Effect of Pb adatom flux rate on adlayer coverage for Stranski-Krastanov growth mode on Si(1 1 1)7 × 7 surface

Lead (Pb) has been a prototypical system to study diffusion and reconstruction of silicon surfaces. However, there is a discrepancy in literature regarding the critical coverage at which island formation takes place in the Stranski-Krastanov (S-K) mode. We address this issue by studying the initial stages of evolution of the Pb/Si(1 1 1)7 × 7 system by careful experiments in ultra-high vacuum with in situ characterization by auger electron spectroscopy, electron energy loss spectroscopy and low-energy electron diffraction. We have adsorbed Pb onto clean Si(1 1 1 )7 × 7 surface with sub-monolayer control at different flux rates of 0.05 ML/min, 0.14 ML/min and 0.22 ML/min, at room temperature. The results clearly show that the coverage of the Pb adlayer before the onset of 3D Pb islands in the S-K mode depends on the flux rates. LEED results show the persistence of the (7 × 7) substrate reconstruction until the onset of the island formation, while EELS results do not show any intermixing at the interface. This suggests that the flux rates influence the kinetics of growth and the passivation of dangling bonds to result in the observed rate-dependent adlayer coverages.     

Effect of H2O chemisorption on passivation of Ge(100) surface studied by scanning tunneling microscopy

The electronic passivation of a Ge(100) surface, via the chemisorption of H₂O at room temperature (RT), and the temperature dependence of H₂O coverage were investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). With a saturation H₂O dose at RT, a highly-ordered structure, due to the dissociative chemisorption of H₂O, was observed on a Ge(100) surface with a coverage of 0.85 monolayers (ML). Annealing the room temperature H₂O-dosed Ge surface to 175 °C decreased the coverage of H₂O to 0.6 ML. Further annealing at 250 °C decreased the coverage of H₂O sites to 0.15 ML, and the surface reconstruction of Ge dimers was observed over much of the surface. Annealing above 300 °C induced Ge suboxide structures, similar to the oxygen-dosed Ge surface. STS measurements confirmed that the surface dangling bond states near Fermi energy are removed by the H₂O chemisorption because the dangling bonds of Ge atoms are terminated by ―OH and ―H. The H₂O pre-dose at room temperature provides a template for the ultrathin passivation of Ge(100) surface via atomic layer deposition (ALD) at RT, since near monolayer nucleation can be obtained with a 1/2 hydroxylated and 1/2 hydrogenated Ge surface.     

Structure and electronic properties of cleaved Si(111) upon Ge adsorption

The effect of ultrahigh vacuum deposition of Ge below and at monolayer coverage onto clean cleaved Si(111) surface held at room temperature is studied by low energy electron diffraction, Auger electron specroscopy and photoemission yield spectroscopy. A well ordered $\text{3}\text{×}\text{3}$ R 30° structure developes at $\text{1}\text{3}$ ML, where it replaces the 2 × 1 initial pattern; it persists at $\text{2}\text{3}$ ML before transforming into a 1 × 1 diagram which fades into increasing background at 1 ML and up. Si surface dangling bonds are replaced at $\text{1}\text{3}$ ML by states associated with Ge-Si bonds and Ge dangling bonds to which states due to Ge-Ge bonds added upon increasing coverage.     

The interaction of carbon with Si(1 1 1):As and Si(1 1 1):H surfaces, a theoretical study

Density functional theory calculations have been performed to determine the adsorption site of carbon at the Si(1 1 1):As and Si(1 1 1):H surfaces at different coverages. The As- and H-passivated surfaces were simulated by replacing the topmost Si layer by As or by saturating the Si dangling bonds with hydrogen atoms, respectively. Different high symmetry sites were considered. Carbon was placed successively in the fourfold (T₄) or threefold coordinated (H₃), the ontop (T₁) sites or substituted for a Si atom in the S₅ position located underneath the Si adatom in the T₄ site. We found that the preferred carbon adsorption site depends on the coverage of the passivated surfaces. At low coverages i.e. at 1/16 ML and 1/3 ML, it prefers a distorted T₄ position whereas at 1 ML, it occupies an H₃ site. This contrasts with the clean surface where the most energetically favored site is the S₅ at all coverages. Carbon adsorption induces a significant change in the structural geometry of the surface atoms, leading to a charge re-arrangement in the surface layers.     

