Manufacturing Bits: Sept. 3

Modeling SiC defects
The Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) has developed a model that reveals the nature of crystal defects in silicon carbide (SiC).

Defectivity is an issue for SiC, a compound semiconductor material based on silicon and carbon. Today, SiC is used to make specialized power semiconductors for high-voltage applications, such as electric vehicles, power supplies and solar inverters.

SiC stands out because it’s a wide band-gap technology. Compared to conventional silicon-based devices, SiC has 10 times the breakdown field strength and 3 times the thermal conductivity.

4H-SiC, the most common polytype used today, is transparent with a high refractive index. During the manufacturing flow, though, SiC substrates are prone to various defect types, such as crystalline stacking faults, micropipes, pits, scratches, stains and surface particles. A micropipe is a type of helical dislocation.

IFJ PAN, meanwhile, has developed a new model, enabling them to study SiC crystals using “ab initio” calculations. Ab initio calculations use quantum mechanics equations. The model enables researchers to look at complex defects like edge dislocations in SiC. It allows them to explain the characteristics during processing at the atomic scale.

SiC crystals consist of several flat layers. They are arranged on top of each other. Each layer resembles a honeycomb, according to IFJ PAN.

The SiC crystal model from the research organization consists of about 400 atoms. “The simulations showed that in the layers of crystals, along the edge of the core of the defect, ‘tunnels’ appear in the form of channels with reduced charge density,” according to IFJ PAN. “They lower the potential barrier locally and cause electric charges to ‘leak’ from the valence band. In addition, in the forbidden gap, which in the insulator guarantees a lack of electrical conductivity, conditions appear which reduce its width and effectiveness in limiting the flow of charge. It was shown that these states originate from atoms located in the dislocation core.”

That’s a possible breakthrough in the arena. “We tried to find the mechanisms responsible at the atomic level for lowering the breakdown voltage in silicon carbide crystals. Our ab initio calculations lead to a qualitative understanding of the problem and contribute to explaining the details of this phenomenon,” said Jan Łażewski, professor at the IFJ PAN.

“The situation can be compared to a deep, steep ravine that a squirrel is trying to cross. If the bottom of the ravine is empty, the squirrel will not get to the other side. However, if there are a number of trees at the bottom that are high enough, the squirrel can jump over their tops to the other side of the ravine. In the crystal we modelled, the squirrels are the electrical charges, the valence band is one edge of the ravine, the conduction band is the other, and the trees are the aforementioned states associated with the atoms of the dislocation core,” said Łażewski.

“When modelling such structures, one of the main problems is computational complexity. A model of pure crystal, devoid of admixtures or dislocations, is characterized by high symmetry and can be calculated even in a few minutes. In order to carry out a calculation for a material with dislocation, we need months working on a high power computer,” added Paweł Jochym, a professor at the IFJ PAN. “The future will verify whether our ideas will be confirmed in their entirety. However, we are confident about the fate of our model and the presented approach to simulating edge dislocations. We already know that the ab initio model has proved its worth in confrontation with certain experimental data.”

Gallium oxide MOSFETs
The Berlin-based Ferdinand-Braun-Institut has developed what the research organization says are gallium oxide power transistors with record values.

Still in R&D, beta gallium oxide is creating a buzz for use in power semiconductor applications. It is a wide-bandgap technology, meaning that it’s faster and provides higher breakdown voltages than traditional silicon-based devices.

Other wide-bandgap technologies are shipping. Two wide-bandgap types—gallium nitride (GaN) and SiC MOSFETs—are ramping up in the power semi market today.

Crystalline beta gallium oxide is also promising. It has a bandgap of 4.8 – 4.9 eV with a high breakdown field of 8 MV/cm. This 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.

Researchers from Ferdinand-Braun-Institut have devised a ß-Ga2O3-MOSFET (metal-oxide-semiconductor field-effect transistor). The research firm obtained gallium oxide substrates from the Leibniz Institute for Crystal Growth. The substrates had an optimized epitaxial layer structure, resulting in a lower defect density level with good electrical properties.

The gallium oxide MOSFET has a breakdown voltage of 1.8 kilovolts with a record power figure of merit of 155 megawatts per square centimeter, according to the organization.

This is close to the theoretical material limit of gallium oxide. “Sub-μm gate length combined with gate recess was used to achieve low ON-state resistances with reasonable threshold voltages above −24 V,” according to Ferdinand-Braun-Institut. “The combination of compensation-doped high-quality crystals, implantation-based inter-device isolation, and SiN x-passivation yielded in consistently high average breakdown field strengths of 1.8–2.2 MV/cm for gate–drain spacings between 2 and 10μm.”

AIN lab
The Leibniz Institute for Crystal Growth has launched a new crystal growth laboratory with three reactors for the sublimation growth of aluminum nitride (AIN) monocrystals.

The lab will develop a process for the production of low-defect AlN substrates with a diameter of 10mm to 1-inch (25.4 mm) and defined properties (doping, orientation, geometry, surface quality).

AIN has a high thermal conductivity. It has good electrical insulation properties. Applications include power modules, LED packages for cooling and protecting circuits, among others.