A review is presented of the recent advances in the study of oxygen precipitation and of the main properties of oxide precipitates in silicon. After a general overview of the system ‘‘oxygen in silicon,’’ the thermodynamics and the kinetics of the precipitate formation are treated in detail, with major emphasis on the phenomenology; subsequently, the most important techniques for the characterization of the precipitates are illustrated together with the most interesting and recent results. Finally, the possible influence of oxygen precipitation on technological applications is stressed, with particular attention to recent results regarding device yield. Actually, the essential novelty of this review rests on the attempt to give an extended picture of what has been recently clarified by means of highly sophisticated diagnostic methods and of the influence of precipitation on the properties of semiconductor devices.

Oxygen precipitation in silicon

Two different boron- and oxygen-related recombination centers are found to be activated in crystalline silicon under illumination or electron injection in the dark, both leading to a severe degradation in the carrier lifetime. While one center forms on a time scale of seconds to minutes, the formation of the second center typically proceeds within hours. In order to reveal the electronic and microscopic properties of both defect centers as well as their formation and annihilation kinetics, we perform time-resolved lifetime measurements on silicon wafers and open-circuit voltage measurements on silicon solar cells at various temperatures. Despite the fact that the two centers are found to form independently of each other, their concentrations exhibit the same linear dependence on the substitutional boron (Bs) and quadratic dependence on the interstitial oxygen (Oi) content. Our results suggest that the fast- and the slowly forming recombination centers correspond to two different configurations of a BsO2i ...

Electronically activated boron-oxygen-related recombination centers in crystalline silicon

Boron-doped crystalline silicon is the most relevant material in today's solar cell production. Following the trend towards higher efficiencies, silicon substrate materials with high carrier lifetimes are becoming more and more important. In silicon with sufficiently low metal impurity concentrations, the carrier lifetime is ultimately limited by a metastable boron–oxygen-related defect, which forms under minority-carrier injection. We have analysed 49 different Czochralski-grown silicon materials of numerous suppliers with various boron and oxygen concentrations. On the basis of our measured lifetime data, we have derived a universal empirical parameterisation predicting the stable carrier lifetime from the boron and oxygen content in the crystalline silicon material. For multicrystalline silicon it is shown that the predicted carrier lifetime can be regarded as a fundamental upper limit. Copyright © 2005 John Wiley & Sons, Ltd.

Fundamental boron-oxygen-related carrier lifetime limit in mono- and multicrystalline silicon

The objective of this article is to review our understanding of the properties of oxygen impurities in Czochralski silicon that is used to manufacture integrated circuits (ICs). These atoms, present at a concentration of ~1018 cm-3, occupy bond-centred sites (Oi) in as-grown Si and the jump rate between adjacent sites defines `normal' diffusion for the temperature range 1325 - 330 °C. Anneals at high temperatures lead to the formation of amorphous SiO2 precipitates that act as traps for fast diffusing metallic contaminants, such as Fe and Cu, that may be inadvertently introduced at levels as low as 1011 cm-3. Without this `gettering', there may be severe degradation of fabricated ICs. To accommodate the local volume increase during oxygen precipitation, there is parallel generation of self-interstitials that diffuse away and form lattice defects. High temperature (T > 700 °C) anneals are now well understood. Details of lower temperature processes are still a matter of debate: measurements of oxygen diffusion into or out of the Si surface and Oi atom aggregation have implied enhanced diffusion that has variously been attributed to interactions of Oi atoms with lattice vacancies, self-interstitials, metallic elements, carbon, hydrogen impurities etc. There is strong evidence for oxygen-hydrogen interactions at T < 500 °C and the formation of fast diffusing O2 dimers. These observations have led to significant advances in understanding the growth and structures of small oxygen clusters, identified with the so-called thermal donor and shallow thermal donor defects. There is a need to improve this understanding because the temperatures of device processing will continue to decrease as the size of future device features decreases below the lower end of the sub-micron range, currently close to 0.18 µm.

https://iopscience.iop.org/article/10.1088/0953-8984/12/25/201/pdf

Oxygen diffusion and precipitation in Czochralski silicon

Reduction in electron lifetime induced by excess electrons is a key effect in degradation of solar cells based on boron-doped and oxygen-containing silicon. Although boron related, degradation is controlled by the concentration of holes and not by the boron concentration. This recent finding is the basis for a new degradation model in which the degradation starts from a latent complex BiO2 of an interstitial boron atom and an oxygen dimer. Electron-induced reconstruction of this defect results in a production of recombination centers. This model provides a detailed explanation of the basic features of the degradation, and of subsequent recovery by annealing in the dark.