Variation of passivation behavior induced by sputtered energetic particles and thermal annealing for ITO/SiO_x/Si system

The damage on the atomic bonding and electronic state in a SiO_x(1.4-2.3 nm)/c-Si(150 μm) interface has been investigated.This occurred in the process of depositing indium tin oxide(ITO) film onto the silicon substrate by magnetron sputtering.We observe that this damage is caused by energetic particles produced in the plasma(atoms,ions,and UV light).The passivation quality and the variation on interface states of the SiO_x/c-Si system were mainly studied by using effective minority carrier lifetime(τ_(eff)) measurement as a potential evaluation.The results showed that the samples' τ_(eff)was reduced by more than 90%after ITO formation,declined from 107 μs to 5 μs.Following vacuum annealing at 200 ℃,the τ_(eff) can be restored to 30 μs.The components of Si to O bonding states at the SiO_x/c-Si interface were analyzed by x-ray photoelectron spectroscopy(XPS) coupled with depth profiling.The amorphous phase of the SiO_x layer and the "atomistic interleaving structure" at the SiO_x/c-Si interface was observed by a transmission electron microscope(TEM).The chemical configuration of the Si-O fraction within the intermediate region is the main reason for inducing the variation of Si dangling bonds(or interface states) and effective minority carrier lifetime.After an appropriate annealing,the reduction of the Si dangling bonds between SiO_x and near the c-Si surface is helpful to improve the passivation effect.     

密度泛函理论方法研究6H-SiC(0001)表面对氧分子和水分子的吸附

本文通过密度泛函方法计算6H-SiC(0001)表面对氧分子和水分子的吸附. 在6H-SiC(0001)表面上吸附的O₂分子自发地解离成O^*,并被吸收在C与Si原子之间的空位上. 吸附的H₂O自发地分解成OH^*和H^*,它们都被吸附在Si原子的顶部,OH^*进一步可逆地转化为O^*和H^*. H^*可以使Si悬键饱和并改变O^*的吸附类型,并进一步稳定6H-SiC(0001)表面并防止其转变为SiO₂.     

Interface modification and trap-type transformation in Al-doped CdO/Si-nanowire arrays/p-type Si devices by nanowire surface passivation

The present work reports the fabrication and detailed electrical properties of Al-doped CdO/Si-nanowire (SiNW) arrays/p-type Si Schottky diodes with and without SiNW surface passivation. It is shown that the interfacial trap states influence the electronic conduction through the device. The experimental results demonstrate that the effects of the dangling bonds at the SiNW surface and Si vacancies at the SiO_x/SiNW interface which can be changed by the Si–O bonding on the energy barrier lowering and the charge transport property. The induced dominance transformation from electron traps to hole traps in the SiNWs by controlling the passivation treatment time is found in this study.     

Interface state density and barrier height improvement in ammonium sulfide treated Al2O3/Si interfaces

The HF treatment removes the native oxide and lays behind the dangling bonds over the Si surface which causes the increment in density of interface traps (D_it) through the direct deposition of high-k dielectric on Si. Here, we propose the facile method for reduction of interface traps and improvement in barrier height with the (NH₄)₂S treatment on Al₂O₃/Si interfaces, which can be used as the base for the non-volatile memory device. The AFM was used to optimize the treatment time and surface properties, while XPS measurements were carried out to study the interface and extract the barrier height (Φ_B). The short period of 20 s treatment shows the improvement in the barrier height (1.02 eV), while the one order reduction in the D_it (0.84 × 10¹² cm²/eV) of sulfur passivated Al/Al₂O₃/Si MOS device. The results indicate the favorable passivation of the dangling bonds over the Si surfaces covered by sulfur atoms.     

