Yulun Xian

Study of Phosphor Thermal-Isolated Packaging Technologies for High-Power White Light-Emitting Diodes

A novel packaging configuration for high-power phosphor-converting white light-emitting diodes (LEDs) application is reported. In this packaging configuration, a thermal-isolated encapsulant layer was used to separate the phosphor coating layer from the LED chip and the submount. Experimental and finite-element method simulation results proved that this thermal management can prevent the heat of LED chip from transferring to the phosphor coating layer. The surface temperature of the phosphor coating layer is a 16.8degC lower than that of the conventional packaging at 500-mA driver current for 1-mm power GaN-based LED chip. Experimental results also show that this packaging configuration can improve the light-emitting power performance and color characteristics stability of the white LED, especially under high current operating condition.

Thermal Study of High-Power Nitride-Based Flip-Chip Light-Emitting Diodes

This paper presents a chip-level thermal study of high-power nitride-based flip-chip (FC) light-emitting diodes (LEDs). In order to understand the thermal performance of the high-power FC LEDs thoroughly, a quantitative parametric analysis of the thermal dependence on the chip contact area, bump configuration, and bump defects was performed by finite-element model (FEM) numerical simulation and thermal infrared (IR) microscopy testing, respectively. FEM numerical simulation results proved that the optimized bump configuration design was essential to get a uniform temperature distribution in the active layer and improve the thermal performance of the FC LED. IR microscopy testing results recognized that bump defects formed in the LED chip solder processing would lead to surface hot spots around the vicinity of these bump defects, particularly under high-current working conditions. In addition, a light-emitting dark zone was also observed in the optical field for FC LEDs with bump defects. In summary, optimized LED FC bump configuration design and good bonding quality in the chip bonding process are proved to be critical for improving the thermal performance and extending the operating longevity of high-power FC LEDs.

Enhanced electrostatic discharge properties of nitride-based light-emitting diodes with inserting Si-delta-doped layers

We report significantly improved electrostatic discharge (ESD) properties of InGaN/GaN-based light-emitting diodes (LEDs) with inserting Si-delta-doped layers between multiple quantum wells and n-cladding layer. The ESD endurance voltage increased from −1200 V to −4000 V with the insertion of delta-doped layers. The mechanism of the enhanced ESD properties was then investigated. According to capacitance-voltage results, the factor of capacitance modulation was ruled out. However, infrared microscopy image proved better current spreading in the LEDs with delta-doped layers. In addition, current-voltage, photoluminescence, and atomic force microscope measurements demonstrated substantial quality improvements. These two reasons were considered as the dominant mechanisms of the enhanced ESD properties.

Contrary luminescence behaviors of InGaN/GaN light emitting diodes caused by carrier tunneling leakage

The luminescence properties of InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with different quantum-well (QW) thicknesses were investigated. It is found that with decreasing the QW thicknesses, the integrated intensities of the photoluminescence (PL) and electroluminescence (EL) peaks demonstrate a contrary changing trend. The PL results show that the luminescence efficiency is improved by using thinner QWs. However, in the EL process, such a positive effect is counteracted by the low carrier injection efficiency in the thin QW LEDs, and consequently leads to a lower light output. Based on our experimental results, it is inferred that the tunneling leakage current associated with dislocations should be responsible for the low carrier-injection efficiency and the observed weaker EL integrated intensity of the LEDs with thin QWs.

Emission enhancement in GaN-based light emitting diodes with shallow triangular quantum wells

GaN-based light emitting diodes (LEDs) with shallow triangular quantum wells (TQW) structure were proposed and investigated. LEDs with shallow TQW demonstrated a lower turn-on voltage and 80% higher lighting efficiency at 20 mA than devices without the shallow QW structure. A more stable emission wavelength and a lower ideality factor were achieved with the proposed structure. X-ray reciprocal space mapping revealed a partial strain relaxation in active region due to the insertion of the TQW structure. The improved performance is ascribed to the weakening of the polarization field in the MQW active region induced by the TQW structure.

Analysis and modeling of the experimentally observed anomalous mobility properties of periodically Si-delta-doped GaN layers

We report the anomalous mobility properties of Si-delta-doped GaN with periodically doping profile. Samples with different delta-doping periods or with varied Si source flow were investigated. It is found that for the short delta-doping-period (<26.5 nm) samples, the Hall mobility increases with decreasing electron concentration; while for the longer-doping-period samples, the situation is just the opposite. To interpret this observation, a two-layer model has been built up for long-period samples based on secondary ion mass spectroscopy measurements. The fitting results using this model are well consistent with the experimental data.

Role of InGaN Insertion Layer on Nitride-Based Light-Emitting Diodes

In this study, we systematically investigate the effect of InGaN insertion layer (IL) on nitride-based light-emitting diodes. First, a series of samples with different InGaN ILs (Si doping level, thickness) were fabricated and investigated. An optimized condition of the IL was obtained based on current–voltage and electroluminescence measurements. Furthermore, in order to investigate the dominant mechanism for the improved performance of the samples with IL, the optimized sample and a control sample without InGaN IL were compared by means of X-ray diffraction, atomic force microscopy, photoluminescence, injection-current-dependent electroluminescence measurements and infrared camera images. Based on the discussion of these measurement results, we conclude that the performance improvements of samples with InGaN IL are due to both the effects of strain relaxation and better current spreading.

Thermal simulation studies of high-power light-emitting diodes

We showed a detailed thermal simulation of an Epi-down flip-chip packaged LED. Simulation results show that chip attachment defects can cause significant thermal gradients across the active layer of chip, leading to premature failures.

High Quality GaN Grown on Si(111) Using Fast Coalescence Growth

In the paper, a fast coalescence growth is introduced to the epitaxial growth of GaN on silicon substrate. With the fast coalescence growth method, a thin low pressure GaN (LP-GaN) layer used as a function layer, the GaN film could coalesce quickly within a thin thickness, additionally, a smooth surface and high crystal quality could be achieved. With further investigation, it was found that the general GaN coalescence thickness was mainly influenced by the thickness and the growth pressure of the LP-GaN interlayer. And the LP-GaN interlayer has a critical thickness, if over the critical thickness, the crystal quality would degrade. At the same time, it is found that the GaN quality was not affected by the coalescence thickness with a thin LP-GaN interlayer under critical thickness.

Performance Improvement of Nitride-Based Light-Emitting Diode With a Thin Mg-Delta-Doped Hole Injection Layer

The performance of InGaN/GaN multiple quantum wells (MQWs) blue light-emitting diodes (LEDs) was improved by inserting a thin Mg-delta-doped hole injection layer at the end of the MQWs. The forward- and reverse-leakage currents were significantly reduced compared with those of the LEDs without the inserting layer. The light output power was enhanced by 13% at a 350 mA injection current. The improved performance could be ascribed to the dislocation suppression and hole concentration enhancement in the p-type GaN by inserting the Mg-delta-doped structure.