Low Energy Nitrogen Ion Bombardment Research Articles

Amorphous Carbon Nitride Films: Surface and Subsurface Composition and Bonding.

To obtain quantitative information about the composition and bonding of atoms located at and beyond the analyzed solid surface nondestructively, we applied angle-resolved X-ray photoelectron spectroscopy aided by the maximum entropy method to air-exposed amorphous carbon nitride films deposited by pulsed laser deposition of diamond-like carbon modified by low-energy nitrogen ion bombardment during film growth. We demonstrate that the composition, chemical bonding, and mass density vary significantly from the top surface to a shallow subsurface region. The analyzed samples, in a shallow surface region of ∼1 nm, are composed of oxygen, nitrogen, hydrogen, and mostly carbon in sp2 hybridization. In a deeper region, the C sp3 content increases substantially going to a maximum, whereas the nitrogen percentage decreases to a minimum, then increases, and tends to saturate. Special attention has been paid to in-depth distributions of carbon atoms in trigonal and tetragonal arrangements because they specify numerous physical and chemical properties of carbon-based materials. These results indicate that the interaction of DLC:N surfaces with surroundings can be influenced, barring oxygen and nitrogen, by sp2-bonded carbon atoms located near the surface of the samples. The obtained results can be useful for developing a deeper understanding of the interaction between DLC:N layer surfaces and their surroundings and particularly with living tissue.

Optical absorption enhancement of CdTe nanostructures by low-energy nitrogen ion bombardment

In this paper we present the fabrication of cadmium telluride (CdTe) nanostructures by means of RF magnetron sputtering followed by low-energy ion implantation and post-thermal treatment. We have thoroughly studied the structural, optical, and morphological properties of these nanostructures. The effects of nitrogen ion bombardment on the structural parameters of CdTe nanostructures such as crystal size, microstrain, and dislocation density have been examined. From x-ray diffractometer (XRD) analysis it could be deduced that N+ ion fluence and annealing treatment helps to form (3 0 0) orientation in the crystalline structure of cadmium-telluride films. Fluctuations in optical properties like the optical band gap and absorption coefficient as a function of N+ ion fluences have been observed. The annealing of the sample irradiated by a dose of 1018 ions cm−2 has led to great enhancement in the optical absorption over a wide range of wavelengths with a thickness of 250 nm. The enhanced absorption is significantly higher than the observed value in the original CdTe layer with a thickness of 3 μm. Surface properties such as structure, grain size and roughness are noticeably affected by varying the nitrogen fluences. It is speculated that nitrogen bombardment and post-annealing treatment results in a smaller optical band gap, which in turn leads to higher absorption. Nitrogen bombardment is found to be a promising method to increase efficiency of thin film solar cells.

Band alignment and Schottky behaviour of InN/GaN heterostructure grown by low-temperature low-energy nitrogen ion bombardment

InN/GaN heterostructure based Schottky diodes are fabricated by low energetic nitrogen ions at 300 °C.

Role of Charged Species on the Growth of GaN Films by Modified Activated Reactive Evaporation

We report the role of charged species N2 + on the growth of GaN films by modified activated reactive evaporation MARE . In MARE technique, substrate is subjected to low energy nitrogen ion bombardment by keeping it in conjunction with the radio frequency cathode. The low energy ion irradiation contributes to the effective formation of GaN as well as in the drastic reduction of the oxygen impurity content by resputtering of the weakly bonded oxygen adatoms. Significant reduction in oxygen impurity can be achieved in the MARE by increasing the concentration of the charged species in the plasma. © 2010 The Electrochemical Society. DOI: 10.1149/1.3512991 All rights reserved.

Characterization of Nitrogen-Doped Carbon Nanotubes by Atomic Force Microscopy, X-ray Photoelectron Spectroscopy and X-ray Absorption Near Edge Spectroscopy

