In situ RHEED control of self-organized Ge quantum dots

In situ registration of high-energy electron diffraction patterns was used for constructing the diagram of structural and morphological states of the Ge film on the Si(100) surface. The following regions identified in the diagram: two-dimensional (2D)-growth, ‘hut’- and ‘dome’-clusters, ‘dome’-clusters with misfit dislocations at the interface. Variations in the lattice constants of the Ge film during the MBE growth on the Si(100) surface were determined. An increase in the lattice constant at the (100) surface was attributed to the elastic deformation at the stage of 2D growth and formation of ‘hut’-clusters and to the plastic relaxation for the ‘dome’-clusters. As a result, epitaxial silicon structures with germanium quantum dots of 15 nm base size at the density of 3×10¹¹ cm⁻² were synthesized. The total electron structure of the hole spectrum of Ge quantum dots in Si was established.     

Thin film analysis via accelerator-based nuclear methods

Methods for depth profiling layered structures in the 10–100 nm range via ion beam analysis, such as Rutherford backscattering spectrometry, channeling, nuclear reaction analysis and elastic recoil detection analysis, are described in connection with ion-beam synthesized silicides (β-FeSi_2, TaSi₂) and epitaxial regrowth of amorphous SiO₂. Methods using implanted radioactive marker isotopes in nanometer thin films, such as perturbed angular correlation spectroscopy, will be sketched.     

Structural and optical characterization of β-FeSi2 layers on Si formed by ion beam synthesis

Structural and optical properties have been investigated for surface β-FeSi₂ layers on Si(100) and Si(111) formed by ion beam synthesis using ⁵⁶Fe ion implantations with three different energies (140–50 keV) and subsequent two-step annealing at 600 °C and up to 915 °C. Rutherford backscattering spectrometry analyses have revealed Fe redistribution in the samples after the annealing procedure, which resulting in a Fe-deficient composition in the formed layers. X-ray diffraction experiments confirmed the existence of /gb-FeSi₂ by annealing up to 915 °C, whereas the phase transformation from the β to phase has been induced at 930 °C. In photoluminescence measurements at 2 K, both β-FeSi₂/Si(100) and β-FeSi₂/Si(111) samples, after annealing at 900–915 °C for 2 h, have shown two dominant emissions peaked around 0.836 eV and 0.80 eV, which nearly coincided with previously reported PL emissions from the sample prepared by electron beam deposition. Another β-FeSi₂/Si(100) sample has shown sharp emissions peaked at 0.873 eV and 0.807 eV. Optical absorption measurements at room temperature have revealed the allowed direct bandgap of 0.868–0.885 eV as well as an absorption coefficient of the order of 10⁴ cm⁻¹ near the absorption edge for all samples.     

Structural properties of swift heavy ion beam irradiated Fe/Si bilayers

The Fe/Si multilayers were prepared by electron beam evaporation in a cryo-pumped vacuum deposition system. Ag⁺ and Au⁺ ions of 100 MeV at two different fluencies such as 1 × 10¹² ions/cm² and 1 × 10¹³ ions/cm² at a pressure of 10^− 7 torr were used to irradiate the Fe/Si samples. The irradiated samples were analyzed by High-Resolution XRD and it reveals that the irradiated films are having polycrystalline nature and it confirms the formation of the β-FeSi₂. The structural parameters such as crystallite size (D), strain (ε) and dislocation density (δ) have been evaluated from the XRD spectrum. The role of the substrates and the influence of swift heavy ions on the formation of β-FeSi₂ have been discussed in this paper.     

Interplay of dislocation network and island arrangement in SiGe films grown on Si(001)

A two-temperature process has been applied to grow 80-nm Si_0.7Ge_0.3 films on Si(001) by molecular-beam epitaxy (MBE). The first 30 nm were deposited at a reduced temperature of only 150–200°C (low-temperature stage). The subsequent growth was performed at 550°C, the temperature range conventionally applied for SiGe MBE. Using atomic-force microscopy, we observed that the misfit dislocation network introduced during sample heating after the low-temperature (LT) stage guides the arrangement of {105}-faceted pyramid-like islands. In the case of a very narrow dislocation network — induced by ion-assisted growth during the LT stage — a checkerboard array of {105}-faceted pits and pyramids evolves with a ‘lattice constant’ of approximately 200 nm.     

