Role of the Metal/Semiconductor interface in quantum size effects: Pb /Si(111)

Self-organized islands of uniform heights can form at low temperatures on metal/semiconductor systems as a result of quantum size effects, i.e., the occupation of discrete electron energy levels in the film. We compare the growth mode on two different substrates [Si(111)- (7x7) vs Si(111)- Pb(sqrt[3]xsqrt[3] )] with spot profile analysis low-energy electron diffraction. For the same growth conditions (of coverage and temperature) 7-step islands are the most stable islands on the (7x7) phase, while 5-step (but larger islands) are the most stable islands on the (sqrt[3]xsqrt[3] ). A theoretical calculation suggests that the height selection on the two interfaces can be attributed to the amount of charge transfer at the interface.     

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.     

Strongly-driven coarsening of height-selected Pb islands on Si(1 1 1)

A theoretical model is proposed to describe the rapid coarsening observed for Pb islands on a Si(1 1 1) surface where classical kinetics breaks down. In this system, quantum size effects produce mesa-like Pb islands with chemical potentials depending strongly on their heights, in addition to the usual dependence on the step curvature. Furthermore, a dense wetting layer enables fast mass transport between islands. Incorporating these features, our theoretical model predicts evolution of the island height distribution in good agreement with experiments.     

Quantum size effect on the diffusion barriers and growth morphology of Pb/Si(111)

An intriguing growth morphology of Pb islands on a Si(111) surface is observed in our STM experiments: the growth of a Pb layer on Pb islands with unstable heights starts from the periphery and moves towards the center, while the nucleation of the next layer on stable Pb islands starts away from the periphery. Using first-principles total energy calculations, we have studied the diffusion barriers of Pb adatoms on a freestanding Pb(111) film as a function of film thickness. The diffusion barriers are found to be very low (<60 meV), and a bi-layer oscillation due to the quantum size effect (QSE) is observed, with a lower barrier on the odd-layered, relatively unstable Pb films. The diffusion barrier difference between the odd- and even-layered film is as large as 40 meV. The observed unusual growth can be attributed to this big difference in the diffusion barriers due to QSE.     

Lead growth on Si(111) surfaces reconstructed by indium

We study the Pb growth on both √3 × √3-In and 4 × 1-In reconstructed Si(111) surfaces at room and low temperature (160 K). The study takes place with complementary techniques, to investigate the role of the substrate reconstruction and temperature in determining the growth mode of Pb. Specifically, we focus on the correlation between the growth morphology and the electronic structure of the Pb films. The information is obtained by using Auger electron spectroscopy, low energy electron diffraction, soft x-ray photoelectron spectroscopy, scanning tunneling microscopy and spot profile analysis-low energy electron diffraction. The results show that, at low temperature and coverage ≤12 ML on the Si(111)√3 × √3-In surface, Pb does not alter the initial semiconducting character of the substrate and three-dimensional Pb islands with poor crystallinity are grown on a wetting layer. On the other hand, for the same coverage range, Pb growth on the Si(111)4 × 1-In surface results in metallic Pb(111) crystalline islands after the completion of a double incomplete wetting layer. In addition, the bond arrangement of the adatoms is studied, confirming that In adatoms interact more strongly with the silicon substrate than the Pb ones. This promotes a stronger Pb-Pb interaction and enhances metallization. The onset of the metallization is correlated with the amount of pre-deposited In on the Si(111) surface. The decoupling of the Pb film from the 4 × 1-In interface can also explain the unusual thermal stability of the uniform height islands observed on this interface. The formation of these Pb islands is driven by quantum size effects. Finally, the different results of Pb growth on the two reconstructed surfaces confirm the importance of the interface, and also that the growth morphology, as well as the electronic structure of the Pb film can be tuned with the initial substrate reconstruction.     

