In the realm of semiconductor technology, Gallium Nitride (GaN)-based high electron mobility transistors (HEMTs) have emerged as quintessential components for advancing high-frequency electronics due to their exceptional electron transport properties. These transistors leverage a heterostructure formed between dissimilar semiconductor materials—commonly GaN and aluminum gallium nitride (AlGaN)—which supports the creation of a two-dimensional electron gas (2DEG). This highly conductive electron layer manifests at the interface, enabling electron mobility far superior to bulk materials and thus, significantly enhancing device performance for applications ranging from high-speed wireless communications to power switching systems.
Recent scientific endeavors have turned the spotlight on scandium aluminum nitride (ScAlN) as a promising barrier material capable of augmenting the already impressive performance metrics of GaN-based HEMTs. ScAlN distinguishes itself through substantial spontaneous and piezoelectric polarization effects, which can drive the electron density within the 2DEG to unprecedented levels. Moreover, ScAlN’s intrinsic ferroelectricity opens up fascinating avenues for dynamically tunable ferroelectric gates, potentially revolutionizing device functionalities by enabling electrical modulation of the 2DEG channel in ways previously unattainable in conventional GaN HEMT architectures.
Traditional techniques for synthesizing ScAlN layers atop GaN substrates entail complex epitaxial methods, often requiring elevated temperatures and expensive equipment, thereby restricting scalability and industrial adoption. Contrastingly, the physical vapor deposition technique known as sputtering emerges as an attractive alternative due to its relative simplicity, rapid deposition rates, and compatibility with lower processing temperatures. Despite its industrial promise, the investigation into sputtering-grown ScAlN films and the influence of deposition parameters on their structural and electrical traits has remained scarce, presenting a crucial knowledge gap for semiconductor researchers and manufacturers alike.
Addressing this gap, a pioneering team of researchers led by Associate Professor Atsushi Kobayashi at Tokyo University of Science (TUS), Japan, has accomplished successful growth of ScAlN films on AlGaN/GaN heterostructures through sputter epitaxy. Their study meticulously explores the pivotal role of substrate temperature during deposition, unveiling how thermal conditions underpin the crystalline quality and the electrical performance of these thin films. This research underscores sputtering’s viability as a cost-effective, scalable method to integrate ScAlN barriers in high-performance GaN HEMTs, thereby propelling these devices closer to widespread commercial utilization.
In their experimental approach, the researchers fabricated ScAlN films with approximately 10% scandium doping on AlGaN/AlN/GaN substrates, systematically varying growth temperatures between 250 °C and 750 °C. Utilizing advanced characterization techniques, including atomic force microscopy and high-energy electron diffraction, the team discerned that epitaxial alignment of ScAlN with the underlying heterostructure was achievable even at the relatively modest temperature of 250 °C. Furthermore, increasing thermal budgets notably enhanced surface morphology, culminating in atomically stepped terraces indicative of superior crystallinity at 750 °C deposition.
Crucially, the investigation into electrical properties via Hall-effect measurements revealed a pronounced dependency on growth temperature. The sample grown at 750 °C exhibited a sheet carrier density in the 2DEG exceeding 1.1 × 10¹³ cm⁻², nearly tripling that observed in comparable heterostructures lacking the ScAlN barrier. This stark enhancement evidences the integral role that improved structural order, fostered by optimized thermal conditions, plays in modulating interfacial electronic characteristics. Lower-temperature-grown films, conversely, demonstrated diminished carrier densities, highlighting the trade-offs associated with insufficient thermal energy to promote lattice perfection.
Despite the boost in carrier concentration, electron mobility experienced a decrement in all ScAlN-containing samples relative to baseline AlGaN/AlN/GaN heterojunctions. This mobility reduction is postulated to stem from increased structural roughness and defect states proximate to the interfaces introduced by the ScAlN layer, underscoring the delicate balance between carrier density augmentation and maintenance of high carrier mobility. These findings illuminate the nuanced interplay between microstructural quality and electronic transport mechanisms, serving as a powerful guide for future optimization of growth protocols.
The implications of this research are manifold. By demonstrating that sputter epitaxy can reliably produce high-quality ScAlN films under industrially relevant conditions, the study paves the way for scalable manufacturing of GaN-based HEMTs with enhanced 2DEG densities. Such advances bear direct relevance to the burgeoning fields of energy-efficient power electronics and ultra-fast wireless communications, where device performance is often constrained by material imperfections and fabrication expenses.
Associate Professor Kobayashi emphasizes the transformative potential of this work, noting that its success could catalyze mass production of next-generation transistor technologies capable of functioning robustly in extreme environments. This encompasses electric vehicles and aerospace applications, domains where high efficiency, reliability, and sustainability are paramount. In this regard, sputtered ScAlN barriers stand to not only advance electronic device capabilities but also contribute materially towards the global push for greener, more resilient technologies.
Looking ahead, the researchers anticipate that their findings will stimulate broader exploration into the integration of ferroelectric ScAlN layers within GaN-based devices. Such devices may harness voltage-controlled modulation of the 2DEG channel, unlocking functionalities such as non-volatile memory, adaptive RF components, or novel logic architectures. The facile and cost-effective nature of sputtering as a deposition technique further amplifies the appeal of this material system, potentially enabling versatile semiconductor platforms well beyond current limitations.
In sum, this study expands the theoretical understanding and practical methodology associated with ScAlN deposition on GaN heterostructures, marking a pivotal step in realizing advanced electronic materials. By meticulously correlating growth temperature with film structure and electronic properties, the work supplies invaluable insight to material scientists and device engineers alike, guiding the refinement of fabrication techniques that harmonize performance, cost, and scalability.
As the electronics industry relentlessly pushes to transcend classical material boundaries, innovations like sputtered ScAlN barriers promise to redefine the limits of high-frequency transistor technology. With this breakthrough, the door opens wider to the widespread adoption of high-performance, scalable GaN devices, poised to accelerate technological frontiers spanning communication, energy systems, and beyond.
Subject of Research: Not applicable
Article Title: Effect of growth temperature on the structural and electrical properties of sputter-epitaxial ScAlN on AlGaN/AlN/GaN heterostructures
News Publication Date: 7-Aug-2025
References: DOI: 10.1063/5.0281540
Image Credits: Dr. Atsushi Kobayashi from Tokyo University of Science, Japan
Keywords
Materials science, Electronics, Electrical engineering, Applied sciences and engineering, Energy
