Neutron Transmutation Doping Process Research Articles

Electrodeposition of Thin Silicon Films for Neutron Transmutation Doping

Thin silicon films were electrodeposited on glassy carbon (GC) from the KF-KCl (2:1)—75 mol% KI—1.5 mol% K2SiF6 melt under potentiostatic condition at 973 K. The synthesized films were single-phase, continuous, dense, and free from unwanted impurities. Neutron transmutation doping (NTD) of the samples was performed in the IVV-2M research reactor (RR) at a thermal neutron flux density of 1.8 × 1013 cm−2s−1 for 7.7 h in order to form the 31P isotope dopant. The irradiated samples were studied by scanning electron microscopy with energy-dispersive X-ray spectroscopy, X-ray diffraction, mass spectrometry, and gamma-ray spectrometry. Some excess of the minimum significant specific activity of the irradiated samples was explained by the formation of the 182Ta isotope due to the presence of tantalum traces in the GC substrate. The formation of the 31P isotope by the NTD process was confirmed. The calculated values of 31P concentration and electrical resistivity were 4.9 × 1016 cm–3 and 0.15 Ω·cm, respectively.

Use of Neutron Absorbers to Influence the Neutron Transmutation Doping Process in Silicon

Neutron transmutation doping is used to create high-quality silicon with a specific target resistivity. By implementing neutron absorbers, it is possible to obtain a broader range of postirradiation resistivities. To develop this method, the influence of neutron absorbers on the reactor spectrum in Belgian Reactor 1 was numerically simulated and experimentally verified. A comparison between the modeled reactor spectrum and the spectrum obtained through activation foils showed good agreement. These data were used to calculate the resistivity of silicon under cadmium and hafnium foils with different thicknesses after neutron irradiation. Experimental four-point probe measurements confirmed the calculated resistivities. Hence, the research shows that tailoring the reactor spectrum using neutron absorbers allows for a large range of final resistivities or doping concentrations in silicon during a single irradiation cycle.

Neutron Transmutation Doping of Thin Silicon Films Electrodeposited from the KF-KCl-KI-K2SiF6 Melt

Neutron transmutation doping (NTD) was applied to produce a stable isotope 31P admixture in thin silicon films electrodeposited on glassy carbon (GC) from the KF-KCl (2:1)—75 mol% KI—0.5 mol% K2SiF6 melt. The irradiation of samples with 5 and 15 μm silicon film thickness was carried out under the neutron flux density of 1.46 × 1012 cm–2 s–1 for 168 h at 318 K in the IVV-2M research reactor (RR). The irradiated samples were studied by methods of gamma-ray and mass spectrometry, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). Impurities the activities of which reach levels above the minimum significant ones (MSA) were detected and identified. The presence of 31P atoms (∼1.1 × 1014 cm–3) in thin silicon films after the NTD process was confirmed. The specific electrical resistance of the irradiated samples was evaluated. Arrangements were defined to optimize the NTD process for obtaining acceptable results, including a higher 31P concentration and lower exposure time to achieve the minimum significant specific activity (MSSA).

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.

Precise Control of Neutron Irradiation Fluence in the Neutron Transmutation Doping Process at HANARO Using SPND and Zirconium Foils

A neutron irradiation control method using a self-powered neutron detector (SPND) and zirconium foils was developed for neutron transmutation doping (NTD) application at HANARO. An SPND was installed at a fixed position of the upper part of the sleeve at the HANARO NTD hole for real-time monitoring of the neutron irradiation. The SPND signal output was significantly affected by the in-core conditions and surroundings of the facility. Furthermore, the SPND signal varied by $\sim 15\%$ throughout the reactor cycle according to the change of the control rod position. However, it was also confirmed that the variation of the neutron flux on the silicon ingots inside the irradiation rig was not significant for the control rod motion. In this procedure, the process for determining the thermal neutron flux by using zirconium foils was established for achieving efficient NTD irradiation. Accordingly, the relationship between the ratio of the neutron flux to the SPND output and the control rod position was deduced. Silicon irradiation experiments for transmutation doping were performed using the established relationship. The irradiated neutron fluence was controlled within ±1.3% of the target value. The mean irradiation/target ratio of the fluence was 0.9992, and the standard deviation was 0.0071. Thus, we confirmed that the produced irradiation was extremely accurate. This procedure can be useful in SPND applications requiring real-time high-accuracy neutron fluence measurements.

