Thermally stimulated surface segregation in sodium chloride crystals

Abstract-Using the methods of X-ray microanalysis (XRMA), Auger electron spectroscopy (AES), highresolution scanning electron microscopy (HRSEM), atomic-force microscopy (AFM), IR specular reflection spectrometry (IRSRS), and quadrupole mass-spectrometry (QMS), the chemical and phase composition, as well as the morphology of the cleaved surfaces of sodium chloride crystals formed as a result of thermally stimulated surface autosegregation (TSAS), is studied. A super-stoichiometric increase in the sodium content in the surface nanolayers with an increase in temperature up to 600 K and a decrease in this content upon further heating is established experimentally. A decrease in the sodium content in the nanolayers virtually in the entire range of annealing temperatures for the reversible segregation process, as well as a decrease in this content on the surface with increasing annealing time under conditions of medium vacuum, is also been found. In addition, we observe the selective sublimation of sodium in the nanolayers at temperatures above 400 K and in the microlayers at temperatures above 600 K.     

Nanoscale peculiarities of the thermally stimulated surface autosegregation of covalent crystals: Gallium arsenide

Nanoscale changes in the morphology and elemental composition of cleavage surfaces of gallium arsenide crystals, which arise due to thermally stimulated surface autosegregation, are investigated in detail and systematized. It is shown that, depending on the annealing conditions (temperature, duration, and evacuation), gallium arsenide dissociates, forming submicron nonstoichiometric layers on the surface or local phases of arsenic, gallium, and gallium oxide. The mechanism and nature of autosegregation are determined by the competing processes of arsenic sublimation and its surface diffusion with an activation energy of ～31 kJ/mol. The migration of arsenic atoms is described with the help of a crystallochemical model. The nanomorphology of the surface phases includes arsenic and gallium nanoparticles with sizes of 10-200 nm, their agglomerates, and gallium oxide nano- and microcrystallites combined in plate- and chainlike configurations.     

Structure and composition of chemically deposited In2S3 thin films

Nanostructured thin indium(III) sulfide thin films 285–756 nm thick are obtained via chemical deposition from aqueous solutions containing indium chloride, thioacetamide, tartaric acid, and hydroxylamine hydrochloride at temperatures of 343–368 K. Oxygen- and carbon-containing impurities, which are not observed in the film bulk (at a depth of 12 nm), are detected in the surface layers of the films. When the synthesis temperature increases, the layer morphology changes substantially and the crystallite size increases from 70 to 150 nm. Upon annealing at a temperature of 573 K, crystallite aggregates are fused and In₂S₃ films are enriched with 6 to 10 at % of oxygen.     

Thermostimulated surface autosegregation in bismuth titanate single crystals

High resolution scanning electron microscopy (HRSEM), atomic force microscopy (AFM), and electron microprobe X-ray analysis (EPMA) were used to study the morphology and local phase composition of (001) faces in single crystals of bismuth titanate Bi₄Ti₃O₁₂ formed as a result of thermostimulated surface segregation (TSAS). One possible mechanism for this phenomenon, generated by the selective internal mass transfer of the matrix’s own atoms to the surface in competition with the processes of selective component evaporation, is discussed.     

Surface self-diffusion of quench condensed hydrogen films

The annealing behaviour of quench condensed H₂-, HD- and D₂-films has been investigated using acoustic surface waves. Upon annealing at temperatures far below the triple point we observe drastic changes of sound velocity and attenuation which can be attributed to a considerable rearrangement of the film structure via surface diffusion. These findings are in good agreement with recent experiments using surface plasmons. From the temperature dependence of the speed of the structural changes we deduce the activation energies for surface self-diffusion as 23 K, 35 K, and 47 K for H₂, HD, and D₂, respectively.     

Sublimation du krypton à basse température

The sublimation of krypton at low temperature (25 ? T ? 50 K) is studied by means of high sentivity vibrating quartz microbalances. One of them supports, on its upper electrode, a condensed krypton crystal and allows the number of sublimated atoms per second to be measured, the temperature varying by steps. The evolution of the sublimation lobes versus temperature is determined by three others, situated in the same plane and respectively at 4°, 30° and 60° to the normal to the krypton crystal surface. The experimental results show the effects of radiation energy, partial pressure and thickness of krypton solid, on the krypton sublimation parameters. From the experimental results of the sublimation rate, the vapor pressure versus temperature and energy of activation of krypton sublimation are calculated and compared to results obtained by other authors. On the other hand for T < 42 K, the deviation of the sublimation lobe from Lambert's law increases as the temperature is lowered, i.e. for a lobe represented by cosⁿθ, n increases as T decreases.     

