Volume 30, Issue 8, 01 August 1959
Index of content:
30(1959); http://dx.doi.org/10.1063/1.1735280View Description Hide Description
A review is given of the mechanisms of radiation damage, and of some of the resulting effects. The discussion is divided into three categories: (1) mechanisms of damage production, (2) nature and mobility of the imperfections produced, (3) effect of the imperfections on the measurement of the properties of the solid. Principal attention is given to metals, with related observations in semiconductors cited.
30(1959); http://dx.doi.org/10.1063/1.1735281View Description Hide Description
The average value of the kinetic energy of recoil following thermal neutron capture and subsequent gamma‐ray emission is 780 ev in silicon and 180 ev in germanium. For every neutron captured in silicon, 0.04 P31 atom (therefore, 0.04 excess electron) are formed by radioactive decay. For every neutron captured in germanium, 0.098 As75, 0.012 Se77 (therefore, 0.122 excess electron), and 0.304 Ga71 atom (therefore, 0.304 excess hole) are ultimately formed, in this time sequence. Analysis of an experiment of J. W. Cleland on the decay of irradiatedn‐type germanium gives 0.8 electron removed from the conduction band per initially recoiling germanium atom.
30(1959); http://dx.doi.org/10.1063/1.1735282View Description Hide Description
The effects of irradiation on the infrared absorption and photoconductivity in silicon are reported. The absorption near the intrinsic edge is increased and drops off more gradually toward longer wavelengths. Several absorption bands are introduced by neutron irradiation with peak absorptions at 1.8, 3.3, 3.9, 5.5, and 6.0 μ, respectively. The observation of each band depends upon the position of the Fermi level. The 1.8‐μ band has also been studied for deuteron irradiated and electron irradiatedsilicon, and the 3.3‐μ band has been observed in electron irradiated samples. The absorption bands arise from electronic excitations of various types of defects and associated photoconductivity has been observed for the 3.9‐μ and 5.5‐μ bands. In addition absorption bands have been observed at long wavelengths: 20.5, 27.0, and 30.1 μ, which are associated with lattice vibration. The significance of these results is discussed.
30(1959); http://dx.doi.org/10.1063/1.1735283View Description Hide Description
The nature of the green and blue emission at 78°K in CdS has been investigated. On the basis of the wavelengths of absorption and emission lines the blue component is assigned as exciton decay. Measurement of the decay of the blue luminescence, following excitation by a 10−8 sec pulse of 1‐Mev electrons, gave an exciton lifetime <10−8 sec. Similar measurements have shown that the green component has a slower decay with emission occurring as long as 20 μsec after the excitation has ended. These results have been used to support the recombination of a free electron with a trapped hole as the mechanism for green edge emission. Heating in sulfur vapor quenches the green luminescence in a controllable fashion. From the infrared reflection spectra of CdS in the region 10 to 50 μ the optical phonon frequencies were determined. The value of 305 cm−1 for the longitudinal phonon is in good agreement with the prediction of Kröger and Meyer from edge emission measurements, showing that the emission is coupled to the lattice through the longitudinal optical phonon. Based on an analysis of the effects on edge emission produced by heat treatment in sulfur vapor and electron irradiation with 0.20 and 1.0‐Mev electrons, a sulfur vacancy is suggested as the recombination center associated with the green edge emission.
30(1959); http://dx.doi.org/10.1063/1.1735284View Description Hide Description
This paper contains a discussion of those diffusion processes commonly referred to as diffusion‐controlled reactions. Attention is focused on those aspects of the phenomena to which special care must be given in developing theoretical analyses. The discussion is documented with several experimental examples drawn from the chemical physics of semiconductors, e.g., annealing of radiation damage, ion pairing, precipitation, and the formation of complexes in solids.
30(1959); http://dx.doi.org/10.1063/1.1735285View Description Hide Description
The use of thermal conduction and thermoelectric measurements in studying radiation damageeffects in semiconductors is discussed. The conclusion is reached that in the present state of knowledge such measurements will probably be more helpful in studying the kinetics of the formation and annealing of radiation‐introduced defects than in characterizing the structure of such defects.
30(1959); http://dx.doi.org/10.1063/1.1735286View Description Hide Description
High‐purity n‐ and p‐type silicon has been irradiated lightly with Co60 γ rays. Analysis of the Hall mobility and magnetoresistance data indicate the introduction of levels near the conduction and valence bands. The analysis suggests the presence of triply ionized acceptors and singly ionized donors. The third ionization state of the acceptor apparently produces a level above the center of the band gap. Under more extensive irradiation a double ionization of the donors can account for the observed high resistivity. An initial increase in the magnetic field dependence of the Hall coefficient in both n‐ and p‐type silicon may be related to the multiple ionization or to radiation annealing. The Hall mobility and transverse magnetoresistance in n‐type gallium arsenide have been studied as a function of impurity concentration and density of defects introduced by fast‐neutron irradiation. The change in the mobility and magnetoresistance with fast‐neutron bombardment suggests the introduction of levels near the band edges and of multiply ionized levels similar to those in silicon.
