Papers by Author: Vladimir V. Voronkov

Abstract: Lifetime-degrading recombination centres those that emerge in the presence of excess carriers in boron and oxygen containing silicon - show a peculiar dependence on the concentrations of the relevant impurities, B and O, and on the hole concentration p₀ (net doping) in materials that contain compensating donors (phosphorus or Thermal Donors) or added Ga acceptors. The data indicate involvement of both substitutional (B_s) and interstitial (B_i) boron atoms in the major recombination centres observed in p-Si. A suggested model ascribes degradation to the presence of a B_iB_sO latent defect inherited from the thermal history in a recombination-inactive atomic configuration. In the presence of excess electrons, this latent defect reconfigures into a recombination-active centre. The defect concentration dependence on the material parameters is reduced, in some special cases, to a proportionality to p₀ [² or to [ [². The essential feature is an involvement of a fast-diffusing species B_i in the defect. This species can be removed to the boron nanoprecipitates thus eliminating the defects responsible for the degradation.

Abstract: Vacancies (and probably also self-interstitials) in silicon appear to exist in several forms (atomic configurations) some of them being fast diffusers and other slow diffusers. The data on enhanced self-diffusivity under proton irradiation, on vacancy and oxide precipitate profiles installed by Rapid Thermal Annealing, and on the self-diffusivity under equilibrium conditions suggest that there are at least two kinds of vacancy: 1) V_w - a fast-diffusing localized vacancy manifested in electron irradiated samples (Watkins vacancy), 2) V_s - a slow-diffusing extended vacancy manifested under hot proton irradiation. In RTA experiments, these two species behave as one equilibrated subsystem of a moderate effective diffusivity intermediate between those of V_w and V_s. There is also strong evidence in favor of a third kind of vacancy: V_f a fast extended species, which controls the grown-in voids in silicon crystals.

Abstract: Transient and quasi-steady-state photoconductance methods were used to measure minority carrier lifetime in p-type Czochralski silicon processed in very clean conditions to contain oxide precipitates. Precipitation treatments were varied to produce a matrix of samples, which were then characterised by chemical etching and transmission electron microscopy to determine the density and morphology of the precipitates. The lifetime component associated with the precipitates was isolated by preventing or factoring out the effects of other known recombination mechanisms. The lifetime component due to unstrained precipitates could be extremely high (up to ~4.5ms). Recombination at unstrained precipitates was found to be weak, with a capture coefficient of ~8 x 10^-8cm³s^-1 at an injection level equal to half the doping level. Strained precipitates and defects associated with them (dislocations and stacking faults) act as much stronger recombination centres with a capture coefficient of ~3 x 10^-6cm³s^-1 at the same level of injection. The lifetime associated with strained precipitates increases with temperature with a ~0.18eV activation energy over the room temperature to 140°C range. The shape of the injection level dependence of lifetime was similar for all the specimens studied, with the magnitude of the lifetime being dependent on the precipitate density, strain state and temperature, but independent of precipitate size.

Abstract: In dislocation-free silicon, intrinsic point defects – either vacancies or self-interstitials, depending on the growth conditions - are incorporated into a growing crystal. Their incorporated concentration is relatively low (normally, less than 1014 cm-3 - much lower than the concentration of impurities). In spite of this, they play a crucial role in the control of the structural properties of silicon materials. Modern silicon crystals are grown mostly in the vacancy mode and contain many vacancy-based agglomerates. At typical grown-in vacancy concentrations the dominant agglomerates are voids, while at lower vacancy concentrations there are different populations of joint vacancy-oxygen agglomerates (oxide plates). Larger plates – formed in a narrow range of vacancy concentration and accordingly residing in a narrow spatial band – are responsible for the formation of stacking fault rings in oxidized wafers. Using advanced crystal growth techniques, whole crystals can be grown at such low concentrations of vacancies or self-interstitials such that they can be considered as perfect.