织构化ZrB2-SiC超高温陶瓷中取向关系的EBSD研究

以放电等离子体烧结的织构化ZrB₂-SiC复相陶瓷为研究对象, 利用SEM和EDS方法对相组成进行分析。研究发现, 由于初始粉体与杂质之间存在多种反应, 陶瓷中出现相当含量的ZrC新相及少量的ZrO₂、BN相。与利用TEM研究新生成相与主相间取向关系的常规方法相比, SEM中EBSD方法不但能研究该取向关系, 还可同时对大量相界进行研究以获得统计性结果, 从而避免人为选择性。利用该方法, 对ZrB₂与ZrC相间可能存在的三种取向关系(011¯0)||(111)&[21¯1¯0]||[101¯]、(112¯0)||( 2¯02)&[0001]||[111]和(1¯21¯0)||(2 2¯0)&[0001]||[110]进行验证, 确定本实验中所得复相陶瓷中两相间不存在特定取向关系, 从而推断ZrC的成相遵循均匀成核模式, 而非外延成核。     

Strain effect in single-crystal silicon-based multilayer surface acoustic wave structures

A method for calculating the characteristics of surface acoustic wave (SAW) propagation in a deformable piezoelectric multilayer medium is presented. The effect of longitudinal and lateral mechanical strain on the SAW phase velocity in a (ZnO or AIN)/SiO₂/Si thin film structure for {001}, {111} and {110} silicon crystal planes within the temperature range 293–673 K is studied. The effects of thickness and internal mechanical stresses in the ZnO or A1N piezoelectric film and SiO₂ dielectric film on the sensitivity of the SAW phase velocity to strains in the structure are investigated. The Si{110}-based SAW structure with the SAW wavevector oriented in the 10 direction is shown to possess maximum operating frequency sensitivity to both longitudinal and lateral strain. The parameters of SAW structure stable to mechanical loads are determined. ZnO and SiO₂ layer deposition on silicon is shown to increase the SAW phase velocity sensitivity to longitudinal strain and to decrease its sensitivity to lateral strain in the structure.     

Semiconducting and structural properties of CrSi2 A-type epitaxial films on Si(111)

Chromium disilicide (CrSi₂) films 1 000 Å thick have been prepared by molecular beam epitaxy on CrSi₂ templates grown on Si(111) substrate. The effect of the substrate temperature on the structural, electrical and optical properties of CrSi₂ films has been studied by transmission and scanning electron microscopies, optical microscopy, electrical resistivity and Hall effect measurements and infrared optical spectrometry. The optimal temperature for the formation of the epitaxial A-type CrSi₂ film have been found to be about 750°C. The electrical measurement have shown that the epitaxial A-type CrSi₂ film is p-type semiconductor having a hole concentration of 1 × 10¹⁷cm⁻³ and Hall mobility of 2 980 cm² V⁻¹ s⁻¹ at room temperature. Optical absorption coefficient data have indicated a minimum, direct energy gap of 0.34 eV. The temperature dependence of the Hall mobility (μ) in the temperature range of T = 180–500 K can be expressed as μ = 7.8 × 10¹⁰T⁻³cm²V⁻¹s⁻¹.     

Heteroepitaxy of CuInSe2 on {111}-oriented germanium

Thin films of CuInSe₂ were deposited onto {111}-oriented germanium substrates by flash evaporation and were investigated by reflection high energy electron diffraction. Epitaxial growth was found in the substrate temperature range 720–820 K. In all cases the epitaxial layers had the chalcopyrite structure except at growth temperatures higher than 795 K where the layers were cubic. Deposition of CuInSe₂ onto {111}-oriented germanium is characterized by one-dimensional epitaxy, and the epitaxial relationship for the chalcopyrite phase is given by {111}_Ge{112}_CuInSe2     

