XRD, XPS and SEM Characterization of Photoconductive CdS LayersDeposited by Vacuum Thermal Evaporation

Peter Shindovl, Philip Philipov2, Rumen Kakanakov3,

Svetlana Kaneval, Theodora Anastasoval1)TU - Sofia, Branch of Plovdiv, e - mail:

Plovdiv, BAS2)TU - Sofia 3) Institute of Applied Physics

Abstract. Thin CdS-layers, formed by vacuum thermal evaporation were put to pulse rate lasertreatment in order to receive a CdO-layer as covering on them, through which activation andrecristalization of the layers were carried out. The so formed hetero structure. CdS- CdO was put tothermal treatment for the purpose of changing its properties. The CdO layer was removed aftercarrying out the activation and recristalization.

1. INTRODUCTION

The polycrystalline layers from CdS, as acomponent of the thin-layer solar cells were put tothermal treatment in fixed medium independentlyfrom the method of their obtaining. The influence ofthe intercrystal barriers was reduced as a result of thistechnological operation, the phase CdO appeared withconductivity of the order of 10-5 Q£lcm-1. Theheterojunction CdS - CdO is a target of variety ofstudies which aim is to determine the possibilities ofits application in the thin layer solar cells. Themanufacturing process optimization will widen thepossibilities of using the CdO layer as a highconductive and transparent layer. When carrying outconventional thermal treating the whole CdS surfacewas covered with a CdO layer, and when carrying outpulse-laser treating - only the locations of thetreatment.

2. EXPERIMENTAL PROCEDURE

A CdS layer, obtained by vacuum thermalevaporation was put to pulse rate laser treatmentaccording to the methodic given in [1-3]. The so

formed two-layer structure CdS - CdO was put toactivation and recristalization in a powder matrixcomposed of CdCl2, CuCl, LiCl, CdS and CdO, attemperature 450 - 500 °C, during 30 min in 02medium. The cooling velocity of the samples was 20aC/1 5min. The layer with CdO composition wasremoved by a solution containing CH3COOH. Thetechnological sequence of activation andrecristalization was repeated for a part of the first setof samples with the purpose of comparing offollowing change of their parameters.

XRD, XPS and SEM measurements

X-ray analysis was carried out with a DRON-2 X-ray diffractometer using a Co anode, K, radiation andNi filter for [ radiation.

The XPS studies were carried out in an EscalabMk II (VG Scientific) electron spectrometer with basepressures in the preparation and analysis chambers of2.10-8 and 1.10-8 Pa, respectively. SEM micrographsof the samples were obtained by a Philips SEM 525MScanning Electron Microscope with a spot size of 100A using a secondary electron (SE) detector, atransmitted electron (TE) detector and signaldifferentiation.

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3. RESULTS AND DISCUSSION

3.1. XRD study

Fig. 1 shows an X-ray diffraction (XRD) pattern ofan as-deposited CdS layer (CdS N° 1) obtained byvacuum thermal evaporation and an annealed CdSlayer (CdS N° 2). As a result of vacuum thermalevaporation of CdS on the cital substrate, columnarhexagonal phase crystallites with (002) orientation aredeposited parallel to the surface. The evaporated CdSlayers are highly faulted. The fault stacking is cubicwhile the planes above and below are hexagonal.Thermal treatment at 450 °C leads to recrystallizationof cadmium sulfide and, whence, to a change inorientation of the CdS crystallites and prevalence ofthe hexagonal phase in the X-ray pattern of sampleCdS N° 2. The high intensity of the (002) peak may bedue not only to the hexagonal CdS but also to thepresence of a (111) peak of cubic CdS.

3.2. XPS study

XP spectra of CdS layers obtained by vacuumthermal evaporation on a cital substrate prior to (1)and after (2) thermal treatment in a powder arecompared in Fig. 2 within the 5 - 1005 eV bindingenergy range. In contrast to the untreated CdS layer(1), the surface of the CdS treated (2) in the abovelayer displays additionally Cl 2p, Cl 2s and Cu 2p.The position of Cd 3d of the untreated CdS layer (1) isat 405.3 eV and corresponds to cadmium present asCdS. The halfwidth of Cd 3d5/2 after heat - treatmentincreases from 1.7 eV to 3.0 eV and the peak positionis shifted to the high binding energies due to theformation of CdCl2 on the surface of the CdS layer.The position of Cl 2p is at 199 eV and corresponds tochlorine in CdCl2 from. As a result of the thermaltreatment, the cadmium chloride concentration on theCdS layer surface increases. It is known from thepreparation of CdS/Cu2S solar cells that the reaction:

2CuCl + CdS -* Cu2S + CdC12

corresponds to sulfur in the form of S042, theprobable sulfates appearing on the surface beingCuSO4 at a binding energy of 169.3 eV and CdSO4.This additional peak is due to oxidation of the sulfuron the CdS layer surface under the effect of oxygenfrom the air during the thermal treatment at 450 °C for15 min. On the surface of as - deposited CdS, 0 Is ispositioned at a binding energy of 532 eV andcorresponds to chemisorbed oxygen formed accordingto the reaction

