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Energy-dispersive X-ray spectroscopy (EDX) is often performed simultaneously with high-angle annular dark-field scanning transmission electron microscopy (STEM) for nanoscale physico-chemical analysis. However, high-quality STEM-EDX tomographic imaging is still challenging due to fundamental limitations such as sample degradation with prolonged scan time and the low probability of X-ray generation. To address this, we propose an unsupervised deep learning method for high-quality 3D EDX tomography of core–shell nanocrystals, which can be usually permanently dammaged by prolonged electron beam. The proposed deep learning STEM-EDX tomography method was used to accurately reconstruct Au nanoparticles and InP/ZnSe/ZnS core–shell quantum dots, used in commercial display devices. Furthermore, the shape and thickness uniformity of the reconstructed ZnSe/ZnS shell closely correlates with optical properties of the quantum dots, such as quantum efficiency and chemical stability. Advanced electron microscopy and spectroscopy techniques can reveal useful structural and chemical details at the nanoscale. An unsupervised deep learning approach helps to reconstruct 3D images and observe the relationship between optical and structural properties of semiconductor nanocrystals, of interest in optoelectronic applications.

Epitaxial SrTiO 3 thin films were grown by molecular beam epitaxy (MBE) using an oxygen plasma source. SrO and TiO2 layers were alternately deposited on 0.5 wt% (001) Nb doped SrTiO3 substrates who...

Scaling down of semiconductor devices requires high- k dielectric materials to continue lowering the operating voltage of field-effect transistors (FETs) and storing sufficient charge on a smaller area. Here, we investigate the dielectric properties of epitaxial BaHf 0.6 Ti 0.4 O 3 (BHTO), an alloy of perovskite oxide barium hafnate (BaHfO 3 ) and barium titanate (BaTiO 3 ). We found the dielectric constant, the breakdown field, and the leakage current to be 150, 5.0 megavolts per centimeter (MV cm −1 ), and 10 −4 amperes per square centimeter at 2 MV cm −1 , respectively. The results suggest that two-dimensional (2D) carrier density of more than n 2D = 10 14 per square centimeter (cm −2 ) could be modulated by the BHTO gate oxide. We demonstrate an n-type accumulation mode FET and direct suppression of more than n 2D = 10 14 cm −2 via an n-type depletion-mode FET. We attribute the large dielectric constant, high breakdown field, and low leakage current of BHTO to the nanometer scale stoichiometric modulation of hafnium and titanium.

Scaling down of semiconductor devices requires high

By controlling the configuration of polymorphic phases in high-k Hf

As the size of the semiconductor device decreases, the importance of the low resistance contacts to devices cannot be overstated. Here, we studied the contact resistance to buried nanometer thick δ-doped Ba1-xLaxSnO3 (BLSO) layers. We have used epitaxial 4% (x = 0.04) BLSO as a contact material, which has additional advantages of forming Ohmic contacts to BaSnO3 and providing thermal stability even at high temperatures. The contact resistance was measured by a modified transmission line method designed to eliminate the contribution from the resistance of the contact material. The upper bound for the contact resistance to a 12 nm thick δ-doped 1% BLSO conductive layer was measured to be 1.25 × 10−1or 2.87 × 10−7 Ω cm2. Our results show that it is possible to provide low resistance epitaxial edge contacts to an embedded nanometer-thick BLSO conductive layer using an ion-milling process. Our low resistance contact method can be easily extended to a two-dimensional electron gas at the oxide interfaces such as LaInO3/BaSnO3.

We outline a method to synthesize ( ATiO3) n AO Ruddlesden–Popper phases with high- n, where the A-site is a mixture of barium and strontium, by molecular-beam epitaxy. The precision and consistency of the method described is demonstrated by the growth of an unprecedented (SrTiO3)50SrO epitaxial film. We proceed to investigate barium incorporation into the Ruddlesden–Popper structure, which is limited to a few percent in bulk, and we find that the amount of barium that can be incorporated depends on both the substrate temperature and the strain state of the film. At the optimal growth temperature, we demonstrate that as much as 33% barium can homogeneously populate the A-site when films are grown on SrTiO3 (001) substrates, whereas up to 60% barium can be accommodated in films grown on TbScO3 (110) substrates, which we attribute to the difference in strain. This detailed synthetic study of high n, metastable Ruddlesden–Popper phases is pertinent to a variety of fields from quantum materials to tunable dielectrics.

