IN

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Journal of Analytical and Applied Pyrolysis 97 (2012) 181–188

Contents lists available at SciVerse ScienceDirect

Journal of Analytical and Applied Pyrolysis

journa l h o me page: www.elsev ier .com/ locate / jaap

nvestigation of structural, optical and luminescent properties of sprayed-doped zinc oxide thin films

.S. Shindea, P.S. Shindea, Y.W. Ohb, D. Haranathc, C.H. Bhosalea, K.Y. Rajpurea,∗

Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, IndiaDepartment of Nano-Engineering, Kyungnam University, Masan 631-701, Republic of KoreaCSIR-National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110 012, India

r t i c l e i n f o

rticle history:eceived 31 March 2012ccepted 16 June 2012

a b s t r a c t

N-doped ZnO (NZO) thin films are synthesized via spray pyrolysis technique in aqueous medium treat-ing zinc acetate and N,N-dimethylformamide as precursors. Influence of N doping on structural, opticaland luminescence properties have been investigated. Films are nanocrystalline having hexagonal crystal

vailable online 1 July 2012

eywords:emiconductorspray pyrolysis-doped ZnO

structure. Raman analysis depicts an existence of N Zn O structure in NZO thin film. XPS spectrum ofN 1s shows the 400 eV peak terminally bonded, well screened molecular nitrogen (�-N2). Lowest directband gap of 3.17 eV has been observed for 10 at% NZO thin film. The UV, blue, and green deep-levelemissions in photoluminescence of NZO films are due to Zn interstitials and O vacancies.

© 2012 Elsevier B.V. All rights reserved.

ptoelectronic properties

. Introduction

Zinc oxide is having a huge impact on materials scienceecause of its high exciton binding energy (60 meV), wide bandap (3.25 eV) and high transparency. It is a promising materialhat could be applied in many fields such as transparent con-uctive contacts, solar cells, laser diodes, ultraviolet lasers, thinlms transistors, piezoelectric devices and photocatalyst [1–4].ature provides a multiplicity of materials, architectures, systemsnd functions with many inspiring properties such as sophistica-ion, miniaturization, hierarchical organizations, adaptability andnvironment-response. Mimicking the elaborate architectures andasic principles to design and make more reliable and efficientaterials or systems is highly appealing. One of the most promisingaterials that has been synthesized is N-doped ZnO (NZO). Since

ts pioneering work, NZO has received a lot of attention because themplantation of nitrogen modifies the electronic structure by intro-ucing localized states to the top of the valence band, narrowinghe band gap.

Transparent conducting oxide (TCO) materials are naturally-type degenerate semiconductors and the lack of a high qual-

ty p-type TCO always has been the main obstacle before the

abrication of a fully transparent devices. Among the candidatesf shallow acceptors, nitrogen is the most tried one due to itsearest-neighbor bond length of 1.88 A that is similar to the Zn O

∗ Corresponding author. Tel.: +91 231 2609435; fax: +91 231 2691533.E-mail address: rajpure@yahoo.com (K.Y. Rajpure).

165-2370/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jaap.2012.06.007

bond length of 1.93 A [5]. Although successful reports have beenmade in the preparation of ZnO by nitrogen using various depo-sition methods such as sputtering [6], chemical vapor deposition,MOCVD [7], PLD [8] and spray pyrolysis [9,10], some doubtsemerge since most of the reported data refer to high resistive films.Moreover, it is difficult to find a metal with a work function highenough to make the proper ohmic contact. So, it is very difficultto prepare a high quality N-doped ZnO due to self compensationeffect and inactivation of the acceptor dopants in the ZnO films[11]. Among the abundant deposition methods, spray pyrolysis isa viable method to prepare stable and conductive films, due to itslow density of oxygen vacancy and consequently limited hydrogencompensation after film formation [9]. Spray parameters such assubstrate temperature, solution concentration, solution quantity,doping concentration play important role on the film properties. Itprovides easy way to dope any element in any proportion into thehost lattice with a meager quantity of solute. Doping concentrationenhances the electrical conductivity due to generation of degen-erate semiconductors, incorporation of defect states etc. In thispaper, the influence of nitrogen doping on the structural, morpho-logical, compositional, optical, luminescence properties of spraydeposited NZO thin films in aqueous media have been discussed.

2. Experimental

Nitrogen doped zinc oxide thin films were prepared onto thecorning glass substrates by using chemical spray pyrolysis tech-nique in aqueous medium. To deposit NZO thin films, zinc acetate

1 l and Applied Pyrolysis 97 (2012) 181–188

(asawaarwrsdhswvsne

torbl–uwrMuprbd1LsfiaomNitrcaratnlato

sZgdFt1wc

A

V

Photoelectrode

hν > EgCounter electrode

Electrolyte

the defect levels responsible for recombination losses, as comparedto the pure ZnO. Upon illumination of junction, the magnitude ofVoc increases with negative polarity towards the NZO thin films,indicating cathodic behavior of photovoltage which confirms

0 5 10 15 200.40

0.42

0.44

0.46

0.48

0.50

0.52

0.54

0.40

0.42

0.44

0.46

0.48

0.50

0.52

0.54

0.56 I

sc

Voc

I sc

(mA

)

