A CEMS investigation of57Fe implanted in Al and Cu at low temperature

A new type of CEM-spectrometer allows in situ measurements on metal foils implanted at low temperatures. It has been used to study defect association and clustering of⁵⁷Fe in Al and Cu. For⁵⁷FeAl, the substitutional fraction (f _s) in samples implanted at 120 K is somewhat smaller than expected for a random impurity distribution but much larger than after room temperature implantation. For⁵⁷FeCu,f _s for samples implanted at 120 K is less than 0.5 of the value expected for a random distribution and it falls to zero after annealing at 600 K, where more extensive Fe clustering occurs. Vacancy trapping in stage III does not contribute significantly to the observed defect sites.     

Improved device performance in nonpolar a ‐plane GaN LEDs using a Ni/Al/Ni/Au n‐type ohmic contact

The authors report upon the increased light‐output power (P_out) via a reduction in the forward voltage (V_f) for nonpolar a ‐plane GaN LEDs using Ni/Al/Ni/Au n‐type ohmic contacts. The specific contact resistivity of the Ni/Al/Ni/Au contact is found to be as low as 5.6 × 10^–5 whereas that of a typical Ti/Al/Ni/Au contact is 6.8 × 10^–4 Ω cm², after annealing at 700 °C. The X‐ray photoelectron spectroscopy results show that the upward surface band bending is less pronounced for the Ni/Al contact compared to the Ti/Al contact, leading to a decrease in the effective Schottky barrier height (SBH). The V_f of the nonpolar LEDs decreases by 10% and P_out increases by 15% when the Ni/Al/Ni/Au scheme is used instead of the typical Ti/Al/Ni/Au metal scheme. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Adsorption and reactivity of SO2 on Ir(1 1 1) and Rh(1 1 1)

The adsorption and reactivity of SO₂ on the Ir(1 1 1) and Rh(1 1 1) surfaces were studied by surface science techniques. X-ray photoelectron spectroscopy measurements showed that SO₂ was molecularly adsorbed on both the Ir(1 1 1) surface and the Rh(1 1 1) surface at 200 K. Adsorbed SO₂ on the Ir(1 1 1) surface disproportionated to atomic sulfur and SO₃ at 300 K, whereas adsorbed SO₂ on the Rh(1 1 1) surface dissociated to atomic sulfur and oxygen above 250 K. Only atomic sulfur was present on both surfaces above 500 K, but the formation process and structure of the adsorbed atomic sulfur on Ir(1 1 1) were different from those on Rh(1 1 1). On Ir(1 1 1), atomic sulfur reacted with surface oxygen and was completely removed from the surface, whereas on Rh(1 1 1), sulfur did not react with oxygen.     

离子注入的铝在Si(100)表面的偏析及其引起的纳米团簇和合金晶粒形成现象的实验研究

通过高温退火注入了铝的Si(100)样品,探讨偏析出来的铝在硅表面的热力学行为.由900℃的退火实验发现,偏析出来的铝原子一方面形成Si(100)基底的外延铝膜和铝岛,另一方面与硅原子结合形成尺度约为2—3nm的铝硅团簇.而1200℃的退火实验显示,铝和硅的快速冷凝形成了立方晶系的Al₄Si合金晶粒、尺度约为20—30nm.细小的铝硅团簇在结构上独立于样品基底并且趋于聚集成团,很可能是在高温退火和快速降温过程中形成铝硅合金晶粒的前驱. 关键词： 硅表面 铝掺杂 团簇 4Si')" href="#">Al₄Si     

Microscopic study of a lattice defect trapped at in impurities in plastically deformed Al

¹¹¹In doped Al specimes (c _In1 ppm) were cold worked by extension at 77 K to several amounts of strain. The PAC method was applied to study the trapping of lattice defects at the indium impurities between 77 K and 293 K. From electron irradiation experiments it is known that the In probes are suitable traps for self-interstitials in Al. Since in the present experiment no such effect is observed, it is concluded that by extension at 77 K in Al essentially no freely migrating interstitials are created. Between 110 K and 210 K an increasing fraction of the indium impurities traps a well defined defect. The symmetry axis of the resulting indium-defect-complex is found to point along 111 crystallographic directions. It is proposed that the In impurity traps a divacancy at nearest neighbour sites and relaxes into the open space, thus forming an equilateral triangle of three vacancies in a {111} plane with the In atom in its centre. Reasons for the formation of this complex at rather low annealing temperatures are discussed.     

