Al Reflector Research Articles

Highly efficient AlGaN-based deep-ultraviolet light-emitting diodes: from bandgap engineering to device craft

AlGaN-based light-emitting diodes (LEDs) operating in the deep-ultraviolet (DUV) spectral range (210–280 nm) have demonstrated potential applications in physical sterilization. However, the poor external quantum efficiency (EQE) hinders further advances in the emission performance of AlGaN-based DUV LEDs. Here, we demonstrate the performance of 270-nm AlGaN-based DUV LEDs beyond the state-of-the-art by exploiting the innovative combination of bandgap engineering and device craft. By adopting tailored multiple quantum wells (MQWs), a reflective Al reflector, a low-optical-loss tunneling junction (TJ) and a dielectric SiO2 insertion structure (IS-SiO2), outstanding light output powers (LOPs) of 140.1 mW are achieved in our DUV LEDs at 850 mA. The EQEs of our DUV LEDs are 4.5 times greater than those of their conventional counterparts. This comprehensive approach overcomes the major difficulties commonly faced in the pursuit of high-performance AlGaN-based DUV LEDs, such as strong quantum-confined Stark effect (QCSE), severe optical absorption in the p-electrode/ohmic contact layer and poor transverse magnetic (TM)-polarized light extraction. Furthermore, the on-wafer electroluminescence characterization validated the scalability of our DUV LEDs to larger production scales. Our work is promising for the development of highly efficient AlGaN-based DUV LEDs.

The composited high reflectivity p-type electrodes with patterned ITO for AlGaN-based ultraviolet light emitting diodes

Composited p-type electrodes with high reflectivity have been investigated in AlGaN-based ultraviolet light emitting diodes (UV-LEDs) to improve the light extraction efficiency, which are composed of a patterned ITO layer and an Al reflector. It is verified that the patterned ITO with a thickness of 30 nm can not only well form Ohmic contact with p-GaN capping layer, but also be nearly 90% transparent to ultraviolet light, and thus presenting a reflectivity of 73% at 280 nm when combined with an Al reflector. Further experimental efforts confirm that the performance of the UV-LEDs is dramatically improved with such p-type electrodes. The maximum light output power and wall plug efficiency in the current range of 0–100 mA are severally increased by 49.8% and 54.2% compared to the device with traditional Ni/Au electrodes.

Enhanced efficiency of deep ultraviolet light-emitting diodes utilizing full-coverage Al reflector and highly reflective Ni/Rh p-electrode

Increasing the reflection of p-side is an effective way to improve the optoelectronic performance of flip-chip light-emitting diodes (FCLEDs). Here, we propose a full-coverage Al reflector (FAR) and a highly reflective Ni/Rh p-electrode to enhance the performance of deep ultraviolet (DUV) FCLEDs. The physical mechanism for the impact of the FAR and Ni/Rh electrode on the light extraction efficiency (LEE) is discussed theoretically. Simulations demonstrate that the combination of the FAR and Ni/Rh electrode improves the LEEs of transverse electric- and transverse magnetic-polarized light by 13.62% and 27.08%, respectively. At an injection current of 100 mA, the fabricated DUV FCLEDs with FAR and Ni/Rh electrode exhibits an external quantum efficiency of 4.01% and a wall plugging efficiency of 2.92%, which are 16.85% and 13.18% higher than those of conventional DUV FCLEDs, respectively. These results support the promise of the FAR and Ni/Rh electrode for high-power DUV LED applications.

