Manufacturing Bits: Feb. 25

Diamond finFETs; finding defects in gallium oxide.


Diamond finFETs
HRL Laboratories has made new and significant progress to develop diamond finFETs.

HRL, a joint R&D venture between Boeing and General Motors, has developed a new ohmic regrowth technique for diamond FETs. This in turn could pave the way towards commercial diamond FETs. Applications include spacecraft, satellites and systems with extreme temperatures.

Still in R&D, diamond field-effect transistors (FETs) have many intriguing properties in power and other applications. These are ultra-wide-bandgap devices with a high breakdown field and thermal conductivity. Diamond, gallium oxide and others have superior performance compared to traditional silicon-based devices as well as wide-band gap materials like gallium nitride (GaN) and silicon carbide (SiC).

Silicon has a bandgap of 1.1 eV. In comparison, SiC has a bandgap of 3.3 eV, while GaN is 3.4 eV. Compared to conventional silicon-based devices, SiC has 10 times the breakdown field strength and 3 times the thermal conductivity. Meanwhile, GaN, a binary III-V material, has 10 times the breakdown field strength with double the electron mobility than silicon.

In comparison, diamond has a wide bandgap (5.45 eV), a high breakdown field (10MV/cm), and a high thermal conductivity (22W/cmK). In diamond FETs, most have focused their efforts on hydrogen-terminated FETs (HFETs), which in turn creates a “p type conductive channel in diamond by hydrogen surface termination,” according to HRL in Scientific Reports, a technology journal.

But there are some reliability issues for these devices when they run in high-temperature conditions. This and other challenges are why diamond devices are still in R&D.

To overcome those issues, HRL two years ago developed a fully-depleted diamond finFET device with a 100nm fin. This was based on a p+/p− diamond thin-film on a diamond substrate, according to HRL. But during the process, the top surface was prone to damage and defects.

To prevent those issues, HRL has devised an ohmic regrowth technology. In the lab, researchers used a 10 × 10mm2 undoped diamond substrate. “The p+ layer was regrown with microwave plasma CVD using a patterned SiO2 mask,” according to HRL in Scientific Reports. “Ti/Pt/Au was evaporated to form a good ohmic contact after 525 °C annealing in argon gas after regrowth and mask removal.”

A gate dielectric was deposited using atomic layer deposition (ALD). “To conformably wrap the gate around the sidewalls of the fins, Al metal was sputtered with a photoresist in place, then the metal was lifted off. Finally the ohmic contact pads were open with wet etching,” according to HRL in Scientific Reports.

“For this new advancement of the technology we were able to use a method called ohmic regrowth to greatly improve the conductivity of the finFET,” said Biqin Huang, HRL’s principal investigator for the finFET project. “When we first conceived the finFET design, we knew we would need this ohmic regrowth process to succeed with a transistor that could perform well enough for practical applications. In our previous paper we demonstrated that the diamond transistor would work; now, we had to show that we could accomplish the ohmic regrowth process with the diamond material. Achieving this is a large step toward optimized finFET devices that can handle many possible practical applications.

“This is a win-win situation for diamond transistors because at room temperature the performance is not good enough, but at those higher temperatures the device actually works better,” Huang said. “Thus, high-temperature applications will be of great interest to us as we go into in the future.”

Finding defects in gallium oxide
A group recently reported on the first observations of point defects in gallium oxide.

Still in R&D, crystalline beta gallium oxide is a promising wide bandgap semiconductor material, which is used for power semiconductor applications. Gallium oxide has a large bandgap of 4.8–4.9 eV with a high breakdown field of 8 MV/cm. The technology has a high voltage figure of merit, which is more than 3,000 times greater than silicon, more than 8 times greater than SiC and more than 4 times greater than that of GaN.

The technology is still in its infancy. To bring the technology closer to the commercial market, researchers must understand the properties of the materials.

Using scanning transmission electron microscopy (STEM), researchers observed the formation of point defects in gallium oxide. “Using STEM, we discovered a new type of point defect complex that involves one cation interstitial atom, which can be positioned at two of the five possible interstitial sites, paired by two cation vacancies. The structure of this unusual cation interstitial-divacancy complex is consistent with the predictions made by density functional theory (DFT),” said Jared Johnson, a graduate research associate at The Ohio State University in Physical Review X, a technology journal.

“Our job is to try to identify why this material, called beta gallium oxide, acts the way it acts at the fundamental level,” Johnson said. “It is important to know why this material has the properties it has, and how it acts as a semiconductor, and we wanted to look at it at the atomic level — to see what we could learn. This material has very good properties for those high-powered technologies. But it is important that we’re seeing this on the fundamental level — we’re almost understanding the science behind this material and how it works, because this defect, these abnormalities, could affect the way it functions as a semiconductor.”

Ohio State, Cornell University, Lawrence Livermore National Laboratory, Thermo Fisher Scientific, and the University of California, Santa Barbara contributed to the work.

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