J. Mater. Sci. Technol., 2010, 26(1), 93-96.
Properties of Gallium Phosphide Thick Films Prepared on Zinc
Sulfide Substrates by Radio-Frequency Magnetron Sputtering
Yangping Li† and Zhengtang Liu
School of Materials Science and Engineering, Northwestern Polytechnical University, Xi0 an 710072, China
[Manuscript received November 3, 2008, in revised form March 23, 2009]
Radio-frequency (RF) magnetron sputtering was employed to prepare gallium phosphide (GaP) thick films on
zinc sulfide (ZnS) substrates by sputtering a single crystalline GaP target in an Ar atmosphere. The infrared
(IR) transmission properties, structure, morphology, composition and hardness of the film were studied. Results
show that both amorphous and zinc-blende crystalline phases existed in the GaP film in almost stoichiometric
amounts. The GaP film exhibited good IR transmission properties, though the relatively rough surface and
loose microstructure caused a small loss of IR transmission due to scattering. The GaP film also showed a
much higher hardness than the ZnS substrate, thereby providing good protection to ZnS.
KEY WORDS: Radio-frequency magnetron sputtering; Gallium phosphide; Thick film; Infrared
transmission
1. Introduction
Zinc sulfide (ZnS) is one of the most promising materials used for long-wave infrared (LWIR, 8–12 µm)
windows and domes owing to its wide transparent
waveband, relatively low cost and availability in large
sizes and complex shapes[1–3] . However, ZnS has
high reflection, low hardness and poor environmental durability in harsh conditions. Thus, efforts have
been made to prepare IR antireflective and protective films for ZnS[4–6] . To improve their protective
performance, the films were usually prepared as thick
films[7] . Among these films, gallium phosphide (GaP)
is a preferred film due to its excellent mechanical, erosion protective and IR transmission properties. Moreover, GaP has frequently been incorporated into antireflective and protective multilayers[8] . So far, the
preparation and properties of GaP thick films have
been extensively studied. Klocek et al.[9] and Wilson
et al.[10] reported the growth of polycrystalline and
† Corresponding author. Ph.D.; Tel.: +86 29 88492178; E-mail
address: liyp@nwpu.edu.cn (Y.P. Li).
epitaxial GaP thick films by metal-organic chemical
vapor deposition (MOCVD). Gibson et al.[11] prepared thick GaP films by reactive sputtering (RS) and
plasma-assisted chemical vapor deposition (PACVD).
Dong et al.[12] employed radio-frequency plasmaenhanced chemical vapor deposition (RF-PECVD) to
deposit thick GaP films. In these preparation methods, PH3 and Ga(CH3 )3 , which are hazardous and
poisonous, were usually utilized as the precursor material. Therefore, extra measures should be taken to
meet stringent health and safety requirements. In
this work, GaP thick films were deposited on ZnS
substrates by RF magnetron sputtering starting from
a single crystalline GaP target in an Ar atmosphere
without the use of toxic reactive gas. The properties
of IR transmission, structure, morphology, composition and hardness of the deposited GaP thick film
were reported.
2. Experimental
Thermo-pressed ZnS discs (φ20 mm×5 mm) and
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Y.P. Li et al.: J. Mater. Sci. Technol., 2010, 26(1), 93–96.
a single crystalline GaP disc (φ50 mm×5 mm) were
used for the substrates and target, respectively. After being cleaned, the ZnS substrate was mounted on
the earthed anode and heated at a fixed temperature
throughout the deposition process. The target was
mounted on the water-cooled cathode, into which a
13.56 MHz RF power was capacitively coupled. As
the working gas, high purity Ar (99.99%) was introduced into the vacuum chamber through a gas flow
controller after the base vacuum had been pumped to
<3.0×10−5 Pa. In order to efficiently prepare thick
GaP films with low IR absorption, the deposition
parameters were optimized to deposit stoichiometric GaP films at a relatively high deposition rate[13] .
