PbS is animportant binary IV-VI semiconductor material with arather small band gap (0.
41 eV at 300K) and relatively large excitation Bohrradius (18-20 nm) 1, which results in good quantum confinementof both holes and electrons in nanosized structures 2. These inherent properties make PbS one of themost important functional materials used in as thin films for severalapplications 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, varioustechniques have been used to deposit PbS thin films includingmicrowave assisted chemical bath deposition 2, successive ionic layer adsorption and reaction(SILAR) technique 14-17, atomic layer epitaxial process18, pulse electro deposition 19, spray pyrolysis 20 and chemical bath deposition.Chemical bath deposition? ?method, alsocalled chemical solution deposition ?technique, has become an attractive method dueto many reasons, including ?low cost, no requirement of sophisticatedinstruments, freedom??to deposit ?materialson a variety of substances, suitability for large scale deposition? ?areas, and ability of tuning thin film properties? ?by adjusting and controlling ?the depositionexperimental? ?parameters. 21 ?It was realized that changing CBD parameterssuch as temperature, ?deposition time and solution composition leads to nanoparticles with ?differentsizes and shapes 22, which change the value of the band gap with ?respectto the effective mass model.
?CBD process uses a controlled chemical reactionto achieve thin film ?deposition by precipitation. It is necessary toeliminate spontaneous ?precipitation in order to form a thin film 23.Chemical deposition of films ?on solid substrates can take place via two majormechanisms.
?The first mechanism is ion-by-ion mechanism, inwhich the film is formed ?by sequential ionic reactions. If the reactionprogresses in alkaline medium, ?a complex agent is required to prevent theformation of hydroxide ?precipitates 24.?? ?If the complex concentration is not adequate to completelyprevent ?formation of metal hydroxides, cluster mechanism occurs. Inthis case, a ?small amount of colloidal hydroxide will beformed, which then reacts with ?anions generated in the bath and produces thefinal product. On the other ?hand, research has shown that the dominantmechanism of deposition is ?dependent on the reaction conditions andchanging the dominant ?mechanism during deposition is possible 24.?Each mechanism ?leads todifferent particle size and morphology, which affects films ?properties.Therefore, for synthesis of a film with specific properties, we ?shouldbe able to predict the effect of deposition parameters on reaction ?mechanism.
We have previously shown that deposition temperature is ?effectivein determining dominant deposition mechanism 26.PbS thin films were deposited on clean,spectroscopic glass substrates at ?differentdeposition time (30 to 150 min). All the reagents were purchased ?from Merck Chemical Co. and wereused without further purification. ?According to previous studies, the aging ofprecursor solutions will affect ?depositionrate 27??.
?? Therefore, fresh precursor solutions wereutilized to ?remove probable noise caused byaging. Prior to deposition, substrates were ?cleaned with the cleaning procedure of Obeid et al. In brief, substrateswere ?washed with hot distilled water,immersed in 20% HCl for 24 hr, and washed ?with acetone.
Then the substrates were cleaned ultrasonically with DIwater for ??20 min 2. ?To prepare the reactive solution, 40 mL of0.146 M NaOH and 100 mL of DI ?waterwere mixed.
After drop wise addition of 8 mL of 0.175 M lead nitrate to ?the stirring mixture, pure N2 waspassed through the reaction solution for 1 hr ?in order to diminish levels of dissolved O2 and CO2. Then 8 mL of 1 M ?thiourea solution was then addedto reaction mixture. Finally, the clean glass ?substrates were placed in the solution at 70ºC with respect to thehorizon using ?the Plaxi holder to prevent largeparticles from adhering to the growing film. ?The samples were taken out after deposition time (30, 60, 90,120 and 150min) ?rinsed with DI water and then airdried. The grayish obtained films were well ?adherent to the substrate and homogenous.
The reactions process forsynthesis ?of lead sulfide films through ionby ion and cluster mechanisms have been ?previously reported28, 29.?Structural characterizations of the filmswere determined by X-ray diffraction ?method using a Philips PW3710 at room temperature with Cu K? radiation (???= 1.5405 ?A, Time/step=0.5S, StepSize=0.
02). In order to determine crystallites ?size from XRD, the Scherrer formula was used. Field emission gun scanning?electron microscopy (FE-SEM)studies were carried out using a HITACHI S-??4160 microscope, in order to determine the morphology of the films. Film ?thickness was measured from crosssections while surface topography was ?observed in plans view. The surface morphology of the thin films was ?characterized with an Auto probeCP (Park Scientific Instruments) scanning ?electron microscope. AFM imaging was performed under ambient conditions ?using commercial Si3N4 cantileversin contact mode at a scan rate of 1 Hz. The ?optical transmittance and reflectance spectrum were recorded on a PerkinElmer ?Lambda950 spectrophotometer in thewavelength range of 200-3100 nm.
