High-energy Electron Diffraction Measurements Research Articles

In segregation influence on properties of InAs quantum dots in dots-in-a-well

We investigated the growth of InAs quantum dots (QDs) in a dots-in-a-well (DWELL) from the perspective of the influence of In segregation from the InGaAs layers in the DWELL. Reflection high-energy electron diffraction (RHEED) measurements during the growth of the lower InGaAs layer indicated that In segregation increased with the In composition of the InGaAs layer. The estimated In segregation values were consistent with the decreases in the critical thickness for the QDs growth and the total volume variations of the grown QDs. These results illustrate that the segregated In from the lower InGaAs layer contributes to the QD growth in the DWELL, and their density increases. Furthermore, RHEED measurements during the growth of the upper InGaAs layer indicated the suppression of the deformation of embedded QDs , which could partially contribute to the longer emission wavelength of the QDs in the DWELL.

Surface chemical states and structures of epitaxial Mg2Sn thermoelectric thin films

Abstract Evaluation of the surface chemical states and structures of thermoelectric thin films is vital for the successful fabrication and operation of microscale thermoelectric devices incorporating such films. The chemical states and structures present in the surface layers of epitaxially grown Mg2Sn thermoelectric thin films were evaluated by Auger electron spectroscopy and reflection high-energy electron diffraction measurements. An oxide layer about 5 Å thick was shown to form on the surface of epitaxially grown Mg2Sn thermoelectric thin films on exposure to air, giving meaningful insights into potential ways of improving the microfabrication of thin-film-based miniaturized thermoelectric devices.

Growth of ultra-thin Cobalt on fullerene (C60) thin-film: in-situ investigation under UHV conditions

With the advent of organic spintronics, material such as fullerene (C60) is drawing immense attention due to the long spin relaxation time. C60 based spin valve structures are always challenging due to the complicated structure at the interfaces of metal (cathode) and C60. In the present work, the growth of e-beam evaporated Co films on the C60 layer has been studied in-situ, starting from a fraction of nanometer to tens of nanometers, using reflection high energy electron diffraction (RHEED), electron transport (ET) and magneto-optic Kerr effect (MOKE) measurements. While RHEED and ET provided information about structure and film morphology, in-situ MOKE provided information about magnetic properties, thus making it possible to correlate the evolution of magnetic properties with that of structure and morphology of the Co thin film. The film grows via island growth, where small isolated islands grow larger to connect with other islands and eventually combine into a continuous film at around 75 Å thickness. Combined analysis of Kerr intensity and resistivity variation with Co thickness revealed the origin of the 23 Å thick magnetic dead layer at the interface, which is attributed to the formation of small clusters with paramagnetic or superparamagnetic properties.

The growth of high-crystalline quality Mn2Au (110) thin films

M n 2 Au is a bimetallic antiferromagnetic with an extraordinarily high Néel temperature of 1600 K. Mn2Au has also been found to demonstrate Néel-order spin–orbit torque, allowing for electrical switching of the antiferromagnetic Néel vector. This has led to much interest in this material for spintronic applications. Here, we detail the growth of high quality Mn2Au (110) thin films. The films were grown on a high quality Pt (111) buffer layer, which was grown on an Al2O3 (0001) substrate. The Mn2Au films have excellent crystalline quality as demonstrated by x-ray diffraction, x-ray reflection, and reflection high energy electron diffraction measurements. The Mn2Au (110) films are found to grow with three equivalent in-plane rotation domains, caused by the Pt/Al2O3 substrate.

Molecular beam epitaxy of TmTe thin films on SrF2 (111)

The odd parity nature of 4f states characterized by strong spin–orbit coupling and electronic correlations has led to a search for novel topological phases among rare earth compounds, such as Kondo systems, heavy Fermions, and homogeneous mixed-valent materials. Our target system is thulium telluride thin films whose bandgap is expected to be tuned as a function of lattice parameter. We systematically investigate the growth conditions of TmxTey thin films on SrF2 (111) substrates by molecular beam epitaxy. The ratio between Te and Tm supply was precisely tuned, resulting in two different crystalline phases, which were confirmed by x-ray diffraction and x-ray photoemission spectroscopy. By investigating the crystalline quality as a function of the substrate temperature, the optimal growth conditions were identified for the desired Tm1Te1 phase. Additional low energy electron diffraction and reflective high energy electron diffraction measurements confirm the epitaxial growth of TmTe layers. X-ray reflectivity measurements demonstrate that homogeneous samples with sharp interfaces can be obtained for varied thicknesses. Our results provide a reliable guidance to prepare homogeneous high-quality TmTe thin films and thus serve as a basis for further electronic investigations.

