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Effect of post-annealing on the properties of copper oxide thin films obtained from the oxidation of evaporated metallic copper
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Applied Surface Science 254 (2008) 3949–3954 www.elsevier.com/locate/apsusc
Effect of post-annealing on the properties of copper oxide thin films obtained from the oxidation of evaporated metallic copper V. Figueiredo a, E. Elangovan a,*, G. Gonc¸alves a, P. Barquinha a, L. Pereira a, N. Franco b, E. Alves b, R. Martins a, E. Fortunato a a
Materials Science Department, CENIMAT-I3N and CEMOP-UNINOVA, FCT-UNL, Campus de Caparica, 2829-516 Caparica, Portugal b LFI, Dep. Fı´sica, Instituto Tecnolo´gico e Nuclear, EN10, 2686-953 Sacave´m, Portugal Received 23 October 2007; received in revised form 12 December 2007; accepted 14 December 2007 Available online 23 December 2007
Abstract Thin films of copper oxide were obtained through thermal oxidation (100–450 8C) of evaporated metallic copper (Cu) films on glass substrates. The X-ray diffraction (XRD) studies confirmed the cubic Cu phase of the as-deposited films. The films annealed at 100 8C showed mixed Cu–Cu2O phase, whereas those annealed between 200 and 300 8C showed a single cubic Cu2O phase. A single monoclinic CuO phase was obtained from the films annealed between 350 and 450 8C. The positive sign of the Hall coefficient confirmed the p-type conductivity in the films with Cu2O phase. However, a relatively poor crystallinity of these films limited the p-type characteristics. The films with Cu and CuO phases show n-type conductivity. The surface of the as-deposited is smooth (RMS roughness of 1.47 nm) and comprised of uniformly distributed grains (AFM and SEM analysis). The post-annealing is found to be effective on the distribution of grains and their sizes. The poor transmittance of the as-deposited films (<1%) is increased to a maximum of 80% (800 nm) on annealing at 200 8C. The direct allowed band gap is varied between 2.03 and 3.02 eV. # 2008 Elsevier B.V. All rights reserved. PACS : X-ray diffraction-crystal structure 61.10.N; Scanning electron-microscopy 61.16.B; Atomic force-microscopy-surface structure 61.16.C; Optical properties 78.66; Electron beam deposition 81.15.E Keywords: X-ray diffraction; Physical vapor deposition processes; Copper oxide thin films; Oxides; Semiconducting materials
1. Introduction Oxides of copper are known to show p-type conductivity and are attracting renewed interest as promising TCO materials in the fabrication of a wide range of optoelectronic devices. Nontoxic, economic and abundant availability and relatively simple formation of oxide makes copper oxide as an interesting material [1]. Two common forms of copper oxide are cuprous oxide or cuprite (Cu2O) and cupric oxide or tenorite (CuO) [1]. Both the CuO (monoclinic) and Cu2O (cubic) are p-type semiconductors with a band gap of 1.9–2.1 and 2.1–2.6 eV respectively [2]. However, CuO is also reported to possess ntype conductivity [3]. Cu2O is one of the oldest semiconducting materials [4] which is useful in solar cell applications [5]. As a
* Corresponding author. Tel.: +351 212948562; fax: +351 212948558. E-mail address: [email protected] (E. Elangovan). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.12.019
p-type semiconductor, conduction arises from the presence of holes in the valence band (VB) due to doping/annealing [6]. Unlike the other oxides, the top states of VB in Cu2O are derived from the fully occupied Cu 3d10 states which are not localized (more mobile) when converted into holes [7]. Formation of Cu vacancy is an often stated mechanism for the origin of p-type conductivity in Cu2O [6]. Stoichiometric Cu2O is a well-studied material and first principles studies can be found elsewhere [8]. CuO is also an interesting candidate in the applications of solar energy converting devices, gas sensors and superconducting devices [9]. A variety of deposition techniques such as pulsed magnetron sputtering [1], electro deposition [10,11], thermal oxidation [6], chemical deposition [12], dc reactive sputtering [13], RF reactive sputtering [14], ion beam sputtering [15], plasma evaporation [16], sol–gel [17], molecular beam epitaxy [18] and chemical vapor deposition [19] have been employed to study copper oxide films. As the development of p-type transparent conducting oxide [20] is one
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V. Figueiredo et al. / Applied Surface Science 254 (2008) 3949–3954
of the key technologies for p–n junction-based oxide devices, the research on copper oxide thin films has attracted the researchers globally. The copper oxide films in the present study were obtained through the conventional and inexpensive thermal oxidation of evaporated metallic copper (Cu) films. The physical properties of the films were studied in order to find the possibility to use them in p–n junction based applications. 