Formation of GaAs1−xNx nanofilm on GaAs by low energy N2 implantation

Nitridation of GaAs (1 0 0) by N₂⁺ ions with energy E_i = 2500 eV has been studied by Auger- and Electron Energy Loss Spectroscopy under experimental conditions, when electrons ejected only by nitrated layer, without contribution of GaAs substrate, were collected. Diagnostics for quantitative chemical analysis of the nitrated layers has been developed using the values of NKVV Auger energies in GaN and GaAsN chemical phases measured in one experiment, with the accuracy being sufficient for separating their contributions into the experimental spectrum. The conducted analysis has shown that nanofilm with the thickness of about 4 nm was fabricated, consisting mainly of dilute alloy GaAs_1−xN_x with high concentration of nitrogen x ∼ 0.09, although the major part of the implanted nitrogen atoms are contained in GaN inclusions. It was assumed that secondary ion cascades generated by implanted ions play an important role in forming nitrogen-rich alloy.     

Nitrogen binding behavior in ZnO films with time-resolved cathodoluminescence

ZnO film with (1 0 0) orientation was produced on silicon substrate and doped with nitrogen using plasma immersion ion implantation. The effects due to N doping were investigated using cathodoluminescence (CL). In the heavily nitrogen-doped ZnO film, the intensity of ultraviolet (UV) band decreases and that of the visible band increases as a function of the electron bombardment cycle i.e. time. Based on the X-ray photoelectron spectroscopy (XPS) analysis, the unstable ZnN bond is responsible for the CL behavior and the experimental results agree well with the first-principle calculation. Our work is helpful to our understanding of the role of p-type dopants in ZnO.     

Investigation of Oxygen Vacancy and Interstitial Oxygen Defects in ZnO Films by Photoluminescence and X-Ray Photoelectron Spectroscopy

ZnO films prepared at different temperatures and annealed at 900^o C in oxygen are studied by photoluminescence （PL） and x-ray photoelectron spectroscopy （XPS）. It is observed that in the PL of the as-grown films the green luminescence （GL） and the yellow luminescence （YL） are related, and after annealing the GL is restrained and the YL is enhanced. The 0 ls XPS results also show the coexistence of oxygen vacancy （Vo） and interstitial oxygen （Oi） before annealing and the quenching of the Vo after annealing. By combining the two results it is deduced that the GL and YL are related to the Vo and Oi defects, respectively.     

The effect of pH and role of Ni in zinc phosphating of 2024-Al alloy: Part I: Macroscopic studies with XPS and SEM

Coatings formed on 2024-T3 aluminum alloy were studied by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) after dipping in zinc phosphating (ZPO) baths at different acidities, for different lengths of time, and with or without Ni²⁺ additive. The overall objective was to learn more about the role of Ni²⁺ on the ZPO coating mechanism, particularly since this additive is believed to improve corrosion protection for the Al alloy. Secondary phosphates dominate the coatings when the Ni-containing solution is adjusted to starting pH values of either 3 or 5, while tertiary phosphate is predominant at pH 4. AlF₃ precipitates during the early stages of the coating process. Ni²⁺ has two main roles in the mechanism. First, the rate of increase in local solution pH is retarded by the slower kinetics of reactions involving Ni²⁺ compared to Zn²⁺, leading to thinner ZPO coatings when Ni²⁺ is present in the coating solution. Second, most Ni²⁺ deposition occurs during the later stages of the coating process, by nickel phosphate deposition and/or by formation of a Ni-rich oxide.     

Spectroscopy in our age

The development of modern spectroscopy is summarized from Bunsen’s detection of atoms as the beginning of spectral analysis to modern molecular spectroscopies including new high resolution techniques for molecular ions. Recent experiments involving long range charge migration in peptides and proteins are outlined. Received: 2 October 2000 / Published online: 11 October 2000     

The growth of Au/Pd/alumina/Cu-Al system studied by SRPES

An ultra-thin alumina layer grown on Cu-9at.%Al (1 1 1) surface was studied using synchrotron radiation photoelectron spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). By deconvolving SRPES spectra of the Al 2p doublet, four components belonging to metallic as well as oxide phases were recognized. Pd-Au alloy formation was confirmed by SRPES measurement during Pd and Au deposition. The study of the system's thermal stability reveals diffusion of Pd and Au atoms through the alumina layer. While Au atoms start to diffuse under the alumina layer at 670 K, Pd atoms are forming Pd-Al surface alloy at this temperature. The diffusion of Pd atoms through alumina occurs when sample was heated over 770 K. Alumina layer was stable even after heating the sample at 870 K, but its structure was corrupted probably due to the diffusion of metal atoms.     

