Pulsed Electron Deposition (PED) is a novel technique  that can be applied for growing high quality thin films. In this technique,  we used an electron beam with a focused diameter of about 1 mm, the energy  up to 15 kV, the frequency of 1-10 Hz, the pulse width of 100 ns and the  total current of 1.5 kA generated in a discharge system. A remarkable  advantage of this technique is the low deviation in composition from bulk to  film. By using the PED technique the transparent coducting ZnO and  Cu(InGa)Se\(_{2 }\) films were prepared. The effect of some deposition  conditions on the properties of film has been examined and discussed. For  Cu(InGa)Se\(_{2 }\), the best film was obtained at the discharge voltage of 12  kV and substrate temperature of 400\(^\circ\)C, while for ZnO, the best film  was grown at the oxygen pressure of 1.3 Pa and at 400(^\circ\)C

Ngoc, Do Quang

Quang, Pham Hong

Sang, Ngo Dinh

Vietnam Academy of Science and Technology: Journals Online

Communications in Physics, Vol. 22, No. 1 (2012), pp. 65-73
PULSED ELECTRON DEPOSITION (PED) – A NOVEL TOOL
FOR GROWTH OF THIN FILMS
NGO DINH SANG
National University of Civil Engineering
PHAM HONG QUANG AND DO QUANG NGOC
Hanoi University of Science, Vietnam National University
Abstract. Pulsed Electron Deposition (PED) is a novel technique that can be applied for growing
high quality thin films. In this technique, we used an electron beam with a focused diameter of
about 1 mm, the energy up to 15 kV, the frequency of 1-10 Hz, the pulse width of 100 ns and the
total current of 1.5 kA generated in a discharge system. A remarkable advantage of this technique
is the low deviation in composition from bulk to film. By using the PED technique the transparent
coducting ZnO and Cu(InGa)Se2films were prepared. The effect of some deposition conditions
on the properties of film has been examined and discussed. For Cu(InGa)Se2, the best film was
obtained at the discharge voltage of 12 kV and substrate temperature of 400˚ C, while for ZnO, the
best film was grown at the oxygen pressure of 1.3 Pa and at 400˚ C.
I. INTRODUCTION
Recently, Pulsed Electron Deposition (PED) has been attracting a great attention
in thin film deposition [1-3]. This technique, also known as Channel-Spark Discharge and
Pulsed Plasma Deposition, is based on a pulsed high power electron beam created in a low
pressure gas discharge. The electron beams are characterized by short pulse widths (∼100
ns), high energy density (∼10 J/cm2), and are delivered to a target surface by relatively
low-energy electrons (∼10–20 keV), penetrating ∼1 mm into a target material, leading
to a non-equilibrium heating that preserves the stoichiometric material transfer from the
target to the substrate. PED shares some of the same advantages that characterize the
well know technique, Pulsed Laser Deposition (PLD) such as easy control of film thickness,
easy set-up, multicomponent film stoichiometry nearly identical to target material, and
a relatively high deposition rate with low consumption of target materials. In addition,
since electrons are charged particles, they interact very effectively via Coulomb interaction
so that PED works for UV-transparent materials where PLD might fail. In fact, PED has
been found to be useful for film deposition of complex materials such as superconductor,
perovskite, oxides, and wide band gap semiconductor [4-6].
The structure of a CIGS based solar cell consists of several layers like the transparent
conducting ZnO and the absorber Cu(InGa)Se2 layers. Growth of pure ZnO films by PED
has been reported by Porter et al. [7], Nistor et al. [8] , and Zhan et al [9]. And
growth of CuInSe2 films by PED has been shown [10]. Andriesh et al. [11] also reported
a two step deposition of CuIn1−xGaxSe2 polycrystalline films with a “e-beam ablation”
66 NGO DINH SANG, PHAM HONG QUANG, AND DO QUANG NGOC
process followed by selenization at 500˚C. In this work, we show a PED system that has
recently been set-up at Hanoi University of Science and its ability in growth of transparent
conducting ZnO and Cu(InGa)Se2 films. Moreover, several issuers concerning the optimal
growth conditions for getting good quality films have been shown.
II. PULSED ELECTRON DEPOSITION TECHNIQUE AND
EXPERIMENTAL DETAILS
For the PED technique the channel spark system is used to produce a pulsed electron
beam. The unit consists of a special type of hollow cathode, known as transient hollow
cathode, a dielectric tube, which acts as an accelerator, and an air-filled spark-gap trigger
tube for triggering the channel spark. For its operation the gas pressure in the system is
maintained in the range of 0.7–2.7 Pa. By triggering the spark gap, constricted discharge
is generated in the trigger tube, which ignites the transient hollow cathode. The electrons