Metallic band structure of CaF2 thin films grown on silicon(1 1 1): Possible formation of CaSi2

We grew epitaxial 10 monolayer (ML) CaF2 films on a Si(111) substrate and measured its electronic structure with angle-resolved photoemission spectroscopy. Reflection high-energy electron diffraction measurements showed the 1×1 streaks of the grown film with no sign of the formation of the Ca-Si surface superstructure at the growth temperature of 400–500 °C. The band dispersion of the films was metallic in contrast to the highly insulating nature of bulk CaF2. The metallic band dispersion did not change with additional Si deposition (1–10 ML) on the CaF2 film. We suggest that CaSi2 may be formed near the surface layers and this shows that the CaF2/Si is not an ideal insulator/semiconductor heterostructure for application in atomically thin electronic devices.

Growth of 3C-SiC(111) on AlN/off-axis Si(110) hetero-structure and formation of epitaxial graphene thereon

A cubic silicon carbide (3C-SiC) film was fabricated by pulsed laser deposition on an aluminum nitride layer grown on an off-axis Si(110) substrate. This is the first report about the use of off-axis Si(110) substrates for manufacturing SiC/AlN/Si multilayers. A high-quality epitaxial 3C-SiC(111) film without rotation domains or twins, as evidenced by reflection high-energy electron diffraction (RHEED) measurements, was successfully obtained by using a wurtzite AlN(0001)/off-axis Si(110) substrate. On this 3C-SiC film, graphene was formed through annealing in ultrahigh vacuum at 1200 °C and its structure and chemical bonding were investigated via in situ RHEED and X-ray photoelectron spectroscopy; the full width at half maximum of the C 1s core level peak and the root-mean-square surface roughness were 0.60 eV and 0.14 nm, respectively, which were substantially lower than those (0.73 eV and 1.44 nm) previously reported for a graphene sample with similar thickness but formed on a SiC/AlN/on-axis Si(110) substrate.

Phase engineering of rare earth nickelates by digital synthesis

We report on the electronic and magnetic properties of a series of [m EuNiO3/p LaNiO3] superlattices (thickness m and/or p = 1 unit cell) epitaxially grown on single crystalline NdGaO3 substrates. The structural symmetry of these films has been investigated by the combination of in-situ reflection high energy electron diffraction and X-ray diffraction measurements. The metal-insulator transition and the magnetic transition temperatures of the short-period superlattices with m ≥ p are modified from the corresponding bulk Eu1–xLaxNiO3 (x=pm+p) composition. In contrast to the corresponding bulk doped compound with x = 0.67, the [1 EuNiO3/2 LaNiO3] film remains metallic down to at least 2 K without signs of electronic or magnetic transitions. These findings demonstrate the power of the digital synthesis approach to realize electronic and magnetic phases of perovskite nickelates, unattainable in bulk.

Structure and magnetic properties of an epitaxial Fe(110)/MgO(111)/GaN(0001) heterostructure

We present the structural and magnetic properties of fully epitaxial Fe(110)/MgO(111)/GaN(0001) tunnel barrier structures grown by molecular beam epitaxy. In-situ reflection high-energy electron diffraction and ex-situ X-ray diffraction measurements indicate epitaxial Fe(110) films on top of an epitaxial 2 nm MgO(111) tunnel barrier on GaN(0001). X-ray reflectivity measurements confirm a roughness of approximately 0.3 nm and 0.7 nm for the MgO/GaN and the Fe/MgO interfaces, respectively. Results of in-situ magneto-optical Kerr effect measurements indicate that 1 nm thick Fe film shows signs of in-plane ferromagnetism at room temperature. Vibrating sample magnetometer measurements determine the saturation magnetisation of the 5 nm thick film to be 1660 ± 100 emu/cm3 and show that this system has a predominant uniaxial anisotropy contribution despite the presence of cyclic twinned crystals. We estimate the values of effective uniaxial (KUeff) and cubic (K1eff) anisotropy constants to be 11700 ± 170 erg cm−3 and −3300 ± 700 erg cm−3 by fitting the angular dependence of the magnetising energy.

