Low-energy Implantation Research Articles

Continuously Controlled Optical Band Gap in Oxide Semiconductor Thin Films.

The optical band gap of the prototypical semiconducting oxide SnO2 is shown to be continuously controlled through single axis lattice expansion of nanometric films induced by low-energy helium implantation. While traditional epitaxy-induced strain results in Poisson driven multidirectional lattice changes shown to only allow discrete increases in bandgap, we find that a downward shift in the band gap can be linearly dictated as a function of out-of-plane lattice expansion. Our experimental observations closely match density functional theory that demonstrates that uniaxial strain provides a fundamentally different effect on the band structure than traditional epitaxy-induced multiaxes strain effects. Charge density calculations further support these findings and provide evidence that uniaxial strain can be used to drive orbital hybridization inaccessible with traditional strain engineering techniques.

Ferromagnetic order in diamond-like carbon films by Co implantation

We report the observation of ferromagnetic order in diamond-like carbon (DLC) films made by mass selective ion beam deposition and after low energy implantation with Co ions. Different Co fluences were studied with a peak concentration of up to 25% at an average Co implantation depth of 30 nm. The saturation moment per Co atom (0.2–0.3 μB) was found to be strongly dependent on temperature and it was significantly lower than that reported in bulk cobalt or cobalt nanoparticles (1.67 μB per Co atom). The observed magnetic moment cannot be attributed to ferromagnetic nanoparticles as no evidence for superparamagnetism was detected. The magnetic order observed may be due to Co bonding in DLC possibly leading to dilute ferromagnetic semiconductor behaviour with an inhomogeneous distribution of cobalt atoms. Raman spectroscopy measurements showed that Co implantation resulted in an increase in the sp2 clustering with increasing Co fluence. Thus, our results show that Co implantation into DLC films increases the graphitic properties of the film and leads to magnetic order at room temperature.

Cracks and blisters formed close to a silicon wafer surface by He-H co-implantation at low energy

We have studied the effect of reducing the implantation energy towards low keV values on the areal density of He and H atoms stored within populations of blister cavities formed by co-implantation of the same fluence of He then H ions into Si(001) wafers and annealing. Using a variety of experimental techniques, we have measured blister heights and depth from the surface, diameter, areal density of the cracks from which they originate as functions of implantation energy and fluence. We show that there is a direct correlation between the diameters of the cracks and the heights of the associated blisters. This correlation only depends on the implantation energy, i.e., only on the depth at which the cracks are located. Using finite element method modeling, we infer the pressure inside the blister cavities from the elastic deformations they generate, i.e., from the height of the blisters. From this, we demonstrate that the gas pressure within a blister only depends on the diameter of the associated crack and not on its depth position and derive an analytical expression relating these parameters. Relating the pressure inside a blister to the respective concentrations of gas molecules it contains, we deduce the areal densities of He and H atoms contained within the populations of blisters. After low-energy implantations (8 keV He+, 3 keV H+), all the implanted He and H atoms contribute to the formation of the blisters. There is no measurable exo-diffusion of any of the implanted gases, in contrast to what was assumed at the state of the art to explain the failure of the Smart-Cut technology when using very low energy ion implantation for the fabrication of ultra-thin layers. Alternative explanations must be investigated.

Ti/Ni/Au contacts to n-SiC after low energy implantation

The effect of low energy implantation of P or C ions in 3 C-SiC on the properties of Ti/Ni/Au contacts has been examined for doses in the range 1013–1015ions/cm2. Measurements of specific contact resistance, ρc, were performed using the two-contact circular test structure. The magnitude of ρc for the Ti/Ni/Au contacts on unimplanted SiC was 1.29×10−6Ωcm2. The value of ρc increased significantly at an implant dose of 1×1015ions/cm2. The dependence of Rsh and ρc on ion dose has been measured using both C and P implant species.

Improving stress stability in low-pressure chemical vapor deposited silicon dioxide films by ion implantation

The stress state of low-pressure chemical vapor deposition (LPCVD) silicon dioxide (SiO2) films fluctuates upon aging due to the ambient moisture. The related variation of the SiO2 films is a key factor of device reliability. In this study, ion implantation is introduced to improve film stress stability. The stress state modified by ion implantation is discussed in detail and an analytical model is presented. The evolution of the stress in as-deposited and P+-, As+-, and B+-implanted LPCVD SiO2 films upon aging is investigated, and the bonding nature of the films is also studied to provide insight into the physical mechanisms involved. It is demonstrated that low implantation energy slightly modifies the stress in films. In addition, P+ ion is the most appropriate impurity ion to improve film stress stability, and the stress in P+-implanted LPCVD SiO2 film can be stabilized with a low tensile stress of 67MPa when the film is implanted at 15keV with a dose of 1×1015cm−2.

