Local environment of silicon in cubic boron nitride
Hidenobu Murata, Takashi Taniguchi, Shunichi Hishita, Tomoyuki Yamamoto, Fumiyasu Oba, and Isao Tanaka
Citation: Journal of Applied Physics 114, 233502 (2013); doi: 10.1063/1.4849015
View online: http://dx.doi.org/10.1063/1.4849015
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JOURNAL OF APPLIED PHYSICS 114, 233502 (2013)

Local environment of silicon in cubic boron nitride
Hidenobu Murata,1,a) Takashi Taniguchi,1 Shunichi Hishita,2 Tomoyuki Yamamoto,3Fumiyasu Oba,4 and Isao Tanaka4,51
Advanced Key Technologies Division, National Institute for Materials Science, 1-1 Namiki Tsukuba,
Ibaraki 305–0044, Japan
2
Environment and Energy Materials Division, National Institute for Materials Science, 1-1 Namiki Tsukuba,
Ibaraki 305–0044, Japan
3
Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169–8555, Japan
4
Department of Materials Science and Engineering, Kyoto University, Yoshida-Honmachi, Sakyo,
Kyoto 606–8501, Japan
5
Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta,
Nagoya 456–8587, Japan(Received 12 November 2013; accepted 2 December 2013; published online 16 December 2013)
Si-doped cubic boron nitride (c-BN) is synthesized at high pressure and high temperature, and the
local environment of Si is investigated using X-ray absorption near edge structure (XANES) andfirst-principles calculations. Si-K XANES indicates that Si in c-BN is surrounded by four nitrogenatoms. According to first-principles calculations, the model for substitutional Si at the B site well
reproduces experimental Si-K XANES, and it is energetically more favorable than substitutional Si
at the N site. Both the present experimental and theoretical results indicate that Si in c-BN prefers
C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4849015]
the B site to the N site. V
I. INTRODUCTION

Cubic boron nitride (c-BN) is a high-density phase of
BN and has a similar crystal structure and properties to diamond. Its applications widely vary from a super-hard material to opt-electronics. Bulk c-BN was first synthesized underhigh-pressure, high-temperature (HPHT) conditions by
Wentorf.1 During syntheses, defects and coexistent elements
are often incorporated into c-BN and behave as the origin ofvarious properties. It has been reported that c-BN shows
both n- and p-type conductivity,2,3 ultraviolet light emission
from its p-n junction,4 and luminescence,5–8 for example.
The electronic and atomistic structures of dopants anddefects in c-BN have attracted much research attention.
Although there have been a number of theoretical studies
reporting on native defects9–14 and dopants,11,15–23 it is difficult to experimentally reveal the atomistic structure of trace
dopants. Recently, a combined scanning transmission electron microscopy (STEM) and first-principles study foundthat luminous Ce3þ is incorporated into c-BN by the following unusual mechanism: Ce3þ in c-BN is not substituted forthe cationic B site but for the anionic N site with surrounding
B-vacancies.23 Therefore, it is necessary to revisit the local
environments of dopants in c-BN.
X-ray absorption near edge structure (XANES) is a
powerful tool to investigate dopants. It reveals local electronic and atomistic structures with elemental selectivity. A
number of dopants in various types of material have been
investigated using XANES.24 Since XANES analyses are
based on the “finger printing” method, theoretical calculations are helpful to interpret the spectra of dopants. Thisa)

Research Fellow of Japan Society for the Promotion of Science. Author towhom correspondence should be addressed. Electronic mail:
MURATA.Hidenobu@nims.go.jp

0021-8979/2013/114(23)/233502/4/$30.00

method enables us to determine the local environment oftrace dopants.25,26
Si is one of the n-type dopants that are found at the initial stage of the development of c-BN.2 In this study, we synthesized Si-doped c-BN under HPHT conditions andinvestigated the local environment of Si using XANES andfirst-principles calculations.
II. EXPERIMENTAL AND COMPUTATIONAL
PROCEDURES
A. Sample preparation

