Ferromagnetic modification of ZnO film by Fe ions implantation

The ZnO-based diluted magnetic semiconductors (DMSs) were achieved by ion implantation. Eighty kilo-electron-volt Fe⁺ ions were implanted into n-type ZnO films at room temperature with doses ranging from 1 × 10¹⁶ cm⁻² to 8 × 10¹⁶ cm⁻² and subsequently annealed at 700 °C for 1 h in air ambient. PIXE was employed to determine the Fe-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters were detected within the sensitivity of XRD. Raman spectrum measurement showed that the Fe ions incorporated into the crystal lattice positions of ZnO through substitution of Zn atoms. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The relationships between the Fe-implanted content and the ferromagnetic property are discussed.     

Cross-section for D(p,p)D elastic scattering from 1.8 to 3.2 MeV at the laboratory angles of 155° and 165°

The D(p,p)D cross-sections for elastic scattering of proton on deuterium over incident proton energy range from 1.8 to 3.2 MeV at both laboratory angles of 155° and 165° were measured. A thin solid state target Ni/TiD_x/Ta/Al used for cross-section measurement was fabricated by firstly depositing layers of Ta, Ti and Ni film on the Al foil substrate of about 7 μm in turn using magnetron sputtering and then deuterating under the deuterium atmosphere. The areal density of metal element in each layer of film was measured with RBS analysis by using a 4.0 MeV ⁴He ion beam, while the areal density of the deuterium absorbed in the Ti film was measured with ERD analysis by using a 6.0 MeV ¹⁶O ion beam. The results show that the cross-sections of p-D scattering under this experimental circumstance were much enhanced over the Rutherford cross-section value. It was found that the enhancement increases linearly as the energy of the incident beam increases. The total uncertainty in the measurements was less than 7.5%.     

Ferroelectricity in Li-implanted ZnO thin films

ZnO:Li thin films were prepared by implantation of ZnO with a Li ion fluence 5 × 10¹⁶ ions/cm² at implantation energies of 50, 100, 200 keV. Ferroelectric characterization of the implanted samples revealed a clear hysteresis in the polarization-field curves. The origin of the ferroelectricity can be attributed to an off-center dipole caused by the large difference in ionic radii between the host Zn (0.74 Å) and the dopant Li (0.60 Å). ZnO:Li films which were implanted at 200 keV and annealed at a temperature of 700 ^°C exhibited a well-defined polarization hysteresis loop, with a remanent polarization of 0.8 μC/cm² and coercive field of 8.2 kV/cm, at room temperature. The dielectric phase transition was observed in the temperature range from 340 to 360 K. It is concluded that this novel ferroelectric phase transition in ZnO:Li results from the small structural distortion induced along the c-axis.     

Depth profiling of Mg by 2010 keV resonance of Mg(p,p′γ)Mg nuclear reaction

Studies on the characteristics of 2010 keV resonance in ²⁴Mg(p,p′γ)²⁴Mg nuclear reaction for depth profiling Mg in thin films are reported. The resonance reaction, based on the detection of characteristic 1368 keV γ-rays, enables interference free measurement of Mg down to 2 × 10²⁰ atoms/cm³ and has a probing depth of about 20 μm. The width of the resonance extracted from excitation curves for thick (>180 nm) thermally grown elemental Mg films, by SPACES is about 350 ± 50 eV. The reaction has been used to depth profile Mg in a Mg/Ti/Mg/Si film which provides interesting information on interfacial mixing involving Ti layer and the underlying Mg layer.     

