Eiji Kamiyama

Density functional theory study on the impact of heavy doping on Si intrinsic point defect properties and implications for single crystal growth from a melt

Density functional theory (DFT) calculations are performed to obtain the formation energies of the vacancy V and the self-interstitial I at all sites within a sphere around the dopant atom with 6 A radius for V and 5 A radius for I in Si crystals. Substitutional p-type (B and Ga), neutral (C, Ge, and Sn), and n-type (P, As, Sb, and Bi) dopants were considered. The results show that the formation energies of V and I around dopant atoms change depending on the types and sizes of the dopants, i.e., depending on the electrical state and the local strain around the dopants. The dependence of the total thermal equilibrium concentrations of point defects (sum of free V or I and V or I around the dopants) at melting temperature on the type and concentration of each dopant is obtained. Further DFT calculations reveal that most of the total incorporated point defects from the melt contribute to pair recombination. An appropriate model of point defect behavior in heavily doped single crystal Si growing from a melt i...

Ab initio study of vacancy and self-interstitial properties near single crystal silicon surfaces

The microscopic model of the Si(001) crystal surface was investigated by first principles calculations to clarify the behavior of intrinsic point defects during crystal growth and thermal annealing. A c(4 × 2) structure model was used to describe the crystal surface in contact with vacuum. The calculations show that a vacancy in the first or second atomic layer has about a 2.0 eV lower formation energy than deeper inside the bulk and that there is a diffusion barrier to penetrate into the deeper crystal region. Furthermore, a vacancy in the first or second atomic layer is stabilized by the fact that Si atoms with dangling bonds attract each other due to ionic and/or covalent bonding. There is, however, no barrier for the diffusion of a vacancy from the first layer to the second one. The tetrahedral (T)-site and dumbbell (DB)-site, in which a Si atom is captured from the surface and forms a self-interstitial, are found as stable sites near the third atomic layer. The T-site has a barrier of 0.48 eV, wherea...

A study on density functional theory of the effect of pressure on the formation and migration enthalpies of intrinsic point defects in growing single crystal Si

In 1982, Voronkov presented a model describing point defect behavior during the growth of single crystal Si from a melt and derived an expression to predict if the crystal was vacancy- or self-interstitial-rich. Recently, Vanhellemont claimed that one should take into account the impact of compressive stress introduced by the thermal gradient at the melt/solid interface by considering the hydrostatic pressure dependence of the formation enthalpy of the intrinsic point defects. To evaluate the impact of thermal stress more correctly, the pressure dependence of both the formation enthalpy (Hf) and the migration enthalpy (Hm) of the intrinsic point defects should be taken into account. Furthermore, growing single crystal Si is not under hydrostatic pressure but almost free of external pressure (generally in Ar gas under reduced pressure). In the present paper, the dependence of Hf and Hm on the pressure P, or in other words, the pressure dependence of the formation energy (Ef) and the relaxation volume (vf),...

Effect of Sn atoms on incorporation of vacancies in epitaxial Ge1−xSnx film grown at low temperature

The anomalous increase and decrease in the S-parameters of Doppler broadening spectroscopy in positron annihilation spectroscopy in a narrow range of Sn atom content were detected in a Ge1−xSnx thin film grown by MBE at low temperatures. The increase can be explained in terms of vacancies when the target content of 1.7% Sn atoms is incorporated in a Ge matrix, owing to the binding nature between them. However, the S-parameters were markedly decreased when the target content of Sn atoms in the film grown at the same temperature was 0.1%. These changes in the S-parameters correspond to the carrier concentrations obtained by Hall measurements.

