Characterization and Processing of CdS/ZnS Thin Layer Films Deposited onto Quartz for Solar Cell Applications

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Energy Procedia 18 ( 2012 ) 85 – 90

Characterization and Processing of CdS/ZnS Thin Layer

Films Deposited onto Quartz for Solar Cell Applications

N Dahbi

a

,D-E Arafah

b,

a*

a_{Laboratory of sem iconductor devices physics (LPDS), university of Béchar n°417 , Béchar (08000), Algeria} b

University of Jordan, Physics Departm ent, Amman 11942; Jordan

Abstract

In this work, synthesis and processing of CdS/ZnS multilayer thin film sy stems are studied; compositional changes within these thin films structures are investigated by the Rutherford backscattering (RBS) technique to determine the depth distributions of the mixed region and stoichiometry according to Rump simulation which is used to estimate layer thickness and compositions even for complex samples; depth profiling using this program is also determined. The RBS data reveal a more homogeneous film could be obtained after annealing in 400°C and stoichiometric film structures with composition Cd1−xZnxS, where x=0.6 are synthesized in ZnS/CdS bilayer.

© 2010 Published by Elsevier Ltd. Select ion and/or peer-review under responsibility of [na me organizer] Keywords: Thin film; ZnS/CdS; Chemical treatment ; Thermal treatment ; RBS; Depth profiles; Vacuum deposition; Solar cell

1. Intr oduc tion

Over the years, CdS and ZnS thin films have been intensively investigated mainly for their applications in la rge-area e lectronic devices such as thin-film field -effect transistors and solar cells. Among several n-type semiconductor materia ls, it has been observed that the large band gap n -type window materia l CdS is the most promising hetero -junction partner for the well-known photovoltaic

* N Dahbi. Tel.: +213-49-81-55-81/91; fax: +213-4-81-52-44. E-mail address: [email protected].

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materia ls, e.g. ZnS [1, 2]. This latter thin film has a vast potential fo r use in short-wavelength optoelectronic such as electroluminescent devices due to his band gap which is of 3.66e V [3, 4]. So by the addition of ZnS, the band gap energy of CdS can be increased, hence combination of the two thins films produce Cd1−xZnxS thin film which has a larger energy gap ma ke it transparent to practically a ll wavelengths of solar spectrum [5] a lso by using this higher band gap materia ls i.e. Cd1−xZnxS as a

heterojunction partner to CdTe can improve the window band gap then improve the short wavelength spectral response of CdTe solar cell.

Cd1−xZnxS can be prepared by a variety of techniques , including spray pyrolysis, ion beam deposition, mo lecular bea m epita xia l growth, che mical bath deposition, therma l evaporation, and screen printing methods [7].

The Rutherford Backscattering technique (RBS) is one of the most powerful methods to investigate the composition, stoichiometry and depth profile of solids and consequently any structural changes in their composition as a result of ion beam modifications. This technique has been considered for the measure ment of dose accumulation in a semiconductor substrate during an imp lant. The RBS involves measuring the nu mber and energy of ions in a bea m (usually He ions) that backscatter after collid ing with atoms at surface and near-surface reg ions of a sa mple at which the bea m was targeted. With this informat ion and the scattering geometry, it beco mes possible to determine quantitatively mass informat ion and ele mental concentrations versus depth below the surface [8].

The most extensive use of the RBS technique finds itself in various fields of electronic and optical materia ls and applications, e.g. special coatings and in the study of various physiochemica l processes on solid surfaces [9].

Our goals in th is work are to study mixing effects of synthesized CdS/ZnS mu ltilayer thin film systems deposited on quartz substrate by using vacuum deposition method, che mica l and therma l treat ment will be used to achieve homogeneous Cd1−xZnxS thin films, also co mpositional changes within the thin films structures will be investigated by the RBS technique to determine the depth distributions of the mixed region and stoichiometry.

2. Experi mental me thods

In the preparation of CdS/ZnS thin film layer, a ZnS film was first deposited on Quartz substrate with thick o f 700A° by vacuu m deposition with a typical working pressures of 10-4 Pa were used during evaporation, this was then dipped in 1% CdCl2 methanol solution for about 30 s, then dried with an infra red la mp, and then rinsed in the Deionized water. This was followed by an additional deposition of CdS thin film with thic k of 500A°; finally the samples we re annealed in a ir at 400 °C for 15min .

Rutherford backscattering (RBS) was used to characterize the films, using 2 M e V D-particles beam supplied fro m the Jordan University Van de Graa ff accele rator (JOVA C). Detection of backscattered particles was done using normal incidence and a scattering geo metry with angle 135° relat ive to the incident beam and the reflected particles were collected using a silicon surface barrie r detector of 12ke V resolutions and 50 active area. To assist in the evaluation, Ru mp simulat ion was used in order to estimate of the layer thic kness, compositions and determine the depth profile of our sa mples [10].

