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

Preirradiation base-line measurements were taken on each device while the test board was in the vacuum chamber and in the final test position. Insitu measurements were taken while the devices were still in the exposure chamber. Additional measurements were performed (over a time interval of O min to 240 min) after stopping the irradiation to study any postirradiation effects. All measurements were done while the devices were in a light and noise insulated vacuum chamber and at room temperature. For a voltage sweep from -5.00 V to 5.00 V, current was measured at both terminals to monitor possible leakage current related to the measurement apparatus. It was found that any small differences in current measured between the terminals were independent of the radiation exposures. The current measurement given on the current-voltage data is that measured at the grounded pin so that the current traversing the device is most accurately represented by the data. This paper focuses on two compositions of IH [(1-x)FeTiO^sub 3^. xFe^sub 2^O^sub 3^], x = 0.1 and x = 0.45, a p- and an n-type material, respectively.

Films deposited on (100) MgO were crystalline and textured, as shown in Fig. 1 for a representative composition of x = 0.45. The films are about 50-nm thick and the roughness of these films is about 1.5 to 2 nm, which is typical for polycrystalline films. All compositions of IH show semiconducting nature, as shown in the inset for x = 0.45 grown at 650�C substrate temperature. Postgrowth annealing also took place at this temperature in situ.

Figure 2 a and b show the current-voltage characteristics of IH with x = 0.1 and x = 0.45 before and after irradiation with 40 MeV protons for fluences up to 5 � 10^sup 10^ p/cm^sup 2^. The data show that the proton irradiation does not significantly influence the I-V characteristics of x = 0.1. The nonlinear coefficient (a), defined as d In I/d In V and the switching voltage or the nonlinear voltage practically remain unchanged after irradiation. The figure of merit of the device is given by the nonlinear coefficient and the voltage at the onset of nonlinearity is referred to as the switching voltage. In the case of x = 0.45, we observed a slight change in the nonlinear region after irradiation. While it is possible that this small effect may be due to a decrease in carrier density due to radiation-induced defects in the materials, the polycrystalline nature of the films likely dominates the I-V response of the devices and the changes are comparable to measurement error. The switching voltage does not seem to be affected; however, we see a very small change in the nonlinear coefficient.

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.