In this work, we perform a comparative study on single-walled carbon nanotubes (CNT) before and after low energy nitrogen ion bombardment (70 eV and 25 x 10-6 A/cm2) at room temperature. The morphology and the mechanical properties were studied by Atomic Force Microscopy (AFM). The bonding configuration of the N-doped CNTs was established by X-ray Photoelectron Spectroscopy (XPS) and X-ray Absorption Near Edge Spectroscopy (XANES). Single-walled carbon nanotubes were prepared using non-intrusive methods and deposited onto silicon substrates. For the spectroscopic studies, samples with a high concentration of CNTs were analyzed. XPS reveals different chemical states for carbon related to the incorporation of nitrogen. XANES confirms the hexagonal structure of the CNTs, resembling the bonding structure of hexagonal carbon nitrides. AFM images confirm that the CNTs were not destroyed after low energy N2+. The morphology of the original nanotubes maintains after nitrogen ion bombardment except for the incorporation of some pearl-shaped decoration, probably due to the adsorption of some contaminants or to deposition of re-sputtered material. Whereas CNTs improve their adherence to the substrate, this extra granularity on the CNT is easily removed even with the AFM tip. In conclusion, spectroscopic measurements and mechanical properties made clear information on the changes produced on CNT after nitrogen incorporation.

Low-temperature growth of polycrystalline GaN films using modified activated reactive evaporation

We report the preparation of polycrystalline GaN films on glass substrates by modified activated reactive evaporation (MARE). In this technique, substrates are kept on cathode instead of ground electrode and hence subjected to low-energy nitrogen ion bombardment. With increase in rf power, nitrogen to gallium (N/Ga) ratio in the films and film resistivity monotonically increases whereas oxygen impurity reduces. All GaN films are of wurtzite structure and films grown at higher powers have preferred orientation towards c-axis. Crystalline quality improves with increase in rf power up to ∼150 W and thereafter it degrades. Improvement in crystalline quality can be attributed to Ga/N stoichiometry and reduction in oxygen concentration, whereas degradation can be attributed to the presence of point defects due to excess nitrogen and nitrogen ion bombardment. Optical emission spectroscopy (OES) is used to investigate the relative concentration of excited species during GaN growth. MARE offers a technique to grow low-temperature group III nitride semiconductors. A high deposition rate (4.3 μm/h) was achieved in MARE for growing polycrystalline films on relatively inexpensive substrates. MARE is relatively less complex and offers a viable alternative for large scale growth for polycrystalline GaN thin films.

Growth of InN thin films by modified activated reactive evaporation

Indium nitride films have been grown using modified activated reactive evaporation (MARE). The films were grown on glass and silicon substrates at room temperatures, i.e. without any intentional substrate heating. In this technique, the substrates were kept on the cathode instead of the grounded electrode and hence subjected to low energy nitrogen ion bombardment leading to highly c-axis oriented films. The photoluminescence (PL) and Raman spectrum shows significant improvement in the quality of the films compared with conventional activated reactive evaporation. The band gap measured from the room temperature PL was found to be 1.9 eV. Very high growth rates can be achieved in the MARE growth technique. The modification in the activated reactive evaporation technique may have a large impact on the growth of various compounds such as metal oxides.

Near-edge X-ray absorption fine-structure studies of GaN under low-energy nitrogen ion bombardment

Abstract The electronic structure of p-type GaN layers exposed to low-energy nitrogen ion bombardment was studied by near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy. It was found that ion bombardment lead to the creation of states lying below the nitrogen absorption edge which posses p-symmetry. These states are attributed to nitrogen interstitials with different local topologies created during ion bombardment. Furthermore, the NEXAFS spectra also shows the development of a strong π ∗ -resonance above the absorption edge with increasing incident nitrogen ion energy. This peak is attributed to the formation of molecular nitrogen at interstitial positions, arising from a build up of nitrogen ions on these sites.

Nitrogen adsorption structures on Cu(1 0 0) and the role of a symmetry-lowering surface reconstruction in the c(2×2)-N phase

Scanning tunnelling microscopy (STM) has been used to study the adsorption structures formed by atomic nitrogen on Cu(1 0 0), produced by low energy nitrogen ion bombardment and annealing. Atomic resolution imaging under varying tip conditions shows that in the c(2×2) phase STM appears to always image as asperities the Cu atoms, and not the N atoms as has been previously supposed. These images also indicate that in this phase the N induces a novel large-amplitude symmetry-lowering rumpling reconstruction of the outermost Cu layer. This conclusion allows one to understand many aspects of the mesoscopic c(2×2) island structures observed in this system and also reported in earlier STM investigations, and specifically the systematic behaviour of inter-island boundary widths which provide an indirect indication of the symmetry reduction. The possible role of adsorbate-induced surface stress changes and strain perpendicular to the surface at both local and longer ranges in the formation and self-organisation of the islands is discussed. Evidence for local strain and information on the anisotropy of step energies in the saturation coverage `trench' phase is also discussed.

