Aberration corrected scanning transmission electron microscopy and electron energy loss spectroscopy studies of epitaxial Fe/MgO/(001)Ge heterostructures

Aberration correction in the scanning transmission electron microscope combined with electron energy loss spectroscopy allows simultaneous mapping of the structure, the chemistry and even the electronic properties of materials in one single experiment with spatial resolutions of the order of one Ångström. Here the authors will apply these techniques to the characterization of epitaxial Fe/MgO/(001)Ge and interfaces with possible applications for tunneling junctions, and the authors will show that epitaxial MgO films can be grown on a (001)Ge substrates by molecular beam epitaxy and how it is possible to map the chemistry of interfaces with atomic resolution.     

Electronic structure of boron-doped diamond with B–H complex and B pair

Abstract

The electronic structure of boron–hydrogen complex and boron pair in diamond are studied by first-principles density-functional calculations with supercell models. The electronic structure calculated for the B–H complexes with C_2v or C_3v symmetry and the nearest-neighbor B pair is used to interpret recent experimental results such as B 1s x-ray photoemission spectroscopy, ¹¹B nuclear quadruple resonance and B K-edge x-ray absorption spectroscopy, which cannot be explained solely by the isolated substitutional boron.     

Silicon carbide and diamond for high temperature device applications

The physical and chemical properties of wide bandgap semiconductors silicon carbide and diamond make these materials an ideal choice for device fabrication for applications in many different areas, e.g. light emitters, high temperature and high power electronics, high power microwave devices, micro-electromechanical system (MEMS) technology, and substrates. These semiconductors have been recognized for several decades as being suitable for these applications, but until recently the low material quality has not allowed the fabrication of high quality devices. Silicon carbide and diamond based electronics are at different stages of their development. An overview of the status of silicon carbide's and diamond's application for high temperature electronics is presented. Silicon carbide electronics is advancing from the research stage to commercial production. The most suitable and established SiC polytype for high temperature power electronics is the hexagonal 4H polytype. The main advantages related to material properties are: its wide bandgap, high electric field strength and high thermal conductivity. Almost all different types of electronic devices have been successfully fabricated and characterized. The most promising devices for high temperature applications are pn-diodes, junction field effect transistors and thyristors. MOSFET is another important candidate, but is still under development due to some hidden problems causing low channel mobility. For microwave applications, 4H-SiC is competing with Si and GaAs for frequency below 10 GHz and for systems requiring cooling like power amplifiers. The unavailability of high quality defect and dislocation free SiC substrates has been slowing down the pace of transition from research and development to production of SiC devices, but recently new method for growth of ultrahigh quality SiC, which could promote the development of high power devices, was reported. Diamond is the superior material for high power and high temperature electronics. Fabrication of diamond electronic devices has reached important results, but high temperature data are still scarce. PN-junctions have been formed and investigated up to 400 ^∘C. Schottky diodes operating up to 1000 ^∘C have been fabricated. BJTs have been fabricated functioning in the dc mode up to 200 ^∘C. The largest advance, concerning development of devices for RF application, has been done in fabrication of different types of FETs. For FETs with gate length 0.2 μ m frequencies f_T = 24.6 GHz, f_{max (MAG)} = 63 GHz and f_{max (U)} = 80 GHz were reported. Further, capacitors and switches, working up to 450 ^∘C and 650 ^∘C, respectively, have also been fabricated. Low resistant thermostable resistors have been investigated up to 800 ^∘C. Temperature dependence of field emission from diamond films has been measured up to 950 ^∘C. However, the diamond based electronics is still regarded to be in its infancy. The prerequisite for a successful application of diamond for the fabrication of electronic devices is availability of wafer diamond, i.e. large area, high quality, inexpensive, diamond single crystal substrates. A step forward in this direction has been made recently. Diamond films grown on multilayer substrate Ir/YSZ/Si(001) having qualities close those of homoepitaxial diamond have been reported recently.     

