Das, Rajesh

Ray, Swati

IACS Institutional Repository

78(9).901-906 (2004)?  U PThickness dependence of the properties of magnetron sputtered ZnO : A1 films andits application in a-Si:H thin film solar cellRajesh Das and Swati Ray*Energy Research Unit, Indian Association for the ('uUivalion of Science. Kolkata-700'032, IndiaE-mail er.sr^iacs res inR e c e iv e d  5 M a tch  2004, a c c e p te d  I I  June 2004\b.struct . Transparent conducting ZnO.Al films with vaiioiis film thicknesses have been prepared under Ai-ambicni at siibsiratc icrnpcraiuic lOOT bv RF-magnetron sputtering The resistivity of ZnO film decreases from 7 2 xlO^ ohm-cm to 8 2x10^ ohm-cm with the incicase of film ihitkness The minimum sheet resistance is ~10 ohm/D and maximum earner concentiation and Hall mobility arc 1 .'^6xl0’^ ‘ cm ' aic 55 7 nr/v. l^tsec respectively. All the films are highly transparent (above 90% visible transmission) m visible wavelength region and IR-transmission is veiy ensiiivc with film thickness. The cry.siallinity has been greatly improved with increase of film thickness upto certain limit, confiimed fiom X lay illiactum pattern (XRD) and Transmission Electron Microscopy (TEM) analysis Surface morphologies arc difteicnt for diffcieni thicknesses onfirnied by Scanning Electron Micrograph (SEM) The effect of variation of ZnO thickne.ss as back reflector on the a-Si solar cell performance is iigndicantKeywords Zinc oxide (ZnO). magnetron sputtering, thickness variation, material pioperiy, solar cell performancel‘A(\S Nos. 73.50.Pz , 73.61.Ga. 73 61.Jc1. IntroductionZnO thin films are technologically important due to their superior electrical prc'perties and high optical transmission over wide range of wavelength, which makes it suitable for thin film solar cell applications. Highly transparent films with a resistivity of the order o f ohm-cm have been prepared by RF-magnetronsputtering by several workers [1-3J. Using thin layer o f ZnO hlms as back reflector in a-Si single and double junction solar cell, the cell performance has been improved significantly 14,5]. Absorption in the i-layer increases due to insertion o f thin ZnO liiyer at the n/metal interface. Therefore, the i-layer thickness can be reduced which will help to increase the electric field across p-n junction and the degradation o f  thin film a-Si:H solar cell reduces. Thus, the ZnO film thickness is an important factor for device applications. The electrical-optical as well as structural properties o f  ZnO:Al film s are strongly dependent on film thickness.Up to date, there are several reports regarding the effect of ^^ygen pressure and growth temperature on the properties o fC orresponding AuthorZnO films grown by sputtering technique. Jae-Min Myoung e t a l [6 ] reported the effect o f thickness variation on the material properties of ZnO films prepared by Pulsed Laser Deposition technique. In this work, resistivity increased and carrier concentration decreased with increase o f  film thickness. However, there has been no report on the effect of film thickness on the properties o f Al doped ZnO films grown by sputtering technique. In this study, the effects o f thickness variation on the electrical, optical properties, surface morphology and structural properties o f ZnO thin films have been discussed. This ZnO films with different thicknesses has been applied as back reflector in a-Si:H single and double junction solar cell. Finally, the cell performance with different ZnO thicknesses has been investigated,2. ExperimentalZnO: Al films were prepared on glass substrate by RF-magnetron sputtering using Ar as sputtering gas. For this purpose, a sintered ceramic disc o f ZnO (purity 99.99%) with 2 wt%. ALO3 (purity 99.99%) was used as target (diameter o f 4 inches). During each deposition, the chamber pressure was kept constant with the help o f a throttle valve and the gas flow was maintained by© 20041ACS902 Rajesh Das and Swaii Raya mass flow controller. Substrates o f  corning 7059 glass were placed parallel to the target surface at a distance o f 7cm. The rf- power is applied between two capacilively coupled electrodes. Before each deposition, the base pressure inside the deposition  unit was brought down to 3" 1 0 '^Torr by water-cooled oil diffusion pump through a liquid nitrogen trap. Prior to sputtering, the substrates were heated at above 2 0 0 ^C for degassing. In order to investigate the effect o f thicknes.ses variation on the properties o f  ZnO, a scries o f  ZnO film s with various thicknesses were prepared with 100 watt rf-power, 4m T chamber pressure at a substrate temperature 1\ = lOO^t.T he e le c tr ic a l r e s is t iv ity , sh ee t