SURFACE AND INTERFACE ANALYSIS
Surf. Interface Anal. 30, 580–584 (2000)
Electron spectroscopy of single-phase (Al,B)N
films
Mirjam Witthaut, Rainer Cremer* and Dieter Neuschütz
Lehrstuhl für Theoretische Hüttenkunde, RWTH Aachen, D-52056 Aachen, Germany
In the present paper the metastable solid solubility between BN and AlN in the wurtzite structure of AlN
has been investigated. Ternary Al–B–N films as well as the binaries AlN and BN have been deposited
by reactive magnetron sputtering on Si(111) wafers. The composition, binding states of the components,
electronic structure, crystallographic structure and texture of the films have been analysed by means of
XPS, x-ray-induced Auger electron spectroscopy (XAES), electron energy-loss spectroscopy (EELS), x-ray
diffraction (XRD) and reflection high-energy electron diffraction (RHEED). Up to a BN content of 33 mol.%,
all films were deposited strongly textured in the single-phase sp3 -bonded wurtzite structure of AlN. At higher
boron nitride contents up to 68 mol.%, the films were still sp3 -bonded but the binding states differed from
those of the more AlN rich films. Only a pure BN film exhibited sp2 bondings, corresponding to the stable
hexagonalBN modification in graphite-like structure. Copyright  2000 John Wiley & Sons, Ltd.
KEYWORDS: AlN; BN; (Al,B)N; solid solutions; magnetron sputtering; group III nitride
INTRODUCTION
Nitridic III–V semiconductors gain increasingly in significance for electronic and optoelectronic applications.
One main aspect is the complete solid solubility of AlN,
GaN and InN, through which the bandgap can be adjusted
between 1.9 and 6.2 eV,1,2 corresponding to wavelengths
from the infrared well into the ultraviolet range. Further
advantages of the group III nitrides are a good thermal
conductivity, a high thermal stability and a high mechanical strength.1,3 Consequently, the group III nitrides are
suitable for high-power, high-frequency electronics and
short-wavelength optoelectronics.1
Owing to its different crystal structure and binding
states, BN does not form solid solutions with the other
group III nitrides under equilibrium conditions. In the
Al–B–N system no ternary phases exist. The deposition
of metastable solution phases would be desirable because
with four group III nitrides the bandgap and lattice parameters could be adjusted more independently.1 This paper
deals with the examination of the metastable solubility
range of BN in AlN under the non-equilibrium conditions
of reactive magnetron sputtering. Special attention was
paid to the existence of single-phase (Al,B)N in the sp3 bonded wurtzite structure of AlN, which is suitable for
electronic applications.
EXPERIMENTAL
Films of Al–B–N were deposited on Si(111) wafers in
a Leybold L 560 magnetron sputtering device (residual
* Correspondence to: R. Cremer, Lehrstuhl für Theoretische
Hüttenkunde, RWTH Aachen, D-52056 Aachen, Germany.
E-mail: [email protected]
Copyright  2000 John Wiley & Sons, Ltd.
gas pressure <1 ð 10 4 Pa) at a substrate temperature
of 100 ° C. The wafers were etched in an ammonium
fluoride mixture to remove the native oxide. Aluminium
was sputtered in d.c. mode and hexagonalBN in r.f.
mode simultaneously in a pure nitrogen plasma (3.4 ð
10 1 Pa). The targets were mounted on PK90 cathodes.
The following sputtering power ratios PDC.Al/ /PRF.hBN/ (W)
were adjusted: 0/300 (film 1), 50/300 (film 2), 100/300
(film 3), 200/300 (film 4), 300/300 (film 5), 300/200
(film 6), 300/100 (film 7), 300/0 (film 8). The deposition
time was 120 min in each case.
The analysis of the films by electron spectroscopy and
electron diffraction was carried out quasi-in situ after the
transfer of the films in a high vacuum, using a lock system,
into the ultrahigh vacuum (UHV) analysis chamber, thus
avoiding severe oxidation of the surface. Both XPS and
x-ray-induced Auger electron spectroscopy (XAES) spectra were taken using a PSP Vacuum Technology TX400
x-ray source operating with non-monochromized Al K˛ xrays at 300 W. Using a DESA 100 energy analyser (Staib
Instruments) in the pulse counting mode, XPS spectra
were recorded in the range of 0–600 eV binding energy,
resolution 3 eV, step 0.5 eV and total acquisition time per
channel 5 s. The XAES spectra were acquired with 3 eV
resolution, step 0.5 and total acquisition time per channel
6.5 s, and the valence-band XPS spectra with 3 eV resolution, step 0.25 eV and total acquisition time 6.5 s. The
electron energy-loss spectroscopy (EELS) spectra in the
reflection mode were excited using an EK-12-R electron
gun (Staib Instrumentes) at a primary electron energy of
1 keV. They were recorded with 3 eV resolution, step
0.5 eV and total acquisition time per channel 5 s. For
quantification, the Al 2s, B 1s, N 1s and O 1s peaks were
considered. The background was subtracted according to
the method of Bishop and the relative sensitivity factors
were determined using AlN, hexagonalBN and Al2 O3 as
standards. For change correction only the weakly visible C 1s peak at 285 eV could be used, because no Ar
Received 15 July 1999
Revised 1 December 1999; Accepted 1 December 1999
SINGLE-PHASE (Al,B)N FILMS
was incorporated during film deposition and no sputter
cleaning was performed. Structure and texture analysis
by reflection high-energy electron diffraction was carried
out in the same device using an EK-35-R electron gun
operating with 35 keV electrons at grazing incidence to
the sample surface. To examine the bulk crystal structure, x-ray diffraction was performed in a Siemens D500
goniometer with grazing incidence attachment at an angle
of incidence of 3° to the surface. Spectra were taken in
the 2 range of 20–80° using 900 W Cu K˛ radiation
(0.154186 nm).
