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epitaxy  industry


Slashing temperatures


for nitride growth Deposition of nitride epilayer stacks by MOCVD requires high temperatures and plenty of ammonia. But these downsides can be sidestepped with an alternative growth process called migration enhanced afterglow,which has been developed by Canadian start-up Meaglow. Richard Stevenson reports.


ales of MOCVD tools have reached staggering levels. According to IMS Research, around 800 of these reactors were shipped in 2010, and this year the figure is forecast to be 4 percent higher. Many of these tools are heading to China, where they will be used to deposit epitaxial stacks of InGaN and GaN layers, which will form the key ingredient in billions and billions of LEDs.


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From a commercial perspective, it is obvious that deposition of nitride layers – partly by chipmakers in China, but predominantly by LED makers in other parts of the world – is a great success. However, that does not mean that there is no room for improvement. The MOCVD growth technique has several downsides, including high temperatures needed for deposition of the nitride layers – typically around 1000 °C. This is a big issue: GaN- based LEDs are grown on silicon, sapphire or SiC, and high growth temperatures exacerbate epiwafer bow that is caused by differences in thermal expansion coefficients between the epilayers and substrate.


This is a particular challenge for leading chipmakers that are migrating to larger diameter substrates to make LED lighting more affordable, because as the epiwafer get bigger, distortion is more pronounced. Complex buffer layers incorporating strain management can combat bowing, but a more attractive solution is to simply grow the epilayers at lower temperatures.


Slashing growth temperature has other benefits too: A greater range of substrates is then available, including ZnO, which is temperature sensitive and closely lattice- matched to GaN; and it is also possible to grow indium- rich InGaN layers at lower growth temperatures. The latter advantage will help expand the spectral range of green LEDs, and also aid the development of other classes of device, such as higher mobility field effect transistors and solar cells covering the entire spectral range.


An alternative, well established deposition technique with the potential to grow nitride films at lower temperatures is MBE. If this is to follow in the footsteps of MOCVD, growth should predominantly be N-face GaN. However, when MBE is used to form layers of such films at low temperatures, they tend to have rough surfaces. That’s because columnar poly-crystals form, which have pyramidal tops. Switching to the Ga-face – which is renowned for yielding better quality material that is suitable for the development of most GaN-based devices including LEDs and laser diodes – is problematic, because it is harder to deposit this class of material at low temperatures. It can be done, but MBE growth of Ga-face material has only been widely successful on MOCVD grown GaN templates, or on AlN buffer layers grown at higher temperatures. Direct growth of Ga-face material on nitrided sapphire is largely unheard of.


Fortunately, another growth option is now available – migration enhanced afterglow. This borrows some insights from MBE, but it is afundamentally different technology, combining far higher pressures of close to a Torr with a CVD-based plasma technique. Trailblazing this novel growth process is a spin-off of Lakehead University in Northwestern Ontario, Canada, called Meaglow. This start-up that was formed in late 2009 is now starting to commercialise its growth technique via a two-pronged approach: It is producing growth tools with low capital cost; and it has plans to offer epiwafer services for the growth of InN films later this year.


The current driving force behind the Thunder Bay start- up is Chief Scientist Scott Butcher, a veteran of InN film growth with a strong academic and industrial background: “We’ve been growing for 6 months with the prototype system, and large leaps are being made forward in these early days. However, we’re still nowhere near reaching the limits of what it can do.”


October 2011 www.compoundsemiconductor.net 37


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