Ravi Bathe

Electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au multilayer Ohmic contacts to n-type GaN

The electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au (30 nm/100 nm/30 nm/30 nm) multilayer Ohmic contacts to n-GaN (doping level 5×1017 cm−3) were studied. The lowest contact resistivity derived from the annealed contact was ρS=3.0×10−6Ω cm2. The contacts were robust and showed high-thermal stability. X-ray diffraction and Auger electron spectroscopy studies were made to investigate the microstructure of the annealed contacts. The key to the success of the contact was the Ti layers placed on both sides of the Al layer. Upon annealing, there occurred both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film. The in-diffusion of this led to the formation of TiN, while the out-diffusion of this led to the formation of Ti–Al alloys. The second Ti layer also in-diffused and out-diffused during annealing. However, due to the presence of Au, the out-diffusion was marginalized, and the in-diffusion was higher than the out-diffusion. The in-diffusion led to th...

Evaluation of manganite films on silicon for uncooled bolometric applications

Pulsed-laser-deposited polycrystalline/amorphous films of mixed-valent manganites [La0.7Ca0.3MnO3 (LCMO), La0.5Sr0.5MnO3 (LSMO), La0.5Ba0.5MnO3 (LBMO), and (La0.6Pr0.4)0.67Ca0.33MnO3 (LPCMO)] grown at low temperature (450 °C) on single crystal (001) silicon substrate are evaluated for uncooled bolometric applications. It is shown that the temperature coefficient of resistance (TCR) and electrical noise (Sv) depend on the chemical composition. The optimum performance is found for LCMO with TCR of ∼7% K−1 and spectral noise ∼8.9×10−13 V2/Hz. In LBMO and LSMO the noise is much lower, but so is the TCR (1.5%–2% K−1). In LPCMO the TCR is high (∼10% K−1) and the noise is as well.

Silver ion implantation in epitaxial La2/3Ca1/3MnO3 thin films: Large temperature coefficient of resistance for bolometric applications

Epitaxial films of La2/3Ca1/3MnO3 were successively implanted with 100 and 200 keV silver ions at fluences of 4.5×1015 and 1×1016 ions/cm2, respectively, to achieve a fairly uniform implant distribution. The as-implanted films are insulating and do not show a metal–insulator transition. Postimplantation annealing at 950 °C shows a recovery of the high structural quality of the films, along with an increase in the metal–insulator transition temperature (Tp), magnetoresistance, and the peak temperature coefficient of resistance (TCR) at the transition. The peak TCR of 23% for manganite films is clearly significant for bolometric applications.

Influence of 90 MeV oxygen ion induced disorder on the magnetotransport in epitaxial La0.7Ca0.3MnO3 thin films

Epitaxial films of La0.7Ca0.3MnO3 have been irradiated with 90 MeV oxygen ions at different dose values ranging from 1011–1014 ions/cm2. The structural, magnetization, and magnetotransport properties have been studied as a function of the ion dose. It is found that the properties change gradually up to a dose of 1013 ions/cm2; however, drastic changes occur when the sample is irradiated at the higher dose of 1014 ions/cm2. Specifically, this sample exhibits a large, nearly temperature independent magnetoresistance in the low temperature regime. The Rutherford backscattering channeling data bring out the presence of defects in the irradiated films. The x-ray diffraction data, the temperature dependence of resistivity and magnetization, and the low temperature magnetic hysterisis data collectively indicate the presence of two different phases in the sample irradiated at 1014 ions/cm2. The surface morphology of this film, observed by atomic force microscopy, exhibits swelling, presumably due to subsurface cl...

Swift heavy ion irradiation effects on transport properties of epitaxial thin films of La1-xCaxMnO3

Abstract The influence of swift heavy ion (SHI) irradiation on transport properties of epitaxial thin films of La0.75Ca0.25MnO3 (LCMO) is studied. The films were irradiated with 90 MeV16O beams and 250 MeV107 Ag beams at different fluence values. In the case of 90 MeV16O ions LCMO specimens were irradiated to 1011-1014 ions/cm2 and for 250 MeV107Ag ions with 1010–1013 ions/cm2. A systematic variation in Curie temperature (T c) or resistivity peak temperature (T p) has been observed. It has been noted that for both types of ions the T p increased for the specimen irradiated at 1011 ions/cm2 fluence. Further increase of fluence decreased the T p value and at higher fluence (1014 ions/cm2 for 90 MeV16O and 1013 ions/cm2 for 250 MeV107 Ag ions) the specimens showed no metal-to-insulator transition even at low temperature down to 77K. They show completely semiconducting behavior with high resistance values. These results are discussed in terms of SHI irradiation-induced structural distrotion and strain on thin...