RESULTS AND DISCUSSION
The composition of the films is given in Fig. 1. A slight
oxidation at the surface of the films could not be avoided
under the high vacuum conditions in the sputtering device
and the lock system. The composition of the binary films 1
and 8 corresponds within an accuracy of 2% relative
to stoichiometric BN and AlN, neglecting the oxygen
content. The nitrogen content remains almost constant
at a ¾50 at.%, whereas the aluminium content increases
continuously from film 1 to film 8.
The XPS and XAES spectra of the different transitions
are shown in Fig. 2. For the BN and the AlN film the line
positions are in good agreement with literature values;
some deviations in the case of the Auger transitions arise
from the differences between direct spectra as measured
in this paper and the differentiated spectra often measured
in the literature. The position of the N 1s peak in AlN is
in the range of 396.4–396.8 eV given by Kovacich.4 In
the case of BN, the peak position corresponds well to
398.0–399.0 eV of different authors.5 – 8 The B 1s peak
in BN measured with 190.5 eV binding energy is in the
interval of 190.0–191.0 eV.6 – 10 For the A1 2p line in
AlN the value of 72.7 eV is lower than common values
of 73.5–74.5 eV.8,11 – 13 The kinetic energy of the N KVV
transition in BN is slightly lower than the 379.6–380.2 eV
581
given in the literature.5,8 The Al KL23 L23 line is placed in
the range of the literature values of 1388.9–1393 eV.8,12,14
The XAES B KLL transitions cannot be evaluated due to
their low intensity in consequence of the low ionization
cross-sections of boron with Al K˛.15
In Fig. 2 a continuous shift of the N 1s peaks and
N KLL Auger transitions can be observed. Assuming
that in solid solutions between BN and AlN boron atoms
would substitute aluminium lattice sites, as in the case of
(Al,Ga,In)N, the different ionic character of the bondings,
proceeding from the difference in electronegativity of Al
and B, would mostly affect the nitrogen electrons. In
the case of the ternary films no two-peak structure of
the N 1s and N KVV lines is detectable, as could be
expected from a two-phase mixture of AlN and BN, but
rather a continuous shift with increasing boron content,
which is a first indication of single-phase ternary films.
A slight broadening of the full width at half-maximum
(FWHM) of the N 1s peaks of films 2 and 3 (¾0.3 eV
compared to films 1 and 4–8) indicates a further binding
state of nitrogen in these films. Because the quantitative
analysis has shown that the boron content increases at a
constant nitrogen content, one can assume that at least
in the case of samples 4–7 (Al,B)N films have been
deposited. The partial ionic character is more pronounced
in AlN than in BN, leading to a decrease of the binding
energy of the nitrogen electrons with increasing AlN
content. Furthermore, the polarizability of AlN is higher
than for BN. The binding states of aluminium and boron
are less influenced by the composition, because there is no
change in the chemical surrounding of these atoms. With
increasing Al content the negative charge at the aluminium
atoms decreases and thus the binding energy of the Al 2p
electrons increases. The shift of the boron peaks can be
explained analogously.
The modified Auger parameter ˛0 of nitrogen [Fig. 3(a)]
can be used for the identification of chemical binding
states, especially of insulating materials, where additional
charge effects lead to peak shifts in electron spectra.16
Figure 1. Composition of the Al B N films as determined by XPS.
Copyright  2000 John Wiley & Sons, Ltd.
Surf. Interface Anal. 30, 580–584 (2000)
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M. WITTHAUT ET AL.
Figure 2. The XPS and XAES spectra of nitrogen, boron and aluminium in the Al B N films.
Figure 3. Modified Auger parameter of nitrogen (a) and amount of ionic character (b) of Al B N films 1 8.
Surf. Interface Anal. 30, 580–584 (2000)
Copyright  2000 John Wiley & Sons, Ltd.
