Diffusion Of Oxygen In Silicon Research Articles

Minimum energy path and atomistic mechanism of the elementary step in oxygen diffusion in silicon: A density-functional study

Using a density-functional scheme, we study the migration of a single O atom in a (110) plane between two adjacent bond-center sites in bulk Si. The minimum energy migration path is found through the nudged elastic band method within a generalized gradient approximation for the electronic structure. The energy barrier is then also evaluated within a hybrid functional scheme. We achieve for the transition barrier a best estimate of 2.3 eV in the generalized gradient approximation and of 2.7 eV in the hybrid functional scheme, both in fair agreement with the commonly accepted experimental value of 2.53 eV. The transition is characterized by a saddle point which does not occur at the midpoint between the two bond-center sites and by a pattern of displacements extending up to the second nearest-neighbor Si atoms. The atomistic mechanism of oxygen migration is analyzed from three complementary viewpoints involving the evolution of the structure, the Wannier centers, and the single-particle energies and wave functions. The diffusion process can be separated into two distinct parts. In one part, the exchange of the Si atoms in the first-neighbor shell of the diffusing O atom occurs through the formation of a threefold coordinated O center and an overcoordinated Si atom. In the other part, the Si--Si bond flips its position through the creation of occupied and unoccupied Si dangling bonds which give rise to states in the band gap.

Density functional theory study of the impact of tin doping on oxygen diffusion in Czochralski silicon

AbstractThe impact of tin (Sn) doping on interstitial oxygen (Oi) diffusion in Czochralski silicon is investigated by density functional theory (DFT) calculations. Two aspects are taken into consideration: (i) Sn induced lattice strain and lattice expansion; (ii) direct and vacancy‐mediated interaction between Sn and Oi atoms. The calculations show that the interstitial oxygen migration energy barrier increases with increasing lattice constant which grows with the concentration of Sn. Structure analysis indicates that the geometric parameters such as the bond length and the angle of SiOSi configuration also increase with the lattice constant, which is considered to be the origin of the increased energy barrier for oxygen diffusion. SnO pair formation is found to be energetically unfavorable, while the SnVO complexes have a large binding energy. Sn is however able to trap Oi atoms by stimulating vacancy generation around it, which increases the oxygen diffusion energy barrier as well. It is thus predicted that oxygen diffusion in silicon with a high concentration of Sn atoms will be retarded.

The effect of impurity-induced lattice strain and Fermi level position on low temperature oxygen diffusion in silicon

Oxygen diffusion in silicon is known to be affected by high concentrations of impurities, although the mechanism underpinning this is poorly understood. We have studied oxygen transport in Czochralski silicon by analyzing data on the locking of dislocations by oxygen as a function of time and temperature. In this paper, we present new data from crystals grown to contain high levels of germanium and arsenic. We analyze these new data, together with our previous data for silicon with a high boron concentration, to further the understanding of the mechanism by which high impurity concentrations affect oxygen transport at temperatures at which the oxygen dimer dominates transport (up to 550 °C). Our results show that a high level of boron doping (∼3 × 1018cm−3) enhances the effective diffusivity of oxygen by a factor of ∼8 to ∼25 relative to low doped material with the same oxygen concentration. High levels of germanium doping (∼8 × 1019cm−3) and arsenic doping (∼2 × 1019cm−3) can both have a slight retardation effect on oxygen transport. The magnitude of the reduction measured is less than a factor of ∼4 in the heavily germanium doped specimens and less than a factor of ∼5 in the heavily arsenic doped specimens, and in most cases is significantly less than this. Germanium doping introduces considerable strain into the silicon lattice without affecting the Fermi level position, so data from these samples show that lattice strain affects oxygen dimer transport. The arsenic and boron doping levels in the materials studied give rise to lattice strain with a smaller magnitude and opposite sign to that in the germanium doped samples. It is therefore suggested that the Fermi level position also affects the transport of oxygen dimers.