Abstract: Illumination-induced degradation of minority carrier lifetime was studied in n-type Czochralski silicon co-doped with phosphorus and boron. The recombination centre that emerges is found to be identical to the fast-stage centre (FRC) known for p-Si where it is produced at a rate proportional to the squared hole concentration, p2. Since holes in n-Si are excess carriers of a relatively low concentration, the time scale of FRC generation in n-Si is increased by several orders of magnitude. The generation kinetics is non-linear, due to the dependence of p on the concentration of FRC and this non-linearity is well reproduced by simulations. The injection level dependence of the lifetime shows that FRC exists in 3 charge states (-1, 0, +1) possessing 2 energy levels. The recombination is controlled by both levels. The proper identification of FRC is a BsO2 complex of a substitutional boron and an oxygen dimer. The nature of the major lifetime-degrading centre in n-Si is thus different from that in p-Si - where the dominant one (a slow-stage centre, SRC) was found to be BiO2 – a complex involving an interstitial boron.

Abstract: Out-diffusion nitrogen profiles measured by SIMS after annealing at 850 and 800oC, have a peculiar minimum at a depth of about 5 m. The profiles are well reproduced by simulations assuming that there is a considerable fraction of nitrogen stored in substitutional clusters VN4. Upon annealing, these clusters lose nitrogen and convert into a stable high-temperature form VN1. This reaction involves a preliminary attachment of a fast-diffusing interstitial trimer, N3. Accordingly, the conversion occurs only in the bulk but not at the surface (due to out-diffusion loss of N3), and the substitutional component decreases from the surface towards the bulk. By fitting the profiles, the two basic parameters of the N2/N1 transport are deduced: P = D1K1/2 (a combination of the monomeric diffusivity D1 and the dissociation constant of dimers, K), and the dissociation time of dimers. With these data, D1(T) and K(T) are specified.

Abstract: The generation of Thermal Donors in Si is a nucleation process controlled by several mobile On clusters. The rate-limiting transitions are found to be O1  O2 and O4  O5. The individual transition rates G12 and G45, and also G23 and G34 are deduced from the experimental data. From the transient variation of the generation rate G(t), the equilibrium concentration of the dimers is found, and with it the dimeric diffusivity is also defined. In samples pre-treated at high T, the G(t) dependence has a maximum, due to quenched-in fast-diffusing oxygen monomers (FDMs). The concentration and diffusivity of FDMs were determined.

Abstract: The use of Rapid Thermal Processing to install lattice vacancy profiles into silicon wafers for the purpose of forming a template for the nucleation and ideal control of oxygen precipitation has become an important materials engineering tool for the microelectronics industry. This paper reviews the principles of the technique and the precise materials/defect engineering that it engenders. It furthermore discusses what has been learned regarding the elusive properties of the intrinsic point defects in silicon through studies of the distributions of vacancies created by use of the technique. Also discussed are recent discoveries about the critical role of the other intrinsic point defect, the self-interstitial and the development of oxygen precipitates and their distributions post-nucleation and the critical importance of what has become to be called the “ninja transformation” in the switching-on of gettering efficiency of oxygen precipitate systems.

Abstract: The time dependence of thermal donor (TD) concentration, N(t), during annealing at 450oC was measured in samples cut from a single slab of silicon containing bands of grown-in microdefects of different types. An enormous impact of the microdefect type on the kinetic curve was observed. Samples from the interstitial region showed simple linear rise in N(t). The samples from an inner part of the vacancy region showed a complicated oscillating variation with an abrupt disappearance of the TDs at some moment followed by an immediate restoration of a linear rise. In samples from the marginal H-band of the vacancy region, an initial anneal does not produce TDs. However if this anneal was followed by a quench, subsequent anneals produce a linear rise in N(t). On the other hand, if the sample was slowly cooled, the subsequent production of TDs remained almost negligible. These observed peculiarities are accounted for by enhanced TD growth in the presence of self-interstitials (I) - due to IO species serving as vehicles for oxygen transport.

Abstract: Nitrogen in silicon is known to affect dramatically the properties of voids. A plausible mechanism could be vacancy trapping by nitrogen interstitial species, mostly by the minor monomeric species (N1) with only a negligible contribution of the major dimeric species (N2). However, a more careful analysis of the published data shows that in Czochralski silicon no vacancy trapping occurs at the void formation stage (around 1100oC). The implication is that the trapping reaction, V + N1, although favoured thermodynamically, is of a negligible rate. Therefore, the nitrogen effect on voids in Czochralski Si is entirely due to nitrogen adsorption at the void surface. Quite a different mechanism operates in Float-Zoned crystals where voids are formed at lower T. Here vacancy trapping by N2 seems to be responsible for void suppression.