Bi Thin Films Research Articles

Ultrafast Surface State Spin-Carrier Dynamics in the Topological Insulator Bi_{2}Te_{2}Se.

Topological insulators are promising candidates for optically driven spintronic devices, because photoexcitation of spin polarized surface states is governed by angular momentum selection rules. We carry out femtosecond midinfrared spectroscopy on thin films of the topological insulator Bi_{2}Te_{2}Se, which has a higher surface state conductivity compared to conventionally studied Bi_{2}Se_{3} and Bi_{2}Te_{3}. Both charge and spin dynamics are probed utilizing circularly polarized light. With a sub-band-gap excitation, clear helicity-dependent dynamics is observed only in thin (<20 nm) flakes. On the other hand, such dependence is observed for both thin and thick flakes with above-band-gap excitation. The helicity dependence is attributed to asymmetric excitation of the Dirac-like surface states. The observed long-lasting asymmetry over 10ps even at room temperature indicates low backscattering of surface state carriers which can be exploited for spintronic devices.

Preparation of (11n) oriented Bi2Sr2CaCu2O8+x thin films by solution methods using NdGaO3 (100) substrates

We have prepared (11n) oriented Bi2Sr2CaCu2O8+x (Bi2212) thin films by KCl flux method which is one kind of solution method. The (100) substrate of NdGaO3 (NGO) was used. Heat treatment was performed with four different temperature profiles. Only the (11n) peak appeared in the θ-2θ X-ray diffraction pattern. From the SEM image, it was found that elongated plate-like crystal grains of Bi2212 were grown. From the viewpoint of lattice matching, this elongated crystal grain is considered to be (010) (or (100)) oriented crystal grains.

P-type transparent conductivity in high temperature superconducting Bi-2212 thin films

In this study, hole-type high-temperature superconductor Bi2Sr2CaCu2Oy (Bi-2212) with strong electronic correlation is proposed as a p-type transparent conducting oxide (TCO) candidate. Phase-pure and c-axis oriented Bi-2212 thin films with different thicknesses were prepared to investigate the optoelectronic properties. High room-temperature conductivity between 1064 and 200 S/cm with visible transmittance over 50% is obtained for the prepared Bi-2212 thin films. The results will provide an arena to search for p-type TCOs in high-temperature superconductors with strong electronic correlation and give a potential application in the field of transparent metallic contacts as well as transparent correlated metal/n-type semiconductor Schottky diodes.

Experiments on hydrolysis of Bi(2201) thin film via atomic force microscope

In this paper, large-scale Bi(2201) nanofilm was mechanically exfoliated. The thin film Bi(2201) was characterized by means of optical image and XRD spectrum analysis. AFM is used to study the stability of Bi(2201) thin film. We found that the Bi(2201) nanofilm was less sensitive to CO2 while it extremely got affected by humidity. With the rise in the humidity, the hydrolytic speed of Bi(2201) film was greatly increased. When the relative humidity of the air is lower than 20%, the Bi(2201) thin film can be stable in two days. However, when the relative humidity is about 90% RH, Bi(2201) film will be hydrolyzed in few minutes.

Study of LIPSS formation on bismuth thin films deposited by Pulsed Laser Deposition

In this work, we report the fabrication of bismuth thin films on glass substrates by Pulsed Laser Deposition. A focused beam of a Nd:YAG laser (9 ns, 10 Hz, 1064 and 532 nm) was utilized to carry out the ablation of a high purity Bi target. The per pulse laser fluence was varied between 0.95 and 1.35 J/cm2, depending on the laser wavelength used. The deposits were characterized by SEM and XRD to analyze the surface morphology of thin films and their crystalline structure, respectively. The SEM micrographs showed differences in roughness, which increased with the laser wavelength. The diffractograms revealed the formation of Bi thin films with a polycrystalline structure or with a preferential orientation, depending on the mean kinetic energy of the plasma ions. The as deposited bismuth thin films were irradiated with a non-focused laser beam at low energies below the ablation threshold. In the laser treated area, it could be observed the formation of periodic structures, known as LIPSS (laser induced periodic surface structures). Depending on the crystallographic structure of the irradiated film, the formed LIPSS showed different characteristics, such as width and shape.

