Effects of implantation defects on the carrier concentration of 6H-SiC

The effects of ion irradiation defects on the carrier concentration of 6H-SiC epitaxial layer were studied by current–voltage (I–V), capacitance.-voltage (C–V) measurements, thermally stimulated capacitance and deep level transient spectroscopy. The defects were produced by irradiation with 10 MeV C⁺ at a fluence of 10¹¹ ions/cm² and subsequent thermal annealings were carried out in the temperature range 500–1700 K under N₂ flux. I–V and C–V measurements reveal the presence of a high defect concentration after irradiation and annealing at temperature lower than 1000 K. Thermally stimulated capacitance measurements show that some of the defects induce a deactivation of the nitrogen donor, while some of the generated defects, behaving as donor-like traps, contribute to increase the material free carrier concentration at temperatures above their freezing point. Deep level transient spectroscopy measurements performed in the temperature range 150–450 K show the presence of several overlapping traps after ion irradiation and annealing at 1000 K: these traps suffer a recovery and a transformation at higher temperatures. The annealing of all traps at temperatures as high as 1700 K allows one to completely restore the n-type conductivity. The defects mainly responsible of the observed change in the carrier concentration are identified. PACS 73.30.+y; 61.80.Jh; 61.82.Fk; 85.30.Hi     

Fluoride-doped MWCNT/Si3N4 composite with improved mechanical and structural properties

The addition of carbon nanotubes (CNT) in ceramic composites has stimulated a substantial interest due to their high mechanical, thermal and electrical properties. This approach used fluoride additives (AlF₃ and MgF₂) to prepare multi-walled carbon nanotubes/silicon nitride (MWCNT/Si₃N₄) composite densified at 1700 °C for 1 h by hot press (HP) sintering. The microstructural analyses of MWCNT/Si₃N₄ composites indicate that the fluoride additives have substantially improved densification and the transformation of α-Si₃N₄ to β-Si₃N₄. As observed, the mechanical properties, i.e. flexural strength, fracture toughness, Young's modulus and hardness of MWCNT/Si₃N₄ composites are improved with an increasing concentration of MWCNT. These results attributed to the highly dense composites, strong interfacial interaction and the pull-out mechanism of MWCNT and β-Si₃N₄. The maximum values of fracture toughness flexural strength, Young's modulus, and hardness were 12.76 ± 1.15 MPa.m^0.5, 883 ±46 MPa, 260 ±9 GPa, and 26.4 ± 1.3 GPa, respectively. The improved mechanical properties also ascribed to the synergistic strengthening and toughening influence of MWCNT and β-Si₃N₄.     

Dielectric response of makrofol-KG polycarbonate irradiated with 145 MeV Ne6+ and 100 MeV Si8+ ions

The passage of heavy ions in a track detector polymeric material produces lattice deformations. These deformations may be in the form of latent tracks or may vanish by self annealing in time. Heavy ion irradiation produces modifications in polymers in their relevant electrical, chemical and optical properties in the form of rearrangement of bonding, cross-linking, chain scission, formation of carbon rich clusters and changes in dielectric properties etc. Modification depends on the ion, its energy and fluence and the polymeric material. In the present work, a study of the dielectric response of pristine and heavy ion irradiated Makrofol-KG polycarbonate is carried out. 40 μm thick Makrofol-KG polycarbonate films were irradiated to various fluences with Si⁸⁺ ions of 100 MeV energy from Pelletron at Inter University Accelerator Centre (IUAC), New Delhi and Ne⁶⁺ ions of 145 MeV from Variable Energy Cyclotron Centre, Kolkata. On irradiation with heavy ions dielectric constant (ɛ′) decreases with frequency where ɛ′ increases with fluence for both the ions. Variation of loss factor (tan δ) with frequency for pristine and irradiated with Si ions reveals that tan δ increases as the frequency increases. Tan δ also increases with fluence. While Ne irradiated samples tan δ shows slight variation with frequency as well as with fluence. Tan δ has positive values indicating the dominance of inductive behavior.      

