Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target

Bi₂Se₃ thin films were deposited on the (100) oriented Si substrates by pulsed laser deposition technique at different substrate temperatures (room temperature −400 °C). The effects of the substrate temperature on the structural and electrical properties of the Bi₂Se₃ films were studied. The film prepared at room temperature showed a very poor polycrystalline structure with the mainly orthorhombic phase. The crystallinity of the films was improved by heating the substrate during the deposition and the crystal phase of the film changed to the rhombohedral phase as the substrate temperature was higher than 200 °C. The stoichiometry of the films and the chemical state of Bi and Se elements in the films were studied by fitting the Se 3d and the Bi 4d5/2 peaks of the X-ray photoelectron spectra. The hexagonal structure was seen clearly for the film prepared at the substrate temperature of 400 °C. The surface roughness of the film increased as the substrate temperature was increased. The electrical resistivity of the film decreased from 1 × 10⁻³ to 3 × 10⁻⁴ Ω cm as the substrate temperature was increased from room temperature to 400 °C.     

Growth and structure characterization of epitaxial Bi2Sr2Co2Oy thermoelectric thin films on LaAlO3 (001)

Epitaxial Bi₂Sr₂Co₂O_y thin films with excellent c-axis and ab-plane alignments have been grown on (001) LaAlO₃ substrates by chemical solution deposition using metal acetates as starting materials. Microstructure studies show that the resulting Bi₂Sr₂Co₂O_y films have a well-ordered layer structure with a flat and clear interface with the substrate. Scanning electron microscopy of the films reveals a step-terrace surface structure without any microcracks and pores. At room temperature, the epitaxial Bi₂Sr₂Co₂O_y films exhibit a resistivity of about 2 mΩ cm and a seebeck coefficient of about 115 μV/K comparable to those of single crystals.     

Fabrication of epitaxial conductive LaNiO3 films on different substrates by pulsed laser ablation

LaNiO₃ (LNO) thin films were deposited on (1 0 0) MgO, SrTiO₃ (STO) and LaAlO₃ (LAO) crystal substrates by pulsed laser deposition (PLD) under 20 Pa oxygen pressure at different substrate temperatures from 450 to 750 °C. X-ray diffraction (XRD), ex situ reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) were employed to characterize the crystal structure of LNO films. LNO films deposited on STO and LAO at a temperature range from 450 to 700 °C exhibit high (0 0 l) orientation. XRD ψ scans and RHEED observations indicate that LNO films could be epitaxially grown on these two substrates with cubic-on-cubic arrangement at a wide temperature range. LNO films deposited at 700 °C on MgO (1 0 0) substrate have the (l l 0) orientation, which was identified to be bicrystalline epitaxial growth. La₂NiO₄ phase appears in LNO films deposited at 750 °C on three substrates. The epitaxial LNO films were tested to be good metallic conductive layers by four-probe method.     

Growth of oriented Ag nanocrystals on air-oxidized Si surfaces: An in-situ reflection high energy electron diffraction study

Growth of Ag nanoislands on air-oxidized Si(001), (111) and (110) surfaces has been investigated by reflection high energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and cross-sectional transmission electron microscopy. We have shown that the oriented nanocrystalline Ag, similar to the epitaxial growth of Ag on clean Si surfaces, can be grown on oxide-covered Si surfaces. A thin oxide layer (~ 2-3 nm thick) is formed on ultra-high vacuum (UHV)-cleaned Si surfaces via exposure of the clean reconstructed surface to air. Deposition of Ag was carried out under UHV at different substrate temperatures and monitored by RHEED. RHEED results reveal that Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while, in spite of the presence of the oxide layer between Ag islands and Si, preferred orientations with an epitaxial relationship with the substrate evolve when Ag is deposited at higher substrate temperatures. STM images of the oxidized surfaces, prior to Ag deposition, apparently do not show any order. However, Fourier transforms of STM images show the presence of a short range order on the oxidized surface following the unit cells of the underlying reconstructed Si surface. It is intriguing that Ag nanoislands follow an epitaxial orientational relationship with the substrate in spite of the presence of a 2-3 nm thick oxide layer between Ag and Si. Apparently, the short range order existing on the oxide surface influences the orientation of the Ag nanoislands.     

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.     

The growth of BaTiO3 films on (001) MgAl2O4 substrates by pulsed laser deposition technique

The microstructural evolution of the BaTiO₃ films grown on (001) MgAl₂O₄ spinel substrates at different temperatures by means of pulsed laser deposition technique is studied via transmission electron microscopy (TEM). The BaTiO₃ film grown at 850 °C consists of columnar grains of random orientations. Once the substrate temperature is over 900 °C, the BaTiO₃ films grow on (001) MgAl₂O₄ substrates epitaxially. The cross-sectional TEM study reveals that the boundaries and interfaces act as the sources to emit stacking faults and twins which are detrimental to the film quality. The quality of epitaxial films increases with the growth temperature, and is optimized at the growth temperature of 1050 °C. The evolution of film microstructures with the growth temperature is discussed in view of the growth temperature, the surface structure of MgAl₂O₄ substrates, and the phase transition of BaTiO₃.     

