Surface alloying and anomalous diffusion of Bi on Cu(1 1 1)

We have used Low-Energy Electron Microscopy (LEEM) to study the formation and structure of the surface alloy of Bi on the Cu(1 1 1) surface. As a function of Bi coverage we find a progression from a dilute surface alloy to a structure, which coexists with a Bi overlayer when the Bi coverage exceeds 1/3 ML. At a temperature of 401 K, an unusual formation of alloy domains is observed. The origin of this effect is traced down to the mobility of Bi adatoms, which is found to strongly depend on the Bi coverage of the surface alloy on which they reside.     

Strain state in bcc Fe films grown on Si(1 1 1)

The strain state of Fe films grown on Si(1 1 1) has been investigated by X-ray diffraction (XRD) in the thickness range between 11 and 304 monolayers. Fe grows tetragonally distorted with the orientation relationship Fe(1 1 1) // Si(1 1 1) . At low coverage, the films grow pseudomorphic. Above 15 monolayers the films are characterized by the coexistence of a pseudomorphic phase with another one which relaxes with the Fe thickness. This relaxation proceeds rapidly in the earlier stages then slowly with the film thickness. The XRD characterization allows one to obtain quantitative information on the in-plane and out-of-plane strains.     

1 550 nm波长PNP型InGaAsP-InP异质结晶体管激光器材料设计与外延生长

基于器件模拟仿真,设计了一种PNP型1.5 μm 波长多量子阱InGaAsP-InP异质结晶体管激光器材料外延结构,并采用金属有机化学气相沉积外延生长.其中基区采用N型Si掺杂.因为扩散系数小,比较P型Zn搀杂具有较高的稳定性,因而较NPN结构外延材料容易获得高质量的光学有源区.由于N型欧姆接触比P型容易获得,基区搀杂浓度可以相对较低,有利于减小基区光损耗和载流子复合,从而获得较低的阈值电流和较高的输出光功率.所获得的外延材料呈现较高光-荧光谱峰值和65.1 nm较低半峰宽.测试结果显示了较高的外延片光学质量.     

Fundamental processes in Si/Si and Ge/Si epitaxy studied by scanning tunneling microscopy during growth

Experimental results on the epitaxy of Si and Ge on Si(0 0 1) and Si(1 1 1) surfaces, which are obtained by scanning tunneling microscopy (STM) imaging during growth, are reviewed. Techniques for simultaneous epitaxial growth and STM measurements at high temperature are described. The ability to access the evolution of the growth morphology during growth down to the atomic level enables the study of the influence of surface reconstruction on the growth. The relatively complete characterization of the growth process facilitates comparison to theoretical models and allows the identification of fundamental growth processes. For instance, the observed transition between different growth modes can be explained by specific growth processes included in a model. The influence of strain on the growth morphology is reviewed for the case of heteroepitaxial growth of Ge on Si. With the method of combining STM imaging and epitaxial growth, the transition from two-dimensional to three-dimensional growth as well as the evolution of size and shape of three-dimensional islands can be studied.     

Photoelectron diffraction study of ultrathin film growth of Ni on Pt(111)

X-ray photoelectron diffraction (PD) based on a forward scattering approach (FS-PD) has been used to study the growth mode of the first few Ni monolayers deposited on the Pt(111) surface, with a particular attention to the initial stages of epitaxy, i.e. the formation of the first atomic layer. Strong evidences for a layer-by-layer (or Frank-Van der Merwe) growth mode are reported, substantiated also by theoretical simulations carried out with the single scattering cluster-spherical wave (SSC-SW) framework. The first Ni monolayer grows strained in-plane to match the substrate pseudomorphically even if there is a 10% mismatch between the lattice parameters of Ni and Pt. The multilayer (up at least to six monolayers) maintains the horizontal strain and consequently shows a vertical spacing contraction (tetragonal distorsion). It retains the overall threefold symmetry and azimuthal orientation of the substrate, indicative of a single-domain epitaxial fcc stacking. There is also some evidence (even if it is not conclusive) for the fact that the Ni atoms of the first monolayer occupy hcp sites of the substrate surface.     