Effect of surface reconstruction on the adsorption of Ge on clean Si(111)

The effect of ultrahigh vacuum deposition of Ge below and at monolayer (ML) coverage onto a 7 × 7 reconstructed clean Si(111) surface held at room temperature is studied by low energy electron diffraction (LEED), Auger electron spectroscopy (AES) and photoemission yield spectroscopy (PYS). The results are compared to those obtained on 2 × 1 reconstructed clean Si(111) : (i) the Si dangling bond states are replaced by Ge dangling bond states at submonolayer coverages in both cases; (ii) the 7 × 7 reconstruction persists below 1 ML, it is not replaced by a ? 3 × ? 3 R30° at 1/3ML as it was on the 2 × 1; and (iii) the coverage below 1 ML is not homogeneous on the 7 × 7 reconstruction. This behaviour can be explained by the influence of the inhomogeneties associated with the 7 × 7 reconstruction.     

Effect of deuterium on passivation of Si surfaces

A properly passivated silicon surface is chemically stable, and all interface properties are constant. The freshly etched Si surface is full of dangling bonds resulting in high surface activity and instability. Hydrogen treatment is a proper procedure to decrease the number of dangling bonds. We demonstrated that deuterium adsorbs on Si surface at room temperature much stronger than hydrogen. Moreover, in case of deuterium-passivated wafers the vacuum storage can be omitted without risking the non-controlled native oxidation of silicon for up to 5 h or more. It could be a suitable and more robust surface cleaning and passivation process for the industry, but heavy water is expensive. As a cheaper procedure, we showed that 1 min vapor phase treatment at 65 °C of heavy water (D₂O) + 50% HF (e.g. 20:1) mixture was enough to remove the native oxide and to passivate the Si surface without any degradation of the atomic surface flatness. The surface evolution of the D-passivated surface was followed by contact angle measurements, spectroscopic ellipsometry (SE), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), infrared absorption spectroscopy (IR), surface potential and surface photovoltage (SPV) mapping and light induced potential transient method. Qualification and the results were compared to the H-passivated, bare and native oxide covered Si surface. Our passivation test results confirmed that using D-passivation on Si surface is a promising method in the MOS technology and the interface engineering processes.     

石墨烯过渡层对金属/SiC接触肖特基势垒调控的第一性原理研究

由于SiC禁带宽度大,在金属/SiC接触界面难以形成较低的势垒,制备良好的欧姆接触是目前SiC器件研制中的关键技术难题,因此,研究如何降低金属/SiC接触界面的肖特基势垒高度(SBH)非常重要.本文基于密度泛函理论的第一性原理赝势平面波方法,结合平均静电势和局域态密度计算方法,研究了石墨烯作为过渡层对不同金属(Ag,Ti,Cu,Pd,Ni,Pt)/SiC接触的SBH的影响.计算结果表明,单层石墨烯可使金属/SiC接触的SBH降低;当石墨烯为2层时,SBH进一步降低且Ni,Ti接触体系的SBH呈现负值,说明接触界面形成了良好的欧姆接触;当石墨烯层数继续增加,SBH不再有明显变化.通过分析接触界面的差分电荷密度以及局域态密度,SBH降低的机理可能主要是石墨烯C原子饱和了SiC表面的悬挂键并降低了金属诱生能隙态对界面的影响,并且接触界面的石墨烯及其与金属相互作用形成的混合相具有较低的功函数.此外,SiC/石墨烯界面形成的电偶极层也可能有助于势垒降低.     

