Materials Sciences and Applicatio ns, 2010, 1, 53-58
doi:10.4236/msa.2010.12010 Published Online June 2010 (
Copyright © 2010 SciRes. MSA
Nonequilibrium Diffusion of Boron in SiC at Low
Iikham G. Atabaev1, Tojiddin M. Saliev1, Erkin N. Bakhranov1, Dilmurad Saidov1, Khimmatali
Juraev1, Chin C. Tin2, Victor Adedeji2, Bakhtiyar G. Atabaev3, Nilufar G. Saidkhanova3
1Physical Technical Institute of Uzbek Academy of Sciences, Tashkent, Uzbekistan; 2Department of Physics, Auburn University,
Auburn, USA; 3Arifov Institute of Electronics, Uzbek Academy of Sciences, Tashkent, Uzbekistan.
Received February 24th, 2010; revised April 24th, 2010; accepted April 27th, 2010.
Nonequilibrium diffusion of Boron in 3C SiC was performed using a flow of carbon vacancies. The temperature of
diffusion was 1150-1250 and concentration of Boron in doped area reached about 1019 to 1020 cm -3. It is shown that
after thermal annealing in vac uum the characteri stics of fabricated str uctures are close to t hose of the structures made by
the conventional technology.
Keywords: Diffusion, Activa tion Energy, Silicone Carb ide, Annealing, Vacancy
1. Introduction
Silicon carbide is widely accepted as an ideal semicon-
ductor material for advanced high power and high fre-
quency electronic devices with potential applications in
spacecraft, aircraft, ships, land vehicles, communication
systems, as well as power generating, conversion, and
transmission systems. These radiation-hard, high per-
formance devices can operate in harsh environment and at
elevated temperature range beyond the capabilities of
conventional silicon devices. A key process step in the
fabrication of silicon carbide devices is controlled impu-
rity diffusion for p-type-conversion of selected n-type
areas in the active region of a device. The conventional
diffusion method for silicon carbide requires a high tem-
perature between 1600-2300. This high temperature
process is complicated, expensive, and can generate more
undesirable structural defects.
It is well known that mobile point defects play an im-
portant role in diffusion processes in semiconductors.
Experiments on doped SiC samples show that diffusion is
mediated by interstitials and vacancies [1,2]. To quanti-
tatively assess the role of interstitials and vacancies, ab
initio investigations of the relevant processes in 3C-SiC
were performed, which showed that carbon vacancies and
silicon interstitials in both p-type and intrinsic materials
were the most abundant mobile defects. Thus the interac-
tion between point defects and the diffusing impurity
atoms can be used to lower the temperature at which the
dopant can be introduced. In this paper, we report on the
diffusion parameters of boron diffusion in 3C-SiC in a
flow of carbon vacancies at low temperature regime in-
terval, 1100-1250, and present some electrophysical
2. Experimental Results and Discussion
The diffusion of boron in 3C-SiC in flow of Carbon va-
cancies was carried out at different diffusion temperatures
between 1100 and 1250 using the method described in
[3,4]. In this low-temperature diffusion technique, a thin
film of borosilicate glass is created on the surface of a
silicon carbide sample. The sample is then annealed under
special conditions in the air. Carbon atoms from the SiC
surface interact with oxygen and leave the SiC surface. As
a result a flow of carbon vacancies was created in the
near-surface region. Thus diffusion of Boron was per-
formed in nonequilibrium conditions in the flow of carbon
vacancies from the surface into volume of crystal.
Initial experiments were conducted on n-type 3C-SiC/
Si structures grown by chemical vapor deposition. The
thickness of the SiC epitaxial layer was about 5-6 m, and
the free electron concentration in the n-type 3C-SiC/Si
layers about 1017-1018 cm-3. Under the conditions of this
experiment, an anomalously high diffusion coefficient
and solubility of B was observed. After diffusion at
1150-1250 for 15-30 minutes, all the 3C-SiC/Si epi-
taxial films of the thickness 3-5 m became р-type. Cal-
culations showed that in this case the diffusion factor of
54 Nonequilibrium Diffusion of Boron in SiC at Low Temperatures
boron, D, was about 10-10 cm2/sec, and the solubility-NB =
1018-1020 сm-3. It should be noted that in the case of or-
dinary diffusion, the magnitude of DB ~ 10-10 cm2/sec can
only be observed at a very high temperature ~ 2100-2300
We suggested two possible reasons for this phenome-
non of an increase of the diffusion rate. The first possible
reason is the interaction of B atoms with a flow of va-
cancies from the surface to the bulk. The second possible
reason is the interaction of B atoms with strain-related
growth defects in the 3C-SiC/Si films. It is well known
that a large concentration of strain-related growth defects
can occur at the 3C-SiC/Si interfaces due to the large
lattice mismatch between 3C-SiC and Si. The large defect
concentration at the 3C-SiC/Si interface can influence the
diffusion processes near the surface in the case of thin
To reveal the role of growth-related defects, we con-
ducted also diffusion studies in homoepitaxial 6H-SiC/
6H-SiC structures, which had no growth defects from
lattice mismatch. In these structures, type conversion from
n-type to р-type was observed also at diffusion tempera-
tures 1150.
