technology LEDs
Inverted epitaxy boosts LED efficiency
Light generation in traditional LEDs is hampered by poor hole doping and an internal electric field that suck carriers out of the active region. The solution: Begin device growth with the p-type layers, because this bolsters carrier capture in the quantum wells and unlocks the door to polarization-induced hole doping, says Crosslight’s Z.Q. Li.
L
EDs only deliver strong performance when they have high internal quantum efficiency – a high ratio of photons generated to carriers injected. However, ensuring that this is the case in nitride LEDs operating over a wide current range is very challenging because in this class of device the internal quantum efficiency tends to plummet as the drive current is cranked up.
Uncertainty surrounds the cause of this efficiency decline that goes by the name of droop. Its origin is the subject of fierce debate, because understanding what
causes it will help to spur the fabrication of droop-busting LED architectures that can underpin a solid-state lighting revolution. At present, some groups are attributing the decline in internal quantum efficiency to various Auger mechanisms, while others are blaming defect recombination or a leakage current. However, no one is disputing that the inclusion of an AlGaN electron- blocking layer (EBL) substantially improves the performance of GaN LEDs.
One way to understand the benefits of the EBL is to first see how it works in a typical LED structure (see Figure 1). In this particular device, just like any other LED, efficiency is maximised by injecting as many electron and holes as possible into the quantum wells (QWs) and enabling incredibly efficient recombination in that trench. However, because electrons have a smaller effective mass and a higher mobility than holes, there is high likelihood that they cross the QW region and reach the p-doped region, rather than recombining to emit light in the QW. This current leakage does not generate any useful photons, and most of the recombination outside the QW is non-radiative.
To prevent electrons escaping over the QW region, engineers can insert a p-type Alx
Ga1-x
Figure 1.A traditional ‘p-side up’nitride LED and a corresponding electron band diagram showing the carrier injection and recombination. Electrons are travelling upwards to the quantum-wells and holes downwards.Electrons with small effective mass cause current leakage while holes with big effective mass result in non-uniform hole density in quantum
wells.Thus the internal quantum efficiency is low
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www.compoundsemiconductor.net November/December 2011 Ga1-x N EBL to impede
N acts as a road-block to electrons travelling outside the QW region. However, in nitride materials holes are about ten times heavier than electrons, and their mobility is roughly 20-30 times lower. Consequently, holes rarely cross the MQW region, leading to non- uniform distributions of hole density in the QWs and ultimately a low internal quantum efficiency.
the progress of these carriers: The larger band gap of Alx
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