2. Experimental details Thin films of Cu were deposited on microscopic glass substrates (25 mm 25 mm 1 mm) by e-beam evaporating the Cu pellets (CERAC; 99.999%). The distance between the source and substrate was 25 cm. The thickness of the films measured using profilometry was found to be 150 nm, which is in agreement with the value obtained through quartz crystal monitor mounted in the evaporation unit. The as-deposited Cu films were post-annealed in a Barnstead Thermolyne F21130 tubular furnace, at temperatures ranging from 100 to 450 8C in air for 30 min. The thickness of the as-deposited films is increased by about 13–20%, to ranging 170–180 nm on annealing due to the difference in lattice parameter of the Cu and copper oxide phases. Similar results have been obtained earlier by Matsumura et al. [21]. The as-deposited and postannealed films were characterized by X-ray diffraction (XRD), Hall measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV–vis spectrophotometer. A radiofrequency power generator (13.6 MHz) from Advanced Energy (Model: RFX 2500) was used for sputtering the target. The thickness of the films was measured using a surface profilometer (Dektak3) with an accuracy of 20 nm. The crystal structure of the films was confirmed using an X-ray diffractometer (DMAX-III C from Rigaku; sealed tube, Cu Ka radiation) in Bragg–Brentano geometry (u/2u coupled). Optical transmittance was measured using a double-beam spectrophotometer (Shimadzu UV-3100). The surface morphology was analyzed using scanning electron microscopy (Hitachi; SU-70). The electrical parameters were estimated using a Hall measurements setup (BioRad HL5500 Hall system) with a permanent magnet of 5 kG in van der Pauw configuration. Tapping mode AFM experiments were performed in a Multimode AFM microscope coupled to a Nanoscope IIIa Controller (Digital Instruments, Veeco). Commercially etched silicon tips with a typical resonance frequency of ca. 300 kHz were used as AFM probes. 3. Results and discussion It is perceptible that the XRD patterns obtained from the films in the 2u ranging 30–608 (Fig. 1) are polycrystalline. The diffraction peaks of (1 1 1)# and (2 0 0)# obtained from the asdeposited films were matched with the standard Cubic Cu phase (ICDD File: 04-0836; space group #225). The reflection from (1 1 1)# plane was found to be the strongest orientation. Annealing at 100 8C partially oxidized the films to copper(I) oxide (cuprous oxide; Cu2O) which lead to the mixed Cu–Cu2O phase. A diffraction peak from the (1 1 1)b plane of cubic Cu2O
Fig. 1. Comparison of XRD patterns of the copper oxide films as a function of annealing temperature (symbol representations of the planes: (#) metallic Cu films, (b) Cu2O films and (~) CuO films).
(ICDD File: 05-0667; space group #224) was obtained along with the other reflections from Cu phase. A single Cu2O phase was obtained for the films annealed at 200 and 250 8C, with a strongest orientation along (1 1 1)b plane. When the annealing temperature is increased to 300 8C, the Cu2O phase transforms into copper(II) oxide (cupric oxide; CuO) phase. However, a small component of (1 1 1)b diffraction peak is retained that resulted in mixed Cu2O–CuO phase. A similar kind of mixed Cu2O–CuO phase for the films annealed at 300 8C is reported D earlier [12]. The diffraction peaks of ð1 1 1Þ and (1 1 1)~ from the monoclinic CuO phase were identified from the standard data (ICDD File: 45-0937 and 48-1548; space group #15). A further increase in annealing temperature (350 8C) leads to the formation of a single CuO phase, retaining the diffraction D peaks of ð1 1 1Þ and (1 1 1)~. The conversion of Cu2O to CuO during heating in the presence of oxygen is well known [22]. It is earlier reported by Nair et al. [12] that the Cu2O phase convert into CuO phase when annealed at 350 8C, which corroborate the present work. The composition of CuO is maintained even for the annealing at 450 8C. It is also reported that at still higher temperature, CuO could revert to Cu2O [20], but however this possibility is not verified in the present work.
V. Figueiredo et al. / Applied Surface Science 254 (2008) 3949–3954
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Table 1 XRD data of the copper oxide films as a function of annealing temperature (symbol representations: (#) metallic Cu films, (b) Cu2O films and (~) CuO films) Annealing temperature (8C)
Phase
hkl
2u
I/I0
#
RT
Cu
(1 1 1) (2 0 0)#
43.37 50.70
100 14
100
Cu
(1 1 1)# (2 0 0)#
43.37 50.70
100 17
Cu2O
(1 1 1)b
36.56
25
200 250
Cu2O Cu2O
b
(1 1 1) (1 1 1)b
36.56 36.56
100 100
300
Cu2O CuO
(1 1 1)b
36.56 35.70
73 64
38.73
100
35.70
73
38.73
100
38.73
100
38.73
100
350
CuO
400
CuO
450
CuO
D
ð1 1 1Þ (1 1 1)~ D
ð1 1 1Þ (1 1 1)~ D
ð1 1 1Þ (1 1 1)~ D
ð1 1 1Þ (1 1 1)~
35.70
75
38.73
100
The obtained XRD peaks are in good agreement with the earlier reports [9–14]. The XRD data are summarized in Table 1. The electrical properties were estimated from the Hall measurements in van der Pauw configuration. The type of conductivity was confirmed from the sign of Hall coefficient. The different electrical parameters are summarized in Table 2 along with a few optical data. The as-deposited and low temperature (100 8C) annealed films showed n-type conductivity. The conductivity is changed to p-type for the films annealed between 200 and 300 8C, where the Cu2O phase is dominant. However, the conductivity is reverted to n-type for the further increase in annealing temperature to 350 8C, where CuO phase is dominant. The variation in type of conductivity reveals that the films are p-type conducting when they crystallize in Cu2O phase. The variation of electrical properties is shown schematically in Fig. 2 as a function of annealing temperature. The bulk resistivity (r) of the films annealed at 100 8C is increased from 2.73 105 to 5.51 102 V cm for the increase in annealing temperature
Fig. 2. Variation of electrical properties of the copper oxide films as a function of annealing temperature.