Transmission electron microscope observation of Si deposited on W(110) surface

The (110) surface of a thin W crystal deposited with Si has been investigated by using a transmission electron microscope (TEM). It was found that an ordered Si–W alloy layer with a regular periodic arrangement of antiphase boundaries (APBs) is formed, as a result of intermixing of deposited Si with the W substrate at room temperature (RT). A crystal structure model of this W–Si superstructure is proposed in this study. After 800°C annealing of Si deposited at RT, grains of tetragonal WSi₂ are formed on the W–Si ordered alloy layer. Grains of WSi₂ are also formed when Si is deposited on a W substrate maintained at 900°C. These WSi₂ grains have epitaxial orientation relationships with the W(110) substrate as follows: (110)W//(01 ) WSi₂ and [001]W//[100]WSi₂.     

Epitaxial growth of gallium nitride by ion-beam-assisted evaporation

The epitaxial growth of gallium nitride thin film was obtained on the inclined Si(111) substrates by the process of ion-beam-assisted evaporation (IBAE) at the low temperature of 500 °C. The film composition determined by Rutherford backscattering spectrometry shows that the synthesized film is a stoichiometric nitride. The epitaxial quality of GaN film is enhanced by minimizing the bombardment-induced film damage by decreasing the ion flux. However, the crystallinity of the GaN film becomes very poor when the ion flux is not sufficient to densify the film. The optimum flux ratio of N⁺₂ to Ga and the energy of incident N⁺₂ ions for the epitaxial growth were found to be 3.4 and 50 eV, respectively. The GaN film deposited on the 4 °-inclined Si (111) with respect to substrate surface shows much better crystalline quality compared with that on the 0 ° inclined Si(111) due to many stable nucleation sites. A thin amorphous layer exists at the interface between GaN and Si(111) substrate and acts as a buffer zone enabling the subsequent epitaxial growth of GaN by relaxing the large misfit strain (23%) in the early stage of film growth. The epitaxial GaN film shows an orientational relation with the Si(111) substrate.     

Formation of thin β-FeSi2 template layer for the epitaxial growth of thick film on Si (111) substrate

For the epitaxial growth of thick β-FeSi₂ films, we fabricated ultrathin β-FeSi₂ template layers (thinner than 20 nm) on Si (111) substrates with different methods. Surface morphology and crystallinity of the template layers were found to be dependent on the surface conditions of the substrate and the fabrication method. It was revealed that to form a smooth and continuous template, a hydrogen-terminated surface was better than that covered with a several-nanometer oxide layer. Using this surface, continuous (110)/(101)-oriented epitaxial template was obtained by depositing 6-nm iron at 400 °C and subsequent in situ annealing at 600 °C in MBE chamber, namely, a reaction deposition epitaxy (RDE) method. Co-deposition of iron and silicon with atomic ratio of Fe/Si=1/2 allowed the forming of template layers at further low temperature. Co-deposited template layers exhibited better crystallinity and morphology than those prepared by RDE. By using the optimized template layer, we succeeded in growing high-quality thick β-FeSi₂ films on Si (111) substrates with sharp β-FeSi₂/Si interface.     

Epitaxy of atomically flat CaF2 films on Si(1 1 1) substrates

The growth of CaF₂ films with a thickness of approximately 3–4 nm on well-oriented Si(1 1 1) substrates by molecular beam epitaxy at temperatures between 410 and 560 °C were investigated by ex vacuo atomic force microscopy. Layer-by-layer growth producing atomically flat CaF₂ surfaces has been observed in a very narrow growth temperature window between approximately 430 and 470 °C. Perfect triangular shaped islands of one CaF₂ layer height are found on the surface with all corners aligned with the Si directions, indicating a pure B-stacking of the CaF₂ film. Surprisingly, also the substrate steps have been overgrown without visible defects. Below 410 °C, two different island orientations revealed a mixture of A- and B-stacking areas in the films. Above 520 °C non-wetting of the CaF interface layer leads to epitaxial films with a rough surface morphology.     

