Rahman, A

Sarmah, P C

IACS Institutional Repository

Indian J. Phys. 64A (1). 21-29 (1990)Studies of rf sputtered CdTe films prepared at different rf powersP C Sarmah and A RahmanDepartment of Physics, Gaiihati University, Guwahali-781 014, India Received 21 February 1989, accepted 11 September 1989A b stract: In view of making solar cells, films of CdTe, a direct gap semicon­ductor having band ga]) of about 1.4v5 eV, have been prepared by rf sputtering. A pellet of In or A1 placed on the CdTe target was simultaneously sputtered  with CdTe to get In-doped or Al-doped CdTe films. Maximum doping concen- t ia lion s  obtained in this way were of the order of 10'* cm~* with In and 10'* cm"* with A1 at taiget-substrate distance 3.8 cm. All the films were studied  to see the effect of i f  power on film structure, electrical and optical properties. Filin.s ])rcparcd at rf p ow eis  abcjve 100 w aits  were polycryslallinc and those  piepated  below this power were amorphous in nature. Rate of deposition  was obscived to be linearly increased with rf power w ith in  about 200 watts and decreased above Ibis. Though resistiv ity  has been observed to increase with  rf power, sheet resistance of films prepared at about 170 watts showed a niini-  nuim. Absorption coefiicient has been found to decrease w ith  increasing rf power. Photo -1 espouse has been observed to be insignificant at the doping  concentrations obtained in the present ca.se In is concluded from these studies  that rf power has a piofouiul effect on film characteristics and suitable dopant material other than Indium and A lum inium  is to be found out before making  photo-voltaic devices.K eyw ords: rf sp u ite i in g  leehiiic[ue, film s l iu c tu ie ,  rate of deposition, sheet resistance, absorption coctlicient.PACS N o s  : 81.13, 73.50, 78.65.1. IntroductionIn recent years, rf sputtering technique is increasingly being used for preparing different kinds of films of metals, semiconductors and insulators (Chetia et ol 1988, Wasa 1985, Das et al 1984, Phahle ct al 1976, Davidse and Maissel 1966, Sinclair and Peters 1962). In the momentum transfer mechanism of rf sputtering, stoichio­metry of compounds or alloys is almost retained and as such it is specially suitable to use in preparing films of compounds and alloys. Unlike dc sputtering, the problem of charge accumulation on the target surface is absent and therefore sputter-deposition of insulating and semiconducting films are also possible.4 21P C Sarmah and A RahmanCdTe is a II-VI group semiconducting compound of band gap energy 1.45 cV (Loferski 1956, Wysocki and Rappaport 1960). This compound being a direct gap semiconductor, can absorb photon of visible region without phonon assisted mechanism. Thus, this compound is particularly suitable to be used as solar energy absorber such as in thin film solar cells. Because of high resistivity in the undoped state, CdTe films cannot be prepared by dc sputtering due to charge accumulation on the target surface. In rf sputtering, these difilculties are absent and so it can conveniently be used in CdTe film preparation. The nature of films prepared by rf sputtering depends on several factors including rf power and gas pressure in the chamber.In the present work, detailed studies on the effect of rf power on film characteristics have been done. In this connection, dielectric properties, sheet resistance and absorption coefficient of H-type CdTe films doped with In and AI atoms have been measured. Effect of rf power on the crystalline structure has also been reported here.2. ExperimentalAn rf sputtering system supplied by M/s Hind High Vacuum Co. (P) Ltd., Bangalore was used for CdTe film preparation. A 2 KW rf generator operating at 13.56 MHz frequency was used whose power could be varied continuously from 0 to 2 KW. Finely controlled pure Argon gas was introduced into the bell jar for Ar ion sputtering. The system has been designed to hold 3 water-cooled targets. The substrate holder was a circular copper disc kept below the targets and could be rotated, water cooled or heated. The perpendicular distance between the target and the substrate holder was 3.8 cm which was kept fixed during all the sputtering operations. For preparation of doped CdTe films, pellet of the doping material (In or Al) of about 2 cm diameter was fixed at a place on the target surface of CdTe target. Glass substrates, thoroughly cleaned by a lOVb solution of RVS-50 (Kratos Analytical, U. K.), were placed on the holder at different places to get different doping levels. In the present case, three sets of samples were obtained in a single cycle by placing substrates radially at 3 