Thin Solid Films 520 (2012) 6368–6374

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Crystallization and electrical resistivity of Cu2O and CuO obtained by thermaloxidation of Cu thin films on SiO2/Si substrates

L. De Los Santos Valladares a,b,c,⁎, D. Hurtado Salinas b,d, A. Bustamante Dominguez d, D. Acosta Najarro e,S.I. Khondaker f, T. Mitrelias a, C.H.W. Barnes a, J. Albino Aguiar c, Y. Majima b,g

a Cavendish Laboratory, University of Cambridge, J.J Thomson Av., Cambridge CB3 0HE, United Kingdomb Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japanc Departamento de Física, Universidade Federal de Pernambuco, 50670-901, Recife-Pe, Brazild Laboratorio de Cerámicos y Nanomateriales, Facultad de Ciencias Físicas, Universidad Nacional Mayor de San Marcos, Ap. Postal 14-0149, Lima, Perue Instituto de Física, Departamento de Materia Condensada, Universidad Nacional Autónoma de México, Ap. Postal 20-364, CP 01000, Mexicof NanoScience Technology Centre and Department of Physics, University of Central Florida, Orlando, FL 32826, USAg CREST, Japan Science and Technology Agency (JST), 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan

⁎ Corresponding author at: Cavendish Laboratory,Thomson Av. Cambridge CB3 0HE, United Kingdom. Tel1223 337200.

E-mail addresses: ld301@cam.ac.uk, luisitodv@yahoo(L. De Los Santos Valladares).

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2012.06.043

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 May 2011Received in revised form 4 June 2012Accepted 11 June 2012Available online 16 June 2012

Keywords:Thin filmsThermal oxidationEvaporationCupric oxideCuprous oxideX-ray diffractionAnnealing

In this work, we study the crystallization and electrical resistivity of the formed oxides in a Cu/SiO2/Si thin filmafter thermal oxidation by ex-situ annealing at different temperatures up to 1000 °C. Upon increasing theannealing temperature, from the X ray diffractogram the phase evolution Cu→Cu+Cu2O→Cu2O→Cu2O+CuO→CuOwas detected. Pure Cu2O films are obtained at 200 °C, whereas uniform CuO films without structuralsurface defects such as terraces, kinks, porosity or cracks are obtained in the temperature range 300–550 °C. Inboth oxides, crystallization improves with annealing temperature. A resistivity phase diagram, which is obtainedfrom the current–voltage response, is presented here. The resistivity was expected to increase linearly as a func-tion of the annealing temperature due to evolution of oxides. However, anomalous decreases are observed at dif-ferent temperatures ranges, this may be related to the improvement of the crystallization and crystallite sizewhen the temperature increases.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Copper is used in a very broad range of electronics applicationsdue to its good electrical conductivity, low cost and non-toxicity [1].However, there are still obstacles which limit the use of copper innano-electronics, mainly because of (i) its diffusion in commonlyused silicon substrates and (ii) its easy oxidation even under vacuumatmosphere. In the first case, it is widely known that copper slowlydiffuses in contact with Si substrates resulting in line-to-line leakagesor electrical shortcut of electronic devices [2]. The chemical physicsunderlying this diffusion is currently under intense study [3,4]. Toslow the diffusion, SiO2 substrates are commonly used. SiO2 is an in-sulator easily produced by thermal oxidation of Si and has great sta-bility and good adherence to copper and other noble metals [5–7].

University of Cambridge, J.J..: +44 7512825533; fax: +44

.es

rights reserved.