Bond cutting in K‐doped tris(8‐hydroxyquinoline) aluminium

A series of Al 2p, K 2p, O 1s and N 1s core‐level spectra have been used to characterize the interaction between potassium (K) and tris(8‐hydroxyquinoline) aluminium (Alq₃) molecules in the K‐doped Alq₃ layer. All core‐level spectra were tuned to be very surface sensitive in selecting various photon energies provided by the wide‐range beamline at the National Synchrotron Radiation Research Center, Taiwan. A critical K concentration (x = 2.4) exists in the K‐doped Alq₃ layer, below which the K‐doped atoms generate a strained environment near the O and N atoms within 8‐quinolinoline ligands. This creates new O 1s and N 1s components on the lower binding‐energy side. Above the critical K coverage, the K‐doped atoms attach the O atoms in the Al—O—C bonds next to the phenoxide ring and replace Al—O—C bonds by forming K—O—C bonds. An Alq₃ molecule is disassembled into Alq₂ and Kq by bond cutting and bond formation. The Alq₂ molecule can be further dissociated into Alq, or even Al, through subsequent formations of Kq.     

Thermoelectric power of Ir(Fe)

Thermoelectric power data between 4.2 and 300°K are presented for Ir containing 0.5 and 1 at.% Fe and for pure Ir. Peaks similar to those found in Pd(Ni) and Rh(Fe) occur in Ir(Fe) at the spin fluctuation temperature T_sf = 28°K, independent of solute concentration. Similarities and differences among the three alloy systems are discussed.     

Metastable structures of Dy layers adsorbed on Mo(112) and their transformations

Ordering of dysprosium on Mo(112) up to 1.5 monolayers has been investigated by LEED and work function analysis after adsorption at 100 K and annealing between 200 and 1000 K. At low annealing temperatures (< 350-600 K) ordered structures are found, which are changed or even destroyed irreversibly by annealing steps to higher temperatures. At coverages, θ, up to 0.3 monolayer a (6×1) not strictly commensurate chain structure is seen, which coexists up to θ = 0.58 with a one-dimensionally incommensurate c( 1.56×2) structure. At higher coverages up to the physical monolayer at θ≈ 0.77, incommensurate ( n×2) followed by oblique ( n×1) structures are seen with n continuously variable with coverage. The second layer forms a p (1.33×1) structure. Annealing to higher temperatures causes irreversible structural transitions with strongly coverage dependent properties. Up to θ = 0.58, only a glass-like disordered phase is formed, which cannot be ordered again. In contrast, the rectangular incommensurate structures between 0.58 < θ < 0.68 remain unchanged upon annealing, whereas the structures at higher coverages and those of the second layer are transformed into commensurate (s×1) structures with integer s. Geometrical models are presented for the non-annealed structures and possible origins for the two-dimensional concentration dependent vitrification of the Dy layers are discussed. Received 15 June 2001     

Magnetic and electric studies of a new Co(II) perovskite-like material

Structural phase transitions in the perovskite-like material [(CH₄)₁₂(NH₃)₂]CoCl₄ have been observed using differential thermal scanning. The material shows an order-disorder transition at T ₁ = 396 ± 5 K with entropy, (ΔS ₁) = 12.8 J/mole/K. A "chain melting" transition with a major endothermic peak at T ₂ = 337 ± 3 K and a minor one at T ′ = 316 ± 2 K, has total entropy ΔS = 28 J/mole/K. At low temperatures, the transitions at T ₃ = 288 ± 3 K and at T ₄ = 188 ± 3 K, have entropies of ΔS ₃ = 14.4 J/mole/K and ΔS ₄ = 2.6 J/mole/K respectively. AC magnetic susceptibility in the temperature range 78-290 K, in a magnetic field of 160 A/m and at a frequency of 320 Hz is presented. The results indicate changes in symmetry at 188 K. Dielectric permittivity has been studied as a function of temperature in the range 300-430 K and frequency range (60 Hz-100 kHz), confirming the observed transitions. The dielectric permittivity reflects rotational and conformational transition for the material. The variation of the real part of the conductivity with temperature is thermally activated with different activation energies in the range of ionic hopping. The temperature dependence of the dc conductivity and that of the ions hopping rate have indicated that the concentration of mobile ions is independent of temperature. The dependence of the conductivity on frequency follows the universal power law, <artwork name="GPHT31040ei1"> in the temperature range 340 K<T<390 K. Values 0 <s ₁ <1 dominate at low frequency and correspond to translational hopping motion and values 1<s ₂<2 dominate at high frequencies and correspond to well localized hopping and/or reorientational motion. For T > 396 K, the AC conductivity was fitted to <artwork name="GPHT31040ei2"> with 0<s<1. Comparison with the corresponding Cu-containing material is discussed.     

