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Solar Energy 83 (2009) 934–939

Influence of dysprosium doping on the electrical and opticalproperties of CdO thin films

A.A. Dakhel *

Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Bahrain

Received 20 May 2008; received in revised form 6 November 2008; accepted 18 December 2008Available online 5 March 2009

Communicated by Associate Editor: Dr. Takhir Razykov

Abstract

Lightly Dy-doped CdO thin films (molar 0.5%, 1%, 2%, and 2.5%) have been prepared by a vacuum evaporation method on glass andSi wafer substrates. The prepared films were characterised by X-ray fluorescence, X-ray diffraction, UV–vis-NIR absorption spectros-copy, and dc-electrical measurements. Experimental data indicate that Dy3+ doping slightly stretchy-stresses the CdO crystalline struc-ture and changes the optical and electrical properties. The bandgap of CdO was suddenly narrowed by about 20% due to a little dopingwith Dy3+ ions. Then, as the Dy doping level was increased, the energygap was also increased. This variation was explained by the effectof Burstein–Moss energy shift (or bandgap widening effect) together with a bandgap shrinkage effect. The electrical behaviour of thesamples shows that they are degenerate semiconductors. However, the 2% Dy-doped CdO sample shows an increase in its mobilityby about 3.5 times, conductivity by 35 times, and carrier concentration by 10 times relative to undoped CdO film. From transparentconducting oxide point of view, Dy is sufficiently effective for CdO doping.� 2009 Elsevier Ltd. All rights reserved.

Keywords: Optical properties; Cadmium–dysprosium oxide; Dy-doped CdO; Mobility; Oxides; Rare earth oxides; TCO

1. Introduction

Cadmium oxide CdO is an n-type degenerate semicon-ductor of high electrical conductivity (10�2–10�4 X cm).Films of CdO are transparent in visible and NIR spectralregions with a direct bandgap in-between 2.2 and 2.7 eVand two indirect bandgaps at 1.18–1.2 eV and 0.8–1.12 eV (Zhao et al., 2002; Carballeda-Galicia et al.,2000; Chopra and Ranjan Das, 1993; Choi et al., 1996;Kondo et al., 1971). As a consequence of this, considerableinterest has been shown in application of CdO material inits pure and doped states for optoelectronic applicationslike transparent conducting oxide (TCO), solar cells, smartwindows, optical communications, flat panel display,photo-transistors, as well as other type of applications like

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doi:10.1016/j.solener.2008.12.015

* Fax: +973 17449148.E-mail address: adakhil@sci.uob.bh

IR heat mirror, gas sensors, low-emissive windows, andthin-film resistors, etc (Zhao et al., 2002; Su et al., 1984;Gomez Daza et al., 2001; Mane et al., 2006; Yan et al.,2001; Santos-Cruz et al., 2006). The procedure of prepara-tion and the type of dopant used are behind the variety ofthe electrical and optical properties. Therefore, the conduc-tivity as well as optical properties of CdO films could becontrolled through doping with different ions. It was foundthat doping of CdO with ions of a smaller ionic radius thanthat of Cd2+, like In, Sn. Al, Sc, and Y improved its elec-trical conductivity and increased its optical energygap,which was explained by application of Moss–Burstein(B–M) effect (Zhao et al., 2002; Yan et al., 2001; Burstein,1954; Freeman et al., 2000; Maity and Chattopadhyay,2006; Shu et al., 2004; Yang et al., 2005). However, theinteresting question arises whether such observation is suit-able for 4f-ions of smaller radius than that of Cd2+. Suchdoping can control the electrical and optical properties as

Fig. 1. X-ray fluorescence of Dy doped CdO film grown on Si substrate.The exciting radiation was Cu Ka of wavelength 0.1543 nm.

Table 1The Bragg angle (2ho

111), the average X-ray grain size perpendicular to[111] direction (D111), and the texture coefficient (TC) for the preparedpure and Dy-doped CdO films on glass substrates.

