FZ Si Research Articles

Diffusion of Magnesium in Czochralski Silicon

The diffusion of magnesium in monocrystalline dislocation‐free Czochralski silicon (Cz–Si) with an oxygen concentration of (3–4) × 1017 cm−3 is studied. Initial silicon wafers are doped with Mg by using the sandwich technique. The impurity diffusion profiles are investigated in n‐Si samples by the differential conductivity method. The diffusivity of electrically active magnesium in the temperature range of 1,100–1,250 °C is two to three orders of magnitude lower than the values observed at doping high‐purity silicon crystals grown by the float‐zone method (Fz–Si). The diffusion in Cz–Si is retarded through the trapping of diffusing Mg atoms by oxygen‐related traps. The observed high value of the activation energy of Mg diffusion in investigated samples (4.1 eV) is due—along with the migration energy of interstitial magnesium in the crystal lattice (1.83 eV) —to the significant binding energy of Mg atoms with traps (2.27 eV). A study of the temperature dependence of the Hall effect in bulk‐doped samples reveals two types of deep double donors: interstitial Mg atoms and MgO complexes.

Study of the Proton Irradiation Effects in n-FZ DSSD for the Si Tracker at R3B Experiment

The R3B collaboration aims to assemble an experimental setup with high resolution and efficiency to perform kinematically complete measurements of reactions with high-energy RIBs (radioactive ion beams). In the R3B experimental setup, the silicon tracker is positioned closest to the target region and can provide high-resolution position measurements of light-charged particles like protons. The three layers of the silicon tracker are constructed with a total of 30 Si double-sidedstrip detectors (DSSD). In this paper, as an option of 23 MeV proton irradiated n-type Float-Zone(Fz)Si double-sidedstrip detector (DSSD) havebeen considered, and by taking the two-trapproton irradiation damage model, the bulk damage effect and the macroscopic performance of the Sidouble-sidedstrip detector (DSSD) have been discussed. The detector is irradiated with three values of proton fluences (equivalent to 1Mev neutron fluence): 2×1014,5×1014 and 8×1014 cm-2. By using the Shockley-Read-Hall recombination (SRH) formulation, the full depletion voltage and leakage current have been measured as a function of the irradiation dose, finally the device and process parameters and specifications for the Si double-sidedstrip detector (DSSD) used in the R3B silicon tracker experimenthavebeen proposed.

Electrically Active Defects Induced by α‐Particle Irradiation in p‐Type Si Surface Barrier Detector

Herein, the investigation of radiation‐induced defects generated in the Al/SiO2/p‐type FZ Si surface barrier detector upon irradiation with α‐particles at room temperature using capacitance−voltage (C−V) and current deep‐level transient spectroscopy (IDLTS) methods is conducted. The carried out C−V measurements indicate the formation of at least 8 × 1012 cm−3 radiation‐induced acceptor traps at the depth fairly close to that where, according to TRIM simulations, the highest concentration of vacancy‐interstitial pairs is created by the incoming α‐particles. The studies conducted by the current DLTS technique allow to relate the observed increase in the acceptor concentration with the near‐midgap level at E V + 0.56 eV. This level can apparently be associated with V2O defects recognized previously to be responsible for the space−charge sign inversion in the irradiated n‐type Si detectors.

Origin of carrier lifetime degradation in floating-zone silicon during a high-temperature process for insulated gate bipolar transistor

We report on the relationship between carrier lifetime degradation of floating-zone silicon (FZ Si) and the guard ring formation process for silicon integrated gate bipolar transistor (Si-IGBT). A clear carrier lifetime degradation was observed through the guard ring oxidation and annealing processes for Si-IGBT and showed interstitial oxygen (Oi) concentration dependence, which was obtained by Fourier transform infrared absorption spectroscopy. Based on the carrier lifetime measurements through the step etching of the FZ Si substrate, it has been suggested that the carrier lifetime degradation is not only caused by the Oi itself near the FZ Si surface region but also the other defects induced by the Oi injection. Diffused interstitial Si atoms kicked-out by the Oi into the FZ Si substrate, which has a longer diffusion length than Oi, can be considered to be the origin of the carrier lifetime degradation.