Regarding oxide formation; like other noble metals, thin semicon-ducting oxide layers easily form on copper surfaces upon contact withair. The two principal types of oxides formed on copper films are cu-prous oxide (Cu2O, cuprite) and cupric oxide (CuO, tenorite) [8,9].Cu2O is a p‐type semiconductor with cubic crystalline structure(a=4.27 Å) and energy gap 2.1 eV [10]. Nowadays Cu2O thin filmsare intensively investigated due to their potential applications inspintronics [11], catalysis [12] and solar cells [13,14]. Whereas, CuOis a p-type semiconductor with monoclinic crystalline structure andcell parameters a=0.4684 nm, b=0.3423 nm, c=0.5128 nm andβ=99.54°. Its energy gap is in the range 1.21–1.51 eV [15]. This isan attractive system for many researchers due to its photoconductiveand photo-thermal applications [16] and for studies of the transportmechanism in cuprates with high-TC superconductivity [17]. To com-pare, Cu atoms in the CuO structure are at the center of an oxygenrectangle forming CuO2 planes, while O atoms are at the center of acopper distorted tetrahedron [10]. This arrangement of atoms resem-bles those of superconductors YBa2Cu3O7 and LaCaBaCu3O7 [18]. Inboth superconductors, the Cu2O are the planes where superconduc-tivity occurs, whereas the nearest O atoms to these planes are respon-sible for charge transfer between the Cu2O planes and the rest of theatoms in the structure.

500 μm

Au Au

Cu oxide

Fig. 1. Optical microscope image of two gold strips evaporated on a Cu oxide film (top),and schematic representation of the configuration for the electrical measurements(bottom).

6369L. De Los Santos Valladares et al. / Thin Solid Films 520 (2012) 6368–6374

Although the formation of Cu2O and CuO is still considered theprincipal obstacle in using copper thin films in nano-electronics, theyhave promising applications in large-area electronics, especially whenthe oxides are high purity and are epitaxially grown. For example, forsolar cell applications, the literature reports different techniquesfor the fabrication of crystalline copper oxide films. Among them,molecular beam epitaxy [19], spray-pyrolysis deposition [20], anodicoxidation [21], electrochemical deposition [22,23], reactive sputtering[24–26] and chemical oxidation [27] are expensive techniques, whichrequire special conditions and they usually result in mixed phases.The simplest and cheapest technique to achieve single phase copperoxide films is thermal oxidation [28–30]. In this technique, copperoxide films are easily obtained by ex-situ annealing of copper thinfilms. The desired type of oxide can be controlled by the annealing tem-perature. Previousworks report thermal oxidation of copper films up to450 °C andmainly using glass substrates [30,31]. In this work, we studythe evolution on copper oxidation thin films up to 1000 °C. We investi-gate the phase formation, crystallization and electrical resistivity of thedifferent types of copper oxides formed on SiO2 substrates.

2. Experimental details

Copper was evaporated on polished SiO2/Si substrates using anEDWARDS 306 evaporator system under base pressure 10−5 Pa. Com-monly, SiO2(60 nm)/Si substrates are used to deposit copper formicro- and nano-electronics applications. However in this work, weused SiO2 (2 μm)/Si, because a thick amorphous SiO2 interlayer slowsthe diffusion of copper into the silicon substrate and it prevents possiblecurrent leakage during the electrical characterization. Moreover, withthis substrate, the use of adhesion promoters such as chromium ortitanium is avoided and thus the formation of alloys during annealingat high temperatures is also avoided. The SiO2 (2 μm)/Si substrateswere uniformly cut into 1×2 cm2 pieces and cleaned with acetone, iso-propyl alcohol and ultrasound following standard procedure. Small Cupellets (99.999% purity) were then evaporated from a tungsten crucibleand the substrates were located 10 cm above this source. The rate ofevaporationwasmaintained at 0.1 nm/s and the thickness of the depos-ited copperwasmeasured by a quartz crystalmicrobalance located nextto the sample. During the deposition process the substrates weremaintained at room temperature in order to characterize only post-annealing oxide formation on the samples. The pristine samples con-sisted of Cu(100 nm)/SiO2(2 μm)/Si structures.