different places on the substrate holder as shown in Figure 1. Thus a set A of samples were obtained by placing substrates at a position A just below the pellet, sets B and C of samples were obtained by placing substrates at the positions B and C respectively. A 5 inch diameter disc-shaped 99.999<X pure CdTe target mounted on the water cooled target holder was procured from M/s Testbourne, U. K. Detailed arrangement inside the chamber has been shown in Figure 1.In the sequence of rf sputtering, at first, the chamber was routinely pumped down to 5 X 10~* torr after which pure Ar gas was introduced to raise the chamberStudies o f  r f  sputtered CdTe films prepared at different r f  powers 23Figure I. Scheinalic  of urrangement inside the vacuum cliainbei : (l) taigetholder ; (2) CdTe la ig e l  ; (3) substrate holder : (4) Base plate ; (5) Belljar : (6) Top plate ; (7) To pump : (8) Ar gas inletA ,  B ,  C, positions of glass substrates . O, pellet for doping.pressure upto 1.5 X 10"® torr. The rf power was switched on after this pressure was steady. The films were prepared at different rf powers. In a single cycle, rf power was maintained constant. In all the cycles substrates were water cooled.The different thicknesses of the rf sputtered films were measured by inter­ference method (Tolansky 1948). CdTe films were deposited on vacuum deposited Indium films on glass substrate. Counter-electrodes of In were vacuum deposited onto In-doped CdTe films and Ni counter-electrodes were deposited onto Al-doped CdTe films. Capacitance was measured at 1 KHz using digital LCR-Q meter (model 4910) procured from M/s Aplab, Bombay. The resistivity P and sheet resistance pQ were calculated by measuring electrical resistance R  using the same LCR-Q meter. For measuring absorption coefficient, monochromatic light of 6000 A obtained from mercury lamp using a filter was focussed on the sample. The transmitted light was seen by a PMT (model RCA 931), the output of which was read in a digital picoammeler (Scientific Equipment, Roorkee). The absoiption coefficient «c was calculated from the relation / —/oC"”* where Tq is the incident light intensity, /  is the transmitted light intensity and t is the film thickness (Streetman 1986). Response of the PMT has been proportional to the light intensity in this part of the spectrum. I-V characteristics were taken both in24 P C Sarmah and A Rahmandark and under illumination for Al-doped CdTe samples. The samples were illuminated with white light from a projector lamp of 500 watts. X-ray diffraction curves were taken for different samples using Philips X-ray diffracto­meter (model PW-1710).3. Results and discussionsThe linearity of deposition rate has been obseived in our case at low power below about 200 watts (Figure 2). At higher rf power, this linearity has been deviated. The growth rate has been observed to decrease with increase in rfF igu re  2. Varilion of deposition rate of CdTe films with sputtering r f  power.power. The possible cause of this deviation may be due to increase of substrate temperature. Higher K.E. carried by the sputtered ions at higher rf power, is released to the lattice of the deposited film increasing thereby its temperature. Higher substrate temperature enhances the mobility of the deposited atoms increasing probability of their reflection without getting bonded to the film thus decreasing the deposition rate. From Figure 2. it is evident that growth rate is higher for films deposited on substrate placed below the centre of the target (B series of the samples).Sheet resistance of different samples prepared at different rf powers have been found to vary significantly with rf power (Figure 3). Sheet resistance has been observed to be minimum in the film prepared at about 170 watts rf power. The possible cause of this minimum may be due to the exact amount of energy carried by the deposited atoms/molecules at this rf power. This energy was sufficient for the atoms/molecules to move to the lattice sites after sputter deposition.Studies o f r f  sputtered CdTe films prepared at different r f  powers 25100 200 300 POWER I WATT)Figure 3. Resistivity ip) anJ slieet resistance (pD) variation of CdTe films prepared at different r/pow ers.Below this power, the deposited atoms/molecules cannot move long distances to occupy lattice sites, thus resulting in higher resistivity. Above minimum, the film may have some defect structures due to high energy carried by the deposited atoms/molecules.X-ray diffraction studies showed that the CdTe films are mainly polycrystalline in structure. Below 100 W amorphous films were produced on water cooled glass substrates. As the rf power