Fe3Si nanodots epitaxially grown on Si(111) substrates using ultrathin SiO2 film technique

Ultrahigh density (> 10¹² cm⁻²) Fe₃Si nanodots (NDs) are epitaxially grown on Si(111) substrates by codeposition of Fe and Si on the ultrathin SiO₂ films with ultrahigh density nanovoids. We used two kinds of methods for epitaxial growth: molecular beam epitaxy (MBE) and solid phase epitaxy. For MBE, low temperature (< 300 °C) growth of the Fe₃Si NDs is needed to suppress the interdiffusion between Fe atoms deposited on the surfaces and Si atoms in the substrate. These epitaxial NDs exhibited the ferromagnetism at low temperatures, which were expected in terms of the application to the magnetic memory device materials.     

Structural, electrical and optical properties of Bi2Se3 and Bi2Se(3−x)Tex thin films

Thin films of Bi₂Se₃, Bi₂Se_2.9Te_0.1, Bi₂Se_2.7Te_0.3 and Bi₂Se_2.6Te_0.4 are prepared by compound evaporation. Micro structural, optical and electrical measurements are carried out on these films. X-ray diffraction pattern indicates that the as-prepared films are polycrystalline in nature with exact matching of standard pattern. The composition and morphology are determined using energy dispersive X-ray analysis and scanning electron microscopy (SEM). The optical band gap, which is direct allowed, is 0.67 eV for Bi₂Se₃ thin films and the activation energy is 53 meV. Tellurium doped thin films also show strong optical absorption corresponding to a band gap of 0.70-0.78 eV. Absolute value of electrical conductivity in the case of tellurium doped thin film shows a decreasing trend with respect to parent structure.     

Selective synthesis of Bi2Se3 nanostructures by solvothermal reaction

Bi₂Se₃ nanobelts, nanoflakes and sheets embedded nanotubes were prepared via solvothermal process with different solvents. The reaction conditions influencing the synthesis of Bi₂Se₃ nanostructures such as solvents, and reaction temperatures were studied and optimized. Results indicated that the mixed solvent composed of triethanolamine and ethanol (TEA-EtOH) or triethanolamine and distilled water (TEA-H₂O) can decrease the threshold temperature (TT) of Bi₂Se₃. With the solvents TEA-EtOH and TEA-H₂O, we originally accomplished the shape-controlled synthesis of Bi₂Se₃ nanocrystals by controlling reaction temperature. Based on the viewpoint of crystallography about Bi₂Se₃, the possible growth mechanisms of Bi₂Se₃ nanostructures were discussed.     

Epitaxial orientation and morphology of β-FeSi2 produced on a flat and a patterned Si(001) substrates

The epitaxial growth of β-FeSi₂ films produced on flat and patterned Si(001) substrates under various substrate temperatures (T_s) with deposition rates of Fe (V_Fe) was investigated by transmission electron microscopy (TEM). In the film deposited on the flat Si(001) substrate, precipitates of flat-bottom shaped β-FeSi₂ and those of round-bottom shaped α-FeSi₂ were formed at T_s = 500 °C and V_Fe = 0.02 nm/s. The β-FeSi₂ adopted the epitaxy to (001)_Si plane, while α-FeSi₂ selected the epitaxy to {111}_Si planes inside the Si matrix. At T_s = 350 °C and V_Fe = 0.01 nm/s, a continuous β-FeSi₂ layer were formed epitaxially on the Si(001) substrate without forming α-FeSi₂. It was found that the lower temperature and the higher Fe-concentration suppress the formation of α-FeSi₂ and promote the formation of β-FeSi₂. In addition, the morphology of β-FeSi₂ changed from fine isolated precipitates (islands) to a continuous layer with increasing the deposition rate and the substrate temperature. In the film deposited on the patterned Si(001) substrate at T_s = 500 °C and V_Fe = 0.02 nm/s, on the other hand, both β- and α-FeSi₂ precipitates were formed on the top-hills and the valleys of the patterned substrate, while only α-FeSi₂ precipitates were formed on the sidewalls. These results demonstrate that not only the growth conditions but also geometric situations affect strongly the epitaxial growth of FeSi₂ precipitates.     

Dislocations in Bi0.4Ca0.6MnO3 epitaxial film grown on (110) SrTiO3 substrate

Bi_0.4Ca_0.6MnO₃ (BCMO) film with a thickness of 110 nm was epitaxially grown on a (110) SrTiO₃ (STO) substrate using pulsed laser ablation technique. The microstructure of the epitaxial films was investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) in details. Two different kinds of dislocations, one being perpendicular to the BCMO/STO interface, the other being parallel to the interface, have been commonly observed. The formation mechanism for these dislocations has been discussed. All the dislocations are thought to relieve the local strain in the epitaxial film.     