Predictions of pressure-induced structural transition, mechanical and thermodynamic properties of α-and β-Si3N4 ceramics: ab initio and quasi-harmonic Debye modeling

The plane-wave pseudo-potential method within the framework of ab initio technique is used to investigate the structural and elastic properties of α-and β-Si3N4.The ground-state parameters accord quite well with the experimental data.Our calculation reveals that α-Si3N4 can retain its stability to at least 40 GPa when compressed at 300 K.The α→β phase transformation would not occur in a pressure range of 0-40 GPa and a temperature range of 0-300 K.Actually,the α→β transition occurs at 1600 K and 7.98 GPa.For α-and β-Si3N4,the c axes are slightly more incompressible than the a axes.We conclude that β-Si3N4 is a hard material and ductile in nature.On the other hand,β-Si3N4 is also found to be an ionic material and can retain its mechanical stability in a pressure range of 0-10 GPa.Besides,the thermodynamic properties such as entropy,heat capacity,and Debye temperature of α-and β-Si3N4 are determined at various temperatures and pressures.Significant features in these properties are observed at high temperature.The calculated results are in good agreement with available experimental data and previous theoretical values.Many fundamental solid-state properties are reported at high pressure and high temperature.Therefore,our results may provide useful information for theoretical and experimental investigations of the Si3N4 polymorphs.     

Effects of high-energy ion-beam irradiation on structural and optical properties of (Mg0.95Co0.05)TiO3 thin films

We report the structural and optical properties of high-energy ion-beam irradiated Co-doped magnesium titanate thin films. (Mg_0.95Co_0.05)TiO₃ (MCT) thin films were deposited on quartz substrates using radio frequency magnetron sputtering. Subsequently, the films were annealed for crystallinity and were irradiated with 100?MeV Ag ions by varying the ion fluence. The X-ray diffraction patterns of the films before and after the irradiation were refined using the Rietveld refinement and the variations in the lattice parameters were correlated with the ion fluence. Although, annealing of thin films results in an enhancement in refractive index and optical bandgap, the ion fluence induces significant changes in the refractive index and optical bandgap. Atomic force microscopy is employed to study the surface morphology of the films. The impact of ion fluence on structural and optical properties of MCT thin films has been investigated.     

The studies of high-temperature and short-time annealing,phase transition process,and magnetic property for LaFe11.7Si1.3 compound

The high-temperature phase transition is analyzed according to the DSC of as-cast LaFe_11.7 Si_1.3 compound and the X-ray patterns of LaFe_11.7Si_1.3 compounds prepared by high-temperature and short-time annealing. Large amount of 1:13 phase begins to appear in LaFe_11.7Si_1.3 compound annealed near the melting point of LaFeSi phase (about 1422?K). When the annealing temperature is close to the temperature of peritectic reaction (about 1497?K), the speed of 1:13 phase formation is the fastest. The phase relation and microstructure of the LaFe_11.7Si_1.3 compounds annealed at 1523?K (5?h), 1373?K (2?h)?+?1523?K (5?h), and 1523?K (7?h) +1373?K (2?h) show that longer time annealing near peritectic reaction is helpful to decrease the impurity phases. For studying the influence of different high-temperature and short-time annealing on magnetic property, the Curie temperature, thermal, and magnetic hystereses, and the magnetocaloric effect of LaFe_11.7Si_1.3 compound annealed at three different temperatures are also investigated. Three compounds all keep the first order of magnetic transition behavior. The maximal magnetic entropy change ΔS_M (T, H) of the samples is 12.9, 16.04, and 23.8?J?kg^?1?K^?1 under a magnetic field of 0–2?T, respectively.     

Defects mediated diffusion in Pt/Co/Pt multilayers induced by dense electronic excitations

