SURFACE AND INTERFACE ANALYSIS, VOL. 24, 671-674 (1996)

Influence of Copper Dopants on the Resistivity of ZnO Films

A. Hartmann,* M. I<. Puchert and R. N. Lamb Surface Science & Technology, School of Chemistry, The University of New South Wales, Sydney 2052, Australia

Copper 2p,,, x-ray photoemission spectroscopy (XPS) measurements and near-edge x-ray absorption spectroscopy (NEXAFS) studies of copperdoped ZnO thin films were carried out in order to understand the role of the dopants in increasing the 6lm resistivity. c-Axis-oriented ZnO films (thickness 120 nm) were deposited by r.f. sputtering on Cr-coated SiOJSi substrates and subsequently implanted with metallic copper (0.5 at.%). After implantation, XPS results indicated that the dopant consists of a mixture of Cu' and Cuo. The c-axis film resistivity was found to decrease to 7 x lo' Q, - cm (prior to implantation: 5 x 10" Q, * cm). Oxygen annealing resulted in complete oxidation of the dopants in the film surface and bulk (Cuz+) as indicated by XPS and bulk-sensitive fluorescence NEXAFS. The film resistivity increased to 2 x 10" 51 - cm, which can be explained in terms of electron trapping by Cu 3d hole states of Cu". The NEXAFS results suggested that the copper dopants after oxygen annealing have preferred orientations in the ZnO matrix. Additional vacuum annealing reduced Cuz' ions to Cu', which demon- strated that the observed decrease in film resistivity from 2 x 101'-107 51- cm is due to the Cu' oxidation state.

INTRODUCTION

Zinc oxide (ZnO) is an optically transparent material (band gap 3.2 eV) which has useful piezoelectric and electrooptical properties.' Characterization and sub- sequent modification of the electrical properties of c-axis-oriented ZnO films, such as resistivity, is impor- tant to a variety of device applications. In a number of applications, especially low-frequency devices,2 the requirement for high electrical field strengths, low driving voltages and minimal leak currents necessitates maximum film resistivity. This can be achieved by doping ZnO films with metallic copper and annealing in air.3 Initially the film resistivity decreases, as expected, while subsequent annealing will actually increase the resistivity. Since very little has been reported concerning the valence state of the copper responsible for increas- ing the film resistivity, we have carried out an investiga- tion of the influence of CuO, Cu+ and Cu2+ on the conducting properties of ZnO.

EXPERIMENTAL

Zinc oxide films were deposited by r.f. magnetron sput- tering on silicon(100) substrates with a multilayer coating consisting of 1 pm of thermally grown SiO, and a 3 nm overlayer of sputtered chromium (i.e. Cr/SiO,/Si). The sputtering gas was chosen to be 100% oxygen (ultrahigh purity) to maximize the oxidation of sputtered zinc during film g r ~ w t h . ~ Films were post- deposition annealed in oxygen at 500°C (1 atm, 2000 min) and subsequently cooled (10 "C min-l) before and after implantation. Film thickness, morphology and

* Author to whom correspondence should be addressed.

CCC 0142-2421/96/090671-04 0 1996 by John Wiley & Sons, Ltd.

structure were determined by high-resolution scanning electron microscopy and x-ray diffraction (XRD). The films had a thickness of 120 nm and were found to be polycrystalline (columnar grains) with a c-axis orienta- tion normal to the substrate. The c-axis resistivity of the resulting film was 5 x lolo !2 * cm, which was deter- mined by measuring the resistance of a known film volume through silver dots applied onto the clean film (diameter 0.5 mm).

Zinc oxide films were implanted with copper after being oxygen-annealed. The implantation of metallic copper was achieved using a metallized vapour vacuum arc, high-current ion source and was carried out at room temperature with an extraction potential of 20 kV. The source was operated at 15 Hz, with a pulse duration of 250 ms, to accumulate a dose 1 f 0.2 x 10l6 ions cm-2. This resulted in a copper concentration of N 0.5 at.%. Cupric oxide reference films were prepared by reactively d.c. sputtering copper in an oxygen atmosphere and then annealing in oxygen (1 atm, 2000 min, 500"C), ensuring that it consisted entirely of CuO.

