Self-ordering of a Ge island single layer induced by Si overgrowth

We provide a direct experimental proof and the related modeling of the role played by Si overgrowth in promoting the lateral ordering of Ge islands grown by chemical vapor deposition on Si(001). The deposition of silicon induces a shape transformation, from domes to truncated pyramids with a larger base, generating an array of closely spaced interacting islands. By modeling, we show that the resulting gradient in the chemical potential across the island should be the driving force for a selective flow of both Ge and Si atoms at the surface and, in turn, to a real motion of the dots, favoring the lateral order.     

Self-organized growth of nanopucks on Pb quantum islands

Electronic Moirè patterns found on lead (Pb) quantum islands can serve as a template to grow self-organized cluster (nanopucks) arrays of various materials. These patterns can be divided into fcc- and hcp-stacked areas, which exhibit different binding strengths to the deposited adatoms. For Ag adatoms, the binding energy can differ substantially and the confined nucleation thus occurs in the fcc sites. Both the size distribution and spatial arrangement of the Ag nanopucks are analyzed and found to be commensurate with the characteristics of the template island, which exhibits a bilayer oscillatory behavior.     

Nucleation and growth of Si on Pb monolayer covered Si(111) surfaces

With a scanning tunneling microscope (STM), we study the initial stage of nucleation and growth of Si on Pb monolayer covered Si(111) surfaces. The Pb monolayer can work as a good surfactant for growth of smooth Si thin films on the Si(111) substrate. We have found that nucleation of two-dimensional (2D) Pb-covered Si islands occurs only when the substrate temperature is high enough and the Si deposition coverage is above a certain coverage. At low deposition coverages or low substrate temperatures, deposited Si atoms tend to self-assemble into a certain type of Si atomic wires, which are immobile and stable against annealing to ~ 200 °C. The Si atomic wires always appear as a double bright-line structure with a separation of ~ 9 Å between the two lines. After annealing to ~ 200 °C for a period of time, some sections of Si atomic wires may decompose, meanwhile the existing 2D Pb-covered Si islands grow laterally in size. The self-assembly of Si atomic wires indicate that single Si adatoms are mobile at the Pb-covered Si(111) surface even at room temperature. Further study of this system may reveal the detailed atomic mechanism in surfactant-mediated epitaxy.     

Minute SiGe quantum dots on Si(001) by a kinetic 3D island mode

We investigated the initial growth stages of Si(x)Ge(1-x)/Si(001) by real time stress measurements and in situ scanning tunneling microscopy at deposition temperatures, where intermixing effects are still minute (< or =900 K). Whereas Ge/Si(001) is a well known Stranski-Krastanow system, the growth of SiGe alloy films switches to a 3D island mode at Si content above 20%. The obtained islands are small (a few nanometers), are uniform in shape, and exhibit a narrow size distribution, making them promising candidates for future quantum dot devices.     

Effect of substrate doping concentration on quantum well states of Pb island grown on Si(1 1 1)

The interaction between the metallic film/island and the semiconductor substrate is important to the electronic properties of metallic nanostructure grown on semiconductor substrate. Here, we report a series of comparison experiments to investigate the effect of doping concentration of Si substrates on the quantum well state (QWS). Using scanning tunneling microscopy, we observed that the apparent QWS energy positions show a strong dependence on the substrate used and on the sample temperature. Further experimental results by varying the height of scanning tunneling microscope tip over the Pb island uncovered that the observed apparent QWS energy position changes mainly come from the partial bias voltage drop on the combined resistance of the Pb wetting layer and the substrate, which is comparable with the vacuum tunneling resistance at low temperatures.     

Coverage-dependent collective diffusion of a dense Pb wetting layer on Si(111)

The dynamics of a dense Pb wetting layer on the Si(111) surface is studied in the framework of a generalized Frenkel-Kontorova model. Instead of the typical diffusion by random hopping processes, a liquidlike collective motion of the Pb atoms within the dense wetting layer is revealed to give rise to ultrafast kinetics of the wetting layer even at low temperatures. A kinetic Monte Carlo simulation including this collective spreading mechanism of the dense wetting layer quantitatively reproduces the experimental observations.     

Site selective growth of Ge quantum dots on AFM-patterned Si substrates

By combining the atomic force microscope (AFM) local anodic oxidation and etching, a periodic array of nanodimples of 40 nm in diameter and 3.5 nm in depth has been made on a Si surface. Ge atoms deposited onto this patterned substrate by the MBE method nucleate preferentially in the dimples and form an array of nano Ge dots of about 50 nm in diameter and 10 nm in height.     