Computer simulation of neutron transmutation doping of isotopically engineered heterostructures

Neutron transmutation doping (NTD) of silicon-based isotopically engineered heterostructures is a powerful technique for the creation of the semiconductor devices with the desired spatial distribution of 31P impurities that is defined by the initial distribution of 30Si nuclei.Computer simulation allows to study dopant distribution changes during NTD process and post-irradiation annealing and to determine appropriate annealing regimes. The initial distribution of the radiation defects was obtained by the dynamic Monte Carlo code DYTRIRS_N, while the subsequent annealing stage was simulated by the rate equations (RE) method. Concentrations of intrinsic defects and depth profiles of phosphorus atoms were obtained.

Investigation of a simple and efficient method for silicon neutron transmutation doping process in Tehran research reactor

Neutron transmutation doping of silicon (NTD) is one of the important applications in industrial utilization of research reactors. In this paper, the proposed irradiation system based on using a reflecting environment around the irradiation channel of the NTD facility was designed and simulated for irradiation of silicon in the Tehran research reactor (TRR) by using codes such as MCNP, WIMS and CITATION. This system is very simple and does not require conventional methods, such as continuous movement and use of absorbing materials and windows, for flattening of neutron flux in the radial and longitudinal direction. The influence of type and dimensions of reflectors on the irradiated silicon crystal was investigated and also the optimum conditions were determined for TRR for silicon ingots with diameter up to 5 cm and up to 10 cm.

New recombination centres in InP:Fe doped by neutron transmutation

Neutron transmutation doping (NTD) has been used to introduce several amounts of tin atoms into semi-insulating InP:Fe samples. Defects related to the doping method which includes the transmutation process and the subsequent annealing procedure necessary to activate the donors have been studied by photoluminiscence (PL) and positron annihilation spectroscopy (PAS). In PL spectra two new donor–acceptor emissions arise. The nearest band-edge emission line has an energy of 1.39 eV, and does not depend on the annealing temperature. However, all the parameters of the second emission band, centred at 1.35 eV, change with temperature, annealing and optical laser power. The results indicate that this band is related to a transition between a donor with a complex structure and residual acceptors. The change in the PAS parameters are related to defects produced during the doping process. The dependence of the PAS parameters on the annealing temperature indicates that the positron trap induced by the NTD process in InP:Fe is formed by an association between iron atoms and indium vacancies.

Defects in NTD InP probed by positron annihilation spectroscopy

Two samples of high purity InP extracted from the same wafer were examined by positron annihilation spectrum analysis after having been, processed by means of thermal Neutron Transmutation Doping (NTD). Compared with the as grown sample with an average positron lifetime of 246 ps at 300 K, the high dose doped one has an average lifetime of 251 ps and the lower dose doped one 248 ps measured under the same condition, indicating that some defects have been introduced in the NTD process. Annealing experimental results show a steady decrease in the average lifetime with increasing annealing temperature up to 550°C. And a peak in lifetime curve around 500°C was observed which may be attributed to defects related structure conversion. Temperature experiments conducted on the low dose doped sample from 150K to 290 K suggest the existence of vacancy-impurity complex which have given rise to an abnormal reduction of average lifetime with increasing temperature. Also a n-type InP sample (A61) was irradiated with thermal neutrons in another reactor and the lifetime results display an increase of 15 ps. Furthermore, to study epithermal neutron irradiation effects on InP, measurements were performed on an n-type InP sample (N119) along with one p-type sample (P118) after having been irradiated with high fluence of epithermal neutrons. The former has an average lifetime of 262 ps and the latter 247 ps after irradiation. The results prove that on some occassion epithermal neutrons can produce sizable defects in InP.

