Atomic layer etching of GaN in chlorinated plasmas requires a detailed understanding of plasma–surface interaction mechanisms at the atomic scale. In this work, the adsorption of radical chlorine on the wurtzite GaN(0001) surface is investigated using density functional theory calculations. The preferred adsorption sites, the adsorption energies, and the diffusion barriers are determined. Charge transfer mechanisms are analyzed using Bader charge analysis and charge density difference calculations. The results indicate that chlorine adsorption occurs preferentially near surface Ga atoms with adsorption energies up to −3.6 eV. Projected density of states calculations reveal the formation of localized surface states associated with Cl–Ga bonding. Surface diffusion barriers toward the most stable Ga adsorption sites are below 0.1 eV, promoting rapid migration of Cl. The influence of chlorine surface coverage is also investigated and shows a progressive decrease in adsorption energy with increasing chlorine coverage. These results provide atomistic insight into chlorine adsorption and surface diffusion mechanisms relevant to the surface modification step of GaN in atomic layer etching processes [T. Rasoanarivo et al., J. Vac. Sci. Technol. A 44, 022608 (2026)].
Nonvolatile memories are crucial for reducing energy consumption in modern computing systems. Ferroelectric random-access memories (FeRAMs) enable data retention without power, offering a promising solution for low-power applications. Hafnium-zirconium oxide (HZO) exhibits robust ferroelectric properties; however, its crystallization requires the presence of a titanium nitride (TiN) top electrode during annealing. Recent studies showed that reducing HZO thickness below 10 nm but such scaling typically requires annealing temperatures above 400 °C, which is not compatible with back-end-of-line (BEOL) integration. In this work, we propose an alternative approach to achieve sub-10 nm ferroelectric HZO films within a BEOL-compatible thermal budget. A 10 nm HZO layer is first crystallized between TiN electrodes at 400 °C, followed by selective removal of the TiN top electrode and controlled plasma thinning of the HZO layer. The selective TiN removal process was developed combining a CH4/N2/Cl2 plasma etching process followed with a subsequent SC1 wet cleaning step, ensuring efficient etching with high selectivity toward HZO and minimal surface degradation. Controlled thinning of crystallized HZO films was optimized using an inductively coupled plasma (ICP) process based on BCl3/Cl2 chemistry. The etching mechanism is discussed and relies on ion-assisted reactions, involving oxygen scavenging by boron species and metal chlorination, leading to the formation of volatile etch products. Minimizing bias conditions further enable improved process control while limiting ion-induced damage. The whole results demonstrate that conventional plasma etching can be adapted for the controlled thinning of crystallized HZO films, enabling the fabrication of ultrathin (<10 nm) ferroelectric layers compatible with BEOL integration.
It has been reported that the relative dielectric constant, refractive index, and density of certain thin dielectric films, such as Al2O3, degrade with decreasing thickness. We investigated the influence of Al2O3 film thickness on the electron energy barriers at the interfaces of Pt/Al2O3/TiN metal/insulator/metal devices with Al2O3 deposited via atomic layer deposition. Using internal photoemission spectroscopy, we found that both the top Pt/Al2O3 and bottom TiN/Al2O3 electron barriers were a function of Al2O3 thickness. While both electron barriers were roughly constant at ∼3.6 eV for films ≥11.6 nm thick, both decreased by ∼0.5 eV as Al2O3 thickness decreased to 4.5 nm. The degradation of electron barriers with insulator thickness impacts the modeling of leakage currents and optimization of performance in devices with ultrathin insulators.
