Se Segregation Research Articles

Stable Carbon–Selenium Bonds for Enhanced Performance in Tremella‐Like 2D Chalcogenide Battery Anode

Abstract2D cobalt selenide based on conversion reaction has attracted much attention due to its open layered structure and high specific capacity. However, effectively suppressing the fast capacity fade caused by the irreversible Se dissolution and serious volume change during the cycling process is still a challenge. Herein, the concentration of dispersion liquid under supercritical conditions is tuned to induce the CoSe crystal to grow along the graphene oxide (GO), and finally obtain the Tremella‐like CoSe–reduced GO (rGO) hybrid. The nature of epitaxial growth leads to the formation of stable CSe bonds, which maintain a favorable conductive connection between CoSe and rGO as well as enhance the mechanical strength of active materials to suppress Se dissolution and volume expansion during Na/Li intercalation and deintercalation. The unique microstructural merits of the hybrid result in superior sodium/lithium storage performance (400.8 mAh g−1 at 1 A g−1 after 100 cycles for sodium‐ion batteries and 769.6 mAh g−1 at 2 A g−1 after 500 cycles for lithium‐ion batteries). Moreover, the transmission X‐ray microscopy technique is first used to directly observe the Se segregation in cobalt selenide and it being suppressed by the CSe bonds.

$\hbox{Ni}(\hbox{Ge}_{1 - x}\hbox{Sn}_{x})$ Ohmic Contact Formation on N-Type $\hbox{Ge}_{1 - x}\hbox{Sn}_{x}$ Using Selenium or Sulfur Implant and Segregation

The physics of ohmic contact formation for nickel stanogermanide [ Ni(Ge1-xSnx)] on n-type germanium-tin (n-Ge1-xSnx) was investigated. Low-resistivity Ni(Ge1-xSnx) was formed on Ge1-xSnx using a 350 °C 30-s anneal. Ion implantation of selenium (Se) or sulfur (S) into n-Ge1-xSnx followed by nickel stanogermanidation led to the segregation of Se or S at the Ni(Ge1-xSnx)/n-Ge1-xSnx interface. Low effective electron Schottky barrier height (ΦBn) of 0.12 and 0.11 eV was achieved for Ni(Ge1-xSnx)/n-Ge1-xSnx contacts with Se and S segregation, respectively. A simulation study was also performed to explain the experimental observations. Se and S atoms could be modeled as donor-like traps near the Ni(Ge1-xSnx)/n-Ge1-xSnx interface, modifying the potential profile near the contact and giving rise to trap-assisted tunneling to increase the reverse leakage current for ohmic contact formation.

Selenium Segregation for Effective Schottky Barrier Height Reduction in NiGe/n–Ge Contacts

In this letter, we report the demonstration of an effective electron Schottky barrier height (ΦBn) reduction technology for NiGe/n-type Germanium (n-Ge) contacts using ion implantation of selenium (Se) followed by its segregation at NiGe/n-Ge interface. Se was found to segregate at NiGe/n-Ge interface after germanide formation. Nickel monogermanide was formed using a 350°C 30-s anneal. Se segregation gives ΦBn as low as ~0.13 eV.

Fe nanoparticles on ZnSe: Reversible temperature dependence of the surface barrier potential

The Fe growth on ZnSe(001) takes place via the initial formation of superparamagnetic nano-islands that subsequently coalesce, giving rise to a continuous film for a nominal thickness of 8 Fe monolayers. For a very low Fe coverage (2 Fe monolayers), we show that the surface barrier potential (i.e. the barrier potential seen by electrons incident on the surface), measured by absorbed current spectroscopy, attains very large values (6.9 eV at room temperature) and dramatically changes as a function of temperature, with an increase of $\ensuremath{\sim}$1.5 eV from room temperature down to 130 K, largely exceeding similar changes observed in both thin films and nanoparticles. This phenomenon disappears as the thickness increases and is fully reversible with temperature. Nonequilibrium phenomena due to the experimental conditions are present, but are not able to explain the observed data. Inverse photoemission, core level photoemission, x-ray photoemission diffraction, and scanning tunneling microscopy are employed in order to find temperature-dependent properties of the Fe islands: while only minor changes as a function of temperature are present in the electronic band structure, the Fe crystal structure, and the morphology of the islands, a noticeable temperature dependence of the Se segregation through the Fe islands is found.

