Ion-Irradiation Effect in Ion-beam-assisted MBE Growth of c-BN Films

Kazuyuki Hirama, Yoshitaka Taniyasu, Hideki Yamamoto, and Kazuhide Kumakura
Materials Science Laboratory

　Cubic boron nitride (c-BN) with sp³-bonding has a large bandgap energy of 6.25 eV, which may further expand the potential of nitride-based semiconductor devices. However, conventional growth methods for group-III nitrides, such as MOVPE, have been unsuccessful in the epitaxial growth of c-BN films because c-BN is a metastable phase in ambient atmosphere and a thermodynamically stable sp²-bonded BN phase is easily formed. Recently, we achieved the epitaxial growth of c-BN films by the ion-beam-assisted molecular beam epitaxy (MBE) method, in which boron atoms are supplied by electron-beam evaporation with a simultaneous irradiation of N₂⁺ and Ar⁺ [1]. Here we establish the growth phase diagram for c-BN thin-film growth and show that the flux intensity ratio of Ar⁺ to boron (F_Ar+/F_B) is a key to controlling the crystal structures of BN films.
　BN films were grown on diamond(001) substrates by electron-beam evaporation of boron with a simultaneous supply of nitrogen radical (N^*) and Ar⁺. Instead of N₂⁺, N^* was used a nitrogen source to investigate the proper ion-irradiation condition of Ar⁺ for c-BN growth. The V/III ratio, defined as the flux intensity ratio of N^* to boron (F_N*/F_B), was set at values > 1. The growth temperature (T_g) was 920°C.
　Figure 1 shows FT-IR absorption spectra of BN films with and without Ar⁺ irradiation. Without Ar⁺ irradiation, absorption peaks attributed to BN sp²-bonding were observed at around 780 and 1380 cm^-1. On the other hand, with Ar⁺ irradiation (F_Ar+/F_B>1), only an FT-IR absorption peak attributed to BN sp³-bonding was observed at around 1070 cm^-1. Other peaks originating from BN sp²-bonding were not observed. From a cross-sectional TEM image and the selective-area electron diffraction (SAED) pattern, we confirmed that the c-BN(001) film was epitaxially grown on the diamond(001) substrate from the initial growth stage (Fig. 2). Accordingly, the Ar⁺ irradiation is a prerequisite for the selective formation of the sp³-bonded c-BN.
　Next, we focused on the effect of F_Ar+ on the structure of BN films. We carried out regrowth of BN films on c-BN(001) film templates on diamond substrates; F_Ar+ and F_B were systematically varied. Figure 3 shows the growth phase diagram plotted as functions of F_Ar+ and F_B. The c-BN(001) films are epitaxially grown at F_Ar+/F_B larger than 1, while sp²-bonded turbostratic BN (t-BN) films are grown at F_Ar+/F_B smaller than 1 even on the c-BN(001) templates. Thus, F_Ar+/F_B larger than 1 is necessary for epitaxial growth of c-BN films, in addition to V/III larger than 1 and T_g above 750°C [1].
　This work was supported by KAKENHI.


Fig. 1. FT-IR spectra of BN films grown with and without Ar⁺ irradiation.	Fig. 2. Cross-sectional TEM image and SAED pattern of epitaxial c-BN(001) film.	Fig. 3. Growth phase diagram for BN as functions of F_Ar+ and F_B.