Hydrogen passivation effect on the conversion efficiency of Si solar cells by low-energy proton implantation

We investigated the hydrogen passivation effect on the conversion efficiency of Si solar cells by including low-energy hydrogen ion implantation in the cell fabrication process. The solar cells were implanted with various proton energies and doses just before the co-firing process. The quantum efficiency specifically in the blue wavelengths of the solar cells increased with a low implantation dose and energy indicating improvement of surface passivation. However, quantum efficiency decreased when the implantation dose and energy were increased, indicating a high defect-assisted surface recombination of minority carriers. The cells implanted with a low dose and energy in the presence of SiNx showed interfacial passivation with enhanced photocurrent, minority carrier lifetime and short-circuit current density for improved conversion efficiency.

Electrical properties of ZnO single nanowires

We have investigated the electrical resistance R(T) of ZnO nanowires of ≈ 400 nm diameter as a function of temperature, between 30 K and 300 K, and frequency in the range 40 Hz to 30 MHz. The measurements were done on the as-prepared and after low-energy proton implantation at room temperature. The temperature dependence of the resistance of the wire, before proton implantation, can be well described by two processes in parallel. One process is the fluctuation induced tunneling conductance (FITC) and the other the usual thermally activated process. The existence of a tunneling conductance was also observed in the current–voltage () results, and can be well described by the FITC model. Impedance spectroscopy measurements in the as-prepared state and at room temperature, indicate and support the idea of two contributions of these two transport processes in the nanowires. Electron backscatter diffraction confirms the existence of different crystalline regions. After the implantation of H+ a third thermally activated process is found that can be explained by taking into account the impurity band splitting due to proton implantation.

Damage accumulation during cryogenic and room temperature implantations in strained SiGe alloys

The amorphization by implantation of strained silicon–germanium epitaxial layers is investigated by experiments and simulation according to the germanium content in the film and the implantation conditions. Experimental results are used to calibrate a numerical model based on a combined approach between a Binary Collision Approximation (BCA) module and a kinetic Monte Carlo (kMC) module. The calibration is implemented in the Synopsys Sentaurus Process simulator and a close agreement between simulations and experiments is shown. Experimental results show that by increasing the germanium concentration, amorphous thickness is reduced for high energy implantations. Germanium content seems to have less impact at low energy implantations.

Doping of Graphene by Low-Energy Ion Beam Implantation: Structural, Electronic, and Transport Properties.

We investigate the structural, electronic, and transport properties of substitutional defects in SiC-graphene by means of scanning tunneling microscopy and magnetotransport experiments. Using ion incorporation via ultralow energy ion implantation, the influence of different ion species (boron, nitrogen, and carbon) can directly be compared. While boron and nitrogen atoms lead to an effective doping of the graphene sheet and can reduce or raise the position of the Fermi level, respectively, (12)C(+) carbon ions are used to study possible defect creation by the bombardment. For low-temperature transport, the implantation leads to an increase in resistance and a decrease in mobility in contrast to undoped samples. For undoped samples, we observe in high magnetic fields a positive magnetoresistance that changes to negative for the doped samples, especially for (11)B(+)- and (12)C(+)-ions. We conclude that the conductivity of the graphene sheet is lowered by impurity atoms and especially by lattice defects, because they result in weak localization effects at low temperatures.

Temperature dependent retention characteristics of ion-beam modified SONOS memories

In this work, we report on the structural and electrical properties of oxide–nitride–oxide (ONO) structures that were formed by low-energy silicon or nitrogen implantation into oxide–nitride stacks. In particular ON stacks (2.5nm/6nm) were formed on n-type Si substrates, further implanted with 1keV Si or N to a fluence of 1016 ions/cm2, and finally wet oxidized at 850°C for 15min. TEM imaging showed that the thickness of the blocking oxide layer, formed during wet oxidation, strongly depends on the implanted species. Charging characteristics revealed that Si implanted stacks can trap either electrons or holes resulting to a memory window as large as 8.5V. In turn N implanted stacks can trap only electrons with a corresponding memory window of 4V. Room-temperature charge retention measurements showed that the electron loss rate is faster in samples implanted with Si (∼0.32V/decade) compared to N-samples (∼0.1V/decade). The 10-year extrapolated memory windows were 1.7 and 2.5V for Si and N-implanted devices, respectively. Retention measurements within the temperature range of 25–150°C indicate that the Si implanted stacks exhibit a thermally activated retention, while N-samples showed a temperature independent behavior. These results are mainly attributed to the different nature of traps generated by ion implantation and wet oxidation processing.

