Fredrik Owman

In situ substrate preparation for high-quality SiC chemical vapour deposition

Abstract In situ preparation of 4H and 6H silicon carbide substrate surfaces in hydrogen and hydrogen-propane etching systems has been studied. The etching of on-axis(0001) 6H-SiC substrates resulted in regular straight terraces and one unit high steps. The etching of on-axis (0001) 4H-SiC substrates resulted in broad terraces interrupted by large step formations. The 4H- and 6H-SiC (0001) off-axis substrates (3.5° towards 〈112¯0〉 yield smooth etched surfaces with the exception of stripe-like defects on the 4H polytype which are shown to be related to stacking-faults. The stacking faults are suggested to be a cause for step-bunching and surface roughening. Hydrogen-etching prior to growth has been shown to improve the epitaxial layer quality both concerning defect formation and step-bunching.

Removal of polishing-induced damage from 6H-SiC(0001) substrates by hydrogen etching

We use atomic force microscopy and ultra-high vacuum scanning tunneling microscopy to show that polishing-induced damage on 6H-SiC(0001) on-axis wafers is efficiently removed by hydrogen etching in a hot-wall chemical vapor deposition reactor. The obtained surfaces exhibit a highly regular step structure with typically 1500 A wide terraces separated by steps with the height of a single unit cell (15 A). The corresponding low-energy electron diffraction pattern is threefold symmetric as expected for a surface with a single preferred domain. These results are compared with results obtained for as-polished and sublimation etched SiC(0001) wafers.

STM study of the SiC(0001) √3 × √3 surface

Abstract We have used scanning tunnelling microscopy to study the √3 × √3 reconstruction of the Si-terminated 6H-SiC(0001) surface. We find that the images are consistent with a structural model composed of 1 3 layer of Si or C adatoms in threefold-symmetric sites on top of the outermost SiC bilayer, similar to the reconstructions observed for 1 3 monolayer of e.g. group-III metals on the Si(111) surface. Additionally, we discuss the nature of the most frequently occurring defects on the √3 × √3 surfaces as well as the stacking sequence of the atomic layers below the reconstructed surface layer.

Morphology, Atomic and Electronic Structure of 6H‐SiC(0001) Surfaces

Recent findings concerning primarily the √3 x √3 and 6√3 x 6√3 reconstructed surfaces of 6H-SiC(0001) are reviewed. First, the morphology of some different types of 6H-SiC crystals is discussed. The scanning tunneling microscopy (STM) and atomic force microscopy (AFM) results presented show that surfaces with a morphology suitable for surface investigations can be prepared using sublimation- or hydrogen-etching. Then results obtained concerning the atomic and electronic structure for the reconstructed surfaces, prepared using an ex situ method for oxide removal and in situ heating, are presented. For the √3 x √3 reconstruction, recent STM and photoelectron spectroscopy (PES) data are discussed in view of available theoretical results. The STM images presented are shown to be consistent with a structural model of Si or C adatoms in threefold symmetric sites. The theoretical results favor Si adatoms in T4 sites as the optimal configuration for this reconstruction. However, the surface shifted components extracted in studies of the C 1s and Si 2p core levels and the location of a surface state band mapped out in angle resolved experiments cannot be explained using this structural model. At present, there is no structural model that satisfactorily can explain all experimental findings for the √3 x √3 reconstruction. A monocrystalline graphite overlayer on top of bulk-terminated or √3 x √3-reconstructed SiC has previously been proposed to explain the 6√3 x 6√3-reconstructed surface. However, STM and PES results are presented that unambiguously show that there is no graphite on the surface when a well developed 6√3 x 6√3 low-energy electron diffraction (LEED) pattern is observed. The STM images recorded during the gradual development of the 6√3 x 6√3 surface show growing fractions of pseudo-periodic 6 x 6 and 5 x 5 reconstructions. These reconstructed regions dominate on the surface, but small √3 x √3-reconstructed regions are still present when a well developed 6√3 x 6√3 LEED pattern is observed. It is shown that the 6 x 6,5 x 5 LEED pattern can be fully explained by scattering from surfaces with a mixture of 6 x 6, 5 x 5 and √3 x √3 reconstructions. Due to the complexity of the STM data, no structural model is proposed for the 6 x 6 and 5 x 5 reconstructions. STM and PES results are presented showing that graphitization of the surface is obtained only after heating at higher temperatures than that required for observing a well developed 6√3 x 6√3 LEED pattern. The STM images then show that the graphite appears as a monocrystalline overlayer on top of the 6 x 6 reconstruction and not on bulk-terminated or √3 x √3-reconstructed SiC(0001).