Growth of a bismuth thin film on the five-fold surface of the icosahedral Ag-In-Yb quasicrystal

We present a study of growth of quasicrystalline bismuth (Bi) thin films on the five-fold surface of the icosahedral Ag-In-Yb quasicrystal using scanning tunnelling microscopy (STM). The main building block of the Ag-In-Yb quasicrystal is a rhombic triacontahedral (RTH) cluster, which is formed by successive shells of atoms. The surface is formed at bulk planes which intersect the centres of the RTH clusters. We show that the deposited Bi atoms occupy vacant sites above the surface where the atoms of the RTH clusters would be and thus Bi grows with three-dimensional quasicrystalline order. Further deposition of Bi yields crystalline Bi islands aligned along high symmetry directions of the substrate.

Reply to “Comment on ‘Quantum transport in the surface states of epitaxial Bi(111) thin films’ ”

In Hirahara and Hasegawa's Comment [Hirahawa and Hasegawa, Phys. Rev. B 97, 207401 (2018)], they question our results of the unusual properties of epitaxial Bi(111) thin films. In this Reply, we show that their Bi samples, with resistivity accuracy three orders of magnitude poorer, may be different from ours. They also lack the most essential results of the Aharonov-Bohm (AB) effect for the quantum transport conclusion.

Comment on “Quantum transport in the surface states of epitaxial Bi(111) thin films”

Zhu et al. [Phys. Rev. B 94, 121401(R) (2016)] recently reported an experimental work that claims that epitaxial Bi(111) films grown on Si(111) are (1) topologically nontrivial, and (2) metallic only at the surface and insulating in the interior bulk, consistent with the old prediction of a semimetal-to-semiconductor (SMSC) transition. Here we point out that although Bi can be nontrivial, the SMSC transition does not happen at the same time. Through the comparison of the thickness and temperature dependence of the film conductivity of Bi with ${\mathrm{Bi}}_{0.9}{\mathrm{Sb}}_{0.1}$ performed in situ, we find no evidence of the insulating bulk states of Bi when the thickness is less than 240 $\AA{}$, which is in accordance with our angle-resolved photoemission spectroscopy measurements and density functional theory calculations.

Electrical contacts to thin layers of Bi2Sr2CaCu2O8+δ

Thin layers of Bi2Sr2CaCu2O8+δ (Bi2212) were fabricated using the mechanical exfoliation technique. Good electrical contacts to the thin Bi2212 films with low contact resistance were realized by depositing Ag and Au electrodes onto the Bi2212 films and annealing them with an oxygen flow at 350 °C for 30 min. We observed cross-section images of the Bi2212 thin film device using a transmission electron microscope to characterize the diffusion of Ag and Au atoms into the Bi2212 thin film.

On the roles of HEC in Pechini sol-gel method: Enhancement of stability, wettability of the sol and surface roughness of Bi2212 film

Bi2212 superconducting thin film was prepared through a modified Pechini sol-gel method called surfactant assisted Pechini. Nitrates were used as reactors, ethylenediamine tetraacetic acid (EDTA) and hydroxyethyl cellulose (HEC) was adopted as chelating agent and surfactant respectively to prepare sol for Bi2212 film. The effects of HEC on the phase formation, crystallization behavior, morphology and superconductivity of the samples were characterized using X-ray diffraction, field emission scanning electron microscope, atomic force microscope and the standard four-probe method. C-axis epitaxial Bi2212 thin film with high Bi2212 phase purity was obtained through the modified method, and the sample had good crystallization properties, smooth surface and excellent electrical transport properties. Furthermore, a model is proposed to clarify the roles of surfactant in the preparation of Bi2212 films.

