Influence of Copper Content and Pre-Treatment on the Structure of ZnS : Cu Thin Films by Sulfidation

ZnS:Cu thin films were prepared at 440 ◦C by sulfuring Zn:Cu thin films which were grown by magnetron sputtering with zinc targets covered with different areas of copper foil to vary the Cu content. As the area of copper increases, the morphology of the ZnS:Cu thin films becomes more uniform and dense. Owning to lower mobility of copper atoms that inhibit the mass aggregation of zinc, the aggregation in Zn:Cu films gradually disappears. After sulfuring the thin films with higher copper content, the concentration of both surface-holes and defects decreases. Annealing the prefabricated Zn:Cu thin films can improve the quality of ZnS:Cu thin films. Notably, the increasing Cu content contributes more to the quality of ZnS:Cu thin films than annealing. Among the samples with increasing cooper content, the defects in the thin films tends to be of only one type.


Introduction
Zinc sulfide (ZnS) is a typical II-VI group compound semiconductor with a direct wide bandgap.For the bulk cubic and hexagonal phases of ZnS, the bandgap E g = 3.54 eV and 3.8 eV, respectively.ZnS film has a high index of refraction and a high transmittance in the visible range [1][2][3].ZnS is doped with impurities like transition metals Ag, Mn, Cu and Al as activator has different energy level luminous centers that show different spectral properties depending on the dopant impurity [4].A green luminous center is always formed in ZnS doped with Cu, this center greatly influences the photoluminescence and electroluminescence properties [5].For films with a Cu content higher than 10.6 at%, the a and c lattice parameters of the crystal cell decrease, leading to a shrinkage of the ZnS:Cu cell.The films are conductive and exhibit a p-type behavior above a copper concentration of 10 at% [6].It is found that the carrier transport behavior is governed by thermionic emission for ZnCuS/n-type Si devices having a high Cu concentration.The sulfur vacancy (VS) is the origin of the conduction behavior conversion and the increased Cu concentration leads to a reduced formation probability of VS [7].The substitutional Cu and S vacancies are two of the most stable impurity species in ZnS due to their small lattice distortion and low formation energy [8].Within the transparent conducting material regime, the hole conduction which makes the conductivity temperature independent is due to substitutional incorporation of Cu onto Zn sites [9].Various synthesis methods of ZnS:Cu thin films, such as chemical vapor deposition (CVD) [10], the sol-gel method [11], RF magnetron sputtering [6] and metal-organic chemical vapor deposition [12] have been explored.In this paper, we found controllable growth for good quality ZnS:Cu thin films by low-temperature sulfidation.We investigated the effect of copper content on surface morphology and defect structure of the ZnS:Cu thin films.

Experimental details
ZnS:Cu thin films were produced by sulfurizing Cu-doped zinc thin films in sulfur vapor.The Cudoped zinc thin films were grown on Si (110) substrates from a 60 mm diameter, 5 mm thick, 99.99 % zinc target by radio frequency (RF) magnetron sputtering at room temperature.High-purity argon was used as a sputtering gas and the distance between the target and substrate was 6 cm.The RF power, deposition time and total pressure were kept constant at 90 W, 2 min and 1 Pa, respectively.The base pressure of the sputtering chamber was pumped down to 1 × 10 −4 Pa before the 15 min pre-sputtering to clean the target surface.The content of Cu in the zinc thin films was controlled by the area of Cu foil on the zinc target.The Cu-doped zinc thin films were annealed in vacuum at a temperature of 400 • C.Then, the different Cu-content Cu-doped zinc thin films, which were labelled samples a, b, c, d, e, f respectively, together with 20 mg sulfur powder (99.5 % of purity) were vacuum sealed in quartz-glass ampoules that had been pumped down to 1×10 −5 Pa.These sealed quartz-glass ampoules were kept at 200 • C for 1 h to ensure the sulfur powder had melted and diffused throughout the whole ampoule, which was then heated for 2 h at a temperature of 440 • C. Subsequently, these sealed quartzglass ampoules were cooled in a furnace.After the cooling, the remaining sulfur gathered in one end of the ampoule in the cold zones.The thickness of ZnS:Cu films was approximately 100 nm.
The surface morphology of the thin films were investigated by AFM and SEM.In addition, positron annihilation Doppler broadening measurements were carried out with a magnetically guided variable-energy positron beam (0.18 keV-20 keV).

