Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects
Phase transformations of SiC crystals and thin films with in-grown original defects have been studied. The analysis of absorption, excitation and low-temperature photoluminescence spectra testifies to formation of new micro-phases during the growth. The complex spectra can be decomposed into simi...
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irk-123456789-1184192017-05-31T03:06:06Z Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects Vlaskina, S.I. Mishinova, G.N. Rodionov, V.E. Svechnikov, G.S. Phase transformations of SiC crystals and thin films with in-grown original defects have been studied. The analysis of absorption, excitation and low-temperature photoluminescence spectra testifies to formation of new micro-phases during the growth. The complex spectra can be decomposed into similar structure-constituting spectra shifted against each other on the energy scale. These spectra are indicative of formation of new nanophases. Taking into account the position of the short-wave edge in the zerophonon part of the SF-i spectra as well as the position of corresponding excitation spectra and placing them on the well-known linear dependence of the exciton gap (Egx) on the percentage of hexagonally in different polytypic structures, one can obtain a hint to the percentage of hexagonally in the new metastable structures appearing in the 6H (33) matrix or in the growth process. The SF spectra are indicative of the appearance of these metastable structures. 2014 Article Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects / S.I. Vlaskina, G.N. Mishinova, V.E. Rodionov, G.S. Svechnikov // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 380-383. — Бібліогр.: 12 назв. — англ. 1560-8034 PACS 64.70.K-, 78.60.Lc, 81.30.-t http://dspace.nbuv.gov.ua/handle/123456789/118419 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| institution |
Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| collection |
DSpace DC |
| language |
English |
| description |
Phase transformations of SiC crystals and thin films with in-grown original
defects have been studied. The analysis of absorption, excitation and low-temperature
photoluminescence spectra testifies to formation of new micro-phases during the growth.
The complex spectra can be decomposed into similar structure-constituting spectra
shifted against each other on the energy scale. These spectra are indicative of formation
of new nanophases. Taking into account the position of the short-wave edge in the zerophonon
part of the SF-i spectra as well as the position of corresponding excitation spectra
and placing them on the well-known linear dependence of the exciton gap (Egx) on the
percentage of hexagonally in different polytypic structures, one can obtain a hint to the
percentage of hexagonally in the new metastable structures appearing in the 6H (33)
matrix or in the growth process. The SF spectra are indicative of the appearance of these
metastable structures. |
| format |
Article |
| author |
Vlaskina, S.I. Mishinova, G.N. Rodionov, V.E. Svechnikov, G.S. |
| spellingShingle |
Vlaskina, S.I. Mishinova, G.N. Rodionov, V.E. Svechnikov, G.S. Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Vlaskina, S.I. Mishinova, G.N. Rodionov, V.E. Svechnikov, G.S. |
| author_sort |
Vlaskina, S.I. |
| title |
Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects |
| title_short |
Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects |
| title_full |
Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects |
| title_fullStr |
Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects |
| title_full_unstemmed |
Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects |
| title_sort |
peculiarities of phase transformations in sic crystals and thin films with in-grown original defects |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2014 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/118419 |
| citation_txt |
Peculiarities of phase transformations in SiC crystals and thin films
with in-grown original defects / S.I. Vlaskina, G.N. Mishinova, V.E. Rodionov, G.S. Svechnikov // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2014. — Т. 17, № 4. — С. 380-383. — Бібліогр.: 12 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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| first_indexed |
2025-07-08T13:56:54Z |
| last_indexed |
2025-07-08T13:56:54Z |
| _version_ |
1837087344629383168 |
| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 380-383.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
380
PACS 64.70.K-, 78.60.Lc, 81.30.-t
Peculiarities of phase transformations in SiC crystals and thin films
with in-grown original defects
S.I. Vlaskina1, G.N. Mishinova2, V.E. Rodionov1, G.S. Svechnikov1
1V. Lashkaryov Institute of Semiconductor Physics, National Academy of Science of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine; e-mail: businkaa@mail.ru
2Taras Shevchenko Kyiv National University, 64, Volodymyrs’ka str., 01033 Kyiv, Ukraine
Abstract. Phase transformations of SiC crystals and thin films with in-grown original
defects have been studied. The analysis of absorption, excitation and low-temperature
photoluminescence spectra testifies to formation of new micro-phases during the growth.
