Journal of Luminescence 134 (2013) 423–428

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Journal of Luminescence

0022-23

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n Tel.:

E-m

journal homepage: www.elsevier.com/locate/jlumin

Sol–gel synthesis of ZnS (O,OH) thin films: Influence of precursor and processtemperature on its optoelectronic properties

Ian Y.Y. Bu n

Department of Microelectronics Engineering, National Kaohsiung Marine University, 81157 Nanzih District, Kaohsiung City, Taiwan, Republic of China

a r t i c l e i n f o

Article history:

Received 18 May 2012

Received in revised form

18 July 2012

Accepted 2 August 2012Available online 10 August 2012

Keywords:

ZnS

Sol–gel

Violet

Photoluminescence

13/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jlumin.2012.08.008

þ886 972506900; fax: þ886 73645589.

ail address: ianbu@hotmail.com

a b s t r a c t

This study investigates the effect of sintering temperature and precursor composition on sol–gel

deposition of ZnS (O,OH) on its structural and optoelectronic properties. ZnS (O,OH) thin films were

prepared from stoichiometric, excess Zn and excess S precursor solutions. It was found that sintering

temperature and precursor composition have strong effect on subsequent optoelectronic properties.

From the combined photoluminance (PL) and EDS analysis it was revealed that beyond critical sintering

temperature, oxygen incorporation occurs and results in PL emission at around 423 nm.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Recently, there have been increased research efforts directedtowards production of zinc sulfide (ZnS) due to its potentialapplication in variety of fields such as field emission [1,2],electroluminescence [3], flat panel displays [4] and solar cells [5].ZnS is one of the first discovered semiconductor material, withremarkable optoelectronic properties [6] such as direct widebandgap (3.58 eV) [7], photoconductive [8], piezoelectric [9], highelectron mobility [10] and thermal stability [11]. Previously, it hasbeen demonstrated that a variety of colors can be producedthrough the addition of suitable activators into the ZnS surfaces.For example, by small addition of manganese into ZnS filmsresults in yellow–orange emission at around 590 nm [12,13],whereas insertion of silver results in bright blue emission at450 nm [14,15]. ZnS thin films have also been proposed aspotential replacement for the window layer in chalcopyrite-based solar cells [16–18]. Currently, the preferred choice ofwindow layer material for chalcopyrite-based solar cells is cad-mium sulfide (CdS), due to its superior electrical performancesand simple set-up[19]. However, cadmium is highly toxic andwould present significant environmental obstacles towards largescale-integration and general public acceptance of chalcopyrite-based solar cell. In contrast, ZnS is deposited using non-toxic,abundant elements that posses higher bandgap than CdS, which

ll rights reserved.

eliminate absorption loss and improve overall solar cell powerconversion efficiency.

ZnS can be deposited by several methods such as pulsed laserdeposition [20], metal organic chemical vapor deposition [21],sol–gel [22] and chemical bath deposition (CBD) [16]. Amongstthese methods, the CBD process is popular due to its large areacapability and simple set-up. Typically, CBD synthesis of ZnS thinfilm involves suspending the substrate within a mixture of Zn, Sand ammonia containing compounds, which contains otherimpurities such as ZnO and Zn(OH)2 and often denoted as ZnS(O,OH) [23–25]. Although, CBD process is popular, it suffers fromdrawbacks such as low material yield and slow deposition rate.Therefore, it would be of interest to develop other depositiontechniques to synthesis ZnS (O,OH) films. Alternatively, ZnS(O,OH) can be derived through the sol–gel process, which is oneof the most commonly used technique in thin film deposition.Sol–gel process is particularly attractive, as it is a solution-basedprocess, offers high material utilization and much higher deposi-tion rate.

In this present study, it was demonstrated that high qualityZnS (O,OH) can be deposited through the sol–gel process. Inaddition, it was found that the origin of emission at 420 nm isrelated to oxygen-passivated vacancies rather than the previousreported zinc or sulfur-related vacancies. The present studysystematically investigates the effects of sintering temperatureand precursor composition on sol–gel produced ZnS (O,OH). Thederived ZnS (O,OH) films were investigated through scanningelectron microscopy (SEM), four probe measurement, photolumi-nience (PL), energy dispersive X-ray spectroscopy (EDS) and X-raydiffraction (XRD).

Fig. 2. Scanning electron microscopy image of the synthesize ZnS (O,H).

