Substrate Termination Research Articles

Electro-optic effect in thin film strontium barium niobate (SBN) grown by RF magnetron sputtering on SrTiO3 substrates

Ferroelectric strontium barium niobate (SrxBa1−xNb2O6 or SBN) is a material with high electro-optic (EO) response. It is currently of interest in low voltage silicon-integrated photonics (SiPh). We have grown strongly textured SBN films with x = 0.65 by radio frequency sputtering on (100)-oriented SrTiO3 substrates where grains with mixed (310) and (001) out-of-plane orientation form. For these mixed orientation films, we observed a maximum effective EO coefficient of 230 pm/V using a transmission EO measurement geometry that is responsive only to the in-plane polarization component coming from the (310)-oriented grains. We also demonstrate that by growing SBN on TiO2-terminated SrTiO3 substrates, we can obtain predominantly (001)-oriented SBN films with out-of-plane polarization. Transmission EO measurements on such (001)-oriented films show a reduced effective Pockels coefficient of 88 pm/V, which is consistent with the overall ferroelectric polarization becoming out-of-plane. This work shows that controlling substrate termination is effective in controlling the grain orientation of SBN films grown on top and that one can readily integrate SBN films on SrTiO3-buffered Si for use in SiPh.

The effect of photodissociation of confined water on photoemission behaviors of monolayer MoS2

The effects of confined water on the photoluminescence of 2D MoS2 depending on the type of isotope, substrate, and surface termination were investigated. The monolayer MoS2 film showed quenched photoluminescence intensity when exposed to water, whereas it was not observed upon exposure to heavy water. This result is attributed to the overtone vibrational energy transfer between 2D MoS2 and water. Furthermore, the investigation of the photoluminescence in 2D MoS2 depending on the surface characteristics indicated that it can be controlled by the adsorbed water on the substrate. Real-time photoluminescence measurements revealed that the two types of adsorbed water affected the variation in the optical properties and that the orientation of the adsorbed water was also significant for reverting to the intrinsic 2D MoS2 by catalytic photodissociation process through exciton-to-vibronic transfer, facilitated in a confined space. Our results indicate that the substrate surface is crucial for controlling the adsorbed water formed during wet-transfer process. Therefore, this study is effective for applications that utilize large-scale two-dimensional materials.

Robust Dipolar Layers between Organic Semiconductors and Silver for Energy-Level Alignment.

The interface between a metal electrode and an organic semiconductor (OS) layer has a defining role in the properties of the resulting device. To obtain the desired performance, interlayers are introduced to modify the adhesion and growth of OS and enhance the efficiency of charge transport through the interface. However, the employed interlayers face common challenges, including a lack of electric dipoles to tune the mutual position of energy levels, being too thick for efficient electronic transport, or being prone to intermixing with subsequently deposited OS layers. Here, we show that monolayers of 1,3,5-tris(4-carboxyphenyl)benzene (BTB) with fully deprotonated carboxyl groups on silver substrates form a compact layer resistant to intermixing while capable of mediating energy-level alignment and showing a large insensitivity to substrate termination. Employing a combination of surface-sensitive techniques, i.e., low-energy electron microscopy and diffraction, X-ray photoelectron spectroscopy, and scanning tunneling microscopy, we have comprehensively characterized the compact layer and proven its robustness against mixing with the subsequently deposited organic semiconductor layer. Density functional theory calculations show that the robustness arises from a strong interaction of carboxylate groups with the Ag surface, and thus, the BTB in the first layer is energetically favored. Synchrotron radiation photoelectron spectroscopy shows that this layer displays considerable electrical dipoles that can be utilized for work function engineering and electronic alignment of molecular frontier orbitals with respect to the substrate Fermi level. Our work thus provides a widely applicable molecular interlayer and general insights necessary for engineering of charge injection layers for efficient organic electronics.

