Cubic boron nitride thin film growth by boron and nitrogen ion implantation

Cubic boron nitride $\left(c\text{−}\mathrm{B}\mathrm{N}\right)$ thin films were deposited on silicon substrates using mass separated ion beam deposition (MSIBD). In order to investigate the influence of the ion energy on the growth of $c\text{−}\mathrm{B}\mathrm{N}$ films, ${}^{11}\mathrm{B}^{+}$ and ${}^{14}\mathrm{N}^{+}$ ions were implanted into $c\text{−}\mathrm{B}\mathrm{N}$ with ion energies ranging from $5\mathrm{keV}\text{to}43\mathrm{keV}$ and substrate temperatures $\left(T_S\right)$ from room temperature (RT) to $250°\mathrm{C}$. A systematic study on the interplay of $E_{\mathit{\text{ion}}}$ and $T_S$ has revealed a characteristic energy-dependent temperature threshold for $c\text{−}\mathrm{B}\mathrm{N}$ growth. This behavior is explained by dynamic annealing of defects caused by a penetrating ion in a collison cascade. In this picture, the suppression of defect accumulation that is crucial for maintaining cubic phase formation is attributed to temperature-driven back diffusion and subsequent annihilation of B and N interstitial recoils. The model is confirmed by analyzing the depth profile of implanted, isotopically pure ${}^{10}\mathrm{B}$, and its application for both $c\text{−}\mathrm{B}\mathrm{N}$ nucleation and growth is discussed.

Figure 1

FTIR absorption spectra of BN samples, grown at a substrate temperature of $250°\mathrm{C}$ with different ion energies on a previously nucleated $c\text{−}\mathrm{B}\mathrm{N}$ film. ${\mathrm{B}}^{+}$ and ${\mathrm{N}}^{+}$ ion energies were adjusted above $10\mathrm{keV}$ with respect to their different ranges due to the mass difference. The labels have to be read as ${\mathrm{B}}^{+}∕{\mathrm{N}}^{+}$ ion energy. The dashed line shows the IR spectrum of a nucleation layer only for comparison. $c\text{−}\mathrm{B}\mathrm{N}$ film growth has continued in all cases.

Figure 2

Dark field TEM image of a BN sample, grown with $15\mathrm{keV}$ ${\mathrm{B}}^{+}$ and $20\mathrm{keV}$ ${\mathrm{N}}^{+}$ ions on a $c\text{−}\mathrm{B}\mathrm{N}$ substrate. Total film thickness is about $210\mathrm{nm}$. Cubic BN crystallites are extending up to the surface. Considering the deposited charge, film area, and taking the sputter yield into account, the thickness of the nucleation layer calculates to about $150\mathrm{nm}$.

Figure 3

(a) High-resolution TEM micrograph of a $c\text{−}\mathrm{B}\mathrm{N}$ crystallite grown with $15\mathrm{keV}$ ${\mathrm{B}}^{+}$ and $20\mathrm{keV}$ ${\mathrm{N}}^{+}$ ions. It reaches up to the surface and is covered with a thin $s{p}^2$-bonded layer. (b) Corresponding FFT image. The sample shows a high amount of disorder, but the $c\text{−}\mathrm{B}\mathrm{N}$ diffraction patterns are clearly visible. The lattice spacing calculated from the diffraction pattern is $d=2.10\left(4\right)\mathrm{\AA }$, which can be associated with the ${\left(111\right)}_c\text{−}\mathrm{B}\mathrm{N}$ planes.

Figure 4

FTIR absorption spectra of $c\text{−}\mathrm{B}\mathrm{N}$ films grown with $15\mathrm{keV}$ ${\mathrm{B}}^{+}$ and $20\mathrm{keV}$ ${\mathrm{N}}^{+}$ ions on a $c\text{−}\mathrm{B}\mathrm{N}$ substrate, in dependence of the substrate temperature during growth. $c\text{−}\mathrm{B}\mathrm{N}$ growth can be maintained at temperatures as low as $100°\mathrm{C}$, but $60°\mathrm{C}$ or below results in a transformation back to $t\text{−}\mathrm{B}\mathrm{N}$ growth.

