IJCRR

IJCRR - 5(20), October, 2013

Pages: 08-15

Date of Publication: 02-Nov-2013

EFFECT OF MN/NI/CU DOPING ON THE STRUCTURAL AND OPTICAL PROPERTIES OF ZNS THIN FILMS

Author: C. Gunasekaran, V. Senthil Kumar

Category: General Sciences

Abstract:ZnS films were deposited by spray pyrolysis at 450 0C on glass substrates. In order to study the influence of manganese, nickel and copper on the properties of ZnS film, undoped and Mn/Ni/Cu \? doped films were synthesized and investigated using x-ray diffraction, scanning electron microscopy, energy dispersive x-ray spectroscopy, ultra violet\?visible absorption transmission spectroscopy and photoluminescence studies. The absorption coefficient was measured and correlated with the photon energy to estimate the energy gap, which varies with the different dopant. x-ray diffraction analysis revealed the polycrystalline zinc sulphide film with cubic structure and a preferential orientation along (111) plane. The size of the zinc crystallites was determined using the full width half maximum values of Bragg peak. The crystallite size and the refractive index of the films increased when the film thickness increased. The absorption edge shifting towards visible region on doping.

Keywords: Zinc sulphide, Spray pyrolysis, Dopant, Optical properties, Refractive index.

Full Text:

INTRODUCTION

Zinc Sulphide (ZnS)  is  a wide-band-gap  semiconductor and n-type conductivity which  crystallises in both  cubic  and  hexagonal  forms,  with a  range  of  potential applications in  optoelectronic devices,  such  as electroluminescent  devices and  photovoltaic cells.  It is  an  excellent  host  material  for  electroluminescent phosphors  and is  being  commercially   used in  emitting  layers  for  electroluminescent  displays[1,2].  In  the opto-electronics, it can be used  as light emitting diode  in  the  blue  to ultraviolet  spectral  region  due  to its  band  gap (3.7 eV) at room temperature.  It  is  Known  that the electrical  conductivity  of ZnS films is  too low to act as a substrate  for  transistors,  however  it can  be  used  as light  source display  screens  and  buffer  layer films  for Cu(In,Ga)(S,Se)₂ solar cells[3,4].  Several  techniques  such  as  sputtering[5], metal-organic vapour phase epitaxy [6], spray pyrolysis [7], and  chemical bath deposition(CBD) [8]  have been  used  to  ZnS  thin  films. 

Among  them,  the chemical  spray  pyrolysis (CSP)  is  especially  suitable,  since it has been proved  to  be  a  simple and  inexpensive  method,  particularly  useful  for large area coating applications.  CSP  technique  has  been developed in 1966  by  Chamberlin and Skarman  for  the  deposition of  CdS  and  CdSe  films.   Nowadays  it  is  widely  used  to synthesize a  variety of  metals oxides as  well  as  binary and  ternary  chalcogenides  in  different  forms  like  dense or porous  thin films  and  powders [14].  Materials  obtained  by  CSP  find  a  wide  range  of  applications  in solar cells,  optoelectronic devices,  and in  emitting,reflective  coatings, sensors, etc.,

Mn/Ni/F  doping  has been  applied  to  some  transparent    semiconducting  films  such as  CdO[9],  CdS[10],  ZnO[11],  SnO₂[12].  It   was   reported  by the references indicated  above  that  the     electro-optical  properties  of  these  films  improved  by  fluorine-doping.  The  use  of  ZnS thin films is  still  limited  to  UV light  due  to  its large  band gap, 3.6 eV [13].  Therefore,  many studies  have  been  carried out  to  develop  a visible –light  active  ZnS  thin films  through  the  doping of   metal  ions.  Doping  of  ZnS  thin films  by  the  transition metal  ions   Mn²⁺[14,15],  Cu²⁺ [16,17]  and Ni²⁺ [18],  has  received  considerable  attention  in  applications  in  electroluminescence  devices,  phosphors, light  emitting  displays  and  optical  sensors.  The  present  research  work deals  with  the  fabrication  and  characterization  of  ZnS  thin films  doped  with  Mn/Ni/Cu  by  spray  pyrolysis technique.  The observed   optical, structural and morphological properties are discussed in detail.

