Tadashi Hisamatsu

Analysis of Radiation Damage to Si Solar Cells under High-Fluence Electron Irradiation

Radiation testing of Si n+–p–p+ space solar cells has revealed an anomalous increase in short-circuit current I sc, followed by an abrupt decrease and cell failure, induced by high-fluence (>1016 cm-2) electron irradiation. A model which can be used to explain these phenomena by expressing the change in majority-carrier concentration p of the base region as a function of the electron fluence has been proposed in addition to the well-known model in which I sc is decreased due to minority-carrier lifetime reduction with irradiation. The reduction in p due to majority-carrier trapping by radiation-induced defects has two effects; one is broadening of the depletion layer which contributes to the increase in the generated photocurrent and that in the recombination-generation current in the depletion layer, and the second is an increase in the resistivity of the base layer resulting in an abrupt decrease of I sc and failure of the solar cells.

Investigation of carrier removal in electron irradiated silicon diodes

We present a detailed study of n+pp+ silicon diodes irradiated with fluences of 1 MeV electrons high enough to cause device failure due to majority carrier removal. Capacitance–voltage (C–V) measurements were used to monitor the change in the carrier concentration of the base of the device as a function of radiation fluence. These were compared to the defect spectra in the same region obtained by deep level transient spectroscopy, and to the current–voltage characteristics of the device, both before and after annealing. We observed the expected deep levels with activation energies of 0.18 and 0.36 eV, but the C–V results imply that other trap levels must play a more important role in the carrier removal process.

ANALYSIS OF STRUCTURE CHANGE OF SI SOLAR CELLS IRRADIATED WITH HIGH FLUENCE ELECTRONS

An anomalous increase in short-circuit current of Si space solar cells, followed by an abrupt decrease and cell failure has been induced by fluences greater than 1016 cm−2 of 1 MeV electrons. This can be explained by a reduction in carrier concentration of the base region, in addition to a decrease of minority-carrier diffusion length. A change in the spectral response has been observed along the change in the short-circuit current. The spectral response has been modeled to account for radiation-induced changes in the cell structure. The results show that the junction depth increases when the degradation occurs. Also, spectral response after cell failure has been explained by conduction-type conversion of the base layer. This conversion is confirmed by a cross-sectional electron-beam-induced current signal. A mechanism for these phenomena has been proposed, which consists of the generation of vacancies and the vacancy-mediated diffusion of phosphorous.

Explanation for carrier removal and type conversion in irradiated silicon solar cells

Heavy doses of radiation in space can cause the failure of n+/p/p+ silicon solar cells due to the gradual introduction of compensating defects into the base layer of the diode. In this letter, we show that the radiation-induced defects, which play the most important role in this process, referred to as “carrier removal,” are probably minority-carrier traps at an energy level approximately 0.18 eV below the conduction band. We conclude that these defects must be positively charged before electron capture, and therefore, act as donor centers which compensate the p-type base layer.

Temperature dependence of electron concentration in type-converted silicon by 1×1017 cm−2 fluence irradiation of 1 MeV electrons

The conduction type of boron (B)-doped silicon (Si) changes from p type into n type by the 1×1017u200acm−2 fluence irradiation (high-fluence irradiation) of 1 MeV electrons. The temperature dependence of the electron concentration n(T) obtained from Hall-effect measurements is reported. From the analysis of n(T), the density and energy level of the defects created by the high-fluence irradiation are determined to be 1.5×1014u200acm−3 and EC−0.30u200aeV, respectively, where EC is the energy level at the bottom of the conduction band. Moreover, the compensated density is 9.5×1013u200acm−3, which is in agreement with the density of B that acts as an acceptor, determined by Fourier-transform infrared spectroscopy.

Evaluation of Hole Traps in 10-MeV Proton-Irradiated p-Type Silicon from Hall-Effect Measurements.