Sample 2ho111 D111(nm) TC

CdO 33.13 32.6 1.540.5% 33.13 29.9 1.611.0% 33.14 26.2 1.632.0% 33.15 25.1 1.662.5% 33.16 25.2 1.69

A.A. Dakhel / Solar Energy 83 (2009) 934–939 935

well as introduce magnetic character so that it might pro-duce magnetic TCO (MTCO) materials. To our bestknowledge, such doping is absence from literature.Recently (Huang et al., 2008), Dy doped ZnO thin filmswas prepared and studied in the potential of TCO applica-tions. Dysprosium ion Dy3+ has a standard ionic radius of0.0912 nm, which is slightly less than that of Cd2+,0.097 nm (Kenneth Barbalace). This means that Dy3+ ionscan substitute Cd2+ ions in its crystalline structure, whichmight lead to an increase in the concentration of conduc-tion electrons and improve the electrical conductivity.The present work reports, for the first time, the effects ofDy-doping on the structural, electrical, and optical proper-ties of CdO films.

2. Experimental

The starting materials were a pure Dy element and CdO(from Fluka A.G./Germany). The alumina coated tungstenbasket filament (Midwest tungsten service/USA) was usedto evaporate the starting materials. The alternating thermaldeposition (layer-by-layer etc.) method was used to depositthe starting materials on ultrasonically clean glass andchemically (using HF) cleaned silicon-wafer substrates heldat about 200 �C in a vacuum chamber of residual oxygenatmosphere of pressure about 1.3 � 10�2 Pa. The as-grownfilms were oxidised and stabilised by annealing in a pureoxygen atmosphere at 400 �C for 2 h. All samples were pre-pared in almost same conditions including the referencepure CdO film. The evaporated mass and thickness werecontrolled by a Philips FTM five thickness monitor andmeasured after annealing with a Gaertner L117 ellipsome-ter to be in the range 0.2–0.3 lm. The structure of the pre-pared films were studied by the X-ray diffraction (XRD)method using a Philips PW 1710 h–2h system with Cu Ka

radiation (0.15406 nm). The energy dispersion X-ray fluo-rescence (EDX) method was used to determine the relativemolar ratio Dy to Cd (r) in the studied samples, to be about0.5%, 1%, 2%, and 2.5%. The spectral optical transmittanceT(k) was measured at normal incidence in UV–vis-NIRspectral region (400–3000 nm) with a Shimadzu UV-3600double beam spectrophotometer. The electrical measure-ments were carried out with a standard Van-der-Pauwmethod with aluminum dot contacts in a magnetic fieldof about 1 T and using a Keithley 195A digital multimeterand a Keithley 225 current source.

3. Results and discussion

3.1. Characterisation by X-rays

(Fig. 1) shows the energy dispersion X-ray fluorescence(EDX) spectrum of the prepared thin Dy-doped CdO filmon Si substrate. The spectrum demonstrates the Cd L-spec-trum (3.133–3.528 keV) and Dy La-spectrum (6.457–6.495 keV) with some signals from the source, collimator,and Si substrate. The ratios of integral intensities of Dy

La-signal (IDy) to Cd L-signal (ICd) or Ir = IDy/ICd wereused to determine the relative weight fraction ratio of Dyto Cd in the film. For that purpose, the known methodof micro radiographic analysis was used (Anjos et al.,2000; Hayakawa et al., 2000). The reference samples werepure Dy2O3 thin film and pure CdO film. The results aregiven in Table 1.

(Fig. 2) shows the X-ray diffraction (XRD) patterns ofthe prepared pure CdO and CdO:Dy films. All the patternsreveal polycrystalline films of cubic CdO structure of spacegroup Fm3m. The lattice constant calculated for a pureCdO film was 0.468 nm, which is almost identical withthe standard value (Powder Diffraction File). The usuallyfavourable (111) preferred orientation growth of CdOfilms, prepared by different techniques (Subramanyamet al., 1998; Reddy et al., 1998; Phatak and Lai, 1992;Tanaka et al., 1969), is studied here through the texturecoefficient (TC). It is defined in Ref. (Barrett et al., 1980)as TC = [nIm(hkl)/I0(hkl)]/[

PIm(hkl)/I0(hkl)], where

Im(hkl) is the measured relative intensity of reflection froma given (hkl) plane, I0(hkl) is the relative intensity of thereflection from the same plane as indicated in a standardsample (Ref. Powder Diffraction File), and n is the totalnumber of reflections observed, which is five in the presentinvestigation. Therefore, the highest value of TC is five fora perfectly oriented film and its value is one for a randomlyoriented one. The calculated values of TC in the present

Fig. 2. X-ray diffraction patterns from pure and Dy-doped CdO filmsprepared at different Dy dopant concentration levels. The used radiationwas Cu Ka-line.

