Optical and electrical properties of doped zinc oxide films prepared by ac reactive magnetron sputtering

]OURNA L OF

ELSEVIER

Journal of Non-Crystalline Solids 218 (1997) 74-80

Optical and electrical properties of doped zinc oxide films prepared by ac reactive magnetron sputtering B.

Szyszka *, S. J~iger

Fraunhofer Institute for Surface Engineering and Thin Films (IST), Bienroder Weg 54E, D-38108 Braunschweig, Germany

Abstract Aluminum-doped and indium-doped zinc oxide films have been prepared by reactive ac magnetron sputtering (twincathode arrangement with ac plasma excitation at frequency of 40 kHz) from metallic targets with different dopant concentrations at substrate temperature of about 573 K. The optical, electrical and structural properties of the sputtered ZnO:A1 and ZnO:In thin films have been investigated by optical spectroscopy (UV-IR), X-ray diffraction, Hall-mobility and conductivity measurements. For aluminum-doped ZnO films a minimum resistivity of 4.0 x 10 -4 ~ cm at high transparency (larger than 89% at film thickness of 530 nm) has been obtained at AI concentration in the target material of 1.2 wt%. Higher resistivity of 8.7 X 10 -4 1"1 cm (85% transmission at film thickness of 400 nm) has been observed for indium-doped zinc oxide films at dopant concentration of about 2 wt% In in the target material. © 1997 Elsevier Science B.V.

1. Introduction Multifunctional coatings with good optical transmission in the visible range and nearly metallic conductivity can be obtained by doping of wide band-gap semiconductive materials (e.g., oxides of indium, tin, zinc and their alloys). These transparent conductive oxide (TCO) films are used, for example, as transparent electrodes in flat panel displays [1], solar cells [2] and other opto-electronic devices [3], as anti-static coatings on cathode ray tubes [4] as well as transparent heating elements [5]. Their infrared reflectance is used for heat reflecting filters, e.g., low-E coatings on architectural glass [6] and on incandescent bulbs [7]. A m o n g the TCO materials available, doped zinc oxide films have promising

* Corresponding author. Tel.: + 49-531 215 5641 ; fax: + 49-531 215 5900; e-mail: [email protected].

properties due to their good electrical and optical properties in combination with low costs and nontoxicity. In this study, aluminum-doped and indium-doped zinc oxide films have been prepared by ac reactive magnetron sputtering. This technique offers important advantages compared to conventional reactive dc magnetron sputtering: (i) The arcing problem due to electrical breakdown of charged layers on the target surface is overcome by the neutralization of these surface charges within one halfcycle of the ac process. (ii) The problem of the disappearing anode is solved, because the ac process offers a metallic anode over the whole target lifetime. (iii) The power density is not limited by arcing any longer. Furthermore, it is possible to drive the cathodes in metallic mode, if the power density is sufficiently large. Thus, an increase of deposition rate by a factor of 3 to 10 compared with reactive dc magnetron sputter-

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B. Szyszka, S. Jiiger / Journal of Non-Crystalline Solids 218 (1997) 74-80

ing is obtained [4,8]. (iv) An intense energetic ion flux to the substrate results as a consequence of the low excitation frequency [9]. Investigations on ac plasma discharges have revealed larger ion energies (up to some tenth of eV) and about 10 times larger ion current densities as in the case of dc magnetron sputtering [ 10].

2. Experimental details Fig. 1 shows schematically the reactive ac magnetron sputtering with the dual magnetron source (TwinMag, Leyboid Systems, Germany). This source consists of an arrangement of two cathodes with target dimension of 488 X 88 mm connected to a matching box and driven by a 10 kW sinusoidal power supply operating at 40 kHz. The tilting angle between the cathodes was optimized to achieve high deposition rate and uniform film thickness in a static deposition mode. The gas inlet system was designed to minimize the oxygen partial pressure at the target without reducing the deposition rate. Thus, no apertures were used. To stabilize the plasma discharge in the transition mode, the reactive gas flow was adjusted by a control ring. Borosilicate (Schott, AF 45) and float-glass substrates of dimensions up to 450 X 50 mm were used as substrates. The substrates were placed at a target-substrate distance of 90 mm and were heated by an electric heater. Further details of the sputtering system have been described elsewhere [10]. Metallic targets with purities better than 99.99% (Degussa AG, Germany) were used for the reactive

