Abrupt Interface Research Articles

Uniformity and repeatability of InAlN-barrier HEMTs growth by high-speed-rotation single-wafer MOCVD tool

We fabricated high-electron-mobility transistor structures with InAlN as a barrier layer on 8- or 6-inch Si substrates by using a recently developed high-speed-wafer-rotation single-wafer MOCVD tool. It has been reported that Ga inclusion in the InAlN layer causes serious problems in the control of group III metal composition in some cases, but the samples grown using the tool exhibited an InAlN layer with an abrupt interface and almost no Ga inclusion. Excellent in-wafer uniformity, repeatability, and wafer-to-wafer uniformity of device structure are also reported. The results shown in this paper indicate high performance of the tool in the real production of devices.

Growth of Ag(1 1 1) on Si(1 1 1) with nearly flat band and abrupt interface

Growth of Ag films of up to 30 nm thickness on Si(1 1 1) 7 × 7 at room temperature is investigated by low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). LEED revealed the coexistence of Ag and Si spots starting with 1 monolayer (ML) of Ag deposited. The Ag lattice constant, starting with 25 ML, is slightly higher than for bulk Ag and increase linearly with Ag thickness, reaching about 4.2 nm for the thickest films. The average terrace widths detected from LEED spot profile analysis are about 30 nm for clean Si(1 1 1) 7 × 7 and about 5.5 nm for the thickest Ag(1 1 1) film, in agreement with STM observations. The intensity variation of core levels analyzed by XPS is taken into account by a model assuming the initial formation of Ag islands with linear variation of coverage vs. the amount of Ag deposited, followed by growth in a quasi layer-by-layer mode. The interface barrier is in the range of 0.4 eV, lower than all values reported previously. Ag deposited on Si(1 1 1) 7 × 7 at room temperature provides flat Ag(1 1 1) for synthesis of 2D materials, and may be used for low barrier Schottky diodes.

Controlling ZnSe/GaAs interface properties: The role of elemental exposure and photon irradiation during growth initiation

Incorporating dissimilar semiconductors into a single platform can offer additional degrees of freedom for optoelectronic device design. However, bonding of elements with mixed valence at heterovalent interfaces often leads to defect formation and poor material quality. Using ZnSe/GaAs as a model system, we investigate the use of above-bandgap photon irradiation in combination with elemental surface treatments as a route to modify the interface properties. We find that this approach produces large changes in the behavior of the interfaces. Specifically, treating the GaAs surface with light and a short exposure to Se flux results in strong excitonic emission from both layers and an abrupt interface between them. We propose that these improvements arise from controlled desorption of As atoms from the GaAs surface and the subsequent Se enrichment of the interface. These results suggest that illumination with above-bandgap photons and proper choice of elemental exposure prior to heterovalent epitaxial layer growth may aid the synthesis of heterovalent semiconductor heterostructures.

Depth-resolved charge reconstruction at the LaNiO3/CaMnO3 interface

Rational design of low-dimensional electronic phenomena at oxide interfaces is currently considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically abrupt interface between paramagnetic $\mathrm{LaNi}{\mathrm{O}}_{3}$ and antiferromagnetic $\mathrm{CaMn}{\mathrm{O}}_{3}$ exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in $\mathrm{LaNi}{\mathrm{O}}_{3}$. Once fully understood, such emergent functionality could turn this archetypal Mott-interface system into a key building block for the above-mentioned future devices. Here, we use depth-resolved standing-wave photoemission spectroscopy in conjunction with scanning transmission electron microscopy and x-ray absorption spectroscopy, to demonstrate a depth-dependent charge reconstruction at the $\mathrm{LaNi}{\mathrm{O}}_{3}/\mathrm{CaMn}{\mathrm{O}}_{3}$ interface. Our measurements reveal an increased concentration of $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{N}{\mathrm{i}}^{2+}$ cations at the interface, which create an electronic environment favorable for the emergence of interfacial ferromagnetism mediated via the $\mathrm{M}{\mathrm{n}}^{4+}\ensuremath{-}\mathrm{M}{\mathrm{n}}^{3+}$ ferromagnetic double exchange and $\mathrm{N}{\mathrm{i}}^{2+}\ensuremath{-}\mathrm{O}\ensuremath{-}\mathrm{M}{\mathrm{n}}^{4+}$ superexchange mechanisms. Our findings suggest a strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states.