AlN thin films deposited by pulsed laser ablation, sputtering and filtered arc techniques

Abstract The present work describes the growth and characterization of AlN thin films deposited by pulsed laser deposition (PLD), DC magnetron sputtering and filtered arc techniques. The focus of this paper is not only on the optimization of process parameters for the production of device quality thin AlN films, but also to investigate deposition techniques that could provide stable and reliable dielectric films wide band-gap power device. We investigated the working gas pressure dependence of the deposition of AlN on the vertical walls of the etched/patterned silicon for device passivation studies. Under high-pressure (13.334–93.343 Pa) conditions, high-density plasma was achieved which produced AlN passivation of a vertically etched Si wall. The films were characterized by X-ray diffraction (XRD), Rutherford backscattering and ion channeling spectrometry, atomic force microscopy (AFM), UV-visible spectroscopy, scanning electron microscopy (SEM).

90 MeV 16O ion irradiation effects on transport and magnetization in epitaxial thin films of La0.75Ca0.25MnO3

The influence of 90 MeV 16O ion irradiation on the electric and magnetic properties of epitaxial thin films of La0.75Ca0.25MnO3 is studied. These films were deposited on LaAlO3(100) substrates by the pulsed excimer laser deposition technique. The thickness of each film was about 3000 A. The high quality of the films was ensured by x-ray diffraction and transport measurements. The films were irradiated with 90 MeV oxygen ions at different doses (1011–1014 ions/cm2). It is observed that at a low dose of 1011 ions/cm2 the resistivity peak temperature (Tp) and the Curie temperature (Tc) increase, however as the dose is increased further, the Tp and Tc decrease. At the higher dose of 1014 ions/cm2, the film becomes insulating and does not show any peak in the resistivity–temperature curve down to 60 K. These results are discussed in terms of the effects of ion induced compositional inhomogeneity and strain on the film properties.

Columnar defect induced phase transformation in epitaxial La0.7Ca0.3MnO3 films

Epitaxial La0.7Ca0.3MnO3 thin films on the SrTiO3(100) surface have been irradiated with 250 MeV Ag17+ ions at different nominal fluence values in the range of 5×1010–4×1011 ions/cm2, resulting in columnar defects. At low fluences these defects cause changes in material properties that are small and scale linearly with dosage. Above a threshold fluence value ∼3×1011 ions/cm2 dramatic changes are observed, including an order of magnitude increase in the resistivity and 50 K drop in the Curie temperature. Transmission electron microscopy measurements show that the changes are associated with a phase transformation of the undamaged region between the columnar defects. The transformed phase has a diffraction pattern very similar to that seen in charge-ordered La0.5Ca0.5MnO3. We propose that above a critical level of ion damage, strains caused by the presence of the columnar defects induce a charge-ordering phase transition that causes the observed dramatic changes in physical properties. We speculate that a c...

Structural, morphological, and electrical characterization of heteroepitaxial ZnO thin films deposited on Si (100) by pulsed laser deposition : Effect of annealing (800 °C) in air

Pulsed laser deposition technique was used for growing thin films of ZnO on Si (100) substrate held at different temperatures (Ts). All the as-deposited films have shown a preferential c-axis orientation associated with varying grain size as a function of Ts ranging from 100to600°C. Current-voltage (I-V) characteristics of these films show Ohmic behavior over the entire Ts range studied. These films were subjected to annealing at 800°C in air ambient for 4h. The grain size was observed to increase after the annealing process for all the films deposited at different Ts. Interestingly, these annealed films show nonlinear variation of current with applied voltage, very similar to the one observed in doped ZnO varistors. The nonlinear parameters such as the asymmetric behavior of change in current on the polarity reversal of voltage, the plateau region, and the break down voltage are observed to depend on Ts. This nonlinear behavior can perhaps be explained on the basis of electronic conduction model proposed...

Electronic transport and 1/f noise studies in 250 MeV 107Ag ion irradiated La0.75Ca0.25MnO3 thin films

The influence of 250 MeV 107Ag ion irradiation on electrical transport and low frequency conduction noise properties of La0.75Ca0.25MnO3 (LCMO) thin films have been investigated. A systematic variation of metal–insulator transition temperature Tp and resistivity as a function of ion fluence has been observed. It has been noted that for the sample irradiated with 1011 ions/cm2, Tp increased by 10 K and the resistivity is decreased in the ferromagnetic metallic state from that of an unirradiated sample, whereas our noise results show a monotonic increase in the normalized noise Sv/V2, as a function of ion fluence. The observed modifications in the noise properties of the irradiated LCMO thin films are consistent with the general expectation concerning the effect of swift heavy ions irradiation induced lattice defects.