid structure has been observed as a result of the nonradiative RET from the nitride emitter to the yellow polymer. A detailed calculation has demonstrated that the efficiency of the nonradiative RET is as high as 73%. The hybrid structure exhibits an extremely fast nonradiative RET with a rate of 0.76 ns(-1), approximately three times higher than the InGaN/GaN MQW nonradiative decay rate of 0.26 ns(-1). It means that the RET dominates the nonradiative processes in the nitride quantum well structure, which can further enhance the overall device performance.

Urban domestic gardens (VII): a preliminary survey of soil seed banks

As part of a larger survey of biodiversity in private gardens in Sheffield, UK, we examined the composition and diversity of the soil seed banks in each of 56 gardens. Six soil cores from each garden revealed 2759 seedlings of 119 taxa, although the real species richness is likely to be much higher than this. By far the most abundant species were weedy natives, while the most common alien was Buddleja davidii. Seeds of perennial herbs were more abundant than hundreds of all other life forms combined. More frequent species were also more abundant, but the relationship was weak. Numbers of species in the seed bank and in the garden flora were positively but very weakly related. Seeds were quite evenly distributed between 0‐5 cm and 5‐10 cm soil layers, and most seeds were of species known to have persistent seed banks. Seeds of some species were largely confined to gardens in which the plant was growing, but others were not.

Great emission enhancement and excitonic recombination dynamics of InGaN/GaN nanorod structures

Excitonic recombination dynamics has been investigated on a series of InxGa1−xN/GaN (0.10 ≤ x ≤ 0.30) nanorod (NR) structures with a diameter of ∼220 nm by time-revolved photoluminescence (PL). The NR structures are fabricated by means of a post-growth etching technique. Compared with their corresponding as-grown samples, the time-integrated PL intensities of the NR samples show a remarkable enhancement with a factor of up to 52 at room temperature. The ratios of the radiative to non-radiative recombination lifetime of the NR structures are much less sensitive to temperature than those of their corresponding as-grown samples. This becomes more prominent with increasing indium composition. A distinct delay in transition temperature, where the dominating emission mechanism changes from radiative to non-radiative recombination, has been observed on the NR structures. The great enhancement in optical properties is attributed to both strain relaxation and extra in-plane excitonic confinement due to the nanostr...

Fabrication of two-dimensional InGaN/GaN photonic crystal structure using a modified nanosphere lithography technique

By means of combining a very cost-effective lift-off process and a nanosphere lithography technique, we have fabricated two dimensional (2D) photonic crystal (PhC) structures on an InGaN/GaN multiple quantum well structure. Significant enhancement in photoluminescence (PL) intensity has been observed when the emission wavelength is within the photonic bandgap. Time-resolved PL measurements have shown that the spontaneous emission rate is strongly reduced by a factor of ∼4 due to the PhC effect. As a consequence, the emission intensity along 2D PhC slab-plane directions is effectively suppressed and redistributed to the direction normal to the 2D PhC slab-plane simultaneously. Temperature-dependent PL measurements have confirmed that the enhanced PL intensity is due to an increase in extraction efficiency as a result of the PhC effect.

Room temperature continuous–wave green lasing from an InGaN microdisk on silicon

Optically pumped green lasing with an ultra low threshold has been achieved using an InGaN/GaN based micro-disk with an undercut structure on silicon substrates. The micro-disks with a diameter of around 1 μm were fabricated by means of a combination of a cost-effective silica micro-sphere approach, dry-etching and subsequent chemical etching. The combination of these techniques both minimises the roughness of the sidewalls of the micro-disks and also produces excellent circular geometry. Utilizing this fabrication process, lasing has been achieved at room temperature under optical pumping from a continuous-wave laser diode. The threshold for lasing is as low as 1 kW/cm2. Time–resolved micro photoluminescence (PL) and confocal PL measurements have been performed in order to further confirm the lasing action in whispering gallery modes and also investigate the excitonic recombination dynamics of the lasing.