Dynamic behavior of helium bubbles at high temperature in Si studied by in situ TEM, STEM-EELS, and TDS

We demonstrated, using in situ transmission electron microscopy (TEM), the random motion of helium bubbles formed by irradiation with helium ions in an FZ (011)Si specimen at high temperatures. Analysis of the random motion yielded the bubble diffusivity, which was dependent on the bubble size and the annealing temperature. The helium density in individual bubbles, measured using scanning transmission electron microscopy-electron energy loss spectroscopy, was as high as 70 ± 14 He/nm3. It was dependent on the irradiation temperature, beam flux, and annealing temperature but was independent of the bubble size. The bubbles retained a high density of helium atoms at around 800 K, but a significant reduction of the helium density within individual bubbles took place after annealing above about 970 K. Thermal desorption of helium from the specimen, measured using thermal desorption spectroscopy, occurred at temperatures between 650 K and 1100 K, centering at around 890 K. TEM revealed the corresponding microstructure. On the basis of these results, the thermal desorption profile was discussed in terms of the release of helium at lower temperatures from migrating and coalescing overpressurized bubbles and bubbles migrating to the specimen surface, and at higher temperatures from stationary large bubbles.

(Invited) A Novel Optical Characterization of a-Si:H/c-Si Interface Microstructures Based on Data of Positron Annihilation Spectroscopy

The power conversion efficiency of amorphous/crystalline silicon (a-Si:H/c-Si) heterojunction solar cells has exceeded 26% recently.1) Such high efficiency is essentially achievable thanks to the notable passivation capability of the several-nm-thick amorphous silicon layer deposited on the c-Si surfaces. For further improvement of the performance, it is necessary to understand and to control the structural properties of the a-Si:H/c-Si heterointerface. Recently, we have found that there is a systematic correlation between the microvoid size determined by the positron annihilation spectroscopy (PAS) and the optical constant of a-Si:H by spectroscopic ellipsometry (SE).2) However, it has been still not clear that this correlation is universally consistent for any a-Si:H thin-films that are fabricated under various preparation conditions using any different apparatus. In this study, we prepared undoped a-Si:H films on H-terminated FZ Si(111) substrates under various chemical vapor deposition conditions including different source gases (SiH4 or Si2H6) and deposition systems. The average diameter of microvoids (D void) and the dielectric function (ε = ε 1 - iε 2) in these a-Si:H films were estimated via PAS and SE measurements, respectively. The peak values of ε 2, ε 2 peak, were extracted from the dielectric function determined based on the Tauc-Lorentz dispersion model.3) It is notable that the D void vs. plots are aligned on a straight line; this result indicates that D void in undoped nm-thin a-Si:H can be estimated readily from SE measurement results using the found D voidーε 2 peak linear correlation. Details in this method and results will be discussed in the presentation.

Reduced radial resistivity variation of FZ Si wafers with Advanced NTD

It is well known that manufactures of power electronic devices rely on FZ silicon wafers of highest possible purity. The manufactures however also have a demand for wafers with lower and lower radial resistivity variation (RRV) across the wafer. The current product on the 200 mm wafer market with the lowest RRV is based on neutron transmutation doping (NTD) where a RRV value lower than 5% is guaranteed for all wafers cut from an ingot. This work presents a novel concept called Advanced NTD which can provide 200 mm wafers with RRV lower than 2%. The radial resistivity profile of an NTD wafer is influenced by the neutron flux in the nuclear reactor. The result is a resistivity that is lower at the edge compared to centre of the wafer. By engineering a gas phase doped profile to be the inverse of the NTD profile, and simultaneously carefully controlling the required NTD doping factor, an almost flat resistivity profile can be achieved by combining the developed gas phase doping process with the NTD process. The capability of the Advanced NTD process is demonstrated in a pilot production of 11 ingots.

Anneal induced transformations of defects in hadron irradiated Si wafers and Schottky diodes

In this research, the anneal induced transformations of radiation defects have been studied in n-type and p-type CZ and FZ Si samples, irradiated with relativistic protons (24GeV/c) and pions (300MeV/c) using particle fluences up to 3 × 1016cm−2. The temperature dependent carrier trapping lifetime (TDTL) spectroscopy method was combined with measurements of current deep level transient spectroscopy (DLTS) to trace the evolution of the prevailing radiation defects. The contactless TDTL technique has been shown to be preferential when the radiation induced trap density approaches or exceeds the dopant concentration and when it is necessary to avoid modification of a detector structure due to annealing processes at elevated temperatures. The deep level spectra were complementarily examined by using DLTS spectroscopy on Schottky diodes made of irradiated Si wafer fragments. The dominant radiation defects and their transform paths under isothermal and isochronal anneals have been revealed. A good agreement between the DLTS and TDTL spectra has been obtained.

