Deactivation of the boron–oxygen recombination center in silicon by illumination at elevated temperature

The boron–oxygen‐related recombination center responsible for the light‐induced degradation of solar cells made on boron‐doped oxygen‐contaminated silicon is deactivated by simultaneously annealing the silicon wafer in the temperature range 135–210 °C and illuminating it with white light. The recombination lifetime after deactivation is found to be stable under illumination at room temperature. The deactivation process is shown to be thermally activated with an activation energy of 0.7 eV. Based on the experimental findings, a defect reaction model is proposed explaining the deactivation of the boron–oxygen center. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interaction of Point Defects with Twin Boundaries in Copper

The interaction between small vacancy clusters and twin boundaries in copper is studied by using many-body potential developed by Ackland et aL for fcc metals. The interaction energies of single-, di- and tri-vacancy clusters with （111） and （112） twin boundaries are computed using well established simulation techniques. For （111） twins the vacancy clusters are highly repelled when they are on the adjacent planes, and are attracted when they are away from the boundary. In the case of （112） twins, vacancy clusters are more attracted to the boundary when they are near the boundary as compared to away from it. Vacancy clusters on both the sides of the boundary are also investigated, and it is observed that the clusters energetically prefer to lie on the off-mirror sites as compared to the mirror position across the twin.     

Minority Carrier Lifetime in As-Grown Germanium Doped Czochralski Silicon

The minority carrier lifetime of as-grown germanium-doped Czochralski （GCZ） silicon wafers doped with germanium concentrations [Ge]=10^16-10^18 cm^-3 is investigated in comparison with conventional CZ silicon samples. It is found that the lifetime distribution along the ingot changes with the variation of[Ge]. There is a critical value of [Ge] = 10^16 cm^-3 beyond which Ge can obviously influence the lifetime of as-grown ingots. This phenomenon is considered to be associated with the competition or combination between the oxygen related thermal donors （TDs） and electrically active Ge-related complexes. The related formation mechanisms and distributions are also discussed.     

Structure of Ge-O complexes in Czochralski silicon

The structures of Ge-O complexes in germanium-doped Czochralski (CZ) silicon wafers have been investigated by means of density functional theory (DFT). The calculations present the fact that the Ge-O complexes can be formed with the absence of vacancy during low-temperature thermal cycles so that they can enhance oxygen precipitation. Furthermore, the total energy of different Ge-O complexes is calculated, and then optimized and stable structure of Ge-O complexes is suggested.     

The silicon vacancy in SiC

The isolated silicon vacancy is one of the basic intrinsic defects in SiC. We present new experimental data as well as new calculations on the silicon vacancy defect levels and a new model that explains the optical transitions and the magnetic resonance signals observed as occurring in the singly negative charge state of the silicon vacancy in 4H and 6H SiC.     

Spin quantum computation in silicon nanostructures

Proposed silicon-based quantum-computer architectures have attracted attention because of their promise for scalability and their potential for synergetically utilizing the available resources associated with the existing Si technology infrastructure. Electronic and nuclear spins of shallow donors (e.g. phosphorus) in Si are ideal candidates for qubits in such proposals because of their long spin coherence times due to their limited interactions with their environments. For these spin qubits, shallow donor exchange gates are frequently invoked to perform two-qubit operations. We discuss in this review a particularly important spin decoherence channel, and bandstructure effects on the exchange gate control. Specifically, we review our work on donor electron spin spectral diffusion due to background nuclear spin flip-flops, and how isotopic purification of silicon can significantly enhance the electron spin dephasing time. We then review our calculation of donor electron exchange coupling in the presence of degenerate silicon conduction band valleys. We show that valley interference leads to orders of magnitude variations in electron exchange coupling when donor configurations are changed on an atomic scale. These studies illustrate the substantial potential that donor electron/nuclear spins in silicon have as candidates for qubits and simultaneously the considerable challenges they pose. In particular, our work on spin decoherence through spectral diffusion points to the possible importance of isotopic purification in the fabrication of scalable solid state quantum computer architectures. We also provide a critical comparison between the two main proposed spin-based solid state quantum computer architectures, namely, shallow donor bound states in Si and localized quantum dot states in GaAs.     