Suppression of silicon (001) surface reactivity using a valence-mending technique

It is experimentally shown that, by terminating dangling bonds on Si(001) with a monatomic layer of selenium, the chemical reactivity of the surface is suppressed. In the case of nickel silicidation, transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy reveal that Se passivation suppresses Ni silicidation by over 100 °C as compared to the bare Si(001) surface. The formation of Ni subsilicide (Ni₂Si) is not observed on Se-passivated Si(001). This interfacial silicidation appears to be linked with changes in electrical behavior of the interface between titanium and Se-passivated Si(001), which we reported previously.     

石墨烯过渡层对金属/SiC接触肖特基势垒调控的第一性原理研究

由于SiC禁带宽度大,在金属/SiC接触界面难以形成较低的势垒,制备良好的欧姆接触是目前SiC器件研制中的关键技术难题,因此,研究如何降低金属/SiC接触界面的肖特基势垒高度(SBH)非常重要.本文基于密度泛函理论的第一性原理赝势平面波方法,结合平均静电势和局域态密度计算方法,研究了石墨烯作为过渡层对不同金属(Ag,Ti,Cu,Pd,Ni,Pt)/SiC接触的SBH的影响.计算结果表明,单层石墨烯可使金属/SiC接触的SBH降低;当石墨烯为2层时,SBH进一步降低且Ni,Ti接触体系的SBH呈现负值,说明接触界面形成了良好的欧姆接触;当石墨烯层数继续增加,SBH不再有明显变化.通过分析接触界面的差分电荷密度以及局域态密度,SBH降低的机理可能主要是石墨烯C原子饱和了SiC表面的悬挂键并降低了金属诱生能隙态对界面的影响,并且接触界面的石墨烯及其与金属相互作用形成的混合相具有较低的功函数.此外,SiC/石墨烯界面形成的电偶极层也可能有助于势垒降低.     

表面悬挂键对GaAs纳米线掺杂的影响及其钝化

利用第一性原理计算方法研究了表面悬挂键对GaAs纳米线掺杂的影响及其钝化．计算结果显示,不论是闪锌矿结构还是纤锌矿结构,GaAs纳米线表面Ga原子上带正电荷的表面悬挂键都是一类稳定的缺陷,并且这种稳定性不会随着纳米线直径的变化而变化．这种表面悬挂键会形成载流子陷阱中心从而从p型掺杂的GaAs纳米线俘获空穴,使得纳米线的掺杂效率下降．和NH₃相比,NO₂ 具有足够的电负性来俘获GaAs纳米线表面悬挂键上的未配对电子,从而有效地钝化GaAs纳米线的表面悬挂键,提高纳米线的p型掺杂效率,并且这种钝化特性不会随着纳米线直径的变化而改变．     

Electronic structure of ultrathin Cs coatings on a Si(100)2×1 surface

The electronic structure and ionization energy of submonolayer Cs coatings on a Si(100)2×1 surface is investigated by threshold photo-emission spectroscopy. Two surface bands induced by Cs adsorption are observed, and their evolution is studied as a function of coverage. It is found that there are two “adsorption locations” for Cs atoms, where they interact with active dangling bonds at the surface. It has been determined that the Cs/Si(100)2×1 interface is semiconducting all the way down to monolayer coverage. The results show that Cs adsorption is predominantly of a covalent character. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 9, 699–702 (10 May 1997)     

Band bending at the Ni/Si(100)-2×1 submonolayer interface

Band bending at the Ni/Si(100)-2×1 interface has been monitored by using Si 2p core level photoemission spectra. Two nickel-induced Si 2p components appear in the initial interaction between Ni and Si(100)-2×1, which is confined at the top surface and the first subsurface layers. At Ni coverage less than 0.0375 ML, Ni atoms prefer the adamantane interstitial sites on the first subsurface, but switch to the pedestal sites on Si dimer rows at higher Ni coverage. The change in the preferred occupation sites of Ni atoms on the Si(100)-2×1 surface strongly affects the amount of band bending shift. The shift towards higher binding energy, when Ni atoms occupy the adamantane interstitial sites, is attributed to metal-induced-gap states. While Ni atoms occupy the pedestal sites, the band bending shift is reduced which is attributed to the passivation of surface states.     

