Features of atomic processes at the formation of a wetting layer and nucleation of three-dimensional Ge islands on Si(111) and Si(100) surfaces

The intermediate stages of the formation of a Ge wetting layer on Si(111) and Si(100) surfaces under quasiequilibrium grow conditions have been studied by means of scanning tunneling microscopy. The redistribution of Ge atoms and relaxation of mismatch stresses through the formation of surface structures of decreased density and faces different from the substrate orientation have been revealed. The sites of the nucleation of new three-dimensional Ge islands after the formation of the wetting layer have been analyzed. Both fundamental differences and common tendencies of atomic processes at the formation of wetting layers on Si(111) and Si(100) surfaces have been demonstrated. The density of three-dimensional nuclei on the Si(111) surface is determined by changed conditions for the surface diffusion of Ge adatoms after change in the surface structure. Transition to three-dimensional growth on the Si(100) surface is determined by the nucleation of single {105} faces on the rough Ge(100) surface.     

Dynamics of bimodal growth in pentacene thin films

Previous studies have established that pentacene films deposited on silicon oxide consist of a substrate-induced "thin-film" phase, with the bulk phase of pentacene detected in thicker films only. We show that the bulk phase nucleates as early as the first monolayer, and continues to nucleate as film growth progresses, shadowing the growth of the thin-film phase. Moreover, we find that the transition between the "thin-film" and the bulk phase is not a continuous one, as observed in heteroepitaxial systems, but rather the two phases nucleate and grow independently.     

The interplay of topography and energy dissipation in pentacene thin films

Nonlinear photoemission electron microscopy was used to study the morphology-dependent lifetime of electronic excitations in pentacene islands on Si(0 0 1) and (√3 × √3)R30°-Ag/Si(1 1 1). After an optical excitation of electrons by a λ = 400 nm femtosecond laser pulse the characteristic decay times were measured with spatial resolution in a pump-probe setup. For pentacene on Si(0 0 1), the observed lifetimes vary by a factor of two between the wetting layer and the fractal-shaped pentacene islands. The measured lifetime difference is explained by a difference in the electronic coupling of the pentacene islands and the wetting layer to the substrate. For pentacene on (√3 × √3)R30°-Ag/Si(1 1 1), similar lifetimes are found, although the orientation of the pentacene molecules in the compact islands is rotated. Our findings suggest that electronic excitations in higher layers of the pentacene islands do not diffuse to the interface before they decay.     

Dynamics of wetting transitions: A time-dependent Ginzburg-Landau treatment

The dynamic behavior at wetting transitions is studied for systems with short-range forces and nonconserved order parameter. From a continuum limit of a purely relaxational lattice model in mean-field approximation, a time-dependent Ginzburg-Landau equation with a time-dependent boundary condition at the surface is derived in the long wavelength approximation. The dynamics of relaxation close to stable and metastable states is treated in linear response. A divergence of the relaxation time occurs both for critical wetting and along the surface spinodal lines (in the case of first-order wetting), although the static surface layer susceptibilities ₁, ₁₁ stay finite at the surface spinodal in the non-wet region of the phase diagram.Also the highly nonlinear relaxation that occurs when a wetting layer forms out of an initially non-wet state is considered. For late times, the thickness of the wetting layer grows proportional to the logarithm of time. A comparison with recent Monte Carlo work shows that the present mean-field theory underestimates the prefactor in this growth law. For early times and states in the metastable region a distance H ₁ away from the first order wetting transition, the formation of the wet layer starts by heterogeneous nucleation of droplets at the surface. The droplets have the shape of (approximately) caps of a sphere and involve a free energy barrier proportional to (H ₁)^–2 as H ₁0. The generalization of this phenomenological approach for the nucleation barrier to the case of long range forces is also discussed and open problems are briefly outlined.     

Structural and interfacial stabilities of epitaxial (11-20)-oriented wurtzite AlN films grown on lattice-matched MnS buffered Si(100)

Epitaxial growth of non-polar wurtzite (11-20) AlN thin films was achieved on a Si(100) substrate by inserting an MnS buffer layer. The a-plane AlN film and MnS buffer layer were fabricated by pulsed KrF excimer laser deposition, and their micro- and interfacial atomic structures were investigated by transmission electron microscopy. The epitaxial relationship between films and substrate was found to be AlN(11-20)MnS(100)Si(100) with in-plane alignment of AlN[1-101]MnS[011]Si[011]. AlN[11-20] grown on Si is perpendicular to AlN[0001] and parallel to MnS[100]. The MnS/Si interface is abrupt enough to inherit the orientation of the Si(100) surface. A sharp interface was also observed for AlN/MnS without any intermediate layer. PACS 81.05.Ea; 81.05.Zx; 81.15.Fg; 68.37.Lp     