Improving solar grade silicon by controlling extended defect generation and foreign atom defect interactions

Multicrystalline silicon has a high commercial potential for solar cell applications. Extended lattice defects, such as dislocations and grain boundaries, are important as recombination and storage centers for metallic impurities. Their control is essential to obtain high efficiencies of the solar cell. Important parameters for the assessment of the final efficiency of the solar cells are the distribution and structure of the defects and their impact on the lifetime of minority carriers. The current understanding of the nucleation mechanisms of the most important defects during crystal growth will be described. Interaction processes between mobile impurities and extended defects are important for the recombination activity. Measurements of the recombination behavior and the contamination level will be presented. Finally, the ramifications on the solar cell efficiency shall be discussed.     

First-Principles Study of Defects in CuGaO2

Using the first-principles methods, we study the electronic structure, intrinsic and extrinsic defects doping in transparent conducting oxides CuGaO₂. Intrinsic defects, acceptor-type and donor-type extrinsic defects in their relevant charge state are considered. The calculation result show that copper vacancy and oxygen interstitial are the relevant defects in CuGaO₂. In addition, copper vacancy is the most efficient acceptor. Substituting Be for Ga is the prominent acceptor, and substituting Ca for Cu is the prominent donors in CuGaO₂. Our calculation results are expected to be a guide for preparing n-type and p-type materials in CuGaO₂.     

Role of vitreous matrix on the optical activity of Ge-doped silica

We report an experimental study on the relationship between the optical activity of Ge-oxygen deficient centers and dynamic properties and conformational heterogeneity of vitreous matrix in silica. We focus our attention on the absorption band at ∼5.2 eV (B_2β) and on the two related emissions at ∼4.2 eV (α_E) and at ∼3.1 eV (β). From the temperature dependence of B_2β band we estimate a mean energy value of 26 meV for local vibrational modes coupled to the electronic transition, suggesting that the chromophore and its surrounding have access to low frequency dynamics. From the thermal behavior of the two emissions we distinguish the two competitive relaxation processes from the first singlet excited state S₁: the radiative one, giving rise the α_E band, and the thermally activated intersystem-crossing process between S₁ and the triplet state T₁, originating the β band. The intersystem-crossing rate increases on increasing the temperature, determining an opposite thermal behavior of the intensity of the two emissions. However, this temperature dependence cannot be rationalized by a simple Arrhenius law and the α_E decay kinetics at high temperatures do not follow a single exponential law, suggesting a complex landscape of configurational energies of the process.     

Tunable band structure in diamond-cubic tin-germanium alloys grown on silicon substrates

Novel chemical methods based on deuterium-stabilized Sn hydrides and ultra-high-vacuum chemical vapor deposition were used to grow Sn_xGe_1−x alloys directly on silicon. Device-quality, strain-free films with a Sn-fraction as high as x=0.2 were obtained. The optical properties provide evidence for a well-defined Ge-like band structure. In particular, the direct band gap E₀ is reduced to a value as low as 0.41 eV for Sn_0.14Ge_0.86. The growth of these high-optical quality infrared materials creates entirely new opportunities for band gap engineering on Si.     

Ferromagnetism dependence on hole carriers in polycrystalline silicon films co-doped with Mn and B

Polycrystalline Si_0.96Mn_0.04:B films were prepared by cosputtering deposition followed by rapid thermal annealing for crystallization. The films are ferromagnetic with Curie temperatures of about 250 K. Through the approach of microwave plasma enhanced chemical vapor deposition, the films were treated by hydrogen plasma and boron plasma. After the plasma treatments, the structural properties of the films did not change, while both the saturation magnetization and hole concentration in the films changed. The correlation between the magnetic properties and the transport properties of the Si_0.96Mn_0.04:B films suggests that free hole carriers play an important role in Si:Mn diluted magnetic semiconductors.     

Electronic structure of light emitting centers in Er doped Si

Ab-initio calculations are carried out for the Er-related electrically active centers in Si. Our proposed microscopic model is consistent with photoluminescence measurements on Si:Er and Si:Er:O samples. For isolated Er, the tetrahedral interstitial site is the stable configuration, being related to the photoluminescence lines in Si:Er. Several configurations containing oxygen and fluorine atoms, surrounding the Er impurities, are proposed to simulate the effects of co-implantation. The results suggest that six oxygen atoms around substitutional Er can stabilize the center, which can be related to the strong photoluminescence lines in Si:Er:O samples. On the other hand, no configuration containing fluorine atoms could explain the stronger photoluminescence lines resulting from fluorine co-implantation. Received: 9 September 2002 / Accepted: 12 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +55-11/3091-5585, E-mail: jjusto@lme.usp.br     

