Materials Science in Semiconductor Processing 5 (2002) 11–15

Structure of ZnO films prepared by oxidation of metallic Zinc

Rohit Kumar Gupta, N. Shridhar1, Monica Katiyar*

Materials and Metallurgical Engineering Department, Indian Institute of Technology, Kanpur 208016, India

Abstract

ZnO is an n-type, wide band gap semiconductor that exhibits electrooptic and piezoelectric properties and it has a

gamut of applications. In this paper, ZnO films were prepared by oxidizing the Zn film deposited by thermal

evaporation on glass substrates. The oxidation of Zn films was carried out at different temperatures in two different

atmospheres namely oxygen and ozone. The oxidized samples were characterized with regard to their microstructure,

phase(s) present, and transmittance in the wavelength range of 200–1000 nm. Zn films have hexagonal phase and flake

like structure. After oxidation, wurtzite phase of ZnO is formed. Texture and structure of ZnO films are shown to be

dependent on the temperature, time for oxidation and type of the oxidant. The XRD pattern of these films shows the

peaks corresponding to both Zn and ZnO phases that indicate incomplete oxidation of the Zn films. Transmittance of

these films is low due to incomplete oxidation and impurities present in the starting material. r 2002 Elsevier Science

Ltd. All rights reserved.

Keywords: ZnO; Film texture; Oxidation

There has been considerable interest in the micro-

structure, texture (crystal orientation) and surface

topology of ZnO thin films. One reason is that rough

vs. flat topology of ZnO makes a difference in the

efficiency of the solar cell by increasing the path length

of the light going through the device [1]. Since ZnO is

also an electrooptic and piezoelectric material, crystal

orientation of the film is important [2]. For these

properties, preferred orientation is that the basal or

(0 0 2) planes of the wurtzite-type ZnO lattice lie parallel

to the substrate. Surface morphology and microstruc-

ture are also important for these films, if they are to be

used as a catalyst and gas sensors [3]. Sun et al. use the

ZnO film as an intermediate layer for improving the

adhesion of metals to different insulating substrates [4].

They also find that surface topology is important for the

adhesion of the Cu metal to ZnO. Indeed, crystal

orientation and/or surface morphology are important

structural properties for various applications. There has

been some research to study the changes in these

properties as a function of processing parameters or

post-deposition annealing [1–7]. In this paper, we have

tried to explore if the structural properties can be

modified by the oxidation process.

The glass substrate was first cleaned with soap

detergent and then dipped in chromic acid solution

(10ml of aqueous K2Cr2O7 in 200ml H2SO4) for some

time. After rinsing in distilled water, they were rinsed in

methanol. Zn films were deposited using thermal

evaporation using Zn powder whose impurity analysis

is given in Table 1. Thickness of the film is determined

by weight gain method assuming uniform thickness.

Oxidation of zinc film was carried out in oxygen and

ozone atmosphere. The first set of zinc films was

oxidized in oxygen atmosphere at 2501C, 3501C and

4001C for different times. The second set of Zn films was

oxidized in ozone atmosphere at 3501C and 5501C.

Films were characterized using RBS, spectropho-

tometer, X-ray diffractometer and SEM. Thickness

measurements by RBS and weight gain method were

in good agreement.

The XRD pattern of Zn film prepared by evaporation

of Zn powder exhibits a single phase of Zn with

*Corresponding author. Fax: +91-512-590260.

E-mail address: mk@iitk.ac.in (M. Katiyar).1Current address: E2, MIDC-WALUJ, Aurangabad 431136,

India.

1369-8001/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 5 0 - 1

hexagonal structure having three high intensity promi-

nent peaks of (1 0 1), (1 0 2) and (1 0 3) planes. As

deposited films exhibit preferred orientation, the relative

intensity of peaks does not match the powder pattern

intensity. These films were oxidized at 2501C, 3501C and

4001C in oxygen for different times to prepare ZnO.