Abrupt SiGe-to-Si interface: influence of chemical vapor deposition processes and characterization by different metrology techniques

Future electronic (gate-all-around transistors) and photonic devices (electro-absorption modulator and laser stack) will require SiGe/Si multilayer stacks with abrupt interfaces. Chemical vapor deposition (CVD) is a proven method to epitaxially grow such multilayer structures. However, the abruptness of the compositional change is limited by Ge segregation occurring during Si layer growth on a (Si)Ge surface. Here, we study the compositional abruptness at the interface between Si0.70Ge0.30 and Si layers epitaxially grown in a CVD reactor. The ultra-thin interface layer between SiGe and Si is characterized by transmission electron microscopy, secondary ion mass spectrometry and spectroscopic ellipsometry (SE). Our results show that a Si layer grown using a chlorinated chemistry produces the most abrupt interface. For Si deposition processes with non-chlorinated chemistry, lower growth temperature and a higher order hydride precursor result in a reduced transition thickness. Furthermore, we show that the interface thickness measured by the different metrology techniques are in good agreement, suggesting that SE can be used to evaluate the ultra-thin interface at the Si0.70Ge0.30-to-Si interface, resulting in considerably shorter development time in a production environment.

Water infiltration into the soil – what do measurements indicate?

Physical properties of top-soil organic materials significantly influence initiation processes of infiltration and runoff generation. This paper deals with the specifics of water infiltration through the top surface organic layer of the forest soil. Three field methods (Guelph permeameter, Tension disk permeameter, Single-ring method) and one laboratory method (Falling head) of hydraulic conductivity (KS) determination are compared and interpreted in the context of their applicability and limitations. The Falling head method provides far different values of KS if sample cylinders are or are not sealed with grease against the wall effect. The Guelf permeameter is very significant to the position of different horizons’ interface, while Tension disc permeameter results are dependent on antecedent soil moisture. The single ring method is applicable with acceptable results only when there is no abrupt interface between horizons in the vicinity of the ring bottom edge.

Efficient ultraviolet photodetector device based on modulated wide band gap Type-II CuO/CdSe core-shell nanowires

A high-performance ultraviolet photodetector has been successfully fabricated based on new CuO/CdSe core-shell nanowires (NWs). The obtained modulated optical band gap due to quantum confinement effect has the capability to detect photons with energies (3.96 eV) significantly larger than the band gap of CuO (1.2 eV) and CdSe (1.7 eV), which is mainly attributed to the abrupt interface between the CuO core and CdSe shell. The fabricated CuO/CdSe core-shell heterostructure NWs as photodetector device showed responsivity 0.63A/W with external quantum efficiency approximately 3.14 × 102% under illumination of wavelength 254 nm. The strong response of CuO/CdSe NWs photodetector is explained by the Type-II band alignment which cause the separation of photo generated electron–hole pairs and their charge transfer at the junction.

Segmented two-phase flow analysis in fractured geological medium based on the numerical manifold method

