Float Zone Silicon Research Articles

Photoluminescence characterization of ultrahigh purity silicon

When photoluminescence (PL) measurements on silicon are calibrated against an established measurement technique, they become a powerful characterization tool capable of quantitatively identifying impurities. PL is particularly useful for characterizing ultrahigh purity silicon, due to its ability to determine the concentrations of the shallow impurities down to ≈5 × 1010 atoms-cm−3. We have utilized variable temperature Halleffect measurements to calibrate PL measurements made at 4.2 K on ultrahigh purity float-zone silicon, establishing calibration relationships to determine boron, phosphorus, and gallium concentrations. The concentrations range from the low −1012 atoms-cm-3 for phosphorus and boron, to 1 × 1014 atoms-cm−3 for boron and 1 × 1016 atoms-cm−3 for phosphorus. The preliminary gallium calibration concentration range is from the high-1012 to the mid-1010 atoms-cm3. Previous PL calibrations based on variable temperature Hall-effect measurements have not produced the theoretically expected proportionality between the impurity concentration and the PL line intensities of the transverse-optical phonon-assisted replicas of the impurity bound exciton normalized to the PL line intensity of the free-exciton. By performing the PL measurements at a fixed free-exciton density, we have achieved the theoretically expected proportionality. The experimental methods and an overview of the theory concerning fixed free-exciton con-ditions, the associated Hall-effect analyses, and the PL calibrations are presented.

On the correlation between high-order bands and some photoluminescence lines in neutron-irradiated FZ silicon

The defects in float-zone silicon irradiated by fast neutron with fluences up to 4.0×1018 n/cm2, followed by various heat treatments, have been studied by low-temperature photoluminescence (PL) and infrared absorption measurement with emphasis upon the high-order band (HOB) and its relationship with the commonly observed PL-lines such as I1 (1.018 eV) and I3(1.039 eV). It has been shown that band 1124 cm−1, unlike the other higher-order bands, is considerably broader for the sample annealed at low temperature (for example, 385 °C) with FWHM as large as 3 meV and is apparently narrowed as the anneal temperature was increased. We have obtained the I3 line and its phonon replicas in the near-infrared absorption measurement, further proving the transition involved in the I3 defect center to be electronic in nature. The combination of luminescence and absorption experiment results demonstrated that the HOB could be well developed after PL lines such as I1 and I3 disappeared completely, or vice versa. PL lines could be observed before the HOB emerged, therefore ruling out the possibility proposed by earlier authors that the HOB could be correlated with some PL lines.

LBIC measurements of the recombining activity of dislocations in float zone silicon

The electrical activity of dislocations has been evidenced by LBIC scanning measurements through two different distributions of dislocations for which theoretical models leading to the recombination velocity could be applied. A close experimental correlation was found between photocurrent and dislocation density. The models led to the same recombination velocity for dislocations Sd ≈ 10 3 cm s −1 . The role of oxygen and of temperature on the activity of dislocations is discussed.

A study of carbon-implanted silicon for light-emitting diode fabrication

An overview is presented of material studies on a wide range of carbon implants, 1 × 10 14 − 1 × 10 17 (35–350 keV C +) cm −2 , in float zone silicon at 77–673 K. The substitutionality, redistribution and precipitation of the isovalent impurity under furnace and rapid thermal annealing are discussed. Strained solid-phase-recrystallized layers containing substitutional carbon in excess of 10 20 atoms cm −3 can be achieved with low levels of residual damage and precipitation. Pre-amorphization with a second group IV dopant (e.g. tin) which expands the lattice is used to counteract the lattice contraction due to carbon and thereby dramatically reduce strain within the implanted layer.

Status and future of silicon crystal growth

An overview of the growth techniques and characteristics of current industrial Czochralski zone (CZ) and float zone (FZ) silicon crystals is given. The origins of the different crystal qualities are described. Particular consideration is given to the impurity situation, to the oxygen content and to the micro- and macro-scopic distribution of dopants, oxygen and other impurities. The reasons for the preferential use of CZ silicon in the manufacture of highly integrated devices and the future requirements of the bulk properties of the silicon crystals are discussed. From this, necessary or possible developments in crystal growth techniques are deduced. The possibilities and limitations of diverse crystal growth techniques and of computer simulations of crystal growth processes are outlined.