Formation of graphene on amorphous SiC film by surface-confined heating with electron beam irradiation

It is demonstrated experimentally that graphene can form on the surface of an amorphous SiC film by irradiating electron beam (e-beam) at low acceleration voltage. As the electron irradiation fluency increases, the crystallinity and uniformity of graphene improve, which is confirmed by the changes of the measured Raman spectra and secondary electron microscopy images. Due to the shallow penetration depth of e-beam with low acceleration voltage, only the region near the surface of SiC film will be heated by the thermalization of irradiated electrons with multiple scattering processes. The thermalized electrons are expected to weaken the bond strength between Si and C atoms so that the thermal agitation required for triggering the sublimation of Si atoms decreases. With these assistances of irradiated electrons, it is considered that graphene can grow on the surface of SiC film at temperature reduced substantially in comparison with the conventional vacuum annealing process.     

Temperature dependences of optical properties,chemical composition,structure,and laser damage in Ta2O5 films

Ta2O5 films are prepared by e-beam evaporation with varied deposition temperatures,annealing temperatures,and annealing times.The effects of temperature on the optical properties,chemical composition,structure,and laserinduced damage threshold(LIDT) are systematically investigated.The results show that the increase of deposition temperature decreases the film transmittance slightly,yet annealing below 923 K is beneficial for the transmittance.The XRD analysis reveals that the film is in the amorphous phase when annealed below 873 K and in thehexagonal phase when annealed at 1073 K.While an interesting near-crystalline phase is found when annealed at 923 K.The LIDT increases with the deposition temperature increasing,whereas it increases firstly and then decreases as the annealing temperature increases.In addition,the increase of the annealing time from 4 h to 12 h is favourable to improving the LIDT,which is mainly due to the improvement of the O/Ta ratio.The highest LIDT film is obtained when annealed at 923 K,owing to the lowest density of defect.     

Self-diffusion of water and benzene molecules adsorbed in synthetic opal samples

The self-diffusion of adsorbed molecules of water and benzene in porous synthetic opals has been studied by pulsed field gradient nuclear magnetic resonance. The study indicates two “phases” of water molecules differing by the self-diffusion coefficients, indicating two types of porosities existing in the synthetic opals. The smallest self-diffusion coefficient characterizes water in ultramicropores found on the surface of nanospheres. The form of the diffusion decay depends on temperature in the region of high temperatures as a result of exchange between pores of different sizes. The ultramicropores are inaccessible for the largest benzene molecules. Water adsorption and self-diffusion data confirm that annealing of the opal samples at 1293 K caused the collapse of ultramicropores.     

Effect of aluminium doping and annealing on structural and optical properties of cerium oxide nanocrystals

Nanocrystalline fluorite-like structures of Ce_1−xAl_xO_2−δ compounds were prepared by the chemical precipitation method using cerium chloride and aluminium chloride as precursors. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS). The effects of aluminium doping concentration and annealing on particle size, lattice parameter and band gap energies were investigated. The particle size of Al-doped CeO₂ samples were found to decrease with Al concentration and it increases from 6 to 20 nm as annealing temperature increases to 900 °C.     

Effect of iron doping and annealing on structural and optical properties of cerium oxide nanocrystals

Nanocrystalline fluorite-like structures of Ce_1−xFe_xO_2−δ compounds were prepared by chemical precipitation method using cerium chloride and iron chloride as precursors. The prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and diffuse reflectance spectroscopy (DRS). The effects of iron doping concentration and annealing on particle size, lattice parameter and band gap energies were investigated. The particle size of Fe-doped CeO₂ samples were found to decrease with iron concentration and it increases from 9 to 26 nm as annealing temperature increases to 900 °C.     

Influence of low-temperature annealing on the morphology of the surface layer of an iron-based amorphous alloy

The influence of isothermal annealing on the chemical composition, microstructure, and surface relief of ribbons of an amorphous Fe₇₇NiSi₉B₁₃ alloy was studied using Auger electron spectroscopy and transmission electron, atomic-force, and scanning tunneling microscopy. It is established that an annealing below the glass transition temperature results in a boron enrichment of the ribbon surface layer on the side of the ribbon that was not in contact with the roller during quenching. On both sides of a ribbon, islands of crystallized material 2–5 μm in size appear consisting of nano-and microcrystals of α-Fe on the roller-contacting side and of iron boride on the opposite side. The surface relief on the roller-contacting side of a ribbon and that on the opposite side are shown to differ in terms of their spectral and fractal characteristics after annealing. The effect of the chemical composition and excess free volume of the surface layer on the formation of islands of crystallized material and surface relief is discussed.     

