Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BAC:I~GROUND OF THE INVENTION
For optoelectronic applications, it is often require to
have polycrystalline thin film materials with high optical
transmission for photons in most part of the solar spectrumO
The high optical transmission can be achieved using
semiconductors with large energy gaps. In addition to the
high optical transmission, it is necessary to have low
electrical resistivity. The low electrical resistivity is
needed in order to reduce ohmic loss of electricity when an
electron device is formed. For example, in a photovoltaic
solar cell, the low resistivity is essential to obtain high
solar ~o electrical energy conversion efficiency. For
display application, the low resistivity is also needed in
order to reduce the power dissipation during the operation.
Apart from the above described requirements, it is
necessary to prepare the large band gap thin films with a
specific crystal orientation and with pre-determined lattice
constants so that the films match well to another
semiconductor (usually absorbing layer with a smaller band
~ap) forming the junction. In general, there is a lattice
mismatch between two different semiconductors. This lattice
mismatch is very important in determining the electronic
quality of junction devices fabricated using the two
semiconductors. The lattice mismatch in the heterojunctions
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will lead to a large density of interface states which act
as recombination centers for charge carriers. The excess
charge carrier recombination in the interface region often
lead to poor devices. For example, in photovoltaic
heterojunction solar cells, the excess interface
recombination will result in a low open circuit voltage and
therefore low solar to electrical energy conversion
efficiency It is therefore beneficial to produce
heterojunction devices with a minimal lattice mismatch.
~mong several other large band gap semiconductors, the
ma~erial CdS has been developed and used with two of the
promising polycrystalline materials CdTe and CuInSe2 for
photovoltaic devices. ~ finite lattice mismatch exists in
devices made using CdS-CdTe and CdS-CuInSe2. In order to
improve the device performance, it is obvious that the
lattice mismatch has to be reduced. The lattice mismatch can
be reduced by adjusting the lattice constants of the
absorbing semiconductor or/and the window material by
adjusting the composition. However, due to the strict
requirements of the carrier concentration and mobility, the
lattice constant adjustment of the absorbing layer appears
to be more difficult to achieve. For the window layer in the
heterojunctions, the requirements of carrier concentration
and mobility are not as critical. Therefore, it is
advantageous to minimize the lattice mismatch by adjusting
the lattice constant o~ the window layer.
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For the optical device applications, it is especially
important to reduce the resistivity of the top window
material. This can be obtained by incorporating a second low
resistivity and large band gap semiconductor on the first
lattice-matched window layer. This second layer is needed in
order to reduce the surface series resistance of the
devices. The second window layer, when fabricated under
appropriate conditions using a suitable material, allows
more photons in the incident light to penetrate through and
to reach the absorbing layer. The increased transmission
will increase the solar to electrical energy conversion
efriciency. Ideal candidates for such applications include
the following materials: indium tin oxide (ITO), indium
oxide (In2O3) and zinc oxide (ZnO). Although low resistivity
indium tin oxide thin films have been very well studied and
used in many optoelectronic devices and display devices, it
consists of about 20% indium. The material indium is a rare
and expensive metal, making the large scale application of
indium tin oxide to be expensive. In order to reduce the
cost of device fabricakion, alternative materials must be
daveloped. The other potential useful large band gap
semiconductor suitable for devices is ZnO. The semiconductor
7nO has an energy band gap value of 3.3 eV~ Therefore most
of the photons in the visible region are allowed to
penetrate through this material. Low resistivity ZnO thin
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~ilms can be prepared by controlling the Zn/O ratio or by
adding impurities. However, unlike CdS and indium tin oxide
films, the low resistivity ZnO films have been found
previously to be relatively unstable and can not be
e~fectively used in devices which require heat treatment
steps during the fabrication. The electrical resistivity of
a undoped low resistivity ZnO film was found to increase by
more than 6 orders of magnitude after a short heat treatment
at 400C in air.
From the above comments, it is quite obvious that a
process is needed in order to prepare oriented thin films
~ased on CdS with predetermined lattice constants in order
to be used with chalcopyrite semiconductor compounds like
CuInSe2. Furthermore, a process is also needed in order to
prepare thermally stable low resistivity ZnO films for the
optoelectronic applications to replace completely or partly
the widely used CdS and indium tin oxide films.
OBJECTS AND STAT~MENT
An object of the invention is to provide an improved
method to produce CdS-based polycrystalline thin ~ilms with
controlled lattice constants and with increased energy band
gaps.
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Another object of the present invention is to provide
an improved method for the deposlting of low resistivity ZnO
films.