Shallow donors in diamond: Be and Mg

The electronic properties of the impurities Be, Mg and the hydrogen complexes Be–H, Mg–H in diamond have been investigated by first-principle calculations. It is found that the interstitial Be- or Mg- doped diamond are of n-type metal conductivity character. Even at low impurity concentration the doped diamond also appears n-type behavior. The further results indicate that the interstitial Be or Mg doping diamond should be synthesized at H-poor conditions to obtain the n-type material because most of hydrogen atom may result in interstitial Be- and Mg- doped diamond p-type semiconductor or insulator. The substitutional Be and Mg show acceptor behaviors and may compensate other interstitial donors in -diamond. Our results are very helpful to the research of n-type doping in diamond for the future experimental work.     

Ab initio density functional simulation of structural and electronic properties of MgO ultra-thin adlayers on the (001) Ag surface

To simulate the properties of ultrathin layers of magnesium oxide epitaxially grown on silver (001), we have adopted a periodic slab model, consisting of six layers of Ag covered on both sides with an MgO monolayer. All calculations have been performed with the 98 program. Several DFT functionals were tried and a rich basis set was adopted. The electronic and structural properties of the two bulk materials (Ag and MgO) were quite accurately reproduced. The presence of the metallic substrate was found to have an appreciable influence on the structural and electronic features of the oxide surface. In the most stable configuration (O ions directly above Ag atoms, Mg ions in the hollow sites), the surface is corrugated, and there is a net transfer of electrons from the overlayer to the metal, leading to a substantial reduction of the work function of the metal and to a decrease of the electrostatic field at the surface. The reactivity properties of the supported oxide surface have been investigated by studying the interaction of the composite material with water molecules.     

Epitaxy of cubic boron nitride on (001)-oriented diamond

Cubic boron nitride (c-BN), although offering a number of highly attractive properties comparable to diamond, like hardness, chemical inertness and a large electronic bandgap, up to now has not found the attention it deserves. This mostly has to do with preparational problems, with easy chemical routes not available and, instead, the necessity to apply ion-bombardment-assisted methods. Hence, most of the c-BN samples prepared as thin films have been nanocrystalline, making the prospect of using this material for high-temperature electronic applications an illusion. Although heteroepitaxial nucleation of c-BN on diamond substrates has been demonstrated using the high-pressure-high-temperature technique, none of the low-pressure methods ever succeeded in the epitaxial growth of c-BN on any substrate. Here, we demonstrate that heteroepitaxial c-BN films can be prepared at 900 degrees C on highly (001)-oriented diamond films, formed by chemical vapour deposition, using ion-beam-assisted deposition as a low-pressure technique. The orientation relationship was found to be c-BN(001)[100]||diamond(001)[100]. High-resolution transmission electron microscopy additionally proved that epitaxy can be achieved without an intermediate hexagonal BN layer that is commonly observed on various substrates.     

Fabrication of conductive SrRuO<Subscript>3</Subscript> thin film and Ba<Subscript>0.60</Subscript>Sr<Subscript>0.40</Subscript>TiO<Subscript>3</Subscript>/SrRuO<Subscript>3</Subscript> bilayer films on MgO substrate

Conductive SrRuO₃ (SRO) thin films have been grown on (100) MgO substrates by pulsed laser deposition (PLD) technique. Effects of oxygen pressure and deposition temperature on the orientation of SRO thin film were investigated. X-ray diffraction (XRD) θ/2θ patterns and the temperature dependent resistivity measurements indicated that oxygen pressure of 30 Pa and deposition temperature of 700 °C were the optimized deposition parameters. A parallel-plate capacitor structure was prepared with the SRO films deposited under optimized condition as an electrode layer and Ba_0.60Sr_0.40TiO₃ (BST) thin film as the dielectric layer. XRD Φ scans indicated a epitaxial relationship between BST and SRO on MgO substrate. The dielectric constant and loss tangent measured at 10 kHz and 300 K was 427 and 0.099 under 0 V bias, and 215 and 0.062 under 8 V bias, respectively. A tunability of 49.6% has been achieved with DC bias as low as 8 V. The C–V hysteresis curve and the P–E hysteresis loop suggested that the BST films epitaxially grown on SRO/MgO have ferroelectricity at room temperature. The induced ferroelectricity was believed to originate from the compressive strain between the epitaxial BST and SRO thin films. These results show the potential application of the BST/SRO heterostructures in microelectronic devices.     