936 A.A. Dakhel / Solar Energy 83 (2009) 934–939

work are presented in Table 1, where one can observe thatthe (111) preferred orientation coefficient increases withincreasing Dy doping level. The mean X-ray grain size(D) perpendicular to [111] direction was estimated by usingScherrer’s relation (F. Kaelble (ed.), 1967): D = 0.9k bcoshwhere k is the X-ray wavelength; b is the full-width at halfmaximum (FWHM) of the diffraction peak calculated by aGaussian fit; and h corresponds to the peak position. Theresults are also given in Table 1. In general, the CdO grainsize was not greatly influenced by Dy doping and it is in therange 25–30 nm. It was also observed that there are a slightshift D(2h111) in the position of the intense CdO(111) peaktowards higher Bragg angle from 33.13� for pure CdO to33.16� (Table 1). This slight peak shift is resulted fromthe created structural strain ðes ¼ �Dhð111Þ cot hð111ÞÞ dueto Dy doping, which is of order �10�3. This strain iscaused by a compressive stress (rst) that can be estimatedby: rst � ð3esÞB, where B is the average bulk modulus ofCdO, which is about 158 GPa (Liu et al., 2004), and theresults are less than about 0.56 GPa. The negative signrefers to the compression nature of the stress leading todecreasing of the lattice parameter that caused by Dy3+

ions doping, which has less ionic radius than that ofCd2+ ion. However, the structural stress is far to be capablecreating a crystal structural transformation, it can onlyproduce a slight decrease in the lattice constant of orderabout 0.1%.

3.2. DC-electrical properties

The room temperature electrical resistivity (q), mobility(lel), and carrier concentration (Nel) were measured with astandard Van-der-Pauw method and the results are pre-sented in Table 2 and (Fig. 3). The main source of experi-mental error is being due to the used technique of Van-der-Pauw, i.e due to the sample size and circular-aluminumcontact spot size, which estimated in the present work tobe about 5%. The measured electrical parameters of pure

CdO film in the present work agree with those data pub-lished for CdO films prepared by different techniques(Zhao et al., 2002; Chu and Chu, 1990; Gurumuruganet al., 1996; Varkey and fort, 1994; Reddy et al., 2003; Liet al., 2001). However, the resistivity of CdO, in the presentwork, is larger than those values mentioned in some otherreferences �10�3–10�4 X cm due to different method andprocedure of preparation. With increasing of Dy dopinglevel, carrier concentration Nel and the mobility lel wasincreased. The resistivity gets minimum value for 2% Dysample. This behaviour can be explained by the variationin Nel and change in carrier scattering by microstructuraldefects, grain boundaries, and ionised impurities, as wellas variation in the intrinsic bandgap (this will be discussedlater). As Dy content was increased, the concentration ofelectrons increases non-linearly, since Dy3+ ions that sub-stitute Cd2+ ions can liberate more conduction electronsin the conduction band. At the same time, the scatteringprobability of conduction electrons by Dy3+ ions residesin Cd2+ places, in interstitial positions, and even in grainboundaries increases. To study the doping, Authors ofRef. (Zhao et al., 2002) have introduced the doping effi-ciency (gDE), which defined as the ratio of the concentra-tion of conduction electrons to the concentration ofincorporated Dy as measured by XRF method. The resultsof calculation are given in Table 2. Generally, gDE trends toa lower value as the Dy concentration increases. Suchbehaviour was also observed in Ref. Zhao et al. (2002).In general, Dy dopant plays almost the same role as thatof other experienced dopants (In, Sn, Al, Sc, and Y). How-ever, one more factor must be taken into the considerationis the 4f-electrons that add a new parameter to the energyband spectrum of CdO, which must qualitatively change itand strongly varies the bandgap, as it will be clear later. Ina summary , the present work discovers that light dopingwith Dy improves the dc-conduction parameters of CdO,so that the 2% Dy-doped CdO sample shows increase itsmobility by about 3.5 times, conductivity by 35 times,and carrier concentration by 10 times, relative to undopedCdO film.

