This dissertation is organized into six chapters. In Chapter 1, the background and motivations of this research and a review of literatures are introduced. Chapter 2 describes the sample preparation procedures and the principles and operations of the experimental apparatus. The following three chapters are presented the three major experiments with detailed results and included two major systems to discuss–ionic crystals/covalent crystal and inter-molecule/semiconductor.
For the ionic/covalent systems, chapter 3 studied heterostructure of ultra-thin KCl films on Si (100)-2 × 1. Chapter 4 studied NaCl on Si(100)-2×1 surfaces. For inter-molecule/semiconductor, in chapter 5 the major study is to understand that the adsorption of prototypical diatomic molecule, ICl (or IBr) gas is excellent framework for the gas-surface reaction of ICl (or IBr) and semiconductor surface.
Finally, in Chapter 6 research results are summarized and the conclusions are made.
Appended at the end of this dissertation is the study of the catalytic oxidation of CO on a Pt(110) surface. This research was conducted when the author was supported by Advanced Light Source of Lawrence Berkeley National Laboratory in USA in 2007. The experiments were carried out by the in-situ ambient pressure X-ray photoelectron spectroscopy at beam-line 9.3.2 in Advanced Light Source of Lawrence Berkeley National Laboratory and the author was instructed by the beam-line scientists of Zhi Lui and Bongjin Simon Mun in 2007.
1. 1 Motivation
The thin films of insulation material with wide band gap oxides and alkali halide on semiconductor or metal surfaces play an important role in microelectronic devices such as field-effect transistors, tunnel junctions, etc. With the coming miniaturization in these fields, it is not only the lateral extension of a device to be made progress, but also the thickness of insulating layer to be made smaller. As device dimensions are further reduced, the more homogeneous epitaxial system with enhanced insulation properties becomes flat necessary and crucial. The thin films of insulation material also serve well as prototypical systems for research in nano-science and nano-technology. Because the non-vanishing electron density extends through the ultrathin (thickness below 1 nm) insulating films, the scanning tunneling microscopy is a powerful tool like eyes for the investigation of the electric property and heteroepitaxial growth processes of wide-gap materials on metals or semiconductors[1, 2].
Alkali halides, such as NaCl, KI, LiF, LiBr, or CaF2, are pure ionic materials and are found to grow epitaxially on the Ge(100), Si(100), and Si(111) surfaces using STM and other techniques[3-6]. However, from previous studies that showed no atomic resolution images, we only can obtain a rough idea about their growth morphology. In my proposed research, I plan not only to obtain the growth morphology but also to grow stable and atomically thin insulating films on semiconductor surfaces with an ordered geometric structure. To achieve these goals, I need to undertake and think about what factors affect the growth mechanism.
From the basic surface science textbook, we learn that the relative energy between substrates and film and lattice mismatch are two key factors to generate three different major growth mode, namely, (a) Layer-by-Layer growth (Frank-van der Merve Mode, FM), (b) Layer-plus-Island growth (Stranski-Krastanov Mode, SK), and (c) Island growth (Volmer-Weber Mode, VW). These two factors affect each other. Therefore, in my current research I plan to vary the lattice mismatch systematically in hope to figure out the complete growth modes and mechanism of pure ionic molecule to grow epitaxially on pure covalent crystal.
An STM measurement from Glöckler et al. suggests that the growth of NaCl on Ge(100) (mismatch = 0.5%) begins with a carpet-like double-layer NaCl film[4]. For LiBr, LiF and KI on Si (100), the lattice mismatch of ionic crystals/covalent crystal are 1.3%, 29%, and 25%
respectively[3, 5, 6]. The STM images showed that well ordered LiBr, LiF and KI films on Si
(100) could not be obtained at 1ML coverage. LiBr, LiF, and KI are adsorbed randomly onto Si(100), and the further growth mode follows the Volmer-Weber mechanism of island growth on Si(100) and Si(111). However, we lack the information of lattice mismatch in the midway between 1 % and 30 %. So, as the first trial, I chose the potassium chloride (KCl) and sodium chloride (NaCl) as the evaporated source. Because the heterostructure of KCl/Si(100) and NaCl/Si(100) have 15% and 5% lattice mismatch respectively, this work could help us to understand the growth mechanism of alkali halide on the group IV semiconductor surfaces.
