P.R. Varekamp

Stimulated desorption of Cl+ and the chemisorption of Cl2 on Si(111)-7×7 and Si(100)-2×1

The chemisorption of Cl2 on Si(111)-7 × 7 and Si(100)-2 × 1 and the mechanism for stimulated desorption of Cl+ from Si are studied with soft X-ray photoelectron spectroscopy (SXPS) and photon-stimulated desorption (PSD). It is shown that Cl2 interacts with Si(111)-7 × 7 at room temperature to form mono-, di- and tri-chlorides, while primarily monochlorides are formed on Si(100)-2 × 1. These differences are explained in terms of the reconstructions of each clean surface. The stimulated desorption of Cl+ ions has a threshold at ~ 20 eV that results from a direct excitation of a Cl 3s electron to an unoccupied Cl antibonding level. No direct desorption of Cl+ ions is observed at the Si 2p edge. Differences between the mechanisms for F+ and Cl+ desorption from Si are discussed.

Substrate disorder induced by a surface chemical reaction: the fluorine-silicon interaction

Abstract Si(111) surfaces etched by XeF 2 are studied with high-resolution soft X-ray photoelectron spectroscopy. After a sufficient exposure to XeF 2 , a thick fluorosilyl reaction layer forms on the surface. An inelastic loss feature in the Si2p core-level spectra collected from these surfaces indicates the presence of Si defects that have been created by the reaction. These defects are located both at the reaction layer/substrate interface and in the near-surface region of the substrate. As suggested by earlier theoretical studies, it is proposed that the defects in the substrate are created by the excess heat liberated from the formation of exothermic Si-F bonds. Reaction-induced defects are an integral and essential part of the etching process, as they enable F to bond with subsurface Si and form the characteristic fluorosilyl reaction layer.

Soft x‐ray photoelectron spectroscopy study of the reaction of XeF2 with GaAs

The room temperature reaction of atomic F (via XeF2 vapor) with GaAs(110) wafers is studied employing synchrotron‐based soft x‐ray photoelectron spectroscopy. Both film growth and etching occur simultaneously in this reaction, as the GaAs substrate is consumed in forming a GaF3 film while the excess As is liberated. The intermediate reaction products GaF, AsF, and/or elemental As, as well as tricoordinate Ga and As atoms, are present at the GaF3–GaAs interface throughout the various stages of film growth.

THE GROWTH OF GAF3 FILMS ON GAAS(110) AT ELEVATED TEMPERATURES STUDIED WITH SOFT X-RAY PHOTOELECTRON SPECTROSCOPY

The growth of GaF3 films on GaAs(110) wafers via exposure to XeF2 is studied as a function of substrate temperature with soft x‐ray photoelectron spectroscopy. For temperatures between 300 and 550 K, a GaF3 film forms with an interface to the substrate consisting of GaF, elemental As and possibly some AsF. In this temperature range, the films thicken with exposure up to a limit of ∼15 A. The initial F uptake rate increases with elevated substrate temperature without altering the limiting thickness. In addition, there is little variation in the thickness or composition of the film–substrate interface over this temperature range. Above 550 K, XeF2 etches GaAs, leaving a GaAs surface covered with ∼1 monolayer of elemental As. A film growth mechanism is discussed to explain these results.

RADIATION DAMAGE OF EPITAXIAL CAF2 OVERLAYERS ON SI(111) STUDIED BY PHOTON-STIMULATED DESORPTION : FORMATION OF SURFACE F CENTERS

The effects of radiation damage at the surfaces of CaF2 films grown on Si(111) substrates have been studied and the electronic transitions responsible for the formation of surface F centers have been identified by photon‐stimulated desorption and soft x‐ray photoelectron spectroscopy. The dominant desorption channel for F+ ions is a direct Auger‐stimulated process following an electronic transition from Ca 3p to 3d‐derived states above the CaF2 conduction band minimum. At the Si 2p edge, there is an indirect x‐ray induced electron‐stimulated desorption process that contributes only slightly to the desorption of F+. The changes in the kinetic energy distributions of the desorbing F+ ions as a function of film thickness are also discussed.

Photon-stimulated desorption of halogens

Photon-stimulated desorption of positive halogen ions is studied for systems than span a range of bond ionicities. It is found that only excitations of cation core-level electrons lead to ion desorption for very ionic systems, while only excitations of anion core electrons produce ions from more covalently bonded materials. For systems of intermediate bonding character, both cation and anion excitations can lead to ion desorption. These observations are explained within the context of Auger-stimulated desorption combined with nuclear motion.