The thermal oxidation was carried out by annealing the samples ina tubular oven (LENTON LTF-PTF Model 16/610) in air atmosphere, atdifferent temperatures from 150 °C to 1000 °C similar with theannealing of gold thin films, reported in our previous publications[32,33]. No sample was re-annealed; the thermal oxidation was car-ried out over each pristine Cu/SiO2/Si sample individually. Theheating rate was set to 10 °C/min. Once the maximum desired tem-perature was reached, it was kept for 3 h to allow free diffusion ofthe copper atoms on the surface. Next, the quenching rate was setto 1.4 °C/min so as to minimize possible stresses of the films.

Phase formation and surface crystallization were analyzed by X-raydiffraction (XRD) using a powder universal diffractometer, Bruker AXSModel D8 FOCUS (Cu-Kα1 radiation), the step size was 0.02° (2θ), andthe counting time for each point was 4 s. The average sizes of the crys-tallites were estimated from the main reflections of the XRD using theDebye–Scherrer formula and neglecting peak broadening caused by re-sidual stresses in the films: [34]

D ¼ 0:916λβhkl cosθhkl

ð1Þ

where D is the average crystallite size, λ is the wavelength of the ap-plied X-ray (λCu-Kα1=0.154056 nm), θhkl is the Bragg's angle and βhkl

is the pure diffraction line broadening (in radians), which can be easily

found bymeasuring the full width at the half maximum (FWHM) of theprincipal hkl reflection for each formed phase. The surface morphologywas observed through a scanning electron microscope (SEM) PhillipsXL-30 with acceleration voltage of 5 kV. Before SEM analysis, most ofthe surfaces were carefully scratched with the help of a needle inorder to reveal part of the SiO2 and to distinguish the copper oxidefilms.

For the electrical characterization, two identical gold strips wereevaporated on the surfaces of the samples (see Fig. 1 (top)). Thestrips are rectangular, 50 nm thickness, 100 μm width and they areseparated by 10 μm. The current–voltage measurements were per-formed in a prober station Grail10-205-LV Nagase Techno-Engineering Co., LTD at room temperature (RT) and in high vacuum8.6×10−3 Pa. Two BeCu electrodes (source (S) and drain (D))touched the center of each gold strips (see Fig. 1 (bottom)). The cur-rent response was sensed in the drain electrode after applyingsource-drain bias in the range −5 to +5 V. Then, the sheet resis-tances (RS) were calculated from the following relation:

R ¼ LW

RS ð2Þ

where R is the measured electrical resistance, L is the width of thestrips, and W is the separation gap between electrodes. After mea-suring the current flow of the SiO2 substrate using the same configu-ration, we obtained random values in the order of 10−13 A,indicating that it is highly insulating. Therefore it does not affectthe resistivity measurements of the copper films and also it will pre-vent current leakage.

3. Results and discussions

Fig. 2 shows the X ray diffraction (XRD) of the samples after annealingat different temperatures up to 1000 °C. Following the deposition, the rawsample shows sharpdiffractionpeaks at 2θ around43.49° and50° belong-ing to the Miller indexes of Cu (PDF-2 card 2-1225) (111) and (200) re-spectively. This confirms that during deposition, the Cu atoms reachedthe substrate with enough thermal energy to form a well orientated Cucrystalline layer. Following annealing at different temperatures, the

28 32 36 40 44 48 52

Cu2O(111)

Cu2O(111)

CuO(111)CuO(111)

Si CuO(111)

SiO2SiO2 Si

Si SiO2CuOCuO

CuO(111)

1000 OC

900 OC

300 OC

275 OC

250 OC

200 OC

CuO(111)CuO(111)

Cu2O(200)

2θ (Ο)

Inte

nsity

(ar

b. u

nits

)

Cu(200)Cu(111) raw

150 OC

CuO(111)

Cu2O(200)Cu2O(111) Cu(200)

CuO(111)

Fig. 2. XRD patterns of copper thin films after annealing at different temperatures up to1000 °C.