Sb 2 Se 3 is an emerging material among alternative light absorbers for photovoltaic applications. Unlike typical chalcogenides, Sb 2 Se 3 is particularly appealing due to its single stable crystal phase, layered structure with loose binding of no dangling bonds. Nevertheless, a cost-effective electrochemical approach for the synthesis of Sb 2 Se 3 compounds has not been identified, and the Sb 2 Se 3 film with the most favorable [001] preferred orientation has only just been developed. In this study, Sb-rich precursors were prepared electrochemically at −950 mV (vs. Ag/AgCl), and homogeneous Sb 2 Se 3 thin films were produced using a pre-thermal treatment process prior to the typical selenization process with additional Se coating. This novel procedure notably suppresses potential Sb dissolution into liquid Se due to the formation of polycrystalline Sb-related crystals. As a result, the Sb-rich precursor was successfully transformed into Sb 2 Se 3 thin films with an enhanced perpendicular orientation of the [001] direction. Unfortunately, a high density of voids was produced in the precursor film with two distinguishable layers, and their size increased after selenization. The voids were formed through evolution of H 2 Se gas after the initial electrochemical reaction. The resulting photovoltaic cells demonstrated an energy conversion efficiency of 1.8% in a substrate structure consisting of Mo/Sb 2 Se 3 /CdS/ZnO/ITO.

4H- and 6H-SiC grown by physical vapor transport method were investigated by transmission electron microscopy (TEM). From the TEM diffraction patterns observed along the [11-20] zone axis, 4H- and 6H-SiC were identified due to their additional diffraction spots, indicating atomic stacking sequences. However, identification was not possible in the [10-10] zone axis due to the absence of additional diffraction spots. Basal plane dislocations (BPDs) were investigated in the TEM specimen prepared along the [10-10] zone axis using the two-beam technique. BPDs were two Shockley partial dislocations with a stacking fault (SF) between them. Shockley partial BPDs arrayed along the [0001] growth direction were observed in the investigated 4H-SiC. This arrayed configuration of Shockley partial BPDs cannot be recognized from the plan view TEM with the [0001] zone axis. The evaluated distances between the two Shockley partial dislocations for the investigated samples were similar to the equilibrium distance, with values of several hundreds of nanometers or even values as large as over a few micrometers.

Native oxide removal prior to poly-Si contact and epitaxial growth of Si is the most critical technology to ensure process and device performances of poly-Si plugs and selective epitaxial growth (SEG) layers for DRAM, flash memory, and logic device. Recently, dry cleaning process for interfacial oxide removal has attracted a world-wide attention due to its superior passivation properties to conventional wet cleaning processes. In this study, we investigated the surface states of Si substrate during and after dry cleaning process, and the role of atomic elements including fluorine and hydrogen on the properties of subsequent deposited silicon layer using SIMS, XPS, and TEM analysis. The controlling of residual fluorine on the Si surface after dry cleaning is a key factor for clean interface. The mechanism of native oxide re-growth caused by residual fluorine after dry cleaning is proposed based on analytical results.

