Nature | Vol 582 | 25 June 2020 | 513diffraction results shown in Fig. 1 reveal that the films are amorphous
with no discernible long-range order; hence, we refer to the material
as amorphous BN (a-BN). This is also supported by the analysis of the
reduced radial distribution function obtained from the electron dif-
fraction data (see Extended Data Fig. 2a, 2b), which shows broad peaks
and a nearest-neighbour distance of 0.144 nm. Cross-sectional chemi-
cal mapping confirms that the films consist of B and N (Extended Data
Fig. 2c). X-ray photoelectron spectroscopy (XPS) was used to obtain
chemical information. The B/N atomic ratio was found to be about
1:1.08 (Fig. 2a, b) with the B 1s and N 1s peaks at 190.4 eV and 397.9 eV,
respectively, indicating that the films are sp^2 -bonded B and N (refs.^17 ,^18 ).
The molecular dynamics simulations shown in Extended Data Fig. 3
confirm the amorphous structure of the BN films, which is consistent
with the result in Fig. 1.
Raman spectra of a-BN and crystalline trilayer hexagonal BN (hBN;
for comparison) reveal that the hBN E2g mode at 1,373 cm−1 is absent in
a-BN (Fig. 2c)^17 ,^18. The Fourier transform infrared spectroscopy (FTIR)
spectrum in Fig. 2d shows an absorption peak near 1,370 cm−1 that
is attributed to the transverse optical mode of BN in a-BN. Another
infrared mode located near 1,570 cm−1 confirms the amorphous nature
of sp^2 -bonded BN (ref.^19 ). FTIR does not show any N–H or B–H bonds
(Extended Data Fig. 4a). Detailed chemical and density analyses were
conducted with Rutherford backscattering spectroscopy (RBS) and
elastic recoil detection analysis (ERDA) and the results are shown in
Extended Data Fig. 4b–d.
Angle-dependent near-edge X-ray absorption fine structure
(NEXAFS) measurements in partial electron yield (PEY) mode were
made at the Pohang Light Source-II 4D beamline to investigate the
chemical and electronic structure of a-BN. In NEXAFS, X-ray absorption
is used to excite core electrons of B and N to unoccupied states— that
is, 1s electrons are excited to empty π* and/or σ* states. In the 1s → π*
transition, the spatial orientation of π orbitals strongly affects the
transition probability. Thus, information pertaining to the relative
orientation of orbitals in hBN layers can be obtained by varying the
incidence angle of X-rays^20. NEXAFS spectra obtained for the a-BN
sample at incident angles of 30°, 55° and 70° are shown in Fig. 2e. The
observed resonance at 192 eV corresponds to the 1s → π* transition in
boron^20. The resonance intensity of the 1s → π* transition in a-BN dem-
onstrates negligible variation with the X-ray incidence angle (Fig. 2e),
strongly indicating that BN planes are randomly oriented throughout
the material. Similar conclusions can be drawn from NEXAFS spectra of
the K edge of N (Extended Data Fig. 4e). Additionally, NEXAFS confirms
that a-BN is completely sp^2 -hybridized^20 ,^21. For completeness, we also
deposited BN films at different remote ICP-CVD parameters such as
power, temperature and pressure. We found that the temperature was
the most important parameter, with ideal a-BN film deposition occur-
ring at 400 °C and plasma power of 30 W. Above this temperature we
obtained nanocrystalline BN (nc-BN), as shown in Extended Data Fig. 5.
We now discuss the dielectric properties of a-BN. The dielectric con-
stant is a physical measure of how easily electric dipoles can be induced01234510 –810 –710 –610 –510 –410 –310 –210 –1100101102104 105 10601234501234502468101214hBNa-BN200 μmCurrent density,J (A cm–2)30 nm5 nmSiCoSiCohBNa-BNDielectric constant, Nabde fRelative dielectric constant,N0123450.00.51.01.52.02.5
BD
FSGSiO 2SiCOH
a-CHPorous HSQPorous MSQcDensity,U (g cm–3)Dielectric constant, Na-BNOSGBreakdown eld (MV cm–1)a-BN SiCOHFSGhBN a-BNa-BN012345012345
Refractive index at 633 nmDielectric constant at 100 kHz,N 3.28 ± 0.421.78 ± 0.162.161.37Frequency (Hz)Bias (V)MSQ OSGBD
SiLKSiO 2hBNFig. 3 | Dielectric properties of a-BN. a, Relative dielectric constant as a function
of frequency for a-BN and hBN. The dielectric constants were determined by
capacitance–frequency measurements on metal–insulator–metal structures
(inset). Thick blue and red lines denote averages. b, Statistical distribution of
dielectric constants measured at 100 kHz and refractive indices (green stars)
obtained by ellipsometry for a-BN and hBN. The box indicates a region with a 25%
and 75% distribution relative to the average value, and the top and bottom bars
mean maximum and minimum values. The number of devices measured in a and
b is 250 for a-BN and 330 for hBN. c, Density versus dielectric constant for low-κ
materials reported in literature (blue circles) and a-BN (red circle). d, Typical
current density–voltage curves for hBN (approximately 1.2 nm thick; blue curve)
and a-BN (3 nm thick; red curve) films. Thick blue and red lines denote averages.
The number of devices measured is 100 for a-BN and 11 for hBN. e, Breakdown
field versus dielectric constant for low-κ materials reported in the literature (blue
circles) and for a-BN (red circle). f, Cross-sectional TEM images of Co/a-BN/Si
interfaces after annealing for 1 h at 600 °C. The bottom image shows a magnified
view of the area marked by the red box in the top image. No diffusion of Co
into Si through a-BN is observed. HSQ, hydrogen silsesquioxane; MSQ,
methylsilsesquioxane; BD, black diamond; FSG, fluorinated silicon glass; OSG,
organosilicate glass; a-CH, amorphous hydrocarbon.