Ultraviolet communication technique and its application

With recent developments of deep ultraviolet(DUV)light-emitting diodes and solar-blind detectors,UV communication(UVC)shows great potential in replacing traditional wireless communication in more and more scenarios.Based on the atmospheric scattering of UV radiation,UVC has gained considerable attention due to its non-line-of-sight ability,omnidirectional communication links and low background noise.These advantages make UVC an ideal option for covert secure communication,especially for military communication.In this review,we present the history and working principle of UVC with a special focus on its light sources and detectors.Comprehensive comparison and application of its light sources and detectors are provided to the best of our knowledge.We further discuss the future application and outlook of UVC.Hopefully,this review will offer valuable insights into the future development of UVC.

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.

Comprehensive study of crystalline AlN/sapphire templates after high-temperature annealing with various sputtering conditions

High-quality AlN/sapphire templates were fabricated by the combination of sputtering and high-temperature(HT)annealing.The influence of sputtering parameters including nitrogen flux,radio frequency power,and substrate temperature on the crystalline quality and surface morphology of annealed AlN films were investigated.With lower substrate temperature,lower power,and lower N2 flux,the full width at half maximum of the X-ray rocking curve for AlN(0002)and(102)were improved to 97.2 and 259.2 arcsec after high-temperature annealing.This happens because the increased vacancy concentration of sputtered AlN films can facilitate the annihilation of dislocations by increasing the recovery rate during HT annealing.Step and step-bunching morphologies were clearly observed with optimized sputtering conditions.

Influence of light absorption on the metallic nanotextured reflectors of GaN-based light emitting diodes

Metallic nanotextured reflectors have been widely used in light emitting diodes(LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structure is often neglected. Here, the influence of absorption loss of metallic nanotextured reflectors on the LED optoelectronic properties were studied. Two commonly used metal reflectors Ag and Al were applied to green GaN-based LEDs. By applying a Ag nanotextured reflector, the light output power of the LEDs was enhanced by 78% due to the improved light extraction. For an Al nanotextured reflector, however,only a 6% enhancement of the light output power was achieved. By analyzing the metal absorption using finite-difference timedomain(FDTD) and the metal reflectivity spectrum, it is shown that the surface plasmon(SP) intrinsic absorption of metallic reflectors with nanotexture structure play an important role. This finding will aid the design of the high-performance metal nanotextured reflectors and optoelectronics devices.

Mitigating fast thermal instability by engineered laser sweep in AlN soliton microcomb generation

Transient thermal instability represents a significant challenge in generating soliton microcombs.Fast laser sweep can be an efficient method to mitigate thermal instability,but it requires an ultrahigh laser sweep rate for crystalline microresonators with fast thermal relaxation.Here,we engineer a laser sweep waveform to generate AlNon-sapphire soliton microcombs with an intermediate sweep speed(<30 GHz∕μs).Two laser sweep methods with backward plus forward tuning or two-step backward tuning added after the fast forward laser sweep were demonstrated to stabilize solitons.Reducing the soliton number is found to be useful to stabilize solitons in fast laser sweep.The effectiveness of the methods was numerically verified.Our measurements and simulations also reveal the impacts of different thermal relaxation processes occurring at quite different time scales on thermal instability.The requirement of the laser sweep protocols is discussed.

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.

Phosphor-free single chip GaN-based white light emitting diodes with a moderate color rendering index and significantly enhanced communications bandwidth

To achieve high quality lighting and visible light communication(VLC)simultaneously,Ga N based white light emitting diodes(WLEDs)oriented for lighting in VLC has attracted great interest.However,the overall bandwidth of conventional phosphor converted WLEDs is limited by the long lifetime of phosphor,the slow Stokes transfer process,the resistance-capacitance(RC)time delay,and the quantum-confined Stark effect(QCSE).Here by adopting a self-assembled In Ga N quantum dots(QDs)structure,we have fabricated phosphor-free single chip WLEDs with tunable correlated color temperature(CCT,from 1600 K to 6000 K),a broadband spectrum,a moderate color rendering index(CRI)of 75,and a significantly improved modulation bandwidth(maximum of150 MHz)at a low current density of 72 A∕cm^2.The broadband spectrum and high modulation bandwidth are ascribed to the capture of carriers by different localized states of In Ga N QDs with alleviative QCSE as compared to the traditional In Ga N/Ga N quantum well(QW)structures.We believe the approach reported in this work will find its potential application in Ga N WLEDs and advance the development of semiconductor lighting-communication integration.

Graphene-driving strain engineering to enable strain-free epitaxy of AlN film for deep ultraviolet light-emitting diode

The energy-efficient deep ultraviolet(DUV)optoelectronic devices suffer from critical issues associated with the poor quality and large strain of nitride material system caused by the inherent mismatch of heteroepitaxy.In this work,we have prepared the strain-free AlN film with low dislocation density(DD)by graphene(Gr)-driving strain-pre-store engineering and a unique mechanism of strain-relaxation in quasi-van der Waals(QvdW)epitaxy is presented.The DD in AlN epilayer with Gr exhibits an anomalous sawtooth-like evolution during the whole epitaxy process.Gr can help to enable the annihilation of the dislocations originated from the interface between AlN and Gr/sapphire by impelling a lateral two-dimensional growth mode.Remarkably,it can induce AlN epilayer to pre-store sufficient tensile strain during the early growth stage and thus compensate the compressive strain caused by hetero-mismatch.Therefore,the low-strain state of the DUV light-emitting diode(DUV-LED)epitaxial structure is realized on the strain-free AlN template with Gr.Furthermore,the DUV-LED with Gr demonstrate 2.1 times enhancement of light output power and a better stability of luminous wavelength compared to that on bare sapphire.An in-depth understanding of this work reveals diverse beneficial impacts of Gr on nitride growth and provides a novel strategy of relaxing the vital requirements of hetero-mismatch in conventional heteroepitaxy.