so generated form a beam and are accelerated through the dielectric tube. The general
scheme of this process is shown in Fig. 1.
Fig. 1. General scheme of the PED process
In the beam, the mutual repulsion between the carriers is overcome by the magnetic
self-field that keeps its trajectory self-focused and allows realization of the high current
density of the beam [12]. The pulsed electron gun used in our experiments is a commercial
source, PEBS-20 manufactured by Neocera, Inc. It has the following technical specifica-
tions: charging potential of 5-20 kV, beam energy of 0.2-0.8 J, pulse duration of 100 ns,
maximum power density of 1.3×108W/cm2, beam cross section of about 6×10−2cm2, rep-
etition rate of 1-10 Hz. In order to enhances the uniform erosion and reduces the ”conic
effect” on the surface of target, a target rastering feature was made by the movement of
the target stage at the X-Y plane about the central axis of the stage. Start angle, end
angle, desired speed of rastering can be selected by the user using computer.
The CIGS films were deposited on glass substrate with the repetition rate of pulses
maintained at 5 Hz and the argon pressure kept at 1 Pa, two series of CIGS films were
prepared. The deposition of the CIGS-1 films was carried out at room temperature and
PULSED ELECTRON DEPOSITION (PED) – A NOVEL TOOL FOR GROWTH OF THIN FILMS 67
at four discharge voltages, viz., 8, 10, 12 and 14 kV. For the CIGS-2 films, the substrate
temperature was ranged from room temperature to 600˚ C while the discharge voltage was
kept at 12 kV. The target used in our work is a high purity commercial Cu(In0.7Ga0.3)Se2
which has a diameter of 2 inch and a thickness of 3 mm. All samples were deposited with
20,000 pulses.
Al-doped ZnO (AZO) thin films were deposited on quartz substrates. During grow-
ing process, the discharge voltage and the pulse frequency were maintained at 14 kV at 5
Hz, respectively. We chose the optimal potential discharge voltage of 14 kV as reported
by Strikovski [13], with this voltage a maximal deposition rate can be obtained.. We also
made two series of films; the series named AZO-1 was deposited at room temperature but
at four pressures, viz., 0.7, 1.3, 2.0, 2.7 Pa of oxygen. The other series named AZO-2
was deposited at 1.0 Pa oxygen pressure but at the substrate temperatures ranged from
150˚ C to 600˚ C. During deposition process, the pressure was maintained by controlling
the balance between the rate of high vacuum pump and the flow of oxygen gas introduced
into the chamber. The target used is a high purity commercial 2 wt % Al2O3doped ZnO
target having a diameter of 2 inch and a thickness of 3 mm. These samples were also
deposited with 20,000 pulses.
The crystallinity of the films was checked by a Bruker-D5005 X-ray. Thickness of
the films was determined by the stylus profiler (Detark-D150) and surface morphology was
examined by SEM (Jeol-JSM5410LV). The optical properties were measured using UV-vis
spectrophometer (UV-2450) in the wavelength range from 200 to 800 nm. The electrical
resistivity was determined by four probe technique at room temperature.
III. RESULTS AND DISCUSSION
III.1. Cu(InGa)Se2 films
The thickness of the CIGS-1 films was found to be 500, 650, 750 and 800 nm
for the films grown at voltages of 8, 10, 12 and 14 kV, respectively. This result can
be expected from the fact that the discharge voltage is the main factor determining the
number of material ions created by ablation. For the CIGS-2 films, the thickness decreases
slightly with increasing substrate temperature, corresponding to the enhancement of the
crystallinity which will be mentioned below.
Fig. 2 shows the SEM images of some typical samples. For the CIGS-1 films, par-
ticulates in the range from 50 to about 100 nm are present on the surface of the films,
whatever the value of discharge voltage. However, the density and the size of the particu-
lates are smaller in the films deposited at lower voltage. The origin of these particulates is
a matter of discussion. They can be directly emitted by the target during electron beam
ablation, or they can be formed in the gas phase, during the transport of species from
the target to the substrate. It is interesting to note that the surface morphology of the
films was improved as increasing the substrate temperature. The film growth at 600˚ C
has isolated grains with uniform size and well-defined boundaries, indicating a very good
crystalinity. We have made an attempt to determine the composition of the films by using
EDS but the result is not reliable due to the influence of the glass substrate.
68 NGO DINH SANG, PHAM HONG QUANG, AND DO QUANG NGOC
Fig. 2. (a),(b),(c) SEM images of CIGS-1 films grown at 8, 12 and 14 kV, respec-
