Improvements still needed to make LEDs more efficient

Osram has grown, literally, an LED in the infrared spectrum and claims to have made huge strides along the path of energy efficiency, although the company's Markus Broell concedes the 25% efficiency improvements recorded in the lab will not be replicated in full production models. Broell, project manager for the development of infrared LED chips at Osram Opto Semiconductors, believes the research team has achieved 'something special'. The first product to use the new technology will be an 850nm wavelength infrared (IR) device. For applications in security, this wavelength provides a good balance between being invisible to the human eye and being detectable by sensors. At the top end of the IR spectrum 700nm a red glow can easily be seen and silicon based sensors are at their most sensitive. At the other end of the spectrum, this sensitivity has faded, but the light becomes completely invisible. An intruder looking for a security system using an 850nm LED would be able to see it just but would need to know where to look and to be looking straight at it. With production set to be underway by this summer, the new die technology will be used in the light source for the high efficiency versions of the company's Dragon range of IR LEDs. The efficiency improvements, according to Broell, are down to the thin film technology used. "We have optimised the current distribution over the chip," he explained, "and have minimised absorption losses at both interfaces not only the bottom interface of the active layer, but also the top interface. By reducing the losses and enhancing the 'outcoupling', we have achieved these performance increases." Outcoupling is a technique which enables more light to escape, rather than be lost through internal reflection. The material used for the crystal is aluminium gallium arsenide with a little bit of indium and traces of doping materials like carbon and tellurium and the film is just 6µm thick. The crystals have been grown epitaxially at the company's Regensburg labs in southern Germany. The wavelength is determined by some of the layers in the centre of this layer stack. These layers, only a couple of nanometers thick, are called quantum wells and convert the electrons and holes within a semiconductor into a photon. So, in the quantum world, the electron drops to a lower energy state and then emits a photon. Broell added: "The basic principle is whenever you make an LED, you want to convert the electron pulse as efficiently as possible into photons, and you want to get the photons out of the crystal as efficiently as possible. We have increased the efficiency in both of these areas; producing more photons, reducing absorption and increasing the efficiency of getting the photons out of the crystal. External Quantum Efficiency is about how many of these photons can escape the crystal before being lost in absorption again." External Quantum Efficiency (EQE) measures the probability of creating a photon and its emission from the LED chip per electron. In the new die, EQE reaches 67% and remains at more than 64% up to an operating current of 1A. These figures, claims Broell, are industry leading and they have particular relevance for engineers designing battery operated equipment. He said: "If I have a battery with a certain mAhr capacity, this is directly related to how many electrons I can push through the circuit. And then it gives you how many photons are generated and how much light you get per electron, which is related to the efficiency, but not to the operating voltage. So it is important for those who, for example, are designing handheld devices." The headline performance figure is the 25% improvement in Wall Plug Efficiency (WPE) to 72%. This figure is the optical power that comes out divided by the electrical power that goes in.













As can be seen in fig 1, it varies depending on the operating current. The peak WPE is 72%, which is reached at about 50 to 100mA. However, at the typical operating condition of about 350mA, WPE falls to 66%. Broell commented: "Today, 50% is considered a very good value, so this is a substantial improvement. This 66% or even the 72% figures are in a special R&D package and the package itself can limit or enhance the performance of the die." LED on board This last point highlights the progress that Philips has made in its latest product – the Luxeon Flip Chip LED, which is based on a newly developed die which produces a blue light with a wavelength from 440 to 460nm. The commodity end (TVs, for example) of the market requires low to mid power LEDs and these are constructed using wire bonding to package the die. Using a flip chip package instead of a wire bonded one brings a number of advantages. These include improved thermal performance, better spreading of current (and therefore light) in the device and the ability to drive the device harder and package them closer together. However, as Rahul Bammi, vice president of products at Philips Lumileds, points out: "Flip chips are more complex to make and are therefore not so widely available, although many players are working on them. They are manufactured at higher cost – although this is outweighed by the higher performance – and are IP protected." In terms of the new die, Bammi said: "This is truly a flip chip. The active epi area is now at the bottom of the chip and the sapphire on top is transparent, so the light shines through. Part of the technology challenge has been to minimise the back reflection/absorption of the light in the sapphire (that is, improve the light extraction) – which we have solved." Previously, the sapphire had been removed to improve light extraction, but this required sophisticated die attach and under fill processes to stabilise and protect the die. This was beyond most packaging houses and so the die was not sold as a standalone product. However, by retaining the sapphire, the device is more robust and can be supplied as a chip scale package (csp) for surface mount application. This reduction to a csp enables it to be used in a number of applications that would have been less efficient with packaged LEDs, principally for high performance LED arrays. CSPs can be packaged more closely together than wire bonded devices, so it is possible to create higher lumen densities in smaller areas. With samples of the new device now available, Bammi suggested the next stage in development: "Creating white die (phosphor coated), or different colours (including green) are the natural next steps under consideration."