Oleg Velichko

Mechanisms of arsenic clustering in silicon

A model of arsenic clustering in silicon is proposed and analyzed. The main feature of the proposed model is the assumption that negatively charged arsenic complexes play a dominant role in the clustering process. To confirm this assumption, electron density and concentration of impurity atoms incorporated into the clusters are calculated as functions of the total arsenic concentration. A number of the negatively charged clusters incorporating a point defect and one or more arsenic atoms are investigated. It is shown that for the doubly negatively charged clusters or for clusters incorporating more than one arsenic atom the electron density reaches a maximum value and then monotonically and slowly decreases as total arsenic concentration increases. In the case of doubly negatively charged cluster incorporating two arsenic atoms, the calculated electron density agrees well with the experimental data. Agreement with the experiment confirms the conclusion that two arsenic atoms participate in the cluster formation. Among all present models, the proposed model of clustering by formation of doubly negatively charged cluster incorporating two arsenic atoms gives the best fit to the experimental data and can be used in simulation of high concentration arsenic diffusion.

Macroscopic description of the diffusion of interstitial impurity atoms considering the influence of elastic stress on the drift of interstitial species

The diffusion equation for non-equilibrium interstitial impurity atoms taking into account their charge states and drift of all mobile interstitial species in the built-in electric field and in the field of elastic stress is obtained. The obtained generalized equation is equivalent to the set of diffusion equations written for the interstitial impurity atoms in each individual charge state. Due to a number of characteristic features the generalized equation is more convenient for numerical solution than the original system of separate diffusion equations. On this basis, the macroscopic description of stress-mediated impurity diffusion due to a kick-out mechanism is obtained. It is supposed that the interstitial impurity atom makes a number of jumps before conversion to the substitutional position. At the same time, local equilibrium prevails between substitutionally dissolved impurity atoms, non-equilibrium self-interstitials, and interstitial impurity atoms. Also, the derived equation for impurity diffusion due to the kick-out mechanism takes into account all charge states of interstitial impurity atoms as well as drift of interstitial species in the electric field and in the field of elastic stress. Moreover, this equation exactly matches the equation of stress-mediated impurity diffusion due to the generation, migration, and dissociation of the equilibrium ‘impurity atom–self-interstitial’ pairs.

Microscopic mechanism responsible for radiation-enhanced diffusion of impurity atoms

Modelling of radiation-enhanced diffusion (RED) of boron and phosphorus atoms during irradiation of silicon substrates respectively with high- and low-energy protons was carried out. The results obtained confirm the previously arrived conclusion that impurity diffusion occurs by means of the ‘impurity atom – intrinsic point defect’ pairs and that the condition of the local thermodynamic equilibrium between substitutional impurity atoms, nonequilibrium point defects created by irradiation, and the pairs is valid. It is shown that using RED, one can form a special impurity distribution in the semiconductor substrate including retrograde profiles with increasing impurity concentration in the bulk of the semiconductor. In addition, modelling of radiation-induced segregation of nitrogen implanted in stainless steel modified by titanium is carried out. It is shown that vacancy-impurity complexes are responsible for nitrogen diffusion in an implanted layer excluding the ‘tail’ region. The calculations performed give clear evidence in favour of further investigation of various doping processes based on RED, especially the processes of plasma doping, to develop a cheap method for forming specific impurity distributions in the near surface region.

Investigation of the Hydrogen Transport Processes in Crystalline Silicon of n-Type Conductivity

The silicon substrates were hydrogenated at approximately room temperature and hydrogen concentration profiles vs. depth have been measured by SIMS. Czochralski grown (CZ) wafers, both n- and p-type conductivity, were used in the experiments under consideration. For analysis of hydrogen transport processes and quasichemical reactions the model of hydrogen atoms diffusion and quasichemical reactions is proposed and the set of equations is obtained. The developed model takes into account the formation of bound hydrogen in the near surface region, hydrogen transport as a result of diffusion of hydrogen molecules 2 H , diffusion of metastable complexes * 2 H and diffusion of nonequilibrium hydrogen atoms. Interaction of 2 H with oxygen atoms and formation of immobile complexes “oxygen atom - hydrogen molecule” (O - H2 ) is also taken into account to explain the hydrogen concentration profiles in the substrates of n-type conductivity. The computer simulation based on the proposed equations has shown a good agreement of the calculated hydrogen profiles with the experimental data and has allowed receiving a value of the hydrogen molecules diffusivity at room temperature.

Clustering of Arsenic Atoms in Silicon during Low-Temperature Annealing

Simulation of arsenic clustering in Si at a temperature of 750 degrees Celsius has been carried out. It has been shown that considering the formation of singly or doubly negatively charged clusters that incorporate one or two arsenic atoms and point defects, one obtains a good fit to the measured values of electron density. It is supposed that we have the initial stage of clustering, when the concentration of complexes with one arsenic atom incorporated is high enough and the diffusion of these mobile particles provides for the formation of more stable clusters incorporating two arsenic atoms.