Vo

c (V

)

82 S.S. Shinde et al. / Journal of Analytica

Zn(CH3COO)2·2H2O, AR grade, 98.8% pure) supplied by HIMEDIAnd N,N-dimethylformamide (HCON(CH3)2, AR grade, 99% pure)upplied by THOMAS BAKER were used as initial ingredients. Tottain nitrogen doping, N,N-dimethylformamide (HCON(CH3)2)as mixed in to the solution. The [N]/[Zn] ratios calculated on

tomic percent used in the starting solution were 1%, 5%, 10%, 15%nd 20%. It was observed that the [N]/[Zn] ratios higher than 20%esulted in degraded films probably due to a reaction of the solutionith excess nitrogen. The actual [N]/[Zn] ratios in the film were

elatively less than in the solution. The resulting 100 cc precursorolution was sprayed onto the preheated corning glass and fluorineoped tin oxide (FTO) substrates of size 0.125 cm × 2.5 cm × 7.5 cmeld at optimized substrate temperature of 450 ◦C through apecially designed glass nozzle. The compressed air (2.3 kg cm−2)as used as carrier gas at a constant spray rate of 5 cc min−1. While

arying the doping concentration, other preparative parametersuch as solution concentration (0.1 M), volume of solution (100 cc),ozzle-to-substrate distance (32 cm) were kept constant for allxperiments.

The structural properties were studied by a Philips X-ray diffrac-ometer PW −1710 (�, 1.5405 A) using Cu-K� radiation in the rangef 20–70◦. Raman-scattering experiments were performed in air atoom temperature with micro Raman system from Jobin Yvon Hori-ra LABRAM-HR visible within 200–1400 cm−1 using Argon 488 nm

aser as an excitation source. The 600 and 1800 lines/mm gratings detector: CCD detector was used. XPS spectra were recorded bysing an ESCA spectrometer (ULVAC-PHI 5600). The measurementsere performed for a freshly cleaved sample surface. The energy

esolution of the spectrometer was about 1 eV at pass energy 50 eV.onochromatized X-ray (K�: 1486.61 eV) from an Al anode was

sed for excitation. An electron flood gun was employed to com-ensate for the sample charging under X-ray irradiation. The overallesolution was about 0.5 eV under our experimental conditions. Thease pressure in the sample chamber was less than 6.0 × 10−8 Pauring the measurements. XPS core-levels were calibrated using Cs peak. The deconvolution of the XPS peaks was performed usingorentzian fitting. Zn 2p, O 1s, N 1s and valence band lines wereystematically recorded. The morphological characterization of thelms was observed by using SEM (Model: JEOL JSM 6360, Japan)nd FE-SEM (Model: JSM-6701F, Japan). The surface topographyf thin films was further analyzed from the AFM images taken byeans of the atomic force microscopy (AFM, Digital Instrument,anoscope III) operated at room temperature, respectively. AFM

mages were collected in contact mode on a molecular imaging sys-em using a silicon nitride cantilever. All values for feature heights,oot mean square (RMS) surface roughness, and surface area werealculated using digital instruments software. Optical transmissionnd reflectance measurements were carried out in the wavelengthange 350–850 nm using Spectrophotometer Systronic Model-119nd Spectroscopic Reflectometer StellerNet Inc., USA. The roomemperature PL spectra were recorded using a Perkin-Elmer lumi-escence spectrometer (model: LS55) equipped with a Xenon flash

amp and a grating to select the source of excitation. The excitationnd emission spectra were recorded in the fluorescence mode overhe wavelength range 350–700 nm using an excitation wavelengthf 225 nm.

The photoelectrochemical (PEC) cell was fabricated by using atandard two-electrode configuration (Fig. 1), comprising of n-typenO deposited onto the FTO substrate as an active photoanode andraphite as a counter electrode. Schematic experimental circuitiagram of PEC cell formed with n-ZnO photoelectrode is shown inig. 1. The aqueous 0.1 M NaOH was used as an electrolyte. These

wo electrodes are fitted in the bakelite holder having thickness

cm and diameter 3.5 cm and fixed in cylindrical corning glass cellith capacity 30 cm3. The distance between photoelectrode and

ounter electrode was 0.5 cm and exposed area of UV illumination

Fig. 1. Schematic diagram of PEC cell.

was 1 cm2. Then we measured the short circuit current (Isc)and open circuit voltage (Voc) between photoanode and counterelectrode in dark and under illumination.

