blue light emitting diodes
Lighting plays a key role in our everyday lives and the global lighting market for 2009 itself was worth USD 75 billion. The lighting market, which has traditionally been dominated by incandescent bulbs have seen a gradual shift towards higher efficiency and more environmental friendly products over recent years. Light emitting diodes (LEDs), which benefits from high efficiency, non-toxic materials, low power consumption, small form factor and long lifetime over incandescent and compact fluorescent lamps are widely predicted to be the dominant light source within the next 10 years.
Current white LEDs are made from nitride-based blue LED chips (~ 445 nm), which are then used to excite a phosphor coating to produce white light. At present, the luminous efficacy of commercially available cool white LED chips is typically ~ 100 - 120 lm/W. The market penetration for LEDs is expected to intensify rapidly once the efficacy breaks the 200 lm/W barrier. Improvements to the blue LED chip performance, such as reduction in the operating voltage, increasing the light extraction efficiency, and the ability to operate the LEDs at high current densities without sacrificing efficiency will be key towards achieving this goal.
Electroluminesence from blue LED chip 
Sharp is one of the main global players in the solid state lighting industry. The high efficiency and environmental friendly tag associated with LEDs are in line with Sharp’s vision as a company that maximizes technology impact and minimizes environmental impact. At SLE, we have a dedicated LED team actively pursuing various approaches towards improving the blue LED chip efficiency. The LED team has a diverse background consisting of physicists, engineers and material scientists, with all-rounded expertise in device simulation, materials growth, device processing and electro-thermo-optical characterization. Our research activities are primarily focused on developing high efficiency next generation LEDs, and current projects include device simulation, novel techniques to increase light extraction efficiency and to reduce the efficiency ‘droop’ effect. The team also works in close collaboration with colleagues in Sharp Japan manufacturing plants.
device simulation
Modeling software such as SpecLED is used to optimize the LED chip design in order to improve operating voltage, light extraction efficiency and junction temperature. FDTD and ASAP are used to simulate the light distribution intensity and far-field profile, which are subsequently calibrated with experimental results via a feedback loop process. Other tools such as Silense are also available to model band diagrams and to understand fundamental theoretical work, such as the piezoelectric effect in nitride-based devices.
LED chip design
using SpecLED
device processing and characterization
SLE is equipped with various state-of-the-art kits that enable the entire LED chip making process to be done in-house. Dry etching capabilities, thin film deposition, nano-patterning, wafer polishing and chip packaging are amongst the facilities available. In terms of device characterization, photoluminescence and electroluminescence setups are available, together with calibrated integrating sphere and goniometers to measure the absolute output power and far-field profile of the LEDs.
Packaged LED chip
on Ag-coated can
fundamental physics
The efficiency of blue LEDs are currently limited by the ‘droop’ effect, which translates to reduced efficiency at high current densities. This phenomenon severely limits the ability to operate the LEDs at higher drive currents and the origin of this problem is still a subject of intense debate. Novel experimental techniques and device structures are designed and tested in SLE as a means to understand the physics behind the origin of the efficiency ‘droop’, and ultimately mitigation of this effect.
high current densities
