Neutron Transmutation Doping of High-Resistivity Silicon
Neutron Transmutation Doping of High-Resistivity Silicon and Development of a Technology for Obtaining of Materials for Discrete Semiconductor Devices
Tech Area / Field
- FIR-MAT/Materials and Materials Conversion/Fission Reactors
- INS-DET/Detection Devices/Instrumentation
- MAN-MAT/Engineering Materials/Manufacturing Technology
- PHY-SSP/Solid State Physics/Physics
8 Project completed
Senior Project Manager
Yakusheva A A
Institute of Physics and Technology, Kazakstan, Almaty
- National Nuclear Center of the Republic of Kazakstan / Institute of Nuclear Physics, Kazakstan, Almaty
- Lehigh University, USA, PA, Bethlehem
Project summaryThe project goal is the development of a method for obtaining of high-resistivity phosphorus-doped silicon with high minority carrier’s lifetime. As the starting silicon samples, we shall take high-resistivity boron doped samples of p-type conductivity, either containing natural boron impurity or lightly doped. The method of neutron transmutation doping of silicon through the reaction
30Si (n, g) 31Si —> 31P + b-
with the involvement of thermal neutrons and 30Si isotopes (3.12 % abundant) will be used. The study of electrical properties of the obtained material (value of an electrical conductance versus a radiation dose, conductance distribution homogeneity) and research of radiation defects in the obtained material will be carried out. The project is focused also on demonstration of adequacy of a designed transmutation doping method for creation of silicon sensors of nuclear radiations and other discrete devices. Though transmutation doping has been under research for a long time, its use for obtaining of high-resistivity silicon and investigation of the silicon properties attract attention. The realization of the project will open up additional possibility to use the nuclear reactor at the Institute of Nuclear Physics (Almaty) for commercial application.
The main objectives of the project are:
- development of a method of homogeneous transmutation doping of monocrystalline silicon with high initial resistivity;
- elaboration of an optimal procedure for annealing of irradiated sample to obtain a maximal lifetime of carriers and maximal activation of phosphorus atoms;
- research of nature and structure of the defects in irradiated samples and transformations of the defects during the annealing;
- preparation of nuclear radiation detectors on the basis of silicon samples obtained to demonstrate the suitability of the material.
Thus, the project has exploratory aspect as well as applied one. The combination of the detector design and the researches of properties of a material obtained by neutron irradiation will allow to carry out the study more effectively, and to find a link between defect structure of irradiated silicon and operating performances of the material.
In the field of research of radiation defects in silicon the scientists of the Institute of Physics and Technology have considerable achievements some of which are results worldwide known and frequently cited. Comparative study of defect formation processes in silicon irradiated by electrons, protons and helium ions in the temperature range of 77-300 K are carried out for the first time by methods of deep level transient spectroscopy (DLTS) and electron paramagnetic resonance (EPR). Important radiation defects like Frenkel pairs and selfinterstitial atoms in silicon are detected and investigated. These defects were not observed earlier despite of numerous attempts of their detection. As such defects are primary ones, processes of their creation, diffusions and the interactions with impurity atoms and other defects determine a resulting spectrum of defects and property of the irradiated material. A number of new radiation defects is also detected; their microscopic structure is defined. It is necessary to note, that the spectrum of defects introduced by the irradiation with helium ions and neutrons is rather similar.
There are also the considerable preliminary results in the field of discrete semiconductor devices preparation (solar cells, magneto-diode sensors, gas sensors and, in particular, nuclear radiations detectors). Earlier a continuous work was carried out at the Institute to construct and calibrate the nuclear radiation sensors based on high-resistivity silicon, small lot of detectors has been made and approved by users.
The project is intended to carry out basic and applied researches on a topical problem of control of crystalline silicon properties by irradiation and doping. It is expected to receive the new data on radiation defect spectrum in transmutation-doped silicon, because this material is investigated insufficiently for the present. Nature of defects, which are introduced by neutron irradiation and responsible for high order optical absorption bands, and also induce the shallow donor appearance, is not clear. At the same time there are data, that these defects have a related nature and are similar to the oxygen thermal donors, therefore research of their nature has great scientific and practical significance.
