Oxygen Precipitation Behavior Research Articles

Oxygen Precipitation Behavior in n-Type Cz-Si Related to Carbon Concentration and Crystal Growth Conditions

The behavior of oxide precipitates during solar cell fabrication processes and the resulting effect on device performance have been investigated by transmission electron microscopy (TEM) observation. Samples were prepared with different carbon concentration and under different crystal growth conditions, namely using the conventional and an advanced process. The density of oxide precipitates increased monotonically with the carbon concentration, while the cell efficiency improved with decreasing oxygen precipitate density. When the carbon concentration was reduced to below 1016 cm−3, the oxide precipitates grew largely and dislocations were introduced. TEM observations confirmed that the morphology of the oxide precipitates clearly differed depending on the crystal growth conditions. Precipitates grown in platelet form introduced high density dislocations in their surroundings, while the dislocation density was relatively lower around polyhedral-type precipitates. These results thus reveal that oxygen precipitation can be controlled by varying the crystal growth conditions, possibly contributing to the production of high-efficiency solar cells.

Evaluation of oxygen precipitation behavior in n-type Czochralski-Si for photovoltaic by infrared tomography: Effects of carbon concentration and annealing process conditions

In this study, we evaluated the effects of carbon and annealing process conditions on the oxygen precipitation in an n-type Czochralski (Cz) wafer for solar cells by infrared light scattering tomography (IR-LST). It was confirmed that precipitates grow larger and denser as carbon concentration increase. Thus, carbon promotes oxygen precipitation. We also evaluated the effect of oxygen precipitation on the minority carrier lifetime by photoluminescence (PL) imaging. It was confirmed that the interface between the precipitate and the Si matrix is a dominant recombination center since the surface area of the precipitate obtained by IR-LST measurement and the PL intensity show good correlation. In addition, it was also confirmed that carbon is involved in the supply of interstitial oxygen to precipitates through the formation and extinction of the thermal donor. We believe that it is important to understand and control the effect of carbon of controlling the oxygen precipitation behavior.

Carbon effect on the survival of vacancies in Czochralski silicon during rapid thermal anneal

Rapid thermal anneal (RTA) at high temperatures can be employed to introduce vacancies to control oxygen precipitation (OP) behavior in Czochralski (CZ) silicon. Such excessive vacancies survive from the recombination of silicon-interstitials and vacancies (V-I recombination) during the RTA. In this work, we aim to elucidate the carbon effect on the survival of vacancies in CZ silicon during the high temperature RTA by means of gold diffusion in combination with deep-level transient spectroscopy. It is revealed that the existence of ∼1017 cm−3 carbon atoms significantly increases the amount of survival vacancies in the form of vacancy-oxygen (VOm, m ≥ 2) complexes in CZ silicon when subjected to the 1250 °C/60 s RTA. Moreover, such an increase in the number of vacancies becomes more significant with the increase in the cooling rate of RTA. The density functional theory calculations suggest that the V-I recombination is to some extent unfavorable as a carbon atom is close to the silicon-interstitial. Alternatively, it is believed that the substitutional carbon (Cs) atoms tend to trap the silicon-interstitials, thus forming Ci complexes (Cs + I → Ci) in CZ silicon during the RTA. In this context, the V-I recombination is suppressed in a manner, leading to the survival of more vacancies, thus generating more VOm complexes. Furthermore, after the 1250 °C/60 s RTA, the oxide precipitate nucleation based on the VOm complexes is more significant in carbon-rich CZ (CCZ) silicon than in the conventional CZ counterpart. Hence, when subjected to the same OP anneal consisting of the nucleation anneal at 650 or 800 °C for 4 h and the subsequent growth anneal at 1000 °C for 16 h, CCZ silicon possesses a higher density of bulk microdefects and therefore stronger internal gettering capability than CZ silicon. However, the nucleation temperature for OP should be carefully selected as 650 °C for CCZ silicon in order to form an oxide precipitate-free denuded zone.