Selective area growth is a promising approach to the seamless integration of dissimilar materials for (opto)electronic devices, yet historically most work in this area has focused solely on conventional SiO2 masks. Expanding the palette of embedded dielectrics could unlock new applications, functionalities, and hybrid structures. Here, we demonstrate selective growth of GaAs over patterned ZrO2 and HfO2 features by molecular beam epitaxy for the first time. Complete selectivity is achieved over both ZrO2 and HfO2 at approximately half of the flux that can be employed for complete selectivity on an SiO2 mask due to higher adatom desorption probabilities on SiO2. Faceting in GaAs grown selectively within ZrO2 gratings is found to depend on the crystallographic orientation of the mask patterns, which is consistent with GaAs grown selectively within nominally identical SiO2 structures. In particular, growth within [010]-aligned structures induces the formation of {101} planes that are well-suited for epitaxial lateral overgrowth and coalescence for seamless mask encapsulation. The identified selective growth conditions enable complete selectivity over ZrO2 and HfO2 features of any arbitrary size and can likely be extended to HfZrO2 alloys of any composition. This work demonstrates the generality of selective molecular beam epitaxy over dissimilar mask materials and highlights the emerging possibilities of seamlessly incorporating functional oxides beyond SiO2 that exhibit novel properties such as the Pockels effect, ferroelectricity, and optical nonlinearity. Specifically, we show a scalable approach to the integration of III–V alloys with HfZrO2 dielectrics toward three-dimensionally structured devices and integrated systems.
Novel two-dimensional nanomaterials have attracted broad interest for both fundamental physics and next-generation device applications because the atomic-layer limit gives rise to properties that are absent in their bulk counterparts. However, conventional approaches for synthesizing atomically thin metals and related non-van der Waals materials are often limited by small lateral dimensions, poor coverage uniformity, and insufficient air stability. Epitaxial graphene on silicon carbide (EG/SiC) serves as a powerful platform for confining metals at the interface to realize large-area, monolayer-to-few-layer, air-stable metals, metal alloys, and metal compounds. In this review, we discuss the mechanisms governing intercalant-layer formation from both experimental and theoretical perspectives, as well as the characterization techniques used to verify intercalation and resolve interfacial superstructures, including low-energy electron diffraction, scanning tunneling microscopy, x-ray photoelectron spectroscopy, Raman spectroscopy, and microscopy methods. Enabled by atomic-scale confinement and the unique asymmetric environment of the host interface, these EG/SiC systems exhibit a range of emergent properties, including metal-to-semiconductor transitions, superconductivity, spin–orbit-related phenomena, and two-dimensional magnetic properties. Finally, recent processing advances toward future device applications and direct epitaxial heterostructures are discussed.
Bubbler-based vaporizers are widely used for chemical delivery in thin-film deposition processes, yet the mechanisms governing outlet saturation remain mostly unexplored. In this work, dry N-2 gas was bubbled through water in a stainless-steel bubbler at atmospheric pressure. Saturation of the outlet stream was tested as a function of inlet gas flow rate and vessel fill level. Using fast x-ray imaging, distinct bubble and coalescence regimes across operating conditions were identified. Despite these differences in bubble dynamics, outlet measurements showed near-complete saturation across all tested conditions. A simplified mass-transfer model indicated that exchange across bubble-liquid interfaces alone cannot account for the observed results. Instead, mass transfer at the liquid-headspace interface, enhanced by surface agitation and droplet entrainment, provides a substantial additional source for mass exchange. These findings highlight the importance of non-bubble interfaces in bubbler operation and suggest that models neglecting these effects may underestimate outlet concentrations.
Inductively coupled plasmas (ICPs) driven by pulsed RF power are used by the semiconductor industry to achieve the process demands for fabricating devices at the nanoscale. Existing global models offer valuable insight into the transient behavior of these plasmas but do not account for the external circuitry needed to deliver RF power from the generator to the plasma; this external circuitry not only distorts the plasma transient compared to idealized step-function power delivery, it also presents a low-barrier pathway for fine-tuning these transients to optimize power delivery within the pulse transient and enable control over the rate of increase of electron density n(e) and electron temperature T-e in the early ON-cycle. This work presents a global plasma model with a coupled equivalent circuit model to elucidate the synergistic effects of a fixed impedance matching network and the time-varying impedance of a plasma load on the plasma transient response during the pulse cycle. The tuning of the impedance matching circuit to provide an ideal match condition at different points in time in the pulsed RF cycle is shown to provide a mechanism for controlling the rate of electron production at the start of the pulse. The delivered power and the centerline n(e) data were experimentally benchmarked against the modeling results in a cylindrical ICP reactor. The results were obtained for 99.999% argon at pressures between 20 and 50 mTorr. The measured densities range from 10(9) to 10(11) cm(-3) and show good agreement with the modeled predictions.
