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16. - 18. 10. 2013, Brno, Czech Republic, EU
THE DEPOSITION OF AMORPHOUS AND AMORPHOUS HYDROGENATED SILICON WITH
EMBEDDED CUBIC MG2SI NANOPARTICLES
The-Ha STUCHLIKOVA1, Jiří STUCHLIK*1, Zdenek REMES1, Radek FAJGAR2,
Nikolay G. GALKIN3, Konstatntin N. GALKIN3, Igor M. CHERNEV3
1 Institute of Physics ASCR, v. v. i., Prague, Czech Republic, EU 2 Institute of Chemical Process Fundamentals of the ASCR, v. v. i., Prague, Czech Republic
3 Institute of Automation and Control Processes of FEB RAS, Vladivostok 5, Russian Federation
ABSTRACT
Our interest is focused on potential applications of Mg2Si for effective solar energy conversion in solar cells
based on hydrogenated silicon thin films (Si:H). Si:H large scale diode structures are successfully deposited
by Plasma Enhanced CVD technique (PECVD) but the reached efficiency of energy conversion for serious
production is too low up to now. It is very well known that in the case of amorphous structure the absorption
coefficient α is high but transport of charge in this structure is not adequate for higher efficiency. From
opposite side the microcrystalline structure is more convenient for transport of charge but α is lower and that
is why the solar cells have been thicker for sufficient absorption of light. That is why we study possibilities
how to increase α by usage of magnesium silicide nanoparticles (Mg2Si-NPs) in structure of Si:H. In this
paper we introduce two technics - combination of PECVD and Vacuum Evaporation (VE) and Reactive Laser
Ablation (RLA) – for preparation of cubic structure of Mg2Si-NPs in amorphous (a-Si) or amorphous
hydrogenated (a-Si:H) silicon matrix. Formation of Mg2Si-NPs was proved by Raman spectroscopy. Likewise
we introduce optical changes measured at absorption edge and the first results on realized NIP structures.
Keywords: Mg2Si nanoparticles, a-Si/a-Si:H matrix, RDA, PECVD, RLA
1. INTRODUCTION
The Plasma Enhanced Chemical Vapour Deposition (PECVD) technique is widely used for deposition of
large scale thin films on convenient surfaces and the technique is used for preparation of different structures.
Final quality of the structures prepared is given by elements which are used and by compounds which we
are able to create in plasma processes and finally by properties of final thin film structures or multi-structures.
One of many examples of PECVD utilization is deposition of hydrogenated silicon (Si:H) thin films (usually in
form of amorphous (a-Si:H) and microcrystalline (µc-Si:H) layers) for large scale solar cells [1]. But the
PECVD technique is limited for preparation of thin layers from volatile precursors.
Another method, the Reactive Deposition Epitaxy (RDE) is known as a technique for formation of silicides of
different elements. The deposition technique is conducted under Ultra High Vacuum (UHV) conditions and
the technique is used mainly in fundamental research for preparation of new materials. The technique allows
to form silicide nanoparticles of different metals (Fe, Cr, Ca, Mg) possesing semiconducting properties and
convenient band gap for advanced applications. For example magnesium silicide (Mg2Si) and magnesium
silicide nanoparticles (/Mg2Si-NPs) possess a narrow band gap of 0.6 – 0.77 eV [2,3] and high absorption
coefficient in the photon energy in range of 0.8-2.0 eV. But the research of this material is focused mainly on
thermoelectric applications up to now. Due to optical properties of silicides, they are studied as dopants in
the hydrogenated silicon thin film for effective solar energy conversion in solar cells.
In this paper we demonstrate a deposition of Si:H diode structures by PECVD technique using silane (SiH4)
as silicon precursor. But at room temperatures there is no volatile and convenient precursor for transport of
magnesium into radiofrequency discharge. That is reason why we combined the PECVD and Vacuum
16. - 18. 10. 2013, Brno, Czech Republic, EU
Evaporation (VE) techniques for deposition of magnesium silicides. Reactive Laser Ablation (RLA) was
applied as an alternative technique of the RDE in this study. It allows preparation of Mg2Si-NPs in
hydrogenated amorphous silicon matrix in one step. The technique is based on laser ablation of elemental
magnesium and interaction of excited magnesium atoms with silane in the gas phase. We introduce here a
new diode structure (NIP) with Mg2Si-NPs embedded in a-Si matrix as well. The multistructures were
deposited ex-situ by RDE technique. Prepared magnesium silicides were analysed by SEM microscopy and
Raman spectroscopy techniques. Optical changes at absorption edge and preliminary results of I-V
characteristics on realized NIP structures are presented.