Sample thickness effect of thermal vibration correction within X-ray dynamical theory for germanium-doped silicon

As of today, highly germanium doped Czochralski silicon crystals are used in applications. However, it is not clear how a germanium content in the range of 1018 atoms/cm3 influences the oxygen precipitation behavior, which is highly relevant for the gettering potential of the wafer. In this context, high energy X-ray diffraction is a promising tool to monitor real-time the strain introduced by the oxygen agglomeration and growth. Nevertheless, in particular, in the dynamical limit of X-ray diffraction and at elevated temperatures, the strain originating from the precipitation process has to be clearly distinguished from the intensity contribution of thermal vibrations. In Laue geometry, dynamical effects can even lead to an increment of the integrated intensity with temperature completely unexpected in the kinematical limit, where temperature is tackled solely by a Debye-Waller factor. In the following, an approach is presented allowing us to completely correct the influence of thermal vibrations in the dynamical limit. This approach is applied to undoped and highly germanium doped silicon crystals, clearly revealing an enhanced inventory of grown-in precipitates with germanium doping and suggesting a morphological transformation towards a phase of higher strain of the grown-in precipitates in the range between room temperature and 800 °C.

Practical Evaluation Method of Oxygen Precipitation in the Czochralski Silicon

Oxygen precipitates in Czochralski silicon (CZ-Si) are still considered as the major process-induced defects found in semiconductor device fabrication process because the role of oxygen precipitates in device process such as gettering of metal impurity, prevention of undesired wafer deformation, and degradation of carrier lifetime is quite significant to manufacture high-performance devices. Until now, various theoretical models to predict the oxygen precipitation in CZ-Si with respect to the interstitial oxygen (Oi), vacancy (CV), and the process temperature T have been suggested. However, the complexity and calculation time of a model strongly depend on the accuracy of the model. Thus, it is difficult to apply them on the evaluation of oxygen precipitation regularly in conventional wafer production. Hence, a practical and simple method to evaluate oxygen precipitation should be proposed. In this work, we derived the parameters that indicate the thermal stability and formation rate of the precipitate nuclei by analyzing as-grown oxygen precipitates formed in a p-type CZ-Si. First, we calculated a cumulative distribution function (CDF) of as-grown precipitate density with respect to the thermal history of CZ-Si crystal from the theoretical nucleation rate J(Oi, CV, T) [1]. After that, we fitted the CDF mathematically into the temperature-dependent precipitate density measured after slow ramped anneals with 1 K/min [2] at various stability temperatures followed by a growth anneal at 1000°C for 16 hours. Moreover, the first derivative Je (T) of the CDF was also calculated which was associated with an effective nucleation rate. The extreme point (T C, Je (TC)) on the Je (T) was determined as a new parameter that represents the precipitate behavior at a measured point. Finally, we correlated the parameters to the experimental results measured in various crystals after the test heat cycle started at 800°C for 4 hours and followed by 1000°C for 16 hours. As shown in Fig. 1, we found that both T C and Je (TC) were strongly correlated to the precipitate density, which means that the parameters were possibly applied to evaluate the behavior of oxygen precipitation in a crystal after various thermal processes. Fig. 1 Correlation of the T C (a) and Je (T ­C) (b) to the precipitate density measured after 2-step annealing starts at 800°C for 4 hours and followed by 1000°C for 16 hours. [1] G. Kissinger, D. Gräf, U. Lambert, and H. Richter, J. Electrochem. Soc. 144, 1447 (1997).[2] G. Kissinger and J. Dabrowski, J. Electrochem. Soc. 155, H448 (2008). Figure 1

The impact of oxygen precipitation on dislocation generation at small angle grain boundaries during seed‐assisted directional solidification of silicon

We have investigated the source of dislocations generated during growth and subsequent cooling of quasi‐monocrystalline silicon. Bi‐crystals of silicon separated by ∼5.2° tilt small angle grain boundary have been directionally solidified at two different pulling rates, i.e., 3 and 13 μm s−1. We observe higher density of grain boundary‐associated dislocations at the ingot pulled at a lower rate. This observation is explained by the impact of oxygen precipitation behavior at the grain boundary.

Oxygen precipitation in 1020 cm−3 germanium-doped Czochralski silicon

We have investigated the effects of germanium (Ge)-doping at the level of 1020 cm−3 on oxygen precipitation (OP) behaviors in Czochralski (CZ) silicon subjected to different low-high two-step anneals without or with prior high temperature rapid thermal processing (RTP). It is found that Ge-doping remarkably suppresses OP in CZ silicon without prior RTP. However, Ge-doping significantly enhances OP in CZ silicon with prior RTP. The suppressed OP in the case of the absence of prior RTP is primarily due to the fact that the 1020 cm−3 Ge-doping introduces compressive strain into silicon crystal lattice, which increases the critical size of oxygen precipitate nuclei for a given nucleation temperature. Moreover, it is revealed that the 1020 cm−3 Ge-doping facilitates the formation of vacancy-oxygen (V-O) complexes and may introduce Ge-V-O complexes in CZ silicon during high temperature RTP. More vacancy-related complexes in CZ silicon not only reduce the critical size of oxygen precipitate nuclei but also provide more precursors for oxygen precipitate nucleation. Therefore, the 1020 cm−3 Ge-doping enhances OP in CZ silicon subjected to the two-step anneals following high temperature RTP.

