EDITION 2
OPTOELECTRONICS Leading the way in even higher performance and lower costs for the LED industry EXTRACTS FROM LAYERS 2016
LAYERS2016
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OPTOELECTRONICS
Optoelectronics
Evatec has enjoyed huge success in Optoelectronics over the last 6-8 years working with the industry & technology leaders in US, EU and in close partnerships with key volume LED manufacturer in Asia. Our equipment & process solutions cover the complete range of LED technologies in production today, all the way from sapphire on 2“ - 6“ to fully automated evaporation & sputter processes for 8“ GaN on Si.
Dr. Reinhard Benz, Head of Strategic Sales and Product Marketing
We deliver sputtering and evaporation equipment used for ITO, metal electrodes, optical reflectors including complex DBR stacks. New additional high performance sputter solutions now mean extended cost effective production solutions for the AlN buffer layer on flat or patterned saphire prior to epi GaN and the SiO/SiN layers required for final packaging. In a hugely competitive market, our focus remains on increased productivity and yield for our customers from new TCOs for higher light utilization and better electrical properties to the capturing of more light by improved reflectors.
LAYERS2016
Boosting LED efficiency with high performance current spreading layers and DBR mirror coatings Both the current spreading layer as well as the mirror contribute substantially to the achievement of high efficiencies in LED devices. Dr. Silvia Schwyn Thoeny explains the choice of deposition techniques and the importance of adapting thin film processes to LED chip design.
After the adoption of LED lighting for backlight units in LCD laptop and television screens, the growth of the market for white LEDs is now mainly driven by general illumination, where LEDs are about to replace the incumbent technologies of incandescent and fluorescent light bulbs and tubes. Deployment of LEDs into these applications is triggered by their major contribution to energy saving, longevity and versatility in terms of design flexibility and color tuning. Both the current spreading layer and mirror contribute substantially to the achievement of high efficiencies in LED devices. Transparent conductive indium-tin oxide (ITO) is widely used in III-nitride light emitting diodes (LEDs) serving functions requiring different ITO properties making it essential to optimize material parameters and overall chip design simultaneously.
DEPOSITING THE RIGHT ITO Evatec electron beam evaporation processes for ITO in a BAK evaporator are already proven in mass production for LEDs, but there is also a strong demand to replace this process by magnetron sputter deposition. The main advantages are a reduced batch time, since in sputtering no heat step is required and simplified integration of automatic wafer handling. The big challenge with sputtering however is to avoid crystal damage of the GaN substrate due to the energetic atoms and ions present in the sputtering plasma. Extensive process optimization on Evatec’s RADIANCE sputter platform was necessary to identify the most suitable process conditions. In a first step, we were able to establish damage free deposition conditions using research grade LED material in a collaboration project with the institute LASPE at the Swiss Federal Institute of Technology in Lausanne. Comprehensive characterization of those films showed low electrical resistivity, high transparency and a work function in the range of 5.2 eV. Thus, tunnel contacts to p-GaN could be formed with a low barrier height in the range of the standard metals used to form the p-contact. Indeed, the devices,
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COMPETENCES IN OPTOELECTRONICS
THREE REASONS TO CHOOSE ITO FOR CURRENT SPREADING LAYERS 1. ITO serves as a contact to p-GaN, with a work function as high as the best metal contact materials
2. Optical losses caused by reflections of the emitted light at the metal-semiconductor interface can be avoided by substituting the metallic electrode by ITO
which were fabricated using these ITO layers showed operating voltages (Vf) as low as reference devices with e-beam deposited ITO and even comparable to devices with standard metallic p-electrodes.
Fig. 1 DBR mirror with excellent reflection over a wide range of indidence anlges at the LED emission (450nm)
However, numerous customer samplings showed that that there is no ONE best condition for depositing ITO layers. Within the parameter space of damage free coating the ITO needs to be tailored to the specific design and doping of the LED. As a general rule the conductivities of p-GaN and ITO need to match for perfect current spreading. If this is the case, competitive / state of the art low operating voltages and high output power can be obtained. Furthermore, annealing of the LEDs, which will be performed at a later stage in the manufacturing chain, will influence the ITO properties and thus has to be taken into account as well.
3. ITO provides uniform current spreading in an LED die
CHOOSING THE RIGHT HIGH REFLECTANCE MIRRORS The mirror on the backside of the LED is an additional key element for excellent energy efficiency of the LED. Light generated in the active region of an LED is evenly emitted in all directions, hence also to the backside of the device. Without a mirror on the back surface of the device half of the light would be lost. In LED terminology this mirror is referred to as a Distributed Bragg Reflector (DBR). There are different schemes of design for this mirror, spanning from metal to metal-dielectric and dielectric only mirrors. A typical example is seen in Fig1. The complexity of the mirror rises in the order they are mentioned, but so does the performance, i.e. the level of reflectance and the wavelength range for which high reflectance can be obtained. The type of mirror used is specific to the LED design and are defined by the LED manufacturer. In this application Evatec benefits strongly from its long standing expertise in deposition of such optical interference coatings requiring techniques for control of film thicknesses in the nm range with high precision. The BAK Evaporator and both the MSP and RADIANCE sputter platforms can be equipped with optical monitoring technology for precise thickness control of layer thicknesses. The highly dense and shift free layers required are obtained by plasma assist naturally present in sputtering processes and by using the high performance plasma source in evaporation processes. Just like for ITO, the choice of system and technology finally depends on the customer’s production environment, the required throughput and the degree of automation of substrate handling.
