Study the Effect of Thermal Treatment and Variation of CdS Thickness on Structural Properties of CdS/CdTe Heterojunction

A polycrystalline CdS/CdTe heterojunction prepared by thermal evaporation technique at substrate temperature of 373 K and heat treatment for annealing temperature 573 K for different duration times of annealing (ta) 60 and 120 min. under vacuum of (10-5) mbar and different thicknesses of CdS layer of about 1000Å, 1500Å and 2000Å. The X-ray diffraction (XRD) technique shows polycrystalline structure and had a mixture of cubic and hexagonal structure.The recrystallization of films enhanced thermally represented by increasing the grain size. The CdS/CdTe heterojunction with the higher thickness of CdS layer shows better crystal structure.


II-VI
semiconductors compounds of considerable interest because of their extensive use in the fabrication of photovoltaic devices cells [1,2].Cadmium telluride (CdTe) is one of the most promising polycrystalline materials for thin film solar cells due to its physical properties: It has a direct band gap (approximately 1.5 eV) with a high absorption coefficient (larger than 105 cm-1 at wavelengths around 700 nm), so that only thin film layers (a few microns) are needed for the absorption of the most of the solar spectra photons with energy higher than the band gap, and it can be obtained as p-type.Some of the commonly used low cost growth techniques for CdTe thin film production include electrodeposition, spray pyrolysis and close-spaced sublimation [3].Irrespective of the growth process the grain size and surface morphology of CdTe films are two of the important parameters that affect the performance of active devices such as solar cells made on such layers.Therefore, understanding the microstructure and evolution in polycrystalline CdTe films is important to further develop an understanding of the performance of devices employing these layers.Polycrystalline films are used in a wide variety of applications in which their average grain size, distribution of grain size and distribution of grain orientations affect their performance and reliability.These grain structure characteristics are often defined by grain formation and growth during the film formation period.However, they may also be modulated through post-deposition process steps involving high temperature annealing [3].

Experimental Work
A polycrystalline CdTe thin films of 8000Å thickness were prepared by thermal evaporation technique (Edward Coating system E306A) under a vacuum of pressure of the order (10-5) mbar.We use a standard CdTe alloy, of purity (99.999) % on corning glass substrate, cleaned by methanol and washing in ultrasonic vibrator with deionized water, and then dipping in 10% HF solution for 2 mints.
A CdS layers with different thicknesses (1000, 1500 and 2000) Å were thermally deposited on the CdTe layer and the CdS 1000Å /CdTe 8000Å hetrojunction annealed for different duration times (60 and 120 min.)under vacuum of pressure of the order (10-5) mbar at 523 K. X-ray diffraction technique was used to investigate the heterojunction structures.The experiments were carried out using Cu Kα radiation of average wavelength 1.54056 Å.It was operated with a target current and voltage of 25 mA and 40 KV respectively.The diffraction pattern was recorded between 2θ = (10°-60°).By comparing the interplaner distance for different planes (dhkl) value with ASTM card for CdTe and CdS have been examined the structure.The grain size (D) was calculated using XRD analysis from Scherrer relation [4] Where λ is X-ray wavelength, β is the full width at half maximum intensity (full width at half maximum intensity (FWHM)) and θ is the diffraction angle (Bragg angle).The grain size was calculated using FWHM of (111) plane.

The effect of variation duration times of annealing on structure of CdS/CdTe heterojunction
The as deposited heterojunction shows identified as polycrystalline CdTe with a cubic structure.It exhibits high preferential orientation along (111) and some low intensities of (002), (110) and (311) peaks.X-ray diffraction spectra of CdS/CdTe heterojunction annealed at 523 K for 60 and 120 min, are shown in Fig. (1).The annealed junctions exhibit a preferential orientation along (111) direction of the cubic structure.The (002), ( 110) and (311) peaks are more intense than those of the as deposited heterojunction.The FWHM of the peaks after annealing are smaller than those of the as deposited heterojunction.These results show that the heat treatment enhances the recrystallization of CdS/CdTe.These results was in agreement with Romeo [16] and Rami et.al. [17].
Such crystallinity improves with increasing the time of annealing have observed by Al-Dhafiri [4] and Enriques and Mathew [5].There is slight shift in peak position was observed in (111) plane towards the higher diffraction angle (smaller lattice parameter), due to thermal stress [16], it may be attributed to the dependence on composition which includes the stochiometry of CdTe compound [6,11].The crystal structure improvement is due to rearrangement in the CdS/CdTe crystallites [4,17].The increasing in intensity with increasing the duration times of annealing may be permitting better grain growth by maintaining the preference for the (111) plane [5].The effect of CdS thickness on structure Fig. (2) Shows the X-ray diffraction patterns of CdS/CdTe heterojunction with different thicknesses of CdS layer.A comparison between the spectra of the three heterojunction shows that there is more crystallization and more orientation of the crystal growth in the case of the thicker CdS layer, espatially in the CdS peaks (002), and (311).Also one can observe a slightly decreasing in the intensities of CdTe peaks.This result are agreement with Shadia et.al. [18] and Ngamnit et.al. [19].The XRD spectra are presented in Fig. (2) showing the influence of variation of thicknesses of CdS layer of about 1000Å, 1500Å and 2000Å on CdS/CdTe heterojunctions.Generally it can be observed that the intensity of (002) plane of CdS have been increased and the FWHM of peaks have been decreased considerably with increasing the CdS thickness this results show that this variation of thickness enhance the recrystallization of the films.

The grain size results
The recrystallization induced by thermal treatment leads to increase grain size in both CdTe and CdS layer as shown in figures (3,4) respectively, and reduction in defects density [16], [17].From Figure (3) and Table (1), the grain size results shows that a small fluctuation in this values were noted as the duration times of annealing was increased.Fig (5) and Table (2) shows the increasing in the grain size values with increasing the CdS thickness.We can observe that the grain size values at different thicknesses are larger than that consisted by annealing as shown in Tables (1,2).The increase in the thickness of CdS layer should lead to increase in the crystallinity of the CdTe layer [20].
XRD spectra for CdS/CdTe heterojunction that annealed under vacuum