Influence of quantum size effects on island coarsening

Surface x-ray scattering and scanning-tunneling microscopy experiments reveal novel coarsening behavior of Pb nanocrystals grown on Si(111)-(7 x 7). It is found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis. This is manifested by the lack of scaling of the island densities. In addition, island decay times tau are orders of magnitude faster than expected from the classical analysis and have an unusual dependence on the growth flux F (i.e., tau is approximately 1/F). As a result, a highly monodispersed 7-layer island height distribution is found after coarsening if the islands are grown at high rather than low flux rates. These results have important implications, especially at low temperatures, for the controlled growth and self-organization of nanostructures.     

Vertical friedel oscillations in interface-induced surface charge modulations of ultrathin quantum islands

Two-dimensional Pb islands of a few atomic layers are grown on the incommensurate Si(111)-Pb surface at low temperatures. Among them, two types of islands having different stacking with the substrate are observed. These islands, respectively, display an alternating image contrast with their thickness. Besides, the contrasts of the islands of different types are complementary to each other layer by layer. These intriguing behaviors do not show significant bias dependence throughout the range from -3 to +3 V and can be explained by the vertical charge oscillation with the growth of a new layer. The charge oscillation in the out-of-plane direction originates from electron scattering by the in-plane potential variation at the Pb/Si interface.     

Modifying Quantum Well States of Pb Thin Films via Interface Engineering

We demonstrate the importance of interface modification on improving electron confinement by preparing Pb quantum islands on Si（111） substrates with two different surface reconstructions, i.e., Si（111）-7 ×7 and Si（111）- Root3×Root3-Pb （hereafter, 7 ×7 and R3）. Characterization with scanning tunneling microscopy/spectroscopy shows that growing Pb films directly on a 7 × 7 surface will generate many interface defects, which makes the lifetime of quantum well states （QWSs） strongly dependent on surface locations. On the other hand, QWSs in Pb films on an R3 surface are well defined with small variations in linewidth on different surface locations and are much sharper than those on the 7 × 7 surface. We show that the enhancement in quantum confinement is primarily due to the reduced electron-defect scattering at the interface.     

Equilibrium model of bimodal distributions of epitaxial island growth

We present a nanostructure diagram for use in designing heteroepitaxial systems of quantum dots. The nanostructure diagram is computed using a new equilibrium statistical physics model and predicts the island size and shape distributions for a range of combinations of growth temperature and amount of deposited material. The model is applied to Ge on Si(001), the archetype for bimodal island growth, and the results compare well with data from atomic force microscopy of Ge/Si islands grown by chemical vapor deposition.     

Interface effects on the quantum well states of Pb thin films

Using scanning tunneling spectroscopy, we have studied the interface effect on quantum well states of Pb thin films grown on various metal-terminated (Pb, Ag, and Au) n-type Si(111) surfaces and on two different p-type Si(111) surfaces. The dispersion relation E(k) of the electrons of the Pb film and the phase shift at the substrate interface were determined by applying the quantization rule to the measured energy positions of the quantum well states. Characteristic features in the phase shift versus energy curves were identified and were correlated to the directional conduction band of the silicon substrate and to the Schottky barrier formed between the metal film and the semiconductor. A model involving the band structure of the substrate, the Schottky barrier, and the effective thickness of the interface was introduced to qualitatively but comprehensively explain all the observed features of the phase shift at the substrate interface. Our physical understanding of the phase shift is critically important for using interface modification to control the quantum well states.     

Element specific surface reconstructions of islands during surfactant-mediated growth on Si (111)

The early stages of surfactant- (As, Sb) mediated homoepitaxial growth on Si (111) are examined by scanning tunneling microscopy and extensive ab initio calculations of Si(n) clusters (n相似文献   

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.     

Interfacial reaction barriers during thin-film solid-state reactions: The crystallographic origin of kinetic barriers at the NiS2/Si(111) interface

