technology GaN transistors
One of the hallmarks of the MORGaN project has been its focus on materials. More than half of all resources have been devoted to substrate development, strain management, heterogeneous semiconductor growth, refractory metals, three-dimensional metal manufacturing and ceramic packaging. This approach has born much fruit: Europe’s first 2-inch silicon- polydiamond composite substrates, the world’s first GaN HEMT grown on single crystal diamond operating in the microwave region, and InAlN/GaN devices operating in continuous wave at 3.5 GHz with an output of 6.6 W/mm and a power added efficiency of 70 percent (see the box “MORGaN’s milestones for more details”).
Many applications are set to benefit from the successes of the MORGaN project. High-power, high-efficiency amplifiers based on InAlN HEMTs promise to cut the carbon footprint of mobile communication base-stations and various forms of power electronics used in consumer applications. In addition, first version pressure sensors capable of operating at temperatures up to 700 °C under pressures of several tens of bar have been fabricated, which should aid oil exploration and space missions, and also permit measurements in automobile and jet engines. And last but by no means least, MORGaN has spawned incredibly robust chemical sensors offering pH measurements over a large dynamic range.
Figure 2.A scanning electron microscopy image of a crack induced by tensile strain that terminates inside the silicon (111) layer
Building on diamond MORGaN’s milestones
The first European demonstration of 2-inch silicon/poly-crystalline diamond composite substrates
The first HEMT operating in the microwave region that was formed by direct growth of a GaN heterostructure on a piece of single crystal diamond
Free-standing epitaxial overgrowth GaN beams and cantilevers The first InAlN/ GaN HEMT coated by nanocrystalline diamond with current gain cut-off frequencies in the 20 GHz range
A top nanocrystalline diamond heat spreader with a thermal conductivity of 500 W m-1
K-1 , indicating that it is possible to preserve
the electrical properties of the device while decreasing its thermal resistance.
The first InAlN/GaN active devices operating in continuous wave with an output of 6.6 W/mm at 3.5 GHz and a power added efficiency of 70 percent
A medium-size amplifier delivering up to 320 W output power, a figure in line with non-linear circuit design expectations
Development of novel, high-temperature (800 °C) diffusion barriers and metallisation technology
Construction of harsh-environment drumskin and cantilever sensors housed in a package.
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www.compoundsemiconductor.net April / May 2012
Leading the development of ultra-high-conductivity platforms is the firm Element Six, which has optimised monocrystalline diamond substrates for the direct growth of GaN on diamond. Although the size of the substrates is relatively small – just 4 mm by 4 mm – electrical performance is promising. Larger sizes are possible by depositing diamond layers on silicon, and this approach has enabled Element Six to produce 2-inch free-standing wafers featuring a 2 µm-thick (111) silicon surface on a 70 µm polycrystalline diamond layer. Scaling to larger substrate sizes, such as 100 mm, will require additional work. One great attribute of the 2-inch polycrystalline composites is a thermal conductivity that reaches 1000 W m-1
K-1 .
Researchers at the University of Bath, UK, have deposited nitride layers on these complex silicon (111)/ polycrystalline diamond composite substrates. Highlights include the formation of high-quality, crack- free AlN and GaN layers, the latter of which is 350 nm thick (see Figure 2).
MBE growth, using either ammonia or RF sources, has been used by engineers at EPFL, CH and FORTH to deposit nitride epi-structures directly onto single crystal diamond. A low-temperature AlN buffer layer was deposited first, followed by strain-engineered interlayers that allowed the subsequent GaN layer to be formed under compressive strain, prior to the growth of an 800 nm-thick GaN layer. On top of this went a HEMT structure, composed of a 24 nm-thick Al0.28 followed by a 2 nm-thick GaN cap (see Figure 3).
Ga0.72 N layer
Hall Effect measurements revealed room-temperature electron mobility of 731 cm2
V-1 s-1 (1740 cm2 V-1 s-1 ) and a
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