1. 2 The Reconstructed Si(100) Surface
The Si(100) surface is used to be the substrates for my follows experiment, and its atomic structure of surface introduced as follows. Silicon is a group in IV element with four electrons in its outer orbit and crystallize in the diamond structure with lattice constant a = 5.43 Å, as shown in Fig. 1.5. In a silicon crystal, each silicon atom has four valance bonds bonded to four neighboring silicon atoms in tetrahedral form.
Figure 1. 1 (a) Tetrahedral bond arrangement of diamond structure. (b) The down view of diamond structure, the fractions denoted the height of the atoms in units of a cubic edge.
As the Si crystal is cleaved along a different crystal orientation, the surface of Silicon reconstruct into new different surface atomic structure. For example, if the crystal is cleaved along the (100) direction, the exposure surfaces reconstruct into 2×1 structure to form a Si(100)- 2×1 structure. If the crystal is cleaved along the direction normal (111) direction, the new surface reconstruct into 7×7 structure to be called Si(111)- 7×7 surfaces. In this section, the detail of the Si(100)-2×1 structure is discussed.
If the silicon crystal is cleaved along the (100) direction, two valence bonds of each Si atom on the exposed surface is broken and transform into dangling bonds. Therefore, every silicon atom in the surface has two dangling bonds and two valence bonds, as shown in Fig.
1.6. Figure 1.7 displays the top view of this unreconstructed Si(100) surface with 1×1 structure. In this 1×1 structure, the surface energy is high since the density of the dangling bonds is high (two dangling bonds per atoms), and then the 1×1 structure is unstable. To reduce the numbers of the dangling bonds, the first layer atoms in the surface will reconstruct.
By this way, the surface energy will be lower and the surface structure will be more stable.
Upon reconstruction, two neighboring atoms form a strong sigma (σ) bond by combined one of the two dangling bonds. The top-layer atoms of the Si(100) surface dimerize (as two surface atoms binding together to form a dimer) to reduce the number of dangling bonds.
These bonded pairs of Si atoms are called dimers. The amount of dangling bonds is reduced by 50 %. This establishes two characteristic directions on the surface, along the dimer row and perpendicular to the dimer. The parallel rows of the dimer bonds also reduce the overall surface energy. These remaining dangling bonds can further form a weak pi (π) bond, as shown in Fig. 1.8. Then the 1×1 structure of the surface have transformed into 2×1 structure, as shown in Fig. 1.9, to be a stable surface.
When preparing the Si(100) surface, the step structure formed by the cleavage along the (100) direction, as shown in Fig. 1.10. The height of the step is about 1.36 Å. The dimer rows on the neighboring terraces are perpendicular, so steps of the terraces divide into two types. SA is the steps where the dimer rows direction on the upper terrace parallel the step edge. SB is the steps where the dimer rows direction on the upper terrace perpendicular the step edge.
Figure 1. 2 The oblique view of the ideal Si(100) surface. Spheres are Si atoms and conoid sticks are dangling bonds. Each silicon atom has two valence bonds and two dangling bonds.
Figure 1. 3 (a) The top view and (b) the side view of the ideal Si(100)-1×1 surface.
Figure 1. 4 The oblique view of the Si (100)-2×1 first layer surface structure.
Figure 1. 5 Top view (a) and side view (b) of the Si(100)-2×1 structure .
Figure 1. 6 Step structures on Si(100)-2×1 surface. (a) STM image of Si(100)-2×1 surface. The size is 15×10 nm2 and Vs = 2 V. (b) Oblique, (c) top and (d) side views of step structures. SA is the steps where the dimer rows direction on the upper terrace parallel the step edge. SB is the steps where the dimer rows direction on the upper terrace perpendicular the step edge.
1. 3 Literature Review
Ultrathin well ordered alkali halide layers have been grown successfully and subsequently studied by STM on various substrates, including germanium, aluminium, copper and silver surfaces.
NaCl islands have indeed been successfully grown on numerous crystalline metal surfaces, such as Cu(111), Cu(110), Cu(311), Ag(111), Ag(100), Al(111), and Al(100)[1, 7-10].