RTA-induced defects: a comparison between lamp and electron beam techniques

Rapid thermal annealing (RTA) techniques are increasingly used in many technological applications. To date, however, only a limited knowledge exists of the effects induced by these thermal treatments on the material characteristics. We have studied the defect equilibria in float zone (FZ) and Czochralski zone (CZ) silicon subjected to annealing at 400°C by employing two different heating techniques, namely electron beam and lamp system. By comparison of the variations induced in the minority carrier lifetime and the crystal quality by the two systems an insight into the defects due to the anneal technique itself is obtained.

Effect of Carbon and Oxygen Precipitation on Gold Diffusion in Silicon

The effect of the precipitation of carbon and/or oxygen on the diffusion of gold in Czochralski(CZ)- and float-zone(FZ) silicon crystals were studied using the neutron activation analysis for gold, infrared spectrophotometry for substitutional carbon and interstitial oxygen and charged-particle activation analysis for total concentration of carbon and oxygen. The diffusion of gold was found to be enhanced by the existence of carbon and/or oxygen precipitates. This enhancement has been related to the precipitation of carbon and oxygen in CZ samples, whereas the gold diffusion in FZ samples, having less oxygen and no carbon precipitation, was not enhanced.

Hydrogen-related electron traps in proton-bombarded float zone silicon

P + n diodes have been irradiated at nominal room temperature by 1.3 MeV H + or 5.0 MeV He 2+ ions to doses in the range 10 8–10 10 cm −2. The diodes were analysed with respect to electron traps in the forbidden band gap by applying deep level transient spectroscopy. It is found that two levels, at approximately 0.32 and 0.45 eV below the conduction band, originate from irradiation-induced defects involving hydrogen. The levels appear at low doses and are presumably caused by low order complexes. On the basis of the experimental results and of computer simulations of the H + implantation profile it is speculated that the two peaks may originate from vacancy-oxygen and divacancy configurations which are partly saturated with hydrogen.

Four-vacancy damage clusters in neutron-irradiated silicon

The production of vacancy clusters in neutron-damaged n-type float zone silicon was investigated using electron paramagnetic resonance (EPR). The irradiation conditions in the reactor channel provided different doses of fast neutrons, Φ N , whereas the thermal neutron dose was kept constant for all the samples studied. The four-vacancy cluster (P3) was found to be the dominant defect centre identified in the EPR spectrum. It was shown that its concentration increased almost linearly with Φ N within the range studied, indicating that these defects are formed in the primary cascades induced by fast neutrons. it is estimated that approximately two P3 centres are produced per primary cascade, irrespective of the fast neutron dose. This suggests that these defects are mainly associated with the displacement spikes which terminate the cascades. Generation of P3 centres was found to depend only on the fast neutron dose and to be quite independent of the fast neutron spectra.

New photoluminescence lines in selenium-doped silicon

After diffusion of selenium at 1250°C for 1 week into phosphorus-doped float zone silicon, the photoluminescence spectrum shows several previously unreported sharp lines. We tentatively describe the dominant line at 1073.1 meV as a transverse optical phonon replica of an exciton bound to a selenium-related donor, the corresponding weak no-phonon recombination line being found at 1131.1 meV. The additional lines observed at lower energies are associated with neutral isoelectronic complexes. The temperature dependence of the intensity of these various lines is presented. Tentative interpretations are given in comparison with results from IR absorption obtained from the same samples.

Structure of thermal donors (NL8) in silicon: A study with electron-nuclear double resonance.

Singly ionized thermal donors [(TD${)}^{+}$], which give rise to the NL8 ESR spectrum, were investigated with electron-nuclear double resonance (ENDOR) in B-doped Czochralski-grown silicon and float-zone silicon into which the magnetic isotope $^{17}\mathrm{O}$ was diffused. TD's were formed by annealing at 460 \ifmmode^\circ\else\textdegree\fi{}C for 2--8 h. ENDOR lines of two shells of $^{17}\mathrm{O}$ and seven shells of $^{29}\mathrm{Si}$ were observed. Five different species of ${\mathrm{TD}}^{+}$'s were identified, which are formed one after another upon annealing. The ESR spectra of these TD's were superimposed in the NL8 spectrum. The smallest ${\mathrm{TD}}^{+}$ measured by ENDOR is probably identical with ${\mathrm{TD}}_{3}^{+}$ as identified from the infrared-absorption bands. The core of the TD's contains four oxygen atoms, two on each of the two (110) mirror planes. All the five ${\mathrm{TD}}^{+}$'s investigated showed ${C}_{2v}$ point-group symmetry. A single Si atom is possibly also part of the TD core structure, while the other Si neighbors are ligands. The hyperfine interactions of all $^{29}\mathrm{Si}$ and $^{17}\mathrm{O}$ nuclei are very small, representing only about 5% of the unpaired-electron spin density. The small size of the interactions cannot be explained by present theories. The compatibility of current TD models with the ENDOR results is discussed.

