2020 17th International Conference on Electrical Engineering, Computing Science and Automatic Control (CCE) Mexico City, Mexico. November 11-13, 2020

978-1-7281-8987-1/20/$31.00 ©2020 IEEE

Study of ZnSxO1-x Prepared by Thermal Oxidation of ZnS Films Deposited from a Chemical Bath

Rosa María Nava-Sánchez Departamento de Ingeniería Electrica

CINVESTAV del IPN line 4: Mexico City, Mexico

rnava@cinvestav.mx

Gaspar Casados-Cruz

Departamento de Ingeniería Eléctrica CINVESTAV del IPN Mexico City, Mexico

gcasados@cinvestav.mx

Arturo Morales-Acevedo Departamento de Ingeniería Eléctrica

CINVESTAV del IPN Mexico City, Mexico

amorales@cinvestav.mx

Abstract—Optical and structural characterization results of ZnSxO1-x films deposited by chemical bath deposition of ZnS with a subsequent thermal oxidation process are presented here. Varying annealing temperatures and times were used to incorporate different amounts of oxygen into the films. The band gap dependence on Sulfur (x) and Oxygen (1-x) composition in the ZnSxO1-x layers was studied. It is shown that the material in the films crystallizes in the hexagonal structure and a maximum oxygen concentration of 9 % is achieved, leading to a maximum energy bandgap reduction (increased affinity) of 0.27 eV which can be used to reduce unfavorable conduction band discontinuities at the ZnS/CIGS interface in solar cells.

Keywords—Zinc sulfide, Oxygen, Thin films, Chemical bath

I. INTRODUCTION

Zinc sulfide (ZnS) and zinc oxide (ZnO) are two important II-VI semiconductor materials. Both, ZnS and ZnO may be used as buffer layers in solar cells. They have been studied because there is a high interest in replacing cadmium sulfide for solar cells because it is toxic and dangerous to the human body [1], while ZnS and ZnO are not toxic. Additionally, they have good chemical stability, low cost and are abundant in nature compared to other chalcogenides [2]. Both semiconductors have several favorable properties such as a wide direct band gap, from 3.4 to 3.8 eV and from 3.3 to 3.4 eV, respectively, with high transparency in the visible and near ultraviolet regions. They have been used for other electronic devices, such as transistors, gas sensors, light-emitting diodes and electro-luminescent displays [3]-[4]. Moreover, in previous works by other researchers, it has been found that ZnSxO1-x alloys are able to combine the favorable properties of ZnS and ZnO to obtain improved solar cells. Alloying these compounds should provide significant changes in the material physicochemical properties, modifying the charge transfer and separation at the hetero-junction interfaces. For example,

junctions of these materials with other semiconductors, such as CuInGaSe2 (CIGS) and CdTe for solar cells, can be optimized avoiding the presence of electronic potential barriers at the interface. Therefore, in this work we intended to introduce oxygen by means of thermal treatments for chemical bath deposited ZnS layers to increase the electrical affinity and hence decrease the interface barrier for electrons in the conduction band of CIGS solar cells. The results show that the oxygen incorporation can be varied, getting ZnSxO1-x layers with different energy bandgaps (and electron affinities) [5]-[8].

II. EXPERIMENTS

Deposition of ZnS thin films on fluorine doped tin oxide (FTO) coated glasses was done by chemical bath deposition (CBD), as explained in a previous work [9]. The substrates were cleaned by first sonicating them in a neutral detergent, followed by degreasing with acetone and isopropyl alcohol, rinsing in deionized (DI) water in every step, and finally dried with nitrogen. Then the CBD solution was prepared as follows: Firstly, a mixture of ZnCl2 (0.02 M) and urea (0.5 M) was prepared at room temperature, and the substrate was immersed in it, placed in a water bath and heated to 353 K. Secondly, in another beaker, thiocetamide (0.2 M) was heated apart and added to the previous solution when it also reached 353 K. Finally, hydrochloric acid (5 M) was added to adjust the pH to 1.8. The final volume of the bath solution was 100 ml. After deposition for 60 minutes, the substrates were taken out, washed with deionized water, and dried with nitrogen.