Thermodynamics and electronic structure of adsorbed and intercalated plumbene in graphene/hexagonal SiC heterostructures

Graphene-covered hexagonal SiC substrates have been frequently discussed to be appropriate starting points for epitaxial overlayers of Xenes, such as plumbene, or even their deposition as intercalates between graphene and SiC. Here, we investigate, within density functional theory, the plumbene deposition for various layer orderings and substrate terminations. By means of total energy studies we demonstrate the favorization of the intercalation versus the epitaxy for both C-terminated and Si-terminated 4H-SiC substrates. These results are explained in terms of chemical bonding and by means of layer-resolved projected band structures. Our results are compared with available experimental findings.

Probing the role of surface termination in the adsorption of azupyrene on copper.

The role of the inorganic substrate termination, within the organic-inorganic interface, has been well studied for systems that contain strong localised bonding. However, how varying the substrate termination affects coordination to delocalised electronic states, like that found in aromatic molecules, is an open question. Azupyrene, a non-alternant polycyclic aromatic hydrocarbon, is known to bind strongly to metal surfaces through its delocalised π orbitals, thus yielding an ideal probe into delocalised surface-adsorbate interactions. Normal incidence X-ray standing wave (NIXSW) measurements and density functional theory calculations are reported for the adsorption of azupyrene on the (111), (110) and (100) surface facets of copper to investigate the dependence of the adsorption structure on the substrate termination. Structural models based on hybrid density functional theory calculations with non-local many-body dispersion yield excellent agreement with the experimental NIXSW results. No statistically significant difference in the azupyrene adsorption height was observed between the (111) and (100) surfaces. On the Cu(110) surface, the molecule was found to adsorb 0.06 ± 0.04 Å closer to the substrate than on the other surface facets. The most energetically favoured adsorption site on each surface, as determined by DFT, is subtly different, but in each case involved a configuration where the aromatic rings were centred above a hollow site, consistent with previous reports for the adsorption of small aromatic molecules on metal surfaces.

GaN power converter and high-side IC substrate issues on Si, p-n junction, or SOI

The lateral GaN power semiconductor technology enables monolithic integration of complete power converter topologies such as half-bridges, multi-phase and multi-level converters. Fabrication on Si substrates enables low-cost and mass production. However, the operation of monolithic GaN power converters on a common conductive silicon (Si) substrate is limited compared to discrete GaN HEMTs, especially at high-voltage operation, due to substrate-biasing effects such as back-gated or trap-related static and dynamic on-resistance increase, and changed effective device capacitances. To circumvent the Si substrate related effects but still using a low-cost large-diameter Si substrate, this paper reviews isolation approaches for GaN ICs such as Si p-n junction isolation or floating Si substrates (GaN-on-Si) and buried oxide isolation using Silicon-on-Insulator substrates (GaN-on-SOI). Published GaN power converter ICs are reviewed. The effect of the isolation approaches on increased output and input transistor capacitances, influencing the switching behavior and switching loss, is calculated and compared for different voltage classes (below 100 V to 600 V-class). Finally, design guidelines for local substrate termination using the Si-based isolation approaches are discussed exemplary for a monolithic half-bridge with driver circuit. Monolithic GaN power converter integration combined with functional integration will result in a new class of advanced power converter ICs, and enable efficient, compact and low-cost power conversion applications.

Layer‐By‐Layer Assembly of Asymmetric Linkers into Non‐Centrosymmetric Metal Organic Frameworks: A Thorough Theoretical Treatment