0 + 4e 202- (2)

the electrons being supplied throughphotogeneration of charge carries. It should be notedthat the intensity of 0 Is increases after the thermaltreatment and is shifted to the high binding energies.This is due to the binding of oxygen on the surface assulfates. The position of Cu 2p at a binding energy of932.5 eV corresponds to Cu+ in the form of Cu2S. Asa result of the thermal treatment of the CdS layer in apowder (CdS, CdCl2 and CuCl), Cu+ with an ionicradius of 0.96 A replaces Cd2+ (0.97 A) in the CdSlayer and diffuses into the depth. The calculatedconcentrations of the elements on the CdS layersurfaces in atomic percents are: Cd: S : Cl: 0 = 46.05: 42.10: 1.20: 10.65 for as-deposited CdS and Cd: S: Cl: 0: Cu 25.75 : 21.50: 17.37: 35.34 : 0.03 forannealed CdS. The stoichiometry of the as-depositedCdS layer is Cd: S = 1.09, while for the annealed CdSlayer Cd : S 1.20. As a result of the thermaltreatment the cadmium amount on the layer surfaceincreases due to oxidation of the sulfur in CdS tosulfates under the effect of the oxygen in the air. Thus,the amount of sulfur present as sulfides on the layersurface is 9.92 at % S, whereas the sulfur amounts to11.58 at % S.

(1)

may proceed on the surface of the CdS layer in thepresence of CuCl. The binding energy of S 2p at161.7 3V corresponds to S present as CdS, formationof Cu2S with a binding energy at 161.3 eV being alsoprobable. The additional peak of S 2p at 169.7 eV

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2 3 4 5 6 7

d (A)

Fig. 1 X - ray diffraction pattern of CdS prepared by thevacuum thermal evaporation:CdS Not as deposited, CdSNo 2 - annealed in powder (CdS, CdSI, CuCL) at 450 °C.

Cd 3d,,,Cd 3dW2

Cd 3pj2 C Is

0 KLL Cd3p,:3"@ cU sp ~Cd 3S i

{~~~~ \ | Ci2SP~~~~~~~~~~~~~~Cd 4dc 4p

t ~~~~~~~~~~~~Cd3di2C: Cd30

46Cd 3

Ju

_) Cd3p,jI0 0KLL Cd3s

: =0 Cd 4dL 0ClsSs S 2p Cd 4P

,Cd 4s

~~ ~ ~

1000 800 600 400 200 0

Binding Energy (eV)

Fig. 2 Cd 3d photoelectron spectra of CdS layers, dots -

experimental spectrum, solid line FFT Filter smoothingspectrum of 2 pts: (1) as- depoisted by vacuum thermal

evaporation: (2) annealed in powder (CdS, CdCl, CuCl) at4500C.

3.3. SEM study

Fig. 3 shows the results from a SEM study of theCdS layers. Fig. 3 (1) presents a SEM picture of a

cital substrate taken with 50 % SE and 50 % TEdetectors. It should be noted that the substrate surfaceis uniform and the grains are small-sized. Fig. 3 (2)shows a picture of the cital substrate surface afterdeposition of a CdS layer by vacuum thermal

evaporation. In this case, 25 % SE and 75 % TEdetectors are used. In addition to large includedparticles, evaporated films can contain nodules. Afterexposure the head - treated CdS layers, the exposedpart of the surface becomes darker quickly due tolayer photosensitivity.

The used method of oxidation of the CdS until it iscovered with a CdO - layer allows removal of layersof the CdS surface, while maintaining thephotosensitivity which coincides with the results in[4].

Fig. 3 SEM photo of treated 2 and not treated 1 surface

4. CONCLUSIONS

Using of vacuum thermal evaporation layers ofCdS on a cital substrate and thermal treatment of thelayers in a powder of CdS, CdCl2 and CuCl, thephotosensitivity of the CdS layers has been improvedby 6 orders of magnitude, which makes them suitablefor photoresistrors.

As a result of thermal treatment of CdS layersobtained by vacuum thermal evaporation, doping withCl- and Cu+ microimpurities occurs, followed byrecrystallization of CdS, its hexagonal fromprevailing. The presence of the elements Cd, S, Cl and

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CdS No2

I ILCdS Nol

I CdS stndard

p1 1 wL CdS standard

Cital .~~~~~~~~~~~~~~~~221 30th ISSE 2007

Cu as CdS, CdCl2, CdSO4 and Cut incorporated in theCdS crystal lattice is established on the surface ofphotosensitive CdS layers.

The enhanced photosensitivity of the CdS layershas a complex character and is due to recrystallizationof CdS accompanied by formation of a small - grainhexagonal structure. The stoichiometry of the maincomponents is: Cd: S = 1.2, chlorine doped with 17 at% Cl, copper doped with 0.03 at % Cu, an enhancedoxygen concentration up to 35 at % 0, and sulfuroxidation to S042.

The results of using a 1Onm thick buffer layer ofCdO show that hetero-junction CdS - CdO is asuitable decision when carrying out activation andrecristalization.References

REFERENCES

[1] V. Serbezov, P. Shindov, Laser applicationin Microelectronic and Optoelectronic Manufacturing V,Proc. SPIE Vol. 4274, San Jose, CA, USA, (2001), pp. 288

[2] S. W. Svechnikov, E. L. Shtrum, Thin solidfilms, 11 (1972),33

[3] P. Chr. Shindov, Electronics ET'2005,Sozopol 2005, pp. 146

[4] P. K. Nair, 0 Gomez Daza, A Arias-Carbajal Readigos, J. Compos and MTS Nair, Semicond.Sci. Tehnol. 16, 2001, pp. 651

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