Identification of carbon‐hydrogen complexes in n‐ and p‐type silicon

AbstractThe origin of several deep traps E42, E90, E262, and H180 in hydrogenated n ‐type and p ‐type FZ and CZ Si is investigated. Comparing the depth profiles of these defects in samples with different C, O, H, and shallow donor (acceptor) concentrations we conclude that they belong to carbon‐hydrogen‐related defects consisting of one C and one H atom. The similar annealing behavior and identical depth profiles of E42 and E262 correlate them with two different charge states of the same defect. From a comparison with earlier calculations we attribute E42 to the double acceptor and E262 to the single acceptor state of the CH1AB complex. In good agreement with the results of previous studies E90 is assigned to the acceptor state of the CH1BC complex. Our preliminary Laplace DLTS studies on SiGe with a Ge content <5% show different local Ge environments for E90 and H180 in the nearest and second‐nearest neighborhood. We interpret this as an indication for a different origin of these defects. After annealing the samples under reverse bias at 320 K another CH‐related defect (CHB) can be detected by Laplace DLTS in n ‐type Si. Previously, this defect was often wrongly assigned to the single donor state of CH1BC. The field dependence of the emission rate of CHB identifies the level as a single donor state. However, the depth distribution of CHB differs clearly from that of E90. Our results on the carbon‐hydrogen complex give a conclusive explanation of previously reported controversial experimental data. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Similarity of Atomic Configurations of Thermally Stable Positron-Sensitive Complexes Produced with 0.9-MeV Electrons and 15-MeV Protons in <i>n–</i>FZ<i>–</i>Si:P Crystals

We observed for the first time the thermally stable point positron-sensitive center of a vacancy type in n–FZ–Si (P) material irradiated at RT by ~ 0.9-MeV electrons. The center that emerges after isochronal annealing at Tanneal.≈ 260 – 280 oC is found to be similar to the vacancy-group-V-atom complex revealed in the same Si material irradiated by 15-MeV protons; the detecting of the centers by the positron trapping is finalized at Tanneal.≥ 520 oC. The annihilation gamma-quanta to be emitted from the positron trap gives rise to a characteristic positron lifetime τ2 (I2 ~ 38–19 %) ≤ 276 – 294 ps which is somewhat longer than the one predicted for unrelaxed single vacancy τV.≈ 254 – 261 ps. Our data suggested a configuration of the complex VopPVop, wherein the atom of phosphorus is tied to a split open vacancy volume 2Vop. It is argued that Vop volume detected by the positron trapping may be formed by extended semi-vacancy, Vs-ext , or by the relaxed inwards vacancy, Vinw , thus resulting in a distorted Vs-extPVs-ext or VinwPVinw configurations.

Deep level contribution to the carrier generation and recombination in high resistivity Si irradiated by neutrons

Deep level spectroscopy in neutron irradiated FZ and MCz Si was performed by extrinsic photoconductivity spectra measurements in the range of temperature of 18–120 K. The Lukovsky model was used, and the Gaussian distribution of deep level energy demonstrated the best fit of simulation and experimental data. The non-monotonous change of deep levels during annealing, and non-monotonous dependence of their concentration on the fluence were observed. The photoconductivity decay was investigated by the transient photo-Hall effect, recombination parameters of the main recombination centre were determined, and the recombination centre model was proposed. The photoconductivity and thermally stimulated current measurements were used to demonstrate the existence of photogeneration of free carriers by a cascade of optical and thermal transitions.

Silicon Dioxide as a Boundary for Oxygen Outdiffusion from CZ-Si

Oxygen out-diffusion from CZ-Si or in-diffusion into FZ Si was studied with Secondary Ion Mass Spectrometry. For anneals up to 1200oC , the value of 16O concentration Csurf that develops at the semiconductor surface in bare wafers with ~2 nm thick natural oxide was found to be comparable to that in thermally oxidized wafers with 260 nm thick SiO2. Beyond this temperature, at 1280oC, Csurf in non-oxidised sample appears 5 times lower that in the oxidised one. This means that the spontaneous oxide becomes permeable to the oxygen species and no longer constitutes a barrier preventing the oxygen atoms from direct out-diffusion into the ambient. The effect becomes more pronounced when the specimens were heated to 1150oC in a chamber evacuated to ~ 10-7 Torr. The resultant bare Cz-Si and, to even greater extent, FZ Si with previously in-diffused oxygen exhibited deep oxygen depletion, 10x below the respective values for annealing under atmospheric pressure. Undoubtedly, it is the removal of residual oxides by sublimation that enables oxygen atoms to freely out-diffuse into vacuum.