Van der Waals epitaxy of SnS film on single crystal graphene buffer layer on amorphous SiO2/Si

Conventional hetero-epitaxial films are typically grown on lattice and symmetry matched single crystal substrates. We demonstrated the epitaxial growth of orthorhombic SnS film (∼500 nm thick) on single crystal, monolayer graphene that was transferred on the amorphous SiO2/Si substrate. Using X-ray pole figure analysis we examined the structure, quality and epitaxy relationship of the SnS film grown on the single crystal graphene and compared it with the SnS film grown on commercial polycrystalline graphene. We showed that the SnS films grown on both single crystal and polycrystalline graphene have two sets of orientation domains. However, the crystallinity and grain size of the SnS film improve when grown on the single crystal graphene. Reflection high-energy electron diffraction measurements show that the near surface texture has more phases as compared with that of the entire film. The surface texture of a film will influence the growth and quality of film grown on top of it as well as the interface formed. Our result offers an alternative approach to grow a hetero-epitaxial film on an amorphous substrate through a single crystal graphene buffer layer. This strategy of growing high quality epitaxial thin film has potential applications in optoelectronics.

Epitaxial growth of superconducting MgB2 thin films with a Mg buffer layer at 110 °C

Since the discovery of MgB2, its application to superconducting electronics has been limited by the absent of proper microfabrication techniques. In this study, we grew crystalline MgB2 thin films using molecular beam epitaxy at a low substrate temperature of 110 °C under ultra-high vacuum of about 10−6 Pa. MgB2 thin films were deposited with an epitaxial Mg buffer layer on c-plane 4H-SiC or sapphire substrates. In spite of the low growth temperature, superior crystallinity and surface flatness were confirmed by in situ reflection high-energy electron diffraction and X-ray diffraction measurements. Moreover, we successfully confirmed the occurrence of a sharp superconducting transition at 27 K.The present growth temperature was lower than any in prior reports on superconducting MgB2 thin films, and is lower than the applicable temperature of an organic-based lift-off resist. Our new MgB2 thin film growth process is promising for the development of an alternative nanofabrication technique for MgB2 thin films by means of a standard lift-off process with an organic resist.

Growth of ordered and disordered ZnSnN2

A series of ZnSnN2 films has been grown by plasma assisted molecular beam epitaxy in order to investigate the possibility of controlled cation sublattice disorder as well as its effects on physical and electronic properties of the material. By varying the growth conditions, specifically either the metal to nitrogen flux ratio or the substrate temperature, the authors have confirmed the existence of both the hexagonal and orthorhombic phases of the material via synchrotron x-ray diffraction and in situ reflection high energy electron diffraction measurements. Here, the authors report the results of an initial mapping and analysis of the growth parameter space, as part of continuing efforts to improve material quality.

Reflection high-energy electron diffraction measurements of reciprocal space structure of 2D materials

Knowledge on the symmetry and perfection of a 2D material deposited or transferred to a surface is very important and valuable. We demonstrate a method to map the reciprocal space structure of 2D materials using reflection high energy diffraction (RHEED). RHEED from a 2D material gives rise to ‘streaks’ patterns. It is shown that from these streaks patterns at different azimuthal rotation angles that the reciprocal space intensity distribution can be constructed as a function of momentum transfer parallel to the surface. To illustrate the principle, we experimentally constructed the reciprocal space structure of a commercial graphene/SiO2/Si sample in which the graphene layer was transferred to the SiO2/Si substrate after it was deposited on a Cu foil by chemical vapor deposition. The result reveals a 12-fold symmetry of the graphene layer which is a result of two dominant orientation domains with 30° rotation relative to each other. We show that the graphene can serve as a template to grow other materials such as a SnS film that follows the symmetry of graphene.

Room-temperature laser annealing for solid-phase epitaxial crystallization of β-Ga2O3 thin films

The epitaxial crystallization of β-Ga2O3 thin films on NiO-buffered α-Al2O3(0001) substrates via the solid-phase crystallization of amorphous Ga2O3 thin films by KrF excimer laser annealing at room temperature (RT) was examined. The results of X-ray and reflection high-energy electron diffraction measurements indicated that the epitaxial β-Ga2O3 thin films were fabricated by RT laser annealing. The optical bandgap of the thin films was estimated to be 4.9 eV from the results of UV/vis transmittance measurements. In the cathodoluminescence spectrum, UV–green luminescence was observed for the thin films. These optical properties are similar to those of bulk β-Ga2O3.