For this study, a set of six ZnS thin films were obtained and a thermal oxidation process was carried out in five of them. The annealing was done at different times and temperatures using an oven in atmospheric air. Table I summarizes the different treatments for each of the films.

The films morphology was characterized using a JEOL scanning electron microscope (SEM), model JSM-6360LV. The optical transmittance was measured using a Shimadzu spectrophothometer model UV-2401PC. The film crystal structure was determined by X-ray diffraction (XRD) with a PANalytical equipment, Xpert-Pro model, using a Cu-Kα source. Thickness measurements were performed using a Tencor KLA profilometer, model P-15.

TABLE I. ANNEALING CONDITIONS FOR EACH FILM

SAMPLE TEMPERATURE (°C) TIME (HOURS)

C51 None None C52 200 2 C53 280 2 C54 200 3 C55 280 3 C56 400 2

III. RESULTS AND DISCUSSION

A. Morphology

The SEM micrographs of ZnSxO1-x deposited on FTO substrates are shown from Fig. 1. to Fig. 6. All samples had a white color characteristic of ZnS. The surface of the ZnSxO1-x films is homogeneous, showing spherical crystalline clusters which covers the substrate surface completely without pinholes. Films had strong adherence to the substrates (i.e no delamination was observed). The estimated average cluster size was 300 nm. Despite the different annealed conditions, size and shape of the clusters are similar. It is likely that the times and temperatures chosen for the films thermal oxidation were not enough to form bigger clusters.

Fig. 1. SEM image of sample C51

Fig. 2. SEM image of sample C52

Fig. 3. SEM image of sample C53

Fig. 4. SEM image of sample C54

Fig. 5. SEM image of sample C55

Fig. 6. SEM image of sample C56

B. X-Ray diffraction.

The crystal structure of the ZnSxO1-x thin films was analyzed by X-ray diffraction, using grazing angle incidence and their profiles are shown in fig. 7. The XRD patterns show that any other oxygen compound is not detected in any of the films because oxygen compounds participate only with a small fraction in the films. ZnS(O) can crystallize in the cubic (Zincblende or Sphalerite) or the hexagonal (Wurzite) structures. The angle position of the peaks associated to the (111) planes of the ZnS cubic structure and the (002) planes of the hexagonal phase nearly coincide, but according to previous works by other authors [5], we have concluded that our material corresponds to the ZnS(O) hexagonal crystalline phase. For all the samples, the X-ray diffraction patterns show 3 peaks in accordance the Joint Committee on Powder Diffraction Standards (JCPDS) card number 036-1450, for wurtzite ZnS. The diffraction peaks at 2θ in degree values were observed around 28.5°, 47.6° and to 56.4°, which corresponds to (002), (110) and (112) planes, respectively, with a preferential orientation (texture) in the (002) plane. The

additional peaks observed in the XRD patterns are due to the FTO layers on the glass substrates.

The sizes for the preferentially oriented crystallites in the films were estimated using the well-known Scherrer equation [10]. The obtained values are shown in Table II, from which an average size of 9 nm was determined. The respective lattice parameters (a) and (c) are also listed in Table II, as determined from the respective peak positions using the interplanar spacing of the (hkl) planes for the hexagonal structure [11].

Fig. 7. X-Ray spectra for ZnSxO1-x thin films

Fig. 8. Optical transmittance versus wavelength

C. UV-Vis optical transmittance

UV-Vis optical transmittance spectra for the samples set are shown in fig. 8 and the corresponding transmittance average values can be consulted in Table II. From these spectra, the bandgap was determined from the corresponding Tauc plots [12], assuming direct band materials. The material bandgap decreased with increasing time or temperature for the thermal oxidation processes. The material bandgap for the film without thermal treatment (C51) was 3.62 eV (typical value for ZnS with hexagonal structure), while the bandgap for the sample with the largest annealing time and temperature (C56) was 3.35 eV. Therefore, the maximum bandgap decrement was 0.27 eV. This bandgap reduction is due to incorporation of oxygen into the ZnS thin film [3]. Because all samples were deposited in similar conditions, thicknesses of the films were expected to be of the same order; in our case, the average was 350 nm. Film non-homogeneous thickness and local roughness differences may explain the observed higher average transmittances for some of the layers with reduced bandgaps.

