Electronic transport at semiconductor surfaces––from point-contact transistor to micro-four-point probes

The electrical properties of semiconductor surfaces have played a decisive role in one of the most important discoveries of the last century, transistors. In the 1940s, the concept of surface states––new electron energy levels characteristic of the surface atoms––was instrumental in the fabrication of the first point-contact transistors, and led to the successful fabrication of field-effect transistors. However, to this day, one property of semiconductor surface states remains poorly understood, both theoretically and experimentally. That is the conduction of electrons or holes directly through the surface states. Since these states are restricted to a region only a few atom layers thick at a crystal surface, any signal from them might be swamped by conduction through the underlying bulk semiconductor crystal, as well as greatly perturbed by steps and other defects at the surface. Yet recent results show that this type of conduction is measurable using new types of experimental probes, such as the multi-tip scanning tunnelling microscope and the micro-four-point probe. The resulting electronic transport properties are intriguing, and suggest that semiconductor surfaces should be considered in their own right as a new class of electronic nanomaterials because the surface states have their own characters different from the underlying bulk states. As microelectronic devices shrink even further, and surface-to-volume ratios increase, surfaces will play an increasingly important role. These new nanomaterials could be crucial in the design of electronic devices in the coming decades, and also could become a platform for studying the physics of a new family of low-dimensional electron systems on nanometre scales.     

Hydrophilicity of amorphous TiO2 ultra-thin films

The UV-light-induced hydrophilicity of amorphous titanium dioxide thin films obtained by radio frequency magnetron sputtering deposition was studied in relation with film thickness. The effect of UV light irradiation on the film hydrophilicity was fast, strong and did not depend on substrate or thickness for films thicker than a threshold value of about 12 nm, while for thinner films it was weak and dependent on substrate or thickness. The weak effect of UV light irradiation observed for the ultra-thin films (with thickness less than 12 nm) is explained based on results of measurements of surface topography, UV-light absorption and photocurrent decay in vacuum. Comparing to thicker films, the ultra-thin films have a smoother surface, which diminish their real surface area and density of defects, absorb partially the incident UV light radiation, and exhibit a longer decay time of the photocurrent in vacuum, which proves a spatial charge separation. All these effects may contribute to a low UV light irradiation effect on the ultra-thin film hydrophilicity.     

Modeling structural metastability of irradiated thin films

The nucleation of crystalline or amorphous phases in irradiated binary metallic, as well as non-metallic compound thin films is interpreted using the atomistic interface migration-charge transfer (IMCT) model. The space and time evolution of dense collision cascades, which form upon ion bombardment of the target, lead to non-equilibrium compositional and electronic density profiles at the interface between each cascade and the surrounding crystalline matrix. This occurs because one of film constituents preferentially migrates to the interface, which becomes enriched in it. Cascade relaxation to metastable equilibrium is simulated by local charge transfer reactions (CTR), each involving a pair of dissimilar atoms of the initial compound, which constitute a dimer of an effective compound. The energy cost to introduce an effective compound dimer into the matrix, the difference of formation enthalpy between effective and initial compound and the local strain suffered by the film as a consequence of ion formation by a CTR are evaluated. Threshold values are found in the above structure stability parameters that allow for a qualitative separation of amorphised from crystalline compounds, both metallic and non-metallic.     

Chemical and electronic properties of sulfur-passivated InAs surfaces

Treatment with ammonium sulfide ((NH₄)₂S_x) solutions is used to produce model passivated InAs(0 0 1) surfaces with well-defined chemical and electronic properties. The passivation effectively removes oxides and contaminants, with minimal surface etching, and creates a covalently bonded sulfur layer with good short-term stability in ambient air and a variety of aqueous solutions, as characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and Hall measurements. The sulfur passivation also preserves the surface charge accumulation layer, increasing the associated downward band bending.     

