Yipin Gong

Influence of strain relaxation on the optical properties of InGaN/GaN multiple quantum well nanorods

Optical investigation has been carried out on InGaN/GaN nanorod structures with different indium compositions, fabricated from InGaN/GaN multiple quantum well (MQW) epitaxial wafers using a self-organized nickel nano-mask and subsequent dry etching techniques. In comparison with the as-grown InGaN/GaN MQWs, the internal quantum efficiencies of the nanorods are significantly improved, in particular, for the green InGaN/GaN nanorods with a high indium composition, the internal quantum efficiency is enhanced by a factor of 8, much larger than the enhancement factor of 3.4 for the blue InGaN/GaN nanorods. X-ray reciprocal space mapping (RSM) measurements have been performed in order to quantitatively evaluate the stain relaxation in the nanorods, demonstrating that the majority of strain in InGaN/GaN MQWs can be relaxed as a result of fabrication into nanorods. The excitation-power-dependent photoluminescence measurements have also clearly shown that a significant reduction in the strain-induced quantum confined stark effect has occurred to the nanorod structures.

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...

InGaN/GaN quantum well structures with greatly enhanced performance on a-plane GaN grown using self-organized nano-masks

Great improvement in crystal quality of a-plane (non-polar) GaN has been achieved using a simple but effective overgrowth technique based on self-organized nano-masks. This has been confirmed by a massive reduction in full width at half maximum of x-ray diffraction rocking curves measured along both symmetrical and asymmetrical directions. Taking the advantage of utilising the nano-masks, a quick coalescence with a thickness of less than 1 μm has been obtained, which is much less than that using any conventional overgrowth techniques. The dislocation density has been significantly reduced by more than one order magnitude compared with a standard a-plane GaN layer on sapphire. An InGaN/GaN multiple quantum well (MQW) structure grown on the high quality a-plane GaN has demonstrated an enhancement with a factor of 7 in optical efficiency, compared with a similar MQW structure grown on a standard c-plane GaN layer. The excitation-power dependent photoluminescence measurements have confirmed that the a-plane I...

Stimulated emission at 340 nm from AlGaN multiple quantum well grown using high temperature AlN buffer technologies on sapphire

It is necessary to further improve crystal quality of AlGaN multiple quantum well (MQW) structures on sapphire in order to achieve ultraviolet (UV) laser diodes. Two buffer technologies have been introduced based on our high temperature AlN buffer technology: modified “GaN interlayer” and “multiple porous AlN buffer.” The Al0.16Ga0.84N/Al0.05Ga0.95N MQWs have been grown on top of the two kinds of buffers on sapphire. High resolution x-ray diffraction measurements have confirmed that the crystal quality has been massively improved. As a result, an UV stimulated emission at 340 nm has been observed via optical pumping with a low threshold power of ∼6.6 kW/cm2 at room temperature. The developed approaches potentially provide a simple way for achieving electrical injection UV (including deep UV) laser.

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...

Influence of high temperature AlN buffer on optical gain in AlGaN/AlGaN multiple quantum well structures

Standard stripe-length dependent optical-pumping measurements have been performed on AlGaN/AlGaN multiple quantum wells (MQWs) on an AlN buffer grown using two different kinds of technologies, i.e., “GaN interlayer” and “porous AlN buffer.” The net modal gains of the two samples along both m- and a-axis have been obtained, showing that the net modal gain of the MQWs on the AlN grown using “GaN interlayer” is higher than that on the AlN grown using “porous AlN buffer.” Reciprocal space mapping measurements have indicated that the MQW structure on the AlN structure grown using “GaN interlayer” is fully strained while that on the AlN grown using “porous AlN buffer” is partially strain-relaxed. The net modal gain along the m-axis is higher than that along the a-axis in both samples, highly reasonably indicating that the most favourable orientation for forming the cavity facets is not 〈11-20〉 direction of c-plane sapphire, along which III-nitride on c-plane sapphire is normally cleaved.

Enhanced polarization of (11–22) semi-polar InGaN nanorod array structure

By means of a cost effective nanosphere lithography technique, an InGaN/GaN multiple quantum well structure grown on (11–22) semipolar GaN has been fabricated into two dimensional nanorod arrays which form a photonic crystal (PhC) structure. Such a PhC structure demonstrates not only significantly increased emission intensity, but also an enhanced polarization ratio of the emission. This is due to an effective inhibition of the emission in slab modes and then redistribution to the vertical direction, thus minimizing the light scattering processes that lead to randomizing of the optical polarization. The PhC structure is designed based on a standard finite-difference-time-domain simulation, and then optically confirmed by detailed time-resolved photoluminescence measurements. The results presented pave the way for the fabrication of semipolar InGaN/GaN based emitters with both high efficiency and highly polarized emission.

Improved Crystal Quality of (112̄2) Semi-Polar GaN Grown on A Nanorod Template

(112) semi-polar GaN with significantly improved crystal quality has been achieved by means of overgrowth on a (112) semi-polar nanorod template. The nanorod template was fabricated on a standard (112) semi-polar GaN layer on m-plane sapphire using a self-organized nickel nano-mask technique. In comparison with any conventional overgrowth technique, the nano-mask approach has demonstrated a very quick coalescence with a thickness of less than 1 µm. X-ray rocking curve measurements as a function of azimuth angle has shown a massive reduction in linewidth for our overgrown GaN. Transmission electron microscope measurements have confirmed a significant reduction in the dislocation density. Dramatic improvement in optical properties has been exhibited by photoluminescence (PL) measurements performed at room temperature, showing that the PL intensity from the band edge emission is 30 times higher than that of the standard semi-polar GaN on sapphire.

Spatially-resolved optical and structural properties of semi-polar (11-22) AlxGa1-xN with x up to 0.56

Pushing the emission wavelength of efficient ultraviolet (UV) emitters further into the deep-UV requires material with high crystal quality, while also reducing the detrimental effects of built-in electric fields. Crack-free semi-polar