We demonstrate the laser mediated atomic layer etching (ALEt) of silicon. Using a nanosecond pulsed 266 nm laser focused loosely over and in a parallel configuration to the surface of the silicon, we dissociate Cl2 gas to induce chlorination. Then, we use pulsed picosecond irradiation to remove the chlorinated layer. Subsequently, we perform continuous wave (CW) laser annealing to eliminate amorphization caused by the picosecond laser etching. Based on atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), we observed strong evidence of chlorination and digital etching at 0.85 nm etching per cycle with good uniformity.

3D printed acrylonitrile butadiene styrene (ABS) and polyvinyl alcohol (PVA) structures were infiltrated by alumina (Al2O3) using a trimethylaluminum(III) and water ALD process at 130 and 80 °C, respectively, to alter their physical properties. Differential scanning calorimetry was used to determine the glass transition temperature (Tg) of the polymers' pre- and post-deposition after varying the number of ALD cycles, resulting in a change of ∼9 and ∼ 27 °C for ABS and PVA, respectively. After one heat cycle, the postdeposition Tg reverted back to its predisposition point indicating reversibility of the deposition effects are possible. Optimal growing patterns, polymer composition, and inhibiting surface coatings—seen by energy-dispersive x-ray spectroscopy mapping—affected the amount of infiltration possible within the polymer substrate and, in turn, Tg. The results achieved provide guidelines to altering the physical and thermal properties of 3D printed polymer architectures.

We have investigated the effect of annealing on the magnetic anisotropy of MBE-grown GaMnAs1−yPy film in which phosphorus content varies from 0% to 24% along the growth direction. Such variation is achieved by growing a series of GaMnAs1−yPy layers in which y is successively increased. Hall effects measurements on an as-grown graded film reveal that the bottom 80% of the film has in-plane easy axes, 10% has both in-plane and perpendicular easy axes, and the remaining 10% has a vertical easy axis. Such gradual change of magnetic anisotropy in the film from in-plane to perpendicular with increasing P concentration is in accordance with the continuous variation of strain from compressive to tensile as the P concentration increases the bottom of the film to tensile toward its tip surface. However, thermal annealing significantly changes the magnetic anisotropy of the graded GaMnAs1−yPy film. In particular, the intermediate region having both in-plane and perpendicular easy axes nearly disappears in the film after annealing, so the film is divided into two types of layers having either only in-plane or only perpendicular anisotropy. These dramatic changes in magnetic anisotropy of the graded GaMnAs1−yPy film introduced by annealing suggest that one can strategically use this process to realize orthogonal magnetic bilayers consisting of in-plane and perpendicular easy axes.

Strain-engineering is a powerful means to tune the polar, structural, and electronic instabilities of incipient ferroelectrics. KTaO3 is near a polar instability and shows anisotropic superconductivity in electron-doped samples. Here, we demonstrate growth of high-quality KTaO3 thin films by molecular-beam epitaxy. Tantalum was provided by either a suboxide source emanating a TaO2 flux from Ta2O5 contained in a conventional effusion cell or an electron-beam-heated tantalum source. Excess potassium and a combination of ozone and oxygen (10% O3 + 90% O2) were simultaneously supplied with the TaO2 (or tantalum) molecular beams to grow the KTaO3 films. Laue fringes suggest that the films are smooth with an abrupt film/substrate interface. Cross-sectional scanning transmission electron microscopy does not show any extended defects and confirms that the films have an atomically abrupt interface with the substrate. Atomic force microscopy reveals atomic steps at the surface of the grown films. Reciprocal space mapping demonstrates that the films, when sufficiently thin, are coherently strained to the SrTiO3 (001) and GdScO3 (110) substrates.

Chemical vapor deposition of indium nitride (InN) is severely limited by the low thermal stability of the material, and, thus, low-temperature deposition processes such as atomic layer deposition (ALD) are needed to deposit InN films. The two chemically and structurally closely related materials—aluminum nitride and gallium nitride (GaN)—can be deposited by both plasma and thermal ALD, with ammonia (NH3) as a nitrogen precursor in thermal processes. InN, however, can only be deposited using plasma ALD, indicating that there might be a limitation to thermal ALD with NH3 for InN. We use quantum-chemical density functional theory calculations to compare the adsorption process of NH3 on GaN and InN to investigate if differences in the process could account for the lack of thermal ALD of InN. Our findings show a similar reactive adsorption mechanism on both materials, in which NH3 could adsorb onto a vacant site left by a desorbing methyl group from the surfaces. The difference in energy barrier for this adsorption indicates that the process is many magnitudes slower on InN compared to GaN. Slow kinetics would hinder NH3 from reactively adsorbing onto InN in the timeframe of the ALD growth process and, thus, limit the availability of a thermal ALD process.

