Study of the morphology evolution of AlN grown on nanopatterned sapphire substrate

This study focused on the evolution of growth front about AlN growth on nano-patterned sapphire substrate by metal-organic chemical vapor deposition.The substrate with concave cones was fabricated by nano-imprint lithography and wet etching.Two samples with different epitaxy procedures were fabricated,manifesting as two-dimensional growth mode and three-dimensional growth mode,respectively.The results showed that growth temperature deeply influenced the growth modes and thus played a critical role in the coalescence of AlN.At a relatively high temperature,the AlN epilayer was progressively coalescence and the growth mode was two-dimensional.In this case,we found that the inclined semi-polar facets arising in the process of coalescence were{112^-1}type.But when decreasing the temperature,the{112^-2}semi-polar facets arose,leading to inverse pyramid morphology and obtaining the three-dimensional growth mode.The 3 D inverse pyramid AlN structure could be used for realizing 3 D semi-polar UV-LED or facet-controlled epitaxial lateral overgrowth of AlN.

Improving the incorporation of indium component for InGaN-based green LED through inserting photonic crystalline in the GaN layer

We report on the effect of inserted photonic crystalline(Ph-C) in the GaN epitaxial layer on the incorporation of the indium component for the InGaN-based green LED. The adoption of Ph-C in the GaN layer shifted the Raman peak value of E2mode of GaN to lower frequency and resulted in a tensive stress relief. The stress relief can be attributed to strained lattices restoring in the matrix of Ph-C and the GaN pseudo-epitaxy over the air-void of the Ph-C. Moreover, the HRXRD rocking curves and AFM results show that the insertion of Ph-C also improves the crystal quality. With the inserted Ph-C, the indium component in the multiple quantum wells of the green LED(Ph-C LED) was enhanced. This resulted in a 6-nm red-shift of the peak wavelength. Furthermore, the LOP of the Ph-C LED was enhanced by 10.65% under an injection current of 20 mA.

Deep ultraviolet light-emitting diodes based on a well-ordered AlGaN nanorod array

The nanorod structure is an alternative scheme to develop high-efficiency deep ultraviolet light-emitting diodes(DUV LEDs). In this paper, we first report the electrically injected 274-nm AlGaN nanorod array DUV LEDs fabricated by the nanosphere lithography and dry-etching technique. Nanorod DUV LED devices with good electrical properties are successfully realized. Compared to planar DUV LEDs, nanorod DUV LEDs present>2.5 times improvement in light output power and external quantum efficiency. The internal quantum efficiency of nanorod LEDs increases by 1.2 times due to the transformation of carriers from the exciton to the free electron–hole, possibly driven by the interface state effect of the nanorod sidewall surface. In addition, the nanorod array significantly facilitates photons escaping from the interior of LEDs along the vertical direction, contributing to improved light extraction efficiency. A three-dimensional finite-different time-domain simulation is performed to analyze further in detail the TE-and TM-polarized photon extraction mechanisms of the nanostructure. Our results demonstrate the nanorod structure is a good candidate for high-efficiency DUV emitters.

Balancing charge injection in quantum dot light-emitting diodes to achieve high efficienciy of over 21%

Quantum dot light-emitting diodes(QLEDs)have attracted considerable attention in displays owing to their high color purity,wide gamut,narrow emission band,and solution-processed characteristics.However,a major problem of the unbalanced carrier(electrons and holes)injection in QLEDs deteriorates their performance.Here,we balanced the charge injection in QLEDs by optimizing the carrier transport layers.Different organic hole transport layers(HTLs)with a suitable thickness were employed to match the electron transport layer(ETL)of ZnO.Mg^(2+) was doped into the ZnO(MZO)ETL to decrease the electron mobility and match the hole mobility of the HTL.Consequently,the QLEDs exhibited an excellent external quantum efficiency(EQE)of 21.10%at a luminance of 4661 cd m^(-2).In the luminance range of 100–30,000 cd m^(-2),EQE roll-off was considerably low,and more than 80%of the initial EQE value could be maintained,indicating less Auger recombination because of the balanced carrier injection.This work reveals that compared with energy level matching,the charge transfer capability of the transport layers is more instrumental in the charge balance regulation of QLED devices.