Growth of Ni-Al alloys on Ni(1 1 1), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni3Al interfacial layer

This paper describes the second part of a study devoted to the growth of thin Ni-Al alloys after deposition of Al on Ni(1 1 1). In the previous paper [S. Le Pévédic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni₃Al(1 1 1). In the present paper we show that this regime is only observed for Al deposits smaller than 8 × 10¹⁵ Al/cm² and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 × 10¹⁵ Al/cm². Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(1 1 0) layer (whose thickness depends on the deposited Al amount), over a Ni₃Al “interfacial” layer (whose thickness—about 18 (1 1 1) planes—is independent of the deposited Al amount). The NiAl overlayer is composed of three variants, at 120° from each other in the surface plane, in relation with the respective symmetries of NiAl(1 1 0) and Ni₃Al(1 1 1). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-L1₂ Ni₃Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin Ni₃Al interfacial layer, to a mixture of Al and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the Al film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate.     

Structural Phase Transition and the Dielectric Permittivity of the Model Lipid Bilayer [(CH2)12(NH3)2]CuCl4

Structural phase transitions in the lipid-like bilayer material [(CH₂)₁₂(NH₃)₂]CuCl₄ have been observed using differential thermal scanning. The compound shows an irreversible thermochromic transition at ? 465 K and three reversible transitions at T ₁ = 433 ± 4 K and T ₂ = 411 ± 2 K and T ₃ = 358 K. The transition at 350 K is ascribed to chain melting. The other two correspond to crystalline phase transformation.

Phase (IV) T₃ = 358 ± 2K Phase (III) T₂ = 411 ± 2K Phase (II) T₁ = 433 ± 4K Phase (I)

Dielectric permittivity is studied as a function of temperature in the range 300-440 K and frequency, range (60 Hz-100 kHz). It confirms the observed transitions. The dielectric permittivity reflects rotational and conformational transitions for the compound. The variation of the real part of the conductivity with temperature is thermally activated in the temperature range above 350 K, with frequency-dependent activation energy, the values of activation energy lie in the range of ionic hopping. The dependence of the conductivity on frequency follows the universal power law σ = σ₀ + A(T) ω ^{s ( T )} with 0<s<1. Comparison of this material with other members of the series is discussed     

Commensurability oscillations and a new phase transition in YBa2Cu3Oy single crystals

Magnetization oscillations due to the commensurability of the vortex and crystal lattice periods in YBa₂Cu₃O_y (y=6.97±0.02) single crystals are investigated using a high angular resolution magnetometer. A sharp peak in the temperature dependence of the oscillation amplitude as well as other features in the behavior of the oscillation amplitude and of the irreversible magnetization are observed at T _f ∼60 K. It is inferred that T _f is the temperature of a transition of the solid vortex state to a smectic phase. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 832–837 (10 June 1999)     

Electric-Field Gradients at Group-III Sites on GaAs and InAs (111)B Surfaces

We report a comparison of electric-field gradients (EFGs) measured at the group-III sites on epitaxially-grown surfaces of 2×2-reconstructed GaAs and InAs crystals. For this purpose, we used ¹¹¹In→¹¹¹Cd perturbed-angular-correlation (PAC) spectroscopy. Sharp spectral lines characterize the perturbation functions corresponding to both surfaces. On GaAs surface, we observe only one well-defined nuclear electric-quadrupole interaction (NQI); and on the InAs surface, we observe a primary and a secondary NQI. Very similar but experimentally-distinguishable values of the EFG-parameters characterize the primary interactions corresponding to the GaAs and InAs surfaces. Specifically, for the GaAs and InAs surfaces, ω _Q =28.0±0.2 Mrad s⁻¹ and η=0.43±0.02 and ω _Q =28.8±0.2 Mrad s⁻¹ and η=0.39±0.02, and the angles between EFG z-axis and the (111) direction are 65°±3° and β=53°±3°, respectively. These unexpected results indicate that the so-called lattice contribution to the EFG is not significant. Moreover, for the primary NQIs on both surfaces, the similar parameter values demonstrate that chemical differences between the ¹¹¹In probe and the indigenous Ga surface atoms cause no large quantitative effects. This information indicates that impurity probes and PAC spectroscopy can be used effectively to investigate III–V surfaces. This revised version was published online in September 2006 with corrections to the Cover Date.     