Ab initio DFT study of ideal shear strength of polytypes of silicon carbide

Ab initio density functional calculations are performed to investigate the ideal shear deformation of SiC polytypes (3C, 2H, 4H, and 6H). The deformation of the cubic and the hexagonal polytypes in small strain region can be well represented by the elastic properties of component Si₄C-tetrahedrons. The stacking pattern in the polytypes affects strain localization, which is correlated with the generalized stacking fault energy profile of each shuffle-set plane, and the ideal shear strength. Compressive hydrostatic stress decreases the ideal shear strength, which is in contrast with the behavior of metals. __________ Translated from Problemy Prochnosti, No. 1, pp. 8–13, January–February, 2008.     

The Study of Half-Metallicity in Zincblende CrBi (001) and VBi (001) Surfaces and CrBi/InSb (001) and VBi/InSb (001) Interfaces

Density functional theory calculations were performed to study the structural, electronic, and magnetic properties of hexagonal NiAs-type, cubic rock salt (RS)-type, and cubic zincblende (ZB)-type CrBi and VBi compounds, free (001) surfaces of ZB CrBi and VBi, and interface of ZB CrBi (001) and VBi (001) with InSb (001). Antiferromagnetic (AFM) NiAs structure was a ground-state structure of bulk CrBi and VBi. The ZB structure of CrBi and VBi was found to be ferromagnetic (FM) half-metals with total moments of 3 B and 2 μ _B per formula unit, respectively. Calculated surface energies in the framework of ab initio atomistic thermodynamics indicated that the Bi-terminated (001) surfaces of CrBi and VBi have the lowest energy over the entire accessible region of chemical potentials with respect to Cr- and V-terminated (001) surfaces. Half-metallic (HM) characteristic was preserved at Cr (001) and V (001) terminations, while half-metallicity disappeared at Bi (001) terminations. Half-metallicity was also retained at CrBi/InSb (001) and VBi/InSb (001) interfaces, introducing ZB CrBi and ZB VBi as promising candidates for possible epitaxial growth on InSb semiconductor substrate in spintronic applications.     

Possible n-type dopants in diamond and amorphous carbon

It has been extremely difficult to produce n-type conducting diamond, whereas it seems that n-type conducting tetrahedrally bonded amorphous carbon (ta-C) can be obtained using phosphorous or nitrogen as dopant. In this work we have studied substitutional group V–VII impurities (N, O and Cl) in diamond and ta-C using first-principles electronic structure methods. Electronic structure calculations reveal the differences between ta-C and diamond in the microscopic level and ease, therefore, the experimental search for producing n-type conducting diamond-like materials.     

Superconductivity of p-type diamond (001) and (111) thin films: Ab initio calculations

The surface structure, electronic properties, lattice dynamics, and the electron-phonon coupling for p-doped diamond (001)-(2 × 1) and (111)-(2 × 1) thin films have been extensively investigated using ab initio methods within the virtual crystal approximation. The calculations of p-doped diamond thin films strongly favor dimer reconstruction of diamond surfaces. The physical origin of superconductivity of diamond (001)-(2 × 1), respectively, and (111)-(2 × 1) thin films which is higher than that of bulk diamond is systematically studied. It is showed that surface vibrational modes of diamond (001)-(2 × 1) surfaces give main contributions to superconducting transition temperature T_c, while T_c of diamond (111)-(2 × 1) surfaces is attributed to the combined action of surface and bulk vibrational modes. Therefore, at the highest concentration (13.98 × 10²¹ cm^− 3) T_c ≈ 56.5 K of diamond (111)-(2 × 1) surfaces is about twice as high as that of bulk and diamond (001)-(2 × 1) surfaces.     

