K. Heinz

LEED and DLEED as modern tools for quantitative surface structure determination

The last decade has seen remarkable new developments in the field of low energy electron diffraction (LEED) which are reviewed in the present paper. The arrival of sophisticated and fast techniques for the measurement of diffraction intensities in the early 1980s was a challenge to theory. Its answer was the development of tensor LEED (TLEED) in 1985. It allows the fast calculation of intensities for structures not too far from a certain reference. This made quantitative surface structure analysis approach new frontiers both with respect to structural complexity and-with the help of the routine use of reliability factors-to precision. Simultaneously, the new experimental techniques allowed access to two-dimensional intensity maps, i.e. the measurement of diffuse intensity distributions. Theory could be modified to calculate such distributions and so gave birth to the diffuse LEED technique (DLEED) which allows the retrieval of the local structure in case of disordered adsorption. The basics and the development of both TLEED and DLEED is reviewed. TLEED is the basis for direct methods and is used in effective search procedures for surface structures structurally close to a certain reference. It can also be used to simulate the substitution of surface atoms by different chemical species as well as to account for surface vibrations. In addition to carrying the information about the local structure, DLEED patterns very recently could be successfully interpreted in a holographic sense yielding real space images directly.

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.

Oxygen diffusion through thin Pt films on Si(100)

We have investigated the diffusion of oxygen through evaporated platinum films on Si(100) upon exposure to air using substrates covered with Pt films of spatially and continuously varying thickness (0–500 Å). Film compositions and morphologies before and after silicidation were characterized by modified crater edge profiling using scanning Auger microscopy, energy-dispersive X-ray microanalysis, scanning tunneling microscopy, and transmission electron microscopy. We find that oxygen diffuses through a Pt layer of up to 170 Å forming an oxide at the interface. In this thickness range, silicide formation during annealing is inhibited and is eventually stopped by the development of a continuous oxide layer. Since the platinum film consists of a continuous layer of nanometer-size crystallites, grain boundary diffusion of oxygen is the most probable way for oxygen incorporation. The diffusion constant is of the order of 10−19 cm2/s with the precise value depending on the film morphology.

Oxidation of low-index FeAl surfaces

Abstract The oxidation of the (100), (110) and (111) surfaces of the intermetallic compound FeAl has been investigated using LEED and XPS. On all three surfaces, oxidation at room temperature leads to the formation of an amorphous oxide film on top of an Al-depleted interlayer. The film growth can be divided into two regions of differing kinetics, i.e. the initial formation of a closed oxide film and a subsequent thickening. In the first region, the oxygen-uptake rate varies significantly with surface orientation, while in the thickening regime the uptake is the same for all surfaces. The maximum thickness as well as the composition of the oxide films were found to depend on the initial oxidation rate. At higher oxidation temperatures, ordered oxide films of around 5–8 A in thickness are formed, very similar to those observed on NiAl. Photoemission spectra from these ordered phases showed evidence for Al atoms in two different chemical environments, i.e. the well-known oxide species in the interior of the film and an additional species present at the oxide/alloy interface.

Fast LEED intensity calculations for surface crystallography using Tensor LEED

The quantitative analysis of intensity spectra from low energy electron diffraction is todays most widely used technique for the extraction of detailed surface crystallographic information. The Erlangen Tensor LEED package TensErLEED provides an efficient computer code for the fast computation of LEED intensity spectra from virtually any periodic surface. For the full dynamic reference calculation, standard methods such as the muffin-tin approach and the layer stacking method are used. Amplitude changes in Tensor LEED are accessible for geometric, vibrational and chemical displacements from the reference structure. The package also contains a structural search algorithm designed for the retrieval of the global R-factor minimum between calculated and measured intensity spectra within a given portion of the parameter space using Tensor LEED.

A new approach to automated structure optimization in LEED intensity analysis

Abstract We present a new structural search concept for low-energy electron diffraction (LEED) intensity analyses. It combines the advantages of an efficient automated optimization scheme with rapid access to model intensity data by tensor LEED. The geometrical, chemical and thermal versions of the latter method are employed to calculate the diffraction amplitudes for structures deviating from a certain preselected reference by atomic shifts, chemical occupation of lattice sites and both isotropic and anisotropic atomic vibrations. The automated optimization of the theory-experiment fit results from a modified random sampling algorithm, which scales as N 2.5 with N the number of parameters, turning out to be a good compromise between global and local search methods. Using simulated intensity spectra for the Ir(110)-(2 × 1) missing-row reconstructed surface as pseudo-experimental data, we test and demonstrate the reliability of the new procedure and its efficiency. Also, we successfully apply the algorithm to real experimental data of two different surface phases of FeAl(100) and to data of Mo 0.95 Re 0.05 (100)-c(2 × 2)-C as examples for multi-parameter fits.

Pseudomorphic growth of Ni films on Cu(001): a quantitative LEED analysis

Abstract We present LEED structure determinations of ultra-thin epitaxial Ni films on Cu(001) for coverages of 3, 5 and 11 ML. From full dynamical intensity analyses, a tetragonal distortion of all films can be deduced, in good accordance with FMR results. The structural parameters of the 5 and 11 ML films are practically identical. So, obviously there are no structural changes at about 7 ML, where the orientation of the magnetization is reported to switch from in-plane to out-of-plane. The film growth is pseudomorphic with an in-plane lattice parameter a p = 2.53 A at coverages of 3 and 5 ML. This value, which is slightly reduced with respect to the copper bulk, was recently also found to produce the best fit for a similarly prepared clean Cu(100) surface. With further increasing coverage, a p tends to approach the value of the nickel bulk (2.49 A) but does not fully reach it even at 11 ML ( a p = 2.51 A ). The films at 5 and 11 ML (and probably also in the regime between) show considerable tetragonal distortions. The top Ni layer shows an outward relaxation of 8% relative to the distances between deeper layers, which, due to the tetragonal distortion, are reduced compared to the ideal value of the nickel bulk.

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.

Hydrogen on W(110): an adsorption structure revisited

Abstract We present a quantitative study of the atomic structure of the clean and H-covered W(110) surface employing an analysis of lowenergy electron diffraction (LEED) as well as density functional theory (DFT) calculations. Our results give no evidence of a noteworthy reconstruction of the W(110) surface upon H-adsorption, and thus discard the widely accepted model of a H-induced lateral shift of the top layer based on earlier LEED data. Moreover, we offer a reinterpretation of the latter which goes along without such a surface reconstruction. In detail, we find good agreement between the LEED analysis and the DFT calculations on a small contraction of the first interlayer distance by about 3% for the clean surface, which is reduced to half this size at full H coverage. Hydrogen itself is found to be adsorbed in quasi-threefold coordinated hollow sites at a height of about 1.20 A above the first substrate layer.

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.