Yet another object is to provide a method for thermally
stable ZnO films.
Still another object is to provide a method for
depositing sandwiched films with a low resistivity top layer
~nd a bottom thin layer with controlled lattice constants.
In the present invention, we present methods to prepare
CdS-based semiconductors with variable lattice constants
which match to other semiconductors like CuInSe2 for thin
film iunction devices. The lattice constants of the layer
are controlled by controlling the stoichiometry of the CdS-
b~sad thin films.
This invention also present a process which allow one
to produce low resistivity and thermally stable ZnO thin
ilms. The low resistivity and thermal stability are
achieved by introducing impurities during the deposition.
The processes can be advantageously used for the preparation
o conducting double-layer thin films with lattice constant
well matched to other semiconductors for optoelectronic
devices or display devices.
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BRIEF D~SCRIPTION OF T~E DRAWING
Fig. 1 is a cross section of photovoltaic device with a
lattice matched first window layer and a thermally stable
low resistivity ZnO layer.
Fig. 2 diagrammatically illustrates the relationship of
lattice spacing of CdS~ films with CdS content in the
tal ~e~ .
Fig. 3 diagrammatically illustrates the relationship of
lattice spacing of CdZnSO films with CdS content in the
target.
DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention relates to polycrystalline CdS
thin films for optoelectronic devices. Referring now to Fi~.
1, the electronic device comprises a supportive substrate
~1), coated with a metal film (2) and a polycrystalline
absorbing material ~CuInSe2 for example) (3) with a
preferred (112) orientation, a (002) oriented CdZnSO or CdSO
layer (4) with lattice constants very close to that of the
CuInSe2 layer, a thermally stable, doped, low resistivity
ZnO layer (5) and a counter electrode (6).
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The important requirements in producing an efficient
thin film photovoltaic solar cell are: producing a CuInSe2
layer with (112) pre~erred orientation, producing a CdZnSO
or CdSO high resistivity layer with lattice constants very
close to that of the absorbing CuInSe2 layer, producing
thermally stable low resistivity ZnO layer and providing low
resistivity contacts.
The processes for CuInSe2 film deposition have been
described in other literature and will not be described
here~
To produce CdS films with pre-determined lattice
constants, the deposition is carried out using a high purity
source (99.999~) consisting of CdS powder and ZnO or CdO
powder. ~lternatively, two or more sources of CdS and ZnO or
Cd~ may be used . The deposition is carried out in a high
vacuum system using an rf sputtering method. Using the
specific method, a target of CdS~ZnO or CdS-~CdO with a
diameter of about 5 cm and a thickness of 0.3 cm is formed~
The vacuum chamber is first evacuated to a pressure of 10-5
torr and maintained at this pressure for several minutes.
High purity argon gas is then introduced into the chamber to
a pressure of about 30 mtorrO The rf power supply is turned
on and the power adjusted to 60 watts. The chamber argon
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pressure is reduced to a value in the range from 1 to 5
mtorr and the deposition is continued for a period of about
30 minutes. A~ter the deposition, the rf power is switched
off and the sample is left in the chamber for about 20
minutes to cool~
The crystalline quality of the films is determined by
X-ray diffraction, optical transmission measurements and
~all effect measurements. The results show that the lattice
constants of the CdS-rich films can be adjusted in a range
to match the absorbing semiconductors like CuInSe2. It is
important to point out that the CdS-rich films have a
~urzite structure and still maintain the (002) preferred
orientation from the X-ray diffraction results. The lattice
constant adjusting effect of the CdS-rich CdSO films is
shown in Fig. 2. Here it is seen that the lattice constant
(d) for the (002) planes decreases from 3.4 angstroms to
about 3.1 angstroms as the CdS content (in the target) is
decreased from 100 to 50%. Below 50% CdS, a rapid decrease
in the lattice constant is seen from the figure.
Similar lattice constant adjustment effect is also seen
in Fig. 3 for CdZnOS filmsO Here, it is seen that the
lattice constant (d) for the (002) planes decreases from 3.4
angstroms to 3.3 angstroms as the CdS content is decreased
from 100 to 40%. For CdS content less than 40%, there is an
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drastic decrease in the lattice constant to about 2.6
angstroms for ZnO~
It is important to emphasize again that all of the
CdZnSO films deposited using targets with more than 40% CdS
and the CdSO films using targets with more than 50~ CdS are
polycrystalline with a (002) preferred orientation.