Si-doped c-BN samples were synthesized with a Si solvent under HPHT conditions.27 Si wafers (99.999%, Raremetallic Co., Ltd.), commercial h-BN sintered disks (type N-1,
Denka Co., Ltd.), and h-BN powder (type GP, Denka Co.,
Ltd.) were used as the starting materials. Before syntheses, deoxidization of h-BN was carried out at 1500  C in N2 flow andat 1950  C in vacuum. Sample assembly for HPHT treatments
was similar to that of Ref. 27; we used a Si wafer and an MgO
capsule instead of Si powder and a Ta capsule. HPHT treatments were performed with a belt-type high-pressure apparatus
FB-30H. Pressure and temperature were determined in thesame manner as that described in Ref. 28. Samples were kept at
7.7 GPa and 1700  C for 1 h and then quenched to room temperature. MgO capsules were removed by nitro hydrochloric
acid. The samples were then soaked in a mixture of HNO3 andHF at 150  C for 12 h in order to remove residual Si and h-BN.
B. Characterization

The samples were characterized using X-ray diffraction(XRD), secondary ion mass spectrometry (SIMS), and
XANES. XRD patterns were collected by Rigaku
RINT2200V/PC with a Cu-Ka target and a graphite countermonochromator. For SIMS measurements, the Si-doped

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c-BN samples were sintered at 7.7 GPa and 2150  C. Detailsof the sintering method have been reported elsewhere.29 The
SIMS measurements were conducted with PHI ADEPT1010
using a Csþ beam. The primary beam acceleration voltage
was 5.0 kV. Undoped c-BN polycrystals for the SIMS standard specimen were prepared by high-pressure reaction sintering from h-BN at 7.7 GPa and 2200  C without additives.
Si-ion implantation was applied to the polished specimen atan energy of 65 keV, a dose of 1  1015 ion/cm2, and an ion
current of 36 nA/cm2. Si-K XANES was measured by the
total electron yield method with an InSb double-crystal
monochromator at BL2A in UVSOR, Okazaki, Japan.
C. First-principles calculations
1. Formation enthalpy

The formation enthalpies and atomistic structures of Si
dopants in c-BN at 0 and 7.7 GPa were calculated with theplane-wave basis projector augmented wave method30
implemented in the Vienna Ab-initio Simulation Package(VASP) code.31,32 The cut-off energy of plane waves was set
to 600 eV. The exchange-correlation functional was given by
the generalized gradient approximation presented by
Perdew, Burke, and Ernzerhof (GGA-PBE).33 Si-dopedc-BN was modeled using a 4  4  4 128-atom supercellof c-BN. The k-point sampling was carried out with
Monkhorst-Pack 2  2  2 mesh.34 Structural parameters
were fully optimized until the residual forces were less than
0.02 eV/Å and the deviations in stress from the target values,i.e., 0 and 7.7 GPa, were less than 0.05 GPa. The calculated
lattice constant at 0 GPa is 3.625 Å, which is close to an experimental value of 3.6159(1) Å.35 Convergence tests with acut-off energy of 800 eV and 4  4  4 k-mesh indicated thatthe total energies converged within less than 3 meV/atom.
In the c-BN crystal, Si might be substituted for both B
and N sites. In order to discuss the stability of these configurations, defect formation enthalpies were estimated.36 The
formation enthalpy of a defect in charge state q is given asXn l þ qeF ;DH ¼ Hdef i i iwhere Hdef , ni , li , and eF are the enthalpy of a defective supercell, the number of atoms and the atomic chemical potential
for element i, and the Fermi level, respectively. The atomic
chemical potentials were determined by equilibrium conditions
based on the experimental observation that c-BN was formedat the boundary between h-BN and Si. In order to simulate thisexperimental situation, we assumed two equilibrium conditions, BN-Si-B and BN-Si-Si3N4, denoted as B- and N-richconditions, respectively. We adopted c-BN, diamond-type Si,
and b-B and b-Si3N4 phases, and their enthalpies were used as
the chemical potentials of these host and reference phases,lBNðbulkÞ , lSiðbulkÞ , lBðbulkÞ , and lSi3 N4 ðbulkÞ . The chemical potential of Si was given as lSi ¼ lSiðbulkÞ under both B- and N-richconditions, and those of B and N were determined by lB ¼
lBðbulkÞ and lB þ lN ¼ lBNðbulkÞ under the B-rich condition,and 3 lSi þ 4 lN ¼ lSi3 N4 ðbulkÞ and lB þ lN ¼ lBNðbulkÞ under
the N-rich condition. The Fermi level in the Si-doped c-BN
sample is unknown, and therefore, it was treated as a variable