30(1959); http://dx.doi.org/10.1063/1.1735287View Description Hide Description
The theory of recombination via defects having energy levels in the forbidden gap is reviewed. Emphasis is given to those aspects which complicate interpretation of lifetime data, such as the inherent difference between steady state and transient measurements, large‐signal behavior, competing recombination mechanisms, trapping, the possible existence of strongly temperature‐dependent cross sections, and the properties of multilevel defects. A summary of the known recombination properties of bombardment‐produced defects is given.
30(1959); http://dx.doi.org/10.1063/1.1735288View Description Hide Description
The properties of recombination centers in germanium are obtained on the basis of lifetime data in conjunction with other information available. For recombination centers introduced by Co60gamma rays and fission neutrons, the recombination energy level position is placed at 0.20 ev below the conduction band. The room temperature hole‐capture cross sections resulting are 1.1×10−15 cm2 and 6×10−15 cm2 for Co60 gammaray and fission neutron irradiation, respectively. For the case of 14‐Mev neutron irradiation the energy level is located 0.32 ev above the valence band. The room temperature hole and electron cross sections are ∼6 ×10−15 cm2 and 2.2×10−17 cm2, respectively. The capture probabilities are assumed to be independent of temperature except for the case of gamma irradiation, for which there is apparently a fairly strong variation corresponding to a change in the activation energy of 0.07 ev. The selection of the values given above is not entirely unique. The assumptions made in their determination are discussed. The values given are directly applicable only in the case of n‐type material, the situation in p‐type material being more complex.
30(1959); http://dx.doi.org/10.1063/1.1735289View Description Hide Description
The rate of change of minority carrier lifetime in germanium crystals bombarded by 1‐Mev electrons has been studied experimentally as a function of the initial resistivity of the material.Analysis of the data leads to the conclusion that a single recombination center controls lifetime in both n‐ and p‐type material, that this level is located at either 0.21±0.005 ev from the conduction band or else at 0.26±0.005 ev from the valence band, and that the ratio of the cross section for capture of minority holes is 18 times the cross section for capture of minority electrons. The absolute value of the hole capture cross section is 4.5×10−15 cm2 if one uses the best available data for the probability of producing a Frenkel defect by a 1‐Mev electron. This large value is compared to the findings of other authors in similar experiments.
30(1959); http://dx.doi.org/10.1063/1.1735290View Description Hide Description
The magnetic susceptibility of semiconductors depends on electronic properties of the lattice, on the motion of free carriers and on the presence of impurity states. Critical points in the work on Ge and Si are reviewed relative to demands they place on theoretical interpretations. Anomalies in the susceptibility of high‐purity specimens, various aspects of valence band carrier susceptibility, and the susceptibility of acceptor states are not as yet explained satisfactorily.
30(1959); http://dx.doi.org/10.1063/1.1735291View Description Hide Description
Magnetic susceptibility measurements above 3°K and Hall effect and resistivity determinations between 50 and 300°K are reported for n‐type silicon samples irradiated with increasingly higher doses of fission neutrons. The paramagnetism due to electronic states in the forbidden gap shows an initial decrease after short irradiation but a reversal, increase, and final saturation at a value less than that originally contributed to the paramagnetism by the filled donors after longer irradiation.
The Hall coefficient shows evidence of a distribution of irradiation‐produced energy levels in the neighborhood of 0.3 ev below the conduction band. The mobility goes through an initial sharp decrease with irradiation but recovers partially after longer irradiations. The results are discussed in terms of several models of radiation damage. It is concluded that a simple model based on uniformly dispersed interstitials and vacancies is not adequate to explain the results and that interactions between centers, and nonuniform distribution of damage will probably have to be taken into consideration.
30(1959); http://dx.doi.org/10.1063/1.1735292View Description Hide Description
Electron spin resonance has been observed in n‐type silicon irradiated with 0.5‐Mev electrons. The particular resonance lines discussed here appear only in pulled crystals which contain about 1018 oxygen atoms per cm3. The lines do not appear in floating zone crystals (<1017 oxygen per cm3). The pattern is anisotropic with respect to the field orientation and can be fitted with a g tensor with components g =2.0029, g =2.0019, g [11̄0]=2.0096.