X-ray photoelectron spectroscopy studies were carried out using a monochromated A1 Ka x-ray source with a hemispherical energy analyser (Fisons Escalab 22OiXL) operated at 10 eV pass energy. The instrumen- tal resolution is at least as small as the full width at half-maximum (FWHM) of the Ag 3d,,, peak, mea- sured to be 0.47 eV. The binding energy scale was cali- brated by setting the adventitious C 1s peak to 284.8 eV. Argon ion etching was performed using a small spot size scanning ion gun (ion energy 3 keV, sample current 0.5 PA, etch area 1 mm').

The NEXAFS data were collected using synchrotron radiation from beamline 20B (Australian national Beamline Facility) at the Photon Factory, Tsukuba, Japan.5 The Cu 1s NEXAFS spectra were obtained from measuring the fluorescence yield in air. Owing to the large escape depth of the photons relative to the film

Received 14 November 1995 Accepted 17 May I996

612 A. HARTMANN, M. K. PUCHERT AND R. N. LAMB

thickness, this method provided data that were largely independent of surface effects and contamination. Separate ion chamber detectors were used to detect intensity variations in the incident photon beam and fluorescence radiation, with the fluorescence detector placed at an angle of 90" with respect to the incoming photon beam. Energy calibration was performed by setting the Cu 1s absorption edge of metallic copper to 8979 eV. The NEXAFS data were normalized with respect to the energy range 10-15 eV below the absorp- tion and 30-40 eV above. The density of empty states which give rise to features at 30-40 eV above the absorption edge is expected to have a continuous char- acter and should therefore be independent of the orien- tation or any small compositional differences6

RESULTS AND DISCUSSION

Detailed surface information concerning the oxidation state of copper ions in ZnO films can be derived by performing Cu 2p XPS and comparing with reference spectra detected under identical measurement condi- tions. Figure 1 shows Cu 2p,/, XPS spectra of metallic copper, Cu,O and CuO reference materials. A clean metallic copper surface was produced by sputter clean- ing a copper polycrystal until no 0 Is signal was detect- able. Cuprous oxide was fabricated by annealing a CuO film in ultrahigh vacuum (UHV) at 800°C. This pro- cedure is known to reduce the Cu2+ ions in CuO to Cu',' and was verified by measuring the copper-oxygen ratio.

Metallic copper (Cu') has a filled 3d band (Cu 3d") and a partly filled Cu 4s band responsible for metallic conductivity. The narrow Cu 2p3,2 XPS peak (FWHM = 0.8 eV) is centred at 932.7 eV binding energy. The Cu 3d band of Cu,O (Cu') is also filled but the 4s band is unoccupied (neglecting hybridization) and the material is a semiconductor with a bandgap of 2.17 eV.' The Cu 2p3,, peak of Cu,O is slightly broader (FWHM = 1.0 eV) compared to that of metallic copper,

l " ' / " ' I " ' l " '

944 940 936 932 928 Binding Energy (eV)

Figure 1. Copper Zp,, XPS spectra of: (a) metallic copper; (b) a Cu,O film; (c) a CuO film.

and slightly shifted towards lower binding energy (932.5 eV). Another more obvious differentiation between metallic copper and Cu,O can be obtained by measur- ing the Cu L3M45M45 Auger feature which was found to be centred at a kinetic energy of 918.6 eV for metallic copper and 916.2 eV for Cu,O (in agreement with Refs 9 and 10, respectively). Cupric oxide (Cu2+) has hole states in the Cu 3d band (Cu 3d9 configuration) and the material is a charge transfer semiconductor with a charge transfer energy of 1.4 eV.8 The Cu 2p3/, spec- trum consists of two broad features: the main peak is centred at 933.7 eV binding energy (FWHM = 3.1 eV) and can be attributed to the Cu 3d"' final state configu- ration which involves a charge transfer transition (Cu 3d9 -+ Cu 3d" L, where L is the ligand hole state), whereas the complex and broad satellite feature between 940 and 945 eV binding energy is associated with the Cu 3d9 final state configuration8

Table 1 lists the resistivities of the ZnO films after different post-deposition processes as discussed below. Figure 2(a) shows the Cu 2p3/, XPS spectrum of a 0.5 at.% Cu-doped film as implanted (no further treatment). The relatively broad peak (FWHM = 2.4 eV) is centred at 932.6 eV binding energy. The low binding energy,

Table 1. Tabulated values of ZnO film resistivity after differ- ent post-deposition processes'

ZnO film Cu dose Approximate P.

