T. C. Leung

Defects in MeV Si‐implanted Si probed with positrons

Vacancy‐type defects produced by implantation of MeV doses of Si ions (1011–1015 atoms/cm2) at room temperature have been probed using depth‐resolved positron annihilation spectroscopy. The defect (divacancy) concentration increases linearly with dose for low doses (<1012 Si/cm2). In situ isochronal annealing was followed for oxygen‐containing Si (10 ppm) and oxygen‐‘‘free’’ Si implanted to doses (5×1012 and 5×1014 Si/cm2). Two main annealing stages were observed at the same temperatures in the studied samples in spite of significant differences in doses and oxygen content. In the first stage (∼200u2009°C) a significant fraction of divacancies was observed to form large vacancy clusters. These clusters were removed in the second stage (∼675u2009°C) after which the oxygen‐free samples returned to pre‐irradiation conditions, whereas oxygen‐defect complexes were formed in the oxygen‐containing samples.

Study of SiO2‐Si and metal‐oxide‐semiconductor structures using positrons

Studies of SiO2‐Si and metal‐oxide‐semiconductor (MOS) structures using positrons are summarized and a concise picture of the present understanding of positrons in these systems is provided. Positron annihilation line‐shape S data are presented as a function of the positron incident energy, gate voltage, and annealing, and are described with a diffusion‐annihilation equation for positrons. The data are compared with electrical measurements. Distinct annihilation characteristics were observed at the SiO2‐Si interface and have been studied as a function of bias voltage and annealing conditions. The shift of the centroid (peak) of γ‐ray energy distributions in the depletion region of the MOS structures was studied as a function of positron energy and gate voltage, and the shifts are explained by the corresponding variations in the strength of the electric field and thickness of the depletion layer. The potential role of the positron annihilation technique as a noncontact, nondestructive, and depth‐sensitive ...

Microvoids and defect chemistry at the SiSiO2 interface studied by positron annihilation depth profiling

Abstract A low energy positron beam (few keV) has been used to study device quality dry thermal SiO 2 layers (∼ 50 nm thick) on Si(100). By tuning the beam energy over the range 0–10 keV to vary implantation depth, and by accounting for positron diffusion as well as annihilation in SiO 2 and Si, depth profiles of material properties are obtained which sense material, defect, and electric field properties. The unusual observation of ortho-positronium 3γ-decay at the interface demonstrates that microvoids > 1 nm in size are present in these oxides, apparently an intrinsic consequence of the thermal oxidation process. The microvoid signal increases with annealing, possibly due to initiation of the interfacial oxide decomposition reaction. Reversible hydrogen uptake and desorption are also observed, illustrating the interface sensitivity of positron studies.

Range of slow positrons in metal overlayers on Al

Polycrystalline Pd and amorphous PdTa films on Al substrates were studied by a variable energy positron beam and by Rutherford backscattering. Since positron diffusion in the overlayers is limited, the range follows directly from the Doppler broadening as a function of incident positron energy. To observe possible effects of positron backscattering, a sandwich of Al/Pd/Al was studied as well. It was found that the mean penetration depth is not described well by z(E)=A(μg/cm2)×En(E), if A and n are assumed to be material and energy independent.

Study of hydrogen interaction with SiO2/Si(100) system using positrons

We describe positron annihilation studies of SiO2/Si(100) structures having 100‐nm‐thick oxide grown by plasma enhanced chemical vapor deposition. A normalized shape parameter is used to characterize the positron annihilation spectra. Activation and passivation of interface states by atomic hydrogen are demonstrated by repeated vacuum anneal and atomic hydrogen exposure. Hydrogen activation energy is derived for one of the samples as ■=2.02±0.07 eV.

Centroid shift of γ rays from positron annihilation in the depletion region of metal‐oxide‐semiconductor structures

Using metal‐oxide‐semiconductor (MOS) structures, the shift of centroid (peak) of γ‐ray energy distributions emitted from positron annihilation has been measured as a function of incident positron energy. The Doppler centroid shift was found to be consistent with the positron motion in the MOS depletion region. The results are described by a one‐dimensional positron diffusion model, and provide information on ‘‘effective’’ positron diffusion length under applied field.

Positron mobility in thermally grown SiO2 measured by Doppler broadening technique

The positron mobility in thermally grown SiO2 is deduced from Doppler broadening lineshape data on a metal‐oxide‐semiconductor sample for positrons implanted into the oxide layer. The fitted mobility is ∼13(10)×10−3 cm2/su2009V. This value is between that of the electron and hole mobilities in the same system and is two orders of magnitude smaller than the previous estimate from positron measurements.

Anomalous deuterium trapping in evaporated iron films studied by positron annihilation

Forward recoil energy spectroscopy showed that thin, evaporated Fe films trap anomalously large quantities of deuterium. Positron annihilation was used to investigate how the film microstructure influenced this trapping. Polycrystalline films trapped more deuterium and contained more open volume defects than single‐crystal films. Annealing reduced both trap and open volume defect concentrations. These results strongly suggest that coalescence voids produced during the thin‐film deposition were the sites for the trapping of deuterium.

The SiO/sub 2//Si interface probed with positrons

The effects of the heat treatment of Si covered with a thermally-grown approx.50 nm overlayer of SiO/sub 2/ were probed by means of measurements of positron annihilation characteristics obtained with a variable-energy positron beam. From measurements at elevated temperature (approx.500/sup 0/C) it was observed that positrons implanted overlapping the SiO/sub 2//Si interface decay from a state with properties distinctively different from the state in Si and in SiO/sub 2/. The nature of the annihilation characteristics indicates the presence of open volume defects. 19 refs., 2 figs., 1 tab.