Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons
Amorphous and microcrystalline silicon are well known materials for thin film large area electronics. The defects in the material are an important issue for the device quality and the manufacturing process optimization. We study defects in thin film silicon with electron spin resonance (ESR). In ord...
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irk-123456789-1106462017-01-06T03:02:47Z Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons Astakhov, O. Finger, F. Carius, R. Lambertz, A. Neklyudov, I. Petrusenko, Yu. Borysenko, V. Barankov, D. Физика радиационных повреждений и явлений в твердых телах Amorphous and microcrystalline silicon are well known materials for thin film large area electronics. The defects in the material are an important issue for the device quality and the manufacturing process optimization. We study defects in thin film silicon with electron spin resonance (ESR). In order to vary the defect density in a wide range 2 MeV electron bombardment at 100 K was applied with dose as high as 10¹⁸ e*cm⁻². Samples were investigated after deposition, after irradiation and between the annealing steps. The spin density (Ns) in the material was varied over 3 orders of magnitude. Strong satellites with g≈2.010 and g≈2.000 were observed on the shoulders of the dangling bond line. The initial Ns and the shape of the resonance line were restored after annealing. Аморфний і мікрокристалічний кремній є широко відомими матеріалами для виробництва тонкоплівкової електроники великої площі. Дефекти у даних матеріалах відіграють вирішальну роль для якості пристроїв і оптимізації виробничих процесів. Ми досліджували тонкоплівковий гідрогенований кремній методом вимірів електронного парамагнитного резонансу (ЕПР). Для зміни щільності дефектів у широкому диапазоні зразки було опромінено електронами з енергією 2 МеВ. Зразки було досліджено після осадження, після опромінення і між етапами відпалу. Щільність спинів (Ns) в матеріалі змінювалась в межах 3-х порядків величини. З обох боків від центрального резонансу, що характеризує обірвані зв’язки кремнію, спостеригались потужні додаткові резонансні лінії (g≈2.010 и g≈2.000). Після відпалу форма резонансних ліній і щільність спинів поверталися до вихідних показників. Аморфный и микрокристаллический кремний являются широко известными материалами для производства тонкопленочной электроники большой площади. Дефекты в данных материалах играют решающую роль для качества приборов и оптимизации производственных процессов. Мы исследовали тонкопленочный гидрогенированный кремний методом измерений электронного парамагнитного резонанса (ЭПР). Для изменения плотности дефектов в широких пределах образцы облучались электронами с энергией 2 МэВ. Образцы исследовались после осаждения, после облучения и между стадиями отжига. Плотность спинов (Ns) в материале изменялась в пределах 3-х порядков величины. По обе стороны от центрального резонанса, характеризующего оборванные связи кремния, наблюдались мощные дополнительные резонансные линии (g≈2.010 и g≈2.000). После отжига форма резонансных линий и плотность спинов возвращались к исходным значениям. 2007 Article Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons/ O. Astakhov, F. Finger, R. Carius, A. Lambertz, I. Neklyudov, Yu. Petrusenko, V.Borysenko, D. Barankov // Вопросы атомной науки и техники. — 2007. — № 2. — С. 39-42. — Бібліогр.: 16 назв. — англ. 1562-6016 http://dspace.nbuv.gov.ua/handle/123456789/110646 538.279:537.533.9 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Физика радиационных повреждений и явлений в твердых телах Физика радиационных повреждений и явлений в твердых телах |
| spellingShingle |
Физика радиационных повреждений и явлений в твердых телах Физика радиационных повреждений и явлений в твердых телах Astakhov, O. Finger, F. Carius, R. Lambertz, A. Neklyudov, I. Petrusenko, Yu. Borysenko, V. Barankov, D. Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons Вопросы атомной науки и техники |
| description |
Amorphous and microcrystalline silicon are well known materials for thin film large area electronics. The defects in the material are an important issue for the device quality and the manufacturing process optimization. We study defects in thin film silicon with electron spin resonance (ESR). In order to vary the defect density in a wide range 2 MeV electron bombardment at 100 K was applied with dose as high as 10¹⁸ e*cm⁻². Samples were investigated after deposition, after irradiation and between the annealing steps. The spin density (Ns) in the material was varied over 3 orders of magnitude. Strong satellites with g≈2.010 and g≈2.000 were observed on the shoulders of the dangling bond line. The initial Ns and the shape of the resonance line were restored after annealing. |
| format |
Article |