This study measured the stress relief extent of a hard coating by a metal interlayer in a bilayer system. An energy-balance model was used to evaluate the energy relief efficiency (ξtot) by the interlayer. The objective of this study was to understand the relationship between plastic deformation and the energy relief efficiency of the metal interlayer in a bilayer thin film system. A TiN/Ti bilayer thin film was chosen as the model system. TiN/Ti samples were prepared with different interlayer thicknesses and under different stress levels in TiN coating using unbalanced magnetron sputtering. The overall stress of the bilayer samples was determined by the laser curvature method, and the stress in the individual layer was measured by average x-ray strain combined with nanoindentation method. For the TiN/Ti sample with Ti interlayer thickness >78 nm, a maximum ξtot was reached at an interlayer thickness about 110 nm; further increasing the interlayer thickness may decrease ξtot. This was mainly due to plastic deformation of the Ti interlayer being localized near the TiN/Ti interface. The results also showed that ξtot increased with increasing stress in the TiN coating. The model analyses revealed that the energy relief was mostly contributed from the TiN coating, while less than 30% was from curvature relaxation of the Si substrate. For the sample with insufficient thickness (52 nm) of an Ti interlayer, the stress of the TiN coating could not be effectively relieved and the interlayer was subjected to compressive stress. In this case, the energy-balance model was not valid, while our previous elastic model could be used to account for the stress state transition. The residual stress state of the Ti interlayer can serve as an index to assess the effectiveness in relieving film stress by the interlayer. The interlayer is functioning by sustaining tensile stress, whereas it is ineffective if the interlayer is subjected to compressive stress.

In this paper, we introduce a vacuum cluster tool designed specifically for studying reaction mechanisms in atomic layer deposition (ALD) and atomic layer etching (ALE) processes. In the tool, a commercial flow-type ALD reactor is in vacuo connected to a set of UHV chambers so that versatile surface characterization is possible without breaking the vacuum environment. This way the surface composition and reaction intermediates formed during the precursor or etchant pulses can be studied in very close to true ALD and ALE processing conditions. Measurements done at each step of the deposition or etching cycle add important insights about the overall reaction mechanisms. Herein, we describe the tool and its working principles in detail and verify the equipment by presenting results on the well-known trimethyl aluminum–water process for depositing Al2O3.

Gallium oxide (Ga2O3) exhibits complex behavior under ion irradiation since ion-induced disorder affects not only the functional properties but can provoke polymorphic transformations in Ga2O3. A conventional way used to minimize the lattice disorder is by doing postirradiation anneals. An alternative approach is to prevent the disorder accumulation from the beginning, by doing implants at elevated temperatures, so that a significant fraction of the disorder dynamically anneals out in radiation-assisted processes. Here, we use these two approaches for the minimization of radiation disorder in monoclinic β-Ga2O3 implanted to a dose below the threshold required for the polymorphic transformations. The results obtained by a combination of channeling and x-ray diffraction techniques revealed that implants at 300 °C effectively suppress the defect formation in β-Ga2O3. On the other hand, in order to reach similar crystalline quality in the samples implanted at room temperature, postirradiation anneals in excess of 900 °C are necessary.

We report the physical properties of Co2CrAl Heusler alloy epitaxial thin films grown on a single-crystalline MgO(001) substrate using a pulsed laser deposition technique. The x-ray diffraction pattern in the θ-2θ mode showed the film growth in a single phase B2-type ordered cubic structure with the presence of (002) and (004) peaks, and the film oriented along the MgO(001) direction. The ϕ scan along the (220) plane confirms the fourfold symmetry, and the epitaxial growth relation is found to be Co2CrAl(001)[100]||MgO(001)[110]. The thickness of about 12 nm is extracted through the analysis of x-ray reflectivity data. The isothermal magnetization (M–H) curves confirm the ferromagnetic (FM) nature of the thin film having significant hysteresis at 5 and 300 K. From the in-plane M–H curves, the saturation magnetization values are determined to be 2.1 μB/f.u. at 5 K and 1.6 μB/f.u. at 300 K, which suggest the soft FM behavior in the film having the coercive field ≈522 Oe at 5 K. The thermomagnetization measurements at 500 Oe magnetic field show the bifurcation between field-cooled and zero-field-cooled curves below about 100 K. The normalized field-cooled magnetization curve follows the T2 dependency, and the analysis reveals the Curie temperature around 335±11 K. Moreover, the low-temperature resistivity indicates semiconducting behavior with the temperature, and we find a negative temperature coefficient of resistivity (5.2×10−4/K).

