首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
Following the upsurge in the study ofSi1  xGex /Si material systems for high-speed applications, we calculate the band offsets using reformulated tight-binding methods. The calculated value of 0.78 eV for the valence band offset (VBO) of pure substances is in excellent agreement with recent experimental measurements. The VBO for alloy interfaces is 0.78 x. We apply the VBO and conduction band offsets (CBO) to shift in photoluminescence and electron confinement in microstructures respectively. The calculated VBO is strongly dependent on the precursor flux ratio.  相似文献   

2.
The interaction of atomic H with Ag(1 1 1)/Si(1 1 1)7 × 7 surfaces was studied by thermal desorption (TD) spectroscopy and scanning tunneling microscopy (STM) at room temperature. TD spectroscopy revealed an intense peak from mono H–Si bonds, even though the Si surface was covered by the Ag atoms. This peak was not observed from Ag-coated SiO2/Si substrates. STM observation showed no clear change of the Ag surface morphology resulting from H exposure. All these results indicate that the atomic H adsorbs at neither the Ag surfaces nor Ag bulk sites, but at the Ag/Si interface by diffusing through the Ag film.  相似文献   

3.
We present a systematic theoretical study of the low-temperature (T =  0 K) quasi-two-dimensional-hole gas mobility in strained p-Si/Si0.8Ge0.2/p-Si selectively doped double heterojunctions. Ionized-impurity (remote and background), interface-roughness and alloy-scattering mechanisms are taken into account. In our calculations we use self-consistently calculated wavefunctions and a multi-subband transport model. We investigate the mobility dependence on the structural parameters, such as the spacer thickness and the well width. We comment on the significance of every scattering mechanism looking for the maximum hole mobility in these systems. Alloy scattering seems to be the main mobility-limiting mechanism resulting in a hole mobility which increases with the spacer thickness and the well width. Our theoretical results are consistent with experiment.  相似文献   

4.
An analysis, by a carrier scattering approach, of the thermionic emission contribution to the dark current is carried out in conventional bound-to-continuum quantum well infrared photodetectors (QWIPs). It is found that the thermionic emission increases with increasing temperature or when extending the detection wavelength from mid- to far-infrared. Considering p-type instead of n-type material, however, the increased effective mass decreases the thermionic emission. Designs for mid- and far-infrared p-type QWIPs based on the Si 1  xGe x/Si system are discussed for both normal and non-normal incident geometries.  相似文献   

5.
Using statistical analysis of the scanning tunneling microscopy images of the 3 × 4-In surface magic cluster (SMC) arrays on Si(100)1 × 2 substrate at low In coverages, the main regularities of the space distribution of the nucleated SMCs have been established. It has been found that a nucleated SMC perturbs the surface potential relief within a limited zone around itself in a way that some of the sites in the zone demonstrate a great preference for nucleating a new cluster, while in the other sites the nucleation is greatly suppressed. Outside the zone, the nucleation probability is close to that of the random nucleation simulated using Monte-Carlo technique. Energetic background of the observed phenomena has been proved using density-functional theory calculations, which clearly demonstrate that the lower the formation energy the higher the occurrence probability of a given cluster-pair configuration.  相似文献   

6.
Motivated by the need to form 1D-nanostructured dopants on silicon surfaces, we have attempted to grow Ga on the high index Si(5 5 12) surface which has a highly trenched (1D) morphology. The evolution of the interface with Ga adsorption in the monolayer regime has been probed by in situ AES, LEED and EELS. Controlling the kinetics by changing the Ga flux rates shows an interesting difference in the 1.0 to 1.5 ML region. The low flux rate (0.03 ML/minute) results in a Frank van der Merwe (layer by layer) growth mode up to 2 ML, while the higher flux rate (0.1 ML/minute) shows a transient island formation after the completion of 1 ML. The low rate shows the formation of 2 × (3 3 7) and (2 2 5) superstructures, while only the 2 × (3 3 7) is observed in a wide coverage range for the higher rate. The results demonstrate the ability to kinetically control the surface phases with different electronic properties of this technologically important interface.  相似文献   