tively and (d) SEM image of CIGS-2 film grown at 12 kV and 600˚ C
The XRD patterns of CIGS-1 and CIGS-2 films are shown in Fig. 3(a) and 3(b),
respectively. We can see that all CIGS-1 films (i.e. the films grown at room temperature)
have an amorphous structure. The (112) peaks of chalcopyrite structure are observed
in the XRD patterns of the CIGS-2 films, indicating the enhancement of crystalinity.
These observations are in accordance with the SEM images. The orientation of the (112)
peak and the grain size estimated by the full width at half-maximum of the (112) peak
increase with increasing the substrate temperature. It is worth to note that the preference
orientation of (112) face is a necessary requirement of the CIGS absorber layer for a high
performance CIGS thin film solar cell. The improvement of the crystallinity is due to as
the increase of the substrate temperature.
The absorption spectra of the CIGS-1 films have a tail in the transparent range
which may be caused by the poor crystallinity of the films. Except the case of film
growth at 150˚ C, the absorption spectra of the CIGS-2 films are typical for a direct
PULSED ELECTRON DEPOSITION (PED) – A NOVEL TOOL FOR GROWTH OF THIN FILMS 69
(a) (b)
Fig. 3. The XRD patterns of (a) CIGS-1 films and (b) CIGS-2 films
band semiconductor which has a steep absorption edge. Fig. 4 shows the photon energy
dependence of the square absorption coefficient for the films grown at 150˚ C, 280˚ C, 400
oC and 600˚ C. The band gap about 1.4 eV was determined by extrapolating the linear
portion of these curves. This value of band gap is in agreement with the expected value for
a bulk Cu(In0.7Ga0.3)Se2 sample [14]. It means that the stoichiometry of target materials
is preserved in the film. It is very difficult to compare our results with those reported
by Andriesh et al. [11] because their results were measured on the samples which have
undergone the selenization process.
Fig. 4. The square of the absorption coefficient versus the photon energy curves
for the CIGS-2 films
70 NGO DINH SANG, PHAM HONG QUANG, AND DO QUANG NGOC
III.2. Al doped-ZnO
The thickness of the ZnO films is strongly dependent on the oxygen pressure. The
thickness of the AZO-1 films is 270, 250, 230 and 220 nm for the films grown at 0.7, 1.3,
2.0 and 2.7 Pa oxygen pressure, respectively. The thickness of the AZO-2 films is almost
identical with the value of about 250 nm. The decrease of the thickness with oxygen
pressure can be can be attributed to the interaction between incoming ions in plasma
flux and gas atoms, resulting in a decrease of current of ions arriving at the substrate.
The SEM images of AZO (not shown) films are quite similar to those of CIGS films with
the particals presenting on surface. Nistor et al also observed the similar problem [8] in
growth of ZnO films and suggested the possibility to reduce it by careful optimization of
the electron beam parameters in relation to the target materials.
(a) (b)
Fig. 5. The XRD patterns of the (a) AZO-1 films and (b) AZO-2 films
Fig. 5(a) shows the XRD patterns of the AZO-1 films. It is clear that the films
grown at lower oxygen pressure (i.e. 0.7 and 1.3 Pa) exhibit a wurzite structure which
has a highly preference for the (002) orientation whereas the films gown at higher oxygen
pressure (i.e. 2.0 and 2.7 Pa) exhibit an amorphous structure. The highest intensity of
(002) peaks for the film grown at 1.3 Pa indicates that this pressure is the most suitable.
The degradation of the crystallinity at high oxygen pressure has also been observed by S.
M. Park et al. in growth of AZO films by using PLD [15] and has been attributed to the
excess oxygen that might induce defects in the films. P. Zhan et al. have also found that
the oxygen pressure of 1.4 Pa is the most favorable value for obtaining the best crystallinity
for the ZnO films grown by PED [9]. In our case, the effect of oxygen pressure seems to
be stronger and more obvious because the films were grown at room temperature, so that
the influence of temperature has been eliminated. The fact that oxygen pressure has an
optimal value originates from the requirement of the average energy per deposited atom
which should be about 10-20 eV. This energy in one hand is strong enough to complete
the disruption of the columnar morphology of the growing film but in other hand do not
damage the surface. It is well-known that in PED and PLD technique, with the presence
PULSED ELECTRON DEPOSITION (PED) – A NOVEL TOOL FOR GROWTH OF THIN FILMS 71
of background gases during ablation, the species arriving at substrate looses their average
velocity as they undergo through scattering, thermalization and deceleration.
Fig. 5(b) shows the XRD patterns of the AZO-2 films. First of all, we can see that