3. Results and discussion

3.1. Photoelectrochemical (PEC) characterization

The variation of short circuit current (Isc) and open circuitvoltage (Voc) as a function of nitrogen doping concentration isas shown in Fig. 2. Both Isc and Voc increases gradually withdoping concentration, attains a maximum value (Isc = 0.52 mA andVoc = 0.562 V respectively) at 10 at% N doping and then decreasesfor higher doping concentrations. This is attributed to the changein metastable N-on-O substitution (NO), which may attract anotherN to form a (N2)O donor or leave the O site to diffuse in the ZnO,at the same time, generate a VO donor [12]. Generally, nitrogencan be substituted inside the ZnO in two forms: atomic nitrogenon oxygen sites (NO) acting as acceptors and molecular nitrogenon oxygen sites (N2)O acting as donors. Formation enthalpy of(N2)O is lower than (NO) which leads to n-type conduction of NZOthin films. The N N atoms have strong bonding energy comparedto N Zn bonding energy [13,14]. The improvement in the PECproperties of the doped thin films is due to (i) morphological mod-ifications that enhance the active surface area and (ii) quenching of

Doping Conce ntra tion (at%)

Fig. 2. Variation of Isc and Voc against doping concentration for N-doped ZnO thinfilms.

S.S. Shinde et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 181–188 183

30 40 50 60 70

1 at% NZO

20 at% NZO

15 at% NZO

10 at% NZO

5 at% NZO

Pure ZnO

(11

2)

(10

3)

(11

0)

(10

2)(1

01

)

(00

2)

(10

0)

2θ (Degr ee)

Inte

ns

ity

(A

.U.)

Fd

noa

3

di((‘fwhehi1easDiseNodt

tmtes[

ˇ

wlXT

ı

200 40 0 60 0 80 0 100 0 120 0 140 05000

10000

15000

20000

25000

30000

35000

NZO

A1(2L O)

A1(2TO)

A1(LA+LO)

A1(TA+LO)

A1(LO)

E2

high

A1

E2

Low

Ra

ma

n I

nte

ns

ity

(C

ou

nts

)

-1

ZnO

ig. 3. XRD patterns of N-doped ZnO thin films prepared for different nitrogenoping concentrations.

-type behaviour of the films. The dependence of conduction typen N doping suggests that the concentration of native defects, suchs oxygen vacancies (VO) or zinc interstitials (Zni) is more.

.2. X-ray diffraction (XRD) study

Fig. 3 shows the X-ray diffraction patterns of NZO thin filmseposited on corning glass substrates for various nitrogen dop-

ng concentrations. Films are nanocrystalline having hexagonalwurtzite) crystal structure with preferred orientation along the0 0 2) and (1 0 1) reflections. Comparison of standard and observedd’ values of NZO thin films is carried out using of Joint Committeeor Powder Diffraction Standards (JCPDS) card No. 05-0664. Someeak reflections such as (1 0 0), (1 0 2), (1 1 0), (1 0 3) and (1 1 2)ave also been observed but with relatively small intensities. Noxtra phases related to Zn or N compounds are observed even atigh [N/Zn] content. The orientation of (0 0 2) and (1 0 1) planes

ncreases with increase in nitrogen doping concentration up to0 at% and then decreases for higher doping concentrations. How-ver, the position of (0 0 2) peak is slightly shifted toward lowerngle due to the fact that atomic radius of N is greater than O andmaller than Zn, it is suggested that N is substituted on O sites.ecrease in intensity for higher doping concentration is due to the

ncorporation of N in to ZnO lattice at interstitial positions or sub-titutional sites producing considerable contraction of c-axis andxpansion of a-axis lattice constants as a result of tensile strain inZO thin films. Compared with the undoped ZnO film, the intensityf the (0 0 2) peak is lower for the N doped ZnO film. This may beue to the incorporation of N dopant into the ZnO film being ableo create more defects in the lattice [15].

The average crystallite size of NZO thin films is calculated usinghe Scherrer’s relation [16]. The average value of full width at half

aximum (FWHM) increases (i.e. crystallite size decreases) upo 10 at% NZO and then decreases continuously (i.e. crystallinitynhances). Average crystallite size varies from 33 to 43 nm. Thetrain generated in deposited films is calculated by using formula17],

=[

�

D cos �

]− [ε tan �] (1)

here D is the crystallite size, is the broadening of the diffractionine measured at half of its maximum intensity (rad) FWHM, � is the

-ray wavelength, � is Bragg’s diffraction angle and ε is the strain.he dislocation density is calculated using the formula [17],

= 1D2

(2)

Raman Sh ift (cm )

Fig. 4. Raman spectra of ZnO and typical 10 at% N-doped ZnO thin films.

where ı is the dislocation density. The dislocationdensity (9.18 × 1014 lines m−2) and strain increases(1.58 × 10−3 lines−2 m−1) with doping concentration up to 10 at%and then decreases for higher doping level.