r e s is ta n ce , carrier  concentration and Hall mobility o f the film were measured at room temperature by the van der Pauw point probe technique using a square configuration. Resistivity has been measured by interchanging the current and potential terminals as per the usual practice in four probe measurements. Relevent expression o f  resistivity for this configuration isp  = (ra//2 ln 2 ) (V ,/ / ,  + V2 / / , ) / a n dwhere Kj, are the measured voltages o f  any two terminals; /j,12 are the current passing through the other two terminals and f is the van der Pauw correction factor. /? . is the sheet resistance o f  the films. The Hall measurements were carried out utilizing a magnetic field (B) o f  several kilo-Gausscs, in the transverse direction o f  the applied current (I). Several Hall voltage readings were taken changing the current and keeping the magnetic field constant. Carrier concentration and Hall mobility have been calculated using the formulasn , = { lB f V „ e d )  and p  -  l / (p  n ^ e ) ,where, and ^  are Hall voltage and mobility respectively. The optical transmission, reflection and absorption coefficient for ZnO film s were measured by a double beam UV-VIS-NIR  spectrophotometer (Hitachi, 330) at room temperature with unpolarised light in the spectral range 185-2600 nm. X-ray diffraction (XRD) pattern and Transmission Electron Micrograph (TEM ) were taken for structural analysis o f  ZnO:Al film s with different thicknesses. For TEM study, carbon coated grid were used as substrate. Crystallite sizes were measured from XRD  data u sin g  S cherrer's form ula. T he ch an ges in surface  m orphology o f the film s were evaluated by Scanning Electron Micrograph (SEM).Single junction (a-Si:H) thin film solar cell (1 cm^) have been fabricated with the following structure in a multichamber PECVD  system. Glass/textured SnO^iF/ p-a-SiC:H/ a-SiC:H/ i-a-Si:H/ n- a-Si:H/AI or ZnO/AI with different ZnO layer thickness. Zinc O xide layers o f  different thicknesses have been deposited on p- i-n structure by magnetron sputtering. Double junction (a-Si/a- Si) solar cells have also been fabricated with the structure Glass/ textured SnO^: F/p-a-SiC: H / a-SiC: H/ i-a-Si: H/n-pc-Si: H/p-a- SiC; H/ a-SiC: H/i-a-Si: H/n-a-Si: H/ZnO/Al. In some cells, ZnO: A1has been introduced in between n-a-SiH and A 1 layers and som e cases, only Al is introduced as back contact. I’ht i \ characteristics o f  these cells w'crc taken under a calibrated /\vj 1 .5G solar simulator.3. Results and discussionThe electrical properties o f ZnO:AI films were measured at room temperature. The substrate temperature was kept fixed at \{){n since the film will be used for the application on amorphous S or microcry.stalline-Si based solar cells as back rellcctot I iolu- 1 shows the variation o f resistivity and carrier conccnthjiK.n wilh film thickness. The values o f resistivity, carrier conccntiain^n and mobility are depicted in T able-1 . There is a sharp decicasc o f resistivity from 7 .2 x 1 0 ’ ohm -cm to4.1  x 1 0 -U)hm-cm film thickness increa.ses from 45nm to 170nm. Then the resisto n, changes slow ly and the lowest resistivity i.e  8.23 x 1 0  S)lmi is achieved at a thickness o f  6 IOnm. But, one interesting tcsuii has been achieved for 1175nm thick film. Here, the rcsisto w\ r o f slightly higher value compared to the former film.Figure 1. Variation of resistivity and carrier conccnlration with him thickness.On the other hand, with increase o f  film thickness, earner concentration increases and al 610nm , it reaches the maxi mum value (shown in Figure 1). The maximum carrier concentratuvi and m obility  are 1.36 x 10*® cm ’ and 55 .7  cmVvolt.sec respectively. The electrical sheet resistance (R )^ o f  ZnO film m Figure 2 shows a sharp fall as the film thickness increases from 85nm tol70nm  and after that the curve continues to fall at a much slower rate, showing near saturation beyond 340nm. 3 he lowest sheet resistance o f ZnO film is 9.75 ohm/ . The thickne.ss dependence o f electrical parameters o f  ZnO film is shown m Table 1.Figure 3 sh ow s the optical transm ission and reflection spectra o f  ZnO films. It is evident that the transmission in visible region is above 90% for very thin films (with thickness <  1 7 0 mn) and between 85-90%  for all other film s. The near intrared transmission is sensitively dependent on film thickness. ThtThickness dependence o f the properties o f magnetron sputtered ZnO etc 903transmission o f 85nm film at 1500nm  is 95% whereas it is below  lor 610nm film . At lOOOnm, the change is much lower i.e fii'ni 95'’/^  lo 85% with increase o f  ZnO film thickness from 85 toHpurc 2. Variation