Effect of Annealing Temperature on Oxygen Precipitation in Fast Neutron Irradiated CZ-Si

The effect of annealing temperature on oxygen precipitation was investigated in various dose fast neutron irradiated Czochralski silicon (CZ-Si). Fourier Transform Infrared Absorption Spectrometer (FTIR) was used to measure the concentration of interstitial oxygen ([Oi]). Bulk microdefects (BMDs) were observed by optical microscope. The behavior of oxygen precipitation depends on the annealing temperature and the concentration of irradiation-induced defects. The mount of oxygen precipitates of irradiated samples is more than that in non-irradiation samples and increases with increasing the irradiation dose. Because of the effect of temperature on critical radius rcand the oxygen diffusivity, oxygen precipitation increase with the increase of temperature at the studied lower temperature range, while decrease with the increase of temperature at the studied higher temperature range. High density dislocation and stacking faults generate in irradiated sample.

Impact of rapid thermal processing on oxygen precipitation in heavily arsenic and antimony doped Czochralski silicon

A comparative investigation is performed on the effects of vacancies induced by rapid thermal processing on oxygen precipitation behavior in heavily arsenic- and antimony-doped Czochralski silicon wafers. It is experimentally found that vacancy-assisted oxide precipitate nucleation occurs at 800, 900, and 1000 °C in the Sb-doped wafers, while it only occurs at 800 °C in the As-doped ones. Density functional theory calculations indicate that it is energetically favorable to form AsVO and SbVO complexes in As- and Sb-doped silicon crystals, respectively. These complexes might act as precursors for oxide precipitate nucleation under appropriate conditions. The difference between the effects of rapid thermal processing -induced vacancies on oxide precipitate nucleation in the heavily As- and Sb-doped Cz silicon crystals is tentatively elucidated based on density functional theory calculations revealing the difference in binding energies of AsVO and SbVO complexes.

Nitrogen Enhanced Oxygen Precipitation in Czochralski Silicon Wafers Coated with Silicon Nitride Films

Oxygen precipitation (OP) behaviors were investigated for Czochralski (Cz) silicon wafers, which were coated with silicon nitride (SiNx) films or not, subjected to two-step anneal of 800C/4 h+1000°C/16 h following rapid thermal processing (RTP) at different temperatures ranging from 1150 to 1250C for 50 s. It was found that OP in the Cz silicon wafers coated with SiNx films was stronger in each case. This was because that nitrogen atoms diffused into bulk of Cz silicon wafer from the surface coated SiNx film during the high temperature RTP. Furthermore, it was proved that the RTP lamp irradiation facilitated the in-diffusion of nitrogen atoms, which was most likely due to that the ultraviolet light enhanced the breakage of silicon-nitrogen bonds.

Effects of high temperature rapid thermal processing on oxygen precipitation in heavily arsenic-doped Czochralski silicon

Oxygen precipitation (OP) behaviors in heavily arsenic-doped Czochralski silicon wafers without and with prior rapid thermal processing at 1100–1250°C, subjected to subsequent two-step anneal of 450, 650 and 800°C/4h+1000°C/16h, have been investigated. It is found that the prior RTP at 1250°C substantially enhances OP in each two-step anneal. Such enhancement effect exhibits most significantly in the two-step anneal with the nucleation at 800°C. However, in order to form the highest density of oxygen precipitates, the most desirable nucleation temperature is 650°C in both cases without and with prior RTP. The OP nucleation mechanisms operating in the three low temperatures as mentioned above have been tentatively discussed.

Oxygen Precipitation in Conventional and Nitrogen Co-Doped Heavily Arsenic-Doped Czochralski Silicon Crystals: Oswald Ripening

Oxygen precipitation (OP) behaviors in conventional and nitrogen co-doped heavily arsenic-doped Czocharalski silicon crystals subjected to low-high two-step anneals of 650 oC/8 h + 1000 oC/4-256 h have been comparatively investigated. Due to the nitrogen enhanced nucleation of OP during the low temperature anneal, much higher density of oxygen precipitates generated in the nitrogen co-doped specimens. With the extension of high temperature anneal, Oswald ripening of OP in the nitrogen co-doped specimens preceded that in the conventional ones. Moreover, due to the Oswald ripening effect, the oxygen precipitates in the conventional specimens became larger with a wider range of sizes. While, the sizes of oxygen precipitates in the nitrogen co-doped specimens distributed in a much narrower range with respect to the conventional ones.

