Manufacturing Bits: May 25

Higher voltage GaN; vertical GaN wafers; GaN defects.


Higher voltage GaN
Imec and Aixtron have demonstrated the ability to extend gallium-nitride (GaN) to new voltage levels in the power semiconductor market, enabling the technology to compete in much broader segments.

Imec and Aixtron have demonstrated epitaxial growth of GaN buffer layers qualified for 1,200-volt applications on specialized 200mm substrates with a hard breakdown exceeding 1,800 volts. This could be a major breakthrough. GaN-based power semiconductors are widely used in applications at 900 volts and below, but it’s been challenging to go beyond those levels due to several issues.

GaN and other technologies are used in power electronics. Using solid-state devices, power electronics control and convert electrical power in systems. These include cars, cellphones, power supplies, solar inverters, trains and wind turbines.

Power semiconductors are specialized transistors that boost the efficiencies and minimize the energy losses in systems. Power semis operate like a switch in systems, allowing the electricity to flow in the “on” state and stop it in the “off” state.

The power semiconductor market is dominated by silicon-based devices. But power semiconductor devices based on GaN as well as silicon carbide (SiC) materials are making significant inroads. GaN and SiC power semis are based on wideband-gap technologies, which are more efficient with higher breakdown electric field strengths than silicon.

GaN, a binary III-V material, is used for LEDs, power semis and RF devices. GaN-based power semis are used in automotive, data centers, military-aerospace and other apps. GaN power semis range from 15 to 900 volts. GaN has a 3.4 electronvolt bandgap, which is higher than SiC. GaN has a breakdown field that is ten times higher than silicon, according to Infineon. The electron mobility for GaN is double as compared to silicon, according to Infineon.

GaN power semis are lateral devices. In the process flow, a thin layer of aluminum nitride (AlN) is deposited on a substrate, followed by a GaN layer. A source, drain and gate are formed on the structure, forming a lateral GaN device.

The problem? In some cases, GaN-based power semis run into issues beyond 650 or 900 volts. It has become difficult to increase the buffer thickness to the levels required for higher breakdown voltages and low leakage levels. This is due to a mismatch in the coefficient of thermal expansion (CTE) between the GaN/AlGaN epitaxial layers and the silicon substrate.

As a result, SiC and silicon-based power semiconductors remain the products of choice for 650 to 1,200-volt applications. Generally, GaN is stuck in markets below that. Imec and Aixtron hope to break down those barriers for GaN, thereby extending the range for the technology.

Vendors developed a process using Qromis’ GaN substrate technology. The substrate technology, called QST, enables power devices at 650 volts and above. The QST substrates from Qromis have a thermal expansion that closely matches the thermal expansion of the GaN/AlGaN epitaxial layers, paving the way for thicker buffer layers – and hence higher voltage operation.

Using these substrates, Imec and Aixtron demonstrated epitaxial growth of GaN buffer layers qualified for 1,200 volts applications. The result comes after the qualification of Aixton’s G5+ C metal-organic chemical vapor deposition (MOCVD) reactor at Imec. The MOCVD tool is used to integrate the material epi-stack.

This opens the door for higher voltage GaN-based power devices in applications such as electric cars and other products. Currently, lateral e-mode devices are being processed to prove device performance at 1,200 volts, and efforts are ongoing to extend the technology towards even higher voltage applications. “GaN can now become the technology of choice for a whole range of operating voltages from 20V to 1200V. Being processable on larger wafers in high-throughput CMOS fabs, power technology based on GaN offers a significant cost advantage compared to the intrinsically expensive SiC-based technology,” said Denis Marcon, senior business development manager at Imec.

Vertical GaN wafers
Today, though, lateral GaN-based power semiconductors are limited in terms of voltages, prompting the need for a next-generation technology like vertical GaN.

In vertical GaN devices, the electrons flow from the top to the bottom. But bulk GaN substrates are limited to small sizes and are expensive.

In response, Kyma has begun offering vertical GaN epiwafers. To developed these wafers, Kyma has devised a hydride vapor phase epitaxy (HVPE) process. HVPE enables the growth of GaN-on-GaN with various free carrier concentrations. This in turn enables the growth of thick films of doped GaN films for power electronics applications.

“Such films are difficult to grow using traditional growth techniques such as MOCVD, due to challenges with doping control and much lower growth rates,” according to Kyma. “With such lightly doped films now available, device manufacturers can develop GaN-based power devices with vertical architectures for applications at 1.2kV and higher such as electric vehicle chargers, onboard DC-DC converters, industrial motors, solar PV inverters, and much more.”

GaN defects
Osaka University has created a new non-destructive technique to characterize and evaluate GaN’s crystalline properties using multiphoton excitation photoluminescence (MPPL).

Researchers published its findings in Applied Physics Express.

Multiphoton excitation photoluminescence evaluates the properties of GaN using lasers that penetrate into the sample. One defect that occurs within GaN is threading dislocations. Threading dislocations are imperfections in the crystalline structure that serve as leakage current paths.

The MPPL laser highlights these defects in the GaN, even deep within the sample, making it ideal for 3D evaluations. MPPL’s method also allowed for statistical classification of defects within the GaN.

GaN has applications in automotive, data centers, and other markets. GaN power switching devices can provide high-power operations, high-speed switching, low on-resistance, and high breakdown voltage. However, the defect density in GaN must be low for it to be advantageous.

MPPL will make it possible to study and analyze GaN samples in-depth in a non-destructive way. It will also make it easier to identify defects that affect the reliability of GaN, improve yields and provide more efficient paths to GaN devices.

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