Joseph Casamento

This study investigates the structural, chemical, and dielectric properties of sub-20 nm Ga2O3 thin films grown by plasma-enhanced atomic layer deposition at 250 °C using triethylgallium and oxygen plasma. The as-deposited films are amorphous and exhibit sub-nanometer surface roughness on c-plane sapphire and Si (100) substrates, with carbon impurity concentrations below the detection limit of x-ray photoelectron spectroscopy. Metal–insulator–metal capacitors fabricated on tungsten-coated Si substrates yield a relative dielectric constant of 11.05 and a maximum intrinsic breakdown field of 7.17 MV/cm, with leakage current remaining below 10−5 A/cm2 at electric fields up to 3.9 MV/cm. Quantitative analysis of the pre-breakdown current–voltage characteristics reveals that injection-limited current is the dominant conduction mechanism, while bulk-limited Poole–Frenkel and space-charge-limited currents are shown not to govern transport.

This paper demonstrates the deposition of GaN on c-plane sapphire by plasma-enhanced atomic layer deposition (PE-ALD) at 250 °C using triethylgallium and forming gas (95% N2, 5% H2) plasma. The films exhibit a wurtzite crystal structure with a full-width at half maximum of 0.31° for the (0002) peak via x-ray diffraction and an abrupt GaN–sapphire interface confirmed by scanning transmission electron microscopy. X-ray photoelectron spectroscopy and secondary-ion mass spectrometry reveal carbon and oxygen concentrations of 1 at. % and less than 3 at. %, respectively. Transfer length method analysis demonstrates a sheet resistance of 1.37 × 108Ω/□. These results provide an understanding for the low-temperature PE-ALD growth of GaN with high crystalline fidelity and minimized impurity incorporation.

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.

The discovery of ferroelectricity in hafnium zirconium oxide (Hf_xZr_1-xO₂) and related fluorite materials has spurred interest in ferroelectric devices suitable for integration with silicon integrated circuits (ICs), especially those that can be embedded in the back-end-of-line (BEOL) process. More recently, ferroelectricity has been found in wurtzite aluminum nitride-based materials, such as scandium and boron-doped aluminum nitride (Al_1-xSc_xN and Al_1-xB_xN). Although these materials currently have undesirably large coercive electric fields, and small breakdown electric field-to-coercive electric field ratio, their low processing temperatures and large remanent polarization offer intriguing possibilities for device applications. Here, we report ferroelectric field-effect transistors (FeFETs) with a 15 nm thick Al_0.88B_0.12N layer and an 11 nm ZnO semiconductor channel, achieving a memory window >1 V and switching voltages (V_switch) <±10 V.

We report a polarization-induced 2D electron gas (2DEG) at an epitaxial AlBN/GaN heterojunction grown on a SiC substrate. Using this 2DEG in a long conducting channel, we realize ultra-thin barrier AlBN/GaN high electron mobility transistors that exhibit current densities > 0.25 A/mm, clean current saturation, a low pinch-off voltage of −0.43 V, and a peak transconductance of ∼0.14 S/mm. Transistor performance in this preliminary realization is limited by the contact resistance. Capacitance–voltage measurements reveal that introducing ∼7% B in the epitaxial AlBN barrier on GaN boosts the relative dielectric constant of AlBN to ɛrAlBN∼16, higher than the AlN dielectric constant of ɛrAlN∼9. Epitaxial high-K barrier AlBN/GaN HEMTs can thus extend performance beyond the capabilities of current GaN transistors.

Unlike naturally occurring oxide crystals such as ruby and gemstones, there are no naturally occurring nitride crystals because the triple bond of the nitrogen molecule is one of the strongest bonds in nature. Here, we report that when the transition metal scandium is subjected to molecular nitrogen, it self-catalyzes to break the nitrogen triple bond to form highly crystalline layers of ScN, a semiconductor. This reaction proceeds even at room temperature. Self-activated ScN films have a twin cubic crystal structure, atomic layering, and electronic and optical properties comparable to plasma-based methods. We extend our research to showcase Sc’s scavenging effect and demonstrate self-activated ScN growth under various growth conditions and on technologically significant substrates, such as 6H–SiC, AlN, and GaN. Ab initio calculations elucidate an energetically efficient pathway for the self-activated growth of crystalline ScN films from molecular N2. The findings open a new pathway to ultralow-energy synthesis of crystalline nitride semiconductor layers and beyond.