Surface copper contamination of as-received float-zone silicon wafers

As-received float-zone Si wafers from two major suppliers are shown to have surface Cu contamination at a level of ∼5×1011 atoms cm−2, detectable by both low-temperature photoluminescence and room-temperature x-ray fluorescence. The effects of selective chemical removal of Cu from, or Cu adsorption onto, the front or rear surfaces of wafers have demonstrated where the majority of the contamination resides, thereby revealing its possible origin.

Defect states in 2.0-MeV electron-irradiated phosphorus-doped silicon

Float-zoned n-type silicon samples with phosphorus concentrations in the range 1014–1017 P atoms cm−3 have been irradiated by 2-MeV electrons at room temperature with the aim of studying the role of phosphorus in the resulting damage. Using deep-level transient spectroscopy we were able to observe a number of previously unreported deep traps. A major post-irradiation bistable defect center detected in highly P-doped (∼1017 cm−3) material has an electron trap at Ec−0.42 eV, and a hole trap at Ev+0.15 eV (Ec and Ev are the conduction- and valence-band edges, respectively), each of which is associated with one of the two configurations of the center. The center, alternating between being acceptorlike to being donorlike as it undergoes configurational transformation, is suggested to be P related. Two metastable electron traps located at Ec−0.32 eV and Ec−0.42 eV are observed immediately after irradiation in moderately P-doped samples (∼1015–1016 cm−3). Interestingly, the configurational transformation rates of these states are observed to be dependent on the magnitude of the bias applied to the diode during cooldown to LN2 temperature. This is taken to indicate that configurational transformations, often thermally activated, can in some cases be influenced by the electric field strength within the depletion region. Upon thermal annealing at 200 °C of the latter two states, a couple of new states emerge; an electron trap at Ec−0.42 eV, stable up to temperatures above 300 °C, and a hole trap at Ev+0.25 eV which anneals out at 300 °C. An electron trap situated at Ec−0.24 eV with a quite small capture cross section for electrons is seen in lightly P-doped material (∼1014 cm−3) following annealing at 300 °C. Tentative propositions for the defect assignments of the observed traps are put forward.

Anomalous diffusion of nitrogen in nitrogen-implanted silicon

A secondary-ion mass spectrometry analysis of the coimplantation of nitrogen, carbon, and oxygen into float-zone silicon followed by rapid thermal annealing for 10 s at different temperatures is used to study the anomalous diffusion behavior of nitrogen in silicon. The results may be only partially explained by a model of paired nitrogen atom diffusion. The complexity of the diffusion of nitrogen in ion-implanted samples, with and without coimplants, and the expectation that the nitrogen after annealing may be in many different forms, suggest that studies which use nitrogen implantation for basic understanding of nitrogen-related defects may be misleading.

Spin-orbit splitting of the valence bands in silicon determined by means of high-resolution photoconductive spectroscopy.

We present the first high-resolution photoconductive observation of the transitions associated with the resonant impurity states of residual boron in very-high-purity float-zone silicon single crystals at different low temperatures. According to the energies of the transitions from the ground state to the impurity states related to the ${p}_{1/2}$ valence band, and taking the nonparabolicity of the split-off valence band into consideration, we determine the ionization energy of the boron acceptor to the spin-orbit split-off valence band very accurately as 88.45\ifmmode\pm\else\textpm\fi{}0.01 meV. The spin-orbit splitting of the valence bands in silicon is also deduced to be 42.62\ifmmode\pm\else\textpm\fi{}0.01 meV.