Table 1Summary for XRD data of copper oxide films obtained by thermal oxidation.

Annealing temperature (°C) Phase (hkl) 2θ (°) Crystallite size (nm)

RT Cu (111) 43.43 19Cu (200) 50.60 –

150 Cu (111) 43.43 21Cu (200) 50.60 –

Cu2O (111) 36.35 6Cu2O (200) 42.70 –

200 Cu2O (111) 36.35 13Cu2O (200) 42.70 –

250 Cu2O (111) 36.74 14CuO (111) 38.63 9CuO 111

� �35.50 –

275 Cu2O (111) 36.63 15CuO (111) 38.74 17CuO 111

� �35.50 –

300 CuO (111) 38.86 21CuO 111

� �35.70 –

900 CuO (111) 38.95 35CuO 111

� �35.70 –

1000 CuO (111) 39.05 40CuO 111

� �35.56 –

6370 L. De Los Santos Valladares et al. / Thin Solid Films 520 (2012) 6368–6374

formation of Cu2O (PDF-2 card 1-1142) with cubic structure followed bythe formation ofmonoclinic CuO (PDF-2 card 2-1040) are detected due tothe thermal oxidation. Initially, annealing the Cu film at 150 °C improvesthe crystallite to orientate along the (111) and (200) planes. However,peaks at 36.4° and 42.4° corresponding to the Miller indexes (111) and(200) of Cu2O respectively, appear. Therefore, together with the crystalli-zation improvement, annealing at this temperature makes some copperatoms to bond oxygen atoms to form Cu2O as the first oxide phase. Thisoxidation could be mediated by the equation [9]

2Cuþ 1=2O2→Cu2O: ð3Þ

At 200 °C, the Cu2O peaks got better defined and the lack of the Cu‐reflections reveals that annealing at this temperature is enough for acomplete oxidation of the copper film. After annealing at 250 °C, the(200) reflection of the Cu2O is hardly detected which means that mostof the Cu2O phase is crystallized in the (111) reflection. Furthermore,the reflections 111

� �and (111) belonging to CuO appear, revealing ini-

tial transformation of Cu2O into CuO. This phase transitionmight beme-diated by the following equation [28]

Cu2Oþ 1=2O2→2CuO: ð4Þ

The transition from Cu2O to CuO is better observed after annealingthe samples at 275 °C, in which the XRD reveals sharp reflections111

� �and (111) belonging to the CuO coexisting with Cu2O. Also, at

this annealing temperature, the Cu2O phase is highly orientated alongthe (111) plane and it may have effects in the electrical characterizationas will be discussed below. At 300 °C, most of the surface is covered byCuO and a very small amount of Cu2O is detected. The transitions fromCu to Cu2O and from Cu2O to CuO in this work are in good agreementwith the oxidation behavior of Cu thinfilms on glass substrates reportedby other authors [16,30,31]. Following further annealing above 300 °C,the CuO stabilizes and formation of other additional type of oxide isnot detected (data not shown here). However, the mean grain size ofthis oxide increases with annealing temperature as we discuss next.At high temperatures such as 900 and 1000 °C, reflections belonging

to the silicon-based substrate appear, predicting uncovered areas ofthe substrate as shown in the SEM images below.