3.3. Optical-electronic properties

The spectral optical absorption method is used to studythe optical properties of the prepared CdO:Dy films oncorning glass substrates. The corrected spectral normaltransmittance T(k) in the UV–vis-NIR region (400–3000 nm) were measured and the results are presented inFig. 4. The spectra show that the investigated films haveno-sharp principal absorption edge. The maximum valueof the transmittance for all investigated samples is lyingin the NIR region. In addition, at NIR region, the trans-mittance curves show a clear damping for all investigatedsamples due to the absorption by the eigen free electrons.Generally, Dy content in the film causes increase in theoptical absorption. The spectral absorption coefficienta(k) is usually estimated by aðkÞ ¼ ð1=dÞ lnð1=T Þ, where d

Table 2Summary of the electrical measurements for pure and Dy-doped CdO films on glass substrates.

Sample q ð� 10�3X cm) lel (cm2/V s) Nel (1020 cm�3) Eg (eV) gDE (%)

0% 20.1 7.03 0.442 2.25 –0.5% Dy 5.31 7.24 1.63 1.72 92.01.0% Dy 2.43 8.23 3.13 1.77 109.72.0% Dy 0.583 24.43 4.38 1.79 86.72.5% Dy 0.642 23.26 4.61 1.81 76.8

Fig. 3. Variation of resistivity, mobility, and carrier concentration forpure and Dy doped CdO films prepared on glass substrates at different Dydopant concentration levels.

Fig. 4. Spectral normal transmittance in the UV–vis-IR regions for pureand Dy doped CdO films prepared on glass substrates at different Dydopant concentration levels.

Fig. 5. Calculated spectral optical absorption coefficient (a) in the UV–vis-IR regions shown in log-log scale for Dy doped CdO films prepared onglass substrates at different Dy dopant concentration levels. The insetshows the calculated relationship a(k) for pure CdO of the present work.

Fig. 6. Calculated (points) spectral optical absorption coefficient a isplotted as (aE)2 vs. photon energy for pure CdO film (in the inset) and Dydoped CdO films. The lines in the bandgap absorption region determinethe direct bandgap.

A.A. Dakhel / Solar Energy 83 (2009) 934–939 937

is the film thickness (the reflectance is very small especiallyat the main absorption region). (Fig. 5) shows the obtaineddependence of a on wavelength k. The optical direct band-gap Eg is evaluated according to the well-known relation(Tauc, 1969; Davis and Mott, 1970):

aE ¼ AðE � EgÞm ð1Þ

where A is a constant and the exponent m is equal to 0.5 or2 for direct and indirect transitions, respectively. Thus, the

plot of (aE)2 vs. E as shown in Fig. 6 gives the value of di-rect energygap (Zhao et al., 2002; Carballeda-Galicia et al.,2000; Gurumurugan et al., 1994; Kawamura et al., 2003).The calculated values of direct energygap for pure anddoped CdO films are given in Table 2. For pure CdO, theobtained energy band is being in the known range (2.2–2.6 eV) obtained for pure CdO films prepared by differenttechniques (Zhao et al., 2002; Kawamura et al., 2003; Ueda

Fig. 7. The dependence of optoelectronic function ðEg � SBGWN 2=3el Þ on the

carrier concentration N 1=3el . The line is being the best fit in accordance with

Eq. (2).

938 A.A. Dakhel / Solar Energy 83 (2009) 934–939

et al., 1998). It is observed that the energy bands of theCdO:Dy films are narrower than that of pure CdO films.The band gap of CdO suffers from a sudden shrinkage(BGN) by about 20% due to the influence of Dy3+ doping.Such BGN is associated with a sudden increasing in thecarrier concentration by around 2.5 times (Nel > 1.63� 1020 cm�3) and is contradictory to the Moss–Burstein(B–M) model. This BGN can be explained as consequenceof a change in the nature and strength of the crystalline po-tential by addition the influence of Dy impurity dopantatoms including the effect of their 4f-electrons on the crys-talline electronic states. So, due to the doping, the bandtailing or impurity band becomes broader and finallyreaches and merges the bottom of the conduction bandcausing the decrease of the mentioned above optical band-gap Eg (A similar explanation was given in Ref. (Lu et al.,2007) for a different case). Then, with increasing of Dy mo-lar doping level to more than 0.5%, a blue shift of absorp-tion edge was observed (called bandgap widening (BGW)).The BGW can be explained by B–M effect, which statedthat the optical absorption edge(direct or indirect mergeland Qiao, 2002; Dua et al., 2008) of a degenerate n-typesemiconductor is shifted towards higher energy by amountproportional to the increase in electron density in the con-duction band through the following relation (Santos-Cruzet al., 2006; Burstein, 1954): DEBM

