Ion-implanted Oxide Research Articles

Room-temperature ferromagnetism in boron-doped oxides: a combined first-principle and experimental study

ABSTRACTThe ferromagnetic properties of boron-doped oxides, namely ZnO, MgO, CdO and TiO2 have been studied using first-principle calculations based on density functional theory. A boron atom, when doped at an anion site, generates a magnetic moment per dopant atom ranging from ∼1 µB in TiO2 to ∼3 µB in ZnO. The source of magnetism in the boron-doped oxides is the 2p-orbital electrons of the dopant atom plus a small contribution coming from the oxygen atoms surrounding the dopant. A spin–spin interaction study reveals that the doping of B at O sites favours ferromagnetic spin coupling in ZnO, MgO, CdO and TiO2. Boron-doped samples have been prepared by ion-beam irradiation with a 10 keV B+ beam implanting on sample pallets. Magnetic measurements have been carried out for B+-ion-implanted oxide samples using a SQUID magnetometer. A stable room-temperature ferromagnetic ordering is observed in boron-ion-irradiated ZnO, MgO, CdO and TiO2 samples.

Charge trapping phenomena in high-efficiency metal-oxide-silicon light-emitting diodes with ion-implanted oxide

This work is a comparative study of the processes of charge trapping in silicon dioxide layers doped with different rare-earth (RE) impurities (Gd, Tb, Er) as well as with Ge. Diode SiO 2–Si structures incorporating such oxide layers exhibit efficient electroluminescence (EL) in the spectral range of UV to IR. Ion implantation was performed over a wide dose range with the implant profiles peaking in the middle of the oxide. Charge trapping was studied using an electron injection technique in constant current regime with simultaneous measurements of the EL intensity (ELI). High-frequency C/ V characteristics were used to monitor the net charge in the oxides. Analysis of the charge trapping and the variation of the EL intensity during electron injection shows that the current density range can be divided in three portions: (i) low injection level, where electron/hole capture at traps with large capture cross-sections and low ELI occurs; (ii) medium injection level corresponding to the main operation mode of the devices (odd hole trapping depending on the injected current level is observed); and (iii) high injection level (electrical quenching of the EL that correlates with electron capture at traps of extremely small capture cross-sections takes place). The nature of specific hole trapping at the medium injection level in RE-doped devices is discussed. Mechanisms of EL quenching at the high injection level are proposed.

Characteristics of low-energy nitrogen ion-implanted oxide and NO-annealed gate dielectrics

In this work we assess the effect of low-energy nitrogen implantation for dual-gate oxide thickness formation. A comparison of transistor performance, 1/ f noise and oxide reliability is made between 3.5 nm thick oxides fabricated by nitrogen ion implantation followed by dry oxidation and a reference oxide grown by dry oxidation followed by annealing in nitric oxide (NO oxide). The results show that the ion-implanted samples have lower oxide charges, lower 1/ f noise, good reliability and sufficient boron penetration immunity.

Carbon diffusion and nanocrystalline diamond formation in carbon ion-implanted oxides studied by nuclear elastic scattering

We have shown that MeV implantation of carbon into fused quartz and sapphire followed by thermal annealing in a suitable environment can result in the formation of diamond. Using cross-sectional transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS), we determined (in a previous paper) that, following annealing, there was a redistribution of carbon from the original implantation depth, depending on the annealing environment, annealing time and annealing temperature. In our search, for the optimum implantation and annealing parameters to maximize the yield of diamond, we have used backscattering spectrometry (BS), with MeV hydrogen, to profile the implanted carbon, taking advantage of the large C(p,p)C scattering cross-section at around 1.73 MeV. We studied samples of fused quartz and sapphire implanted with carbon to a range of doses and annealed in forming gas, oxygen and argon. We show that in an oxygen environment, there is significant carbon loss in fused quartz but not in sapphire while in the other environments no significant loss is reported. We conclude that redistribution of carbon, the formation of nanocrystalline diamond (as seen in cross-sectional TEM) and possible carbon loss is determined both by the mobility of carbon in the host matrix at the prevailing annealing temperatures and, most importantly, the annealing ambient.

Radiation dose measurment using MOSFETs

The MOS transistor as a radiation dose detector has been presented. MOS transistors present advantages such as low cost, small volume and weight, robustness, accuracy, large measurable dose range, and sensitivity to low-energy radiation (10 keV). They are useful in real-time measurements or post-irradiation read-out, while they retain information after reading. The sensitivity of unbiased MOSFETs has been improved, and further improvement is possible by increasing the oxide thickness via dual dielectrics or by using ion-implanted oxides and stacked MOSFET configurations. The stacked-transistor configuration is a very promising solution to reach the mRad range (personnel dosimetry). MOSFETs are already used in various application fields with increasing interest for use in specific cases of in-vivo dosimetry.