Advances in methods to obtain and characterise room temperature magnetic ZnO

We report the existence of magnetic order at room temperature in Li-doped ZnO microwires after low energy H+ implantation. The microwires with diameters between 0.3 and 10 μm were prepared by a carbothermal process. We combine spectroscopy techniques to elucidate the influence of the electronic structure and local environment of Zn, O, and Li and their vacancies on the magnetic response. Ferromagnetism at room temperature is obtained only after implanting H+ in Li-doped ZnO. The overall results indicate that low-energy proton implantation is an effective method to produce the necessary amount of stable Zn vacancies near the Li ions to trigger the magnetic order.

Free Volume Profiles at Polymer–Solid Interfaces Probed by Focused Slow Positron Beam

The properties of materials involving polymer–solid interfaces are determined not only by the properties of the constituents but also by the interfacial region of the polymer near the solid, the so-called interphase. Here we report on positron annihilation lifetime spectroscopy (PALS) measurements of the free volume near the interface with a focused positron beam. The measurements were performed at an interface between a Teflon AF 1600 film and a 30 nm thick SiN membrane from the backside of a Si substrate which was locally etched away to expose the ultrathin SiN membrane. We were able to focus the positron beam onto the exposed region and to probe the SiN–Teflon AF 1600 interfacial region with low implantation energies, resulting in very narrow implantation profiles and excellent depth resolution. We found a pronounced decrease in orthopositronium lifetime close the interface. On the basis of modeling, we could estimate the interphase width and the average density increase in the interfacial region as ∼12 nm and 5–7%, respectively.

Analysis of ROP signaling in the leaf epidermis of mutant tomato with low-energy ion beam.

The importance of the ROP small GTPase signaling pathway in the regulation of cellular polarity growth in eukaryotes has been thoroughly studied. In this study, we examined the LeROP small GTPase (related to Arabidopsis thaliana genome LeROP GTPase in tomato) signaling of cell polarity growth in the mutant (M-1) tomato. Interestingly, we detected expansive growth of epidermis cells in M-1, in which the leaves appeared slightly lobed shaped. However, we observed jigsaw puzzle shaped and deeply lobed shaped leaves in wild-type leaf epidermis cells. The t-test showed significant difference (P < 0.05). Based on previous studies of the AtROP gene in Arabidopsis leaf epidermis cells, we hypothesized that the growth of mutant M-1 tomato leaf epidermis cell is related to AtROP gene signal transmission. The results of reverse transcription-polymerase chain reaction showed the expression of LeROP2, LeROP4, and LeROP7 in M-1 mutants were stronger than in wild-type cells. At the flowering stage, LeROP2 GTPase showed no expression in wild-type cells, but was expressed in mutant cells. This study revealed a link between the low-energy ion beam and the ROP GTPase signaling pathway in tomato. In addition, the ROP gene changes analyzed suggest a new mechanism for mutations following low-energy ion beam implantation.

Simple General Purpose Ion Beam Deceleration System Using a Single Electrode Lens

Ion beam deceleration properties of a newly developed low-energy ion beam implantation system were studied. The objective of this system was to produce general purpose low-energy (5 to 15 keV) implantations with high current beam of hundreds of μA level, providing the most wide implantation area possible and allowing continuously magnetic scanning of the beam over the sample(s). This paper describes the developed system installed in the high-current ion implanter at the Laboratory of Accelerators and Radiation Technologies of the Nuclear and Technological Cam-pus, Sacavem, Portugal (CTN).

Effect of Low Energy Implantation on the Properties of Ti/Ni/Au Contacts to n-SiC

ABSTRACTThe effect of low energy implantation of P or C ions in 3C-SiC on the properties of Ti/Ni/Au contacts has been examined for doses in the range 1013-1015 ions/cm2. Measurements of specific contact resistance, ρc, were performed using the two-contact circular test structure. The magnitude of ρc for the Ti/Ni/Au contacts on unimplanted SiC was 1.29 x 10−6 Ω.cm2. The value of ρc increased significantly at an implant dose of 1 x 1015 ions/cm2. The dependence of ρc on ion dose has been measured using both C and P implant species.

