Zhu Yanxu

Graphene transparent electrodes grown by rapid chemical vapor deposition with ultrathin indium tin oxide contact layers for GaN light emitting diodes

By virtue of the small active volume around Cu catalyst, graphene is synthesized by fast chemical vapor deposition (CVD) in a cold wall vertical system. Despite being highly polycrystalline, it is as conductive and transparent as standard graphene and can be used in light emitting diodes as transparent electrodes. 7-10 nm indium tin oxide (ITO) contact layer is inserted between the graphene and p-GaN to enhance hole injection. Devices with forward voltage and transparency comparable to those using traditional 240 nm ITO are achieved with better ultraviolet performances, hinting the promising future for application-oriented graphene by rapid CVD.

Reliability of AlGaInP light emitting diodes with an ITO current spreading layer

Three aging experiments were performed for AlGaInP light emitting diodes (LED) with or without indium tin oxide (ITO), which is used as a current spreading layer. It was found that the voltage of the LED with an ITO film increased at a high current stressing, while there was little change for that of the LED without the ITO. The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability. The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.

Effects of rapid thermal annealing on ohmic contact of AlGaN/GaN HEMTs

Ohmic contacts with Ti/Al/Ti/Au source and drain electrodes on AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated and subjected to rapid thermal annealing (RTA) in flowing N2. The wafer was divided into 5 parts and three of them were annealed for 30 s at 700, 750, and 800 ?C, respectively, the others were annealed at 750 ?C for 25 and 40 s. Due to the RTA, a change from Schottky contact to Ohmic contact has been obtained between the electrode layer and the AlGaN/GaN heterojunction layer. We have achieved a low specific contact resistance of 7.41 ? 10?6 ??cm2 and contact resistance of 0.54 ??mm measured by transmission line mode (TLM), and good surface morphology and edge acuity are also desirable by annealing at 750 ?C for 30 s. The experiments also indicate that the performance of ohmic contact is first improved, then it reaches a peak, finally degrading with annealing temperature or annealing time rising.

High power and high reliability GaN/InGaN flip-chip light-emitting diodes

High-power and high-reliability GaN/InGaN flip-chip light-emitting diodes (FCLEDs) have been demonstrated by employing a flip-chip design, and its fabrication process is developed. FCLED is composed of a LED die and a submount which is integrated with circuits to protect the LED from electrostatic discharge (ESD) damage. The LED die is flip-chip soldered to the submount, and light is extracted through the transparent sapphire substrate instead of an absorbing Ni/Au contact layer as in conventional GaN/InGaN LED epitaxial designs. The optical and electrical characteristics of the FCLED are presented. According to ESD IEC61000-4-2 standard (human body model), the FCLEDs tolerated at least 10 kV ESD shock have ten times more capacity than conventional GaN/InGaN LEDs. It is shown that the light output from the FCLEDs at forward current 350mA with a forward voltage of 3.3 V is 144.68 mW, and 236.59 mW at 1.0 A of forward current. With employing an optimized contact scheme the FCLEDs can easily operate up to 1.0 A without significant power degradation or failure. The life test of FCLEDs is performed at forward current of 200 mA at room temperature. The degradation of the light output power is no more than 9% after 1010.75 h of life test, indicating the excellent reliability. FCLEDs can be used in practice where high power and high reliability are necessary, and allow designs with a reduced number of LEDs.

Rapid thermal annealing effects on vacuum evaporated ITO for InGaN/GaN blue LEDs

8 mil × 10 mil InGaN/GaN blue LEDs with indium tin oxide (ITO) emitting at 460 nm were fabricated. A vacuum evaporation technique was adopted to deposit ITO on P-GaN with thickness of 240 nm. The electrical and optical properties of ITO films on P-GaN wafers, as well as rapid thermal annealing (RTA) effects at different temperatures (100 to 550 °C) were analyzed and compared. It was found that resistivity of 450 °C RTA was as low as 1.19 × 10−4 Ω·cm, along with a high transparency of 94.17% at 460 nm. AES analysis indicated the variation of oxygen content after 450 °C annealing, and ITO contact resistance showed a minimized value of 3.9 × 10−3 Ω·cm2. With 20 mA current injection, it was found that forward voltage and output power were 3.14 V and 12.57 mW. Furthermore, maximum luminous flux of 0.49 lm of ITO RTA at 550 °C was measured, which is the consequence of a higher transparency.

