Structural and optical properties of ZnO thin films deposited onto ITO/glass substrates

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Structural and optical properties of ZnO thin ﬁlms deposited onto ITO/glass substrates

M. Rusu

^a

, G.G. Rusu

^a,^*

, M. Girtan

^a,b

, S. Dabos Seignon

^b

aAl.I. Cuza University, Faculty of Physics, Carol I Blvd. No. 11, 700506 Iasi, Romania

bLaboratoire POMA, Université d’Angers, Lavoisier Bd, No. 2, 49035 Angers, France

a r t i c l e i n f o

Article history:

Available online 3 August 2008

PACS:

81.15.Ef 81.65.Mq 61.05.cp 78.40.Fy Keyword:

Films and coatings

a b s t r a c t

ZnO films were prepared by post deposition thermal oxidation in the ambient atmosphere of metallic Zn films (d= 100–170 nm) vacuum evaporated onto unheated indium tin oxide (ITO)-coated glass substrates. To study the effect of the substrate position during the Zn film deposition on the microstructure and optical properties (transmittance, reflectance and absorbance) of as obtained ZnO films, two set of Zn samples simultaneously deposited onto horizontally and obliquely arranged substrates were prepared.

The as obtained ZnO films had a polycrystalline wurtzite structure, those obtained from normally deposited Zn films having a higherc-axis preferred orientation and a lower optical transmittance in the visible wavelength range. The optical band-gap was found to be of 3.14 eV for oxidized normally deposited vir- gin Zn films and of 3.16 eV for those obliquely deposited.

1. Introduction

In the last years, among other oxides in thin films, the zinc oxide (ZnO) thin films have received considerable attention due to their applications as active semiconductor compounds in transparent electronic devices. The quality of these devices is strongly influenced by the physical properties of component films which in turn are dependent on the deposition technology used for film preparation. For this reason the interest regarding the correlation between the physical properties of ZnO films and the conditions of their preparation has increased recently.

A deposition parameter which can influence the structure and physical properties of ZnO films is the nature of the used film substrates. ZnO films grown by different methods on silicon, sapphire, GaAs, quartz, glass, stainless steel, alumina plate, Mo, etc. were obtained and their structural, electrical and optical properties were investigated[1–14].

In this paper, the structural and optical properties of ZnO thin films obtained by thermal oxidation of Zn films, vacuum evaporated onto indium tin oxide (ITO)-coated glass substrates, have been studied. Such ZnO films deposited onto ITO substrates are useful in technology of different optoelectronic devices.

2. Experimental

Metallic zinc films were obtained by thermal evaporation of pure Zn in standard vacuum equipment at the residual pressure of about 10³Pa onto unheated commercial ITO-coated glass substrates. The quasi-closed volume technique was used [15]. The temperature of the evaporation source, maintained constant during film deposition, was 723 K. Our previous investigations had revealed that the substrate position relative to the incident vapor during film deposition, influenced the structure of the respective samples[16]. For this reason, in this work, the incidence angle,b, of the incident vapor onto film substrate was taken as the deposition parameter. Two sets of samples, simultaneously deposited onto horizontally (b= 0^°) and obliquely (b= 60^°) arranged substrates were prepared. The film thicknesses,d,determined with an interference microscope ranged between 100 nm and 170 nm.

The thickness of the ITO ﬁlm on the glass substrate was 900 nm.

After preparation, the as-deposited Zn ﬁlms were oxidized by heating under ambient conditions with a rate of 12 K/min from room temperature to ﬁnal 550 K.

The crystalline structure of the as-deposited and oxidized samples was investigated by an X-ray diffraction (XRD) analysis using Cu Karadiation (k= 1.5418 Å) in the range of 2h= 20–70^°. The surface topography of the obtained ZnO ﬁlms was analyzed using atomic force microscopy (AFM).

doi:10.1016/j.jnoncrysol.2008.06.070

* Corresponding author. Tel.: +40 232 201165.