AbstractLayer‐by‐layer synthesis of surface‐coordinated metal–organic frameworks (SURMOF) enables the assembly of asymmetric, dipolar linkers into non‐centrosymmetric pillar‐layered structures. Using appropriate substrate terminations can yield oriented growth with the dipoles aligned perpendicular to the surface. The aligned pillar linkers give rise to a built‐in electrostatic field. In addition, the non‐centrosymmetric structure of the SURMOF gives rise to intriguing nonlinear optical features, such as second harmonic generation. Previous research with methyl‐functionalized bipyridine pillar linkers have demonstrated that this approach works in principle, but so far the total degree of alignment is only very small. Herein, a multiscale modelling approach is presented for in‐silico SURMOF assembly to identify and overcome limitations in the growth of pillar‐layered SURMOFs and to develop a strategy to maximize linker alignment. Using master equation models and kinetic Monte Carlo simulations, it is found that the formation of a highly ordered state corresponding to the thermodynamic equilibrium is often prevented by long‐lasting transient effects. Based on ab initio binding energies for a wide selection of hypothetical pillar linkers, a fast‐binding, slow‐relaxation scheme is able to be identified during the SURMOF growth for a range of different pillar linkers. These observations allow them to derive a rational strategy for the design of novel linkers to yield SURMOF‐based non‐centrosymmetric materials with substantially improved properties.

Control of Self-Polarization in Doped Single Crystalline Pb(Zr0.5Ti0.5)O3 Thin Films

We report the control of self-polarization of monocrystalline Lead Zirconate Titanate thin films grown by Pulsed laser deposition by doping and bottom interface. Doped and undoped films grown on ScO2 terminated surface of polar PSO substrate have developed polarization oriented toward the substrate due to negative polarity (ScO−) of the substrate termination. For films grown on the strontium ruthenate (SRO) bottom electrode, only undoped and 1% Nb doped PZT films have developed upwards oriented self-polarization. The introduction of strontium ruthenate bottom electrode have made the work function difference between films and bottom electrode to be the dominant mechanism for self-polarization.

Visualization of defects in single-crystal and thin-film PdCoO2 using aberration-corrected scanning transmission electron microscopy

Single-crystal delafossite ${\mathrm{PdCoO}}_{2}$ is known to have an extremely low intrinsic impurity concentration of \ensuremath{\sim}0.001%, demonstrating extraordinarily high conductivity with a mean free path of \ensuremath{\sim}20 \ensuremath{\mu}m at low temperatures. However, when grown as thin films, the resistivity at room temperature increases by a factor of 3--80 times, depending on the film thickness. Using scanning transmission electron microscopy, we identify different classes of defects for the single crystal vs epitaxial thin film. The dominant defect for single-crystal ${\mathrm{PdCoO}}_{2}$ is found to be ribbonlike defects. For the thin films, we identify different types of defects arising in epitaxial thin films mainly due to substrate termination that disrupt the lateral connectivity of the conducting planes. Our results are consistent with the high conductivity of single crystals and increased electrical resistivity of the thin films compared to that of single crystals, suggesting that selecting a proper substrate, improving surface quality, and reducing the step density are the keys to enhance the film quality for utilizing ${\mathrm{PdCoO}}_{2}$ as a platform for future applications.

Fe3O4 thin films epitaxially growth model on TiO2-terminated SrTiO3(100)

The advanced interface of Fe 3 O 4 /SrTiO 3 , popular in spintronic, has attracted considerable attention. Some experiments have given controversial results on the structure at the Fe 3 O 4 /SrTiO 3 (100) interface. One opinion suggests the formation of interfacial antiferromagnetic FeO layers, while another opinion suggests that are γ-Fe 2 O 3 layers. Here, we propose a theoretical model that what kind of iron oxide stacking sequence forming on the substrate depends on the substrate termination charge. We give a tentative and easy-to-understand growth model for epitaxy, which agrees with the opinion of γ-Fe 2 O 3 layers formation at the Fe 3 O 4 /SrTiO 3 (100) interface. This theoretical model agrees well with the experimental curve and gives a prediction for the different Fe ions concentration depending on thickness. The calculation results of the Fe ions concentration adjacent to the substrate surface show that there are almost no B-site Fe ions at the interface. It indicates that there are unusual electric and magnetic properties at interface. Our growth model illustrates that the possible magnetically dead layer at the interface is responsible for the decrease of the magnetization in the thin films. • This growth mode give credit to the results of interfacial ferrimagnetic γ-Fe 2 O 3 layer in Fe 3 O 4 /SrTiO 3 (100). • The theoretical model curve agrees with the experimental curve, gives prediction for the Fe ions concentration depending on thickness. • The calculation result shows there are almost no B-site Fe at interface. This indicates that there are unusual electric and magnetic properties at interface. The present growth illustrates there exists possible magnetically dead layer at the interface. • It is proposed that the growth mode and the oxidation level are determined by the TiO 2 -termination charge. • The phenomenon that magnetite film grown on SrTiO 3 (001) exhibited vertical compressive and lateral tensile strain, which contradicted the expected behavior, has been explained.