10 nm-thick Hf1−xZrxO2 (HZO) films were deposited on TiN by atomic layer deposition at 250 °C using H2O2 or H2O as an oxidant with various exposure time. X-ray diffraction results showed that all as-grown films exhibited an overlapping peak associated with the orthorhombic (O)/tetragonal (T)/cubic (C) phases, while no notable peaks of the monoclinic phase were observed. With extending oxidant exposure time, both H2O2- and H2O-based films exhibited enhanced O/T/C phases peak area and reached saturation under sufficient oxidation. In the saturated region, the O/T/C phases peak area of H2O2-based films was ∼30% higher than that of H2O-based films. Carbon and nitrogen impurities were considered to remain in the films as residual precursor-ligand fragments due to incomplete reaction. With evaluation of impurity levels, both H2O2- and H2O-based HZO films exhibited a trend that crystallinity increases as impurity concentration of carbon and nitrogen decreases. Moreover, H2O2-based films had carbon and nitrogen concentrations approximately one order of magnitude lower than those of H2O-based films. These results suggest that impurity removal during the oxidation step is a critical factor for enhancing the crystallization of as-grown HZO films.
The two-dimensional semiconductor HfSe2 is a promising candidate for future electronic, optical, and sensing applications due to its moderate bandgap (similar to 1.1 eV) and the spontaneous formation of a native high-kappa dielectric in ambient conditions. In this study, a range of experimental techniques was employed to investigate the intrinsic surface properties of bulk HfSe2 grown by chemical vapor transport and to directly track oxidation in air. Scanning tunneling microscopy reveals a high defect density across the HfSe2 surface caused by Se vacancies and impurities, confirmed by XPS, inductively coupled plasma mass spectrometry, and high-resolution Rutherford backscattering spectrometry. Furthermore, XPS highlights the highly reactive nature of the HfSe2 surface in air, where the formation of HfOx is detected after a 1 min exposure to air, resulting in changes to the electronic properties, such as the work function and the valence band offset. Understanding the intrinsic surface properties, air stability, and material quality will facilitate the integration of 1T-HfSe2 in novel devices.
Atomic layer etching (ALE), an etch method capable of precisely removing atomic monolayers, is gaining renewed interest as an enabling technology across a wide range of applications. However, the long cycle times in conventional ALE, imposed by the need to purge between reactant doses, can make the process time-intensive. Recently, bias-pulsed atomic layer etching (BP-ALE) has been shown to improve throughput while maintaining atomic precision in material removal. BP-ALE differs from conventional ALE by pulsing only the plasma DC bias in an inductively coupled plasma-reactive ion etching reactor, thereby reducing cycle duration. Here, we demonstrate the application of BP-ALE to diamond etching using Ar/Cl-2 plasma, reporting an etch rate of 0.90 +/- 0.02 & Aring;/cycle with 6 s cycles. We study the transition from BP-ALE to conventional ALE as a function of purge time. Furthermore, we observe a smoothing effect on diamond surfaces, with clear reductions in surface roughness across multiple single-crystal diamond substrates. Additionally, we use optical emission spectroscopy to elucidate plasma behavior in the BP-ALE process. This work presents a new perspective on reactive ion etching of diamond, one of the premier materials in semiconductor and quantum technologies.
van der Waals heterostructures (vdWHs), built by stacking atomically thin layers of two-dimensional materials, offer unprecedented control over electronic and optical properties. This review outlines advances in mechanical assembly methods, from traditional exfoliation-based stacking to modern dry-transfer techniques with precise twist-angle control. We highlight strategies to improve interface cleanliness, mitigate interlayer bubbles, and scale fabrication. Special emphasis is placed on emerging autonomous robotic systems that integrate computer vision and machine learning to enable high-throughput, wafer-scale vdWH assembly. Finally, we discuss key challenges and future directions toward scalable, reliable, and contamination-free manufacturing of vdWH-based technologies.
