Theoretical analysis of the impurity distribution in single-crystal silicon

Results are presented for the total energies calculated for oxygen and carbon impurities in silicon at T=0 K. The equilibrium positions of these point defects are determined at low (10⁻³–10⁻² at. %) concentrations. Fiz. Tverd. Tela (St. Petersburg) 39, 1384–1385 (August 1996)     

New mechanism of impurity conduction in lightly doped crystalline noncompensated silicon

It is found that the plastic deformation of lightly doped crystalline silicon samples (N<6×10¹⁶ cm^?3) with a low compensation (K～3×10^?2) gives rise to nonohmic conduction σ_M in electric fields that differs radically from conventional hopping conduction via the ground states of impurities (σ₃). The values of σ_M can exceed values of σ₃ by a factor of 10³?10⁵. The value of σ_M and its dependence on the electric (E) and magnetic (H) fields can be controlled by varying the density of dislocations and the mode of thermal sample treatment. A strong anisotropy of σ_M is observed in samples with oriented dislocations: the conductivities along and across dislocations can differ by a factor of 10⁴. The results are explained by the occurrence of conduction via the H^?-like states of impurities concentrated in the vicinity of dislocations. The levels of these states lie between the upper and the lower impurity Hubbard bands.     

Magnetic field effects on the nonohmic impurity conduction of uncompensated crystalline silicon

The impurity conduction of a series of crystalline silicon samples with the concentration of major impurity N ≈ 3 × 10¹⁶ cm^?3 and with a varied, but very small, compensation K was measured as a function of the electric field E in various magnetic fields H-σ(H, E). It was found that, at K < 10^?3 and in moderate E, where these samples are characterized by a negative nonohmicity (dσ(0, E)/dE < 0), the ratio σ(H, E)/σ(0, E) > 1 (negative magnetoresistance). With increasing E, these inequalities are simultaneously reversed (positive nonohmicity and positive magnetoresistance). It is suggested that both negative and positive nonohmicities are due to electron transitions in electric fields from impurity ground states to states in the Mott-Hubbard gap.     

Theoretical modeling of the structure of tilt grain boundaries in crystalline silicon

This paper reports on the results of the calculation of the structures of tilt grain boundaries in crystalline silicon. The calculation has been performed using the classical molecular mechanics method with the Tersoff potential. The calculation of the phonon spectrum has demonstrated that there are several frequency ranges corresponding to vibrations localized along the grain boundary.     

Theoretical study of the diffusion of lithium in crystalline and amorphous silicon

The effect of the lattice deformation on potential barriers for the motion of a lithium atom in crystalline silicon has been studied through ab initio density functional calculations. A new universal method of calculating the diffusion coefficient of an admixture in amorphous solid media through the activation mechanism has been proposed on the basis of these data. The method is based on the calculation of the statistical distribution of potential barriers for the motion of an admixture atom between minima depending on the position of neighboring atoms. First, the amorphous structure, which is generated by annealing from the crystalline structure with vacancies, has been simulated. Then, the statistical distribution of the potential barriers in the amorphous structure for various local environments of the admixture atoms has been calculated by means of linear regression with the parameters determined for barriers in crystalline silicon subjected to different deformations. The diffusion coefficient of the admixture has been calculated from this distribution by using the Arrhenius formula. This method has been tested by the example of crystalline and amorphous silicon with admixture of lithium atoms. The method demonstrates that the diffusion of lithium in amorphous silicon is much faster than that in crystalline silicon; this relation is confirmed experimentally.     

晶体硅太阳电池金属电极光学损失的理论分析与实验研究

本文基于丝网印刷和丝网印刷后光诱导电镀太阳电池,分析了太阳电池前表面金属电极引起的光学损失的各种情况.考虑到空气-玻璃界面和金属电极两侧边缘区域的反射,通过将金属电极截面近似为半椭圆形模拟了电极的光学损失,计算得到的有效宽度比约为金属电极几何宽度的40%.通过对不同类型样品反射谱的测量计算,同时在理论模拟和实验测量上得到了太阳电池前表面金属电极的光学损失,相应的理论与实验结果相符合. 关键词： 光学损失 有效宽度比 光诱导电镀 反射谱     

Hexavacancies in crystalline silicon

The kinetics of disorientation of B ₈₀ ⁴ centers is studied. The reorientation energy of the centers is determined to be ～1.5 eV. Based on the fact that this value is close to the energy of hexavacancy transition from the ground state to the first metastable state, it is concluded that the B ₈₀ ⁴ center is a ring hexavacancy. The reorientation mechanism is explained by the hexavacancy transition to the metastable state and, then, to a state with a new orientation.     

Theoretical analysis of photoelectron angular distributions in hydrogen chloride

Multiple-scattering calculations for the ≈X² II and ≈A²Σ⁺ ionizations of HCl are analysed by a partitioning procedure to explain the angular distributions around the Cooper minima. A formal analogy to resonances is pointed out. The Cooper minima in the valence ionizations of Cl₂ are discussed qualitatively.     