脉冲激光原位辐照对InAs/GaAs(001)量子点生长的影响

在InAs/GaAs(001)量子点生长过程中, 当InAs沉积量为0.9 ML时, 利用紫外纳秒脉冲激光辐照浸润层表面, 由于高温下In原子的不稳定性, 激光诱导的原子脱附效应被放大, 样品表面出现了原子层移除和纳米孔. 原子力显微镜测试表明纳米孔呈现以[110]方向为长轴(尺寸: 20-50 nm)、[110]方向为短轴(尺寸: 15-40 nm)的表面椭圆开口形状, 孔的深度为0.5-3 nm. 纳米孔的密度与脉冲激光的能量密度正相关. 脉冲激光的辐照对量子点生长产生了显著的影响: 一方面由于纳米孔的表面自由能低, 沉积的InAs优先迁移到孔内, 纳米孔成为量子点优先成核的位置; 另一方面, 孔外的区域因为In原子的脱附, 量子点的成核被抑制. 由于带有纳米孔的浸润层表面具有类似于传统微纳加工技术制备的图形衬底对量子点选择性生长的功能, 该研究为量子点的可控生长提供了一种新的思路.     

Pulsed laser deposition of c * axis oriented pentacene films

Pulsed laser desorption and film deposition behaviors have been investigated on pentacene as an organic molecule primarily due to its applications in electronics. The laser desorption time-of-flight (LDTOF) mass spectrum exhibited a single parent peak when a pressed pentacene pellet was ablated by an N₂ laser beam of its fluence lower than 100 mJ/cm², indicating that pentacene could be evaporated without an appreciable photodecomposition by the pulsed laser beam. Nd:YAG pulsed laser deposition of pentacene films was performed using such optimization parameters as laser fluences and wavelength (second, third and forth harmonic generations (SHG, THG, FHG)). The analyses with AFM, XRD and UV-Vis spectroscopy revealed the fabrication of c^* axis oriented pentacene films on quartz, silicon, and CaF₂ substrates by the SHG. The SHG films have a surface morphology superior to those of films deposited by THG and FHG. PACS 81.15.Fg; 81.05.Lg; 82.80.Rt     

Surface and interface properties of thin pentacene and parylene layers

To improve Organic Thin Film Transistor (OTFT) properties we study OTFT semiconductor/dielectric interfacial properties via examination of the gate dielectric using thin Parylene C layer. Structural and morphology properties of pentacene layers deposited on parylene layer and SiO₂/Si substrate structure were compared. The surface morphology was investigated using atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM topography of pentacene layer in non-contact mode confirmed the preferable pentacene grain formation on parylene surface in dependence on layer thickness. The distribution of chemical species on the surfaces and composition depth profiles were measured by secondary ion mass spectroscopy (SIMS) and surface imaging. The depth profiles of the analyzed structures show a homogenous pentacene layer, characterized with C or C₂ ions. Relatively sharp interface between pentacene and parylene layers was estimated by characteristic increased intensity of CCl ions peak. For revealing the pentacene phases in the structures the Micro-Raman spectroscopy was utilized. Conformal coatings of parylene and pentacene layers without pinholes resulted from the deposition process as was confirmed by SIMS surface imaging. For the pentacene layers thicker than 20 nm, both thin and bulk pentacene phases were detected by Micro-Raman spectroscopy, while for the pentacene layer thickness of 5 and 10 nm the preferable thin phase was detected. The complete characterisation of pentacene layers deposited on SiO₂ and parylene surface revealed that the formation of large grains suggests 3D pentacene growth at parylene layer with small voids between grains and more than one monolayer step growth. The results will be utilized for optimization of the deposition process.      

Alternating layer and island growth of Pb on Si by spontaneous quantum phase separation

Real-time in situ x-ray studies of continuous Pb deposition on Si(111)-(7x7) at 180 K reveal an unusual growth behavior. A wetting layer forms first to cover the entire surface. Then islands of a fairly uniform height of about five monolayers form on top of the wetting layer and grow to fill the surface. The growth then switches to a layer-by-layer mode upon further deposition. This behavior of alternating layer and island growth can be attributed to spontaneous quantum phase separation based on a first-principles calculation of the system energy.     