SINGLE-PHASE (Al,B)N FILMS
Except for the boron-rich films 2 and 3, the Auger
parameter increases continuously with increasing Al content from 776.2 eV in BN to 777.7 eV in AlN. This
obviously indicates the continuous change of the chemical binding state of nitrogen and thus the single-phase
structure of the ternary films 4–7. The energetic distance between the N KVV and the N KLV transition is
a measure of the amount of ionic character of nitridic
bonds. As revealed by Wagner17 using the O KLL transition of oxides, this distance decreases with increasing ionic character of the bond. In the case of the
Al–B–N films a continuous shift to lower values with
increasing Al content is visible [Fig. 3(b)], only films 2
and 3 exist in another bonding, which also differs from
pure BN.
The EELS spectra of the low-loss region are shown
in Fig. 4. They are dominated by the bulk plasmon,
which shifts with increasing boron content to higher loss
energies. The –Ł transition at 8.0 eV is detectable
only in the case of the pure boron nitride, i.e. evidently only in this film do sp2 -bondings exist, typical
for the graphite-like structure of hBN. This transition
originates from the shake-up of delocalized -electrons,
which are not present in sp3 -bonded structures. The
–Ł transition is also detectable at the high binding
energy side of the N 1s and B 1s peaks of film 1 in
Fig. 2.
Figure 5 shows the valence-band spectra of the films.
The background was subtracted according to the method
of Bishop.18 In the case of the AlN film 8, the peaks
are designated according to French.19,20 The state at the
higher binding energy side is often indicated as Al 3s.
In the sp3 -bonded AlN the four hybridized orbitals form
four equivalent bonds between the different atoms in the
Figure 4. The EELS spectra of the Al B N films.
Copyright  2000 John Wiley & Sons, Ltd.
583
Figure 5. Valence-band XPS spectra of the Al B N films.
hexagonal lattice. Elemental aluminium has three valence
electrons and elemental nitrogen has five. According to
Pauling,21 nitrogen transfers one electron partially to aluminium, enabling the formation of four sp3 -hybridized
orbitals and stabilizing the compound. Consequently it
is assumed that in the valence-band spectra the original valence electrons of aluminium and those obtained
by electron transfer can be distinguished. In the case of
the hBN film 1, the states are designated corresponding
to the results of Siegbahn and co-workers.7 The upper
valence band is formed by the N 2p -electrons of
the sp2 -hybridized hexagonalBN. The peak at 10.8 eV
often is designated as B 2s.22 The valence-band spectra
of the ternary Al-rich films 4–7 show two states similar to AlN. In the case of the B-rich films 2 and 3,
an additional state is observable, interpreted analogously
as pure BN or B–N bonding. Because this third state
is not detectable in spectra 4–7, boron is probably dissolved in the AlN lattice in these films. In accordance
with the XPS and XAES results, the electronic structure of the B-rich films 2 and 3 obviously differ from
that of the other ternary phases, as well as from that
of hexagonalBN. Presumably these two films consist of
a heterogeneous mixture of (Al,B)N or AlN, on the
one hand, and wBN (BN in wurtzite structure) on the
other hand.
Reflection high-energy electron diffraction (RHEED)
and x-ray diffraction (XRD) analyses of the near-surface
and bulk structure of the films confirmed the results of
electron spectroscopy. The RHEED pattern of film 5 is
shown in Fig. 6. The diffraction patterns of films 5–8 correspond to the hexagonal wurtzite structure of AlN. Film 4
also shows a diffraction pattern but it is too weak for
evaluation. In the case of films 1–3 no structure or strong
charge effects were observed. Films 4–8 are strongly [h0l]
textured. The XRD spectra (Fig. 6) revealed that films 4–8
Surf. Interface Anal. 30, 580–584 (2000)
584
M. WITTHAUT ET AL.
Figure 6. Structure analysis of the films by RHEED and XRD.
are deposited in wurtzite structure. The peak positions of
the pure AlN film correspond well (š0.0003 nm) with
the positions given in the ICDD card 25–1133. The lattice parameters decrease with increasing boron content
due to the substitution of Al by the smaller boron atoms.
The spectrum of film 4 reveals a less pronounced crystallinity compared to the more Al-rich films. This was
confirmed by SEM images. This effect probably is associated with the maximum solubility of BN in the AlN lattice,
which in the case of film 4 is almost reached under the
present deposition conditions. Films 1–3 are nearly x-ray
amorphous; broad peaks also correspond to the wurtzite
structure.
CONCLUSIONS
The results have shown that for the first time single-phase
ternary (Al,B)N films with a maximum BN content of
at least 33 mol.% could be deposited on Si(111) wafers
by reactive magnetron sputtering. These films have been
grown at 100 ° C in the strongly textured wurtzite-type
structure of AlN.
Acknowledgement
The financial support granted by the Deutsche Forschungsgemeinschaft,
contract No. Ne351/23-1 is gratefully acknowledged.
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Copyright  2000 John Wiley & Sons, Ltd.