Growth and structure of MBE grown TiO2 anatase films with rutile nano-crystallites

We have explored the systematics of TiO₂ polymorph nucleation during film growth by molecular beam epitaxy on perovskite substrates. The accidental lattice match between anatase (0 0 1) and LaAlO₃(0 0 1) or SrTiO₃(0 0 1) typically results in anatase nucleation at the interface. However, the growth conditions dictate whether or not rutile also nucleates, and the associated morphological and structural properties of the composite film. Four symmetry equivalent epitaxial orientations of rutile on anatase are observed when rutile nucleates as discrete particles on LaAlO₃(0 0 1). Such films constitute model systems for studying the anatase/rutile interface, which is of considerable current interest in photochemistry.     

RHEED observation of BaTiO3 thin films grown by MBE

Ferroelectric BaTiO₃ thin films with a thickness of 10 monolayers (ML) were epitaxially grown on SrTiO₃(0 0 1) substrates by very slow deposition using molecular beam epitaxy (MBE). The investigations were carried out by two growth methods: (i) codeposition and (ii) alternate deposition of the metal elements in an oxygen atmosphere. In situ observation of reflection high-energy electron diffraction confirmed that an epitaxial cube-on-cube structure was prepared. After the deposition, X-ray diffraction measurements were carried out. The 10-ML-thick BaTiO₃ films were highly c-axis oriented single crystals with good film quality.     

Theoretical study of conductance in stretched monatomic nanowires

Recent experiments showed that the last, single channel conductance step in monatomic gold contacts exhibits significant fluctuations as a function of stretching. From simulations of a stretched gold nanowire linked to deformable tips, we determine the distribution of the bond lengths between atoms forming the nanocontact and analyze its influence on the electronic conductance within a simplified single channel approach. We show that the inhomogeneous distribution of bond lengths can explain the occurrence and the 5% magnitude of conductance fluctuations below the quantum conductance unit g₀=2e²/h.     

Surface-state conduction through dangling-bond states

The surface-state conduction through dangling-bond states on the (1 1 1) ideal surfaces of group-IV semiconductors is studied theoretically. Localization strength of wave functions is an important factor determining the surface-state conduction. Approximate expressions for the decay constants of the dangling-bond states are presented using a tight-binding model. The origin of the difference in the decay constants of diamond, Si, and Ge is clarified in terms of linear combination of the states at the top of valence bands and the bottom of conduction bands. The ballistic conductance from a probe to the surfaces is calculated using the Landauer formalism. The surface-state conductance is much larger than the bulk-state one, which is due to small band dispersions of the dangling-bond states.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

Texture of molybdenum thin films on SrTiO3(1 0 0): A RHEED study

The structure of ultrathin Mo films on SrTiO₃(1 0 0) was studied by in situ reflection high-energy electron diffraction (RHEED). A different structure was observed for films less than 20 Å thick than for thicker films. These films were epitaxial and had a metastable structure. Thicker films had the dimensions of equilibrium bcc Mo(1 1 0). Relaxation processes transformed the metastable Mo into bcc Mo, resulting in the following orientation relationships between Mo and SrTiO₃: (1 1 0)[0 0 1]_bcc Mo ∥ (1 0 0)[0 0 1]_SrTiO3 and (1 1 0)[1 1 1]_bcc Mo ∥ (1 0 0)[0 1 1]_SrTiO3. The formation of such specific orientations is related to transformations via the Bain and Needle Path, respectively.     

Reactivity of (H)(e) color centers at the MgO surface: formation of O2 and N2 radical anions

We have considered the interaction of O₂ and N₂ molecules with electrons trapped at the surface of MgO by performing embedded cluster DFT calculations. Trapped electrons at the surface of MgO react instantaneously with O₂ and N₂ leading to the formation of the corresponding O₂⁻ and N₂⁻ molecular anions stabilized at specific surface sites. So far, the atomistic model for this process was based on the idea that the electrons are trapped at oxygen vacancies, the F_S⁺ centers. Recently, it has been shown that morphological sites at the MgO surface like a reverse corner, a step or a corner, can adsorb hydrogen and stabilize (H⁺)(e⁻) pairs giving rise to a new class of paramagnetic color centers. Here we show that these centers exhibit reactivity towards O₂ and N₂, which is fully consistent with the experimental observations, providing further support to the new model of electron traps at the MgO surface.     