An application of gold diffusion for defect investigation in silicon

The application of gold diffusion for defect investigation in Si is illustrated by the diffusion experiments carried out on crystals containing grown-in or specially introduced defects. The efficiency of gold diffusion experiments for monitoring the concentration and spatial distribution of these defects is shown. The effect of vacancy type defects on gold diffusion is illustrated by investigations of nitrogen-doped FZ Si and of Cz Si after rapid thermal annealing. In both these cases the gold depth profile is distinctive for trap limited diffusion. The effect of sinks for self-interstitials on gold diffusion is illustrated by the results obtained on the plastically deformed Si. It is shown that in silicon deformed at relatively low temperatures the gold diffusion is to a great extent determined by the defects in the dislocation trails while in high temperature deformed Si the sinks for self-interstitials are associated with dislocations themselves.

Recombination characteristics in 2–3 MeV protons irradiated FZ Si

Combined analysis of the carrier recombination and generation lifetime as well as reverse recovery durations ( τ RR ), dependent on proton irradiation fluence in the range of 7×10 12−7×10 14 p/cm 2, has been performed in FZ silicon PIN diodes and wafer structures. A δ-layer and triangle profiles of radiation induced defects were formed by varying energy of protons in the range 2–3 MeV. Carrier decay constituents and values of recombination lifetime have been evaluated by employing a microwave probed photoconductivity transient technique, while deep levels spectra ascribed to generation lifetime variations have been examined by exploiting capacitance deep-level transient (DLTS) spectroscopy. Recombination lifetime decreases from several μs to few ns, while DLTS spectra show an increase in the amplitude of a DLTS peak at 170 K with irradiation fluence. Transforms of DLTS spectra and a decrease in density of the majority carrier traps have been revealed after 24 h isochronal anneals in the range of temperatures of 80–420 °C. Inhomogeneous depth distribution of recombination lifetime in proton irradiated samples has been revealed from the cross-sectional scans of the excess carrier lifetime measured by MW-PC technique and compared for δ-layer and triangle profiles of radiation induced defects. After isochronal anneals, the τ RR changes its behaviour as a function of irradiation fluence.

Surface passivation at low temperature of p- and n-type silicon wafers using a double layer a-Si:H/SiNx:H

Surface passivation of bare silicon or emitter region is of great importance towards high efficiency solar cells. Nowadays, this is usually accomplished by depositing an hydrogenated amorphous silicon nitride (a-SiNx:H) layer on n +p structures that serves also as an excellent antireflection layer. On the other hand, surface passivation of p-type silicon is better assured by an hydrogenated amorphous silicon (a-Si:H) layer but suffers from optical properties. In this paper, we reported the surface passivation of p-type and n-type silicon wafers by using an a-Si:H/SiNx:H double layer formed at low temperature (50–400 °C) with ECR-PECVD technique. We first investigated the optical properties (refraction index, reflectance, and absorbance) and structural properties by FTIR (bonds Si–H, N–H) of the deposited films. The hydrogen content in the layers was determined by elastic recoil detection analysis (ERDA). The passivation effect was monitored by measuring the minority carrier effective lifetime vs. different parameters such as deposition temperature and amorphous silicon layer thickness. We have found that a 10–15 nm a-Si film with an 86 nm thick SiN layer provides an optimum of the minority carriers’ lifetime. It increases from an initial value of about 50–70 μs for a-Si:H to about 760 and 800 μs for a-Si:H/SiNx:H on Cz-pSi and FZ-nSi, respectively, at an injection level 2 × 10 15 cm −3. The effective surface recombination velocity, S eff, for passivated double layer on n-type FZ Si reached 11 cm/s and for FZ-pSi—14 cm/s, and for Cz-pSi—16–20 cm/s. Effect of hydrogen in the passivation process is discussed.

The Unusual Temperature Shift of Dislocation Related D1/D2 PL Bands in Donor Doped Silicon

The samples of p- and n-doped Fz Si were deformed in 3-point bending mode in the temperature range 800-950flC. Dislocation related PL (DRL) spectra were measured at temperature in the range 4.2 – 200K. Several features of DRL turned out to be sensitive to donor level doping. First, the low energy components of D1/D2 bands disappear at middle and high doping level. Second, the intensity of D1 band showed much more dependence on the donor doping level than other DRL bands and almost disappeared at the concentration of donors around 1017cm-3. Finally, it has been found that the temperature variation of the D1/D2 line positions depend on the donor doping level. Namely, at the donor concentration higher than 1015cm-3 the D1/D2 bands demonstrate the blue shift when the sample temperature goes up from 4K. At higher temperature the band positions more or less follow the temperature behavior of the band gap. The effect does not depend on chemical nature of donor except a value of ionization energy. No such behavior has been observed for different levels of acceptor doping. Taking into account that the ionization of shallow donors happens in the temperature range of above 30K, the effect of blue shift has been attributed to the influence of free electrons released from donors.

