Electronic sputtering of CuO films by high-energy ions

Nuclear Instruments and Methods in Physics Research B 314 (2013) 55–58

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Electronic sputtering of CuO films by high-energy ions N. Matsunami a,⇑, Y. Sakuma a, M. Sataka b, S. Okayasu b, H. Kakiuchida c a

Nagoya University, EcoTopia Science Institute (ESI), Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan Japan Atomic Energy Agency, Tokai 319-1195, Japan c National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan b

a r t i c l e

i n f o

Article history: Received 30 November 2012 Received in revised form 9 March 2013 Accepted 4 April 2013 Available online 20 April 2013 Keywords: CuO Electronic sputtering Modifications of electronic and atomic structures

a b s t r a c t We have studied the electronic sputtering, electronic and atomic structure modifications of CuO films under high-energy ion impact, for comparison with the other materials such as cuprite oxides (Cu2O) and further understanding of the electronic-excitation effects. It is found that the sputtering yields are much larger (by a factor of 100–1000) than those of the elastic collisions, confirming that the electronic excitations play a dominant role in the sputtering. The electronic sputtering yield Y of CuO is well fitted , Se being the electronic stopping power (keV/nm). This is exceptionally close to linear by Y = 4.0S1:08 e dependence on Se, in contrast to the super linear dependence for other oxides. The direct bandgap is determined to be 2.1(±0.1) eV for unirradiated films and no appreciable modification of the bandgap is observed by the 100 MeV Xe ion impact. Disordering and lattice compaction were observed by the ion impact. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Atomic displacement induced by electronic excitations in nonmetallic solids is one of keys for understanding of material modifications under high-energy-heavy ions impacts. Sputtering due to electronic excitations, called electronic sputtering is a direct measure of atomic displacements by electronic excitations near surface. It has been extensively studied for oxides and nitrides, and suggested that the bandgap is an important factor for the electronic sputtering, larger electronic sputtering yields with larger bandgap [1]. However, data is scarce for the bandgap less than 3 eV. One of materials with small bandgap such as Cu2O (cuprite) with the bandgap of 2.54 eV has been investigated [2]. Cuprite is known as p-type semiconductor [3,4] and widely studied for applications to, e.g., solar cells [5]. It is found that the electronic sputtering yields of Cu2O are somewhat larger than those estimated from the suggested bandgap-dependence [2]. The sputtering yields of Cu3N (bandgap = 1.4 eV) [6] and WO3 (bandgap = 3 eV) [7] are comparable with that of SiO2 [1] and much larger than those estimated from the bandgap dependence [1]. Hence, it is desired to extend such studies to materials having smaller bandgap (< 3 eV) in order to further examine the bandgap dependence of the electronic sputtering and compare with the results of the other materials such as Cu2O for further understanding of the electronic excitation effects, i.e., the electronic-excitation induced material modifications and atomic-displacement of non-metallic solids. ⇑ Corresponding author. E-mail address: [email protected] (N. Matsunami). 0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.04.029

In this study, we have investigated the electronic sputtering yields of cupric oxides (CuO) films by high-energy ions and modifications of optical absorption and bandgap as well as crystalline structure in terms of X-ray diffraction (XRD). This material has been chosen, because the optical bandgap is 2 eV [8] and smaller than that of Cu2O. The present results are compared with those of other oxides such as Cu2O [2]. It is found that the electronic sputtering yields of CuO are larger than those of Cu2O and the suggested bandgap-dependence of the electronic sputtering yields is discussed. 2. Experimental Sample preparation method is similar to that of Cu2O [2]. Briefly, CuO films were prepared on MgO (0 0 1)-substrates at 700 °C by using a reactive-RF-magnetron-sputter-deposition (offaxis) method with a Cu disk target (99.99% purity) in Ar and O2 mixture gas. Total gas pressure was 13 Pa, and the flow rates of Ar and O2 were approximately 3 and 1.3 cc/min (CuO films are easily grown by increasing the O2 pressure employed for Cu2O preparation). Crystalline quality and orientation of films were examined by XRD (RIGAKU RAD-II with Cu-Ka X-rays at Radioisotope Research Center, Nagoya University). XRD shows that the crystal structure of the films is monoclinic with preferential orientation of (1 1 1) [9]. The substrate temperature was optimized so that the XRD intensity takes its maximum. The (1 1 1) planar spacing (d), 0.2339 nm of the film is larger by 0.7% than the bulk value, 0.2323 nm [9]. According to Rutherford backscattering spectroscopy (RBS) of 1.8 MeV He+ ions, the film thickness is 60–100 nm

