Neutron Transmutation Doping Research Articles

Gamma-ray diffraction in the study of silicon

Abstract Diffraction of high-monochromaticity (δλ/λ⩽10 −6 ) , short-wavelength (λ=0.003 nm ) gamma radiations was used to study the defect structure changes arising during different treatments of silicon single crystals and p−n structures. Absolute values of the integrated reflectivity for a few orders of reflection as well as rocking curves were derived by ω-step scanning in transmission geometry. The theoretical foundations relevant to the interpretation of the experimental data and characterization of the samples are given. The main advantages of γ-ray diffraction, in particular high sensitivity to lattice distortions, have been demonstrated in a study of the defect structure changes taking place during crystal growth, neutron transmutation doping and device structure formation. The investigations showed high efficiency of the heat treatment in chlorine-containing atmosphere as a means of decreasing the structural defect concentration in single crystals and p−n structures. The role of non-equilibrium intrinsic point defects in defect formation during thermochemical treatments is discussed.

Semiconductor Isotope Engineering

Isotopic control of semiconductor crystals offers a wide range of scientific and technical opportunities. We review neutron transmutation doping of natural and isotopically controlled semiconductor structures, special properties of isotope superlattices, the effect of host isotopes on local vibrational modes of low mass impurities, and intrinsic properties which depend on isotope mass and isotopic composition of single crystals.

Low temperature hopping conduction in neutron transmutation doped isotopically enriched70Ge:Ga single crystals

The temperature dependence of variable range hopping resistivity ρ in neutron transmutation doped (NTD) isotopically enriched70Ge:Ga samples is reported. Five samples with compensation ratios K less than 0.001 and Ga concentrations between 3×1016 and 1.77×1017 cm−3 were studied. All samples investigated show the ln ρ∝T−1/2 dependence in the temperature range below 1.5K. As thermistor materials NTD70Ge:Ga samples are found to have more than factor of two higher sensitivity than commonly used natural NTD Ge in the temperature range between 0.2K and 1K. Our results are compared with theoretical predictions for variable range hopping conduction.

Cryogenic micro-calorimeters for low energy beta spectroscopy

We are using calorimeters with completely encapsulated sources to study low energy beta spectra. Our devices work as true calorimeters in the sense that they totally absorb and sum all the various quanta released in a decay into a single temperature pulse with amplitude proportional to the total energy deposited. We use a neutron transmutation doped (NTD) germanium thermistor as the temperature sensor and superconducting NbTi leads, which form a weak thermal link to the cold stage (≈80 mK). We have been using superconducting tin as our absorber material, and we discuss various techniques for source and absorber preparation. Annealing the absorber leads to shorter pulses with larger amplitude, and significantly improves detector performance. The production of a device to study the beta decay of107Pd presented special difficulties due to the low specific activity of this isotope (halflife 6.5×106 y), and the low enrichment (15%) that was available. This meant it was necessary to incorporate a large amount of palladium into our tin absorber. We found we could avoid a corresponding increase in heat capacity by forming a superconducting PdSn alloy.

Neutron transmutation doping of gallium arsenide

Neutron transmutation doping (NTD) was studied as a means of compensating p-type Cd-doped GaAs. By introducing specific donor concentrations, the net acceptor level was measured and showed a progressive reduction. The NTD constant K = 0.32 donor atoms cm 3/n cm 2 was also measured. Radiation damage caused by neutron bombardment was annealed and no additional traps were generated.

Direct dark matter search program with bolometers at the University of Tokyo

Bolometers are being developed at the University of Tokyo aiming at a search for galactic dark matter. We have succeeded in constructing a small-sized bolometer with 2.8 gram of lithium fluoride crystal with an rms energy resolution of 3.8 ke V for 60-ke V γ rays. Both lithium and fluorine are expected to be suitable for the detection of possible particle dark matter with a spin-dependent interaction like supersymmetric neutralinos. Fabrication of Neutron Transmutation Doped (NTD) germanium is also in progress to make a number of thermistors with uniform sensitivities to be used for multi-module bolometers. When the development of a full-sized detector with a total mass of order one kilogram is completed, we plan to install it in the Kamioka underground laboratory.

