固体C60/Si异质结的电学表征——整流特性、能带模型与温偏效应

研究了非掺杂固体C₆₀与n-Si和与p-Si接触的电学性质,电流-电压(J-V)特性测量表明两种接触的导电极性相反,且都具有很强的整流作用,表明在两种接触界面附近存在着阻挡载流子输运的、性质不同的势垒.电流-温度(J-T)测量表明,电流与温度的倒数呈指数依赖关系,从中估算出C₆₀/n-Si和C₆₀/p-Si异质结的有效势垒高度分别为0.30和0.48eV.引进异质结的能带模型,成功地解释了上述测量结果,由能带模型和测量数据估算出以硅为衬底的 关键词：     

Preferential regrowth of indium–tin oxide (ITO) films deposited on GaN (0001) by rf-magnetron sputter

Effects of thermal treatments on the electrical properties and microstructures of indium–tin oxide (ITO)/GaN contacts have been investigated using a rf-magnetron sputter deposition followed by rapid thermal annealing. ITO films annealed at 800 °C revealed Schottky contact characteristics with a barrier height corresponding to ITO’s work function of 4.62 eV. The evolution of electrical properties of ITO/GaN contacts was attributed to the preferential regrowth of In₂O₃ (222)//GaN (0001) with an ideal metal–semiconductor Schottky contact. The feasible use of ITO/GaN as a transparent Schottky contact would be realized by the enhanced regrowth of In₂O₃ at high temperature. Received: 1 September 2000 / Accepted: 15 November 2000 / Published online: 28 February 2001     

Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E_F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10³ L of oxygen, the electronic surface state band localized just below the Fermi level E_F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10⁴L and 10⁵L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.     

Electrical behavior of the Au/MoS2 interface studied by light emission induced by scanning tunneling microscopy

Light emitted in the tunneling junction of a scanning tunneling microscope has been used to establish the electrical characteristics of nanojunctions made of Au islands deposited on flat MoS₂ surfaces. It is shown that these characteristics are those of rectifying contacts when the gold islands are isolated and that they evolve toward those of ohmic contacts when the island density increases. It is observed that the rectifying behavior also evolves over time as on infinite metal/semiconductor contacts. Using the STM tip, single gold islands can be manipulated on the MoS₂ surface so that their electrical behavior can be changed depending on their position with regard to the other islands.     

A.C. electrical conductivity of potassium perchlorate

A.C. electrical conductivity of pottasium perchlorate (KP) has been measured in the temperature range 25–325°C at frequencies ranging from 50–500 Hz using an automated technique. Three regions with activation energies. 1.08 ± 0.02, 0.66 ± 0.01 and 0.39 ± 0.04eV are observed in ln(σT) vs 1/T plots at frequencies < - 50 Hz. Dielectric loss measurements on KP samples doped with CrO₄^2? and SO₄^2? ions show the presence of impurity-vacancy complexes with reorientation energies of 0.99 ±0.02 and 1.07 ± 0.01 eV respectively. The results are interpreted in terms of a defect model which highlights the role of various conduction and relaxation mechanisms in the a.c. electrical properties of the material.     

Investigation of surface self-diffusion of Ni atoms on the Ni(100) plane by means of work function measurements and Monte Carlo calculations