Enhanced formation of oxygen-related thermal donors in Ge crystals exposed to hydrogen plasma

Enhanced thermal donor (TD) formation and oxygen diffusion in silicon exposed to hydrogen plasma are well known and have been widely studied. Similar phenomena are expected to occur in oxygen-rich Ge crystals as well, but they have not been studied in detail yet. In this work the effects of hydrogenation in H +-plasma on the TD formation in Ge are investigated. Both oxygen-rich and oxygen-lean Ge crystals of n- and p-type conductivity were used. The samples were treated in dc H +-plasma in the temperature range 200–350 °C for different durations. The TD depth profiles were obtained by spreading resistance probe measurements on the angle-lapped surfaces of the samples. In oxygen-rich Ge crystals an enhanced formation of TDs has been found to occur upon hydrogenation as well as upon subsequent annealing at 350 °C deeply in the bulk up to depth of about 1 mm. The depth distribution of the TDs formed in enhanced way depends on the hydrogenation procedure as well as on the conditions of the subsequent heat-treatments. A possibility of the deep p–n-junction formation by H +-plasma treatment of the oxygen-rich Ge crystals has been demonstrated. An enhancement factor of the TD formation is estimated and a comparison with similar phenomena in silicon has been carried out.

Defect Formation Behaviors in Heavily Doped Czochralski Silicon

To reveal a difference of defect formation behaviors,i.e. grown-in void formation during crystal growth and oxide precipitation in n- and p-type silicon, we have investigated by using heavily boron- and arsenic-doped silicon crystals. The density of void defects in heavily boron doped silicon was decreased with a shrinking OSF-ring, but in arsenic doped silicon were increased with resistivities below 3.3mΩcm. On the other hand, for oxygen precipitation, the nucleation rate in boron doped silicon was enhanced with increasing resistivities, while decreased by one tenth in reference to lightly doped silicon for resistivities up to 4.4mΩcm in arsenic doped silicon. These contrastive phenomena between n- and p-type cannot be explained with a growth model of precipitates by an accelerated diffusion of oxygen in silicon. We believed that the nucleation rate of oxide precipitates related to a dependence of point defects on fermi level closely.

Enhanced diffusion of oxygen in silicon due to resonant laser excitation of local vibrational mode

We have found that the diffusion of the oxygen impurity atoms in silicon crystals is significantly enhanced by irradiation with infrared laser light whose wave number is close to the frequency of the local vibrational mode of the oxygen. This effect is considered to be ascribed to the optical excitation of the oxygen-localized vibrations at the Si–O–Si units in the silicon crystals.

The Build-Up of Strain Fields in Czochralski-Si Observed in Real Time by High Energy X-Ray Diffraction

The build-up of strain fields caused by the precipitation of oxygen in Czochralski-silicon during annealing up to 1200°C and for process times up to 70 hours has been observed in real time by high energy x-ray diffraction. Five different processes are distinguished in the temperature evolution of the intensity and of the rocking width of the silicon 220-reflection. These features are attributed to different precipitation mechanisms. A fit to part of the data with a diffusion limited precipitation model leads to an activation energy for oxygen diffusion in silicon of 2.2 eV in the temperature range from 700°C to 950°C.

General Model of Diffusion of Interstitial Oxygen in Silicon, Germanium and Silicon - Germanium Crystals

A theoretical modelling of the oxygen diffusivity in silicon, germanium and Si1-xGex (O) crystals both at normal and high hydrostatic pressure has been carried out using molecular mechanics, semiempirical and ab initio methods. It was established that the diffusion process of an interstitial oxygen atom (Oi) is controlled by the optimum configuration of three silicon (germanium) atoms nearest to Oi. The calculated values of the activation energy Ea (Si) = 2.59 eV, Ea(Ge) = 2.05 eV and pre-exponential factor D0(Si) = 0.28 cm2 s−1, D0(Ge) = 0.39 cm2 s−1 are in good agreement with experimental ones and for the first time describe perfectly the experimental temperature dependence of the Oi diffusion constant in Si crystals (T = 350–1200 °C). Hydrostatic pressure (P ≤ 80 kbar) results in a linear decrease of the diffusion barrier (∂P Ea (P) = −4.38 × 10−3 eV kbar−1 for Si crystals). The calculated pressure dependence of Oi diffusivity in silicon crystals agrees well with the pressure-enhanced initial growth of oxygen-related thermal donors. The simulation (PM5) has revealed that in Si1-xGex crystals there are two mechanisms of variation of Oi diffusion barrier. The increase of lattice constant leads to the linear increase of the diffusion barrier. Strains around Ge atoms decrease the diffusion barrier. Formation of gradient of diffusion barrier in the volume of Si1-xGex may be responsible for the experimentally observed suppression of generation of TD in Si1-xGex (O) crystals.