Stabilisation of Ce-Cu-Fe amorphous alloys by addition of Al

The present work describes the formation of amorphous alloys in the (Al_1?xCe_x)₆₂Cu₂₅Fe₁₃ quaternary system (0 ≤ x ≤ 1). When the amount of Ce falls in the range 0.67 ≤ x ≤ 0.83, the alloys obtained exhibit a completely amorphous structure confirmed by powder X-ray diffraction. Otherwise, at compositions x = 0.5, 0.58, 0.92 and 1, a primary crystalline phase forms together with an amorphous matrix. The crystallisation temperature (T_x) decreases with increasing Ce content, varying from 593 K for x = 0.5–383 K for x = 1. Composition x = 0.75 is considered as the best glass former, exhibiting a large supercooled liquid region of 40 K width that precedes crystallisation. In order to form bulk amorphous alloys, ribbons with this later composition were consolidated into few millimetre thick discs using pulsed electric current sintering at different temperatures, yet preserving the amorphous structure. Meanwhile, increasing temperature above 483 K triggers crystallisation of a primary phase isostructural to AlCe₃. Further increase in the temperature up to 573 K yields a higher fraction of the crystalline phase. Testing mechanical properties, using nanoindentation, revealed that both elastic modulus (E) and hardness (H) depend on the Al content, ranging from E = 85.6 ± 3.7 GPa and H = 6.2 ± 0.7 GPa for x = 0.5 down to E = 39.8 ± 1.0 GPa and H = 3.1 ± 0.2 GPa for x = 0.92.     

Growth of Ni-Al alloys on Ni(1 1 1): (I) Formation of epitaxial Ni3Al from ultra-thin Al deposits

In this paper we describe the alloying process of ultra-thin Al layers (below 8 × 10¹⁵ Al/cm²) deposited on Ni(1 1 1). For this purpose Auger electron spectroscopy, low energy electron diffraction, and ion beam analysis-channelling measurements have been performed in situ in an ultra-high vacuum chamber. Al deposits formed at low temperature (about 130 K) are strained defective crystalline layers retaining the substrate orientation. Alloying takes place, with very progressive Ni enrichment, in a very broad temperature range between 250 K and 570 K. This feature shows that diffusion of the alloy species is more and more difficult when the Ni concentration increases. At 570 K a crystallographically and chemically ordered Ni₃Al phase is formed, and its order continuously improves upon annealing, up to 750 K. We have shown by ion beam methods that this alloy is three-dimensional, extending up to 16 (1 1 1) planes for the thickest deposits. The Ni₃Al phase can also be obtained directly by Al deposition at 750 K, but its crystalline quality is lower and the layer is probably formed of grains elongated along 〈1 1 −2〉 directions. The Al content of the thin Ni₃Al layers formed mostly dissolves in the bulk above 800 K. However a small amount of Al remains segregated at the Ni crystal surface.     

Specific features of the electronic structure of cerium and its 4d and 5d partners in CeM2 Laves phases (M=Fe, Co, Ni, Ru, Rh, Os, Pt, Mg, Al)

The x-ray line shift method has been used to study the electronic state of Ce (the 4f population) and of its 4d and 5d partners in the CeM₂ Laves phases (M=Fe, Co, Ni, Ru, Rh, Os, Pt, Mg, Al). It is shown that the valence of Ce in CeM₂ decreases monotonically from the limiting value m≈3.35 to m≈3 with decreasing intracrystalline compression of Ce atoms. The population of the outer 4d and 5d orbitals of Ru, Rh, and Os in the Laves phases has been found to be larger than that in metals. Fiz. Tverd. Tela (St. Petersburg) 40, 1397–1400 (August 1998)     

Infrared and Raman Spectra,conformations, ab initio calculations and spectral assignments of 1,3‐disilabutane (SiH3CH2SiH2CH3)

