Evidence of metastability with athermal ionization from defect clusters in ion-damaged silicon (original) (raw)
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Charge redistribution among defects in heavily damaged silicon
Physical Review B, 1998
We have studied trapping kinetics of defects during carrier capture in heavily damaged silicon, where damage was induced by MeV heavy ions at doses near but below the amorphization threshold. Using spectroscopic junction transient measurements, we provide unambiguous evidence of charge redistribution between defects. These results imply that changes in the occupancy of gap states are responsible for the deepening of emission energies with filling time, as is commonly observed in transient experiments in disordered silicon. This is in contrast to its usual explanation in terms of deepening of energy states due to hierarchical defect relaxation. ͓S0163-1829͑98͒01323-X͔
MRS Proceedings, 1998
We have carried out electrical characterization of defects in heavily damaged silicon, where damage is created by MeV heavy ions at doses near but below amorphization threshold. Trapping kinetics over several orders of magnitude in time have been monitored using isothermal spectroscopy called Time Analyzed Transient Spectroscopy (TATS). Two distinct effects regarding the nature of changes in density of states in the gap have been demonstrated. Firstly, we show that charge redistribution among multiple traps occur such that only the occupancy of the deeper states increase at the cost of shallower ones for long time filling. Secondly, a novel defect relaxation mechanism is observed for samples with relatively lower damage. A trap is seen to exhibit progressive deepening in energy with increase in filling time, finally stabilizing for large filling times. From the athermal nature of associated TATS peaks, it is argued that the relaxation involves large entropic contribution to free ene...
2002
We review our results on trapping kinetics studies at defect clusters in ion damaged silicon studied by depletion layer capacitance transient spectroscopic techniques. Conventional deep level transient spectroscopy (DLTS) studies on as-implanted and low temperature annealed Si show two major peaks corresponding to a divacancy trap and an interstitial cluster related trap. Kinetics of trapping at the clusters has been monitored over several orders of magnitude in time using an isothermal transient spectroscopic technique called time analyzed transient spectroscopy (TATS). Two distinct effects have been observed regarding the metastability of defect clusters in as-implanted and partially annealed samples. Firstly, with the help of higher order TATS used for monitoring the trap occupancy as a function of filling time, we show that charge redistribution among multiple traps occurs at low temperature in as-implanted samples. A detailed analysis of the relative trap occupancies reveals that the interstitial cluster related major trap exists in two metastable configurations, perhaps with negative U (Hubbard correlation energy), and the stable configuration of the defect is a midgap compensating trap. Secondly, in partially annealed samples, we observe a novel metastability of the defect clusters near room temperature where the trap energy progressively deepens with increasing filling time, finally stabilizing for large filling times at a fixed temperature, and the emission rate of carrier from any relaxed state is nearly temperature independent. From the athermal nature of the associated TATS spectra obtained at different temperatures, it is argued that the defect metastability is driven by change in configurational entropy associated with multiple trapping/detrapping process. These results constitute direct experimental evidence of metastability for small interstitial clusters in silicon and opens up opportunities for further studies on this new class of defect. The necessity of using a time domain relaxation spectroscopy such as TATS in the study of metastabilty is demonstrated.
Journal of Applied Physics, 1998
We have studied electrical activity of defects created by high-dose MeV heavy-ion implantation in n-silicon. Heavy damage induced by Ar ϩ and Au ϩ ions is embedded within depletion layers of Schottky diodes. The defects are characterized using capacitance-voltage (C -V), current-voltage (I -V), deep-level transient spectroscopy ͑DLTS͒ and time analyzed transient spectroscopy techniques. Large concentration of defects in the depletion layer of as-implanted device lead to unusual features in C -V and I -V characteristics. The damage layer is found to extend several microns beyond the ion range or the damage profile predicted by standard Monte Carlo simulation packages. The dominance of a single trap in the damaged region is established from hysteresis effect in C -V, space-charge-limited conduction in forward I -V and DLTS spectrum. With annealing in the temperature range of 400-600°C, the observed changes in the defect profile indicate that the effective electrical interface between damaged and undamaged layer moves progressively towards the surface. From transient spectroscopic analysis the major defect is found to be a midgap trap whose energy is sensitive to the degree of disorder in the damaged layer. The experimental features in C -V characteristics have been simulated using model charge profiles taking into account crossing of the Fermi level with the midgap trap within the depletion layer. The simulations suggest the presence of a compensated region and a sharp negatively charged defect profile at a distance much larger than that expected from ion range. Our results constitute experimental evidence, in qualitative agreement with recent predictions of molecular dynamics simulations, of defect migration and clustering of interstitial related defects even at room temperature in the case of high-dose irradiation.