Present study compares the effects of 200 MeV Ag¹⁵⁺ and 100 MeV O⁷⁺ ion irradiations on the structural, interfacial mixing and magnetic properties of annealed Pt/Co/Pt layers fabricated by DC magnetron sputtering. X-ray diffraction analysis shows that ion irradiations coupled with post annealing results in the formation of the face centred tetragonal L1₀ CoPt phase. Irradiation using 200 MeV Ag¹⁵⁺ ions having higher ionizing energy transfer to the film was found to be more efficient in causing structural phase transition as compared with that using 100 MeV energy O⁷⁺ ions having lower ionizing energy transfer at similar fluence. Rutherford back scattering analysis reveals the role of defect mediated inter-atomic diffusion in tailoring the alloy composition of the film irradiated by different energetic ions. A broad magnetic switching field distribution for O⁷⁺ ion irradiated films compared to Ag¹⁵⁺ ion irradiation was evident from the magnetic measurements. The contribution of alloy composition to switching field distribution has been discussed in details. Above results showed that the electronic energy loss and fluence dependent defects, generated by irradiation, played an important role in tuning the structural, atomic diffusion and magnetic reversal properties of Pt/Co/Pt.     

Crystallisation kinetics of amorphous Si–C–N ceramics: Dependence on nitrogen partial pressure

The crystallisation kinetics of amorphous precursor-derived ceramics of composition Si₂₆C₄₁N₃₃ is investigated as a function of temperature and nitrogen partial pressure using X-ray diffractometry. Isothermal annealing at a pressure of 1 bar leads to simultaneous crystallisation of Si₃N₄ and SiC, while only crystalline SiC is formed with annealing at a reduced pressure of 1 mbar. Rate constants of crystallisation are determined using the Johnson–Mehl–Avrami–Kolmogorov (JMAK) formalism. For temperatures below 1700°C, crystallisation rates are significantly higher for annealing at 1 mbar compared to 1 bar. For an explanation of the results, a model is proposed, which is based on diffusion-controlled nucleation and growth of crystalline Si₃N₄ and SiC in an amorphous matrix combined with thermal decomposition of Si₃N₄ at high temperatures.     

Structural and optical properties of Ge nanocrystals obtained by hot ion implantation into SiO2 and further ion irradiation

In this work, SiO₂ layers containing Ge nanocrystals (NCs) obtained by the hot implantation approach were submitted to an ion irradiation process with different 2 MeV Si⁺ ion fluences. We have investigated the photoluminescence (PL) behavior and structural properties of the irradiated samples as well as the features of the PL and structural recovery after an additional thermal treatment. We have shown that even with the highest ion bombardment fluence employed (2×10¹⁵ Si/cm²) there is a residual PL emission (12% from the original) and survival of some Ge NCs is still observed by transmission electron microscopy analysis. Even though the final PL and mean diameter of the nanoparticles under ion irradiation are independent of the implantation temperature or annealing time, the PL and structural recovery of the ion-bombarded samples have a memory effect. We have also observed that the lower the ion bombardment fluence, the less efficient is the PL recovery. We have explained such behavior based on current literature data.     

Ion induced silicide formation in niobium thin films

MeV ⁴He backscattering and x-ray diffraction analysis were used to examine the intermixing of niobium thin films on single crystal silicon during ²⁸Si⁺ ion bombardment. The ambient temperature dependence of the intermixing is reported. The dependence cannot be explained by either radiation-enhanced diffusion or cascade mixing alone. The silicides. NbSi₂ and Nb₅Si₃, were both observed. Silicide growth was found to be proportional to the square root of the fluence for the case in which the ion range exceeds the film thickness.     

The effects of Si3N4 interlayer on the thermal stability and hardness of Ti/TiNx (x = 0.5-1) nanolayered coatings

The polycrystalline Ti/TiN_x multilayer films were deposited by magnetron sputtering, and the as-deposited multilayer coatings were annealed at 500-800 °C for 2-4 h in vacuum. We investigated the effects of annealing temperature and annealing time on the microstructural, interfacial, and mechanical properties of the polycrystalline Ti/TiN_x multilayer films. It was found that the hardness increased with annealing temperature. This hardness enhancement was probably caused by the preferred crystalline orientation TiN(1 1 1). The X-ray reflectivity measurements showed that the layer structure of the coatings could be maintained after annealing at 500 °C and the addition of the Si₃N₄ interlayer to Ti/TiN_x multilayer could improve the thermal stability to 800 °C.     

