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GaN Substrates To Challenge Silicon

Falling bulk gallium nitride costs could soon see the high bandgap substrates displacing cheap silicon wafers. Compound Semiconductor investigates.

It's no secret that nitride-based devices pack a powerful performance compared to silicon-based counterparts, but overwhelmingly high substrate costs have left the industry floundering.

While today's six inch silicon wafer costs as little as US$25, a relatively modest two inch GaN substrate will set a device maker back by at least US$1900. But this could soon change.

US-based Lux Research analyst, Pallavi Madakasira, recently forecast that bulk gallium nitride costs will fall by 60% come 2020. As she highlights, plummeting prices coupled with the promise of up to 380% performance gains means GaN substrates will at last be ready to displace its cheaper silicon competitor. But how, exactly, can manufacturers slash costs?

Madakasira believes better use of materials is crucial. As she highlights, wafer wastage levels during GaN substrate production are currently high, but manufacturers could drive costs down by using waste material in subsequent production runs.

And of course, rising demand for the substrates will boost production capacities. “This is the chicken and egg problem... but cost will come down as the capacity goes up," says Madakasira.

But assuming rising demand kick-starts production, can quality substrates actually be produced? Today's GaN substrates are typically manufactured by a HVPE process but the material suffers from high dislocation densities triggered by the use of non-native seeds during crystal growth. Dislocation densities in good quality HVPE-grown substrates come in at around 106cm-2 but in practice, many free-standing HVPE GaN substrates are highly stressed and bowed.

Rival technology, ammonothermal growth, produces better quality material - with dislocation densities down to 102cm-2 - but crystal growth takes longer. And, despite best attempts from industry players, such as Poland-based Ammono, the process hasn't been proven at scale.

However, a recent announcement from Japan-based Hitachi Cable raises a question mark over the need to fabricate virtually defect-free crystals. In September, the electrical manufacturing giant unveiled the world's first GaN vertical diode, with a high reverse breakdown voltage of 3000V and a low on-resistance of 1mΩcm² in the forward direction.

As part of a manufacturing trial, GaN epitaxy layers were deposited via MOVPE onto a HVPE-grown GaN substrate, with the company highlighting how free-standing GaN substrates made in this way have the potential to produce higher performing power devices than those grown on silicon or silicon carbide. But perhaps more noteworthy is that the GaN substrate used in the sample diode had a relatively high dislocation density of 106cm-2.

Industry has not yet established the defect densities required for specific applications, but some players have suggested dislocation densities as low as 102cm-2  may be necessary. Hitachi Cable's diode could change this.

“[This] recent device is a proof of concept at this point," says Madakasira. “But perhaps it also demonstrates to industry that you don't need an ammonothermal process for every application in the world."

As such, the Lux analyst believes less price-sensitive devices, such as high-end laser diodes, ultra-high brightness LEDs and very high voltage power electronic devices, will be fabricated on the highest quality substrates grown via ammonothermal processes. Meanwhile, HVPE-grown substrates will be better suited to lower performing power electronic devices as well as high brightness LEDs, especially as the industry matures and manufacturers accept they don't need to have the lowest defect density substrate for all devices.

And as Madakasira points out, we could see these market developments emerging sooner rather than later. “I think our analyses have been pretty conservative," she concludes. “Maybe the price of these substrates will come down even faster that we have forecast."

Good quality GaN substrates, with low defect densities, are considered crucial for fabricating high performance LEDs, laser diodes and power electronics devices. But how low do substrate makers need to go?

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