Electrodeposition of Bi Thin Films on n-GaAs(111)B. II. Correlation between the Nucleation Process and the Structural and Electrical Properties

The surface morphology and the crystal structure of 40 nm thin Bi films electrodeposited on GaAs(111)B at different growth overpotentials have been studied by means of atomic force microscopy and X-ray diffraction, respectively. The Bi/GaAs interface has also been electrically characterized by means of current–voltage curves that have been analyzed with the thermionic-field emission theory. Taking into account the results presented in Part I, we can conclude that the structural and electrical properties of the Bi layers are correlated with the nucleation process and, therefore, with the energy band diagram of the semiconductor–electrolyte interface. We have found that surface morphology is directly dependent on the amount of protons adsorbed on the GaAs surface, whereas the crystal quality and the interfacial properties also depend on the nucleation mechanism (instantaneous or progressive).

Electrodeposition of Bi Thin Films on n-GaAs(111)B. I. Correlation between the Overpotential and the Nucleation Process

Bismuth thin films constitute a promising nanostructure for the fabrication of spin-based devices. To achieve this goal, it is necessary to obtain high-quality Bi layers with controlled and reproducible properties. Therefore, studies focused on the understanding of the nucleation process and the correlation between the growth conditions and the film properties are of great interest. In this work, we have studied the electrodeposition of Bi thin films onto GaAs(111)B substrates at different overpotentials. In Part I, we have analyzed the nucleation of the films by means of potentiostatic curves. The current density transients have been deconvoluted into individual processes taking into account the energy band diagram of the semiconductor–electrolyte interface. The deconvolution of the current density transients indicates that Bi electrodeposition follows a 3D nucleation controlled by diffusion, accompanied by concurrent processes such as both proton adsorption and reduction. The competition of these proces...

Metal-semimetal Schottky diode relying on quantum confinement

Quantum confinement in a semimetal thin film such as bismuth (Bi) can lead to a semimetal-to-semiconductor transition which allows for the use of semimetals as semiconductors when patterned at nanoscale lengths. Bi native oxide on Bi thin film grown by molecular beam epitaxy (MBE) is investigated using X-ray photoelectron spectroscopy (XPS) to measure the elemental composition of the oxide. Also, an in-situ argon plasma etch step is developed allowing for the direct coating of the surface of thin Bi films by a metal contact to form a Schottky junction. Model structures of rhombohedral [111] and [110] bismuth thin films are found from density functional theory (DFT) calculations. The electronic structure of the model thin films is investigated using a GW correction and the formation of an energy band gap due to quantum confinement is found. Electrical characterization of the fabricated Bi-metal Schottky diode confirms a band gap opening in Bi thin film for a film thickness of approximately 5 nm consistent with the theoretical calculations.

Scanning tunneling microscopy research of Bi(110) thin films grown on NbSe2

Due to the novel physical properties induced by the strong spin orbit coupling and band inversions in the energy band structure, two-dimensional topological insulator has become a hot research point in the field of condensed matter physics and material science in recent years. Particularly, two-dimensional topological insulator may host exotic Majorana fermionic excitations in its edge state if superconductivity is introduced. Bi thin film with (111) orientation proves to be a two-dimensional topological insulator both in theory and in experiment. However, the topological nature of Bi thin film with (110) orientation has not yet been confirmed. In this study, high quality Bi(110) thin films are successfully prepared on superconductor NbSe2 surfaces, by the molecular beam epitaxial technology at ambient temperature and a low deposition rate (~24℃,~3 min/bilayer). The morphologies and electronic properties of the samples are studied by using scanning tunneling microscopy and spectroscopy. The experimental results reveal that the growth mode changes from bilayer (BL) in BL mode to monolayer (ML) in ML mode. Such transition takes place at a critical height of about 4 BLs. The mechanism of the growth mode transition is believed to be induced by the drastic variation of the surface energies of the thin films with different thickness values. Due to the large coverage of Bi(110) film on the NbSe2 substrate, it is almost impossible to find the exposed areas of NbSe2 substrate surface in practice. Especially on the sample with a large number of layers of Bi thin film, it is hard to directly determine the number of layers for each film. Hence, the critical thickness could be only estimated by controlling the deposition time and growth rate combining with the measurements of stage height of the film. The nearly identical local density of states wherever measured in the interior of a terrace or at the step edges can be discerned from the dI/dV spectra, which is thus hard to corroborate with non-trivial topology in either BL or ML thick Bi(110) film. The superconductivity induced by proximity effect from the superconducting substrate NbSe2 is also observed on the thin films. Through Bardeen-Cooper-Schrieffer type data fitting, the superconducting gap on the Bi thin film is estimated at about 0.5 meV. In addition, the quantum well state, which is often observed in thin films, is also revealed from the Bi(110) thin films, whose characteristic is equal energy spacing between peaks in dI/dV spectra. Noticeably, the spectral shapes of BL and ML are similar, and the local density of states from adjacent film layers displays an approximate πup phase shift.