In general, from the XRD, the sequence of the oxide phase for-mation on the copper thin films by thermal oxidation is Cu→Cu+Cu2O→Cu2O→Cu2O+CuO→CuO. Note that the formation of pureCu2O occurs at smaller interval of annealing temperature (around200 °C) than that required to obtain pure CuO films (300–1000 °C).Moreover, no copper silicides were detected by the XRD even at thehighest temperatures, which means that the SiO2 interface is thickenough to prevent diffusion of copper into the silicon substrate. Theannealing temperatures for the formation of each phase are clearlydistinguished in the XRD (Fig. 2) and listed in Table 1. The table alsolists the mean crystallite sizes obtained from the FWHM of the princi-pal directions by using the Scherrer's formula (Eq. (1)) and othercharacteristics provided by the XRD. From the table, the mean crystal-lite size of the pristine Cu sample is around 19 nm, and it slightly in-creases after annealing at 150 °C. Cu2O also appears after annealing atthis temperature with crystallite average size (c.a.) 6 nm, howeverthis size increases up to 15 nm at 275 °C. At 250 °C, CuO starts to nu-cleate with crystallite diameters of (c.a) 9 nm, and it grows withannealing temperature up to 40 nm at 1000 °C. The growth of CuOcrystallites at higher annealing temperatures such as 800–1000 °Ccauses porosity and cracks on the film surface as it is discussed inthe SEM micrographs below.

The kinetics of the oxidation mechanism depends on many factorssuch as temperature, oxygen partial pressure, time of annealing, etc. Itcannot be described solely by the techniques used here. Note that in thepresent work, annealing was performed on air and thus the oxygen pres-sure could be considered constant (atmospheric pressure). However, thephase transformation Cu→Cu+Cu2O→Cu2O→Cu2O+CuO→CuO ob-tained in this work, follows the typical thermal phase diagrams verywell [8,9]. In addition, similarly to the oxidation of common metals, themechanism of oxidation copper should occur by transference of electronsfrom the surface to find the oxygen molecules (O2) present in the sur-rounding air.

A possible mechanism of the oxidation is schematically representedin Fig. 3. Note that oxidation starts immediately after evaporation, as aslow and natural process, due to the exposition of copper on air. Likein mostmetals, the surrounding oxygen atoms are chemically adsorbedto form an invisible oxide layer on the copper surface [35], which is rep-resented in Fig. 3 (a). In addition, nucleation of oxide appears on thesurface and they begin expanding laterally to high speed ratio (Fig. 3(b)). The nucleation sites could originate from structural defects suchas grain boundaries, dislocations, impurities or particles. These islands

O2 O2O2 O2

Cu+O-

Cu Cu

Cu

O

SiO2

O2 O2 O2

O2 O2O2 O2

O2 O2O2 O2

O-

O2

Cu+

a b

c d

SiO2

Cu2OCu2O

CuO

Cu2O

Fig. 3. Representation of the oxidation mechanism of the surface on a copper film.

6371L. De Los Santos Valladares et al. / Thin Solid Films 520 (2012) 6368–6374

grow rapidly to form a Cu2O film protecting the Cu surface from oxygen.Annealing accelerates the growth of this protective film and the oxida-tion reaction (Eq. (3)) is controlled thermally.

If we assume that Cu-ions are more mobile in the oxide than theO-ions, then, the Cu ions diffuse outward to ionize the oxygenatoms from the interface to be incorporated into the oxide network.Thus new oxide layers are formed at the interface oxide/oxygen andthe Cu2O thickness increases (Fig. 3 (c)). However, the distance atwhich the ions must diffuse increases and it is expected that the ox-idation rate decreases. In order to maintain the oxidation rate con-stant, in this work the annealing temperature was increased tohigher temperatures. For instance, the coexistence of Cu2O and Cuphases has been detected at 150 °C, and at 200 °C, the copper con-sumed to become completely Cu2O. Since, there is not more purecopper to oxidize, annealing at higher temperatures transformCu2O to CuO in the same way but following the oxidation reactiondescribed in Eq. (4) (see Fig. 3 (d)).

SEMmicrographs of the copper oxide films obtained by thermal ox-idation after annealing the copper thin films at different temperaturesare shown in Fig. 4. As mentioned in the experimental section, someparts of the surfaces were carefully scratched with a needle in order toreveal the SiO2 substrate and to compare it to the copper oxide films.After annealing at 200 °C a grained surface seems to cover the substrate.After annealing at 250 °C, the grains agglomerate to form a dense andrough surface perhaps caused by the transition from Cu2O to CuO. At300 °C most of the component is CuO with a small amount of Cu2O. Italso contains small agglomerations making the surface rougher. Thesample annealed at 350 °C shows fine grains as the main componentof the film. At 400 °C, the surface becomes rougher than in the previouscases and the grained nature of the CuO surface is better appreciated.