I.Y.Y. Bu / Journal of Luminescence 134 (2013) 423–428424

2. Experimental

ZnS (O,OH) thin films were deposited on Corning Eagle 2000glass substrates by sol–gel deposition technique as shown in theflow chart in Fig. 1. Zinc acetate 99.999%, aqua ammonia (30%),isopropanol (IPA) and thiourea (99.0%) were purchased fromSigma Aldrich and used without further purification.

Three types of samples were synthesized with initial precursorsol at different ratio of Zn/S. The concentrations of stoichiometricsamples (sample A) were synthesized by mixing equimolar of0.7 M zinc acetate and thiourea in IPA and aqua ammonia. Forzinc rich (sample Z) and sulfur rich ZnS (O,OH) (sample S), theamount of zinc and sulfur was doubled, respectively, as comparedwith standard stoichiometric sample.

The chemical compounds were magnetically mixed for over2 h at 80 1C and left to age for at least a day to form the sol.Corning glass substrates were sequentially cleaned in acetone, IPAand water, respectively, and blow dry with N2.

The derived sols were spin onto a pre-cleaned Corning glasssubstrate at spinning speed of 3000 rpm. The deposition cyclewas repeated until thicknesses of around 250 nm were formed onthe Corning glass substrate. Each coating was dried using ahotplate through two stage heat treatments at 250 1C to removethe solvents and at 400 1C, 450 1C, 500 1C and 550 1C for 1 h tosinter the film.

In order to determine the structure of the films, SEM images ofthe sol–gel derived films were acquired by using a FEI Quanta 400F Environmental Scanning Electron Microscope (ESEM). Energydispersive spectroscopies analysis was conducted within thesame ESEM chamber. The crystal orientation was investigatedby using a Siemens D5000 X-Ray Diffractometer using Cu Karadiation. The photoluminescence (PL) measurements were per-formed by a 325 nm He/Cd laser (Dongwoo Macro Raman)spectrometer. The Hall measurements were carried out tocharacterize the electrical properties of all films with an EcopiaHMS-3000 Hall effect measurement system to determine mobi-lity, resistivity and carrier concentration. Zn/Al contacts weresputtered on the four corner of the sample to ensure low contactresistances.

IPA+ Zinc Acetate dehydrateAqua ammonia + thiourea

Stir at 80°C for 2 hour to yield clear solution

Spin coating @ 3000 rpm

Preheat 250°C for 10 min

Annealed 400, 450, 500 and550°C for 60 min

Fig. 1. Flow chart process for the sol–gel synthesis of ZnS(O,H).

Fig. 3. (a) EDS compositional analysis of ZnS(O,H) thin film from stoichiometric

precursor and (b) extracted compositional data of the ZnS(O.H) thin film as a

function of sintering temperature.

I.Y.Y. Bu / Journal of Luminescence 134 (2013) 423–428 425

3. Results and discussion

Fig. 2 shows the top view of the typical SEM micrograph of thesol–gel derived ZnS (O,OH) (sample A) that are quite smooth,without visible pinholes. Typically, the absorber layer inchalcopyrite-based solar cells exhibit high surface roughness[26,27]. If left untreated, the high surface roughness can lead tosevere degradation of solar cell performance due to formation ofundesirable phases of materials and non-conformal coatings [28].Usually, the high surface roughness is reduced via conformalcoating of CdS buffer layer and through post-annealing treatment.The sol–gel derived ZnS (O,OH) is quite smooth and thereforewell-suited to serve as buffer layer in chalpoyrite-based solarcells.

In order to confirm the composition of the ZnS (O,OH) thinfilms, EDS analysis was performed. Typical EDS composition(Au and Si were eliminated from the elemental analysis) andelemental ratio as a function of temperature are shown in Fig. 3(a)and (b), respectively. Under stoichiometric solution precursor, the

Fig. 4. XRD pattern for ZnS (O,H) deposited under (a) stoichiometric precu

as-synthesized ZnS (O,OH) (sample A) thin films consist of Znwith near equal amount of S and O. It is clear from Fig. 3(b) thatthe composition of the thin films are significantly affected by postsintering temperature, especially at 4500 1C. The ZnS (O,OH) filmundergoes substantial S loss as post sintering temperature raiseabove 5001C. The loss of S does not significantly affect the Zn/Oratio, which suggests only S has diffused out of the thin films.