AlScN is a new wide bandgap, high-k, ferroelectric material for radio frequency (RF), memory, and power applications. Successful integration of high-quality AlScN with GaN in epitaxial layer stacks depends strongly on the ability to control lattice parameters and surface or interface through growth. This study investigates the molecular beam epitaxy growth and transport properties of AlScN/GaN multilayer heterostructures. Single-layer Al1−xScxN/GaN heterostructures exhibited lattice-matched composition within x = 0.09–0.11 using substrate (thermocouple) growth temperatures between 330 and 630 °C. By targeting the lattice-matched Sc composition, pseudomorphic AlScN/GaN multilayer structures with ten and twenty periods were achieved, exhibiting excellent structural and interface properties as confirmed by x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). These multilayer heterostructures exhibited substantial polarization-induced net mobile charge densities of up to 8.24 × 1014/cm2 for twenty channels. The sheet density scales with the number of AlScN/GaN periods. By identifying lattice-matched growth condition and using it to generate multiple conductive channels, this work enhances our understanding of the AlScN/GaN material platform.

We report the properties of molecular beam epitaxy deposited AlBN thin films on a recently developed epitaxial nitride metal electrode, Nb2N. While a control AlN thin film exhibits standard capacitive behavior, distinct ferroelectric switching is observed in the AlBN films with increasing Boron mole fraction. The measured remnant polarization Pr∼15μC/cm2 and coercive field Ec∼ 1.45 MV/cm in these films are smaller than those recently reported on films deposited by sputtering, due to incomplete wake-up, limited by current leakage. Because AlBN preserves the ultrawide energy bandgap of AlN compared to other nitride hi-K dielectrics and ferroelectrics, and it can be epitaxially integrated with GaN and AlN semiconductors, its development will enable several opportunities for unique electronic, photonic, and memory devices.

This report demonstrates Al0.93B0.7N thin films grown epitaxially on n-type GaN (0002)/c-plane sapphire substrates by reactive magnetron sputtering at 300 °C. At 200 nm film thickness, the Al0.93B0.07N layers exhibit partially relaxed substrate-induced epitaxial strain, a 0.16° wide (0002) rocking curve, in-plane crystallographic registry, and sub-nanometer surface roughness. Electrically, the stack shows robust hysteresis over three frequency decades, a remanent polarization of ∼125 μC/cm2, a strongly frequency dependent coercive field, highly uniform dc leakage currents, and endurance >106 field cycles. This report validates possibilities for ferroelectric nitride integration into conventional III-nitride heterostructures with high crystalline fidelity, high electrical resistivity, and persistent hysteresis. Such materials are synthesizable at thermal budgets and temperatures compatible with back-end-of-the-line boundary conditions.

Wurtzite ferroelectrics are an emerging material class that expands the functionality and application space of wide bandgap semiconductors. Promising physical properties of binary wurtzite semiconductors include a large, reorientable spontaneous polarization, direct band gaps that span from the infrared to ultraviolet, large thermal conductivities and acoustic wave velocities, high mobility electron and hole channels, and low optical losses. The ability to reverse the polarization in ternary wurtzite semiconductors at room temperature enables memory and analog type functionality and quasi-phase matching in optical devices and boosts the ecosystem of wurtzite semiconductors, provided the appropriate combination of properties can be achieved for any given application. In this article, advances in the design, synthesis, and characterization of wurtzite ferroelectric materials and devices are discussed. Highlights include: the direct and quantitative observation of polarization reversal of ∼135 μC/cm2 charge in Al1−xBxN via electron microscopy, Al1−xBxN ferroelectric domain patterns poled down to 400 nm in width via scanning probe microscopy, and full polarization retention after over 1000 h of 200 °C baking and a 2× enhancement relative to ZnO in the nonlinear optical response of Zn1−xMgxO. The main tradeoffs, challenges, and opportunities in thin film deposition, heterostructure design and characterization, and device fabrication are overviewed.