E-mail address:rusugxg@uaic.ro(G.G. Rusu).

Journal of Non-Crystalline Solids 354 (2008) 4461–4464

Contents lists available atScienceDirect

Journal of Non-Crystalline Solids

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j n o n c r y s o l

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1400 nm. An ITO-coated glass substrate was used as reference forT andRrecords.

3. Results and discussion 3.1. Structural characteristics

Representative XRD patterns obtained for typical as-deposited and thermally oxidized Zn ﬁlms are shown inFigs. 1 and 2. Some deposition parameters for these ﬁlms are listed inTable 1.

As follows fromFigs. 1(a) and 2(a), the as-deposited Zn films are polycrystalline, having a wurtzite Zn structure [17]. The intense (0 0 2) peak at 2hffi36.4^° shows that in respective films, the Zn crystallites grow preponderantly with (0 0 2) plane parallel to the substrate surface. In the case of a normally deposited Zn sample (Fig. 1(a)), this preferred orientation is much stronger in comparison with that of the obliquely deposited samples. This conclusion also results fromTable 2, where the calculated relative intensityIhkl

of diffraction peaks for as-deposited typical Zn samples are listed.

The relative intensityI002/I101ffi16, calculated for normally deposited Zn films is greater about five times than the value of 3.05 obtained for obliquely deposited Zn films, and much greater than the standard value of 0.53 for hexagonal bulk Zn. This indicates that, in normally deposited samples, practically all crystallites grow with theirc-axis normally to the substrate.

The full-width at half maximum (FWHM) of the (0 0 2) peak from Fig. 1(a), lower than the FWHM of the same peak from Fig. 2(a), indicates that crystallites have a greater size in normally deposited Zn films. The values of the crystallite size,D, listed in Table 2, calculated using the Debye–Scherrer formula[18], relative to the (0 0 2) Zn peak, for each XRD pattern, confirm this conclusion. The different crystallite size values of obtained for two typical Zn films cannot be attributed to the different thicknesses of the respective films because sampleA has a lower thickness and the crystallite size usually increases with film thickness. Therefore, the observed difference between crystallite sizes can be attributed to the different positions of the film substrate during the deposition process. The lower distance to the film substrate during film deposition and the greater growth rate for sampleBcan influence the lower crystallite size of sampleB.A preferred (0 0 2) orientation of the crystallites was also observed for Zn film normally evapo-

rated onto glass substrates[16], but this is not so strong as in this case. This fact indicates that, in the analyzed case, the intermediary ITO ﬁlm substrate favors much more the respective texture.

The XRD patterns for the same two typical samples, recorded after their heat-treatment up to 550 K, are presented inFigs. 1(b) and 2(b). The diffraction peaks at 2h= 31.8^°, 34.4^°, 36.3^°, 47.6^°, 56.6^°and 62.9^°from the respective figures are characteristic for a polycrystalline wurtzite structure of bulk ZnO[19], indicating the formation of a respective compound as a consequence of thermal oxidation of Zn films. The substrate position during film deposition also influences the preferential orientation of crystallites in the obtained ZnO films. As follows fromFigs. 1(b), 2(b) and (Table 3), oxidized Zn films present also a (0 0 2) texture which is more pronounced in case of a normally deposited sample.

The value of the I002/I101 ratio calculated for respective ZnO films is 3.10 for normally deposited films and 0.96 for obliquely deposited films, respectively, both these values being greater than standard 0.56 value. This indicates that the oxidation process re- duces partially the preferred orientation of ZnO crystallites, more strongly for normally deposited films. The crystallite size values

20 30 40 50 60 70

0 20 40 60 80

100 20 30 40 50 60 70

0 20 40 60 80 100

2 (deg)

Zn (002)

Zn (100) Zn (101)

Sample A

as-deposited

a

Intensity

ZnO (112) ZnO (103) ZnO (110)

ZnO (102) ZnO (101) ZnO (100)

ZnO (002) Sample A

thermally oxidized

b

θ

Fig. 1.Typical XRD patterns for normally (b= 0^°) deposited Zn ﬁlms: (a) as- deposited; and (b) after thermal oxidation. (The dots mark the peaks corresponding to ITO substrate).

