Temperatura de superficie

As shown in section 2.3, dislocation-free crystals contain various types of intrinsic point defects and their agglomerates. These point defects and microdefects will affect the performance of silicon devices in many different ways. Understanding the structures and properties of these defects is important for silicon technology. Lattice vacancies and interstitials are the fundamental building blocks of vacancies and interstitials aggregates. Therefore, understanding the individual vacancy and interstitial defects is the first logical step toward unraveling the structures and properties of the many complex grown-in and process-induced defects involved in crystalline silicon. Since electron irradiation can produce isolated single vacancies and interstitials for study, the

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study of irradiated silicon has been crucial. Watkins [103] used electron irradiation ~1- 3MeV at cryogenic temperatures to displace lattice atoms by Rutherford scattering of the high-energy electrons. The advantage of that is, the low mass of the electron will assure simple damage, since the recoiling nucleus only obtains a small amount of excess kinetic energy, preventing further displacements of lattice atoms by it. The cryogenic temperature will freeze out the displacement products. In addition, electrons are not an impurity and only results in the displacement of the host atoms, but do not cause any contamination. In this section, the various types of vacancies and vacancy-impurity pairs studied in irradiated silicon are reviewed. These results will be the guideline for the investigation of vacancy-related as-grown defects studied later in this chapter.

2.4.1 Various types of vacancy-impurity pairs complexes

Vacancies can be trapped by different types of impurities to form complexes. For example, interstitial oxygen, isoelectronic substitutional impurities (Ge,Sn), substitutional donors (P, As, Pb), substitutional acceptors (B, Al) and other vacancies to produce divacancies, 3-Vacancies and 4-Vacancies [142-144]. The EPR and LVM methods have been used to investigate the chemical constituents and their atomic lattice structures. Watkins [103] summarized the energy and charge state of different types of vacancies and vacancy complexes, which is shown in Figure 2.8 below.

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Watkins also investigated the stability of the vacancy and several of the vacancy- defect pairs in ~15-30 min isochronal annealing studies. Figure 2.9 shows the results from this study, revealing that most vacancy and vacancy-defect pairs are unstable and can be annihilated below 500o_C.

Figure 2.9: Schematic of vacancy and vacancy-impurity pair annealing in ~15 – 30min isochronal conditions [103]

The samples used for the studies in this chapter are Cz n-type silicon, therefore, if any vacancy defect pairs limit the lifetime phosphorus and oxygen vacancy complexes are the most likely candidates. The possible configuration of vacancy-phosphorus (PV) and vacancy-oxygen pairs are briefly discussed next.

2.4.2 Vacancy-phosphorus pair

Phosphorus is used as dopant atoms in n-type silicon due to its high segregation coefficient and low cost. In section 2.4.1, it has been shown that dopant atoms like phosphorus can actually be trapped by vacancies to form VP pairs, which has an energy level about 0.47eV below the conduction band. It also has two charge states: negative and neutral [145]. The VP complexes in silicon were first identified by Watkins and Corbett [146] using EPR and electron nuclear double resonance. However, due to the lack of conclusive experimental observations, the formation mechanism is still unclear.

Phosphorus atoms occupy substitutional lattice sites in silicon. Chen et al. [109] suggested that vacancy and phosphorus form clusters in silicon and has the form of PnV,

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denoting a vacancy pairing with n of its nearest phosphorus neighbors (1 ≤ n ≤ 4). Figure 2.10 illustrates the structure of a P4V cluster.

Figure 2.10: Schematic of a P4V cluster containing 4 phosphorus atoms (pink/dark) and

a vacancy (grey/light). Yellow spheres denote silicon atoms [109].

2.4.3 Vacancy-oxygen pair

Oxygen is the main residual impurity in Cz silicon crystals. As mentioned in section 2.3.3.3, oxygen can be trapped by vacancies easily and form vacancy-oxygen (VO) complexes, these complexes are also called silicon A-centers. As shown in Figure 2.8 above, the VO complex has an energy level of 0.17eV below the conduction band and has two charge states: negative and neutral. As in the case of VP, VO complexes can have different atomic configurations, which can be written as VOn (1 ≤ n ≤ 4) [147].

Figure 2.11 (a) and (b) shows two types of VO complexes. In both case, the vacancy is trapped by interstitial oxygen in silicon to form VO complex.

In this section, different vacancy-impurity pairs have been briefly illustrated. It is seen that both VP and VO pairs can introduce deep levels, which may significantly contribute to recombination in n-type silicon. In the experimental section, evidence will be provided to validate the presence of these defects in as-grown n-type Cz silicon.

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(a) (b)

Figure 2.11: (a) Schematic representation of VO defect. (b) Schematic representation of VO2 defect [147].

2.5

Recent studies on vacancy-related defects in as-grown