Abstract A segmented two-phase flow model is developed to describe immiscible two-phase flow in discrete fracture networks; i.e., there is an abrupt interface separating the two immiscible fluids. The capillary pressure on the fluid material interface and gravity are considered. For two-phase flow in fracture networks it is inevitable to form a mixed fluid in some fracture segments. The mixed fluid is described by a homogeneous model, and parameter γ is introduced as a critical value controlling new fluid formation in mixed fluid. The governing equations for two-phase flow are solved with a developed numerical manifold method (NMM) which is verified using the finite element and analytical methods with two examples. Two-phase flow through a single fracture intersection acting as one of the basic elements for complex fracture networks is then analysed. Different inlet and outlet configurations are considered including one-inlet/two-outlets, two-inlets/one-outlet and one-inlet/multi-outlets. For the second configuration, parametric studies of defined parameter γ and boundary conditions are carried out. In the third configuration, capillary pressure and gravity effects are discussed. The developed NMM is applied to the analysis of two-phase flow in a complicated fracture network showing that it is an efficient method for simulating segmented two-phase flow in fracture networks. Complex two-phase flow characteristics in the fracture networks that cannot be addressed in continuum models, e.g., the change in flow direction and transition from static to flow, are also revealed and analysed.

(Invited) Epitaxy of (SiGe/Si) Superlattices for the Fabrication of Horizontal Gate-All-Around Nanosheet Transistors

Stacked horizontal Gate All Around Nanosheet transistors were recently proposed as a replacement of FinFET for the sub-7nm device nodes. In order to stack Nanosheet channels, a specific SiGe/Si superlattice epitaxy is employed, with SiGe being used as a sacrificial layer that will define the suspension thickness (Tsus) between the channels. In this paper, we will consider the optimization of SiGe/Si multilayer epitaxy for Nanosheet channel definition (Fig. 1). Stacks with different Ge compositions are physically characterized and studied. With strain characterization and x-ray diffraction measurements on (100) substrates, we will show that it is possible to form pseudomorphic defect-free SiGe/Si multilayers over a wide range of Ge concentrations in the sacrificial SiGe (Fig. 2 and 3). The uniformity obtained on blanket 300mm substrates will be presented (Fig. 4), with a discussion on the SiGe/Si interface abruptness. In the context of CMOS device integration, it is necessary to employ a low temperature STI process to mitigate the Ge inter-diffusion in the silicon channel and to avoid the possible generation of defects from plastic relaxation of SiGe. The SiGe and Ge-Si intermixed regions are removed with appropriate selective etch process in two Nanosheet specific modules: inner spacer formation in the extension regions (Fig. 5) and post channel release in the gate regions (Fig. 6), leaving only 5nm intrinsic silicon Nanosheets. We will report the etch rate and selectivity of our in-house vapor-phase channel release process performed at low temperature. This process enables us to remove SiGe versus Si with a great selectivity and a limited amount of silicon loss during channel release, before gate stack deposition around the stacked Nanosheets. From a device perspective, we will show that the optimization of the silicon thickness in the SiGe/Si superlattice plays an important role in the Nanosheet device performance and electrostatics (Fig. 7). Finally, with a good SiGe/Si multilayer uniformity control, a reduced thermal budget and after optimization of our in-house SiGe selective etch process, we will show the capability to form 5nm silicon Nanosheet channels with small within wafer variation, low device variability and good process control, compatible with the fabrication of very large scale integrated circuits for future CMOS device nodes. Acknowledgements: This work was performed by the Research Alliance Teams at various IBM Research Facilities. Figure 1

Analysis of plasma enhanced pulsed laser deposition of transition metal oxide thin films using medium energy ion scattering

In this study, plasma-enhanced pulsed laser deposition (PE-PLD), which is a novel variant of pulsed laser deposition that combines laser ablation of metal targets with an electrically-produced oxygen plasma background, has been used for the fabrication of ZnO and Cu2O thin films. Samples prepared using the PE-PLD process, with the aim of generating desirable properties for a range of electrical and optical applications, have been analysed using medium energy ion scattering. Using a 100 keV He+ ion beam, high resolution depth profiling of the films was performed with an analysis of the stoichiometry and interface abruptness of these novel materials. It was found that the PE-PLD process can create stoichiometric thin films, the uniformity of which can be controlled by varying the power of the inductively coupled plasma. This technique showed a high deposition rate of ∼0.1 nm s−1.