Achieving Multiwavelength Optical Amplification Based on Polymer Waveguides Doped with NaYF4:Er3+, Yb3+ Nanoparticles under Commercial and Convenient Led Pumping

AbstractRelying on a reverse energy transfer mechanism between erbium (Er3+) and ytterbium (Yb3+) ions, optical gains at wavelengths of 1550, 980, and 1067 nm are achieved in NaYF4: Er3+, and Yb3+ nanoparticles‐doped polymer waveguides under excitation of low‐power light‐emitting diodes (LEDs) instead of 976 or 1480 nm semiconductor laser as pump sources. The optical gains at 1550, 980, and 1067 nm are demonstrated in five waveguide structures: active‐rectangular, evanescent‐field with and without an aluminium (Al) reflector, and dual‐active hybrid with and without an Al reflector. The maximum relative optical gains of 6.6, 4.0, and 3.8 dBcm−1 are realized at 1550, 1067, and 980 nm, respectively, in a 5 mm long dual‐active hybrid waveguide with cross‐section of 4 × 10 µm, and a ≈100 nm‐thick Al reflector grown under the lower cladding layer under excitation of a 395 nm LED. This work proves the feasibility of using a low‐cost LED to realize multi‐wavelength amplification in a dense wavelength‐division multiplexing system and is thus valuable for the development of complex multi‐functional photonic‐integrated‐module mixed‐band optical communication systems.

240 nm AlGaN-based deep ultraviolet micro-LEDs: size effect versus edge effect

240 nm AlGaN-based micro-LEDs with different sizes are designed and fabricated. Then, the external quantum efficiency (EQE) and light extraction efficiency (LEE) are systematically investigated by comparing size and edge effects. Here, it is revealed that the peak optical output power increases by 81.83% with the size shrinking from 50.0 to 25.0 μm. Thereinto, the LEE increases by 26.21% and the LEE enhancement mainly comes from the sidewall light extraction. Most notably, transverse-magnetic (TM) mode light intensifies faster as the size shrinks due to the tilted mesa side-wall and Al reflector design. However, when it turns to 12.5 μm sized micro-LEDs, the output power is lower than 25.0 μm sized ones. The underlying mechanism is that even though protected by SiO2 passivation, the edge effect which leads to current leakage and Shockley-Read-Hall (SRH) recombination deteriorates rapidly with the size further shrinking. Moreover, the ratio of the p-contact area to mesa area is much lower, which deteriorates the p-type current spreading at the mesa edge. These findings show a role of thumb for the design of high efficiency micro-LEDs with wavelength below 250 nm, which will pave the way for wide applications of deep ultraviolet (DUV) micro-LEDs.

Miniature GaN optoelectronic temperature sensor.

The combination of plastic optical fiber (POF) with monolithically integrated transmitter and receiver is becoming increasingly attractive for the development of miniature optoelectronic sensing systems. Here, we propose a temperature sensing system by integrating a GaN optoelectronic chip with a POF and aluminum (Al) reflector. Owing to the overlap between electroluminescence and responsivity spectra of multiple quantum well (MQW) diodes, both the transmitter and the receiver having identical MQW structures are monolithically integrated on a tiny GaN chip by using the same fabrication process flow. Environmental temperature change leads to thermal deformation in the Al reflector, which reflects the transmitted light back with a light pulse. The reflected light is coupled into the guided POF again and sensed by the on-chip receiver. Finally, the temperature information is read out as electrical signals. When the ambient temperature changes from 20.1°C to 100°C, the optically induced electrical signal decreases from -3.04 µA to -3.13 µA. The results suggest that the monolithically integrated GaN device offers a promising option for optoelectronic temperature sensing systems.

Enhanced dual-band absorption of graphene mediated by an aluminum metastructure

Graphene absorption from the visible to infrared spectrum has great potential and broad applications in miniature of modern optoelectronic biosensors and photodetectors. However, graphene has zero bandgap energy, which limits its absorption to 2.3% in the visible and infrared spectrums. Here, we propose a metastructure to optimize graphene absorption in the visible to near-infrared frequency regions. The metastructure, comprising an array of aluminum square blocks (Al-SBs) on a graphene layer, a silica spacer, and an Al reflector, is investigated for absorption enhancement. This work deciphers the effect of the periodicity of decorated Al-SBs on the evolution of dual-band absorption in single-layer graphene under normal incidence. The electromagnetic signatures of two excited modes indicate that surface plasmons and magnetic dipole plasmons are mediators of absorption. The investigation into the impact of geometrical parameters illustrates that the coexisting phenomena of a relative broad peak and a relative sharp peak have been achieved simultaneously with high efficiency. The dynamic manipulation of surface plasmons and magnetic dipole plasmons presents great potential for a diverse range of applications, such as sensing and imaging. By controlling the periodicity of Al-SBs, it is possible to achieve active control of surface plasmon resonance, and a detection range of 300 nm is observed. Dynamic control of the magnetic dipole plasmon is successfully achieved by modifying the electrical environment of the graphene layer, which is realized by altering the underlying spacer material. Collectively, the findings of this study demonstrate the significant potential of the suggested metastructure for its prospective applications in optoelectronic devices, including biosensors, photovoltaics, and photodetectors that rely on the dynamic control of surface and magnetic plasmon resonances.