Physical Review B, 2000
We have considered five different models of charge transfer among coupled defect states in semiconductors where the free-carrier density is limited by the density of unoccupied trap levels, as in the case of defectdominated materials. To determine the time dependence of the trap occupancy features, we formulate a set of coupled differential equations that govern charge capture and emission processes for two defect states. A numerical solution assuming model parameters for traps provides features of the trap occupancy as a function of time. A critical comparison is made in occupancy features for different models, primarily categorized as serial ͑hierarchical͒ and parallel mechanisms of charge transfer. The model predictions are successfully applied to a study of trapping kinetics of defects observed in heavily damaged n-type silicon. We show that, in addition to the occurrence of charge redistribution among multiple traps, the major trap in the damaged silicon exists in two metastable configurations, perhaps with negative U ͑Hubbard correlation energy͒, and the stable configuration refers to a midgap compensating center related to a small cluster of self-interstitials. The applicability of our model simulations can be extended to more complex defect systems using a combination of these simple models.
2000
We have investigated the electrical characteristics of defects in heavily damaged silicon induced by MeV ion implantation at high doses, by extending the scope of depletion layer capacitance transient techniques. The heavily damaged layer is embedded in the depletion layer of a Schottky diode and high-frequency capacitance measurements are carried out to evaluate charge relaxation kinetics of defects specific to high-dose implantation. Deep-level transient spectroscopy of as-implanted silicon shows presence of the divacancy trap (V 2) and relatively high concentration of a damage-related trap (D1) with unusual spectral lineshape. Thermally stimulated capacitance spectra show large capacitance step even without application of a trap-filling pulse. Constant-capacitance time-analysed transient spectroscopy studies of the D1 peak reveal that the skewed peakshape is due to premature termination of the transient signal during trap emission. Strong temperature dependence of spectral lineshape, ranging from broad to narrow peak (stronger than that expected from exponential transient), and trap occupancy points to a dynamic interdependence of trap occupancy and quasi-Fermi level. Unusual features in spectral lineshape are simulated by introducing a time-dependent capture term into the rate equations for trapping dynamics for a single trap level and provide strong support for our model on the dynamic interdependence of quasi-Fermi level and trap occupancy. The defect parameter is found to be sensitive to implantation dose and low-temperature annealing. The D1 trap is ascribed to small self-interstitial clusters.
Comprehensive model of damage accumulation in silicon
Journal of Applied Physics, 2008
Ion implantation induced damage accumulation is crucial to the simulation of silicon processing. We present a physically based damage accumulation model, implemented in a nonlattice atomistic kinetic Monte Carlo simulator, that can simulate a diverse range of interesting experimental observations. The model is able to reproduce the ion-mass dependent silicon amorphous-crystalline transition temperature of a range of ions from C to Xe, the amorphous layer thickness for a range of amorphizing implants, the superlinear increase in damage accumulation with dose, and the two-layered damage distribution observed along the path of a high-energy ion. In addition, this model is able to distinguish between dynamic annealing and post-cryogenic implantation annealing, whereby dynamic annealing is more effective in removing damage than post-cryogenic implantation annealing at the same temperature.
