J. F. Morar

Surface oxidation states of germanium

Abstract High resolution Ge 3d photoelectron spectra obtained with synchrotron radiation are used to determine the surface oxidation states of Ge(100) and Ge(111) surfaces. For both orientations four discrete chemically shifted core levels are observed that yield a chemical shift of 0.85 eV per Ge-O bond. With this interpretation, Ge forms 1+, 2+, 3+ oxidation states when exposed initially. Upon annealing predominantly the 2+ state is formed. The stability of this 2+ oxidation state is compared within the neighboring elements.

Soft X-ray photoemission study of the silicon-fluorine etching reaction

High resolution soft X-ray photoemission spectra of Si(111) surfaces subjected to steady-state etching are reported. The reaction is shown to proceed via the formation of a thick, highly fluorinated reaction layer, dominated by trifluorosilyl moieties. In addition, SiF4, the major reaction product has been observed in the spectra, indicating that it may be trapped within the reaction layer. The implications of these measurements for previously proposed reaction mechanisms are discussed.

Synchrotron‐radiation excited carbon 1s photoemission study of Cr/organic polymer interfaces

The chemical nature of the adhesive interface between Cr and three polymers has been investigated using synchrotron‐radiation excited photoemission from the polymer carbon 1s core levels. PMDA–ODA polyimide (PI), an important polymer in the electronics industry, was studied along with two simple polymers, PMPO and PVMK, which contain structural fragments of the complicated PI protomer. The carbon 1s spectra were decomposed into distinct peaks which were assigned to specific chemical species and monitored as a function of Cr coverage. Our results confirm that the carbonyl groups on these polymer surfaces are targets for reactive metals during the initial stages of metallization. This initial reaction stage was followed by gradual formation of an electron‐rich, carbidelike, carbon species.

Synchrotron photoemission investigation: Fluorine on silicon surfaces

High resolution core level photoemission spectroscopy has been used to obtain the first direct identification of the chemical species remaining on silicon surfaces after exposure to fluorine. Both Si(111) 2×1 and Si(111) 7×7 were exposed to fluorine via the dissociative chemisorption of XeF2. For fluorine coverages in the monolayer regime, SiF1, SiF2, and SiF3 were all present although their relative abundance varied significantly between the two surfaces. No evidence for the existence of unreacted interestitial fluorine was found. These results suggest the need for modification of current models describing plasma and reactive ion etching of silicon.

Structure and bonding at the CaF2/Si (111) interface

High resolution core level spectroscopy with synchrotron radiation is used to determine the bonding at the epitaxial CaF2/Si (111) interface. It is found that both Ca and F bond to Si at the interface inducing core level shifts of +0.4 eV and −0.8 eV, respectively. Structural models with an atomically sharp interface are proposed where Ca bonds to the first layer Si and F to the second layer.

Oxygen removal from Si via reaction with adsorbed Ge

Silicon surface optimization prior to film growth is central to the fields of chemical vapor deposition and molecular beam epitaxy. We have examined a method for low‐temperature in situ cleaning of the Si (100) surfaces utilizing a submonolayer coverage of germanium. Synchrotron excited x‐ray photoemission data indicate that Ge atoms arriving at a Si (100) surface can break silicon–oxygen bonds, thereby producing new chemical species which sublimate at 625 °C. In the absence of Ge, the observed silicon oxide species were stable at temperatures well in excess of 750 °C. These results are used to investigate the mechanisms by which adsorbed Ge can be used to produce oxide‐free Si (100) surfaces at 625 °C.

Early stages in the formation of the oxide-InP(110) interface

Abstract The early stages in the formation of the InP(110)-oxide interface have been studied using high energy resolution photoemission with synchrotron radiation. Both unexcited and excited oxygen were used. Changes were observed in P 2p, In 4d and valence-band spectra. The oxidation of InP appears spatially inhomogeneous. Three stages were observed: a precursor chemisorption stage, a nucleation process, the formation of an oxide layer.

High resolution photoemission investigation: The oxidation of W

Surface‐sensitive core‐level photoelectron spectra excited by synchrotron radiation are presented for surface oxide phases of W in the monolayer range. Discrete chemically shifted core levels are observed and are shown to be associated with specific formal oxidation states by comparison with bulk reference compounds. Slight increases in the value of the core‐level shift for each oxidation state as the surface oxygen content increases are attributed to changes in the screening response of the system. Decomposition of the spectra into the separate contributions from each oxidation state provides specific and quantitative chemical information about the surface oxides. This and similar studies should be particularly useful for characterizing the disordered systems typical in applied surface physics where clarity of interpretation is very important.

Spectroscopic characterization of fluorinated silicon single crystal surfaces

Spectroscopic studies of Si(100) fluorinated by the dissociative chemisorption of XeF2 at 80 K are reported. LEED and XPS measurements suggest the formation of a disordered fluorosilyl layer after low fluorine doses. Separate EELS experiments identified a fluorine induced 800 cm−1 loss, suggesting that SiF is the major surface fluorosilyl species. Vibrational evidence for coadsorbed hydrogen, due to side reactions during fluorination, is also found. Recent synchrotron soft x‐ray photoemission spectra support the SiF identification and also demonstrate dramatic crystallographic effects in comparative Si(111) studies.Spectroscopic studies of Si(100) fluorinated by the dissociative chemisorption of XeF2 at 80 K are reported. LEED and XPS measurements suggest the formation of a disordered fluorosilyl layer after low fluorine doses. Separate EELS experiments identified a fluorine induced 800 cm−1 loss, suggesting that SiF is the major surface fluorosilyl species. Vibrational evidence for coadsorbed hydrogen, due to side reactions during fluorination, is also found. Recent synchrotron soft x‐ray photoemission spectra support the SiF identification and also demonstrate dramatic crystallographic effects in comparative Si(111) studies.

Conduction band structure of GexSi1-x using spatially resolved electron energy-loss scattering

Transmission electron energy‐loss spectroscopy has been used to follow the positions of the Δ1 and L1 conduction‐band minima, and the L3 saddle point, as a function of Ge content in GexSi1−x (x=0–0.95) alloys. By analyzing the shape of the Si 2p→conduction‐band (CB) spectra we find that L3 and Δ1 shift largely together, as the band‐structure compatibility relations predict. L1 shifts rapidly downwards with respect to Δ1 as the Ge content increases. Measurements were carried out in a scanning transmission electron microscope at a spatial resolution of better than 2 nm. Based on these results, this technique can be used to identify the composition of thin GexSi1−x alloys with a precision of better than 5%.