Vertically aligned carbon nanotube forest growth uses a thin-film iron catalyst on an alumina support. The iron catalyst thickness (typically, 1-10 nm) strongly affects forest morphology. We explored the use of spectroscopic ellipsometry (SE) as a rapid, sensitive, and nondestructive metrology method for these films. SE does have challenges, however, as it is difficult to break the correlation in the analysis between fitted optical constants and thickness of ultrathin films. Partial oxidation and optical absorption in the iron-iron oxide films add further complexity. We performed a multisample SE analysis of thermally evaporated iron films with target thicknesses of 1-14 nm. To improve sensitivity, we used interference enhancement by incorporating a 350 nm silica film on a silicon substrate beneath the iron film and alumina support. We used a consecutive-layer approach, collecting SE data and fitting the optical constants and thickness of each film before depositing the next. The iron-iron oxide film was modeled with an effective medium approximation layer. The model fit the data well with a mean squared error of 25. From the SE results, we estimated the thickness of the iron film before oxidation ("equivalent iron thickness"). We found that SE is highly sensitive to equivalent iron thickness and yields repeatable thickness measurements (ca. +/- 0.015 nm). We determined that the equivalent iron thickness variation we observed across different measurement locations on the same sample can be explained by error propagation from uncertainty in the underlying alumina thickness.
We investigated the initial adsorption states of three aminosilane precursors, tris(dimethylamino)silane (TDMASi), bis(diethylamino)silane (BDEASi), and di(isopropylamino)silane (DIPASi), on an –OH-terminated ALD alumina surface using in situ x-ray photoelectron spectroscopy with vacuum transfer. After a 0.5 s precursor pulse without oxidant, the scaled Si 2p/O 1s saturation values, corresponding to Si atoms adsorbed per 1000 O atoms in alumina, were nearly identical for all three precursors, indicating that the saturated Si coverage itself in the first half-cycle is essentially the same. Thus, the reported differences in growth per cycle cannot be explained simply by differences in the amount of adsorbed Si during the initial precursor exposure. In contrast, N 1s, C 1s, and compositional analyses revealed pronounced precursor-dependent ligand retention. TDMASi and BDEASi left substantial nitrogen- and carbon-containing species, whereas DIPASi showed no detectable N 1s signal and a much smaller C contribution. Deconvolution of the N 1s spectra showed that the nitrogen species remaining after TDMASi and BDEASi dosing consist mainly of N–C and N–Si components with a smaller N–O component, indicating the coexistence of undecomposed amino ligands bound to Si and nitrogen-containing ligands re-adsorbed on surface –OH sites. Quantitative comparison of the N–Si and N–O components further demonstrated that TDMASi retains more Si-bonded and readsorbed nitrogen-containing ligands than BDEASi, whereas the single bulky diisopropylamino ligand in DIPASi favors reaction with surface –OH groups and suppresses readsorption. These results show that precursor-dependent ligand removal and readsorption, rather than initial Si coverage, govern the first-half-cycle chemistry of aminosilane-based SiO2 atomic layer deposition.
Substrate crystal facet-driven self-assembly of porphyrin derivatives on metal substrates provides a versatile platform for understanding structure-property relationships at molecule-surface interfaces. In this work, we investigate the adsorption and supramolecular organization of a porphyrin derivative on Ag(100) and Ag(110) single-crystal surfaces using ultrahigh vacuum (UHV) scanning tunneling microscopy, and tip-enhanced Raman spectroscopy (TERS). On Ag(100), the molecules form nearly isotropic, close-packed two-dimensional assemblies characterized by a quasi-square unit cell, consistent with weak surface corrugation and intermolecular-interaction-dominated ordering. In contrast, adsorption on the anisotropic Ag(110) surface leads to pronounced directional self-assembly, where molecules align along the close-packed atomic rows to form rectangular unit cells and quasi-one-dimensional domains. Quantitative analysis reveals that this transition from isotropic to anisotropic packing originates from enhanced molecule-substrate interactions and direction-dependent diffusion barriers on Ag(110), which impose strong surface-templating effects. Furthermore, UHV-TERS measurements reveal distinct variations in spectral response between the two surfaces, reflecting differences in local adsorption environments. These experimental observations are further supported by phonon calculations, which capture the specific modifications of vibrational modes and provide insight into the origin of the observed spectral variations. These findings highlight the fundamental role of surface symmetry and corrugation in directing molecular organization and provide insight into the interplay between structural anisotropy and spectroscopic response at metal-organic interfaces.
