COL 12(7), 072201(2014) CHINESE OPTICS LETTERS July 10, 2014
Improvement of thickness uniformity and elements
distribution homogeneity for multicomponent films
prepared by coaxial scanning pulsed laser
deposition technique
Juguang Hu (
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, Yi Liu (
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College of Physics Science and Technology, Shenzhen University, Shenzhen 518060, China
∗
Corresponding author: linxd@szu.edu.cn
Received Novemb er 27, 2013; accepted April 29, 2014; posted online June 25, 2014
In conventional pulsed laser deposition (PLD) technique, plume deflection and composition distribution
change with the laser incident direction and pulse energy, then causing uneven film thickness and com-
position distribution for a multicomponent film and eventually leading to low device quality and low rate
of final products. We present a novel method based on PLD for depositing large CIGS films with uni-
form thickness and stoichiometry. By oscillating a mirror placed coaxially with the incident laser beam,
the laser’s focus is scanned across the rotating target surface. This arrangement maintains a constant re-
flectance and optical distance, ensuring that a consistent energy density is delivered to the target surface by
each laser pulse. S cann in g the laser spot across the target suppresses the formation of micro-columns, and
thus the plume deflection effect that red uces film uniformity in conventional PLD technique is eliminated.
This coaxial scanning PLD method is used to deposit a CIGS film, 500 nm thick, with thickness uniformity
exceeding ±3% within a 5 cm d iameter, and exhibiting a highly homogeneous elemental distribution.
OCIS codes: 220.0220, 220.4830, 310.0310, 310.1860.
doi: 10.3788/COL201412.072201.
Pulsed laser deposition (P L D) is a powerful and versatile
technique for growing films of a wide range of materials.
It is especially suitable for depo siting multicomponent
films such as YBCO (usually YBa
2
Cu
3
O
6+x
) sup e rcon-
ductor films
[1,2]
or CIGS (usually CuIn
0.7
G
0.3
S
2
) films
for solar cells
[3]
, or La
0.7
Sr
0.3
Mn
0.5
Fe
0.5
O
3
(LSMFO)
thin films
[4]
. However, film thickness a nd elements dis-
tribution inhomogeneities in multicomponent films lead
to band-gap fluctuations, which have a detrimental effect
on the device performance
[5−9]
; thus, maintaining a high
level of uniformity of both thickness and stoichiometry
is crucial, especially for large area films produce or ex-
periments.
Unfortunately, ensuring such spatial uniformity with
conventional PLD techniques is complicated by interac-
tions between the laser and the target material. The
composition, structural quality, surface roughness, and
optical band gap value of the film may be affected by
variations in the intensity of the incident laser. This is
because the spatial distributions of the plume and the
sp e cies it carries change as a function of laser fluence
[10]
.
The plume is characterized by a highly polar, forward-
peaked distribution, which results in thickness inho-
mogeneity for large-area films
[11]
. During conventional
deposition, the target and substrate are rotating and the
ablated spot is stationary with respect to the axis of
the rotating substrate; over long periods of irradiatio n,
micro-columns form on the target surface, which cause
the plume to deflect towards the incident beam
[12]
. This
effect makes it difficult to predict the final film thick-
ness and contributes to inho mogeneity of the thickness
and the local ratio of atomic species.
To obtain films with uniform thickness, a number of
techniques have been introduced: rotational/ transla-
tional PLD and offset PLD
[13,14]
; the matrix-assisted
pulsed laser evaporation (MAPLE) technique
[15,16]
; in-
verse pulsed laser deposition (IPLD)
[17]
; the shadow-
mask technique
[18]
; dynamic deposition configuration
[19]
;
the multi-beam approach
[20]
. Unfortunately, these tech-
niques present various drawbacks, such as inefficient use
of target materials, low depositio n ra te, difficulties in
preparing large- area films, and increased complexity. In
this letter we report a PLD configuration, designed to
allow the straightforward deposition of large, homoge-
neous films; we name the method coaxial scanning PL D
(CSPLD).
In this configuration (Fig. 1), the axis of the scan-
ning mirror is coaxial with the direction of incident laser
beam. The plane of the mirror is tilted with respect to
the beam, such that the laser is reflected at a glancing
angle onto the target in the vacuum chamber. As the
axis of the mirror oscillates, the mirror changes its nor-
mal direction, and the laser spot is scanned back and
forth across the surface of the targ et.
The principle behind this configuration is illustrated in
Fig. 2. The angle α between the incident laser beam and
the mirror remains constant dur ing scanning, as does the
position of the laser spot on the mirror, which ensures
that the reflectance does not vary. This avoids energy
variations in the reflected non-polarized laser pulse as
the mirr or oscillates. Assuming the incident laser direc-
tion (or, eq uivalently, the rotating axis of the mirror) is
1671-7694/2014/072201(4) 072201-1
c
2014 Chinese Optics Letters