The optimized parameters were: RF power 80W, Ar
gas flow ratio 6.0 sccm (standard-state cubic centimeter per minute), working gas pressure 0.07 Pa, substrate temperature 390◦ C and target-substrate distance 60 mm. The deposition rate was approximately
14 nm/min.
The IR transmittance spectra of the specimens
were measured with a Nicolet 60 SXR Fourier transform infrared (FTIR) spectrometer. X-ray diffraction (XRD) was conducted for structure analysis on a
PANalytical X0 Pert HighScore diffractometer (CuKα,
λ=0.154 nm) at a grazing incidence angle of 0.5◦ .
The morphologies of the GaP films were investigated
by using a JSM5800 scanning electron microscope
(SEM), and the elemental content in the films was
studied by using an energy dispersive X-ray spectrometer (EDS) fitted to the SEM . The hardness of the
GaP film was tested with an HX-1000 Vickers hardness tester.
3. Results and Discussion
3.1 IR transmission performance of the GaP film
For IR transmission performance evaluation, the
IR transmittance spectra of GaP/ZnS (i.e., GaP film
on ZnS substrate) and the uncoated ZnS were analyzed and the results are shown in Fig. 1. The spectra were analyzed between 400 and 5000 cm−1 (2–
25 µm waveband) and the insets were between 833
and 1250 cm−1 (8–12 µm). The spectrum of GaP/ZnS
was below that of ZnS because the refractive index of
GaP (typically 2.9 for bulk GaP) is larger than that of
ZnS (typically 2.2 for bulk ZnS). Due to interference
between the light reflected from the film surface and
the GaP/ZnS interface, the spectrum for GaP/ZnS
was a smooth wavy curve having many maxima and
minima. The maxima did not contact the spectrum
curve for uncoated ZnS, indicating an IR transmission
loss caused by absorption or scattering in the GaP
film. The IR transmission loss can be estimated by
the average transmission loss, ∆T , over the 8–12 µm
waveband. ∆T is expressed as ∆T =∆T 0 − ∆T 00 ,
where ∆T 0 is the average transmittance of uncoated
ZnS and ∆T 00 is the average value of the envelope of
Fig. 1 Infrared transmittance spectra of uncoated ZnS
and GaP/ZnS samples
Fig. 2 XRD pattern of the GaP thick film
the maxima over the 8–12 µm waveband. For the GaP
film, the value ∆T is equal to 1.42%. Through fitting the transmittance spectrum of GaP/ZnS[14] , the
film thickness and refractive index at a wavelength of
10 µm were found to be 14.83 µm and 3.02, respectively. Since the IR transmission loss caused by absorption or scattering was small, the absorption and
scattering of the film were neglected for simplification
of the fitting. Thus, the drop in the transmittance
of GaP/ZnS caused by absorption or scattering is ascribed to the rise in refractive index of the film. As a
result, the obtained refractive index was a little larger
than that of the bulk GaP.
3.2 Structure, morphology and composition of the
GaP film
The optical performance of films is determined by
their properties of structure, morphology and composition. Therefore, these properties of the GaP film
were investigated. Figure 2 shows the XRD pattern of
the GaP film. The X-ray diffractogram was characterized by the superposition of broad humps and peaks.
The humps indicated the existence of an amorphous
phase in the GaP film. The peaks are attributed to
the (111), (220) and (311) diffraction planes of the
zinc-blende crystalline GaP and these peaks are in
good agreement with the standard JCPDS data file
No. 32-0397. The average size of the crystalline grain
Y.P. Li et al.: J. Mater. Sci. Technol., 2010, 26(1), 93–96.
95
Fig. 3 Surface (a) and cross-section (b) SEM images, EDS spectrum (c) and cross- section line-scan analysis (d)
of the GaP thick film
calculated by the (111) peak using Scherrer0 s formula
was approximately 10 nm.
Figure 3(a) and (b) show the surface and crosssection morphologies of the GaP film, respectively.
