Effects of Atomic Oxygen Irradiation on ... - ScienceDirect.com

Chinese

Chinese Journal of Aeronautics 20(2007) 464-468

Journal of Aeronautics www.elsevier.com/locate/cja

Effects of Atomic Oxygen Irradiation on Transparent Conductive Oxide Thin Films Wang Wenwen, Wang Tianmin* Center of Material Physics and Chemistry, School of Science, Beijing University of Aeronautics and Astronautics, Beijing 100083, China Received 13 December 2006; accepted 5 June 2007

Abstract Transparent conductive oxide (TCO) thin film is a kind of functional material which has potential applications in solar cells and atomic oxygen (AO) resisting systems in spacecrafts. Of TCO, ZnOAl (ZAO) and In2O3:Sn (ITO) thin films have been widely used and investigated. In this study, ZAO and ITO thin films were irradiated by AO with different amounts of fluence. The as-deposited samples and irradiated ones were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Hall-effect measurement to investigate the dependence of the structure, morphology and electrical properties of ZAO or ITO on the amount of fluence of AO irradiation. It is noticed that AO has erosion effects on the surface of ZAO without evident influences upon its structure and conductive properties. Moreover, as the amount of AO fluence rises, the carrier concentration of ITO decreases causing the resistivity to increase by at most 21.7%. Keywords: transparent conductive oxide thin film; ZnO:Al; In2O3:Sn; atomic oxygen; erosion; electrical properties

1 Introduction* With a wide forbidden band gap, high optical transmittance in visible range, low electrical resistivity and high reflectivity to the infrared, transparent conductive oxide (TCO) has found wide applications in various fields including transparent electrodes in solar cells, flat panel displays[1] and heat reflecting mirrors[2]. Furthermore, TCO possesses a latent capability to be protection coatings against atomic oxygen (AO) for spacecrafts[3]. In2O3:Sn (ITO) thin film, as it is known, is the most widely used TCO and has been utilized as conductive thermal control films on the surface of satellite[4]. However, ZnO:Al (ZAO), as a potential and optimal substitute for In2O3:Sn (ITO) thin film, has attracted *Corresponding author. Tel.: +86-10-82317931. E-mail address: [email protected] Foundation item: National Natural Science Foundation of China (50471004)

much attention due to the cheapness and abundance of its raw material, its non-toxicity and good stability in hydrogen plasma[5]. Many spacecrafts, such as space shuttles, spaceships and space stations, are all traveling in low earth orbit (LEO). The previous studies indicated that the LEO environmental effects, such as AO, VUV, thermal cycling etc, might influence the normal operation and the lifetime of a spacecraft. As a predominant and most active element, AO might cause many kinds of spacecraft materials to erode and degrade, which further shorten the lifetime of a spacecraft. Although a great deal of research work relevant to AO effects was carried out and some important conclusions were drawn[6-10], the effects of AO irradiation on ITO and ZAO films have rarely been known. During the past two decades, numerous re-

Wang Wenwen et al. / Chinese Journal of Aeronautics 20(2007) 464-468

searchers have been exploring the deposition methods, optimizing the preparation parameters, clarifying the doping and scattering mechanisms, and investigating the optical and electrical properties of TCO thin films[1-3, 11-12]. Although various kinds of preparation and characterization technology have been considered, the effect of aerospace irradiation on TCO is often ignored. Actually, when applied in the fields mentioned above, especially in LEO environment, TCO thin films will unavoidably be exposed to AO irradiation. Consequently, investigating the AO effects on TCO thin films is not only essential for their application in spacecrafts, but also new to clarify their conductive mechanisms and transform their properties. In this paper, the erosion effects of AO upon TCO thin films and the dependence of the electrical properties of ITO thin films on the amount of fluence of AO irradiation are studied. As ZAO thin films have better stability than ITO under plasma conditions, the effects of high-fluence AO irradiation (1.2×1020 atoms/cm2) on the structure, morphology and conductivity of ZAO thin film samples with different sheet resistivity are also clarified.

2 Experimental Facilities and Details The AO effect experiments were carried out on a ground-based simulation device in which AO is generated by discharging the cathode filament confined by a multiple magnetic field[13]. The discharge voltage was 100 V with a 100 mA discharge current. Uniformly distributed in the vacuum chamber, the amount of AO fluence was approximately 1016 atoms/cm2·s. Standard material Kapton was used to define the accurate amount of AO fluuence onto the samples. A piece of uniform ITO conductive glass manufactured by Shenzhen Laibao High Technology Corporation was cut into five samples, square in cross-section of 10 mm×10 mm. Before AO irradiation, each sample had a 20 nm film thick and sheet resistivity of 115 ȍ/ƶ. The amounts of AO fluence applied onto different samples are listed in Table 1.