Mössbauer study of the stability of Fe5Si95 produced by inert gas condensation

Fe₅Si₉₅ specimens with an enhanced solubility of Fe in Si by about twenty-one orders of magnitude relative to crystalline Si at room temperature were prepared by the inert gas condensation technique. The thermal stability of the Fe₅Si₉₅ sample was investigated by Mössbauer spectroscopy. The spectrum of the as-prepared Fe₅Si₉₅ exhibited a broadened many different iron sites in the sample. This material was thermal stable up to temperatures of 673k for one hour. After annealing at 773K for one hour, the intermetallic compound α-FeSi₂ was formed in the annealed sample, probably via the precipitation process. The amount of the α-FeSi₂ phase increased with the annealing temperature. No β-FeSi₂ phase was observes in any of the annealed samples up to an annealing temperature of 1273K For one hour.     

Investigation of SiO2/Si3N4 films prepared on sapphire by r.f. magnetron reactive sputtering

Sapphire is a desired material for infrared-transmitting windows and domes because of its excellent optical and mechanical properties. However, its thermal shock resistance is limited by loss of compressive strength along the c-axis of the crystal with increasing temperature. In this paper, double layer films of SiO₂/Si₃N₄ were prepared on sapphire (α-Al₂O₃) by radio frequency magnetron reactive sputtering in order to increase both transmission and high temperature mechanical performance of infrared windows of sapphire. Composition and structure of each layer of the films were analyzed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. Surface morphology and roughness of coated and uncoated sapphire have been measured using a talysurf. Flexural strengths of sapphire sample uncoated and coated with SiO₂/Si₃N₄ have been studied by 3-point bending tests at different temperatures. The results show that SiO₂/Si₃N₄ films can improve the surface morphology and reduce the surface roughness of sapphire substrate. In addition, the designed SiO₂/Si₃N₄ films can increase the transmission of sapphire in mid-wave infrared and strengthen sapphire at high temperatures. Results for 3-point bending tests indicated that the SiO₂/Si₃N₄ films increased the flexural strength of c-axis sapphire by a factor of about 1.4 at 800 °C.     

Electrically active defect annealing in neutron and in ion-damaged Si

Hall effect and electrical conductivity measurements of defect annealing in 1 ohm-cm n-type and 2 ohm-cm p-type silicon were made following neutron irradiation at ～50°C. Measurements were also made following 400-keV B¹¹ ion implantation into a 100 ohm-cm n-type Si substrate. As the neutron fluence is increased the electrical effects of the damage eventually outweigh those of the chemical dopants, and further changes in the electrical properties become small. Conversely, significant electrical recovery upon annealing begins only when the electrical effects of the remaining damage become comparable to those of the chemical dopants. This condition will occur at higher anneal temperatures for higher fluence irradiations. The neutron fluence dependence of the damage and the annealing is interpreted in terms of the neutron energy per cm³. E, spent in atomic processes divided by the number/cm³, N, of electrically active dopants. When E/N ≤ 0.5 keV the electrical measurements show that the predominant defect annealing occurs below 400°C. However, when E/N > 0.5 keV electrical measurements emphasize the annealing at temperatures > 400°C. After 500°C annealing, energy levels in neutron damaged Si are observed at E_v +0.1 and E_v +0.15 eV in p-type and at E_c -0.33 eV in n-type Si. Application of the E/N criteria to room temperature implant-doped Si predicts that the electrical effects will be dominated by lattice damage even if all the implanted ions are substitutional.     

Structural properties, magnetic order and electronic transport in single crystalline UPt2Si2

We present a detailed comparison of the physical properties of as-cast and annealed single crystalline UPt₂Si₂, a compound whose properties we have shown to be governed by strain disorder on the Pt/Si ligand sites. Contrary to common knowledge, and to our surprise, from our data we do not observe a significant improvement of the physical properties of UPt₂Si₂ upon annealing at 900 °C for one week. We attribute this to the specific way the strain disorder is produced in UPt₂Si₂ by presenting evidence that it results from a first order phase transition at ambient temperatures. We discuss the implications of such phase transitions occurring at comparatively low temperatures for the ground state properties of heavy fermion systems and related correlated electron materials.     