J. Appl. Phys. 114, 233502 (2013)

FIG. 1. XRD pattern of a Si-doped c-BN sample after acid treatments.

between the valence band maximum (VBM) and conduction
band minimum (CBM). In order to correct the finite-size
effects of the supercells with charged defects, we employed approximate third-order image charge corrections reported inRef. 37 in conjunction with electrostatic potential alignment
suggested in Ref. 38.
2. XANES simulations

Theoretical Si-K XANES spectra were calculated using
the augmented plane-wave þ local orbitals (APW þ lo)
method implemented in the WIEN2k code.39 RMTKMAX was
set to 6.0 bohrs Ry1/2. RMTs were set to 1.30, 1.30, and 1.50
bohrs for B, N, and Si, respectively. A core-hole effect was
included in the final (excited) state calculations. The samemodels and k-mesh as those in the formation enthalpy calculations were employed. Calculated spectra were broadened by
Lorenz functions with a natural width of Si-K shell.40
Convergence tests on RMTKMAX and k-mesh confirmed thatthe transition energy of Si-K XANES was converged within
0.01 eV. The transition energy of Si-K XANES was corrected
via the alignment of the strongest peak of a-Si3N4 between thetheoretical and experimental spectra. In other words, all the
calculated spectra were rigidly shifted by 9.1 eV.
III. RESULTS AND DISCUSSIONS

After the HPHT treatment, the samples were recovered
as aggregate form. Figure 1 shows the powder XRD pattern

FIG. 2. SIMS depth profiles of a Si-doped c-BN sample and references. Red
and blue thick lines show the concentration of Si and the secondary ion intensity of B for the c-BN sample. Orange and black thin lines denote thoseof the references.

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J. Appl. Phys. 114, 233502 (2013)

FIG. 3. (a) Experimental Si-K XANESof Si-doped c-BN and references. Notethat the Si powder is partially oxidized
and a SiO2 peak is detected at 1855 eV
in its spectrum. The vertical dashedlines indicate the main peak in the reference spectra. (b) Experimental(black) and calculated (blue) Si-K
XANES of Si-doped c-BN. The calculated spectra are rigidly shifted by9.1 eV (see text) and broadened by
Lorenz functions with a natural
width.40