75

magnetron sputtering process. The targets consist of four differently doped segments, which were bonded on a copper plate. The dopant concentration of the Zn:A1 and Zn:In target segments ranged from 1 to 4 wt% A1 and In, respectively. The maximum power density was limited to 2 W / c m 2 due to indirect cooling of the targets. Prior to deposition, the AF 45 substrates were cleaned by flowing air. No further treatment of the substrates was found to be necessary to achieve good reproducibility. The chamber was evacuated to a pressure less than 3 X 10 - 4 Pa and the substrates were heated to a temperature between 553 and 573 K for 1 h. To remove target contamination, the surfaces were sputtered in an Ar plasma with power density of 1 W / c m 2 and afterwards for 5 min in a gas flow and with the power used for deposition. After deposition, the substrates cooled to 373 K. At this temperature, the chamber was opened. Some properties of the films were measured by electrical, optical and structural techniques. The film thickness was usually determined by conventional stylus measurements and compared with optical transmission data. The sheet resistance at room temperature was measured by a linear four point probe. Hall investigations were performed using the vander-Pauw method. Optical transmission was measured by a double-beam spectrophotometer in the spectral range between 250 and 2500 nm. The structure and morphology were studied by X-ray diffraction (XRD, copper K~ radiation, Bragg-Brentano setup) and by atomic force microscopy (AFM).

3. Experimental results

substrate

3.1. Deposition p r o c e s s

reactivega~'LOJ

~

0

powersupply,40kHz Fig. 1. Schematic diagram of the ac magnetron source (TwinMag).

The deposition parameters used are summarized in Table 1. The most important parameters are oxygen flow and substrate temperature [11,12]. The substrate temperature was fixed between 553 and 573 K. Fig. 2 shows typical results for the effects of oxygen flow on target voltage, deposition rate and film resistivity in case of reactive ac magnetron sputtering of ZnO:A1 films. A transition of target voltage from high voltage in metallic mode (metallic target surface) to low voltage in oxide mode (oxidized

B. Szvszka S. Jiiger / Journal of Non-Co'stalline Solids 218 (1997) 74-80

76

target surface) is shown in Fig. 2(a). By adjustment of oxygen flow using the control ring it was possible to stabilize the discharge even in the transition mode. The corresponding data on deposition rate and resistivity are displayed in Fig. 2(b). The deposition rate as a function of oxygen flow has a m a x i m u m near the oxide mode, whereas the deposition rate in metallic mode was less. This behavior is explained by the vapor pressure of Zn at substrate temperature of 570 K, which gives rise to partial re-evaporation of the

500

> &

metallic mode

400

o >

350 I-

300

1'0

(a)

1'5

2'0

2'5

Oxygen flow [sccm]

I

~1.0

_

t/

=

g

= 0.6 o

10.2>

0.4

°

.

o.o (b)

~

LJ

;o

Total pressure Ar flow/O 2 flow Power density Substrate temperature Target-substrate distance Deposition mode Deposition rate

ZnO:A1

ZnO:In

0.13-0.15 Pa 60 sccm/18 sccm 1 W/cm 2 553-573 K 90 mm oxide 1.1 nm/s

0.13-0.15 Pa 60 sccm/40 sccm 2 W/cm 2 553-573 K 90 mm metallic 1.7 nm/s

mode oxide mod(

transition

450

Table 1 Deposition parameters on ac reactive magnetron sputtering of ZnO:AI and ZnO:In films

;5

2'o

I

2'5

t

Oxygen flow [sccm]