of sheet resistance with the film thickness.OlOnm. For m icrocrystalline-Si solar cell, transmission upto 101K)nm IS important. The reflection spectra o f the ZnO films are also shown in Figure 3. It is alm ost same for film s o f  differentTahic 1. Variation of electrical parameter o f ZnO films with cliffcient thickness deposited at substrate temperature 7 = 100 CSiibsuatelunperatuie•' [ )VZnO film thickness (nm)Resistivity (ohm -cm )Carrierconcentration  (c m ')Mobility(cmV sec. volt)100 85 7.2x10^ 6.2x10'“ 13 0170 4 .1 x 1 0 ' 3 4x10*'' 45340 .1x10’ 4.33x10'* 48.5425 1 ,9 x 1 0 ' 6 2x10"' 5 3610 8 .2x10* 1.36x10’" 55 71175 8 .8 x 1 0-* 1,32x10“ 53.8thicknesses in visible range and varies within to 2 0 %. However, % reflection at longer w avelength  (A > 1300n m )  changes with film  thickness. P lasm a resonant frequency is I^ctined by the wave number at which transmission and reflection 5»pectra intersect. It is proportional to the carrier concentration, which is corroborated by the recent observation. The plasma resonant freq u en cy  h as b een  sh ifte d  tow ard s shorter  wavelength side initially with increase o f  film  thickness upto ^lOnm but after further increase o f  film  thickness, plasma h^quency shifts towards longer wavelength. The variation o f  reflectance is not the function o f  thickness, but the function ot ^^ rrier concentration. From Hall effect studies, it was also found h^at carrier concentration increases upto a film thickness ot ^HOnm. The band gap o f  ZnO film  deposited under Ar-ambienl substrate temperature F = 1 0 0  has been calculated fromVA' f iv  plot. The value o f band gap o f ZnO film is 3.5eV and it does nut vary with film thickness significantly.Figure 3. Variation of optical transmission with the thickness T 1 S,Snm. T'2 170nm. T 3 340nm, T-4 425nm and 'I’ 5 I175nm and rcflcciion with the film thickness R-3 340nm, 1< 4 425nm and R 5 1175nmA scries of X-ray diffraction pattern of ZnO film with diflercnl thicknesses have been shown in Figure 4. AH the films exhibit the <002> diffraction since ZnO film grows with strong < 0 0 1> preferred orientation as the surface free energy is minimum |7).36002800<002>(a)------ 1 . 1 3<004 >1<002>2 1 1_ i _ jy <002> 120 30 40 50 60Angle (2o)70 80Angle (2e) in degreeFigure 4. X-Ray diffraction pattern of ZnO.Al films of thickness (a) (1) nonrn, (2) 340nm, (3) 425nm and (b) 1175nm.904 Rajesh Das and Swati RayThe film with thickness 85nm exhibits the weak ZnO <0 0 2 >  diffraction but with increase o f  film thickness, the intensity sharply increases. For all the films, the ZnO <002> peak position slightly shifts towards the higher angle sides than that o f the bulk ZnO and it clearly implies that the film s are under tensile strain. As glass is amorphous material, the film strain has been originated due to the mismatch o f thermal expansion coefficient o f  ZnO and glass. The strain values ( ^ ) has been calculated by using the equation, = [ A /5 c o s 0 ) “ /3 ]x l/ta n 0  [8,9], where A , <5 and /3 are the wavelength o f  the x~ray, crystallite size and Full Width H alf Maxima (FW HM) respectively. The values of microstructural parameters are enlisted in the Table 2. With increase o f  the film thickness from 170nm to 425nm, the FWHM decreases and as a result, the crystallite size increases. The average crystallite sizes vary from 19.8nm to 24.2nm. Further increase o f  the film thickness does not affect the value o f  FWHM. So crystallite sizes get saturated. The film strain decreases with increase o f film thickness. On the other hand, dislocation density decreases with increase o f  crystallite size and saturates at 610nm  thickness. The dislocation density (D), which is defined as the length o f  dislocation lines per unit volum e, has been calculated from the formula, D  -  l/<5^ [9]. Lattice spacing {d )  is obtained from  the B ragg’s law given  by /^ = A /2 s in 0 .  The lattice p aram eters h ave b een  ca lc u la te d  by the eq u a tio n s , \ / d ^  = -f and v /d ^  = 4(/?