Materials Sciences and Applicatio ns, 2011, 2, 1205-1211
doi:10.4236/msa.2011.29163 Published Online September 2011 (
Copyright © 2011 SciRes. MSA
Influence of Defects on Low Temperature
Diffusion of Boron in SiC
Ilkham G. Atabaev1, Tojiddin M. Saliev1, Dilmurad Saidov1, Vadim A. Pak1, Khimmatali Juraev1,
Chin-Che Tin2, Bakhtiyar G. Atabaev3, Vyacheslav N. Giryansky3
1Physical Technical Institute of Uzbek Academy of Sciences, Tashkent, Uzbekistan; 2Department of Physics, Auburn University,
Alabama, USA; 3Arifov Institute of Electronics, Uzbek Academy of Sciences, Tashkent, Uzbekistan.
Received January 18th, 2011; revised March 17th, 2011; accepted May 30th, 2011.
The low temperature diffusion of Boron in bulk SiC crystals is investigated and simplified model of such diffusion is
presented. The method of UV stim ulated etching by a queous solution of KO H is proposed and some exp erimental data
on influence of defects on quality of prepared p-n junctions are presented.
Keywords: Diffusion, Activation Energy, Silico ne Carbide, Annealing, Vacancy
1. Introduction
Due to its unique material properties, silicon carbide (SiC)
is very prospective material for fabrication of high power,
high temperature electronic devices that can operate in
harsh environment. One of the constraining factors in the
fabrication of silicon carbide devices is the necessity of
using high temperature of about 1800˚C - 2200˚C to in-
troduce impurity by thermal diffusion. Due to the rela-
tively low melting temperature of silicon (1412˚C), con-
ventional high temperature diffusion of shallow impuri-
ties in SiC epilayers grown on Si is impossible. In the
case of doping by ion implantation, a post-implantation
dopant activation annealing at ~1600˚C is needed. High-
temperature thermal processing can cause surface dete-
rioration resulting in poor device characteristics. There-
fore, a reduction in processing temperature for impurity
introduction in SiC is important.
To reduction of temperature of diffusion of impurity in
SiC up to 1250˚C - 1 300˚C we are proposed and investi-
gated new method of low-temperature diffusion of boron
[1-4]. In this method, on surface of SiC crystal creates a
layer of borosilicate glass, then sample annealed in air.
As we have expected, in these conditions, there is the
flow of carbon vacancies, which allows to reduce the
temperature of diffusion. Experiments have shown that
the temperature of diffusion decreased to 1150˚C -
1250˚C. Boron concentration reached a high value to the
1020–21 cm–3.
Although this technique has been demonstrated to be
applicable to the fabrication of SiC devices [4], further
work needs to be done to optimize the technique in order
to improve the surface morphology and reduce crystal-
line defects and to improve p-n junction characteristics.
To achieve these goals, further research of mechanism
of low-temperature diffusion of boron is necessary. As
mentioned above, we have assumed that on the surface of
SiC crystal a flow of carbon vacancies creates. In this
case, the diffusion area is saturated by carbon vacancies
and the reducing of diffusion temperature related with
interaction of impurity with VC. However, it seen from
experiments, a significant part of Boron atoms after dif-
fusion was in the electrically active state in the silicon
sublattice of SiC. It means that during the diffusion con-
centration of VSi vacancies should be large. Concentra-
tion of Boron was also very high! Thus the need to find
out—how occurs the large concentration of silicon va-
cancies during low-temperature dif fusion.
In the defect enhanced diffusion the method of proc-
essing the crystal surface is very important. It is always
important, but in our case, diffusion is stimulated by de-
fects (VC, VSi, VSiC) that interact with structural defects.
So, the quality of the crystal surface in our method has
played a key role. That is why it is necessary to conduct
our research together with the study of etching of the
In a previous article [4] the results of research carried
out on SiC/Si and SiC/SiC structures were presented. In
Influence of Defects on Low Temperature Diffusion of Boron in SiC
this paper, we report on the simplified model of low-
temperature diffusion of impurity in bulk SiC crystals,
some experimental data on etching and influence of sur-
face morphology on IV characteristics are presented.