OA 0 0 5 x 10'O O A + I 1 O l e (0.5 at.%) 0.5 at.% 7 x 1 0 ' OA + I + OA 10" (0.5 at.%) 0.5 at.% 2 x 10''

I x107 OA + I + VA

processing (ions cm-') Cu concentration (a ' cm)

0.5 at.% 1 017 (0.5 at.%)

"OA=annealing in oxygen at 500°C (1 atm, 2000 rnin); VA =annealing in UHV at 800°C for 300 min; I = implantation with copper ions; pc =electrical resistivity measured perpendicular to the film.

944 940 936 932 928

Binding Energy (eV)

Figure 2. Copper 2p,,, XPS spectra of 0.5 at.% copper implanted in ZnO: (a) as-implanted; (b) after oxygen annealing; (c) after further annealing in UHV.

INFLUENCE OF COPPER DOPANTS ON THE RESISTIVITY OF ZnO FILMS 673

and the fact that virtually no higher binding energy satellite is detectable, indicates that mainly Cuo and Cu' ions are present in the surface. Owing to the prob- ability of a slightly oxidized surface, it is expected that relatively more Cuo is present in the bulk of the film. X-ray diffraction measurements indicated that the film is still crystalline and c-axis-oriented after implantation. The Cu 2p,,, peak is much broader than those of either the metallic copper or the Cu,O reference (see Fig. l), or a combination of both, which may be due to intersti- tial lattice positions and a range of different atomic environments for the dopants in the ZnO matrix. X-ray photoelectron spectroscopy depth profiling indicated a drop in copper concentration about halfway through the film, with copper only detectable in trace amounts near the substrate/film interface. The c-axis film resis- tivity was reduced from 5 x 10" 5 2 . cm prior to implantation to 7 x lo7 R . cm after implantation. This can at least partly be attributed to the presence of metallic copper, which is expected to donate 4s elec- trons and therefore contribute to the conductivity. The influence of other copper ions such as Cu+ cannot be isolated in this film as the amount present is unknown.

Figure 2(b) shows Cu 2p3,, XPS spectra of the 0.5 at.% Cu-doped film after oxygen annealing (1 atm, 2000 min, 500°C). The broad main peak (FWHM = 3.0 eV) is centred at 933.7 eV binding energy which, together with the intense higher binding energy satellite, indi- cates that the copper is oxidized to Cu2+. X-ray photo- electron spectroscopy depth profiling indicated that after oxygen annealing the copper distribution was uniform down to the substrate/film interface. The film resistivity increased to 2 x 10" R * cm, which is almost an order of magnitude higher than the resistivity of the unimplanted ZnO reference film annealed under identi- cal conditions. This would suggest that the Cu 3d hole states of Cu2+ act as traps for free electrons. However, XPS is a surface-sensitive technique and gives no infor- mation concerning the copper oxidation deep within the crystallites of the doped ZnO film, and depth profiling is known to reduce Cu2+.'' In order to confirm that Cu2+ is indeed responsible for the increased film resis- tivity and to obtain information concerning the orienta- tions of the dopants, we performed bulk-sensitive NEXAFS measurements.

The NEXAFS intensity is related to both the density and symmetry of empty electronic states above the Fermi level. Figure 3 shows Cu 1s NEXAFS spectra of a copper foil sample and a polycrystalline CuO film (no preferred orientation). Comparable measurements for Cu,O could not be performed as a well-defined Cu,O standard sample could not be prepared in air. Due to the dipole selection rules, Cu 1s NEXAFS reflects the empty p-density of states at the copper sites. Therefore, Fig. 3(a) reflects the density of empty p-states of metallic copper, and the different shape of the curve shown in Fig. 3(b) is related to the different orbital occupation and to the additional oxygen (hybrid) states of CuO. Figure 4 shows Cu 1s NEXAFS spectra of the 0.5 at.% copper-doped and oxygen post-annealed ZnO film obtained, with the angle between the photon beam polarization and the c-axis of the films being 10" and 80°, respectively. Due to the orientation dependency of NEXAFS,1Z,'3 the spectra are closely related to the c- components [Fig. 4(a)] and a,b-component [Fig. 4(b)]