Diffusion of oxygen and silicon in silicon: Silicon monoxide model

The diffusion of oxygen in silicon was modeled to result from the diffusion of dissolved silicon monoxide. The SiO molecule dissolved in the largest space in the diamond lattice of silicon, oriented in a 〈111〉 direction, with the oxygen lightly bonded to a network silicon atom. This configuration is consistent with infrared spectra, dichroism of infrared spectral lines, and internal friction of oxygen in silicon. The anomalous rapid diffusion of oxygen below 700 °C could result from the diffusion of molecular water or oxygen in silicon, but more evidence is needed to test these possibilities. Diffusion and exchange of silicon tracer in SiO with lattice silicon can possibly explain tracer diffusion of silicon in silicon.

Early stages of oxygen precipitation in silicon: The effect of hydrogen

The formation of small oxygen clusters upon heat treatment at 280–375°C was studied in crystalline silicon doped with hydrogen by high-temperature in-diffusion. The presence of hydrogen in concentrations of 1015–1016 cm−3 significantly enhances (by up to a factor of 106 at T≤300°C) the oxygen diffusivity in Si crystals. Possible mechanisms of interaction between O and H impurity atoms and the origin of hydrogen-enhanced oxygen diffusion in silicon are discussed.

Oxygen precipitation in Czochralski grown silicon heattreated at 550 °C

The diffusion of oxygen in silicon at 550 °C is studied using small-angle neutron scattering (SANS) andthe diffusion coefficient, D, calculated from Ham's theory isfound to be ~10 times higher than that expected byextrapolation of higher- and lower-temperature data(D = 0.13exp (-2.53 eV kT-1) cm2 s-1). This resultconfirms previous observations of enhanced diffusion atintermediate temperatures (400-650 °C) althoughthe magnitude of theenhancement we find is much smaller than that reported by someothers. At 550 °C the precipitates observedinitially by SANS appear to be spherical. For longerannealing times, the scattering becomes anisotropic, suggestingthat a cuboidal morphology is developing.

Microfluctuations of oxygen impurity concentration as a reason of accelerated oxygen diffusion in silicon

Microfluctuations of oxygen impurity concentration as a reason of accelerated oxygen diffusion in silicon

"Ab initio" studies of hydrogen-enhanced oxygen diffusion in silicon

A novel microscopic mechanism for hydrogen-enhanced oxygen diffusion in p-doped silicon is proposed. A path for joint diffusion of O and H is obtained from an ab-initio molecular dynamics "kick" simulation. The migration pathway consists of a two-step mechanism, with a maximum energy of 1.46 eV. This path represents a 0.54 eV reduction in the static barrier when compared with the diffusion of isolated O in Si, in excellent agreement with experiments.

Mechanism for hydrogen-enhanced oxygen diffusion in silicon

Oxygen diffuses in silicon with an activation energy of 2.53--2.56 eV. In hydrogenated samples, this activation energy is found to decrease to 1.6--2.0 eV. In this paper, a microscopic mechanism for hydrogen-enhanced oxygen diffusion in p-doped silicon is proposed. A path for joint diffusion of O and H is obtained from an ab initio molecular-dynamics simulation in which the O atom is ``kicked'' away from its equilibrium position with a given initial kinetic energy. After reaching a maximum potential energy of 1.46 eV above the ground state, the system relaxes to a metastable state on which a Si-Si bond is broken and the H atom saturates one of the dangling bonds. With an extra 0.16 eV, the Si-H bond is broken and the system relaxes to an equivalent ground-state configuration. Therefore, the migration pathway is an intriguing two-step mechanism. This path represents a 0.54-eV reduction in the static barrier when compared with the diffusion of isolated O in Si, in excellent agreement with experiments. This mechanism elucidates the role played by the H atom in the process: it not only serves to ``open up'' a Si-Si bond to be attacked by the oxygen, but it also helps in reducing the energy of an important intermediate state in the diffusion pathway.

Loss of oxygen and carbon during donor formation in CZ-silicon

Due to a lack of proper understanding about the formation mechanism of oxygen related donors during the transition temperature range (465–540°C) which exhibits the transition of TDs to NDs, an attempt has been made to study their behaviour in the present context. We have found the formation and diffusion of molecule like oxygen at low temperature annealing of silicon and observed that second order kinetics of oxygen diffusion holds good. A relatively low value of 0·6 eV has been estimated to be the activation energy for the diffusion of oxygen in silicon which is supposed to be due to the hydrogen passivation.

