Cubierta vegetal (Land Cover)

vacancy-type. During the cooling stage of the crystal, the vacancy concentration becomes strongly supersaturated as the temperature decreases. The vacancies tend to agglomerate into microdefects, such as bulk defect voids, at a progressively increasing rate. Due to the fast increase of the nucleation rate upon cooling, appreciable nucleation occurs only within a narrow range of temperature around a certain nucleation temperature Tn [97]. A typical nucleation temperature is around 1100oC [134] for a Cz

silicon crystal grown under typical conditions. In this section, the various vacancy-type agglomerates shown in Figure 2.6 (b) will be discussed.

Vacancy-related recombination active defects in as-grown n-type Czochralski Silicon

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2.3.3.1

Voids

Voids are common bulk defects that form by the aggregation of vacancies during the cooling of crystals grown in vacancy mode from a supersaturated vacancy solution [119, 135]. The density of voids is proportional to the factor q1.5_C

V-0.5 [97], where q is the

cooling rate at the temperature at which the nucleation occurs, and CV is the local

concentration of vacancies. The nucleation temperature of voids depends on CV. The

range lies at about 1100 oC as mentioned above. The nucleation occurs over a narrow temperature range of ΔT~5K. The voids have energetically favored octahedral shape. Various techniques have been used to characterize this type of defect. Therefore different names are given according to the technique used before people come to the conclusion that they are characterizing the same defect, for instance, D-defects, COPs, Flow Pattern Defects (FPDs), Light Scattering Tomography Defects (LSTDs) and Gate Oxide Integrity (GOI) [120, 136-138].

2.3.3.2

Vacancy-oxygen agglomerates

In addition to self-agglomerates of vacancies to form voids, vacancies forms complex with oxygen or can be involved in oxygen precipitation process to accommodate the requested extra volume. In Figure 2.6 (b), at the periphery of the central voids region there are three bands: H-band, P-band and L-band [132, 133]. The middle one is called the P-band, that is, particle-band. In this band, large oxide particles are formed with typical density of 108 cm-3 [132]. Vacancies and oxygen atoms are two crucial constituent to produce oxide particles. As the oxide particles become twice as large in volume after adding oxygen atoms, the role of vacancies is to provide space for an oxide particle and to release the significant strain energy created. The formation of oxide particles is actually in competition with the formations of voids. The nucleation rate of voids and oxide particles decreases dramatically upon reducing CV. However, the

reduction of the particle nucleation rate is of smaller extent. Since vacancies are not the only constituent involved in the oxide particle formation, the particle nucleation rate is not as sensitive to vacancy concentration as that of void formation. Therefore, at sufficiently low CV, the dominant agglomeration path is switched from voids to oxide

particles. The formation of oxide particles consumes a large number of vacancies and the residual vacancy density in the P-band is very low. The well-known OISFs-ring in

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the wafer after oxidation is a result of the oxide particle in the P-band, which is a stacking fault formed at those oxide particles by nucleating interstitials injected from the growth of a surface oxide [112, 114].

Besides the P-band, there are another two bands in Figure 2.6 (b), the L-band and H- band. The L-band appears when the initially incorporated CV is even lower than in the

P-band. L stands for low CV. Instead of producing large oxide particles, very small high

density oxide particles are formed [139]. The nucleation temperature in the L-band is shifted to a lower temperature T. Thus, the oxygen diffusivity is reduced and the oxide particles grow very slowly, meaning that vacancy consumption becomes insignificant. The vacancy concentration remaining in the L-band is therefore much higher than in the P-band, and close to the originally incorporated value.

At a slightly higher initially incorporated CV than that in the P-band, the vacancies

agglomerate mostly into voids, but this occurs at relatively low T. This band is called the H-band (H refers to the peak in Figure 2.7 located at the side of higher initially incorporated vacancy concentration). Due to the formation of voids, the vacancy consumption is limited and the remaining vacancy concentration in the H-band is comparable to the value in the L-band. The oxide particle formation is similar to the L- band and results in high density of very small oxide particles. The difference from the L-band is that the oxide particles in the H-band coexist with small voids.

Figure 2.7: Residual (normalized) vacancy concentration versus the starting vacancy concentration, solid Curve: combined effect of voids and oxide particles; dashed curve:

negligible particle contribution [139]

The residual vacancy concentrations in the P-band, L-band and H-band can be summarized in Figure 2.7. The two peaks corresponds to the L-band and H-band

Vacancy-related recombination active defects in as-grown n-type Czochralski Silicon

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respectively, L and H also represents the lower and higher peaks, the P-band is located at zero residual vacancy concentration. The residual concentration above the higher peak decreases due to the formation of voids.

2.3.3.3

Binding of vacancies by oxygen

Another important effect of vacancies is a reversible trapping of oxygen into Vacancy- Oxygen (VO) and VO2 Defects. The binding occurs below some binding temperature Tb,

which is estimated around 1050oC [140].The binding of vacancies with oxygen reduces the effective diffusivity of vacancies, thus, preventing vacancies from complete consumption by voids.

2.3.4 Defects incorporated in interstitial mode crystal growth