Characterization of Multi-Hollow Cathode Discharge and Its Application
of a(μc)-Si Deposition
Lizhen Yang, Shouye Zhang, Jiuxiang Chen, Zhenduo Wang and Zhongwei Liu, Qiang Chen*
Laboratory of Plasma Physics and Materials, Beijing Institute of Graphic Communication, 102600, Beijing, China
Abstract: High-density capacitively coupled plasma was produced with the effect of the multi-hollow
cathode. In order to obtain the stable and uniform large area plasma, the various power supplies such as
direct current (DC) power, and radio frequency (27.12 MHz) power were utilized to investigate the I-V
characteristics, hollow cathode discharge effect, the spatial and temporal of electron temperature and
electron density as well as the discharge uniformity. In the case of applications of this multi-hollow
cathode discharge, the hybrid a(μc)-Si films were then deposited. The rate of as-deposited a(μc)-Si thin
films, the deficit, the chemical and crystal structure varying with the discharge parameters, likely the
ratio of Si source to diluted hydrogen gas, working pressure, the deposition temperature were also
explored in this work.
Introduction
Thin film silicon, especially amorphous silicon (a-Si:H), as semiconductor channel layer for the
solar cell is widely adopted in purpose of reducing cost and lightening the mass production. The a-Si:H
cells, however, have unstable, degradation due to a Stabler–Wronski (SW)-like effect in the
environment and the relative low efficiency. The alternative of poly-Si thin film, which demonstrates a
high efficiency, is expensive cost [1- 4].
Recently, hydrogenated microcrystalline silicon (μc-Si:H) prepared from silane and hydrogen
mixture using plasma enhanced chemical vapor deposition (PECVD) is an attractive trial in such as thin
film solar cell application. The hydrogen dilution is considered to be a source of atomic hydrogen which
affects the crystalline formation [5]. In future, using the hydrogenated μc-Si:H film these problems
about the unstable operation property of a-Si:H and the high running cost of poly-Si shall be solved. The
hybrid μc-Si solar cell because of its high efficiency and inexpensive might be essential selection for the
solar calls.
The normal chemical vapor deposition provided a low growth rate of μc-Si film. With plasma
enhanced CVD, on the other hand, the growth rate was significantly increased but not the quality.
Although there have been many approaches for the high rate growth and high-quality μc-Si:H film
fabrication [6–8], the quality of the films deposited at high rates is still left unsatisfied in terms of
photovoltaic application. A high rate growth of the μc-Si:H is usually accompanied by a great number of
dangling-bond defects in the resulting film, which act as recombination centers for photo-excited
carriers, resulting in a deterioration in the device performance. In previous works [6, 9], the number of
dangling-bond in μc-Si:H films prepared at high deposition rate has been reduced from the very high
values obtained under conventional radio-frequency (RF) PECVD low pressure conditions to the lower
values obtained by the use of VHF and high pressure conditions with optimization of SiH4-depletion
[10]. This is attributed to a reduction in Te of the plasma and the optimized SiH4-depletion. The hollow
cathode discharge (HC), especially the micro-hollow cathode discharge (MHC) therefore, can
remarkably reduce the Te and improve the density of plasma due to the advantage of operation at high
working pressure. With a cathode having specific surface structure, which enables the production of
uniformly distributed and stable high density plasma spots near the cathode surface, aiming to introduce
the radical separation effect into μc-Si:H deposition at high deposition rate [11].
In this paper, we employ MHC discharge to prepare hybrid μc-Si:H film with controllable
crystallization. The SiH2Cl2 instead of SiH4 was used as precursor for the μc-Si:H film deposition. It
obtains the crystallization of μc-Si:H films arrived at 70% in the optimal conditions of MHC plasma
source.
Experiments
The MHC discharge adopted for μc-Si:H film-growth, more specifically, was a capacitively
coupled RF-PECVD reactor with micro-hollow electrode constructed the discharge system. The
numerous micro-hollows arranged on the cathode surface are also as the gas injection.
