Compound Technologies article: Porotech: A different kind of GaN

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A relatively unknown material from a newly minted UK start-up is set to makes waves in the rapidly growing GaN market.

With a host of novel properties that open the door to high performance optoelectronic devices, ‘porous GaN’ is attracting more and more interest in compound semiconductor circles. And thanks to £1.5 million in investment funds, University of Cambridge spin-off, Poro Technologies – or Porotech – is on course to be one of the first companies to bring this mesoporous version of GaN to market

Porous GaN can be regarded as a semiconductor composite of solid GaN and air. As Professor Rachel Oliver, Co-founder and Chief Scientific Officer of Porotech, and Director of the Cambridge Centre for Gallium Nitride puts it: “Porous GaN is basically GaN with holes in it that are a few tens of nanometres across.”
“With porous GaN we can engineer a wide range of material properties… and offer a new material platform to build semiconductor devices on,” she adds. The Porotech team creates the nanoscale porosity in GaN wafers using electrochemical etching. The etch is conductivity selective and responds differently to the material depending on its doping density.

Porosity is created in doped layers while undoped layers are left undamaged, allowing complex 3D nanostructures to be created.

According to Oliver, the etchant flows to and from the doped layers via the many nanometre-scale channel-like defects – dislocations – that exist within any GaN wafer.

“Even your best quality GaN wafer will still have around 105 dislocations per square centimetre,” she explains. “So the etchant will flow down a dislocation and when it hits the doped layer will etch it very quickly to create the porosity before continuing down that channel to the next doped layer.”

“We can take an entire wafer and using this conductivity selective etching mechanism to create GaN [structures] with a whole new set of properties that haven’t been available before,” she adds. “It’s very cool from a commercial perspective.”

Indeed, both Oliver and Porotech chief executive, and co-founder, Dr Tongtong Zhu, are certain their porous GaN fabrication process lends itself to commercial exploitation. The method has been tried and tested wafers up to eight inches in diameter and Zhu reckons it will seamlessly scale to even larger wafer sizes.
“The first thing that inspired us to pursue this process commercially is that it works with wafers,” says Zhu. “What’s more, during the process we preserve the surface quality and the integrity of the materials so anyone can take the porous GaN wafer and insert it into their production processes without any disruption.”