Fig. 2. Influence of oxygen flow on target voltage (a) and on deposition rate and resistivity (b), respectively (ac sputtering of ZnO:AIfilms, PAr = 0.15 Pa, constant power mode, P = 1000 W, P / A = 1 W/cm 2, dopant content of the target material: 1.2 wt% AI; the error bar shown in (a) is representativefor the voltage drift and the adjustmentof oxygen flow during deposition;error bars in (b): see Table 2. The lines drawn through the data are a guide for the eye).

metal atoms. At larger oxygen flux, the formation of oxide improved and, thereby, the probability of reevaporation decreased. The deposition rate in the oxide mode was limited by the small sputtering yield of the oxidized target. The resistivity data, as shown in Fig. 2, were obtained at a dopant content of about 1.2 wt% AI in the target material. A dependence of resistivity on oxygen flow over several orders of magnitude was observed. Small resistivity of 4.0 × 10 -4 ~-~ cm was obtained at an optimum oxygen flow corresponding to the largest deposition rate. At larger oxygen fluxes, surplus oxygen is incorporated at grain boundaries, thus the free electrons, which are responsible for the electrical conductivity, are trapped at the grain boundaries. Our experiments have shown that it is possible to obtain high quality films in metallic mode if the power density is increased. This approach was successful in case of indium-doped ZnO films at power density of 2 W / c m 2. In case of ZnO:A1 films further increase of power density was found to be necessary due to the high enthalpy of formation of A120 3 which promotes target poisoning [ 12].

3.2. Physical properties o f ZnO:Al and ZnO:ln films Ac sputtered ZnO:A1 and ZnO:In films with high conductivity and good optical transparency have been prepared at optimized deposition conditions with dopant concentration of about 1.2 wt% AI and 2.0 wt% In in the target material, respectively. Fig. 3 shows the dependence of the electrical properties of ZnO:AI films as a function of the aluminum content in the target. The film resistivity increases

77

B. Szyszka, S. Jiiger / Journal of Non-Crystalline Solids 218 (1997) 74-80

60 .

-. . . . .

10

N

50

~P-

"t

4o 20,-,~

10: 10 p = 4 . 0 1 0 .4 n c m

...... 0

e,t. T.~.t[ w t - %1

Fig. 3. Dependence of Hall mobility /x, carrier density N and resistivity p on dopant concentrationof ac sputtered ZnO:A1films (as prepared; d ---500 nm, error bars: see Table 2, the lines drawn through the data are a guide for the eye).

monotonously by more than one order in magnitude with increase of the AI content of the target from 1 to 4 wt%. At a doping level of about 1.2 wt% A1, small film resistivity of 4.0 × 10 .4 ~ cm is obtained. Both, the mobility and the carrier density decrease monotonously with increasing A1 content. In the range between 1 and 3 wt% A1, the Hall mobility decreases linearly from 32 to 11 c m 2 / V s. At a high doping level of 4 wt% AI in the target material, a minor decrease down to 9 c m 2 / V s is observed. Minor dependence of carrier density on A1 concentration is obtained at low doping levels. Between 1 and 2 wt% A1, the carrier density decreases by a factor of 0.8 from 4 . 9 × 102o to 4 . 0 X 102o cm 3, indicating ineffective incorporation of A1 atoms even at low doping level of 2 wt% AI in the target material. A steep decrease of carrier density

down to 3.8 × 1019 cm -3 is observed when the A1 content is raised to 4 wt%. To explain these strong dependencies, the following effects have to be considered: films with a small dopant content of 1.2 wt% A1 in the target material exhibit good crystal structure in terms of grain size and grain orientation (see Fig. 6). The dopant atoms are effectively incorporated substitutionally at Zn sites in the ZnO lattice. Thus, ionized impurity scattering becomes dominant in comparison to grain boundary scattering [13,14]. The dependence of mobility on the carrier density in case of ionized impurity scattering is given by the formula ix ct N 2/3, which is not sufficient to describe the strong decrease of mobility at low doping levels. Further scattering centers are formed by the incorporation of alumina precipitations, as shown by the decrease of carrier density, if the doping level is raised. Both the mobility and the carrier density are affected by the oxidization of the dopant atoms. The results of Hall-measurements after post annealing for 1 h at 450°C in air are given in Table 2. A strong increase of resistivity by a factor of 20 is observed. This breakdown of conductivity may be caused by the formation of A1203, which lowers the carrier density by a factor of ten and the Hall-mobility by a factor of three. The dependence of electrical properties on dopant concentration in case of ZnO:In films is shown in Fig. 4. Only a minor change of resistivity is observed in the range between 1 to 4 wt% In in the target material. The m i n i m u m resistivity of 8.7 X 10 -4 f~ cm is reached at dopant concentration of 2 wt% In due to high carrier density of 6.2 × 102o cm -3 at low Hall-mobility of 11 c m 2 / V s. Monotonous decrease of Hall-mobility from 14 to 8.9 c m 2 / V s is observed when the In concentration is raised from 1 to 4 wt%. The carrier concentration increases between 1 to 2 wt% In by a factor of two, indicating