to 300 8C. The r is then decreased slightly with increasing annealing temperature by an order of magnitude to 3.1 101 V cm when annealed at 450 8C. The carrier concentration (n) of the films (dominating metallic Cu phase) annealed at 100 8C decreased from 3.75 1022 cm3 by about six orders of magnitude to 3.69 1016 cm3 when annealed at 200 8C. The n is then gradually increased with increasing annealing temperature to reach 7.19 1017 cm3 at 450 8C. The mobility (m) of the films annealed at 100 8C decreased unexpectedly from 6.11 to 1.56 cm2/V s for the increase in annealing temperature to 200 8C. The m then decreased gradually to 0.28 cm2/V s at 450 8C. The evident aspect of electrical properties is that the films with Cu2O show p-type conductivity. However, relatively poor crystallinity of these films leads to reduced electrical characteristics. It may be noteworthy that the Hall measurements were carried out again after about 7 months and the electrical properties are found to be the same. This indicates that the deposited films have high environment stability. The surface microstructure of the films was analyzed by AFM. The surface of the as-deposited Cu films is very smooth and the grains are distributed uniformly. The RMS roughness of these films is found to be 1.47 nm. The RMS roughness of the films is increased to 11.78 nm on annealing at 100 8C, presumably due to the random distribution of the grains. The RMS roughness is increased to a maximum of 14.42 nm on annealing at 200 8C but then decreased gradually to a value of
Table 2 Optical and electrical data of the copper oxide films as a function of annealing temperature T (8C)
RT 100 200 250 300 350 400 450
t (mm)
0.15 0.15 0.17 0.17 0.17 0.18 0.18 0.18
AVT (%)
0.1 5.1 65.6 49.4 38.0 44.4 43.0 36.4
Band gap (eV)
– 2.70 3.02 2.57 2.85 2.03 2.79 2.80
Carrier type
n n p p p n n n
Resistivity Rsh (V/&)
r (V–cm)
0.29 1.82 6.37 106 1.48 107 3.24 107 1.07 107 3.22 106 1.73 106
3.51 106 2.73 10-5 108 251 551 192 58 31
Mobility (cm2 V1 s1)
Carrier concentration (cm3)
18.30 6.11 1.56 1.27 0.21 0.26 0.22 0.28
9.72 10 22 3.75 10 22 3.69 10 16 1.95 10 16 5.45 10 16 1.25 10 17 4.98 10 17 7.19 10 17
T—annealing temperature; t—film thickness; AVT—average visible transmittance; Rsh—sheet resistivity; r—electrical resistivity.
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Fig. 3. AFM microstructures of the copper oxide films as a function of annealing temperature.
Fig. 4. SEM microstructures of the copper oxide films as a function of annealing temperature.
V. Figueiredo et al. / Applied Surface Science 254 (2008) 3949–3954
10.26 nm when annealed at 450 8C. The typical AFM microstructures obtained from the as-deposited and those annealed at 100, 250 and 450 8C films are shown in Fig. 3. The surface of the as-deposited films is comprised of uniformly distributed and tightly packed grains with the size ranging 10–20 nm. However, there appear few bigger grains with the size as big as 40 nm. When the films are annealed at 100 8C, the surface show randomly distributed cluster of grains. Further, the grains vary in size between 10 and 30 nm with few agglomerated grains as big as 60 nm on top of smaller grains. There appear few drains throughout the surface of the films. The drains seem to contain grains which are not distinguishable presumably due to the technical limitation of AFM. The drains are suppressed for the 250 8C annealing, which probably suggest that the grains are distributed uniformly due to the increased surface energy. The grains are cubical in shape and the size of the grains varies between 15 and 30 nm. The grains are held together by a uniformly distributed bunches, which gives a cauliflower-like morphology. There appear few agglomerated grains as big as 70 nm on top of this morphology. The size of the grains increases to a range 20–40 nm when the films are annealed at 450 8C, with fewer agglomerated grains on the surface. The typical microstructures of the films analyzed by SEM are shown in Fig. 4. The surface of the as-deposited films is very smooth and well packed with tiny grains. The films annealed at 100 8C comprise cubical shaped grains size of which varying between 10 and 40 nm, on top of very fine grains. The size of the grains is increased to ranging 40–90 nm when annealed at 250 8C. The grains are agglomerated and it is very hard to identify the shape of the grains. The grains appear to grow as stacking of layers and correspondingly the surface looks rough. The stacking of grains as layers is clearly visible when the films are annealed at 450 8C, which is akin to the etched surface. The size of grains decreased to a range 20–70 nm. The transmittance spectra recorded in the wavelength ranging 350–2500 nm are shown in Fig. 5 as a function of annealing temperature. The as-deposited films showed poor transmittance (<1%), which is understandable due to the high absorption of the metallic Cu. The transmittance is increased on annealing, and the transmittance of the films annealed at 200 8C is varying between 70 and 80%. The average visible transmittance (AVT) calculated in the wavelength ranging 600– 800 nm is varied between 50 and 66%. The minimum AVT of 5 obtained for the films annealed at 100 8C is increased with increasing temperature to 66% at 200 8C. The AVT is then decreased to 50% for the films annealed at 250 8C. The AVT of the films annealed at 300 8C is reduced to be around 40%. The transmittance data are summarized along with electrical properties in Table 2. The optical absorption in the UV region is dominated by the optical band gap (Eg) of the semiconductor that is related to the optical absorption coefficient (a) and the incident photon energy (hn) by the relation a = (Eg hn)n [23], where ‘n’ depends on the kind of optical transition that prevails. Specifically, n is 1/2, 3/2, 2 and 3 when the transition is direct-allowed, direct-forbidden, indirect-allowed, and indirect-forbidden respectively. The copper
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Fig. 5. Comparison of transmittance spectra of the copper oxide films as a function of annealing temperature.