Further annealing at 550 °C and 800 °C makes the grains grow,confirming the results obtained from the XRD. It was reported byJeong and Aydil that the improvement in grain size and crystallizationincreases the stress in copper oxide films [36]. This effect has been de-duced from this work by the presence of low porosity and cracksformed after annealing at 800 °C. After annealing at 900 °C, the gran-ular morphology is clearly distinguished. They have edged bordersand cover the whole surface and the degree and size of the porosityalso increase. Eventually, at 1000 °C, multiple fractures cover most

of the CuO surface. These fractures originate due to the growth ofthe grains and may be orientated parallel or perpendicular to the{111} planes and follow crystal dislocations. It is expected that theseporosity and fractures affect the electrical measurements of the filmas we discuss next.

The electrical characterization is presented in Fig. 5. In Fig. 5 (a), theID–VDSmeasurements taken in the configuration described in the previ-ous section (Fig. 1) are shown. As shown in the figure, ohmic responses,which in all cases pass through the zero coordinate point, are observedin the ID for all annealing temperatures. This behavior implies constantresistances in all the copper oxide films fabricated in this work. For clar-ity only the I–V characteristics of some annealing temperatures aredepicted. The highest slope corresponds to the sample annealed at150 °C, which indicates the lowest resistance of the annealed samples.At higher annealing temperatures, the slopes decrease dramatically in-dicating higher resistances.

Table 2 summarizes the analytical results of the electrical proper-ties of the formed oxides obtained by thermal oxidation. As discussedabove, annealing increases oxidation and thus, large decays in theconductivity were expected. However, in the temperature range250–275 °C and 300–900 °C anomalous decreases of the resistivityare detected. This effect is better appreciated in Fig. 5 (b), in whichthe variation of the resistivity as a function of the annealing temper-ature is depicted to obtain a resistivity phase diagram. This diagram issimilar to that obtained by Figueiredo et al. after annealing copperfilms on glass substrates for 30 min [30]. The colored areas indicatethe dominant phases for each interval of temperature. The main re-gions are Cu+Cu2O, Cu2O, Cu2O+CuO and CuO. It is not possible todetermine the exact transition temperature because the oxidation oc-curs progressively. Therefore, the changes in color contrasts are roughestimations for each transition. The main zones of the diagram arediscussed next:

– Zone Cu+Cu2O: initially, after evaporation, the surface consists ofa strained copper layer with resistivity around 6×10−5 Ωcm. Dueto the Cu2O formation, the resistivity increases exponentially up to(c.a) 1.6×104 Ωcm when annealing at 200 °C. This is because theamount of pure copper decreases considerably with thermal oxi-dation, and at 200 °C probably there is no pure copper to oxidize.

200°C 250°C 300°C

350°C 400°C 550°C

800°C 900°C 1000°C

SiO2

Cu2O SiO2

Cu2O+

CuO

SiO2

Cu2O+

CuO

SiO2

CuO

CuO CuO

SiO2

SiO2

CuO

SiO2

CuO CuO

2 μm 1 μm 2 μm

2 μm 2 μm 2 μm

2 μm 2 μm 5 μm

Fig. 4. SEM micrographs of the oxide formed on copper thin films on SiO2/Si substrates after annealing at different temperatures. The scratched areas reveal a regular, compact andsoft substrate surface.

6372 L. De Los Santos Valladares et al. / Thin Solid Films 520 (2012) 6368–6374

– Zone Cu2O: after metal copper consumption, the resistivity re-mains almost constant from 200 °C up to 250 °C.