The complex spectra can be decomposed into similar structure-constituting spectra
shifted against each other on the energy scale. These spectra are indicative of formation
of new nanophases. Taking into account the position of the short-wave edge in the zero-
phonon part of the SF-i spectra as well as the position of corresponding excitation spectra
and placing them on the well-known linear dependence of the exciton gap (Egx) on the
percentage of hexagonally in different polytypic structures, one can obtain a hint to the
percentage of hexagonally in the new metastable structures appearing in the 6H (33)
matrix or in the growth process. The SF spectra are indicative of the appearance of these
metastable structures.
Keywords: phase transformation, absorption spectrum, excitation spectrum, low-
temperature photoluminescence spectrum, polytypic structure, SiC crystal.
Manuscript received 11.07.14; revised version received 26.08.14; accepted for
publication 29.10.14; published online 10.11.14.
1. Introduction
The correlations between the crystal structure and
optical properties (photoluminescence, absorption) are
under discussion for polytypic materials. Silicon carbide
polytypes are the best choice for these investigations.
Actual atomic structures, the accompanying lattice
vibrations, properties of layered combinations of
polytypes can be considered from the optical spectra.
In this work, we report phase transformations of
SiC crystals and thin films with in-grown original
defects. SiC crystals were grown applying the Tairov
method, films were obtained using the “sandwich”-
method and chemical vapor deposition.
The analysis of absorption, excitation and low-
temperature photoluminescence spectra testifies to
formation of new microphases during the growth. The
complex spectra can be decomposed into similar
structure-constituting spectra shifted against each other
on the energy scale. These spectra are indicative of
formation of new nanophases.
Joint consideration of photoluminescence spectra,
excitation photoluminescence spectra and absorption
spectra testifies to the uniformity of different spectra and
the autonomy of each of them. Structurally, the total
complex spectra are correlated with the crystal
imperfection and the peculiarities of one-dimensional
disorder. Three different types of spectra have three
different principles of construction and behavior.
Nanostructure-indicator’s spectra are placed on the
wide donor–acceptor pair (DAP) spectrum in crystals
and films in case of higher concentrations of non-
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 380-383.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
381
Fig. 1. LTPL spectra according to the structural imperfection and impurity concentration in α-SiC crystals.
compensated impurities. Spectra of excitation,
photoluminescence and absorption spectra indicate
formation of nanostructures SiC (8H-, 10H-, 14H-SiC).
In this paper, the D-layers with in-grown
disordering in α-SiC crystals and films (mainly 6H-SiC)
as well as zones of β→α solid-phase transformations
were investigated. Low-temperature photoluminescence
(LTPL) spectroscopy was used as a highly sensitive and
accurate method to the structure changes [1-3].
2. Experimental
LTPL spectra were registered by the DFS-12
spectrograph with the photodetector (FEU-79). In
photoluminescence (PL) experiments a nitrogen LGI-21
(337 nm, 3.68 eV) or helium-cadmium LG-70
(441.6 nm, 2.807 eV) laser were used. Also a mercury
ultrahigh pressure lamp SVDSh-1000 with the UV-2
filter, and a xenon lamp DKSSh-1000 were used.
The group of monocrystalline α-SiC crystals grown
using Lely’s (Tairov’s) method under consideration were
separated by three groups:
1. NSF–samples (colourless): pure crystals with the
impurity concentration of ND – NA 316 cm107...6 ,
ND 316cm108...7 , NA < 316cm101 .
2. NDL–samples (doped, light-green): with ND – NA
316 cm108...2 , ND 317 cm108...5 , and ND – NA
> 317 cm103 , ND > 318cm101 ..
3. Cubic crystals of Nβ–samples of the n-type with
ND ≤ 317 cm101 (light yellow). Undoped SiC single
crystals with the impurity concentration of ND – NA
316 cm108...2 , NA 317 cm108...2 , and ND – NA
317 cm105...1 , ND ≥ 318cm101 were investigated.
Films were prepared using the sublimation
“sandwich” and CVD methods.
