J. W. Cleland

Nature of Bombardment Damage and Energy Levels in Semiconductors

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 neutron irradiated n‐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...

Observation of Irradiation‐Induced Interstitial Copper Impurity in Germanium

The possibility that substitutional impurities can be displaced into interstitial sites in germanium by electron irradiation was investigated by electrical property measurements. The impurity chosen was copper since it is a triple acceptor in a lattice site, but is a donor when interstitial; hence, ejection of a single copper atom adds four electrons to the conduction band. Specimens of n‐type germanium doped with both copper and antimony (the ratio of Cu to Sb was ∼1:4) were irradiated with 1.6‐MeV electrons at 77°K along with control specimens that contained no copper. The decrease in electron concentration n was appreciably less in the copper‐doped specimen than in the control specimen, as would be anticipated if copper impurities were displaced into interstitial sites. Isochronal annealing at several temperatures between 77° and 260°K after irradiation produced a further decrease in n (reverse annealing) in the copper‐doped specimens, whereas the control specimens showed a normal recovery of n. A like...

Radiation damage in neutron transmutation doped silicon: Electrical property studies

Radiation damage in neutron‐transmutation‐doped (NTD) silicon, irradiated to introduce 5×1013 to 6×1016 phosphorus cm−3, has been studied by electrical property measurements. The experimental results indicate that thermal‐neutron‐induced (n,γ) recoil‐type damage can be annealed at 400 °C. The nature of any remaining lattice defects and their annealing behavior above 400 °C is a function of the fast‐neutron fluence. Small defect clusters are present in Si irradiated with a light‐to‐moderate fast‐neutron fluence (?5×1018 n cm−2), and temperature‐dependent Hall coefficient measurements indicate that at least two deep acceptor levels and one deep donor level are formed during annealing. One of these acceptor levels anneals at ∼450 °C, and the other two levels anneal at ∼550 °C. A shallow acceptor level near the valence band that anneals at 750 °C is also observed. Larger defect clusters which reduce the electron mobility tremendously and distort the band structure are formed in heavily irradiated Si (5×1018 t...

Effect of Irradiation on the Hole Lifetime of N‐Type Germanium

The minority carrier lifetime of germanium is very sensitive to certain types of irradiation. The results of irradiating large single crystal samples of n‐type germanium with fast neutrons and Co60 gamma rays are presented and discussed. Lifetime was determined from a transient measurement of the decay of holes following an injection pulse. The effect of initial carrier concentration has been considered. According to the James‐Lark‐Horovitz model, four energy levels are introduced by irradiation into the forbidden band of germanium, one above and three below the center of the band. Using the upper level, a one level recombination model serves to explain the experimental results approximately. The determination of the recombination level agrees well with the position of the first ionization level of the interstitial produced by irradiation at about 0.2 ev below the conduction band. On this basis, the effective cross section for hole capture by these recombination centers is found to be about 4×10−15 cm2 an...

Pulsed excimer laser annealing of ion implanted silicon: Characterization and solar cell fabrication

A pulsed ultraviolet excimer laser (XeCl, 308‐nm wavelength, ∼41‐ns pulse duration) has been sucessfully used for laser annealing of both boron‐ and arsenic‐implanted silicon, and for formation of high quality p‐n junctions. Transmission electron microscopy, secondary ion mass spectroscopy, and sheet electrical properties measurements are used to characterize ion implanted and XeCl laser annealed specimens. Predictions of thermal melting model calculations of the annealing process are also compared with results of these measurements. Finally, we demonstrate the first use of high repetition rate, scanned, overlapping excimer laser pulses to fabricate large area photovoltaic solar cells with good performance characteristics.