| author |
Astakhov, O. Finger, F. Carius, R. Lambertz, A. Neklyudov, I. Petrusenko, Yu. Borysenko, V. Barankov, D. |
| author_facet |
Astakhov, O. Finger, F. Carius, R. Lambertz, A. Neklyudov, I. Petrusenko, Yu. Borysenko, V. Barankov, D. |
| author_sort |
Astakhov, O. |
| title |
Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons |
| title_short |
Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons |
| title_full |
Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons |
| title_fullStr |
Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons |
| title_full_unstemmed |
Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons |
| title_sort |
paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 мev electrons |
| publisher |
Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| publishDate |
2007 |
| topic_facet |
Физика радиационных повреждений и явлений в твердых телах |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/110646 |
| citation_txt |
Paramagnetic centers in amorphous and microcrystalline silicon irradiated with 2 МeV electrons/ O. Astakhov, F. Finger, R. Carius, A. Lambertz, I. Neklyudov, Yu. Petrusenko, V.Borysenko, D. Barankov // Вопросы атомной науки и техники. — 2007. — № 2. — С. 39-42. — Бібліогр.: 16 назв. — англ. |
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Вопросы атомной науки и техники |
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UDC 538.279:537.533.9
PARAMAGNETIC CENTERS IN AMORPHOUS
AND MICROCRYSTALLINE SILICON IRRADIATED
WITH 2 MeV ELECTRONS
A. Astakhov1,2, F. Finger2, R. Carius2, A. Lambertz2, I. Neklyudov1, Yu. Petrusenko1,
V. Borysenko1, D. Barankov1
1National Science Center “Kharkov Institute of Physics & Technology”,
Institute of Material Science & Technology, 1, Akademicheskaya st.,
61108, Kharkov, Ukraine;
2Forschungszentrum Jülich, Institute of Photovoltaics, 52425 Jülich, Germany
Amorphous and microcrystalline silicon are well known materials for thin film large area electronics. The de-
fects in the material are an important issue for the device quality and the manufacturing process optimization. We
study defects in thin film silicon with electron spin resonance (ESR). In order to vary the defect density in a wide
range 2 MeV electron bombardment at 100 K was applied with dose as high as 1018 e*cm-2. Samples were investigat-
ed after deposition, after irradiation and between the annealing steps. The spin density (Ns) in the material was var-
ied over 3 orders of magnitude. Strong satellites with g≈2.010 and g≈2.000 were observed on the shoulders of the
dangling bond line. The initial Ns and the shape of the resonance line were restored after annealing.
INTRODUCTION
Amorphous hydrogenated silicon (a-Si:H) is a wide-
ly used material for production of large area electronics,
TFTs and photovoltaics [1]. Microcrystalline hydro-
genated silicon (µc-Si:H) is a new promising material
for this area of applications, providing high carrier mo-
bility and effective doping. Low substrate temperature
during deposition (200...300 oC) together with possibili-
ty of large area depositions make these materials attrac-
tive for commercial thin films electronics. Great
progress has been made in the last decades in under-
standing of the properties of the silicon based disordered
semiconductors [2, 3]. Nevertheless many questions still
have to be answered in order to improve nowadays tech-
nologies. In particular, clear understanding of the role of
defects in the electronic properties of a-Si:H and µc-
Si:H is crucial. The Electron Spin Resonance (ESR)
technique was long ago found to be a suitable tool for
investigation of defects in thin film silicon [2, 3]. ESR
being sensitive to the nearest neighborhood of the elec-
tron in paramagnetic state could give valuable informa-
tion on the nature and configuration of the given defect
[4]. Unfortunately natural disorder in the investigated
material smears out fine structure of the spectrum [2, 3].
Slightly asymmetric ESR lines at g-values in the range
of 2.0045...2.0055 with the width of 6-8 gauss charac-
terize the intrinsic thin film silicon [2, 3, 5, 6, 7, 8]. The
resonance is commonly assigned to the silicon dangling
bonds (db) in different environments [5], but a more ac-
curate identification of the defects in the material is still
missing. Defect density management with the post-
preparation treatment is required to gain more informa-
tion on the defect structure. Low temperature bombard-
ment with high-energy electrons was shown to be suit-
able tool for the reversible enhancement of the defect
density in the hydrogenated silicon samples with a mi-
crostructure ranging from microcrystalline to amor-
phous [9]. In the report we present results on a-Si:H and
µc-Si:H material were the defect density is varied by
electron irradiation and subsequent annealing.