The SiC(0001)6√3 × 6√3 reconstruction studied with STM and LEED

Abstract We have used scanning tunneling microscopy (STM) to study the 6√3 × 6√3 reconstruction obtained by heat treatment of 6H-Sic(0001) samples at temperatures above 1100°C. For surfaces showing a well-developed 6√3 × 6√3 low-energy electron diffraction (LEED) pattern, we observe with STM two pseudo-periodic reconstructions with approximate periodicities of 6 × 6 and 5 × 5, respectively, in addition to √3 × √3-reconstructed regions. The fraction of the surface exhibiting the √3 × √3 reconstruction in STM images and the intensity of the √3 × √3 spots in the 6√3 × 6√3 LEED pattern decrease with increasing annealing temperature and time. The 5 × 5 reconstruction is observed on a small fraction of the surface (⪅ 10%) and the 6 × 6 reconstruction becomes dominating upon annealing at temperatures above 1200°C. For the 6 × 6 reconstruction, a Fourier analysis of the STM images reveals an underlying incommensurate 2.1 × 2.1-R30° lattice with long-range order. The features defining the 6 × 6 periodicity have well-defined positions with respect to this lattice. A comparison between the Fourier transforms of the STM images and the 6√3 × 6√3 LEED pattern shows that the LEED pattern can be fully explained by scattering from surfaces with a mixture of the √3 × √3, 5 × 5 and 6 × 6 reconstructions. For surfaces heated above 1250°C, we observe a partial graphitization which results in a modification of the 6 × 6 structure observed in STM.

Surface state on the SiC(0001)-(√3 × √3) surface

An angle resolved photoemission study of a surface state on the SiC(0001)-(√3 × √3) surface is reported. Experiments carried out on the 6H and 4H polytypes give essentially identical results. A surface state band with semiconducting occupation is observed, centered around 1.0 eV above the valence band maximum (VBM) and with a width of about 0.2 eV. Recently calculated results for a Si-adatom-induced √3 × √3 reconstruction give a metallic surface state band centered about 1.2 eV above the VBM and with a width of 0.35 eV. The dispersion determined experimentally is smaller than calculated but exhibits the same trend, the surface state disperses downwards towards the VBM with increasing parallel wavevector component along both the \qGM and \qGK directions of the √3 × √3 surface Brillouin zone. The VBM is determined to be located at about 2.3(±0.2) eV below the Fermi level. The results indicate that Si adatoms on top of an outermost SiC bilayer may be an inadequate structural model for explaining recent experimental findings for the SiC(0001)-(√3 × √3) surface.

STM study of structural defects on in situ prepared Si(111) 1 × 1-H surfaces

Abstract We have used scanning tunneling microscopy to study the structural defects occurring on Si(111)1 × 1-H surfaces prepared by in situ hydrogen exposure of Si(111)7 × 7 surfaces at elevated temperatures. These defects can be divided into four categories: point-like defects, holes in the first bulk bilayer, islands, and stacking-faulted regions. The two most frequently occurring point-like defects are identified as adatom trihydrides and top-layer atoms with a missing hydrogen atom. The islands observed on the 1 × 1-H surfaces are single-layer linear islands and bilayer-high linear and triangular islands. We have identified three distinct types of island edges and proposed structural models for each type. These models, which involve different arrangements of tilted monohydride edge species, are also applicable for the step edges on the created surfaces. The stacking-faulted regions, triangular in shape, are found to have a double periodicity along the borders which is explained in terms of a dimerization of the surrounding second-layer atoms.