Raman spectra of 1,3‐disilabutane (SiH₃CH₂SiH₂CH₃) as a liquid were recorded at 293 K and as a solid at 78 K. In the Raman cryostat at 78 K an amorphous phase was first formed, giving a spectrum similar to that of the liquid. After annealing to 120 K, the sample crystallized and large changes occurred in the spectra since more than 20 bands present in the amorphous solid phase vanished. These spectral changes made it possible to assign Raman bands to the anti or gauche conformers with confidence. Additional Raman spectra were recorded of the liquid at 14 temperatures between 293 and 137 K. Some Raman bands changed their peak heights with temperature but were countered by changes in linewidths, and from three band pairs assigned to the anti and gauche conformers, the conformational enthalpy difference Δ_confH(gauche–anti) was found to be 0 ± 0.3 kJ mol⁻¹ in the liquid. Infrared spectra were obtained in the vapor and in the liquid phases at ambient temperature and in the solid phases at 78 K in the range 4000–400 cm⁻¹. The sample crystallized immediately when deposited on the CsI window at 78 K, and many bands present in the vapor and liquid disappeared. Additional infrared spectra in argon matrixes at 5 K were recorded before and after annealing to temperatures 20–34 K. Quantum chemical calculations were carried out at the HF, MP2 and B3LYP levels with a variety of basis sets. The HF and DFT calculations suggested the anti conformer as the more stable one by ca 1 kJ mol⁻¹, while the MP2 results favored gauche by up to 0.4 kJ mol⁻¹. The Complete Basis Set method CBS‐QB3 gave an energy difference of 0.1 kJ mol⁻¹, with anti as the more stable one. Scaled force fields from B3LYP/cc‐pVQZ calculations gave vibrational wavenumbers and band intensities for the two conformers. Copyright © 2007 John Wiley & Sons, Ltd.     

Adsorption and reaction of nitric oxide and oxygen on Rh(111)

Adsorption of NO and O₂ on Rh(111) has been studied by TPD and XPS. Both gases adsorb molecularly at 120 K. At low coverages (θ_NO < 0.3) NO dissociates completely upon heating to form N₂ and O₂ which have peak desorption temperatures at 710 and 1310 K., respectively. At higher NO coverages NO desorbs at 455 K and a new N₂ state obeying first order kinetics appears at 470 K. At saturation, 55% of the adsorbed NO decomposes. Preadsorbed oxygen inhibits NO decomposition and produces new N₂ and NO desorption states, both at 400 K. The saturation coverage of NO on Rh(111) is approximately 0.67 of the surface atom density. Oxygen on Rh(111) has two strongly bound states with peak temperatures of 840 and 1125 K with a saturation coverage ratio of 1:2. Desorption parameters for the 1125 peak vary strongly with coverage and, assuming second-order kinetics, yield an activation energy of $\text{85 ± 5}\text{kcal}\text{mol}$ and a pre-exponential factor of 2.0 cm² s^?1 in the limit of zero coverage. A molecular state desorbing at 150 K and the 840 K state fill concurrently. The saturation coverage of atomic oxygen on Rh(111) is approximately 0.83 times the surface atom density. The behavior of NO on Rh and Pt low index planes is compared.     

Spectrophotometric and 27‐Al NMR Characterization of Aluminum(III) Complexes with l‐Histidine

Abstract

The complex formation between l‐histidine (HHis) and aluminum(III) ion in water solutions was studied by UV spectrophotometric and 27‐Al NMR measurements at 298 K. UV spectra were measured on solutions in which the total concentration of histidine was from 15.0 to 50.0 mmol/dm³ and the concentration ratio of histidine to aluminum was varied from 3∶1 to 10∶1 in the pH range between 4.2 and 6.0. The spectra were taken in the wavelength interval 240–340 nm. Nonlinear least‐squares treatment of the spectrophotometric data indicates the formation of the complexes Al(HHis)³⁺, Al(His)²⁺, Al(HHis)His²⁺, and Al₂(OH)His⁴⁺ with the overall formation constants β_p,q,r: log β_1,1,1=11.90±0.04, log β_1,1,0=7.25±0.08, log β_1,2,1=20.1±0.1, and log β_2,1,1=5.92±0.12 (p, q, r are stoichiometric indices for metal, ligand, and proton, respectively). ²⁷Al‐NMR spectra were taken on solutions with the concentration of aluminum 50 mmol/dm³ and that of histidine 250 mmol/dm³. In the pH interval 5.0–6.1, two resonances at 9.5 ppm and 12.0 ppm were assigned to Al(HHis)²⁺ and Al(HHis)(His)²⁺ (or Al(OH)(HHis)₂ ²⁺), respectively.     

Electrical resistivity and length variations of iron and nickel during low temperature electron irradiations

Abstract

Low concentrations of Frenkel pairs are produced in polycrystalline iron and nickel targets during irradiations with 1.9 to 2.5 MeV electrons at 5 or 20 K. During irradiation (and subsequent thermal annealing for the nickel target) we simultaneously measure on the targets the changes of electrical resistivity, and of length with two different linear systems: a capacitive and an inductive device.

The measured ratios of relative length to electrical resistivity increases are [(Δl/l)/Δρ]_Fc = and [(Δl/l)/Δρ]_Nj = 1020 ±120(Ωcm)^?1. Values of Frenkel pair formation volumes of ν _Fc = 1.35 ± 0.25 and ν _Ni = 2.06 ± 0.30 atomic volumes are proposed.