Determination of Hydrogen in Semiconductors and Related Materials by Cold Neutron Prompt Gamma-ray Activation Analysis
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Determination of Hydrogen in Semiconductors and Related Materials by Cold Neutron Prompt Gamma-ray Activation Analysis Rick L. Paul Analytical Chemistry Division National Institute of Standards and Technology Gaithersburg, MD 20899 ABSTRACT An instrument for prompt gamma-ray activation analysis (PGAA) at the NIST Center for Neutron Research has proven useful for the measurement of hydrogen and other elements in a variety of materials. The sample is irradiated by a beam of low energy neutrons. Gamma-rays emitted by atomic nuclei upon neutron capture are measured and elemental concentrations determined by comparison with appropriate standards. The detection limit for hydrogen is < 5 mg/kg in most materials, and 2 mg/kg for hydrogen measured in silicon. The instrument has been used to measure hydrogen mass fractions of < 100 mg/kg in high purity germanium, and < 10 mg/kg in quartz. More recently PGAA has been used to measure hydrogen in 1 µm thick porous thin films on a silicon substrate, and in crystals of silicon carbide and cerium aluminate. INTRODUCTION The presence of hydrogen in semiconductor materials is known to alter electrical properties. However, quantitation of hydrogen at the levels present in these materials (mg/kg levels or lower) is difficult by most analytical methods. We have used cold neutron prompt gamma-ray activation analysis to measure hydrogen in a wide variety of materials, including semiconductors. The sample is irradiated by a beam of low energy neutrons; gamma-rays emitted by atomic nuclei upon neutron capture are measured using a high purity germanium detector. The analysis is both multielement and nondestructive, and the analyte is measured in situ. Furthermore, because both neutrons and gamma-rays penetrate the sample, the entire sample is analyzed. The presence of hydrogen is indicated by a 2223 keV gamma ray. EXPERIMENT The cold neutron PGAA spectrometer (Fig. 1) is located in the cold neutron guide hall of the NIST Center for Neutron Research (NCNR). This instrument has been described previously.1-3 Neutrons from the reactor core, moderated by passage through liquid hydrogen at 20 K, pass through a 58Ni coated guide to the sample position of the PGAA station. The neutron beam is collimated to a diameter of 2 cm or smaller before striking the sample. The neutron fluence rate at the sample position is 1 x 109 cm-2 s-1.
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Samples for irradiation are normally sealed into bags of FEP Teflon and mounted between Teflon strings suspended between the prongs of an aluminum fork. The optimum sample size is normally between 0.1 g and 1 g, although samples smaller or larger than this may be analyzed. Samples may be irradiated in air, or inside an evacuated magnesium sample chamber that reduces the hydrogen background by a factor of 2.5. Due to the presence of another neutron guide, 3 cm above the center of the PGAA beam, the size of the sample that can be mounted is limited. Furthermore, the interior areas of samples larger than about 6 cm diameter cannot presently be analyzed. Gamm
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