Ali Yassine

This study explores the effects of a sputtered Al0.7Sc0.3N layer deposited onto metalorganic chemical vapor deposition-grown Al0.3Ga0.7N/GaN heterostructures, with a focus on structural and electrical properties, including DC characteristics of high-electron-mobility transistors (HEMTs). Structural analysis reveals that a 25 nm AlScN layer preserves surface roughness and barrier/channel interface quality, although relaxation occurs due to non-ideal growth conditions. Capacitance–voltage profiling indicates a sharp and uniform depletion, with reduced hysteresis post-deposition, suggesting effective passivation. Incorporation of AlScN increases the sheet carrier density from 1.4 to 1.89 × 1013 cm−2, although this is accompanied by lower mobility and a modest rise in sheet resistance from 406 to 441 Ω/sq. Devices with a 0.7 μm gate length demonstrate a saturation current of 0.98 A/mm, approximately 30% higher than reference AlGaN/GaN HEMTs. The composite barrier configuration further exhibits reduced leakage and improved insulation, but results in decreased transconductance and a pronounced threshold voltage shift from −0.8 to −7.8 V, attributed to increased barrier thickness. On-resistance also rises from 2 to 2.5 Ω mm likely due to sputter-induced contact damage. Optimization with a thinner 15 nm AlScN layer achieves a favorable balance, enhancing saturation current and soft-breakdown voltage while minimizing drawbacks in transconductance and on-resistance. Overall, sputtered AlScN serves as both a high-quality barrier and passivation layer, yielding enhanced device performance and robustness, and highlighting the future potential of sputter epitaxy to further advance GaN technology.

AlScN-based bulk acoustic wave resonators with enhanced performance have become attractive for meeting the demanding requirements of the 5G telecommunication industry. For resonators to have better performance, AlScN films exhibiting epitaxy, high crystalline quality, high electric insultation, and low number of abnormally oriented grains are desired. In this study, we systematically change the total sputter power and investigate the structural, optical, and electrical properties of the resulting Al1−xScxN films. Our findings demonstrate that increasing the total sputter power results in Al1−xScxN films with low abnormally oriented grains, reduced sub-bandgap absorption, and lower leakage currents—predominantly due to reduced nitrogen vacancies. Based on these results, we argue that a higher growth rate promotes the growth of Al1−xScxN with reduced impurity levels. Furthermore, by depositing Al0.7Sc0.3N on epitaxial AlN, we demonstrate that epitaxial AlScN can be achieved while improving its crystal quality by ≈36%.

The electrical properties of SiNy/Al1−xScxN/GaN-based metal–insulator–semiconductor (MIS) structures, including electron sheet density, electron mobility, depletion behavior, and off-state leakage current, were systematically investigated as a function of Sc concentration (x = 0.06–0.17). A reduction in polarization-induced charge with increasing Sc concentration was observed, evidenced by lower electric fields needed to deplete the two-dimensional electron gas (2DEG) confined close to the Al1−xScxN/GaN interface and the corresponding decreases in electron sheet density from 2.5 × 1013 to 2.0 × 1013 cm−2. These measured values are at least two times lower than theoretical predictions, likely due to expected high carbon and oxygen impurity levels in the grown barrier layers. A pinch-off instability was found to occur after SiNy passivation removal. The electron mobility was found to be inversely correlated with Sc concentration declining from 950 to almost 750 cm2/Vs, attributed to interactions between the 2DEG and the barrier. Furthermore, the off-state leakage current investigations revealed a minimum in leakage current at x = 0.12. Accordingly, an optimal Sc concentration range between x = 0.10 and 0.12 was identified that balances depletion behavior, mobility, electron sheet density, and leakage current, potentially due to near lattice-matched conditions between Al1−xScxN barrier and GaN-channel layer. These findings provide crucial insights for the development of high-performance Al1−xScxN/GaN-based transistors for next-generation high-frequency and high-power electronic applications.

YxAl1−xN in its wurtzite phase has been identified as a prospective wide bandgap semiconductor and a promising competitor of ScxAl1−xN in application devices. Notwithstanding theoretical predictions of the high stability of YxAl1−xN in the wurtzite structure even at high alloy concentrations, experimental studies have revealed significant challenges in achieving the requisite high concentration films. This study demonstrates that strain is an effective parameter on the growth of wurtzite YxAl1−xN, which can be tuned by engineering growth methods, such as the introduction of different buffer layers. Conversely, difficulties have been encountered in achieving Y concentrations above x = 0.4, despite the incorporation of Y atoms into the layers, with the formation of amorphous structures occurring prior to the predicted structural phase transition to the rock salt crystal. A comprehensive grasp of the structural characteristics of YxAl1−xN thin films offers invaluable insight that will prove to be beneficial for future research on this material system.