Selenium Segregation for Lowering the Contact Resistance in Ultrathin-Body MOSFETs With Fully Metallized Source/Drain

We report the first integration of selenium (Se) segregation contact technology in ultrathin-body (UTB) n-MOSFET featuring Ni fully silicided source and drain. During the Ni silicidation process, the implanted Se segregated at the NiSi-n-Si interface, leading to significant reduction of Schottky barrier height and contact resistance. The UTB n-MOSFETs integrated with Se segregation (SeS) contact technology show significant external series resistance reduction and drive current performance enhancement. Drain-induced barrier lowering and gate leakage current density are not adversely affected by the SeS process.

Low Schottky barrier height for silicides on n-type Si (100) by interfacial selenium segregation during silicidation

The electron Schottky barrier height ΦBn modulation for NiSi and PtSi formed on selenium-implanted n-type Si (100) has been experimentally investigated. Selenium (Se) segregation is observed at the silicide/n-Si(100) interface during silicidation process. ΦBn of 83 and 120 meV were achieved for Se segregated NiSi and PtSi on n-Si (100) interfaces, respectively. Contrary to previously reported Fermi level depinning effect in monolayer Se-passivated n-Si (100), the low ΦBn achieved in this work points to metal silicide Fermi level pinning near to conduction band EC of n-Si (100).

Novel Nickel Silicide Contact Technology Using Selenium Segregation for SOI N-FETs With Silicon–Carbon Source/Drain Stressors

We explore a novel silicide contact technology for effective Schottky barrier height PhiBn and contact resistance reduction, which is compatible with an advanced silicon-carbon (Si1-xCx) source/drain (S/D) stressor technology. The new silicide contact technology incorporates selenium (Se) that is coimplanted with S/D dopants into the silicon-carbon S/D prior to nickel silicidation, leading to the segregation of Se at the NiSi:C/n-Si0.99 C0.01 interface and the achievement of excellent ohmic contact characteristics. We demonstrate that the Se-coimplantation process contributes to a 23% drive current enhancement in a strained silicon-on-insulator n-MOSFET. The enhancement is attributed to the decrease of external series resistance which is primarily due to the reduction of silicide contact resistance.

Investigation of optical parameters of Ag–In–Se thin films deposited by e-beam technique

The optical properties of the Ag–In–Se (AIS) thin films deposited by e-beam technique were investigated by means of the optical transmittance measurements. The optical absorption coefficients of the films were found to vary from 10 3 to 10 5 cm −1 over the wavelength range of 300–1100 nm. The real and imaginary parts of the refractive index and dielectric constant for the as-grown and annealed films in between 100 and 400 °C were evaluated by means of both envelope method (EM) and continuous wavelet transform (CWT) method and the results were in quite good agreement with each other. The refractive index, n, dispersion over the measured wavelength range was explained by applying single-oscillator model (SOM) and the related parameters were calculated. The optical absorption process for the AIS thin films was characterized by three direct transitions from three closely spaced valence bands to a single conduction band due to the splitting of the valence band under the influence of the tetragonal crystalline field and spin–orbit interaction. The direct optical band gap energy decreases as the annealing temperature increases because of the increase in the width of band tail states near valence-band edge caused by the Se segregation. The two distinct parameters of the quasicubic model; crystal-field splitting, Δ CF , and spin–orbit splitting, Δ SO , were calculated for as-grown and annealed AIS thin films.

CoPt alloy grown on the WSe 2(0 0 0 1) van der Waals surface

The structural and magnetic properties of 3-nm-thick CoPt alloys grown on WSe 2(0 0 0 1) at various temperature are investigated. Deposition at room temperature leads to the formation of a chemically disordered fcc CoPt alloy with [1 1 1] orientation. Growth at elevated temperatures induces L1 0 chemical order starting at 470 K accompanied with an increase in grain size and a change in grain morphology. As a consequence of the [1 1 1] growth direction, the CoPt grains can adopt one of the three possible variants of the L1 0 phase with tetragonal c-axis tilted from the normal to the film plane direction at 54°. The average long-range order parameter is found to be 0.35(±0.05) and does not change with the increase in the deposition temperature from 570 to 730 K. This behavior might be related to Se segregation towards the growing facets and surface disorder effects promoted by a high surface-to-volume ratio. Magnetic studies reveal a superparamagnetic behavior for the films grown at 570 and 730 K in agreement with the film morphology and degree of chemical order. The measurements at 10 K reveal the orientation of the easy axis of the magnetization lying basically in the film plane.