Preparation of (11n) oriented Bi2Sr2CaCu2O8+x thin films without c-axis twin structure by the metal-organic decomposition method using vicinal SrTiO3 (110) substrates

We have prepared (11n) oriented Bi2Sr2CaCu2O8+x (Bi2212) thin films by metal-organic decomposition method. The vicinal (110) substrate of SrTiO3 (STO) inclined toward the [1¯10] direction was used for suppressing “c-twinning”. In the sample prepared in the O2 atmosphere, only the (11n) peak appeared in the θ-2θ X-ray diffraction pattern. When the miscut angle of the substrate was φ = 10°, it was shown by the scanning electron microscope images and the (008) pole figures that the c-axis was inclined by about 38° and the c-twinning was substantially suppressed. ρab and ρc of this sample were calculated by the corrected van der Pauw method and component calculation of the two-dimensional resistivity tensor. The superconducting transition temperatures were Tc = 81 K. The temperature dependence of ρab was a typical metallic behavior reflecting the characteristics of the ab-plane of Bi2212. On the other hand, that of ρc did not become a typical semiconductor behavior in the c-axis direction of Bi2212, but it showed a metallic behavior. The anisotropic parameter γ was about 10.

Size effect in the electronic transport of thin films of Bi2Se3

Thin films of a topological insulator (TI) Bi2Se3 of various thicknesses from 20 nm to 75 nm were obtained. The resistivity measurements were carried out according to the conventional 4-contact DC technique. This allows to “separate” the bulk and surface conductivities at different temperatures and magnetic fields. It was suggested that similar effects should be observed in other TIs and systems with inhomogeneous distribution of dc-current on sample cross section.

Sputtering of bismuth thin films under MeV Cu heavy ion irradiation: Experimental data and inelastic thermal spike model interpretation.

The sputtering of bismuth (Bi/Si) thin films deposited onto silicon substrates and irradiated by swift Cuq+ heavy ions (q = +4 to +7) was investigated by varying both the ion energy over the 10 to 26‐MeV range and the ion fluence ϕ from 5.1 × 1013 cm−2 to 3.4 × 1015 cm−2. The sputtering yields were determined experimentally via the Rutherford backscattering spectrometry technique using a 2‐MeV He+ ion beam. The measured sputtering yields versus Cu7+ ion fluence for a fixed incident energy of 26 MeV exhibit a significant depression at very low ϕ‐values flowed by a steady‐state regime above ~1.6 × 1014 cm−2, similarly to those previously pointed out for Bi thin films irradiated by MeV heavy ions. By fixing the incident ion fluence to a mean value of ~2.6 × 1015 cm−2 in the upper part of the yield saturation regime, the measured sputtering yield data versus ion energy were found to increase with increasing the electronic stopping power in the Bi target material. Their comparison to theoretical predicted models is discussed. A good agreement is observed between the measured sputtering yields and the predicted ones when considering the contribution of 2 competitive processes of nuclear and electronic energy losses via, respectively, the SRIM simulation code and the inelastic thermal spike model using refined parameters of the ion slowing down with reduced thermophysical proprieties of the Bi thin films.