Theoretical analysis of impurity precipitation in nanopores in crystals. II: Kinetics of impurity cluster growth in pores

The kinetics of the formation of impurity clusters in subsurface nanopores in crystals is studied theoretically. A physical model of precipitation of the impurity phase in nanopores in a sample with sinks of various types is developed. This model forms the basis for the calculation of the annealing kinetics of copper containing subsurface pores and cobalt impurity atoms. The optimal annealing conditions are determined in which cobalt atoms diffuse predominantly into pores and form impurity clusters in them.     

Understanding the current-voltage characteristics of industrial crystalline silicon solar cells by considering inhomogeneous current distributions

Solar cells made from multi- or mono-crystalline silicon wafers are the base of today’s photovoltaics industry. These devices are essentially large-area semiconductor p-n junctions. Technically, solar cells have a relatively simple structure, and the theory of p-n junctions was established already decades ago. The generally accepted model for describing them is the so-called two-diode model. However, the current-voltage characteristics of industrial solar cells, particularly of that made from multi-crystalline silicon material, show significant deviations from established diode theory. These deviations regard the forward and the reverse dark characteristics as well as the relation between the illuminated characteristics to the dark ones. In the recent years it has been found that the characteristics of industrial solar cells can only be understood by taking into account local inhomogeneities of the dark current flow. Such inhomogeneities can be investigated by applying lock-in thermography techniques. Based on these and other investigations, meanwhile the basic properties of industrial silicon solar cells are well understood. This contribution reviews the most important experimental results leading to the present state of physical understanding of the dark and illuminated characteristics of multi-crystalline industrial solar cells. This analysis should be helpful for the continuing process of optimizing such cells for further increasing their energy conversion efficiency.     

Donor-hydrogen complexes in crystalline silicon

Summary Experimental results are presented on the study of Sb-H complexes in crystalline silicon, employing¹¹⁹Sb→¹¹⁹Sn source M?ssbauer spectroscopy and a low-energy H implantation technique. In addition to a visible component, we observe a large decrease of the M?ssbauer intensity associated with the trapping of hydrogen, even at low temperatures. This is interpreted as the formation of a component with a negligible recoilless fraction. The different M?ssbauer components were studied as a function of H dose, H-implantation temperature and annealing temperature. The data show that the visible component is associated with the well-known SbH complex, whereas the invisible component is associated with the formation of SbH _n (n≥2) complexes. We show that these complexes are in thermal equilibrium with a larger hydrogen reservoir (H ₂ ^* ), which governs their thermal stability. No Sb-H complexes are observed inp-type Si after H-implantation, in agreement with the current belief that hydrogen has a deep donor level in the gap. The microscopic structure of the various Sb-H and Sn-H complexes was studied with first-principles calculations using the pseudopotentialdensity-functional approach. The structure of the Sb-H complex is found to be similar to the P-H complex, with the H in an antibonding site of a Si atom neighbouring the Sb impurity. For SbH₂ three configurations are found with energies differing by less than ≈ 0.1 eV. We find that the reaction SbH+H≠SbH₂ is exothermic. We argue that the SbH₂ complexes are shallow donors, irrespective of the structure. Therefore, the formation of SbH₂ may depassivate the sample. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Theoretical analysis of the formation of impurity precipitates in nanocavities. I. thermodynamic analysis

The formation of the impurity phase in materials containing nanopores is investigated theoretically. The formation of impurity clusters on the inner surface of pores is studied using the thermodynamic approach. The most advantageous states of metal impurities in silicon are determined, and comparison with available experimental data is carried out. The possibility of the formation of cobalt nanoclusters in subsurface cavities in copper is substantiated theoretically.     

States of hydrogen in crystalline silicon

The thermal transformation of deuterium in p-type crystalline silicon is studied with a variety of experimental techniques. It is found that D-atoms initially trapped at acceptor sites can be transferred by low temperature annealing to a different state tentatively ascribed to interstitial D₂ molecules. Diffusion of D out of the passivated sample only occurs at temperatures significantly higher than this transformation temperature. This fact allows us to produce Si samples with extremely high deuterium concentrations (several at%) by a suitable passivation-annealing sequence. With increasing D-concentration, a number of characteristic Si-D defect complexes have been observed by vibrational spectroscopy.     

Ionization mechanisms of aluminum acceptor impurity in silicon

Processes of ionization of shallow acceptor centers (ACs) in silicon are studied. In crystalline silicon samples with phosphorus (1.6×10¹³, 2.7×10¹³, and 2.3×10¹⁵cm^?3) and boron (1.3×10¹⁵cm^?3) impurities, _μAl impurity atoms were produced by implantation of negative muons. It is found that thermal ionization is the main mechanism for ionizing the Al acceptor impurity in both p-type and n-type silicon with an impurity concentration of ?10¹⁵cm^?3 at T>45 K. The thermal ionization rate of Al ACs in Si varies from ～10⁵ to ～10⁶s^?1 in the temperature range 45–55 K.