Fig. 6. Comparison of the a2 vs. hn plots of the copper oxide films as a function of annealing temperature.
oxide film is known to be a direct-allowed semiconductor, and hence a graph is plotted (Fig. 6) with a2 versus photon energy (hn) as a function of annealing temperature. Intersect of the xaxis from the extrapolation of the linear portion of the curve to a2 = 0 gives the value of Eg. The calculated direct-allowed Eg estimated from the plot is varied ranging 2.03–3.02 eV. The obtained values are in agreement with the previous reports [2– 6]. Due to the poor transmittance and undistinguishable absorption edge, the Eg of undoped films (metallic Cu) could not be estimated. The Eg of the films annealed at 100 8C is 2.70 eV, which is increased to 3.02 eV when annealed at 200 8C but then decreased significantly with the further increase in annealing temperature. The variation of Eg is oscillatory when correlated with the annealing temperature, which is quantitatively presented in Table 2. To the best of authors’ knowledge the value of 3.02 eV obtained in the present study is the highest reported Eg value for copper oxide thin films. A relatively low Eg value (2.03 eV) obtained for the films annealed at 350 8C may probably suggest that the films have structural defects which needs to be studied in detail.
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4. Conclusion Thin films of copper oxide were obtained through oxidization of the metallic Cu films deposited on glass substrates by e-beam evaporation. It is demonstrated that the cubic Cu phase of the asdeposited films changes into single cubic Cu2O phase for the annealing between 200 and 300 8C, and whereas the films annealed between 350 and 450 8C show a single monoclinic CuO phase. The films with dominating Cu2O phase are p-type conducting, but however, a relatively poor crystallinity of these films limited the p-type characteristics. The films with Cu and CuO phases show n-type conductivity. The surface of the asdeposited films is smooth (RMS roughness of 1.47 nm) and is comprised of uniformly distributed grains. The post-annealing is found to be effective on the distribution of grains and their sizes. The poor transmittance of the as-deposited films (<1%) is increased to a maximum of 80% (800 nm) on annealing at 200 8C. The direct allowed band gap is varied between 2.03 and 3.02 eV. The p-type characteristics need to be improved to make these films useful for applications like transparent p–n junction based devices. Acknowledgements This work was partially funded by the Portuguese Science Foundation (FCT-MCTES) through projects POCI/CTM/ 55945 and POCI/CTM/55942 and by European contract NMP3-CT-2006-032231 (Multiflexioxides project). References [1] E.M. Alkoy, P.J. Kelly, Vacuum 79 (2005) 221.
[2] S.C. Ray, Sol. Energy Mater. Sol. Cells 68 (2001) 307. [3] L.O. Grondahl, Rev. Mod. Phys. 5 (1993) 141. [4] Y.S. Chaudharya, A. Agrawala, R. Shrivastava, V.R. Satsangib, S. Dassa, Intl. J. Hydrogen Energy 29 (2004) 131. [5] B.P. Rai, Solar Cells 25 (1988) 265. [6] A.O. Mousa, T. Akomolafe, M.J. Carter, Sol. Energy Mater. Sol. Cells 51 (1998) 305. [7] J. Ghisjen, L.H. Tjeng, J. van Elp, H. Eskes, J. Westerink, G.A. Sawatzky, M.T. Czyzyk, Phys. Rev. B 38 (1988) 11322. [8] M. Nolan, S.D. Elliott, Phy. Chem. Chem. Phys. 8 (2006) 5350. [9] C. Arijit, G. Vinay, K. Sreenivas, Rev. Adv. Mater. Sci. 4 (2003) 75. [10] X. Mathew, N.R. Mathews, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 70 (2001) 277. [11] T. Mahalinga, J.S.P. Chitra, J.P. Chu, H. Moon, H.J. Kwon, Y.D. Kim, J. Mater. Sci.: Mater Electron. 17 (2006) 519. [12] M.T.S. Nair, L. Guerrero, O.L. Arenas, P.K. Nair, Appl. Surf. Sci. 150 (1999) 143. [13] P. Samarasekara, M.A. Mallika Arachchi, A.S. Abeydeera, C.A.N. Fernando, A.S. Disanayake, R.M.G. Rajapakse, Bull. Mater. Sci. 28 (2005) 483. [14] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, G.K. Paul, T. Sakurai, Sol. Energy 80 (2006) 715. [15] K.H. Yoon, W.J. Choi, D.H. Kang, Thin Solid Films 372 (2003) 250. [16] K. Santra, C.K. Sarker, M.K. Mukherjee, B. Ghosh, Thin Solid Films 213 (1992) 226. [17] A.Y. Oral, E. Mensur, M.H. Aslan, E. Basaran, Mater. Chem. Phys. 83 (2004) 140. [18] K.P. Muthe, J.C. Vyas, S.N. Narang, D.K. Aswal, S.K. Gupta, D. Bhattacharya, R. Pinto, G.P. Kothiyal, S.C. Sabharwal, Thin Solid Films 324 (1998) 37. [19] T. Maruyama, Sol. Energy Mater. Sol. Cells 56 (1998) 85. [20] J. Wang, V. Sallet, F. Jomard, A. Rego, E. Elangovan, R. Martins, E. Fortunato, Thin Solid Films 515 (2007) 8780. [21] H. Matsumura, A. Fujii, T. Kitatani, Jpn. J. Appl. Phys. 35 (1996) 5631. [22] A.F. Trotman-Dickenson (Ed.), Comprehensive Inorganic Chemistry, vol. 3, Pergamon, Oxford, 1973, pp. 16–39. [23] J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, NJ, 1971, p. 34.