Xenon-ion-induced and thermal mixing of Co/Si bilayers and their interplay

Studies on ion-irradiated transition-metal/silicon bilayers demonstrate that interface mixing and silicide phase formation depend sensitively on the ion and film parameters, including the structure of the metal/Si interface. Thin Co layers e-gun evaporated to a thickness of 50 nm on Si(1 0 0) wafers were bombarded at room temperature with 400-keV Xe⁺ ions at fluences of up to 3 × 10¹⁶ cm⁻². We used either crystalline or pre-amorphized Si wafers the latter ones prepared by 1.0-keV Ar-ion implantation. The as-deposited or Xe-ion-irradiated samples were then isochronally annealed at temperatures up to 700 °C. Changes of the bilayer structures induced by ion irradiation and/or annealing were investigated with RBS, XRD and HRTEM. The mixing rate for the Co/c-Si couples, Δσ²/Φ = 3.0(4) nm⁴, is higher than the value expected for ballistic mixing and about half the value typical for spike mixing. Mixing of pre-amorphized Si is much weaker relative to crystalline Si wafers, contrary to previous results obtained for Fe/Si bilayers. Annealing of irradiated samples produces very similar interdiffusion and phase formation patterns above 400 °C as in the non-irradiated Co/Si bilayers: the phase evolution follows the sequence Co₂Si → CoSi → CoSi₂.     

Raman investigation of incident N-, Xe-ions induced effects in ZnO thin films

Highly c-axis orientation ZnO thin films with hundreds nanometers in thickness have been deposited on (1 0 0) Si substrate by RF magnetron sputtering. These films are implanted at room temperature by 80 keV N-ions with fluences from 5.0 × 10¹⁴ to 1.0 × 10¹⁷ ions/cm², implanted by 400 keV Xe-ions with 2.0 × 10¹⁴ to 2.0 × 10¹⁶ ions/cm², irradiated by 3.64 MeV Xe-ions with 1.0 × 10¹² to 1.0 × 10¹⁵ ions/cm², or irradiated by 308 MeV Xe-ions with 1.0 × 10¹² to 5.0 × 10¹⁴ ions/cm², respectively. Then the ZnO films are investigated using a Raman spectroscopy. The obtained Raman spectra show that a new Raman peak located at about 578 cm⁻¹ relating to simple defects or disorder phase appears in all ZnO films after ion implantation/irradiation, a new Raman peak at about 275 cm^-1 owing to N-activated zinc-like vibrations is observed in the N-implanted samples. Moreover, a new Raman peak at about 475 cm⁻¹ is only seen in the samples after 400 keV and 3.64 MeV Xe-ions bombardment. The area intensity of these peaks increases with increasing ion fluence. The effects of ion fluence, element chemical activity, atom displacements induced by nuclear collisions as well as energy deposition on the damage process of ZnO films under ion implantation/irradiation are discussed briefly.     

Mixing of Cr and Si atoms induced by noble gas ions irradiation of Cr/Si bilayers

Cr/Si bilayers were irradiated at room temperature with 120 keV Ar, 140 keV Kr and 350 keV Xe ions to fluences ranging from 10¹⁵ to 2 × 10¹⁶ ions/cm². The thickness of Cr layer evaporated on Si substrate was about 400 Å. Rutherford backscattering spectrometry (RBS) was used to investigate the atomic mixing induced at the Cr-Si interface as function of the incident ion mass and fluence. We observed that for the samples irradiated with Ar ions, RBS yields from both Cr layer and Si substrate are the same as before the irradiation. There is no mixing of Cr and Si atoms, even at the fluence of 2 × 10¹⁶ ions/cm². For the samples irradiated with Kr ions, a slight broadening of the Cr and Si interfacial edges was produced from the fluence of 5 × 10¹⁵ ions/cm². The broadening of the Cr and Si interfacial edges is more pronounced with Xe ions particularly to the fluence of 10¹⁶ ions/cm². The interface broadening was found to depend linearly on the ion fluence and suggests that the mixing is like a diffusion controlled process. The experimental mixing rates were determined and compared with values predicted by ballistic and thermal spike models. Our experimental data were well reproduced by the thermal spikes model.     