N. Matsunami et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 55–58

ENERGY (MeV)

0.6

8 O 6

2

-2

cm )

Cu ATOMS IN C-FILM (10

14

99 M e V Xe 0.4

89 M e V Ni

0.2 60 M e V Ar

0

0

1

2

3 12

4

-2

ION FLUENCE (10 cm ) Fig. 1. Cu in carbon-foil collector vs ion fluence for 198 MeV Xe (D), 99 MeV Xe (), 89 MeV Ni (s) and 60 MeV Ar (h) ion impacts on CuO films.

1.4

10

The amount of Cu in the C-foil collector after high-energy ion impact is shown in Fig. 1 as a function of the ion fluence and one sees that the amount of Cu is proportional to the ion fluence. A small amount of the energy loss in the C-foil collector (2 MeV for 200 MeV Xe, 1 MeV for 100 MeV Xe and 90 MeV Ni ions) was subtracted for further analysis. From the linear relationship, the sputtering yield of Cu per ion is evaluated using the C-foil collection efficiency of Cu (0.3) [2], and total sputtering yield, i.e., sum of sputtering yields of Cu and O is obtained by multiplying the sputtering yield of Cu by 2 (sputtering yield means total sputtering yield hereafter). Here, stoichiometric sputtering is assumed, based on the fact that the composition observed in the RBS (Fig. 2) remains nearly stoichiometric and no appreciable modifications of optical absorption and no drastic change in XRD patterns after the ion impact, as described later, meaning no decomposition by ion impact, in contrast to an exception of the ion-induced decomposition of Cu3N [6]. Furthermore, no transformation from CuO to Cu2O was observed in contrast to high-energy He ion irradiation at a quite high fluence [11]. The results are summarized in Table 1 and the sputtering yields by low energy ions, i.e., 100 keV Ne and N are also given. Since ions hit the films after passing through C-foil, the energy loss of ions in the C-foil is subtracted as mentioned above and the equilibrium charge state (Qe = 30, 25, 19 and 13 for 199 MeV Xe, 99 MeV Xe, 89 MeV Ni and 60 MeV Ar in C [12]) is assumed for further analysis and discussion. Firstly, it is noticed that the sputtering yields are much larger (by a factor of 100–1000) than those of the elastic col-

198 M e V Xe

1.2

Cu

4

CuO

1

12

3. Results and discussion

0.6

0.8

14

3

and the composition is close to stoichiometric (Cu:O = 1:1) within an estimated accuracy of 10%. Here, the stopping power [10] and film density of 4.85  1022 Cu cm 3 (6.4 g cm 3) were employed. Optical absorption was measured by using a conventional spectrometer (JASCO V570). Irradiation of high-energy ions (200 MeV Xe, 100 MeV Xe, 90 MeV Ni and 60 MeV Ar) was performed using a Tandem accelerator at Japan Atomic Energy Agency at Tokai. Beam current of high-energy ions was 1 nA cm 2 and temperature rise was estimated to be a few ten degrees as described in [6]. The carbon (C)-foil (100 nm) collector method with RBS [2] and the stopping powers [10] was applied to analyze Cu sputtered from CuO films.