Comparison of structure and solar-selective absorbance properties of Al2O3 thin films with Al and Ni reflector interlayers

This work aims at evaluating the selective solar absorbance (SSA) of Al2O3 thin films deposited by physical vapor deposition (PVD) with two different metallic reflector interlayers. Al2O3 thin films are deposited on the same substrate stainless steel (SS) 304L, with two different interlayers between the thin film and the substrate, namely, nickel (Ni) and aluminum (Al) are used as the reflector interlayer SS304L/Al/Al2O3 and SS304L/Ni/Al2O3. A scanning electron microscope (SEM) was utilized to characterize the chemical composition by energy dispersive X-ray analysis (EDX) and surface morphology of the deposited thin films. The phases of the thin films were analyzed and identified by X-ray diffraction (XRD) to detect the present phases. The surface topography and the thickness of the deposited thin films were investigated using an atomic force microscope (AFM). The optical properties of the substrate and the deposited thin films (absorbance & emittance) in two conditions were identified by Fourier transform infrared spectroscopy (FTIR) and spectrophotometer. The obtained results demonstrate that both SS304L/Al/Al2O3 and SS304L/Ni/Al2O3 show good performance, such as high solar absorbance and low thermal emittance. However, the Al/Al2O3 thin film provides high selectivity (absorbance/emittance (α/ε)) of 0.916/0.05, compared to 0.913/0.15 for the Ni/Al2O3 coating. The effect of different properties and microstructure on the efficiency of deposited thin films showed that the SS304L/Al/Al2O3 has higher absorbance (92%) in visible and ultraviolet (UV) regions; and lower emittance (5%) than the SS304L/Ni/Al2O3. This work shows that the intermediate IR layer has a more pronounced effect on the emittance properties rather than the absorbance properties of the Al/Al2O3 layer.

Sub-Micron Monolithic Full-Color Nanorod LEDs on A Single Substrate

The extended reality (XR) display, including augmented reality (AR) and virtual reality (VR), requires ultrahigh-resolution and high-luminance pixel technology. GaN-based micro-LEDs are the most promising for high-density pixel applications. However, it is necessary to simultaneously fabricate full-color R/G/B emitters on a single substrate. Moreover, the efficiency droop at long wavelengths with high-indium InGaN still remains a challenge. The InGaN nanorod has the potential to realize monolithic R/G/B LEDs on a single substrate, and the Al reflector has a light reflectance that can dramatically improve light extraction efficiency. Here, we designed sub-micron monolithic full-color R/G/B nanorod LEDs with an Al core-shell reflector on single chip. The optimal structures for light confinement and vertical light extraction by single InGaN nanorod R/G/B LED structures were identified. The single nanorod with Al core-shell structure dramatically increased the light extraction efficiency, and showed an improvement of more than 20%, especially in the red wavelength. As a result, by introducing a long-length nanorod structure, we have successfully designed a new submicron pixel with full-color R/G/B LEDs operating within 1 μm × 1 μm size. This unique design will be a key factor in enabling the next generation of AR and VR displays that require ultra-small and compact pixels.