Oxygen precipitation heterogeneously nucleating on silicon phosphide precipitates in heavily phosphorus-doped Czochralski silicon

Through comparison between the oxygen precipitation (OP) behaviors in heavily and lightly phosphorus (P)-doped Czochralski silicon (CZ-Si) crystals subjected to low-high two-step anneal of 600, 650, or 750 °C/8 h+1050 °C/16 h, we have found that in heavily P-doped CZ-Si, OP is much stronger in the case with the nucleation anneal at 600 or 650 °C while it is to some extent suppressed in the case with the nucleation anneal at 750 °C in contrast to lightly doped CZ-Si where nucleation is enhanced at 750 °C. Transmission electron microscopy investigation reveals that silicon phosphide precipitates of face-centered-cubic SiP form during the nucleation anneal at temperatures 650 °C and below. The SiP precipitates act as the heterogeneous nuclei for OP during the subsequent high temperature anneal while the oxygen precipitate nuclei containing certain amounts of P atoms generate during the nucleation anneal at 750 °C. They are further coarsened to be larger oxygen precipitates during the subsequent high temperature anneal. Of significance is that the oxygen precipitate nucleation mechanism operating at certain temperatures for heavily P-doped CZ-Si is found to be completely different from that of lightly P-doped CZ-Si. We believe that the present work provides a further insight into OP in CZ-Si.

Oxygen Precipitation in Heavily Phosphorus-doped Czochralski Silicon

Oxygen precipitation in heavily phosphorous (P)-doped Czochralski silicon (CZ-Si) subjected to the ramping or two-step anneals was investigated. It was revealed that the grown-in oxygen precipitates exerted significant effect on oxygen precipitation behaviors and then resulted in distinctive oxygen precipitation for the wafers form various positions of the ingot. Moreover, it was found that both the seed and tang-end wafers processed significantly intense nucleation at 650 oC without the influence of grown-in oxygen precipitates. It was considered that the heavily P-doping introduced phosphorus related precipitation at 650 oC. When the annealing temperature increased, the capability for nucleation of oxygen precipitation by heavily P-doping would be gradually weakened but the one by oxygen clustering became pronounced. In this case, it was understandable that the tang-end wafer processed higher density of oxygen precipitates than the seed-end wafer at around 750 oC, while a lower one at 1000 oC.

Effects of low-temperature annealing on oxygen precipitate nucleation in heavily arsenic-doped Czochralski silicon

Through the comparative investigation on oxygen precipitation behaviors in the heavily and lightly arsenic-doped n-type Czochralski (CZ) silicon wafers subjected to the two-step annealing successively at low temperature (450—800℃) and high temperature (1000℃), the effects of low-temperature annealing on oxygen precipitate nucleation in heavily arsenic-doped CZ silicon wafer have been elucidated. It was found that for the heavily arsenic-doped CZ silicon the oxygen precipitate nucleation during the 450 and 650℃ annealing was more significant than that during the 800℃ annealing, which was contrary to the case for lightly-doped CZ silicon. Moreover, in comparison with the lightly-doped CZ silicon, the oxygen precipitate nucleation at 450 and 650℃ was enhanced while that at 800℃ was suppressed in the heavily arsenic-doped CZ silicon. It is believed that in the heavily arsenic-doped CZ silicon the As-V-O complexes can be formed during the annealing at 450 and 650℃ so as to enhance the oxygen precipitate nucleation; while, during the 800℃ annealing the As-V-O complexes are not stable enough to act as the precursors of nuclei and, moreover, the heavy arsenic-doping leads to compressive lattice stress, therefore the oxygen precipitate nucleation is noticeably suppressed. Furthermore, it is revealed that the nitrogen-doping facilitates the oxygen precipitate nucleation during the annealing at low temperatures especially at 800℃, which is believed to be due to the heterogeneous nucleation centers induced by nitrogen-doping.