Production, structure, and microhardness of nanocrystalline Ni-Mo-B alloys

Radiography, differential scanning calorimetry, luminescence and high-resolution electron microscopy are used to study the production, nanocrystalline structure, stability, and microhardness of alloys from the Ni-Mo-B system containing from 27 at. % to 31.5 at. % Mo and 10 at. % B. All studies of these alloys indicated that annealing at 600 °C leads to the creation of a granular phase consisting of FCC nanocrystallites with average grain sizes of 15–25 nm, depending on the chemical composition of the alloy. Annealing these nanocrystalline samples isothermally at a temperature of 600 °C has no appreciable effect on the grain size. Structurally, the nanocrystalline phase consists of grains of an FCC solid solution of Mo and B in Ni, dispersed in an amorphous matrix that isolates them from one another. The lattice parameters of the FCC nanocrystallites depend on the alloy composition and the duration of their isothermal anneal. Within this latter time, molybdenum and boron atoms diffuse from the FCC solid-solution lattice into the surrounding amorphous matrix. The stability of the nanocrystalline structure is determined by the thermal stability of the amorphous matrix, whose crystallization temperature increases with the isothermal annealing time due to enrichment by boron and molybdenum. As the structure forms, the alloy becomes harder as the nanocrystalline grains grow in size. This relation between hardness and grain size, which is opposite to the Hall-Petch law, is explained by hardening of the amorphous matrix due to changes in its chemical composition. Fiz. Tverd. Tela (St. Petersburg) 40, 10–16 (January 1998)     

Effect of phase transition and particle size on thermoluminescence characteristics of nanocrystalline K2Ca2(SO4)3:Cu+ phosphor

A new K₂Ca₂(SO₄)₃:Cu⁺ nanocrystalline TLD phosphor was prepared by chemical coprecipitation method using calcium chloride, potassium chloride and ammonium sulphate as reactants and copper chloride (Cu₂Cl₂, as impurity) in the presence of ethanol. The samples were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Differential thermal and thermogravimetric analysis (DT-TGA) techniques. XRD and DTA data show that there is a phase transformation from orthorhombic to cubic occurring at around 550 K. The effects of impurity concentration, annealing temperature and γ rays dose on TL characteristics of the material have been studied. Different glow curves have been theoretically deconvoluted using computerized glow curve deconvolution (CGCD method) into simple glow peaks to determine their trapping parameters. The results show that different phases existing in the material on annealing might have reorganized the electron-traps (energy levels) and hole-traps (luminescence centers) and are responsible for changing glow curve structures. This is very important to know as any change in the glow curve structure would cause errors in estimation of doses of high-energy radiations and also may lose its reusability. The photoluminescence (PL) studies show that there are no redox reactions (Cu⁺

Cu²⁺) taking place and Cu remains in its Cu⁺ form after annealing and even after irradiation.     

The influence of potassium and the role of coadsorbed oxygen on the chemisorptive properties of Pt(100)

The adsorption of K on Pt(100) has been followed by thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES); carbon monoxide was used as a probe for the modification of the chemical properties of K promoted surfaces. The role of subsequent adsorption of oxygen on the K modified surfaces has also been measured. For low potassium coverage (θ_K = 0 to 0.35), the mass-28 TDS peak temperature of adsorbed CO increases continuously with the K coverage, indicating an increase of the adsorption energy of CO which has been explained by a substantial charge donation from K into the $\text{2π}{}^1$ orbitals of CO via long range interactions through the platinum substrate. No oxygen uptake was detected after oxygen exposure at room temperature. For high potassium content (θ_K = 0.45 to 1), the mass-28 TDS peak temperature of coadsorbed CO is very narrow and remains constant at 680 K. We propose the formation of a COKPt surface complex which decomposes at 680 K, since K desorption is detected concomitantly to CO. On such K covered surfaces, the oxygen uptake is promoted, and it cancels the modifications of the surface properties induced by potassium.     