2. Experimental Results and Discussion
2.1. Physical Approaches for the Implementation
of Low-Temperature Diffusion
It is well known that mobile point defects play an impor-
tant role in diffusion processes in semiconductors. And to
reduce the temperature of diffusion the increased con-
centration of mobile point defects can be used. One of
the possible ways of introducing point defects in crystal
is the irradiation of samples during diffusion process.
In [5] described the results of studies of radiation-
stimulated diffusion of boron in silicon and silicon car-
bide irradiated with protons. It is shown that in silicon
the temperature of diffusion falls, however, in silicon
carbide does not. As shown in [5] a significant decrease
in temperature diffusion of cesium (up to temperatures of
1150˚C - 1300˚C) occurs when the amorphization of SiC
samples. Of course, this path can not be regarded as op-
timal for the manufacturing technology of semicon ductor
On our opinion, the surface oxidation of silicon car-
bide is a next possible way to effective introduction of
point defects (vacancies) in the crystal. Our method of
diffusion [1-4] was a modification of the surface oxida-
tion. We then assumed that the crystal surface of silicon
carbide is formed vacancy flux of carbon. However, we
recently found a great article of S. C. Singhal on oxida-
tion kinetics of silicon carb ide [6].
In this work [6] is described that at temperatures
1100˚C - 1400˚C SiC exhibits two types of oxidation
behavior, “active” and “passive”, depending upon the
ambient oxygen potential. At high oxygen pressures,
“passive” oxidation occurs wherein a protective film of
SiO2(s) is formed on the surface by the reaction:
2SiC(s)3O (g)2SiO (s)2CO(g) 
At low oxygen potentials, severe “active” oxidation
occurs due to the formation of gaseous products accord-
ing to reactions:
SiC(s)2SiO (s)3SiO(g)CO(g) (2)
SiC(s) O(g)SiO(g) CO(g) (3)
Active oxidation of SiC occurs only at oxygen pres-
sures lower than ~3·10–4 atm at 1400˚C.
As seen from Equations (2-3) surface oxidation of
silicon carbide in this co nditions lead s to produ ction flow
of both as Carbon an d Silicon vacancies from th e surface
into the bulk of the crystal (May be mostly of carbon
vacancies). Obviously, the vacancy concentration in this
case can be much higher as compared with the introduc-
tion of vacancies by irradiation.
2.2. Distribution of Boron Impurity in Samples
after Low—Temperature Diffusion
The diffusion of Boron in SiC was performed in non-
equilibrium conditions in the flow of vacancies from the
surface into volume of crystal using the method de-
scribed in [1,2]. In this low-temperature diffusion tech-
nique, a thin film of borosilicate glass is created on the
surface of a silicon carbide sample. Then sample is an-
nealed in the air. Temperature of diffusion was different
between 1150˚C and 1300˚C.
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 crystal. Then
the distribution of specific resistance along the surface of
the SiC film was measured to obtain the distribution pro-
file of boron, NB(x).
Diffusion in crystals of silicon carbide of various
polytypes was carried out as well. Since diffusion is
stimulated by defects the inhomogeneity of the distribu-
tion of impurities and defects in the crystals play an im-
portant role. As a result, the surface and depth distribu-
tion of impurity in p-area is nonun iform. Figure 1 shows
the distribution of the impurity measured in different
points of the 3C-SiC crystal and distribution of Boron in
Estimates have shown that for bulk crystals of silicon
carbide activation energy of diffusion Ea~ 0.9 - 1.2 eV as
was determined in experiments with SiC/SiC het-
erostructures in [4].
2.3. Some Remarks on Possible Mechanism of
Low-Temperature Diffusion of Boron
In our previous work [4], based on an analysis of litera-
ture on diffusion mechanisms in SiC [7-11] has been
assumed that the accelerated diffusion takes place via
mobile (Bc-Vc) associates.
However, according to expressions (2,3) silicon va-
cancy are also formed on the surface of the crystal. Thus
it can be assumed that the diffusion of impurities goes
via Si and C vacancies at different rates and as described
in [8] with a different solubility in Si and C-sublattices.
