PbS thin ?lms were grown onglass substrates by chemical bath deposition (CBD) using lead nitrate, thioureaand sodium hydroxide in aqueous solutions for different deposition time (30-150min). The microstructure and morphology evolution of the ?lms were investigatedusing X-ray diffraction, scanning electron microscopy and atomic forcemicroscopy. As the deposition time increases, besides increasing the thicknessof the film from 100 to 600 nm, the ?shape of ?the ?particles changesfrom round (with typicalsize of 100 nm) to cubic (withtypical size of 500 nm).? Also simultaneously with completeshift of particle shape, the roughness ?value of thefilm increases sharply. The results indicate that deposition time is animportant parameter in determining the dominant mechanism of deposition andconsequently the characteristics of the film. The active deposition mechanismchanged from cluster to ion-by-ion mechanism during deposition reaction, andconsequently, ?lm properties such as shape, size, roughness and preferredorientation changed completely.PbS is an important binary IV-VI semiconductor materialwith a rather small band gap (0.
41 eV at 300K) and relativelylarge excitation Bohr radius (18-20 nm) 1, which results in good quantum confinement of both holes and electrons in nanosizedstructures 2. These inherent propertiesmake PbS one of the most important functional materials used in as thin filmsfor several applications such as IR detectors 3, photovoltaic cells 4, thin films transistors 5, LED 6, gas and biosensors 7-12 and photonic crystals 13.In recent years, various techniques have been used todeposit PbS thin films including microwaveassisted chemical bath deposition 2, successive ionic layeradsorption and reaction (SILAR) technique 14-17, atomic layer epitaxial process 18, pulse electro deposition 19, spray pyrolysis 20 and chemical bath deposition.
Chemicalbath deposition? ?method,also called chemical solution deposition ?technique, has become an attractive method due to manyreasons, including ?low cost, no requirement ofsophisticated instruments, freedom? ?todeposit ?materials on a variety ofsubstances, suitability for large scale deposition? ?areas, and ability of tuning thin filmproperties? ?byadjusting and controlling ?thedeposition experimental? ?parameters.21 ?Itwas realized that changing CBD parameters such as temperature, ?deposition time and solution composition leads tonanoparticles with ?different sizes and shapes22, which change the value of the band gap with ?respect to the effective mass model.?CBDprocess uses a controlled chemical reaction to achieve thin film ?deposition by precipitation. It isnecessary to eliminate spontaneous ?precipitation in order to form a thin film 23. Chemicaldeposition of films ?on solid substrates can takeplace via two major mechanisms.?Thefirst mechanism is ion-by-ion mechanism, in which the film is formed ?by sequential ionic reactions. If thereaction progresses in alkaline medium, ?a complex agent is required to prevent the formation ofhydroxide ?precipitates 24.
?? ?If the complex concentration isnot adequate to completely prevent ?formation of metal hydroxides, cluster mechanism occurs. In thiscase, a ?small amount of colloidalhydroxide will be formed, which then reacts with ?anions generated in the bath and produces the finalproduct. On the other ?hand,research has shown that the dominant mechanism of deposition is ?dependent on the reaction conditions andchanging the dominant ?mechanismduring deposition is possible 24.?Eachmechanism ?leads to different particlesize and morphology, which affects films ?properties. Therefore, for synthesis of a film withspecific properties, we ?shouldbe able to predict the effect of deposition parameters on reaction ?mechanism. We have previously shown thatdeposition temperature is ?effectivein determining dominant deposition mechanism 26.? Materials and methodPbS thin films were deposited on clean,spectroscopic glass substrates at ?different deposition time(30 to 150 min). All the reagents were purchased ?from Merck Chemical Co.
andwere used without further purification. ?According to previous studies, the aging ofprecursor solutions will affect ?deposition rate 27??.?? Therefore, fresh precursorsolutions were utilized to ?remove probable noise causedby aging. Prior to deposition, substrates were ?cleaned with the cleaningprocedure of Obeid et al. In brief, substrates were ?washed with hot distilledwater, immersed in 20% HCl for 24 hr, and washed ?with acetone. Then the substrateswere cleaned ultrasonically with DI water for ??20 min 2. ?To prepare the reactive solution, 40 mL of0.