Figure 3(a) reveals a relatively rough but uniform film
surface. Micro holes and grains were found to exist in
the GaP film. From Fig. 3(b), it can be seen that the
part of the GaP film adjacent to the substrate was featureless and compact, while with increasing thickness,
the GaP film tended to have a columnar growth and
became a little loose. Micro-holes were also found in
the cross-section micrograph. Columnar growth of a
film always leads to a rough surface and the presence
of micro-holes between the columns in the film. Moreover, when the deposition rate was high, the atoms
deposited onto the film-growth surface would easily
meet other atoms to form stable clusters prior to full
diffusion. Hence, on one hand, the micro-holes that
had formed on the film-growth surface would be covered and embedded within the film; while on the other
hand, grains would be formed in the film. Therefore,
the rough surface and micro-holes were attributed to
the high deposition rate to some extent. Figure 3(c)
shows the EDS spectrum of the GaP film, and the inset shows the atomic content of the film. The elements
found were mainly Ga and P, plus a slight amount of
O, which was incorporated into the GaP film due to
the leakage of air into the vacuum chamber during
deposition. The content of P and Ga in the film was
50.26% and 46.76% (in at. pct), respectively, showing
that the GaP film was almost stoichiometric. Figure
3 (d) shows the cross-section line-scan analysis of the
GaP film. The
Lscanning path was shown in Fig. 3(b),
beginning at . It can be seen in Fig. 3(d) that the
composition of the GaP film was homogeneous in the
thickness direction. At the GaP/ZnS interface, all
the curves for S, Zn, P and Ga changed in a sloping
manner, indicating the diffusion of P and Ga into the
ZnS substrate. The diffusion can help to improve the
adhesion of the film to the substrate.
IR transmission loss may be caused by scattering
and absorption. However, it is hard to distinguish the
contributions of these two factors to the transmission
loss. The content of Ga and P in the GaP film is almost stoichiometric, but the film has a relatively loose
structure and rough surface. Hence, the major source
of its IR transmission loss was due to the scattering
occurring both at the grains and holes in the bulk and
at the surface of the film, rather than caused by the
absorption.
3.3 Hardness of the GaP film
The hardness of films is essential to their protective performance. So the Vickers hardness of the GaP
film and the uncoated ZnS was tested and the results
are shown in Fig. 4. The hardness of the GaP film
decreased with increasing load, but the variation of
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Y.P. Li et al.: J. Mater. Sci. Technol., 2010, 26(1), 93–96.
phases existed in the film. Although a small amount
of IR transmission loss was caused by scattering, the
GaP film had good IR transmission properties.
(3) The hardness of the GaP film was much higher
than that of the ZnS substrate; hence the film is anticipated to give good protection to ZnS.
Acknowledgement
This work was supported by the Aviation Science
Foundation of China under grant No. 2008ZE53043.
Fig. 4 Vickers hardness of the GaP thick film
the hardness became moderate at relatively high load.
This phenomenon can be explained as follows. When
the load was relatively large, the ZnS substrate was
also indented to some extent despite the large thickness of the GaP film. Hence the hardness tested at
relatively high load reflected the integrative hardness
of the GaP film and the ZnS substrate and such an
integrative hardness tended to become constant with
increasing load. Yet when the load was relatively low,
the substrate was not indented. Thus the hardness
tested at small load was close to the real hardness of
the GaP film. After all, the hardness of the GaP film
was much higher than that of the ZnS substrate according to Fig. 4. Furthermore, the GaP film could
not be peeled from the ZnS substrate in the qualitative adhesion test with peeling method, suggesting
the film well adhered to the substrate. Therefore, the
GaP film could provide good protection to the ZnS
substrate.
4. Conclusions
(1) Stoichiometric GaP thick films were successfully prepared on ZnS substrates by pure RF magnetron sputtering.
(2) Both amorphous and zinc-blende crystalline
Volume 26 Number 1 pp 1-96 2010
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