Solution-Growth ZnO Nanorods for Light Extraction in GaN-Based Flip-Chip LEDs

A simple and low-cost successive ionic layer adsorption and reaction and hydrothermal method was used to form ZnO nanorods sapphire surface for GaN-based power flip-chip (FC) light-emitting diodes (LEDs). With 350-mA current injection, it was found that the output powers were 361.7 and 448.2 mW for the FC LED without ZnO nanorods and with ZnO nanorods, respectively. The FC LED with ZnO nanorods was 24% larger than that of the FC LED without ZnO nanorods. Furthermore, it was also found that the formation of ZnO nanorods on the surface of sapphire will not degrade the electrical properties. © 2015 The Electrochemical Society. [DOI: 10.1149/2.0061504ssl] All rights reserved. Manuscript submitted December 4, 2014; revised manuscript received January 15, 2015. Published February 7, 2015 III-NITRIDE compound semiconductors, which include GaN, InGaN, and AlGaN, have become the most important material system for short-wavelength light emitters in recent years. Indeed, GaN-based LEDs have attracted extensive attention for many important applications, especially for solid-state lighting. 1-4 Compared with conventional light sources, GaN-based LEDs are compact in size and consume less power. Lifetime of GaN-based LEDs is also much longer. However, output power of current LEDs is still low. Conventional GaN-based LEDs emit photons from the p-side indium-tin-oxide (ITO) contact. 5 With this configuration, however, a significant amount of photons will be obscured by the bonding pads and wires. One effective way to enhance LED output power is to use flip-chip (FC) technology. 6-8 Without the blocking of bonding pads and wires, photons can be emitted freely from the sapphire substrates. By flipping the LEDs, one can also shorten the thermal path between the active light-emitting region and the heat sink. To fabricate FC LEDs, it is necessary to solder the flipped LED chips onto a Si submount prior to packaging. Electrical connections are then made to the FC LEDs via the submount. It is known that thermal conductivity of Si (149 W/m K) is much larger than that of sapphire. 9 Together with the shorter thermal path, the higher thermal conductivity of Si submount thus results in much better thermal property of GaN-based FC LEDs as compared with conventional GaN-based non-flip-chip (NFC) LEDs. For practical solid-state lighting, one needs to deliver large power into large-size devices. The inputted electrical power will be partially converted into output light while the remaining power will be converted into heat. Without a good thermal property, the generated heat can easily fail the devices. Thus, it is extremely important to enhance thermal properties of high-power LEDs. However, the reflective index of sapphire is higher than that of air. This will result in Fresnel reflections, most of the generated lights in the active layer are absorbed inside LEDs and then converted into heat. Rough sapphire surface is a simple technique and has been used to destroy the total internal reflection. 10 It has been reported that one can increase the light extraction efficiency of GaN-based LEDs by using SiO 2 , ITO, and SiNx nanopillars. 11-13 Recently, using ZnO nanopillars to increase the light extraction of GaN-based LEDs has been reported due to simplicity and cost effectiveness. 14-15 Among these, the solution approach based on soft chemical technique has attracted increasing attention in recent years. Although the ZnO zeed layer can be grown by rf magnetron sputtering, 15 the rf magnetron sputtering needs the long-time and vaccum process. This process is complicated and expensive. In this paper, we reported the solution-growth ZnO nanorods on the surface of sapphire to enhance light extraction of GaN flip-chip LEDs. The ZnO nanorods were formed by SILAR-based and Hydrothermal method. It should be noted that the ZnO zeed layer was formed by SILAR due z E-mail: [email protected] to the fact that the SILAR can shorten the growth time. The combination of SILAR and Hydrothermal presents a simple, low-temperature, low-cost, high-reliability and large-area growth. This will not destroy the Ni/Ag mirror property for GaN-based FC LEDs. The detailed procedure will be reported. The samples used in this experiment were all grown by a metallorganic chemical vapor deposition on c-face 2 in. sapphire Al 2 O 3 (0001) substrates. The LED structure consists of a 30 nm thick GaN nucleation layer grown at 550 • C, a 3 μm thick Si-doped n-GaN buffer layer grown at 1050 • C, an unintentionally doped InGaN/GaN multiquantum well (MQW) active region grown at 770 • C, a 50 nm thick Mg-doped p-Al 0.15 Ga 0.85 N electron-blocking layer grown at 1050 • C and a 0.25 μm thick Mg-doped p-GaN layer grown at 1050 • C. The InGaN/GaN MQW active region consists of five pairs of 3 nm thick In 0.23 Ga 0.77 N well layers and 7 nm thick GaN barrier layers. The samples were subsequently annealed at 750 • C in N 2 ambient to active Mg in the p-type layers. After annealing to activate Mg in the p-type layers, we partially etched surfaces of the LED samples until the n + -GaN layers were exposed. We subsequently deposited a 15-nm-thick ITO layer, a 1-nm-thick Ni layer, and a 200-nm-thick Ag layer onto the sample surfaces. Here, the 15-nm-thick ITO layer and the 1-nm-thick Ni layer serve as the transparent ohmic contact while the 200-nmthick Ag layer serves as the reflective mirror. Rapid thermal annealing was then performed at 300 • C for 35 s to improve electrical properties of the p-contacts. On the other hand, Cr/Pt/Au was deposited onto the exposed n + -GaN layers to serve as the n-contacts. We then deposited SiO 2 films as passivation onto the wafers by plasma-enhanced chemical vapor deposition. Photolithography and hydrofluoric (HF) solution etching were subsequently performed to define the P/N pad pattern for bump electroplating. Sn/Au (15 μm/5 μm) layers were then electroplated onto the wafers before the bumps were formed by liftoff. We then thin the sapphire substrate to around 90 μm by back side grinding without polishing. The samples were subsequently chemically treated in HCl:DI water (1:5) at 50 • C for 90 s to remove the remaining contaminant. The ZnO was grown on the top of sapphire as the seed layer by successive ionic layer adsorption and reaction (SILAR) method. The detailed procedures of a ZnO seed layer in four cycles described as follows. The rinsing procedure by SILAR method

