As modern electrical systems demand higher efficiency, power density, and operating temperatures, traditional silicon-based semiconductors are increasingly reaching their physical limits. This has driven intense research into wide-bandgap materials, such as aluminium gallium nitride (AlGaN), which can handle significantly higher voltages and thermal loads. However, the performance of these advanced semiconductor devices is heavily constrained by the quality of the substrate upon which they are grown. For the crystalline layers of AlGaN to deposit cleanly and function reliably, they require a substrate with a closely matching crystal lattice. If the spacing between the atoms in the substrate differs from that of the active semiconductor film, the resulting physical strain introduces structural defects, such as dislocations and cracks, which degrade electrical performance and compromise device longevity.
Scandium diboride (ScB2) is an ultrahigh-temperature ceramic that has emerged as a highly promising substrate candidate for aluminium-rich AlGaN power microelectronics due to its excellent lattice matching. However, because it is an ultrahigh-temperature ceramic, growing high-quality single crystals of scandium diboride has historically been exceptionally difficult. The extreme temperatures and complex conditions required for its synthesis have typically demanded specialised, high-pressure equipment, which limits the scale and increases the cost of production. To make scandium diboride a viable option for commercial microelectronics, researchers must find a way to grow these crystals under more manageable, scalable conditions.
Ambient-pressure crystal growth via optical heating
In a new study, which is currently available as a preprint and has not yet undergone peer review, researchers have demonstrated a method to grow single-crystalline scandium diboride at ambient pressure. To achieve this, the team utilised a laser-heated Optical Floating Zone (OFZ) system and employed the travelling solvent method. The floating zone technique is a crucible-free crystal growth method where a narrow molten zone is suspended between a feed rod of raw material and a growing crystal seed. Because the molten material is held in place solely by its own surface tension and heated by focused lasers, it never comes into contact with the walls of a physical container. This is a significant advantage when working with ultrahigh-temperature ceramics, as it completely eliminates the risk of chemical contamination from crucible materials.
To lower the required processing temperatures and control the crystallisation process, the researchers used a self-flux approach, which acts as a travelling solvent. They investigated two different starting compositions to see how the ratio of elements influenced crystal quality: a scandium-rich self-flux containing 55 to 65 atomic percent scandium, and a boron-rich self-flux containing 80 to 83 atomic percent boron. Throughout these experiments, the crystals were grown at steady rates ranging from 0.2 to 2 millimetres per hour. Conducting this entire process at ambient pressure represents a major step forward, as it avoids the need for complex high-pressure containment vessels, making the manufacturing process far more viable for industrial scale-up.
Structural differences and defect profiles
The resulting crystals were found to adopt an AlB2-type layered hexagonal phase, belonging to the P6/mmm space group. The choice of starting flux had a subtle but measurable impact on the final lattice parameters of the material. For the crystals grown under the scandium-rich conditions, the ‘a’ lattice constant was measured at 3.1423(2) Ångströms, and the ‘c’ lattice constant was 3.5084(3) Ångströms. For the crystals grown using the boron-rich flux, the ‘a’ lattice constant was slightly larger at 3.1502(3) Ångströms, while the ‘c’ lattice constant was slightly smaller at 3.5041(3) Ångströms. Regardless of the chemical composition of the flux, the crystals naturally grew along the in-plane [100] crystallographic direction.
While both methods produced scandium diboride, electron backscattered diffraction (EBSD) analysis revealed critical differences in their internal structures. The boules grown from the scandium-rich flux were found to contain multiple crystal domains. Furthermore, these boules exhibited metallic scandium inclusions within their structure, although the individual domains themselves remained highly aligned. In a semiconductor substrate, such metallic inclusions and domain boundaries are highly undesirable, as they can cause electrical short circuits and scatter charge carriers.
In contrast, the boules grown from the boron-rich flux proved to be single-domain structures once they passed the initial nucleation region. To confirm the structural integrity of these boron-flux crystals, the researchers performed X-ray rocking-curve measurements for the (h000) and (000l) reflections. The measurements produced single, sharp peaks, which proved that the crystals were entirely free of grain boundaries. The researchers did, however, observe a slight asymmetry in the tails of the scattered-intensity peaks. This asymmetry suggests the presence of point defects—such as individual missing atoms or slight lattice distortions—rather than larger, device-ruining grain boundaries.
Electronic properties and surface quality
To ensure that the grown crystals could perform effectively in power electronics, the researchers evaluated their surface chemistry and electrical characteristics. Using surface X-ray photoemission spectroscopy (XPS), they compared the electronic environments of the crystals grown under the different flux conditions. The analysis showed that the boron-flux crystals possessed a superior electronic environment compared to their scandium-flux counterparts. The boron-flux samples yielded highly resolved binding-energy peaks for both the boron 1s and scandium 2p electronic states, indicating a clean, chemically ordered surface that is highly suitable for the subsequent deposition of AlGaN layers.
The researchers also measured the work function of the crystals on the (11-20) crystallographic plane, obtaining a value of approximately 5 electron volts (eV). The work function is a fundamental material property representing the energy required to remove an electron from the surface of the solid. A work function of this magnitude is entirely consistent with the highly electrically conductive nature of scandium diboride. This combination of high electrical conductivity, excellent surface quality, and a single-domain crystal structure free of grain boundaries confirms that ambient-pressure growth can yield scandium diboride substrates of sufficient quality for demanding microelectronic applications.
Sources
- Enabling the Ambient Pressure Growth of ScB2 Crystals for AlGaN Power Electronics, Satya K. Kushwaha et al., arXiv preprint, not yet peer reviewed, 2026-09-29
The R&D takeaway
For R&D leaders planning next-generation power semiconductor roadmaps, this research demonstrates that ambient-pressure, crucible-free crystal growth can successfully produce high-quality ultrahigh-temperature ceramic substrates like scandium diboride. Strategically, investing in alternative growth techniques such as laser-heated optical floating zones can bypass the high-pressure manufacturing bottlenecks that typically limit wide-bandgap substrate development. Furthermore, the findings highlight that tuning the precursor flux composition—specifically utilising boron-rich self-fluxes—is a critical lever for eliminating grain boundaries and achieving single-domain crystal quality early in the materials design cycle.
The R&D Innovate desk