Sub-surface retention of Pb atoms in silicon after low-energy ion implantation and electron beam annealing

Low-energy Pb ion implantation into (1 0 0) Si and subsequent high-vacuum electron beam annealing was used to study the potential of sub-surface retention of Pb atoms after applying a high temperature annealing process. 7 keV Pb⁺ ions were implanted into p-type (1 0 0) Si at room temperature with a fluence of 4 × 10¹⁵ ions cm⁻². The implantation results in a Pb depth distribution that has a calculated Pb peak concentration of 23.9 at.% at a depth of 8.0 nm. The Pb implanted Si substrates were annealed with a high-current 20 keV electron beam at 200-700 °C for 15 s. The Pb loss by out-diffusion was measured with RBS. Key results are: (i) minimal Pb loss in samples annealed up to 400 °C, (ii) emerging out-diffusion above 400 °C, (iii) retention of Pb atoms in the near-surface region in samples annealed up to 700 °C. Comparison of the RBS data with the calculated evaporation rate of Pb under similar conditions reveals two distinctive temperature ranges in which the measured Pb loss of the implanted samples disagrees with the calculated Pb loss: (1) Pb atoms diffused out of the samples at a higher rate in the temperature range up to 400 °C and (2) the Pb atoms diffused out of the samples at a much slower rate above 450 °C. Both phenomena are attributed to the ion implantation process.     

Growth, characterization and effect of substrate bias on ZnO prepared by reactive capillaritron ion beam sputtering deposition

ZnO thin films have been deposited on Si substrates by reactive ion beam sputtering deposition utilizing a capillaritron ion source. All the as-deposited ZnO films exhibit a preferred (0 0 2) growth direction while the grain size increases as oxygen partial flow rate increases. An optimum photoluminescence result was achieved using oxygen partial flow rate of ∼40% that the ratio of integrated deep level emission to that of integrated near-band-edge emission is less than 1.5. The atomic percentage ratio of Zn:O remains at 55/45, regardless of oxygen partial flow rates. With the applied substrate bias, the atomic percentage ratio of Zn/O changes to 40/60, indicating that stoichiometric ZnO may be achieved by selecting appropriate substrate bias. With the substrate bias applied, increased amount of oxygen atoms were found located in oxygen deficient matrixes, likely due to the bombardment of secondary ions.     

Nanoscratch study of ZnO thin films deposited using radio frequency magnetron sputtering

In this study, a radio frequency magnetron sputtering system was used to deposit zinc oxide (ZnO) thin films onto langasite substrates. The thickness of the ZnO film increased from 0.3 to 1.2 μm upon increasing the deposition power from 100 to 200 W. The predominant growth orientation was along the c-axis (0 0 2); the intensities of the signals in the X-ray diffraction spectrum increased significantly upon increasing the film thickness. Scanning electron microscopy images revealed columnar structures in the ZnO films and the morphology of ZnO grains is found to be continuous and dense. It is attributed that oxygen chemisorbs on the target and cases a surface layer of adsorbed oxygen. We suggest that the more neutral ion bombardment on the growing film which induces the higher sputtering rate of the growing film. From in situ imaging of scratched tracks, measurement of the coefficient of friction was an effective means of detecting the occurrence of structural defects in the microstructures. We also found that the chemical compositions of ZnO films prepared under various deposition powers could be investigated using X-ray photoelectron spectroscopy.     

Modelling of deposition processes on the TiO2 rutile (1 1 0) surface

Deposition of Ti_xO_y clusters onto the rutile TiO₂ (1 1 0) surface has been modelled using empirical potential based molecular dynamics. Deposition energies in the range 10-40 eV have been considered so as to model typical deposition energies of magnetron sputtering. Defects formed as a function of both the deposition energy and deposition species have been studied.The results show that in the majority of cases Ti interstitial atoms are formed, irrespective of whether Ti was contained within the deposited cluster. Furthermore that the majority of these interstitials are formed by displacing a surface Ti atom into the interstitial site. O surface atoms are also relatively common, with Ti and TiO₂ surface units often occurring when the deposited cluster contains Ti but becoming less frequent as the deposition energy is increased. Structures that would give rise to the growth of further layers of rutile are not observed and in the majority of the simulations the energy barriers for diffusion of the end-products is high.     

