Influence of Proton Radiation on the Nonlinear Current-Voltage Characteristics of Pulsed Laser Deposited Ilmenite-Hematite Thin Films

Journal of Electronic Materials,  Aug 2005  by Padmini, P,  Tompkins, F,  Shojah-Ardalan, S,  Kale, P,  Et al

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The same samples that were irradiated with 40 MeV protons were subsequently subjected to 10 MeV protons. The lower energy protons deposit more energy per unit length and are therefore more likely to cause displacement damage, while also producing ionization damage within the materials.

The effect of 10 MeV protons on the nonlinear characteristics of x = 0.1 and x = 0.45 are shown in Fig. 3a and b. The samples received a total fluence of 5 × 10^sup 10^ p/cm^sup 2^; and, as before, I-V measurements were taken at different fluences. The 10 MeV protons such as the 40 MeV protons do not significantly affect the characteristics of x = 0.1; whereas in the case of x = 0.45, we do see a slight change in the nonlinear region. This observation, however, is not significant enough to cause a substantial change in the figure of merit of the device; therefore, the IH devices are radiation tolerant to the irradiation conditions described previously. Wide bandgap semiconductors such as ilmenite are commonly attributed to be "radiation tolerant" with regard to ionization effects as compared to silicon because of their higher ionization energies for electron-hole pair creation.19 However, the electron-hole pairs created per unit length of an ionizing particle's path through a material are also strongly dependent on the density of the material.19 In the case of ilmenite, E^sub g^ is about 2.58 eV. This is over 2.5 times the value for silicon, and the density of ilmenite is about 4.72 g/cm^sup 3^ compared to silicon at 2.32 g/cm^sup 3^. One has some reason then to expect ilmenite devices to be more radiation tolerant compared to silicon devices. Nevertheless, many diode devices made from wide bandgap semiconductors do not exhibit significant radiation damage until exposure to high fluences (>10^sup 13^ particles/cm^sup 2^) of even lower energy protons (typically, 3-5 MeV protons).1,19,20 The effect of material density is most important in devices where charge collection volumes play a role such as in single event effects. Experiments are planned to expose IH devices similar to those described in this paper to high fluences of 3 MeV protons. While these experiments would not necessarily simulate the expected space environment, they would help elucidate the relative contributions of ionization and displacement damage mechanisms in the IH devices.

For a given voltage, the current handling capability of x = 0.45 is much larger than that of x = 0.1 as a result of its lower resistivity. We have observed that the resistivity decreases as we go from p to n type.7 The switching voltage of these devices is on the order of 1-2 V, which makes them ideal for low voltage varistor applications in semiconductor electronics.

In summary, we have studied the radiation tolerance of a IH-based low voltage varistor device to proton irradiation. We have found the device to be tolerant to 10 MeV and 40 MeV proton irradiations up to the fluence of 5 × 10^sup 10^ p/cm^sup 2^, making it a potentially viable material for application in the radiation-dominant environment. It should be noted that both p- and n-type materials exhibit good varistor behavior and that the devices show no significant radiation degradation in their I-V characteristics within measurement error. Further work needs to be carried out to study the influence of film thickness and other growth parameters on the device characteristics.