3. Results and discussion
Fig. 1 shows LTPL spectra according to structural
imperfection and the impurity concentration in α-SiC
crystals. Structurally perfect 6H-SiC crystals (or perfect
blocks of the crystal which coherently coalesce with
disordering layers) show a typical spectrum of nitrogen-
bound exciton complexes (PRS) together with the linear
ABC-spectrum related to Ti [(δ-0) – (a)] and emission
spectra of the donor-acceptor pairs.
Topical for this paper are the spectra of [(δ-I) –
(a)], [(δ-II) – (a)] NSF [4-6] as well as [(δ-I) – (b)], [(δ-II)
– (b)] NDL samples [7, 8].
The peculiarity of the photoluminescence spectra
related to the zone of disorder to a great extent depends
on the impurity concentration in the matrix as a whole.
In pure samples NSF (case (а) i = 1, 2, 3, …) at low
temperatures the SF-i spectra are dominant, whereas the
intensity of the DL-i spectra is very low. On the
contrary, in the doped samples the DL-i spectra are
dominant and located on a broad DAP emission band,
while SF-i spectra are practically invisible.
Peculiarities of the Laue patterns of the D-layers in
6H-SiC crystals are the same as for the 3C-SiC crystals
after phase transformation [9]. Pure samples with the
SF-i and DL-i series, AD NN ~ 316cm108...2 ,
AN ~ 317 cm108...2 and samples with only DL-i
spectra, AD NN ~ 317 cm107...2 , DN < 318cm101
show a linear dependence of intensity of the DL-i
spectra on the impurity concentration [9].
For the absorption spectra of DL samples [9], it
was shown that absorption is spread far to the lower
energy region relative to H
gxE6 (exciton band gap in 6H
structure). This can serve as an evidence of formation of
structures with a higher percentage of cubic α-SiC than
6H-SiC.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 380-383.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
382
General panorama of the PL spectra of the samples
consists of a certain set of the SF-spectra with different
intensities. Sometimes, it is peculiar to the different parts
of the same crystal. It was shown that all the SF-spectra
have the identical character, principal of structure,
independent of the position on the energy scale.
Decoding the SF-spectrum (as an example the most
frequent SF-1 spectrum) shows that the total structure of
the spectrum can be obtained by additive summation of
the phonon replica of some zero-phonon parts. In this
case, phonons of the edge of the extended Brillouin zone
of SiC (ТА – 46 meV, LA – 77 meV, ТО – 95 meV, LO
– 104 meV) were involved. While the zero-phonon part
itself is not detected, it was “redesigned” according to
the structure of the TA replica and transferred as a whole
by the TA phonon energy 46 meV to the high energy
region. Thin linear structure and its readability in case of
different D-layers is different. The minimum halfwidth
of this structure is about 1.5 meV. After defining the
position of the zero-phonon part in the SF-1 spectrum on
the energy scale, it appears that the short-wave part of
the spectrum coincides with the position of the exciton
band gap of 21R polytype at Т = 4.2 K (2.853 eV).
Investigating the photoluminescence excitation
spectra (ηi) reveals that each SF-1 spectrum has its own
excitation spectrum [6]. This may be the evidence of
some kind of autonomy of the SF-1 spectrum, its
independence from the special part of crystal disorder,
and the explanation of their different contributions to the
total spectra. It was shown that the long-wave edge of
each ηi spectrum coincides with the short-wave edge of
the zero-phonon part of corresponding SF-1 spectrum.
For the given sample, the overall excitation spectrum
(∑ηi) corresponds to the variation of the absorption
coefficient in the D-layer [2].
Consequently, it is possible by choosing suitable
photon energy of the exciting radiation to completely
eliminate the more short-wave DL-i spectra from the
total complex panorama of the spectrum, i.e. resolve it
into the independent components (Fig. 2).
We found that SF-1 and SF-2 spectra appeared
under excitation by using the Аr+ laser (hν = 2.54 eV),
while these spectra are in the higher energy region. It
may be due to the two-photon excitation with transfer of
carriers to the higher energy bands through the
intermediate states. The identical character of the SF-i
spectra is backed up by the identical temperature
behavior and peculiarity of this behavior made it
possible to decode the thin structure of the spectra [6].