Influence of impurities on the surface structures and fault generation in homoepitaxial Si (111) films

In situ LEED studies of the homoepitaxial growth of Si(111) films by uhv sublimation, indicate a strong correlation between the type of surface structure generated and the metallic impurity content of the silicon substrates as estimated from minority carrier lifetimes. The development of the familiar Si (111)−7 × 7 structure is favored by the presence of lifetime-killing impurities in the substrate. Experiments where Fe is introduced on high lifetime substrates prior to annealing and film growth, suggest that this impurity species plays a role in the generation of the 7 × 7 surface structure. Electron microscopy reveals that homoepitaxial Si(111) layers are generally faulted, the number density of which increases progressively as the growth temperature is lowered and the deposition rate increased. Films deposited on high lifetime silicon contain substantially fewer stacking faults than those grown on low lifetime substrates. These results suggest that the faults originate at the substrate surface at microprecipitates consisting of fast diffusing, low solubility impurity species.     

Mechanisms of radiation-induced surface composition modification in VCr alloys at high and low temperature

The effect of 0.5 to 3 keV argon ion sputtering on the surface composition of several alloys of vanadium and chromium has been investigated at temperatures in the range 300 to 950 K. At room temperature, sputtering depletes the alloys in chromium and the relative V/Cr sputter rate constant ratio is found to be 1.20, consistent with the ratio of the pure element sputter yields. Annealing clean alloys at temperatures in the range 550 to 670°C further enriches the surface in vanadium and also oxygen segregates to the surface. This behaviour is attributed to co-segregation of oxygen and vanadium, an interpretation supported by the observation of vanadium chemical shifts analogous to those observed in vanadium oxides, the depth profiles of the segregated surfaces and the bond energies in the VCrO system. Sputtering the alloys at temperatures in the range 410 to 620°C enriches the surface in chromium to a depth which increases with sputter annealing time. Similar behaviour has been reported on sputtering a V/Cr alloy at these temperatures but with much higher energy V⁺ ions, and has been attributed to radiation-induced segregation of chromium. The results presented in this paper offer a different interpretation. Annealing enriches the surface in vanadium which co-segregates with oxygen. Thus on sputter- annealing the surface will become depleted in vanadium at a rate which is probably enhanced by an increased sputter yield of VO over either V or Cr. This interpretation is further supported by the observation that the alloy is still depleted in vanadium when the sputtering occurs at room temperature when performed in a cycle with intervening annealing intervals.     

Sublimation of atomic layers from a chromium surface

We employ low-energy electron microscopy to study the kinetics of thermal etching, or sublimation, of Cr(001) at approximately 1100 K. Atomic layers are removed from the surface by spontaneous nucleation and growth of two-dimensional vacancy islands, by rotation of spiral steps, and by island decay. The growth rates of vacancy islands and the rotation frequencies of double spirals are measured as a function of temperature, and the results are correlated with activation barriers of surface processes. Mass transport between the surface and bulk is shown to be unimportant.     

Structural and phase analysis of rapidly solidified Al-Fe alloys

The structure and phase composition of lightly-doped Al-Fe alloys obtained by ultrarapid quenching from the melt are investigated. The surface of foils was studied using scanning electron microscopy, atomic-force microscopy, and Rutherford backscattering technique. The variation in the phase composition of alloys during annealing was studied by x-ray diffraction technique and by resistivity and microhardness measurements. The Al-Fe alloys have microcrystalline structure with a nonuniform iron content in the near-surface region of the samples. A correlation of depth profiles of iron and phase composition of the foils is observed. It is found that decomposition of the supersaturated α solid solution proceeds in the temperature range 250–350°C. As the annealing temperature increases, a metastable Al₆Fe phase is precipitated. In the range 300–500°C, the metastable Al₆Fe phase decomposes, and a stable Al₃Fe phase is precipitated.     

InGaN/GaN p-i-n Photodiodes Fabricated with Mg-Doped p-InGaN Layer

Mg-doped p-InGaN layers with In composition of about 10% are grown by metalorganic chemical vapor deposition (MOCVD). The effect of the annealing temperature on the p-type behavior of Mg-doped InGaN is studied. It is found that the hole concentration in p-InGaN increases with a rising annealing temperature in the range of 600-850°C, while the hole mobility remains nearly unchanged until the annealing temperature increases up to 750°C, after which it decreases. On the basis of conductive p-type InGaN growth, the p-In_0.1Ga_0.9N/i-In_0.1Ga_0.9N/n-GaN junction structure is grown and fabricated into photodiodes. The spectral responsivity of the InGaN/GaN p-i-n photodiodes shows that the peak responsivity at zero bias is in the wavelength range 350-400nm.