– Zone Cu2O+CuO: Cu2O and CuO coexist. Interestingly, at 275 °C, adecrease in the resistivity is detected. This behavior in the resistiv-ity of Cu2O has been observed for many years and it is still notcompletely understood. It was initially believed that this effectwas exclusively dependent on the excess of oxygen content inthe copper oxide (See for example the study of the temperaturedependency of the electrical conductivity of copper oxide plateswith different oxygen contents reported by Zhuze and Kurchatov[37].). This assumption was improved since Cu2O was found tobe a p-type semiconductor, and thus the conduction should origi-nate due to the presence of holes in the valence band and it couldbe enhanced by doping [38]. Furthermore, it has been confirmedthat in this material, the carriers are generated by Cu-vacanciesin the crystalline structure [39,40]. Drobny and Pulfrey [25] sug-gest that during the transition from Cu2O to CuO, the CuO sitesprobably act as electrically neutral defects which replace the elec-trically active copper vacancies resulting in the decrease of the re-sistivity. In addition, Figueiredo et al. observed that this decreaseof resistivity is accompanied by an increase of the carrier concen-tration [30]. Note that, at 275 °C Cu2O crystallite reaches thehighest crystallization and the biggest size, suggesting that theseeffects might also contribute in the decrease of resistivity. Never-theless, further annealing up to 300 °C also accelerates the transi-tion of Cu2O to CuO which eventually results in an excess of thelatest and the resistivity to continue to increase.

– Zone CuO: a decrease of the resistivity is observed again in the sta-bilized CuO film. This effect has also attracted the interest of manyresearchers, especially in the field of superconductivity. CuO is ofspecial interest for its electronic structure and electron–electroninteraction, since the basic structural unit is similar to the Cu2O

planes in the high-TC superconductors [41]. The effect is still notwell understood, but similar to the Cu2O case, it might be causedby an increase of the carrier concentration. In the present work,we observed that annealing produces better crystallization andbigger sizes of the CuO crystallites leading in a decrease of the re-sistivity, suggesting that recrystallization might also contribute inthis effect. Despite, the Hall‐effect measurements have not beenperformed to determine the type of the films, previous studiesdemonstrated the p‐type nature of bulk CuO [42–44]. Eventually,the increase of the resistivity observed at temperatures higherthan 800 °C may be caused by the crystal defects, porosity and dis-locations detected above.

Due to the fact that pure Cu2O and CuO thin films have been foundto be formed at 200 °C and above 300 °C by following the thermal ox-idation technique described here, copper oxide film hetero-structurescan be easily fabricated and tested for potential electrical applica-tions. For instance, recently copper oxide hetero-junctions havebeen reported to be good candidates for solar cell applications[45–47]. Copper oxide hetero-structures Cu2O/CuO on SiO2/Si or onother type of substrates could be inexpensively fabricated as follows.A deposited copper thin film on SiO2/Si can be transformed into CuO/SiO2/Si by annealing above 300 °C. In a subsequent step, coppercould be deposited on this system and annealed at 200 °C to formthe next Cu2O layer. Alternatively, a Schottky barrier can be formedon the latest by direct deposition (e.g. by evaporation) of copper orother metal on top. Thus the top layer can be used as an electrodefor electrical connections. Eventually, since both Cu2O and CuO arep-type semiconductors, by n-doping any of them, p–n copper oxidehetero-junctions similar to those reported by other authors can beeasily fabricated and tested for solar cell applications [13]. Moreover,investigations of these materials which present various electrical

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

I D (A

)

VDS (V)

150 oC200 oC300 oC400 oC500 oC900 oC1000 oC

x 3x10-5

x 10-6

x 10-7

Cu+Cu2O

Cu2O

CuO

a

0 100 200 300 400 500 600 700 800 900 1000110010-6

10-4

10-2

100

102

104

106

108

1010

1012

CuOCu

2O

+CuO

Cu2O

Bul

k R

esis

tivity

(Ω

cm

)

Annealing Temperature (°C)

Cu+

Cu2O

b

Fig. 5. a) I–V responses of the copper oxide films obtained at different annealing tem-peratures and b) resistivity phase diagram of the thermally oxidized copper thin films.

