The most successful example of large lattice-mismatched epitaxial growth of semiconductors is
the growth of III-nitrides on sapphire, leading to the award of the Nobel Prize in 2014 and great
success in developing InGaN-based blue emitters. However, the majority of achievements in the
field of III-nitride optoelectronics are mainly limited to polar GaN grown on c-plane (0001)
sapphire. This polar orientation poses a number of fundamental issues, such as reduced quantum
efficiency, efficiency droop, green and yellow gap in wavelength coverage, etc. To date, it is still
a great challenge to develop longer wavelength devices such as green and yellow emitters. One
clear way forward would be to grow III-nitride device structures along a semi-/non-polar
direction, in particular, a semi-polar orientation, which potentially leads to both enhanced indium
incorporation into GaN and reduced quantum confined Stark effects. This review presents recent
progress on developing semi-polar GaN overgrowth technologies on sapphire or Si substrates,
the two kinds of major substrates which are cost-effective and thus industry-compatible, and also
demonstrates the latest achievements on electrically injected InGaN emitters with long emission
wavelengths up to and including amber on overgrown semi-polar GaN. Finally, this review
presents a summary and outlook on further developments for semi-polar GaN based
optoelectronics.

/pdf/topical-review-development-of-overgrown-semi-polar-gan-for-26lrrtws0c.pdf

Topical Review: Development of overgrown semi-polar GaN for high efficiency green/yellow emission

We demonstrate semipolar InGaN single-quantum-well light emitting diodes (LEDs) in the green, yellow-green, yellow and amber spectral region. The LEDs are grown on our overgrown semipolar (11-22) GaN on micro-rod array templates, which are fabricated on (11-22) GaN grown on m-plane sapphire. Electroluminescence measurements on the (11-22) green LED show a reduced blue-shift in the emission wavelength with increasing driving current, compared to a reference commercial c-plane LED. The blue-shifts for the yellow-green and yellow LEDs are also significantly reduced. All these suggest an effective suppression in quantum confined Stark effect in our (11-22) LEDs. On-wafer measurements yield a linear increase in the light output with the current, and external quantum efficiency demonstrates a significant improvement in the efficiency-droop compared to a commercial c-plane LED. Electro-luminescence polarization measurements show a polarization ratio of about 25% in our semipolar LEDs.

(11-22) semipolar InGaN emitters from green to amber on overgrown GaN on micro-rod templates

A significant enhancement in solar hydrogen generation has been achieved using a GaN-based nanorod array structure as a photoelectrode in comparison with a planar one fabricated from the same parent wafer. The nanorod array structure was formed by dry etching using a self-organised nickel nanomask. Under identical illumination conditions in hydrochloric acid solution, the photoelectrode with the nanorod array structure has demonstrated a photocurrent enhancement with a factor of 6 and an enhancement in the rate of hydrogen generation with a factor of 7. The enhancement in solar hydrogen generation is attributed to a massive improvement in light absorption area, reduced travelling distance for the migration of the photogenerated carriers to the semiconductor/electrolyte interface, and surface band bending.

Enhancement in solar hydrogen generation efficiency using a GaN-based nanorod structure

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...

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

Visible light communications (VLC) require III-nitride visible micro-light-emitting diodes (μLEDs) with a high-modulation bandwidth. Such μLEDs need to be driven at a high injection current density on a kA/cm2 scale, which is about 2 orders of magnitude higher than those for normal visible LED operation. μLEDs are traditionally fabricated by dry-etching techniques where dry-etching-induced damages are unavoidable, leading to both a substantial reduction in performance and a great challenge to viability at a high injection current density. Furthermore, conventional biasing (which is simply applied across a p-n junction) is good enough for normal LED operation but generates a great challenge for a single μLED, which needs to be modulated at a high injection current density and at a high frequency. In this work, we have proposed a concept for an epitaxial integration and then demonstrated a completely different method that allows us to achieve an epitaxial integration of a single μLED with a diameter of 20 μm and an AlGaN/GaN high-electron-mobility transistor (HEMT), where the emission from a single μLED is modulated by tuning the gate voltage of its HEMT. Furthermore, such a direct epitaxial approach has entirely eliminated any dry-etching-induced damages. As a result, we have demonstrated an epitaxial integration of monolithic on-chip μLED-HEMT with a record modulation bandwidth of 1.2 GHz on industry-compatible c-plane substrates.

Direct Epitaxial Approach to Achieve a Monolithic On-Chip Integration of a HEMT and a Single Micro-LED with a High-Modulation Bandwidth.

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...

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

(1120) non-polar and (1122) semi-polar GaNs with a low defect density have been achieved by means of an overgrowth on nanorod templates, where a quick coalescence with a thickness even below 1 μm occurs. On-axis and off-axis X-ray rocking curve measurements have shown a massive reduction in the linewidth for our overgrown GaN in comparison with standard GaN films grown on sapphire substrates. Transmission electron microscope observation demonstrates that the overgrowth on the nanorod templates takes advantage of an omni-directional growth around the sidewalls of the nanostructures. The dislocations redirect in basal planes during the overgrowth, leading to their annihilation and termination at voids formed due to a large lateral growth rate. In the non-polar GaN, the priority 〈0001〉 lateral growth from vertical sidewalls of nanorods allows basal plane stacking faults (BSFs) to be blocked in the nanorod gaps; while for semi-polar GaN, the propagation of BSFs starts to be impeded when the growth front is changed to be along inclined 〈0001〉 direction above the nanorods.

Efficient reduction of defects in (1120) non-polar and (1122) semi-polar GaN grown on nanorod templates

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.

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

(11−22) semi-polar GaN overgrown on nanorod templates with different nanorod diameters has been systematically studied in terms of crystal quality, strain relaxation, wafer bowing and electrical properties. With increasing nanorod diameter from 300 to 827 nm, the full width at half maximum (FWHM) of the X-ray rocking curve reduces from 0.20° to 0.15° along (1−100) direction and from 0.15° to 0.10° along (11−2−3) direction, respectively. The strain relaxation of the overgrown GaN layers has been significantly enhanced, leading to a reduction in wafer bowing from 0.0298 to 0.0091 cm−1. The electron mobility in the overgrown GaN layers increases from 83 to 228 cm V−1 s−1 and from 52 to 124 cm V−1 s−1 along (1−100) and (11−2−3) directions, respectively.

Study of high‐quality (11−22) semi‐polar GaN grown on nanorod templates

Non-polar a-plane AlN epitaxial films on r-plane sapphire with greatly reduced defect densities obtained by high-temperature annealing

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