all films exhibit a wurzite structure with a highly preference for the c-axis orientation
perpendicular to the substrate surface. The crystallinity evaluated from the intensity
and full width at half-maximum of the (002) peak improves with increasing substrate
temperature up to 400˚ C. Further increasing the substrate temperature leads to a slightly
degradation in crystalinity. The grain size estimated from Scherrer equation is about 30
nm for the best film, i.e. the one deposited at 400˚ C. Hirata et al. [16], and Zhan et al. [9]
have reported the similar effect of substrate temperature on the crystalinity of the ZnO
films grown by PLD and PED. P. Zhan attributed the promotion of the (002) peak to the
increase of adatom mobility as increasing substrate temperature. P. Zhan also explained
the degradation of (002) peak at high temperature by structure zone model proposed by
Thormton [17]. According to this model, the films deposited at high temperature (> 320˚ C
for his case) have zone three structures which contain much more randomly oriented grains
formed by secondary nucleation and recrystallization.
The electrical resistivity measured on the AZO-1 films is very high. However, it is
still possible to realize that the resistivity is lower for the films deposited at lower oxygen
pressure. The substrate temperature dependence of electrical resistivity of the AZO-2
films was presented in Table 1. As can be seen from this table, the resistivity decreases
dramatically with increasing the substrate temperature up to 400˚ C, and then increases
slightly again. Park et al. [9] have observed a similar phenomenon on undoped ZnO films
grown by PLD and attributed the decrease of resistivity to the increase of both carrier con-
centration and carrier mobility. The slight increase in resistivity of the films grown at high
temperature was explained by the contamination of C from quartz substrate. Hirata et al.
[16] reported that resistivity of Ga-doped ZnO films deposited by PLD decreases with in-
creasing deposited temperature up to 300˚ C which is the highest investigated temperature.
Since the facts that there is a close relation between the crystallinity and conductivity and
that our films are ZnO doped with Al, we suggest that the enhancement of conductivity
is mainly due to the increase of number of Al atoms really activated in matrix oxide to
produce extra electrons in the band. The resistivity of 3.4×10−5Ω×m obtained at the op-
timal temperature is still quite high for requirement of TCO films. An additional process
for improving electrical resistivity is necessary and will be studied in next future.
Table 1. The substrate temperature dependence of electrical resistivity of the
AZO-2 films
Substarte temperature Electrical resistivity
25˚ C ∼ 10−2Ω×m
150˚ C 5×10−3Ω×m
400˚ C 3.4×10−5Ω×m
600˚ C 8×10−5Ω×m
The optical transmittance of the films in both AZO-1 and AZO-2 series are very high
(more than 80%) in the range of 350-900 nm. It is worth to note that there is a relation
72 NGO DINH SANG, PHAM HONG QUANG, AND DO QUANG NGOC
between the crystallinity and the transmittance, e.g. the highest value of transmittance
(∼90%) has been obtained for the film which has the best crystallinity. This value meets
the application requirement of transmittance exceeding 80%. The relation between the
crystallinity and the optical property can be seen more clearly from the absorption spectra.
The absorption spectra of AZO-1 and AZO-2 films are shown in Fig. 6(a) and 6(b),
respectively. The absorption spectra of AZO-1 films have a tail in the transparent zone
while the absorption spectra of all AZO-2 films have a sharp absorption edge. The value
of band gap about 3.2 eV deduced from absorption spectra is somewhat lower than the
value of 3.37 eV measured on pure and bulk ZnO materials [18].
(a) (b)
Fig. 6. The square of the absorption coefficient versus the photon energy curves
for the (a) AZO-1 and (b) AZO-2 films
IV. CONCLUSION
By using a PED equipment that has been set-up at Hanoi University of Science the
transparent conducting ZnO and Cu(InGa)Se2 absorber films have been done. Several
issuers concerning the optimal growth conditions for getting high quality films have been
shown. For Cu(InGa)Se2, the best film was obtained at the discharge voltage of 12 kV
and substrate temperature of 400˚ C, while for ZnO, the best film was grown at the oxygen
pressure of 1.3 Pa and at 400˚ C.
ACKNOWLEDGEMENTS
This work was supported by the Vietnam National Foundation for Science and Tech-
nology Development (NAFOSTED) in the period 2010-2011 (Project Code 103.02.59.09).
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Received 05 September 2011.


Pulsed Electron Deposition (PED) - a Novel Tool for Growth of Thin Films

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