Taking into account the position of the short-wave
edge of the zero-phonon part of the SF-i spectra: 2.853,
2.712, 2.611, …, 3.002 eV, i.e. SF-1, SF-2, SF-3, …,
SF-5, as well as the position of corresponding excitation
spectra and placing them on the well known linear
dependence of the exciton gap (Egx) on the percentage of
hexagonally in different polytypic structures [3, 6]
(Fig. 3), one can obtain a hint to the percentage of
hexagonally of the new metastable structures appearing
in the 6H (33) matrix or in the growth process.
SF-1 – 14H134 – 17.5 Å – 28.5 % (h)
SF-2 – 10H255 – 25 Å – 20%
SF-3 – 14Н277 – 35 Å – 14.3%
In several cases, the SF-5 spectra with the motive
to build the quasi-polytype 33R (3332) – 37% (h) were
observed. Formation of the structures 10H255 and
14H277 but not 10H13223 40% and 14H14334
(corresponds to 34 in 21R) with the same periods, but
with the higher percentage of hexagonality is backed by
the fact that exactly these structures are characterized by
the percentage of hexagonality at which the
correspondent values of Еgx are pointed to.
The motive of construction 3223 (40% h) of the
structure on the metastable microlevel corresponds to the
motive of 15R polytype construction that occurs in a
stable condition. The motive of 4334 (28.6% h)
corresponds to the well-known stable 21R polytype.
Low-temperature photoluminescence spectra of
pure (Al, N) α-SiC crystals and pure β-SiC crystals
(zones of β→α solid-phase transformations) are
represented by the same SF-i spectra, which are
indicators of the intermediate metastable micro- and
nanostructures with the medium intermediate percentage
of hexagonality regarding 6H and 3C phases, namely
21R34, 10H255, 14H277. In pure SiC crystals, the
SF-i spectra are dominant [6]. In crystals with a higher
impurity concentration, the DL-i spectra appear. The
spectra of DL-i type are different from the SF-i spectra
and have other principles of construction and behavior.
They are located on a broad DAP emission band in
crystals with higher concentrations of non-compensated
impurities (Fig. 1). Structurally, the general complexity
of the DL-i spectra correlated with the degree of disorder
of the crystal and was related with the one-dimensional
disorder, the same as in the case of the stacking fault
(SF-i) spectra.
Fig. 2. DL-i spectra at T = 77 K.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 380-383.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
383
Fig. 3. SF-i spectrum allocation with respect to the linear
dependence of the exciton gap (Egx) on the percentage of
hexagonally to different polytypic structures. The long-wave
edge of the excitation spectra is shown. Edge excitation spectra
coincide with short edges of SF-i spectra at the point
corresponding to definite values of Egx.
The analysis of absorption, excitation and low-
temperature photoluminescence spectra suggest
formation of a new microphase during the growth
process and appearance of the deep level (DL) spectra.
The complex spectra of the crystals can be decomposed
into the so-called DL-i (i = 1, 2, 3, 4) spectra. All the
spectra of the DL type demonstrate identical behavior of
the thin structure. If SF-i and DL-i spectra are reconciled
at the long-wave part, this combination of spectra are
along the line of the dependence Eg = f (percentage of
hexagonality). Herein, SF-i and DL-i spectra exist
independently, they have different way to behave,
however, match the same nanostructure transformations
at and transformations [10-12].
4. Conclusions
The peculiarity of the photoluminescence spectra related
to the zones of disorder to a great extent depends on the
impurity concentration in the matrix as a whole. In pure
samples, NSF (case (а) I = 1, 2, 3, …) at low temperatures
the SF-i spectra are dominant, whereas the intensity of
the DL-i spectra is very low. On the contrary, in the
doped samples the DL-i spectra are dominant and
located on a broad DAP emission band, while SF-i
spectra are practically invisible.
Taking into account the position of the short-wave
edge of the zero-phonon part in the SF-i spectra as well
as the position of corresponding excitation spectra and
placing them on the well-known linear dependence of
the exciton gap (Egx) on the percentage of hexagonally in
different polytypic structures, one can obtain a hint to
the percentage of hexagonally of the new metastable
structures appearing in the 6H (33) matrix or in the
growth process. The SF spectra are indicative of the
appearance of these metastable structures.
References
1. Fei Yan, Low Temperature Study on Defect
Centers in Silicon Carbide, dissertation, University
of Pittsburgh, 2009 (Dissertation LTPL-Choyke
Pittsburg 2009, etd-08052009-012630).