Normal incidence X-ray standing wave analysis of thin gold films

Normal incidence X-ray standing wave (NIXSW) analysis has been successfully performed on epitaxial gold films on mica substrates using reflection from the (1 1 1) planes parallel to the surface. We show that NIXSW can be used to monitor the decrease in order within the gold film caused by annealing, and the position of sulfur within a monolayer of methyl thiolate (CH₃S-) on the surface. The Au-S layer spacing was found to be 2.54 ± 0.05 Å, in close agreement with previous work on a single crystal system.     

Growth mechanism and morphology of Ge on Pb covered Si(111) surfaces

We study the mechanism and surface morphology in epitaxial growth of Ge on Pb covered Si(111) using a scanning tunneling microscope (STM). We find that Ge adatoms have a very large diffusion length at room temperature. The growth is close to perfect layer-by-layer for the first two bilayers. Surface roughness increases gradually with the film thickness, but no 3D islands are found at room temperature. For growth at 200°C, 3D Ge islands appear after completion of the second bilayer. At room temperature, we believe, the Pb layer enhances surface diffusion and the descending-step motion of Ge adatoms, but the ascending-step motion is hindered and thus 3D island growth is suppressed.     

Epitaxial growth of bcc Fe films on Al(1 0 0) surfaces using Ti as an interface stabilizer

An approach is described to promote epitaxial growth of thin metal films on single-crystal metal substrates by stabilizing the interface with an extremely thin metallic interlayer. A single atomic layer of a metal is deposited at the interface, Ti on Al(1 0 0) in this case, prior to the growth of the metal film of interest to produce an epitaxial interface in a system that is otherwise characterized by interdiffusion and disorder. The stabilized interface reduces interdiffusion and serves as a template for ordered film growth. Using Rutherford backscattering and channeling techniques along with low-energy electron diffraction and low-energy He⁺ scattering, it is demonstrated that an atomically thin layer of Ti metal deposited at the Fe-Al interface, a system well known for considerable intermixing at room temperature, reduces interdiffusion and promotes the epitaxial growth of Fe films on the Al(1 0 0) surface. The decrease in ion scattering yield for Al atoms, Fe-Fe shadowing and long-range order of the surface suggest that the epitaxial growth of Fe is greatly improved when the Ti interlayer is introduced prior to Fe deposition. Off-normal ion channeling experiments provide clear evidence for the bcc structure of Fe on the Ti/Al(1 0 0) template with the measured average (1 0 0) interplanar distance of 1.44 Å in the Fe overlayer.     

Theoretical Studies on the Reaction Mechanism of AsCl_3 with H_2 in the Vapor Phase Epitaxy of GaAs

The reaction mechanism of AsCl3 with H2 has been studied by using the method of BHandHLYP in Density Functional Theory (DFT) at the 6-311G** basis set. The transition state of each reaction is verified via the analysis of vibration mode and Intrinsic Reaction Coordinate (IRC). Meanwhile,single-point energy has been calculated at the QCISD(T)/6-311G** level,and the zero-point energy correction has been made to the total energy and reaction energy barrier. It shows that AsCl3 reacts with H2 to first result in AsHCl2 which may incline to self-decompose and finally afford the product As2,or continue to react with H2 to provide the product AsH3. The computing result demonstrates that the former is the main reaction channel.     

Fabrication of 2D ordered arrays of cobalt silicide nanodots on (0 0 1)Si substrates

Two-dimensional (2D) periodic arrays of Co metal and Co silicide nanodots were successfully fabricated on (0 0 1)Si substrate by using the polystyrene (PS) nanosphere lithography (NSL) technique and thermal annealing. The epitaxial CoSi₂ was found to start growing in samples after annealing at 500 °C. The sizes of the Co silicide nanodots were observed to shrink with annealing temperature. From the analysis of the selected-area electron diffraction (SAED) patterns, the crystallographic relationship between the epitaxial CoSi₂ nanodots and (0 0 1)Si substrates was identified to be [0 0 1]CoSi₂//[0 0 1]Si and (2 0 0)CoSi₂//(4 0 0)Si. By combining the planview and cross-sectional TEM examination, the epitaxial CoSi₂ nanodots formed on (0 0 1)Si were found to be heavily faceted and the shape of the faceted epitaxial CoSi₂ nanodot was identified to be inverse pyramidal. The observed results present the exciting prospect that with appropriate controls, the PS NSL technique promises to offer an effective and economical patterning method for the growth of a variety of large-area periodic arrays of uniform metal and silicide nanostructures on different types of silicon substrates.