1. Transparent Conducting Films of Cu doped ZnS
Alexander G.J. Tallon & Christopher E. Wilshaw - HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, BS8 1TL, UK.
5. Conclusions & Further Work
6. References & Acknowledgements
[1]. M.J Alam et al., Optical and electrical properties of transparent conductive ITO thin
films deposited by sol–gel process, Thin Solid Films, 377–378, 455-459, (2000).
[2]. A.M. Diamond et al. Copper-alloyed ZnS as a p-type transparent conducting
material. Physica Status Solidi (a), 209 (11): 2101-2107, (2012).
[3]. K. Jayanthi et al., Structural, optical and photoluminescence properties of ZnS: Cu
nanoparticle thin films as a function of dopant concentration and quantum confinement
effect, Crystal Research and Technology, 42 (10): 976-982, (2007).
[4]. K. Wang., Laser Based Fabrication of Graphene, Advances in Graphene Science,
InTech, Chapter 4, Page 82, (2013).
[5]. S. Yilmaz et al., Pulsed laser deposition of stoichiometric potassium-tantalate-
niobate
films from segmented evaporation targets, Applied physics letters, 58 (22): 2479–2481,
(1991).
With thanks to: Dr Neil Fox for ideas, guidance and support throughout. Dr James Smith
for assistance at practical setup and work. Prof David Cherns for advice and guidance.
1. Motivation
Introduction
• Transparent Conducting Materials (TCMs) are wide band gap
semiconductors exhibiting low electrical resistivity and high
transparency to visible light.
• The majority of TCMs consist of thin films of n-type
semiconductors in which electrons from dopant ions act as charge
carriers. The current industry standard is Indium Tin Oxide (ITO),
capable of around 80% transmittance in the vsible, and a resistivity
of ~10−3
Ω. 𝑐𝑚 1 .
• Until now these n-type TCMs have met industry demands, mostly
acting as electrodes in optical devices such as photovoltaics, LEDs
and flat panel displays.
• The development of a p-type TCM, in which holes act as charge
carriers, would facilitate the fabrication of transparent active
devices, such as PN-junctions, transistors and diodes. These in turn
open up the field of Transparent Electronics.
Cu:ZnS as a p-type TCM
• Density Functional Theory (DFT) calculations carried out by
Diamond et al. indicated that under Sulphur rich conditions 𝐶𝑢1+
will replace 𝑍𝑛2+
in the ZnS lattice, leading to a surplus of holes
and thus a p-type semiconductor [2].
• The thermal stability, high abundance and low toxicity of the
elements, along with its wide band gap make Cu:ZnS an ideal
candidate for a p-type TCM.
• At RTP ZnS has two stable lattice structures; Sphalerite (cubic) and
Wurtzite (hexagonal), and while p-type TCMs have been made with
the cubic structure there has yet to be a Wurtzite p-type TCM. But
with good lattice matching with several n-type TCM’s and an ideal
band gap of ~3.9𝑒𝑉 [3], the Wurtzite structure is highly desirable.
• Prior research on such a technique failed to control the percentage
of Cu in the films and only obtained conducting films with high
concentrations of Cu (~25% as a mole fraction of the film) [2]. This
doping level is several orders of magnitude greater than most
degenerately doped semiconductors, raising questions concerning
the deposition method and the efficiency of the substutional
process.
2. Aims & Applications
Aim
This project aimed to use Pulsed LASER Deposition (PLD) to make thin
films of transparent, conducting Cu doped ZnS with a Wurtzite
structure. In particular it aimed to control the concentration of Cu in
each film as well as ensure the correct substitution of Cu for Zn.
Applications
• Al:ZnO, as an established n-type TCM with Wurzite structure, could
be joined with the p-type Cu:ZnS to form a transparent PN-junction
from highly abundant materials. The abundance of all 6 base
materials would make the device cheap to manufacture
plume
3. Fabrication Process
Fig. 1 [4] Pulsed LASER Deposition was used to deposit the films:
A rotating target is hit repeatedly by a high power LASER pulse. Each
pulse ablates the target material into a plume, which then travels across
the chamber and deposits as a thin layer on a heated Sapphire
substrate. The system is pumped down to UHV before a
background gas is introduced for the deposition.
• PLD conventionally uses a mixed target containing a constant
composition mix of each material throughout.
• In this project however, a new segmented target design was used
in which thin strips of Cu were wrapped around a pure ZnS target
(a). Thus during deposition, as the target rotated, the LASER
alternately ablated ZnS and Cu (c). The concentration of Cu
ablated in each deposition was dependent upon the Cu strip
width and the LASER focus radius, and was varied from 1% to 20%
as a mole fraction of ablated material.