The S loss not only affected the composition of the films butalso caused significant alteration to the crystal structure. Fig. 4(a)shows the XRD pattern of the ZnS (O,OH) (sample A) thin filmas a function of sintering temperature. It is interesting to notethat no significant diffraction peaks were observed at sinteringtemperature below 500 1C. As the sintering temperatureincreased above 500 1C, several peaks become detectable at31.441, 34.411 and 35.911, respectively, which correspond to(1 0 0), (0 0 2) and (1 0 1) in ZnO hexagonal wurtzite structure.In order to determine the effect of elemental composition onsol–gel derived ZnS (O,OH), the concentration of Zn and S wasadjusted. Two separate sols, Znþ (sample Z) and Sþ (sample S),

rsor, (b) zinc rich precursor, and (c) sulfur rich precursor, respectively.

Fig. 5. (a) mobility, (b) carrier density, and (c) resistivity of ZnS (O,H) deposited

under stoichometric, zinc rich and sulfur rich conditions.

I.Y.Y. Bu / Journal of Luminescence 134 (2013) 423–428426

were prepared via sol–gel process. The sols were prepared withexact method as standard ZnS (O,OH), with Znþ double theamount of Zinc and Sþ double the amount of S, respectively.Fig. 4(b) shows the XRD pattern for Zþ ZnS (O,OH), it is clear theaddition of Zn results in standard wurtzite structured ZnO even attemperature below 5001C. The additional Zn readily forms ZnOthrough oxygen from air or water and results in wurtzite ZnO.Fig. 4(c) shows the XRD peak under excessive S precursorcondition. The XRD peak reveals formation of ZnS (0 0 2) crystalplane at around 28.51 at temperature 4450 1C under S richprecursors. Such peak disappears as the sample was heated at550 1C due to S diffusion out of the film. The result is significantand reveals that sol–gel synthesis of ZnS can only occur atexcessive S condition. During the synthesis of ZnS, the solutioncontains NH2, NH3, S2- and OH-. The precipitation of Zn(OH)2 isexpected along with ZnNH3 species. As sintering temperatureincreases, ZnOH preferentially transforms to ZnO. Whereas theammonia (NH3) detaches from the ZnNH3 and leave the Zn toreact with S, to form ZnS. The sol–gel process is in goodagreement with CBD synthesis of ZnS (O,OH), where excessamount of thiourea (four times) is required to incorporate S intoZnS synthesized as compared to CdS. In the case of Sþ ZnS(O, OH), the ZnO formation does not occur until sintering tem-perature reaches 550 1C. Our data suggests that thiourea not onlysupplies the S for ZnS formation but also plays a role in suppressthe ZnO formation through attachment of NH3 species onto Zn.

From the XRD data, we can extract shift of 2.21 for S rich filmsand �0.191 for Zn rich films. The ionic radius of O2- (0.14 nm) issmaller than S2 (0.18 nm). Consequently, the shift in XRD peakscan be attributed to the degree of S incorporation in ZnS (O, OH).It is known that crystallinity of thin films improves with highersintering temperature, thus depends on S incorporation, thecrystallization temperature shifts.

All the films were electrically characterized through four probemeasurements to determine its resistivity, mobility and carrierdensity concentration. Several measurements were taken andaveraged to provide meaningful data. Fig. 5(a) shows the mobilitywith a function of annealing temperature with standard, zinc richand sulfur rich ZnS (O, OH). The general trend is that the mobilitydecreases with annealing temperature for all ZnS (O, OH) samples.Fig. 5(b) exhibits the carrier density of the ZnS (O, OH) films. Thesintering temperature appears to have no significant effect untilannealed at 550 1C.

At 550 1C, the carrier density increases, with stoichometric ZnS(O, OH) possess the highest carrier density. From the XRD data inFig. 4(b) indicates for Zn rich ZnS (O, OH) the film forms ZnO.Therefore, the increase in carrier density cannot simply beattributed to ZnO formation. Fig. 5(b) also displays the additionof excessive Zn and S leads to loss of carrier density, probably dueto structure disorder and vacancy related defects. The carrierdensity is also an indication of dopant S activation, our datareveals S dopant is not activated until sintering temperaturereaches above 550 1C. Fig. 5(c) shows the effect of annealingtemperature on the resistivity of ZnS (O, OH). Regardless of thefilm composition (Zn or S rich), the lowest resistivity of the ZnS(O, OH) films was obtained at sintering temperature of around550 1C, with range around KOhm� cm range. We believe this isdue to combination of improved crystallinity of the film and Sremoval from the sample.