A novel method is presented to study migration of adsorbed metal atoms on a clean (hkl) surface of a metal substrate. The system used was Ni on Ni(100). We make use of the assumption that each single step of random walk motion of an individual adatom on the metal substrate refers to an activation energy ΔE_if (indices i and f giving the number of occupied nearest neighbour sites in the initial and final position respectively). Furthermore, we assume that the dipole moment p_v of an adatom is determined by the number v of occupied nearest neighbour sites. Via their dipole moments the adsorbed atoms induce a change of work function ΔΦ of the substrate metal, ΔΦ being related to the average dipole moment per unit area by the Helmholtz equation. To measure ΔΦ we use a pendulum device. At fixed low coverage θ the variation of ΔΦ with temperature T has been calculated numerically within the framework of a Monte Carlo simulation (MCS) model, using various activation energies ΔE_if and dipole moments p_v as input parameters. By fitting these data to the experimental curves we could derive the following dipole moments and activation energies: p₀ = 0.45 ± 0.05 D, p₁ = 0.36 ± 0.05 D, p₂ = 0.27 ± 0.05 D, p₃ = 0.18 ± 0.05 D, ΔE₀₀ 0.60 ± 0.02 eV, ΔE₁₀ = 0.70 ± 0.025 eV, ΔE₂₁ = 0.80 ± 0.05 eV, ΔE₂₀ = 0.90 ± 0.05 eV. We compare these results with those of other workers.     

Dielectric behavior of sodium borate glasses

The dielectric constant and the electrical conductivity of the transparent glasses in the composition 3Na₂O-7B₂O₃ (NBO) were investigated in the 100 Hz–10 MHz frequency range at various temperatures. The activation energy associated with the electrical relaxation determined from the electric modulus spectra was found to be 0.76 ± 0.02 eV, close to that (0.74 ± 0.02 eV) obtained from DC conductivity studies. The frequency-dependent electrical conductivity was analyzed using Jonscher’s power law. Temperature-dependent behavior of the frequency exponent (n) suggested that the correlated-barrier hopping model was the most appropriate to rationalize the electrical transport phenomenon in NBO glasses.     

Untersuchung der vier ersten angeregten Zustände des6Li-Kernes durch Elektronenstreuung

Inelastic electron scattering cross sections were measured for energies below 60 MeV and momentum transfersq between 0.2 and 0.6 fm^?1. Ground state radiative widths Γ _γ ⁰ and transition radiiR _tr were deduced. 2.18 MeV: Γ _γ ⁰ (E2)=(4.40±0.34) · 10^?4 eV,R _tr=(4.28±0.39)fm; 3.56MeV: Γ _γ ⁰ (M1)=(8.31±0.36)eV,R _tr=(2.90±0.10)fm; 4.27 MeV: Γ _γ ⁰ (E2)=(5.4±2.8) · 10^?3 eV,R _tr=(3.4±1.2) fm. The excitation of the 5.37 MeV level shows a transverse angular dependence.     

Measurements of the work function of single-walled carbon nanotubes encapsulated by AgI,AgCl, and CuBr using kelvin probe technique with different kinds of probes

We report the results on the measurements of the work function of single-walled carbon nanotubes encapsulated by Agl (AgI@SWCNT), AgCl (AgCl@SWCNT), and CuBr (CuBr@SWCNT) by the local Kelvin probe technique. We found the values of the work function of tubes encapsulated with AgI and AgCl (Φ(AgI@SWCNT) = 5.08 ± 0.02, Φ(AgCl@SWCNT) = 5.10 ± 0.02 eV) to exceed substantially that of pristine carbon nanotubes, and the value of the work function of carbon nanotubes encapsulated with CuBr is Φ(CuBr@SWCNT) = 4.89 ± 0.03 (eV). The measurements are carried out using different kinds of microscope probes including multi-walled carbon nanotube tips.     

Dependence of work function on coverage for aluminum/tungsten in the field electron microscope

Aluminum adsorption on tungsten has been investigated by field electron microscope techniques. Changes in average work function and work functions of the (001) plane and (111) region were determined as a function of aluminum deposition. The average work function exhibits a minimum of about (4.11 × 0.04)eV (relative to an assumed value of 4.50 eV for the tungsten substrate) and reaches a value of (4.21 ± 0.04) eV for thick deposits. The (001) plane and (111) region exhibit complicated work function dependences on aluminum coverage. Some qualitative observations of aluminum on tungsten surface diffusion are also reported.     