Table 1

· 465 ·

The amount of AO fluece onto different ITO samples

Sample No.

1

2

3

4

5

AO flux /(1020atoms·cm–2)

0

0.95

1.88

2.91

3.66

The ZAO thin films were prepared by D.C. magnetron sputtering. The average thickness of a film was about 850 nm. The amount of AO fluence was 1.2×1020 atoms/cm2 which was kept constant all the time. Four ZAO samples each with a different value of sheet resistivity were exposed to AO for about 5 h. The sheet resistivity of each ZAO sample is listed in Table 2. Table 2

Sheet resistivity of different ZAO samples

Sample No.

11

12

13

14

Sheet resistivity /(ȍ·ƶ–1)

2.15

5.38

14.04

212.46

The thickness of the films was monitored by an D-step surface profiler (Dektak IIA). XRD with a Cu KD source was used to determine the crystalline structure of the samples. The surface morphology was determined with a Hitachi S-3500 SEM using secondary electrons. Hall measurement was carried out to determine the resistivity, carrier concentration and Hall mobility of the thin films. All the operations were performed at room temperature.

3 Experimental Results and Discussion 3.1 AO effects on electrical properties of ITO The dependence of electrical properties of ITO thin films on the amount of AO irradiation fluence is shown in Fig.1, which illustrates the variation of resistivity (U), Hall mobility (P) and carrier concentration (Ne). Their relationship can be expressed by[14] 1 PU RH (1) fN e e where RH represents the Hall coefficient of semiconductor and f is f-factor. It can be observed from Fig.1 that all the irradiated samples have higher Hall mobility and lower carrier concentration than the as-deposited ITO thin films. It also indicates that AO irradiation could slightly improve the crystallin-

· 466 ·


ity of ITO, probably because of the strong oxidation ability of AO to decrease the number of O2– vacancy defects in the films. Moreover, the decrease of O2– vacancy and the oxidation of the dissociated Sn might result in the decrease of the carrier concentration. According to Eq.(1), the resistivity, U, of the samples is determined by P, Ne and f. As the extent of decrease of Ne is much larger than that of increase of P, the resistivity goes higher up as the amount of AO fluence rises.

(a) Resistivity and carrier concentration

study is under way with the help of ITO samples 350 nm thick. 3.2

AO effects on ZAO

(1) Structure and surface morphology The XRD patterns of ZAO sample No.11 before and after AO erosion are illustrated in Fig.2, which indicates that both the as-deposited ZAO thin film and the irradiated one are polycrystalline and highly textured with the c-axis perpendicular to the substrate surface. Fig.2 shows the typical results of the comparison between the as-deposited samples and the irradiated ones from No.11 to No.14. The strongest peak at 34.380° is attributed to the (002) line of the hexagonal zincite phase, and the other smaller peak at 72.5° represents the (004) diffraction. Differences only exist in the intensity and the full width at half maximum (FWHM), which could partly be contributable to the crystallinity of the thin films. From the comparison, a conclusion can be drawn that AO irradiation of 1020 atoms/cm2 could hardly influence the hexagonal structure of ZAO thin films. Meanwhile, the crystallinity of the thin films is slightly weakened by AO erosion.

(b) Hall mobility

Fig.1

Resistivity, carrier concentration and Hall mobility of ITO before and after AO irradiation.

This study has preliminarily clarified the AO-resisting ability of ITO thin films. When the amount of AO fluence reaches 3.66×1020 (atmos/ cm2), the resistivity of ITO increases by 21.7% without any visible changes in its appearance and color. As the thickness of the ITO thin film used in this experiment is only 20 nm, it proves too thin for XRD and SEM examinations. Therefore, further

Fig.2

XRD patterns of ZAO sample No. 11 before and after AO irradiation.

Fig.3 shows the surface morphology of the as-deposited ZAO thin film samples on the left and the irradiated ones on the right. Under the observation of SEM, three kinds of dissimilar surface morphology exist in the four samples presented in Fig.3: tangling strings (a), honeycomb (b),(d) and intermediate state between honeycomb and graininess (c).


(a) Sample No.11

· 467 ·

AO irradiation of 1.2×1020 atoms/cm2 are listed in Table 3. From the comparison, no obvious influences of AO erosion upon the electrical properties of ZAO samples can be found though; it indicates that the ZAO probably owns AO resisting capability superior to that of ITO. Afterwards, an experiment with even higher amount of fluence of AO irradiation will be carried out to further clarify the stability of ZAO in AO circumstances. Table 3

Sample

Electrical parameters of ZAO samples before and after AO irradiation State/AO irradiation

U/(ȍ·cm)

Ne /cm–3

P/(cm2· V–1·V–1

Before

1.80×10–4

1.27×1021

27.40

After

1.83×10–4

1.26×1021

27.30

Before

4.68×10–4

8.65×1020

15.50

After

4.69×10–4

8.63×1020

15.60

Before

1.18×10–3

4.41×1020

12.00

After

1.20×10–3

4.30×1020

12.10

Before

1.88×10–2

5.02×1019

6.65

After

1.90×10–2

5.01×1019

7.00

No.11

(b) Sample No.12 No.12

No.13

No.14

(c) Sample No.13

(d) Sample No.14

Fig.3 SEM micrographs of ZAO samples from No.11 to No. 14 before (left) and after (right) AO irradiation.