Effective Schottky Barrier Height Reduction Using Sulfur or Selenium at the NiSi/n-Si (100) Interface for Low Resistance Contacts

We explore a novel integration approach that introduces valence-mending adsorbates such as sulfur (S) or selenium (Se) by ion implantation and prior to nickel silicidation for the effective reduction of contact resistance and Schottky barrier (SB) height at the NiSi/n-Si interface. While a low SB height of ~0.12 eV can be obtained for NiSi formed on S-implanted n-Si, the insertion of a 1000degC anneal prior to silicidation leads to S out-diffusion and loss of SB modulation effects. We demonstrate that Se-implanted Si does not suffer from Se outdiffusion even after a 1000degC anneal, and subsequent Ni silicidation formed an excellent ohmic contact with a low SB height of 0.13 eV. Se segregation at the NiSi/n-Si (100) interface occurred. Implantation of Se and its segregation at the NiSi/n-Si interface is a simple and promising approach for achieving reduced SB height and contact resistance in future high-performance n-channel field-effect transistors.

Structural characterization of polycrystalline Ag–In–Se thin films deposited by e-beam technique

The Ag–In–Se thin films were deposited by e-beam evaporation of the Ag 3 In 5 Se 9 single crystal powder under high vacuum without intentional doping. Energy dispersive X-ray analysis (EDXA) showed the decreasing behavior of Se and Ag in the structure depending on the annealing. X-ray diffraction (XRD) analysis showed that as-grown films have amorphous structure while annealing the films under nitrogen environment at 200 °C transformed from the amorphous to polycrystalline structure. The crystallinity of the films improved as annealing temperature increases up to 400 °C by 100 °C-step. The polycrystalline films show mixed binary and ternary crystalline phases. Each phase was determined by comparing XRD patterns with complete data cards as Ag 3In 5Se 9, AgInSe 2, In 4Se 3, In 2Se 3, InSe, Se 6 and Se. The existence of Se segregation was supported by the formation of Se aggregates in crystalline phases of Se 6 and Se. The X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analysis have been carried out in order to obtain detailed information about the atomic composition, chemical states and morphology of the thin film surface. The decomposition of In 4d, Se 3d and Ag 3d photoelectron peaks revealed the existence of In–In, In–Se, In–Ag, Se–Se and Ag–Ag bondings in as-grown thin films. After annealing the thin films at different temperatures, the concentration of In–Se and In–Ag bonds decreases significantly, which results in an In-rich, but Ag- and Se-deficient thin film structure. The roughness of the film surface as a result of application of post-annealing in between 200 and 400 °C monitored by AFM technique was observed to change from 1.81 to 22.89 nm.

Self-assembling of FePt nanostructures by quasi-van der Waals epitaxy

We report on the growth of FePt nanostructures by self-assembling on the van der Waals surface ofWSe2(0001) under ultra-high vacuum conditions. The morphology and crystalline structureof nanostructures were investigated by reflection high energy electrondiffraction (RHEED), scanning tunnelling microscopy (STM) and x-ray diffraction.The FePt nanostructures grow with the (111) plane azimuthally aligned to theWSe2(0001) plane with a narrow size distribution centred around 4.5 nmshowing a rounded shape for deposition temperatures in the300–500 °C range. Theydevelop the L10-type structure starting at a relatively low deposition temperature of about200 °C with the occurrence of three possible variants. Moreover, segregation ofSe at the growing surface was observed even in films deposited at roomtemperature. However, such a surfactant effect does not prevent theL10 ordering and could explain the perpendicular magnetic anisotropy observed in previously studiedCoPt3 films grown at roomtemperature on WSe2. Magnetic measurements in 3 nm thick FePt(111) deposits have revealed an easyaxis of magnetization in the film plane, with a coercivity strongly enhanced withL10 order.