To test this hypothesis in samples without the boro-
silicate glass layer a boron diffusion was performed in
vacuum with low oxygen pressure (as in work [6]). After
diffusion at 1200˚C in SiC crystal the p-type layer was
created. Thus, according to expressions (2,3) a flow of Vc
and VSi vacancies is creates and the stimulated diffusion
occurs. However, in the obtained samples p-region was a
Copyright © 2011 SciRes. MSA
Influence of Defects on Low Temperature Diffusion of Boron in SiC1207
NB, arb.u.
depth, micron
NB, arb.u.
depth, micron
4H SiC
Figure 1. (a) Distribution of NB in different points of p-area
of 3C SiC after diffusion in nonequilibrium conditions at
1200˚C; (b) Distribution of NB versus x after diffusion in 4H
SiC crystals in nonequilibrium conditions at 1200˚C.
very inhomogeneous, most of the p-n junctions has al-
most linear characteristic. Detailed results on diffusion of
SiC in vacuum will be published in the next article after
some additional experiments.
Let us return to our experiments. Since the diffusion
occurs at a constant generation of vacancies at the crystal
surface installs a quasi-steady state in which the vacancy
concentration has maximum value at the surface and
tends to zero at some depth. The diffusion of impurity
occurs only in the area saturated by nonequilibrium va-
cancies. So the impurity distribution is determin ed by the
distribution of vacancies. Obviously, in this case, the
diffusion depth weakly depends on the diffusion time.
Consider the distribution of vacancies at generation of
VC and VSi on the surface with a constant rates (It was
assumed that the mechanisms of diffusion in all
polytypes of silicon carbide are similar). Diffusion equa-
tions can b e w rite as:
Si SiC
SiSi SiSiC
Here, VC, VSi VSiC concentration of carbon and silicon
vacancies and [VCVSi] divacancies; D diffusion coeffi-
cients; (VSiC) —term accounts for decrease in VC, Vsi of
the formation of divacancies [VsiVc]; G—secondary gen-
eration of vacancies due to the disintegration of VSiC di-
vacancies. Additionally to main reactions (CSi SiC
and SiCCSi ) there are following reactions of
annihilation of vacancies: interaction with impurities and
the formation of impurity-vacancy complexes, annihila-
tion reactions with dislocations, microcracks and other
point and 2D, 3D defects of SiC crystal. These reactions
are varied and their consideration is very complicated.
In the work [12] considering the radiation defect for-
mation in SiGe to account for different mechanisms of
annihilation of point defects (except the main reaction)
introduce parameters
having the dimension of time -
“effective lifetime”. In our case, as all above mentioned
reactions leads to annihilation of vacancies we also in-
troduce the effective parameters . These parameters
describe the interaction with defects of crystal in diffu-
sion area. Parameters can vary widely, depend
sharply on mechanical (cutting, polishing) and thermal
treatments, which performed on a sample.
Assuming that the formation of divacancies is propor-
tional to the product of vacancy concentration CSi
and secondary generation is proportional to the concen-
tration of divac a nc i e s SiC
SiSi SiCSi SiC
In the steady state (C0
, Si0
, SiC 0
2SiSiCSi SiC
Assuming that in the near-surface area of sample the
main factor is the formation of divacancies obtain the
followin g sol ution for dist ri b u t i on of VSi and VC:
Copyright © 2011 SciRes. MSA
Influence of Defects on Low Temperature Diffusion of Boron in SiC
Si 2
Integration constants CC and CSi can be found fro m the
initial conditions: the generation rate of vacancies on the
surface is constant (the derivative at zero is equal -GC,
The maximum concentration of vacancies at the sur-
face is proportional to G2/3:
1/3 2/3
1/3 2/3
Si 2/3
The analysis shows that the influence of other mecha-
nisms of annihilation of vacancies (taking into account
effective lifetime of the vacancies) leads to a
change in the shape of the distribution of vacancies. But
maximal value Vc(o) and Vsi(0) on the surface almost not
changes and depends only from G. If the defect distribu-
tion is nonuniform on depth (distribution
also) the
distribution of impurities becomes irregular (not smooth)
as in Figure 1.
2.4. UV Etching of SiC
As can be seen from Figure 1, the distribution of boron
impurities in various locations of the crystal is different.
Amplitude of boron concentration in distribution along
surface is changes. Measurement of the distribution of
specific resistance along the crystal surface shows that
the p-region is inhomogeneous. As a result, only 25% -
30% of produced p-n structures have a nonlinear I-V
characteristics, reverse breakdown voltage is low (up to
5Volt). Thus, the value of
inhomogeneously distrib-
uted in di ffusio n a re a of s amples.
