Effect of Mn doping concentration on the electronic transitions of ZnO thin fi lm

FONDAZIONE GIORGIO RONCHIhttp://ronchi.isti.cnr.it

Estratto da: Atti della Fondazione Giorgio RonchiAnno LXVIII, n. 5 - Settembre-Ottobre 2013

Tip. L’Arcobaleno s.n.c. - Via Bolognese, 54 - Firenze2013

SAAD FARHAN OBOUDI, SAMI SALMANN CHIAD,SABAH H. JUMAAH, NADIR FADHIL HABUBI

Effect of Mn doping concentration on the electronic transitions of ZnO thin film

ANNO LXVIII SETTEMBRE-OTTOBRE 2013 N. 5

EDITORIAL BOARD

Pubblicazione bimestrale - Prof. LAURA RONCHI ABBOZZO Direttore ResponsabileLa responsabilità per il contenuto degli articoli è unicamente degli Autori

Iscriz. nel Reg. stampa del Trib. di Firenze N. 681 - Decreto del Giudice Delegato in data 2-1-1953

Tip. L’Arcobaleno - Via Bolognese, 54 - Firenze - Ottobre 2013

Prof. Roberto BuonannoOsservatorio Astronomico di RomaMonteporzio Catorne, Roma, Italy

Prof. Ercole M. GloriaVia Giunta Pisano 2, Pisa, Italy

Prof. Franco GoriDip. di Fisica, Università Roma IIIRoma, Italy

Prof. Vishal GoyalDepartment of Computer SciencePunjabi University, Patiala, Punjab, India

Prof. Enrique Hita VillaverdeDepartamento de OpticaUniversidad de Granada, Spain

Prof. Irving KaufmanDepartment of Electrical EngineeringArizona State University, TucsonArizona, U.S.A.

Prof. Franco LottiI.F.A.C. del CNR, Via Panciatichi 64Firenze, Italy

Prof. Tommaso MaccacaroDirettore Osservatorio Astronomico di Brera,Via Brera 28, Milano

Prof. Manuel MelgosaDepartamento de OpticaUniversidad de Granada, Spain

Prof. Alberto MeschiariScuola Normale Superiore, Pisa, Italy

Prof. Riccardo PratesiDipartimento di FisicaUniversità di Firenze, Sesto Fiorentino, Italy

Prof. Adolfo PazzagliClinical PsychologyProf. Emerito Università di Firenze

Prof. Edoardo ProverbioIstituto di Astronomia e Fisica SuperioreCagliari, Italy

Prof. Andrea RomoliGalileo Avionica, Campi BisenzioFirenze, Italy

Prof. Ovidio SalvettiI.ST.I. del CNRArea della Ricerca CNR di Pisa, Pisa, Italy.

Prof. Mahipal SinghDeputy Director, CFSL, Sector 36 AChandigarh, India

Prof. Marija StrojnikCentro de Investigaciones en OpticaLeon, Gto Mexico

Prof. Jean-Luc TissotULIS, Veurey Voroize, France

Prof. Paolo VanniProfessore Emerito di Chimica Medicadell’Università di Firenze

Prof. Sergio VillaniLatvia State University, Riga, Lettonia

A T T IDELLA «FONDAZIONE GIORGIO RONCHI»

Effect of Mn doping concentration on the electronic transitions of ZnO thin fi lm

SAAD FARHAN OBOUDI (*), SAMI SALMANN CHIAD (**),SABAH H. JUMAAH (**), NADIR FADHIL HABUBI (**)

SUMMARY. – We have prepared Manganese-doped Zinc oxide thin fi lms on glass substrates by spray pyrolysis technique. The atomic percentage of Mn dopant in solution was 2% and 4%. The effect of Mn doping on the electronic transitions of ZnO fi lms was studied. The band gap of the ZnO fi lms slightly widens with increasing the Mn concentration. The band gap of un-doped sample was 3.2 eV, this value increased slightly with increasing doping concentration and became 3.28 eV for ZnO: 2% Mn and 3.34eV for ZnO: 4% Mn.