YIELD (10 counts)

56

0

300

400

500

600

700

CHANNEL NUMBER Fig. 2. Rutherford backscattering spectra of CuO film on MgO for unirradiated () and irradiated with 100 MeV Xe at 2.5  1012 cm 2 (s), obtained using 1.8 MeV He. The incident and outgoing angles of He ions were 40° and 60° from the surface normal. For visible clarity, irradiated spectrum is shifted by 1. Thickness of this film is 63 nm.

Table 1 Electronic (Se) and nuclear (Sn) stopping powers in CuO (keV/nm) [10], sputtering yield Y, the calculated sputtering yield YC of the elastic collisions and projected range Rp (lm). Sputtering yields by low energy ions (100 keV Ne and N, the elastic collision dominant) are also given for comparison and from which YC are estimated. Ions

Se

Sn

Y

YC

Rp

198 MeV Xe 99 MeV Xe 89 MeV Ni 60 MeV Ar 100 keV Ne 100 keV N

31.2 24.0 17.1 9.64 0.334 0.421

0.126 0.221 0.0349 0.0154 0.276 0.124

173 120 80 48.3 0.88 0.4

0.40 0.71 0.11 0.050

12 8.3 8.0 8.0 0.11 0.14

lisions (YC), confirming that the electronic excitations play a dominant role in the sputtering. Here, YC is calculated assuming that it is proportional to the nuclear stopping power with the sputtering yield of 0.88 and 0.4 by 100 keV Ne and N ion impact (see Table 1). Secondly, it appears that the electronic sputtering yields of CuO (173, 120 and 80 for 198 MeV Xe, 99 MeV Xe and 89 MeV Ni ions) are larger than those of Cu2O (48, 38 and 10 for the corresponding ions) [2], even though the electronic stopping powers are similar for both CuO and Cu2O. Fig. 3 shows the sputtering yields vs electronic stopping power Se. The electronic sputtering yield Y is well fitted by Y = 4.0Se1:08 , Se being the electronic stopping power (keV/ nm). This is exceptionally close to linear dependence on Se, in contrast to the super linear dependence for other oxides and nitrides [1,2,6,7]. For instance, Y = 0.006S2:78 for Cu2O. e Now, ion-induced modifications of optical property and atomic structure studied by means of XRD are described. No appreciable change in the optical absorption spectra is observed, as shown in Fig. 4, except for the absorption at 0.25 lm due to color center generation of MgO substrate by the ion impact [13]. The direct bandgap is determined to be 2.1(±0.1) eV for unirradiated films as shown in the inset in Fig. 4 and this value agrees with that in [8]. It appears that the bandgap remains unchanged under the ion impact, in spite of lattice disordering in terms of XRD. The (1 1 1) diffraction intensity is reduced to approximately half at the fluence of 1.3  1012 cm 2 by 100 MeV Xe ions as shown in Fig. 5(a) and no

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stress-relaxation, i.e., lattice parameter is getting closer to the bulk value by ion impact (as mentioned in Section 2, the lattice parameter of films is larger by 0.7% than the bulk value). Another possibility is more vacancy generation than interstitial, because vacancies and interstitials tend to lead to lattice compaction and expansion, respectively. The half-width of the rocking curves of unirradiated films shown in Fig. 5(b) is obtained to be 1 deg. indicating that crystalline quality of the films are reasonably good and increased by 40% by the ion impact, meaning again disordering of the crystalline lattice. Finally, bandgap dependence of the electronic sputtering yields is discussed. The experimental sputtering yield interpolated at Se = 15 keV/nm is 75 and 11 for CuO and Cu2O. The corresponding sputtering yield is estimated to be 2 and 4 from the suggested bandgap dependence (0.1 E4g at Se = 15 keV/nm) [1]. Hence, the suggested bandgap dependence does not hold. The suggestion is based on the idea that the available energy for atomic displacement from electron-hole pairs generated by electronic excitations is proportional to the bandgap and the efficiency of atomic displacement is larger for larger the available energy Eg, which seems to work for large bandgap. The deviation from the suggested bandgap dependence is considerable for Eg < 3 eV, as seen in Fig. 6, where the sputtering yields at Se = 15 keV/nm are plotted as a function of the bandgap. The results imply that for small bandgap, the number of electron hole pairs, which is inversely proportional to the bandgap, becomes more important factor in the electronic sputtering and the electronic sputtering yields super-linearly depend on the number of electron-hole pairs. Nevertheless, the sputtering yields of WO3 are exceedingly large and other factors are required for explanation of WO3 results.