2. EXPERIMENTAL
A standard radio frequency PECVD with 13.56 MHz for excitation of glow discharge at two electrodes
configuration was used for decomposition of SiH4 ( Fig. 1). Samples with Mg2Si nanoparticles on substrates
(a-Si:H) were grown by combined PECVD/VE techniques (ex-situ) followed by vacuum annealing or plasma
treatment. All processes were performed after standard 12 hour heating, pumping and degassing of a
chamber and samples down to pressure about 10-5 Pa. For the deposition of N-type Si:H thin films a mixture
of silane with 1% of phosphane (PH3) was used. Undoped (intrinsic) I-type layers were deposited using high
resistivity silane (5.0 purity) and for P-type layers a mixture of silane with 1% of diborane (B2H6) was used.
All the gas precursors where diluted by hydrogen in the volume ratio 4.3 : 50. The pressure in chamber was
kept at 70 Pa and density of RF power was about 0.01 W/cm2. Magnesium evaporation was conducted ex
situ in a four boat vacuum apparatus (SENVAC GmbH) with INFICON XTC/2 Deposition Controller. The
stimulation of surface reaction was tested by annealing in vacuum up to 400 °C and by plasma treatment in a
hydrogen or helium glow discharge.
Fig. 1 The vacuum chamber and scheme of apparatus for deposition of hydrogenated silicon thin films by
PECVD technique
In the RLA technique, a magnesium target was irradiated by a focused beam of the ArF excimer laser (193
nm, 50 mJ/pulse, repetition frequency 10Hz, irradiation time 10 minutes) in a glass cell filled by 0,5 - 10 Pa of
silane (SiH4). Magnesium/silicon layers were deposited on various substrates (glass, quartz, tantalum) and
analysed by microscopy, spectroscopy and diffraction techniques.
The RDE growth of Mg2Si multilayers on two different glass substrates were carried out in an UHV chamber
(OMICRON, base pressure of 2.10-10 Torr) equipped with LEED and AES-EELS spectrometers, Si and Mg
16. - 18. 10. 2013, Brno, Czech Republic, EU
sublimation sources and a film thickness quartz sensor. Glass, ITO/glass and Si:H/ITO/glass wafers were
used as the substrates for growth experiments. The glass substrates were cleaned in ethanol and then
loaded in UHV device. In the first step a thin amorphous Si layer (30 or 60 nm thickness) was deposited on
the glass substrates with ITO and Si:H layers at 70 - 80 oC. Then magnesium (1 nm) and amorphous silicon
(5 nm) thin layers were deposited. The depositions of Mg/a-Si were repeated 15 or 30 times and then top
amorphous Si layer with thickness from 30 to 60 nm was deposited. Samples at growth temperatures 92 oC
were prepared and analyzed. The temperature was optimized in previous study [4]. Deposition rates (1
nm/min for Si and 0.25-0.30 nm/min for Mg) were measured by quartz sensor before the deposition. The
morphology of the grown samples with embedded Mg2Si nanolayers was investigated ex-situ by atomic force
microscopy (AFM) in tapping mode. The optical reflectance spectra of the grown samples were registered
using spectrophotometer Hitachi U-3010. The vacuum FT-IR spectrometer VERTEX 80v (Bruker) in the
energy range of 0.5- 6.2 eV (with integrating sphere at 1.5 – 6.2 eV) was used for collecting the reflectance
spectra. An optical absorbance of samples was measured by the Photothermal Deflection Spectroscopy
(PDS).
Raman spectra of the grown samples were measured using Nicolet Almega XR dispersive spectrometer with
excitation 473 nm, equipped with Olympus BX 51 microscope. Scanning electron microscopy (SEM) was
performed on a Tescan FR-TI2/736 and Tescan Indusem microscope equipped with EDX detector Bruker
Quantax 125eV.
3. RESULTS AND DISCUSSION
It is already known that a sticking factor of magnesium on the silicon surface is low. Thus a control of the
very thin (few nanometers thick) magnesium film deposition brings problems. It is reason why both
reproducibility of the evaporation and stimulation of magnesium surface for reaction with silicon are difficult.