Compositional control of gold-catalyzed ternary nanowires and axial nanowire heterostructures based on IIIP1−xAsx

Ternary IIIP1−xAsx nanowires and axial heterostructures within such nanowires are perfect building blocks for optoelectronics in an important wavelength range. However, the full control over the morphology and composition of IIIP1−xAsx nanowires as well as the interfacial abruptness of axial In(As)P/InP nanowire heterostructures is not achieved so far. Here, we present a fully self-consistent model for the growth and composition of gold-catalyzed IIIP1−xAsx nanowires and nanowire heterostructures that explains and predicts interesting phenomena. In particular, we find that the phosphorous concentration in the quaternary Au-III-As-P droplet should be much smaller than that of arsenic under the typical growth conditions. This suppresses the indium crystallization rate when passing from the InAs to InP segment, leading to the droplet swelling. Special care should be taken to keep the droplet on top of the InP segment. We show that the InAs/InP interfacial abruptness is challenging, while the reverse InP/InAs switch should always be atomically sharp. The governing equations connecting thermodynamics and kinetics of ternary nanowires based on IIIP1−xAsx are used to formulate clear routes for the necessary compositional control, with possible extensions to other material systems.

Epitaxial mosaic-like Mn5Ge3 thin films on Ge(001) substrates

Epitaxial mosaic-like Mn5Ge3 thin films were grown on Ge(001) substrates using reactive deposition epitaxy process from Mn deposition or Mn-Ge co-deposition at a substrate temperature of 250 °C using magnetron sputtering. The cross-sectional transmission electronic microscopy analyses reveal an abrupt interface at the atomic scale; two equivalent epitaxial relationships are found between the substrate and the Mn5Ge3 crystallites: Ge(001)[110] and [11¯0]∥ Mn5Ge3(11¯1)[1¯1¯0]. The chex axis of Mn5Ge3 forms an angle of 45° with the substrate plane. Rietveld analysis from a synchrotron 2-dimensional diffraction pattern revealed that remanent deformations of about 1% exist in the film. M-H measurements of 50-nm thick films elaborated by co-deposition revealed a saturation magnetization, Ms, of 636 kAm−1, whereas the films elaborated by Mn deposition saturate at different values depending on the orientation of the applied magnetic field: Ms⊥ = 545 kAm−1 and Ms∥ = 774 kAm−1. This Ms difference is attributed to shape anisotropy of crystallites and interface quality of the films.

CO2 electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface.

Carbon dioxide (CO2) electroreduction could provide a useful source of ethylene, but low conversion efficiency, low production rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alkaline electrolyte reduces CO2 to ethylene with 70% faradaic efficiency at a potential of -0.55 volts versus a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO2 reduction and carbon monoxide (CO)-CO coupling activation energy barriers; as a result, onset of ethylene evolution at -0.165 volts versus an RHE in 10 molar potassium hydroxide occurs almost simultaneously with CO production. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between separate hydrophobic and conductive supports, providing constant ethylene selectivity for an initial 150 operating hours.

Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit.

Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe x) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that, in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe2) monolayer, while for thicker films it could originate from a combination of vdW MnSe2 and/or interfacial magnetism of α-MnSe(111). Magnetization measurements of monolayer MnSe x films on GaSe and SnSe2 epilayers show ferromagnetic ordering with a large saturation magnetization of ∼4 Bohr magnetons per Mn, which is consistent with the density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe2. Growing MnSe x films on GaSe up to a high thickness (∼40 nm) produces α-MnSe(111) and an enhanced magnetic moment (∼2×) compared to the monolayer MnSe x samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveals an abrupt and clean interface between GaSe(0001) and α-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe2 monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.