103

SPUTTERING YIELD

CuO 4Se1.08 198 MeV Xe 99 MeV Xe

102

89 MeV Ni 60 MeV Ar

10

100 Se (keV/nm)

Fig. 3. Sputtering yields of CuO vs electronic stopping power Se. The dotted line is the fit to the data.

amorphization was observed for the fluence less than 3  1012 cm 2 employed for obtaining the sputtering yields (Fig. 1). The reduction of the XRD intensity indicates disordering of the atomic structure and according to preliminary results, the disordering appears to be exceptionally independent of the ion species or Se, in contrast to the case of Cu2O, where the disordering is more pronounced for 100 MeV Xe ion impact than for 90 MeV Ni ion impact [2]. A possibility of the difference of disordering between CuO and Cu2O is that defects induced by ion impact might be different between them, though the defect structures are not known. It also appears that the (1 1 1) planar spacing or lattice parameter is reduced by 1% at 1.3  1012 cm 2 for 100 MeV Xe ions. The lattice parameter modification of CuO is similar to that in Cu2O [2]. An explanation for lattice compaction is

4. Summary Electronic sputtering of CuO has been investigated and the results are compared with those of Cu2O. The electronic sputtering yields Y are well fitted by Y = 4.0S1:08 , Se being the electronic stope ping power (keV/nm). This dependence is exceptionally close to

2.5

CuO

(ABSORBANCE*E)

2

10

ABSORBANCE

2

1.5

8 unir.

6

4

2 1

0 1.5

2

2.5

3

PHOTON ENERGY (e V)

unir.

0.5

0

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

WAVELENGTH (µm) Fig. 4. Optical absorption spectra of CuO before and after irradiation with 100 MeV Xe ion at 1.3  1012 cm illustrating the bandgap determination.

2

. Inset shows square of absorbance times photon energy E vs E,

58

N. Matsunami et al. / Nuclear Instruments and Methods in Physics Research B 314 (2013) 55–58

10 4

20

unir.

15 XRD INTENSITY (a. u.)

SPUTTERING YIELD

(a)

10 100 MeV Xe 12

1.3x10 cm

WO 3

10 3

Eg4

a-SiO2 c-SiO2

Cu3N

10 2

CuO Y2O3 Cu2O

10 1

TiO2

SCO Si3N4 AlN

STO CeO2

Al2O3 MgAl2O4

ZnO

10 0

ZrO2(YSZ)

MgO

-2

10 -1

Se=15 keV/nm

5 1

3

5

10

BAND GAP Eg (eV) Fig. 6. Sputtering yields at the electronic stopping power of 15 keV/nm vs bandgap (Eg). Data of Cu2O, Cu3N and WO3 are after [2], [6] and [7], and other data except for CuO and E4g -dependence are taken after [1].

0 37

38

39

40

2θ (DEG.)

linear with Se. Under the high-energy ion impact, optical absorption shows little change and the bandgap of CuO is unchanged. XRD intensity degrades, i.e., disordering takes place but does not amorphize and lattice compaction is observed by the ion impact.

(b) 6

XRD INTENSITY (a. u.)

unir.

References

4

2

0 17

18

19

20

21

22

θ (DEG.) Fig. 5. XRD patterns of CuO before and after irradiation with 100 MeV Xe ion at 1.3  1012 cm 2 (a) and rocking curves (b).

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