Multicomponent amorphous InGaZnO thin films with several metal cations have been synthesized with flexible chemical composition control based on a sol–gel process, and a combinatorial approach through a sol–gel process enables us to perform a systematic survey to fluently find the best film properties. Contrary to amorphous films, crystalline InGaO3(ZnO)m requires a refined chemical composition ratio among metal cations. These ratios are expected to affect the growth evolution and thermoelectric properties of two-dimensional InGaO3(ZnO)m superlattice structures with various compositional combinations. Here, we explore a combinatorial approach to the ratio of metal cations using various mole fractions of metal precursors in InGaZnO sol for amorphous InGaZnO films fabricated on an epitaxial ZnO buffer layer, and then, they were crystallized with various chemical compositions. The crystallized InGaO3(ZnO)m films can be classified as strong single-phase InGaO3(ZnO)m, double-phase InGaO3(ZnO)m/InGaO3(ZnO)m+1, and weak single-phase InGaO3(ZnO)m with excess metal ions. Among them, the strong single-phase InGaO3(ZnO)m films with superlattice structures showed superior thermoelectric power factors. The detailed microstructural growth evolution of single- and double-phase InGaO3(ZnO)m films was investigated using transmission electron microscopy.

The SrRuO3 (SRO) thin films were epitaxially grown on (0 0 1) SrTiO3 (STO) substrates using plasma assisted molecular beam epitaxy (MBE). In order to identify how the cation flux affects the surface characteristics, films were grown at different Sr/Ru flux ratios. As Ru flux increased, SRO thin film exhibited a flatter but more bunched stepped terrace implying the prevalence of 2-D growth mode during the film formation, whereas excessive Sr flux led to 3-D film growth and resulted in a rough surface. In addition, XRD and STEM analysis provided that samples grown at Sr/Ru flux ratio higher than 2.7 showed volume expansion and crystal disorder by Ru vacancy which caused higher resistivity. But the samples grown at the ratio lower than 2.9 maintained the resistivity close to that of stoichiometric SRO.

For practical device applications, monolayer transition metal dichalcogenide (TMD) films must meet key industry needs for batch processing, including the high-throughput, large-scale production of high-quality, spatially uniform materials, and reliable integration into devices. Here, high-throughput growth, completed in 12 min, of 6-inch wafer-scale monolayer MoS2 and WS2 is reported, which is directly compatible with scalable batch processing and device integration. Specifically, a pulsed metal-organic chemical vapor deposition process is developed, where periodic interruption of the precursor supply drives vertical Ostwald ripening, which prevents secondary nucleation despite high precursor concentrations. The as-grown TMD films show excellent spatial homogeneity and well-stitched grain boundaries, enabling facile transfer to various target substrates without degradation. Using these films, batch fabrication of high-performance field-effect transistor (FET) arrays in wafer-scale is demonstrated, and the FETs show remarkable uniformity. The high-throughput production and wafer-scale automatable transfer will facilitate the integration of TMDs into Si-complementary metal-oxide-semiconductor platforms.

Typically, three kinds of ZnO nanostructures, nanowires, nanosheets, and nanorods, are synthesized by changing the reactor pressure in the chemical vapor transport method. The photoelectrochemical (PEC) properties of ZnO nanostructures were investigated by using an ultraviolet lamp with a wavelength of 365 nm and a measured intensity of 0.4 mW/cm22. Photocurrent densities of 0.61, 0.47, and 0.37 mA/cm2 were obtained for nanowires, nanosheets, and nanorods, respectively. The photoconversion efficiencies of these ZnO nanostructures under ultraviolet illumination were calculated as 73.1, 57.3, and 41.8%, respectively. The different PEC results were explained by using the effects of dimension in the nanostructures. The separation of light-induced charge formed near the surface, leading to the transfer of electron holes toward the surface and electrons toward the bulk at the photo-anode, will be higher as the dimension is decreased. The difference in the optical absorption in the PEC process could be neglected because the absorption intensities were nearly the same for the three kinds of samples, independent of both the morphology and the density. Therefore, the different PEC efficiencies could be thought to be a result of the difference in the nanostructures with different dimensions, not the result of the density of the nanostructure.

