STM studies of overlayers on semiconductor or insulator surfaces

Abstract: Scanning tunneling microscopy (STM) operated in ultra-high vacuum (UHV) has been used to study overlayers on semiconductor or insulator surfaces. The overlayers have been deposited by magnetron sputtering, thermal evaporation or by gas exposure, and the investigated material systems include Mo on MgO, Ni on SiC, Sn on Si(lll), C on Si(lO0) and O on Si(lll). Besides STM, the surfaces have been characterized by using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), reflection high energy electron diffraction  RHEED) and atomic force microscopy ( AFM).Thin Mo films were grown on MgO(00l) substrates by ultra-high vacuum DC magnetron sputter deposition. The generated surfaces were characterized with LEED, RHEED and STM and it was found that oxygen is present on the surfaces during the initial stages of growth. Oxygen-derived reconstructions of T e Mo(lO0) surfaces are found for film thicknesses below 50 nm and it is shown that the oxygen originates from the MgO substrates.Ni has been deposited on 6H-SiC(0001) substrates kept at 600° and subsequently annealed to 800, 900 and 1000°C. Characteristic islands are formed by the annealing. Most islands feature extremely sharp edges and exceptionally flat tops containing a peculiar "stitched" pattern on top. The islands formed at 1000°c were rv300 nm in diameter and rv30 nm high and separated rv5μm from each other. AES revealed that the islands contain Ni and C and that the C is graphitically bound, as opposed to in the substrate where it is carbidically bound. From comparison with literature, it is concluded that these islands are containing, or are made up from, a graphite intercalation compound (GIC) with the Ni as the intercalant.The ( )3 x v'3)-Sn/Si(lll) surface has been studied with STM and AES, and it is found that it contains five different types of defects originating from substitutional atoms and vacancies. The influence the defects have on their immediate neighbourhood is investigated and found to extend to the third nearest ( )3 x }3) adatom neighbours. Close to some defect configurations, a very local (3 x 3) periodicity in the apparent height in the STM images is found. This periodicity is shown to be a linear superposition of the perturbations generated by the individual defects in the configurations.Submonolayers of C and Si have been deposited on a Si(lO0) surface and  ubsequently annealed to form the Si(100)-c(4x4) reconstruction. AES, LEED and STM investigations reveal that a sufficient amount of C to get a full coverage of the c( 4 x 4) reconstruction is equivalent to 0.07 ML. A deposition of 0.035 ML C produces a coverage of only 19%, indicating that there is not a complete separation of the C between the c(4x4) and 2xl reconstructed areas. We conclude that the c( 4x4) reconstruction is strain induced, and that a combination of the mixed ad-dimer and buckled ad-dimer models accurately explains the observed features.Initial oxidation stages of the Si(lll)-(7x7) surface have been studied and the first observation of metastable oxygen in STM has been made. Comparison between bias-dependent STM images and previous photoemission studies indicates that the metastable oxygen appears (as a protrusion) only in STM images with tip biases larger than 2.1 V.A software package called ProbeMaster for SPM image acquisition and processing is presented. The features include a plugin system for specialized SPM image processing, file format translations, tip movement, microscope drivers and general extensions.Included in the thesis, there is also a discussion of a model for the STM tip etching process. The possibility that the time evolution of the etching current can be basis for estimating the sharpness of tips is considered. The speed of the etching process and the timing needed to resolve tip-sized features are some of the issues that are discussed.