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INDUSTRY MOCVD


Figure 1: Process gases enter the horizontal flow reactor through a three-layered gas-injection nozzle


produced per day from an MOCVD system. With a conventional reactor, the growth rate for the buffer is limited to 1 - 3 µm/h due to parasitic reactions.


However, with our systems, far faster growth rates are possible, thanks to the combination of a laminar high flow velocity and a specially designed three-layer-flow, gas-injection nozzle. Equipped with these attributes, engineers using our tools have a better control over vapour phase reactions and can realise shorter process cycle times.


Growth rates for III-Ns depend on the constituents, with


maximum values of 27 μm/h, 3.8 μm/h, and 11.4 μm/h for GaN, AlN and AlGaN, respectively (see figure 2). Employing a


very high growth rate, we have deposited a 3 μm-thick HEMT test structure on 6-inch silicon, using a growth rate of 8.5 μm/h for the AlGaN/AlN strained layer superlattice and 7.5 μm/h for GaN. The net time for film growth, which excludes temperature


ramping and reactor purging, is only 41 minutes, compared to 88 minutes for our standard growth time.


This trimming of the growth time by just over 50 percent did not lead to a significant deterioration in electrical properties. According to Van der Pauw measurements, in the high-growth- rate epiwafer the typical electron mobility was 1530 cm2 at a sheet carrier density of 8.9×1012


V-1 cm-2 s-1 . It is possible that


further optimisation of the growth process could enable even shorter growth times, and thus deliver an additional hike in the productivity of III-N wafers for electronic applications.


Superior doping control For a power switching device, the usual approach is to heavily dope the GaN buffer layer with carbon so that it is highly resistive, while employing a pure (undoped) GaN layer for the channel. Research groups have shown that increasing carbon concentration boosts breakdown voltage, while increasing growth pressure cuts yellow luminescence in GaN and suggests suppression of current collapse.


With conventional reactors, the range of carbon concentrations at a given growth rate is held back by the limited range of V/III ratios and growth pressure. It is possible to deposit GaN layers with very low carbon concentrations, but this requires low growth rates, and that leads to long growth times. In stark contrast, with our tools the range of V/III ratios and growth pressures that can be used is far wider, and this allows engineers to obtain the carbon concentration they wish at a high GaN growth rate. For example, it is possible to produce GaN films at carbon concentrations from 1016


cm-3 to 1020 with growth rates in excess of 3 μm/h (see Figure 3).


An alternative, attractive architecture for GaN-based electronics is the vertical diode. This device, which offers easy wiring and packaging and high area efficiency, consists of n-type and p-type GaN layers that are grown on a conductive GaN substrate. Challenges for manufacturing this device include: a reduction in growth time, because the n-type GaN needs to be tens of microns thick; and uniform silicon doping of GaN from concentrations of 1015


cm-3 to 1019 cm-3 . Producing films with cm-3


Figure 2. Growth rates for III-N binary and ternary films deposited in a Taiyo Nippon Sanso reactor 42 www.compoundsemiconductor.net March 2014


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