J. J. Joyce

Quantitative analysis of synchrotron radiation photoemission core level data

Abstract A procedure for quantitative analysis of photoemission core level energy distribution curves is presented which includes the determination of the Fermi level, the background function, and phonon broadening, as well as properties unique to evolving metal/semiconductor interfaces. The fitting algorithm based on a non-linear least squares routine is discussed in detail. Core level data are represented by computer-generated Lorentzian (or Doniach-Sunjic) lineshapes convoluted with Gaussians. An approximation to the convolution representation using summations of a Lorentzian (or Doniach-Sunjic) with a Gaussian is presented and its limitations are discussed. Constraints on the fitting parameters are discussed in light of the physical descriptions of clean surfaces, interfaces, and the photoemission process. The Si2 p and As3 d (in GaAs) core levels are considered as prototype shallow core levels. This quantitative analysis has been used successfully on a wide range of group IV, III-V and II-VI semiconductors as well as a number of metals.

Metal–anion bond strength and room temperature diffusion at metal/GaAs interfaces: Transition versus rare‐earth versus Au metal overlayers

We discuss synchrotron radiation photoemission results for several room temperature transition metal and rare–earth metal GaAs(110) interfaces (Ce, Sm, V, Cr) and Au. Analysis of normalized core intensity attenuation curves shows anion trapping that varies with, but is not entirely controlled by, the ionicity of interface bonds. In particular, very narrow reacted regions were observed for Ce and Sm [3–5 monolayers (ML)]. The reacted region is much wider for the transition metals (9 ML for V, 19 ML for Cr), reflecting a less effective barrier for intermixing. For Au, the amount of As and Ga in the near surface region greatly exceeds that for the transition metals.

Systematics of electronic structure and local bonding for metal/GaAs(110) interfaces

We present high‐resolution synchrotron radiation core level photoemission results which reveal the evolving electronic structures and morphologies of metal/GaAs interfaces for the metals Ce, Sm, Ti, V, Cr, Fe, Co, Cu, and Au. By studying a wide range of overlayer metals we sought to identify common interface characteristics and discriminate between chemical and morphological effects. Quantitative fitting of the Ga and As 3d core level emission provided insight into the stages of interface development. Our results indicate that Ga is found in solution at these interfaces with chemical shifts from −0.40 eV for Au to −1.78 eV for Ce (relative to Ga in GaAs). In contrast, the results for As indicate well‐defined local chemical environments and, in some cases, the possibility of surface segregation. A direct correlation between overlayer electronegativity and core level shifts is observed.

Modeling of interface reaction products with high‐resolution core‐level photoemission

High‐resolution photoemission studies make it possible to distinguish different atomic configurations at evolving interfaces by monitoring chemical shifts. Hence, it is possible to determine the coverage at which reactions are triggered and are effectively completed, the species that outdiffuses into the metal overlayer, and the bonding of surface‐segregated species. Core‐level deconvolutions and plots of the concentration of substrate, reacted, and segregated species as a function of coverage are discussed for Ce/Si(111), Ce/GaAs(110), and Cr/GaAs(110). Our results show that distinct Ce/As and Ce/Si species form but that no distinct Cr/Ga bonding configuration is established at the interface.

Reaction at a refractory metal/semiconductor interface: V/GaAs(110)

Synchrotron radiation photoemission spectroscopy has been used to study the formation of the reactive V/GaAs(110) interface. Valence‐band and core‐level results indicate that metal deposition produces an extended intermixed phase involving the formation of both V–Ga and V–As bonds. In this reacted region the Ga 3d core line exhibits a continuous shift to lower binding energy (total shift 1.55 eV over band bending) indicative of a variable chemical environment, while analysis of the As 3d line shape suggests that As is present in two well‐defined chemical states. Core‐level intensity profiles show preferential outdiffusion of arsenic, with As present at ∼6% of the original level with coverages of 110 A. Comparison to previous results for Cr/GaAs(110) shows similar Ga and As attenuation profiles.

V/Ge(111): Temperature dependent intermixing studied with high resolution photoemission and quantitative modeling

High resolution, temperature dependent core level photoemission results for the V/Ge(111) interface show the interplay between atomic diffusion and trapping in the boundary layer. Room temperature results show strong intermixing, and core level analysis allows us to identify well‐defined chemical environments for Ge atoms (chemical shifts of −0.5 and −0.95 eV). Through modeling of the interface, we have determined the composition and extent of each reacted species. Studies at higher temperature showed that the extent and composition of the reacted overlayer could be varied. Indeed, by increasing the substrate temperature, we could produce an overlayer which became chemically homogeneous by 475 K. In this range, we find a diffusion activation energy of 5 kcal/mol which controls the width of the reacted layer.

High-Resolution Core Level Study of the Co/Si(111) Interface

There has recently been considerable interest in the reaction between Co and a clean Si surface. This interest stems from the epitaxy of CoSi 2 and NiSi 2 on Si and its potential for the construction of reliable and stable metal-semiconductor structures. In fact, the fabrication of a Si/CoSi 2 /Si transistor has been recently reported.[l] On a more fundamental side, it has been possible to address the problem of the relation between Schottky barrier height and structure at the NiSi 2 /Ni interface, which exhibits both a rotated (B-type) and unrotated (A-type) geometry.[2] For CoSi 2 /Si only the 180° rotated, B-type disilicide is formed. By studying the room temperature interface, we have attempted to describe the nature and physical extent of reaction products; such knowledge is important to understand the formation of interface silicides which ultimately control the nature of the high temperature epitaxial interface.