3.3. Raman spectroscopic analysis

We present an overview of the first-order Raman scattering inZnO films, and compare our results with previously reported data.We have performed polarized Raman scattering measurements inall Raman active modes of ZnO (see Table 1). The Raman tensors foreach of these modes are given by,

E(1)2 =

(d 0 0

0 −d 0

0 0 0

)E(2)

2 =

(0 d 0

d 0 0

0 0 0

)

E1(x) =

(0 0 c

0 0 0

c 0 0

)E1(y) =

(0 0 0

0 0 c

0 c 0

)A1(z) =

(a 0 0

0 a 0

0 0 b

) (3)

For polar modes, the coordinate in parentheses denotes thedirection of phonon polarization. As derived from the Raman ten-sors of the respective modes, all Raman active modes can beobserved in backscattering geometry except for the E1(LO) mode,for which only the polarizability components involving z and thedirection of phonon polarization change during the vibration. Inorder to prove the existence of N Zn O structure in NZO thin film,Raman spectroscopy measurements are performed. Fig. 4 showsthe room temperature micro-Raman spectra of the pure and typi-cal 10 at% NZO thin film. It shows the peaks at ∼100, 216, 287, 330,357, 382, 402, 437 and 488 cm−1 in the low wave-number regionhaving acoustic combinations. On the other hand, peaks located at993, 1037, 1099, 1134 and 1161 cm−1 are found in the high wave-number region from 820 to 1200 cm−1 having optical combinationsand overtones. The 571, 667, 762, 781 and 803 cm−1 peaks are foundin the intermediate frequency region from 540 to 820 cm−1 havingoptical and acoustic phonon combinations.

No other higher order peaks are observed in the region above1200 cm−1. The results and its comparison with the previous workare listed in Table 1, from which a good agreement is evident [18].The presence of prominent E2 modes in film indicates that thenitrogen doping does not change the wurtzite structure of ZnO lat-tice. The EHigh

2 mode observed at 437 cm−1 is accompanied with

ELow

2 mode at 100 cm−1 and 2E2 (M) at 330 cm−1. The character-istic modes observed at 216 and 330 cm−1 are originated frommulti phonon processes [19]. Raman modes located at 287, 571and 667 cm−1 are attributed to nitrogen related local vibrational

184 S.S. Shinde et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 181–188

Table 1Wave number (in cm−1) and symmetries of the modes found in Raman spectrum of typical 10 at% N-doped ZnO thin film and their assignments.

Observed frequency(cm−1)

Frequency from Ref.[18] (cm−1)

Symmetry Process Brillouin zone points/lines

NZO ZnO

100 100 101 E2 Elow2 �

216 227 208 A1, (E2) 2TA; 2Elow2 L, M, H; �

287 293 – A1 Bhigh1 − Blow

1 �

330 331 332 A1 (E2, E1) Ehigh2 − Elow

2 �

357 – 332 A1 (E2, E1) Ehigh2 − Elow

2 �382 385 380 A1 A1 (TO) �402 409 408 E1 E1 (TO) �

437 438 437 E2 Ehigh2 �

488 485 – A1 2LA M–K571 570 574 A1 A1 (LO) �667 667 – A1 TA + LO M762 795 – A1 LA + TO L–M781 – – A1 LA + TO M, K803 – – A1 LA + LO L, M993 995 990 A1 2TO L–M–K–H

1037 1052 – A1 TO + LO A, H1099 1099 – A1 2LO H, K1134 – 1160 A 2A1(LO), 2E1(LO), 2LO � ; A–L–M

2A1(LO), 2E1(LO), 2LO � ; A–L–M

mavatldinemLTtotpstnotiwbtapp2

tc

�

wtHr

Table 2Phonon lifetimes of the Ehigh

2 , A1(LO), A1(2LO) modes of N-doped ZnO thin film.

Phonon mode � (cm−1) , (10−12 s)

NZO ZnO NZO ZnO

Ehigh 5.67 6.56 0.94 0.81

XPS measurements are conducted to identify the compositionand chemical bonding states. A typical survey scan spectrum of10 at% NZO thin film is as shown in Fig. 5. It shows various corelevels such as Zn 2p, O 1s, Zn 3p, Zn 3d, C 1s, N 1s, valence

0

50000

100000

150000

200000

250000

300000

N 1

s

Zn 3

dZ

n 3

pZ

n 3

s

C 1

s

Zn L

MM

Zn L

MM

O 1

sZ

n L

MM

O K

LL

Zn 2

p3

/2

Zn 2

p1

/2

Co

un

ts/s

ec

1

1161 – 1160 A1

odes [20,21]. In addition to the strong E2 modes, a new peakppears at 382 cm−1, which can be assigned to the A1 (TO) trans-erse optical mode slightly shifted from pure ZnO (380 cm−1). Andditional peak emerges at 402 cm−1 in the spectrum (E1 symme-ry), which is assigned to the E1 (TO) mode. It is shifted towardsow wave number regime because ionic radii of N is less than Zn oroped N combines to O. Also the phonon scattering will not be lim-

ted to the center of the Brillouin zone and the phonon dispersionear the zone center must also be considered for shifting, broad-ning, and the asymmetry of the first order optical phonon. Theode detected at 488 cm−1 exhibits A1 symmetry and assigned to

A overtones along M-K, where phonon dispersion is less intense.he peak at 571 cm−1 is clearly of A1 symmetry and is attributedo LO overtones along � . This mode is ascribed to the formationf the defects, such as oxygen vacancy, interstitial Zn, and lack ofhe free carrier [22]. The TA + LO combinations give rise to the Moint at 663–676 cm−1 with A1 symmetry. The highest density oftates (DOS) for combinations in this frequency range is found athe L, M, and H points. Both at L and H points the TA + LO combi-ation has symmetry E1, E2, whereas at the M point combinationsf the type Mi ⊗ Mi ⊃ A1, E2 occur. This suggests the assignment ofhe 667 cm−1 mode to be TA + LO combinations at the M point. Lowntensity mode of mainly A1 symmetry is detected at 803 cm−1,

hich can be attributed to acoustic and optical combinations. Aand observed at about 993 cm−1 can be attributed to TO over-one is attributed to the presence of N2O defects [23]. The peakst 1037 and 1099 cm−1 can be assigned to A1 (TO + LO) and 2LOhonon modes at the A, H and H, K point of the Brillouin zone. Theeaks at 1134 and 1161 cm−1 are attributed to 2A1(LO), 2E1(LO),LO phonon modes at � ; A–L–M point of Brillouin zone.