p-Cu2O-shell/n-TiO2-nanowire-core heterostucture photodiodes

This study reports the deposition of cuprous oxide [Cu2O] onto titanium dioxide [TiO2] nanowires [NWs] prepared on TiO2/glass templates. The average length and average diameter of these thermally oxidized and evaporated TiO2 NWs are 0.1 to 0.4 μm and 30 to 100 nm, respectively. The deposited Cu2O fills gaps between the TiO2 NWs with good step coverage to form nanoshells surrounding the TiO2 cores. The p-Cu2O/n-TiO2 NW heterostructure exhibits a rectifying behavior with a sharp turn-on at approximately 0.9 V. Furthermore, the fabricated p-Cu2O-shell/n-TiO2-nanowire-core photodiodes exhibit reasonably large photocurrent-to-dark-current contrast ratios and fast responses

Fabrication of Compact Microstrip Line-Based Balun-Bandpass Filter with High Common-Mode Suppression

A new type of balun-bandpass filter was proposed based on the traditional coupled-line theory and folded open-loop ring resonators (OLRRs) configuration. For that, a new device with both filter-type and balun-type characteristics was investigated and fabricated. Both magnetic and electric coupling structures were implemented to provide high performance balun-bandpass responses. The fabricated balun-bandpass filters had a wide bandwidth more than 200 MHz and a low insertion loss less than 2.51 dB at a center frequency of 2.6 GHz. The differences between the two outputs were below 0.4 dB in magnitude and within 180 ± 7° in phase. Also, the balun-bandpass filter presented an excellent common-mode rejection ratio over 25 dB in the passband. An advanced design methodology had been adopted based on EM simulation for making these designed parameters of OLRRs, microstrip lines, and open stubs. The measured frequency responses agreed well with simulated ones

Selected Papers from IEEE ICASI 2018

This Special Issue on “Selected Papers from IEEE ICASI 2018” includes excellent papers presented at the IEEE ICASI 2018 regarding the “applied system innovation” topic. Mechanical engineering and design innovations are both academic and practical engineering fields, which involve systematic technological materialization through scientific principles and engineering designs. Technological innovations in mechanical engineering include IT-based intelligent mechanical systems, mechanics and design innovations, and applied materials in nanosciences and nanotechnology. The aim is to encourage the attendees at the IEEE ICASI 2018 to publish their experimental and theoretical research relating to applied system innovation

Selected papers from IEEE ICASI 2018

This Special Issue on "Selected Papers from IEEE ICASI 2018" includes excellent papers presented at the IEEE ICASI 2018 regarding the "applied system innovation" topic. Mechanical engineering and design innovations are both academic and practical engineering fields, which involve systematic technological materialization through scientific principles and engineering designs. Technological innovations in mechanical engineering include IT-based intelligent mechanical systems, mechanics and design innovations, and applied materials in nanosciences and nanotechnology. The aim is to encourage the attendees at the IEEE ICASI 2018 to publish their experimental and theoretical research relating to applied system innovation.</p

High Response of Ethanol Gas Sensor Based on NiO-Doped Apple Pectin by the Solution Process

Novel gas sensor devices, based on biomaterial apple pectin film (APN) doped with NiO, were fabricated for the first time using a solution processing technique. The device was then annealed in a microwave chamber. The structural, elemental, and surface morphology of the device was investigated, using TEM, XPS, and AFM, respectively. The as-fabricated film sensor possessed a superior sensing performance regarding ethanol gas, compared to the pure apple pectin film sensor. The response of the device was recorded at a maximum efficiency of 161. For a 10 ppm concentration of ethanol gas at an operational temperature of 250 °C, the response time was 1.379 s. Nevertheless, the sensing mechanism for the sensor device is also described thoroughly

Design of LTE/Sub-6 GHz Dual-Band Transparent Antenna Using Frame-Structured Metal Mesh Conductive Film

This paper proposes a dual-band transparent antenna using frame-structured metal mesh conductive film (MMCF). The frame-structured metal mesh conductive film is based on the conductive-coated thin film and forms a narrow strip surrounding the edge of the antenna. The frame-structured metal mesh conductive film can resist considerable current leakage on the edge of the conductive strip to improve the antenna’s efficiency by 51% at 2.1 GHz and 53% at 3.6 GHz. As a result, the transparent dual-band antenna has an operating bandwidth of 1.9–2.4 GHz and 3.2–4.1 GHz with a high transparency of 80%, which make it valuable to the applications of biomedical electronic components, wearable devices, and automobile vehicles

High Responsivity MgZnO Ultraviolet Thin-Film Phototransistor Developed Using Radio Frequency Sputtering

We investigated the electrical and optoelectronic properties of a magnesium zinc oxide thin-film phototransistor. We fabricate an ultraviolet phototransistor by using a wide-bandgap MgZnO thin film as the active layer material of the thin film transistor (TFT). The fabricated device demonstrated a threshold voltage of 3.1 V, on–off current ratio of 105, subthreshold swing of 0.8 V/decade, and mobility of 5 cm2/V·s in a dark environment. As a UV photodetector, the responsivity of the device was 3.12 A/W, and the rejection ratio was 6.55 × 105 at a gate bias of −5 V under 290 nm illumination