XRD results, in Fig. 1, show formation of wurtzite ZnO

(hexagonal) after oxidation at 2501C. ZnO film texture

can be inferred from the XRD results, for example,

dominant (0 0 2) means c-axis perpendicular to the

substrate; (1 1 0) means c-axis parallel to the substrate;

(1 0 1) means c-axis inclined to the substrate; (1 0 0)

means (10.0) texture of the film [1]. At 2501C oxidation,

ZnO grows with a dominant (1 0 1) texture. Fig. 2 gives

the XRD results at 3501C for different times; once again

the dominant texture is (1 0 1). Fig. 3 gives the XRD

results of oxidation at 4001C, and now all the three

peaks from (1 0 0), (0 0 2) and (1 0 1) are seen, but

the intensity ratio is not what will be expected from

random orientation of the crystals [2]. There is

dominance of (0 0 2) type structure. In all cases, we find

the peaks for Zn and ZnO coexisting which indicates

incomplete oxidation of the Zn film. We observe that the

texture of the film is dependent on the oxidation

temperature. It is also observed that the intensity ratio

of different ZnO peaks is changing with oxidation time

indicating multiple mechanism of nucleation and growth

of ZnO crystals in these films. Cho et al. were able to

produce the complete oxidation of Zn films of 2000 (A

thickness after annealing at 10001C [8]. They also

reported the presence of Zn peaks for the samples

oxidized at 4001C, but they were able to completely

oxidize the 2000 (A Zn films at higher temperatures. They

obtained ZnO films with no preferred orientation. This

indicates to us that by controlling the texture of Zn film,

we can control the texture of ZnO by oxidation

technique.

We also oxidized the Zn film in the O3 atmosphere at

3501C for 7 h. It is clear from diffraction results in Fig. 4

that the ozone atmosphere is a more effective oxidizing

agent, still there is a small Zn peak present indicating the

presence of elemental Zn even in these films. At the sameFig. 1. The XRD pattern of ZnO film of thickness 3.1 mmoxidized after oxidation at 2501C.

Table 1

Impurities present in the Zn powder

Element Percentage

Lead 1.5

Tin 3.6

Iron 0.9

Copper 0.3

Cadmium 0.02

Arsenic 1.5

Fig. 2. The XRD pattern of ZnO film of thickness 2 mm oxidized in oxygen atmosphere at (a) 3501C for 350min (b) 3501C for 830min.

R.K. Gupta et al. / Materials Science in Semiconductor Processing 5 (2002) 11–1512

oxidation temperature of 3501C, the texture developed

in ozone is (0 0 2) dominant and oxygen is (1 0 1)

dominant. Therefore, the oxidant type also decides the

texture developed in the film.

Optical measurements confirm the XRD results. Due

to the presence of the elemental Zn and other impurities

in the film, optical transmission from these films was

o20% in the 200–1000 nm region.

SEM pictures of Zn films are shown in Fig. 5. The

microstructure of Zn film consists of small flakes.

Existence of flake like grains points to growth of

preferred orientation in agreement with the XRD

results. Fig. 6 shows the microstructure of films oxidized

in oxygen atmosphere. At 2501C, we see some hexagonal

grains due to the formation of ZnO on the surface. At

3501C, ZnO films show completely different morphol-

ogy. In Fig. 6(b), we see long flake type grain showing

preferred orientation of original films. This film on

further oxidation at the same temperature shows

random orientation of the grains (Fig. 6(c)). The ZnO

films, which were oxidized at 4001C, have fine micro-

structure when compared with the other films. Fig. 7

shows the microstructure of the film oxidized in ozone at

3501C. This structure is much finer compared to the

oxidation by oxygen.

Microstructure and XRD analysis show that there

is a difference in oxidation process when films are

annealed in ozone as opposed to in oxygen. Our results

indicate that by controlling the oxidation temperature,

time and oxidant type, we can control the texture and

microstructure of the ZnO films. Furthermore, texture

of the Zn film may also play a role, which can be

modified by the directional deposition using physical

vapour deposition techniques. Although we see some

amount of elemental Zn in our films, that can be

eliminated by using high purity thin films of Zn for

oxidation or thinner films [8], detailed research is

required to characterize the oxidation process in terms

of kinetics and nucleation and growth of the ZnO

phase.

Fig. 3. The XRD pattern of ZnO film of thickness 2mm oxidized in oxygen atmosphere at (a) 4001C for 410min (b) 4001C for

820min.

Fig. 4. The XRD pattern of ZnO film oxidized in ozone

atmosphere at 3501C. Fig. 5. SEM micrograph of Zn film prepared by thermal

evaporation of Zn powder.

R.K. Gupta et al. / Materials Science in Semiconductor Processing 5 (2002) 11–15 13

Acknowledgements

N. Shridhar will like to acknowledge the financial

support from the Council for Scientific and Industrial

Research.

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