成形压力对SiC/TiB2复合材料组织与性能的影响

基于熔融Si浸渗法制备出较致密的SiC/TiB₂复合材料, 并研究了坯体成形压力对SiC/TiB₂复合材料致密度、相组成、显微组织和力学性能的影响。实验结果表明, 复合材料由TiB₂、SiC和Si相组成。SiC/TiB₂复合材料的显微组织特征为: TiB₂相和SiC相均匀分布, 游离Si填充在TiB₂相和SiC相的空隙处, 且形成了连续相。随成形压力的增大, 复合材料中游离Si含量降低, TiB₂颗粒尺寸减小, 复合材料的力学性能先增加后降低。坯体最佳成形压力为200 MPa, 对应SiC/TiB₂复合材料的体积密度、开口气孔率、抗弯强度、断裂韧性和维氏硬度分别为3.63 g/cm³、0.90%、(354±16) MPa、(6.8±0.2) MPa·m^1/2和(21.0±1.1) GPa。     

The Dislocation Structure of a Single-Crystal γ + γ′ Two-Phase Alloy After Tensile Deformation

The dislocation structures of an industrial single-crystal γ + γ′ two-phase alloy DD3 after tensile deformation from room temperature to 1273K were studied by transmission electron microscopy. The strength of this alloy decreased with an increase in the temperature, and showed a strength peak at 1033K. At room temperature, the dislocations shearing the γ′ particles were found to be 1/3<112> partial dislocations on the dodecahedral slip system <112>{111}. Some dislocation pairs on the cubic <110>{100} system that blocked the glide of dislocations were found at a medium temperature of 873K. As a result, dislocation bands were formed. Shearing of γ′ particles by 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} was also found at this temperature. At the peak temperature of 1033K, because of the strong interaction between dislocations on the {111} and {100} planes, the extent of dislocation bands with high dislocation densities was extensive. The 1/3<112> partial dislocations on the dodecahedral slip system <112>{111} also existed. When the temperature reached the high temperature of 1133K, the range of dislocation bands was limited. The γ′ particles were sheared by <110> dislocation pairs on the octagonal <110>{111} system and the cubic <110>{100} system. At 1273K, the regular hexagonal dislocation networks were formed in the γ matrix and at the γ/γ′ interface. The Burgers vectors of the network were found to be b₁ = 1/2[110], b₂ = 1/2[1–10], b₃ = [100], and the last one was formed by the reaction of b₁ + b₂ → b₃. Dislocations shearing the γ′ particles were found to be <110> dislocation pairs on the octagonal system <110>{111} and cubic slip system <110>{100} at 1273K.     

Low-temperature growth and orientational control in RuO2 thin films by metal-organic chemical vapor deposition

For growth temperatures in the range of 275°C to 425°C, highly conductive RuO₂ thin films with either (110)- or (101)-textured orientations have been grown by metal-organic chemical vapor deposition (MOCVD) on both SiO₂/Si(001) and Pt/Ti/SiO₂/Si(001) substrates. Both the growth temperature and growth rate were used to control the type and degree of orientational texture of the RuO₂ films. In the upper part of this growth temperature range ( 350°C) and at a low growth rate (< 3.0 nm/min.), the RuO₂ films favored a (110)-textured orientation. In contrast, at the lower part of this growth temperature range ( 300°C) and at a high growth rate (> 3.0 nm/min.), the RuO₂ films favored a (101)-textured orientation. In contrast, higher growth temperatures (> 425°C) always produced randomly-oriented polycrystalline films. For either of these low-temperature growth processes, the films produced were crack-free, well-adhered to the substrates, and had smooth, specular surfaces. Atomic force microscopy showed that the films had a dense microstructure with an average grain size of 50–80 nm and a rms. surface roughness of 3–10 nm. Four-probe electrical transport measurements showed that the films were highly conductive with resistivities of 34–40 μΩ-cm (at 25°C).     

