J. M. M. de Nijs

VEPFIT applied to depth profiling problems

Abstract The modelling and fitting program VEPFIT has been employed in recent years for resolving defect depth profiles and depth structures of deposited layers. Recent activities concerning program development include the testing of a new model of MOS systems for implementation into VEPFIT and a study into decomposition of Doppler-broadened photo-peaks. Further methods are proposed using VEPFIT for analysis of lifetime measurements and for modelling of positron transport with multi-energy groups.

Hydrogen induced donor‐type Si/SiO2 interface states

Study of donor‐type Si/SiO2 interfaces states shows that these centers anneal at room temperature when neutral but are stable when charged positively. Moreover, the anneal process is accompanied by the release of H. We propose that the donor states are related to H attached to interfacial network sites, most likely O atoms, constituting electrically active complexes. When positively charged, H is strongly bonded; when neutral H is only weakly attached. In the latter case it can escape and dimerize so that the states disappear. Similar complexes tying down H in a positively charged state would constitute small cross‐section hole traps in the bulk of the oxide.

Systematic and random errors in rotating-analyzer ellipsometry

Errors and error sources occurring in rotating-analyzer ellipsometry are discussed. From general considerations it is shown that a rotating-analyzer ellipsometer is inaccurate if applied at P = 0° and in cases when π = 0° or where Δ is near 0° or 180°. Window errors, component imperfections, azimuth errors and all other errors may, to first order, be treated independently and can subsequently be added. Explicit first-order expressions for the errors δΔ and δπ caused by windows, component imperfections, and azimuth errors are derived, showing that all of them, except the window errors, are eliminated in a two-zone measurement. Higher-order errors that are due to azimuth errors are studied numerically, revealing that they are in general less than 0.1°. Statistical errors are also discussed. Errors caused by noise and by correlated perturbations, i.e., periodic fluctuations of the light source, are also considered. Such periodic perturbations do cause random errors, especially when they have frequencies near 2ωA and 4ωA.

Analysis of positron beam data by the combined use of the shape‐ and wing‐parameters

An improved approach is presented for the analysis of positron beam Doppler broadening data. Instead of analyzing the energy‐dependent shape parameter, the so‐called S(E) data, we combined the shape S(E) and wing W(E) data by plotting them as a trajectory in the S–W plane, using the implantation energy as a running parameter. It is shown that this plot is of particular interest for the qualitative interpretation of the data. Furthermore, it allows the independent determination of the characteristic shape and wing parameters of the different positron trapping layers without the use of a numerical simulation and fitting program. The method and its advantages and limitations are illustrated for three cases: a silicon sample implanted with helium, a metal–oxide–silicon system subjected to a bias voltage and a bare oxide layer on silicon.

Degradation of the thermal oxide of the Si/SiO2/Al system due to vacuum ultraviolet irradiation

The generation of point defects in Si/SiO2/Al capacitors due to exposure to vacuum ultraviolet (VUV) (10 eV) radiation has been investigated by studying the electron and hole trapping properties and electron‐spin‐resonance spectroscopy for exposures ranging from 1014 up to 1019 photons cm−2 absorbed in the oxide. At low VUV exposures, the generation of hydroxyl groups and electron traps with cross section ≥10−16 cm2 is observed; however, for exposures larger than 1017 cm−2 these centers are subsequently eliminated. For exposures larger than 1018 cm−2 the oxide network is gradually destroyed; oxygen atoms are removed from their network positions and decorated with hydrogen atoms, thus producing water molecules and trivalent silicon centers in comparable numbers. At an exposure of 1019 cm−2, of the order of 1014 cm−2 of oxygen atoms are removed, without any indication that the process would saturate. A tentative model is presented in which the degeneration process takes place at regular network sites; a def...

Nature of defects in the Si‐SiO2 system generated by vacuum‐ultraviolet irradiation

We have used high‐frequency and quasi‐static capacitance‐voltage measurements to study the properties of interface states generated upon vacuum‐ultraviolet irradiation under positive gate bias followed by neutralization of the holes trapped in the oxide. The data indicate the exclusive generation of fast donor‐type states that anneal at room temperature. We propose that these states explain the turn‐around effect and annealing of positive charge.

Fabrication of Co/Si nanowires by ultrahigh-vacuum scanning tunneling microscopy on hydrogen-passivated Si(100) surfaces

We have fabricated nanometer width Co/Si metal lines on Si(100) surfaces by ultrahigh-vacuum scanning tunneling microscopy (UHVSTM) based nanolithography on the hydrogen-passivated surface, combined with vapor deposition of Co at room temperature and subsequent annealing. The STM tip was used to define depassivated lines (<10 nm in width) by electron stimulated hydrogen desorption, and subsequently Co was deposited at a submonolayer coverage. Annealing of the substrate at 410 °C (just below hydrogen desorption) improves the structure of the wire due to silicidation, whereas the as-deposited wire is very granular (comparable to other materials in previous studies).

Diffusion, nucleation and annealing of Co on the H-passivated Si(100) surface

We investigate the diffusion, nucleation and annealing behaviour of Co on H-passivated Si(100) surfaces by scanning tunneling microscopy (STM). Due to the absence of nucleation sites for silicide formation, the nucleation and growth mode is dominated by the formation of non-epitaxial islands which merge by increasing Co coverage θ. The island number density N shows a power law dependence on coverage N∝θc (c=0.29±0.03) for room temperature deposition. Annealing at temperatures up to ~400°C results in small changes of the Co clusters, while deposition at elevated substrate temperatures (~400°C) results in the formation of fewer but coarser Co islands. Finally, at higher annealing temperatures (~490°C) where H desorption takes place, the formation of two-dimensional islands occurs which are surrounded by an irregular 2×n (n>1) reconstructed surface due to interstitial diffusion of Co into Si.

Transport of positrons in the electrically biased metal-oxide-silicon system

This paper describes a study of the effect of an external electric field on the behavior of positrons in metal-oxide-silicon (MOS) systems. Doppler broadening measurements of the annihilation radiation were performed on capacitors with identical thermally grown SiO2 layers and with Al, W and Au layers as a gate. The data were analyzed by the combined use of the shape- and wing-parameters of the photo peak. The observed effects of the electric field are due to the field-driven transport of positrons through the SiO2, silicon and the interfaces. By applying a field of the order of 1 MV/cm the positrons can be efficiently transported through the approximately 100 nm thick SiO2 layer. From the transport behavior of the positrons it is concluded that the positron affinity is higher for SiO2 than for silicon and for the gate metal. By properly choosing the direction of the field, the positrons implanted into the SiO2 layer are collected either at the Si/SiO2 interface or at the SiO2/gate interface. For negative...

SiO2 hole traps with small cross section

SiO2 hole traps with small cross section (σ≤10−15 cm2) in Al gated metal–oxide–silicon capacitors have been studied using vacuum ultraviolet hole injection in combination with capacitance voltage measurements. The data show that small σ hole traps are related to H/hole pairs or protons trapped in the oxide. Since accumulated positive charge and H vanish simultaneously from the oxide in times of the order of 102 s, it is proposed that they migrate together through the oxide as a proton. The small σ hole traps are not associated with defects in the as‐grown oxide; they are generated by radiation induced release of atomic H. Their number is governed by the rates of release and of removal (dimerization) of atomic H.