Surface-induced charge at a Ge (100) dimer surface and its interaction with vacancies and self-interstitials

The behavior of intrinsic point defects near the Ge (100) surface was investigated by using ab initio calculations. A thin plate model with clean Ge surfaces including a dimer structure on both sides was examined. A decrease of the formation energies of both types of intrinsic point defects near the surface is obtained similar as was reported before for Si. An important difference, however, is that the impact of the electric charges at the Si surface vanishes around the fifth layer, while for Ge, the effect of negative charges near the surface remains and positive charges are observed even deeper than the fifteenth layer from the surface. In bulk Ge, negatively charged vacancies are reported to be stable. Opposite to this, the neighbouring atoms around a vacancy near the Ge surface have a strong positive charge, compared with the case of Si. Taking these facts into consideration, the difference of the charge state of a vacancy in the bulk and that near the surface can be explained by band bending due to t...

First principles analysis on interaction between vacancy near surface and dimer structure of silicon crystal

We investigated the interactions between the dimer structures in a reconstructed Si (100) surface and a vacancy to understand the behavior of the point defects near the surface using first principles calculations. The calculated results showed that even a vacancy set in the ninth layer from the surface affects the charges around the dimers. This effect intensifies if the vacancy moves to the surface. Similar to the vacancies in a bulk, Jahn-Teller distortion occurred at atoms around the vacancy. When the introduced vacancy approaches to the fourth layer from the bottom, this distortion increased, and thus decreases the total energy. The third layer, in which forming a vacancy requires the highest energy, becomes a singularity layer in a Si (100) surface with dimer structures. The model, in which a vacancy was introduced into the fifth layer from the surface, came to have a vacancy in the fourth layer as an optimized structure due to a Si atom moving from the fourth to the fifth layer. We can see a clear i...

Thermal stress induced void formation during 450 mm defect free silicon crystal growth and implications for wafer inspection

When pulling large diameter Si crystals from a melt close to the Voronkov criterion, small changes in pulling speed and thermal gradient can lead to the formation of voids leading to detrimental pits on the polished wafer surface. The creation of voids is mainly due to the lowering of the vacancy formation energy due to increased thermal compressive stress. The small size and low density of the formed voids when pulling crystals close to the Voronkov criterion conditions are a challenge for wafer surface inspection tools and possible solutions are discussed.

Impact of the Formation of Dimer Structures at the Surface on the Internal Atoms of Si Thin Film

The impact of dimer structures forming at the surface on the internal atoms of the Si thin film was examined by using two types of models: silicon-on-insulator (SOI) and plate. In SOI models, a dimer formation was modeled at one side of the Si thin film. Plate models had two dimers at each surface, which had been considered as a Si bulk model in previous studies. First principles calculation showed that the deviations of Si atoms from first to fourth layers and total energies of SOI models did not differ remarkably from those of plate models. The internal atoms deeper than the fifth layer showed no deviation in some SOI models and had nonzero deviation in the other SOI models. All SOI and plate models showed the lower self-energy of a Si atom than that in the Si bulk. The layer-to-layer distance of internal atoms in the film became longer than that of atoms in the Si bulk. These results indicated that (i) Si films with dimer surfaces are relaxed in deviations of the whole film and (ii) the plate models correspond not to Si bulk but to silicon-on-nothing structures or Si thin film.

Growth and crystalline properties of Ge 1−x−y Sn x C y ternary alloy thin films on Ge(001) substrate

We achieved the worlds first epitaxial growth of a Ge_1-x-ySn_xC_y layer, and investigated the effect of Sn incorporation on the growth of Ge_1-xC_x. Sn incorporation can decrease the epitaxial temperature of Ge_1-xC_x layer. Also, Sn incorporation can make C atoms stable at the substitutional site. This Ge_1-x-ySn_xC_y layer is expected to realize the energy band engineering independently on the lattice parameter and promises to extend the potential of group-IV semiconductor materials for nanoelectronics and optoelectronic applications.

Intrinsic point defect behavior close to silicon melt/solid interface

The impact of various crystal pulling process and silicon material parameters on the so called Voronkov criterion for “perfect” crystal pulling is revised. It is shown that thermal stress effects should be taken into account in particular for the development of the 450 mm diameter single crystal silicon pulling technology. An improved Voronkov criterion is proposed and its application illustrated showing that all published experimental results on grown-in defects dependence on doping and crystal pulling conditions can be explained at least semi-quantitatively.