MATERIALS AND METHODS

Computerized  spray pyrolysis technique  was  employed  for  the  synthesis  of  zinc sulphide thin  films  in  air.   For  undoped  ZnS  films, the  initial  solution  is  prepared  from zinc  chloride (Zncl₂)  at 0.2 M  and  thiourea [SC(HN₂)₂]  at  0.2 M in deionized  water.  One drop of ammonia is added   for   good mixing of solution.  The solution is stirred   for two hours using magnetic stirrer.   The  prepared  solution  was  sprayed  with  a  steel  needle  onto  the  glass  substrates with  a spray  rate  of  about  2ml/min  and  a  growth  rate  in  the  range  of  few  nm/min  using  air  as  a  carrier  gas.  The needle-to-substrate   distance was maintained at approximately 15 cm.  The substrate temperature was maintained at 450^? C   at  atmospheric pressure.    The  thermocouples  and  heating  elements  are  connected  with  a  temperature  controller.   The  compressed  air  was allowed  to  atomize  the  solution  containing  the  precursor  compounds  through  a  needle  onto  the  heated  glass  substrate.  The  motion  of  the  needle  was  controlled  by stepper  motor,  which  is  connected  to  the  computer  system.   The  harmful  fumes  evolved  during  the  deposition  were  expelled  out  using  external  exhaust  system.  Undoped  ZnS  films  obtained  in  this  manner  had  thickness  ranging  from  approximately  500 – 800 nm  and  exhibited  good  adherence  to  the  substrate  surfaces. 

Synthesis  of  Mn/Ni/Cu  doped  Zns  thinfilms:  For  this  doped  ZnS films,  (i) Mn:  The  cationic  solution  is  prepared  from  zinc  chloride(0.2 M)  and  thiourea(0.2M)  and  manganous  chloride  at  6 wt%  were  dissolved  separately  in de-ionised   water.   One drop of ammonia is added   for complete dissolution.   The solution is stirred   for  two  hours  using  magnetic  stirrer.  (ii) Ni:The solution  is  prepared  from  zinc  nitrate(0.1M),  nickel  nitrate (0.1M), ammonium nitrate(0.1M, buffer  solution),  sodium  citrate(0.1M, complexing  agent)  and  thiourea (0.1M)  were  added   sequentially  under  constant  stirring  for  two hours  using  magnetic  stirrer.  (iii)Cu:  The  initial  solution  is  prepared  from  1.36g  Zinc  chloride  and  0.76g thiourea  in  100ml  of  deionized  water. Copper-doping was achieved  by  adding copper sulphate(Cu₂SO₄) to  the  starting  solution.  Thus  prepared  solutions  are  sprayed   onto  the  glass  substrates  using  the  above  procedure.

The  crystal  structural  study  of  these  films  were  examined  by  the  XPERT  PRO diffractometer  using  Cu K_α radiation(k = 1.5406 A⁰).  The  scanning  angle  2θ was  varied  in  the  range  of  10-80  in  steps  of  0.05⁰  at  room  temperature.   From  the  X-Ray  Diffraction  studies,   grain  size, dislocation  density,  strain,  interplanar  spacing,   absorption  coefficient (α)  of  these  thin  films  were  determined  using  a  Perkin-Elmer  Lambda  35  UV/Vis  Spectrometer  with  300-1100  nm  wavelength  range  using  non-polarized  light   by  recording  the  absorption  spectrum   in  the  above  wavelength  range.  The  spectral  data  was  used  to  determine  the  type  of  optical  transition, the extinction  coefficient,  refractive  index, optical  conductivity   and  the   band  gap   present  in  the  sample.  The surface morphology was studied using    JEOL JSM-6390 Scanning   Electron Microscope (SEM).  Scanning Electron Microscope operation voltage was 3 kV.  The  thickness  of  the  films  are  measured  using   PYROHELIOMETER , the  calibration  is  carried  out  at  the  room  temperature.

The  compositional analysis  of  the  films   were   carried  out   using  Energy  Dispersive  X-Ray  analysis (EDAX)  equipment ,  which works  as  an integrated  feature of  JEOL  JSM-6390  Scanning Electron Microscope  system.

RESULTS AND DISCUSSION

Growth Mechanism of ZnS

The formation  of  ZnS  thin films  using  computerized  spray  pyrolysis  method  can  be  explained  that  Zn²⁺,  resulting  from  the  dissociation  of   ZnCl₂  complex  ions, would  combine  with  S^2- ions,  resulting  from  the  hydrolysis  of  the  thiourea  in  a basic  aqueous  solution,  to  form  ZnS  on  the  substrate,  followed  by  a heterogeneous  reaction  and  precipitation[19].   After   the  deposition,   a bright  silvery  ZnS  film  was  observed  on  the  glass  substrates.