Applied Surface Science 254 (2008) 3949–3954 www.elsevier.com/locate/apsusc
Effect of post-annealing on the properties of copper oxide thin films obtained from the oxidation of evaporated metallic copper V. Figueiredo a, E. Elangovan a,*, G. Gonc¸alves a, P. Barquinha a, L. Pereira a, N. Franco b, E. Alves b, R. Martins a, E. Fortunato a a
Materials Science Department, CENIMAT-I3N and CEMOP-UNINOVA, FCT-UNL, Campus de Caparica, 2829-516 Caparica, Portugal b LFI, Dep. Fı´sica, Instituto Tecnolo´gico e Nuclear, EN10, 2686-953 Sacave´m, Portugal Received 23 October 2007; received in revised form 12 December 2007; accepted 14 December 2007 Available online 23 December 2007
Abstract Thin films of copper oxide were obtained through thermal oxidation (100–450 8C) of evaporated metallic copper (Cu) films on glass substrates. The X-ray diffraction (XRD) studies confirmed the cubic Cu phase of the as-deposited films. The films annealed at 100 8C showed mixed Cu–Cu2O phase, whereas those annealed between 200 and 300 8C showed a single cubic Cu2O phase. A single monoclinic CuO phase was obtained from the films annealed between 350 and 450 8C. The positive sign of the Hall coefficient confirmed the p-type conductivity in the films with Cu2O phase. However, a relatively poor crystallinity of these films limited the p-type characteristics. The films with Cu and CuO phases show n-type conductivity. The surface of the as-deposited is smooth (RMS roughness of 1.47 nm) and comprised of uniformly distributed grains (AFM and SEM analysis). The post-annealing is found to be effective on the distribution of grains and their sizes. The poor transmittance of the as-deposited films (<1%) is increased to a maximum of 80% (800 nm) on annealing at 200 8C. The direct allowed band gap is varied between 2.03 and 3.02 eV. # 2008 Elsevier B.V. All rights reserved. PACS : X-ray diffraction-crystal structure 61.10.N; Scanning electron-microscopy 61.16.B; Atomic force-microscopy-surface structure 61.16.C; Optical properties 78.66; Electron beam deposition 81.15.E Keywords: X-ray diffraction; Physical vapor deposition processes; Copper oxide thin films; Oxides; Semiconducting materials
1. Introduction Oxides of copper are known to show p-type conductivity and are attracting renewed interest as promising TCO materials in the fabrication of a wide range of optoelectronic devices. Nontoxic, economic and abundant availability and relatively simple formation of oxide makes copper oxide as an interesting material [1]. Two common forms of copper oxide are cuprous oxide or cuprite (Cu2O) and cupric oxide or tenorite (CuO) [1]. Both the CuO (monoclinic) and Cu2O (cubic) are p-type semiconductors with a band gap of 1.9–2.1 and 2.1–2.6 eV respectively [2]. However, CuO is also reported to possess ntype conductivity [3]. Cu2O is one of the oldest semiconducting materials [4] which is useful in solar cell applications [5]. As a
* Corresponding author. Tel.: +351 212948562; fax: +351 212948558. E-mail address: [email protected] (E. Elangovan). 0169-4332/$ – see front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.12.019
p-type semiconductor, conduction arises from the presence of holes in the valence band (VB) due to doping/annealing [6]. Unlike the other oxides, the top states of VB in Cu2O are derived from the fully occupied Cu 3d10 states which are not localized (more mobile) when converted into holes [7]. Formation of Cu vacancy is an often stated mechanism for the origin of p-type conductivity in Cu2O [6]. Stoichiometric Cu2O is a well-studied material and first principles studies can be found elsewhere [8]. CuO is also an interesting candidate in the applications of solar energy converting devices, gas sensors and superconducting devices [9]. A variety of deposition techniques such as pulsed magnetron sputtering [1], electro deposition [10,11], thermal oxidation [6], chemical deposition [12], dc reactive sputtering [13], RF reactive sputtering [14], ion beam sputtering [15], plasma evaporation [16], sol–gel [17], molecular beam epitaxy [18] and chemical vapor deposition [19] have been employed to study copper oxide films. As the development of p-type transparent conducting oxide [20] is one
3950
V. Figueiredo et al. / Applied Surface Science 254 (2008) 3949–3954
of the key technologies for p–n junction-based oxide devices, the research on copper oxide thin films has attracted the researchers globally. The copper oxide films in the present study were obtained through the conventional and inexpensive thermal oxidation of evaporated metallic copper (Cu) films. The physical properties of the films were studied in order to find the possibility to use them in p–n junction based applications. 