20 30 40 50 60 70

0 20 40 60 80

1000 20 30 40 50 60 70

20 40 60 80 100

2θ (deg)

Zn (002)

Zn (102) Zn (101)

Zn (100)

Sample B as-deposited

a

Intensity

ZnO (112) ZnO (103) ZnO (110)

ZnO (102) ZnO (101) ZnO (002) ZnO (100)

Sample B

thermally oxidized

b

Fig. 2.Typical XRD patterns for obliquely (b= 60^°) deposited Zn ﬁlms: (a) as- deposited; and (b) after thermal oxidation. (The dots mark the peaks corresponding to ITO substrate).

Table 1

Growth data for typical as-deposited Zn ﬁlms

Sample Tev(K) d(nm) b(°) l(cm) r(nm/s)

A 723 102 0 6 0.5

B 723 170 60 5.5 0.9

Tev– temperature of Zn evaporation source;d– ﬁlm thickness;b– incidence angle;

l – source–substrate distance;r– growth rate.

Table 2

Relative intensityIhklof XRD peaks for two representative as-deposited Zn samples (D – average crystallite size)

Sample Ihkl(%) D(nm)

(0 0 2) (1 0 0) (1 0 1) (1 0 2)

A 100 5.9 6.2 – 25.6

B 100 6.9 32.8 5.4 20.4

Zn hexagonal phase[17] 53 40 100 28

The data for Zn ﬁlms have been normalized relative to maximum intensity Zn peak for each pattern.

4462 M. Rusu et al. / Journal of Non-Crystalline Solids 354 (2008) 4461–4464

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calculated relative to their three main maxima fromFigs.1(b) and 2(b) for both representative ZnO films are listed inTable 3. It can be observed that these values are lower in comparison with those of the unheated Zn films. Such decreasing in the crystallite size in Zn films in consequence of thermal oxidation was also observed in case of Zn films evaporated onto glass substrates[16]. It can be also concluded fromFigs. 1 and 2that the annealing process does not affect the crystalline structure of the used ITO substrates.

The AFM images for representative heat-treated obliquely deposited Zn films are shown in Fig 3. It can be observed that the respective ZnO film has a grain-like surface morphology. Sim- ilar AFM images were obtained for normally deposited films. The calculated values of average roughness,Ra, are 55 nm for normally deposited samples and 32 nm for those deposited obliquely, respectively. The greater value ofRaobtained for oxidized theA sample may be in correlation with a higher preferential (0 0 2) orientation of crystallites in the original Zn film and with their higher size in the respective film.

3.2. Optical properties

The reflection and transmission spectra for representative oxi- dizedAandBsamples are plotted inFig. 4. The presence of interference maxima and minima can be observed which indicates a good optical quality of respective films. In case of the normally deposited sampleA, both the transmittance and reflectance are lower in comparison with those for the ZnO film obliquely deposited, sampleB. This indicates a higher incident light absorption in sampleA. This can be due to the greater surface roughness of the sample which determines a multiple light reflection inside the film surface.

It can be also observed inFig. 4that both the transmission and reﬂection interference spectra become weaker in the 600–700 nm wavelength range, a fact that indicates an increase in light absorption in the respective domain. This fact can be associated with the recently reported red luminescence band attributed to double ion- ized oxygen vacancies.[20].