The ratio between second- and first-order scattering cross sec-ion is found to be a very sensitive function of the electron–phononoupling strength,

2 = e2

a

(24�

)1/3 1�ωLO

(1

ε∞− 1

ε0

)1w

w∫0

x4(2 + x2)2

(1 + x2)4dx (4)

here w = (3�2)1/2

(a0/a) and ao, are the exciton radius and lat-ice parameter, respectively. � is known to be related to theuang–Rhys parameter S by the relation S = �2. For bulk ZnO mate-

ials, � is calculated to be 2.85 as reported earlier. The calculated

2A1(LO) 39.67 39.36 0.13 0.14A1(2LO) 78.73 70.84 0.07 0.075

second-first order ratio is 1.48 for 10 at% NZO thin film. The phononlifetime () can be derived from the Raman spectra via the energy-time uncertainty relation. It is seen that the phonon life time () isalmost comparable to pure ZnO thin films (Table 2). The prominentEhigh

2 phonon mode has highest life time for both films corroborat-ing better crystallinity.

3.4. X-ray photoelectron spectroscopy (XPS) study

1400 120 0 100 0 80 0 60 0 40 0 20 0 0

Bind ing Ener gy (eV)

Fig. 5. XPS survey scan spectrum of typical 10 at% N-doped ZnO thin film.

S.S. Shinde et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 181–188 185

1055 105 0 104 5 104 0 103 5 103 0 102 5 102 0 101 5

80000

100000

120000

140000

160000

180000

200000

220000C

ou

nts

/se

c

Bind ing Ener gy (eV)

Ffi

bselabe

ts5Zcpwltvt01ewc

Z

Ffi

0 5 10 15 205000

10000

15000

20000

25000

Valence band

Co

un

ts/s

ec

Zn 3d

ig. 6. Narrow scan XPS spectrum of Zn 2p core level for 10 at% N-doped ZnO thinlm.

and regime and Auger transitions. Fig. 6 shows the pronouncedplitting of the Zn 2p spectral line into the 2p1/2 and 2p3/2 core lev-ls having binding energy difference of about 23.1 eV. The doubletine of Zn corresponding to 2p3/2 and 2p1/2 are observed at 1022.2nd 1045.3 eV, respectively. The Zn (2p3/2) line has been shiftedy EZn = 0.7 eV from the binding energy position of 1021.5 eV forlemental zinc [24].

Fig. 7 shows O 1s core level XPS spectra of typical 10 at% NZOhin film. The O 1s peak is broad and asymmetric. The spectrumhows mainly a peak at 531 eV with splitting at about 532.5 and30 eV. The peak is assigned to oxygen atoms bound to Zn innO. The N-doped film shows O 1s peak at 531 eV due to theontribution of N O bonds as well as that of ZnO. The strongereak at 530 eV may be attributed to O2− ions in Zn O bonds,hile another peak at 532.5 eV is usually associated with the

oosely bound oxygen (e.g. adsorbed O2, OH) chemisorbed onhe surface and/or grain boundary of polycrystalline film [25]. Thealence band (VB) spectra revealed the bottom of the VB, abovehe Zn 3d line, whereas the FWHM value of the VB was about.71 eV (Fig. 8). The DOS in the VB has two maxima at 4.7 and.5 eV. The theory of the ZnO VB indicates that the lower bindingnergy portion of the VB consists mainly of O 2p and Zn 4p levels,

hereas the higher BE portion of the VB has major O 2p and Zn 4s

haracter.Fig. 9 shows the N 1s core level spectra of typical 10 at% N doped

nO thin film. The literature shows that the binding energy of the N

540 53 8 53 6 53 4 53 2 53 0 52 8 52 645000

50000

55000

60000

65000

70000

75000

80000

530

532.5

Co

un

ts/s

ec

Binding Ene rgy (eV)

O 1s

531

ig. 7. Narrow scan XPS spectrum of O 1s core level for 10 at% N-doped ZnO thinlm.

Binding Energy (eV)

Fig. 8. XPS spectrum of the Zn 3d and valence band region for 10 at% N-doped ZnOthin film.