Review of technologies for high-voltage integrated circuits

High-Voltage power Integrated Circuits (HVICs) are widely used to realize high-efficiency power conversions (e.g., AC/DC conversion), gate drivers for power devices and LED lighting, and so on. The Bipolar-CMOS-DMOS (BCD) process is proposed to fabricate devices with bipolar, CMOS, and DMOS modes, and thereby realize the single-chip integration of HVICs. The basic integrated technologies of HVICs include High-Voltage (HV) integrated device technology, HV interconnection technology, and isolation technology. The HV integrated device is the core of HVICs. The basic requirements of the HV integrated device are high breakdown voltage, low specific on-resistance, and process compatibility with low-voltage circuits. The REduced SURFace field (RESURF) technology and junction termination technology are developed to optimize the surface field of integration power devices and breakdown voltage. Furthermore, the ENhanced DIelectric layer Field (ENDIF) and REduced BULk Field (REBULF) technologies are proposed to optimize bulk fields. The double/triple RESURF technologies are further developed, and the superjunction concept is introduced to integrated power devices and to reduce the specific on-resistance. This work presents a comprehensive review of these technologies, including the innovation technologies of the authors' group, such as ENDIF and REBULF, substrate termination technology prospective integrated technologies and HVICs in wide band gap semiconductor materials are also discussed.

Modulated Photoluminescence of Single-Layer MoS2 on Various Rutile TiO2 Surfaces: Implications for Photocatalytic Applications

Hybridization with oxide semiconductors provides a versatile strategy for tailoring physicochemical properties of two-dimensional materials (2DMs). However, the direct impacts of specific interface interactions have not yet been very well categorized, in particular at an atomic level. In the present work, through a chemical vapor deposition (CVD) method, we successfully grew monolayer MoS2 flakes on an atomically smooth rutile TiO2 single crystal with (100), (110), and (001) terminations. We found that the fabrication of comparable high-quality MoS2 on all of the TiO2 substrates can only be achieved via finely varying the growth parameters. Moreover, the photoluminescence of MoS2 also changes against the substrate terminations, showing a gradually reduced A0/A– exciton ratio following the sequence of (100) > (110) > (001). Detailed X-ray photoelectron spectroscopy measurements showed the same tendency for the binding energy shifts of both Ti and O in the MoS2/TiO2 samples, which were attributed to the varied dipole fields established at the MoS2/TiO2 interfaces. Our work not only reinforces the important role of interface charge redistribution in tailoring the properties of hybridized systems but also emphasizes that the facet effect may be applied as an efficient strategy for optimizing the photocatalytic activities of compositional systems.

The formation and role of the SiO2 oxidation layer in the 4H-SiC/β-Ga2O3 interface

Although 4H-SiC has been regarded as an excellent substrate for the epitaxial growth of β-Ga2O3, an additional oxidation layer could be formed at the substrate during the high temperature growth process. First-principles calculations were employed to investigate the formation and role of the oxidation layer, which was vital for fabricating high quality β-Ga2O3. Through inserting O atoms and emitting CO molecules, the 4H-SiC substrate was spontaneously oxidized, and the oxidation site and process were dependent of the substrate termination. Such inconsistent characters disappeared for the 4H-SiC/β-Ga2O3 interface, and the SiO2 tetrahedron was preferably formed at all the 4H-SiC/β-Ga2O3 interfaces. Moreover, the 4H-SiC/β-Ga2O3 interfaces with Si-terminations and OGa1-termiantions were easier oxidized due to the lower migration energies of O atoms, in particular the O1 atoms. In addition, the oxidization layer accumulated abundant electrons at the SiO2/β-Ga2O3 contact region and turned the type-I band alignment for 4H-SiC/β-Ga2O3 interface into the type-II band alignment, which might be useful to the charge transport of electronic device. However, many interfacial gap states were introduced into the band gaps of 4H-SiC/β-Ga2O3 interfaces, which were harmful to the optoelectronic device.