2. I.S. Gorban and G.N. Mishinova, Basics of
luminescent diagnostics of the dislocation structure
of SiC crystals // Proc. SPIE, 3359, p. 187 (1998).
3. W.J. Choyke, H. Matsunami, G. Pensl, Silicon
Carbide: Recent Major Advances. Springer, 2004.
4. S.I. Vlaskina, G.N. Mishinova, V.I. Vlaskin, V.E.
Rodionov, G.S. Svechnikov, 6H-3C transformation
in heated cubic silicon carbide 3C-SiC //
Semiconductor Physics, Quantum Electronics and
Optoelectronics, 14(4), p. 432-437 (2011).
5. S.I. Vlaskina, G.N. Mishinova, V.I. Vlaskin,
G.S. Svechnikov, V.E. Rodionov, S.W. Lee,
Silicon carbide phase transition in as-grown 3C-6H
– polytypes junction // Semiconductor Physics,
Quantum Electronics and Optoelectronics, 16(2),
p. 132-136 (2013).
6. S.I. Vlaskina, G.N. Mishinova, V.I. Vlaskin,
V.E. Rodionov, G.S. Svechnikov, 8H-, 10H-, 14H-
SiC formation in 6H-3C silicon carbide phase
transitions // Semiconductor Physics, Quantum
Electronics and Optoelectronics, 16(3), p. 272-278
(2013).
7. S.I. Vlaskina, D.H. Shin, 6H to 3C polytype
transformation in silicon carbide // Jpn. J. Appl.
Phys. 38, p. 27 (1999).
8. S.W. Lee, S.I. Vlaskina, V.I. Vlaskin,
I.V. Zaharchenko, V.A. Gubanov, G.N. Mishinova,
G.S. Svechnikov, V.E. Rodionov, S.A. Podlasov,
Silicon carbide defects and luminescence centers in
current heated 6H-SiC // Semiconductor Physics,
Quantum Electronics and Optoelectronics, 13(1),
p. 24-29 (2010).
9. S.I. Vlaskina, G.N. Mishinova, L.I. Vlaskin,
V.E. Rodionov, G.S.Svechnikov, Nanostructures in
lightly doped silicon carbide crystals with polytypic
defects // Semiconductor Physics, Quantum
Electronics and Optoelectronics, 17(2), p. 155-159
(2014).
10. S. Shinozaki, K.R. Kisman, Aspects of “one
dimensional disorder” in silicon carbide // Acta
Metallurgica, 26, p. 769-776 (1978).
11. L.U. Ogbuji, T.E. Mitchell, A.H. Heuer, The β→α
transformation in polycrystalline SiC: The
thickening of α plates // J. Amer. Ceram. Soc.
64(2), p. 91-99 (1981).
12. Kozuaki Kobayashi, Shojiro Komatsu. First-
principle study of 8H-, 10H-, 12H- ND 18H-SiC
polytypes // J. Phys. Soc. Jpn. Appl. 024714 (2012).
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2014. V. 17, N 4. P. 380-383.
PACS 64.70.K-, 78.60.Lc, 81.30.-t
Peculiarities of phase transformations in SiC crystals and thin films with in-grown original defects
S.I. Vlaskina1, G.N. Mishinova2, V.E. Rodionov1, G.S. Svechnikov1
1V. Lashkaryov Institute of Semiconductor Physics, National Academy of Science of Ukraine,
41, prospect Nauky, 03028 Kyiv, Ukraine; e-mail: businkaa@mail.ru
2Taras Shevchenko Kyiv National University, 64, Volodymyrs’ka str., 01033 Kyiv, Ukraine
Abstract. Phase transformations of SiC crystals and thin films with in-grown original defects have been studied. The analysis of absorption, excitation and low-temperature photoluminescence spectra testifies to formation of new micro-phases during the growth. The complex spectra can be decomposed into similar structure-constituting spectra shifted against each other on the energy scale. These spectra are indicative of formation of new nanophases. Taking into account the position of the short-wave edge in the zero-phonon part of the SF-i spectra as well as the position of corresponding excitation spectra and placing them on the well-known linear dependence of the exciton gap (Egx) on the percentage of hexagonally in different polytypic structures, one can obtain a hint to the percentage of hexagonally in the new metastable structures appearing in the 6H (33) matrix or in the growth process. The SF spectra are indicative of the appearance of these metastable structures.
