R. P. Vasquez

Local atomic and electronic structure of oxide/GaAs and SiO2/Si interfaces using high‐resolution XPS

The chemical structures of thin SiO2 films, thin native oxides of GaAs (20–30 A), and the respective oxide–semiconductor interfaces, have been investigated using high‐resolution x‐ray photoelectron spectroscopy. Depth profiles of these structures have been obtained using both argon ion bombardment and wet chemical etching techniques. The chemical destruction induced by the ion profiling method is shown by direct comparison of these methods for identical samples. Fourier transform data‐reduction methods based on linear prediction with maximum entropy constraints are used to analyze the discrete structure in oxides and substrates. This discrete structure is interpreted by means of a structure‐induced charge‐transfer model (SICT).

Visible luminescence from silicon wafers subjected to stain etches

Etching of Si in a variety of solutions is known to cause staining. These stain layers consist of porous material similar to that produced by anodic etching of Si in HF solutions. We have observed photoluminescence peaked in the red from stain‐etched Si wafers of different dopant types, concentrations, and orientations produced in solutions of HF:HNO3:H2O. Luminescence is also observed in stain films produced in solutions of NaNO2 in HF, but not in stain films produced in solutions of CrO3 in HF. The luminescence spectra are similar to those reported recently for porous Si films produced by anodic etching in HF solutions. However, stain films are much easier to produce, requiring no special equipment.

Electronic structure of light‐emitting porous Si

Characterization of light‐emitting porous Si films with x‐ray photoelectron spectroscopy is reported. Only traces of O are detected on HF‐etched samples, in contradiction to an earlier report that oxides are a significant component of porous Si. Si 2p and valence‐band measurements demonstrate that the near‐surface region of high porosity films which exhibit visible luminescence consists of amorphous Si.

X-ray photoelectron spectroscopy study of Sr and Ba compounds

Abstract X-ray photoelectron spectroscopy (XPS) measurements of core level binding energies and kinetic energies of X-ray-induced Auger transitions for a comprehensive set of 25 Sr and Ba compounds are reported, including many for which there have been no previous XPS measurements. The majority of the Sr and Ba compounds measured here have cation core level binding energies which are lower than that of the metal, with the negative shifts largest for high temperature superconductors. These data are interpreted within a point charge model, which has previously been used to interpret XPS data from alkali halides and other ionic compounds. Data reported in the literature reveal that, rather than being anomalous, small or negative binding energy shifts are common to many ionic compounds. It is suggested that the large negative shifts observed for the high temperature superconductors are a consequence of large Madelung energies at the cation sites, and may be a measure of the hole distribution on the Cu-O planes. Measurements of the energy loss regions from the core levels of the Sr compounds are also reported. A peak which has previously been assigned as a configuration interaction satellite of the Sr4s peak is shown to be a plasmon loss from the Sr4p peak.

Nonaqueous chemical etch for YBa2Cu3O7−x

A nonaqueous chemical etch, with Br as the active ingredient, is described which removes the insulating hydroxides and carbonates that form on high‐temperature superconductor surfaces as a result of atmospheric exposure. X‐ray photoemission spectra have been recorded before and after etching YBa2Cu3O7−x films. It is found that, after the etch, the high binding energy O 1s and Ba 3d peaks associated with surface contaminants are greatly reduced, the Y:Ba:Cu ratio is close to the expected 1:2:3, and the oxidation state of the Cu(2+) is not affected. The resistance of an etched film reaches zero at 78 K, compared to 81 K for a similar unetched film. The suitability of other nonaqueous halogen‐based etches is discussed, as is the applicability of this etch to other high Tc superconductors.

A novel x‐ray photoelectron spectroscopy study of the Al/SiO2 interface

We report on the first nondestructive measurement of the chemical and physical characteristics of the interface between bulk SiO2 and thick aluminum films. Both x‐ray photoelectron spectroscopy (XPS) and electrical measurements of unannealed, resistively evaporated Al films on thermal SiO2 indicate an atomically abrupt interface. Post metallization annealing (PMA) at 450 °C induces reduction of the SiO2 by the aluminum, at a rate consistent with the bulk reaction rate. The XPS measurement is performed from the SiO2 side after the removal of the Si substrate with XeF2 gas and thinning of the SiO2 layer with HF:ETOH. This represents a powerful new approach to the study of metal‐insulator and related interfaces.