STM studies of alkali halide thin films grown on semiconducting substrates have been carried out for the systems; NaCl/Ge(001),LiBr/Si(001),LiF/Si(001),and KI/ Si(001)[3, 4, 6].
In a study on the initial growth of NaCl overlayers on Ge(100), Glöckler et al reported STM images for NaCl films up to a thickness of three atomic layers in Figure 1. 7, giving evidence for the earlier proposed carpet-like growthmode of the NaCl layer overmonatomic Ge steps, even for small NaCl islands at submonolayer coverage. The authors report that they were only able to perform STM images by imaging occupied sample states using tip voltages of U=1.5–2.7 V. The authors discuss their observation of lateral atomic resolution for the initial double layer[4].
As UPS data indicate that the band structure of the NaCl double layer, at least for k parallel to the surface, is similar to that of the bulk, with the valence band maximum at about 4.2 eV, Glöckler et al concluded that the tunnelling current is most likely predominantly due to emission from Ge states through the NaCl layer. They suggest therefore that the lateral contrast, showing protrusions at either the Na+ or the Cl− positions, is due to a perturbation and interaction of the Ge wavefunction(s) by the NaCl layer causing a lateral variation of the tunnelling barrier. In Figure 1. 8, the reported apparent heights of the NaCl layers on Ge(001) are positive, but smaller than the corresponding geometric heights (3.8±0.3 Å versus 5.6 Å for the first (double) layer and 2.0±0.3 Å for the second (single) versus 2.8 Å NaCl layer)[4].
Figure 1. 7 STM images (a) Large-area STM scan (1130×1130 Å2) of NaCl/Ge (100) (integral coverage about 0.15 DL). Nearly all NaCl islands have a height of one double layer (VTIP=2.7 V, IT =0.7 nA). (b) STM image (98×81 Å2) with atomic resolution of a NaCl layer of one double layer. The square lattice has a lattice constant of 4.0 Å, and is oriented along the [110] and [-110] directions of the underlying Ge (100) surface (VTIP
=2.7 V, IT=1.8 nA, 1/f filter used)[4].
Figure 1. 8 (a) Close-up (200×200 Å2) of an interesting area of Figure 1. 7(a), and (b) line scan from (a) extracted between the two arrows[4].
LiBr/Si(001) heterostructure has been investigated by scanning tunneling microscopy and spectroscopy (STM and STS). In the initial stage of LiBr growth, rectangular islands are observed consisting of accumulation of about 0.2 nm-thick unit layers. The STM results
indicate that LiBr grows on Si(001) in a single layer fashion. The STS measurement shows a wide band gap region in I-V curve and the energy gap of the LiBr film shows no thickness dependence down to a nominal thickness of 1.2 monolayer (ML)[3].
Figure 1. 9 (a) An STM image after 0.4 ML LiBr deposition. 50×25 nm2. Vs = +3.3 V, IT=0.2 nA. (b) A section profile of the solid line A-A ’ in (a). (c) Rigid spheres model of single and double layer growth of LiBr. (d) Top view of LiBr dimer on Si(001) in flat-lying configuration. Thick lines mean chemical bonds between Br and Si atoms[3].
The growth of submonolayer KI on Si(111) and Si(100) surfaces at room temperature has been studied with ultrahigh vacuum scanning tunneling microscopy (STM). The STM results show that KI on these surfaces essentially follows island-growth with a reactive interface, and that the morphology of the KI adsorbates at submonolayer coverage critically depends on the atomic structure of the surfaces. On the Si(111) surface, KI initially (coverage below 0.4 monolayer) Tends to coalesce into clusters of size smaller than the 7×7 unit-cell. With further deposition the clusters grow into well-defined islands. On the Si(100) surface, the KI initially shows dissociative adsorption; distinctive islands appear with coverages above 0.4 monolayer, preferentially clustering at the steps and growing into islands with less regularity of size and shape. This comparative study enables us to evaluate the effects of the adsorbate–substrate and adsorbate–adsorbate interactions on the adsorbing behavior and the morphological evolution of the KI/silicon systems[5].
Figure 1. 10 STM images of KI on Si(111) surface. (a) The image (30×30 nm2) obtained with VS=3.0 V and IT=0.08 nA at 0.1 ML of KI. (b) The image (20×20 nm2) obtained with VS=3.0 V and IT=0.15 nA at 0.3 ML of KI[5].