Using the temperature dependence of the hole concentration p(T) obtained from Hall-effect measurements, we attempt to uniquely determine the densities and energy levels of hole traps in 10-MeV proton-irradiated p-type silicon. Since the function p(T) exp (Eref/kT)/kT has a peak at the temperature corresponding to each hole trap level, the trap densities and energy levels can be determined uniquely and accurately. Here, k is the Boltzmanns constant, T is the absolute temperature and Eref is a newly introduced parameter which shifts the peak temperature of the function within the measurement temperature range. Three types of hole trap levels (about 0.09 eV, 0.13 eV and 0.19 eV above the top (EV) of the valence band) are observed, and these hole trap densities increase with increasing 10-MeV-proton fluence. The hole traps at EV + 0.09 eV and EV + 0.19 eV have been reported and their origins have been discussed. However, the hole trap at EV + 0.13 eV has not yet been reported. The quantitative relationship between the decrease in hole concentration and the increase in densities of certain traps due to proton irradiation has been clarified for the first time.

Evolution of defect complexes in silicon single crystals with heavy fluence 10 MeV proton irradiation

We have investigated the defect structure of 10 MeV proton irradiated Czochralski-grown Si single crystals and space solar cells with boron-doped p-Si base layer using deep level transient spectroscopy measurements to characterize both vacancy interstitials and their complex-type defects and to monitor their evolution upon annealing at temperatures ⩽500u200a°C. We have observed quite different annealing behavior of the deep levels for conduction-type converted samples (n-type) irradiated at 1×1014 p/cm2 as compared to an intermediate dose of 3×1013 p/cm2. The observed concentrations of the minority carrier level at EC−0.20 eV and the new electron level at EC−0.71 eV that can be seen in type converted samples, have been found to be enough to account for the carrier removal effects. The present study also throws light on the fact that heavy proton irradiation not only changes the structure of the device (from p to n type) but also makes the defect structure complex as compared to the simple defect structure in ...

Effects of Annealing on Type Converted Si and Space Solar Cells Irradiated with Heavy Fluence 1 MeV Electrons

We present a detailed quantitative study of the thermal annealing characteristics of deep level defects in Si and space solar cells with boron-doped p-Si base layer, introduced by 1 MeV electrons irradiation. Present isochronal annealing provides an overall different annealing behavior of the defects in type converted (n-type) heavy dose electrons (=1×1017 e/cm2) irradiated samples, contrary to earlier low dose electrons studies. Isochronal annealing provides evidence that the minority carrier trap (EC-0.18 eV) and majority carrier trap (EC-0.71 eV) that appear after type conversion play a dominant role in carrier removal and are the major defects responsible for the type conversion as well as the severe degradation of space solar cells. This study also sheds light on the fact that heavy electron irradiation not only changes the structure of the device (p to n-type) but also makes the defect structure more complex as compared to simple defects structure in low dose samples.

Analysis of 10 years' flight data of solar cell monitor on ETS-V

Abstract This paper describes the analysis of NASDAs various space solar cells by analyzing the data measured in space. NASDA launched the Engineering Test Satellite-V (ETS-V) on August 27, 1987 and put into the geostationary orbit at a longitude 150°E. This satellite was in operation until electronics stopped on September 12, 1997. On this satellite, the solar cell monitor (SCM) was equipped as a part of the Technical Data Acquisition Equipment (TEDA) for observation of the space environment. SCM consisted of 24 kinds of solar cells including silicon (Si) cells and gallium arsenide (GaAs) cells as shown in Table 1. The short-circuit current (Isc) characteristic of each cell were measured in the radiation environment of geostationary orbit for 10 years. The degradation data due to a solar flare, occurred in October 19, 1989, were also recorded. All flight data agree qualitatively with ground test data.

Photoluminescence study of silicon solar cells irradiated with large fluence electrons or protons

Abstract In order to investigate the anomalous degradation of space silicon solar cells which was found in large fluence region, photoluminescence measurements are carried out for the cells irradiated with 1 MeV electrons with a fluence exceeding 1×10 16 e/cm 2 and 10 MeV protons with a fluence exceeding 1×10 13 p/cm 2 . For both irradiation, the intensity of boron-related bound exiton line decreases with fluence and it disappears at the fluences where the anomalous degradation occurs. The dominant defect is a complex of an interstitial carbon and an interstitial oxygen (C I –O I ). The generation of five-vacancy-defects was also observed for the proton irradiation. Variations of photoluminescence line intensity are discussed in terms of displacement damage dose calculated based on non-ionizing energy loss (NIEL).