146 M NaOH and 100 mL of DI ?water were mixed. After dropwise addition of 8 mL of 0.175 M lead nitrate to ?the stirring mixture, pureN2 was passed through the reaction solution for 1 hr ?in order todiminish levels of dissolved O2 and CO2. Then 8 mL of 1 M ?thiourea solutionwas then added to reaction mixture. Finally, the clean glass ?substrates wereplaced in the solution at 70ºC with respect to the horizon using ?the Plaxi holderto prevent large particles from adhering to the growing film. ?The samples weretaken out after deposition time (30, 60, 90,120 and 150 min) ?rinsed with DIwater and then air dried. The grayish obtained films were well ?adherent to thesubstrate and homogenous.
The reactions process for synthesis ?of lead sulfidefilms through ion by ion and cluster mechanisms have been ?previouslyreported28, 29.?Structural characterizations of the filmswere determined by X-ray diffraction ?method using a PhilipsPW3710 at room temperature with Cu K? radiation (? ??= 1.5405 ?A,Time/step=0.5S, Step Size=0.02). In order to determine crystallites ?size from XRD,the Scherrer formula was used.
Field emission gun scanning ?electronmicroscopy (FE-SEM) studies were carried out using a HITACHI S-??4160 microscope,in order to determine the morphology of the films. Film ?thickness wasmeasured from cross sections while surface topography was ?observed in plansview. The surface morphology of the thin films was ?characterized with an Autoprobe CP (Park Scientific Instruments) scanning ?electron microscope. AFMimaging was performed under ambient conditions ?using commercial Si3N4cantilevers in contact mode at a scan rate of 1 Hz. The ?opticaltransmittance and reflectance spectrum were recorded on a Perkin Elmer ?Lambda950spectrophotometer in the wavelength range of 200-3100 nm.
? Results and DiscussionFigure 1 shows the XRD pattern of a PbS film depositedon a glass substrate ?at room temperature for 30 to 150minutes. As shown in Fig. 1, with ?increasingdeposition time, the intensity of the peaks and the crystallinity of ?the films increased. This issue can be attributed toincreased thickness and ?increased particle size withincreasing reaction time.?? ?According to theidentification with X’pert HighScore software, all reflections ?corresponding to rocksalt phase of PbS (JCPDS powderdiffraction file #5-??0592).
The absence of any otherdiffraction peaks indicates that no other ?crystallinephases, such as oxides or carbonates of Pb, exist with detectable ?concentration within the layers.?The XRD spectra indicate anincrease in grain size with increasing deposition ?time,and a gradual transition to <100> texture, which likewise strengthens ?with deposition time. Fig. 1. XRD pattern of film PbS deposited on glass substrate atroom temperature and 30 to 150 ?minutes?The evolution of the filmtopography with deposition time is illustrated by ?AFMsurface plot images shown in Fig.2??.
? Fig.2a displays the initial nucleation stage, whereas the subsequent images ??(Fig. 2b–d) show films which gradually developed with increasing?deposition time.
The plot of the surface roughness vs.deposition time for ?layers deposited at roomtemperature shown in Fig. 2f indicates that simultaneously with complete shiftof particle shape, the roughness ?value of the filmincreases sharply from 20 nm to 65 nm;?? which fallsback to around 30 nm?, with further increasingdeposition time. Similar behavior ?has been reportedfor samples deposited at lower deposition temperatures (10 ??ºC)on the GaAs (100) substrate, however, due to the lower deposition ?temperature, these changes occurred over longer periods oftime30. ?In the period from 90 to 120 minutes, island growth has occurred,resulted in a ?significant increase surface roughnessof the film, but over a period of 120 to ??150 minutes,the growth process has progressed through layer by layer growth ??(Frank–van der Merwe) resulted in a significant reductionin RMS.?Sample deposited for 30 min(Fig. 3a) showed a discontinuous nano-crystalline ?filmconsisting of round particles with typical size of 100 nm.