In this work, the value and the polarity of the spontaneous and piezoelectric polarization have been investigated, as the use of two different reference structures for wurtzite-type group-III nitrides, namely, the zinc-blende and the layered-hexagonal crystal lattice, have resulted in different predictions. It was found that although the differences in value and polarity of the polarization for heterostructures such as wurtzite Al1−xScxN/GaN lead to similar interface sheet charges, a significant mismatch is observed when polarization reversal is considered. The interface sheet charge predicted before and after the polarization reversal in the wurtzite Al1−xScxN layer on GaN using the zinc-blende lattice as a reference predominantly shows a change in sign. When using the layered-hexagonal lattice as a reference, not only is the same polarity of the interface sheet charge maintained after polarization reversal, but it is even 30 times larger. In this case, the giant and positive spontaneous polarization values for metal-polar Al1−xScxN extracted from the ferroelectric switching, as well as the alignment of the piezoelectric polarization to it, were observed to be consistent with the predictions referenced to the layered-hexagonal lattice. Thus, it is concluded that the layered-hexagonal reference is not only more suitable for predicting the ferroelectric properties of wurtzite Al1−xScxN but should also be the correct reference when considering polarization reversal in heterostructures. If the significant increase in the interface sheet charge after polarization reversal is experimentally detected, it will allow the design and fabrication of novel devices for future high-frequency and power electronics applications.

High quality, uni-polar, epitaxial AlN with minimum oxygen content promises excellent surface acoustic wave and bulk acoustic wave resonator characteristics such as high electromechanical coupling coefficient and power handling capabilities, which is particularly useful for RF filter applications. By systematically varying the growth temperature, the study investigates its impact on the oxygen levels, defect states, and crystallographic texture of the AlN thin films using a combination of atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, spectroscopic ellipsometry, scanning transmission electron microscopy, as well as room temperature and temperature dependent I–V measurements. The research demonstrates that the films grown at a temperature of 700°C exhibit the most favorable results. These films exhibit the lowest oxygen levels, possess epitaxial growth, and display the highest crystalline quality (XRD AlN 0002 ω−FWHM=1.3°). Additionally, these films demonstrate a significant reduction in sub-bandgap absorption. By comparing the cathode current measured during deposition, we suggest that the presence of an impurity layer formed during idle time between depositions as a possible source of oxygen in the sputter chamber. In addition, the study presents a possible model to explain the mixed polarity observed in AlN and proposes various ways to achieve uni-polar AlN on silicon substrates.

A review of the structural, elastic, and thermodynamic properties of cubic and hexagonal ScxAl1−xN crystals over the range of possible random alloys is provided. Based on measured and simulated lattice and internal cell parameters of NaCl (B1), CsCl (B2), and α-ZnS (B3) type cubic ScxAl1−xN lattices as well as of β-ZnS (B4), lh-MgO (Bk), and NiAs (B81) type hexagonal ScxAl1−xN crystals, their atomic positions, distances to nearest neighbor atoms, geometric dimensions of crystal cells, mass density, as well as their average bond length and bond angles are presented in dependence on the alloy composition. The understanding gained about the crystal lattices is used to provide a model for the transitions from the β-ZnS to the lh-MgO or NaCl lattice induced by the alloying of AlN with ScN. Based on published data sets of stiffness coefficients, the compliance coefficients, Young's modulus, shear modulus, Poisson's ratio, compressibility, and the sound velocities are presented in relation to the orientation of representative crystal planes and axes for rock salt, layered hexagonal, and wurtzite ScxAl1−xN crystals. Particular attention is paid to the directional anisotropies of elastic properties of the different crystal lattices if Sc atoms substitute an increasing number of Al atoms. Based on sound velocities determined, an overview of the fundamental thermodynamic properties of cubic and hexagonal ScxAl1−xN alloys is provided, such as the Debye temperature, heat capacity, minimum heat conduction, and melting temperature.

The structural, elastic, and basic thermodynamic properties of hexagonal Sc_xAl_1−xN crystals are calculated and discussed over the whole range of possible random alloys, including the transition from wurtzite to the layered hexagonal structure. Based on a review of lattice and internal parameters in combination with complete datasets of stiffness coefficients published in the literature, differing in the considered alloying intervals and the predicted structural transitions, changes in the crystal lattices caused by the substitution of aluminum by scandium atoms are discussed and illustrated. Crystal properties like the mass densities, average bond angles, and bond lengths are calculated, and the compliance coefficients, Young's modulus, shear modulus, Poisson's ratio, compressibility, and sound velocities are determined depending on the alloy composition and in relation to the orientation of crystal planes and axes. Particular attention is paid to the occurring directional anisotropies and the changes in structural and elastic properties in the alloy region of the structural transition between wurtzite and layered hexagonal Sc_xAl_1−xN crystals. The acoustic velocities determined are used to calculate basic thermodynamic properties such as the Debye temperature, heat capacity, and minimum heat conduction, as well as to evaluate both the influence of the alloying and the structural transition on these properties.