6373L. De Los Santos Valladares et al. / Thin Solid Films 520 (2012) 6368–6374

behaviors are promising for a better understanding of the conductiv-ity mechanism in semiconductor hetero-junctions.

4. Conclusions

Thin films of Cu2O (cubic structure) and CuO (monoclinic struc-ture) were obtained by thermal oxidation following annealing Cuthin films at different temperatures. At low annealing temperatures

Table 2Electrical characteristics of the Cu2O and CuO obtained by thermal oxidation of copperthin films. t: film thickness (measured by SEM), Rsh: sheet resistance and ρBulk: bulkresistivity.

Annealing temperature °C Phases t (μm) RSh (Ω/□) ρBulk (Ω cm)

RT Cu 0.1 0.06 6×10−5

150 Cu+Cu2O 0.12 36 4.2×10−2

175 Cu2O 0.12 1.3×106 1.6×103

200 Cu2O 0.12 1.4×107 1.6×104

250 Cu2O+CuO 0.12 2.4×107 3.0×104

275 Cu2O+CuO 0.12 8.0×106 9.0×103

300 CuO 0.12 2.3×108 2.7×105

400 CuO 0.13 1.6×108 2.0×105

500 CuO 0.13 5.2×107 6.8×104

550 CuO 0.13 3.7×107 4.8×104

800 CuO 0.13 1.7×108 2.2×105

900 CuO 0.14 6.1×109 8.6×106

1000 CuO 0.14 1.3×1014 1.8×1011

some copper atoms bond oxygen atoms to form Cu2O as the firstoxide phase. It is not possible to determine the exact temperaturetransition because oxidation occurs progressively. However, thephase transition upon increasing the annealing temperature is Cu→Cu+Cu2O→Cu2O→Cu2O+CuO→CuO. No diffusion of the Cu ofCu-oxides to the SiO2 substrate was detected after annealing up to1000 °C. Following the technique presented here, pure Cu2O filmsare obtained at 200 °C, whereas, CuO films are obtained above theannealing temperature 300 °C. Annealing also improves crystalliza-tion and increases the crystallite sizes of both copper oxides. The re-sistivity phase diagram obtained by I–V characterization, presentsanomalous decrease of the resistivity at the annealing temperatureintervals 250–275 °C and 300–600 °C. In addition to the hole-dopinginduction mechanism, the improvement of crystallization might alsobe related with this effect. However, more investigation is necessaryto elucidate the cause of the decrease of the resistivity. The thermaloxidation technique described here can be used to fabricate copperoxide hetero-structures which are very useful for their potential ap-plications in electrical devices.

Acknowledgments

This work was supported by the Academic Vice-Council of theUniversidad Nacional Mayor de San Marcos through its Scientific Ini-tiation Program (contract no. PIC — RR 04572-R-10). The work inCambridge was supported by the European Union Program ALBAN(Grant no. E06D101257PE) and the Cambridge Overseas Trust. Thework in Japan was supported by the Japan Society for the Promotionof Science (Grant no. P12030), Grants-in-Aid for Scientific Researchon Innovative Areas (No. 20108011, π-Space) from the Ministry ofEducation, Culture, Sports, Science and Technology (MEXT), Collabo-rative Research Project of the Materials and Structures Laboratory(Tokyo Institute of Technology) and the World Class University Pro-gram, Sunchon National University, Korea. The work in Brazil hasbeen supported by a CAPES grant. The authors are indebted to Dr. J.Llandro, from the University of Cambridge and Drs. Y. Azuma and N.Okabayashi from the Tokyo Institute of Technology for their help dur-ing different stages of the experiments.

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