H. Feldermann

Electrical properties and thermal stability of ion beam deposited BN thin films

Abstract Boron nitride (BN) thin films were grown in UHV by alternating deposition of low energy mass separated 11 B + and 14 N + ions. Depending on the deposition conditions films are disordered (t-BN), hexagonal (h-BN) or cubic (c-BN). These films were annealed in various gas environments and characterized by infrared spectroscopy as a function of annealing temperature. Disordered BN films (t-BN) show a gradual structural change towards crystalline h-BN upon annealing, whereas for c-BN films only relaxation of compressive stress is observed. c-BN films are stable up to 1200 K for annealing in air. Electrical measurements on various BN-films deposited on metal and silicon substrates reveal Frenkel-Poole emission as the dominant conduction mechanism at high bias voltages. The current-voltage characteristics of BN/Si heterojunctions, studied between room temperature and 500 K, are almost symmetric, i.e. do not show significant rectifying behavior. Current-voltage curves were found to be reversible in temperature but we find irreversible behavior after applying high bias voltages.

Room temperature growth of cubic boron nitride

Boron nitride thin films were deposited at room temperature with various ion energies by mass selected ion beam deposition on cubic boron nitride (c-BN) previously nucleated on Si (100) substrates at a higher temperature. Selective area diffraction, electron energy loss, and infrared spectroscopy results reveal continued growth of the cubic phase. The reported temperature threshold of about 150 °C for c-BN film formation is therefore unmistakably related to the initial nucleation of c-BN, whereas the growth of c-BN appears to be temperature independent. The latter is in accordance with predictions of the cylindrical thermal spike growth model recently proposed by our group.

Cubic boron nitride thin film heteroepitaxy

In this study we investigate the possibility of nucleating nanocrystalline cubic boron nitride (c-BN) thin films directly onto suitable substrates without the soft turbostratic BN (t-BN) interlayer that is usually observed. This would open a path to the epitaxial growth of c-BN films which is essential particularly for practicable applications in electronic devices. Appropriate substrates are required to exhibit a lattice that matches the c-BN crystallite structure, survives the ion bombardment imperative for c-BN film formation, and is not disturbed by the development of a heterogeneous interface layer. In accordance with these criteria, monocrystalline AlN is selected and employed as a potential substrate for direct c-BN film growth using mass selected ion beam deposition. A detailed examination of the BN/AlN interface microstructure by cross-sectional high-resolution transmission electron microscopy reveals that the AlN crystallinity is indeed retained, with no amorphous layer next to the BN film as co...

Ion beam deposition of fluorinated amorphous carbon

We have studied the growth and the properties of (t)a-C:F films prepared by the deposition of mass separated 12C+ and 19F+ ions as a function of the F concentration. The films are always strongly F deficient due to the formation of volatile F2 and CFx molecules during the deposition process. A maximum F content of about 25 at. % is obtained for an ion charge ratio of C+:F+=1:1. The observed mechanical, optical, electrical, and structural properties as well as the thermal stability of the films are strongly influenced by the F content. A three step progression of the film structure is evident for increasing F concentration: the amorphous three-dimensional network of tetrahedrally bonded carbon atoms of pure carbon films (ta-C) with diamondlike properties is doped for very low F concentrations (ta-C:F). A further increase of the F content results first in transformation to a graphitelike amorphous structure (a-C:F) before the deposited films become porous and to a polymerlike one for the highest F content.

Hydrogen-plasma etching of ion beam deposited c-BN films: An in situ investigation of the surface with electron spectroscopy

In the present study nanocrystalline c-BN films deposited with a mass selected ion beam were subjected to a hydrogen plasma or atomic hydrogen produced by the hot filament method. Film composition and electronic properties of the surface were subsequently analyzed in situ by photoelectron spectroscopy in the x-ray and ultraviolet regime, and by electron energy loss spectroscopy. The sp2-bonded surface layer, which is native to ion beam deposited c-BN films, is rapidly removed by hydrogen plasma etching and the almost phase pure c-BN layer uncovered. Continuation of hydrogen plasma exposure leads to a removal of the c-BN layer at an etch rate of about 0.65 nm/min until finally the sp2-BN nucleation layer is detected. No preferential etching of either B or N is observed and an equal concentration of the constituents is maintained throughout the experiment. The large atomic hydrogen flux from the plasma is held responsible for the efficient etching, which could not be achieved with the hot filament method. T...

Surface Brillouin scattering of cubic boron nitride films

Surface Brillouin scattering has been used to determine the elastic properties of thin hard submicron cubic boron nitride (cBN) films grown on silicon by mass selected ion beam deposition. The elastic properties of the films have been determined by fitting experimental data to theoretical dispersion curves. A Green’s function method was used to predict Brillouin scattering spectra of the acoustic excitation at the free surface. Our results demonstrate that the effect of the thin hexagonal boron nitride interlayer located between cBN film and the Si substrate on the velocity of the surface acoustic wave does not exceed 2% for a thin (16 nm) film and is negligible for cBN films thicker than 100 nm. The elastic properties of the cBN films are not softer than those of bulk cBN.

Film growth using mass-separated ion beams

Mass-separated low energy ion beams provide clean, well defined, and controllable deposition conditions for film growth and are thus particularly suited to study basic mechanisms of thin film growth. The recent advances and emerging applications of mass-separated ion beam deposition (MSIBD) are discussed. During the last years MSIBD has been extensively applied to study the growth of tetrahedral amorphous carbon (ta-C) and cubic boron nitride (c-BN) films. The nucleation and growth conditions are discussed and compared with model predictions. The microstructure and properties of F-, B- and N- containing carbon films are briefly discussed.

Quantitative analysis of chemically-enhanced sputtering during ion beam deposition of carbon nitride thin films

The sputter losses during growth of carbon nitride thin films using mass selected ion beam deposition of C{sup +} and N{sup +} ions with energies between 20 eV and 500 eV are studied. Depending on the ion energy 35--100% of C{sup +} but only 3--35% of N{sup +} ions are incorporated in the films. Thus the films are always strongly nitrogen-deficient. To suppress the preferential loss of nitrogen the authors introduce the concept of continuously growing surface protective layers. Starting from a diamond-like carbon film as substrate, carbon nitride films are deposited using 100 eV {sup 12}C{sup +} and 1 keV {sup 14}N{sup +} ions, so that the growing films are always covered with a 1--2 nm thick protective layer of amorphous carbon. In this case they observe an increased nitrogen incorporation yielding to films with average film composition of C{sub 2}N.

Thin Film Growth of Group III Nitrides by Mass Separated Ion Beam Deposition

The authors have grown gallium nitride (GaN), aluminum nitride (AlN) and boron nitride (BN) thin films by mass separated ion beam deposition. All deposited films were found to be almost stoichiometric. AlN and GaN films are crystalline even after room temperature deposition whereas for the formation of crystalline boron nitride temperatures above 150 C are necessary. The influence on the phase formation and the film structure of ion energy and substrate temperature on the one hand, and bond ionicity on the other hand, was investigated for these three systems.