Table 2 Electrical properties of ac sputtered ZnO:A1 and ZnO:In films (SnO2:Sb data taken from Ref. [10]; experimental errors: Ad= 5 nm, AR~h/R~h= 1%, A p / p = 2 % , A N / N = 15~, A/z//z= 15%) Film d (nm) R~h (~]) p (~~ cm) N (cm 3) /x (cme/V s) ZnO:AI ZnO:A1post annealed ZnO:In SnO2:Sb

530 480 400 250

7,5 270 22 67

4.0 x l0 4 1.3 × 10 2 8.7 X 10-4 1.7 x 10 3

4.9 × 4.6 × 6.2 × 4.0 ×

102o 1019

102o 102o

32 11 I1 9.2

B. Szyszka, S. Jiiger / Journal of Non-Crystalline Solids 218 (1997) 74-80

78

10-:

1.0

16

;';

---4--- N .%

.... g

1' ,

08

12

,,

~

'-,

'

.......

--

t

10z "=

-'"'""-..

""" ........ "'J"

ZnO:AI as propared,

",\, ,. ,~ ~ o : , ,

.~ 0.6

0=4.0X 10

.. p,~.~,, o=8.7x ~o

i 0.4

0.2 10

~"

"~.~,

p = 8.7 104 ncm

2

0.0

cm,T,m.[wt. %] Fig. 4. Dependence of Hall mobility /x, carrier density N and resistivity p on dopant concentration of AC sputtered ZnO:In films (as prepared; d = 400 nm, error bars: see Table 2, the lines drawn through the data are a guide for the eye).

efficient incorporation of In atoms in this range. At higher doping levels, a decrease of carrier concentration is obtained but a slight increase is to be seen in the range from 3 to 4 wt% In. This increase is attributed to structural modifications of the Z n O matrix due to incorporation of indium [15]. The electrical properties of ac sputtered ZnO:In films are comparable to SnOz:Sb films (see Table 2). Optical transmission spectra of ZnO:A1 and ZnO:In films are shown in Fig. 5. Films were prepared at a dopant content of 1.2 wt% A1 and 2 wt% In in the target material. High transmission in the visible range of more than 89% is obtained for ZnO:AI films of 530 n m thickness. A lower transmission of 85% is observed in case of ZnO:In films of 400 nm thickness. The decrease of transmission almost to zero in the near infrared region indicates high infrared reflectance due to the low sheet resistance smaller than 8 ~ / s q in case of ZnO:A1. Post annealed ZnO:A1 films show a high transmission of 91% in the visible range, which is nearly the transmission of the uncoated substrate. The increase of transmission after post annealing treatment is attributed to lower absorption losses due to the decreased carrier density. Furthermore, an increase of transmission in the near infrared region between 1200 and 2500 nm is observed, corresponding to the

.

.

.

.

,

500

,

1000

1500

2000

2500

Wavelength [nm]

Fig. 5. Transmissionof aluminum-dopedand indium-dopedZnO films (as prepared and post annealed at 723 K for 1 h in air; dopant content of the target material: 1.2 wt% AI and 2 wt% In, respectively, d = 400-530 nm). higher film resistivity. For all ZnO:AI and ZnO:In films examined, no significant dependence of optical transmission on dopant content was observed. Investigations on crystal structure and morphology of films with optimal electrical properties have revealed highly textured crystallites with c-axis orientation of the hexagonal ZnO phase normal to the (oo2)

ZnO:AI

(a)

20t .m.