Figure 1. 11 STM images of KI adsorbates on Si(100) surface. (a) The image (25×25 nm2) was obtained with VS=2.0 V and IT=0.15 nA at 0.4 ML of KI. (b) The image (167×
167 nm2) was obtained with VS=3.5 V and IT=0.15 nA at 0.8 ML[5].
The surface morphology of Si(100)- 2×1 with submonolayers of LiF adsorbate and its annealing behavior are studied using scanning tunneling microscopy. LiF adsorbs randomly on the Si(100)-2×1 surface at room temperature (RT), and the 2×1 structure disappears when the coverage of LiF is close to 1 monolayer. Interaction of the Si surface and the LiF adsorbate is enhanced by specimen annealing, which causes dissociation of the LiF and fluorination of the Si surface. Desorption of SiFx (x=1, 2, 3, 4) results in surface etching.
After annealing at 700℃ for 5 min, fluorine on the surface x decreased below the limit of the detection by X-ray photoelectron spectroscopy, and the Si surface is reconstructed to 2×
1 at about 800℃[6].
Figure 1. 12 STM patterns of imaging size 20×20 nm2. (a) The clean Si(100)- 2×1 surface. VS=-2.2 V, IT=0.1 nA. (b) The surface with 0.2 ML LiF. VS=-2.8 V, IT=0.2 nA. (c) The surface with 0.8 ML LiF. VS=2.8 V, IT=0.08 nA[6].
A carpet-like growth mode of NaCl at submonolayer coverage is also found on the aluminium, copper and silver substrates. Atomic resolution images of the NaCl(001) layers on all of these substrates also show a periodicity that corresponds to the positions of one type of ion. The NaCl islands appear for all of these substrates, also with positive apparent height in the STM images. For ultrathin insulating NaCl layers on Al(111) and Al(100), atomic resolution has been obtained at negative sample bias voltages (−0.5 to −3.0 V), imaging the Cl anions as confirmed by spatially resolved ab initio calculations of the local density of states. Decreasing NaCl–NaCl step heights were observed with increasing layer thickness and a maximum thickness of three layers for successful imaging was inferred.
However, the gap width of the NaCl films has not been investigated. Apart from the energetically favoured (001) orientation of the NaCl layers, Hebenstreit et al reported recently on the growth of polar NaCl islands on Al(111)[10].
Figure 1. 13 STM constant current topographs of Al(111) after adsorption of 0.35 ML Na and a dose of Cl2 corresponding to 0.2 ML. (a) 50×50 nm2, -1.2 V, 0.06 nA. (b) NaCl(111) island with atomic resolution (7×7 nm2, -1.2 V, 0.3 nA). (c) Structure model of an NaCl(111) island[10].
NaCl on Cu(111) has been studied by Repp et al. Similar to the observations on Ge and Al substrates, the islands are imaged with bright contrast in STM images. Atomic resolution images show, even for bias voltages within the band gap of bulk NaCl, the square lattice corresponding to one type of ion of NaCl(001). dI/dU measurements on the NaCl films show a shift of the prior Cu(111) surface state onset towards higher energies upon adsorption of NaCl which is interpreted as an interface state due to NaCl adsorption[11, 12].
In a recent paper, the adsorption properties of NaCl monolayers and bilayers adsorbed on Cu(311) and Cu(100) have been also investigated theoretically by Olsson et al using density functional calculations. These investigations include the calculation of adsorption energies and workfunctions for adsorbed NaCl monolayers and bilayers and are compared to the experimental observations of NaCl layers on copper surfaces. For the Cu(311) surface a direct covalent interaction between Cl 3p and Cu 3d states has been identified.
Figure 1. 14 (a) The Cl ions of a NaCl island (appearing as protrusions, right) are located above the intrinsic steps of the substrate surface (left) as indicated by the dashed line; image size 40×18 Å. (b) Electron bombardment creates single Cl vacancies (circle);
size 38×18 Å. (c) A single layer of NaCl shows alternatingly a c(2×2) and a p(1×1) structure; size 88×27 Å. (d) Defects are observed (dotted lines), which consist of two neighboring Cl ions that appear darker due to a missing Cu atom underneath; size 62×19 Å[11].