technology industry
While stress engineering may be the most challenging aspect of
forming GaN-on-silicon devices, it is arguably not the hottest topic. That accolade goes to passivation of surface states
Armed with this method of working, we were able to simultaneously improve: Material aspects, such as channel conductivity and defect density; device related aspects, including in-situ passivation, dynamic performance, enhancement-mode devices, high-voltage operation, power density and operating frequency; and last, but by no means least, reliability. During this effort we paid careful attention to the reproducibility of the results. This approach brought us much success, including state-of-the-art device performances and the unveiling of a device concept for an enhancement-mode 600 V HEMT.
The commercial promise of these achievements has helped us to attract a strong, balanced consortium of investors. Our vision is shared by Robert Bosch Venture Capital, Capricorn Cleantech fund and LRM. Their funding in 2011 has been used to set-up a new plant for producing GaN epiwafers by MOCVD in Hasselt, Belgium.
Taking the strain
Our growth process deals with the strain that arises when GaN is deposited on silicon. These two materials have significantly different crystalline properties and thermal expansion coefficients. Left unchecked, this can lead to strain in the epilayer and substrate that can ultimately cause the wafer to bow and even crack.
By carefully managing this strain, we can make wafers that are suitable for passing through silicon processing lines. Day-in, day-out, we can manufacture 150 mm
epiwafers with a bow well below 50 µm – it is typically 20-30 µm, depending on wafer specs. Uniformity, in terms of standard deviation of either layer thickness or electrical characteristics, is typically better than 3 percent.
These epiwafers are optimized for high voltage/high frequency operation. This requires the formation of a buffer layer that withstands a very high voltage. It is possible to realise this: We have observed that in devices formed with high-quality GaN layers, the upper breakdown voltage is actually limited by the silicon substrate. The epiwafers that we produce for high- voltage devices have a buffer leakage current well below µA/mm at 600 V. Operating frequencies in excess of 100 GHz are also possible with our material, thanks to a reduction of buffer traps.
While stress engineering may be the most challenging aspect of forming GaN-on-silicon devices, it is arguably not the hottest topic. That accolade goes to passivation of surface states. GaN is a piezoelectric material that features an excellent high-electron-concentration – associated with high electron mobility – that is obtained ‘for free’, without the need for any extra doping. But there is a significant price to pay: An extreme sensitivity governing device characteristics, such as current density and threshold voltage, on the filling of those surface states, which have a density comparable to that found in the channel. If passivation is poor, the device’s dynamic behaviour suffers. To combat this so-called dispersion problem, devices must be processed in a carefully controlled manner using high-quality epiwafers, because this leads to optimised buffers and controlled surface states.
Figure 2. Electrical uniformity mapping of 150 mm
GaN-on-silicon wafer,
measured contact-less sheet resistivity
16
www.compoundsemiconductor.net March 2012
Fail to do this, and uncontrolled charging or discharging of these surface states – which can be modified during processing and device operation – can severely degrade the dynamic properties of the device. To prevent this from happening we deposit a unique in-situ SiN capping layer, which is grown by MOCVD as part of the epitaxy process, on top of HEMT epiwafers. The interface between this capping layer and the top nitride surface is incredibly smooth, and it enables perfect passivation of surface states (see Figure 1).
We have shown that this capping layer can properly control the filling of the surface states during device operation. In fact, it is believed that SiN can provide enough charge to neutralize the surface charge of the
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