As follows from our simplified mechanism the diffu-
sion of impurities strongly modulated by defects in the
surface-damaged layer of SiC samples. In conventional
semiconductors the depth of surface-damaged layer
reaches a 5 - 6 microns and depends from method of
technological processing. In this regard, it is important
to develop a method of etching silicon carbide is not
only to clean the surface, but also to remove the surface-
damaged layer.
Silicon carbide is a chemically stable compound. It is
not etched by conventional acids. They are used just for
surface cleaning of SiC. And when it is necessary to re-
moving of SiC layer an ion etching or special types of
etchants are used. For example, the melt of KOH at tem-
peratures 500˚C - 900˚C [13,14] used for etching for the
manufacture of SiC devices. An anodic etching at room
temperature in a solution (HF + C2H5OH + HNO3 ) at the
current density of 20 - 120 mA/cm2 was used for the
manufacture of porou s silicon carbide [15,16].
Etching of silicon carbide by liquid metals, for exam-
ple, Manganese at 1200˚C, was carried out in [17] and by
liquid Aluminu m [18].
As it is evident from the literature, this process for
silicon carbide is very time-consuming and complicated,
leading to increased cost of SiC devices. Widely used
now methods of surface sputtering are introduced defects
into a thin surface layer, which in our method of diffu-
sion plays a crucial role. That is why it is important to
develop a new method of etching, which does not intro-
duce defects and provides a smooth surface.
In this section, the etching of silicon carbide in a KOH
solution stimulated by electrical current and UV radiation
at room temperature was investigated. Our objective was
to obtain a mirror-smooth surface of silicon carbide
without the use of high-temperature processes.
Well known it is very difficult to make selective etch-
ing using the high-temperature processes. That is why we
tried to do etching of silicon carbide in aqueous KOH
(30% - 50%) at room temperature, stimulating by elec-
trical current and UV radiation.
Since the process occurs at room temperature in a rela-
tively inactive medium for selective etching as a mask
could be used ordinary chemically resistant lacquer or
even masking tape.
Well known [19] at the border of semiconductor—
electrolyte forms a double barrier layer. And application
of the electrical current can make a significant impact in
the electrolytic etching mechanism [15]. The light if it is
generate a surface holes is also can be used for stimula-
tion of etching [20]. As is known, [21] for etching of
semiconductors can play an important role excitation of
surface states, when a surface holes or two-hole state are
created. Time of localization of the hole in the bulk of
crystal is ~ 10–15 seconds and th e bonds in crystal do not
manage to break off. On the surface time of localization
of holes considerably higher ~10–14–13 sec. And stimula-
tion by light quanta with energy above the energy band-
gap leads to formation surface two-hole states, resulting
to creation of a weak and dangling bonds on the surface.
Thus, ultraviolet light (UV) can induce etching of silicon
Initially in our experiments the etching was carried out
at room temperature without active mixing of the solu-
tion. For n-type crystals at 300 K a very slow etching of
Copyright © 2011 SciRes. MSA
Influence of Defects on Low Temperature Diffusion of Boron in SiC
Copyright © 2011 SciRes. MSA
about 0.05 - 0.1 micron/hour is observed. Rate of etching
of SiC crystals with high-resistance p-layer is also slow.
Etched surface is uneven, has a matte color.
Experiments have shown that stimulation by electrical
current does not give positive result: the surface of the
crystal is not uniform and not smooth. Etching highlights
areas with a high resistivity or with non-uniform thick-
ness of the oxide layer. The etching rate reaches 1 mi-
Further experiments showed that under UV illumina-
tio n o f t h e e t ch in g r at e a ls o reaches 1 - 2 micron/h (which
is 10 times higher than without light stimulation). UV
stimulated etching with active mixing of solution allowed
to obtain a sufficiently smooth surface of SiC crystal.