Keywords: phase transformation, absorption spectrum, excitation spectrum, low-temperature photoluminescence spectrum, polytypic structure, SiC crystal.
Manuscript received 11.07.14; revised version received 26.08.14; accepted for publication 29.10.14; published online 10.11.14.
1. Introduction
The correlations between the crystal structure and optical properties (photoluminescence, absorption) are under discussion for polytypic materials. Silicon carbide polytypes are the best choice for these investigations. Actual atomic structures, the accompanying lattice vibrations, properties of layered combinations of polytypes can be considered from the optical spectra.
In this work, we report phase transformations of SiC crystals and thin films with in-grown original defects. SiC crystals were grown applying the Tairov method, films were obtained using the “sandwich”-method and chemical vapor deposition.
The analysis of absorption, excitation and low-temperature photoluminescence spectra testifies to formation of new microphases during the growth. The complex spectra can be decomposed into similar structure-constituting spectra shifted against each other on the energy scale. These spectra are indicative of formation of new nanophases.
Joint consideration of photoluminescence spectra, excitation photoluminescence spectra and absorption spectra testifies to the uniformity of different spectra and the autonomy of each of them. Structurally, the total complex spectra are correlated with the crystal imperfection and the peculiarities of one-dimensional disorder. Three different types of spectra have three different principles of construction and behavior.
Nanostructure-indicator’s spectra are placed on the wide donor–acceptor pair (DAP) spectrum in crystals and films in case of higher concentrations of non-compensated impurities. Spectra of excitation, photoluminescence and absorption spectra indicate formation of nanostructures SiC (8H-, 10H-, 14H-SiC).
In this paper, the D-layers with in-grown disordering in α-SiC crystals and films (mainly 6H-SiC) as well as zones of β→α solid-phase transformations were investigated. Low-temperature photoluminescence (LTPL) spectroscopy was used as a highly sensitive and accurate method to the structure changes [1-3].
2. Experimental
LTPL spectra were registered by the DFS-12 spectrograph with the photodetector (FEU-79). In photoluminescence (PL) experiments a nitrogen LGI-21 (337 nm, 3.68 eV) or helium-cadmium LG-70 (441.6 nm, 2.807 eV) laser were used. Also a mercury ultrahigh pressure lamp SVDSh-1000 with the UV-2 filter, and a xenon lamp DKSSh-1000 were used.
The group of monocrystalline α-SiC crystals grown using Lely’s (Tairov’s) method under consideration were separated by three groups:
1. NSF–samples (colourless): pure crystals with the impurity concentration of ND – NA (
(
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3
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×
, NA <
3
16
cm
10
1
-
×
.
2. NDL–samples (doped, light-green): with ND – NA (
(
)
3
16
cm
10
8
...
2
-
×
, ND (
(
)
3
17
cm
10
8
...
5
-
×
, and ND – NA >
3
17
cm
10
3
-
×
, ND >
3
18
cm
10
1
-
×
..
3. Cubic crystals of Nβ–samples of the n-type with ND ≤
3
17
cm
10
1
-
×
(light yellow). Undoped SiC single crystals with the impurity concentration of ND – NA (
(
)
3
16
cm
10
8
...
2
-
×
, NA (
(
)
3
17
cm
10
8
...
2
-
×
, and ND – NA (
(
)
3
17
cm
10
5
...
1
-
×
, ND ≥
3
18
cm
10
1
-
×
were investigated.
Films were prepared using the sublimation “sandwich” and CVD methods.
3. Results and discussion
Fig. 1 shows LTPL spectra according to structural imperfection and the impurity concentration in α-SiC crystals. Structurally perfect 6H-SiC crystals (or perfect blocks of the crystal which coherently coalesce with disordering layers) show a typical spectrum of nitrogen-bound exciton complexes (PRS) together with the linear ABC-spectrum related to Ti [(δ-0) – (a)] and emission spectra of the donor-acceptor pairs.
Topical for this paper are the spectra of [(δ-I) – (a)], [(δ-II) – (a)] NSF [4-6] as well as [(δ-I) – (b)], [(δ-II) – (b)] NDL samples [7, 8].