The results of the project will have the importance for physics of radiation defects in semiconductors and will promote more deep understanding of a nature of defects in silicon. Besides scientific importance, the issue of contemporary hi-tech production is very topical for Kazakhstan and Central Asia countries. The successful implementation of the project will enable project participants to produce high-quality silicon for discrete structures and to enable commercial use of nuclear reactor that will respond to Kazakhstan’s national priorities to develop domestic manufacture of various equipment and spare parts, in particular, for needs of companies working in export branches.
The project will undoubtedly respond to the ISTC goals and objectives:
- 29 weapon scientists and engineers from IP&T and INP will get the opportunity for peaceful basic and applied research activity in the field of radiation defects physics and development of silicon detectors widely used for civil production;
- Researches of defect structure of semiconductors and silicon properties management methods are now at the leading edge of science and engineering and will undoubtedly assist to the integration of scientists from IP&T and INP into the international scientific community;
- The project is intended to basic and applied researches and development of silicon detector technology. For Kazakhstan with ecology exposed by various deleterious man-caused effects, it is important to develop sensing devices used for control of radiation level.
The project has potential to contribute to conversion efforts of former weapon companies in Kazakhstan that are interested to produce science-based competitive products for civil needs. Methods for silicon detectors obtaining, expected to be developed in the course of project implementation, may be utilized by these companies to manufacture various equipment, tools, constructions etc.
It is expected that foreign collaborators will:
- provide comments to the technical reports (quarterly, annual, final etc.) submitted by project participants to the ISTC;
- help to perform coordinated experiments with using of certain equipment of collaborator especially with using of transmittance electron microscope if it will be available;
- participate, in case of need, in cross-checks of results obtained in the course of project implementation;
- participate in technical monitoring of project activities performed by ISTC staff (this has to be preliminary coordinated with collaborator);
- assist project participants to attend at international meetings and to search foreign partners for further project development;
- assist in conduction of joint seminars and workshops.
The following approaches and methods will be applied to achieve the project goals:
1. For obtaining a uniformly doped material the high-resistivity silicon samples will be irradiated with thermal neutrons produced by experimental nuclear reactor.
2. The conductivity and electrical homogeneity of the sample will be determined by electrical measurements – a four-probe technique and Hall effect method, and also method of photo-induced voltage. The lifetime of charge carriers will be determined by a photoconduction method, the concentration of defects will be estimated by optical absorption spectra in a wide range of wavelength and by measurements of EPR signal.
3. The facility for rapid thermal processing and usual thermal furnaces with precision regulation of temperature will be applied to anneal the radiation defects and to activate the phosphorus impurity atoms.
4. Radiation defects spectrum will be studied by methods of optical absorption in the range of 25000-200 cm-1, Raman reflection and photoluminescence will be measured at temperatures ~10 K, additional methods of researches will be methods of DLTS and EPR at 77-300 K.
5. Shallow planar p-n-junctions will be formed by ion implantation of boron with energies in the range of 5-40 keV with the subsequent rapid impulse annealing.
6. Electric contact systems will be prepared by using the methods of magnetron sputtering and ion beam assisted deposition, the obtained structures will be passivated by an ion-beam deposition of SiO2 or other dielectrics thin films.
7. The field-performance data of detectors (volt-ampere and volt-farad characteristics, leakage currents, noise performances, resolution) will be analyzed by specialized tools.
8. The design-experiment works for choice of irradiated samples location and definition of spatially – energetic distribution of neutrons in irradiated samples will be carried out on experimental test bench with usage of verification design condition.
9. Irradiation of the samples will be carried out in neutron reactor VVR-K experimental channels with known neutron distribution characteristics and at required temperature.
10. Clearing of the irradiated samples from surface radioactive contaminations up to acceptable level will be carried out by equipment of neutron reactor.
11. The control of irradiated samples radio-activity level will be carried out by radiation safety services.