Impact of (111)-oriented TiN conductive layers for the growth of vanadium dioxide films and the effect of grain boundary diffusions

We report on advantage of (111)-oriented TiN conductive layers for realizing low temperature crystalline growth of vanadium dioxide (VO2) films with phase transition. Comparison between (111)-oriented TiN and random polycrystalline TiN layers was performed to clarify the effect of oriented TiN for succeeding VO2 growth. Oriented growth of VO2 (010) on (111)-oriented TiN suggests feasibility of most dense TiN (111) surface with pseudo-hexagonal lattice fitted against VO2 (010) surface with monoclinic angle β of 122°. Abrupt resistance change nearly three orders of magnitude was achieved in the films prepared at substrate temperatures of 250 ～ 300 °C. XPS depth profiles clearly showed abrupt interface between VO2 and TiN at deposition temperatures lower than 300 °C. On the other hand, it was found that (111)-oriented TiN layers enhance diffusions of Ti and O atoms through the grain boundaries, which act as direct path for diffusion at temperatures higher than 350 °C. Strictly unchanged composition of TiN layers confirms the superior property as anti-diffusion barrier material, however, grain boundaries arrangements are supposed to be crucial factors for the maintanance of layerd structure. Obtained results tell us the impact of (111)-oriented TiN layers not only for succeeding growth of VO2 films, but also for design of layerd device introducing TiN as electrodes.

Clinicopathological and Ultrasonic Features of Triple-Negative Breast Cancers: A Comparison with Hormone Receptor-Positive/Human Epidermal Growth Factor Receptor-2-Negative Breast Cancers

The purpose of this study was to analyze the clinicopathological and ultrasound characteristics of triple-negative breast cancers (TNBCs) and compare these findings with those for hormone receptor-positive (HR-positive)/human epidermal growth factor receptor-2-negative (HER-2-negative) tumors. Seventy-five TNBCs and 135 HR-positive/HER-2-negative breast cancers were reviewed. Data from conventional ultrasound, Doppler vascularity and elastography were included in the analysis. TNBCs had a higher histologic grade and Ki-67 level. On ultrasound, TNBCs often appeared as microlobulated, markedly hypo-echoic masses with an abrupt interface boundary, posterior acoustic enhancement, absence of calcifications and more characteristics of surrounding tissue. Results from multivariate regression analysis revealed that margin, posterior acoustic features and surrounding tissue features of tumors were independent predictive factors in differentiating TNBCs from HR-positive/HER-2-negative tumors. Our results suggest that a thorough evaluation of sonographic findings might be useful in discriminating between TNBCs and HR-positive/HER-2-negative tumors, which may provide accurate evidence for clinical early diagnosis.

Optimizing phonon scattering by tuning surface-interdiffusion-driven intermixing to break the random-alloy limit of thermal conductivity

We investigate the evolution of the cross-plane thermal conductivity $\kappa$ of superlattices (SLs) as interfaces change from perfectly abrupt to totally intermixed, by using non-equilibrium molecular dynamics simulations in combination with the spectral heat current calculations. We highlight the role of surface-interdiffusion-driven intermixing by calculating the $\kappa$ of SLs with changing interface roughness, whose tuning allows for the $\kappa$ values much lower than the "alloy limit" and the abrupt interface limit in same cases. The interplay between alloy and interface scattering in different frequency ranges provides a physical basis to predict a minimum of thermal conductivity. More specifically, we also explore how the interface roughness affects the thermal conductivities for SLs materials with a broad span of atomic mass and bond strength. In particular, we find that (i) only when the "spacer" thickness of SLs increases up to a critical value the $\kappa$ of rough SLs can break the corresponding "alloy limit". (ii) Whether the $\kappa$ changes monotonically as interface roughness strongly depends on the period length and intrinsic behavior of phonon transport for SLs materials. Especially, for the SL with large period length, there exists an optimal interface roughness which can minimize the thermal conductivity. (iii) Surface-interdiffusion-driven intermixing is more effective in achieving the low $\kappa$ below the alloy limit for SL materials with large mass mismatch than with small one. (iv) It's possible for SLs materials with large lattice mismatch (i.e., bond strength) to design an ideally abrupt interface structure with $\kappa$ much below the "alloy limit". These results have a clear implications for optimization of thermal transport for heat management and for the development of thermoelectric materials.