EXPERIMENT
SAMPLE PREPARATION
Samples were prepared using Very High Frequen-
cy Plasma Enhanced Chemical Vapour Deposition
(VHF-PECVD) (95 MHz) from silane-hydrogen mix-
tures. The deposition parameters were constant for all
samples in the series: gas pressure 300Torr, discharge
power 0.1W/cm2, and substrate temperature 200 oC.
Only the silane to hydrogen ratio (SC =
[SiH4]/[SiH4]+[H2]) was varied in the gas mixture
from run to run in a range of 3...100% leading to
structure variation from highly crystalline to com-
pletely amorphous. Several samples were deposited
with a supplement of PH3 in the deposition gas mix-
ture in order to achieve n-type doping as high as
5...13 ppm.
The crystalline volume fraction (ICRS) was semi-
quantitatively determined from Raman measurements
as a ratio between intensities of the Raman signals at
520 cm-1 and 500 cm-1 (attributed to the crystalline
phase) and the Raman signal at 480 cm-1 (attributed
to the disordered phase), i.e. ICRS = (I500 + I520)/(I480 +
I500 + I520) [10].
Films with 4...7 µm thickness were deposited on
the Mo substrate. The substrate was bent after deposi-
tion and peeled off flakes of the deposited films were
collected and sealed in quartz tubes at 0.5bar He at-
mosphere for ESR measurements.
The ESR was measured in X-band (F≈9,3GHz) at
temperature of 40 K using lock-in detection tech-
nique. The samples preparation and ESR measure-
ments were carried out in the Institute of Photo-
voltaics (Forschungszentrum Juelich, Germany).
_______________________________________________________________________________
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2007. № 2. 39
Серия: Физика радиационных повреждений и радиационное материаловедение (90), с. 39-42.
IRRADIATION
The electron bombardment was carried out using the
ELIAS Van de Graaf electron accelerator in National
Science Center “Kharkov Institute of Physics & Tech-
nology” (Ukraine) with energy of 2 MeV and beam cur-
rent density of 5µA*cm-2. A dose of 1018 e*cm-2 was ap-
plied for all samples in the present experiment. The irra-
diation was carried out at a temperature of around 100K
in order to reduce self-annealing and exclude the heat
damage to the sample. During irradiation samples were
cooled with the flow of high-purity N2.
Note, that after irradiation samples were transported
to the Institute of Photovoltaics (Germany). Therefore to
exclude room temperature annealing, samples were
stored in the LN2-cooled dry transport cryostat. Hence
the room temperature exposure was minimized to
2...5 minutes. After the measurements of the irradiated
samples an annealing procedure was applied in a step-
wise manner with the following sequence: 50, 80, 120,
160 oC, each step was 30 minutes long. The maximum
annealing temperature was chosen well below the depo-
sition temperature (Ts=200 oC) in order to avoid changes
of the microstructure of the samples.
RESULTS
We investigated ESR of the intrinsic thin film sili-
con with different structural composition extensively
before the electron irradiation experiment [6, 11, 12,
13]. Fig. 1 shows the dependence of the Ns and g-value
on the SC during sample preparation.
Fig. 1. g-value and the spin density of the samples pre-
pared at various SC. Dashed line approximately indi-
cates the transition from microcrystalline to amorphous
structure according to the Raman data
The transition from the microcrystalline to amor-
phous growth at our deposition conditions is found at
SC≈7% according to the Raman data i.e. samples with
SC>7% do not show a crystalline peak in the Raman
spectra (Raman amorphous). There is a clear systematic
shift of the g-value between microcrystalline and
amorphous material. The g-value increases in the vicin-
ity of transition (with increase of the amorphous phase
fraction) from g≈2.0047 to g≈2.0050. Note, that the g–
value increases further beyond the transition to amorph-
ous structure (SC=8…15%). The saturation was found
only at SC>50% at g≈2.0054. The g-value shift is ex-
pected due to the difference of the dangling bond envir-
onment in μc-Si:H and a-Si:H. The shift of the g-value
after transition to amorphous growth, where no crystal-
line peak could be seen in the Raman spectra, however
is surprising. The region is interesting because the Ns
has a minimum here and the material prepared within
the region is known to be the best absorber layer for the
amorphous thin film silicon solar cells.
The Fig. 2,a shows the ESR spectra before and after
irradiation. Note that all spectra are normalized to the
same peak-to-peak height for the lineshape comparison.