We report on the structural and optical properties of non polar a-plane MgxZn1−xO (0 ⩽ x ⩽ 0.57) films on r-plane sapphire substrates grown by plasma-assisted molecular beam epitaxy. Reflection high energy electron diffraction (RHEED) revealed a formation of cubic MgO phase when an Mg concentration increases. Room temperature (RT) photoluminescence (PL) and transmission electron microscopy consistently revealed the formation of cubic MgO phase from the Mg0.21Zn0.79O film. The Mg0.11Zn0.89O film showed a band edge emission at ∼360 nm, which is a shorter wavelength than the ZnO (∼373 nm), from the RT PL measurements. Photoluminescence excitation (PLE) measurements at RT showed that band-gap energies of MgxZn1−xO films could be tuned up to ∼4.65 eV (∼270 nm) although cubic MgO phase were mixed for high Mg concentration. For the single phase wurtzite MgZnO film, band-gap energy of 3.48 eV was obtained from the Mg0.11Zn0.89O film.

The convenient synthesis of one-dimensional nanostructures of chalcogenide compounds with a visible band-gap is an essential research topic in developing next-generation photoelectronic devices. In particular, the design of a theoretically predictable synthesis process provides great flexibility and has a considerable ripple effect in nanotechnology. In this study, a novel rational growth approach is designed using the spinodal decomposition phenomenon for the synthesis of the Sb2Se3 nanowires, which is based on the thermodynamic phase diagram. Using a stacked elemental layer (Sb/Sb–Se/Se) and heat treatment at 623 K for 30 min under an N2 atmosphere, the vertically inclined one-dimensional nanostructures are experimentally demonstrated. An additional annealing process at 523 K in a vacuum effectively removed excess Se elements due to their high vapor pressure, resulting in highly dense single crystal Sb2Se3 nanowire arrays. Adaption of our synthesis approach enables significantly improved photocurrent generation in the vertically stacked structure (glass/ITO/Sb2Se3 nanowires/ITO/PEN) from 6.4 (dark) to under 690 μA (at 3 V under AM 1.5G). In addition, a photoelectrochemical test demonstrated their p-type conductivity and robust photocorrosion performance in 0.5 M H2SO4.

InGaN layers with controlled In composition up to 30 at.% are grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. By considering the growth rate differences in GaN and InN caused by different vapor pressure and sticking coefficient, factors of the Ga and In source fluxes for the targeted In composition are determined. By applying the factors, InGaN layers with the almost same In compositions are grown. Before the growth the substrates were nitrided by rf-nitrogen plasma, which resulted in the formation of epitaxial AlN layer. The growth of thin GaN on this AlN surface shows strong streaky reflection high energy electron diffraction pattern with a specular spot, however, InGaN layers on the GaN layer show spotty patterns. Surface morphology of the InGaN layers shows island-like granules and the granule-like morphology is getting clear as the In composition and roughness are increased, too. The InGaN layers with In composition up to 30 at.% do not show formation of InN and only InGaN peaks are detected from the X-ray diffraction. Crystal quality of the InGaN layer with In composition of 15 at.% is worse than that of 30 at.%-In layer as addressed by larger broadening of X-ray rocking curves.

The growth of high-quality indium (In)-rich InXGa1−XN alloys is technologically important for applications to attain highly efficient green light-emitting diodes and solar cells. However, phase separation and composition modulation in In-rich InXGa1−XN alloys are inevitable phenomena that degrade the crystal quality of In-rich InXGa1−XN layers. Composition modulations were observed in the In-rich InXGa1−XN layers with various In compositions. The In composition modulation in the InXGa1−XN alloys formed in samples with In compositions exceeding 47%. The misfit strain between the InGaN layer and the GaN buffer retarded the composition modulation, which resulted in the formation of modulated regions 100 nm above the In0.67Ga0.33N/GaN interface. The composition modulations were formed on the specific crystallographic planes of both the {0001} and {0114} planes of InGaN.

A dislocation-eliminating chemical control method for high-quality GaN nanostructures together with various types of InGaN quantum well structures are demonstrated using a chemical vapor-phase etching technique. Unlike chemical wet etching, chemical vapor-phase etching could efficiently control the GaN and form various shapes of dislocation-free and strain-relaxed GaN nanostructures. The chemically controlled GaN nanostructures showed improved crystal quality due to the selective etching of defects and revealed various facets with reduced residual strain via the facet-selective etching mechanism. These structural properties derived excellent optical performance of the GaN nanostructures. The chemical vapor-phase etching method also showed possibilities of the fascinating applications for high-efficiency InGaN quantum well structures, such as InGaN quantum well layer on void embedded GaN layer, InGaN quantum well embedded GaN nanostructure, and InGaN/GaN core/shell nanostructure.