Magnetic anisotropic properties in Fe3O4 and CoFe2O4 ferrite epitaxy thin films

Epitaxial thin films of Fe₃O₄ and CoFe₂O₄ on MgO (0 0 1) substrates were grown by molecular beam epitaxy at low temperature growth process. Magnetization and hysteresis loop of both films were measured to investigate magnetic anisotropic properties at various temperatures. Anomalous magnetic properties are found to be correlated with crystalline, shape, and stress anisotropies. The Fe₃O₄ film below Verwey structural transition has a change in crystal structure, thus causing many anomalous magnetic properties. Crystalline anisotropy and anomalous magnetic properties are affected substantially by Co ions. The saturation magnetization of Co–ferrite film becomes much lower than that of Fe₃O₄ film, being very different from the bulks. It indicates that the low temperature growth process could not provide enough energy to have the lowest energy state.     

Growth of ZnO thin films on c-plane Al2O3 by molecular beam epitaxy using ozone as an oxygen source

The growth of c-axis oriented ZnO thin films on c-plane Al₂O₃ via molecular beam epitaxy (MBE) using dilute ozone (O₃) as an oxygen source was investigated. Four-circle X-ray diffraction (XRD) indicates that films grown at 350 °C are epitaxial with respect to the substrate, but with a broad in-plane and out-of-plane mosaic. The films were highly conductive and n-type. Epitaxial film growth required relatively high Zn flux and O₃/O₂ pressure. The growth rate decreased rapidly as growth temperature was increased above 350 °C. The drop in growth rate with temperature reflects the low sticking coefficient of Zn at moderately high temperatures and limited ozone flux for the oxidation of the Zn metal. Characterization of the films included atomic force microscopy (AFM), X-ray diffraction, photoluminescence, and Hall measurements. These results show that molecular beam epitaxy of ZnO using ozone is rate limited by the ozone flux for growth temperatures above 350 °C.     

The surface science of xerography

Over the past four decades xerography, the dry ink marking process developed by the photocopy industry, has grown from nothing into a $170 billion industry worldwide. This amazing commercial success is due to the fact that during this period, xerographic technology experienced constant and often-dramatic improvement created by sustained industrywide research and development. Indeed, the development of the xerographic copying and printing industry is one of the great applied surface science successes of all time. In this article we outline the story of the advances in xerographic technology during the past four decades, describe the profound dependence on these advances of the control of surface and interface properties of increasingly sophisticated multi-component materials systems, and indicate the potential impact on the industry of the continuing development of the surface and interface science of the multi-component materials packages used in xerographic technology.     

Growth and structural characterization of semiconducting Ru2Si3

Recent band structure calculations indicate that ruthenium silicide (Ru₂Si₃) is semiconducting with a direct band gap. Electrical measurements lead to a band gap around 0.8 eV which is technologically important for fiber communications. This makes Ru₂Si₃ a promising candidate for silicon based optical devices, namely LEDs. We present first results on the epitaxial growth of ruthenium silicide films on Si(1 0 0) and Si(1 1 1) fabricated by the template method, a special molecular beam epitaxy technique. Orientational relationships on Si(1 1 1) have been determined. We characterized the films by Rutherford Backscattering and Channeling, X-ray diffraction and transmission electron microscopy.     

Structural characterization of Ga-doped Mg0.1Zn0.9O layers grown on ZnO/α-Al2O3 templates by P-MBE

In this study, structural properties of epitaxial Ga-doped Mg_0.1Zn_0.9O layers grown on ZnO/α-Al₂O₃ templates by plasma-assisted molecular beam epitaxy have been investigated by high-resolution transmission electron microscopy (HRTEM), and high resolution X-ray diffraction (HRXRD). From analysis of the diffraction pattern, the monocrystallinity of the Mg_0.1Zn_0.9O layer with hexagonal structure is confirmed. The orientation relationship between Mg_0.1Zn_0.9O and the template is determined as (0 0 0 1)_Mg0.1Zn0.9O(0 0 0 1)_ZnO(0 0 0 1)_Al2O3 and [ [ ]_ZnO[ . The density of dislocations near the top surface layers measured by plan-view TEM is about 3.610¹⁰ cm⁻², one order of magnitude higher than the value obtained for ZnO layers on α-Al₂O₃ with a MgO buffer. Cross-sectional observation revealed that the majority of threading dislocations are in the [0 0 0 1] line direction, i.e. they lie along the surface normal and consist of edge, screw, and mixed dislocations. Cross- sectional TEM and X-ray rocking curve experiments reveal that most of dislocations are edge dislocations. The interface of Mg_0.1Zn_0.9O and ZnO layers and the effect of excess Ga-doping in these layers have been also studied.     

