substrates technology
AlN: can it become a universal
substrate for III-nitrides? Physical vapor transport can produce high-quality 2-inch AlN crystals with low dislocation densities. Substrates sliced from these crystals provide an ideal platform for the growth of ultraviolet LEDs, lasers and RF devices, says a team from Nitride Crystals.
I
II-nitrides are undoubtedly a remarkable family of semiconductors. Unlike their semiconductor cousins, high-quality films of this material can be grown on almost any substrate - sapphire, silicon and even glass.
However, despite their propensity for forming single crystals on practically anything, no III-nitrides occur naturally. Consequently, there has been continuing interest in manufacturing a native, single crystal substrate for these nitrides since the first demonstration of epitaxial growth of GaN by HVPE in 1969 by Paul Maruska and co-workers at RCA Sarnoff. Over the intervening decades nitride devices have kicked on to demonstrate unexpectedly good performance characteristics despite high dislocation densities. Blue LEDs, for example, can realize excellent reliability and power at dislocation densities of 5 x 109
/cm2 , a value that would kill light
emission in their GaAs and InP longer wavelength cousins. However, in general, semiconductor devices have shown highest performance on low-defect native substrates, so there is no reason to suspect that the III-nitrides, even with their unique growth habit, will be any different.
There are three options for native substrates: AlN, GaN and InN. To date, no one has produced bulk crystal InN; GaN has been grown from solution at high pressure; and AlN has been produced using relatively straightforward physical vapor transport (sublimation). Another strength of AlN is that it is the most promising universal substrate for epitaxy of a wide variety of nitride devices, including LEDs, lasers, RF and surface acoustic wave (SAW) devices. Of all the III-nitrides, it has the largest bandgap; highest thermal conductivity, breakdown electric field and SAW velocity; and the smallest a-lattice parameter. What’s more, deposition of high-quality epilayers on this platform is relatively easy, because all AlInGaN compositions are in compression when grown on AlN, thus minimizing cracking probability.
However, although the PVT process used to grow true bulk AlN crystals was first identified by Glen Slack and co-workers at GE Research Lab more than 35 years ago,
it has proven extraordinarily difficult to grow large diameter, low-defect crystals, even with well established experience in SiC growth. Initially, there was little interest in scaling crystal dimensions of AlN, but this has now changed thanks to the dramatic success of the nitrides.
Today, a handful of companies have reported success in developing at least small, high-quality AlN substrates. These include the US firms Crystal IS, Hexatech and Fairfield Semiconductor; the Japanese materials specialist Sumitomo Electric Industries; the German outfit CrystAlN; and ourselves, Nitride Crystals, which has bases in both the US and Russia. To our knowledge, we, along with Fairchild, are the only companies shipping AlN substrates on a commercial basis. We focus on sales of round substrates, while Fairchild ships 10 mm squares. Of the other players, CrystAlN has recently entered the market, and Crystal IS and Hexatech have reportedly established internal production of AlGaN devices such as deep UV LEDs. Perhaps the fact that both Crystal IS and Hexatech have decided to focus their AlN wafer manufacturing toward their own device products is the clearest indication of the potential importance of that material.
Our approach to operating in the AlN substrate and device market is based on this belief: The AlN substrate and device markets will never be significant unless major device players and substrate manufacturers adopt the technology. Therefore, we place no restrictions on how
Figure 1 AlGaN epitaxy on AlN
November / December 2010
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