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

Ilmenite-hematite (IH) [(1-x)FeTiO^sub 3^.xFe^sub 2^O^sub 3^] solid solutions are unique classes of materials showing both magnetic and semiconducting properties, which make them potential candidates for novel applications in microelectronics and spintronics. This paper focuses on their varistor behavior before and after exposure to 40 MeV and 10 MeV proton radiations up to a fluence of 5 � 10^sup 10^ p/cm^sup 2^. The IH films are tolerant to these irradiations with little significant change to the nonlinear current-voltage characteristics. The switching voltage of the devices is in the regime of practical applications, and the radiation tolerance makes these materials suitable for aerospace applications.

Key words: Ilmenite-hematite (IH), varistor, protons, radiation

Radiation-induced damage has become an important field of study and continues to gain importance because of the need to protect electronic components, devices, and microelectronic circuits from the potentially catastrophic effects of radiation on their performance.1 Space systems often require electronics that can operate in a high-radiation environment. This radiation may result from particles trapped in planetary magnetic fields, galactic cosmic rays, or high-energy protons from solar events. Exposure to high energetic particles can degrade device performance and ultimately lead to component failure. The primary effects of natural space radiation on spacecraft electronics are total ionizing dose effects, displacement damage, and single event upsets.2

Radiation can alter the characteristics of electronic materials and the performance of electronic devices made from these materials by the mechanisms of ionization and atomic displacement. The displacement damage takes place by nuclear interactions and the ionization damage by atomic interactions in which energy deposited by the ionizing radiations creates electron-hole pairs. Semiconductor- based sensors, such as varistors and other current sensitive sensors, have applications in aerospace and other hostile environments for power and other applications. The radiation effects on device performance are important in these applications because the elevated radiation levels associated with these environments can produce both ionization and displacement damage. The performance of the device will clearly depend on the radiation tolerance of the material, which will determine the concentration and electronic properties of the radiation-induced defects. Substantial effort is being put forth to attain radiation hardness in semiconductors.1-4

Radiation damage studies in semiconductor materials usually focus on the effect on minority carrier lifetimes and carrier mobilities. Devices made from these materials are generally characterized by some figure of merit or performance parameter that is specific to that device. For example, some varistor devices have been studied for radiation tolerance where the devices were characterized by current-voltage characteristics, breakdown voltage, and nonlinear coefficient as a function of radiation fluence.5 It has been reported that ZnO varistors show remarkable tolerance to gamma rays.6 However, no studies on the effects of neutron and proton radiation to varistor devices made from any semiconductor material have been made. Proton and neutron radiation are directly relevant to aerospace applications.

This paper discusses the effect of proton radiation on the low voltage varistor behavior of thin films of ilmenite-hematite (IH). Our work on the influence of neutron and proton radiation on the varistor characteristics of the bulk ceramics of IH has been reported elsewhere.7'8 This family of materials has a unique set of attributes. They are wide bandgap semiconductors (Eg > 2.58 eV); they can be tailormade as p- or n-type by carefully selecting the mole fraction of hematite in ilmenite.9 The solid solution exhibits a p-type behavior for hematite mole fraction of less than 0.2, whereas for fractions greater than 0.2, the material is n type.10 They exist as both antiferromagnetic or ferrimagnetic with the magnetic transition point usually far above room temperature for some compositions.11

The 1-in. targets for pulsed laser deposition were prepared in-house by a standard ceramic processing method, the details of which can be found in our earlier article.12 Thin films of IH were grown by ablating targets of desired composition using a KrF excimer laser of wavelength 248 nm operated at 8 Hz and 500 mJ/pulse. The films were deposited onto MgO single-crystal substrate with a (100) orientation. The ablation was carried out in vacuum at a base pressure of 10^sup -5^ niT and a substrate temperature of 650�C. The thickness of the films was determined by profilometry and the roughness by atomic force microscope. The structure of the films was also examined by x-ray diffraction. The resistivity of the films was determined by the van der Pauw method. A two part silver epoxy is generally used to make contacts on the sample. The resistivity measurements were done in the temperature range of 20 K to 350 K.

Selected samples were installed on a test board and were proton irradiated at the Texas A&M University Cyclotron Institute using a K500 superconducting cyclotron. Two terminals of each device were connected to a HP4145 semiconductor parametric analyzer (Agilent Technologies). The irradiation took place at room temperature in a vacuum of about 3 � 10^sup -2^ mT. The beam had a square cross section with area 5 cm � 5 cm and a uniformity of about 90% or better. These samples received different fluences of proton radiation up to 5 � 10^sup 10^ p/cm^sup 2^, equivalent to 9.55 Krad (Si) for the 40 MeV protons and 28.35 Krad (Si) for the 10 MeV protons. The proton energies and the total fluence are consistent with the expected long-term radiation exposure in low earth orbit (LEO).13'14 Protons found in orbits designated as LEO (usually considered