The composition modulation of in In-rich InxGa1−xN layers with an indium content as high as ∼67% was suppressed in plasma-assisted molecular beam epitaxy on c-sapphire substrates. It was found that the higher nitrogen plasma flow rate was very effective in suppression of composition modulation in In-rich InGaN layers. X-ray diffraction, X-ray energy dispersive spectroscopy, and transmission electron microscopic images clearly showed disappearance of composition modulation with increasing the nitrogen flow rate.

The effects of growth temperatures (in a wide range from 100 to 800 °C) on properties of a-plane (11 2 ¯ 0) ZnO films grown on r-plane (1 1 ¯ 02) sapphire substrates by plasma-assisted molecular beam epitaxy were investigated. The film was grown as polycrystalline at 100 °C, but grown as single crystalline at the temperatures from 200 to 800 °C without any mixture of c-plane (0001) ZnO and m-plane (10 1 ¯ 0) ZnO. The single crystalline ZnO films showed anisotropic surface morphology with the conglomerated granules or striations along the [0001]ZnO direction. The ZnO film grown at 400 °C showed better crystal quality than others. It showed the smallest full width at half maximums of 0.450° and 0.297° for (11 2 ¯ 0) x-ray omega rocking curves under the measuring configuration with the omega axis parallel and perpendicular to the ZnO and directions, respectively, and 0.387° for (10 1 ¯ 1) omega rocking curve. The threading dislocation and the stacking fault densities were determined to be ~ 3.7 × 1010 cm−2 and ~ 8.5 × 104 cm−1 for the ZnO film grown at 400 °C.

As complementary metal-oxide-semiconductor technology nodes are scaled down, lowering the contact resistance has become a critical problem for continued scaling. In this study, we suggested the reduction method of the Schottky barrier height, one of the main causes of contact resistance, by insertion of atomically thin two-dimensional (2D) materials between the metal and Si interface. Also, we found that the inserted 2D materials could modulate the work function of the metal and mitigate the Fermi level pinning, leading to reduced barrier height and, hence, reduced contact resistance of the metal–semiconductor junction. With the insertion of MoS2 and WS2 materials a two-layer thick, we achieved 160 meV reductions in the Schottky barrier height and increased the current density by 14 times for titanium contact to the n-type silicon. Finally, we suggested a modified band diagram of Ti/n-Si contacts with the 2D interfacial layer. Our results showed that employing 2D materials can be an alternative route for overcoming the contact resistance challenges in modern transistors.

The convenient synthesis of one-dimensional nanostructures of chalcogenide compounds with a visible band-gap is an essential research topic in developing next-generation photoelectronic devices. In particular, the design of a theoretically predictable synthesis process provides great flexibility and has a considerable ripple effect in nanotechnology. In this study, a novel rational growth approach is designed using the spinodal decomposition phenomenon for the synthesis of the Sb2Se3 nanowires, which is based on the thermodynamic phase diagram. Using a stacked elemental layer (Sb/Sb-Se/Se) and heat treatment at 623 K for 30 min under an N2 atmosphere, the vertically inclined one-dimensional nanostructures are experimentally demonstrated. An additional annealing process at 523 K in a vacuum effectively removed excess Se elements due to their high vapor pressure, resulting in highly dense single crystal Sb2Se3 nanowire arrays. Adaption of our synthesis approach enables significantly improved photocurrent generation in the vertically stacked structure (glass/ITO/Sb2Se3 nanowires/ITO/PEN) from 6.4 (dark) to under 690 μA (at 3 V under AM 1.5G). In addition, a photoelectrochemical test demonstrated their p-type conductivity and robust photocorrosion performance in 0.5 M H2SO4.