• The reasons for this technique were twofold, firstly it provides a
simple, versatile way of adjusting Cu concentration and
distribution within the film. Secondly, it has been proven to help
control the transfer of materials for PLD of elements of drastically
differing volatility’s, as in the case of Cu, Zn and S [5].
• A direct consequence of the segmented target approach
was layering of Cu and ZnS in the deposited films. Three
approaches were used to reduce this effect:
• Firstly, a revised ‘bicycle wheel’ target design was used
to increase the frequency that Cu was ablated, but not
percentage (b).
• Secondly, rapid thermal annealing (RTA) was used to
diffuse Cu throughout the films, leading to more
isotropic doping and higher conductivity.
• Finally, a background gas and lower LASER power were
used to deposit far less than one atomic layer per
LASER pulse, leading to less single component layering.
4. Key Results & Discussion
A wide range of characterisations were performed in order to determine the properties of the deposited films and establish the
degree to which each of the three main criteria were met: transparency, conductivity & Wurtzite lattice structure.
RTA
a) b) c) d)
• Films were deposited which demonstrated transmittance of 70% on
average which is close to that of ITO films.
• The films had a Sphalerite structure upon deposition but were
demonstrated to adjust to the desired Wurtzite structure after RTA.
This would allow the films to be compatible with an Al:ZnO film to
produce a PN-junction suitable for a solar cell.
• Both the magnitude and uniformity of the conductivity in the films
was poor. The introduction of the ‘bicycle wheel’ target design (b)
coupled with RTA and a reduced laser power did improve the
conductivity of the films, thought to be the result of reduced layering
of Cu and ZnS in the films.
• EDX of the samples revealed that the stoichiometric transfer of
ablated materials using the segmented target was inconsistent, with
little correlation between the amount of Cu ablated and the amount
present in the films.
• EDX data also showed that the films tended to be Zn rich rather than
S rich as required for 𝐶𝑢1+ ions to substitute for 𝑍𝑛2+ ions. For this
reason it is thought that the Cu was taking up interstitial positions
between crystal grains, and thus failing to act as a hole donor.
• Further work should therefore concentrate on achieving S rich
conditions, perhaps through the use of a second target or different
deposition conditions, as well as reducing layering in the film..
Transparency Conductivity Lattice Structure
• Hall measurements revealed that
while the films were p-type, most
had very high resistances.
• Implementation of the steps to
reduce layering, discussed in
section 3, led to more conductive
films. The lowest resistivity
achieved was 2𝑘Ω. 𝑐𝑚 with a
carrier mobility of
0.18 𝑐𝑚2
𝑉−1
𝑠−1
.
• Energy Dispersive X-ray (EDX)
Spectroscopy was performed on
the samples to investigate the
concentration of Cu in the films.
This was found to vary
substantially both across the films
and from the value expected from
the deposition process used.
Generally the films contained
between 0-10% Cu as a mole
fraction.
• EDX also revealed that all the
samples contained slightly more
Zn than S. As Density Functional
Theory suggests S rich conditions
are necessary to make the Cu
substituting process energetically
favourable. Therefore the Zn rich
nature could be the major reason
behind the low conductivity of the
films.
• The X-ray diffraction data also revealed an
improvement in crystallinity could be achieved through
annealing, whilst a decrease in crystallinity was seen in
films of higher Cu content, as shown below.
• Ultraviolet-Visible spectroscopy
showed that pre-annealed films had
band gaps between 3.2-3.5eV, with
a decrease in bandgap with
increasing Cu content. This trend is
thought to be due to the Cu
introducing mid-bandgap states.
• UV-vis. of post-annealed samples
revealed higher band gaps of
around 3.9eV, suggesting a
transition from Sphalerite to the
desired Wurtzite structure.
• The thin thickness and wide
bandgap of the films led to an
average transmittance in the visible
wavelength range of around 70%.
• X-ray Diffraction revealed that RTA to 550 ⁰C for 30
minutes led to the desired Wurtzite structure, as is
clear from the characteristic splitting of the main peak
into 3 separate peaks.
200 nm
• Focussed Ion Beam spectroscopy of
the films revealed film thicknesses
of around 200nm, as shown below.
The image demonstrates the
uniformity of the films, which was
confirmed by Atomic Force
Microscopy.
• Such a PN-junction could act as a
functional solar window, which
transmits the visible portion of
solar radiation yet generates
electricity by the absorption of the
UV part. Glass in almost any
setting could then double as an
electronic device.
• One prospective application is a
transparent solar cell.