XPS and ToF-SIMS analysis of natural rubies and sapphires heated in an inert (N2) atmosphere

Advanced surface analysis techniques: X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, have been employed in the study of heat treatment of natural corundum as ruby and sapphire. The stones were heat treated in an inert (N₂) atmosphere. The setting temperatures were: 1000, 1100, 1200, 1300, 1400, 1500 and 1600 °C. The XPS studies and the parallel ToF-SIMS experiments revealed diffusion behavior of Fe and Ti in the as-mined stones as evidenced by surface observations. Both metals exhibited broad maxima in surface concentration near 1300 °C. Owing to its superlative detection limit, ToF-SIMS spectra are able to provide the temperature-dependent concentration profiles of trace transition metals such as Cr, Cu and V at a level not detectable by XPS. Visible appearance of the stones is clearly affected by heat treatment. Interestingly, the ruby stones did not exhibit cloudy inclusion (“silk”) on heating, contrary to previous experiments under atmospheric conditions.     

An XPS study of CrOx on a thin alumina film and in alumina supported catalysts

We have investigated chromium layers evaporated onto a thin alumina film at room temperature. The oxidation and reduction behavior of this model catalyst was compared to atomic layer deposition (ALD) and impregnated alumina supported catalysts using X-ray photoelectron spectroscopy (XPS) with a detailed analysis method utilizing asymmetric peak shapes to represent both metallic and oxidic states. The ALD and impregnated catalysts were measured after calcination in air and after reduction with several gases at 850 K. Both catalysts show Cr³⁺ and Cr⁶⁺ species after calcination and mostly Cr³⁺ after reduction. The chromium layers deposited in vacuum show initially small partial oxidation due to the interaction with the oxygen terminated alumina film. These model catalysts can be oxidized in vacuum to Cr³⁺ species but not to higher oxidation states. The model catalysts were also subjected to calcination and reduction treatments after deposition in vacuum. Under these conditions the model systems exhibit similar oxidation/reduction behavior as the supported catalysts. Photoreduction of Cr⁶⁺ during the measurements was also studied and found to be very slow having a negligible effect on the results.     

Non-equilibrium grain boundary cosegregation of Mo and P

The segregation of P at grain boundaries is believed to be an important cause of temper embrittlement in steels. As an alloy element, Mo may reduce the embrittlement. However, the concentration measured by Auger electron spectroscopy at the grain boundary in 2.25Cr1MoV and 12Cr1MoV showed that the concentration of P increased with that of Mo, which indicates that Mo and P cosegregated to the grain boundary in Cr-Mo steels.     

Surface segregation in HAYNES 230 alloy

The surface segregation in the Ni-based alloy HAYNES 230 was studied by Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy between 400 and 1100 °C. The qualitative variations of the surface contents of S, P, W, Mo, N, Si, and Mn were determined as a function of annealing temperature and time. It was found that at 925 °C the maximum coverage of sulphur at the alloy surface is in the range 0.06-0.15 monolayers. Chromium evaporation from the HAYNES 230 surface under UHV conditions is clearly evidenced for annealing at 1100 °C.     

Surface characterisation and interface studies of high-k materials by XPS and TOF-SIMS

High-k dielectric LaAlO₃ (LAO) films on Si(100) were studied by TOF-SIMS and XPS to look for diffusion processes during deposition and additional thermal treatment and for the formation and composition of possible interfacial layers. The measurements reveal the existence of SiO₂ at the LAO/Si interface. Thermal treatment strengthens this effect indicating a segregation of Si. However, thin LAO layers show no interfacial SiO₂ but the formation of a La-Al-Si-O compound. In addition, Pt diffusion from the top coating into the LAO layers occurs. Within the LAO layer C is the most abundant contamination (10²¹ at/cm³). Its relatively high concentration could influence electric characteristics. XPS shows that CO₃²⁻ is intrinsic to the LAO layer and is due to the adsorption of CO₂ of the residual gas in the deposition chamber.     

