Growth and Characterization of Deposited Nitride Thin Films by
Ion and Neutral Assisted Molecular Beam Epitaxy

Abstract

Boron nitride (BN), carbon nitride (CN), and boron carbonitride (C-B-N) thin films have been prepared by electron cyclotron resonance (ECR), ion and neutral beams assisted deposition (IBAD, NBAD) techniques. The use of these techniques allowed for both nitrogen ions and neutrals to be used. Most of the BN films contained a mixture of phases except the ones deposited using ECR. A necessary though not sufficient condition, is bombardment (energetic ions) induced stabilization of the cubic phase. ). Fourier transform infrared spectroscopy (FTIR) measurements indicated that the BN films deposited using the neutralized nitrogen beam have a very high percentage of cubic phase (80 %), those deposited with ions are mixed hexagonal-cubic with only 10-20 % cubic, and those deposited using ECR-assisted growth were pure hexagonal. The appearance of the cubic phase in the BN films grown using the Kaufman source confirm the effect of ion energy on the kinetic of the c-BN formation. The electrical properties of the BN films were investigated using Hall measurements. It was found that the films grown by NBAD technique where p-type, those grown by ECR technique where n-type, whereas those grown by IBAD where either n- to p-type depending on the ion energy. A proposed model based on these experimental results suggests nitrogen vacancies for n-type films and boron antisite or impurities for p-type. In this issue, we reported for the first time, the major chemical trends in the deep-energy levels of isolated sp³-bonded substitutional native defects in boron nitride. Band structure calculations, based on the second-neighbor tight-binding theory, are presented and the results are obtained for the electronic energy states of substitutional (isolated) defects in the Green’s-function framework. Our calculations suggest that the energies of the bound states (T₂-symmetry) of B (V_B) and N (V_N) vacancies are near E_V +0.28 eV and E_V +4.03 eV, respectively. In other words the B- and N- vacancies in c-BN behave as acceptors and donors, respectively. The other necessary conditions for c-BN stabilization are an adequate substrate temperature during growth (~ 450 ^oC in our case) and a film stoichiometry close to the ideal 1:1. The detailed understanding of the atomic-scale mechanisms whereby c-BN is stabilized is still unknown and remains an area of active scientific interest. What is much more encouraging is that the three techniques used (ECR, IBAD, and NBAD) provide easy control of deposition parameters, particularly the arrival rate ratio of boron to nitrogen. When all stabilizing factors are present, how well the processes used are able to maintain the ideal B/N ratio in the films how much the cubic phase will be promoted with the adequate n or p conductivity. Our search for optimal conditions of depositing epitaxially grown cubic boron nitride with n and p type conductivity region control still be our ongoing research effort.

We presented preliminary results of C-B-N compounds deposited at low temperature (100 - 400 ^oC) using ECR-assisted vapor deposition. The C-B-N films were either carbon rich or boron rich depending on the initial boron rate. The composition was determined by using electron probe microanalysis (EPMA). The present study was undertaken to determine the suitability of our technique to synthesize ternary C-B-N compounds. The effect of boron introduction into CN materials was investigated using Auger depth profiling (ADP). At low boron rate the films have a composition C_0.59B_0.10N_0.31 and are semi-metallic, and at high boron rate the films have a composition C_0.33B_0.42N_0.25 and are insulating type. X-ray photoelectron spectroscopy (XPS) measurement showed the existence of bonding between the main elements C, B, and N and suggested that a single-phase C-B-N may be present in the film with higher B content. The results reflect the change in the electrical properties of the C-B-N composite as the boron content changes the material from a semi-metallic graphitic material to a BN-like insulating material.

The strongest evidence we have regarding the synthesis of carbon nitride films for coating purpose is that there was a one to one correlation between the nitrogen flow and the nitrogen content and the sp³ type bonding signatures present in the films. These observations are mainly based on our Rutherford backscattering spectroscopy (RBS) and XPS measurement data. Our preliminary investigation using the transmission electron microscope (TEM) under the electron diffraction mode revealed the films to be non-crystalline. Homogeneous, non-crystalline materials (like these films) do not exhibit any interesting contrast features in the image mode. In other words, there is very little that one can learn from these images. Occasionally, one can observe so-called "turbostratic" structures in thin non-crystalline materials. However, we did not attempt this observation yet. The diffraction patterns exhibit diffused rings as opposed to the sharp rings that one would see from a polycrystalline material. A series of deposited carbon nitride films at room temperature (~80 ^oC) have been investigated after a post growth rapid thermal annealing (RTA) up to 600 ^oC. Both ECR and Ion beam sources have been used to grow these films at a constant carbon rate of 0.2 Å/s and a different ion beam current up to 270 mA. The films were too thin for RBS investigation and spectra from FTIR and diffraction patterns from TEM were featureless. The higher amount of nitrogen incorporation did not exceed 30% as determined by electron energy loss spectroscopy (EELS). Consequently, we believe that, at least, a temperature much higher than 80 ^oC, is required for the synthesis of carbon nitride films with a higher nitrogen concentration.

The ground state properties of both c-BN and diamond were investigated using "fhi96md" computer code. Our preliminary studies are considered as a test of the feasibility and reliability of this new developed computer code. At equilibrium volume, the calculated total energies for c-BN and diamond are –348.70 eV/pair and –304.35 eV/pair, respectively. These values are quit close to what is published in the recent literature and make us comfortable for further extended investigations.