1s line is very sensitive to the chemical environment of the nitrogenatom. One peak of binding energy at about 400 eV which corre-sponds to the N 1s core level is observed. It varies from 396 to 408 eV[26]. Generally, peaks detected at 396, 398, 400 and 405 eV in theN 1s XPS spectra have been assigned to different types of bond-ing configuration for chemisorbed nitrogen. The peak at 396 eV isassigned to atomic nitrogen (�-N), the 398 eV peak to well screenedmolecular nitrogen (�-N2), the 400 eV peak to terminally bondedwell screened molecular nitrogen (�-N2), and the 405 eV peak toterminally bonded poorly screened molecular nitrogen (�-N2) [27].

3.5. Morphological study

Fig. 10(a–d) shows the scanning electron micrographs of NZOthin films deposited for different N doping concentrations from1 to 15 at% and Fig. 10(e and f) shows respectively typical FESEMand AFM images of 10 at% NZO thin film. The deposited NZO thinfilms are uniform, compact and adherent. With increase of dopingconcentration from 1 to 15 at% N doping, the grain size increasesfrom 100 to 150 nm. Deposited films show progressive transitionof the grain shape from spherical to plume-like from 1 to 10 at%N. The roughness and grain size increases with increase in dop-ing concentration ascribing the enhancement in crystalline quality.

AFM image shows that films have plume-like structure with fewpits and voids. This has well reflected in FESEM and AFM images(Fig. 10e and f). It can be explained that the different morphologiesof the films should be related to the lattice structure and defects

412 41 0 40 8 40 6 40 4 40 2 40 0 39 8 39 618000

18400

18800

19200

19600

20000

Co

un

ts/s

ec

Bind ing Ener gy (eV)

N 1s

Fig. 9. XPS spectrum of the N 1s core level.

186 S.S. Shinde et al. / Journal of Analytical and Applied Pyrolysis 97 (2012) 181–188

F at%, ((

gs

3

fiacTat1twdlifitathNi

3.7. Photoluminescence properties

PL spectroscopy is a very sensitive tool to characterize radia-tive and non-radiative processes of carriers. Room temperature PL

0

20

40

60

80

100

(e')(d')

(c')(b')

R, T

(%

)

(a') R 1at% NZO

(a) T 1at% NZO

(b') R 5at% NZO

(b) T 5at% NZO

(c') R 10at% NZO

(c) T 10at% NZO

(d') R 15at% NZO

(d) T 15at% NZO

(e') R 20at% NZO

(e) T 20at% NZO(a')

(e)(d)

(c)(b)

(a)

ig. 10. Scanning electron micrographs of N-doped ZnO thin films deposited at (a) 1f) AFM images of 10 at% N-doped ZnO thin film.

enerated during deposition governing the chemical adsorption,ubsequent nucleation and growth.

.6. Optical properties

The optical reflectance and transmittance spectra of NZO thinlms over the spectral range 350–850 nm for the films depositedt different N doping concentrations are shown in Fig. 11. It is per-eptible that the average visible transmittance is around 80–85%.he perceptive interference patterns in R and T show that the filmsre specular to a great extent. As doping concentration increases,he average transmittance enhances up to 89% (at � = 550 nm) for0 at% N doping and further decreases for higher doping concen-ration. The increase in transmittance might be attributed to theell-crystallization of films. The amplitude of interference fringesecreased for higher doping concentration and this indicates the

oss in surface smoothness leading to a slight scattering loss whichs in corroboration with the SEM study. Average thickness of thelm is about 230 nm. It can be observed that the value of reflec-ion decreases from 10% to 3%, when the concentration of nitrogentoms in the ZnO films increases from 1 to 10 at%. Meanwhile,

he result also shows that the concentration of nitrogen atomsas influence on the reflection and band gap. Direct band gap ofZO thin films decreases up to 10 at% NZO (3.17 eV) and further

ncreases for higher concentrations.

b) 5 at%, (c) 10 at%, (d) 15 at% nitrogen doping concentration, (e) typical FESEM and

400 500 600 700 800

Wavelength (nm)

Fig. 11. Optical transmittance and reflectance spectra of N-doped ZnO thin filmsdeposited at various doping concentrations.

S.S. Shinde et al. / Journal of Analytical and

350 400 450 500 550 600 650 7000

4000

8000

12000

16000

20000

24000

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(a) 1at% NZO

(b) 5at% NZO

(c) 10at% NZO

(d) 15at% NZO

(e) 20at% NZO

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ig. 12. PL spectra of N-doped ZnO thin films as a function of nitrogen dopingercentage.

mission spectra of NZO thin films as a function of N doping concen-rations measured in the wavelength range 350–700 nm excited by25 nm wavelength is illustrated in Fig. 12. Inset shows Gaussiantting of the 10 at% NZO PL spectrum. Three emission peaks at 398,47 and 559 nm appeared in the PL spectra of the NZO films. Theffect of enhancement in doping concentration on PL spectra cane seen by the growth of detected emission intensities. The peak at98 nm is originated due to the electron transition from the con-uction band to the valence band and it is tentatively assigned theonor acceptor pair (DAP) emission, suggesting the formation ofcceptor and donor states in the NZO film. The high-energy UVeak is slightly blue shifted from 398 to 388 nm at high dopingoncentration.