Temporally and spatially resolved photoluminescence investigation of (112¯2) semi-polar InGaN/GaN multiple quantum wells grown on nanorod templates

By means of time-resolved photoluminescence (PL) and confocal PL measurements, temporally and spatially resolved optical properties have been investigated on a number of InxGa1−xN/GaN multiple-quantum-well (MQW) structures with a wide range of indium content alloys from 13% to 35% on (112¯2) semi-polar GaN with high crystal quality, obtained through overgrowth on nanorod templates. With increasing indium content, the radiative recombination lifetime initially increases as expected, but decreases if the indium content further increases to 35%, corresponding to emission in the green spectral region. The reduced radiative recombination lifetime leads to enhanced optical performance for the high indium content MQWs as a result of strong exciton localization, which is different from the behaviour of c-plane InGaN/GaN MQWs, where quantum confined Stark effect plays a dominating role in emission process.

Stokes shift in semi-polar ( 112¯2) InGaN/GaN multiple quantum wells

The mechanism for the large Stokes Shifts of InGaN/GaN structures is under debate. Here, we report a systematic study on the Stokes shift of semi-polar ( 112¯2) InGaN/GaN multiple quantum wells (MQWs) with a wide spectral range from green (490 nm) to yellow (590 nm) by means of both photoluminescence excitation and time resolved PL measurements in comparison with their c-plane counterparts. The semi-polar samples exhibit a lower Stokes shift than their c-plane counterparts, although they show stronger localization effect than their c-plane counterparts. In the long wavelength region, the Stokes shift of the semi-polar MQWs shows a linear relationship with emission energy, but with a smaller gradient compared with their c-plane counterparts. The time-resolved PL measurements confirm a significant reduction in piezoelectric field of the semi-polar samples compared with the c-plane counterparts. It is suggested that the piezoelectric field induced polarization is the major mechanism for causing the large Sto...

Defect reduction in overgrown semi-polar (11-22) GaN on a regularly arrayed micro-rod array template

We demonstrate a great improvement in the crystal quality of our semi-polar (11-22) GaN overgrown on regularly arrayed micro-rod templates fabricated using a combination of industry-matched photolithography and dry-etching techniques. As a result of our micro-rod configuration specially designed, an intrinsic issue on the anisotropic growth rate which is a great challenge in conventional overgrowth technique for semi-polar GaN has been resolved. Transmission electron microscopy measurements show a different mechanism of defect reduction from conventional overgrowth techniques and also demonstrate major advantages of our approach. The dislocations existing in the GaN micro-rods are effectively blocked by both a SiO2 mask on the top of each GaN micro-rod and lateral growth along the c-direction, where the growth rate along the c-direction is faster than that along any other direction. Basal stacking faults (BSFs) are also effectively impeded, leading to a distribution of BSF-free regions periodically spaced...

Coherent nanocavity structures for enhancement in internal quantum efficiency of III-nitride multiple quantum wells

A “coherent” nanocavity structure has been designed on two-dimensional well-ordered InGaN/GaN nanodisk arrays with an emission wavelength in the green spectral region, leading to a massive enhancement in resonance mode in the green spectra region. By means of a cost-effective nanosphere lithography technique, we have fabricated such a structure on an InGaN/GaN multiple quantum well epiwafer and have observed the “coherent” nanocavity effect, which leads to an enhanced spontaneous emission (SE) rate. The enhanced SE rate has been confirmed by time resolved photoluminescence measurements. Due to the coherent nanocavity effect, we have achieved a massive improvement in internal quantum efficiency with a factor of 88, compared with the as-grown sample, which could be significant to bridge the “green gap” in solid-state lighting.

Microstructure investigation of semi-polar (11-22) GaN overgrown on differently designed micro-rod array templates

In order to realize semi-polar (11-22) GaN based laser diodes grown on sapphire, it is necessary to further improve the crystal quality of the (11-22) GaN obtained by using our overgrowth approach developed on regularly arrayed micro-rod templates [T. Wang, Semicond. Sci. Technol. 31, 093003 (2016)]. This can be achieved by carefully designing micro-rod templates. Based on transmission electron microscopy and photoluminescence measurements, it has been found that the micro-rod diameter plays a vital role in effectively reducing both the dislocation density and the basal staking fault (BSF) density of the overgrown (11-22) GaN, but in different manners. The BSF density reduces monotonically with increasing the micro-rod diameter from 2 to 5 μm, and then starts to be saturated when the micro-rod diameter further increases. In contrast, the dislocation density reduces significantly when the micro-rod diameter increases from 2 to 4 μm, and then starts to increase when the diameter further increases to 5 μm. F...