Epitaxial growth of Sr x Ba1−x Nb2 O6 (SBN) thin films on Pt coated MgO substrates: the determining control of platinum crystallographic orientation

The growth of (001) oriented Sr _x Ba_1−x Nb₂ O₆ (SBN:x) thin films on MgO single crystals substrates has focused attention due to its potential application to integrated signal waveguiding and electro-optic processing. This paper addresses the possible insertion of a platinum bottom electrode to implement the electro-optic effect, and examines the influence of Pt orientation on the subsequent growth of SBN thin films deposited by R.F. magnetron sputtering. We have first investigated the optimal sputtering conditions of a (001) Pt oriented growth. Experimental evidence is provided for an orientation switching temperature sensitive to R.F. power but insensitive to deposition rate which suggests that kinetic phenomena are not involved in the orientation development of Pt films. Seeding the MgO substrate with Fe, Cu or Cr inhibits the competitive (111) Pt growth without modifying the optimal temperature for (001) growth. Annealing at a temperature sufficient to crystallise the top SBN film causes a drastic evolution of the initial (111) and (001) Pt volume fractions, making particularly critical the deposition conditions for a final dominant (001) Pt texture on unseeded substrates. The (001) oriented growth of SBN is shown to occur exclusively on (001) Pt crystallites stable against subsequent annealing steps. As a final result, (001) SBN films have been obtained epitaxial on both Pt coated and not coated (001) MgO substrates. They display two in-plane orientations mirror symmetric (±α) to the axis of (001) MgO, with α shifted from 31° to 18° by (001) Pt coating.     

Atomic and electronic structures of Li4Ti5O12/Li7Ti5O12 (001) interfaces by first-principles calculations

We have performed density-functional theory calculations for Li₄Ti₅O₁₂/Li₇Ti₅O₁₂ (LTO/Li-LTO) interfaces and made a detailed analysis of the local atomic and electronic structures. In the bulk regions of the supercell, the atomic and electronic structures are well reproduced to be similar to those of the LTO and Li-LTO bulk crystals. The present (001) interface models show abrupt structural changes between the cubic spinel-based LTO and ordered rock-salt Li-LTO phases, while there occur no substantial strains around the interface due to the little volume change or lattice mismatch. Thus, the calculated interfacial energy is very small. The calculated O–K electron energy-loss near-edge structure/X-ray adsorption near-edge structure (ELNES/XANES) spectra in the bulk regions are similar to those of the bulk crystals, while the O–K edge spectra at the two kinds of interfaces have specific shapes, differently from the simple superposition of the bulk spectra. The preferential occurrence of the (001) interface can be understood from the preferential Li diffusion along the [110] direction in LTO and the small interfacial energy.     

Electronic structure of boron-doped diamond with B–H complex and B pair

The electronic structure of boron–hydrogen complex and boron pair in diamond are studied by first-principles density-functional calculations with supercell models. The electronic structure calculated for the B–H complexes with C_2v or C_3v symmetry and the nearest-neighbor B pair is used to interpret recent experimental results such as B 1s x-ray photoemission spectroscopy, ¹¹B nuclear quadruple resonance and B K-edge x-ray absorption spectroscopy, which cannot be explained solely by the isolated substitutional boron.     

Epitaxial growth of MgO/TiN multilayers on Cu

The growth of epitaxial MgO/TiN multilayer films on (001) Cu has been investigated. In particular, epitaxial structures were grown on (001) Cu layers that were epitaxial on (001) SrTiO₃. X-ray diffraction and reflection high-energy electron diffraction indicate that the multilayer structures are epitaxial on the (001) Cu surface. The motivation is the use of crystalline MgO/TiN multilayers as a diffusion barrier to both copper and oxygen. MgO/TiN multilayers are potentially useful as diffusion barriers for Cu interconnects on semiconductors as well as for superconducting wires based on the epitaxial growth of cuprate superconductors on biaxially textured copper.     

Phase Engineering of 2D Tin Sulfides

Tin sulfides can exist in a variety of phases and polytypes due to the different oxidation states of Sn. A subset of these phases and polytypes take the form of layered 2D structures that give rise to a wide host of electronic and optical properties. Hence, achieving control over the phase, polytype, and thickness of tin sulfides is necessary to utilize this wide range of properties exhibited by the compound. This study reports on phase‐selective growth of both hexagonal tin (IV) sulfide SnS₂ and orthorhombic tin (II) sulfide SnS crystals with diameters of over tens of microns on SiO₂ substrates through atmospheric pressure vapor‐phase method in a conventional horizontal quartz tube furnace with SnO₂ and S powders as the source materials. Detailed characterization of each phase of tin sulfide crystals is performed using various microscopy and spectroscopy methods, and the results are corroborated by ab initio density functional theory calculations.     

