Dissociative Diffusion Mechanism Research Articles

Phenomenon of ultra-fast tracer diffusion of Co in HCP high entropy alloys

An experimental-ab initio approach is applied to investigate solute diffusion in hexagonal high-entropy alloys. The radioisotope 57Co is selected to probe the diffusion behaviour in a series of HCP alloys, from equiatomic binary (HfZr) and ternary (HfTiZr) to quinary Al5Sc20Hf25Ti25Zr25 at.% and Al15Sc10Hf25Ti25Zr25 at.% high-entropy alloys. Diffusion of Co in the present alloys is found to be strongly anisotropic and exceptionally fast (by 5 to 10 orders of magnitude faster than self-diffusion) which can be explained in terms of a dissociative diffusion mechanism. The origin of ultra-fast diffusion of Co in the given HCP high-entropy alloys and the impact of the multi-element environment on the interstitial positions are analyzed in the framework of ab initio computations. Our results show that solute diffusion can be used as a tool to probe potential short range order in HCP multi-principal element alloys on very short time scales with respect to the jump rates of matrix atoms.

The interaction between lithium acceptors and gallium donors in zinc oxide

Diffusion of lithium (Li) in uniformly gallium (Ga)-doped monocrystalline bulk zinc oxide (ZnO) is studied over a wide temperature range (500–1150°C) and is demonstrated to be dictated by the distribution of Ga. Below 800°C, the indiffusion of Li from a Li-doped ZnO sputtered film into n+ single crystalline ZnO yields an abrupt and compensated Li-doped box region with the Li concentration matching the free-electron concentration, in accordance with several previous experimental and theoretical reports. However, experimental observations of Li-diffusion at higher temperatures reveal a dissociative diffusion mechanism for heat treatments up to 1150°C. By employing a reaction-diffusion model that includes both Li and Ga, a dissociation energy of 4.6 eV is obtained from the experimental Li diffusion data. This is in excellent agreement with theoretical results for the dissociation of (LiZnGaZn)0 (4.8 eV) into Lii+ and (GaZnVZn)− and suggests that this neutral and stable acceptor-donor pair prevails in Li- and Ga-doped ZnO.

Dissociative diffusion mechanism in vacancy-rich materials according to mass action kinetics

Two sets of diffusion-reaction numerical simulations using a finite difference method (FDM) were conducted to investigate fast impurity diffusion via interstitial sites in vacancy-rich materials such as Cu(In,Ga)Se2 (CIGS) and Cu2ZnSn(S, Se)4 (CZTSSe or CZTS) via the dissociative diffusion mechanism where the interstitial diffuser ultimately reacts with a vacancy to produce a substitutional. The first set of simulations extends the standard interstitial-limited dissociative diffusion theory to vacancy-rich material conditions where vacancies are annihilated in large amounts, introducing non-equilibrium vacancy concentration profiles. The second simulation set explores the vacancy-limited dissociative diffusion where impurity incorporation increases the equilibrium vacancy concentration. In addition to diffusion profiles of varying concentrations and shapes that were obtained in all simulations, some of the profiles can be fitted with the constant- and limited-source solutions of Fick’s second law despite the non-equilibrium condition induced by the interstitial-vacancy reaction. The first set of simulations reveals that the dissociative diffusion coefficient in vacancy-rich materials is inversely proportional to the initial vacancy concentration. In the second set of numerical simulations, impurity-induced changes in the vacancy concentration lead to distinctive diffusion profile shapes. The simulation results are also compared with published data of impurity diffusion in CIGS. According to the characteristic properties of diffusion profiles from the two set of simulations, experimental detection of the dissociative diffusion mechanism in vacancy-rich materials may be possible.

Experimental Evidence of Multiple Diffusion Mechanisms in Thin-Film Cu(In,Ga)Se2

Lattice and grain boundary diffusions of cadmium in copper indium gallium diselenide (Cu(In,Ga)Se2 or CIGS) thin films were investigated by annealing cadmium into samples of 700-nm CIGS thickness at temperatures between 250 and 300 °C. Diffusion profiles of cadmium were analyzed by dual-beam time-of-flight secondary ion mass spectroscopy (TOF-SIMS). In addition to fast cadmium grain boundary diffusion, experiments revealed cadmium diffusion profiles with two distinct lattice diffusion stages, which could be indicative of simultaneous vacancy and dissociative diffusion mechanisms.

