Author: Gary Tompa Structured Materials Industries (SMI) GaN LEDs are a great success. They have created a multi-billion dollar market that is now widely used, including mobile phone displays and buttons, small screens and architectural lighting systems. However, if LED sales continue to grow rapidly, this is bound to lead to the next profitable application, the $60 billion solid-state lighting market. To succeed in this area, cheaper and even more efficient LEDs are needed, which can only be achieved through improvements in high-volume manufacturing technology. Figure 1. LED design includes improved light extraction efficiency, but all forms have the same basic epitaxial layer structure (a). Metal electrodes can be applied to this structure, but will block light generation in the active region and reduce the effective radiation area (b). Transparent ITO overcomes this shortcoming and forms the top electrode of conventional design (c) and flip-chip structure (d). At SMI, the MOCVD equipment manufacturer based in the Piscataway area of ​​New Jersey, we developed this technology, a production method compatible with deposited ZnO electrodes. These electrodes are transparent, and when they replace those based on light-shielding metal films, the light output of the LEDs will increase by 80%. LED design Figure. ZnO thin films of different morphologies grown by MOCVD, First generation LED devices, such as alloyed electrodes, are characterized by contact with p-type layers and n-type layer electrodes, which can impede or attenuate light output and reduce the radiation area of ​​the device (Figs. 1a and 1b). Due to the low conductivity of the p-type electrode layer, such devices are also capable of producing weaker radiation away from the electrodes. Both of these defects can be solved by replacing the metal layer on the surface to increase the current propagation velocity inside the structure; however, while obtaining a more uniform optical radiation, the radiation attenuation caused by the new electrode also follows. The only way to deal with the inherent disadvantages of metal electrodes is to replace it with a transparent conductor (Fig. 1c). Tin indium oxide (ITO) is an obvious choice because it has been widely used in flat panel displays and optoelectronic devices. LEDs using this material will increase their output power by 30-50%, but deposition techniques for electrode fabrication, electron beam evaporation or sputtering do not consistently achieve high quality films, which is a major aspect of high volume production. problem. However, when we look further, we will find that ZnO can overcome this problem and show some other advantages. Figure 2. Transparency of ZnO thin films grown by MOCVD ZnO and ITO contest A key advantage in the manufacture of ZnO over ITO is the better, renewable growth process. ITO is deposited by a PVD process, such as MBE and electron beam evaporation, or by sputtering. All of these techniques tend to produce inferior films on differently shaped surfaces, as those found on the upper surface of the LED. Due to the drawbacks of the above steps, poor electrode reliability is produced to limit the yield of the device. It is also difficult to carry out large-capacity production of ITO by MBE and electron beam evaporation, and the sputtering process actually impairs device performance. MOCVD is the method we use to deposit ZnO in SMI and is not affected by all the events in Table 2 (Table 2). Since these growth methods are versatile, they are also suitable for mass production and are extremely compatible with the fabrication process of GaN devices. LEDs produced by this method eliminate the post-growth annealing process used to activate p-type doping because MOCVD itself is a thermally driven process. ZnO thin films grown by MOCVD also produce a series of topography, including highly crystalline continuous films, closely packed crystal columns, and nanowire columns. This allows the surface of the LED electrode to have some characteristic (roughening), which can further improve the extraction efficiency. We have developed the MOCVD process to produce three different types of ZnO surface topography and plan to investigate the effects of different electrode topography on LED performance. We demonstrate the benefits of ZnO electrodes by depositing ZnO electrodes on GaN epitaxial wafers using our own high-speed rotating disk reactor. The reactor produces a wafer with a capacity of 38 × 2 inches (see the front page, "SMI's ZnO MOCVD Reactor" "detail). These aluminum-doped ZnO electrodes have a certain percentage of thickness uniformity, a deposition growth rate of 10-20 nm/min, and finally an ohmic contact having a resistivity of less than 10-3 Ω/cm, which is effective. Leading to a higher output path <br>These initial results illustrate the potential of ZnO electrodes, but we believe that many LEDs can be changed by changing the bandwidth of the ZnO alloy and by combining photonic crystal structures and direct deposition of illuminant structures. Performance is likely to increase. CdZnO and MgZnO alloys can also be grown by MOCVD, which will help to adjust the bandwidth and optical properties of the electrodes to optimize the precise design of the LEDs. Rare earth elements, such as lanthanum, manganese and cerium, can also be added to the growth process of ZnO, using a single or dual deposition system to produce luminescent electrodes. Uninterrupted vacuum growth and passivation of an optical emitter is undoubtedly an attractive option. We have created a simple structure that includes a green illuminant, ZnSiO:Mn, which produces cathodoluminescence, electroluminescence and photoluminescence. . Figure 3. LEDs using ZnO electrodes show higher light extraction efficiencies than LEDs with equivalent metal and ITO electrodes. At 40 mA, the luminescence intensity of the GaN LED (a) of the nickel/gold electrode is 191 mcd, and the equivalent of the ZnO electrode (b) produces a light intensity of 355 mcd, which saturates the camera signal. The light intensity of the LED of the ITO electrode is 271 mcd, but it is not shown in the figure. SMI ZnO MOCVD reactor
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The drawbacks of various forms of metal electrodes are now exposed in many LEDs, which are typically fabricated on transparent insulating sapphire substrates. A buffer layer is grown on the sapphire, followed by n-type GaN, multiple quantum well active regions, and a p-type layer that is difficult to do with high doping, and LEDs are produced. The electrodes must be added before the LED chip package. This design promises to break through in efficiency and even release efficiencies as high as 200 lm/W. However, today's LEDs are capable of releasing only half or less of this value, in part because a large portion of the optical radiation exiting the active region is captured in the structure.