Preparation of Composite Film from Non-Suspended Solution By Electrochemical Technique

A composite plating method is very useful to add new function in plated films. However, the composite plating method has serious problems due to particles in the bath, deterioration of bath stability and difficulty of filtering to maintain quality of plated films. Oue et. al.1) reported a composite films, Zn-Al2O3, plated from non-suspended solution bath without particles by using quaternary ammonium salts. In this study, two type composite films, Zn-Al2O3 and Zn-TiO2, have been prepared from non-suspended solution by electrochemical technique. First, results of Zn-Al2O3 composite films were reported. The zinc matrix with aluminum oxide particles composite films from non-suspended solution containing quaternary ammonium salts were investigated. The solution contains 520 mol/m3 ZnSO4·7H2O and 79 mol/m3 Al2(SO4)3·14-18H2O as metal source. A quaternary ammonium salts of Benzyldimethyltetradecylammonium chloride dehydrate (BDTAC, Wako Co.,Ltd.) was used in this study. Transmission electron microscopy (TEM) was used to evaluate the composite films. Bath pH at 2.5 is the best of all and high aluminum content ratio values, Al/(Al+Zn), more than 30at% are obtained. The aluminum content ratio values increase with increasing current density under the condition of bath pH at 2.5 and 3.0. However, the films plated bath whose bath pH adjusted at 2.0 showed almost no aluminum content. Since the codeposition mechanism is pH jump at the cathode electrodes, pH value of 2.0 is too low. From transmission electron microscopy (TEM) measurement, two different deposits were observed as shown in Fig.1, one is needle-shaped deposit and the other is closely packed fine particle area. Magnified photograph shows closely packed fine particle area and fine particles are clearly observed. From results of composition analyses, the D area, needle-shaped deposit, is composed of 82.6 at% zinc, 17.0 at% aluminum and 15.7at% oxygen. As the same matter, the E area, closely packed fine particle area, is composed of 45.6 at% zinc, 29.2 at% aluminum, 20.6 at% oxygen and 4.5 at% sulfur. The zinc content of area D, needle-shaped deposit, is much higher than that of area E, closely paced fine particle area. On the other hand, the aluminum content of area E is much higher than that of area D. It is concluded based on above results that the needle-shaped deposit is mainly composed of Zn or ZnO, and the fine particle is mainly composed of Al2O3 from electron diffraction measurement. From results of transmission high energy electron diffraction (THEED) measurement, the fine particles were determined as θ-Al2O3. Therefore, first, aluminum ion formed Al(OH)3 near the cathode electrode by increasing pH value. Then it was dehydrated and it changed to θ-Al2O3. From X-ray diffraction (XRD) measurement, no peaks due to θ-Al2O3 have been observed. The size of θ-Al2O3 particles, whose diameter is about 5nm, is too small to detect by XRD measurement. However, transmission high energy electron diffraction (THEED) measurement shows higher sensitivity and the crystal structure is determined as θ-Al2O3. The origin of sulfur is not clear. Second, the results of Zn-TiO2 composite films were reported. The solution contains 520 mol/m3 ZnSO4·7H2O and 79 mol/m3 TiSO4·1-2H2O as metal source. A quaternary ammonium salts of Benzyldimethyltetradecylammonium chloride dehydrate (BDTAC, Wako Co.,Ltd.) was also used in this study. The plating bath temperature was controlled at 40 oC. The titanium was difficult to plate because it is easy to precipitate as a hydroxide. A complexing agent was very important to prevent from precipitating. In this study, six kinds of complexing agents ethylenediamintetraacetic acid (EDTA), glycine, formic acid, trisodium citrate, ascorbic acid and gluconic acid, have been investigated. A critical pH value to precipitate was evaluated by changing complexing agent. The critical pH values by different complexing agents, ethylenediamintetraacetic acid (EDTA), glycine , formic acid, trisodium citrate, ascorbic acid and gluconic acid, were 1.65, 1.85, 2.06, 2.65, 2.67 and 5.50, respectively. The gluconic acid is one of the most promising candidates of theses 6 complexing reagent. The 0.5g or 1.0g gluconic acid added to the plating baths and bath pH was controlled at 0.5 or 1.0. A current density was controlled from 50 to 500 A/m2. The highest TiO2 content value, 1.53 at%, was obtained at following condition, gluconic acid concentration was 1.0 g/l, bath pH was 0.5, and current density was 100 A/m2. Therefore, the two types of composite films, Zn-Al2O3 and Zn-TiO2, have been prepared by using quaternary ammonium salts, BDTAC, from non-suspended solution. Acknowledgement: This work was partially aided by MEXT-supported Program for the Strategic Research Foundation at Private Universities. [1] S. Oue, H. Nakano, S. Kobayashi, T. Akiyama, and K. Okumura, J. Surface Finishing Soc. Jpn.,53 920 (2002) . Figure 1