Radiation effects on ion-implanted silicon-dioxide films

The effects of radiation on SiO/sub 2/ films implanted with high ion doses are studied. The interface-state buildup is suppressed by the ion-implanted oxide. The amount of suppression depends on the ion dose, the gate bias during irradiation, and the annealing atmosphere. A radiation-hardening technique for field oxide is proposed, using the ion-implanted oxide. Radiation-induced interface-state density is suppressed by one order of magnitude using arsenic implantation. By applying this technique to a conventional bipolar process, current gain reduction is suppressed to within 10% after 10/sup 6/ rad(SiO/sub 2/) irradiation. >

Electrocatalytic properties of ion-implanted oxide films

Ion implantation is tested as a technique to modify TiO 2 films on titanium and to form electrocatalytically interesting systems. Implantation of the while film with Pd + and Ru + creates states within the band gap (radiation damage and doping) and surface states. Implantation of Xe + through the whole oxide yields only the effects of radiation damage. Outer-sphere electron transfer reactions (ETRs) give information about the electronic behaviour of the film. The electron transfer current of the redox couple Ce 3+/4+ is enhanced at the implanted electrodes. The electron transfer rate of platinum, however, is not reached. The values for the activation energy and the rate constant of the oxidation of Ce 3+ at Pd +-implanted TiO 2 film indicate rate-determining tunneling processes via the palladium states. On the cathodic side, the results show the large contribution of tunneling processes at the implanted electrodes. The electrocatalytical activity of modified layers is tested with technically most interesting innersphere ETR: Cl 2-evolution and O 2 reduction. TiO 2 implanted with a metal shows that the adsorption of O 2 or O 2 − at the surface is important. This indicates the influence of surface states. The Cl 2 evolution at Ru +-implanted TiO 2 is very slow and corrosion of the implanted metal takes place. This behaviour is compared with the results obtained from electrodes prepared by other techniques.

Structural, electrical and catalytic properties of ion-implanted oxides

The potential application of ion implantation to modify the surfaces of ceramic materials is discussed. Changes in the chemical composition and microstructure result in important variations of the electrical and catalytic properties of oxides.

Electrochemical investigations of ion-implanted oxide films

Oxide films (passive films) of 40–50 nm thickness were prepared by anodic polarization of hafnium and titanium electrodes up to 20 V. Multiple-energy ion implantation of palladium, iron and xenon was used in order to obtain modified films with constant concentration profiles of the implanted ions. Rutherford backscattering, X-ray photoelectron spectroscopy measurements and electrochemical charging curves prove the presence of implanted ions, but electrochemical and photoelectrochemical measurements indicate that the dominating effect of ion implantation is the disordering of the oxide film. The capacity of hafnium electrodes increases as a result of an increase in the dielectric constant D. For titanium the Schottky-Mott analysis shows that ion implantation causes an increase in D and the donor concentration N. Additional electronic states in the band gap which are created by the implantation improve the conductivity of the semiconducting or insulating films. This is seen in the enhancement of electron transfer reactions and its disappearance during repassivation and annealing. Energy changes in the band gap are derived from photoelectrochemical measurements; the absorption edge of hafnium oxide films decreases by approximately 2 e V because of ion implantation, but it stays almost constant for titanium oxide films. All changes in electrochemical behavior caused by ion implantation show little variation with the nature of the implanted ion. Hence the dominating effect seems to be a disordering of the oxide.

Formation of subsurface Al 2O 3 layers in aluminum by oxygen ion implantation

The use of high-dose oxygen ion implants to create buried, insulating SiO 2 layers in silicon has been reported by many groups. In contrast, only a few groups have studied ion-implanted oxide layers on and in aluminum films. We have investigated the formation of subsurface Al 2O 3 layers in bulk, polycrystalline aluminum. In particular, we implanted (1–16) × 10 17 atoms cm −2 using low current densities of 180 keV O 2+ near room temperature. Depth distributions of oxygen and aluminum were determined using both Auger-electron spectroscopy combined with argon-ion sputtering and helium-ion backscattering. For oxygen fluences greater than about 8 × 10 17 atoms cm −2 our analyses revealed subsurface layers of Al 2O 3 were formed with thicknesses that increased with implanted dose. We combined our results with those from the aluminum film literature to propose a formation process.

Radiation-Induced Charge Transport and Charge Buildup in SiO2 Films at Low Temperatures

Studies of the temporal, temperature, and electricfield dependences of radiation-induced charge transport have been performed for radiation-hardened SiO2 films. At room temperature for high applied fields, nearly all electrons and holes generated in the oxide by a pulse of ionizing radiation (5-keV electrons) drift to the interfaces, whereas at low temperatures only electrons contribute to observed transport for relatively low fields. Below ~130°K at high fields, field-induced emission of trapped holes occurs, giving rise to collection within seconds of a significant fraction of the total number of holes generated. The present hole transport data are accounted for quite well in terms of a multiple-trapping model with a spread in trap levels ranging from ~0.3 to ~0.5 eV from the valence band. Comparison with the stochastic hopping transport model is made and that model is found to be less satisfactory in explaining these data. Charge buildup was examined in a Co60 environment and it is demonstrated that oxides exhibiting radiation tolerance at room temperature display severe radiation-induced changes at 77°K. It is also demonstrated that low-temperature charge buildup problems can be alleviated either by employing an ion-implanted oxide or by applying a relatively high field to the oxide during irradiation.