2. Experimental details Thin films of Cu were deposited on microscopic glass substrates (25 mm 25 mm 1 mm) by e-beam evaporating the Cu pellets (CERAC; 99.999%). The distance between the source and substrate was 25 cm. The thickness of the films measured using profilometry was found to be 150 nm, which is in agreement with the value obtained through quartz crystal monitor mounted in the evaporation unit. The as-deposited Cu films were post-annealed in a Barnstead Thermolyne F21130 tubular furnace, at temperatures ranging from 100 to 450 8C in air for 30 min. The thickness of the as-deposited films is increased by about 13–20%, to ranging 170–180 nm on annealing due to the difference in lattice parameter of the Cu and copper oxide phases. Similar results have been obtained earlier by Matsumura et al. [21]. The as-deposited and postannealed films were characterized by X-ray diffraction (XRD), Hall measurements, atomic force microscopy (AFM), scanning electron microscopy (SEM) and UV–vis spectrophotometer. A radiofrequency power generator (13.6 MHz) from Advanced Energy (Model: RFX 2500) was used for sputtering the target. The thickness of the films was measured using a surface profilometer (Dektak3) with an accuracy of 20 nm. The crystal structure of the films was confirmed using an X-ray diffractometer (DMAX-III C from Rigaku; sealed tube, Cu Ka radiation) in Bragg–Brentano geometry (u/2u coupled). Optical transmittance was measured using a double-beam spectrophotometer (Shimadzu UV-3100). The surface morphology was analyzed using scanning electron microscopy (Hitachi; SU-70). The electrical parameters were estimated using a Hall measurements setup (BioRad HL5500 Hall system) with a permanent magnet of 5 kG in van der Pauw configuration. Tapping mode AFM experiments were performed in a Multimode AFM microscope coupled to a Nanoscope IIIa Controller (Digital Instruments, Veeco). Commercially etched silicon tips with a typical resonance frequency of ca. 300 kHz were used as AFM probes. 3. Results and discussion It is perceptible that the XRD patterns obtained from the films in the 2u ranging 30–608 (Fig. 1) are polycrystalline. The diffraction peaks of (1 1 1)# and (2 0 0)# obtained from the asdeposited films were matched with the standard Cubic Cu phase (ICDD File: 04-0836; space group #225). The reflection from (1 1 1)# plane was found to be the strongest orientation. Annealing at 100 8C partially oxidized the films to copper(I) oxide (cuprous oxide; Cu2O) which lead to the mixed Cu–Cu2O phase. A diffraction peak from the (1 1 1)b plane of cubic Cu2O
Fig. 1. Comparison of XRD patterns of the copper oxide films as a function of annealing temperature (symbol representations of the planes: (#) metallic Cu films, (b) Cu2O films and (~) CuO films).
(ICDD File: 05-0667; space group #224) was obtained along with the other reflections from Cu phase. A single Cu2O phase was obtained for the films annealed at 200 and 250 8C, with a strongest orientation along (1 1 1)b plane. When the annealing temperature is increased to 300 8C, the Cu2O phase transforms into copper(II) oxide (cupric oxide; CuO) phase. However, a small component of (1 1 1)b diffraction peak is retained that resulted in mixed Cu2O–CuO phase. A similar kind of mixed Cu2O–CuO phase for the films annealed at 300 8C is reported D earlier [12]. The diffraction peaks of ð1 1 1Þ and (1 1 1)~ from the monoclinic CuO phase were identified from the standard data (ICDD File: 45-0937 and 48-1548; space group #15). A further increase in annealing temperature (350 8C) leads to the formation of a single CuO phase, retaining the diffraction D peaks of ð1 1 1Þ and (1 1 1)~. The conversion of Cu2O to CuO during heating in the presence of oxygen is well known [22]. It is earlier reported by Nair et al. [12] that the Cu2O phase convert into CuO phase when annealed at 350 8C, which corroborate the present work. The composition of CuO is maintained even for the annealing at 450 8C. It is also reported that at still higher temperature, CuO could revert to Cu2O [20], but however this possibility is not verified in the present work.
V. Figueiredo et al. / Applied Surface Science 254 (2008) 3949–3954
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Table 1 XRD data of the copper oxide films as a function of annealing temperature (symbol representations: (#) metallic Cu films, (b) Cu2O films and (~) CuO films) Annealing temperature (8C)
Phase
hkl
2u
I/I0
#
RT
Cu
(1 1 1) (2 0 0)#
43.37 50.70
100 14
100
Cu
(1 1 1)# (2 0 0)#
43.37 50.70
100 17
Cu2O
(1 1 1)b
36.56
25
200 250
Cu2O Cu2O
b
(1 1 1) (1 1 1)b
36.56 36.56
100 100
300
Cu2O CuO
(1 1 1)b
36.56 35.70
73 64
38.73
100
35.70
73
38.73
100
38.73
100
38.73
100
350
CuO
400
CuO
450
CuO
D
ð1 1 1Þ (1 1 1)~ D
ð1 1 1Þ (1 1 1)~ D
ð1 1 1Þ (1 1 1)~ D
ð1 1 1Þ (1 1 1)~
35.70
75
38.73
100
The obtained XRD peaks are in good agreement with the earlier reports [9–14]. The XRD data are summarized in Table 1. The electrical properties were estimated from the Hall measurements in van der Pauw configuration. The type of conductivity was confirmed from the sign of Hall coefficient. The different electrical parameters are summarized in Table 2 along with a few optical data. The as-deposited and low temperature (100 8C) annealed films showed n-type conductivity. The conductivity is changed to p-type for the films annealed between 200 and 300 8C, where the Cu2O phase is dominant. However, the conductivity is reverted to n-type for the further increase in annealing temperature to 350 8C, where CuO phase is dominant. The variation in type of conductivity reveals that the films are p-type conducting when they crystallize in Cu2O phase. The variation of electrical properties is shown schematically in Fig. 2 as a function of annealing temperature. The bulk resistivity (r) of the films annealed at 100 8C is increased from 2.73 105 to 5.51 102 V cm for the increase in annealing temperature
Fig. 2. Variation of electrical properties of the copper oxide films as a function of annealing temperature.