3. Results and discussion

Typical and theoretica l results of Rutherford backscattering (RBS) e xperiment for our mu ltilayer ZnS\CdS samp les are reported in figure 1 for two cases: before and a fter che mica l t reat ment, the calculated (theoretical) spectrum which is obtained from Ru mp simulat ion and is shown as a solid curve appear to be in good agreement with the measured spectrum (e xperimental) which is represented by open

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circ les. The simulat ion reproduces the nomina l thickness of the top surface lay er name ly, 50n m (CdS) layer, the interface layer on the other hand was found to be about 14% thic ker. Such a layer was found to contain quite a large percentage, 80%, of o xygen; this may be expected, and this work is indeed consistent with other work [11] in wh ich o xygen is present in large concentration and noted to be active.

Fig. 1 Typical 2 MeV RBS spectrum of ZnS/CdS thin film deposited onto quartz glass: (a) before chemical treatment; (b) after chemical treatment.

However, one source of its presence is air during sample preparation, in addition a sma ll a mount of silicon was noted to be present within the layer;this indicate that the mat rix is an active ingredient that may mix within the film co mponents at room te mpe rature Ho wever, this observation is noted sooner than e xpected, and silicon can cross the interface layer by its higher diffusivity wh ich dominates all other matrix ele ments present in the sa mple . The process which can justify the t ransform of silicon is the presence of a high concentration gradient.

Close exa mination of the signal revea ls sloping on the front edges of sulfur, o xygen and silicon as we ll as the back edge of Zn. After che mica l treat ment, wh ich involves dipping the samples in a saturated solution of Cd Cl2 at room te mperature for 30s in air, represent e xcellent match with the measured spectra. Once again, the input data for calcu lation are based on the experimental conditions which makes in the results obtained more reliable. Exa minat ion of figure 1 reveals wide variations and marked changes and can be summarized in the following points: First, the ratio of Cd after che mical treat ment is doubled due to incorporation of Cd fro m the solution. Second, the Cd is e xtending over layer that is 12% wider with larger concentration at large depths such that its peak is detected at deeper depth by about 30%. Third, the Zn signal appears to be drastically reduced by about 30% fro m its origina l peak value, acco mpanied by move ment towards the surface by an a lmost the same proportion. The a mount of material is e xtending over a layer that is 13% larger, the motion of Zn towards the surface appears to conserve about 0.3 % of Zn to be retained in the sample; perhaps, if larger dipping times were used, then the amount of Zn retained would be sma ller. Forth, a similar observation to Zn was noted with the sulfur signal; in particular, sulfur was observed to exhib it movement towards the surface such that its peak concentration has moved by 13% and has increased by an almost 15% with a more de fined and narro w region. Fifth, film co mponents are homogeneous being that the signals distributions are not widely varying. Fina lly,

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both oxygen and silicon with in the bulk a re ho mogenous but their concentration near the surface region e xhibit variation fro m stoichio metry by an a mount not exceeding 50 %( See Fig. 3).

Figure 2 shows the measured and simu lated spectra of the sample after annealing at 400 °C for 15min, wh ich indicates a clea r and marked change in the signals .

Fig. 2 Typical 2 MeV RBS spectrum of ZnS/CdS thin film deposited onto quartz glass after annealing at 400 °C

The computer simulat ion of the spectrum shows an excellent agree ment with measured one based on the formation of mixed layer Cd1−xZnxS, Isaiah et a l. [12] found that a homogeneous Cd1−xZnxS can be fabricated fro m ZnS\ CdS structure.

Therma l annealing activated diffusion appears to o and has intimate mixing within the layers, this latter process has resulted in about 40% change in the Cd wh ile that due to Zn is large r and reaches about 60% . Heat and Cd Cl2 treat ments exhib it ma rked changes; however, annealing in a ir fo llo wing che mical treatment acts to drive chlorine to the interior, rather than to the surface. Such that it is t rapped at vacancy type defects and comple xes [13].

The lattice mismatch between ZnS and CdS p roposes the presence of Cd1−xZnxS mixed layer at the ZnS\CdS interface reg ion to reduce the structural disconnection. In addition, the interdiffusion between the two layers appears to reduce the interface defect density [14, 15]. The depth profiles of the structure obtained from the simu lation by RUMP calculat ion are a lso shown in Fig.3 and it confirm the above results.

Finally, we have de monstrated that using chemica l and therma l activated diffusion that Cd1−xZn xS, with x=0.6 thin films can be synthesized fro m vacuum deposited ZnS/ CdS mult ilayer. Structural study by RBS depth profiles reveals that Cd diffuses within ZnS film more readily than Zn diffuses into CdS film.

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Acknowle dge me nts

This work is partly supported by Algerian min istry of higher education and research (CNEPRU) project N° D03820070006. We would like to acknowledge the University of Jo rdan .

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