STM study of Si(111)1 × 1-H surfaces prepared by in situ hydrogen exposure

Abstract We have used scanning tunneling microscopy and low-energy electron diffraction to study the preparation of hydrogen-terminated Si(111)1 × 1 surfaces by in situ atomic-hydrogen exposure of Si(111)7 × 7 surfaces. We find that exposure at sample temperatures of 350–480°C with a hydrogen dose above 1000 L results in the complete transformation of the 7 × 7 structure to the H-terminated 1 × 1 structure. The highest quality Si(111)1 × 1-H surfaces are obtained for doses of 5000 L hydrogen at a temperature around 380°C. These surfaces have less than 5% of a monolayer stacking faults, approximately 1% point-like defects, and less than 0.5% of contamination. For larger hydrogen doses the amount of stacking faults is further reduced, but the surfaces become rough due to the formation of holes in the first bulk double layer. A discussion of the temperature dependence of the removal of the stacking faults is presented as well as a discussion on the origin of the most frequently occurring point-like defects.

A photoemission study of 4HSiC(0001)

Abstract A photoemission study using synchrotron radiation of the (0001) surface of 4H-SiC is reported. The investigations were concentrated on the (√3 × √3)-R30° and (6√3 × 6√3)-R30° reconstructed surfaces, prepared by resistive heating at a temperature of about 1000°C and 1250°C, respectively. Results from surfaces heated at intermediate temperatures, exhibiting a mixture of these reconstructions, and after heating at a higher temperature, when graphitisation is clearly observed, are also presented. The √3 and 6√3 reconstructed surfaces exhibit characteristic core level and valence band spectra. High resolution core level spectra show unambiguously the presence of surface shifted components in both the Si 2p and C 1s core levels. For the √3 reconstruction, two surface shifted components are observed both in the Si 2p and C 1s level. For the 6√3 reconstruction, the surface region is found to contain a considerably larger amount of carbon. This carbon is found not to be graphitic since surface C 1s components with binding energies different from a graphitic C 1s peak are observed. Graphitisation, as revealed by the appearance of a graphitic C 1s peak, is observed only after heating to a higher temperature than that required for obtaining a well developed 6√3 diffraction pattern.

STM study of hydrogen exposure of the Si(111)√3 × √3-In surface

Abstract We have used scanning tunneling microscopy to study Si(111)√3 × √3-In surfaces exposed to atomic hydrogen at temperatures in the range 350–450°C. For hydrogen doses as low as 2 L a considerable disorder is induced in the √3 × √3 structure. Increasing the dose to 10 L results in the formation of two-dimensional islands exhibiting 2 × 2, 4 × 1 and √7 × √3 reconstructions, exposing small areas of the underlying surface with the hydrogen-terminated 1 × 1 structure. Our data for the 4 × 1 reconstruction corroborate proposed models consisting of 1 ML of In atoms in two distinct layers. After 50 L exposure, most of the surface exhibits the hydrogen-terminated 1 × 1 structure with a small amount of isolated defects while about 15% of the surface is covered with two-dimensional islands. For the majority of these islands, showing a weak corrugation with √7 × √3 periodicity, we propose a structure consisting of two pseudomorphic In layers on top of the Si(111) surface. Further increase of the hydrogen dose results in the formation of three-dimensional In islands. We observe a substantial loss of In from the surface during hydrogen exposure, which increases with increasing dose and increasing temperature. For surfaces exposed to 5000 L above 400°C, all In is removed but some stacking-faulted regions have formed on the resulting hydrogen-terminated 1 × 1 surfaces.