Chandrashekhar Savant

Alloys of AlN have rapidly emerged as a material platform for nonlinear optics. In this paper, we measure the second-order optical nonlinearity of AlScN films grown directly on nitrided c-plane sapphire by molecular beam epitaxy. This direct growth approach, which bypasses a thick AlN buffer layer, substantially reduces the influence of the buffer layer on the measured nonlinear response of the AlScN film. Our results show a large enhancement of d 31 , while no statistically significant enhancement of d 33 is resolved within the experimental uncertainty. We observe that d 31 can be as high as 4.92 pm V − 1 , approximately 60 times larger than the d 31 value of AlN. The development of AlScN-based photonic devices can enable energy-efficient nonlinear optical operations that can be epitaxially integrated with electronic and photonic devices based on Si, GaN and AlN.

Multiple-channel field-effect transistors based on aluminum scandium nitride (AlScN)/gallium nitride (GaN) heterostructures stand as a promising avenue for next-generation high-frequency applications. In this work, we demonstrate prototype AlScN/GaN super-lattice castellated field-effect transistors (SLCFETs) employing three-channel heterostructures grown by molecular beam epitaxy (MBE) and report their direct-current (DC) and small-signal radio-frequency (RF) characteristics. Normalized by active channel width, peak on-current of 3.83 A mm − 1 , cut-off frequency ( f T ) of 31.2 GHz and maximum oscillation frequency ( f MAX ) of 17.7 GHz are measured. These results establish AlScN/GaN SLCFETs as a viable platform for next-generation RF devices, motivating further optimization of substrate and epitaxial growth strategies.

Ternary aluminum scandium nitride (AlScN) and aluminum yttrium nitride (AlYN) are emerging wide-bandgap piezoelectric and ferroelectric materials with significant potential for next-generation electronic devices. This work investigates the impact of post-growth annealing on the structural, electrical, and piezoelectric properties of AlScN and AlYN thin films grown by plasma-assisted by molecular beam epitaxy on tungsten electrodes. Annealing is performed in vacuum, air, and nitrogen atmospheres at temperatures ranging from 750 to 1100 °C. It is observed that annealing in air and nitrogen lowers the leakage current in AlScN and AlYN films. Piezoelectric coefficient (d33) enhancements as high as 137 pm/V for AlScN and 8.05 pm/V for AlYN are observed for less leaky, nitrogen-annealed samples at 750 °C. Preservation of ferroelectric switching properties post-annealing is confirmed by piezo-response force microscopy and PUND measurements. These findings highlight the role of thermal treatments on the performance of AlScN and AlYN for piezoelectric, ferroelectric, and high-K dielectric device applications.

We report step-flow homoepitaxial growth of Al2O3 on m-plane sapphire at a relatively low substrate temperature of 650 °C using molecular beam epitaxy (MBE). The films exhibit atomically smooth terraces with step heights of 0.58 nm, corresponding to two-thirds of the m-plane lattice spacing. A decrease in growth rate of sapphire with increasing aluminum flux beyond a certain value is observed, a growth regime not previously reported for sapphire homoepitaxy. Oxygen plasma surface pre-treatment is found to produce highly uniform step heights and terrace widths upon MBE, enabling reproducible step-flow growth. The m-plane orientation stabilizes the growth of (110)-oriented TiN, eliminating the twin domains observed in TiN grown on c-plane sapphire. TiN films grown on homoepitaxial m-plane Al2O3 show superconductivity below Tc = 5.4 K, room-temperature resistivity of 31.9 μΩ cm, and residual resistivity ratio (RRR) of 2.7, compared to 18.2 μΩ cm and RRR = 3.3 grown directly on as-received substrates. The findings indicate that for qubit applications, m-plane sapphire homoepitaxial layers/epitaxial TiN superconductor interfaces without air exposure are feasible.

We present a technique of patterned in situ molecular beam epitaxy (MBE) selective area sublimation etching of p-type GaN and direct MBE regrowth of n-type GaN to form lateral pn junction structures without air exposure of the junction region. The Mg-doped p-GaN has 2.1×1018/cm3 mobile holes of mobility 4 cm2/V s and the Si-doped n-GaN has 3×1018/cm3 mobile electrons of mobility >100 cm2/V s, both at room temperature. The resulting lateral pn junction exhibits rectification and GaN band-edge electroluminescence at 3.4 eV at forward bias, in addition to blue electroluminescence associated with Mg-doped GaN. We systematically describe the entire MBE etch and regrowth process and expect this vacuum-based technique to enable lateral structures of higher complexity than shown in this initial demonstration.

Al1−xScxN (AlScN) has emerged as a promising ferroelectric material for next-generation memory and logic devices; yet its performance remains limited by poor understanding of nanoscale structural instabilities. Here, we uncover intrinsic short-range chemical ordering and strain–defect coupling in molecular-beam-epitaxy grown Al0.66Sc0.34N using a combination of atom probe tomography and transmission electron microscopy. The heterostructure exhibits wurtzite structure with a sharp and clean AlScN/AlN interface with negligible interdiffusion. Frequency distribution and radial distribution analyses reveal statistically significant Sc–Sc enrichment and Sc–N depletion, indicating nanoscale clustering accompanied by local nitrogen deficiency. Corresponding lattice-spacing variations observed by inverse FFT mapping confirm strain variation correlated with Sc-rich domains. These results provide the first atom-by-atom experimental evidence of scandium clustering and its coupling to nitrogen-vacancy formation in the ferroelectric composition regime. These findings identify short-range ordering and strain–defect coupling as intrinsic structural features of ferroelectric AlScN, with important implications for polarization switching behavior and reliability.