Al-rich AlGaN is essential for ultra-wide-bandgap (UWBG) electronics and deep-ultraviolet optoelectronics, yet its processing is limited by the lack of damage-free etching methods. Plasma-based approaches introduce defects, while anisotropic wet etching for GaN often fails at high Al content due to the larger bandgap and stable oxide formation. Here, we demonstrate the first, to our knowledge, plasma-free hν-MacEtch of Al0.6Ga0.4N using a modified HF/(NH4)2S2O8 chemistry that enables stable etching and remains compatible with GaN, allowing a single process across AlGaN/GaN heterostructures. Ohmic contacts formed on hν-MacEtched surfaces show comparable contact resistance but significantly improved uniformity (<5% variation versus >15% using inductively coupled plasma reactive ion etching). These results establish a viable pathway for damage-free, reproducible processing of UWBG nitride materials and devices.
As dielectric metasurfaces move toward CMOS-compatible wafer-scale manufacturing, the plasma-etch transfer of high-aspect-ratio (HAR) silicon pillars on oxide-containing stacks has become a key challenge for process integration. Here, we investigate ∼10:1 Si pillar etching on a SiO2 underlayer in a 12-in. CMOS production-line environment under continuous-wave (CW) and pulsed-bias conditions. Under CW operation, aspect-ratio-dependent etching (also termed RIE)-lag-induced asynchronous etch-front arrival causes isolated pillars to reach the Si/SiO2 interface earlier than dense regions. The resulting local interface-exposure dwell time promotes foot loss, charge-assisted ion deflection, and mask-related profile degradation, leading to notching and collapse. Pulsed bias modifies this late-stage evolution nonmonotonically: 50% duty preserves vertical propagation and suppresses CW-type collapse, whereas lower duty cycles reduce continuous interface exposure but can induce taper or height-deficient profile loss. To rationalize these observations, we develop a mechanism-oriented two-dimensional feature-scale simulator that tracks etch-front timing, local interface-exposure dwell time, bottom-critical dimension evolution, and charge-assisted foot degradation. Guided by these results, a stage-dependent pulsed recipe using 50% duty for the main-etch step and 30% duty for the over-etch step yields substantially improved vertical pillar profiles. This work provides a process-level framework for interpreting HAR silicon pillar etching on oxide-containing stacks, with implications for silicon photonics, metasurface manufacturing, and other CMOS-compatible nanophotonic platforms.
A detailed analysis of all of the extant data for the N4,5 x-ray absorption spectroscopy of α-plutonium is presented here. Branching ratios and 5f populations (n) are produced and discussed. The impact of the Ni L2,3 (2p3/2,1/2) spectral structure caused by the beamline optical components is presented and quantified.
This study introduces a new concept of the quantum well formed on the homogeneous silicon nanosheet by the charged oxide walls. The “homogeneous nanosheet single quantum well” (HNSQW), sandwiched by the fixed charges originated from the oxides, forms homogeneous band well without any heterostructures. The HNSQW is available on extremely thin silicon-on-insulator technology, and numerical simulation verified the concept of this new idea. The numerical simulation revealed that the HNSQW neglects the doping type and forms the single-side QW region on either the conduction (negative fixed charge) and the valence (positive fixed charge) band. When the silicon layer thickness is thin enough, the HNSQW structure provides the increased on-current and minimized off-current, respectively, and its PN-junction is advantageous in terms of the low on-resistance and minimized leak current. State-of-the-art field-effect-transistors (FETs) of gate-all-around and complementary FET contain the similar structure within the channel region; therefore, the knowledge found in this study will be effective for further shrinking of FETs.