Adhesion of binary giant vesicles containing negative spontaneous curvature lipids induced by phase separation

We report the adhesion of binary giant vesicles composed of two types of phospholipids, one has negative spontaneous curvature which tends to bend toward the head group and the other has zero spontaneous curvature. In a homogeneous one-phase region, the giant vesicles do not adhere to each other, whereas in a coexisting two-phase region, the giant vesicles show adhesion. A fluorescence microscope observation reveals that the adhesion takes place through the domains rich in phospholipids having negative spontaneous curvature. We propose a phase separation induced hemifusion model where two apposed monolayers of adjacent vesicles are hemifused in order to reduce the bending energy of monolayers with negative spontaneous curvature and the boundary energy between the domains and matrix. We provide a strong evidence for the hemifusion model by lipid transfer experiments.     

Spontaneous domain switching during phase separation of Pb on Ge(111)

Using low-energy electron microscopy (LEEM), we have discovered a novel phase separation mechanism for Pb on Ge(111). When the low Pb coverage (1 x 1) phase coexists with the high coverage beta phase, the surface consists of approximately 100 nm sized domains that spontaneously switch from one phase to the other. We argue this striking mechanism occurs because nanometer-scale domains can have density fluctuations comparable to the density difference between the two phases.     

应变补偿层对量子点生长影响的理论研究

量子点的光学特性与量子点的大小均匀性、密度、内部应变以及隔离层的厚度等有密切关系.文中从理论角度定量研究了GaNXAs1-X应变补偿层对InAs/GaAs量子点生长质量的改善作用,分析了应变补偿层对隔离层厚度减小的作用.讨论了应变补偿层的补偿位置和补偿层N组分X对量子点生长时局部应变和体系应变的补偿作用.分析了应变补偿层对体系应变的减少作用,并计算了相邻层量子点的垂直对准概率.研究结果对实验中应变补偿的优化和高质量量子点阵列的生长实现提供了理论依据.     

ZnO:Ag film growth on Si substrate with ZnO buffer layer by rf sputtering

ZnO buffer layers were deposited on n-Si (1 0 0) substrate by rf magnetron sputtering at a lower power of 40 W. Then Ag-doped ZnO (SZO) films were deposited on buffered and non-buffered Si at a higher sputtering power of 100 W. The effects of buffer layer on the structural, electrical and optical properties of SZO films were investigated. The three-dimensional island growth process of ZnO buffer layer was discussed. The energy band diagram of p-SZO/n-Si heterojunction was constructed based on Anderson's model. Results show the ZnO buffer layer leads to better properties of SZO film, including larger grain size, smoother surface, higher carrier mobility, better rectifying behavior, lower interface state density, and weaker deep-level emission. It is because the ZnO buffer layer effectively relaxes the partial stress induced by the large lattice mismatch between SZO and Si.     

Oscillatory electron phonon coupling in Pb/Si（111）deduced by temperature-dependent quantum well states

Photoemission study of atomically flat Pb films with a thickness from 15 to 24 monolayers （ML） have been performed within a temperature range 75-270K. Well-defined quantum well states （QWSs） are observed, which exhibit interesting temperature-dependent behaviours. The peak position of the QWSs shifts towards higher binding energy with increasing substrate temperature, whereas the peak width broadens linearly due to enhanced electron-phonon coupling strength （λ）. An oscillatory A with a period of 2ML is deduced. Preliminary analysis shows that the oscillation can be explained in terms of the interface induced phase variations, and is thus a manifestation of the quantum size effects.     

Effect of nitrogen gas flow and growth temperature on extension of GaN layer on Si

The effect of nitrogen flow and growth temperature on extension of GaN on Si substrate has been studied. By increasing the nitrogen flow whose outlet is located in the center of the MOCVD (metal-organic chemical vapor deposition) gas/particle screening flange and by increasing the growth temperature of HT-AlN and AlGaN buffer layers near the primary flat of the wafer, the GaN layer has extended more adequately on Si substrate. In the meantime, the surface morphology has been greatly improved. Both the AlN and GaN crystal quality uniformity has been improved. X-ray diffraction results showed that the GaN (0002) XRD FWHMs (full width at half maximum) decreased from 579 arcsec～ 1655 arcsec to around 420 arcsec.     

Exchanges between Si and Pb adatoms on Si(111)

Real-time observation by high-temperature scanning tunneling microscopy of exchanges between Si and Pb atoms on a Si(111)-√3 × √3 surface is reported. The exchange rate is obtained as a function of the temperature. The activation energy of the exchange is about 1.2 eV, and the prefactor, shown to depend on the Pb coverage, is from 2 × 10¹⁰ to 8 × 10¹¹ s⁻¹. This prefactor is much larger than that for the exchange between Pb and Ge adatoms on a Ge(111)-c(2 × 8) surface, indicating that the adatom arrangement greatly influences the exchange mechanism. We also report that metastable 9 × 9 reconstruction appears during Pb desorption.