The growth of high-quality indium (In)-rich In(X)Ga(1-X)N alloys is technologically important for applications to attain highly efficient green light-emitting diodes and solar cells. However, phase separation and composition modulation in In-rich In(X )Ga(1-X)N alloys are inevitable phenomena that degrade the crystal quality of In-rich In(X)Ga(1-X)N layers. Composition modulations were observed in the In-rich In(X)Ga(1-X)N layers with various In compositions. The In composition modulation in the In X Ga1-X N alloys formed in samples with In compositions exceeding 47%. The misfit strain between the InGaN layer and the GaN buffer retarded the composition modulation, which resulted in the formation of modulated regions 100 nm above the In(0.67)Ga(0.33)N/GaN interface. The composition modulations were formed on the specific crystallographic planes of both the {0001} and {0114} planes of InGaN.

One of the top design priorities for semiconductor chemical sensors is developing simple, low-cost, sensitive and reliable sensors to be built in handheld devices. However, the need to implement heating elements in sensor devices, and the resulting high power consumption, remains a major obstacle for the realization of miniaturized and integrated chemoresistive thin film sensors based on metal oxides. Here we demonstrate structurally simple but extremely efficient all oxide chemoresistive sensors with ~90% transmittance at visible wavelengths. Highly effective self-activation in anisotropically self-assembled nanocolumnar tungsten oxide thin films on glass substrate with indium-tin oxide electrodes enables ultrahigh response to nitrogen dioxide and volatile organic compounds with detection limits down to parts per trillion levels and power consumption less than 0.2 microwatts. Beyond the sensing performance, high transparency at visible wavelengths creates opportunities for their use in transparent electronic circuitry and optoelectronic devices with avenues for further functional convergence., Korea (South). Ministry of Intelligence and Economy (Contract K0004114), Korea Institute of Science and Technology (Grant 2E22121), Korea (South). Ministry of Environment (Research Program)

Some materials exhibit ‘morphotropic phase boundaries’ at which their crystal structure abruptly changes from a tetrahedral to a rhombohedral lattice. Such phase boundaries are created under high pressure or strain and are accompanied by a change in the material's characteristics. For example, there is great interest in controlling mixed-phase domains composed of ‘T’ and ‘R” phases in thin films of bismuth ferrite (BiFeO3, also known as BFO) for electromechanical applications. Now, a South Korea-Australia-based team led by Chan-Ho Yang has used an atomic force microscope tip to create - and further manipulate or erase - well-aligned, alternating straight stripes of the T and R' domains on a BFO thin film doped with lanthanum. In the absence of lanthanum, the stripes are curved rather than straight. The T-R' phase boundary shows enhanced electronic conduction compared to the bulk phases. These precise and rewritable nanostructures might serve to develop functional materials.

Using in situ transmission electron microscopy, we demonstrated that gold nanoparticles are unified via "oriented attachment" assisted either by nanoparticle rotation or grain boundary migration at the attachment interface. We also observed that the combined nanoparticle changes shape with stable facet planes via surface diffusion, along with recrystallization.

We report growth of epitaxial AlN thin films on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. To achieve two-dimensional growth the substrates were nitrided by nitrogen plasma prior to the AlN growth, which resulted in the formation of a two-dimensional single crystalline AlN layer. The formation of the two-dimensional AlN layer by the nitridation process was confirmed by the observation of streaky reflection high energy electron diffraction (RHEED) patterns. The growth of AlN thin films was performed on the nitrided AlN layer by changing the Al beam flux with the fixed nitrogen flux at 860. The growth mode of AlN films was also affected by the beam flux. By increasing the Al beam flux, two-dimensional growth of AlN films was favored, and a very flat surface with a root mean square roughness of 0.196 nm (for the 2 2 area) was obtained. Interestingly, additional diffraction lines were observed for the two-dimensionally grown AlN films, which were probably caused by the Al adlayer, which was similar to a report of Ga adlayer in the two-dimensional growth of GaN. Al droplets were observed in the sample grown with a higher Al beam flux after cooling to room temperature, which resulted from the excessive Al flux.