Neutron transmutation doping of GaP: optical studies

An unambiguous proof for successful neutron transmutation doping (NTD) of GaP is presented on the basis of optically detected magnetic resonance (ODMR). GaP:S samples grown by the liquid encapsulated Czochralski method were irradiated with thermal neutrons and subsequently annealed at 800°C. In the ODMR experiments the transmuted Ge substitutional on Ga sites was detected. The NTD process was also found to create deep acceptors, the nature of which will be tentatively discussed.

Neutron Transmutation Doped GaP: Optically Detected Magnetic Resonance Studies

The first direct proof for successful neutron transmutation doping (NTD) of GaP is presented on the basis of optically detected magnetic resonance . (ODMR). GaP:S samples grown by the liquid encapsulated Czochralski method were irradiated with thermal neutrons and subsequently annealed at 800° C. In the ODMR experiments the transmuted Ge substitutional on Ga sites was detected. The NTD process was also found to create deep acceptors, the nature of which will be tentatively discussed.

Optically detected magnetic resonance studies of neutron-transmutation-doped GaP

A direct proof of neutron transmutation doping (NTD) of GaP is presented on the basis of optically detected magnetic resonance (ODMR). GaP:S samples grown by the liquid- encapsulated Czochralski method were irradiated with thermal neutrons and subsequently annealed at 800 °C. In the ODMR experiments the transmuted Ge substitutional on Ga sites was detected. The NTD process was also found to create deep acceptors; these are tentatively identified as associates of gallium vacancies (VGa) and germanium donors on gallium sites (GeGa). Such identification requires that some of the structural defects (vacancies) created by β and γ recoil during transmutation are stabilized by forming VGa- GeGa complexes.

Infrared absorption study of neutron-transmutation-doped germanium

Using high-resolution far-infrared Fourier transform absorption spectroscopy and Hall effect measurements we have studied the evolution of the shallow acceptor and donor impurity levels in germanium during and after the neutron transmutation doping process. Our results show unambiguously that the gallium acceptor level concentration equals the concentration of transmutated 70 Ge atoms during the whole process indicating that neither recoil during transmutation nor gallium-defect complex formation play significant roles. The arsenic donor levels appear at full concentration only after annealing for 1 h at 450 °C. We show that this is due to donor-radiation-defect complex formation. Again recoil does not play a significant role.

Influence of fast neutrons on electrical properties in neutron transmutation doped GaAs: New annealing stage

In neutron transmutation doping (NTD) to the undoped semi-insulating GaAs, a new annealing stage related to the tunneling assisted hopping conduction was found around 400 °C for fast neutron fluences of ≥7.0×1017 n/cm2. It is suggested that this stage is based on the enhancement in the hopping conduction by the activated dopant in the NTD process. The stage was not observed for the irradiation with a small amount of fast neutrons. The activation energy for the annihilation of As antisite defects (AsGa) was found to be 0.9 eV. The annealing temperature to achieve the desired carried concentration increased with the fast neutron fluence.

A Review of Transmutation Doping in Silicon

The neutron transmutation doping (NTD) process in silicon is based upon nuclear reactor thermal neutron irradiation which induces the neutron capture reaction 30Si (n,?) 31Si ? 31P + s-. The transmutation product, phosphorus, becomes electrically active after suitable annealing of the accompanying radiation damage which is caused by a number of displacement processes. Because of the superior doping homogeneity which results from the NTD process, a number of device applications have evolved resulting in a significant fraction of the world's float zone being neutron doped. This paper will present a review of basic research and production techniques which have evolved at MURR as a result of work associated with this new radiation effects technology.