Ultrawide bandgap β-(AlxGa1−x)2O3 vertical Schottky barrier diodes on (010) β-Ga2O3 substrates are demonstrated. The β-(AlxGa1−x)2O3 epilayer has an Al composition of 21% and a nominal Si doping of 2 × 1017 cm−3 grown by molecular beam epitaxy. Pt/Ti/Au has been employed as the top Schottky contact, whereas Ti/Au has been utilized as the bottom Ohmic contact. The fabricated devices show excellent rectification with a high on/off ratio of ∼109, a turn-on voltage of 1.5 V, and an on-resistance of 3.4 mΩ cm2. Temperature-dependent forward current-voltage characteristics show effective Schottky barrier height varied from 0.91 to 1.18 eV while the ideality factor from 1.8 to 1.1 with increasing temperatures, which is ascribed to the inhomogeneity of the metal/semiconductor interface. The Schottky barrier height was considered a Gaussian distribution of potential, where the extracted mean barrier height and a standard deviation at zero bias were 1.81 and 0.18 eV, respectively. A comprehensive analysis of the device leakage was performed to identify possible leakage mechanisms by studying temperature-dependent reverse current-voltage characteristics. At reverse bias, due to the large Schottky barrier height, the contributions from thermionic emission and thermionic field emission are negligible. By fitting reverse leakage currents at different temperatures, it was identified that Poole–Frenkel emission and trap-assisted tunneling are the main leakage mechanisms at high- and low-temperature regimes, respectively. Electrons can tunnel through the Schottky barrier assisted by traps at low temperatures, while they can escape these traps at high temperatures and be transported under high electric fields. This work can serve as an important reference for the future development of ultrawide bandgap β-(AlxGa1−x)2O3 power electronics, RF electronics, and ultraviolet photonics.

Electron-stimulated etching of surfaces functionalized by remote plasma is a flexible and novel approach for material removal. In comparison with plasma dry etching, which uses the ion-neutral synergistic effect to control material etching, electron beam-induced etching (EBIE) uses an electron-neutral synergistic effect. This approach appears promising for the reduction of plasma-induced damage (PID), including atomic displacement and lateral straggling, along with the potential for greater control and lateral resolution. One challenge for EBIE is the limited selection of chemical precursor molecules that can be used to produce functionalized materials suitable for etching under electron beam irradiation. In this work, we studied a new experimental approach that utilizes a remote plasma source to functionalize substrate surfaces in conjunction with electron beam irradiation by an electron flood gun. Etching rates (ERs) of SiO2, Si3N4, and poly-Si are reported in a broad survey of processing conditions. The parametric dependence of the ER of these Si-based materials on the operating parameters of the flood gun and the remote plasma source is evaluated. We also identified the processing parameters that enable the realization of material selective removal, i.e., the etching selectivity of Si3N4 over SiO2 and poly-Si over SiO2. Additionally, surface characterization of etched materials is used to clarify the effects of the co-introduction of particle fluxes from the remote plasma and flood gun sources on surface chemistry.

Two atomic layer etching (ALE) methods were studied for crystalline GaN, based on oxidation, fluorination, and ligand exchange. Etching was performed on unintentionally doped GaN grown by hydride vapor phase epitaxy. For the first step, the GaN surfaces were oxidized using either water vapor or remote O2-plasma exposure to produce a thin oxide layer. Removal of the surface oxide was addressed using alternating exposures of hydrogen fluoride (HF) and trimethylgallium (TMG) via fluorination and ligand exchange, respectively. Several HF and TMG super cycles were implemented to remove the surface oxide. Each ALE process was monitored in situ using multiwavelength ellipsometry. X-ray photoelectron spectroscopy was employed for the characterization of surface composition and impurity states. Additionally, the thermal and plasma-enhanced ALE methods were performed on patterned wafers and transmission electron microscopy (TEM) was used to measure the surface change. The x-ray photoelectron spectroscopy measurements indicated that F and O impurities remained on etched surfaces for both ALE processes. Ellipsometry indicated a slight reduction in thickness. TEM indicated a removal rate that was less than predicted. We suggest that the etch rates were reduced due to the ordered structure of the oxide formed on crystalline GaN surfaces.

