S. Dreiner

Structure of the interface between ultrathin SiO{sub 2} films and 4H-SiC(0001)

The interface between ultrathin SiO{sub 2} films and 4H-SiC(0001) has been studied by x-ray photoelectron spectroscopy (XPS) and photoelectron diffraction. The investigation was performed for two different films. An ordered silicate layer showed a clear ({radical}(3)x{radical}(3))R30 deg. reconstruction, whereas a second film showed no long-range order. The comparison of the photoelectron diffraction data from these two films reveals that the local atomic environments of the Si atoms at the interface are very similar in both films. Further, a comparison of the experimental data with simulation calculations within a comprehensive R-factor analysis shows that also the local environments around near-interface Si atoms inside the SiO{sub 2} film are similar, but some modifications to the model are necessary. The use of the cluster radius as a fitting parameter in the simulation allowed to estimate the size of locally ordered regions in the film without long-range order to be about 4.5 to 5.0 A ring . It turns out that the transition from SiC to SiO{sub 2} is abrupt and therefore the occurrence of defects in the SiO{sub 2} film near the interface is probable. These defects may be oxygen vacancies, oxygen dangling bonds or silicon interstitials.

Hafnium silicide formation on Si(100) upon annealing

High dielectric constant materials, such as HfO{sub 2}, have been extensively studied as alternatives to SiO{sub 2} in new generations of Si based devices. Hf silicate/silicide formation has been reported in almost all literature studies of Hf based oxides on Si, using different methods of preparation. A silicate interface resembles close to the traditional Si/SiO{sub 2}. The silicate very likely forms a very sharp interface between the Si substrate and the metal oxide, and would be suitable for device applications. However, the thermal instability of the interfacial silicate/oxide film leads to silicidation, causing a dramatic loss of the gate oxide integrity. Despite the importance of the Hf silicide surface and interface with Si, only a few studies of this surface are present in the literature, and a structural determination of the surface has not been reported. This paper reports a study of the Hf silicide formation upon annealing by using a combination of XPS, LEED, and x-ray photoelectron diffraction (XPD) analyses. Our results clearly indicate the formation of a unique ordered Hf silicide phase (HfSi{sub 2}), which starts to crystallize when the annealing temperature is higher than 550 deg. C.

Investigation of carbon contaminations in SiO2 films on 4H-SiC(0001)

SiO2 films can be grown on SiC by oxidation of the clean SiC surfaces. During the oxidation process carbon atoms have to leave the crystal. This occurs by outdiffusion of CO molecules from the reaction front through the growing film. Carbon atoms remaining at the interface or in the oxide film lead to an increased density of states in the band gap, and therefore lower the quality of the SiO2∕SiC interface. In this work photoemission spectroscopy and photoelectron diffraction were used to study the carbon contamination in ultrathin SiO2 films on 4H-SiC(0001). The contaminations were produced during oxidation at high temperatures and low oxygen pressure. Due to their chemical shift carbon atoms from the contaminations and from the substrate can be distinguished in the C 1s photoemission spectrum. A combined photoelectron spectroscopy and photoelectron diffraction study shows that these carbon agglomerations are similar to carbon enrichments observed after heating of clean SiC surfaces and that they are eith...

Surfactant mediated heteroepitaxial growth of Ge/Si(111) probed by X-ray photoelectron diffraction

X-ray photoelectron diffraction (XPD) is used as a tool to investigate the growth of Ge on Si(111) with and without surfactant (Sb) mediation. Structural information is extracted directly from the forward scattering maxima of recorded photoelectron diffraction patterns. The result of a quantitative analysis of the patterns taken for Ge layers of different coverage are used for a structural growth model of Ge on Si(111). Without Sb, a compression of the top Si and the first Ge double layers is found followed by a relaxation of the Ge structure. Under the presence of Sb an intermixed interface is formed followed by compressed Ge double layers and relaxed distances between double layers.

THE TRANSITION FROM AMORPHOUS SILICON OXIDE TO CRYSTALLINE SILICON STUDIED BY PHOTOELECTRON DIFFRACTION

Angle-scanned photoelectron diffraction patterns of the Si 2p signal of oxidized Si(111) and Si(001) surfaces have been recorded and compared with simulations. The chemically shifted components of the Si 2p signal were deconvoluted by least squares fitting. The different oxidation states exhibit individual diffraction patterns for both surface orientations, indicating a different environment for each suboxide. For the Si(111) surface the results agree with a previously proposed sharp interface, whereas a graded interface is indicated for the Si(001) surface.

Photoelectron diffraction as a tool for the study of a buried interface during heteroepitaxial growth of Sb/Ge/Si(111)

Abstract X-ray photoelectron diffraction is used to analyze the structure of a buried Ge Si interface below a covering Sb layer. The Si(111) crystalline structure below Ge and Sb adsorbate layers is clearly seen in the diffraction pattern. The epitaxial growth of 2–10 Ge layers on Si(111) is mediated by a monolayer of Sb terminating the interface to the vacuum. The Sb photoelectron diffraction pattern shows no intense forward scattering maximum, indicating no Sb below the surface. The Ge film grows to form an epitaxial double layer at the interface, followed by layer-by-layer growth. For two Ge layers, the average vertical lattice constant of Si(111) is compressed due to adsorbate-induced stress. Also, the average vertical Ge lattice constant is compressed.

The role of the Si-suboxide structure at the interface: an angle-scanned photoelectron diffraction study

Photoelectron diffraction patterns of the different silicon sub-oxides have been recorded and compared with simulated patterns for various model structures. Each silicon sub-oxide is embedded in an ordered environment since individual diffraction patterns and differences among them were exhibited. In particular, the intensity maxima are located at different angles. In the simulation the silicon-oxide/silicon interface was assumed to be abrupt and within one atomic layer. Excellent agreement between experimental and calculated patterns was achieved. At the interface, horizontally compressed SiO2 was found. Furthermore, the highest oxidation state of silicon, Si4+, displays a diffraction pattern indicating an ordered structure for this chemical state.

Investigation of the (√3×√3)R30° Sb/Si(111) structure by means of X-ray photoelectron diffraction

Abstract X-ray photoelectron diffraction was used to investigate the structure of 1 ML (√3×√3)R30° Sb/Si(111). The Sb 3d 5/2 signal was recorded at an electron kinetic energy of E kin =720 eV varying the polar and azimuth angle. The Sb diffraction pattern displayed intensity maxima at low polar angles indicating Sb atoms below the surface. In a structure model Si atoms below the surface were substituted by Sb atoms in order to study the forward scattering maxima in the Sb diffraction pattern for electron emission from Sb atoms in various layers below the surface.

Investigation of the SiO2/Si(111) interface by means of angle-scanned photoelectron diffraction

Abstract Photoelectron diffraction measurements of the Si 2p core level were carried out in order to investigate the interface structure of a SiO 2 film thermally grown on Si(111). The photoemission spectra show chemically shifted components derived from the individual oxidation states, which exhibit different diffraction patterns. These diffraction patterns are compared with those obtained by multiple scattering calculations for a simple chemical abrupt interface structure model. In this model each oxidation state occurs in various bonding configurations. Thus the experimental data are compared with superpositions of simulated patterns for possible model clusters. As a result of a multipole R -factor analysis we obtain a parameter set that reproduces the experimental data well.