The typical deposition conditions include: plasma excitation frequency of 27.12 MHz, the flow rate
of SiH2Cl2 was varied from 20 to 70 sccm, H2 gas flow rate was 300 or 700 sccm. Initially, the substrate
holder temperature was set at 200 °C. Corning glass was used as substrates. And the other controllable
dispersion parameters were identified.
Results and discussion
55
80
RMS Voltage(V)
50
RMS Voltage (V)
70
60
45
40
50
35
40
30
0
200
400
600
800
1000
RMS Current (A)
1200
1400
200
400
600
800
1000
1200
1400
1600
RMS Current (A)
a
b
Fig.1 the characteristic of I-V in MHC Ar plasma. a-the gap is 7 mm,20 Pa; b-the gap is 18 mm, 20 Pa
The characterization of I-V in MHC discharge was measured by and a digital oscilloscope
(Tektronix DPO 4104). The current and voltage across the electrodes were measured by a wide band
current probe (Tektronix 6021AC), a wide band voltage probe (Tektronix P6015A). The configuration of
I-V in Fig.1 illustrates that the discharge in RF MHC is a typical hollow cathode discharge, especially in
the case of short gap of 7 mm where the curve is similar to the configuration generated in direct current
supply. While the gap between the RF electrode and the grounded one was larger, 18mm in here, the
configuration of hollow cathode discharge is still visible, but the region of uniform glow discharge was
narrow. It quickly transfers into arc discharge from the positive differential impedance.
The evident uniform discharge in HMC is shown in Fig. 2 photos for (a) Ar and (b) H2,
respectively. As is demonstrated that plasma is rather uniform in the whole electrode, in particular, when
the plasma was generated in H2 gas environment. The dense electric field at the edge of hollows caused
a high illumination in Fig. 3(c), which is essential for the much more active radicals generated in HMC.
a
b
Fig. 2 The photos of HMC in a-Ar and b-H2
According to the measurement of the electron temperature Te and density Ne, it results that MHC
can provide a high efficient dissociation rate of species than that in parallel panel electrodes. In Fig. 3
one can see that the Ne arrived at 1.0 ×1012 cm-3 in H2 plasma of MHC, over two orders of magnitude
higher than that in the same structural capacitively coupled plasma, whereas the Te was remained almost
identical. Te was reduced with working pressure after compared Fig. 3(a) with (b) based on Langmuir
probe measurement. As aforementioned, a low Te is benefit for the defects control in μc-Si:H films. It
means that MHC shall be prior to using for hybrid μc-Si film deposition.
5.0
3
3.8
Electronic temperature
Electronic density
1
3.2
3.0
3
3.5
2
3.0
1
2.5
0
-3
-3
4.0
10
10
0
2.8
4
Electronic density (10 cm )
3.4
Electronic density (10 cm )
2
Electronic temperature (eV)
4.5
3.6
Electronic temperature (eV)
5
Electronic temperature
Electronic density
2.6
2.0
-1
0
50
100
150
200
250
-1
0
300
50
100
150
200
250
300
RF Power (W)
RF Power (W)
a
b
Fig. 3 Te and Ne in HMC H2 plasma dependent on the RF applied power. a-150 sccm; b-200 sccm
With regard to the deposition of μc-Si film films in MHC, the discharge parameters, such as the ratio
of the gas flow rate, the RF power, temperature of the substrate holder, the working pressure, diluted Ar
gas flowing rate, all influenced the film structure and morphology. However, the working pressure was
emphasized in this work.
Fig. 4 shows that the film structure varied with the pressures. One can see that in FTIR spectra (Fig.