Deposition of polycrystalline Si thin films on glass substrates by direct negative Si ion beam deposition

Polycrystalline Si (Poly-Si) thin films were deposited on a glass substrate by direct negative Si (Si⁻) ion beam deposition. The glass substrate temperature was kept constant at 500 °C for all depositions. Prior to deposition, the ion energy spread and ion-to-atom arrival ratio were evaluated as a function of the ion beam energy.The Si⁻ ion energy spread was less than 10% regardless of the ion energy, while the ion-to-atom arrival ratio increased proportionally from 1.3 to 1.6 according to the ion beam energy.Atomic force microscopy images showed that a relatively rough surface was obtained at 50 eV of Si⁻ ion energy and it is also concluded that the Si⁻ ion beam irradiation at 50 eV is effective to deposit Si thin film with small grains as shown in Fig. 3.     

AFM characterization of model nuclear fuel oxide multilayer structures modified by heavy ion beam irradiation

This work explored a potential new model dispersion fuel form consisting of an actinide material embedded in a radiation tolerant matrix that captures fission products (FPs) and is easily separated chemically as waste from the fuel material. To understand the stability of this proposed dispersion fuel form design, an idealized model system composed of a multilayer film was studied. This system consisted of a tri-layer structure of an MgO layer sandwiched between two HfO₂ layers. HfO₂ served as a surrogate fissile material for UO₂ while MgO represented a stable, fissile product (FP) getter that is easily separated from the fissile material. This type of multilayer film structure allowed us to control the size of and spacing between each layer. The films were grown at room temperature by e-beam deposition on a Si(1 1 1) substrate and post-annealed annealing at a range of temperatures to crystallize the HfO₂ layers. The 550 °C annealed sample was subsequently irradiated with 10 MeV Au³⁺ ions at a range of fluences from 5 × 10¹³ to 3.74 × 10¹⁶ ions/cm². Separate single layer constituent films and the substrate were also irradiated at 5 × 10¹⁵ and 8 × 10¹⁴ and 2 × 10¹⁶, respectively. After annealing and irradiation, the samples were characterized using atomic force imaging techniques to determine local changes in microstructure and mechanical properties. All samples annealed above 550 °C cracked. From the AFM results we observed both crack healing and significant modification of the surface at higher fluences.     

Influence of ion irradiation on internal residual stress in DLC films

The dependence of internal residual stress in thin diamond-like carbon films grown on Si substrate by PECVD technique on most important growth parameters, namely RF-power, DC bias voltage and substrate temperature, is described. Results show that compressive stress reaches the highest value of 2.7 GPa at low RF-power and DC bias. Increase of substrate temperature from 250 to 350 °C leads to nonlinear increase of stress value. Inhomogeneity of residual stress along the film surface disappears when film is deposited at temperatures above 275 °C. Post-growth film irradiation by P⁺ and In⁺ ions cause decrease of compressive stress followed by its inversion to tensile. For all ion energy combinations used residual stress changes linearly with normalized fluence up to 0.2 DPA with slope (8.7 ± 1.3) GPa/DPA.     

Enhancement of ZnO thin film transistor performance by high-dose proton irradiation

The object of this study was to improve the performance of ZnO thin film transistors (TFTs) by exposing them to high-dose proton irradiation. A rapid thermal annealing (RTA) process was necessary to improve the interface characteristics between the source-drain electrodes and the channel layer for the high performance of ZnO-TFTs. However, this affected the resistivity of the ZnO channel layer; it was dramatically decreased during the RTA process. As a result, the RTA-treated ZnO-TFTs did not show the proper off-state characteristics. In order to control the electrical properties of the channel layer, we exposed the RTA-treated ZnO-TFTs to 6.1 MeV of proton irradiation beam energy at fluences from 6.7 × 10¹² cm⁻² to 6.5 × 10¹⁴ protons-cm⁻². The resulting resistivity of the ZnO thin film increased after the high-dose proton irradiation. In addition, we studied the structural and electrical properties and the variations in the native defects of ZnO thin films. The field effective mobility of ZnO-TFTs increased from 1.65 to 4.12 cm²/V s after both the RTA and the high-dose proton irradiation. We obtained an enhancement of ZnO-TFT performance using high-dose proton irradiation.     