Epitaxial thin films of topological insulator Bi2Te3 with two-dimensional weak anti-localization effect grown by pulsed laser deposition

We have grown high quality epitaxial topological insulator Bi₂Te₃ thin films on silicon (111) substrates by pulsed laser deposition. Systematic structural characterization of the films using X-ray diffraction and transmission electron microscopy has demonstrated that a low laser pulse rate is the key to achieving epitaxial films. The films show n-type metallic behavior, consistent with Te deficiency as determined by Rutherford backscattering spectrometry measurements. The A_1g longitudinal optical phonon mode of Bi₂Te₃ was detected by time-resolved reflectivity measurements. A 2-dimensional (2-D) weak-antilocalization effect was also observed at low temperatures, which indicates the existence of topologically protected 2-D surface states in our thin films. This growth and characterization effort paves the way to fabricate multi-layer heterostructures of topological insulators along with ferromagnetic oxides and high temperature superconductors by the same growth technique in the search for physics arising from their interfacial couplings.     

Growth mechanism of epitaxial CoSi2 on Si and reactive deposition epitaxy of double heteroepitaxial Si/CoSi2/Si

The growth mechanism of epitaxial CoSi₂ was studied using Co/Ti/Si multilayer solid phase reaction. Results showed that phase formation was controlled by diffusion of Co through the growing CoSi_x, although at the early stage of CoSi₂ growth the diffusion of Co could be controlled by a Ti layer. A reactive deposition technique was also evaluated by using a conventional magnetron sputtering system. Results showed that an epitaxial CoSi₂ layer was formed by controlling the Co sputtering rate not to exceed the Co diffusion rate through CoSi_x. However, the surface of CoSi₂ became rough when the deposition rate was much slower than the Co diffusion rate through CoSi_x. The roughness was caused by the formation of CoSi₂ (111) facets at the interface between CoSi₂ and the Si substrate. Si/CoSi₂/Si double heteroepitaxial structures were fabricated when Si and Co were sequentially sputter-deposited on a Si (100) substrate.     

Biaxial CdTe/CaF2 films growth on amorphous surface

A continuous and highly biaxially textured CdTe film was grown by metal organic chemical vapor deposition on an amorphous substrate using biaxial CaF₂ nanorods as a buffer layer. The interface between the CdTe film and CaF₂ nanorods and the morphology of the CdTe film were studied by transmission electron microscopy (TEM) and scanning electron microscopy. Both the TEM and X-ray pole figure analysis clearly reveal that the crystalline orientation of the continuous CdTe film followed the {111}<121> biaxial texture of the CaF₂ nanorods. A high density of twin faults was observed in the CdTe film. Furthermore, the near surface texture of the CdTe thin film was investigated by reflection high-energy electron diffraction (RHEED) and RHEED surface pole figure analysis. Twinning was also observed from the RHEED surface pole figure analysis.     

High-quality carbon-doped β-type FeSi2 films synthesized by ion implantation

Carbon-doped β-FeSi₂ films synthesized by ion implantation is investigated with the aim to fabricate high-quality semiconducting β-FeSi₂ layer on silicon substrate. According to our TEM cross-section observations, the carbon-doped films are of better quality than the non-doped ones for their improved uniform film thickness, smooth β/Si interface and high thermal stability. In particular, annealing at 500-700 °C leads to the formation of a flat and continuous β-type silicide layer. Optical absorption measurements show that the carbon doping does not influence the band structure. We further point out that the presence of multiple and incoherent orientation relationships between β and Si, discussed within the framework of the near coincident site lattice theory, is a key factor responsible for the difficulty in obtaining high-quality epitaxial β films.     

Ferroelectric properties of epitaxial Bi3.15Nd0.85Ti3O12 films on SiO2/Si using biaxially oriented MgO as templates

High quality epitaxial Bi_3.15Nd_0.85Ti₃O₁₂ (BNT) thin films with thicknesses from 30 to 80 nm have been integrated on SiO₂/Si substrates. MgO templates deposited by ion-beam-assisted deposition and SrRuO₃ (SRO) buffer layers processed by pulsed laser deposition have been used to initiate the epitaxial growth of BNT films on the amorphous SiO₂/Si substrates. The structural and ferroelectric properties were investigated. Microstructural studies by X-ray diffraction and transmission electron microscopy revealed high quality crystalline with an epitaxial relationship of (001)_BNT||(001)_SRO||(001)_MgO and [100]_BNT||[110]_SRO||[110]_MgO. A ferroelectric hysteresis loop with a remanent polarization of 3.1 μC/cm² has been observed for a 30 nm thick film. The polarization exhibits a fatigue-free characteristic up to 1.44 × 10¹⁰ switching cycles.     