(O04)

ZnO:.AI

-

q 33.6

.8

'

a

.8

34.0

'

34.2

'

34.4

'

.6

34.8

'

35.0

'

Fig. 6. X-ray diffractionpatterns of ZnO:AI and ZnO:In films (as prepared).

B. Szvszka S. Jiiger / Journal of Non-Crystalline Solids 218 (1997) 74-80

79

Fig. 7. Atomic force micrograph of an ac sputtered ZnO:AI film (as prepared, RMS roughness of 2.0 nm, d = 550 nm).

substrate [16] in both cases, ZnO:A1 and ZnO:In. A typical X R D spectrum o f a Z n O : A I film is g i v e n in Fig. 6(a). The grain sizes, which w e r e estimated f r o m the full width at the half m a x i m u m of the (002)

reflexes are 30 nm in case o f ZnO:A1 and 20 nm in case o f ZnO:In, respectively. The smaller grain size o f Z n O : I n films is attributed to an increased density o f nucleation sites due to the incorporation o f indium

Fig. 8. Atomic force micrograph of an ac sputtered ZnO:ln film (as prepared, RMS roughness of 3.4 nm, d = 400 nm).

80

B. Szyszka, S. J~iger/ Journal of Non-C~stalline Solids 218 (1997) 74-80

[15]. Fig. 6(b) and (c) show a shift of the (002) peak from the bulk position, due to lattice widening in case of ZnO:In films and lattice shrinking in case of ZnO:AI films. The lattice constant (c-axis) is 0.520 nm in case of ZnO:A1 films and 0.523 n m in case of ZnO:In, respectively, the bulk value is 0.5215 nm. Figs. 7 and 8 show typical AFM micrographs of ZnO:A1 and ZnO:In films. Low RMS roughness of 2.0 n m is observed in the case of ZnO:A1 films. ZnO:In films show a higher RMS roughness of 3.4 nm. Non-circular shaped crystallites can be observed in ZnO:In films which have been never found in ZnO:A1 films. Furthermore, a transition of film texture from (002) to (100) orientation was observed by other groups in case of sprayed ZnO:In films while the In dopant level was changed from low to high values [15,17]. The formation of the non-circular shaped grains observed is attributed to that effect. The content of these grains in the film may be too small to be detected by XRD.

good optical transparency of more than 89% in the visible range at film thickness of 530 nm. ZnO:In films with a m i n i m u m resistivity of 8.7 × 10 4 f~ cm have been prepared at a dopant concentration of 2 wt% In in the target material. These films with a thickness of 400 nm show an optical transmission of more than 85% in the visible range.

Acknowledgements The authors would like to thank their colleagues K. Schiffmann for AFM micrographs and U. Bringmann for helpful discussions. Also, we are pleased to acknowledge the financial support by the BMBF under contract 13N6520.

References 4. Discussion The electrical properties of ZnO films are modified by In and AI incorporation in different ways: as a consequence of the smaller enthalpy of formation of I n 2 0 3 compared to A120 3 a higher maximum carrier density is observed in case of ZnO:In films. On the other hand, the In incorporation results in poorer crystalline quality and, as a consequence, low Hall-mobility. The higher resistivity of ZnO:In films (and SnO2:Sb films) compared to ZnO:A1 films is caused by the low Hall-mobility of these films.

5. Conclusions High quality, transparent and conductive ZnO:AI and ZnO:In films have been prepared by ac reactive magnetron sputtering from metallic targets with different dopant concentrations and substrate temperatures of about 573 K. ZnO:A1 films with good electrical properties films have been prepared at a dopant concentration of about 1.2 wt% A1 in the target material. A m i n i m u m resistivity of about 4.0 X 10 4 f~ cm has been reached. These films show

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