Effects of substrate termination on R on increase under stress in 650 V GaN power devices

Buffer related electron trapping and hot electron injection are responsible for R on degradation in devices, but the effects of substrate termination are still uncertain. In this work, both positive and negative substrate bias are applied to investigate the different vertical trapping mechanisms in 650 V gallium nitride (GaN) power devices. R on shows an instant and significant increase under vertical bias stress, and the magnitude of downward buffer electron trapping induced R on increase is relatively larger than that induced by upward trapping. Additionally, the substrate floated and grounded GaN devices are also submitted to both off-state and semi-on-state stresses to investigate the effects of substrate termination on hot electron injection induced R on increase. The intensity of the upward electron trapping increases faster with temperature resulting in a higher R on increase than the downward situation. The hot electron effect is only obvious when the substrate is grounded, suggesting that the main injection destination is not in the buffer. The substrate floated device exhibits a lower R on increase after both off-state and semi-on-state stresses at elevated temperatures. Substrate floated packages are suggested to ensure reliable dynamic performance of device, especially for high voltage application design.

Biosynthetic Cyclization Catalysts for the Assembly of Peptide and Polyketide Natural Products.

Many biologically active natural products are synthesized by nonribosomal peptide synthetases (NRPSs), polyketide synthases (PKSs) and their hybrids. These megasynthetases contain modules possessing distinct catalytic domains that allow for substrate initiation, chain extension, processing and termination. At the end of a module, a terminal domain, usually a thioesterase (TE), is responsible for catalyzing the release of the NRPS or PKS as a linear or cyclized product. In this review, we address the general cyclization mechanism of the TE domain, including oligomerization and the fungal C-C bond forming Claisen-like cyclases (CLCs). Additionally, we include examples of cyclization catalysts acting within or at the end of a module. Furthermore, condensation-like (CT) domains, terminal reductase (R) domains, reductase-like domains that catalyze Dieckmann condensation (RD), thioesterase-like Dieckmann cyclases, trans-acting TEs from the penicillin binding protein (PBP) enzyme family, product template (PT) domains and others will also be reviewed. The studies summarized here highlight the remarkable diversity of NRPS and PKS cyclization catalysts for the production of biologically relevant, complex cyclic natural products and related compounds.

The interface between chloroaluminum phthalocyanine and titanium dioxide: the influence of surface defects and substrate termination.

Interface properties of chloroaluminum(iii) phthalocyanine (AlClPc) on two different rutile titanium dioxide (TiO2) single crystal surfaces ((100) and (001)) have been studied using X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). It is shown that the strength of the interaction clearly depends on the substrate termination and preparation. Generally, the (001) surface is more reactive compared to the (100) surface. The most important interaction channel involves the nitrogen atoms of the phthalocyanine macrocycle. An exposure to oxygen during the annealing steps of the preparation procedure allows diminishing the extent of interaction of nitrogen with titanium dioxide. The work function of AlClPc/TiO2 is rather independent of the substrate, indicating a pinning regime at all interfaces, where the HOMO of the molecule is aligned at the maximum of the defect states of the substrate.