Defects in pulsed laser and thermal processed ion implanted silicon

The evolution of the nature and concentration of the defects produced by 100 or 300 keV As ions at fluences 1 to 4×10^–12 cm^–2 inn-type, Fz Silicon doped with 10¹⁵ to 10¹⁶ cm^–3 has been studied as function of thermal treatments (in the range 500°–900 °C) and of the energy density (in the range 0.3–0.6 J cm^–2) of a light pulse from a ruby laser (15 ns, 0.69 m). Deep-level transient spectroscopy (DLTS) combined with capacitance — voltage (C-V) measurements were used to get the characteristics (energy level, crosssection for the capture of majority carriers) of the defects and theirs profiles. The difficulties encountered in the analysis of the results, due to the large compensation of free carriers in the implanted region and to the abrupt defect and free carrier profiles, are discussed in detail and the corrections to apply on the C-V characteristics and the DLTS spectra are described. The defects resulting from the two types of treatments are found to be essentially the same. Only, for laser energies higher than 0.5 J cm^–2, the laser treatment appears to introduced new defects (atE0.32 eV) which should result from a quenching process. The fact that a laser energy smaller than the threshold energy for melting and recrystallization is able to anneal, at least partially, the defects produced by the implantation, demonstrates that the annealing process induced by the laser pulse is not a purely thermal process but is enhanced by a mechanism involving ionization.     

Iron contamination in silicon technology

This article continues the review of fundamental physical properties of iron and its complexes in silicon (Appl. Phys. A 69, 13 (1999)), and is focused on ongoing applied research of iron in silicon technology. The first section of this article presents an analysis of the effect of iron on devices, including integrated circuits, power devices, and solar cells. Then, sources of unintentional iron contamination and reaction paths of iron during device manufacturing are discussed. Experimental techniques to measure trace contamination levels of iron in silicon, such as minority carrier lifetime techniques (SPV, μ-PCD, and ELYMAT), deep-level transient spectroscopy (DLTS), total X-ray fluorescence (TXRF) and vapor-phase decomposition TXRF (VPD-TXRF), atomic absorption spectroscopy (AAS), mass spectrometry and its modifications (SIMS, SNMS, ICP-MS), and neutron activation analysis (NAA) are reviewed in the second section of the article. Prospective analytical tools, such as heavy-ion backscattering spectroscopy (HIBS) and synchrotron-based X-ray microprobe techniques (XPS, XANES, XRF) are briefly discussed. The third section includes a discussion of the present achievements and challenges of the electrochemistry and physics of cleaning of silicon wafers, with an emphasis on removal of iron contamination from the wafers. Finally, the techniques for gettering of iron are presented. Received: 16 November 1999 / Accepted: 7 January 2000 / Published online: 5 April 2000     

Iron and its complexes in silicon

This article is the first in a series of two reviews on the properties of iron in silicon. It offers a comprehensive summary of the current state of understanding of fundamental physical properties of iron and its complexes in silicon. The first section of this review discusses the position of iron in the silicon lattice and the electrical properties of interstitial iron. Updated expressions for the solubility and the diffusivity of iron in silicon are presented, and possible explanations for conflicting experimental data obtained by different groups are discussed. The second section of the article considers the electrical and the structural properties of complexes of interstitial iron with shallow acceptors (boron, aluminum, indium, gallium, and thallium), shallow donors (phosphorus and arsenic) and other impurities (gold, silver, platinum, palladium, zinc, sulfur, oxygen, carbon, and hydrogen). Special attention is paid to the kinetics of iron pairing with shallow acceptors, the dissociation of these pairs, and the metastability of iron–acceptor pairs. The parameters of iron-related defects in silicon are summarized in tables that include more than 30 complexes of iron as detected by electron paramagnetic resonance (EPR) and almost 20 energy levels in the band gap associated with iron. The data presented in this review illustrate the enormous complexing activity of iron, which is attributed to the partial or complete (depending on the temperature and the conductivity type) ionization of iron as well as the high diffusivity of iron in silicon. It is shown that studies of iron in silicon require exceptional cleanliness of experimental facilities and highly reproducible diffusion and temperature ramping (quenching) procedures. Properties of iron that are not yet completely understood and need further research are outlined. Received: 14 December 1998 / Accepted: 22 February 1999 / Published online: 26 May 1999     

