Divacancy annealing in Si: Influence of hydrogen

Abstract

We have performed comparative studies of divacancy (V 2 ) annealing in hydrogenated and nonhydrogenated Si by deep level transient spectroscopy. It is shown that the nonhydrogenated samples demonstrate the formation of divacancy-oxygen (V 2 O) complex during annealing of V 2 , while the hydrogenated samples demonstrate annealing of V 2 without correlated growth of electrically active centers. No substantial formation of divacancy-hydrogen (V 2 H) complexes is observed in the hydrogenated samples. It is suggested that the dominant mechanism of V 2 annealing in hydrogen-rich Si is the interaction with hydrogen molecules (H 2 ) that results in the formation of the V 2 H 2 complex.The discussion on the mechanisms of impurity-assisted annealing of divacancy (V 2 ) in Si has attracted a renewed interest. This is due to recent reports on the observation of a double acceptor center identified as the divacancy-oxygen center (V 2 O). 1-3 These reports have considerably improved the understanding of the mechanisms of V 2 annealing and, at the same time, raised new questions concerning the origin of the dominant mechanism.From the first experimental observations of V 2 by electron paramagnetic resonance ͑EPR͒, it has been established that V 2 is more stable in float-zone ͑FZ͒ Si as compared to Czochralski ͑Cz͒ Si. 4 This difference has been explained by assuming the existence of two competing mechanisms: ͑i͒ the dissociation of V 2 and ͑ii͒ the migration of V 2 with subsequent annihilation/passivation at an impurity trap. It has been estimated that the V 2 dissociation has an activation energy of ϳ1.6 eV and the V 2 migration has an activation energy of ϳ1.3 eV. Since FZ-Si contains relatively small amounts of the impurity traps, most of the V 2 's are stable up to the temperature of dissociation. In contrast, Cz-Si is known to contain relatively high concentrations of impurities, and most of the V 2 's can be annealed by the migration mechanism with lower activation energy. It has been found in further studies that a fraction of the V 2 's can anneal in Cz-Si by interacting with interstitial oxygen (O i ), forming V 2 O. 5 A different situation can be observed in studies of the V 2 annealing by electrical characterization techniques such as deep level transient spectroscopy ͑DLTS͒. The thermal stability of V 2 in FZ-Si has been found to be higher than that in Cz-Si, in accordance with the EPR results. However, in contrast to EPR, the DLTS studies have yielded almost identical values of the activation energy for the V 2 annealing in FZ-Si and Cz-Si ͑with a lower annealing rate by a factor of ϳ2 in FZ-Si due to a smaller pre-exponential factor͒, which favors the migration mechanism in both of the materials. 6 It has also been established ͑see Ref. 6 and references therein͒ that the annealing of V 2 in both FZ-and Cz-Si does not result in the formation of electrically active centers. These observations exclude the interaction V 2 ϩO i →V 2 O as a dominant mechanism since the oxygen content is at least an order of magnitude hig...

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“…6) one observes a decrease of the well-known peaks E1 (VO+C i C s ), E2 (V 2 ( ¼ /À) ) and E4 (V 2 (À/0) ) and an increase of the signal E5 which is attributed to the hydrogen-related VOH complex. Similar measurements on oxygen-enriched and hydrogenated FZ samples had been performed by Monakov et al [34], revealing the same annealing behavior as shown in Fig. 6.…”

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“…It appears, therefore, reasonable to assume that reaction (4) together with reaction (5) Figure 6 shows DLTS spectra of sample C annealed isochronally for 15 min from 175 • C to 400 • C in steps of 25 • C. Sample C was heat treated for 1 h at 450 • C after the HF and prior to the electron irradiation; in such pretreated samples, the generation of V OH * still exists, but is strongly suppressed. This is attributed to the fact that the 450 • C treatment distributes hydrogen more homogeneously and the locally high hydrogen concentration promoting reaction (1) in samples without preannealing diminishes. The amplitude decrease substantially, the V OH level emerges, and two new levels, labeled E2 and E3, appear, with energy positions of E c − 0.17 eV and about E c − 0.58 eV, respectively.…”

“…It is known, for instance, that the vacancy-oxygen (V O) center and the divacancy (V 2 ) center disappear at temperatures below 300 • C in the presence of hydrogen. 1 The V O center transforms into a V OH center, [2][3][4][5][6][7][8][9][10] while the annealing mechanism for the V 2 center is less documented, although there have been several suggestions that the hydrogen-mediated annealing may give rise to a divacancy-hydrogen (V 2 H) center. 2,9,11 Reported values for the migration energy of monatomic hydrogen in Si range from 0.48 eV (Ref.…”

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“…Thus, in order to form a complex between VO and hydrogen, the reconstructed SiSi bond must be broken, which requires energy and slows down the reaction rate. This consideration is supported by the fact that the hydrogen-assisted annealing of VO takes place at higher temperatures than that of V 2 [27]. At these temperatures, the concentration of free atomic hydrogen increases because of release of trapped H from complexes and, possibly, also dissociation of H 2 , and the interactions VO+H→VOH and VOH+H→VOH 2 are promoted.…”

“…Thus, the reaction V 2 +H→V 2 H can be ruled out as a dominant one. Instead, it can be argued that the hydrogen dimer (H 2 ) is a common form of mobile hydrogen at 200-300 °C [27]. The V 2 center has two dangling bonds [12,13], and similar to the interaction of H 2 with the mono-vacancy [28], it may be energetically favorable to break up the H-H bond and saturate the two dangling Si bonds in V 2 .…”