Measurement of electron-phonon decoupling time in neutron-transmutation doped germanium at 20 mK

We have studied the electron-phonon decoupling in Neutron-Transmutation Doped (NTD) Germanium thermistors below 50 mK, and measured a characteristic time constant of this phenomenon. We will discuss how our decoupling model accounts for observed non-linearities in I–V characteristics of NTD Ge and for the time structure of phonon pulses detected in a Ge crystal operated at 20 mK.

Neutron transmutation doping of silicon by magnesium

We have demonstrated that irradiation of silicon by fast neutrons and subsequent annealing lead to doping due to formation of Mg-related centers. The impurity introduction rates were calculated as a result of transmutation reactions during irradiation in the IBR-30 reactor. The behaviour of the resistivity of irradiated samples during annealing was studied.

Density of states near the Fermi level from measurements of variable-range-hopping magnetoresistance in germanium.

The variable-range-hopping (VRH) resistance of n- and p-type neutron-transmutation-doped (NTD) Ge was measured at temperatures down to 30 mK and in magnetic fields up to 7 T. It is shown that the temperature dependence of the VRH resistance can be reduced to a universal curve for all samples and fields, from which one can determine the density of states (DOS) near the Fermi level. Temperature-dependent measurements show that VRH conductivity at low energies is in agreement with the existence of a soft Coulomb gap at the Fermi level. The tendency to a constant DOS at higher energies is shown: the so-called crossover phenomenon in VRH conductivity which reduces the effect of the Coulomb gap at higher magnetic fields and temperatures is discussed.

An irradiation rig for neutron transmutation doping of silicon in the IEA-R1 research reactor

An irradiation rig with simple design has been constructed and installed in the IEA-R1 research reactor for neutron transmutation doping (NTD) of silicon with phosphorus. Crystal ingots with 3 and 4 in. diameter can be irradiated in this device. By adopting a procedure in which two ingots of 20 cm long each are irradiated simultaneously and their positions interchanged at a point when precisely half the total necessary neutron dose has been received, it has been possible to achieve the desired axial uniformity of the neutron dose. This method avoids the use of neutron absorbing shields around the crystals which necessarily compromise the overall irradiation capacity of the reactor. Test irradiations were performed with 50 float zone silicon crystals, and the results of radial and axial uniformities in the final resistivity values as well as the doping accuracy obtained in the test irradiations show an excellent doping quality achieved.

Vacancy-Type Defects in Neutron-Irradiated Silicon

Positron annihilation measurements show that the radiation defects and secondary defects in neutron-transmutation-doped (NTD) Si irradiated by two neutron doses can be removed at 600-650°C. The concentration and annealing behavior of V-type and V2 defects are related to the neutron doses. V4 appears at 150°C and 450°C-550°C in NTD Si irradiated by higher neutron dose.

Physics with Chemically and Isotopically Pure Semiconductors

Chemically and isotopically pure semiconductors offer a wealth of interesting physics. We review a number of impurity complexes which were discovered in ultrapure Germanium. The have led the way to the widely pursued studies of hydrogen in numerous semiconductors. Isotope related effects and processes include neutron transmutation doping, a technique used for a number of silicon and germanium devices. Isotopically pure and deliberately mixed crystals of germanium have been grown recently and have been used to study the dependence of the indirect bandgap and phonon properties on the mass and mass disorder of the nuclei. The large number of stable isotopes of the various semiconductors present a great potential for basic and applied studies. Semi-conductor isotope engineering may become a reality because of the new economic and political world order.

Germanium-doped gallium phosphide obtained by neutron irradiation

Results of electrical, optical, electron spin resonance and optically detected magnetic resonance studies of thermal neutron irradiated and annealed at 800 °C n-type GaP are presented. Evidence is found to support the view that the main dopant introduced via transmutation of GaP, germanium, occupies cation sites and forms neutral donors. This confirms the possibility of neutron transmutation doping of GaP. Simultaneously, it is shown that germanium is absent at cation sites. Presence of other forms of Ge-related defects is deduced from luminescence and absorption data. Some of them are tentatively identified as VGa-GeGa acceptors leading to the self-compensation process. This observation means that the neutron transmutation as a doping method in application to GaP is not as efficient as for Si.

