Float Zone Silicon Research Articles

Generation of 1.2 ps electrical pulses through parallel gating in ultrathin silicon photoconductive switches

We report on a parallel-gating photoconductive switching technique which is capable of generating ultrashort electrical pulses. Carrier-lifetime-independent pulses as short as 1.2 ps are produced using a long-lifetime intrinsic float-zone (FZ) silicon substrate. The technique utilizes a thin FZ-silicon layer as the light-activated medium, where photoexcitation electrically contacts the switch output to both the bias line and the substrate ground plane. Model calculations based on transmission-line theory and lumped-element analysis accurately describe the experimental results.

In-situ HRTEM observation of the melting-crystallization process of silicon

Behaviors in the melting-solidification process of float-zone silicon (Fz-Si) were examined by in-situ high-resolution transmission electron microscopy (HRTEM). Point defect clusters were observed to form at high temperatures. {1 1 1} liquid–solid interfaces were confirmed to proceed by the lateral movement of a pair of steps having several atomic heights, while {1 0 0} liquid–solid interfaces were flat and moved between both end facets. A crystallized {1 1 1} liquid–solid interface was observed at about 20 layers. A twin band was observed at an interface close to silicide formed by surface diffusion.

Very low bulk and surface recombination in oxidized silicon wafers

Bulk and surface processes determine the recombination rate in crystalline silicon wafers. In this paper we report effective lifetime measurements for a variety of commercially available float-zone silicon wafers that have been carefully passivated using alnealed silicon oxide. Different substrate resistivities have been explored, including both p-type (boron) and n-type (phosphorus) dopants. Record high effective lifetimes of 29 and 32 ms have been measured for 90 Ω cm n-type and 150 Ω cm p-type silicon wafers, respectively. The dependence of the effective lifetime has been measured for excess carrier densities in the range of 1012–1017 cm−3. These results demonstrate that very low bulk and surface recombination rates can be maintained during high-temperature oxidation (1050 °C) by carefully optimizing the processing conditions.

Low temperature muonium behaviour in Cz-Si and Cz-Si 0.91Ge 0.09

Observation of muon and muonium behaviour in semiconducting materials provides valuable insight into analogous hydrogen states and dynamics. We have used muon spin rotation, relaxation and resonance techniques to observe the low-temperature muon behaviour in Czochralski-grown silicon and bulk, Czochralski-grown silicon-germanium alloy materials. Low temperature relaxation of the rapidly moving tetrahedral muonium species and its conversion to a diamagnetic species is seen in the Cz material but not in the float zone silicon, suggesting site change and subsequent ionisation are promoted by the oxygen impurity. This is also the first time muon behaviour in a Si 1− x Ge x alloy material has been presented.

High-energy proton radiation induced defects in tin-doped n-type silicon

The formation of radiation defects in Sn-doped Float-Zone (FZ) and Czochralski (Cz) n-type silicon after a high-energy proton irradiation is studied by a combination of Deep Level Transient Spectroscopy (DLTS) and Photoluminescence (PL). Besides the Sn-V levels, evidence is found for at least three new Sn-related radiation defects, two electron traps close to the conduction band and one hole trap close to the valence band. No new PL lines have been identified so far in Sn-doped material. It is furthermore concluded that the Sn-related radiation defects do not show pronounced recombination activity, in the concentration range studied.

New electron spin resonance spectra from iron–vacancy pair in silicon: II. Hyperfine interactions and isotopic effect

We studied the TU2 and TU3 ESR spectra from an iron–vacancy pair in iron-doped and electron-irradiated float-zone grown silicon samples. Isotopic shifts in the fine structure term and hyperfine and superhyperfine interaction terms of the spin Hamiltonian are considered in detail. We obtained that the change in the spin value of the defect is accompanied by a modification of the iron atom position. The existing model fails to explain observed large anisotropy of the isotopic effect.

DLTS study of defects in hydrogen plasma treated p-type silicon

A deep level transient spectroscopy (DLTS) study of defects found in float-zone p-type silicon exposed to a DC hydrogen plasma is reported. DLTS measurements of these samples revealed three deep levels. Two of the levels are broad, with E T– E V in the range 0.34–0.39 eV (H2) and 0.40–0.44 eV (H3); these appear as bands in the Arrhenius plot. The third level has an activation energy of 0.09 eV (H1). The variations in the capture cross-sections of H2 and H3 are believed to be strain-related. The concentration of H3 exceeds the other two levels and decreases rapidly into the samples with ∼10 15 cm −3 at a depth of 0.20 μm. H3 is tentatively ascribed to an extended defect.

New electron spin resonance spectra from iron-vacancy pair in silicon: I. Defect with two values for the spin

New ESR spectra from an iron-vacancy pair were detected in electron-irradiated float-zone grown silicon samples pre-doped with iron. The paramagnetic center reveals trigonal symmetry. The center is in a neutral state and has two spin values: S=1 when the defect axis direction is close to that of the static magnetic field and S=3/2 for other orientations. The obtained anisotropies of the ESR peak position and intensity suggest a transformation process for the spin magnitude.