The distribution of B in the SiC/SiC samples was
measured by spreading current method. In this method,
the samples were polished by diamond paste at a gradient
of 1-3 degrees to the surface of the SiC film. Then the
distribution of specific resistance along the surface of the
SiC film was measured to obtain the distribution profile of
boron, NB(x).
On the basis of our experimental data, shown in Fig-
ures 1 and 2, the following parameters of B diffusion in
the presence of vacancy flow were evaluated: DB about
5.5 × 10-11-5.0 × 10-10 cm2/s in the temperature interval of
1150-1250; activation energy of B diffusion, a
0.9-1.15 eV. This activation energy of diffusion is typical
for interstitial mechanism of diffusion in ordinary semi-
Secondary ion mass spectroscopy (SIMS) is a direct
method to confirm the penetration of Boron atoms into
SiC in our diffusion experiments. SIMS measurements
showed (see Figure 3) that both atomic B1 and molecular
boron B2 were detected on the surface of SiC and also
deep within the bulk [5], as well as the peaks of boron
trimer, B3. In Table 1, the depth distribution of boron and
other elements and clusters measured by SIMS is pre-
sented. It is shown that atomic boron is present in the
whole diffusion area. Relative concentration of carbon
and silicon atoms is constant for all depths, concentration
of oxygen atoms lowered and that of lattice cluster ions,
, , , and , increased.
It is necessary to note that boron clusters are registered
on an initial surface of SiC<B>, and B2 and B3 are not
registered at the depth of about 0.024 mkm. Boron dimers
Table 1. Depth distribution of Boron and other elements and
clusters measured by SIMS
Depth, μm 0,024 0,12 0,24 0,36
В 17 20 20 20
C 40 40 45 45
O 120 100 100 80
OH 25 30 25 20
B2 0 5 5 10
C2 35 30 25 35
Si 40 40 45 45
O2 15 10 5 0
SiC 25 30 30 35
SiC2 15 15 20 20
Si2 20 25 25 25
Intensity of peaks, arb.u.
B3 0 0 5 10
1150 C
x, m
NB, arb.u.
115 0
Figure 1. Distribution of NB versus x after diffusion in non-
equilibrium conditions at 1150оС.
1200 C
NB, arb.u.
x, m
Figure 2. Distribution of NB versus x after diffusion in non-
equilibrium conditions at 1250оС.
B2 appear at the depth of 0.12 mkm, and boron trimmers B3
at 0.24 mkm. B2 and B3 intensity increased up to 10 at the
depth of 0.36 mkm. To perform semiquantitative analysis of
the relation between atomic and cluster boron, we need to
take into account an electron affinity of B, B2, B3.
The interesting feature of the spectra is the presence of
peaks of boron trimmer B3. The B2 boron dimers in silicon
are known from the literature, whereas the data for B3 are
Copyright © 2010 SciRes. MSA
Nonequilibrium Diffusion of Boron in SiC at Low Temperatures 55
0 10203040506
- or +10B11B2
I(h), arb. units
Figure 3. Mass spectra of negative ions from SiC<B> surface
spattered by Cs+ (E=2 keV at 300 C, time of spattering 1
8,10 8,15 8,20 8,25
(A) x, 10-8 m
N, cm-3
3C SiC<B>
2,40 2,41 2,42 2,43 2,44 2,45
3C SiC<B>
N, cm-3
x, 10-8 m
Figure 4. Distribution of Boron in the near surface area
obtained from CV data (3C SiC samples).
absent. In our opinion, this is due to nonequilibrium con-
ditions of diffusion and modified surfaces of SiC. The
clustering of boron obtained from SIMS data shows that
the boron diffusion in SiC can be provided by interstitial
boron diffusion mechanism with suppression of formation
of BX complex of boron with lattice atoms C and Si.