6H-SiC(0001)表面graphene逐层生长的分子动力学研究

利用经典分子动力学方法和模拟退火技术分析研究了6H-ＳiC（0001）表面graphene的逐层生长过程及其形貌结构特点．研究表明,经过高温蒸发表面硅原子后,6H-ＳiC（0001）表面的碳原子能够通过自组织过程生成稳定的局部单原子层graphene结构．这种过程类似于6H-ＳiC（0001）表面graphene的形成,其生长和结构形貌演化主要取决于退火温度和表面碳原子的覆盖程度． 研究发现,当退火温度高于1400K时,6H-ＳiC（0001）表面碳原子能形成局部的单原子层graphene结构．这一转变温 关键词： graphene 碳化硅 分子动力学     

A STUDY ON ABSORPTION OF Na ATOMS ON Si(100) 2×1 SURFACES WITH DV-Xα METHOD

The Na absorption on Si(100) 2×1 surface is studied with quantum chemistry molecular cluster method. The calculated results show that the most favourable absorption site of Na is the cave site and the charge transfer of Na atom to Si is large when the Na coverage is smaller than 0.5 monolayer (ML). A Na chain is formed along the cave sites at the 0.5 ML Na coverage, the charge transfer then becomes small. The calculated density of states show that the Na atoms are metallic along the chain. At 1 ML coverage, the Na atoms occupy both the cave and pedestal sites and form a double-layer. There is a charge transfer of 0.5e from each Na atom to the Si surface. The calculated surface energy shows that the saturation absorption of Na on Si surface is 1 ML.     

Ab initio calculations of surface structure and electronic properties caused by adsorption of Ca atoms on a Si(1 1 0) surface

The adsorption of Ca metals onto a Si(1 1 0) surface has been theoretically investigated by first-principle total-energy calculations. We employed a local density approximation of the density functional theory as well as a pseudopotential theory to study the atomic and electronic properties of the Ca/Si(1 1 0) structure. The (1×1) and (2×1) surface structures were considered for Ca coverages of 0.5 and 0.25 ML, respectively. It is found that the (1×1) phase is not expected to occur even for rich Ca regime. It was found that Ca adatoms are adsorbed on top of the surface and form a bridge with the uppermost Si atoms. The most stable structure of Ca/Si(1 1 0)-(2×1) surface produces a semiconducting surface band structure with a direct band gap that is slightly smaller than that of the clean surface. We have observed one filled and two empty surface states in the gap region. These empty surface states originated from the uppermost Si dangling bond states and the Ca 4s states. Furthermore, the Ca-Si bonds have an ionic nature with almost complete charge transfer from Ca to the surface Si atoms. The structural parameters of the ground state atomic configuration are detailed and compared with the available results of metal-adsorbed Si(1 1 0) surface, Ca/Si(0 0 1), and Ca/Si(1 1 1) structures.     

A STUDY ON ABSORPTION OF Na ATOMS ON Si(100) 2×1 SURFACES WITH DV-Xα METHOD

The Na absorption on Si(100) 2×1 surface is studied with quantum chemistry molecular cluster method. The calculated results show that the most favourable absorption site of Na is the cave site and the charge transfer of Na atom to Si is large when the Na coverage is smaller than 0.5 monolayer (ML). A Na chain is formed along the cave sites at the 0.5 ML Na coverage, the charge transfer then becomes small. The calculated density of states show that the Na atoms are metallic along the chain. At 1 ML coverage, the Na atoms occupy both the cave and pedestal sites and form a double-layer. There is a charge transfer of 0.5e from each Na atom to the Si surface. The calculated surface energy shows that the saturation absorption of Na on Si surface is 1 ML.