The following chemical reaction leads to the formation of ZnS:

The decomposition of the zinc chloride is given by,

ZnCl₂  à Zn²⁺ + Cl^2-

The  decomposition of  the  thiourea  is  given by,

SC(HN₂)₂  +OH^-àSH^-  +CH₂ N₂ +H₂O

SH^-  +OH^-à S^2- + H₂O

The formation of  ZnS  is  as  follows:

Zn²⁺+ S^2-àZnS

Fig.1 (a) – Fig.1 (d)  shows the  x-ray  diffraction pattern  of  undoped   and  Mn/Ni/Cu  doped ZnS  thin films  grown  by  spray  pyrolysis  technique.  The  diffraction   pattern  arising  from  the film  has  a  single  intense  peak  at  _~ 29^?  due  to  the  zinc  blende  (111)  reflection.  For Mn/Ni/Cu doped  ZnS  thin  films ,  the  peaks  are  at  around  29^?, 29.6^?, 28.7^?  respectively. Additional   x-ray  diffraction  peaks  have  been  found  to  correspond  to  (200),(220) and  (311)  planes  of   the  pure  ZnS   cubic  phase(JCPDS  05-0566).  The  lattice   constant   a,   is  calculated  from  the  peak  position  and  is  determined  to be a=5.4060 A^?.  This  indicates  that  the  crystallites   in  the  film  have  a  single  preferred  orientation (111)  and  the  zinc  blende  (111)  plane  was  parallel   to  the  substrate  surface.  This  was  expected  since  the  cubic (111)  lattice  planes  has  the  lowest surface  energy [20].   In  general,  for  a   ZnS  crystal,  the  zinc  blende  structural  phase  is  stable  at  low  temperature  and  the  wurtzite structural  phase  is   stable  at  temperature  higher  than  1023⁰ C [21]. According to Scherrer  formula,  D = 0.94λ / βcosθ,  Where D  is  the  grain  diameter,  λ is  the wavelength  of  the  x-ray,  β  is  the  full  width  half  maximum   of  the  diffraction  peak  expressed  in  radians   and  θ  is  the  Bragg  diffraction  angle.  Using  this  expression,  one  can  calculate  the  grain size  for  undoped  and  Mn/Ni/Cu  doped   ZnS  thin films.  The  magnitude  of  the  crystallite  size  does  not  change  significantly  with   Mn/Cu   doping  in  the  ZnS  thin films.   The  grain  size  values  are  on  the  order  of  8 nm  in  all  cases.  The  grain  size  increases  on  Ni  doping  and  is  of   the  order  of  57 nm.  Since  the  ionic  radius  of Ni²⁺ (0.69 A⁰ ) is  smaller  than  that  of   Zn²⁺ (0.74 A⁰),  it  can  be  inferred  that  nickel  ions  might  insert  into  the  structure  of  ZnS  and  located  at interstices  or  occupied  some  of  the  lattice  sites  of  ZnS.  From  the  x-ray  diffraction  studies,  the  following  values  are  calculated  using   the  corresponding   formulas,  and  tabulated  in  the  below  table.

To  study  the optical properties  of  the  thin films, the  optical  absorption spectra of  the  film  is  recorded  in  the wavelength  range  300-1100 nm  from which the  absorption coefficient,  refractive  index, extinction coefficient, reflectance, optical conductivity and  the band gap  are calculated  from the  respective  formulas  and plotted(fig  2).  The determined values are tabulated in table 2.  To quantify  the  optical  band gap(E_g)  of  films,  the  following  formula  is  employed  in the  high  absorbance region  of  the  transmittance  spectra,

(αhγ)ⁿ   = A(hγ - E_g)

Where  α is  the  absorption coefficient,  A  is  a  constant  which  is  independent  of  photon  energy  and  hγ  is  the  photon  energy,   E_g  is  the  optical  band  gap  and  n has numeric  values (1/2  for  allowed   direct, 2  for  allowed  indirect,  3  for  forbidden  direct  and  3/2  forbidden  indirect  optical  transitions).  In  this  work,  direct  bandgap  was  determined  by plotting   (αhγ)²  versus  hγ  curves  respectively,  with  the  extrapolation of  linear  region  to  low  energies.The following values relating to optical properties are calculated using the corresponding formulas and are tabulated.

The crystallographic structure of doped Ni/Cu/Mn may result in a change of electronic structure, which enables the absorption edge to shift towards longer wavelengths after the addition of Ni/Cu/Mn. The absorption edge shifting towards visible region explained the yellowish colour for the Ni doped ZnS thin films. The red shift in the absorption spectra may be due to larger grain size. The bandgap value of Ni/Mn doped ZnS thin films decreases thus makes it to use it in the visible region. The bandgap value of Cu doped ZnS thin films does not change appreciably.