Ferroelectric and dielectric multilayer heterostructures based on KTa0.65Nb0.35O3 and Bi1.5-xZn0.92-yNb1.5O6.92-1.5x-y grown by pulsed laser deposition and chemical solution deposition for high frequency tunable devices

Epitaxial growth of Bi_1.5-xZn_0.92-yNb_1.5O_6.92-1.5x-y (BZN) thin films was achieved on (100)_pc LaAlO₃ substrate by pulsed laser deposition (PLD) and by chemical solution deposition based on Pechini process. Effect of bismuth and zinc deficiency on the BZN thin films obtained by PLD was discussed, in relation with the starting target composition. Dielectric permittivity and bandgap values were determined from electrical and spectroscopic ellipsometry measurements performed on randomly oriented films grown on Pt/Si substrate. BZN thin films obtained by PLD exhibit, at 100 kHz, a dielectric constant of ε_r = 203 and quite low dielectric losses of tanδ = 5 × 10^− 2. Epitaxial ferroelectric − dielectric KTa_0.65Nb_0.35O₃ (KTN) − Bi_1.5-xZn_0.92-yNb_1.5O_6.92-1.5x-y (KTN on BZN and BZN on KTN) bilayers were obtained by PLD on (100)_pc LaAlO₃ with the insertion of a suitable buffer layer of KNbO₃ in the case of KTN on BZN. Such multilayer heterostructures with an epitaxial growth control of each layer are promising candidates for potential integration in microwave devices.     

Self-patterned Nano Structures in Structurally Gradient Epitaxial La0.5Ba0.5CoO3 Films

Highly epitaxial La_0.5Ba_0.5CoO₃ (LBCO) thin films with sharp interface and a thickness of 200 nm were epitaxially grown on (001) SrTiO₃ substrates using pulsed laser deposition. High-resolution transmission electron microscopy and electron diffraction analysis revealed that the films have a triple-layered structure. The first layer, close to the film/substrate interface, has a thickness of ~ 6 nm and is a defect free single crystal disordered cubic structure (a = 3.882 Å) which has a lattice mismatch of − 0.59% with respect to the substrate. The second layer which dominates the film structure has a single crystal disordered cubic structure (a = 3.854 Å) which has a lattice mismatch of − 1.31% with respect to the substrate. The third layer located on the top of the film has a thickness of several nanometers and consists of 112-type ordered tetragonal structure. The cubic structures in the first and second layer have an orientation relationship of (001)_LBCO//(001)_STO and < 100 > _LBCO//< 100 > _STO with respect to the substrate. Self-patterned 3-dimensional nano structures with a dimension range from 2 to 10 nm were formed in the second and third layers. These nano structures were formed by the enclosure of anti-phase boundary planes which are parallel to the {100} of the cubic structure. Epitaxial LBCO thin films with such nano structures are hard ferromagnetic with a large coercive field value and magnetoresistance effect value (~ 24%), and exhibit semiconductor behavior at temperatures < 300 K.     

La-substitution Bi2Ti2O7 thin films grown by chemical solution deposition

(La_0.05Bi_0.95)₂Ti₂O₇ (LBTO) thin films had been successfully prepared on P-type Si substrate by chemical solution deposition method. The structural properties of the films were studied by X-ray diffraction. The phase of (La_0.05Bi_0.95)₂Ti₂O₇ is more stable than the phase of Bi₂Ti₂O₇ without La substitution. The films exhibited good insulating properties with room temperature resistivities in the range of 10¹²-10¹³ Ω cm. The dielectric constant of the film annealed at 550 °C at 100 kHz was 157 and the dissipation factor was 0.076. The LBTO thin films can be used as storage capacitors in DRAM.     

Effect of target compositions on the crystallinity of β-FeSi2 prepared by ion beam sputter deposition method

Targets with the elemental composition of Fe, Fe₂Si and FeSi₂ were employed in the present study to grow β-FeSi₂ film on Si (100) substrate by means of ion beam sputter deposition (IBSD) method. The results revealed that when FeSi₂ target was employed, a Si-rich phase, α-FeSi₂ (Fe₂Si₅), was predominant at temperatures above 973 K, while β-FeSi₂ phase was observed only in the limited temperature range at around 873 K. In this case, Si was originated both from the sputtered target and the substrate, thus, the supply of Si was considered to be excessive to sustain β structure. On the other hand, the films prepared with Fe target became polycrystalline as they grow thicker than 100 nm. In order to optimize the supply of Fe and Si for epitaxial growth, Fe₂Si target was employed, where highly (100)-oriented β-FeSi₂ layer of 120 nm in thickness was obtained at 973 K.