On the nature of hydrogen-related centers in p-type irradiated silicon

Hydrogen interaction with the radiation defects is studied by the DLTS and MCTS techniques. Hydrogenation was carried out during wet chemical etching in acid solutions and following reverse bias annealing at 380 K. The levels H4= E v+0.28 eV and E4= E c–0.31 eV are formed as a result of the hydrogenation in all samples irradiated with electrons at room temperature. The levels are not found in irradiated samples which were annealed at 620–650 K before the hydrogenation. Another prominent level H3= E v+0.51 eV is introduced by hydrogenation of as-irradiated float-zone silicon. Czochralski samples have to be annealed at 450 K before hydrogenation to form the H3 level. The H3 center is not formed in the samples annealed at ∼650 K before hydrogenation. The nature of the hydrogen-related centers is discussed.

Evolution of copper–hydrogen-related defects in silicon

Copper-implanted silicon and floating-zone silicon doped with copper during crystal growth have been studied by means of capacitance spectroscopy (deep-level transient spectroscopy, DLTS) and photoluminescence (PL). The hydrogen was incorporated by either wet-chemical etching or hydrogen-plasma treatment. The evolution of defects after the incorporation of the impurities and further annealing steps was investigated. PL in combination with DLTS was used to unambiguously identify the copper–copper pair and clarify a recent controversy about the identification of the 1.014 eV luminescence line. Drift and diffusion experiments provide an insight into the structure of this center.

DLTS and PL studies of proton radiation defects in tin-doped FZ silicon

In this paper, deep level transient spectroscopy (DLTS) is applied to study the deep levels in tin-doped and high-energy proton irradiated n-type float-zone (FZ) silicon. The results will be compared with irradiated tin-free FZ reference material, in order to evaluate the hardening potential. It will be shown that in Sn-doped silicon (FZ:Sn), a number of additional deep levels can be observed, two of which have been identified as acceptors associated with Sn–V. Furthermore, optically active recombination centres have been probed by photoluminescence (PL) spectroscopy. The PL results confirm the reduction of electrically active radiation-defect formation in FZ:Sn. At the same time, no Sn-related optically active centres have been found so far.

Carrier Lifetime Analysis by Photoconductance Decay and Free Carrier Absorption Measurements

High-power devices require a thick silicon layer with high resistivity to achieve high breakdown voltages. In order to optimize the turn-on and turn-off behavior of these devices, precise adjustment of the carrier lifetime is necessary, together with knowledge of its dependence on process parameters and operation conditions (e.g., temperature and injection level). We have therefore analyzed the carrier lifetime of the starting material (float-zone silicon) as well as processed devices that have both been either doped with transition metals (Pt, Au, Fe) or irradiated with electrons, using two different measurement techniques based on photoconductivity decay and free carrier absorption measurements. © 2001 The Electrochemical Society. All rights reserved.

Hydrogen enhanced thermal donor formation in oxygen enriched high resistive float-zone silicon

Hydrogen supported thermal donor (TD) formation was observed in oxygen enriched high resistive float zone (FZ) silicon being used as substrates for detectors in the Large Hadron Collider (CERN). TD formation was provided by a “2-step-process”, consisting of a plasma hydrogenation at 250 °C (60 min) and subsequent annealing at 450 °C in air (typically for 20–30 min). The samples were analyzed by spreading resistance probe (SRP), C( V) and DLTS measurements. Doping by TDs in the oxygen enriched layers of FZ Si samples might be a promising method for the creation of very deep (∼ 100 μm) electrical field gradients for an improved performance of Si radiation detectors.

Dose-rate influence on the defect production in MeV proton-implanted float-zone and epitaxial n-type silicon

The production of stable vacancy-related point defects in proton-implanted float-zone and epitaxial silicon has been studied in the low dose range (⩽10 10/cm 2) as a function of dose-rate. The well-known “inverse dose-rate” effect has been observed in both types of materials with a decrease in the concentration of vacancy-related defects as the dose-rate increases. The effect is less pronounced in oxygen lean epitaxial silicon. Moreover, a continuous decrease of the vacancy-related defect concentration as a function of the flux was measured while a threshold was expected according to previous studies. Both of these results can be explained by a simple calculation, taking into account the influence of the oxygen concentration as well as the influence of the diffusion coefficient of point defects on the “inverse dose-rate” effect.