Ab initio study of magnetism at the TiO<Subscript>2</Subscript>/LaAlO<Subscript>3</Subscript> interface

In this article, we study the possible relation between the electronic and magnetic structures of the TiO₂/LaAlO₃ interface and the unexpected magnetism found in undoped TiO₂ films grown on LaAlO₃. We concentrate on the role played by structural relaxation and interfacial oxygen vacancies. LaAlO₃ has a layered structure along the (001) direction with alternating LaO and AlO₂ planes, with nominal charges of +1 and −1, respectively. As a consequence of that, an oxygen-deficient TiO₂ film with anatase structure will grow preferently on the AlO₂ surface layer. We have therefore performed ab initio calculations for superlattices with TiO₂/AlO₂ interfaces with interfacial oxygen vacancies. Our main results are that vacancies lead to a change in the valence state of neighbor Ti atoms but not necessarily to a magnetic solution and that the appearance of magnetism depends also on structural details, such as second neighbor positions. These results are obtained using both the local spin density approximation (LSDA) and LSDA + U approximations.     

Oxygen treatment effects on properties of MgO thin films grown on single-crystalline diamond (1 0 0)

Oxygen post-treatment effects on the electronic structure and electrical properties of MgO films grown on homoepitaxial single-crystalline (1 0 0) diamond have been studied. MgO films examined were deposited at room temperature (RT) using an electron beam evaporator and were subsequently either annealed at 573-773 K for 12 h in oxygen ambient or treated by O₂ plasma for 10-40 min. RT resistivities remarkably increased after the O₂ annealing and plasma treatment, indicating that the post treatments play an essential part on the formation and positioning of bandgap states. Cathodoluminescence (CL) spectra had a broad band feature in a wavelength region from 360 to 530 nm, which were decomposed to several peaks originating mainly from the oxygen-vacancy-related F and F⁺ centers and the interstitial vacancies of MgO film. A prominent rectifying behavior of I-V property was observed for a Au/MgO/p-diamond layered structure. Based on temperature dependences of the electrical properties in a temperature region from RT to 600 K, the electrical conduction mechanism in the MgO films is discussed in relation to polaron-related conduction as well as the ionic conduction.     

Epitaxial growth of Ni films sputter-deposited on a GaAs(001) surface covered with a MgO film

We studied the epitaxial growth of a Ni film prepared on a GaAs(001) substrate covered with a thin epitaxial MgO buffer film, assuming that this buffer film plays a key role in the epitaxial growth of the Ni film. The MgO and Ni films were deposited by radio-frequency magnetron sputtering of the MgO and Ni targets in pure Ar gas. First, a MgO film of thickness ranging from 78 to 4.4 nm was deposited on the GaAs(001) substrate at a temperature ranging from ambient temperature to 700 °C, and then, a 136-nm-thick Ni film was deposited on the MgO/GaAs substrate at a temperature range 300-500 °C. Using transmission electron microscopy and X-ray diffractometry, we showed that the MgO film grows with the epitaxial relationship MgO(001)[001]//GaAs(001)[001] on GaAs(001) at 500 °C, and that the structure of the Ni film depends on three factors: the MgO/GaAs substrate temperature, the MgO thickness, and the annealing condition of the MgO/GaAs substrate before the Ni deposition. In conclusion, we proved that the Ni film grows with the epitaxial relationship Ni(001)[001]//MgO(001)[001]//GaAs(001)[001] on MgO/GaAs with the 4.4-nm-thick MgO film when the MgO/GaAs substrate is annealed in situ at room temperature before the Ni deposition and maintained at 300 °C during the Ni deposition.     

第一性原理研究PDP放电单元MgO保护层各种空缺对二次电子发射系数的影响

基于密度泛函理论的第一原理赝势法,研究了PDP放电单元中MgO保护层在形成氧空缺后的电子结构的变化.通过对能带结构和态密度分布的计算,可以看到MgO形成氧空缺后在禁带中引入了能级.本文计算了完整MgO以及含F、F~+、F~(2+)空缺的MgO晶体,得到不同能带结构和态密度分布,同时计算了相应的二次电子发射系数.结果表明空缺的形成,可有效提高二次电子发射系数,其中形成F空缺的MgO晶体的二次电子发射系数最大.