Like sapphire wafers and phosphors, metal-organic precursors are essential materials for the LED manufacturing process.
Look inside a typical high-brightness white LED package and you’ll find plenty of interesting materials. These might include silicone, various plastics, copper, aluminum, thermal greases, and lots more besides. Look closer and you’ll find an LED chip containing elements such as nitrogen, aluminum, gallium and indium, sitting on top of a substrate that might be made of sapphire, silicon or a suitable metal. Arguably the most exotic materials are found in the phosphors, which might contain “rare-earth” elements such as yttrium, cerium and europium, among many others.
In this article, we looked at two critical materials in the LED manufacturing supply chain, namely sapphire wafers (used as the starting template for most LED chip growth) and phosphors, a group of different materials that are used to create white LED light.
Metal-organic (MO) precursors
Metal-organic (MO) chemicals or precursors are used in the epitaxial growth of LED semiconductor materials to provide metals such as gallium (Ga), indium (In) and aluminum (Al) to the growing layers (see “Epitaxial growth” section – below).
Trimethyl gallium (TMG) is most in demand, not only for growth of most types of LEDs but also for many other compound-semiconductor devices including gallium arsenide (GaAs) electronic devices, and DVD and Blu-ray laser diodes.
During 2010, in the face of growing demand from the LED market, several major MO suppliers took steps to increase their production capacity (see “MO suppliers address demand” section – below).
Joe Reiser, Dow Electronic Materials’ global business director for Metalorganic Technologies, told us that, for the LED market as a whole, backlighting applications took off more rapidly than expected last year. This meant there was “some tightness in the market” in terms of MO supply, he said.
This situation resulted in the capacity-expansion announcements by various suppliers. However, said Reiser, “Dow’s plans were already in place before our formal announcements were made.”
In Reiser’s opinion, the tightness should resolve itself in the next couple of quarters. He explained that Dow keeps a close eye on the market for LEDs, particularly in backlighting and lighting applications, so that it can address the market demand looking several years ahead.
While Reiser was not free to discuss specifics related to pricing, it’s clear that prices have risen during the last year, as a consequence of the supply tightness. However, there have not been any significant price spikes.
When considering starting materials, MO suppliers are primarily concerned with the price and availability of Ga and In, while Al is much more widely available. Reiser said that Dow has contacted its key suppliers of these materials, and feels the company is in a “pretty good position” with substantial agreements in place. Of course, Dow is a very large, global company that enjoys substantial leverage with its suppliers.
Epitaxial growth
Epitaxy is the process of depositing a high-quality crystalline layer of material onto a crystalline substrate. In the fabrication of LED chips, epitaxy takes place inside a metal-organic chemical vapor deposition (MOCVD) system or reactor. For LEDs based on gallium nitride (GaN), the substrate is usually sapphire, and multiple layers are grown to create the specific LED structure.
Metal-organic (MO) chemicals or precursors provide the source of metallic elements such as gallium (Ga), indium (In) and aluminum (Al). In trimethyl gallium (TMG), for example, the metal Ga is attached to methyl groups (the “organic” part). In the MOCVD reactor, TMG breaks apart and Ga is supplied to the semiconductor layer growing on top of the heated substrate wafer. To grow GaN, nitrogen is also required, and this typically comes from ammonia. Other elements called dopants are also introduced to influence the current-carrying ability of different layers, creating either p- or n-type material.
All the materials introduced into the MOCVD system – MO precursors, dopants, ammonia, As and P sources (for GaAs-based LEDs), and hydrogen carrier gas – must be supplied with high purity, to prevent impurities being incorporated into the epitaxial LED layers, which would then affect the performance of the processed LED chips.