WORKING IN PARTNERSHIP IS KEY TO SUCCESS In conclusion, samplings are of utmost importance to match the ITO and DBR to the specific LED material. Evatec applications specialists are here to assist customers to identify the optimal conditions. With this approach, we manage to deliver unsurpassed performance and are proud to see our tools for ITO deposition and DBR mirror coatings on the production floor of world leading LED manufacturers.
LAYERS2016
Innovation in HBLED production using PVD of Aluminium Nitride Dr. Dominik Jaeger shows how AlN Nanosmooth™ deposition by PVD on flat sapphire (FS) and on patterned sapphire substrates (PSS) prior to epi GaN growth can be a successful alternative to MOCVD of AlN for HB-LED production. Light emitting diodes (LEDs) with a high brightness (HB) require a high light extraction efficiency (LEE) and high internal quantum efficiency (IQE) of the device. Patterned sapphire substrates (PSS) are often used as a substrate for HB-LEDs since PSS improves the LEE and the IQE. Within a HB-LED a stack of highly crystalline layers forms the active region where photons are created and the light is emitted. This stack is called a multi quantum well (MQW) and contains metalorganic vapour phase epitaxy (MOVPE)-GaN layers. In order to obtain a high IQE it is essential to grow defect free GaN-MQWs, since defects in the crystal cause both photon scattering and absorption.
Figure 1: SEM image of a cross section of a patterned sapphire substrate (PSS).
Patterned substrates typically have an array of cones that are separated by a few micrometers (see Figure 1). Light is refracted when passing through an interface. The diffraction depends on the refractive index and the angle of incidence. PSS have both an enhanced surface area and tilted cones, resulting in a change of the angle of incidence, and hence allow higher LEE compared to the flat substrates.
Additionally the patterned structures reduce the threading dislocations density (TDD) in the GaN MOCVD process [Shin2007]. Adding an AlN layer to the PSS allows for a further defect reduction [Lee2014] and wavelength binning [Preble2011]. This results in a 5-15% increase in LED brightness for AlN films on patterned structures compared to plain structures [Preble2011].
GaN GROWTH HABITS ON PSS Different growth types of GaN on PSS are known [Shen2011], e.g. side-wall growth or bottom growth (Figure 2). The growth type depends on the substrate properties (e.g. roughness and pattern geometry), GaN MOCVD process parameters and, if used, on the AlN film properties. In the case of a side-wall growth (Figure 2a) the crystals grow on the cone sides in multiple directions until they touch each other. This results in a high TDD and a rough, hazy GaN surface. This polycrystalline GaN material is not suitable for HB-LEDs. In order to obtain a defect free crystalline GaN structure it is essential that the GaN growth occurs only at the bottom (groove growth, Figure 2b). The combination of a vertical and horizontal GaN MOCVD-growth allows for nearly defect free GaN structures.
CONTROLLING THE GaN GROWTH HABIT WITH AlN PVD PROCESSES Nanosmooth™ AlN films were deposited on PSS using a CLUSTERLINE® equipped with a carbonheater module, allowing for uniform deposition temperatures of up to 900°C. The quality of the AlN film deposited is critical - resulting in either in a side-wall growth (Figure 3) or a purely groove growth (Figure 4) under the same GaN MOCVD conditions.
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COMPETENCES IN OPTOELECTRONICS
Figure 5 (left): Excellent temperature distribution with carbon heater achieved
a)
b)
Figure 2 (above): side view sketch of a thin AlN film (black) on PSS (grey) covered with GaN (orange) with two growth types: a) side-wall growth and b) groove growth. Figure 3 (right-top): top view of GaN growing on the sidewall of the structure of an AlN PSS template. GaN was colored in orange on one cone for a better illustration. Figure 4 (right): top view of GaN purely growing in the grooves of the structure of an AlN PSS template. GaN was colored on the top left side of the image for a better illustration.
Due to the low lattice mismatch between Al2O3 and AlN, growth is epitaxial on c-oriented sapphire surfaces. The AlN lattice is strained due to the epitaxial growth (Koehler stress) and also due to thermal expansion (thermal stress). XRD measurements show that the lattice distance of AlN is influenced by the deposition temperature and the film thickness [Kong2015]. The deposition process has to be fine tuned in such a way that the AlN film deposited at the PSS is highly crystalline in the PSS grooves and at the same time of poor quality at the side-walls of the cones. Modifying the AlN growth process allows us to control growth of the GaN only in the grooves and not on the sides (Figure 4).
CLUSTERLINE® IS READY PVD of AlN on CLUSTERLINE® is an ideal precursor to the subsequent groove growth of GaN by MOVCD essential for the successful manufacture of HBLEDs. The CLUSTERLINE® platform is ready to process 2, 4, 6 and even 8 inch substrates using carriers or direct substrate handling at typical throughputs of up to 432 x 2 inch wafers per hour according platform configuration.
LAYERS2016
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