Ferromagnetic metals on II–VI semiconductors: epitaxial growth, and structural and magnetic properties

We have studied the growth conditions to obtain the chemically well ordered L1 0 phase of the FePd alloy deposited on ZnSe and CdZnMgTe II–VI semiconductors. We have observed that the segregation of Se or Te atoms at the surface involved a dramatic effect on the formation of the L1 0 phase. The strong anisotropy exhibited along the growth direction allows to observe, by magnetic force microscopy, the perpendicular magnetic configuration with up and down domains. Comparably, we have grown, on the same materials, Cu/Ni multilayers showing a clear perpendicular magnetic anisotropy. Optical measurements on a semimagnetic heterostructure covered by a ferromagnetic metal layer suggest fluctuations of the Fermi level at the interface between the semiconductor and the metal.

In situ high-resolution X-ray photoelectron spectroscopy of light-induced changes in As–Se glasses

To establish the mechanism of reversible photoinduced changes such as in the recently discovered opto-mechanical effect in chalcogenide glasses, we have measured the effect of He–Ne laser light on the electronic structure of As–Se glasses using high-resolution X-ray photoelectron spectroscopy (XPS). We present the first results during in situ irradiation of As–Se bulk glass and amorphous films in the spectrometer, as well as after ex situ exposure in air. The non-reversible light-induced changes are due to the segregation of Se on the surface, increased homogeneity for As, and a greater heterogeneity in the chemical environment of Se. In addition, there are reversible changes in the valence band (VB), indicating transfer of lone-pair states of Se to a new kind of bonding state deeper in the band, which have a smaller population in the absence of light.

Effects of selenium surface segregation on the texture of a selenium‐doped FeSi alloy

High-resolution Auger electron spectroscopy has been used to study selenium surface segregation on an Se-doped FeSi alloy. The surface segregation of Se and impurities such as C, P and S was measured in situ in the analysis chamber of an Auger spectrometer under ultrahigh vacuum conditions in the temperature range 200–850 °C. At elevated temperatures (T≥850 °C), co-segregation of Se and S was observed to occur by means of x-ray photoelectron spectroscopy. It was also found that the surface segregation of selenium critically affected reconstruction of the (110) surface microstructure of the FeSi alloy, resulting in the formation of (100) planes at 850 °C. Copyright © 2000 John Wiley & Sons, Ltd.

Effects of selenium surface segregation on the texture of a selenium-doped FeSi alloy

High-resolution Auger electron spectroscopy has been used to study selenium surface segregation on an Se-doped FeSi alloy. The surface segregation of Se and impurities such as C, P and S was measured in situ in the analysis chamber of an Auger spectrometer under ultrahigh vacuum conditions in the temperature range 200-850 °C. At elevated temperatures (T ≥ 850 °C), co-segregation of Se and S was observed to occur by means of x-ray photoelectron spectroscopy. It was also found that the surface segregation of selenium critically affected reconstruction of the (110) surface microstructure of the FeSi alloy, resulting in the formation of (100) planes at 850°C.

Effect of Al-rich surface on Se δ-doped GaAs grown by low-pressure metalorganic chemical vapor deposition

We study Se δ-doped GaAs grown by low-pressure metalorganic chemical vapor deposition using hydrogen selenide as a doping precursor. The results of capacitance–voltage measurements show that the very sharp doping profile can be obtained by Se δ-doping on an Al-rich surface and Se segregation is also reduced by making an Al-rich surface after δ-doping. It is essential to utilize the δ-doping sequence which does not have a post-δ-doping purge step to minimize the Se evaporation.

Se Segregation and Chemical Bonding in Pd/Se/GaAs

The chemical bonding and surface segregation of the Pd/Selenium-passivated GaAs interface is monitored by synchrotron radiation photoemission spectroscopy (SRPES). Pd deposition on GaAs(100) and (111)A is accompanied by Se segregation into the Pd overlayer preventing As segregation. Moreover, Se atoms are found to remain at the Pd/GaAs(111)B interface and may play a role in suppressing GaAs substrate disruption during metal deposition.

Anisotropic Segregation in InSb

The anisotropic segregation of Se in has been studied using radioactive Se as a tracer element. A core consisting of a high concentration of Se has been found in the center of crystals pulled in the [111] direction. This phenomenon is thought to result from an extremely rapid lateral growth occurring on the (111) facet. Well‐defined striations were also observed. Crystals grown from seeds oriented in directions other than the [111] are found to have high concentrations of Se in the (111) facets near the edge of the crystal, thus removal of the edges leaves the major portion of the crystal relatively homogeneous and with lower impurity content.