Crystal growth and oxygen precipitation behavior of 300 mm nitrogen-doped Czochralski silicon

A 300 mm, p-type, 〈1 0 0〉-oriented nitrogen-doped Czochralski (NCZ) silicon crystal was grown in a Keyex MCZ 150 crystal puller equipped with cusp magnetic field. In order to dope nitrogen into the crystal, the primary protection gas of argon was mixed with high-purity nitrogen gas at a flow rate ratio of 10:1. Due to the long time growth of crystal, any perturbation due to the SiO and cristobalite particles at the crystal/melt interface would generate dislocation in the crystal. In order to avoid this ‘disaster’ during the crystal growth, a thermal shield in connection with a protection gas director was used, thereby generating a sweeping flow over the melt to drive the SiO and cristobalite particles out of the crystal/melt interface. The oxygen precipitation behavior of a near-seed-end wafer subjected to 800 °C/8 h+1000 °C/16 h was investigated. It was found that oxygen precipitation within the P-band was stronger than that in the region outside of the P-band, which is generally not the case for the conventional Czochralski (CZ) wafer with P-band. Based on this result, it is deduced that the grown-in oxygen precipitates existing in the P-band are relatively smaller in size but with higher density compared to those in the CZ crystal.

Germanium effect on as-grown oxygen precipitation in Czochralski silicon

The behavior of as-grown oxygen precipitation in Czochralski (Cz) silicon (Si) with the germanium (Ge)-doping has been investigated. It is found that the as-grown oxygen precipitation in Ge-doped Cz (GCz) Si can be enhanced in comparison with the conventional Cz Si at high temperatures, even above the formation temperature of void. It has been ascribed to the effect of heterogeneous nucleus sites supplied by the Ge-related complexes generated in GCz Si. Meanwhile, more large-sized as-grown oxygen precipitates can be formed in GCz Si is revealed through both a controlled ramping annealing and an isothermal annealing performed at high temperatures. This phenomenon is considered to be associated with the reduction in the critical radius of oxygen precipitates for the elevated temperatures.

Effect of carbon doping on oxygen precipitation behavior in internal gettering processing for Czochralski silicon

The effect of carbon doping on oxygen precipitation behavior in internal gettering processing for Czochralski silicon (Cz-Si) has been investigated. Through high–low–high anneals in argon ambient, it has been shown that both good-quality denuded zone and high-density bulk microdefects (BMDs) could be formed in high carbon content (H[C]) Cz-Si wafer. Meanwhile, the density of BMDs in H[C] Cz-Si crystal was a little higher compared with that in the Cz-Si crystals with lower carbon content or without carbon doping. The concentration of substitutional carbon almost remained during the whole thermal cycles. Moreover, the [C i–O i]C(3) centers with low concentration were detected by infrared spectroscopy. Weak effect of carbon on oxygen precipitation in H[C] Cz-Si during internal gettering processing is considered. Based on the experimental results, H[C] Cz-Si crystal is suggested to be one of silicon materials used for larger scale integrated circuit fabrication.

Oxygen precipitation and improved internal gettering in heavily As-doped silicon crystal

The behavior of oxygen precipitation and its induced defects in heavily As-doped silicon was investigated through annealing experiments. Comparison to similar investigations on heavily B- and Sb-doped silicon reveals the formation temperature of the oxygen precipitates dependence of dopant type. The experimental results showed that nucleation temperature of the oxygen precipitates in heavily As-doped Si is at 900 °Cand it is higher than for the other heavily doped silicon. And the density and size will be varied with the annealing temperature or time. The oxygen precipitates formed at moderate temperature can be the nucleation centers of the faults and dislocation loops. According to the annealing results, an internal gettering process was suggested for the heavily As-doped silicon and an ideal IG structure with 45 μm width was obtained.

Quality evaluation of resistivity-controlled silicon crystals

The segregation phenomenon of dopants causes a low production yield of silicon crystal that meets the resistivity tolerance required by device manufacturers. In order to control the macroscopic axial resistivity distribution in bulk crystal growth, numerous studies including continuous Czochralski method and double crucible technique have been studied. The simple B–P codoping method for improving the productivity of p-type silicon single-crystal growth by controlling axial specific resistivity distribution was proposed by Wang [Jpn. J. Appl. Phys. 43 (2004) 4079]. In this work, the quality of Czochralski-grown silicon single crystals with a diameter 200 mm using B–P codoping method was studied from the chemical and structural points of view. It was found that the characteristics of B–P codoped wafers including the oxygen precipitation behavior and the grown-in defects are same as that of conventional B-doped Czochralski crystals.