Identification of the dissociative and kick-out diffusion mechanisms of Zn diffusion in GaAs by photoluminescence analysis

Abstract Most investigations on Zn diffusion in GaAs were processed using the Zn–As alloy sources to prevent the As atoms from escaping GaAs wafers, while we found that the Zn diffusion would change fundamentally if Zn–Ga alloy sources were used. The Ga atoms from the diffusion sources suppressed the formation of the high-concentration surface region in Zn profiles, thus converting a kink-and-tail profile into a box profile. The photoluminescence (PL) analysis was used to identify the diffusion mechanisms. The Ga vacancy defects were found in the surface region of the kink-and-tail profile, indicating that the dissociative mechanism dominated; the PL spectrum in the tail region of kink-and-tail profile and the main region of box profile showed the same signals, no Ga vacancy defects were found, thus the kick-out mechanism dominated.

A combined kick-out and dissociative diffusion mechanism of grown-in Be in InGaAs and InGaAsP. A new finite difference-Bairstow method for solution of the diffusion equations

Experimental results on the diffusion of grown-in beryllium (Be) in indium gallium arsenide (In0.53Ga0.47As) and indium gallium arsenide phosphide (In0.73Ga0.27As0.58P0.42) gas source molecular beam epitaxy alloys lattice-matched to indium phosphide (InP) can be successfully explained in terms of a combined kick-out and dissociative diffusion mechanism, involving neutral Be interstitials (Bei0), singly positively charged gallium (Ga), indium (In) self-interstitials (IIII+) and singly positively charged Ga, In vacancies (VIII+). A new numerical method of solution to the system of diffusion equations, based on the finite difference approximations and Bairstow's method, is proposed.

Radiation-enhanced self- and boron diffusion in germanium

We report experiments on proton radiation-enhanced self- and boron (B) diffusion in germanium (Ge) for temperatures between 515 ${}^{\ensuremath{\circ}}$C and 720 ${}^{\ensuremath{\circ}}$C. Modeling of the experimental diffusion profiles measured by means of secondary ion mass spectrometry is achieved on the basis of the Frenkel pair reaction and the interstitialcy and dissociative diffusion mechanisms. The numerical simulations ascertain concentrations of Ge interstitials and B-interstitial pairs that deviate by several orders of magnitude from their thermal equilibrium values. The dominance of self-interstitial related defects under irradiation leads to an enhanced self- and B diffusion in Ge. Analysis of the experimental profiles yields data for the diffusion of self-interstitials ($I$) and the thermal equilibrium concentration of B$I$ pairs in Ge. The temperature dependence of these quantities provides the migration enthalpy of $I$ and formation enthalpy of B$I$ that are compared with recent results of atomistic calculations. The behavior of self- and B diffusion in Ge under concurrent annealing and irradiation is strongly affected by the property of the Ge surface to hinder the annihilation of self-interstitials. The limited annihilation efficiency of the Ge surface can be caused by donor-type surface states favored under vacuum annealing, but the physical origin remains unsolved.

Defect reactions in gallium antimonide studied by zinc and self-diffusion

Extrinsic diffusion of zinc (Zn) in gallium antimonide (GaSb) under Ga-rich conditions was analyzed on the basis of the kick-out and the dissociative diffusion mechanism. It is concluded that the changeover of interstitial Zn to substitutional gallium (Ga) sites is mainly mediated by Ga interstitials ( I Ga ). Fitting of the Zn profiles provides the relative contributions of I Ga to Ga diffusion. This contribution is lower than the directly measured Ga diffusion coefficient indicating that Ga diffusion in GaSb is rather mediated by Ga vacancies than by Ga interstitials even under Ga-rich conditions. This finding supports transformation reactions between native point defects that are confirmed by first-principles total-energy calculations. In addition Ga and Sb diffusion experiments under H 2 2 atmosphere were performed to reconcile the controversial data on self-diffusion in GaSb published by Weiler et al. and Bracht et al.

Radiotracer study of cobalt diffusion and solubility in electronic-grade germanium wafers

The diffusion rate of Co in Ge is found to be as fast as 2 × 10 - 6 cm 2 s - 1 at 900 °C, whereas the Co solubility comes close to 1 × 10 16 cm - 3 at the same temperature. Based on these properties and its acceptor activity, Co may cause serious contamination problems during the fabrication of Ge-based electronic devices. In contrast to an early radiotracer study [L.Y. Wei, J. Phys. Chem. Solids 18 (1961) 162], we observe common diffusion profiles of complementary error function type, which are indicative of a constant diffusivity depending only on temperature. A preliminary analysis of the data points to the dissociative diffusion mechanism involving interstitial–substitutional exchange via vacancies. However, in contrast to Cu, which migrates via the vacancy-controlled mode of the dissociative mechanism, the diffusion of Co may be rate-limited by the transport properties of its interstitial modification ( Co i ) .