Key words: Mn doped ZnO, spray pyrolysis, transparent conducting oxide (TCO).

1. Introduction

Zinc oxide (ZnO) is a well-known optoelectronic material which belongs to the wide-band-gap semiconductor family having an energy gap of 3.37 eV with a relatively large exciton binding energy (~ 60 meV), which makes it a good can-didate that has potential applications in various fi elds such as varistors (1), opto electronic applications (2), liquid crystal displays, heat mirrors and multilayer photo-thermal conversion systems (3), transparent transistors (4,5). ZnO thin fi lm is also used in the fabrication of solar cells (6), catalysers (7) and gas sensors (8). In the form of thin fi lm, ZnO is a very promising alternative in fl at display screens (9). When it is doped, ZnO fi lm presents promising second-order nonlinear opti-cal properties, which achieve the giant nonlinear optical effects about 50 pm/V (10, 11).

(*) Baghdad University, College of Science, Physics Department, Baghdad, Iraq.(**) Al_Mustansiriyah University, College of Education, Physics Department, Baghdad,

Iraq; e-mail: [email protected]

ATTI DELLA “FONDAZIONE GIORGIO RONCHI” ANNO LXVIII, 2013 - N. 5

S.F. Oboudi - S.S. Chiad - S.H. Jumaah - N.F. Habubi718

ZnO thin fi lms have been deposited on different substrates by using dif-ferent techniques such as RF magnetron sputtering (12), spray pyrolysis method (13), electro-deposition process(14), pulsed laser deposition (PLD) (15), sol-gel process (16), and molecular beam epitaxy (17). Spray pyrolysis, among these methods, is an excellent method for the deposition of thin fi lms of metallic oxides. The wide band-gap ZnO semiconductor doped with transition metals (Mn, Co, Fe, V) became one of the most extensively studied materials since the prediction of Dietl et al. (18) of a Curie temperature above the room temperature and of a spin polarization of 100%. In this work, undoped and Mn-doped ZnO fi lms have been prepared by using spray pyrolysis technique. The doping is achieved by re-placing Zn2+ atoms with Mn2+ atoms, and the effects of Mn doping concentration on the electronic transitions of ZnO fi lms are reported. It is intended the determi-nation of optimum conditions that leads to the manufacturing of well crystallized, conductive and transparent ZnO:Mn thin fi lms.

2. Experimental details

Thin fi lms of zinc oxide have been prepared by chemical pyrolysis meth-od. The spray pyrolysis was done by using a laboratory designed glass atomizer, which has an output nozzle about 1 mm. The fi lms were deposited on preheated glass substrates at a temperature of 450°C, the starting solution was achieved by an aqueous solution of 0.1M zinc acetate dehydrate (Zn(CH3COO)2.2H2O) provided from Merck company/Germany and 0.1M manganese chloride MnCl2 from BDH/England, used as a doping agent with a concentration of 2% and 4%. These materials were dissolved in deionized water and ethanol, few drops of HCl were added to speed up the dissolving, formed the fi nal spray solution and a total volume of 50 ml was used in each deposition. With the optimized conditions that concern the following parameters, spray time was 10 sec and the spray interval (3 min) was kept constant. The carrier gas (fi ltered compressed air) was maintained at a pressure of 105 Nm–2, the distance between nozzle and the substrate was about 28 cm ±1 cm, the solution fl ow rate 5 ml/min. Thickness of the sample was measured using the weighting method and was found to be around 0.3 µm. Optical transmittance and absorbance were recorded in the wavelength range 300-1000nm using UV-visible spectrophotometer (Shimadzu Company Japan). Optical transmittance and absorbance were reported in order to fi nd the effect of doping on the parameters under investigation.