Figure 5

[a and b] SIMS depth profiles of $2\times {10}^{15}{}^{10}\mathrm{B}^{+}∕{\mathrm{cm}}^2$ implanted into a $c\text{−}\mathrm{B}\mathrm{N}$ sample with an ion energy of $10\mathrm{keV}$. (a) Implantation at room temperature and subsequent annealing at $250°\mathrm{C}$ for $1\mathrm{h}$ under UHV conditions. (b) Implantation at room temperature (up triangles) and at $250°\mathrm{C}$ (circles). (c) ${}^{10}\mathrm{B}$ depth profile as calculated by SRIM, when implanted with $10\mathrm{keV}$ into a $c\text{−}\mathrm{B}\mathrm{N}$ sample (down triangles). Squares represent the SRIM calculated number of ${}^{11}\mathrm{B}$ target vacancies created by the incident ion beam. The data have been scaled down in order to be comparable to the calculated depth profile.

Figure 6

Projected range of ${}^{11}\mathrm{B}$ ions implanted into $c\text{−}\mathrm{B}\mathrm{N}$, as a function of the ion energy (triangles). Circles represent the position of the maximum of the projected boron target vacancy profile as created by the incident ion beam. Data have been calculated by SRIM.

Figure 7

Mininum temperature required to maintain $c\text{−}\mathrm{B}\mathrm{N}$ growth on a previously nucleated $c\text{−}\mathrm{B}\mathrm{N}$ film at a given ${\mathrm{B}}^{+}$ ion energy (circles). The data points have been fitted using Eq. (3) with $t=0.1\mathrm{s}$, $x=x\left(E\right)$, and $D_0=1.5\times {10}^{-3}{\mathrm{cm}}^2∕\mathrm{s}$. A migration energy of $W_m=0.5\left(1\right)\mathrm{eV}$ was extracted from the fit.

Figure 8

(a) FTIR absorption spectra of BN films, directly deposited on silicon substrates with ion energies ranging from $500\mathrm{eV}\text{to}5\mathrm{keV}$, at substrate temperatures of $125–250°\mathrm{C}$. No $c\text{−}\mathrm{B}\mathrm{N}$ signal is visible. Instead, all samples show a strong $t\text{−}\mathrm{B}\mathrm{N}$ absorption with broad peaks at $\sim 1380{\mathrm{cm}}^{-1}$, indicating a highly disordered film structure. (b) Phase diagram for ion-beam-deposited boron nitride films, as published by Hofsäss et al. (Ref. 4). The $c\text{−}\mathrm{B}\mathrm{N}$ content is plotted as a function of ion energy and substrate temperature. Filled symbols indicate successful $c\text{−}\mathrm{B}\mathrm{N}$ formation, whereas hollow symbols represent $t\text{−}\mathrm{B}\mathrm{N}$ samples. Contrary to previous assumptions (indicated by the dashed line), the temperature and energy thresholds are not independent of each other.

Figure 9

BN phase diagram for BN films deposited via MSIBD, plotted as ${\log }_{10}E$ vs. $1∕T$. $c\text{−}\mathrm{B}\mathrm{N}$ nucleation is only possible in a narrow energy window ranging from $125\mathrm{eV}$ to around $2\mathrm{keV}$. For higher ion energies the temperature has to be increased slightly in order to nucleate the cubic phase. $c\text{−}\mathrm{B}\mathrm{N}$ growth, on the other hand, is possible for a wider range of both substrate temperature and ion energy. The low-energy threshold is at $E_{\mathit{\text{ion}}}\approx 60\mathrm{eV}$. For ion energies up to $5\mathrm{keV}$, $T_S\approx \mathrm{RT}$ is sufficient to grow $c\text{−}\mathrm{B}\mathrm{N}$, but for higher energies the temperature has to be increased according to Eq. (3).