Hyperfine structure due to interactions with Si29 nuclei is also observed. The hyperfine structure tensor exhibits a  symmetry indicating that the electron is not centered on the silicon nucleus with which it is interacting. A model is suggested, consistent with the observed symmetries, according to which vacancies produced during irradiation diffuse to the distorted regions around oxygen atoms.
30(1959); http://dx.doi.org/10.1063/1.1735293View Description Hide Description
The spin resonance behavior in room temperature irradiatedn‐type silicon is observed to be significantly different for silicon grown in quartz crucibles from that grown by the floating zone method. The dominant spectrum in each is discussed. The defects giving rise to the spectra are interpreted as containing impurity atoms and as having formed when the impurities trap mobile interstitials and/or vacancies. In quartz crucible grown silicon, the impurity may be oxygen. In the floating zone material, the impurity appears to be the phosphorus used in the doping. A 20°K irradiation and anneal is described which suggests the temperatures at which this defect motion is occurring. Features of the spin resonance spectra suggest that the vacancy may be the mobile effect.
30(1959); http://dx.doi.org/10.1063/1.1735294View Description Hide Description
The different effects of Co60 gamma ray and fast neutron bombardment on the electrical behavior of germanium are discussed in terms of different local distributions of lattice defects expected for these two types of radiation. For the first of these, which is expected to introduce randomly distributed pairs of interstitials and vacancies, the state at 0.20 ev below the conduction band has been found to increase the concentration of ionized scattering centers on becoming occupied with conduction electrons. This state has been ascribed to a mobile interstitial because of its annealing behavior at moderate temperatures. Energy levels in p‐type Ge and their annealing behavior are also discussed. In neutronirradiatedn‐type germanium the behavior of Hall mobility and the apparent energy distribution of defect levels are discussed in terms of a tentative model for the potential distribution around regions of high local lattice disorder expected to result from neutron bombardment. This model postulates that the disordered region is p type and is surrounded by a positive space charge region in the n‐type matrix which insulates it from the matrix. This configuration of electrostatic potential around the disordered region in effect produces an insulating region whose radius is ∼10 times that of the disordered region itself. The behavior of Hall mobility in irradiatedn‐type specimens is consistent with the predictions of this model.
30(1959); http://dx.doi.org/10.1063/1.1735295View Description Hide Description
The width and depth of the potential wells surrounding disordered regions in neutronirradiatedn‐type germanium and extrinsic silicon are estimated. Numerical examples of well dimensions for a wide range of sample characteristics are presented. Some effects of disordered regions, e.g., (a) scattering of conduction electrons, (b) absorption of holes, and (c) polarizability, are discussed.
30(1959); http://dx.doi.org/10.1063/1.1735296View Description Hide Description
The energy levels found in germanium irradiated by different particles seem at first to be mutually inconsistent. It is possible tentatively to reconcile the differences by consideration of clustering and association of defects. Four levels are ascribed to single vacancies and interstitials. A crude theory is constructed to explain these levels, particularly their asymmetrical distribution in the energy gap, and to assign each to a definite defect. This theory differs somewhat from a previous one due to James and Lark‐Horovitz; some differences in experimental predictions are discussed in particular.
30(1959); http://dx.doi.org/10.1063/1.1735297View Description Hide Description
Radiation‐induced energy levels in silicon have been detected, positioned, and evaluated with the help of many techniques. In this paper, the presently available body of information has been organized, summarized, and analyzed. A short historical review precedes a critical discussion of evidence recently reported for electron‐, deuteron‐, and neutron‐irradiated silicon. Three conclusions emerge from such a confrontation: (1) the energy level pattern is essentially independent of the damaging agent; (2) the pattern seems to present a remarkable symmetry with respect to the middle of the gap; (3) shallow levels close to the band edges are introduced in similar concentrations and at higher rates than the deep traps. On this basis, an attempt is made to interpret the infrared absorption and photoconductivity data of Fan and Ramdas. Problems such as those associated with the charge state of radiation‐induced defect centers are examined in the light of a novel experimental approach.
30(1959); http://dx.doi.org/10.1063/1.1735298View Description Hide Description
Electron‐bombardment damage in oxygen‐free silicon is compared with that found in pulled crystals which have been previously reported. It is found, in agreement with conclusions drawn from spin resonance experiments, that the level 0.16 ev below the conduction band is associated with oxygen. In oxygen‐free silicon a deep level, 0.38 ev below the conduction band is produced in dominant density, and a broad shallow level in lesser concentration. The 0.16‐ev level is not produced. The difference between this level scheme and that found in pulled crystals which contain oxygen points up the hitherto unsuspected importance of impurities in the formation of the stable defects, during the rearrangement of the vacancies and interstitials produced by the primary displacement.