Molecular Beam Epitaxy of Boron Nitride Thin Films and Their Analytical Characterization
- PDF / 3,373,998 Bytes
- 11 Pages / 414.72 x 648 pts Page_size
- 95 Downloads / 183 Views
contrast, recent research strongly indicates that a high stress level in the film is a necessary condition for the formation of the cubic phase [ 15,16]. An area of research where significant progress has been made is in determining the deposition conditions necessary for c-BN growth. The work of Kester and Messier,[J7] established that for a given substrate temperature, the momentum transferred into the growing film by the bombarding ions is the single parameter which controls the formation of c-BN. As long as sufficient N is present, a threshold value of momentum transfer exists for the formation of c-BN. This controlling parameter incorporates the values of ion energy, ion flux, and ion species. As a result of the present study described below we have developed a better understanding of both the conditions required for c-BN growth and of the mechanisms responsible for that growth. EXPERIMENTAL PROCEDURES The substrates used in this study were (100) single crystal infrared-transparent, high resistivity (r>50 Q-cm) on-axis Si; cut and polished single crystal natural diamond; as well as polished single crystal Cu and Ni. The Si substrates were cleaned using a standard RCA procedure [ 18] of which the final step was a 5 min dip in 10% HF. This left the surface H terminated, as determined by XPS. The diamond substrates were etched in a boiling 3:4:1 H2 SO4 :HNO 3 :HC10 4 solution for 45 minutes to remove any graphitic phase. The Cu and Ni substrates were decreased ultrasonically using a acetone, methanol, and de-ionized water sequence. The substrates were attached to Mo holders using Ag paint which acted both as an adhesive and a thermal conductor. This assembly was subsequently loaded through a vacuum load lock into the UHV deposition chamber. Base pressures in the chamber were typically 5-8 x 10-1° Torr. Each substrate was heated using a W wire coil located behind the Mo substrate holder. They were baked under UHV conditions at 700°C for 20 mim to remove residual H 2 0 and hydrocarbon species. The substrate temperature during deposition was monitored using thermocouples near the heater, the readings of which had been previously calibrated to several temperatures of the substrate surface. Boron was deposited using a constant deposition rate of 0.5 A/s, which was monitored using a quartz crystal rate monitor. Fluxes of N and Ar ions were obtained from a 3 cm Ion Tech Kaufman source. Argon and nitrogen gases were used with a gas flow ratio of 1:1. Because of the close ionization potentials of these two gases, the ion ratio was also close to 1:1. The ion source was operated at 500 eV; the N ions were predominantly N:+, based on the studies of Van Vechten et al.[19]. The discharge voltage of this source was maintained at a sufficiently low value to prevent double ionization of either species. Since it has been previously shown that c-BN can be successfully deposited using an Ar:N 2 ratio of 1:1 and an ion energy of 500 eV [16,17], these values were kept constant throughout this study. Both the ion beam and the boron ev
Data Loading...