Deep‐Ultraviolet LEDs Incorporated with SiO2‐Based Microcavities Toward High‐Speed Ultraviolet Light Communication

AbstractOptical wireless communication (OWC) in the deep‐ultraviolet (DUV) band requires an efficient DUV light source with large bandwidth characteristics. In this work, a feasible approach is reported to enlarge the light output power as well as the bandwidth of a DUV light‐emitting diode (LED) by embedding a SiO2‐based microcavity on which an aluminum (Al) reflector is simultaneously deposited. Consequently, on the one hand, the microcavity with the Al‐reflector can facilitate photon escape from the LED to increase the light extraction efficiency, thus enhancing the light output power of the devices. On the other hand, the LED incorporated with a microcavity structure exhibits a reduced resistance–capacitance time constant, leading to an increase in the modulation bandwidth of the LED. Strikingly, the DUV LED incorporated with microcavities represents a significant enhancement of light output power by nearly 30% at 80 mA while exhibiting a higher modulation bandwidth of 12% in comparison to the conventional LED without microcavities. Thus, the implementation of the microcavity and Al reflector on top of a classic LED can enlarge the light output power and modulation bandwidth, eventually facilitating to establish viable high‐speed OWC systems.

Highly stable semitransparent multilayer graphene/LaVO3 vertical-heterostructure photodetectors

A heterostructure composed of a combination of semi-metallic graphene (Gr) and high-absorption LaVO3 is ideal for high-performance translucent photodetector (PD) applications. Here, we present multilayer Gr/LaVO3 vertical-heterostructure semitransparent PDs with various layer numbers (L n ). At L n = 2, the PD shows the best performance with a responsivity (R) of 0.094 A W−1 and a specific detectivity (D*) of 7.385 × 107 cm Hz1/2 W−1 at 532 nm. Additionally, the average visible transmittance of the PD is 63%, i.e. it is semitransparent. We increased photocurrent (PC) by approximately 13%, from 0.564 to 0.635 μA cm−2 by using an Al reflector on the semitransparent PD. The PC of an unencapsulated PD maintains about 86% (from 0.571 to 0.493 μA cm−2) of its initial PC value after 2000 h at 25 °C temperature/30% relative humidity, showing good stability. This behavior is superior to that of previously reported graphene-based PDs. These results show that these PDs have great potential for semitransparent optoelectronic applications.

Origin of operating voltage increase in deep UV light-emitting diodes with ITO/Al reflector

In an attempt to further elucidate the operating voltage increase in a vertical UV-C LED with p-electrode composed of transparent conducting Sn-doped indium oxide (ITO)/Al reflector, the interface formation between ITO and Al thin film was studied by using scanning transmission electron microscopy in combination with electron energy loss spectroscopy. It was confirmed that the oxidized layer was formed at the interface of the ITO/Al electrode in accordance with the thermal annealing. It was found that not only the thickness of oxide formation grew with the increased annealing temperature, the content of oxygen also increased. Moreover, it was also ascertained that the prolonged annealing time at high temperature induced the indium diffusion into the Al-oxide layer.

Light extraction efficiency of AlGaN nanowire deep ultraviolet light-emitting diodes on Si with different photonic structures

The low light extraction efficiency (LEE) stemming from the transverse-magnetic dominant emission is one of the major factors limiting the performance of aluminum gallium nitride (AlGaN)-based deep ultraviolet (UV) light-emitting diodes (LEDs) when the light emission wavelengths are close to 200 nm. These wavelengths, nonetheless, are pivotal to applications including sensing and sterilization and are considered human safe. We investigate the LEE of AlGaN/AlN nanowire (NW) deep UV LEDs emitting at 225 nm, with a focus on the top-surface LEE by considering different NW arrangements, typical NW spacings and radii from AlGaN NWs grown by selective area epitaxy, the influence of Si on the LEE in comparison to an Al reflector, and graphene as the top electrode. Our results show that given the selected range of design parameters, the top-surface LEE for honeycomb, square, and hexagonal lattices can be up to around 42% for complete devices on Si with the graphene top electrode; and compared to using an Al reflector, the Si substrate does not reduce LEE considerably. In the end, the light extraction mechanism is discussed by simulating the two-dimensional photonic crystal band structures.