Thickness Measurements

Thickness is one of the most important thin film parameter since it largely determines the properties of the films. Any physical quantity related to film thickness can in principle be used to measure film thickness.  Methods of monitoring thickness can be categorized as mechanical, electrical, magnetic, radiation, optical, mass diffraction methods etc., of which mass difference methods are most commonly used for measuring thickness of chemically deposited. Electrical methods of film thickness measurements involve film resistance method, capacitance monitor method and ionization method. Important optical methods are photometric, ellipsometric, spectrometer beam interferometer, including  fizeau, FECO and Michelson beam interferometer and polarization interferometer methods. Stylus methods and sectioning are important mechanical methods for thickness determination. Two important mass methods for film thickness measurements are microbalance (gravimetric) method and crystal oscillator methods. In crystal oscillator method thickness measurement depends on the oscillation of quartz crystal when excites and the frequency of its oscillation depend on thickness change in frequency is due to the change in mass due to the deposition of a film on the quartz surface. For our study the thickness of the samples are measured with the help of Pyroheliometer. The calibration is carried out at the room temperature.  In our study, the thickness of the undoped ZnS and Ni,Cu and Mn doped ZnS thin films are 853nm, 1.5μm, 53nm and 279nm respectively.

Scanning Electron Microscope Studies

Figure (3a-d)  shows that the surface of the undoped ZnS thin film is composed of clusters and few particles are found in some ware of cubic and hexagonal shaped nano crystals different dimensions and sizes. The surface of the Ni/Mn doped ZnS film is composed of the rods and sticks shaped nano crystals of different dimensions and sizes. The surface of the Cu doped ZnS film is composed of the small clusters   and small rods. It should be noted that the average D values deduced from the scherrer formula are much lower than the sizes observable in the scanning electron microscope pictures. However, grains in the sizes less than a 100nm are also evident in the micrographs. The observed discrepancy can be probably due to the non spherical geometry of the nano crystallities. Infact, it is anticipated in the case of nano crystalline thin films that the domains have a tendancy to increase its size near the film surface, thus scanning electron microscope images representing the surface features of the film give the maximum  possible  size  of  the grains. On  the other hand, the  D values calculated using the  x-ray  diffraction data is thickness averaged magnitude, which usually dominated by the smallest crystallities. Similar discrepancy has been reported for various thin films. Grain sizes of these images are not homogeneously distributed as seen from this figure. Larger clusters of about 360nm and 650nm in size are formed; the clusters themselves are aggregate of grains. The apparent smaller size of ZnS grain and clusters in the thin films compared to that of the precipitate may be attributed to the geometric restriction in the aggregation process in the case of the film growth and that the smaller grains or clusters have high energy surface enough to stick to the surface of the substrate, due to this reason, the boundaries can’t be seen in the distinguished manner. It is good looking to note that the particles are regular and are of uniform sized distribution of about some nanometers.  The morphological characterization revealed that the deposition temperature has an important role on the growth mechanism of both the thin films, so that with increasing temperature, the size of the building units(grains and clusters) of the films increases. It is to note that  x-ray  diffraction analysis reaveals the fact that grains themselves must be coalescene of nanocrystals of about 4-12 nm in the size as given in table.

Composition Analysis [EDAX]

Chemical composition of undoped and Ni doped  ZnS thin films was analyzed by Energy Dispersive X-Ray Analysis[EDAX]. Composition Analysis conforms the presence of Zn and S in Undoped ZnS thin films and also conforms the presence of Ni/Cu/Mn in the doped ZnS thinfilms. The atomic ratio(x) of Ni to ZnS determined from the relation, X= [(Ni/Mn/Cu)/(ZnS+(Ni/Mn/Cu))] to be x=0.1% for Ni/Mn and x=0.02% for Cu. Energy Dispersive X-Ray analysis shows how much amount of Ni atom has really entered into the ZnS thin films. Both atomic percentage(at%) and weight percentage(wt%) shown in the  table   proves the ratio of Ni/Mn/Cu dopant in the film. Therefore composition analysis conforms the incorporation of Ni element in the film. The composition of the pure ZnS films there is found to be more or less the same as doped ZnS and there was no significant variation in the composition.

CONCLUSION

Thin films of undoped and Ni/Mn/Cu doped ZnS are successfully prepared on glass substrates using spray pyrolysis technique. The films are characterized by means of scanning electron microscopy, x-ray diffraction, ultraviolet visible spectroscopy, composition analysis(energy dispersive x-ray analysis) and thickness measurements. x-ray diffraction studies show that ZnS films grown on glass preferentially orientation along the (111) direction. scanning electron microscope images confirm the formation of uniform grains for all samples. It has been established that the addition of Ni/Mn/Cu affects the growth mechanisms of films, the crystallographic structure and the absorption spectra. In the case of undoped ZnS thin film, sphere, hexagonal like branches are found, with the cubic structure have   been   absorbed  by  scanning  electron  microscope  and  x-ray  diffraction. The composition analysis confirms the presence of Ni/Mn/Cu, Zn and S. The grain size is found to be 8nm and 57nm for undoped and Ni doped thin films  and remained unchanged on doping with Cu/Mn, from x-ray diffraction analysis. The band gap is found to be 3.94eV and 3.23eV, 3.18eV, 3.96eV for undoped and Ni,Mn,Cu doped thin films  respectively as calculated from UV studies.

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