Heavy ion induced damage in crystalline silicon and diodes
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1999
N-type crystalline silicon samples (preferred á1 1 1ñ oriented), after irradiation using dierent¯uences (from 1´10 10 to 5´10 14 ions/cm 2) of 45 and 65 MeV boron ions and 60 and 80 MeV oxygen ions, were studied by the grazing angle X-ray diraction (XRD), and the optical radiation re¯ectivity techniques. In these samples the lifetime of the minority carriers was also measured with a pulsed electron beam and damage coecients were estimated. The XRD peak intensity at 28.4°(2h), the re¯ectivity of 200±700 nm radiation and the lifetime of minority carriers were found to decrease continuously with increasing ion¯uence. These results reveal that a fraction of the near surface silicon atoms were displaced from their lattice positions. Moreover, the degree of the damage induced by 65 MeV boron ions was higher than that of 45 MeV boron ions, even though the energy deposited by 45 MeV boron ions was greater than that deposited by 65 MeV boron ions. Similar results were also obtained with 60 and 80 MeV oxygen ions. The feasibility of reducing the turn-o time of silicon diodes with heavy ions was examined by irradiating a large number of diodes with dierent¯uences (from 10 10 to 10 12 ions/cm 2) of 35 MeV lithium, 50 MeV boron and 104 MeV silicon ions. Among the ions used, only 35 MeV lithium ions could reduce the turn-o time from 1000 to 50 ns, with marginal increase in the forward voltage drop. In each diode the lifetime of minority carriers was estimated by measuring the turn-o time. The variations of the damage coecient in the n-type silicon and the lifetime of minority carriers in the diodes, with the ion uence indicate that a fraction of the induced damage in silicon was annealed out with the energy deposited through the electronic energy loss process by heavy ions.
Charge emission and precursor accumulation in the multiple-pulse damage regime of silicon
Journal of the Optical Society of America B, 1985
To contribute to further understanding of damage mechanisms in silicon induced by picosecond Nd:YAG 1.06-,um laser pulses, the damage at laser intensities below the one-shot damage threshold has been studied. By using a biased charge-collection wire or electron-multiplier tube, one can detect the charge emitted during small pit formation, which may be considered as the initial damage morphology. The investigation of the incubation period has been conducted by observing the first charge emission, which indicates its termination. This incubation time is analogous to the lifetime of the solids subject to a repeated mechanical load and is characterized by processes that are irreversible for at least 3 sec. This suggests that the damage precursors are long-lived excitations or an accumulation of permanent states. The heterogeneous nature of the nucleation of damage suggests that these precursor states act as nucleation seeds to the laser-induced damage. Charge emission after the incubation period was also studied. Positive and negative particles are emitted equally after damage initiation, probably because of thermal evaporation of silicon from small regions. The charge emission follows the Arrhenius relation. Experimentally, this result was independent of pulse-repetition period up to at least 10 sec. Damage morphology was compared to charge emission and suggests that charges are emitted mainly from pits, grooves, and holes within the damaged area.
Damage evolution in low-energy ion implanted silicon
Physical Review B, 2007
The annealing of damage generated by low-energy ion implantation in polycrystalline silicon ͑poly-Si͒ and amorphous silicon ͑a-Si͒ is compared. The rate of heat release between implantation temperature and 350-500°C for Si implanted in both materials and for different ions implanted in poly-Si shows a very similar shape, namely, a featureless signal that is characteristic of a series of processes continuously distributed in terms of activation energy. Nanocalorimetry signals differ only by their amplitude, a smaller amount of heat being released after light ion implantation compared to heavier ones for the same nominal number of displaced atoms. This shows the importance of dynamic annealing of the damage generated by light ions. A smaller amount of heat is released by implanted poly-Si compared to a-Si, underlining the effect of the surrounding crystal on the dynamic annealing and the relaxation of the defects. Damage accumulation after 30-keV Si implantation is also characterized by Raman scattering and reflectometry, featuring a similar trend in a-Si, poly-Si, and monocrystalline silicon ͑c-Si͒ with a saturation around 4 Si/ nm 2. Considering these results together with other recent experiments in c-Si and molecular dynamic simulations, it is concluded that the damage generated by low-energy ion implantation that survives dynamic annealing is structurally very similar if not identical in both crystalline and amorphous silicon, giving rise to the same kind of processes during a thermal anneal. However, the damage peak obtained by channeling saturates only above 10 Si/ nm 2. This suggests that between 4 and 10 Si/ nm 2 , further damage occurs by structural transformation without the addition of more stored energy.