Study of the Band Alignment between Atomic-Layer-Deposited High-κ Dielectrics and MoS2 Film

In this study, we determined the energy-band alignment of various amorphous high-κ dielectrics (Al2O3, HfO2, ZrO2, and TiO2) that were deposited on a molybdenum disulfide (MoS2) film through atomic layer deposition by using high-resolution X-ray photoelectron spectroscopy. The uniformity and continuity of the MoS2 films were inspected by physical characterizations. The roughness of MoS2 film deposited by chemical vapor deposition was evaluated using atomic force microscopy. Moreover, cross-sectional transmission electron microscopy images confirmed that all high-κ dielectrics were uniformly and continuously deposited on the MoS2 film, leading to superior high-κ dielectric–MoS2 stacks. An abrupt interface between the high-κ dielectrics and MoS2 film was also observed. Finally, the valance and conduction band offsets between the Al2O3, HfO2, ZrO2, and TiO2 and the MoS2 film interface were measured to be 3.14, 1.81, 1.56, and 0.18 eV and 2.36, 1.77, 1.88, and 1.68 eV, respectively. Our study demonstrated the direct deposition of numerous commonly used high-κ dielectrics on top of a MoS2 film. This top–down scheme is very useful for applying two-dimensional materials to complementary metal–oxide–semiconductor field-effect-transistor circuits.

Sonographic features of invasive ductal breast carcinomas predictive of malignancy grade.

Context:Assessment of individual sonographic features provides vital clues about the biological behavior of breast masses and can assist in determining histological grade of malignancy and thereby prognosis.Aims:Assessment of individual sonographic features of biopsy proven invasive ductal breast carcinomas as predictors of malignancy grade.Settings and Design:A retrospective analysis of sonographic findings of 103 biopsy proven invasive ductal breast carcinomas.Materials and Methods:Tumor characteristics on gray-scale ultrasound and color flow were assessed using American College of Radiology (ACR) Breast Imaging Reporting and Data System (BI-RADS) Atlas Fifth Edition. The sonographic findings of masses were individually correlated with their histopathologic grades.Statistical Analysis Used:Chi square test, ordinal regression, and Goodman and Kruskal tau test.Results:Breast mass showing reversal/lack of diastolic flow has a high probability of belonging to histological high grade tumor (β 1.566, P 0.0001). The masses with abrupt interface boundary are more likely grade 3 (β 1.524, P 0.001) in comparison to masses with echogenic halos. The suspicious calcifications present in and outside the mass is a finding associated with histologically high grade tumors. The invasive ductal carcinomas (IDCs) with complex solid and cystic echotexture are more likely to be of high histological grade (β 1.146, P 0.04) as compared to masses with hypoechoic echotexture.Conclusions:Certain ultrasound features are associated with tumor grade on histopathology. If the radiologist is cognizant of these sonographic features, ultrasound can be a potent modality for predicting histopathological grade of IDCs of the breast, especially in settings where advanced tests such as receptor and molecular analyses are limited.

Single Atomic Layer Ferroelectric on Silicon.

A single atomic layer of ZrO2 exhibits ferroelectric switching behavior when grown with an atomically abrupt interface on silicon. Hysteresis in capacitance-voltage measurements of a ZrO2 gate stack demonstrate that a reversible polarization of the ZrO2 interface structure couples to the carriers in the silicon. First-principles computations confirm the existence of multiple stable polarization states and the energy shift in the semiconductor electron states that result from switching between these states. This monolayer ferroelectric represents a new class of materials for achieving devices that transcend conventional complementary metal oxide semiconductor (CMOS) technology. Significantly, a single atomic layer ferroelectric allows for more aggressively scaled devices than bulk ferroelectrics, which currently need to be thicker than 5-10 nm to exhibit significant hysteretic behavior (Park, et al. Adv. Mater. 2015, 27, 1811).