Reaction of nonaqueous halogen solutions with YBa2Cu3O7−x

X‐ray photoelectron spectroscopy (XPS) has been used to investigate the reaction of YBa2Cu3O7−x films with solutions of HF, HCl, Br2, HBr, I2, and HI in absolute ethanol (EtOH). The XPS core level and x‐ray excited Auger spectra from untreated and halogen‐treated surfaces are used to identify surface species by comparison with XPS data from the literature and with XPS spectra from more than 20 Y, Ba, and Cu halides, oxides, hydroxides, and carbonates measured in this work. XPS measurements on a number of these materials are being reported for the first time. Treatment of films with HF/EtOH results in the formation of an oxyfluoride with Y:Ba:Cu relative concentrations of 1:4:3. Additional features in the XPS spectra from HF‐treated films are also consistent with the formation of CuF, a compound which does not exist in bulk form. Treatment of films with HCl/EtOH results primarily in the formation of BaCl2 (∼75%), with smaller amounts of YCl3, CuCl, and CuCl2. Treatment of films with Br2/EtOH or HBr/EtOH re...

Effects of lattice distortion and Jahn-Teller coupling on the magnetoresistance of La0.7Ca0.3MnO3 and La0.5Ca0.5CoO3 epitaxial films

Studies of La0.7Ca0.3MnO3 epitaxial films on substrates with a range of lattice constants reveal two dominant contributions to the occurrence of colossal negative magnetoresistance (CMR) in these manganites: at high temperatures (T → TC, TC being the Curie temperature), the magnetotransport properties are predominantly determined by the conduction of lattice polarons, while at low temperatures (T ≪ TC/, the residual negative magnetoresistance is correlated with the substrate-induced lattice distortion which incurs excess magnetic domain wall scattering. The importance of lattice polaron conduction associated with the presence of Jahn–Teller coupling in the manganites is further verified by comparing the manganites with epitaxial films of another ferromagnetic perovskite, La0.5Ca0.5CoO3. Regardless of the differences in the substrate-induced lattice distortion, the cobaltite films exhibit much smaller negative magnetoresistance, which may be attributed to the absence of Jahn–Teller coupling and the high electron mobility that prevents the formation of lattice polarons. We therefore suggest that lattice polaron conduction associated with the Jahn–Teller coupling is essential for the occurrence of CMR, and that lattice distortion further enhances the CMR effects in the manganites.

Composition variations in pulsed-laser-deposited Y-Ba-Cu-O thin films as a function of deposition parameters

The composition of pulsed-ultraviolet-laser-deposited Y-Ba-Cu-O films was examined as a function of position across the substrate, laser fluence, laser spot size, substrate temperature, target conditioning, oxygen pressure and target-substrate distance. Laser fluence, laser spot size, and substrate temperature were found to have little effect on composition within the range investigated. Ablation from a fresh target surface results in films enriched in copper and barium, both of which decrease in concentration until a steady state condition is achieved. Oxygen pressure and target-substrate distance have a significant effect on film composition. In vacuum, copper and barium are slightly concentrated at the center of deposition. With the introduction of an oxygen background pressure, scattering results in copper and barium depletion in the deposition center, an effect which increases with increasing target-substrate distance. A balancing of these two effects results in stoichiometric deposition.

Chemical depth profiles of the GaAs/native oxide interface

The final‐state oxidation products and their distribution in thin native oxides (30–40 A) on GaAs have been studied using x‐ray photoelectron spectroscopy in conjunction with chemical depth profiling. Extended room‐temperature‐oxidation conditions have been chosen to allow the native oxide to attain its equilibrium composition and structure. The work emphasizes the use of chemical depth‐profiling methods which make it possible to examine the variation in chemical reactivity of the oxide structure. A minimum of two distinct regions of Ga2O3 with differing chemical reactivity is observed. Arsenic oxide species show one distinct phase at the outer oxide surface. Intensity analysis indicates an oxidized Ga to oxidized As composition ratio of 2:1 at the outer surface, which changes to approximately 10:1 at the oxide/GaAs interface. Chemical shift data indicate the presence of As2O3 in the oxide together with an elemental As overlayer at the interface. A change in relative charge transfer between oxygen and bot...