7.
The adsorption of coronene (C24H12) on the Si(1 1 1)-(7 × 7) surface is studied using scanning tunneling microscopy (STM). Upon room temperature submonolayer deposition, we find that the coronene molecules preferentially adsorb on the unfaulted half of the 7 × 7 unit cell. Molecules adsorbed on different sites can be induced to move to the preferential sites by the action of the tip in repeated image scans. Imaging of the molecules is strongly bias dependent, and also critically depends on the adsorption site. We analyze the results in terms of differential bonding strength for the different adsorption sites and we identify those substrate atoms which participate in the bonding with the molecule.  相似文献   

8.
Synchrotron radiation based photoemission spectroscopy (SRPES) and low energy electron diffraction (LEED) are used to study the interaction between Ag atoms and the Si(1 1 1)1 × 1–H surface. At an Ag coverage of 0.063 monolayers (ML) on the Si(1 1 1)1 × 1–H surface, the Si 2p component corresponding to Si–H bonds decreases, and an additional Si 2p component appears which shifts to a lower binding energy by 109 meV with respect to the Si bulk peak. The new Si 2p component is also observed for 0.25 ML Ag on the Si(1 1 1)7 × 7 surface. These findings suggest that Ag atoms replace the H atoms of the Si(1 1 1)1 × 1–H surface and form direct Ag–Si bonds. Contrary to the widely accepted view that there is no chemical interaction between Ag particles and the H-passivated Si surface, these results are in good agreement with recent first-principles calculations.  相似文献   

9.
Wei Jie Ong  Eng Soon Tok 《Surface science》2012,606(13-14):1037-1044
Using Scanning Tunneling Microscope (STM), we show that the surface undergoes phase transformation from disordered “1 × 1” to (7 × 7) reconstruction which is mediated by the formation of Si magic clusters. Mono-disperse Si magic clusters of size ~ 13.5 ± 0.5 Å can be formed by heating the Si(111) surface to 1200 °C and quenching it to room temperature at cooling rates of at least 100 °C/min. The structure consists of 3 tetra-clusters of size ~ 4.5 ? similar to the Si magic clusters that were formed from Si adatoms deposited by Si solid source on Si(111)-(7 × 7) [1]. Using real time STM scanning to probe the surface at ~ 400 °C, we show that Si magic clusters pop up from the (1 × 1) surface and form spontaneously during the phase transformation. This is attributed to the difference in atomic density between “disordered 1 × 1” and (7 × 7) surface structures which lead to the release of excess Si atoms onto the surface as magic clusters.  相似文献   

10.
Optical second harmonic generation spectra have been experimentally obtained from a clean Si(111) 7 × 7 in two different polarization configurations isolating the rotational anisotropic and isotropic contributions. The energy of the fundamental photon is varied from 0.8 eV to 2.5 eV. For comparison, we also use a microscopic formulation based on the semi-empirical tight binding method to evaluate the nonlinear surface susceptibility tensor χ(2ω). Good agreement between theory and experiment is obtained with respect to the number of resonances, their position in energy, and surface or bulk character.  相似文献   

11.
12.
The chemisorption of the allylamine molecule, which contains two functional groups (ethenyl and hydroxyl), on a Si(001) ? (2 × 1) surface was studied using density functional theory (ab-initio DFT) based on the pseudopotential approach. In particular, we focused on the determination of the most stable position of the CC double bond in the ethenyl group and observation of the passivation effect of allylamine on the electronic structure of the clean Si(001) ? (2 × 1) phase. For this purpose, all of the possible interaction mechanisms occurring at the interface were considered: (i) dissociative bonding where the CC bond is parallel to the silicon surface, (ii) dissociative bonding where the CC bond is perpendicular to the silicon surface, and (iii) the [2 + 2] CC cycloaddition reaction. From our total energy calculations, it was found that the bifunctional allylamine molecule attached to the Si(001) ? (2 × 1) surface through the amino functional group, by breaking the N–H bond and forming a Si–H bond and Si–NHCH2CHCH2 surface fragments. During this process, the ethenyl functional group remains intact, and so can be potentially used as an extra reactive site for additional chemical interactions. In addition to these findings, the nudged elastic band method (NEB) calculations related with the reaction paths showed that the parallel position of the CC bond with respect to the surface of the substrate is more favorable. In order to see the influence of the chemisorbed allylamine molecule on the surface states of the clean Si(001)  (2 × 1), we also plotted the density of states (DOS), in which it is seen that the clean Si(001)  (2 × 1) surface was passivated by the adsorption of allylamine.  相似文献   