Impact of fast-neutron irradiation on the silicon p–n junction leakage and role of the diffusion reverse current

The p–n junction leakage ( I R) at reverse voltage, which sets the leakage and power consumption of state of the art integrated circuits (ICs) and the DRAM retention time, is mostly related to the peripheral leakage ( I R,p), which is poorly controlled by the defect engineering techniques, opposite to the planar regions. These ICs from their processing till applications cope with irradiation damages, which are side effects of the ICs manufacturing, due to the plasma, e-beam, X-ray and implantation processing, etc. On the other hand, ICs may be submitted to a significant natural radiation level, as e.g. in applications for space satellites and airplane equipment. Therefore, a proper analysis of the I R,p degradation by high-energy irradiation in silicon technology is proposed. It is found that I R,p increases slower for low fluences of neutron irradiation as compared to the leakage of the p–n junction planar regions, so that the I R,p contribution to the total I R decreases. However I R,p still dominates in irradiated small-size junctions. Moreover, for higher fluences the I R,p again strongly increases its contribution to the overall leakage. Deep insight into the underlying physics is obtained by an analysis of the gated diodes current. This method saves test-chip area and testing-time and omits the assumption of the same parasitic leakage for all junctions. Results are reported for n +p gated diodes and p–n junctions in Czochralski, epitaxial and float zone silicon, exposed to 1 MeV fast neutrons with fluences in the range 5×10 11–5×10 13 n/ cm 2 .

EPR studies of neutron-irradiated n-type FZ silicon doped with tin

N-type float zone (FZ) silicon doped with tin ( 2–10×10 18 cm −3 ) was irradiated by neutrons in a nuclear reactor (fluence of 1×10 17 cm −2 for 2 h). The irradiation-induced paramagnetic defects were studied by continuous wave (cw) and pulsed electron paramagnetic resonance (EPR) spectroscopy. The EPR measurements were carried out between liquid helium and room temperature. The two- and three-pulse electron spin echo envelope modulation (ESEEM) experiments were performed in order to inspect the hyperfine couplings between the electron spin and its surrounding nuclei. This study also revealed the existence of a weak coupling between the paramagnetic centre and nearby protons.

Numerical simulation of point defect transport in floating-zone silicon single crystal growth

This work compares simulations of intrinsic point defect transport for a 0.8″ crystal for two different values of the point defect recombination factor, thereafter utilizing the model for two more commercially relevant 4″ crystals, grown with the needle-eye technique. Using this approach to study defect transport in floating-zone configurations is the novelty of this work. The simulation of the thin 0.8″ crystal is compared to the qualitative Voronkov model and DLTS experiments, to give some insights on the applied recombination factors. The work concludes that for large crystals, grown in the vacancy region, the influence of an uncertain recombination factor is not as significant as for smaller crystals grown with a near critical growth parameter.

Three-dimensional simulation of Marangoni flow and interfaces in floating-zone silicon crystal growth

Three-dimensional (3D) simulation is conducted for the floating-zone (FZ) growth of silicon in an ellipsoid mirror furnace, for the first time, simultaneously considering the time-dependent Marangoni flow, heat transfer, and moving interfaces. The numerical method is based on an efficient multigrid finite-volume method with front tracking. The growth of an 8 mm diameter silicon crystal under microgravity is considered. A half-zone configuration is also used for benchmarking, and the calculated bifurcation points and flow structures are in good agreement with previous results. However, quasi-periodic modes and angular waves are observed at higher Marangoni number. For full zone calculations, including the feeding and growth interfaces, the bifurcation behavior is similar, but the primary bifurcation is found to be subcritical and the 3D one-fold flow mode is dominant. Significant growth rate fluctuations and back melting are found for a typical growth condition as well. The major fluctuation frequency ranges from 0.1 to 0.3 Hz, and the severe back melting may be related to the angular waves.

Oxygen precipitation in floating-zone silicon grown in hydrogen ambience and its application

Oxygen precipitates in the floating-zone silicon in hydrogen ambience [FZ(H) Si] and in the neutron transmutation doping (NTD) FZ(H) Si were investigated by infrared (IR) spectroscopy at room temperature. In the intermediate temperature range, 600–850°C, the apparent activation energies of 1.4 and 1.2 eV were derived from Arrhenius plots of the product of the absorbance at 1230 cm −1 and the half-peak breadth for the formation of oxygen precipitates in the FZ(H) Si and in the NTD FZ(H) Si, respectively. The high temperature stability of the oxygen precipitates was only in the NTD FZ(H) Si. A denuded zone was obtained by denuding annealing and precipitating annealing the NTD FZ(H) Si wafers.

Photoluminescence method for detecting trace levels of iron in ultrapure silicon

A nondestructive technique is presented for the determination of trace levels of interstitial iron contamination in ultrapure silicon. This approach is based on the well-known ability of iron to undergo a reversible pairing reaction with boron near room temperature. A variety of float-zoned silicon samples with low concentrations of boron (∼1011 cm−3) were subjected to thermal annealing treatments to study changes in the apparent boron concentration as determined by the standard method of comparing the photoluminescence intensity of the boron bound exciton to that of the free exciton. Changes in the apparent boron concentration were attributed to the formation or dissociation of iron–boron pairs, allowing us to estimate the interstitial iron concentration in these samples. Remarkably, relatively mild thermal treatments can change the apparent boron concentration in some of these samples by up to a factor of ten.