GaN Integrated Bridge Circuits on Bulk Silicon Substrate: Issues and Proposed Solution

A discrete GaN power transistor’s substrate is typically connected to its source electrode. However, on the GaN-on-Si power IC platform, the high-side transistor (HS-transistor) and low-side transistor (LS-transistor) share a common substrate that cannot be simultaneously connected to both source electrodes of the two transistors. Thus, the termination of the common substrate remains an undecided issue. In this work, comprehensive TCAD simulations are exploited to reveal the influences of various substrate termination schemes. It is found the common substrate inevitably leads to severe degradation in the dynamic ${R} _{\mathrm{ ON}}$ due to back-gating effects. The mechanisms for the degradations vary with the substrate termination scheme, and will be discussed in detail. To address these issues, we propose a new GaN power IC platform on an engineered bulk silicon substrate, and study the new platform with TCAD simulations. The proposed platform provides a local electrical substrate (a p+ island) for each GaN power transistor. The source electrode of each GaN transistor is connected to its local electrical substrate, while all devices share a common mechanical substrate. The junctions between the local substrates and the underlying n-layer provide an effective isolation between GaN transistors. The back-gating effects are completely suppressed for the GaN integrated bridge circuit.

A Novel Ultralow R ON,sp Triple RESURF LDMOS With Sandwich n-p-n Layer

This article proposes a novel ultralow specific ON-resistance (RON,sp) triple reduced surface field (RESURF) lateral double-diffused MOSFET (LDMOS) with sandwich np-n layer. Compared with the conventional triple RESURF LDMOS with n-type top layer (NTTR LDMOS), the new structure is characterized by an additional N-buried layer below the P-buried layer. Due to the introduction of N-buried layer, the concentration of N-top layer will decrease to ensure charge balance, thus making the surface electric field distribution better and the higher additional doping introduced by N-top and N-buried layers can be achieved. The dose and implantation energy of n-p-n layers were simulated and optimized. Furthermore, in order to eliminate the premature avalanche breakdown occurring in the source-centered termination region, substrate termination technology (STT) is applied to avoid the high electric field peaks generated in the curved source region with small curvature radius. But the use of STT makes it easy to breakdown in the transitional region compared with the straight region. Therefore, the transitional region is studied and the crucial parameter AL in the layout of transitional region is also optimized. The fabricated device experimentally demonstrated improved performance with low R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON,sp</sub> of 78.3 mΩ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and high breakdown voltage (BV) of 795 V, and the experimental R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON,sp</sub> is 26.8% lower than that of the conventional triple RESURF LDMOS (CTR LDMOS) and 10% lower than that of the NTTR LDMOS based on L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> = 67 μm, which can break the silicon limit of CTR and NTTR LDMOS and meet the demand of 700-V high-voltage integrated circuit applications.

In situ thermal preparation of oxide surfaces

Substrate surfaces terminated with a specific surface reconstruction are a prerequisite for the controlled epitaxial growth of most materials. Focusing on SrTiO3 (001) substrates, it has recently been shown that in situ substrate termination by thermal annealing has decisive advantages over standard termination methods. We report here that in situ substrate termination is a generally applicable method not restricted to SrTiO3 crystals. We specifically demonstrate the successful surface preparation of doped SrTiO3 (001), LaAlO3 (001), NdGaO3 (001), DyScO3 (110), TbScO3 (110), MgO (001), and Al2O3 (0001) surfaces.

Hole-Induced Degradation in ${E}$ -Mode GaN MIS-FETs: Impact of Substrate Terminations

We conducted reliability characterization under reverse-bias stress (i.e., stress at OFF-state with high ${V}_{\text {DS}}$ ) on the ${E}$ -mode GaN metal–insulator–semiconductor field-effect-transistors (MIS-FETs) with various substrate terminations. The MIS-FETs with floating substrate (FS) show worse threshold voltage ( ${V}_{\text {TH}}$ ) stability than that with a grounded substrate. A non monotonic dependence of ${V}_{\text {TH}}$ shifts and OFF-state time-to-breakdown ( ${t}_{\text {BD}}$ ) on the positive substrate bias ( ${V}_{\text {sub}}$ ) was also observed. The underlying mechanisms are the different impacts of positive ${V}_{\text {sub}}$ on the drift of electrons and holes during the long-term stress. An important indication is that positive-biased and FS terminations should be restricted at OFF-state in order to obtain good ${V}_{\text {TH}}$ stability in applications of the GaN MIS-FET.