Electron and positron work functions of Cr(100)

We report measurements of the electron and positron work functions of submonolayer contaminated single crystal surfaces of Cr(100) in ultra high vacuum. The positron work function ø₊ is obtained by measuring the spectrum of slow positrons reemitted by the Cr(100) surface when it is bombarded with keV energy positrons. The electron work function ø_- is measured relative to Al(100) by comparing the target biases at which the slowest emitted positrons are recollected by the target. We obtain ø₊ = ?1.76(10) eV and ø_- = 4.46(6) eV for our Cr(100) surface using the value ø_- = 4.41(3) eV for Al(100) reported by Grepstad, Gartland and Slagsvold. The ø₊ value is in agreement with the ?2.2 eV calculated by Nieminen and Hodges. The positronium work function for Cr implied by these results is ?4.10(10) eV; the positronium negative ion (Ps^-) work function for this surface is calculated to be + 0.37(7) eV. A search for Ps^- showed that at a 90% confidence level less than one in 10³ thermalized positrons reaching the Cr surface are emitted as Ps^-. The slow positron emission spectrum was observed not to change over the 70–300 K range in contrast to recent theoretical predictions.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Thermal stability of W2B and W2B5 contacts on ZnO

Rectifying contact formation on n-type bulk single crystal ZnO using novel W₂B or W₂B₅ metallization schemes was studied using current-voltage, scanning electron microscopy and Auger electron spectroscopy (AES) measurements. When a single Au overlayer was used to reduce the metal sheet resistance, the contacts were ohmic for all annealing conditions due to outdiffusion of Zn through the metal. By sharp contrast, when a bilayer of Pt/Au was used on top of the boride layers, rectifying contacts with barrier heights of ∼0.4 eV for W₂B were obtained. The highest barrier height of 0.66 eV was achieved for W₂B₅ annealed at 600 °C, although at this condition the contact showed a reacted appearance and AES showed almost complete intermixing of the metallization.     

Band alignment at sputtered ZnSx O1–x/Cu(In,Ga)(Se,S)2 heterojunctions

Valence band offsets ΔE_VBM at ZnS_x O_1–x/Cu(In,Ga)(Se,S)₂ (CIGSSe) heterojunctions have been studied by photoemission spectroscopy (XPS, UPS) as a function of composition x in sputtered ZnS_x O_1–x films. In the composition range from ZnO to ZnS we found ΔE_VBM between –(2.1 ± 0.3) eV and –(0.8 ± 0.4) eV, respectively. Considering the optical band gaps, the conduction band offsets ΔE_CBM range from –(0.1 ± 0.3) eV to +(1.4 ± 0.4) eV. These results suggest that sputtered ZnS_x O_1–x is suitable as substitution for the CdS buffer and ZnO window layers in standard chalcopyrite‐based solar cells. Current–voltage characteristics of the solar cells have been investigated as a function of the composition x. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of laser irradiation with <Emphasis Type="Italic">h</Emphasis>ν=1.96 eV on the properties of titanium-silicon contact under thermal annealing in nitrogen

The effect of laser irradiation with hν=1.96 eV (λ=0.63 μm) on the properties of titanium-silicon contacts subjected to steady-state thermal annealing in a nitrogen atmosphere is studied. It is found that treatment-induced changes in the phase composition of the contact modify its electrophysical parameters. The applicability of laser irradiation to the formation of rectifying titanium-silicon junctions with desired parameters is demonstrated.     