The peculiarity of the photoluminescence spectra related to the zone of disorder to a great extent depends on the impurity concentration in the matrix as a whole. In pure samples NSF (case (а) i = 1, 2, 3, …) at low temperatures the SF-i spectra are dominant, whereas the intensity of the DL-i spectra is very low. On the contrary, in the doped samples the DL-i spectra are dominant and located on a broad DAP emission band, while SF-i spectra are practically invisible.
Peculiarities of the Laue patterns of the D-layers in 6H-SiC crystals are the same as for the 3C-SiC crystals after phase transformation [9]. Pure samples with the SF-i and DL-i series,
A
D
N
N
-
~
(
)
3
16
cm
10
8
...
2
-
×
,
A
N
~
(
)
3
17
cm
10
8
...
2
-
×
and samples with only DL-i spectra,
A
D
N
N
-
~
(
)
3
17
cm
10
7
...
2
-
×
,
D
N
<
3
18
cm
10
1
-
×
show a linear dependence of intensity of the DL-i spectra on the impurity concentration [9].
For the absorption spectra of DL samples [9], it was shown that absorption is spread far to the lower energy region relative to
H
gx
E
6
(exciton band gap in 6H structure). This can serve as an evidence of formation of structures with a higher percentage of cubic α-SiC than 6H-SiC.
General panorama of the PL spectra of the samples consists of a certain set of the SF-spectra with different intensities. Sometimes, it is peculiar to the different parts of the same crystal. It was shown that all the SF-spectra have the identical character, principal of structure, independent of the position on the energy scale.
Decoding the SF-spectrum (as an example the most frequent SF-1 spectrum) shows that the total structure of the spectrum can be obtained by additive summation of the phonon replica of some zero-phonon parts. In this case, phonons of the edge of the extended Brillouin zone of SiC (ТА – 46 meV, LA – 77 meV, ТО – 95 meV, LO – 104 meV) were involved. While the zero-phonon part itself is not detected, it was “redesigned” according to the structure of the TA replica and transferred as a whole by the TA phonon energy 46 meV to the high energy region. Thin linear structure and its readability in case of different D-layers is different. The minimum halfwidth of this structure is about 1.5 meV. After defining the position of the zero-phonon part in the SF-1 spectrum on the energy scale, it appears that the short-wave part of the spectrum coincides with the position of the exciton band gap of 21R polytype at Т = 4.2 K (2.853 eV).
Investigating the photoluminescence excitation spectra (ηi) reveals that each SF-1 spectrum has its own excitation spectrum [6]. This may be the evidence of some kind of autonomy of the SF-1 spectrum, its independence from the special part of crystal disorder, and the explanation of their different contributions to the total spectra. It was shown that the long-wave edge of each ηi spectrum coincides with the short-wave edge of the zero-phonon part of corresponding SF-1 spectrum. For the given sample, the overall excitation spectrum (∑ηi) corresponds to the variation of the absorption coefficient in the D-layer [2].
Consequently, it is possible by choosing suitable photon energy of the exciting radiation to completely eliminate the more short-wave DL-i spectra from the total complex panorama of the spectrum, i.e. resolve it into the independent components (Fig. 2).
We found that SF-1 and SF-2 spectra appeared under excitation by using the Аr+ laser (hν = 2.54 eV), while these spectra are in the higher energy region. It may be due to the two-photon excitation with transfer of carriers to the higher energy bands through the intermediate states. The identical character of the SF-i spectra is backed up by the identical temperature behavior and peculiarity of this behavior made it possible to decode the thin structure of the spectra [6].
Taking into account the position of the short-wave edge of the zero-phonon part of the SF-i spectra: 2.853, 2.712, 2.611, …, 3.002 eV, i.e. SF-1, SF-2, SF-3, …, SF-5, as well as the position of corresponding excitation spectra and placing them on the well known linear dependence of the exciton gap (Egx) on the percentage of hexagonally in different polytypic structures [3, 6] (Fig. 3), one can obtain a hint to the percentage of hexagonally of the new metastable structures appearing in the 6H (33) matrix or in the growth process.