Zinc and gallium diffusion in gallium antimonide

Extrinsic diffusion of zinc (Zn) in gallium antimonide (GaSb) under Ga-rich conditions was analyzed on the basis of the kick-out and the dissociative diffusion mechanism. Accurate modeling of the experimental profiles by means of continuum theoretical calculations reveals that Zn diffusion proceeds via singly positively charged Zn interstitials $({\mathrm{Zn}}_{i}^{+})$. The changeover of ${\mathrm{Zn}}_{i}^{+}$ to substitutional gallium (Ga) sites, thereby forming the acceptor dopant ${\mathrm{Zn}}_{\mathrm{Ga}}^{\ensuremath{-}}$, is concluded to be mainly mediated by neutral ${I}_{\mathrm{Ga}}^{0}$ and singly positively charged Ga interstitials ${I}_{\mathrm{Ga}}^{+}$ via the kick-out mechanism. Fitting of the Zn profiles provides the reduced ${\mathrm{Zn}}_{i}^{+}$-mediated Zn diffusion coefficient and the relative contributions of ${I}_{\mathrm{Ga}}^{0}$ and ${I}_{\mathrm{Ga}}^{+}$ to Ga diffusion. These contributions to Ga diffusion are lower than the directly measured Ga diffusion coefficient, which indicates that Ga diffusion in GaSb is rather mediated by Ga vacancies than by Ga interstitials even under Ga-rich conditions. This finding supports the transformation reaction between native point defects in GaSb that was previously proposed to explain the Ga-vacancy-mediated diffusion of Ga in GaSb under Ga-rich conditions [H. Bracht et al. Nature (London) 408, 69 (2000)].

Distribution of Substitutional Nickel Atoms in Dislocated Silicon Crystal

Diffusion profiles of substitutional nickel atoms in dislocated silicon crystal are investigated using deep-level transient spectroscopy (DLTS). The profiles are analyzed on the basis of the theory of the dissociative mechanism of diffusion. It is found that dislocations play an important role as the sinks and sources of vacancy annihilation and generation, respectively, giving rise to the flat distribution of the U-shaped distribution observed in the crystal bulk.

Release and Diffusion Rate of Helium in Neutron-Irradiated SiC

Helium release measurements and diffusion coefficient calculations of helium in a neutron-irradiated α-SiC have been carried out in the temperature range of 750 1,260°C. The powder was obtained from SiC ceramics containing B4C, of which 10B concentration was 4.2×1016atom/mg. Neutron irradiation was conducted up to a fluence of 2.8×1024n/m2 (E>0.1 MeV) ~300°C. The diffusion coefficients were calculated for the temperature region of 750-1,060°C with time range of 0-10 min. The diffusion coefficient of helium for SiC containing B4C was D (cm2/s)=1.38×1010exp[—0.91±0.07 (eV/atom)/kT]. Dissociative and interstitial diffusion mechanisms were supposed. Diffusion coefficients were reduced above ~1,160°C probably due to accelerated migration of vacancies.

The dissociative diffusion mechanism with charge effects II: Two-dimensional in-diffusion

The dissociative diffusion mechanism with charge effects II: Two-dimensional in-diffusion

Release and Diffusion Rate of Helium in Neutron-Irradiated SiC

Helium release measurements and diffusion coefficient calculations of helium in a neutron-irradiated α-SiC have been carried out in the temperature range of 750 1,260°C. The powder was obtained from SiC ceramics containing B4C, of which 10B concentration was 4.2×1016atom/mg. Neutron irradiation was conducted up to a fluence of 2.8×1024n/m2 (E>0.1 MeV) ~300°C. The diffusion coefficients were calculated for the temperature region of 750-1,060°C with time range of 0-10 min. The diffusion coefficient of helium for SiC containing B4C was D (cm2/s)=1.38×1010exp[—0.91±0.07 (eV/atom)/kT]. Dissociative and interstitial diffusion mechanisms were supposed. Diffusion coefficients were reduced above ~1,160°C probably due to accelerated migration of vacancies.