On the other hand deposition of magnesium films of 100 nm thickness is very simple. Thus we prepared
thick magnesium film by evaporation and subsequent etching process was applied to reduce the film
thickness and stimulate reaction between Si and Mg. For this purpose helium and hydrogen plasma at
pressures about 50 Pa with high power density of 1.5 W/cm2 was used. Magnesium silicide was formed on
the surface in the form of nanosized (50-100 nm) islands (nanoclusters) with density 109 cm-2. It is well
known that in the case of amorphous and microcrystalline silicon the role of hydrogen is important for
dangling bonds saturation. We suppose that the role of hydrogen is important on the boundary formation
between crystalline form of Mg2Si NPs and the silicon matrix.
In the RLA technique a magnesium target was irradiated by a focused beam of the excimer ArF laser.
Ablation was accompanied with bright green emission due to highly excited magnesium atoms and clusters,
interacting with SiH4 molecules in a gas phase. Because of excess energy of magnesium species, silane
decomposed and a dark brown material was deposited on the substrates. Ablation of magnesium was
studied in a broad pressure range 0.5 – 10 Pa of silane and elemental composition of the deposit was
investigated by an EDX technique. At 1 Pa we revealed the composition 2:1 (Mg:Si) and further deposition
was carried out at this silane pressure. The deposited thin layer was revealed to be amorphous (by electron
diffraction techique) after deposition at room temperature, while deposition carried out with the substrates
heated to 200 oC afforded nanocrystalline deposits, as revealed by both electron diffraction and Raman
spectroscopy.
Crystalline structure of all grown samples was measured by the Raman spectroscopy. In the PECVD/VE and
RLA samples the Raman spectroscopy revealed two peaks 259-261 cm-1 and 336 cm-1 (Fig. 2a), belonging
to the Mg2Si cubic structure [5,6]. Raman spectra confirm formation of semiconducting Mg2Si nanoclusters
on the /a-Si:H/ layers in samples prepared by PECVD/VE and deposits prepared by RLA on the glass
substrate. Ratios between intensity of crystalline Si (517 cm-1) and Mg2Si bands (centered at 262 and 340
cm-1) depend on conditions of technological process. Both Mg2Si bands are very broad, which is an evidence
for presence of small nanocrystallites. Moreover, in the case of PECVD/VE process the strong peak of
16. - 18. 10. 2013, Brno, Czech Republic, EU
crystalline silicon appears (Fig. 2a, red and green line). It corresponds to conversion of the hydrogenated
amorphous silicon into nanocrystalline form, since the intensity of Raman peak strongly increases and peak
position shifts. Fig 2a (blue line) represents well crystallized magnesium silicide prepared by PECVD/VE
technique. Thus the results presented are a challenge for further plasma parameters optimization in the
PECVD/VE process and in subsequent plasma treatment.
A typical SEM image of Mg2Si-NPs on the surface of Si:H thin film is presented on the Fig. 2b. It
demonstrates the morphology of the surface formed by nanoparticles with diameter up to 200 nm.
Corresponding Raman spectrum of this surface is represented by the green line in the Fig. 2a. It means that
both magnesium silicide and silicon crystallites are present in the sample.
a) b)
Fig. 2a Raman spectra of Mg2Si-NPs samples, prepared by the PECVD/VE technique followed by vacuum
annealing and plasma treatment (red, blue and green lines) and prepared by RLA (black line)
Fig. 2b Typical SEM image of Mg2Si-NPs/Si on the surface of Si:H thin film
According to AFM, the surface of multilayer samples with embedded Mg2Si layers prepared by RDE at
92 oC consisted of small (100-150 nm) granules without faceting (Fig. 3a), square roughness is 3.03 nm.
Mg/Si multilayers were capped by Si layer in the most samples. Therefore, after reloading the samples from
growth chamber an oxidation of amorphous Si surface occurs, as observed by Raman spectroscopy bands
centred at 480 cm-1.