Early precipitation stages of aluminum alloys—The role of quenched-in vacancies

We present preliminary results on the structure of nano-sized particles (clusters of solute atoms or Guinier-Preston-Bagaryatsky (GPB) zones) in aluminum alloys. Hindering the motion of dislocations, these GPB zones (precipitates) are responsible for the strength of AlCuMg-alloys - used e.g. as AA2024 - for the outer shell plates in aircraft industry. We will discuss the role of quenched-in vacancies for the formation and growth of the precipitates. Using positron annihilation (Doppler broadening together with the recently developed tool of high-momentum analysis (HMA)) and X-ray absorption spectroscopy measurements, we are able to characterize the local atomic environment in the vicinity of vacancies and selected elements (Cu) forming the precipitates. The interpretation will be based on a comparison to numerical calculations of positron lifetimes and momentum distributions as well as simulated X-ray absorption spectra. Often reliable numerical calculations of experimental quantities are possible only if relaxed atomic positions - calculated employing an ab-initio code like SIESTA - are used as input.     

Current-voltage behaviour of Schottky diodes fabricated on p-type silicon for radiation hard detectors

Current-voltage (I-V) measurements were carried out on Schottky diodes fabricated on undoped and on metal-doped p-type silicon. The metals used are gold, platinum, erbium and niobium. The I-V data were used to extract the saturation current, the ideality factor and the Schottky barrier height for each of the five diodes. These parameters were correlated to the defect levels generated by the metals in silicon. The results show that in all cases the silicon has become relaxation-like after doping since the device current is Ohmic. This is in agreement with the existence of the midgap defect in all the doped devices as compiled from the literature. Such metal doped (or relaxation) devices have been found to perform better as radiation-hard particle detectors.     

Charge-state effects of deep centres in semiconductors on non-radiative capture of carriers by multiphonon processes

A set of simplified analytical expressions for carrier capture coefficients, including quantatively the charge-dependent effect, have been obtained for easy physical examination and comparison with experiments. The temperature-related charge-state-dependent factorF(T) thus calculated could be used to present more accurately the nature and magnitude of the charge state of a trap centre. The ranges of values ofF(T) valid for attractive, repulsive and neutral centres are also obtained. In addition, we show that the thermal ionization energy for theB centre in GaAs is a function of temperature. The importance of the data of capture cross-section at low temperatures in determining the charge state and characteristic of a deep centre is also manifested. Both the absolute magnitude and the temperature-dependent behaviour of the calculated capture cross-section are well-supported by the very good fits to the experimental electron cross-sections forA andB centres in GaAs reported by Lang [7] and Wang et al. [22] and for Cu centre in Ge reported by Zhdanova and Kalashnikov [23].     

Studies on the origins of the absorption spectra in PbWO4 crystal

The electronic structures and absorption spectra for both the perfect PbWO₄ (PWO) crystal and the three types of PWO crystals, containing V_Pb²⁻, V_O²⁺ and a pair of V_Pb²⁻-V_O²⁺, respectively, have been calculated using CASTEP codes with the lattice structure optimized. The calculated absorption spectra indicate that the perfect PWO crystal does not occur absorption band in the visible and near-ultraviolet region. The absorption spectra of the PWO crystal containing V_Pb²⁻ exhibit seven peaks located at 1.72 eV (720 nm), 2.16 eV (570 nm), 2.81 eV (440 nm), 3.01 eV (410 nm), 3.36 eV (365 nm), 3.70 eV (335 nm) and 4.0 eV (310 nm), respectively. The absorption spectra of the PWO crystal containing V_O²⁺ occur two peaks located at 370 nm and 420 nm. The PWO crystal containing a pair of V_Pb²⁻-V_O²⁺ does not occur absorption band in the visible and near-ultraviolet region. This leads to the conclusions that the 370 and 420 nm absorption bands are related to the existence of both V_Pb²⁻ and V_O²⁺ in the PWO crystal and the other absorption bands are related to the existence of the V_Pb²⁻ in the PWO crystal. The existence of the pair of V_Pb²⁻-V_O²⁺ has no visible effects on the optical properties. The calculated polarized optical properties are well consistent with the experimental results.     

Computer simulation of intrinsic defects in YAlO3 single crystal

Computer simulation techniques were used to investigate intrinsic defects in YAlO₃ single crystal. A set of short-range potential parameters were derived using a relaxed fitting procedure incorporating with the known crystal properties. These parameters were then applied within the framework of the shell model. The simulation results reveal that oxygen Frenkel disorder and the antisite defect of Al ion substituting the Y ion dominate the intrinsic defects in YAlO₃. An analysis of redox reactions corroborate that the oxidation is most likely to occur via forming interstitial oxygen, while the oxidation via filling oxygen vacancies and reduction reaction may predominate at high temperature. The activation energy of oxygen vacancy migration on conduction was also studied.