Influence of Cu contamination on dislocation related luminescence

AbstractThis work is devoted to investigation of Cu contamination on dislocation related luminescence (DRL) in Si. Cu is known to be a very fast diffuser with high solubility. Fz Si samples were plastically deformed at 950 °C up to dislocation density of about 107 cm–2. Cu diffusion was performed by diffusion at 950 °C from a quartz ampoule contaminated with Cu. The presence and concentration of luminescent copper (Cu PL) centers was evaluated from the excitation dependence of the copper bound exciton no‐phonon line (BE NP) line at 1.014eV, which was activated after annealing at 1100 °C and rapid quenching in the water. 10 minutes isochronous annealing of the samples in the temperature range 300–600 °C revealed quite a different behavior of D4 and D1/D2 lines. It was assumed that a strong decrease of D1/D2 line intensity in annealing interval 400–500 °C could be attributed to passivation of D1/D2 luminescence centers by Cu atoms. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanocavity Buffer Induced by Gas Ion Implantation in Silicon Substrate for Strain Relaxation of Heteroepitaxial Si1-xGex/Si Thin Layers

AbstractTo weight the importance of a nanocavity buffer in a SiGe deposition substrate, some P type (001) FZ Si wafers are implanted (A samples) or not (B samples) at room temperature with 5×1016 He+ cm–2 at 10keV. They are annealed at 700°C for one hour to form a nanocavity layer close to the Si surface. Then, the wafers are carefully chemically cleaned in a clean room to remove both organic and metallic impurities from the surface. They are coated either by 210 nm (A) or 430 nm (B) Si1−xGex (x=0.20±0.02) alloy grown at 575°C for 0.42 hour by low pressure chemical vapor deposition (LP-CVD) with a growth rate of 8 to 17 nm.mn−1. Both kinds of samples are studied by cross section transmission electron microscopy, X-rays diffraction, Rutherford backscattering, atomic force microscopy and etch pit counts. The association of these techniques demonstrates that the thin SiGe layer which is deposited on sample A is fully relaxed and that the threading dislocation density (estimated to hardly reach 4×103cm−2) is at least one order of magnitude lower than what is obtained so far using ion implantation assistance in SiGe layer growth on Silicon. The roughness of the SiGe surface is low enough to stand a further Si epitaxy. Nevertheless, the mechanism involved responsible for the threading dislocation annihilation and/or confinement is still unclear.

Effect of implanted O on gettering of Au at dislocations in Si

The effect of implanted O on gettering of Au at dislocations in Si has been studied. FZ Si(1 1 1) wafers, implanted with Au (1.5 MeV, 2.2 × 1 0 15 atoms cm −2), were annealed for 1 h at 850 ° C and subsequently implanted with MeV O or Si ions. The gettering behavior of the damage layers in Si has been studied as a function of annealing temperature and time using Rutherford backscattering spectrometry. The concentration of Au gettered in the O implanted layer has been found to increase with annealing time at 850 ° C whereas that in the Si implanted layer remained constant. A drastic reduction in the gettering efficiency of the O implanted layer has been observed for annealing temperatures higher than 850 ° C. Our results indicate that gettering of Au at dislocations in Si is strongly influenced by the presence of O atoms.

Reactive ion etching of dielectrics and silicon for photovoltaic applications

This paper investigates the reactive ion etching of SiO2 ,S i3N4, and Si using CHF3/O2 plasma. In particular, we have characterized the time and rf power dependence of the carrier lifetimes in n- and p-type FZ Si. The time dependence of reactive ion etching (RIE) at different rf powers provide insight into the two competing processes of damage accumulation and damage removal in the near-surface region of the Si during plasma etching. The carrier lifetime, measured using the quasi-steady-state photoconductance (QSSPC) technique, has a quadratic dependence on the rf power, which can be related to changes in the dc self-bias generated by the plasma at different rf powers. The change in carrier lifetime is similar in both n- and p-type Si of the same doping concentration. Using this fact, together with the electronic properties of defects obtained by deep level transient spectroscopy (DLTS), we have modeled the injection-dependence of the measured carrier lifetimes using the Shockley– Read–Hall model. The isochronal annealing behavior of plasma etched Si has also been studied. Copyright # 2006 John Wiley & Sons, Ltd.