A study of copper adsorption on (100) tungsten by high field microscopy

An increase in the work function of a copper layer approximately three monolayers deep on tungsten (100) can be thermally induced at temperatures above 540 K. Helium ion microscopy reveals a consequent but small increase in structural perfection of the tungsten substrate surface, which is akin to the rearrangement of gold, but the change in work function of the copper layer appears not to depend upon the rearrangement. Detailed investigation of changes in work function at (100) using a probe-hole field emission microscope shows that copper in the first monolayer increases φ₁₀₀ by 0.42 eV at monolayer coverage, probably by formation of an array of dipoles of moment μ₀ = (1.6 ± 0.5) × 10^?30 C m with polarizability α = 3.80 ± 1.8 ų. A large apparent increase in φ₁₀₀ of 1.6 eV takes place when a thicker copper layer is spread so that it surrounds and approaches (100), and this is ascribed to a reduction in the field enhancement factor β produced by the surrounding copper. Comparison of the present findings with those of Bauer et al. reveals substantial agreement on the behaviour of the first monolayer and two major differences in the behaviour of thicker layers: (i) an increase in φ₁₀₀ from 4.25 to 7.1 eV which we observe above 540 K, and (ii) absence of any evidence for breakup of third and higher layers to form crystallites. It is tentatively suggested that (i) results from band-structure changes accompanying structural reorganisation within the adsorbed layer, which take place under experimental conditions not used by Bauer et al., and that (ii) is due to the absence of steps in the probed area of (100) which can act as nucleating centres for crystallite formation on a macroscopic (100) surface.     

CeAlO<Subscript>3</Subscript> crystals: Preparation and study of their electrical and optical characteristics

Crystals of cerium aluminate with perovskite structure were obtained using the cold-crucible technique. The electrical and optical properties of cerium aluminate were studied in air in the range 300–1300 K. The main characteristics of CeAlO₃ at T=300 K are a follows: electrical conductivity σ=10^?7 S/cm, dielectric permittivity ?=3000–10000 (both measured at a frequency of 1000 Hz), thermal band-gap width ΔE=2.3±0.5 eV, and optical width δE=2.65±0.25 eV, which decreases at a rate of ?0.62×10^?3 eV/K with increasing temperature in the 300-to 1500-K interval.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.     

The surface ionization of gadolinium,terbium and dysprosium on the tungsten (110) surface

A study of the surface ionization of gadolinium, terbium, and dysprosium (Gd, Tb, and Dy) is described and the results compared with the predictions of the Saha-Langmuir equation. A tentative measurement of holmium is also included. The experimental index parameter, E, which should be equal to the difference between the first ionization potential of the element and the apparent work function of the surface, was found to exceed the predicted value by the following amounts: Gd, 0.66 ± 0.14 eV; Tb, 0.27 ± 0.13 eV; and Dy, 0.24 ± 0.12 eV. The absolute ionization efficiencies were estimated by comparison of our gadolinium results with those reported by Dresser and Hudson. The preexponential coefficients are found to exceed the theoretically predicted values by an order of magnitude. Surface ionization efficiency of these rare earths is larger than predicted by the Saha-Langmuir equation by factors ranging from 5 to 25 at 2500 K. The (110) face of a tungsten single crystal was used as the ionizing surface, and its work function was determined to be 5.46 ± 0.11 eV in an auxiliary study of the emission of tungsten ions.     

Photoemission study of oxygen chemisorption on tin

Photoemission measurements have been made at photon energies from 5–12 eV and at 21.2 eV on evaporated Sn films and the same films with varying room temperature exposure to oxygen. For hν ? 9 eV the quantum yield for Sn with exposures of as much as 4000 L oxygen (1 L = 1 Langmuir = 10^?6 Torr sec) differs only slightly from the clean metal. For hν ? 9 eV no change in yield is observed with oxygen exposure. The energy distribution of photoemitted electron (EDC's) from Sn with increasing exposure to oxygen above ? 20 L are characterized by the growth of two peaks which were not present in the EDC's for the clean metal, located 2.9 ± 0.1 eV and 4.8 ± 0.1 eV below the Fermi level. We associate this structure with the presence of SnO₂. No sharp resonance which could be associated with adsorbed oxygen was seen. Uniformly reduced emission from metallic Sn states and a Fermi level as sharp as for the clean metal is observed in the EDC's at all oxygen exposures. In addition, no change in work function with oxygen exposure was detected. The effects of oxygen saturate for exposures ? 4000 L. We suggest that under the conditions used in this experiment, the oxygen penetrates beneath the surface forming SnO₂ and leaving metallic Sn on the surface.