D. Variation of the (x) fraction in the annealed films

In order to determine the composition and its dependence with the material bandgap for our thin films, we depart from the fact that the bandgap of ZnSxO1-x layers can be described by [3]

here EgZnS and EgZnO are the energy bandgaps for the respective binary compounds at 300 K, and b is the optical bowing parameter. For our calculations we assumed EgZnS to be 3.62 eV, EgZnO as 3.3 eV, and 3.0 eV for the bowing parameter, as reported by other researchers [5]. The determined x represents the sulfur fraction in the alloy, while (1-x) refers to the oxygen incorporated due to thermal oxidation. The obtained x values from 1 (Sample C51) to 0.91 (Sample C56) are given in Table II.

We also observe that, as the bandgap is decreased, due to the oxygen incorporation, the hexagonal lattice parameter c is smaller. This can be explained by the Vegard´s law [13] for two constituent solid mixtures.

Fig. 9. Dependence of lattice parameter c on sulfur x fraction

In fig. 9, the lattice parameter c versus x is plotted. Notice that there is a linear relation, in agreement with the Vegard´s law.

TABLE II. SUMMARY OF FILM PROPERTIES

Property Sample C51 C52 C53 C54 C55 C56

Thickness (nm) 379 335 320 371 332 368

Transmittance (%)

47.3 54.0 56.1 50.1 54.7 52.8

Bandgap (eV) 3.62 3.56 3.52 3.48 3.45 3.35

Crystallite size (nm)

8 8 9 8 9 9

Lattice parameter a

(nm)

0.3805 0.3802 0.3796 0.3799 0.3795 0.3797

Lattice parameter c

(nm)

0.6227 0.6225 0.6225 0.6221 0.6216 0.6212

Fraction x 1 0.982 0.969 0.956 0.946 0.912

E. Photoluminescence measurements

The photoluminescence (PL) spectra, measured at low temperature (10 K), are plotted in fig. 10 for three samples: C51, C52 and C56 which correspond to the as-deposited film, without annealing, and for the lowest and the highest annealing temperatures, respectively. For the as-deposited sample C51 there were two PL emission bands at 461 and 511 nm, while for the C52 sample there was only one emission band at 506 nm and for the C56 sample also there was only one emission band at 545 nm. Other researchers have reported that these emission bands would be due to zinc vacancies [14]-[18]. In other words, the zinc vacancies cause the presence of up to three different deep energy levels within the ZnSxO1-x bandgap. From the PL results, the respective energy levels within the bandgap can be determined, and they are shown in fig. 11. Notice that two possibilities are displayed since we cannot establish whether the values correspond to levels referred to the conduction band or to the valence band.

Fig. 10. Photoluminescence spectra for samples (a) C51, (b) C52 and (c) C56

IV. CONCLUSIONS

In the present work, we obtained ZnSxO1-x thin films, deposited by chemical bath deposition of ZnS layers and their subsequent thermal oxidation. The obtained films had good adherence to the substrate, nanometric crystallite sizes and no pinholes. Oxygen incorporation in the thin films up to 9% was achieved, leading to a maximum energy bandgap reduction (increased affinity) of 0.27 eV which can be used to reduce unfavorable conduction band discontinuities at the interface of ZnS/CIGS. We also observed the presence of different energy levels due to Zinc vacancies, depending upon the oxidation temperature and time.

ACKNOWLEDGMENT

The authors thank Adolfo Tavira-Fuentes for X-Ray diffraction measurements and Miguel Luna-Arias for film thickness determination.

Fig. 11. Energy sublevels referred to the conduction band (a), or to the valence band (b).

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