J. I. Flege

Cleaning of GaN(2¯110) surfaces

The cleaning of GaN(2¯110) surfaces was investigated by x-ray photoelectron spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction. Two different two-step cleaning methods, performed under ultrahigh-vacuum conditions, were carried out and compared. The first cleaning step of both methods is thermal degassing. The second step is either the deposition of metallic gallium followed by redesorption or an exposure to active nitrogen from a radio frequency nitrogen plasma source. Upon storage in a glovebox (N2 atmosphere) and transfer to ultrahigh vacuum under dry nitrogen, carbon and oxygen were identified as the major contaminants. A significant decrease in oxygen and carbon was achieved by thermal degassing at 750 °C under ultrahigh-vacuum conditions. By applying a subsequent Ga deposition/redesorption or N2-plasma cleaning step, a further reduction in oxygen and carbon could be achieved. In comparison, the Ga deposition/redesorption cleaning showed a better performance in oxygen rem...

Control of Epitaxial Graphene Thickness on 4H-SiC(0001) and Buffer Layer Removal through Hydrogen Intercalation

We report graphene thickness, uniformity and surface morphology dependence on the growth temperature and local variations in the off-cut of Si-face 4H-SiC on-axis substrates. The transformation of the buffer layer through hydrogen intercalation and the subsequent influence on the charge carrier mobility are also studied. A hot-wall CVD reactor was used for in-situ etching, graphene growth in vacuum and the hydrogen intercalation process. The number of graphene layers is found to be dependent on the growth temperature while the surface morphology also depends on the local off-cut in the substrate and results in a non-homogeneous surface. Additionally, the influence of dislocations on surface morphology and graphene thickness uniformity is also presented.

Origin of X-ray photon stimulated desorption of Cl+ and Cl2+ ions from Cl/Si(1 1 1)-(1×1)

In this paper we investigate minority adsorption sites on Cl/Si(1 1 1)-(1 � 1) with the X-ray standing wave technique. By a combination with X-ray photon stimulated desorption,we show that the coordinates of the desorption-active chlorine minority adsorption site can be determined analytically if standing wave difference spectra are recorded at the chlorine 1s absorption edge. Furthermore,the direct and indirect contributions to the total Cl þ and Cl 2þ ion desorption yields above the chlorine K edge can be separated and the ratio of the atomic desorption cross-sections can be estimated. 2002 Elsevier Science B.V. All rights reserved.

Photon‐stimulated Desorption and X‐ray Standing Waves

Introduction X-ray standing waves (XSW) using electrons and fluorescence photons as secondary signals has become a standard technique to determine adsorbate structures. However, when a sample is irradiated by intense light, desorption of ions is also a frequently observed phenomenon which is known as photon-stimulated desorption (PSD). But, this signal is of a fundamentally different physical nature. The identification of this process has been a persistent topic in surface science throughout the last decades [1]. By virtue of the inherent periodic time structure of synchrotron radiation, an efficient detection of positive ions emitted from the surface is feasible using time-of-flight spectroscopy. In this review, we will survey the relevant desorption mechanisms which initiate the prevailing processes. We will show that the combination of x-ray standing waves with x-ray PSD (XPSD) is a unique tool to identify the underlying desorption processes in a site specific manner. In general, two distinct types of stimulated desorption processes were identified in the past, i. e. direct and indirect desorption processes. This classification is based on the nature of the initial excitation which sequentially leads to ion emission. For direct processes, the desorption-active photoabsorption is located at the atom which is finally ejected. For indirect processes, the primary photons are absorbed either at the bonding partner of the desorbing ion, which results in so-called nearest-neighbor PSD, or in the underlying bulk crystal, leading to the creation of secondary electrons which cause the ion desorption by valence excitation (x-ray induced electronstimulated desorption (XESD)). Therefore, three primary desorption processes can be distinguished. While the details of the desorption mechanisms are in general highly specific to the system under investigation, it has to be emphasized that the dominating desorption processes can be identified by using synchrotron radiation. This is feasible through applying a combination of XSW and XPSD (XSW-PSD), since all three types can be related to distinct Fourier components of the atom distribution function. In the course of developing this method we have applied it to several systems, e. g. H/Pd(111), K/Ge(001), CsCl/Si(111) [2] as well as Cl/Si(001) and Cl/Si(113) [3]. In the following, we will deal with the systems Si(111)-(7x7), H/Si(111)-(1x1), Ge/Si(111)-(1x1):H [2], and Cl/Si(111) [4]. This choice serves to illustrate the peculiarities of the different kinds of desorption processes and to demonstrate the general approach to the interpretation of XSW-XPSD data.

CoPt3 nanoparticles adsorbed on SiO2: a GISAXS and SEM study

Ultra-thin CoPt 3 nanoparticle films have been prepared on SiO 2 surfaces using a Langmuir-Blodgett (LB) deposition technique. The structural properties of the overlayers have been investigated by grazing-incidence small-angle x-ray scattering (GISAXS) and high-resolution scanning electron microscopy (SEM) for the first time. Self-assembly of the nanoparticles is found and with GISAXS an average particle-particle distance of (8.23 ± 0.06) nm is determined, in good agreement with the SEM results. A particle correlation length of (22.3 ± 1.2) nm was derived which is shown to be independent of the surface coverage. The latter quantity may be controlled by choice of a suitable retraction speed during the LB step.