Tantalum-based films with tailored composition, density, and electrical resistivity are of intere... more Tantalum-based films with tailored composition, density, and electrical resistivity are of interest for next generation hohlraums for magnetized indirect-drive inertial confinement fusion. Here, we use reactive direct-current magnetron sputtering to deposit tantalum suboxide films with O content in the range of 46–71 at.%. In contrast to a common approach involving varying reactive gas contents, compositional control is achieved kinetically by changing the total chamber pressure and the deposition rate, while keeping the working gas mix of Ar-5%O2 constant. The resultant films are X-ray amorphous with electrical resistivity varying by over seven orders of magnitude. The dominant conduction mechanism changes from metallic to activated tunneling above ∼55 at.% of O, which is characterized by a sharp increase in resistivity and a decrease in the carrier density at low temperatures.
Sputter deposition of B 4 C films with tailored physical properties remains a challenge. Here, we... more Sputter deposition of B 4 C films with tailored physical properties remains a challenge. Here, we systematically study how substrate temperature influences properties of B 4 C films deposited by direct current magnetron sputtering onto planar substrates held at temperatures in the range of 100 − 510 • C. Results show that all films are amorphous stoichiometric B 4 C, with low O content of ∼ 1 at.%. Films deposited onto substrates at 100 • C exhibit high compressive residual stress and decreased mechanical properties. For elevated substrate temperatures in the range of 180 − 510 • C, film mass density, surface roughness, Young's modulus, and hardness are weakly dependent on substrate temperature. However, in this temperature range, an increase in substrate temperature leads to larger residual compressive stress, accompanied by a corresponding reduction in the concentration of nanoscale inhomogeneities. At least for the landing atom ballistics conditions studied here, a substrate temperature in the range of ∼ 185 − 250 • C is optimum for growing films with near-zero intrinsic residual stress. The overall weak substrate temperature dependence of film properties revealed in this work is favorable for the development of a robust deposition process, particularly for the case of deposition onto non-planar substrates where temperature control is often challenging.
The influence of pre-existing lattice disorder on radiation defect dynamics in SiC remains unexpl... more The influence of pre-existing lattice disorder on radiation defect dynamics in SiC remains unexplored. Here, we use a pulsed ion beam method to study dynamic annealing in Ar-ion-bombarded 3C-SiC at 200 • C with different levels of pre-existing lattice disorder. Results reveal a non-monotonic dependence of the defect relaxation time constant on the level of pre-existing disorder, exhibiting a maximum of ∼ 4 ms at a level of relative initial disorder of ∼ 0.4, while crystals without pre-existing damage are characterized by a time constant of ∼ 1.4 ms. These observations demonstrate that radiation defect dynamics in SiC can be controlled by defect engineering.
At moderately elevated temperatures, radiation defects in SiC exhibit pronounced dynamic annealin... more At moderately elevated temperatures, radiation defects in SiC exhibit pronounced dynamic annealing, which remains poorly understood. Here, we study 3C-SiC bombarded at 100 • C with pulsed beams of 500 keV Ar ions. Radiation damage is monitored by a combination of X-ray diffraction, Raman scattering, and ion channeling. Similar damage buildup behavior but with different defect relaxation time constants, ranging from ∼ 1 to ∼ 6 ms, is observed for the different types of lattice defects probed by these techniques. A correlation between relaxation times and the nature of the defects is proposed. These results reveal additional complexity of radiation defect dynamics in SiC and demonstrate that results of different defect characterization techniques are needed for a better understanding of dynamic annealing processes in solids.
Gallium arsenide under ion bombardment at room temperature and above exhibits pronounced dynamic ... more Gallium arsenide under ion bombardment at room temperature and above exhibits pronounced dynamic annealing that remains poorly understood. Here, we use a pulsed beam method to study radiation defect dynamics in GaAs in the temperature range of 20-100 C irradiated with 500 keV Xe ions. Results show that, with increasing temperature, the defect relaxation time constant monotonically decreases from $5.2 to $0.4 ms. A change in the dominant dynamic annealing process occurs at a critical temperature of $60 C, as evidenced by a change in the activation energy. A comparison with the other semiconductors studied by the pulsed beam method (Si, Ge, and 4H-SiC) reveals that both the high-temperature activation energy and the temperature below which dynamic annealing becomes negligible scale with the melting point.
