G.J. Shaw

Radiation-induced reverse dark currents in In0.53Ga0.47As photodiodes

The reverse dark current‐voltage (dark I‐V) curves of InGaAs photodiodes have been measured as a function of temperature following irradiation with 1‐MeV electrons. Prior to irradiation, the I‐V curves are well described by a diffusion term alone indicating that the junctions are of good quality. Irradiation produces a large increase in the generation current which can be modelled as resulting from a single defect center with an energy Ec−0.29 eV. Such a defect center called E2 has been detected using deep level transient spectroscopy.

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.

Radiation effects in Ga/sub 0.47/In/sub 0.53/As devices

The effects of irradiating Ga/sub 0.47/In/sub 0.53/As solar cells and p-i-n photodiodes with 1-MeV electrons were measured using deep level transient spectroscopy (DLTS) and both dark and illuminated (1 sun, air mass zero (AM0)) I-V measurements. Fits of the dark I-V data to the two-term diode equation before irradiation were satisfactory, yielding an estimated bandgap energy of 0.79 eV. The DLTS detected two radiation-induced defect levels, one shallow (0.10 eV) and one near mid-gap (0.29 eV). Temperature coefficients of the Ga/sub 0.47/In/sub 0.53/As photovoltaic parameters follow the same general behavior as for other solar cell materials (e.g., Si and GaAs). However, a sharp decrease in the short circuit current is observed above approximately=375 K. This temperature is reduced by irradiation. Isochronal thermal annealing induced recovery in the photovoltaic parameters at approximately=400 K, coinciding with an annealing stage of the near mid-gap defect level. >

Time‐resolved photoluminescence of undoped InP

Energy and time‐resolved photoluminescence data have been obtained for nominally undoped (n 4.5×1015 cm−3) bulk InP grown by the vertical‐gradient freeze method. The data were taken as a function of temperature, from 80 to 290 K, and analyzed using a solution to the continuity equation. The resulting lifetime values range from 300 ns to 3.2 μs, and surface recombination velocities were fund to be on the order of 103 cm/s. The temperature dependence can be explained by assuming a radiatively limited recombination with a resulting B coefficient ≥5.9×10−11 cm3/s at 300 K.

Continued study of the photo-injection annealing of thermally diffused InP solar cells

The authors present more results on the photo-injection annealing of irradiated diffused junction cells. Dark IV measurements were incorporated into the experiments, and the changes in the diffusion and recombination currents in the cell junction due to the irradiation and annealing were measured along with the corresponding changes in the illuminated IV curves and deep level transient spectra. Also, the results of thermal annealing in the dark are included to further investigate the annealing of V/sub oc/ and the H5 defect. The effect of different base dopant concentrations on the annealing of H5 is presented as well. These additional data allow for a more detailed description of the annealing of these cells. The results are a more complete model of the mechanism of the radiation response of the diffused junction solar cells.<<ETX>>

Low temperature proton irradiation of GaAs MESFETs

GaAs MESFETs and resistors were irradiated with 3 MeV protons at irradiation temperatures (T/sub IRR/) in the 100 K >

Minority-carrier lifetime damage coefficient of irradiated InP

Minority-carrier lifetime damage coefficients for 1 MeV electron, 3 MeV proton, and 6 MeV alpha particle irradiation of n-type (4.5×1015 and 1.3×1017 cm−3) and p-type (2.5×1017 cm−3) InP have been measured using time-resolved photoluminescence. These values are relatively insensitive to carrier type and show a slight increase with increasing carrier concentration. Evidence of comparable electron and hole capture lifetimes is found for the dominant recombination defect. The effect of 3 MeV proton and 6 MeV alpha particles relative to 1 MeV electrons is an increase in the lifetime damage coefficient by factors of about 104 and 105, respectively.

The effect of base dopant level and thickness on the radiation response of Ga/sub 0.47/In/sub 0.53/As solar cells

The dependence of the radiation response of Ga/sub 0.47/In/sub 0.53/As solar cells on the thickness of the cell base was investigated. The major conclusion of the Ga/sub 0.47/In/sub 0.53/As radiation testing done to date is that the degradation of open circuit voltage (V/sub /spl prop//) is the controlling factor. This seems to be due to the low base dopant level of the cells which have been studied, and the suggestion is that an increase in the dopant level will decrease the sensitivity of V/sub /spl prop//. However, an increase in the dopant level may increase the sensitivity of the short circuit current density (J/sub sc/) to irradiation, so in contrast to V/sub /spl prop//, a cell with a radiation hard J/sub sc/ is lightly doped and thin. Nevertheless, previous results have shown that a 5/spl mu/m thick Ga/sub 0.47/In/sub 0.53/As base doped to 2 /spl times/ 10/sup 16/ cm/sup -3/ shows significantly more resistance in V/sub /spl prop// than observed presently with no change in the sensitivity of J/sub sc/. Thus, the optimum design for a radiation hard Ga/sub 0.47/In/sub 0.53/As solar cell seems to be a 3-4/spl mu/m thick base with a dopant concentration of > 2 /spl times/ 10/sup 16/ cm/sup -3/.<<ETX>>

Irradiation of monolithic InP/Ga/sub 0.47/In/sub 0.53/As tandem solar cells

Prototype three-terminal, two-junction, monolithic InP/Ga/sub 0.47/In/sub 0.53/As tandem solar cells were grown by the National Renewable Energy Laboratory (NREL) using metal-organic chemical vapor deposition (MOCVD). The tandem cells have been irradiated at room temperature with incremental fluences of 1-MeV electrons. One sun, AM0, 25 degrees C I-V measurements and deep level transient spectroscopy (DLTS) measurements are reported for each fluence step. The results are presented and compared with similar measurements on electron-irradiated single-homojunction (SHJ) InP solar cells. Even though the tandem cells were prototype cells, they displayed excellent radiation responses. The InP top cell of the tandem device degraded no more than the SHJ InP. Also, the Ga/sub 0.47/In/sub 0.53/As bottom and the InP top cell were nearly current-matched throughout the fluence range studied. The preliminary conclusion is that the InP/Ga/sub 0.47/In/sub 0.53/As tandem cell will display the radiation resistance of an SHJ InP cell while producing the high power output of a tandem solar cell.<<ETX>>