Fig. 6(a) shows the PL spectra for stoichiometric ZnS (O, OH)sintered at different temperature. Fig. 6(a) revealed as thetemperature increases the PL spectra intensity also increases.The increased PL spectra are believed to be enhanced crystal-lization of the film with increased annealing temperature. As thethin film transforms from amorphous zinc acetate complexes intocrystalline ZnS (O, OH) thin film. The data correlate with XRD data

presented in Fig. 4 which shows ZnS (O,OH) films crystallizes withhigher sintering temperature. Predominantly, two peaks can beobserved from our stoichiometric film, centered at 480 and670 nm, respectively.

Fig. 6(b) displays the PL spectra for stoichiometric, Zn rich andS rich ZnS (O, OH), respectively. Unlike the previous PL studies onZnS/ ZnS (O, OH), which reported dual peak at 428 nm and515 nm, respectively [29], only single peak at 423 nm wasobserved in our experiments. Fig. 6(b) clearly shows the PLspectra shift from 501 nm to 423 nm as the film switch from Znrich to S rich composition. The occurrence of single emission peakat 423 nm is often attributed to transition of electron fromshallow donor levels to valence band due interfacial traps withinthe grain boundaries from S and ZnO grains [30]. Zhang and

Fig. 6. photoluminience of (a) ZnS (O,H) as a function of sintering temperature

and (b) using different precursor concentrations.

Fig. 7. (a) EDS compositions analysis as a function of sintering temperature for

sulfur rich precursor and (b) corresponding evolution of PL spectra.

I.Y.Y. Bu / Journal of Luminescence 134 (2013) 423–428 427

co-workers [31] attributed the PL emission at around 423 nm toS vacancies and interstitial lattice defects. It can also be observedfrom Fig. 6(a), the sample A thin films annealed at 400 1C exhibitadditional broad PL peak at around 423 nm. As indicated by XRDmeasurements, sample A deposited at below 500 1C is not crystal-line and therefore due to aforementioned interfacial latticedefects.

In order to gain understanding of the origin of 423 nmemission, we performed EDS chemical analysis on Sþ ZnS sam-ples. Fig. 7(a) shows EDS composition analysis of the Sþ ZnSsamples. From Fig. 7(a) it can be observed that for S rich ZnS(O, OH) films, the composition has changed substantially, withexpected Zn decrease. However, upon closer examination one canobserve unexpected increase O content and substantial loss in S atZnS (O, OH) thin films sintered above 450 1C. The results indicatedthat sulfur (boiling point 444.6 1C) and sulfur-related derivati-ves,such as H2S, with lower evaporation point have diffused out ofthe thin film. It is believed that the vacancies from S are replacedby O. Fig. 7(b) shows the evolution of the PL emission of sample Sas a function of annealing temperature. Regardless of the anneal-ing temperature, there is a prominent PL peak at around 423 nm.Clearly, there is a trend of increasing PL spectra intensity astemperature increases. Previous studies suggested that the PL

emission at 423 nm is related to S vacancies, which is probablesince significant amount of S is removed from the ZnS(O, OH)network. However, it is well-known that such vacancies readilyscavenges oxygen to form ZnO. Therefore, the PL spectra at423 nm are tentatively assigned to vacancies filled by oxygen,rather than just Zn vacancy or S related defects, as previouslyreported.

4. Conclusion

In this study we have examined the opto-electrical propertiesof ZnS (O, OH) films as a function of post sintering temperatureand various composition of the films. It was found that ZnS (O,OH) films crystallize at post sintering temperature above 500 1Cresulting in formation of ZnS (O, OH) films. Furthermore, it wasfound concentration of as-prepared precursor solution stronglyaffects subsequent optoelectronic properties of the film. Forinitially S rich precursor film, the subsequent out-diffusion of Sresulting in vacancies, which are filled by oxygen. Such replace-ment is evident in the increase of oxygen content and decrease inS content. The result is important in understanding the incorpora-tion of S into ZnO films and maybe of research interest into violetemission LEDs.

I.Y.Y. Bu / Journal of Luminescence 134 (2013) 423–428428

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