Increase of the ?deposition time to 60 min (Fig. 3b) resulted in relativelycontinuous and dense ?film. In addition nuclei withtypical 20-30 nm have appeared on the primary ?film.Within 90 minutes a well adherent, dense compact layer which covers the ?entire substrate surface was achieved (Fig. 3c) the firstsigns of change in ?particle shape have appeared inthis stage. Due to the compactness of the film, ?distinguishingof particle boundaries and determination of particle size are ?difficult.Further increase in deposition time to 120 min (Fig.
3d) results in ?complete transition to faceted cubic particles with typicalsize of 500 nm. The ?boundaries of particles are quitedistinctable, it can be attributed to ?the ?dominance of columnar growth (versus layer by layer growth)at this stage. ?Further increase in deposition time to150 minutes, was not varied the film ?morphologysignificantly.? Fig. 2. AFM surface plot images of PbS films deposited atroom temperature for (a) 30 min, (b) 60 ?min, (c) 90min, (d)120 min, (e) 150 min, (f) surface RMS roughness as a function of ?deposition time ?Figure 3f shows the thicknessthe film as a function of the deposition time. ?Changein the growth rate with the reaction time is illustrated by this curve.
?Different slope of the graph represent the different stagesof the reaction. The ?initial slope can be attributedto the nucleation stage or incubation time; at this ?stage,as ?the time increases, the thickness increasesslightly because the ?primary ?nucleiare forming. The formation of these nuclei provides fast growth ?rate of the film during the next stage. In the third stage,due to the depletion of ?the reaction solution from thereactants, the deposition rate is less than the ?previousstage.? Each deposition mechanism has a characteristicgrowth rate, grain size and shape, which directly affect the nature andproperties of the films 21.
Hence, it can be concludedthat the changes in the deposition rate and particle shape is due to thetransition in deposition mechanism. On the other hand, it is well understoodpreviously that cluster mechanism has higher growth rate; so, the high rate ofdeposition in the second stage (60-90 min) can be attributed to the dominanceof cluster mechanism.The deposition rate declines,tendency to form larger particles and columnar ?growthin the third stage of deposition are evidences to transition from the ?cluster growth mechanism in the initial stages of growth toion-by-ion growth.
? Fig. 3. Field Emission Scanning Electron Microscopy (FESEM) Imagesof PbS films ?de?-?posited on glass at room temperature (a) for 30 min, (b) for 60 min , (c) for90 min, (d) for 120 min (e) for 150 min,(f) film thickness as a function of deposition time. In fact, this time-dependenttransition from cluster to ion-by-ion mechanism is ?expecteddue to depletion of lead ions in solution (e.
g. increase in complex-to-?metal ion concentration ratio) as the reaction proceeds.?The gradual change in filmmorphology, accompanied by enhancement of (200) ?preferredorientation, occurs with increasing film thickness.
? Thisobservation consistent with the AFM results; with increasing deposition ?time from 90 to 120 minutes, roughness of the samplesincreases sharply (from ?about 20 to about 65 nm),which can be attributed to the columnar growth, ?whereasthe columnar growth is characteristic of the ion-by-ion mechanism, it ?would be suggested than after 90 min from the beginning of the reaction active ?mechanismaltered to ion-by-ion.?Deposition mechanism dependson the reaction conditions and specifies the ?productcharacteristics. Previous studies on the PbSe films deposited via CBD, ?revealed that texture development which observed withincreasing thickness is ?also related to the change ofthe dominant mechanism 31. The results of this ?studyshow that increasing deposition time leads to (200) texture ?developments.?ConclusionsThe thin film oflead sulfide was deposited on a glass substrate using the CBD ?method fordifferent deposition times. It was observed that morphology of the ?samplesdepends on deposition time. As the deposition time increases, the ?shape of the ?particleschanges from round to cubic, texture ??(200) ?develops Furthermore the roughness of the film changes ?during thedeposition.
These ?changesare attributed to the change in the dominant deposition mechanism. ?This studyshowed that deposition time is an important parameter in ?determining the dominant mechanism ofdeposition and consequently the ?characteristics of the film.? References