Crystal growth of SrTiO3 films on H-terminated Si(1 1 1) with SrO buffer layers

Growth of epitaxial SrTiO₃ (STO) films has been examined on H-terminated Si(1 1 1) with SrO buffer layers. The epitaxial SrO buffer layers have reduced stress on H-terminated Si substrates. On the SrO buffer layers, the STO films grow epitaxially with triple domains at low temperature. Each STO domain has equivalent epitaxial relationship to SrO buffer layers, STO(1 1 0)∥SrO(1 1 1) and .     

High-resolution X-ray diffraction analysis of SnTe/Sn1−xEuxTe superlattices grown on (1 1 1) BaF2 substrates

A set of SnTe/Sn_1−xEu_xTe superlattice (SL) samples with increasing nominal Eu content x up to 0.28 was successfully grown on (1 1 1) BaF₂ substrates by molecular beam epitaxy. A complete structural characterization was performed by triple-axis X-ray diffractometry and reciprocal space mapping. The X-ray results showed that, despite the phase separation that normally occurs for unstrained Sn_1−xEu_xTe layers with x0.02, an SL stack with homogeneous individual layers can be formed for SL samples with a nominal Eu content up to 0.16. No SL satellite peak structure could be identified for samples with x values higher than 0.24. The structural parameters of the individual layers that compose the SL were determined using a best-fit simulation procedure which compared the calculated X-ray spectra to the measured (2 2 2) ω/2Θ scans. The strain information used in the simulation was obtained from the reciprocal space maps measured around the (2 2 4) lattice point.     

Band offsets at the epitaxial anatase TiO2/n-SrTiO3(0 0 1) interface

We have used high-energy resolution X-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO₂(0 0 1)/n-SrTiO₃(0 0 1) heterojunction prepared by molecular beam epitaxy. The valence band offsets range between −0.06 ± 0.05 and +0.16 ± 0.05 eV for anatase thicknesses between 1 and 8 monolayers and three different methods of substrate surface preparation, with no systematic dependence on film thickness. The conduction band offset (CBO) varies over a comparable range by virtue of the fact that anatase and SrTiO₃ exhibit the same bandgap (∼3.2 eV). In contrast, density functional theory predicts the VBO to be +0.55 eV. The lack of agreement between theory and experiment suggests that either some unknown factor in the interface structure or composition excluded from the modeling is influencing the band offset, or that density functional theory cannot accurately calculate band offsets in these oxide materials. The small experimental band offsets have important implications for the use of this interface for fundamental investigations of surface photocatalysis. Neither electrons nor holes are likely to become trapped in the substrate and thus be unable to participate in surface photocatalytic processes.     

Cu-MgF2复合纳米金属陶瓷薄膜的电导特性研究

用射频磁控共溅射法制备了Cu体积分数分别为10%，15%，20%和30%的Cu-MgF₂复 合金属陶 瓷薄膜.用x射线衍射、x射线光电子能谱和变温四引线技术对薄膜的微结构、组分及电导特 性进行了测试分析.微结构分析表明：制备的Cu-MgF₂复合薄膜由fcc-Cu晶态纳 米微粒镶嵌 于主要为非晶态的MgF₂陶瓷基体中构成，Cu晶粒的平均晶粒尺寸随组分增加从1 1.9nm增 至17.8nm.50—300K温度范围内的电导测试结果表明：当Cu体积 关键词： 2复合纳米金属陶瓷膜')" href="#">Cu-MgF₂复合纳米金属陶瓷膜 微结构 组分 电导特性 激活能 渗透阈