Gettering in high resistive float zone silicon wafers for silicon detector applications

An intrinsic gettering technique for float-zone highly resistive silicon using TCA+O/sub 2/ has been described. The capacitance-voltage technique was used to determine the flat-band voltage and stretch-out of MOS (metal oxide semiconductor) structures made on various oxides. It has found that this intrinsic getting process improves minority-carrier-generation lifetime and SiO/sub 2//Si interface properties, leading to the reduction of leakage current in the p-i-n detector configuration. Direct comparisons of intrinsic gettering and extrinsic gettering using As ion-implantation have been made. Intrinsic gettering has been found to be the dominant process. >

On the yield point of floating-zone silicon single crystals III. The evolution of the dislocation structure through the yield point at low temperature

Abstract The dislocation structures in silicon single crystals deformed up to different points in the yield region, at 823 K and at the shear strain rate γ of 2 × 10−5 s−1, have been estimated by transmission electron microscopy. In addition to the total dislocation density ρt, the mobile dislocation density ρm can be measured with reasonable accuracy, thanks to the characteristic shape of dislocation loops. Local shear stresses are estimated from the radius of curvature at dislocation bends. Estimation of the effective stress τeff is difficult before the lower yield point because of micro-scale inhomogeneities. The internal consistency of the measured τeff and ρ has been checked using Orowan's relation, leading to the conclusion that, in the experimental conditions realized, τeff is very close to the applied stress and nearly all dislocations are mobile from the onset of plastic deformation up to the lower yield point.

Measurements of Enhanced Diffusion of Buried Layers in Silicon Membrane Structures During Oxidation

AbstractReported calculations of the diffusion coefficient of silicon self interstitials vary over several orders of magnitude at temperatures of interest for integrated circuit fabrication. In this work, we measure the enhanced diffusion of phosphorus buried layers while interstitials are injected at a wafer surface via thermal oxidation. The starting substrates were either float-zone silicon or Czochralski silicon with a pre-treatment to precipitate excess oxygen. The samples were prepared by implantation of phosphorus followed by the growth of a 40-60µm epitaxial layer. They were then etched anisotropically from the frontside or backside to yield membrane structures. Local oxidation was performed at 1100°C on either the frontside or backside of each wafer and buried layer diffusion was monitored, yielding information about interstitial diffusion in silicon and it’s dependence on bulk properties.

Electrical and optical properties of gold-doped n-type silicon

Different measurement techniques, both electrical and optical, were utilized in this work to characterize gold diffusion in n-type, float-zoned silicon in the temperature range 600–1150 °C. In the lower temperature region (≤750 °C), the gold diffusion is observed by the introduction of the Au acceptor state at 0.53 eV below the conduction band, and is correlated to the electrical behavior of the samples deduced from Hall effect and resistivity data. Also, the effects of Au diffusion on the free-carrier concentration and mobilities are discussed. It was shown that high temperatures and long times for gold diffusion change the conductivity type in the samples from n to p. In the samples that converted to p type, a limiting room-temperature resistivity of 2.0×103 Ω cm was attained, when the conduction is mainly influenced by the Au-related deep electronic states in the band gap. In this case, the diffusion mechanism is also investigated by secondary ion mass spectroscopy data determining the equilibrium Au solubility, which is close to the equilibrium solubility of interstitial gold. Low-temperature photoluminescence measurements have shown that the intensity of the lines often attributed to dislocations, increases significantly by gold diffusion in the lower temperature region. At higher diffusion temperatures, a decrease of the dislocation-related lines was found, associated with formation of gold-related precipitates. Introducing an inhomogeneous internal stress distribution in the Si matrix, these precipitates cause line shifts as well as line broadenings of the free exciton, the phosphorus bound exciton, and the electron-hole droplet photoluminescence emissions. The concentration of substitutional phosphorus is found to decrease with increasing diffusion temperatures.

Investigations for the improvement of the radial doping homogeneity of dislocation‐free floating zone silicon crystals

AbstractIn order to improve the doping inhomogeneities of dislocation free FZ Silicon crystals with diameters ≧ 75 mm investigations were carried out to influence the shape and curvature of the crystallization interface during the crucible‐free floating zone crystal growth process. In this works shapes and geometries of the applied melting inductors and the relative positions of these inductors to the seed axis were examined concerning the values of radial macroscopic and microscopic doping inhomogeneities in dependence on other growth parameters, for instance crystal rotation rate. The application of eccentric inductor‐seed positions effects a decreasing “bending value” of the crystallization interface, that means an improvement of the macroscopic radial doping inhomogeneity with crystal rotation rate in the range between 4 and 8 min−1.