Fig. 2,b is a plot of the spin density vs. silane
concentration before and after irradiation. The spin
density after irradiation increases by 3 orders of
magnitude and qualitatively repeats the Ns vs. SC
dependence before irradiation. No significant shift of
the resonance line from the initial position of the db line
was detected after irradiation, but significant change of
the lines shape is clearly seen (Fig. 2,a). Remarkable
additional features appear on the shoulders of the central
db line at g-values around 2.000 and 2.010. These
features were found in the spectra of all irradiated
samples having different configuration for different
material structures.
Fig. 2. ESR spectra of a-Si:H and c-Si:H withμ
different silane concentration before (thin) and after
(bold) irradiation (a); Spin density of the material
prepared with different SC before and after irradiation
(b)
After irradiation annealing was applied to the
samples. In Fig. 3,a spin densities of the intrinsic
samples after annealing steps are presented. The Ns of
the db line was increased after irradiation as was
mentioned above. During annealing all samples show
return of the Ns close to as-deposited value.
_______________________________________________________________________________
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2007. № 2. 40
Серия: Физика радиационных повреждений и радиационное материаловедение (90), с. 39-42.
Fig. 3. Spin density of the intrinsic samples with
different structure vs. treatment steps (a); Spin density
of the PH3-doped material vs. treatment steps (b)
In Fig. 4 (a&b) some of the spectra are shown for
intrinsic µc-Si:H and a-Si:H. As one can see the
satellites being pronounced after irradiation are more
sensitive to the annealing and already at 50 oC their
relative contribution is clearly reduced. In the majority
of cases spectra of annealed material return to the initial
line shape.
In the Fig. 3,b Ns of n-doped samples are presented.
One has to mention, that in the initial state the db
resonance in the n-doped material is reduced or not
detectable at all, but the conduction electron (CE) line
arises with doping level increase [12], as shown on the
Fig. 4,c (top line).
Fig. 4. Spectra after deposition, after irradiation and on
two annealing points: a – intrinsic μc-Si:H; b –
intrinsic a-Si:H; c – 10 ppm PH3-doped μc-Si:H
The reduction of the db line is caused by the shift of
the Fermi level towards the conduction band. In this
situation the db states become doubly occupied and are
not detectable any more in the ESR experiment. Instead
tail states of the conduction band get populated. After
irradiation the resonance of the n-doped sample is
identical to the intrinsic samples i.e. only db line (with
above mentioned satellites) was observed (Fig. 4,c bold
line). During annealing the CE resonance re-appears and
after annealing at 120 oC becomes dominant again in the
spectrum. Therefore we can observe a minimum of Ns
when the position of the Fermi level is already above
the midgap but is still below the level where majority of
the donor states are situated. After annealing the return
of the lineshape and the spin density was observed for
the n-doped samples as well as for the intrinsic ones.
DISCUSSION
The increase of the defect density in the middle of
the gap in the same sample was the central idea of the
irradiation experiment. One of the important
requirements for this approach is maintenance of the
material microstructure during the whole experiment.
From the reversibility of the irradiation effect we can
conclude that the structure of the samples was not
significantly affected by the treatment. Another
important issue is that the position of the created defects
is within the bandgap of the material. The fact is
supported with two observations: (i) the resonance of
the intrinsic samples was not shifted after irradiation (ii)
the observation of the db resonance in the n-doped
samples indicates the Fermi level shift in the middle of
the gap.
One should consider now the appearance of the new
features in the resonance of irradiated samples
(Fig. 2,a). This observation was not reported by other
groups and was an unexpected outcome of the
experiment. The structure of the features is different for
μc-Si:H and a-Si:H. For the a-Si:H samples prepared
with different SC there is no significant difference in the
structure of satellites but their relative contribution is
different. The origin of the satellites is not identified at
the moment therefore the line shape of these satellites
could not be evaluated unambiguously. There are
number of possible origins of the satellites: hyperfine
interaction with hydrogen nuclei, creation of centres
with high anisotropy which lead to a complex powder
spectrum line, the spin-spin interaction in the local areas
with high Ns, creation of the new defects in a special
environment, and of course the combination of these
reasons could not be excluded. The choice of the model
will affect the estimation of the contribution of
satellites. We have estimated the contribution of the
satellites for the a-Si:H after irradiation assuming, for
simplicity, the constancy of the central line shape. The
simulation procedure is presented in Fig. 5.