RADIATION-INDUCED RECOMBINATION CENTERS IN GERMANIUM

An attempt has been made to provide a better understanding of minority carrier recombination processes in irradiated germanium. To this end, studies have been made of the minority carrier lifetime of both n‐ and p‐type material, following fast neutron and Co60 γ irradiations. The effect of carrier concentration and temperature has been determined. The primary conclusion drawn from these investigations is that the recombination process is associated with an energy level located 0.20 ev below the bottom of the conduction band. From measurements of the lifetime in n‐type specimens, the hole capture cross section has been calculated and found to be σp=3×10−15 cm2 and σp=4×10−16 cm2 for neutron and γ‐induced centers, respectively. If a second, lower lying level is associated with the second ionization of the defect responsible for recombination, then the effective number of recombination centers is reduced in p‐type material as the Fermi level approaches the valence band, due to this ionization. Such a process...

Low‐Temperature Irradiation of n‐Type Germanium

The rate of change of conductivity of n‐type single crystal plates of germanium at a temperature of ∼16°K during fast neutron irradiation is enhanced over that previously observed at higher temperatures. Annealing studies subsequent to irradiation reveal no evidence of thermally unstable defect states between ∼10 and ∼95°K. Annealing above ∼95°K indicates the presence of thermally unstable minority carrier traps of the type previously observed following irradiation at 120°K. The enhanced decrease in conductivity at 16°K compared to that observed at higher temperatures is believed to reflect a greater rate of introduction of defects, many of which anneal at temperatures above ∼95°K.

MONOENERGETIC NEUTRON IRRADIATION OF GERMANIUM

A study has been made on 14‐Mev neutron‐irradiated germanium, using lifetime, Hall, and resistivity measurements to determine the nature of the radiation‐induced defects and to compare the damage with that produced by neutrons from a fission spectrum. The electron removal rate in high‐resistivity, n‐type material is ∼8/cm−3 per incident neutron/cm2, measured at 77°K. Lifetime measurements have been made on n‐ and p‐type material. On the basis of simple recombination theory, assuming no variation of capture probabilities with temperature, the results for n‐type material indicate that a recombination level is located 0.32 ev above the valence band near the center of the energy gap. Assuming an introduction rate of recombination centers equal to one‐half the electron removal rate in n‐type material, the following values of recombination capture cross sections are obtained: σn=2.2×10−17 cm2; σp=6×10−15 cm2, the latter value being correct only within about a factor of two. The ratio of the cross sections, σp/σ...

Lattice Defect Production in Thermal Neutron Shielded Materials

It is noted that in reactor irradiation procedures using a thermal neutron shield, capture gamma rays produced in the shield may have an appreciable effect on the material on which the irradiation is being carried out. The capture gamma spectrum of Cd is discussed as an example, and the gamma-produced damage to Al and Cu is outlined for gamma energies of 1 to 5 Mev. Finally, in an attempt to ascertain the mechanism for production of lattice defects by gamma radiation, the damage produced in Ge and Si by gamma radiation at 0 to 5 Mev and by thermal neutrons is compared.

Pulsed Excimer Laser (308 nm) Annealing Of Ion Implanted Silicon and Solar Cell Fabrication

A pulsed ultraviolet excimer laser (XeCl, 308 nm wavelength, 40 nsec FWHM pulse duration) has been successfully used for laser annealing of both boron- and arsenic-implanted silicon. TEM, SIMS, and sheet electrical measurements are used to characterize specimens. C-V and I-V measurements demonstrate that near-ideal p-n junctions are formed (diode perfection factor A = 1.2). Electrical activation of implanted ions by single laser pulses is essentially complete for energy densities E/sub l/ greater than or equal to 1.4 J/cm/sup 2/, far below the threshold for substantial surface damage --4.5 J/cm/sup 2/. Melting model calculations are in good agreement with observed thresholds for dopant redistribution and for epitaxial regrowth. Changes in annealing behavior resulting from multiple (1,2,5) laser pulses are also reported. Finally, the authors demonstrate the use of scanned overlapping excimer laser pulses for fabrication of large area (2 cm/sup 2/) solar cells with good performance characteristics. In contrast to pulsed ruby laser annealing, high open circuit voltages can be obtained without the use of substrate heating.