SF-1 – 14H1(34( – 17.5 Å – 28.5 % (h)
SF-2 – 10H2(55( – 25 Å – 20%
SF-3 – 14Н2(77( – 35 Å – 14.3%
In several cases, the SF-5 spectra with the motive to build the quasi-polytype 33R (3332) – 37% (h) were observed. Formation of the structures 10H2(55( and 14H2(77( but not 10H1(3223( 40% and 14H1(4334( (corresponds to (34( in 21R) with the same periods, but with the higher percentage of hexagonality is backed by the fact that exactly these structures are characterized by the percentage of hexagonality at which the correspondent values of Еgx are pointed to.
The motive of construction (3223( (40% h) of the structure on the metastable microlevel corresponds to the motive of 15R polytype construction that occurs in a stable condition. The motive of (4334( (28.6% h) corresponds to the well-known stable 21R polytype.
Low-temperature photoluminescence spectra of pure (Al, N) α-SiC crystals and pure β-SiC crystals (zones of β→α solid-phase transformations) are represented by the same SF-i spectra, which are indicators of the intermediate metastable micro- and nanostructures with the medium intermediate percentage of hexagonality regarding 6H and 3C phases, namely 21R(34(, 10H2(55(, 14H2(77(. In pure SiC crystals, the SF-i spectra are dominant [6]. In crystals with a higher impurity concentration, the DL-i spectra appear. The spectra of DL-i type are different from the SF-i spectra and have other principles of construction and behavior. They are located on a broad DAP emission band in crystals with higher concentrations of non-compensated impurities (Fig. 1). Structurally, the general complexity of the DL-i spectra correlated with the degree of disorder of the crystal and was related with the one-dimensional disorder, the same as in the case of the stacking fault (SF-i) spectra.
Fig. 2. DL-i spectra at T = 77 K.
Fig. 3. SF-i spectrum allocation with respect to the linear dependence of the exciton gap (Egx) on the percentage of hexagonally to different polytypic structures. The long-wave edge of the excitation spectra is shown. Edge excitation spectra coincide with short edges of SF-i spectra at the point corresponding to definite values of Egx.
The analysis of absorption, excitation and low-temperature photoluminescence spectra suggest formation of a new microphase during the growth process and appearance of the deep level (DL) spectra. The complex spectra of the crystals can be decomposed into the so-called DL-i (i = 1, 2, 3, 4) spectra. All the spectra of the DL type demonstrate identical behavior of the thin structure. If SF-i and DL-i spectra are reconciled at the long-wave part, this combination of spectra are along the line of the dependence Eg = f (percentage of hexagonality). Herein, SF-i and DL-i spectra exist independently, they have different way to behave, however, match the same nanostructure transformations at ((( and ((( transformations [10-12].
4. Conclusions
The peculiarity of the photoluminescence spectra related to the zones of disorder to a great extent depends on the impurity concentration in the matrix as a whole. In pure samples, NSF (case (а) I = 1, 2, 3, …) at low temperatures the SF-i spectra are dominant, whereas the intensity of the DL-i spectra is very low. On the contrary, in the doped samples the DL-i spectra are dominant and located on a broad DAP emission band, while SF-i spectra are practically invisible.
Taking into account the position of the short-wave edge of the zero-phonon part in the SF-i spectra as well as the position of corresponding excitation spectra and placing them on the well-known linear dependence of the exciton gap (Egx) on the percentage of hexagonally in different polytypic structures, one can obtain a hint to the percentage of hexagonally of the new metastable structures appearing in the 6H (33) matrix or in the growth process. The SF spectra are indicative of the appearance of these metastable structures.
References
1.
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S.I. Vlaskina, G.N. Mishinova, L.I. Vlaskin, V.E. Rodionov, G.S.Svechnikov, Nanostructures in lightly doped silicon carbide crystals with polytypic defects // Semiconductor Physics, Quantum Electronics and Optoelectronics, 17(2), p. 155-159 (2014).
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L.U. Ogbuji, T.E. Mitchell, A.H. Heuer, The β→α transformation in polycrystalline SiC: The thickening of α plates // J. Amer. Ceram. Soc. 64(2), p. 91-99 (1981).
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Kozuaki Kobayashi, Shojiro Komatsu. First-principle study of 8H-, 10H-, 12H- ND 18H-SiC polytypes // J. Phys. Soc. Jpn. Appl. 024714 (2012).
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Fig. 1. LTPL spectra according to the structural imperfection and impurity concentration in α-SiC crystals.
© 2014, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
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