4(a)) SiHn (n=1, 2) stretching mode at 630 and 2100 cm-1 are visually observed. The two sharp FTIR
peaks composed of 2080 and 2100 cm-1 correspond to the surface SiH2 bonds of a nanocrystallite grain
boundary and bulk SiH2 bonds, respectively. The peak intensities, corresponding to the residual
hydrogen content in the film systematically increase with decreasing working pressure, a significant
change of the SiHn configuration is observed besides the peak intensity. The film crystallization is
enhanced reversely with working pressure.
0.07
8000
-1
640cm
0.05
2000cm
-1
2090cm
(a) 230Pa
(b) 600Pa
(c) 800Pa
6000
-1
Intensity (a.u.)
(a) 230Pa
0.04
ABS
500
-1
520cm
7000
0.03
(b) 600Pa
0.02
0.01
(c) 800Pa
0.00
5000
400
Intensity (a.u.)
0.06
-1
480cm
4000
3000
300
Xc=71.4 %
200
100
2000
(a)
1000
(b)
0
(c)
-100
0
-0.01
500
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumber (cm )
a
200
400
600
800
-1
Raman shift (cm )
b
1000
400
450
500
550
-1
600
650
700
Raman shift (cm )
c
Fig. 4 the influence of working pressure on a-FTIR, b-Raman of the deposition μc-Si films; c- Raman spectroscopy of
deposited μc-Si and the Gaussian simulation
Raman spectroscope was used to evaluate the crystallization of μc–Si:H. Fig. 4(b) compares the
Raman spectra of the as-deposited μc–Si:H layers at different working pressures of 230, 600 and 800 Pa,
respectively. Concerning the MHC μc–Si:H layers, sample c is a pure amorphous structure (peak at 480
cm-1), whereas sample b is a heterogeneous structure composed of nanocrystalline material embedded in
an amorphous matrix (peaks at 480 and 513 cm-1). Raman spectrum of sample a exhibits a large peak at
520 cm-1 compared to little one at 480 cm-1 revealing a microcrystalline structure with small amorphous
component as Fig. 4 (c) the Gaussian simulation. A crystallization of 71.4% can be achieved when the
μc-Si film was deposited at gas ratio of SiH2Cl2/H2=3/200sccm, 80W of RF power, 200~300℃ of
temperature, and the working pressure of 230 Pa.
The fast deposition of highly crystallized μc–Si:H:Cl film with lower defect density is expected
because the efficient termination of dangling bond by efficient supply of SiHxCly is accelerated with the
abstraction of H and Cl as HCl at the depleted growing surface at the higher working pressure in MHC.
A sufficient supply of deposition precursors such as SiH3 at the growing surface under an appropriate
ion bombardment is effective for the fast deposition of highly crystallized μc-Si films as well as for the
suppression of the amorphous incubation and transition interface layers at the initial growth stage.
a
b
Fig. 5 the relationship of morphology of μc-Si with the plasma power a-80W, 20min; b-200W, 20min (the root mean
square (RMS) roughness is 1.33nm for 80W and 3.39nmfor 200W)
Fig. 5 shows the AFM surface images of as-deposited μc-Si film at various applied powers. The
root mean square (RMS) of roughness was measured in 80 W applied power in Fig. 5 (a) to be 1.33 nm.
The RMS was 3.39 nm in Fig. 5 (b) when the sample was deposited in 200 W. There is clearly
substantial roughening induced through the high applied power. According to the results of surface
morphology measurement, it can assume that the films were grown in island model, and the size of the
grains will be grown along with the increase of the input power.
Conclusions
In this paper a novel plasma source HMC for the stable and uniform large area plasma was reported.
The characteristics of I-V in the radio frequency (27.12 MHz) power were typical hollow cathode
discharge. With HMC plasma source a high crystallization of the hybrid a(μc)-Si films were deposited.
A crystallization of 71.4% μc-Si can be achieved in optimal conditions. The crystallization of
as-deposited a(μc)-Si thin films was seriously dependent on the working pressure.
Acknowledgments
This work was financially supported Beijing Natural Science Foundation (No.1112012),
KM201110015008, and PHR20110516.
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