^ - k - h k j ? > a ^I which arc valid for hexagonal system  |9j.Tabic 2. Variation of microstructural parameters o f ZnO:AI films with film thickness deposited at 100"C'ZnO film XRD , Peak C'rystallite Micro- Dislocation Lattice thickness peak position size (6) structural density constant(nm ) (20) in (A) strain (4) (D) (d)in degree lines/m'x in (A)1 0 ‘"340nm. Here, grain size is bigger, granular and round shaped with different sizes (80- 120nm). The above fact clearly illustntes that with increases o f  film thickness, surface roughness hisjfJrT jV V  * 'v ; * ^  > -■r.t:;.’170 <002> 34 26 19.8 2 93x10  ' 2 .55x10’ 2.617340 <()02> 34.3 22 2 2 63x10 ' 2 03x10’ 2 614425 <002>,<004>34 32 24 2 2 3 7 x 1 0 ' 1.7x10’ 2 6121175 <002>,<004>.34.4 24.5 2 .3 4 x 1 0 ' 1.66x10’ 2.606Figure 5 shows the TEM micrographs o f  ZnO film s o f  thicknesses 45nm ahd 80nm. For both the film s, the crystallite sizes are small, but it is clear that with increase o f  film thickness, the num ber density o f  crysta llite  increases. The average  crysta llite  s ize  as estim ated from  T ransm ission  Electron  Micrograph is around 15nm.Figure 6  shows the surface m orphologies o f  ZnO film s with thicknesses 85nm, 3 ^ h m  and i 175nm as observed by Scanning Electron Micrographs. The film with thickness 85nm (shown in Figure 6 a) shows the non-uniform distribution o f  small grains. The density o f  grains increases as the o f  ZnO film  thickness isFigure 5. Transmission electron micrograph of ZnO him \utli liln' thickness (a) 43nm and (b) S.'S nmincreased. For the film  o f  th ick n ess 1175nm  (shown m Figure 6 c), the grains arc closely packed with average grain .ma 6Qnm.On the basis o f  SEM, TEM and XRD analysis, we can explant the variation o f  electrical properties with film thicknes.s f)t ZnO film  with thickness. Initially, the growing film  is discontinuous and non-uniform. During the film growth, som e islands come into mutual contact and coalescence like two droplets. Large islands grow faster and small ones partly disappear due to the coalescence with larger ones. In this way, uniform film s aie formed. That means, very thin film s are inhomogeneous where spatially separated crystallites are formed. Due to incomplete co a lescen ce  o f  the crysta llite , the current carriers cannot com plete the percolation path and as a result, the film shosvt' high resistivity. With increase o f  film thickness, proper percolationpaihT h ic k n e ss  d e p e n d e n c e  o f  th e  p r o p e r t ie s  o f  m a g n e tro n  s p u t te r e d  Z n O  e tc  formed as the number density o f  carriers and grain size905ncteases- The increase in m obility is observed from Hall effect measurement with thickness. This together with the increase in •a rrie i concentration explains the sharp fall o f  resistivity as well ihc sheet resistance o f  the film with increase o f thicknessx2 0 k 0024 2 u»(b)risk 0031 25kV a(c)•^Rure 6. Scanning electron micrograph of ZnO film with film thickness 85 nm (b) 340 nm and (c) 1175nm.85nm to 340nm, A gain , with further increase o f  film  ^ i^ckness, variation o f  carrier concentration is not significant.  ^or the ZnO film with thickness =  lOOOnm, there is no further ‘ncrease o f carrier concentration. The film  resistivity depends both carrier concentration as w ell as mobility. M obility  ^^ Pends on grain boundary scattering, impurity scattering and surface roughness scattering. For the com bined effect o f  them, carrier mobility in 1175nm thick film decreases which causes  ^ b^ght decrease o f  resistivity.This ZnO: Al films with different thicknesses have been used as back reflector of a-wSi:H single and double junction solar cells. The cell performances have been investigated in details. The I- V characteristics o f these solar cells have been shown in Figure- 7a. In Figurc-7a, curve- 1 shows the current-voltage characteristic of a single junction a-Si solar cell with Al as back contact. Here, Open circuit voltage (V^ )^, Short circuit cunent (1^ )^, Fill factor OF.F) and efficiency o f the cell are 0.87 V, 14.85 m'X, 0.667 and $.629?’ respectively which are shown in Table-3. Then the ZnO lnyers of different thicknesses have been applied before Al asVoltage (Volts)Figure 7. IV characteristics of (a) single junction and (b) double junction a-Si.H solar cell.back reflector o f solar cell. These film s are deposited under Ar- ambience. For 43 nm thick ZnO interface layer, increases by 12.7% which further increases by 20% when 8 6  nm ZnO layer was used between n/AI interface. But, % increase o f is much lower for 120nm thick ZnO layer (not shown in the graph) as back reflector at n/AI interface. The improvements of performance o f  solar cell using ZnO as back contact arc shown in the Table 3.The FV  characteristics o f  double junction a*Si:H solar cells are depicted in Figure-7b. The double junction solar cell with Al as back contact (shown in Figure-7b, curve-1) shows open circuit

Thickness Dependence of the Properties of Magnetron Sputtered Zno : A1 Films and its Application in a-Si:H Thin Film Solar Cell

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Thickness Dependence of the Properties of Magnetron Sputtered Zno : A1 Films and its Application in a-Si:H Thin Film Solar Cell

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