The enhanced light diffuse reflection of laser marking Al substrate for the back reflector purpose

This study set out with the aim of assessing the possibilities of incorporating the laser marking strategies for the purpose of diffuse back reflector fabrication. The correlation between the laser process parameters of fabricated samples’ microstructure and their optical properties has been determined. The laser texturizing was conducted on the aluminum substrate throughout the Nd:YAG (1064 nm) laser varying the laser process parameters, i.e. scanning speed (v) and pulse repetition rate (f). The effect of laser irradiation on surface nanostructure was investigated by means of confocal optical microscopy (COM) and scanning electron microscopy (SEM). In order to evaluate the phase composition and optical properties of fabricated nanotextures, the X-ray diffraction (XRD) and spectrophotometric measurements were performed. The obtained reflectance spectra of samples were used to perform the colorimetry analysis and calculate the luminous reflectance. The morphological evaluation of patterned surfaces gives evidence of periodic crater-shaped structures formation influenced by chaotic radial-ligaments patterns resulting from splashing liquid metal solidification. Laser-textured Al reflectors show almost similar total reflectance as the untreated substrate whereas the significant enhancement of diffuse reflectance of patterned sample series can be observed. The different hues of white colour appear on all laser-patterned Al surfaces. The calculated luminous reflectance of proceeded samples ranging from 65% to 75%. The overall results show the effectiveness of the laser marking process in order to obtain highly diffusive scattering layers on Al substrate without employing any additional coating.

Optimized Aluminum Reflector for Enhancement of UVC Cathodoluminescence Based-AlGaN Materials with Carbon Nanotube Field Emitters.

The far ultraviolet C (UVC) light sources based on carbon nanotube (CNT) field emitters as excitation sources have become promising light sources for sterilization, disinfection, and water purification. However, the low light extraction efficiency of UVC–CNT light sources still hinders the practical application of these structures. Herein, we report an optimized aluminum (Al) reflector to enhance the light extraction efficiency of UVC–CNT light sources. Optical analysis of UVC-CNT light sources covered by the Al reflectors with various thicknesses ranging from 30 to 150 nm was performed to realize the optimized reflector. The UVC-CNT light sources exhibit the highest light extraction efficiency when the Al reflector layer has an optimized thickness of 100 nm. For comparison, the cathodoluminescence (CL) spectra were recorded for UVC–CNT light sources with and without the optimized Al reflector. The measured light output power and the estimated power efficiency of the UVC–CNT light-source-tube with Al reflector were enhanced by about 27 times over the reference. This enhancement is mainly attributed to the outstanding reflection effect of the Al reflector.

Decomposition of dimethyl methylphosphonate vapor on ultrathin-film titania photocatalytic light absorber

The decomposition of chemical warfare agent simulant, dimethyl methylphophonate (DMMP) vapor, was investigated on an ultrathin film titania (TiO2) photocatalytic light absorber. The light absorber contains an aluminum (Al) reflector and the TiO2 thin film with different thicknesses, sequentially deposited on a supportive glass substrate. The designed structure constructs a nanocavity that exhibits strong light absorption within the photocatalytic TiO2 ultrathin film. Thus, the intrinsic trade-off between optical absorption and charge carrier extraction efficiency, i.e., a light absorber should be thick enough to absorb the light allowable by its band gap but thin enough to allow charge carrier extraction for catalytic reactions, is conquered. The TiO2/Al light absorber significantly boosted TiO2 photocatalytic activity compared to the benchmark Aeroxide®P25 catalyst (i.e., up to 2013 times increase in reaction rate). The effects of reactant (i.e. DMMP, water and oxygen, respectively) partial pressure and reaction temperature on photocatalytic decomposition of DMMP by the ultrathin-film TiO2 photocatalytic light absorber were studied. Kinetic data of the DMMP decomposition can be described by the Langmuir-Hinshelwood model.

Development of a Phosphor Screen with Mirror Coating to Improve Optical Properties in Radiography.