Although each sample shows none of essential transformation in surface morphology, the AO irradiation have apparent corrosive effects on all of the four ZAO samples. (2) Electrical properties U, P and Ne of ZAO thin films before and after

4 Conclusions ITO samples were irradiated with increasing amount of fluence of AO from 0.95×1020 to 3.66× 1020 atoms/cm2. ZAO samples having different sheet resistivity were exposed to AO irradiation with the amount of fluence being 1.2×1020 atoms/cm2. By means of advanced characterization methods, the effects of AO irradiation on ITO and ZAO have been clarified and concluded as follows. The strong oxidizing ability possessed by AO could decrease not only the O2– vacancy defects resulting in the increase of Hall mobility, but also the carrier concentration of ITO thin films. The changes of mobility and carrier concentration accordingly influence the conductivity properties of ITO samples. However, it is also noticed that the structure and electrical properties of ZAO could hardly be influenced by AO irradiation, although the AO has an


· 468 ·

evident erosion effects on the surface morphology of the ZAO.

[9]

in low earth orbit. Nuclear Instruments and Methods in Physics Research B 2003; 208: 48-57.

References [10] [1]

Beneking C, Rech B, Wieder S, et al. Recent developments of

[2]

Physica C 2003; 388-389: 301-302. [11]

Hao X T, Ma J, Ma H L, et al. Characterization of ZnO:Al films

films. Journal of Beiijng University of Aeronautics and Astro-

in China A 2002; 45(3): 394-399. Liu Z X. Space physics. China: Harbin Technology University

nautics 2005; 31(2):236-241. [in Chinese] [12]

Press, 2005. [in Chinese] [4]

ZnO:Al films. Thin Solid Films 2005; 491: 54-60. [13]

thermal control film. Vacuum & Cryogenics 2005; 10: 152-158. [5]

bound of magnetic field. Acta Aeronautica et Astronautica Sinica

of ZnO:Al deposited by reactive magnetron sputtering. J Vac Sci

[6]

Zhao X H, Shen Z G, Wang Z T, et al. Experimental investigations

Shen Z G, Zhao X H, Wang Z T, et al. Ground-based atomic oxygen effects simulation facility with the filament discharge and

Zafar S, Ferekides C S, Morel D L. Characterization and analysis

Technol A 1995; 13: 2177-2182.

Wang W W, Diao X G, Wang Z, et al. Preparation and characterization of high-performance direct current magnetron sputtered

Wang Z M, Lu Y S, Feng Y D, et al. Influence of space radiation and atomic oxygen environment on performance of conductive

Wang W W, Diao X G, Wang Z, et al. Optical, electrical and infrared emissing properties of D.C. magnetron sputtered ZnO:Al thin

deposited by r.f. magnetron-sputtering at low temperature. Science

[3]

Ueno S, Kashiwaya S, Terada N, et al. STM/STS studies on YBa2Cu3O7-į thin films treated with an atomic oxygen beam.

silicon thin film solar cells on glass substrates. Thin Solid Films 1999; 351: 241-246.

Grossman E, Gouzman I. Space environment effects on polymers

2000; 21(5): 425-430. [in Chinese] [14]

Ashcroft N W. Solid state physics. Beijing: World Publishing Corp, 2004.

of atomic oxygen, temperature, ultraviolet radiation effects on a spacecraft material polytetrafluoroethylene. Acta Aeronautica et Astronautica Sinica 2001; 22(3):235-239. [in Chinese] [7]

Zhao X H, Shen Z G, Xing Y S, et al. Experimental study of vacuum ultraviolet radiation effects and its synergistic effects with atomic oxygen on a spacecraft material polytetrafluoroethylene. Chinese Journal of Aeronautics 2004; 17(3): 181-186.

[8]

Chambers A R, Harris I L, Roberts G T. Reactions of spacecraft materials with fast atomic oxygen. Materials Letters 1996; 26: 121-131.

Biography: Wang Wenwen Born in Oct. 1980, she received B.S. from Beijing University of Aeronautics and Astronautics in 2002, and earned a Ph.D. degree in material physics and technology in 2007. Her academic interest covers properties and irradiation effects upon transparent conductive oxide thin films. E-mail: [email protected]