of a Si-doped c-BN sample after the acid treatment. All of
the peaks can be assigned to c-BN (ICDD PDF 35-1355). Sidoped c-BN powder was treated at 7.7 GPa and 2150  C so
as to sinter as a disk shape without any additives. The concentration of Si in the sintered body was characterized using
SIMS. As shown in Figure 2, there are 1020 atoms/cm3 of Si.
The concentration of Si does not depend on the depth fromthe surfaces to the inner parts, suggesting a homogeneous
distribution of Si in the c-BN bodies.
In order to analyze the local environment of Si in c-BN
samples, Si-K XANES was measured. Figure 3(a) shows the
Si-K XANES of Si-doped c-BN and reference materials. SiK XANES clearly exhibits chemical shifts. Note that Si3N4
has a lower transition energy than SiO2 although the formal
charge of Si is the same. The spectrum of Si-doped c-BN has
a similar feature to a-Si3N4, but the fine structure is different
in that the shoulder peak of Si-doped c-BN lies at a lower
energy than that of a-Si3N4 at 1855 eV, for example. The
chemical shift of Si-doped c-BN is almost the same as
a-Si3N4. These results imply that there are Si-N bonds andSi4þ in c-BN.
For a detailed analysis of the experimental spectra, Si-K
XANES were simulated by first-principles calculations. In
our models, Si was substituted for the B or N sites. Since theexperimental Si-K XANES indicates that Si in our samples
has a similar chemical shift to a-Si3N4 with tetravalent Si,
we adopted charged defect models, Si•B (singly positively0charged) and SiN (singly negatively charged), which wererepresented in Kr€
oger-Vink notation. Figure 3(b) shows thecalculated Si-K XANES of Si-substituted c-BN. These models present clearly different features. Si•B has similar features
to the experimental spectrum: the strongest peak A, shoulder
B, and weak peak C in the calculated spectrum. In addition,0
this model reproduces the chemical shift. In contrast, SiN has
an absorption edge around 1845 eV and the strongest peak at
1863 eV. These features are different from those of the experimental spectrum.
Figure 4 shows the calculated formation enthalpies of
Si-doped c-BN at 7.7 GPa as a function of the Fermi level.
Under both of the B- and N-rich conditions and any Fermi
level position, the models for substitutional Si at the B site
are more stable than those for the N site. Lowther also
reported that SiB is more stable than SiN based on the resultsof first-principles calculations using the local density approximation.20 In our results, there are two stable models, Si•B
(singly positively charged) and SiB (neutral), depending on
the Fermi level. It is worth mentioning that the calculated
Si-K XANES spectra of these models have qualitatively thesame shapes. The Fermi level that changes the charge statecorresponds to the ionization energy when measured from•the VBM or CBM. The ionization energy from Si
B to SiB is0.18 eV at 7.7 GPa and 0.15 eV at 0 GPa, which underestimates the experimental value at ambient pressure, 0.24 eV.41
For more precise discussions, however, we need a larger celland a more accurate but computationally demanding approximation such as GW or a hybrid functional, which is beyond
the scope of this study. In fact, our previous work using a
432-atom cell and the PBE0 hybrid functional yielded an

FIG. 4. Formation enthalpies of substitutional Si in c-BN as a function of the
Fermi level under (a) B-rich and (b)N-rich conditions. Solid red, dashedblue, and bold black lines indicate thevalues for the B and N sites and theminimum at 7.7 GPa, respectively. The
minimum at 0 GPa is also indicated by
the dotted black line. Note that the calculated band gap only slightly depends
on pressure: 4.45 and 4.47 eV at 0 and
7.7 GPa, respectively.

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ionization energy of 0.18 eV at 0 GPa.22 The formation enthalpy of SiB has a tendency to slightly decrease at high pressure. This stabilization mainly results from a relatively large
increase in lSiðbulkÞ given by diamond-type Si due to its largest volume and hence PV term in enthalpy among the substances considered in this study.
IV. CONCLUSION

Si-doped c-BN was synthesized under HPHT conditions, and the local environment of Si was investigated. The
XRD pattern can be assigned to c-BN without any secondary phases. According to the SIMS measurements, there
were 1020 atoms/cm3 of Si in the c-BN sample. The Si-K
XANES of the Si-doped c-BN sample has a similar feature
to a-Si3N4, which implies that Si is bonded to N. According
to the first-principles calculations, the model for substitutional Si at the B site, Si•B , reasonably reproduces the experimental Si-K XANES spectrum. From an energetic point ofview, the model for substitutional Si at the B site is morefavorable than that for the N site. The experimental andtheoretical results consistently indicate that Si in c-BN issubstituted for B.
ACKNOWLEDGMENTS

This work was supported by JSPS KAKENHI Grant No.
2410639 (Grant-in-Aid for JSPS Fellows), the Use-ofUVSOR Facility Program (BL2A, No. 24-502) of theInstitute for Molecular Science, and MEXT Elements
Strategy Initiative to Form Core Research Center.
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