Effective n-type doping of InP by the neutron transmutation technique

The neutron transmutation doping technique has been used to introduce controlled amounts of shallow donor tin impurities in nominally undoped InP substrates. Both optical (Raman scattering) and electrical (Hall effect) methods were applied for characterization purposes. From the analysis of the Raman features, the resistivity of as-irradiated samples is shown to increase with the neutron fluence as a direct consequence of the inherent lattice damage (mainly point defects). After annealing at 550°C, Hall measurements show that electron concentration values up to 10 18 cm −3 can be readily achieved in a controlled manner.

A nuclear relaxation study of hopping in p-type InSb

The authors present a low-temperature study of the charge and spin dynamics of holes, at density approximately 1016 cm-3, in the narrow-band semiconductor InSb. The conductivity and Hall coefficient are used to characterize the charge dynamics, and nuclear spin-lattice relaxation is used to monitor the electron spin dynamics. They have two samples; the first is cadmium-doped, p-type, uncompensated, of density 1.5*1016 cm-3 and the other is an initially identical sample, but additionally compensated by neutron transmutation doping to reduce the hole density to 7.2*1015 cm-3. This dramatically increases the mobility in the hopping regime, by reducing correlation effects. An unusual nuclear relaxation rate enhancement at low field and at low temperatures is monitored for all three nuclei, 115In, 121,123Sb, over a wide field range; high field quenches the effect. The authors discuss possible nuclear relaxation mechanisms and speculate that the effect is due to electron spin fluctuations associated with the amorphous antiferromagnetic nature of the sample in which the effect is observed, which has a dopant density close to the critical density for the metal-non-metal transition.

A positron annihilation study of defects in neutron transmutation-doped float-zone (Ar)-Si

Annealing of defects introduced by neutron transmutation doping of float-zone silicon has been investigated by positron lifetime spectroscopy and Doppler-broadening measurements. It is shown that the main defects anneal out at about 150 and 500 °C. During annealing, the formation of bigger defect complexes can be seen.

DX Centers in AlGaAs and Pressurised GaAs

ABSTRACTIn this paper, we present results of our investigations on some aspects of the DX centers. It is shown from the low temperature Hall mobility measurements that the charge state of the DX center is neutral supporting the positive U model for the DX center. Hall and DLTS measurements on Al-rich Al0.8Ga0.2As:Te sample show a reversal in the ordering of the energies of the hydrogenic and deep states of the Te donor, with the DX state lying higher than the X valley related effective mass state. Thermal emission properties of the pressure induced DX states of Ge and Se donors in neutron transmutation doped (NTD) GaAs are discussed.

Neutron Transmutation Doping of Isotopically Controlled Ge

Neutron Transmutation Doping of Isotopically Controlled Ge

Hydrogen-related donor in silicon crystals grown in a hydrogen atmosphere

Neutron transmutation doped (NTD) silicon crystals grown in a hydrogen atmosphere have been investigated by infrared absorption spectroscopy at a low temperature (10 K). An effective-mass-like donor state HD0/+ has been found at 110.8 meV below the conduction band bottom after rapid thermal annealing (RTA). The HD0/+ formation mechanism after NTD and RTA is briefly discussed, and tentatively attributed to H atoms present in the vicinity of some residual irradiation defects, like a complex of a H atom and a H-saturated vacancy.

Transmutation Doping and Fermi-Level Stabilization in Neutron-Irradiated InP

N-type conductivity and the ultimate Fermi-level position Flim ≈ Ev + 1.0 eV are obtained in neutron bombarded InP. In the heavily defective samples the de-conductivity is measured in the temperature range from 350 to 77 K. The neutron transmutation doping eficiency of InP is estimated after post-irradiation annealing up to 900 °C. [Russian Text Ignored.]