The absorption coefﬁcient,

a

, was calculated from transmission and reﬂection spectra. Assuming the direct allowed transitions, the absorption coefﬁcient dependence on the photon energyht was Table 3

Relative intensityIhklof XRD peaks for obtained ZnO ﬁlms (D– average crystallite size)

Sample Ihkl(%) D(nm)

(1 0 0) (0 0 2) (1 0 1) (1 0 2) (1 1 0) (1 0 3)

Athermally oxidized 14.4 100 32.8 7.7 8.6 10.2 20.7

Bthermally oxidized 60 96.4 100 21.5 24.5 17.3 17.8

ZnO hexagonal phase[19] 71 56 100 29 40 35 –

The data for ZnO ﬁlms have been normalized relative to maximum intensity ZnO peak for each pattern.

Fig. 3.AFM images for ZnO ﬁlm originate in obliquely deposited Zn ﬁlm.

400 500 600 700 800 900 1000 1100

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

(a) (a)

(b) (b)

Reflectance Transmittance

Reflectance/Transmittance

λ (nm)

Sample A Sample B

Fig. 4.Transmission and reflection spectra for ZnO films originate in: (a) normally deposited Zn films; and (b) obliquely deposited Zn films.

2. 0 2.5 3.0 3 .5 4. 0 0

1 2 3 4 5 6 7 8

(thermally oxidized)

EgA=3.14 eV EgB=3.16 eV

(

αhν) (10 cm eV )211-22

hν (eV)

Sample A Sample B

Fig. 5.Typical (ahm)²=f(ht) dependence for ZnO ﬁlms.

M. Rusu et al. / Journal of Non-Crystalline Solids 354 (2008) 4461–4464 4463

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analyzed using the expression for allowed direct band-to-band transitions[21]:

a

¼ A h

m

^h

m

Eg

1=2

ð1Þ

whereEgdenotes the optical energy band-gap andAis a parameter characteristic for respective transitions. (Fig. 5) shows the dependence (

a

ht)²=f(ht) for the two representative ZnO thin ﬁlms.

The linear portions of the plots near the fundamental band edge in respective figure confirm the direct nature of the band-to band transitions for respective films. By extrapolating the linear seg- ments of these curves to (

a

ht²) = 0, the optical band gap values have been determined. The obtained values ofEgare very similar (3.14 eV for a normally deposited Zn film and 3.16 eV for an obliquely deposited Zn film), which suggests that the position of the substrates during Zn films deposition does not influence signifi- cantly the optical energy band gap of the obtained ZnO films.

As shown in the literature[22,23], the direct optical band gap of pure polycrystalline ZnO films is 3.28–3.30 eV. The slightly lower band gap of our ZnO films can be attributed to a greater density of the donor states near a conduction band determined by oxygen vacancies[24]and to the small grain size from the films[11].

It can be also observed inFig. 5that the ZnO films, obtained by oxidation of normally deposited Zn films, have a greater optical absorbance. This is in concordance with the same conclusion reached above resulting from analyzing the reflectance and transmittance spectra for the two types of ZnO films.

4. Conclusions

ZnO films were obtained by heating the post deposition vacuum evaporated Zn films onto unheated ITO-coated glass substrates up to 550 K. The incidence angle of vapor onto film substrates during the Zn film deposition was taken as the deposition parameter. The obtained ZnO films present a polycrystalline hexagonal (0 0 2) ori- ented structure, more noticeable in case of normally deposited samples. The position of substrates during Zn films deposition influences the transmission and reflection spectra of the obtained ZnO films. The ZnO films obtained by oxidation of obliquely deposited Zn films present a higher transmittance of about 85%, whereas those obtained by oxidation of normally deposited Zn films have a grater absorbance in the visible wavelength range. An absorption

peak at the wavelength of about 650 nm in the transmission and reﬂection spectra of the studied ZnO ﬁlms was revealed. The calculated values of the optical band gap for ZnO samples were found to be 3.14 eV for normally deposited samples and 3.16 eV for obliquely deposited samples, respectively.

Acknowledgements

A part of this work originates from the research conducted under Grant CEEX 89/2006 and 2CEx 06-11-51-1/2007, CNCSIS, Romania with ﬁnancial support received from those sources.

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