to 300 8C. The r is then decreased slightly with increasing annealing temperature by an order of magnitude to 3.1 101 V cm when annealed at 450 8C. The carrier concentration (n) of the films (dominating metallic Cu phase) annealed at 100 8C decreased from 3.75 1022 cm3 by about six orders of magnitude to 3.69 1016 cm3 when annealed at 200 8C. The n is then gradually increased with increasing annealing temperature to reach 7.19 1017 cm3 at 450 8C. The mobility (m) of the films annealed at 100 8C decreased unexpectedly from 6.11 to 1.56 cm2/V s for the increase in annealing temperature to 200 8C. The m then decreased gradually to 0.28 cm2/V s at 450 8C. The evident aspect of electrical properties is that the films with Cu2O show p-type conductivity. However, relatively poor crystallinity of these films leads to reduced electrical characteristics. It may be noteworthy that the Hall measurements were carried out again after about 7 months and the electrical properties are found to be the same. This indicates that the deposited films have high environment stability. The surface microstructure of the films was analyzed by AFM. The surface of the as-deposited Cu films is very smooth and the grains are distributed uniformly. The RMS roughness of these films is found to be 1.47 nm. The RMS roughness of the films is increased to 11.78 nm on annealing at 100 8C, presumably due to the random distribution of the grains. The RMS roughness is increased to a maximum of 14.42 nm on annealing at 200 8C but then decreased gradually to a value of
Table 2 Optical and electrical data of the copper oxide films as a function of annealing temperature T (8C)
RT 100 200 250 300 350 400 450
t (mm)
0.15 0.15 0.17 0.17 0.17 0.18 0.18 0.18
AVT (%)
0.1 5.1 65.6 49.4 38.0 44.4 43.0 36.4
Band gap (eV)
– 2.70 3.02 2.57 2.85 2.03 2.79 2.80
Carrier type
n n p p p n n n
Resistivity Rsh (V/&)
r (V–cm)
0.29 1.82 6.37 106 1.48 107 3.24 107 1.07 107 3.22 106 1.73 106
3.51 106 2.73 10-5 108 251 551 192 58 31
Mobility (cm2 V1 s1)
Carrier concentration (cm3)
18.30 6.11 1.56 1.27 0.21 0.26 0.22 0.28
9.72 10 22 3.75 10 22 3.69 10 16 1.95 10 16 5.45 10 16 1.25 10 17 4.98 10 17 7.19 10 17
T—annealing temperature; t—film thickness; AVT—average visible transmittance; Rsh—sheet resistivity; r—electrical resistivity.
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Fig. 3. AFM microstructures of the copper oxide films as a function of annealing temperature.
Fig. 4. SEM microstructures of the copper oxide films as a function of annealing temperature.
V. Figueiredo et al. / Applied Surface Science 254 (2008) 3949–3954
10.26 nm when annealed at 450 8C. The typical AFM microstructures obtained from the as-deposited and those annealed at 100, 250 and 450 8C films are shown in Fig. 3. The surface of the as-deposited films is comprised of uniformly distributed and tightly packed grains with the size ranging 10–20 nm. However, there appear few bigger grains with the size as big as 40 nm. When the films are annealed at 100 8C, the surface show randomly distributed cluster of grains. Further, the grains vary in size between 10 and 30 nm with few agglomerated grains as big as 60 nm on top of smaller grains. There appear few drains throughout the surface of the films. The drains seem to contain grains which are not distinguishable presumably due to the technical limitation of AFM. The drains are suppressed for the 250 8C annealing, which probably suggest that the grains are distributed uniformly due to the increased surface energy. The grains are cubical in shape and the size of the grains varies between 15 and 30 nm. The grains are held together by a uniformly distributed bunches, which gives a cauliflower-like morphology. There appear few agglomerated grains as big as 70 nm on top of this morphology. The size of the grains increases to a range 20–40 nm when the films are annealed at 450 8C, with fewer agglomerated grains on the surface. The typical microstructures of the films analyzed by SEM are shown in Fig. 4. The surface of the as-deposited films is very smooth and well packed with tiny grains. The films annealed at 100 8C comprise cubical shaped grains size of which varying between 10 and 40 nm, on top of very fine grains. The size of the grains is increased to ranging 40–90 nm when annealed at 250 8C. The grains are agglomerated and it is very hard to identify the shape of the grains. The grains appear to grow as stacking of layers and correspondingly the surface looks rough. The stacking of grains as layers is clearly visible when the films are annealed at 450 8C, which is akin to the etched surface. The size of grains decreased to a range 20–70 nm. The transmittance spectra recorded in the wavelength ranging 350–2500 nm are shown in Fig. 5 as a function of annealing temperature. The as-deposited films showed poor transmittance (<1%), which is understandable due to the high absorption of the metallic Cu. The transmittance is increased on annealing, and the transmittance of the films annealed at 200 8C is varying between 70 and 80%. The average visible transmittance (AVT) calculated in the wavelength ranging 600– 800 nm is varied between 50 and 66%. The minimum AVT of 5 obtained for the films annealed at 100 8C is increased with increasing temperature to 66% at 200 8C. The AVT is then decreased to 50% for the films annealed at 250 8C. The AVT of the films annealed at 300 8C is reduced to be around 40%. The transmittance data are summarized along with electrical properties in Table 2. The optical absorption in the UV region is dominated by the optical band gap (Eg) of the semiconductor that is related to the optical absorption coefficient (a) and the incident photon energy (hn) by the relation a = (Eg hn)n [23], where ‘n’ depends on the kind of optical transition that prevails. Specifically, n is 1/2, 3/2, 2 and 3 when the transition is direct-allowed, direct-forbidden, indirect-allowed, and indirect-forbidden respectively. The copper
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Fig. 5. Comparison of transmittance spectra of the copper oxide films as a function of annealing temperature.
