LEDs ♦ news digest
Figure 1. Electron microscope image of InP nanowires
Controlling the direction of the light is vitally important for increasing LED efficiency. The direction in which a LED emits light is mainly determined by the surface between the LED and the surrounding air. As light can only escape from the LED at small angles, the direction of emission is usually straight on (perpendicular to the surface).
Technology Foundation STW.
Further details of this work can be obtained from the paper entitled, ‘Controlling the directional emission of light by periodic arrays of heterostructured semiconductor nanowires’, by Diedenhofen et al in ACS Nano (2011), doi: 10.1021/nn201557h.
SETi takes lifetime of 280nm UV LEDs to another level
The firm has achieved more than 10,000 hours lifetime on its aluminium gallium nitride UVTOP ultraviolet LEDs.
Figure 2. Diagram of the light intensity. Measurement of the intensity of the light emitted by the nanowires as a function of the wavelength (i.e. colour) and the angle of the light emitted. The maximum depends on the emitter, which is located in the nanowires. In this case it is at 950 nm and 55 degrees.
However this can be influenced by nanostructures in the surface of the LED. Inspired by these nanostructures, the researchers have developed a new technology with which the direction of the light can be changed.
The new method consists of growing partially- emitting nanowires in an ordered pattern. This pattern forms a ‘photonic crystal’ that sends the light in specific directions. Furthermore, the researchers have shown that the emission can be optimised by a smart positioning of the emitting part within the nanowire.
This knowledge could lead to an increased efficiency of LEDs. Moreover, the researchers say it provides opportunities for a next generation of LEDs, based on semiconducting nanowires.
This research is part of the Industrial Partnership Programme ‘Improved solid-state light sources’ of the Foundation for Fundamental Research on Matter (FOM) and Philips and the FOM Programme ‘NanoPhotovoltaics’. It also received support from
Figure: SETi’s UVTOP LEDs
The lifetime demonstration has been performed on a statistically valid group of LEDs from several batches which are representative of UVTOP275 products; the LEDs were packaged in TO-39 metal- glass packages with flat windows and emit at a wavelength of 280nm ±5nm at an average power of 0.8mW after burn-in.
Lifetime analysis was performed to SETi standard procedure; driven with a current of 20mA DC at room temperature (approximately 23⁰C ambient) with no heatsink or thermal management. The LEDs were physically life tested to approximately 2,000 hours and L50 levels (50% of the LED’s original power) modelled from these tests.
August/September 2011
www.compoundsemiconductor.net 85
Under its continuous improvement program, Sensor Electronic Technology, Inc. (SETi) has demonstrated lifetimes of over 10,000 hours on its UVTOP275 LEDs.
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