On orientation relationship of the Ti5Si3 precipitates in a TiAl alloy

The orientation relationship (OR) and interface structure between ζ-Ti₅Si₃ precipitates and γ-TiAl phase in a Ti–Al based alloy composed of γ-TiAl and ₂-Ti₃Al lamellae have been investigated using transmission electron microscopy. Various orientation relationships defined by a pair of parallel directions and planes are discussed with the method of basic vector transformation matrix in the reciprocal space from γ-TiAl to ζ-Ti₅Si₃ precipitate phase and two new kinds of orientation relationships between ζ-Ti₅Si₃ and γ-TiAl phases have been found. Periodical interface fringes at γ-TiAl/ζ-Ti₅Si₃ interface are analyzed according to the Moiré fringes and interface misfit dislocations.     

Adsorption-Controlled Growth of BiFeO3 by MBE and Integration with Wide Band Gap Semiconductors

BiFeO3 thin films have been deposited on (001) SrTiO3, (101) DyScO3, (011) DyScO3, (0001) AlGaN/GaN, and (0001) 6H-SiC single crystal substrates by reactive molecular beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth over-pressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO₃ to control stoichiometry in accordance with thermodynamic calculations. Four-circle x-ray diffraction and transmission electron microscopy reveal phase-pure, epitaxial films with rocking curve full width at half maximum values as narrow as 7.2 arc seconds (0.002°). Epitaxial growth of (0001)-oriented BiFeO3 thin films on (0001) GaN, including AlGaN HEMT structures, and (0001) SiC has been realized using intervening epitaxial (111) SrTiO₃ / (100) TiO₂ buffer layers. The epitaxial BiFeO3 thin films have 2 in-plane orientations: [1120] BiFeO>sub>3 || [1120] GaN (SiC) plus a twin variant related by a 180° in-plane rotation. This epitaxial integration of the ferroelectric with the highest known polarization, BiFeO3, with high bandgap semiconductors is an important step toward novel field-effect devices.     

Bias sputtered Ta modified diffusion barrier in Cu/Ta(Vb)/Si(111) structures

In this investigation, we have fabricated Ta(V_b)/Si(111) and Cu/Ta(V_b)/Si(111) systems using a DC bias sputtering technique at high Ar pressure (100 mTorr). For Ta/Si(111) system, tantalum layer was formed under various bias voltages ranging from 0 to −150 V. The films were characterized by Rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and four-point probe sheet resistance measurements (R_s). From electrical resistivity and SEM data, a minimum resistivity (99 μΩ cm) and well surface morphology at an optimum bias voltage (V_b=−50 V) was obtained for the Ta(V_b)/Si(111) system. The Ta films deposited under these conditions with 50 nm thickness are then used as a diffusion barrier in the Cu/Ta(V_b)/Si(111) multilayer structure. According to our RBS, SEM and R_s analysis, the Ta barrier layer formed under the controlled bias sputtering at high Ar pressure has demonstrated an improved Ta structure with excellent thermal stability up to 650°C for the Cu/Ta(V_b)/Si(111) system annealed in N₂ environment for 30 min. Formation of TaSi₂ was observed at 700°C after the barrier failure using RBS spectra.     

Low-temperature epitaxial growth of conductive LaNiO3 thin films by RF magnetron sputtering

Epitaxial growth of LaNiO₃ (LNO) thin films was successful on CeO₂/YSZ/Si(100), MgO(100) and SrTiO₃ (STO)(100) substrates by RF magnetron sputtering at 300 °C, although pulsed laser deposition requires 600 °C to prepare epitaxial LNO films according to the literature. Epitaxial LNO films deposited on CeO₂/YSZ/Si(100) and STO(100) had single orientation of LNO[100]//CeO₂[110]//YSZ[110]//Si[110]) and LNO[100]//STO[100], respectively. On the other hand, epitaxial LNO films deposited on MgO(100) had mixed orientations of LNO[100]//MgO[100] and LNO[100]//MgO[110]. The lattice parameter, composition and resistivity of the LNO thin films were strongly dependent on the substrate temperature. The minimum resistivity of LNO films was approximately 5×10⁻⁶ Ω m, which value almost agrees with the resistivity in the literature. It was found that the temperature to achieve minimum resistivity was 200 °C, irrespective of the type of substrate. The surface of the LNO films was smooth and flat.