Phosphor screens have attracted increasing global interest because they can aid the acquisition of high-quality images while simultaneously reducing exposure. However, although increasing the thickness of the phosphor screen increases exposure efficiency due to scattered light, it also leads to a broader light spread, which results in poorer resolution. Hence, in this study, we implemented a reflector using a mirror-coating technique on the surface of a phosphor screen and analyzed its characteristics in terms of luminescence intensity and resolution. We present the fabrication and measurement of the phosphor screen based on a reflector containing Gd₂O₂S:Tb. The phosphor screen containing the reflector can be fabricated via the screen-printing method and roll-to-roll sputtering method. In particular, when compared to the condition without a reflector, Al and Cu reflectors showed improvements in luminescence intensity of 15.7% and 12.2%, respectively, as well as lower full widths at half maximum of 11.45 and 9.08, respectively. This quantitatively demonstrates that Cu and Al are suitable reflector materials to improve exposure efficiency while maintaining resolution in radiography systems. Moreover, because we did not utilize an optical assembly, which improves the transmission efficiency by matching the different refractive indices of the phosphor screen and photo-detector, we believe that the quantity of photons could be further improved if the reflectors were applied to a commercial product. Thus, future studies using single-layer anti-reflector coating techniques with optical assemblies are warranted.

Development and Characterization of Solar Simulator for Solar Cells

The solar simulator is crucial instrument to determine the performance of solar cells and modules which are core components of solar power plants. Therefore, a precise performance measurement of solar cells is very important to design a cost effective solar stimulator with high accuracy. This paper proposes the development of a precise and measurable innovative solar simulator technique. The designed solar stimulator is a continuous type for 6 inch solar cell measurement in which a reflector system is added for improving the uniformity of result. In addition, 2∼6 light emitting diodes (LEDs) are attached to the one side of the reflector. For reliable evaluation, the standard measurement of IEC60904-3 was applied and matched the uniformity, temporal instability, and spectrum values under 1000 W/m2 light intensity condition. The uniformity of the solar simulator was recorded to 1.43% by using Al reflector and LED, which was highly comparable to 3.06 before its utilization (without reflector). Thus, designed stimulator improved the temporal instability and spectrum match values, which might be considered as a class A.

Integrating remote reflector and air cavity into inclined sidewalls to enhance the light extraction efficiency for AlGaN-based DUV LEDs.

In this work, we propose and demonstrate the concept of remote reflections, which help to multiply the photon propagations for increasing the light extraction efficiency (LEE) for both transverse magnetic (TM)- and transverse electric (TE)-polarized light. The remote reflection is enabled by using a remote-metal-reflector-based air cavity extractor. According to our study, the remote reflections can significantly avoid the optical absorption when compared with the conventional inclined-sidewall-shaped deep-ultraviolet light-emitting diodes with the metal Al reflector on the inclined sidewalls. As a result, the optical power for our proposed devices has been significantly enhanced by 55% experimentally. Numerical simulations further reveal that the remote metal reflector not only favors more total internal refection on the inclined sidewalls but also supports additional light escaped channels for enhancing the LEE.

High Efficiency Flip Chip-Based UV Emitter with Indium Tin Oxide Nano Grains/Al Reflector and Periodic Microhole Arrays.

We propose a high efficiency flip chip-based ultraviolet (UV) emitter with aluminum (Al) reflector that includes indium tin oxide (ITO) nano grains for current injection between the Al and p-AlGaN layer. Al has attracted attention as a reflector for high efficiency UV emitters because of its high reflectance in the UV region. To improve the efficiency of UV emitter, we generated periodic microhole arrays on the p-AlGaN layer, which serve as a scattering center in the flip chip structure and enhance the light extraction efficiency. The light output power of the fabricated flip chip-based UV emitter with ITO nano grains/Al reflector and microhole arrays on the p-AlGaN layer is significantly improved by 72% and 45% at an injection current of 20 mA, compared to that of UV emitter with only Al reflector and ITO nano grains/Al reflector.