Geoffrey P. Summers

Displacement damage analogs to ionizing radiation effects

Abstract We show that concepts, such as effective equivalent dose and the quality factor, which have long been found useful in comparing the effects of different kinds of ionizing radiation, are also applicable in correlating displacement damage effects in semiconductors. In the case of displacement damage, the energy deposition process is determined by the nonionizing energy loss (NIEL), instead of linear energy transfer (LET), as an ionization.

Deep level transient spectroscopy of irradiated p-type InP grown by metalorganic chemical vapor deposition

Results are presented of a deep level transient spectroscopy study of radiation‐induced defects in p‐type (Zn‐doped) InP grown by metalorganic chemical vapor deposition. Three major hole traps (H3, H4, and H5) and two electron traps (EA and EB) were observed. The electron trap structure in particular is significantly different from that reported in the literature for p‐type InP grown by other methods. Activation energies of 0.22 eV (EA) and 0.76 eV (EB) have been measured, and capture cross sections (σ∞) of 4.4×10−15 cm2 (EA), and 1.4×10−12 cm−2 (EB) have been determined. The H5 center has a thermally activated capture cross section with an energy barrier of 0.35 eV. The measured injection annealing rate of the primary hole trap (H4) was different than previously observed.

Correlation of Electron Radiation Induced-Damage in GaAs Solar Cells

GaAs solar cells with different structures and polarities were irradiated with 1 and 5 MeV electrons. The energy dependence of the electron damage coefficients for the photocurrent, photovoltage, and maximum power were found to vary approximately linearly with NIEL in contrast to what has been found for other GaAs cells.

Probability distributions of energy deposition and ionization in sub-micrometer sites of condensed media

A general analytic approach is developed for calculating both the energy deposition distributions and the ionization distributions produced by ions incident on target sites with dimensions that range from micrometers down to a few nanometers. The effect of target geometry, energy-loss straggling, transport of energy from the site by secondary electrons and Fano fluctuations on the ionization distribution is clearly delineated as a function of target size. As an example, the ionization distributions produced by 1 MeV protons randomly incident on small spherical sites of silicon are calculated. The approach should be useful in a wide variety of research areas including radiation effects in microelectronics, microlithography, radiation biology and thermoluminescence dosimetry.

Electron and proton irradiation-induced degradation of epitaxial InP solar cells

Abstract The degradation of epitaxial, shallow homojunction n+p InP solar cells under 1 MeV electron and 3 MeV proton irradiation is presented. The data measured under 3 MeV proton irradiation are analyzed in terms of displacement damage dose which is the product of the particle fluence and the calculated non-ionizing energy loss (NIEL)[1]. A characteristic proton degradation curve is derived from which the cell degradation under any energy proton irradiation can be calculated. The data measured under 1 MeV electron irradiation is also analyzed in terms of displacement damage dose. The electron irradiation-induced degradation is correlated with the proton degradation curve by determining electron to proton dose ratios for each of the photovoltaic (PV) parameters. A comparison of the characteristic degradation curves for InP and GaAs/Ge solar cells, which was determined previously, shows InP to be intrinsically more resistant to displacement energy deposition. The base carrier concentration was measured during the irradiations, and significant carrier removal was observed. When analyzed as a function of displacement damage dose, the reduction in carrier concentration under both the 1 MeV electron and the 3 MeV proton irradiation is shown to follow the same degradation curve. From this common degradation curve, a characteristic carrier removal rate is calculated for InP under any irradiation. The junction dark current was also measured during both irradiations, and the data were fit to a three-term diode dark current equation. From the fits, the diffusion current is determined as a function of particle fluence. Changes in the diffusion current under electron and proton irradiation are shown to correlate in terms of displacement damage dose in the same way as the cell maximum power. The junction recombination current is also determined from the dark current data, and the results show the energy level of the dominant radiation-induced recombination center to be approximately the same in both the electron and proton irradiated samples. In addition, the dark current analysis indicates that the relative changes in the hole and electron lifetimes are essentially the same under both the electron and the proton irradiations. Based on these results and the overall correlation between the electron and proton damage, a detailed description of the mechanism of the radiation response of InP is developed which describes the cell degradation under any particle irradiation.

Time dependence of radiation‐induced generation currents in irradiated InGaAs photodiodes

The annealing behavior of the reverse bias current‐voltage curves of 1 MeV electron irradiated In0.53Ga0.47As photodiodes has been measured at 300 K. The observed decay is shown to be correlated with the reduction of the E2 peak height with time, as measured by deep level transient spectroscopy. The reverse current is found to decay with a logarithmic time dependence, which can be explained by a model in which the annealing of the E2 defects is controlled by a distribution of thermal energy barriers.

Use of displacement damage dose in an engineering model of GaAs solar cell radiation damage

In this study we have combined a method of calculating radiation induced damage to solar cells using Non-Ionizing Energy Loss (NIEL), with models of Earth orbiting radiation in arbitrary orbits, to assess the lifetime of solar cells. This paper provides a comparison of the NIEL technique to results from the JPL Radiation Handbook, and to actual space experimental damage results. In addition, we discuss ways of extending the calculation to newer solar cell materials, as well as environments outside of the Earths orbit.

Effect of carrier concentration on the properties of irradiation‐induced defects in p‐type indium phosphide grown by metalorganic chemical vapor deposition

Deep‐level transient spectroscopy has been used to monitor the effect of carrier concentration on the properties of radiation‐induced defects in InP n+p mesa diodes grown by metalorganic chemical vapor deposition. The activation energy Ea for hole emission from H4 and H5 centers and the injection‐enhanced annealing rate of H4 at 200 K have been measured as a function of carrier concentration NA over the range ∼1×1016– 4×1017 cm−3. The measured values of Ea decrease with increasing NA in a way that can be semi‐quantitatively explained by a combination of the Frenkel–Poole effect and phonon‐assisted tunneling produced by the electric field in the junction. The results suggest that hole emission from H4 and H5 centers takes place to maxima in different valence bands. The injection‐enhanced annealing rate of H4 centers increases with increasing NA at low concentrations, but approaches a maximum value near NA ∼ 1017 cm−3, indicating a limiting dopant (Zn) concentration for impurity‐enhanced defect annealing.

Energy dependence of majority carrier defect introduction rates in p+n GaAs photodiodes irradiated with protons

Traps introduced into p+n GaAs diodes, grown by molecular beam epitaxy, by room temperature irradiation with 1, 4, 10, 50, and 53MeV protons, have been studied using deep--level transient spectroscopy. Five distinct majority carrier (electron) traps were observed after irradiation. The activation energies, capture cross sections, and introduction rates of the traps were measured. Good agreement was found between the energy dependence of the introduction rates for each trap and the calculated elastic nonionizing energy loss for protons incident upon GaAs (i.e., excluding the effects of inelastic nuclear interactions).

Optically and thermally stimulated luminescence in MgO

Abstract Following illumination with 250 nm light, samples of neutron irradiated and thermochemically reduced MgO emit either green luminescence (≈ 540 nm), when heated above room temperature, or blue luminescence (≈ 400 nm), when stimulated with 1064 nm light from a Nd: YAG laser. The luminescence mechanisms appears to involve F -type centers and traps such as Fe.