The Harrison diffusion kinetics regimes in solute grain boundary diffusion

Knowledge of the limits of the principal Harrison kinetics regimes (Types A, B and C) for grain boundary diffusion is very important for the correct analysis of depth profiles in a tracer diffusion experiment. These regimes for self‐diffusion have been extensively studied in the past by making use of the phenomenological lattice Monte Carlo (LMC) method with the result that the limits are now well established. However, the relationship of these self‐diffusion limits to the corresponding ones for solute diffusion in the presence of solute segregation to the grain boundaries remains unclear. In the present study, the influence of solute segregation on the limits was investigated with the LMC method for the well‐known parallel grain boundary slab model by showing the equivalence of two diffusion models. It is shown which diffusion parameters are useful for identifying the limits of the Harrison kinetics regimes for solute grain boundary diffusion. It is also shown how the measured segregation factor from the diffusion experiment in the Harrison Type‐B kinetics regime may differ from the global segregation factor.     

X-ray photoelectron spectroscopy study of sputter-annealed Ni2.1Mn0.9Ga surface

The modifications in surface composition of Ni_2.1Mn_0.9Ga ferromagnetic shape memory alloy have been investigated using X-ray photoelectron spectroscopy (XPS) under various sputtering and annealing conditions. XPS core-level spectra show that sputtering makes the surface Ni rich. However, by annealing, the Mn content at the surface increases and at about 390 °C the bulk composition is restored. The valence band spectra show evidence of Ni related extra states for the sputtered surface, which decrease with annealing. This behavior is in agreement with the change in surface composition derived from the core-level spectra.     

Quantitative XPS imaging—new possibilities with the delay-line detector

X-ray photoelectron spectroscopy (XPS, ESCA) is an ideal tool for identifying differences in surface chemistry. In the past, it has lacked the spatial resolution as well as the performance of elemental or even chemical state imaging, to be of significant use in detecting most microscopic surface phenomena. The recent development of improved micro- or small spot-XPS systems with near-micron spatial resolution as well as outstanding chemical state image performance has opened up a significant opportunity to undertake more detailed studies of micro-structured or micro-patterned surfaces or technical samples with locally distributed impurities. The introduction of a new detector type, the delay-line detector (DLD), to XPS-equipments allows for the first time the acquisition of quantifiable XPS images. This study is intended primarily to explore the capabilities of quantitative ESCA-imaging with respect to the possibilities and limits.     

XPS investigation of the PTFE induced hydrophobic properties of electrodes for low temperature fuel cells

In electrodes of low temperature fuel cells like polymer electrolyte membrane fuel cells (PEFC) or alkaline fuel cells (AFC) the reactants and the water must be transported. For this purpose the pore system in the electrodes needs a hydrophilic character for the transport of the water and a hydrophobic character for the transport of the gases. The degree of the hydrophobicity determines whether the pore system will be flooded by the reaction water. In the case of PEFC, this is also determined by the degree of the required humidification of the reaction gases. In AFC hydrophobicity determines the extension of the three-phase reaction zone. Caused by the strong influence of hydrophobicity on the transport processes, the electrochemical performance and the optimized operation conditions are also affected by hydrophobicity.Typically polytetrafluoro-ethylene (PTFE) is used to make the electrodes hydrophobic, because PTFE has a high chemical stability. Hydrophobicity depends on the concentration of PTFE on the electrode surface. The PTFE concentration, which is related to the hydrophobic character, can be determined by XPS. The changes in the PTFE content and structure of the electrode of a PEFC was investigated by cyclic voltammetry and XPS and correlated with the performance of the cell in long-term operation. With both methods an initial significant increase in free and electrochemically active surface platinum area is observed. This activation is associated with a degradation of the PTFE in the electrode which is responsible for the hydrophobic properties of the electrode. With further operation the performance of the cell decreases because the water management becomes more critical. Generally, it is shown that XPS can be used for the investigation of the hydrophobicity of electrodes prepared by various manufacturing techniques as well as of changes in their hydrophobicity induced by the electrochemical operation.     