13.
The adsorption of the small amounts of tantalum on Si (111)-7 × 7 reconstructed surface is investigated systematically using scanning tunneling microscopy and tunneling spectroscopy combined with first-principles density functional theory calculations. We find out that the moderate annealing of the Ta covered surface results in the formation of clusters of the butterfly-like shape. The clusters are sporadically distributed over the surface and their density is metal coverage dependent. Filled and empty state STM images of the clusters differ strongly suggesting the existence of covalent bonds within the cluster. Tunneling spectroscopy measurements reveal small energy gap, showing semiconductor-like behavior of the constituent atoms. The cluster model based on experimental images and theoretical calculations has been proposed and discussed. Presented results show that Ta joins the family of adsorbates, that are known to form magic clusters on Si (111)-7 × 7, but its magic cluster has the structural and electronic properties that are different from those reported before.  相似文献   

14.
Using the experimental data obtained mainly with the scanning tunneling microscopy observations, density functional theory calculations have been applied to examine an atomic structure of the Ag/Si(100)-c(6 × 2) reconstruction. A set of structural models has been proposed having a similar Si(100) substrate reconstruction which incorporates rows of top Si atom dimers and troughs in between the rows. Stability of about twenty models with various Ag coverage ranging from 1/6 to 1 ML has been tested, that allows reducing the number of plausible models to four. Two of these four models have been attributed to the “regular” intrinsic Ag/Si(100)-c(6 × 2) reconstruction, while the other two to its defect-induced modification. The latter is observed in the local areas near defects and domain boundaries and exhibits 3 × 2 periodicity. Comparing the results of calculations with the experimental STM images, it has been concluded that while the Si(100) substrate reconstruction is solid, the Ag subsystem is flexible due to the presence of the lightly bonded mobile Ag atoms.  相似文献   

15.
Excitonic lifetimes in Cd1  xMnUe2Te, Cd1  xMgxTe epilayers and CdTe/Cd1  xMnxTe, Cd1  xMnxTe/Cd1  vMgyTe single quantum wells with different well widths and Mn, Mg compositions are investigated. The excitonic lifetimes are found to reduce drastically by applying external magnetic fields to samples with giant Zeeman splittings. The observed phenomenon is interpreted in terms of the PL decay time contribution from the long-life dark excitons which can convert to excitons for recombinations by a spin-flip process. We attribute the lifetime reduction to the depletion of dark excitons due to their crossing over the exciton energies for dipole allowed transitions in magnetic fields.  相似文献   

16.
高鸿楷  朱作云 《光子学报》1993,22(2):189-192
用自制常压MOCVD装置,在Si衬底上生长GaAs和AlGaAs外延层,在高温去除Si衬底表面氧化膜之后,采用两步法,即低温生长过渡层,再提高温度生长外延层。得到了表面镜面光亮的优质GaAs和AlGaAs外延层。X射线双晶衍射仪测试GaAs外延层,其回摆曲线半峰宽是200孤秒,GaAs和AlGaAs外延层在77K温度下,PL谱半峰宽分别是17meV和24meV。  相似文献   

