Optimization of internal quantum efficiency (IQE) for InGaN quantum wells (QWs) light-emitting diodes (LEDs) is investigated. Staggered InGaN QWs with large electron-hole wavefunction overlap and improved radiative recombination rate are investigated for nitride LEDs application. The effect of interface abruptness in staggered InGaN QWs on radiative recombination rate is studied. Studies show that the less interface abruptness between the InGaN sub-layers will not affect the performance of the staggered InGaN QWs detrimentally. The growths of linearly-shaped staggered InGaN QWs by employing graded growth temperature grading are presented. The effect of current injection efficiency on IQE of InGaN QWs LEDs and other approaches to reduce dislocation in InGaN QWs LEDs are also discussed. The optimization of both radiative efficiency and current injection efficiency in InGaN QWs LEDs are required for achieving high IQE devices emitting in the green spectral regime and longer.

Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

The main challenge for light-emitting diodes is to increase the efficiency in the green part of the spectrum. Gallium phosphide (GaP) with the normal cubic crystal structure has an indirect band gap, which severely limits the green emission efficiency. Band structure calculations have predicted a direct band gap for wurtzite GaP. Here, we report the fabrication of GaP nanowires with pure hexagonal crystal structure and demonstrate the direct nature of the band gap. We observe strong photoluminescence at a wavelength of 594 nm with short lifetime, typical for a direct band gap. Furthermore, by incorporation of aluminum or arsenic in the GaP nanowires, the emitted wavelength is tuned across an important range of the visible light spectrum (555–690 nm). This approach of crystal structure engineering enables new pathways to tailor materials properties enhancing the functionality.

http://pubs.acs.org/doi/pdf/10.1021/nl304723c

Direct band gap Wurtzite gallium phosphide nanowires

This paper reviews the device physics and technology of optoelectronic devices based on semiconductors of the GaN family, operating in the spectral regions from deep UV to Terahertz. Such devices include LEDs, lasers, detectors, electroabsorption modulators and devices based on intersubband transitions in AlGaN quantum wells (QWs). After a brief history of the development of the field, we describe how the unique crystal structure, chemical bonding, and resulting spontaneous and piezoelectric polarizations in heterostructures affect the design, fabrication and performance of devices based on these materials. The heteroepitaxial growth and the formation and role of extended defects are addressed. The role of the chemical bonding in the formation of metallic contacts to this class of materials is also addressed. A detailed discussion is then presented on potential origins of the high performance of blue LEDs and poorer performance of green LEDs (green gap), as well as of the efficiency reduction of both blue and green LEDs at high injection current (efficiency droop). The relatively poor performance of deep-UV LEDs based on AlGaN alloys and methods to address the materials issues responsible are similarly addressed. Other devices whose state-of-the-art performance and materials-related issues are reviewed include violet-blue lasers, 'visible blind' and 'solar blind' detectors based on photoconductive and photovoltaic designs, and electroabsorption modulators based on bulk GaN or GaN/AlGaN QWs. Finally, we describe the basic physics of intersubband transitions in AlGaN QWs, and their applications to near-infrared and terahertz devices.

http://iopscience.iop.org/article/10.1088/1361-6633/aa7bb2/pdf

Optoelectronic device physics and technology of nitride semiconductors from the UV to the terahertz.

A novel approach to enhancing the emission effi ciency of InGaN/GaN multiple quantum wells via coupling to surface plasmons (SPs) in a periodic two-dimensional silver array is demonstrated. A higher internal quantum effi ciency and a higher light extraction effi ciency are simultaneously achieved by engraving an array of nanoholes into the p-GaN cladding layer, followed by partial fi lling with silver. By top excitation and collection from the top of the Ag-incorporated light emitting diodes (LEDs), a 2.8-fold enhancement in peak photoluminescence intensity is demonstrated. The proposed nanoengraving technique offers a practical approach to overcoming the limitation of the exponentially decayed SP fi eld without sacrifi cing the thickness of the p-GaN layer and to controlling the effective coupling energy. The approach is feasible for high-power lighting applications.

Enhancement of Green Emission from InGaN/GaN Multiple Quantum Wells via Coupling to Surface Plasmons in a Two-Dimensional Silver Array

This review addresses the recent development and future prospects for Sc-based III-nitride alloys in energy-efficient device applications. Wurtzite-structure ScAlN and ScGaN alloys are wide band-gap semiconductors which can be stabilised at the low Sc contents relevant to devices, can be grown epitaxially, include the lattice-matched Sc0.18Al0.82N/GaN system, retain direct band gaps in the near-UV region up to 25% ScN and 50% ScN respectively, and should offer significantly higher exciton binding energies and piezoelectric coefficients compared to other III-nitrides. These properties greatly expand the options for band gap and polarisation engineering required for efficient optoelectronic devices, high-electron mobility transistors and energy-harvesting devices.

ScGaN and ScAlN: emerging nitride materials

Green GaInN/GaN quantum well light-emitting diode (LED) wafers were grown on nanopatterned c-plane sapphire substrate by metal-organic vapor phase epitaxy. Without roughening the chip surface, such LEDs show triple the light output over structures on planar sapphire. By quantitative analysis the enhancement was attributed to both, enhanced generation efficiency and extraction. The spectral interference and emission patterns reveal a 58% enhanced light extraction while photoluminescence reveals a doubling of the internal quantum efficiency. The latter was attributed to a 44% lower threading dislocation density as observed in transmission electron microscopy. The partial light output power measured from the sapphire side of the unencapsulated nanopatterned substrate LED die reaches 5.2 mW at 525 nm at 100 mA compared to 1.8 mW in the reference LED.

https://homepages.rpi.edu/~wetzel/Preprints/APL2011_Li_DefectReducedGreenLED_NanopatternedSapphire,Yufeng_Li_Appl.Phys.Lett,98(15),(2011).pdf

Y. Taniguchi

Papers

Defect-reduced green GaInN/GaN light-emitting diode on nanopatterned sapphire