Besides the UV emission, films exhibit blue and green emissionst 447 and 559 nm. The broad blue emission peak at 447 nm cane referred to the singly ionized oxygen vacancy in the ZnO andhe emission results from the irradiative recombination of a photo-enerated hole with an electron occupying the oxygen vacancy28]. The most commonly investigated deep level emission in ZnOnown as green emission appears at 559 nm. This emission peaks presumably due to emission from vacancies or defects that arentrinsic to most synthesis methods. It is well known that the broadeep level emission is closely related to intrinsic defects such as Zn

nterstitials and O vacancies which are believed to act as donorefects [29]. More recently, the green emission has been explaineds originating from more than one deep level defects such as, VO andZn, which have different optical characteristics, are both found toontribute to the green emission [30]. The green emission at 559 nmight be attributed to the OZn on theoretical basis [31]. PL spec-

rum especially in higher doping concentrations evidently can beivided into two peaks. Splitted peaks at 393–388 nm seems to bessociated to free electron to neutral acceptor transition or some LOhonon replicas that are usually followed by free electron-acceptorransitions [32]. This also can be attributed to donor–acceptor pairransitions (Do, Ao) that can be acceptable due to the presence ofcceptor and native donor defects [33].

. Conclusions

N-doped ZnO thin films exhibit hexagonal (wurtzite) crystal

tructure. The existence of N Zn O structure in NZO thin film haseen studied using Raman analysis. The presence of prominent E2odes in film indicates that the nitrogen doping does not change

he wurtzite structure of ZnO lattice. The intensity of UV emission

[

Applied Pyrolysis 97 (2012) 181–188 187

band increases with the concentration of activated nitrogen impu-rities. Shifting of PL peak from 393 to 388 nm seems to be associatedwith free electron to neutral acceptor transition or some LO phononreplicas followed by free electron-acceptor transitions.

Acknowledgements

The authors are very much thankful to the Defense Researchand Development Organization (DRDO), New Delhi, for the finan-cial support through its project No. “ERIP/ER/0503504/M/01/1007”.Authors also thank to UGC-DSA-I, DST-PURSE and DST-FIST-II pro-grams for financial support. Authors are also thankful to Dr. VasantSathe and Dr. S.R. Barman, UGC-DAE Consortium for ScientificResearch, Indore for Raman and XPS characterization.

References

[1] H. Von Wenckstern, H. Schmidt, M. Brandt, A. Lajn, R. Pickenhain, M. Lorenz,M. Grundmann, D.M. Hofmann, A. Polity, B.K. Meyer, H. Saal, M. Binnewies, A.Borger, K.D. Becker, V.A. Tikhomirov, K. Jug, Anionic and cationic substitutionin ZnO, Progress in Solid State Chemistry 37 (2009) 153–172.

[2] S.K. Hazra, S. Basu, Stable p-ZnO thin films by oxygen control using reversespray dynamics, Solid State Communications 133 (2005) 245–248.

[3] P. Wang, N.F. Chen, Z.G. Yin, P-doped p-type ZnO films deposited on Si substrateby radio-frequency magnetron sputtering, Applied Physics Letters 88 (2006)152102.

[4] J.G. Lu, Y.Z. Zhang, Z.Z. Ye, L.P. Zhu, L. Wang, B.H. Zhao, Q.L. Liang, Low-resistivity,stable p-type ZnO thin films realized using a Li–N dual-acceptor doping method,Applied Physics Letters 88 (2006) 222114.

[5] C.H. Park, S.B. Zhang, Su-Huai Wei, Origin of p-type doping difficulty in ZnO:the impurity perspective, Physical Review B 66 (2002) 073202.

[6] K.S. Ahn, Y. Yan, M. Al-Jassim, Band gap narrowing of ZnO:N films by vary-ing rf sputtering power in O2/N2 mixtures, Journal of Vacuum Science andTechnology B 25 (2007) L23–L26.

[7] J. Lu, Z. Ye, L. Wang, J. Huang, B. Zhao, Structural, electrical and optical propertiesof N-doped ZnO films synthesized by SS-CVD, Materials Science in Semicon-ductor Processing 5 (2002) 491–496.

[8] J.H. Leem, D.H. Lee, S.Y. Lee, Properties of N-doped ZnO grown by DBD-PLD,Thin Solid Films 518 (2009) 1238–1240.

[9] J.L. Zhao, X.M. Li, J.M. Bian, W.D. Yu, C.Y. Zhang, Growth of nitrogen-doped p-type ZnO films by spray pyrolysis and their electrical and optical properties,Journal of Crystal Growth 280 (2005) 495–501.

10] S. Golshahi, S.M. Rozati, R. Martins, E. Fortunato, P-type ZnO thin film depositedby spray pyrolysis technique: the effect of solution concentration, Thin SolidFilms 518 (2009) 1149–1152.

11] E.C. Lee, Y.S. Kim, Y.G. Jin, K.J. Chang, Compensation mechanism for N acceptorsin ZnO, Physical Review B 64 (2001) 085120.

12] L.G. Wang, A. Zunger, Cluster-doping approach for wide-gap semiconductors:the case of p-type ZnO, Physical Review Letters 90 (2003) 256401.