We report the molecular beam epitaxial growth and characterization of wurtzite InScN thin films. We find that wurtzite InScN can be grown on GaN substrates under metal-rich conditions, resulting in smooth surfaces with <1 nm roughness and a two-dimensional growth mode. We further find that InScN can be stabilized in the wurtzite crystalline phase for Sc content up to at least 10.4%. We measure and report the lattice parameters of wurtzite InScN as a function of Sc content and find that between ∼5% and 8% Sc mole fraction InScN has a similar a lattice constant to InN. The c/a ratio deviates from the ideal tetrahedral value of 1.633 as Sc content increases. The optical absorption edge of wurtzite InScN monotonically increases over a Sc content range of 0%–10.4% from 0.8 to 1.05 eV, accompanied by a shift in the peak index of refraction and extinction coefficient. The E2high Raman peak of InScN shows negligible shift with increasing Sc content compared to a gradual increase in the A1 (LO) peak position. These fundamental observations can facilitate the future design of InN-based devices.

Wurtzite aluminum scandium nitride thin films are a leading candidate material for the development of next-generation semiconductor electronic, photonic, and acoustic devices. The integration of high-quality AlScN thin films on a silicon platform is of scientific and commercial interest. Here, we report the successful epitaxial growth of crack-free metal-polar AlScN thin films on (111) silicon substrates by plasma-assisted molecular beam epitaxy. We identify critical nucleation conditions for smooth morphology and growth parameters for high crystal quality and stress mitigation. Using an AlN nucleation layer grown under nitrogen-rich conditions and compositionally graded AlScN buffer layers, we obtain single-phase films with narrow 002 rocking curve FWHM as low as 0.51°, crack-free films up to 150 nm thick with ScN fraction as high as 31%, and surface RMS roughness as low as 0.54 nm over 400μm2, which follow the morphology of the underlying silicon substrates with a mixed 2D–3D growth mode. The lattice parameters, d33 piezoelectric coefficients, and optical bandgap are measured as a function of Sc composition. With increasing Sc composition, the piezoelectric coefficient is enhanced to as high as 34.7 pm/V.

Self-activated growth of ScN was recently reported owing to the ability of scandium to self-catalyze N2 bond breaking, facilitating nitride film growth from molecular N2 without plasma. In this report, we expand this novel synthesis method to explore the possibility of self-activated growth of aluminum–scandium–nitride, an emerging ternary nitride system with enhanced functionalities. We find that the rock salt AlScN film growth of low Al composition from 0% to 48% is possible by self-activated growth. The cubic rock salt AlScN films are phase pure up to 21% Al, whereas mixed rock salt AlScN and intermetallic cubic AlxSc phases appear in films with 28% to 48% Al. Self-activated nitride crystal growth was not observed for films grown with high Al content of 93.7% and 100% Al, which showed cubic AlxSc and FCC α-Al phases, respectively, without any sign of nitrogen incorporation. Lattice parameters of crystalline rock salt AlScN films decrease with increasing Al composition. The self-activated AlScN films, as well as cubic AlxSc and FCC α-Al phase, exhibit twinning and are epitaxial to the 6H-SiC substrate. The findings open up new avenues for the ultralow-energy synthesis of ternary nitrides and beyond.

Aluminum scandium nitride (AlScN) is a promising barrier material for gallium nitride (GaN)-based transistors for the next generation of radio-frequency electronic devices. In this work, we examine the transport properties of two-dimensional electron gases (2DEGs) in single- and multi-channel AlScN/GaN heterostructures grown by molecular beam epitaxy and demonstrate the lowest sheet resistance among AlScN-based systems reported to date. Assorted schemes of GaN/AlN interlayers are first introduced in single-channel structures between AlScN and GaN to improve conductivity, increasing electron mobility up to 1370 cm2/Vs at 300 K and 4160 cm2/Vs at 77 K, reducing the sheet resistance down to 170 and 70 Ω/□, respectively. These improvements are then leveraged in multi-channel heterostructures, reaching sheet resistances of 65 Ω/□ for three channels and 45 Ω/□ for five channels at 300 K, further reduced to 21 and 13 Ω/□ at 2 K, respectively, confirming the presence of multiple 2DEGs. Structural characterization indicates pseudomorphic growth with smooth surfaces, while partial barrier relaxation and surface roughening are observed at high scandium content, with no impact on mobility. This first demonstration of ultra-low sheet resistance multi-channel AlScN/GaN heterostructures places AlScN on par with state-of-the-art multi-channel Al(In)N/GaN systems, showcasing its capacity to advance existing and enable new high-speed, high-power electronic devices.