increased film structure changes from amorphous to polycrystalline nature (Figure 4).Effect of bias voltages on capacitance were studied with increasing bias. Capacitance values of many samples increased suddenly to higher values and then26P C Sarmah and A RahmanStudies o f  r f  sputtered CdTe films prepared at different r f  powers 27got short circuited. This is not possibly due to dielectric breakdown, but may be due to migration of conducting electrode materials into the bulk of the sample through holes and vacant sites present in the films. C-V plots of two typical Al-doped CdTe samples have been shown in Figure 5. Doping concentration was calculated from the slope using relation (SZe 1969).where « is dielectric constant, e is the electronic charge, A is the contact area and Vjy is the diffusion potential. Co is the capacitance at zero bias condition. The doping concentrations found from the two C-V plots were of the order of 10** cm"“. In case of In-doped films the highest concentration found was of the order of 10^^ cm”®. Hall effect could not be performed on these films as the resistivity values were quite high to send any appreciable current through the samples.Absorption coeflScient < has been found to vary with rf power for all the samples. Effect of rf power on < has been shown in Figure 6 for In-doped CdTeF igu re 6. Absoiplion coefficient vaiiatiou vpith r f  poet of a few Tn-doped CdTe iihuB. ^films. Its variation with rf power may be due to the variation of defect concentra­tion which also caused resistivity variation with rf power as already has been discussed.Photo-response of these structures was very low. Typical //F cu iv cso la  sample with Al-doped CdTe film have been shown in Figure 7. The //K curve of the sample under illumination was taken at 500 lx light intensity. The nature of28 P C Sarmah and A Rahmanthe iUiiminaled curve shows that series resistance of the device is very high. It heeds further improvement for using as an efficient photovoltaic device.Figure 7. I jV  clia ii ic le iisUcs of an Al-iloiica CaT',; film in dark end al 500 lx  i iuensity  of illumination (Area p.79X 10"* s<i- cm).4. ConclusionsSome Properties of rf sputtered CdTe films have been studied in the present case. Doping of CdTe films by In or A1 from a pellet on the CdTe target during sputte­ring, could be achieved upto 10*^  cm"® or 10^“ cm"®. There has been profound effect of rf power on deposition rate, structure, film resistivity and absorption coefficient. A minimum sheet resistance has been obtained in the films prepared at about 170 watts rf power. This power may be suitable to prepare CdTe films for solar cell application. Photo-response of these films was not significant enough at these doping concentrations for using them in PV devices. Further woik are in progress in our laboratory.AcknowledgmentThe authors wish to thank DST (Govt, of India) for using the equipments procured for a different research project funded by them. The authors are indebted to A Buragohain and B Chetia for their help in some experimental set up. One ofStudies o f r f  sputtered CdTe films prepared at different r f  powers 29the authors (PCS) is thankful to CSIR for granting him leave to carry out this work.ReferencesClietia B, Biiragoliain A and Rahman A 1988 J. A s s a m  Sci* Soc .  30 33D asM  B, Krishnaswamy S V, Petk ic  Ronald, Swab R Slid Vedam K 1984 S o l id  S t a t e  E le c t r o ­n i c s  27 329Davidse P D and Maissel I 1966 J .  A p p l .  P h y s .  37 574 Loferski J j  1956 J .  A p p i .  P h y s .  2 7  777Phahle A M, Hill A U ,  Ruzinsky M and Calderwood J H 1976 T h i n  S o l id  F i l m s  38 73 Sinclair W R and Peters P O 1962 R e v .  S e t ,  I n s t r .  33 744Streetman B G 1986 S o l i d  S t a t e  E l e c t r o n i c  D e v ic e s  (New Delhi : Printice Hall ot India) 95 SZe S M 1969 P h y s i c s  o j  S e m ic o n d u c t o r  D e v i c e s  (New York : John W iley and Sons) p 370 TolanskyS \9AS M u l t i p l e  B e a m  I n t e r f e r o m e t r y  o f  S u r f  a c e  a n d  f i l m s  (Oxford : University Press) Wasa K 1985 T h i n  F i l m  T e c h n o lo g y  a n d  A p p h c a l i o n s  cds. K. D. Chopra and D* K. Malhotra (New Delhi : Me Graw Hill) p 27 Wysocki J j  and Rappaport P 1969 J , A p p l ,  P h y s .  31 571

Studies of rf Sputtered CdTe Films Prepared at Different rf Powers

http://arxiv.iacs.res.in:8080/jspui/bitstream/10821/4354/1/Studies%20of%20rf%20Sputtered%20CdTe%20Films_By%20P%20C%20Sarmah%20and%20A%20Rahman.pdf

Studies of rf Sputtered CdTe Films Prepared at Different rf Powers

Abstract

Similar works

Full text

Available Versions

IACS Institutional Repository