To define the concentration of Boron in the near surface
area, the Schottky barriers on doped layer of samples were
fabricated and their characteristics measured. Figures 4
and 5 shows the distribution of Boron obtained from CV
data of Ni-p-3C SiC<B>, p-6Н SiC<B>, and Ni-p-4Н
3,0 3,5 4,0 4,5 5,0
1E20 N, cm-3
x, 10-8 m
6H SiC
2,5 3,0 3,5
N, cm-3
x, 10-8 m
4H SiC
Figure 5. Distribution of Boron in the near surface area
obtained from CV data (6H and 4H SiC samples)
SiC<B> structures. As seen from figures, under diffusion
of B in a flow of carbon vacancies, an anomalously high
solubility of B in SiC is observed. The distribution of
doped impurity near the surface for samples on the basis
of 3C polytype of silicon carbide has a “quasiperiodical”
character. We believe that “quasiperiodical” distribution
of Boron is related to synergy processes, which can be
observed due to interaction between impurity and point
defects of crystal at high concentration [6].
2.1 Mechanism of Diffusion
The mechanism of a conventional high-temperature dif-
fusion of a Boron in a silicon carbide is rather complex
and there exist various points of view about this mecha-
In the work [7] (published in 2001), the diffusion ex-
periments with B-implanted 6H-SiC at temperatures be-
tween 1700 and 2100 were performed. After diffu-
sion, B concentration profiles were measured by means of
secondary ion mass spectrometry. Accurate modeling of B
diffusion was achieved on the basis of a kick-out diffusion
mechanism, which involves only point defects in the Si
In the work [8] (published in 2003), the authors, on the
basis of diffusion experiments at 2000-2200 in 6H SiC,
suggest that the boron impurity possesses two different
solubilities depending on occupancy at either Si or C
lattice sites. The solubility (6 – 9 × 1019 cm-3) in a Si va-
Copyright © 2010 SciRes. MSA
56 Nonequilibrium Diffusion of Boron in SiC at Low Temperatures
cancy is about one order of magnitude higher than that (1
– 10 × 1018 cm-3) in a C vacancy. According to [9], the
concentration of carbon vacancies in SiC at high tempera-
tures can be as high as 3 – 5 × 1017 cm-3, and, according to
authors of [8], the carbon vacancies will contribute to the
diffusion of boron in SiC. It can be speculated that the
boron atoms migrate into SiC bulk by occupying substi-
tutional carbon sites (BC) or forming associates of boron
and carbon vacancies (B-VC). The diffusion of boron via
carbon vacancies has a much higher diffusion rate due to
the existence of high-density carbon vacancies in SiC,
which lead to a boron profile with a fairly long tail.
In the work [10] (published in 2009), for a modeling of
diffusion of Boron in SiC, it is also supposed, that the
diffusion occurs both on carbon and silicon sublattices.
Reported profiles of high-temperature (500)-implanted
boron ions diffused in 4H-silicon carbide at 1200-1900
for 5-90 min were simulated through a “dual-sublattice”
modeling, in which a different diffusivity is assigned for
diffusion via each sublattice.
These approaches are related to conclusions of the work
[11], where the authors claim that most mobile states of
Boron are (ВC-VC) associates. These mobile associates
dissociate substantially during diffusion-annealing proc-
ess with formation of more stable (BSi-VC) associates.
On the basis of abovementioned, for our experiments
with a flow of carbon vacancies, we can speculate that the
nonequilibrium concentration of VC in the near surface
area of sample is very high. Most likely, the diffusion in
this area proceeds via mobile (ВC-VC) associates.
As to the low energy of activation about 0.9-1.15 eV,
determined in our experiment, we can mention the fol-
lowing. In the work [12], ab initio theory of point defects
and defect complexes in SiC was developed. It was shown
that for interstitial-mediated boron diffusion in 3C-SiC,
the migration and kick-out/kick-in barriers depended on
the charge state and started from 0.7 eV and higher.
Certainly, the further researches are necessary for the
specification of the mechanism of diffusion and large
solubility of Boron in our conditions.
2.2 Applicability of This Technology
Since the low temperature diffusion is performed in a flow
of carbon vacancies, the obtained p-SiC layers contain
defects and their characteristics are worse in comparison
with the structures fabricated by the conventional tech-
nology. Applicability of this technology is subject to
availability of the efficient ways of removing of defects in
the structures. Annealing experiments showed that an-
nealing of above mentioned Schottky barriers at 500, 700,
and 900 led to an improvement of IV and CV charac-
To study in more details the possibility of removing of
defects in SiC<B> samples, the influence of thermal an-
nealing on the electroluminescence spectra was investi-
gated [13].
