J. Schardt

Epitaxially ideal oxide–semiconductor interfaces: Silicate adlayers on hexagonal (0001) and (0001̄) SiC surfaces

The preparation of hexagonal {0001} 4H and 6H silicon carbide surfaces by hydrogen plasma or etching in hydrogen flow produces highly ordered monolayers of silicon dioxide. Their structure and epitaxial relationship to the SiC substrate were analyzed by quantitative low-energy electron diffraction and Auger electron spectroscopy. The bond angles and distances retrieved agree with those of bulk SiO2. Due to the saturation of all dangling bonds the semiconductor surface is passivated and preserves its perfect order also in air. The practically ideal oxide monolayers may serve as a seed for growing epitaxial oxides with low defect density and only few structural distortions at the interface to the SiC substrate.

SiC SURFACE RECONSTRUCTION: RELEVANCY OF ATOMIC STRUCTURE FOR GROWTH TECHNOLOGY

Growth of SiC wafer material, of heterostructures with alternating SiC crystal modifications (polytypes), and of oxide layers on SiC are of importance for potential electronic device applications. By investigation of hexagonal SiC surfaces the importance of atomic surface structure for control of the respective growth processes involved is elucidated. Different reconstruction phases prepared by ex situ hydrogen treatment or by Si deposition and annealing in vacuum were analyzed using scanning tunneling microscopy (STM), Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) crystallography. The extremely efficient dangling bond saturation of the SiC(0001)-(3×3) phase allows step flow growth for monocrystalline homoepitaxial layers. A switch to cubic layer stacking can be induced on hexagonal SiC(0001) samples when a phase is prepared. This might serve as seed for polytype heterostructures. Finally, we succeeded in preparing an epitaxially well matching silicon oxide monolayer with periodicity on both SiC(0001) and SiC. This initial layer promises to facilitate low defect density oxide films for MOS devices.

Functional surface reconstructions of hexagonal SiC

The basal surfaces of hexagonal SiC exhibit a large variety of surface reconstructions that develop under a similarly rich variety of sample preparations. A subset of these surface phases, which have been investigated in structural detail using scanning tunnelling microscopy and quantitative low-energy electron diffraction, is described and shown to offer the scope to be used for the formation of SiC-based semiconductor devices. The phases discussed are the (3 × 3) and reconstructions for the (0001) surfaces of 4H- and 6H-SiC and the oxygen-uptake-driven reconstructions of these polytypes for both the (0001) and the (000) surface orientations. We show that the (3 × 3) reconstruction corresponds to a highly passivated surface that facilitates hexagonal single-crystal growth, while suitable preparation of the reconstruction favours a switch to cubic growth and hence to the formation of a heterojunction. The reconstructions promise to form defect-free interfaces to insulating silicon oxide films, which is important for device applications.

Reconstructed oxide structures stable in air: Silicate monolayers on hexagonal SiC surfaces

Ultrathin oxide layers on hexagonal SiC surfaces were studied using low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). SiC(0001) and SiC(0001) samples were ex situ prepared using thermal hydrogen etching or a microwave powered hydrogen plasma treatment. A well ordered (3×3)R30° reconstructed surface is observed by LEED immediately upon introduction into vacuum. The samples contain oxygen of approximately one layer equivalent bonded to Si atoms as indicated by AES. From a full dynamical LEED structure analysis carried out for the SiC(0001) surface the crystallographic structure is determined: The silicon oxide is arranged as a silicate (Si2O3) layer on top of the SiC substrate, forming rings of (3×3)R30° periodicity with twofold coordinated oxygen atoms in the topmost position. The oxygen incorporation into the surface presumably proceeds via rapid oxidation in air of the well ordered topmost substrate bilayer. The extreme stability of the resulting surface reconstruction is ca...

Surface structure of hexagonal SiC surfaces: key to crystal growth and interface formation?

Abstract The atomic structure of the SiC(0001) surface was analysed using low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Dependent on the preparation procedure, the surface assumes different complex reconstruction phases. Using an ex situ hydrogen etching procedure, the sample surfaces can be passivated and are covered by a silicon oxide monolayer on top of the SiC bulk. In this state, the surface could serve as a seed to grow epitaxial oxide films for MOS device applications. Subsequent preparation in ultra high vacuum (UHV) by annealing under simultaneous silicon deposition results in a complex (3×3) reconstruction which proves to be almost free of dangling bonds. This surface structure favours the homoepitaxial single polytype growth by enabling incoming particles to diffuse to steps, thus, allowing for an efficient step flow growth mechanism. By further annealing, this phase can be transformed into a ( 3 × 3 )R30° phase, which is characterized by a Si adatom geometry. Variation of the preparation procedure for this structure allows the controlled switch of the surface stacking sequence from hexagonal to cubic stacking, which might be useful as a starting point to grow heterostructures of different SiC polytypes.

LEED holography applied to a complex superstructure: A direct view of the adatom cluster on SiC(111)- ( 3 × 3 )

For the example of the SiC(111)-

ATOMIC STRUCTURE OF HEXAGONAL 6H AND 3C SiC SURFACES

(3\ifmmode\times\else\texttimes\fi{}3)

Interface structure of ultrathin CoSi2 films epitaxially grown on Si(111)

reconstruction we show that a holographic interpretation of discrete low energy electron diffraction (LEED) spot intensities arising from ordered, large unit cell superstructures can give direct access to the local geometry of a cluster around an elevated adatom, provided there is only one such prominent atom per surface unit cell. By comparing the holographic images obtained from experimental and calculated data we illuminate validity, current limits and possible shortcomings of the method. In particular, we show that periodic vacancies such as cornerholes may inhibit the correct detection of the atomic positions. By contrast, the extra diffraction intensity due to slight substrate reconstructions, as for example buckling, seems to have negligible influence on the images. Due to the spatial information depth of the method the stacking of the cluster can be imaged down to the fourth layer. Finally, it is demonstrated how this structural knowledge of the adcluster geometry can be used to guide the dynamical intensity analysis subsequent to the holographic reconstruction and necessary to retrieve the full unit cell structure.

Structure and morphology of SiC surfaces studied by LEED, AES, HREELS and STM

The crystallography of hexagonal SiC surfaces prepared ex situ by chemical methods was investigated by low energy electron diffraction (LEED) structure analysis. The surface morphology was analyzed by considering mixtures of domains with different surface layer stacking geometries. On the 6H–SiC(0001) surface ABCACB stacking is the dominating termination, covering about 80% of the surface. On all three possible surface stacking sequences are present. The (111) surface of a 3C–SiC film sample shows linear stacking of layers with only one domain orientation present. In contrast to the carbon-terminated 6H sample, the silicon-terminated surfaces are covered by an oxygen layer with the adatoms bound on top of silicon.

Tailoring the SiC Subsurface Stacking by the Chemical Potential

The interface structure of ultrathin CoSi2 films grown on Si(111) was investigated by quantitative low-energy electron diffraction. Codeposition of the elements leads to a film composed of domains with two and three Si–Co–Si trilayers in CaF2 structure. As within the film, Co atoms at the interface are eightfold coordinated. The lateral unit cells of the film and substrate are mutually rotated by 60° (B-type orientation). The interfacial trilayer is substantially distorted, its distance to the substrate expanded, and its sublayer spacings considerably modified from the bulk. Also, the substrate’s top spacing is expanded. The results compare almost quantitatively with recent density-functional calculations.