Interband absorption edge in the topological insulators Bi2(Te1−xSex)3

We have investigated the optical properties of thin films of topological insulators Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$ and their alloys Bi$_2$(Te$_{1-x}$Se$_x$)$_3$ on BaF$_{2}$ substrates by a combination of infrared ellipsometry and reflectivity in the energy range from 0.06 to 6.5 eV. For the onset of interband absorption in Bi$_2$Se$_3$, after the correction for the Burstein-Moss effect, we find the value of direct bandgap of $215\pm10$ meV at 10 K. Our data supports the picture that Bi$_2$Se$_3$ has a direct band gap located at the $\Gamma$ point in the Brillouin zone and that the valence band reaches up to the Dirac point and has the shape of a downward oriented paraboloid, i.e. without a camel-back structure. In Bi$_2$Te$_3$, the onset of strong direct interband absorption at 10 K is at a similar energy of about 200 meV, with a weaker additional feature at about 170 meV. Our data support the recent GW band structure calculations suggesting that the direct interband transition does not occur at the $\Gamma$ point but near the Z-F line of the Brillouin zone. In the Bi$_2$(Te$_{1-x}$Se$_x$)$_3$ alloy, the energy of the onset of direct interband transitions exhibits a maximum near $x=0.3$ (i.e. the composition of Bi$_2$Te$_2$Se), suggesting that the crossover of the direct interband transitions between the two points in the Brillouin zone occurs close to this composition.

Exploring the performance of thin-film superconducting multilayers as kinetic inductance detectors for low-frequency detection

We have solved numerically the diffusive Usadel equations that describe the spatially varying superconducting proximity effect in Ti–Al thin-film bi- and trilayers with thickness values that are suitable for kinetic inductance detectors (KIDs) to operate as photon detectors with detection thresholds in the frequency range of 50–90 GHz. Using Nam’s extension of the Mattis–Bardeen calculation of the superconductor complex conductivity, we show how to calculate the surface impedance for the spatially varying case, and hence the surface impedance quality factor. In addition, we calculate energy-and spatially-averaged quasiparticle lifetimes at temperatures well-below the transition temperature and compare to calculation in Al. Our results for the pair-breaking threshold demonstrate differences between bilayers and trilayers with the same total film thicknesses. We also predict high quality factors and long multilayer-averaged quasiparticle recombination times compared to thin-film Al. Our calculations give a route for designing KIDs to operate in this scientifically-important frequency regime.

Structural evolution of Bi thin films on Au(111) revealed by scanning tunneling microscopy

We present the structural evolution of Bi on Au(111) from monolayer to multilayer regimes explored mainly by scanning tunneling microscopy. At the monolayer regime, Bi clusters distribute homogeneously to make an array with $5\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{4pt}{0ex}}5$ periodicity. Further increase of the coverage converts these clusters to the two-dimensional periodic $(\sqrt{37}\ifmmode\times\else\texttimes\fi{}\sqrt{37})\mathrm{R}25.{3}^{\ensuremath{\circ}}$ and $(\mathrm{p}\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ structures. We propose a model of the $(\sqrt{37}\ifmmode\times\else\texttimes\fi{}\sqrt{37})\mathrm{R}25.{3}^{\ensuremath{\circ}}$ structure based on the STM measurements and density-functional theory calculations. In the multilayer regime, superstructures at first appear with long-range periodicities arising from the moir\'e structures due mainly to the stacking of Bi layers. Then, the thicker films grow whose lattice constants are close to those of the (110) surface of rhombohedral Bi crystal. The Bi(110) films of more than 60 layers grow stably. Thus, this system provides a good stage for further investigation of the peculiar electronic properties of Bi(110).