Enhanced modes of oxygen diffusion in silicon

Experiments have established two modes of enhanced oxygen diffusion in silicon, one involving only oxygen atoms and the other catalyzed by hydrogen. We report state-of-the-art first-principles calculations and determine migration pathways for a self-enhanced mode via oxygen dimers and for the H-enhanced mode via oxygen-hydrogen complexes. The concerted atomic motions are physically transparent and the corresponding reduced activation energies agree with experimental data.

Formation Kinetics of the Al-Related Shallow Thermal Donors: A Probe for Oxygen Diffusion in Silicon

Formation Kinetics of the Al-Related Shallow Thermal Donors: A Probe for Oxygen Diffusion in Silicon

Kinetics of Formation and Metastability Mechanism for Thermal Donor-Related Defects in Al-Doped Silicon

For almost 40 years, since their discovery in the 1950's [1], thermal donors (TD) in silicon are certainly among the most studied defect systems in solids. Although much effort has been put into the investigation of this defect, the microscopic stucture and the generation kinetics of thermal donors are not fully understood [2]. Even less is known about the TD-related defects, shallow thermal donors (STD), observed for the first time in the mid eighties by photo-thermal ionization spectroscopy [3]. In the Al-doped samples a new member of the STD-family has been found, called the K-donor [4]. It is generated simultaneously with other shallow thermal donors. It has a very similar binding energy, but, in contrary to other STD, it exhibits a metastable behaviour. As it has been found only in Al-doped samples, it is believed to contain aluminium [4]. In this paper we present results of the generation kinetics of the K-donor. The creation process , of the donor is linked to a more general problem of oxygen diffusion in silicon at TD-annealing temperatures. We also attempt to explain the unusual metastable behaviour of the K-donors in terms of an entropy-driven transition. The large entropy change accompanying the metastable transition of the K-donors at low temperatures can in turn be the physical reason of the very efficient diffusion processes leading to the creation of the defect. The K-donor generation experiment was carried out with boron and aluminium doped Si samples (WASO-grade) in numerous annealing steps at 470°C

Ctivation Energies Of Interstitial Oxygen Diffusion In Silicon Containing Hydrogen

AbstractThe activation energies of interstitial oxygen diffusion in silicon containing hydrogen were derived from the results obtained from thermal donor generation experiments using numerical fitting based upon the classical nucleation rate theory and the time-dependent reduction of interstitial oxygen concentration resulting from oxygen aggregation. By using a new thermal equilibrium concentration of hydrogen at a pressure of 1 atm of 2×1021 exp(- 1.75 eV/kT), and the expression for the dependence of hydrogen thermal equilibrium concentration on the partial pressure P given as P0.65 times the hydrogen thermal equilibrium concentration at a pressure of 1 atm, the correlation between the activation energies of interstitial oxygen diffusion and hydrogen saturation is observed to be linearlogarithmic. In the hydrogen saturation range between 103 and 106, the activation energies of interstitial oxygen diffusion are estimated to be between 0.03 and 0.21 eV lower than the normal value of 2.53 eV. This implies that enhanced oxygen diffusion may occur primarily under hydrogen saturation conditions.

Thermal configurations of oxygen in silicon

The central position and the infrared absorption coefficient of the 9 μm band of Si samples were measured with Fourier transform infrared spectroscopy (FTIR) at temperatures from T=77 K to 775 K. The infrared absorption coefficients were corrected by considering background absorption and free carrier absorption calculated from the increased free carrier concentration and from the resistivity determined from Hall effect measurements. We found the central position of the 9 μm band to shift to longer wavelengths with increasing temperature. The concentration [Oi] of interstitial oxygen is almost constant for T 600 K. These results verified there are two types of thermal configurations of oxygen in silicon: The bonded Si2O configuration with a binding energy E b≈0.8 to 1.0 eV at T≈77 K to 600 K, and the Si2O configuration coexists with a quasi-free interstitial oxygen (QFIO) state for T>600 K. The lattice potential barrier E L, which retards QFIO atoms from migrating in the lattice, is estimated to be 1.5 to 1.6 eV. From these configurations the anomalous diffusivity of oxygen in silicon can be explained quite well.