Stoichiometry as key to ferroelectricity in compressively strained SrTiO3 films

While strain is a powerful tuning parameter for inducing ferroelectricity in thin film oxides, the role of stoichiometry control is critical, but far less explored. A series of compressively strained SrTiO3 films on (001) (LaAlO3)0.3(Sr2AlTaO6)0.35 substrates were grown by hybrid molecular beam epitaxy where the Ti cation was supplied using a metal-organic titanium tetraisopropoxide molecule that helps systematically and precisely control Sr:Ti stoichiometry in the resulting films. A stoichiometric growth window is located through X-ray diffraction and in-situ reflection high-energy electron diffraction measurements, which show a minimum out-of-plane lattice parameter as well as constant growth rate within the stoichiometric growth window range. Using temperature dependent optical second harmonic generation (SHG) characterization, a ferroelectric-to-paraelectric transition at T ∼ 180 K is observed for a stoichiometric SrTiO3 film, as well as a higher temperature structural transition at T ∼ 385 K. Using SHG polarimetry modeling, the polar point group symmetry is determined to be tetragonal 4mm with the polarization pointing out-of-plane of the film. The SHG coefficients, d31/d15=3 and d33/d15=21, were determined at 298 K. The ferroelectric transition disappears in films grown outside the growth window, thus proving the critical role of stoichiometry control in realizing strain-induced ferroelectricity.

Ambient temperature epitaxial growth of MgB2 thin films with a Mg buffer layer

We grew crystalline MgB2 thin films using molecular beam epitaxy at a low substrate temperature of 110 °C under an ultrahigh vacuum of about 10−6 Pa. MgB2 thin films were deposited on the (001) surface of a 4H-SiC substrate with an epitaxial Mg buffer layer. Superior crystallinity and surface flatness were confirmed by in situ reflection high-energy electron diffraction and X-ray diffraction measurements. The growth temperature was lower than any in prior reports on superconducting MgB2 thin films. Moreover, we successfully confirmed the occurrence of a sharp superconducting transition at 27.2 K.

Cubic ZnO films obtained at low pressure by molecular beam epitaxy**Project supported by the National Natural Science Foundation of China (Grant Nos. 11204253, U1232110, U1332105, 61227009, and 91321102), the Fundamental Research Funds for Central Universities, China (Grant No. 2013SH001), and the National High Technology Research and Development Program of China (Grant No. 2014AA052202).

A zinc oxide thin film in cubic crystalline phase, which is usually prepared under high pressure, has been grown on the MgO (001) substrate by a three-step growth using plasma-assisted molecular beam epitaxy. The cubic structure is confirmed by in-situ reflection high energy electron diffraction measurements and simulations. The x-ray photoelectron spectroscopy reveals that the outer-layer surface of the film (less than 5 nm thick) is of ZnO phase while the buffer layer above the substrate is of ZnMgO phase, which is further confirmed by the band edge transmissions at the wavelengths of about 390 nm and 280 nm, respectively. The x-ray diffraction exhibits no peaks related to wurtzite ZnO phase in the film. The cubic ZnO film is presumably considered to be of the rock-salt phase. This work suggests that the metastable cubic ZnO films, which are of applicational interest for p-type doping, can be epitaxially grown on the rock-salt substrates without the usually needed high pressure conditions.

Room-temperature fabrication of highly oriented β-Ga2O3 thin films by excimer laser annealing

Highly oriented crystalline Ga2O3 thin films were fabricated at room temperature (RT) by excimer laser annealing. Amorphous Ga2O3 thin films were grown on α-Al2O3 (0001) substrates at RT by the pulsed laser deposition method using a focused KrF excimer laser and a sintered β-Ga2O3 target. Amorphous precursor films were irradiated by a non-focused KrF excimer laser (100–250mJ/cm2) at RT. The results of x-ray and reflection high-energy electron diffraction measurements indicated that highly (101)-oriented crystalline β-Ga2O3 thin films were obtained after RT laser annealing. The optical bandgaps of the crystalline thin films were approximately 4.7–4.9eV, as determined from the UV/Vis transmittance. The film surfaces after laser annealing revealed slight planar grain growth, indicating a high degree of crystallinity and showed the root mean square roughnesses of 0.28–0.48nm.

Lattice relaxation studies in strained epitaxial Fe-Co-C films

Aiming at high magnetocrystalline anisotropies, small additions of carbon have been proposed to stabilize a tetragonal lattice distortion in Fe-Co. In the present work, the relaxation mechanisms of Fe-Co films containing 2 at. % C are examined in detail. Film growth on buffers with different lattice constants is compared by a combination of in situ reflection high energy electron diffraction and ex-situ X-ray diffraction measurements. It is shown that relaxation starts at a thickness around 2 nm and ends at a distortion of about 4%, which is independent from the chosen buffer. Integral XRD and magnetization measurements suggest that the relaxation occurs over the complete film thickness.