Three metastable compounds predicted to be kinetically stable using an “island” approach were successfully synthesized from designed modulated elemental reactants. Fe0.8V0.2Se2 was synthesized by depositing ultrathin elemental layers in a V|Fe|Se sequence to control the local composition. An alloyed rock salt structured Pb3Mn2Se5 constituent layer, which does not exist as a bulk compound, was synthesized in the heterostructure (Pb3Mn2Se5)0.6VSe2 by depositing a precursor with a V|Se|Pb|Se|Mn|Se|Pb|Se|Mn|Se|Pb|Se sequence of elemental layers that mimicked the compositional profile of the targeted heterostructure. The heterostructure (PbSe)1+δ(FeSe2)2 was prepared by depositing a precursor with a repeating layering sequence of Fe|Pb|Fe|Se, where each sequence contains the number of atoms required to form a single unit cell. In all three systems, the local compositions in the layer sequence kinetically favored the nucleation and growth of the targeted products during the deposition. The diffusion lengths to form the targeted compounds were short, and the diffusion was limited by postdeposition low temperature annealing to favor the growth of the targeted compounds and avoid the decomposition into a mixture of thermodynamically stable compounds.

Hybrid molecular beam epitaxy (MBE) growth of Sn-modified BaTiO3 films was realized with varying domain structures and crystal symmetries across the entire composition space. Macroscopic and microscopic structures and the crystal symmetry of these thin films were determined using a combination of optical second harmonic generation (SHG) polarimetry and scanning transmission electron microscopy (STEM). SHG polarimetry revealed a variation in the global crystal symmetry of the films from tetragonal (P4mm) to cubic (Pm3¯m) across the composition range, x = 0 to 1 in BaTi1−xSnxO3 (BTSO). STEM imaging shows that the long-range polar order observed when the Sn content is low (x = 0.09) transformed to a short-range polar order as the Sn content increased (x = 0.48). Consistent with atomic displacement measurements from STEM, the largest polarization was obtained at the lowest Sn content of x = 0.09 in Sn-modified BaTiO3 as determined by SHG. These results agree with recent bulk ceramic reports and further identify this material system as a potential replacement for Pb-containing relaxor-based thin film devices.

We report a new type of structural defect in β-Ga2O3 homoepitaxial thin films grown by metalorganic vapor phase epitaxy, which we have dubbed as “sympetalous defects.” These consist of a line defect (for example, a nanotube defect) in the underlying substrate combined with a multi-faceted inverted polycrystalline pyramid in the epitaxial film, which may also be decorated with twinned polycrystalline grains. In plan-view atomic force, scanning electron, or optical microscopies, the sympetalous defects appear similar in shape to polygonal etch pits observed for single crystals. Photoluminescence microscopy exposed spots of polarization-dependent luminescence at these defects, different from the single crystal films' luminescence. Furthermore, some of the defects exhibited circular dichroism in their luminescence that we correlated with partial helices formed within the pits by the arrangement of linearly dichroic polycrystalline grains. Finally, the density of sympetalous defects agrees with the etch pit densities of the substrates. Understanding and controlling these defects will be of importance as they modify the local properties of films, affect fabricated device yields, and influence characterization experiments.

Thin films of ferromagnetic (Fe70Ni30)96Mo4 were grown via molecular beam epitaxy. Their magnetic properties and magnetocaloric effects were investigated. X-ray diffraction and vibrating sample magnetometry measurements confirmed the crystalline ferromagnetic (Fe70Ni30)96Mo4 phase, with a Curie point near room temperature. To determine the suitability of this material for magnetocaloric applications, we observed a large magnetic entropy change with a large temperature span as well as high relative cooling power near Curie temperature comparable to rare-earth-based materials operating near room temperature.

As the sizes of semiconductor devices continue to shrink, the fabrication of nanometer-scale device structures on material surfaces poses unprecedented challenges. In this study, molecular dynamics simulations of CF3+ ion beam etching of SiO2 were performed with carbon masks to form holes with a diameter of 4 nm. It is found that, when the ion energy is sufficiently high and the etching continues, tapered holes are formed by the ion beam etching. This is because the etching under these conditions is essentially due to physical sputtering, so that tapered surfaces having high etching yields appear as the sidewalls and sputtered Si-containing species are redeposited. Furthermore, preferential removal of oxygen from SiO2 surfaces occurs, which leads to the formation of Si-rich sidewall surfaces. It is also found that, with simultaneous irradiation of CF3 radicals, the etching yield of a flat SiO2 surface by energetic CF3+ ion beams can double, but too large a flux of CF3 radicals causes etch stop.