Therefore, the present process allows very well oriented
transparent ~ilms to be deposited. Furthermore, by adjusting
tl1e content of CdS in the films, a very small lattice
mismatch can be achieved between the window layer and the
bottom absorbing layer. For example, if the bottom absorbing
layer is polycrystalline CuInSe2 with a (112) orientation,
then films of CdZnSO with a ZnO concentration of about 30%
or films of CdSO with 20% CdO can be used.
Another aspect of the invention is based on a discovery
that the thermal stability of ZnO thin films can be greatly
improved by introducing suitable impurities and by
increasing the film thickness. The impurities are added to
the target and incorporated in the films during the
deposition process. According to the present invention, the
impurities added to the ZnO material create donor centers
which are more difficult to remove by the heat treatment in
an oxygen containing atmosphere than those created by zinc
or oxygen defects.
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The preferred process for the deposition o-f low
resistivity and thermalLy stable ZnO films is as follows. A
target consisting of zinc oxide (ZnO) and indium o~ide
(In2O3) is first prepared by mixing high purity ZnO and
In203 powder and pressing in an aluminum target holder. The
target is then mounted to a target support of a conventional
vacuum sputtering system with a radio frequency sputtering
power source. Glass substrates are selected and mounted on
an aluminum substrate holder placed parallel to the target
surface~ The sputtering is performed by first evacuating the
deposition chamber and filling the chamber with high purity
argon. After this step, the RF power supply is turned on and
argon ions are created in the vacuum chamber and accelerated
and directed towards the ZnO target. The atoms Zn, O and In
in the target are knocked out by the accelerated argon ions
and deposited on the glass substrates in a polycrystalline
thin film orm. The deposition is allowed for a period of
time beore the RF power is switched off. After the
deposition, the films are removed for subsequent device
abrication processing. Alternately, plates already
deposited with metal or semiconductor films like CuInSe2 and
Cd~SO may be used as the substrates.
In order to obtain low resistivity ZnO films with good
thermal stability, it is preferred to placed the substrates
in positions away from the vertical projection of the target
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center. These positions are preferred in order to reduce the
surface temperature during the deposition. Furthermore, it
is preferred to adopt a deposition time so that a ~ilm
thickness of at least 0.~ micron is achieved. This ZnO film
thickness is important for maintaining the thermal stability
when the film is subjected to post deposition heat treatment
in air or other oxygen containing atmospheres~
Low resisti~ity and thermally stable II-VI compound
thin ~ilms also can be achieved by adding In203 or A1203 in
the sputtering target and sputtering in a ~acuum system with
argon gas on a substrate placed on a substrate holder
mountèd parallel to the target surface and controlling the
thickness value of the films. The film electrical
resistivity can be further reduced by positioning the
substrate so that the angle between the normal of the target
and the line through the center of the substrate and the
center of the target is about 40 degrees.
EXAMPLE 1
High purity ZnO (99.999~) and In203 (99.999) are
weighted (ZnO 25 gm and In203 0.5 gm) and mixed and then
pressed in an aluminum holder (diameter 5 cm) to form a
target. The target is mounted on the target support o~ a
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high vacuum system with a diffusion pumpO Clean glass
substrates with a dimension 2.5x3x0.1 cm3 are mounted in a
substrate holder placed in a position parallel to the tar~et
surface. The distance between the target and the substrates
is about 5 cm and the horizontal distance between the
vertical projection of the target center (on the substrate
holder) and the center of the substrate is about 3 cmO The
deposition chamber is first evacuated by the pumping system
to a pressure of about 10-5 torr. This pressure is
malntained for several minutes before introducing high
purity argon into the chamber. ~he argon pressure is about
30 mtorr and the RF power is switched on in order to
initiate plasma in the chamber. After this stage, the argon
pressure is reduced to a value in the range from 1 to 5
mtorr ~or the film deposition~ The incident RF power is
maintained at about 60 watts and the deposition time is
about 1 hour. During the deposition, no intentional
substrate heating is used. However, a substrate heater may
be added in order to improve further the film quality by
controlling the substrate temperature. After the deposition,
the substrates are allowed to cool in the vacuum system for
a period of about 20 minutes. The average thic]cness of the
deposited ZnO films is about 1 micron using the above
described conditions and the resistivity about 10-3 ohm-cm.
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The deposited ZnO thin films using the above conditions
~re thermally stable in an oxygen containing environment at
temperatures below 250C. Therefore they are useful for
device fabrication where a post fabrication heat treatment
is needed.
While the invention has been described with reference
to CdS and ZnO thin film deposition, deposition of other II-
VI compounds like CdO may also be achieved as well. The
techni~ue can be used to create donor states by
incorporating impurities in CdO to produce much better
thermal stability of electrical properties than undoped CdO
films.
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