Effects of the collision cascade density on radiation damage in SiC remain poorly understood. Her... more Effects of the collision cascade density on radiation damage in SiC remain poorly understood. Here, we study damage buildup and defect interaction dynamics in 3C-SiC bombarded at 100 °C with either continuous or pulsed beams of 500 keV Ne, Ar, Kr, or Xe ions. We find that bombardment with heavier ions, which create denser collision cascades, results in a decrease in the dynamic annealing efficiency and an increase in both the amorphization cross-section constant and the time constant of dynamic annealing. The cascade density behavior of these parameters is non-linear and appears to be uncorrelated. These results demonstrate clearly (and quantitatively) an important role of the collision cascade density in dynamic radiation defect processes in 3C-SiC.
Understanding response of solids to particle irradiation remains a major materials physics challe... more Understanding response of solids to particle irradiation remains a major materials physics challenge. This applies even to SiC, which is a prototypical nuclear ceramic and wide-band-gap semiconductor material. The lack of predictability is largely related to the complex, dynamic nature of radiation defect formation. Here, we use a novel pulsed-ion-beam method to study dynamic annealing in 4H-SiC ion-bombarded in the temperature range of 25-250 °C. We find that, while the defect recombination efficiency shows an expected monotonic increase with increasing temperature, the defect lifetime exhibits a non-monotonic temperature dependence with a maximum at ~100 °C. This finding indicates a change in the dominant defect interaction mechanism at ~100 °C. The understanding of radiation defect dynamics may suggest new paths to designing radiation-resistant materials.
Above room temperature, the buildup of radiation damage in SiC is a dynamic process governed by t... more Above room temperature, the buildup of radiation damage in SiC is a dynamic process governed by the mobility and interaction of ballistically-generated point defects. Here, we study the dynamics of radiation defects in 3C-SiC bombarded at 100 • C with 500 keV Ar ions, with the total ion dose split into a train of equal pulses. Damage-depth profiles are measured by ion channeling for a series of samples irradiated under identical conditions except for different durations of the passive part of the beam cycle. Results reveal an effective defect relaxation time constant of ∼ 3 ms (for second order kinetics) and a dynamic annealing efficiency of ∼ 40% for defects in both Si and C sublattices. This demonstrates a crucial role of dynamic annealing at elevated temperatures and provides evidence of the strong coupling of defect accumulation processes in the two sublattices of 3C-SiC.
Tantalum-based films with tailored composition, density, and electrical resistivity are of intere... more Tantalum-based films with tailored composition, density, and electrical resistivity are of interest for next generation hohlraums for magnetized indirect-drive inertial confinement fusion. Here, we use reactive direct-current magnetron sputtering to deposit tantalum suboxide films with O content in the range of 46–71 at.%. In contrast to a common approach involving varying reactive gas contents, compositional control is achieved kinetically by changing the total chamber pressure and the deposition rate, while keeping the working gas mix of Ar-5%O2 constant. The resultant films are X-ray amorphous with electrical resistivity varying by over seven orders of magnitude. The dominant conduction mechanism changes from metallic to activated tunneling above ∼55 at.% of O, which is characterized by a sharp increase in resistivity and a decrease in the carrier density at low temperatures.
Sputter deposition of B 4 C films with tailored physical properties remains a challenge. Here, we... more Sputter deposition of B 4 C films with tailored physical properties remains a challenge. Here, we systematically study how substrate temperature influences properties of B 4 C films deposited by direct current magnetron sputtering onto planar substrates held at temperatures in the range of 100 − 510 • C. Results show that all films are amorphous stoichiometric B 4 C, with low O content of ∼ 1 at.%. Films deposited onto substrates at 100 • C exhibit high compressive residual stress and decreased mechanical properties. For elevated substrate temperatures in the range of 180 − 510 • C, film mass density, surface roughness, Young's modulus, and hardness are weakly dependent on substrate temperature. However, in this temperature range, an increase in substrate temperature leads to larger residual compressive stress, accompanied by a corresponding reduction in the concentration of nanoscale inhomogeneities. At least for the landing atom ballistics conditions studied here, a substrate temperature in the range of ∼ 185 − 250 • C is optimum for growing films with near-zero intrinsic residual stress. The overall weak substrate temperature dependence of film properties revealed in this work is favorable for the development of a robust deposition process, particularly for the case of deposition onto non-planar substrates where temperature control is often challenging.