Fig. 5. The simulation of the satellites contribution with
two approaches: contributions assumed to be Gaussian
lines (a); additional spectrum assumed to be the powder
pattern of the anisotropic state (b)
CONCLUSIONS
_______________________________________________________________________________
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2007. № 2. 41
Серия: Физика радиационных повреждений и радиационное материаловедение (90), с. 39-42.
The first important outcome of the work is the suc-
cessful application of MeV electron bombardment as a
tool for reversible increase of the defect density in μc-
Si:H and a-Si:H. The defects were created in the band
gap of the material and likely have the same origin as
defects before irradiation. The experiment being applied
for the single layers of μc-Si:H and a-Si:H as well as to
devices on their basis could result in new information
on the role of defects in the electronic properties of the
material.
New features were observed in the irradiated materi-
al appearing as satellites on the central (db) line. Their
shape and contribution is dependent on the particular
material structure. The origin of the satellites is cur-
rently not identified. The states responsible for the satel-
lites are less stable at elevated temperatures than the db
like defects thus the low temperature irradiation and
sample storage is critical for the observation of the
satellites. Possibly due to this reason no reports on the
satellite appearance was found despite many irradiation
experiments were done by other groups [15, 16].
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16.W. Bronner, M. Mehring, R. Brüggemann //Phys.
Rev. B. 2002, v. 65, p. 165212.
ПАРАМАГНИТНЫЕ ЦЕНТРЫ В АМОРФНОМ И МИКРОКРИСТАЛЛИЧЕСКОМ КРЕМНИИ,
ОБЛУЧЕННОМ 2 МэВ ЭЛЕКТРОНАМИ
А. Астахов, Ф. Фингер, Р. Кариус, A. Ламбертз, И. Неклюдов, Ю. Петрусенко,
В. Борисенко, Д. Баранков
Аморфный и микрокристаллический кремний являются широко известными материалами для производства тонко-
пленочной электроники большой площади. Дефекты в данных материалах играют решающую роль для качества прибо-
ров и оптимизации производственных процессов. Мы исследовали тонкопленочный гидрогенированный кремний мето-
дом измерений электронного парамагнитного резонанса (ЭПР). Для изменения плотности дефектов в широких пределах
образцы облучались электронами с энергией 2 МэВ. Образцы исследовались после осаждения, после облучения и между
стадиями отжига. Плотность спинов (Ns) в материале изменялась в пределах 3-х порядков величины. По обе стороны от
центрального резонанса, характеризующего оборванные связи кремния, наблюдались мощные дополнительные резо-
нансные линии (g≈2.010 и g≈2.000). После отжига форма резонансных линий и плотность спинов возвращались к исход-
ным значениям.
ПАРАМАГНИТНІ ЦЕНТРИ У АМОРФНОМУ І МІКРОКРИСТАЛІЧНОМУ КРЕМНІЇ,
ОПРОМІНЕНОМУ 2 МеВ ЕЛЕКТРОНАМИ
O. Астахов, Ф. Фінгер, Р. Каріус, A. Ламбертз, І. Неклюдов, Ю. Петрусенко,
В. Борисенко, Д. Баранков
_______________________________________________________________________________
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2007. № 2. 42
Серия: Физика радиационных повреждений и радиационное материаловедение (90), с. 39-42.
Аморфний і мікрокристалічний кремній є широко відомими матеріалами для виробництва тонкоплівкової
електроники великої площі. Дефекти у даних матеріалах відіграють вирішальну роль для якості пристроїв і оптимізації
виробничих процесів. Ми досліджували тонкоплівковий гідрогенований кремній методом вимірів електронного
парамагнитного резонансу (ЕПР). Для зміни щільності дефектів у широкому диапазоні зразки було опромінено
електронами з енергією 2 МеВ. Зразки було досліджено після осадження, після опромінення і між етапами відпалу.
Щільність спинів (Ns) в матеріалі змінювалась в межах 3-х порядків величини. З обох боків від центрального резонансу,
що характеризує обірвані зв’язки кремнію, спостеригались потужні додаткові резонансні лінії (g≈2.010 и g≈2.000). Після
відпалу форма резонансних ліній і щільність спинів поверталися до вихідних показників.
_______________________________________________________________________________
ВОПРОСЫ АТОМНОЙ НАУКИ И ТЕХНИКИ. 2007. № 2. 43
Серия: Физика радиационных повреждений и радиационное материаловедение (90), с. 39-42.
REFERENCES
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