INDUSTRY MBE FOR NITRIDES
Working with ammonia Our tools are not based on the most common form of nitride MBE growth, PAMBE, but are based on an alternative approach known as ammonia MBE. We are not the sole pioneers of this deposition technology, but use far higher temperatures and III-V ratios than is normal with this technique.
The typical approach is to use growth temperatures that are 100-200 °C higher than those associated with PAMBE and III-V ratios a little higher. With these conditions, growth rates are 1 µm/hr or more. In this regime, researchers don’t tend to crank up the III-V ratios, which restricts growth temperatures to levels considerably lower than those found in MOCVD tools, where they are often above 1000 °C.
A major downside of keeping the temperatures below this is the high density of dislocations arising in nitride films – they tend to be more than an order of magnitude higher than those produced in epiwafers grown by MOCVD.
In comparison, our tools can get very close to MOCVD growth temperatures. This was a primary goal of ours when we started developing an MBE reactor for nitride growth in the late 1990s. Back then, we already had a great deal of experience in the growth of epiwafers for high power InAlGaAs laser diodes, and we were well aware that higher growth temperatures lead to better devices.
We were also aware of a ‘gap in the market’ when we commenced our development of a GaN tool. At that stage, there were no commercially available MBE systems that were capable of the growth conditions that we believe are best for nitride growth. That led us to think that there were sizeable rewards available for trying to realise an MBE system that could operate reliably in such extreme process conditions.
Initially, we focused on increasing the growth temperature of GaN that is, of course, limited by thermal decomposition on the wafer’s surface. Since nitrogen is more volatile than gallium, thermal decomposition of GaN is governed by the ammonia flux that flows onto the surface of the growing film.
To realise extremely high ammonia flows, we developed a special MBE system that features an increased area
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of liquid-nitrogen-cooled cryopanels. The pumping system based on these cryopanels, which is used in conjunction with a high-speed turbo molecular pump, can produce a high vacuum in the growth chamber even with very high ammonia flow rates.
Thanks to these features, our prototype system enabled the GaN growth temperature without thermal decomposition to increase to 970 °C, while using an ammonia flow of 400 sccm. This temperature is even significantly higher than the highest values reported for ammonia MBE – about 900 °C – while the vacuum in the growth chamber is typical for ammonia MBE, staying below 5 x 10-3
Pa. Armed
with these new growth conditions, engineers can improve the structural quality of GaN. However, the dislocation density is still higher than that for MOCVD-grown layers.
Slashing dislocation densities To significantly slash the dislocation density with ammonia MBE, we performed a series of experiments that revealed that the key is to grow an AlN buffer layer at an extremely high temperature – more than 1000-1100 °C. This type of buffer improves material quality by increasing coalescence, so nitride film deposition quickly moves to the two-dimensional growth mode.
Our tools show that with ammonia MBE, rather than plasma- assisted MBE, it is possible to grow AlN
and high-aluminium-content AlGaN with a high substrate temperature and V/III ratio in excess of unity (N-rich mode). Alternately, growth at high temperatures in PAMBE is very tricky, because the aluminium-rich mode is essential for two-dimensional growth of the AlN buffer, while aluminium desorption is significant at substrate temperatures of 900 °C or more.
To realise the extremely high substrate temperatures for AlN buffer layer growth we had to develop a specialized MBE system. In this system, ammonia flows are lower for the growth of an optimal AlN layer than for GaN growth, while large area cryopanels enable typical vacuum levels for MBE, even at extremely high substrate temperatures.
This vacuum level, combined with an aluminium effusion cell, rules out the possibility of unwanted, difficult-to- address parasitic reactions that typically occur in the MOCVD reactor between tri-methyl-aluminium and ammonia.
Another of our tool’s features is that it opens the door to the growth of high quality structures for microelectronics or optoelectronics that feature an active region grown by either ammonia or PAMBE. Such structures, which can be grown in a single run, can be used to make ultraviolet emitters and detectors and microwave transistors. These devices demand very high quality layers of AlN and high-aluminum-content AlGaN.
SemiTEq’s has recently released its advanced compact MBE System, the STE75
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