?Results and DiscussionFigure 1 shows the XRD pattern of a PbS filmdeposited on a glass substrate ?at room temperature for30 to 150 minutes. 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 the identification with X’pertHighScore software, all reflections ?corresponding torocksalt phase of PbS (JCPDS powder diffraction file #5-??0592).The absence of any other diffraction peaks indicates that no other ?crystalline phases, such as oxides or carbonates of Pb,exist with detectable ?concentration within the layers.?The XRD spectra indicate an increase in grainsize with increasing deposition ?time, and a gradualtransition to <100> texture, which likewise strengthens ?withdeposition time.
The evolution of the film topography withdeposition time is illustrated by ?AFM surface plotimages shown in Fig.2??.?Fig.
2a displays the initial nucleation stage, whereas the subsequent images ??(Fig. 2b–d) show films which gradually developed withincreasing ?deposition time. The plot of the surfaceroughness vs. deposition time for ?layers deposited atroom temperature shown in Fig. 2f indicates that simultaneously with completeshift of particle shape, the roughness ?value of thefilm increases sharply from 20 nm to 65 nm;?? whichfalls back 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, islandgrowth has occurred, resulted in a ?significantincrease surface roughness of the film, but over a period of 120 to ??150 minutes, the growth process has progressed throughlayer by layer growth ??(Frank–van der Merwe) resultedin a significant reduction in RMS.?Sample deposited for 30 min (Fig.
3a) showed adiscontinuous nano-crystalline ?film consisting ofround particles with typical size of 100 nm. Increase of the ?depositiontime to 60 min (Fig. 3b) resulted in relatively continuous and dense ?film.
In addition nuclei with typical 20-30 nm have appearedon the primary ?film. Within 90 minutes a welladherent, dense compact layer which covers the ?entiresubstrate surface was achieved (Fig. 3c) the first signs of change in ?particle shape have appeared in this stage. Due to thecompactness of the film, ?distinguishing of particleboundaries 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 quite distinctable,it can be attributed to ?the ?dominanceof columnar growth (versus layer by layer growth) at this stage. ?Further increase in deposition time to 150 minutes, was notvaried the film ?morphology significantly.?Figure 3fshows the thickness the film as a function of the deposition time. ?Change in the growth rate with the reaction time isillustrated by this curve.
?Different slope of thegraph represent the different stages of the reaction. The ?initialslope can be attributed to the nucleation stage or incubation time; at this ?stage, as ?the time increases, thethickness increases slightly because the ?primary ?nuclei are forming. The formation of these nuclei providesfast growth ?rate of the film during the next stage. Inthe third stage, due to the depletion of ?the reactionsolution from the reactants, the deposition rate is less than the ?previous stage.
? Each depositionmechanism has a characteristic growth rate, grain size and shape, whichdirectly affect the nature and properties of the films 21. Hence, it can be concluded that the changesin the deposition rate and particle shape is due to the transition indeposition mechanism. On the other hand, it is well understood previously thatcluster mechanism has higher growth rate; so, the high rate of deposition inthe second stage (60-90 min) can be attributed to the dominance of clustermechanism.
Thedeposition rate declines, tendency to form larger particles and columnar ?growth in the third stage of deposition are evidences totransition from the ?cluster growth mechanism in theinitial stages of growth to ion-by-ion growth.?In fact,this time-dependent transition from cluster to ion-by-ion mechanism is ?expected due to depletion of lead ions in solution (e.g.increase in complex-to-?metal ion concentration ratio)as the reaction proceeds.
?Thegradual change in film morphology, accompanied by enhancement of (200) ?preferred orientation, occurs with increasing filmthickness.? This observation consistent with the AFMresults; with increasing deposition ?time from 90 to120 minutes, roughness of the samples increases sharply (from ?about20 to about 65 nm), which can be attributed to the columnar growth, ?whereas the columnar growth is characteristic of theion-by-ion mechanism, it ?would be suggested than after90 min from the beginning of thereaction active ?mechanism altered to ion-by-ion.?Depositionmechanism depends on the reaction conditions and specifies the ?product characteristics. Previous studies on the PbSe filmsdeposited via CBD, ?revealed that texture developmentwhich observed with increasing thickness is ?alsorelated to the change of the dominant mechanism 31. The results of this ?study show that increasing deposition time leads to (200)texture ?developments.?ConclusionsThe thin film of lead sulfide was deposited ona glass substrate using the CBD ?method for different deposition times.
It was observedthat 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.? ?