Fig. 6. Comparison of the a2 vs. hn plots of the copper oxide films as a function of annealing temperature.
oxide film is known to be a direct-allowed semiconductor, and hence a graph is plotted (Fig. 6) with a2 versus photon energy (hn) as a function of annealing temperature. Intersect of the xaxis from the extrapolation of the linear portion of the curve to a2 = 0 gives the value of Eg. The calculated direct-allowed Eg estimated from the plot is varied ranging 2.03–3.02 eV. The obtained values are in agreement with the previous reports [2– 6]. Due to the poor transmittance and undistinguishable absorption edge, the Eg of undoped films (metallic Cu) could not be estimated. The Eg of the films annealed at 100 8C is 2.70 eV, which is increased to 3.02 eV when annealed at 200 8C but then decreased significantly with the further increase in annealing temperature. The variation of Eg is oscillatory when correlated with the annealing temperature, which is quantitatively presented in Table 2. To the best of authors’ knowledge the value of 3.02 eV obtained in the present study is the highest reported Eg value for copper oxide thin films. A relatively low Eg value (2.03 eV) obtained for the films annealed at 350 8C may probably suggest that the films have structural defects which needs to be studied in detail.
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4. Conclusion Thin films of copper oxide were obtained through oxidization of the metallic Cu films deposited on glass substrates by e-beam evaporation. It is demonstrated that the cubic Cu phase of the asdeposited films changes into single cubic Cu2O phase for the annealing between 200 and 300 8C, and whereas the films annealed between 350 and 450 8C show a single monoclinic CuO phase. The films with dominating Cu2O phase are p-type conducting, but however, a relatively poor crystallinity of these films limited the p-type characteristics. The films with Cu and CuO phases show n-type conductivity. The surface of the asdeposited films is smooth (RMS roughness of 1.47 nm) and is comprised of uniformly distributed grains. The post-annealing is found to be effective on the distribution of grains and their sizes. The poor transmittance of the as-deposited films (<1%) is increased to a maximum of 80% (800 nm) on annealing at 200 8C. The direct allowed band gap is varied between 2.03 and 3.02 eV. The p-type characteristics need to be improved to make these films useful for applications like transparent p–n junction based devices. Acknowledgements This work was partially funded by the Portuguese Science Foundation (FCT-MCTES) through projects POCI/CTM/ 55945 and POCI/CTM/55942 and by European contract NMP3-CT-2006-032231 (Multiflexioxides project). References [1] E.M. Alkoy, P.J. Kelly, Vacuum 79 (2005) 221.
[2] S.C. Ray, Sol. Energy Mater. Sol. Cells 68 (2001) 307. [3] L.O. Grondahl, Rev. Mod. Phys. 5 (1993) 141. [4] Y.S. Chaudharya, A. Agrawala, R. Shrivastava, V.R. Satsangib, S. Dassa, Intl. J. Hydrogen Energy 29 (2004) 131. [5] B.P. Rai, Solar Cells 25 (1988) 265. [6] A.O. Mousa, T. Akomolafe, M.J. Carter, Sol. Energy Mater. Sol. Cells 51 (1998) 305. [7] J. Ghisjen, L.H. Tjeng, J. van Elp, H. Eskes, J. Westerink, G.A. Sawatzky, M.T. Czyzyk, Phys. Rev. B 38 (1988) 11322. [8] M. Nolan, S.D. Elliott, Phy. Chem. Chem. Phys. 8 (2006) 5350. [9] C. Arijit, G. Vinay, K. Sreenivas, Rev. Adv. Mater. Sci. 4 (2003) 75. [10] X. Mathew, N.R. Mathews, P.J. Sebastian, Sol. Energy Mater. Sol. Cells 70 (2001) 277. [11] T. Mahalinga, J.S.P. Chitra, J.P. Chu, H. Moon, H.J. Kwon, Y.D. Kim, J. Mater. Sci.: Mater Electron. 17 (2006) 519. [12] M.T.S. Nair, L. Guerrero, O.L. Arenas, P.K. Nair, Appl. Surf. Sci. 150 (1999) 143. [13] P. Samarasekara, M.A. Mallika Arachchi, A.S. Abeydeera, C.A.N. Fernando, A.S. Disanayake, R.M.G. Rajapakse, Bull. Mater. Sci. 28 (2005) 483. [14] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, G.K. Paul, T. Sakurai, Sol. Energy 80 (2006) 715. [15] K.H. Yoon, W.J. Choi, D.H. Kang, Thin Solid Films 372 (2003) 250. [16] K. Santra, C.K. Sarker, M.K. Mukherjee, B. Ghosh, Thin Solid Films 213 (1992) 226. [17] A.Y. Oral, E. Mensur, M.H. Aslan, E. Basaran, Mater. Chem. Phys. 83 (2004) 140. [18] K.P. Muthe, J.C. Vyas, S.N. Narang, D.K. Aswal, S.K. Gupta, D. Bhattacharya, R. Pinto, G.P. Kothiyal, S.C. Sabharwal, Thin Solid Films 324 (1998) 37. [19] T. Maruyama, Sol. Energy Mater. Sol. Cells 56 (1998) 85. [20] J. Wang, V. Sallet, F. Jomard, A. Rego, E. Elangovan, R. Martins, E. Fortunato, Thin Solid Films 515 (2007) 8780. [21] H. Matsumura, A. Fujii, T. Kitatani, Jpn. J. Appl. Phys. 35 (1996) 5631. [22] A.F. Trotman-Dickenson (Ed.), Comprehensive Inorganic Chemistry, vol. 3, Pergamon, Oxford, 1973, pp. 16–39. [23] J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, NJ, 1971, p. 34.