β-detected NMR ofLi8+in Bi, Sb, and the topological insulatorBi0.9Sb0.1

We report the NMR Knight shift and spin-lattice relaxation of $^{8}\mathrm{Li}{}^{+}$ implanted $\ensuremath{\sim}100$ nm into single crystals of semimetallic Sb, Bi, and topologically insulating ${\mathrm{Bi}}_{0.9}{\mathrm{Sb}}_{0.1}$. We find small negative shifts (of order 100 ppm) in all three. In the insulator, the shift is nearly temperature independent, while in Bi and Sb it becomes more negative at low temperature without following the bulk susceptibility, suggesting two distinct temperature dependent contributions, possibly from the orbital and spin response. However, a simple model is unable to account for the observed shift. The spin-lattice relaxation differs in both scale and temperature dependence in all three. It is Korringa-like in Bi and remarkably is fastest in the insulating alloy and slowest in Sb with the highest bulk carrier density. These surprising results call for detailed calculations, but phenomenologically demonstrate that $\ensuremath{\beta}$-detected NMR of implanted $^{8}\mathrm{Li}{}^{+}$ is sensitive to the magnetic response of low-density carriers. The prospects for depth-resolved studies of conventional and topological surface states at lower implantation energies are good.

Defect removal after low temperature annealing of boron implantations by emitter etch-back for silicon solar cells

Ion implantation offers new possibilities for silicon solar cell production, e.g. single side doping that can be structured in-situ with shadow masks. While phosphorus implantations can easily be annealed at low temperature, the annealing of boron implantations is challenging. In this study, we use low energy implantations of boron (1 keV and 5 keV) with a projected range of 5.6 nm and 21.2 nm that form defects causing charge carrier recombination after a low temperature anneal (950 °C, 30 min). An ozone-based wet chemical etching step is applied to remove this near surface damage. With increasing chemical etch-back the electrical quality (i.e. emitter saturation current density, J0e) improves continuously. The calculated limit for J0e was reached with an abrasion of 35 nm for 1 keV and 85 nm for 5 keV implantations, showing that the relevant defects causing charge carrier recombination are located very close to the surface, corresponding to the as-implanted profile depth. This emitter etch-back allows for the fabrication of defect free boron doping profiles with good sheet resistance uniformity (standard deviation 5 × 1019 cm–3, J0e < 30 fA/cm2), these boron profiles are well suited for silicon solar cells. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

On the synthesis of a compound with positive enthalpy of formation: Zinc-blende-like RuN thin films obtained by rf-magnetron sputtering

4d- and 5d-transition metal nitrides are of interest both because of their importance for the understanding of mechanisms of phase formation in systems that under ambient conditions present positive enthalpies of formation and because of their appealing structural and electronic properties. In this study, we report the synthesis of thin films of ruthenium mononitride (RuN) in the zinc-blende structure by radio-frequency-magnetron sputtering. Films present a characteristic structure of packed columns ending with tetrahedral tips. The effect of changing the synthesis parameters was investigated in detail. It was found that RuN can be formed if the nitrogen partial pressure exceeds a minimum value and that the addition of argon has the major effect of increasing the deposition rate because of its higher sputter ability. Temperature plays an important role: if it is too high, decomposition/desorption effects overcome those leading to the formation of the compound. Phenomena resulting in the formation of RuN occur at the surface of the growing films and are related to the interactions of ruthenium with energetic nitrogen ions, or atoms, which can penetrate the first atomic layers by low energy implantation. Because of its properties and structure, this material is a promising candidate for applications like sensing, catalysis, and electrode material for energy-storage devices.

Influence of ion bombardment on the profile of the depth distribution of impurity atoms in Si used for solar cells and diode structures

The chemical composition of the surfaces and surface layers of Si samples used in solar cells are studied using Auger electron spectroscopy (AES). It is shown that the low-energy implantation of Ba and O ions leads to the redistribution of impurity atoms in the surface layer and can prevent impurity atoms adsorbed onto the surface from penetrating inside the sample.

Formation of Shallow n-p Junctions in Cz-Si by Low-Energy Implantation of Carbon Ions

A low-temperature method for shallow n-p junction formation in thin subsurface layers of р-type Cz-Si wafers is presented. Junctions are formed due to the enhanced formation of thermal donor centers by C ion implantation. It is shown that by C ion implantation with doses between 0.6 and 1.0×1014 cm-2 and subsequent annealing at temperatures in the range between 600 and 650 oC, n-p junctions with breakdown voltages of 100 to 150 V and reverse currents below 1×10-10 A at 20 V bias, can be formed. The highest breakdown voltages and lowest leakage currents were obtained for a C ion implantation dose of 1x1014 cm-2 and for annealing at 600 oC.