Results and discussion
Figure 1 shows the morphology of zinc thin films with different Cu content obtained by AFM.From top to bottom, the area of the copper foil on the target material is 4.4 % (line), 9.3 % (cross), 14.6 % (double cross), with images taken before and after annealing at 400 • C in vacuum for 1 hour, followed by cooling in a furnace.With increasing copper foil area the surface morphology of the thin films gradually become smoother.Also, the quality of surface morphology improves a lot after annealing at 400 • C. At 400 • C, zinc or copper atoms diffuse in the as-deposited films which leads to a decrease in the number of defects [13].Figure 1(a) shows that a structure of ribbons exists in most areas which may be due to the low rotating rate of the substrate platform and compound target.According to Fig. 1(b) and Fig. 1(c), increasing copper content has a greater effect in comparison to annealing in the condition of lower copper content.With the increasing of copper proportion covering on the zinc target, more copper atoms are deposited and well-distributed in the films.Although annealing has a significant effect on improving the surface roughness, aggregations still exist in the films.On the other hand, the copper atomic radius is greater than zinc, so the diffusion mobility of zinc atoms is higher than copper when the temperature reaches 400 • C which is near the melting point of zinc [14].When the distribution of copper atoms tends to uniformity, the migration of zinc atoms becomes stable.Subsequently, the aggregations almost disappear.
Figure 2 shows the SEM morphology of ZnS:Cu thin films which were prepared by sulfurizing the Zn:Cu films shown in Fig. 1.Due to the silicon substrate orientation, if there is low lattice matching between substrate and thin film, the thin film grain size is not easy to grow too large [15].Also, with increasing amounts of copper, the growth of film grains is restrained.In Fig. 2(a), because of the existence of a banded structure in the Zn:Cu thin film, the surface morphology of the ZnS:Cu thin film is irregular and mainly consists of coarse grains and large holes.Without the effect of annealing, there are aggregations in the prefabricated films.In the subsequent process of sulfurization, the reaction will be concentrated in these area of aggregations.In the remaining areas of the thin films, there are mainly diffusion of atoms by the thermal effects.Because the growth direction of grains along with the grain boundaries [16], the diffusion of zinc atoms will concentrate on the grain boundary too.The introduction of copper atoms which have larger radius and lower rate of migration than zinc atoms will suppress the migration of zinc atoms.When the copper atoms are well distributed after the thermal effect, the migration of zinc atoms will not concentrate on one part of thin films in the subsequent process of sulfurization.The prefabricated films have a great influence on the morphology of thin films prepared by sulfurization.The well-distributed copper atoms will make the ZnS:Cu more uniform and compact.
Figure 3 shows the S -E curves (a) and W-E curves (b) of ZnS:Cu thin films prepared by different conditions.The large deviation in sample-a may be because the surface includes large voids and disordered grains such that the grains size is very variable.Some part of incident positrons may reach the single crystal Si substrate which has a lower density of defects.On the one hand, with the effect of annealing, the aggregations are reduced to some extent and the S parameter is significantly reduced.Because of the lower migrating rate of copper atoms, the migration of zinc atoms may be restrained to some extent and the formation of clusters inhibited.When the amount of clusters in the prefabricated thin films reduces, the occurrence of concentrated reactions in the process of sulfuration become less and the amount of holes in the ZnS:Cu thin films is further decreased.On the other hand, when the content of copper atoms increases in the prefabricated thin films, the surface roughness improves and the S parameter decreases.In the process of sulfurization, the reaction will be concentrated in the area of aggregations.With increasing copper covering on the zinc target, more copper atoms are deposited and well-distributed in the films.The area of aggregations reduces and the surface of ZnS:Cu prepared by sulfurization becomes more uniform as shown in Figs.2(b)(d) and (f).The S parameter of samples-c, -d, and -e decreases, demonstrating that the effect of annealing is less than the effect of the number of copper atoms and distribution mode.When the copper atoms are not welldistributed, annealing may improve the surface of prefabricated thin films to a limited extent and some apparent holes still exist in the ZnS:Cu thin films.Subsequently, with a combination of both, most of the holes in the ZnS:Cu thin films disappear and the roughness of surface reaches the highest level among all samples.
The slope of S -W plot represents the mechanism of positron annihilation after trapping.S -W plots of all the samples are shown in Fig. 4. The S -W curve has been used to identify the number of defect types in materials [17].In Fig. 4, when the content of copper is low, there are different size of grains and holes that make the type of defect in the films variable.Because sample-a includes the void and the grains are in disorder.The part of incident positrons may reach at the single crystal Si substrate.So the S -W curves cannot identify the defect types of sample-a.From sample-b, the S -W curve shrinks apparently.Although the defect of sample-b is still complicated, the type of defect changes clearly and curve exhibits some characteristic of thin films.The S -W curves of sample-b is similar to sample-c.According to Fig. 2, the surface morphology of these two samples is differ from each other, but both have a large area of holes.This indicates that the existence of holes has a significant influence on positron annihilation.From sample-b to sample f, with the effect of annealing and increasing copper content on the prefabricated thin films, the S -W curves are close to straight line.The variation of S -W curves means that defects in ZnS:Cu thin films tend to be of one type.According to Fig. 1 and Fig. 2, the growth of the ZnS:Cu films is mainly affected by the concentration and type of defects.The introduction of copper atoms restrains the migration of zinc atoms and ■■■ 011106-5 JJAP Conf.Proc., 011106 (2018) reduces the occurrence of aggregations in the process of sulfurization.When the copper atoms are well-distributed, the reaction between zinc atoms and sulfur extends over the whole film uniformly, causing a significant reduction in the concentration of holes.

Conclusion
ZnS:Cu thin films were prepared by sulfuring prefabricated Zn:Cu thin films.The density of films is connected with the compactness of prefabricated Zn:Cu thin films which is affected by the area of the copper foil on the target i.e. the copper content and the effect of annealing.Since copper atoms have a larger radius than zinc atoms, the copper atoms may restrain the migration of zinc atoms in the process of sulfuring.When the copper atoms are well-distributed, the reaction between zinc atoms and sulfur may occur throughout the whole area of the prefabricated thin films.The type of defect tends to be of only one type after sulfuring the prefabricated films with a uniform surface.

Fig. 2
Fig. 2 SEM images of ZnS:Cu prepared by sulfuring the Zn:Cu thin films shown in Fig. 1.The labels are the same in both figures.

3 ■■■Fig. 3
Fig.3The S -E (a) and the W-E (b) curves of ZnS:Cu thin films prepared by different conditions.

Fig. 4
Fig.4The S -W curves of ZnS:Cu thin films prepared by different conditions.