Distribution of Substitutional Nickel Atoms in Dislocation-Free Silicon Studied by Deep Level Transient Spectroscopy and Theoretical Analyses Based on the Dissociative Mechanism of Diffusion

By means of deep level transient spectroscopy (DLTS), the diffusion profiles of substitutional nickel atoms in silicon are investigated in dislocation-free silicon at 980°C for the in-diffusion process and at 950°C for the annealing process. The results are analyzed on the basis of the dissociative mechanisms of diffusion. It is shown that nickel distribution in dislocation-free silicon can be explained by the dissociative mechanism with the model that sinks and sources of vacancies are present in the bulk in addition to the surfaces.

Shallow p-n junctions formed in silicon using pulsed photon annealing

Shallow and ultrashallow p-n junctions were formed in Si by stimulated diffusion of P from phosphosilicate glass and B from borosilicate glass under pulsed photon annealing. Electrical, photoelectric, and optical properties of these junctions were investigated. Special features of stimulated diffusion of P and B in surface layers of Si under pulsed photon annealing were revealed. The obtained results are discussed in terms of kick-out, pair vacancy-interstitial, and dissociative diffusion mechanisms. The features of the dopant concentration profiles are explained in terms of the vacancy-interstitial mechanism and the stimulated diffusion model with allowance made for the time dependence of the dopant surface concentration and the concentration dependence of the diffusivity.

Microscopic defects in silicon induced by zinc out-diffusion

We have performed Zn out-diffusion experiments in homogeneously Zn-doped Si in the temperature range from 850 to 1207°C. Diffusion profiles of Zn measured by spreading-resistance profiling are accurately described on the basis of the simultaneous occurrence of the kick-out and the dissociative diffusion mechanisms. Compared to Zn in-diffusion, Zn out-diffusion tends to be dominated by the dissociative mechanism, i.e. preferentially in regions of high Zn concentration. Numerical modeling of the out-diffusion profiles yields fairly reliable data for the vacancy transport coefficient CVeqDV. In contrast, an accurate determination of the vacancy equilibrium concentrations CVeq presumably suffers from vacancy agglomerates whose formation is driven by the vacancy supersaturation arising from the dissociative out-diffusion of Zn. Microscopic defects decorated with Cu3Si have been detected after the out-diffusion process by preferential etching and transmission electron microscope analysis. A tentative explanation for the observed Cu3Si precipitates invokes their nucleation on vacancy clusters.

Transient enhanced diffusion of aluminum in SiC during high temperature ion implantation

6H–SiC wafers were implanted at room temperature (RT) and at 1700 °C high temperature (HT) with 50 keVAl+ ions to doses from 1.4×1014 to 1.4×1016 cm−2. Compared to samples implanted at RT, the samples implanted at high temperature display considerable aluminum redistribution. The diffusion of Al is shown to be a transient effect with different decay times in the near-surface region and in the bulk. Investigation of the crystalline structure indicated that in the near-surface region dislocation loops grow in size and Al precipitates are formed as the dose of Al implanted at HT is increased. Changes in the structure of the implanted layer may have a strong effect on the redistribution of Al. The observed redistribution can be explained by a dissociative diffusion mechanism during the high-temperature implantation.

Out-diffusion of Zn from Si: A method to study vacancy properties in Si

We report out-diffusion experiments of Zn from homogeneously Zn-doped Si samples which were performed at 1107 °C. Depth distributions of Zn recorded by spreading-resistance profiling are accurately described on the basis of simultaneous contributions of the kick-out and dissociative diffusion mechanism. Analysis of the profiles reveals that Zn out-diffusion is mainly mediated by the dissociative mechanism. Fitting of Zn profiles yields data for the transport capacity of vacancies CVeqDV in Si and their thermal equilibrium concentration CVeq.

Ultrashallow p +-n junctions in silicon (100): Electron-beam diagnostics of the surface zone

Ultrashallow p +-n junctions formed in silicon (100) under nonequilibrium impurity diffusion conditions are analyzed by electron-beam diagnostics of the surface zone using a probe of low-to medium-energy electrons. The energy dependence of the radiation conductivity is investigated, along with its distribution over the area of the p +-n junction. This procedure can be used to determine the depth distribution (in the crystal) of the probability of separation of electron-hole pairs by the field of the p-n junction; the experimental results show that this distribution differs according to whether the kick-out mechanism or the dissociative vacancy mechanism of impurity diffusion is predominant as the basis of formation of the ultrashallow p +-n junctions. Also reported here for the first time are the results of investigations of the distribution of secondary point centers formed near the boundary of the ultrashallow diffusion profile, which exert a major influence on the transport of nonequilibrium carriers. The data obtained in the study demonstrate the possibility of improving the efficiency of photodetectors, α-particle detectors, and solar batteries constructed on the basis of ultrashallow p-n junctions.