Subsequent thermal annealing of the p-n structures was
made at temperatures between 550 and 600 for 5 min-
utes. Electroluminescence spectra were measured at room
temperature and reverse biases (15-20 V) were applied for
p-n samples before and after annealing. Density of reverse
current was about 2-3 A/cm2.
Figures 6 and 7 shows the electroluminescence spectra
of p-n junctions made by a low temperature diffusion of
boron in 3C-SiC epilayer with various thicknesses (1 and
12 microns). As shown in figures, the electrolumines-
cence spectra correlate strongly with the amount of de-
fects in the epitaxial films.
The deep Boron level in SiC is related to (BSi-VC)
center [11], and, in our case of diffusion in a flow of
carbon vacancies, we have to observe luminescence peaks
related with Bdeep: the peak at about 810 nm (nitro-
gen-deep- boron transitions) and the peak at about 760 nm
(EC-Bdeep transitions) (see Figures 6-8). There is also the
luminescence peak at 630 nm connected with carbon-
silicon divacancy [14]. It is clear that all these defects
were formed during the low temperature diffusion proc-
As is well known, the amount of growth-related defects
in SiC/Si structures decreases with an increase of the
thickness of the epitaxial layer. The thickness of this
strained transition layer is about 1.5-3 microns. Due to this,
400 500 600 700 800 900
25 I, a.u.
, nm
LTDB(1250C,30 min)
p-n 3C SiC/n 6H SiC
d=1 micron
Figure 6. Spectrum of an electroluminescence of p-n junc-
tion made by low temperature diffusion of Boron in 3C SiC
film (thickness-1 micron)
400 500 600 700 800 900
1,0 p-n-3C SiC/6H SiC
d=12 micron
, nm
I, a.u.
Figure 7. Spectrum of an electroluminescence of p-n junc-
tion made by low temperature diffusion of Boron in 3C SiC
film (thickness-12 micron)
Copyright © 2010 SciRes. MSA
Nonequilibrium Diffusion of Boron in SiC at Low Temperatures 57
the peaks at 630 and 760 nm in a thin (1 μm) epilayer look
more distinctly than in a thick epilayer (12 μm).
The optical absorption spectra of the p-n structures
were also measured at room temperature. The optical
absorption data were in good agreement with the elec-
troluminescence data. To study the effect of annealing on
the defect structure, we annealed the p-n junctions at the
temperature about 550-600 during 5 minutes. The re-
sulting luminescence spectrum of an annealed sample is
given in Figure 8. A reduction in the intensity of the
luminescence peaks connected with deep Boron related
defects (760 and 810 nm), and carbon-silicon divacancies
(630 nm), entered during low temperature diffusion, was
observed. For a comparison, an electroluminescence
spectrum of the epitaxial p-n junction fabricated on 3C
SiC by the conventional chemical vapor deposition (CVD)
technology is shown in Figure 9. In this case, the crys-
talline quality of the sample is better with a lower con-
centration of growth defects. A luminescence peak is
observed at about 530 nm corresponding to an energy gap
about 2.35 eV for 3C-SiC polytype.
A comparison of Figures 8 and 9 shows that the lu-
minescence spectra of p-n junctions fabricated by low
temperature diffusion followed by thermal annealing are
close to the spectrum of epitaxial junction fabricated by
the conventional CVD. At the same time, it is necessary to
note that the peak at 490-500 nm is clearly observed for
non-annealed samples and weakly visible on samples after
annealing. We believe that this peak is due to transitions
of hot electrons accelerated at applied reverse bias from
minimum of conductivity band in L1 direction to the top
of the valence band.
Thus it is shown that after thermal annealing in vacuum
the characteristics of fabricated structures are close to
those of the structures made by the conventional tech-
3. Conclusions
Nonequilibrium diffusion of Boron in 3C, 4H SiC was
400 500 600 700 800 900
, nm
I, a.u.
p-n 3C-SiC/n-6H-SiC
1 d=3 micron
2 d=6 micron
~550-600C,5 min)
Figure 8. A spectrum of electroluminescence of p-n junctions
for SiC films with a thickness of 3 and 6 microns after an
400 500 600 700 800 900
70 I a.u.
3C-SiC (6 mcm)/6H-SiC
p-n junction
fabricated by CVD
Figure 9. A spectrum of an electroluminescence of p-n 3C
SiC/6 H SiC junction fabricated by growth of p and n – SiC
layers by CVD.
performed using a flow of Carbon vacancies. The tem-
perature of diffusion was 1150-1250, and the concen-
tration of Boron -1019-20 cm-3. It is shown that thermal
annealing in vacuum can be used to improve the charac-
teristics of fabricated structures.
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