Analysis on electrical characteristics of high-voltage GaN-based light-emitting diodes

In order to investigate their electrical characteristics, high-voltage light-emitting-diodes (HV-LEDs) each containing four cells in series are fabricated. The electrical parameters including varying voltage and parasitic effect are studied. It is shown that the ideality factors (IFs) of the HV-LEDs with different numbers of cells are 1.6, 3.4, 4.7, and 6.4. IF increases linearly with the number of cells increasing. Moreover, the performance of the HV-LED with failure cells is examined. The analysis indicates that the failure cell has a parallel resistance which induces the leakage of the failure cell. The series resistance of the failure cell is 76.8 ?, while that of the normal cell is 21.3 ?. The scanning electron microscope (SEM) image indicates that different metal layers do not contact well. It is hard to deposit the metal layers in the deep isolation trenches. The fabrication process of HV-LEDs needs to be optimized.

AlGaInP thin-film LED with omni-directionally reflector and ITO transparent conducting n-type contact

In this paper a novel AlGaInP thin-film light-emitting diode (LED) with omni-directionally reflector (ODR) and transparent conducting indium tin oxide (ITO) n-type contact structure is proposed, and fabrication process is developed. This reflector is realized with the combination of a low-refractive-index dielectric layer and a high reflectivity metal layer. This allows the light emitted or internally reflected downwardly towards the GaAs substrate at any angle of incidence to be reflected towards the top surface of the chip. ITO n-type contact is used for anti-reflection and current spreading layers on the ODR-LED with ITO. The sheet resistance of the ITO films (95 nm) deposited on n-ohmic contact of ODR-LED is of the order 23.5Ω/□ with up to 90% transmittance (above 92% for 590–770 nm) in the visible region of the spectrum. The optical and electrical characteristics of the ODR-LED with ITO are presented and compared to conventional AS-LED and ODR-LED without ITO. It is shown that the light output from the ODR-LED with ITO at forward current 20 mA exceeds that of AS-LED and ODR-LED without ITO by about a factor of 1.63 and 0.16, respectively. A favourable luminous intensity of 218.3 mcd from the ODR-LED with ITO (peak wavelength 620 nm) could be obtained under 20 mA injection, which is 2.63 times and 1.21 times higher than that of AS-LED and ODR-LED without ITO, respectively.

Efficiency-enhanced AlGaInP Light-Emitting Diodes with Thin Window Layers and Coupled Distributed Bragg Reflectors

A new structure of high-brightness light-emitting diodes (LED) is experimentally demonstrated. The thin window layer is composed of a 300-nm-thick indium-tin-oxide layer and a 500-nm-thick GaP layer for both current spreading and light anti-reflection. The two coupled distributed Bragg reflectors (DBRs) with one for reflecting normal incidence light and the other for reflecting inclined incidence light which is emitted to the GaAs substrate are employed in the LED fabrication. The coupled DBRs in the LED can provide high reflectivity with wide-angle reflection. The efficiency-enhanced AlGaInP LED has the luminance intensity increase of more than 50% compared with conventional LEDs and high reliability with the saturation current 130 mA.

Flip-Chip GaN-Based Light-Emitting Diodes with Mesh-Contact Electrodes

GaN-based light-emitting diodes (LEDs) with mesh-contact electrodes have been developed. The p-type ohmic contact layer is composed of oxidized Ni/Au mesh and NiO overlay (20 A). An Ag (3000 A) omni-directional reflector covers the p-type contact. The n-type contact is a Ti/Al planar film with a 10-μm-width Ti/Al stripe. The Ti/Al stripe surrounds the centre of LED mesa. With a 20-mA current injection, the light output power of GaN-based LEDs with mesh-contact electrodes is 23% higher than that of the conventional LEDs.

Single-Fundamental-Mode 850 nm Surface Relief VCSEL

The performance of the oxide-confined surface-relief (SR) structure vertical-cavity surface-emitting laser (VCSEL) is simulated and analyzed by using the three-dimensional finite-difference time-domain (FDTD) method. The impacts of the device structure parameters on the far-field characteristics are researched. A single-fundamental-mode SR VCSEL with an oxide-aperture of 15 ?m is designed and produced. The single-mode power of the VCSEL is 5mW, the threshold current is 2.5 mA, far-field divergent angles range from 7.8? to 10.8? and the side-mode suppression ratio is over 30 dB. The optical and electrical properties of the device are in agreement with the results of FDTD simulation, which shows that the SR technology can effectively suppress the higher-order-mode lasing, and make the SR VCSEL work in a single mode under a larger oxide aperture.