16. - 18. 10. 2013, Brno, Czech Republic, EU
Fig. 3a AFM images of samples grown by RDE at 92 °C deposited on the a-Si:H/ITO/corning glass substrate
Fig. 3b Dependence of absorption coeficient α of a-Si multistructure with embedded Mg2Si-NPs on photon
energy
Thickness of the RDE grown samples with alternating Mg/Si multilayers is 135 - 210 nm, as measured by
AFM technique. Calculations in the frame of two-layer model of the absorption coefficient shows (Fig. 3b)
that a main contribution in the energy range of 0.5 – 1.0 eV is caused by Mg2Si possessing a direct band gap
about 0.40 eV but the main contribution in the spectra is given by both the second direct transition with band
gap 0.72 eV and high density of states. We can propose that in the case of amorphous Mg2Si some changes
in its electronic structure occur. So, the main contribution to the absorption spectra of samples with the Mg2Si
nanoparticles is caused by two strong direct interband transitions.
An optical absorbance of samples was measured by the Photothermal Deflection Spectroscopy (PDS). The
Mg2Si nanoparticles have been embedded in the about 440 nm thick a-Si:H layer deposited on the Corning
glass substrate. Fig. 4 compares the optical absorbance spectra of a-Si:H (blue line) with that of 13 layered
Mg2Si nanoparticles embedded in a-Si:H (black line). The a-Si:H thickness 440 nm has been estimated from
the interference fringes. Figure 4 shows that the embedded Mg2Si nanoparticles significantly increase the
optical absorption in a-Si:H in the red region. Therefore, the growth of Mg2Si multilayers (about 1 nm of Mg in
each layer) together with Si interlayers is perspective for an increase of the absorption coefficient in the
photon energy range of 1.5 – 2.5 eV as required for fabrication of solar cells based on hydrogenated silicon.
SEM image (Fig 4, inset) shows morphology of the sample surface. Nanoparticles show a broad size
distribution from 50 to 200 nm.
Fig. 4 Optical absorbance spectra of the intrinsic a-Si:H layer (blue line) and 13 layered of Mg2Si
nanoparticles in a-Si (black line). Inset: Morphology of 13 layered Mg2Si in a-Si by SEM
The deposition of diodes on the base of Si:H thin films was performed by a standard deposition technique.
First, a transparent and conductive thin film of indium tin oxide (ITO) was deposited by magnetron sputtering
on the Corning glass. The NIP diode structure was deposited subsequently by PECVD technique through
decomposition of SiH4 with PH3, SiH4 and SiH4 with B2H6. The I-V characteristics for the first realised diode
structures are shown in Fig. 5. The curves difference is caused by presence of magnesium silicide
nanoparticles and by oxidation process on the diode surface. But the I-V characteristic was held with a
rectification factor about 104 - 105. This rectification factor of the standard diode structures is 107 for an
applied voltage +/- 2-3V. For a direct measurement of quantum efficiency the NIP structures prepared by the
in-situ grown are needed and this work is in progress.
16. - 18. 10. 2013, Brno, Czech Republic, EU
Fig. 5 I-V characteristics for first realised diode structures on the base of a-Si:H thin films deposited by
standard PECVD technique. Scheme of the realized diode structures A and B
4. CONCLUSIONS
Two new technological processes (PECVD/Vacuum Evaporation combined with annealing and plasma
treatment and RLA) have been successfully used for the deposition of magnesium silicide (Mg2Si) layers and
magnesium silicide nanoparticles (Mg2Si-NPs). Morphology of the deposited Mg2Si-NPs was observed by
SEM and the cubic structure of prepared magnesium silicide has been confirmed by the Raman
spectroscopy. The 15-30 layers Mg2Si/α-Si heterostructures on Si:H/ITO/Corning glass substrates were
prepared by RDE method and substrate deposition temperature was optimized (90 - 92 oC). Increased
infrared absorption of the hydrogenated /Si:H/ layers with embedded Mg2Si in the photon energy range of
0.5-2.0 eV has been firstly demonstrated due to the formation of strong direct transitions in the Mg2Si
nanoparticles inside amorphous silicon. The multistructure was successfully integrated into NIP a-Si:H diode
structure and their I-V characteristics of the a-Si:H and Mg2Si /a-Si:H were presented .
ACKNOWLEDGEMENTS
The work was performed with partial financial support by LH12236 (MSMT KONTAKT II, Czech
Republic), Grant Agency of the Czech Republic (No. 13-25747S) and the Russian Foundation of Basic
Researches grant No 13-02-00046_a
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