13] Q. Ou, K. Shinji, A. Ogino, M. Nagatsu, Enhanced photoluminescence of nitrogen-doped ZnO nanoparticles fabricated by Nd:YAG laser ablation, Journal ofPhysics D: Applied Physics 41 (2008) 205104.

14] S. Limpijumnong, X.N. Li, S.H. Wei, S.B. Zhang, Substitutional diatomicmolecules NO, NC, CO, N2, and O2: their vibrational frequencies and effectson p doping of ZnO, Applied Physics Letters 86 (2005) 211910.

15] S.H. Park, J.H. Chang, H.J. Ko, T. Minegishi, J.S. Park, I.H. Im, M. Ito, D.C. Oh, M.W.Cho, T. Yao, Lattice deformation of ZnO films with high nitrogen concentration,Applied Surface Science 254 (2008) 7972–7975.

16] S.S. Shinde, P.S. Shinde, C.H. Bhosale, K.Y. Rajpure, Optoelectronic propertiesof sprayed transparent and conducting indium doped zinc oxide thin films,Journal of Physics D: Applied Physics 41 (2008) 105109.

17] S. Thanikaikarasan, T. Mahalingam, A. Kathalingam, Y.D. Kim, T. Kim, Growthand characterization of electrosynthesized iron selenide thin films, Vacuum 83(2009) 1066–1072.

18] J.M. Calleja, M. Cardona, Resonant Raman scattering in ZnO, Physical Review B16 (1977) 3753–3761.

19] T.C. Damen, S.P.S. Porto, B. Tell, Raman effect in zinc oxide, Physical Review 142(1966) 570–574.

20] L.L. Kerr, X. Li, M. Canepa, A.J. Sommer, Raman analysis of nitrogen doped ZnO,Thin Solid Films 515 (2007) 5282–5286.

21] H.Q. Le, S. Tripathy, S.J. Chua, The influence of nitrogen plasma treatment on thelattice vibrational properties of hydrothermally grown ZnO nanorods, AppliedPhysics Letters 92 (2008) 141910.

22] R. Ferro, J.A. Rodriguez, O. Vigil, A. Morales-Acevedo, Chemical composition

and electrical conduction mechanism for CdO:F thin films deposited by spraypyrolysis, Materials Science and Engineering B 87 (2001) 83–86.

23] F. Berg Rasmussen, S. Oberg, R. Jones, C. Ewels, J. Goss, J. Miro, P. Deak, Thenitrogen-pair oxygen defect in silicon, Materials Science and Engineering B 36(1996) 91–95.

1 l and

[

[

[

[

[

[

[

[

[

88 S.S. Shinde et al. / Journal of Analytica

24] B.A. de Angelis, On the surface reduction of some chromium compounds dur-ing X-ray photoelectron spectroscopy, Journal of Electron Spectroscopy andRelated Phenomena 9 (1976) 81–84.

25] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films, Applied Surface Science 158 (2000) 134–140.

26] J. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, J. Chastain, Handbook of X-rayElectron Spectroscopy, Perkin-Elmer, 1992.

27] N. Tabet, M. Faiz, A. Al-Oteibi, XPS study of nitrogen-implanted ZnO thin filmsobtained by DC-magnetron reactive plasma, Journal of Electron Spectroscopyand Related Phenomena 163 (2008) 15.

28] J.C. Sun, H.W. Liang, J.Z. Zhao, J.M. Bian, Q.J. Feng, L.Z. Hu, H.Q. Zhang, X.P.Liang, Y.M. Luo, G.T. Du, Ultraviolet electroluminescence from n-ZnO:Ga/p-ZnO:N homojunction device on sapphire substrate with p-type ZnO:N layerformed by annealing in N2O plasma ambient, Chemical Physics Letters 460(2008) 548–551.

[

Applied Pyrolysis 97 (2012) 181–188

29] S.B. Zhang, S.H. Wei, A. Zanger, Intrinsic n-type versus p-type dopingasymmetry and the defect physics of ZnO, Physical Review B 63 (2001)075205.

30] T.M. Børseth, B.G. Svensson, A.Y. Kuznetsov, P. Klason, Q.X. Zhao, M. Willander,Identification of oxygen and zinc vacancy optical signals in ZnO, Applied PhysicsLetters 89 (2006) 262112.

31] P. Klason, T.M. Børseth, Q.X. Zhao, B.G. Svensson, A.Y. Kuznetsov, M. Willan-der, Temperature dependence and decay times of zinc and oxygen vacancyrelated photoluminescence bands in zinc oxide, Solid State Communications145 (2008) 321–326.

32] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, B. Liu, Y. Che, L.P. Zhu, B.H. Zhao, Study on the

Hall-effect and photoluminescence of n-doped p-type ZnO thin films, MaterialsLetters 61 (2007) 41–44.

33] F. Zhuge, L.P. Zhu, Z.Z. Ye, J.G. Lu, H.P. He, B.H. Zhao, Al concentration depen-dence of electrical and photoluminescent properties of co-doped ZnO films,Chemical Physics Letters 437 (2007) 203–206.