Epitaxial NiSi₂ islands have been grown on Si(111) substrates by the direct reaction of nickel vapour with the silicon substrate in ultra-high vacuum at 400° C. Growth kinetics was shown to depend on the orientation of the islands: A-oriented islands grow about ten times faster than B-oriented ones, with the ratio of the advance rates of the main growth fronts even reaching 30. Applying plan-view transmission electron microscopy and high-resolution electron microscopy of cross sections, a corresponding difference was found in the structure of the NiSi₂/Si(111) growth front: Steps at the B-oriented growth front were of three or six interplanar (111) spacings in height, whereas at the A-oriented growth front step-like defects of less than one interplanar (111) spacing in height were observed. These observations are explained by an atomic-scale model of the solid-state reaction, which involves the diffusion of nickel to the interfaces and the nucleation and subsequent lateral propagation of interfacial steps. The difference in the reaction kinetics originates from the presence of kinetic reaction barriers at the NiSi₂/Si(111) growth fronts, the barrier at the B-front being higher owing to the lower formation rate of steps of triple atomic height than that of steps of lower height at the A-NiSi₂/Si(111) growth front.     

Surface diffusion of Pb on clean Si surfaces

Pb diffusion on clean Si(111), (100), and (110) surfaces was studied by Auger electron spectroscopy and low energy electron diffraction in the temperature range from 100 to 300°C. It is shown that lead transport along sillicon surfaces takes place via the mechanism of solid-phase spreading with a sharp moving boundary. The temperature dependence of the Pb diffusion coefficients on Si(111), (100) and (110) surfaces have been obtained. A Si(110)-4×2-Pb surface structure has been observed for the first time.     

Electronic properties and interfacial coupling in Pb islands on single-crystalline graphene

Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.     

Imaging subsurface reflection phase with quantized electrons

Lead quantum wells (QW) epitaxially grown on annealed Pb/Si(111) interface form a model system for the study of interactions between quantized electrons and adiabatically modulated boundaries. Tunnel spectra of this system reveal a previously unknown adiabatic shift of QW resonances due to lateral variations of the electronic reflection phase at the buried interface. With this effect, lateral distribution of the subsurface reflection phase can be probed, using scanning tunneling microscopy.     

Interface behavior of Mn/PbTe(111) studied by scanning tunneling microscopy and X-ray photoemission spectroscopy

Mn overlayers growth on PbTe(111) have been investigated by using scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). The strong chemical interactions were found during the formation of Mn/PbTe(111) interface. At the initial deposition of Mn, one part of Mn adatoms substitute Pb atoms on the PbTe(111) surface, forming a (√3 × √3)R30° MnTe phase, and the other part of Mn adatoms, together with the kicked-out Pb atoms, nucleate at the boundaries of the MnTe islands, forming loop islands around the MnTe islands as an intermediate state. Finally, they develop into regular 3D Pb capped Mn islands upon further Mn deposition. For Mn growth on the PbTe surface where Pb atoms are almost completely substituted by Mn, the deposited Mn atoms either cooperate into the 3D Pb capped Mn islands promoting the upright growth of the 3D Pb capped Mn islands, or nucleate and grow on the MnTe superstructure areas. Free Pb layer always floats on the top of surface, indicating that Pb layer has smaller surface energy, and Mn adatoms always exchange the positions with the underneath Pb atoms during the growth.     

Spectrum of electron-hole states of the Si/Ge structure with Ge quantum dots

The lateral photoconductivity spectra of Si/Ge multilayer structures with Ge quantum dots of various sizes are investigated. We observed optical transition lines between the hole levels of quantum dots and electronic states of Si. This enabled us to construct a detailed energy level diagram of the electron-hole spectrum of the Si/Ge structures. It is shown that the hole levels of Ge quantum dots are successfully described by the “quantum box” model using the actual sizes of Ge islands. It I found that the position of the longwavelength photosensitivity boundary of Si/Ge structures with Ge quantum dots can be controlled by changing the growth parameters.     

Decay of silicon mounds: scaling laws and description with continuum step parameters

The decay of mounds about a dozen layers high on the Si(111)-(7x7) surface has been measured quantitatively by scanning tunneling microscopy and compared with analytic predictions for the power-law dependence on time predicted for a step-mediated decay mechanism. Conformably, we find an exponent 1/4 associated with the (3D) decay of the mound height and exponent 1/3 associated with the (2D) decay of top-layer islands. Using parameters from a continuum step model, we capture the essence of the kinetics. Qualitative features distinguish these mounds from multilayer islands found on metals.     

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.