Implementation of slow and smooth etching of GaN by inductively coupled plasma

Slow and smooth etching of gallium nitride（GaN） by BCl;/Cl;-based inductively coupled plasma（ICP）is investigated in this paper. The effects of etch parameters, including ICP power, radio frequency（RF） power, the flow rate of Cl;and BCl;, on GaN etch rate and etch surface roughness RMS are discussed. A new model is suggested to explain the impact mechanism of the BCl;flow rate on etch surface roughness. An optimized etch result of a slow and smooth etch surface was obtained; the etch rate and RMS were 0.36 ?/s and 0.9 nm, respectively.

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.

Performance improvement of light-emitting diodes with double superlattices confinement layer

In this study, the effect of double superlattices on GaN-based blue light-emitting diodes(LEDs) is analyzed numerically. One of the superlattices is composed of InGaN/GaN, which is designed before the multiple quantum wells(MQWs). The other one is AlInGaN/AlGaN, which is inserted between the last QB(quantum barriers) and p-GaN. The crucial characteristics of double superlattices LEDs structure, including the energy band diagrams, carrier concentrations in the active region, light output power, internal quantum efficiency, respectively,were analyzed in detail. The simulation results suggest that compared with the conventional AlGaN electronblocking layer(EBL) LED, the LED with double superlattices has better performance due to the enhancement of electron confinement and the increase of hole injection. The double superlattices can make it easier for the carriers tunneling to the MQWs, especially for the holes. Furthermore, the LED with the double superlattices can effectively suppress the electron overflow out of multiple quantum wells simultaneously. From the result, we argue that output power is enhanced dramatically, and the efficiency droop is substantially mitigated when the double superlattices are used.

The effect of AlN/AlGaN superlattices on crystal and optical properties of AlGaN epitaxial layers

We investigate the effect of AlN/AlGaN superlattices（SLs） on crystal and optical properties of AlGN epitaxial layers. The result indicates that the crystal quality of AlGaN layers is consistent within a wide range of SLs thicknesses, while the optical properties are opposite. With SLs thickness decreasing from 20/44 to 17/36 and 15/29 nm, the full-width at half maximum of X-ray rocking curves for （0002）- and（1012）-plane of n-AlGaN layers grown on SLs are consistent of around 250 arcsec and 700 arcsec, respectively. Meanwhile, the center of the low optical transmittance band decreases from 326 to 279 nm and less than 266 nm as the SLs thickness decreases.280 nm deep ultraviolet light-emitting diodes（DUV-LEDs） structures are further regrown on the n-AlGaN layers.The electroluminescent intensities of samples are 30% higher than that of the sample whose low optical transmittance band appears around 279 nm. Optical simulations reveal that the SLs acts as distributed Bragg reflectors, thus less photons of the corresponding wavelength escape from the sapphire backside.

Producing deep UV-LEDs in high-yield MOVPE by improving AlN crystal quality with sputtered AlN nucleation layer

High-quality AlN layers with low-density threading dislocations are indispensable for high-efficiency deep ultraviolet light-emitting diodes（UV-LEDs）. In this work, a high-temperature AlN epitaxial layer was grown on sputtered AlN layer（used as nucleation layer, SNL） by a high-yield industrial metalorganic vapor phase epitaxy（MOVPE）. The full width half maximum（FWHM） of the rocking curve shows that the AlN epitaxial layer with SNL has good crystal quality. Furthermore, the relationships between the thickness of SNL and the FWHM values of（002） and（102） peaks were also studied. Finally, utilizing an SNL to enhance the quality of the epitaxial layer, deep UV-LEDs at 282 nm were successfully realized on sapphire substrate by the high-yield industrial MOVPE. The light-output power（LOP） of a deep UV-LED reaches 1.65 mW at 20 mA with external quantum efficiency of 1.87%. In addition, the saturation LOP of the deep UV-LED is 4.31 mW at an injection current of 60 mA. Hence, our studies supply a possible process to grow commercial deep UV-LEDs in high throughput industrial MOVPE, which can increase yield, at lower cost.

Exploration of photosensitive polyimide as the modification layer in thin film microcircuit

Positive type photosensitive polyimide is used as the modification layer in the thin film transistors production process. The photosensitive polyimide is not only used as the second insulating layer, it can also be used instead of a mask because of the photosensitivity. A suitable curing condition can help photosensitive polyimide form the high performance polyimide with orderly texture inside, and the performance of imidization depends on the precise control of temperature, time, and heat control during the curing process. Therefore, experiments of different stepped up heating tests are made, and the ability of protecting silicon dioxide is analyzed.