Including high crystalline continuous film (a),
Tightly aligned crystal columns (b) and nanowire columns (c).
Thin film electrodes also increase the chance of catastrophic burnout under high current conditions. Thickening the electrode layer can reduce this risk, but at the same time reduce the efficiency of light extraction (or light extraction). While some techniques, such as the introduction of photonic crystals or surface roughening, can reduce light trapping in the device and improve light extraction efficiency, LEDs of this construction will not operate at their theoretical maximum.
85-90%, it is easy to etch.
Some manufacturers have completely deviated from the traditional metal electrodes and replaced them with a high mirror. This includes LEDs that take light through a sapphire substrate (Figure 1d). This method does reduce the current spreading limitations of the p-type layer and enhances the uniformity of optical radiation across the cross-section of the LED. However, the efficiency of light extraction is still far from ideal. As the chip size increases, current spreading in the n-type layer begins to limit its performance. Some LED developers have used "drilling" methods to form a variety of n-type electrode holes, but each new electrode hole is a black dot, which reduces the total light output according to the theory of optical radiation propagation.
The incorporation of mirrors into a flip-chip structure is also a feature of commercial design. The p-type layer is once again uniformly connected, but this time it provides a mechanical support for the chip with a thicker metal layer. The sapphire is then removed, allowing direct connection to the n-type layer. This exposed film will have a high defect density buffer layer on its surface, which will cause light loss, but this will be overcome by polishing. The top electrode is still required, and the transparent electrode is used again for maximum light extraction efficiency. Surface roughening and photonic crystal structures have also been added to this camp, all of which contribute to improved light extraction efficiency.
The different LED designs we have just described are all trying to overcome the problems associated with metal electrodes. But switching to a high-quality, high-conductivity transparent electrode is still the best way to overcome this problem. We believe that this method, combined with photonic crystal structure or surface roughness, will come at the brightest and most efficient LED. Play a major role in the process.
ZnO is an ideal electrode material because it is transparent to the entire visible spectrum; in the ultraviolet range, it is often used to pump phosphorus from white LEDs to produce optical radiation. ITO can do this too, but ZnO has several advantages, including good thermal conductivity, a small lattice mismatch with GaN, and superior high temperature stability (Table 1). In addition, ZnO is capable of dry and wet etching and is doped with aluminum, indium and gallium to improve conductivity.
Table 1. Selection of LED electrodes
Table 2. Different ZnO deposition techniques
We compared the LEDs of these ZnO electrodes with two different control devices made with conventional nickel gold films and ITO. In the drive current range of 10-80 mA, the light output from the LED using the ZnO electrode is 80% and 30% higher than that of the metal and ITO, respectively (Fig. 3). In a conventional burn-in test, ZnO-based LEDs also produced gains of five times or more over their lifetime. By adding ZnO electrodes, the power and lifetime of LEDs have been greatly improved, and we will apply for patents to protect our technology.
Our results demonstrate that ZnO electrodes can improve the performance of GaN LEDs. We are working with several companies and research groups on ZnO electrode material sampling, including companies or research groups in China, Taiwan, Japan, Korea, and the United States. We work with these organizations to develop methods that can release the advantages of ZnO electrodes, especially in special device structures that are completely different from each other. We hope this will drive the sale of our multi-wafer ZnO MOCVD reactors, radically improving the performance of commercial LEDs containing phosphor transparent electrodes and photonic crystal structures.
About the Author
Gary Tompa is the President of SMI and was founded in 1994. He is grateful to the company's SSun, GProvost, DMentel, BWillner and NSbrockey, as well as Philip Chan, Keny Tong, Raymond Wong and ALee from Podium Photonics for their efforts in the development of ZnO.
Reference reading
JO Song et al. 2003 Appl. Phys Lett. 83 479.
JH Lim et al. 2004 Appl. Phys Lett. 85 6191.
CJ Tun et al. 2006 Proc. SPIE 6121 287.
K Nakahara et al. 2006 Proc. SPIE 6122 79.
GS Tompa 2007 Mat. Res. Symp. Proc. 957 K09-05.
Structured Materials Industries produces ZnO reactors characterized by high-speed rotating discs of various sizes, 1 × 1, 1 × 2, 3 × 2, 6 × 2, 8 × 2, 19 × 2 And 38 x 2 inches. These rotatable CVD devices use a uniformly heated deposition platform to grow a large area of ​​uniform film at a deposition rate of 10-20 nm per minute.
This uniform film deposition is derived from a four-band heating system that is compatible with conditions associated with ZnO growth such as high temperature, thermal expansion, and oxidizing environments. With this design, the largest reactor can also achieve a uniform temperature on the wafer, the wafer temperature non-uniformity is less than 1%, and the resulting doped and undoped film can vary in thickness by less than 4%. Even in a wafer coming out of a furnace, the thickness unevenness between the sheets is less than 5%.
The ZnO growth conditions of the LED electrode are: growth pressure of 0.05 atm; temperature ranging from 450 ° C to 650 ° C; flow rates of diethyl zinc, trimethyl aluminum and oxygen are 80, 15 and 200 standard cubic centimeters per minute, respectively ( Sccm) for providing a Zn source, an Al dopant, and an O source. The flow rate of argon ranges from 4,000 to 7,000 sccm and is used as an inert carrier gas. The source material is uniformly injected in a radial shape distribution, and the downward direction transport airflow is used to obtain high quality film growth.
Fig. Resistance heating is used in the reactor with a rotation speed of hundreds of discs per minute. This multi-wafer growth device is capable of depositing ZnO on silicon, quartz and GaN.