17.
Topmost-surface-sensitive Si-2p photoelectron spectra of a clean Si(1 0 0)-2 × 1 surface have been measured using Si-2p photoelectron Si-L23VV Auger coincidence spectroscopy (Si-2p–Si-L23VV PEACS). The escape depth of the PEACS electrons is estimated to be ~1.2 Å. The results support the assignments of the Si up-atoms, the Si down-atoms, the Si 2nd-layer, and the Si bulk proposed in previous researches. The Si-2p component with a binding energy of ?0.23 eV relative to the bulk Si-2p3/2 peak, is shown to originate mainly from the topmost surface. Site selectivity of PEACS is indicated to be achieved to some degree by carefully selecting the kinetic energy of the Auger electrons. Since PEACS can be applied to any surface, the present study opens a new approach to identify PES components.  相似文献   

18.
卢江  吴自勤 《物理学报》1989,38(6):981-986
本文用横截面电子显微镜法分析了Si-W/Si/SiO2/Si(100)在440—1000℃退火后的晶化过程,以及各个界面的变化情况.发现Si-W合金膜中,WSi2并未优先在表面、界面处形成晶核.当退火温度不高于700℃时,反应在合金膜内发生,表面、界面起伏和缓.退火温度高达800—1000℃时,界面、表面出现原子扩散,造成剧烈的界面起伏;表面则出现小的热沟槽,Si/SiO2界面也出现高分辨电子显微镜才能观察到的起伏.表面、界面的原子迁移的动力来源于晶界与表面、界面张力.由于SiO2中Si—O键很稳定,不易发生Si和O在界面处的互扩散,所以Si/SiO2界面起伏很小. 关键词:  相似文献   

19.
《Surface science》2003,470(1-2):L840-L846
Chemisorption of a family of six chloroethylenes (C2H3Cl, 1,1-C2H2Cl2, cis-1,2-C2H2Cl2, trans-1,2-C2H2Cl2, C2HCl3, and C2Cl4) on Si(1 1 1)7 × 7 at room temperature (RT) has been investigated by vibrational electron energy loss spectroscopy (EELS). The characteristic vibrational EELS features have been used to identify the prominent surface species upon RT adsorption. Like ethylene, C2H3Cl has been found to predominantly adsorb in a di-σ bonding geometry to the Si surface, while 1,1-C2H2Cl2, cis- and trans-1,2-C2H2Cl2, C2HCl3 and, to a lesser extent, C2Cl4 appear to undergo dechlorination upon adsorption to form chlorinated vinyl adspecies involving single-σ bonding structures. Evidence of vinylidene (>CCH2) has been obtained for the first time on a semiconductor surface for the adsorption of 1,1-C2H2Cl2. The present work illustrates that the molecular structure and the Cl content of chloroethylenes play a crucial role in controlling not only the adsorption geometry but also the extent of dechlorination and the resulting adspecies upon RT adsorption on Si(1 1 1).  相似文献   

20.
《Surface science》2003,470(1-2):9-18
First principles total energy studies are performed to investigate the energetics, and the atomic structure of the adsorption of germane (GeH4), and digermane (Ge2H6) on the Si(0 0 1)-c(2 × 4) surface. It has been observed experimentally that adsorption of Ge2H6 is a dissociative process, which first yields GeH3 and then GeH2 fragments as products. We first study the adsorption of GeH2 considering two different models; the intra-row and the on-dimer geometries. Our results show that the on-dimer site is more stable than the intra-row geometry by 0.44 eV. This is not a surprise since in the absence of H atoms, adsorption in the on-dimer site leaves no dangling bonds. In contrast, when the GeH2 fragment is considered together with two H atoms, the intra-row geometry is favored energetically as compared with the on-dimer site, in good agreement with experiment. Similar results have been previously obtained for the adsorption of SiH2 on Si(0 0 1). Digermane adsorption is explored according to two different geometries. In the first one, we have considered the adsorption as two GeH3 fragments, while in the second, we have considered the adsorption as two GeH2 fragments plus 2 H fragments. In good agreement with experiments, it is found that the latter geometry is energetically more favorable.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号