We introduce a novel technique, masked-assisted radial-segmented target pulsed-laser deposition (MARS-PLD) for unprecedented capabilities in area-selective physical vapor deposition. The MARS-PLD setup consists of a conventional PLD chamber with mechanical feedthrough for a laterally movable mask or mask set. By this means and, in principle, the arbitrary choice of a shadow mask layout, any desired area on a substrate can be masked in order to create multinary lateral and vertical material composition gradients using radially segmented targets already described in the literature [Kneiß et al., ACS Comb. Sci. 20, 643–652 (2018)]. To illustrate the capabilities of this method, we fabricated material gradients in (Mg,Zn)O thin films with a nearly linear spatial variation of the cation composition of 15at.%mm−1. Additionally, we fine-tuned our setup to fabricate a material gradient on a predefined two-dimensional lateral pattern to demonstrate the versatile capabilities of the MARS-PLD technique.

The thermal atomic layer etching (ALE) of VO2 was demonstrated using sequential exposures of BCl3 and SF4. The VO2 etch rate measured by quartz crystal microbalance investigations at 250 °C was 2.3 Å/cycle. The mass losses during individual BCl3 and SF4 reactions were nearly self-limiting versus BCl3 and SF4 exposures. The VO2 etch rates were also dependent on temperature and varied from 0.05 Å/cycle at 150 °C to 2.3 Å/cycle at 250 °C. Fourier transform infrared (FTIR) spectroscopy studies observed VO2 etching by monitoring the decrease in absorbance from V—O stretching vibrations in the VO2 film. The FTIR spectra during the initial BCl3 exposures on the VO2 film observed the growth of absorbance from B—O stretching vibrations from B2O3 and the concurrent loss of V=O vibrational features. These changes were consistent with BCl3 converting VO2 to B2O3. The FTIR difference spectra during subsequent SF4 and BCl3 reactions also observed the growth and loss of absorbance features that were attributed to F3V=O and V—F stretching vibrations, respectively. These changes indicate that SF4 fluorinates VO2 to form a VOF3 surface layer and then BCl3 undergoes ligand-exchange with VOF3 to volatilize the VOF3 surface layer as VOCl3. There was also evidence for conversion of VO2 to B2O3 during BCl3 exposures and then removal of B2O3 by SF4 exposures. In addition, quadrupole mass spectrometry (QMS) measurements observed that the SF4 exposures produced ion intensities for SOxFyClz products in oxidation states greater than 4+. These SOxFyClz products indicate that SF4 is being oxidized and acting as a deoxyfluorination reactant. Concurrently, the QMS analysis also monitored ion intensity for S8+, S7+, S6+, S5+, and S4+. These S8 electron impact ionization products argue that SF4 oxidation occurs concurrently with SF4 reduction. The QMS also observed ion intensities corresponding to VCl4+ and VOCl3+. The presence of VOCl3+ indicates that the oxidation state of vanadium has increased to 5+ in some of the volatile etch products. The QMS also detected trichloroboroxin (B3O3Cl3) during BCl3 exposures. B3O3Cl3 is a known etch product of B2O3 during BCl3 exposures. BCl3 can convert VO2 to B2O3 and then proceed to etch the converted B2O3. Thermal VO2 ALE using BCl3 and SF4 reveals the rich complexity of surface etching reactions that can proceed by multiple pathways including conversion, ligand-exchange, and oxidation state changes.

Tungsten oxide–silicon dioxide (WOx–SiOy) composite thin films were deposited for the first time via the remote oxygen plasma-enhanced atomic layer deposition (ALD) process using a novel metal-organic heteronuclear and heteroleptic precursor, bis(tert-butylimido)bis(trimethylsilylmethyl)tungsten. Self-limiting ALD growth was demonstrated over a wide temperature window of 203–328 °C with growth per cycle decreasing with increasing temperature from 0.75 to 0.4 Å/cycle, respectively. Residual gas analysis revealed ligand competition and showed that ligand reaction during ALD nucleation and growth was a function of deposition temperature, thereby affecting the film composition. As the temperature increased from 203 to 328 °C, the film composition [W/(Si + W)] ranged from 0.45 to 0.53. In addition, the carbon impurity content was reduced and the refractive index increased from 1.73 to 1.96, the density increased from 4.63 to 5.6 g/cm3, and the optical bandgap decreased from 3.45 to 3.27 eV. Grazing angle x-ray diffraction indicated that as-deposited films were amorphous. Upon annealing in O2 at 500 °C or higher, depending on deposition temperature, films are crystalized into the triclinic WO3 phase. At the same time, WO3 is sublimed from the surface and films are reduced in thickness.