The Figure 2 shows photographs of surface of 4H SiC
with 3D defect taken by interference microscope: (a)
the surface of 4H SiC crystal before etching; (b)—after
etching during 5 - 6 hours (befor e the diffusion process);
(c)—after secondary etching during 5 hours for forma-
tion of mesa-structure. As can be seen from the figures,
the surface remains smooth, despite to double etching
and region with 3D defect becomes much smoother: Ac-
cording evaluation on the base of Figure 2(c) after sec-
ond etching the roughness height is ~0.4 - 0.6 micron.
2.5. Influence of Thermal Annealing on IV
Characteristics of p-n SiC Junctions
Prepared by Low Temperature Diffusion
Since the low temperature diffusion is performed in a
flow of carbon vacancies, the obtained p-SiC layers con-
tain defects. Applicability of this techno logy is subject to
availability of the efficient ways of removing of defects
in the structures.
In a previous article [4] it was shown that thermal an-
nealing in vacuum (at 500˚C, 700˚C, 900˚C during 10
minutes) allows to reduce concentration of defects in
SiC/SiC structures. The following are comparative data
on the effect of annealing on the I-V characteristics of
p-n junctions prepared on bulk samples with conven-
tional SiC chemical treatment, without UV etching—
group 1) and crystals in which the surface-damaged layer
was remove d (with UV et ching—g roup 2).
Group 1: For crystals from group1 - most of p-n junc-
tions (up to 75%) have almost linear I-V characteristics
(as well as in experiments on SiC/SiC and SiC/Si struc-
tures [4]). In the samples with non-linear I-V the break-
down voltage was low (up to 5 - 6 Volt), with a coeffi-
cient of nonlinearity k = 4 - 10 at 2 volts. (Coefficient of
nonlinearity k is the ratio of the forward and reverse cur-
rents for structure). The thermal annealing in vacuum at
500˚C, 700˚C and 900˚C lead to worsening of quality of
p-n junctions and for most samples the IV becomes linear
(Figure 3). In some samples with a linear I-V the non-
linearity is appeared after annealing, but coefficient of
nonlinearity k was very low 2 - 4.
Thus, after low temperature diffusion in samples with
surface-damaged layer the distribution of impurities is
very nonuniform. This distribution is changed under
thermal annealing (T = 500˚C - 900˚C). As a result,
character of IV is also changing. However, the quality
and uniformity of p-type regions is so low that annealing
practically does not improve situation.
Group 2: After removing of surface-damaged layer by
UV etching the low-temperature diffusion of boron in
silicon carbide of various polytypes was conducted. Di-
ameter of p-area was 2 - 3 mm.
Measurement of the distribution along the surface of
the crystals showed the homogeneity of the p-region has
become much better: most of structures have nonlinear
IV characteristics (more 80%), the reverse breakdown
voltage increased up to 30 - 40 Volt, the coefficient of
nonlinearity k was 103 - 106 at 2 Volt. (Figure 4). An-
nealing at 500˚C - 900˚C not change IV characteristics.
Remind that only 20% - 30% of junctions were a nonlin-
ear and reverse breakdown voltage was up to 5 Volt. As
the average flatness of SiC surface not changed the shape
(a) (b) (c)
Figure 2. Photo of surface of 4H SiC with 3D defect taken by interference microscope: (a)—the surface of SiC crystal before
etching; (b)—after etching during 5 - 6 hours (before the diffusion process); (c)—after secondary etching during 5 hours for
formation of mesa-structure.
Influence of Defects on Low Temperature Diffusion of Boron in SiC
Figure 3. Influence of annealing on IV—characteristics of
p-n 4H SiC sample from group 1: IV after diffusion at
1300˚C; IV after annealing at 500˚C; IV after annealing
at 700˚C.
Figure 4. Influence of annealing on IV-characteristics of p-n
4H SiC sample from group 2: IV after diffusion at 1300˚C;
IV after annealing at 500˚C; IV after annealing at
700˚C; IV after annealing at 900˚C.
of p-n border of samples from group 1 and 2 the same.
Apparently, improving of IV characteristics is connected
with the removal of the surface-damaged layer contain-
ing increased conce nt rati o n o f de fects.
3. Conclusions
Thus the low temperature diffusion of Boron in bulk SiC
crystals is investigated, the simplified model of diffusion
is presented, the method of UV stimulated etching by
aqueous solution of KOH is proposed and some experi-
mental data on influence of defects on quality of pre-
pared p-n juncti ons are presen ted.
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