9005 9000 8995 8990 8985 8980 8975 8970 Photon Energy (eV)

Figure 3. Copper 1 s NEXAFS spectra of: (a) a metallic copper foil; (b) a CuO film.

of empty p-density of states at the copper sites. Both spectra appear to be very similar to that of CuO [Fig. 3(b)], which would indicate that the copper dopants in the bulk of the film are fully oxidized, and the increased film resistivity can in fact only be attributed to the pres- ence of CU" ions. The small relative intensity differ- ences at the pre-edge area and at the peak maxima positions of the normalized spectra may indicate that the copper p-orbitals and therefore the C k 2 + ions have some preferred orientation in the ZnO matrix, probably driven thermodynamically by annealing.

The fully oxidized 0.5 at.% Cu-doped film was then annealed in UHV (800°C) in order to reduce the Cu2+ ions to Cu'. The corresponding Cu 2p3,, XPS spec- trum was taken in situ and is shown in Fig. 2(c). After the annealing, the FWHM of the main peak width was reduced to 1.6 eV and no Cu2+ satellite feature was

9005 9000 8995 8990 8985 8980 8975 8970 Photon Energy (eV)

Figure 4. Copper 1s NEXAFS spectra of 0.5 at.% copper implanted in ZnO after oxygen annealing obtained at two different angles between the photon beam polarization and the c-axis of the films: (a) 10"; (b) 80".

614 A. HARTMANN, M. K. PUCHERT AND R. N. LAMB

detectable. The Cu 2p,/, peak is centred at 932.5 eV binding energy, which is identical to the Cu 2p3/, binding energy of the Cu,O reference sample [Fig. l(a)], and the Cu L3M4,M4, Auger feature was found to be centred at 916.1 eV kinetic energy, which also clearly indicates that the copper oxidations state is + 1.l' The fact that the Cu 2p,/, peak of the doped film is still broader than that of Cu,O may again indicate intersti- tial positions and a range of atomic environments for the Cu+ dopants in the ZnO matrix. The film resistivity decreased to lo7 Q . cm, which is more than three orders of magnitude below the film resistivity of an unimplanted film annealed under identical conditions. The film was initially fully oxidized (Cu") and, since the UHV annealing treatment is known not to reduce Cu2+ to Cue: the observed resistivity changes can only be related to the influence of Cu' ions. It follows that Cu+ dopants do not cause, as suggested earlier,, increased film resistivity but in fact decrease the film resistivity. The fact that the film resistivity is even lower than that of the as-implanted film (7 x lo7 R . cm) with metallic copper present may be explained by the more uniform copper distribution after annealing.

was investigated using XPS and NEXAFS. The Cu 2p3,, XPS and resistivity measurements indicated that after implantation (no further treatment) thc average copper valence is low (Cu' and Cu'). The film resis- tivity decreased to 7 x lo7 R . cm (prior to implanta- tion: 5 x 10" SZ - cm), which is at least partly related to the presence of metallic copper. The XPS surface analysis of oxygen post-annealed films has shown that the copper dopants are oxidized to Cu2 + . Bulk-sensitive Cu 1s NEXAFS studies were employed to confirm that there is no difference between the copper oxidation states in film surface and bulk. The observed resistivity increase to 2 x 10" 0 - cm after oxygen post-annealing is explained in terms of electron trapping by Cu 3d hole states associated with Cu2 +. Furthermore, NEXAFS studies indicate that the copper dopants in the ZnO matrix may have a preferred orientation. Vacuum annealing reduces the Cuz+ ions to Cu' and cause the film resistivity to decrease from 2 x 1011-107 R . cm. This confirms that doping with Cu+ ions reduces the ZnO film resistivity, and only Cu2+ dopants result in the desired resistivity increase important for device applications.

SUMMARY AND CONCLUSIONS Acknowledgements

The authors thank J. W. Martin for the copper implantation of the The electronic structure of copper dopants ZnO films. This work was supported by a grant from the Australian (concentration = 0.5 at.%) in c-axis-oriented ZnO films Research Council.

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