The influence of pre-existing lattice disorder on radiation defect dynamics in SiC remains unexpl... more The influence of pre-existing lattice disorder on radiation defect dynamics in SiC remains unexplored. Here, we use a pulsed ion beam method to study dynamic annealing in Ar-ion-bombarded 3C-SiC at 200 • C with different levels of pre-existing lattice disorder. Results reveal a non-monotonic dependence of the defect relaxation time constant on the level of pre-existing disorder, exhibiting a maximum of ∼ 4 ms at a level of relative initial disorder of ∼ 0.4, while crystals without pre-existing damage are characterized by a time constant of ∼ 1.4 ms. These observations demonstrate that radiation defect dynamics in SiC can be controlled by defect engineering.
At moderately elevated temperatures, radiation defects in SiC exhibit pronounced dynamic annealin... more At moderately elevated temperatures, radiation defects in SiC exhibit pronounced dynamic annealing, which remains poorly understood. Here, we study 3C-SiC bombarded at 100 • C with pulsed beams of 500 keV Ar ions. Radiation damage is monitored by a combination of X-ray diffraction, Raman scattering, and ion channeling. Similar damage buildup behavior but with different defect relaxation time constants, ranging from ∼ 1 to ∼ 6 ms, is observed for the different types of lattice defects probed by these techniques. A correlation between relaxation times and the nature of the defects is proposed. These results reveal additional complexity of radiation defect dynamics in SiC and demonstrate that results of different defect characterization techniques are needed for a better understanding of dynamic annealing processes in solids.
Gallium arsenide under ion bombardment at room temperature and above exhibits pronounced dynamic ... more Gallium arsenide under ion bombardment at room temperature and above exhibits pronounced dynamic annealing that remains poorly understood. Here, we use a pulsed beam method to study radiation defect dynamics in GaAs in the temperature range of 20-100 C irradiated with 500 keV Xe ions. Results show that, with increasing temperature, the defect relaxation time constant monotonically decreases from $5.2 to $0.4 ms. A change in the dominant dynamic annealing process occurs at a critical temperature of $60 C, as evidenced by a change in the activation energy. A comparison with the other semiconductors studied by the pulsed beam method (Si, Ge, and 4H-SiC) reveals that both the high-temperature activation energy and the temperature below which dynamic annealing becomes negligible scale with the melting point.
Effects of the collision cascade density on radiation damage in SiC remain poorly understood. Her... more Effects of the collision cascade density on radiation damage in SiC remain poorly understood. Here, we study damage buildup and defect interaction dynamics in 3C-SiC bombarded at 100 °C with either continuous or pulsed beams of 500 keV Ne, Ar, Kr, or Xe ions. We find that bombardment with heavier ions, which create denser collision cascades, results in a decrease in the dynamic annealing efficiency and an increase in both the amorphization cross-section constant and the time constant of dynamic annealing. The cascade density behavior of these parameters is non-linear and appears to be uncorrelated. These results demonstrate clearly (and quantitatively) an important role of the collision cascade density in dynamic radiation defect processes in 3C-SiC.
Understanding response of solids to particle irradiation remains a major materials physics challe... more Understanding response of solids to particle irradiation remains a major materials physics challenge. This applies even to SiC, which is a prototypical nuclear ceramic and wide-band-gap semiconductor material. The lack of predictability is largely related to the complex, dynamic nature of radiation defect formation. Here, we use a novel pulsed-ion-beam method to study dynamic annealing in 4H-SiC ion-bombarded in the temperature range of 25-250 °C. We find that, while the defect recombination efficiency shows an expected monotonic increase with increasing temperature, the defect lifetime exhibits a non-monotonic temperature dependence with a maximum at ~100 °C. This finding indicates a change in the dominant defect interaction mechanism at ~100 °C. The understanding of radiation defect dynamics may suggest new paths to designing radiation-resistant materials.
Above room temperature, the buildup of radiation damage in SiC is a dynamic process governed by t... more Above room temperature, the buildup of radiation damage in SiC is a dynamic process governed by the mobility and interaction of ballistically-generated point defects. Here, we study the dynamics of radiation defects in 3C-SiC bombarded at 100 • C with 500 keV Ar ions, with the total ion dose split into a train of equal pulses. Damage-depth profiles are measured by ion channeling for a series of samples irradiated under identical conditions except for different durations of the passive part of the beam cycle. Results reveal an effective defect relaxation time constant of ∼ 3 ms (for second order kinetics) and a dynamic annealing efficiency of ∼ 40% for defects in both Si and C sublattices. This demonstrates a crucial role of dynamic annealing at elevated temperatures and provides evidence of the strong coupling of defect accumulation processes in the two sublattices of 3C-SiC.
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Papers by Bayu Aji