Real-time in situ spectroscopic ellipsometry investigation of the amorphous to crystalline phase transition in Mo single layers

Molybdenum single layers were grown by ion beam sputter deposition onto [001] Si substrates. Argon or xenon was used as sputter gas. The layer growth was monitored by real-time in situ spectroscopic ellipsometry in the visible spectral region. A volume phase transition from amorphous to polycrystalline molybdenum layer growth was observed during the deposition process. The time regime of the phase transition as well as the layer thickness at which the phase transition occurs, depends on the sputter regime, especially on sputter species and deposition-pressure range. The thermodynamic approach of energy minimisation is discussed as the driving force for the Mo phase transition. A moderate backscattered particle bombardment of the growing molybdenum film provides the activation energy for the recrystallisation process. A self-diffusion-like process is made responsible for atomic rearrangement of the entire as-deposited thin-film volume. The molybdenum phase transition is connected to thin-film densification and therefore volume contraction. PACS 68.65.Ac; 81.15.Cd; 78.67.Pt     

“In vivo” STM studies of the growth of Germanium and Silicon on Silicon

A scanning tunneling microscope (STM) capable of imaging during crystal growth from the vapour is described. This method (MBSTM) opens the possibility to follow the growth process of semiconductor molecular beam epitaxy (MBE) in vivo. The ability of the microscope to access the evolution of specific features during growth is demonstrated by images of the Si homoepitaxy. The transition from initial multilayer to layer-by-layer growth was imaged in Si(1 1 1) homoepitaxy. In Si/Si(1 0 0) homoepitaxy the fractional coverage of non-equivalent terraces was studied as function of coverage and a theoretically predicted transient growth mode was observed. In Ge on Si(1 1 1) heteroepitaxy the nucleation of 3D Ge islands was observed. When 3D islands occurred on the surface an etching of the 2D Stranski-Krastanov layer was observed.     

Surface mobility difference between Si and Ge and its effect on growth of SiGe alloy films and islands

Based on first-principles calculations of surface diffusion barriers, we show that on a compressive Ge(001) surface the diffusivity of Ge is 10(2)-10(3) times higher than that of Si in the temperature range of 300 to 900 K, while on a tensile surface, the two diffusivities are comparable. Consequently, the growth of a compressive SiGe film is rather different from that of a tensile film. The diffusion disparity between Si and Ge is also greatly enhanced on the strained Ge islands compared to that on the Ge wetting layer on Si(001), explaining the experimental observation of Si enrichment in the wetting layer relative to that in the islands.     

Role of surface electronic structure in thin film molecular ordering

We show that the orientation of pentacene molecules is controlled by the electronic structure of the surface on which they are deposited. We suggest that the near-Fermi level density of states above the surface controls the interaction of the substrate with the pentacene pi orbitals. A reduction of this density as compared to noble metals, realized in semimetallic Bi(001) and Si(111)(5 x 2)Au surfaces, results in pentacene standing up. Interestingly, pentacene grown on Bi(001) is highly ordered, yielding the first vertically oriented epitaxial pentacene thin films observed to date.     

Heteroepitaxial praseodymium sesquioxide films on Si(1 1 1): A new model catalyst system for praseodymium oxide based catalysts

The structure, growth and stoichiometry of heteroepitaxial Pr₂O₃ films on Si(1 1 1) were characterized by a combined RHEED, XRD, XPS and UPS study in view of future applications as a surface science model catalyst system. RHEED and XRD confirm the growth of a (0 0 0 1) oriented hexagonal Pr₂O₃ phase on Si(1 1 1), matching the surface symmetry by aligning the oxide in-plane direction along the Si azimuth. After an initial nucleation stage RHEED growth oscillation studies point to a Frank-van der Merwe growth mode up to a thickness of approximately 12 nm. XPS and UPS prove that the initial growth of the Pr₂O₃ layer on Si up to ∼1 nm thickness is characterized by an interface reaction with Si. Nevertheless stoichiometric Pr₂O₃ films of high crystalline quality form on top of these Pr-silicate containing interlayers.     

Effect of modifying a methyl siloxane-based dielectric by a polymer thin film for pentacene thin-film transistors

We report on the fabrication of pentacene thin-film transistors (TFTs) utilizing a spun methyl siloxane-based spin-on-glass (SOG) dielectric and show that these devices can give a similar electrical performance as achieved by using pentacene TFTs with a silicon dioxide (SiO₂) dielectric. To improve the electrical performance of pentacene TFTs with the SOG dielectric, we employed a hybrid dielectric of an SOG/cross-linked poly-4-vinylphenol (PVP) polymer. The PVP film was deposited onto the spun SOG dielectric prior to pentacene evaporation, resulting in an improvement of the saturation field effect mobility (μ_sat) from 0.01 cm²/(V s) to 0.76 cm²/(V s). The good surface morphology and the matching surface energy of the SOG dielectric that was modified with the polymer thin film allow the optimized growth of crystalline pentacene domains whose nuclei are embedded in an amorphous phase.     