Microstructural effects on the formation and degradation of zinc phosphate coatings on 2024-Al alloy

The formation of zinc phosphate (ZPO) coatings on 2024-T3 aluminum alloy was studied using scanning electron microscopy (SEM), scanning Auger microscopy (SAM) and X-ray photoelectron spectroscopy (XPS), with an emphasis on microstructural effects involving second-phase particles and the alloy matrix. Surface polishing results in an Al-Cu-Mg particle surface that contains metallic Cu as well as an overlayer of aluminum and magnesium oxide, while larger amounts of aluminum oxide are present on the Al-Cu-Fe-Mn particle and matrix. When dipped in an acidic ZPO coating solution, the oxide covering the Al-Cu-Mg particle is etched most easily, and metallic Cu near the surface makes that region most cathodic, allowing more coating deposition compared with the other regions. The oxides on the Al-Cu-Fe-Mn and matrix regions are similar, thereby confirming that the observed differences in ZPO coating characteristics at these two regions arise from their underlying electrochemical characteristics. Immersion of a coated 2024-Al sample in corrosive NaCl solution for extended periods indicates that the ZPO provides better protection to the second-phase particles than to the matrix.     

Study of the surface contamination of copper with the improved positron annihilation-induced Auger electron spectrometer at NEPOMUC

The high intensity positron source NEPOMUC at the FRM-II in Munich enables measurement times for positron annihilation-induced Auger electron spectroscopy (PAES) of only 2.4 h/spectrum, in contrast to usual lab beams with measurement times up to several days. The high electron background due to surrounding experiments in the experimental hall of the FRM-II has been eliminated and hence background free experiments have become possible. Due to this, the signal to noise ratio has been enhanced to 4.5:1, compared to 1:3 with EAES. In addition, a long-term measurement has been performed in order to observe the contamination of a polycrystalline copper foil at 150 °C.     

Dopant diffusion and segregation in semiconductor heterostructures: Part III, diffusion of Si into GaAs

We have mentioned previously that in the third part of the present series of papers, a variety of n-doping associated phenomena will be treated. Instead, we have decided that this paper, in which the subject treated is diffusion of Si into GaAs, shall be the third paper of the series. This choice is arrived at because this subject is a most relevent heterostructure problem, and also because of space and timing considerations. The main n-type dopant Si in GaAs is amphoteric which may be incorporated as shallow donor species Si_Ga ⁺ and as shallow acceptor species Si_As ^-. The solubility of Si_As ^- is much lower than that of Si_Ga ⁺ except at very high Si concentration levels. Hence, a severe electrical self-compensation occurs at very high Si concentrations. In this study we have modeled the Si distribution process in GaAs by assuming that the diffusing species is Si_Ga ⁺ which will convert into Si_As ^- in accordance with their solubilities and that the point defect species governing the diffusion of Si_Ga ⁺ are triply-negatively-charged Ga vacancies V_Ga ^3-. The outstanding features of the Si indiffusion profiles near the Si/GaAs interface have been quantitatively explained for the first time. Deposited on the GaAs crystal surface, the Si source material is a polycrystalline Si layer which may be undoped or n⁺-doped using As or P. Without the use of an As vapor phase in the ambient, the As- and P-doped source materials effectively render the GaAs crystals into an As-rich composition, which leads to a much more efficient Si indiffusion process than for the case of using undoped source materials which maintains the GaAs crystals in a relatively As-poor condition. The source material and the GaAs crystal together form a heterostructure with its junction influencing the electron distribution in the region, which, in turn, affects the Si indiffusion process prominently. Received: 19 April 1999 / Accepted: 3 May 1999 / Published online: 4 August 1999     

High resolution quantitative SIMS analysis of shallow boron implants in silicon using a bevel and image approach

Secondary ion mass spectrometry (SIMS) is frequently used as the preferred tool for dopant profiling due to its sensitivity and depth resolution. However, as dopant profiles become shallower most, if not all of the implant profile lies in the pre-equilibrium or transient region of an SIMS depth profile. In this region sputter yield and ionisation rate vary making accurate quantification of the implant profile very difficult. These problems can be reduced through the use of much lower beam energies or oxygen flooding of the sample. However, most SIMS instruments do not have these capabilities. In this paper an alternative technique for producing an accurate depth profile of a shallow implant, using existing SIMS technology is presented.Through the fabrication of bevels with very small slope angles on a shallow boron implanted silicon via a chemical etch, SIMS ion imaging is performed on the exposed surface. Ion image data is then summed, and in conjunction with accurate measurement of the bevel morphology, a shallow boron implant profile produced. The ‘bevel-image’ profile compares very well with a profile obtained using a 1 keV oxygen beam. To ensure a good dynamic range on the ‘bevel-image’ profile it is important to clean the bevel with a HF etch, prior to imaging.