Latent complexes of interstitial boron and oxygen dimers as a reason for degradation of silicon-based solar cells

Optical characterization of the oxygen dimer in silicon has been performed for the first time. The vibrational IR absorption bands at 1012, 1060, and $1105{\mathrm{cm}}^{\ensuremath{-}1}$ are shown to arise from this complex. Using heat-treatment studies, the dimer binding energy is determined to be about 0.3 eV. Indications of the high migration ability of the dimer predicted earlier are found as well.

Experimental evidence of the oxygen dimer in silicon

Computer simulations of the generation kinetics of thermal double donors (TDD's) in Czochralski-grown silicon have been performed and compared with experimental data for samples heat treated at temperatures between 350 and 420 \ifmmode^\circ\else\textdegree\fi{}C for durations up to 500 h. The experimental data were obtained by Fourier-transform infrared spectroscopy exploring the recent finding that local vibrational modes can be associated with the individual TDD's. A model assuming sequential generation of the TDD's and a fast diffusing oxygen dimer has been found to quantitatively reproduce the experimental data. The diffusivity of the oxygen dimer was estimated to be $\ensuremath{\sim}{10}^{6}$ times the value of interstitial oxygen at 400 \ifmmode^\circ\else\textdegree\fi{}C, with an activation energy of \ensuremath{\sim}1.3 eV and a preexponential factor of $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{\mathrm{cm}}^{2}{\mathrm{}\mathrm{s}}^{\mathrm{\ensuremath{-}}1}.$ The transformation from TDD1 to TDD2 is well described by a first-order reaction having an activation energy of \ensuremath{\sim}2.5 eV, strongly indicating that the process involves motion of interstitial oxygen atoms $({\mathrm{O}}_{i}).$ This conclusion is further supported by the deduced value for the preexponential factor, being very close to that for the jump frequency of ${\mathrm{O}}_{i}.$

Kinetic study of oxygen dimer and thermal donor formation in silicon

Infrared absorption studies of defect formation in Czochralski silicon irradiated with fast electrons in a wide range of temperatures (80-900 K) have been performed. The samples with different contents of oxygen (O-16, O-18) and carbon (C-12, C-13) isotopes were investigated, The main defect reactions are found to depend strongly on irradiation temperature and dose, as well as on impurity content and pre-history of the samples. Some new radiation-induced defects are revealed after irradiation at elevated temperatures as well as after a two-step (hot + room-temperature (RT)) irradiation.

Defect engineering in Czochralski silicon by electron irradiation at different temperatures

The structure and dynamic properties of the interstitial oxygen dimer in silicon are found using a combination of infrared spectroscopy and ab initio modeling. We find that the stable dimer consists of a pair of inequivalent weakly coupled interstitial oxygen atoms separated by a Si-Si bond. Two high frequency modes are decoupled in one ${}^{16}\mathrm{O}{\ensuremath{-}}^{18}\mathrm{O}$ combination but are strongly mixed in the other combination. A third lower lying mode involves the compression of the Si-Si bond joining the oxygen atoms and gives distinct modes in the mixed ${}^{16}\mathrm{O}{\ensuremath{-}}^{18}\mathrm{O}$ case.

https://www.ewels.info/science/publications/papers/oxygendimer.prl.pdf

First Stage of Oxygen Aggregation in Silicon: The Oxygen Dimer

The precipitation of oxygen has been investigated for 2 MeV electron irradiated silicon samples, with irradiation doses 1015–1018 cm−2, at an annealing temperature of 900 °C for up to 444 h. The samples initially contained either different concentrations of the vacancy‐oxygen (VO) center created at the irradiation, or the vacancy‐dioxygen (VO2) center created by annealing at 350 °C after the irradiation. It was found that the incubation time and oxygen decay rate for the precipitation process was irradiation dose dependent. Among the VO samples this could be caused by a supersaturation of vacancies, which would both decrease the critical precipitate radius and enhance oxygen diffusion. It was also found that an enhanced precipitation among VO samples appeared as a transient process, which has not been observed at this high temperature before. A different mechanism could account for the enhanced precipitation rate for samples with VO2. For these samples results from transmission electron microscopy studies...

T. Hallberg

Papers

Experimental evidence of the oxygen dimer in silicon

Kinetic study of oxygen dimer and thermal donor formation in silicon

Defect engineering in Czochralski silicon by electron irradiation at different temperatures

First Stage of Oxygen Aggregation in Silicon: The Oxygen Dimer

Enhanced oxygen precipitation in electron irradiated silicon