Contemplations on the impressions of MeV swift heavy ion irradiation on nonlinear optical dimethyl-amino-pyridinium-4-nitrophenolate-4-nitro-phenol (DMAPNP) single crystals

Organic nonlinear optical crystal dimethyl-amino-pyridinium-4-nitrophenolate-4-nitro-phenol was subjected to 100 MeV Ag⁸⁺ ions and 50 MeV Si⁸⁺ ions. The radiation effects are studied in terms of processes observed with the pristine samples and in comparison with them. The dielectric properties of the crystals were studied before and after irradiation from 100 Hz to 5 MHz at various temperatures (308–383 K). A drastic increase in the dielectric constant is seen due to irradiation. The dielectric constant and conductivity increases with the increase of irradiation fluence for the samples. The observed results are discussed in detail.     

Tailoring soft and hard magnets by annealing Co-based metallic glass

Co₆₇Fe₄Mo₂Si₁₇B₁₁ metallic glass ribbon has been subjected to the isothermal annealing at temperatures in the range 250–600°C so as to produce a series of samples with gradually coarser microstructure. For this series of samples a giant increase of the coercivity, exceeding five orders of magnitude, is observed. It shows a possibility to tailor soft or hard magnets using the same parent material. An abrupt increase of the coercivity occurs in a relatively small range of annealing temperatures between 480 and 520°C, and is mainly due to a strengthening of the pinning effect of the precipitates (fine crystalline structure) on the domain walls. Samples annealed at higher temperatures become fully crystallized. First, the metastable phase(s) is created which decomposes to the stable phases at still higher temperature. Coercivity for fully crystallized samples shows first a narrow plateau and afterwards a gradual decrease of its value with increasing temperature of annealing. Magnetic and microstructural properties of the samples, annealed at various temperatures, were investigated applying a number of complementary techniques including DSC and TGM methods, X-ray diffraction, TEM, strain-modulated FMR spectroscopy as well as conventional magnetic measurements.     

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe₁₃, URu₂Si₂ and U₆Fe. The magnetic field penetration depth in the superconducting state of UBe₁₃ is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowT _c . A sharp increase in the precession frequency is seen, starting atT _c . This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U₆Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atT _c=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.     

Mechanical alloyed Ho doping in CoFe2O4 spinel ferrite and understanding of magnetic nanodomains

We doped Ho³⁺ in CoFe_1.95Ho_0.05O₄ spinel ferrite by mechanical alloying and subsequent annealing at different temperatures (600-1200 °C). We understood the structural and magnetic properties of the samples using X-ray diffraction, SEM, Thermal analysis (TGA and DTA), and VSM measurement. The samples have shown structural stabilization within cubic spinel phase for the annealing temperature (T_AN)≥800 °C. Thermal activated grain growth kinetics has been accompanied with the substantial decrease in lattice strain. The gain size dependent magnetism is evident from the variation of magnetic moment, remanent magnetization and coercivity of the material. The paramagnetic to ferrimagnetic transition temperature T_C (∼805 K) seems to be grain size independent in the present material. The magnetic nanograins, either single domain/pseudo-single domain (50-64 nm) or multi-domain (above 64 nm) regime, showed superparamagnetic blocking below T_m, which is below T_C (805 K) and also well above the room temperature.     

Effects of low temperature irradiation with heavy ions on superconductor niobium

To study the effects of heavy ion irradiation at low temperature on type II superconductor Nb, the transition temperatureT _c , the normal state residual resistivityρ _B , the transition widthΔT _ph using oxygen ions of 25 MeV and subsequent thermal annealing were measured. The samples were held at temperatures <20 K during irradiation in a cryostat for in situ measurements. The maximum oxygen fluence was about 2·10¹⁵ cm^?2 corresponding a relatively high defect concentration. The heavy ion irradiation experiments are described. The critical temperatureT _c decreases with increasing residual resistivityρ _B . In agreement with the theory and experiments, the gap anisotropy parameter is 〈a ²〉=0.008, subsequent annealing shows a hysteresis ofT _c versusρ _B . The resistivity saturation value Δρ_BS = 2.55 μΩ cm was obtained and different recovery stages were found. Significant broadening of transition width during irradiation was observed.T _c andΔT _ph anneal to 60% in the temperature interval of (60–90) K. Oxygen induced effects as a simulation method of high neutron damage are compared with irradiation measurements using neutrons and deuterons.