Ferromagnetic modification of GaN film by Cu ions implantation

The structural and magnetic properties of Cu⁺ ions-implanted GaN films have been reported. Eighty kiloelectron-volt Cu⁺ ions were implanted into n-type GaN film at room temperature with fluences ranging from 1 × 10¹⁶ to 8 × 10¹⁶ cm⁻² and subsequently annealed at 800 °C for 1 h in N₂ ambient. PIXE was employed to determine the Cu-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters were detected within the sensitivity of XRD. Raman spectrum measurement showed that the Cu ions incorporated into the crystal lattice positions of GaN through substitution of Ga atoms. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The experimental result showed that the ferromagnetic signal strongly increased with Cu-implanted fluence from 1 × 10¹⁶ to 8 × 10¹⁶ cm⁻².     

The range distribution and annealing behavior of Er ions implanted in silicon-on-insulator

The range distribution for energetic 400 keV Er ions implanted in silicon-on-insulator (SOI) at room temperature were measured by means of Rutherford backscattering followed by spectrum analysis. The damage distribution and annealing behavior of implanted Er ions in SOI at the energy of 400 keV with dose of 5 × 10¹⁵ cm⁻² were obtained by Rutherford backscattering technique. It has been found that the damage around the SOI surface had been almost removed after annealed in nitrogen atmosphere at 900 °C, and a lot of Er atoms segregate to the surface of sample with the recrystallization of surface Si of SOI sample after annealing at 900 °C.     

Nuclear reaction analysis of helium migration in silicon carbide

4H-SiC and 6H-SiC single crystals were implanted at room temperature with 3-MeV ³He ions at a fluence of 1 × 10¹⁶ cm⁻². Analysis of helium migration was carried out with the ³He(d, p)⁴He nuclear reaction. No clear thermally-activated migration in the end-of-range (EOR) region is found below 1100 °C, meaning that helium is strongly trapped probably in helium-vacancy clusters. At 1100 °C and above, a fraction of ³He atoms remains trapped in the clusters, but a significant fraction is detrapped into a broad distribution, which is slightly shifted towards the sample surface. Helium detrapping from the EOR region increases with increasing annealing time and temperature. Moreover, the helium content is not conserved, since a significant fraction of ³He atoms is released out of the sample. Helium out-gassing actually increases with increasing annealing time and temperature, up to about 40% at 1150 °C. No clear difference is found between the 4H-SiC and 6H-SiC polytypes.     

Observation of high ferromagnetic ordering in Fe implanted ZnO at room temperature

Room temperature ferromagnetic ordering has been observed in ZnO by implanting 1.63 MeV ⁵⁶Fe⁶⁺ ions. The total number of implanted Fe ions in ZnO is 1 × 10¹⁶, which is effectively 1 at.% doping of Fe in ZnO. The amount of saturation magnetic moment in Fe implanted ZnO, annealed to 500 °C, has been found to be two orders of magnitude more than that observed earlier in chemically prepared Zn_0.98Mn_0.02O and Zn_0.96Mn_0.02Fe_0.02O samples.     

Effects of composition and structure on hydrogen incorporation in tungsten oxide films deposited by sputtering

The effects of composition and structure on hydrogen incorporation in tungsten oxide films were investigated. Films were deposited on carbon and SiO₂ substrates using a reactive sputtering by varying the substrate temperature from 30 to 600 °C in argon and oxygen mixture. The films were characterized using X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA) and Raman scattering. XRD patterns showed amorphous structure in the films deposited below 400 °C and (0 1 0) oriented monoclinic WO₃ in the films deposited beyond 400 °C. The results of RBS and ERDA indicated that hydrogen concentration in the amorphous films increased from 0.1 to 0.7 H/W with changing the composition from WO_0.25 to WO₃. The hydrogen concentration in WO₃ films decreased to 0.4 H/W with increasing the substrate temperature during deposition. The Raman spectra of the WO₃ films revealed that decreasing of W⁶⁺O terminals was related to decreasing of the hydrogen concentration. It was considered that the incorporated hydrogen in tungsten oxide films was bonded at the end of W⁶⁺O terminals.