Cu doped ZnS was investigated as a potential p-type transparent conducting material. Thin films of Cu:ZnS were deposited via pulsed laser deposition using a segmented target to control Cu concentration between 1-20% mole fraction. The films exhibited 80% visible light transparency and crystallized in the desired wurtzite structure after annealing. However, conductivity was low due to most Cu occupying interstitial rather than substitutional sites in the ZnS lattice. Achieving sulfur-rich deposition conditions is needed to promote Cu substitution for Zn and improve hole concentration and conductivity.

1. Cu doped ZnS as a P-type Transparent Conducting Material
By Chris Wilshaw
Lab partner: Alex Tallon
Supervisor: Neil Fox

2. Transparent Conducting Materials
What are Transparent Conducting Materials (TCMs)?
Transparent across visible wavelengths
Electrically conductive
How?
Bandgaps above ~3𝑒𝑉 transmit visible
wavelengths but absorb UV
Low resistivity comes from doped charge carriers
𝐸 𝑣
𝐸𝑐
UV

3. Uses of Transparent Conducting Materials
The majority of TCMs are n-doped metal oxides
Industry standard is Tin-doped Indium Oxide (ITO), capable of
around 80% transmittance in the visible, and a conductivity of
~104
𝑆. 𝑐𝑚−1
Scarcity of Indium has led to research into alternatives using
abundant, cheap materials such as Al doped ZnO (AZO).
Generally used as electrodes in LED’s, PV and flat panel
displays p-Silicon
n-Silicon
ITO

4. P-type TCMs and their Potential
Applications of these n-type TCMs are limited
A p-type TCM would allow transparent pn-junctions, transistors, diodes
etc
Opens up the field of ‘Transparent Electronics’
These have however proved far harder to create than their n type
counterparts.

5. Cu doped ZnS as a P-type TCM
Zinc Sulphide has a wide, direct, bandgap of around 3.5eV
Calculations using the ‘supercell modelling method’ show 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
Stable, non-toxic, abundant elements
ZnS has two stable crystalline phases: Zincblende (cubic) is most common at
room temperature, but Wurtzite (hexagonal) can be achieved at higher
temperatures.
Several good n-type TCMs with Wurzite structure, including AZO, but no p-
type ones
Zincblende
Wurzite

6. Prior work and Project Aims
Prior Work
Diamond et al. used Pulsed Laser Deposition to successfully fabricate thin films of
Cu:ZnS achieving 65% transmittance at 550nm, and a conductivity of 102
𝑆. 𝑐𝑚−1
.
However, they failed to control the Cu concentrations in the films and required
very high doping levels of Cu-about 25% as a mole fraction of the film. Raising
questions concerning the transfer of Cu into the film, as well as the efficiency of
the substitutional process.
The films also failed to exhibit clear Wurzite structure.
Thus our research aimed to make thin films of Cu:ZnS with high transparency, low
resistivity and a Wurzite structure in such a way that accurately controlled the
amount of Cu deposited in the films, and ensured efficient substitution of Cu for Zn.

7. Pulsed Laser Deposition (PLD)
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.
Under UHV

8. Pulsed Laser Deposition of our Samples
Conventionally use a mixed target containing a constant composition mix of each material throughout
(Technique used in prior work)
Instead, we used a new segmented target design
During deposition, as the target rotated, the LASER alternately ablated ZnS and Cu.
The concentration of Cu 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.
Target Plume Film growth
Substrate

9. Results: Crystal Structure
X-ray Diffraction revealed that annealing at
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.
Xrd data also showed that the films had
good crystallinity, with average crystal sizes
up to 50nm, around double that reported
by Diamond et al.
Zincblende
Wurzite
(111)
(200)
(100)
(002)
(101)

10. Results: Transparency
The transparency of the films rely mainly on
thickness, bandgap and crystallinity
Focussed Ion Beam spectroscopy of the films
revealed film thicknesses of around 200nm, as
shown to the right. The image demonstrates
the uniformity of the films, which was
confirmed by Atomic Force Microscopy.
Ultraviolet-Visible spectroscopy showed that
post-annealed samples had band gaps of
around 3.8eV.
Average transmittance in the visible
wavelength range of ~80%.
200 nm
0
10
20
30
40
50
60
70
80
90
100
200 300 400 500 600 700 800
Transmittance
Wavelength (nm)

11. Results: Conductivity
The conductivity of the films will depend mainly upon the carrier density and mobility as well as the
thickness of the sample.
Energy Dispersive X-ray spectroscopy of the samples showed that the concentration of Cu in the films varied
substantially from the amount expected from the deposition conditions, with most films containing between
2 and 10% Cu as a mole fraction.
Hole Density
(𝑐𝑚−3
)
Hole Mobility
(𝑐𝑚2
𝑉−1
𝑠−1
)
Thickness
(nm)
Conductivity
(S. 𝑐𝑚−1
)
Cu:ZnS 2 × 1013
2 × 102 200 5 × 10−4
ITO ~1021 50 200 ~104
Hall measurements of our best film,
containing 5% Cu, compared to ITO.
These are surprisingly low given the amount of Cu in the film, suggesting that the vast majority of Cu took up
interstitial positions in the lattice rather than substitutional ones, resulting in relatively few holes.
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.

12. Conclusions and Further Work
Thin films of Cu:ZnS were successfully fabricated with 80% transparency, good crystallinity and the
desired Wurzite structure.
The segmented target approach, whilst quick and easy, did not lead to control of the Cu entering
the films
The conductivities of the films were poor, thought to be the result of layering in the film, or Zn rich
deposition conditions preventing Cu from substituting for the Zn
Further Work
Further work should concentrate on reducing layering within the film, and achieving Sulphur rich
deposition conditions.