Surface modifications in diamond-like carbon films submitted to low-energy nitrogen ion bombardment

Amorphous hydrogenated carbon films (a-C:H) films deposited by plasma enhanced chemical vapor deposition in CH 4 atmosphere were submitted to nitrogen r.f.-plasma treatment. The influence of several parameters was investigated: the chamber pressure, ranging from 0.5 to 10 Pa, the self-bias voltage in the range between −50 and −500 V and the exposure time. The erosion rate was determined by profilometry. It increases as the self-bias increases and tends to saturate for higher pressures. The nitrogen incorporated in the films was measured by nuclear reaction analysis. The results indicate a decrease in the N content as the pressure or the self-bias increase. The chemical bonds were probed by XPS and two chemical environments can be identified, one with N atoms surrounded by C atoms and the other one with N atoms binding to H. Surface roughness and friction coefficient modifications were followed by atomic force and lateral force microscopies. The surface roughness is independent on the value of the self-bias voltage, while the friction coefficient decreases.

X-ray photoelectron spectroscopy characterization of CN x thin films deposited by electron beam evaporation and nitrogen ion bombardment

Abstract Carbon nitride thin films have been deposited on Si(100) substrates by electron beam evaporation of graphite and simultaneous low energy nitrogen ion bombardment. The layers were analyzed by X-ray photoelectron spectroscopy. The synthesized layers are tetrahedrally bonded and amorphous, consisting of sp 3 -coordinated carbon with one nitrogen atom between its nearest neighbors. About 27% of the nitrogen is in C N bonds and some nitrogen is in C N bonds.

Raman and X-ray photoelectron spectroscopy study of carbon nitride thin films

Carbon nitride thin films were deposited on Si(100) substrates by electron beam evaporation of graphite and simultaneous low energy nitrogen ion bombardment. They were analysed by Raman and X-ray photoelectron spectroscopy. The formed amorphous layers are tetrahedrally bonded and consist of sp 3 carbon bonds with one nitrogen atom among its nearest neighbours. Substitution of the tetrahedrally bonded carbon atom by nitrogen leads to decrease of the percentage weight of the nanocrystalline diamond phase and formation of a CN x phase embedded in the amorphous carbon layer. By changing the deposition conditions, redistribution of sp 2 and sp 3 bonded C–N occurs.

Study of the growth of thin nitride films under low-energy nitrogen-ion bombardment

Bombardment of silicon surfaces by low-energy (300–1000 eV) nitrogen ions has been investigated as a potential process for growing ultrathin films at relatively low temperatures (<500°C). The thicknesses and chemical states of the obtained films are analysed using ellipsometry, X-ray Photoelectron Spectroscopy (XPS), and Auger Electron Spectroscopy (AES). All the analyses show that ultrathin (∽ 60 A) silicon nitride films have been directly grown on silicon substrates. Detailed studies of the influence of different process parameters on the obtained films have been carried out. The thicknesses of the obtained films appear to increase with ion energy. The nitridation is found to be a rapid process which can be divided into two steps. The thicknesses are also observed to vary slightly with substrate temperature. The average active energy of nitridation rates is about 3.5 meV which indicates nonthermal process kinetics. For AES analysis, the films are found to be nitrogen-rich ones with the stoichiometric factor x≈1.7 larger than that of pure silicon nitride (x=1.33). In both AES and XPS studies, the chemical state of the silicon atoms resembles the existence of silicon oxynitride films of low oxygen concentration. The growth mechanism is also discussed briefly.

Thermochemical surface treatment of titanium and titanium alloy Ti6Al4V by low energy nitrogen ion bombardment

In this paper the basic processes of the thermochemical surface treatment of titanium and titanium alloy by low energy (0.6−0.9 keV) nitrogen ion bombardment and the characteristic structures of the layers produced are dealt with. The effect of the process parameters gas composition, temperature and treatment time on the microstructure and the growth rate of the surface layer has been investigated by means of X-ray diffraction, electron microscopy and microprobe analysis. It has been found that for titanium and titanium alloy the growth of the nitrided layer is controlled essentially by a diffusion mechanism. The growth depth can be approximately estimated using a t 1 2 rule. It has been shown that not only ion implantation and reactive sputtering but also chemisorption of the nitrogen on the surface play a significant role in the layer formation.