3. Results and discussion

The absorption coeffi cient (α) could be calculated using the following rela-tion (19):

Effect of Mn doping concentration … 719

[1]

where A is the absorption and t is the fi lm thickness.Figure 1 shows the dependence of the absorption coeffi cient α on the pho-

ton energy of Zn1-xMnxO fi lm prepared with a Mn composition of 2% and 4%, deposited at 450ºC. The absorption coeffi cient decreases with the increasing of Mn dopant concentration. This decrease could be due to the presence of struc-tural disorder and/or formation of defects caused by the substitution of Mn ions in ZnO lattice. In general, structural defects or disorder led to the creation of local electric fi elds that affect the band tails. Similar observations were reported in the literatures (20, 21). The optical absorption studies revealed that the absorption coeffi cient α increased with the increase in composition up to 4%.

α =

2.303 At

FIG. 1

Absorption coeffi cient versus wavelength of ZnO and ZnO:Mn thin fi lms.

In order to calculate the band gap of the un-doped and Mn doped Zno fi lms, we used the Tauc’s relationship (22) as follows:

[2]

where α is the absorption coeffi cient, A a constant, h is Planck’s constant, υ the photon frequency, Eg the optical band gap and n is 1/2 for a direct band gap semiconductor. An extrapolation of the linear region of a curve of (αhυ)2 on the y-axis against the photon energy (hυ) on the x-axis gives the value of the band gap Eg. According to the absorption spectra measured, (αhυ)2 versus hυ curves of un-doped and doped ZnO fi lms were plotted and the band gap values were evaluated. Figures (2-4) shows the dependence of the band gap values on the dop-ing concentration of Mn in the ZnO fi lms. Band gap value of 3.2, 3.28, 3.34 eV

αhν( )2

= A hν −Eg( )n


were obtained for the un-doped and Mn doped ZnO fi lms. It can be seen that the energy band gap of the un-doped fi lm is about 3.2 eV and this increases to 3.34 eV with the increase in the Mn content. The variation of band gap of Zn1-xMnxO fi lms with increasing Mn content was found to be linear, which is due to the fact that the band gap of MnO (4.2 eV) is greater than that of ZnO (3.3 eV) (23).

FIG. 2

(αhυ)2 for undoped ZnO thin fi lm versus photon energy.

FIG. 3

(αhυ)2 for ZnO:2%Mn thin fi lms versus photon energy.

The optical conductivity was calculated using the relation (24):

[3]

where c is the velocity of light.Figure 5 shows the variation of optical conductivity with the wavelength. It is

observed that the optical conductivity decreases with the increasing of Mn doping concentration to 4%. It can be noticed from the fi gure that the optical conductiv-

σ =

αnc4π


ity for all fi lms increases in the high photon energies region and decreases in the low photon energy region: this decrease is due to the low absorbance of the fi lms in that region.

FIG. 4

(αhυ)2 for ZnO:4%Mn thin fi lms versus photon energy.

FIG. 5

Optical conductivity versus wavelength for ZnO and ZnO:Mn fi lms.

The skin depth could be calculated using the following relation (25):

[4]

where λ is the wavelength of the incident photon, and k is the extinction coef-fi cient. Fig. 6 shows the variation of skin depth as a function of wavelength for all fi lms. It is clear from the fi gure that the skin depth increases as the wavelength increases: this behavior could be seen in all samples, but the skin depth increases as the doping concentration increases, so the skin depth is a transmittance related.

χ =

λ

2πk


4. Conclusion

Transparent oxide semiconductor thin fi lms of zinc oxide and Mn-doped zinc oxide have been successfully prepared onto a glass substrate by the chemical spray pyrolysis technique. Both doped and undoped samples were characterized using UV-VIS technique and the results were systematically presented. The ab-sorption coeffi cient and the optical band gaps were calculated. The optical band gap is found to increase with the increasing of Mn dopant concentration and has the values of 3.2, 3.28 and 3.34 eV for ZnO, ZnO:Mn 2% and ZnO:Mn 4% fi lms respectively.

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FIG. 6

Skin depth versus wavelength of ZnO and ZnO:Mn thin fi lms.


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Atti dellA “FondAzione GiorGio ronchi” Anno lXViii, 2013 - n. 5

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Effect of Mn doping concentration on the electronic transitions of ZnO thin fi lm

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