内嵌CuO薄膜对并五苯薄膜晶体管性能的改善

制备了基于内嵌氧化物铜(CuO)薄膜的并五苯薄膜晶体管器件. 将3 nm CuO薄膜内嵌入到并五苯(pentacene)中, 作为空穴注入层, 降低电极与并五苯之间的空穴注入势垒. 相对于纯并五苯薄膜晶体管器件, 研制的晶体管的迁移率、阈值电压(V_TH)、电流开关比(I_on/I_off) 等参数都有明显改善. X射线光电子能谱数据表明, 这种空穴注入势垒的降低源自并五苯向CuO的电子转移.     

Delayed phase separation in growth of organic semiconductor blends with limited intermixing

Binary mixed thin films of picene (C₂₂H₁₄, PIC) and pentacene (C₂₂H₁₄, PEN) consist of crystallites with a statistical occupation of the lattice sites by either PEN or PIC and unit cell parameters continuously changing with the mixing ratio. For high PIC ratios a PIC phase forms which corresponds to a limited intermixing of the two compounds. The growth behavior of these mixtures is investigated in situ and in real‐time using grazing incidence X‐ray diffraction. We observe a delayed phase separation in PIC‐rich blends, i.e. complete intermixing in the monolayer range and the nucleation of a pure PIC‐phase in addition to the intermixed phase starting from the second monolayer.

Growth scenario of picene‐rich pentacene‐picene blends.     

Preparation of single-crystal-like MgO films on Si and orientation control of platinum films on MgO/Si

Growth of a Pt/MgO bilayer on Si(100) was investigated by pulsed-laser deposition. The growth modes of both MgO and platinum films are layer-by-layer growth, which were revealed by in situ reflection high energy electron diffraction observations. Two kinds of orientations of platinum films, viz. epitaxially (100) and (111)-oriented platinum films, were obtained on the same MgO(100)/Si(100) substrate only by varying the laser fluence. The effect of laser fluence on the orientation of platinum films is briefly discussed. The platinum films prepared in our experiments are epitaxially grown and exhibit atomic-scale surface flatness. It is believed that the improvement in the quality of platinum films can be attributed to the perfectly single-crystalline quality of the MgO buffer layer, which was further confirmed by the excellent dielectric properties. For a 150 nm thick MgO film, the leakage current density was found to be 10^-7 Acm^-2 with an electric field of 8×10⁵ Vcm^-1 and the relative dielectric constant (_r) was 10.6. PACS 68.55.Jk; 81.15.Fg; 85.50.Gk     

Theoretical investigation of Mn adsorbates aside self-organised Bi nanolines on hydrogenated Si(0 0 1) surface

We have theoretically investigated the atomic structure, magnetic behaviour, and electronic properties of Mn adsorbates on hydrogen passivated self-organised Bi nanolines on the Si(0 0 1)surface. It is found that the most stable geometry for monolayer (ML) coverage of Mn is just underneath the first Si(0 0 1) surface layer. The Mn atoms in the optimised configuration are seven-fold coordinated with their neighbouring Si atoms. Total energy calculations suggest that the Mn adsorbates form a degenerate state of ferromagnetic and anti-ferromagnetic lines parallel and adjacent to the self-assembled Bi lines. The density functional band structure calculation within the local-spin density approximation shows that the ferromagnetic system behaves like a metal in both spin channels. On the other side, the anti-ferromagnetic phase exhibits a half-metallic phenomenon with semiconducting character for the majority spin channel and semi-metallic character for the minority spin channel.     

The effects of ambient oxygen pressure on the 1.54 μm photoluminescence of Er-doped silicon-rich silicon oxide films grown by laser ablation of a Si+Er target

We fabricated Er-doped silicon-rich silicon oxide (SRSO:Er) films by pulsed laser deposition. A Si+Er target consisting of an Er metallic strip and a silicon disk was adopted with a goal to achieve a convenient control of the Er and oxygen density in the film. We found that the photoluminescence (PL) at 1.54 m is highly dependent on the ambient oxygen pressure, which determines the relative ratio of Si-Si, SiO_x, and SiO₂ phase in the film. The PL intensity increased drastically with increase in the annealing temperature and reached the maximum intensity at 500 °C. PACS 81.15.Fg; 81.07.-b