g ¼ ð�h2=2cmeÞ ð3p2N elÞ2=3

¼ SBGW N 2=3el , where SBGW ¼ ð�h2=2cmeÞ ð3p2Þ2=3, �h is the

Plank constant and c ¼ m�vc=me is the ratio for of reducedeffective mass to free-electron mass, which is equal to0.274 for pure CdO (Kawamura et al., 2003; Ueda et al.,1998; Asahi et al., 2001). Therefore, the theoretical valueof the coefficient is Sth

BGW ¼ 1:348� 10�18 eV m2. But, thebest linear fit for the experimental data of Eg vs. N 2=3

el givesSBGW ¼ 3� 10�19 eV m2, which is far from the theoreticalvalue. This discrepancy can be explained by introducing acorrection factor, which appears accompanying with a highconcentration of electrons (Nel > 1019 cm�3 Sernelius et al.,1988). This correction was constructed theoretically frommany-body-interactions that leading to introduce a band-gap narrowing (BGN) factor, which is described simplyas BGN � N 1=3

el (Wolff, 1962; Lu, 2006). Thus, the totaleffective bandgap shift is determined by a combination ofBGW and BGN effects, the former giving a blueshift andthe latter an offsetting redshift:

Eg � Eg0 ¼ ðBGW� BGNÞ

¼ SBGWN 2=3el � SBGNN 1=3

el þ Cf ð2Þ

where Cf is a fitting parameter. The plot of ðEg�SBGWN 2=3

el Þvs:N 1=3el gives a straight line, as seen in Fig. 7 with

SBGN ¼ 1:351� 10�9 eV m. This value is comparable tothat one obtained by Wolff calculations (Zhao et al.,2002; Wolff, 1962):BGNðWolffÞ ¼ ðe=2pe0erÞð3=pÞ1=3N 1=3

el

¼ ð3:836� 10�9=erÞN 1=3el ¼ 1:375� 10�9N 1=3

el , where e0 isthe permittivity of free space, e is the electronic charge,and for the dielectric constant er we used e1.

It is well known that the absorption in the NIR spectralregion is mainly executed by the eigen free carriers (Qiaoet al., 2006) and depends on the type of carrier scatteringmechanism. Therefore, the absorption coefficient a is pro-portional to k according to the power law: a � kp, wherep varies from 1.5 to 3.5 depending on the dominant scatter-ing mechanism in the material in a certain wavelengthrange. The scattering by acoustic phonons gives p � 1:5,scattering by optical phonons gives p � 2� 2:5, and scat-tering by ionised impurities is described by p � 3� 3:5singh and Mehra, 2001; Xu et al., 2006. In the presentwork, a good proceed to the power law was observed forwavelengths k > 2200 nm, where the value of p variesbetween 3.0 and 3.3., which means that the scattering byionised impurities is dominant in these samples.

4. Conclusions

The optical, structural, and dc-electrical properties ofCdO films lightly doped with rare-earth element Dy werereported in the present work. It was observed that a lightdoping with Dy3+ ions suddenly decreases the bandgapof the host CdO by about 20%. Then, as the Dy dopinglevel was increased, the energygap was also increased (blueshift). This variation was explained by the effect of Bur-stein–Moss energy shift (or bandgap widening effect)together with a bandgap shrinkage effect. The dc-electricalconduction parameters (q, lel, and Nel) measured as a func-tion of the molar fraction of dopant Dy show that all theinvestigated films are degenerate n-type semiconductors.However, the 2% Dy-doped CdO sample shows an increasein its mobility by about 3.5 times, conductivity by 35 times,and carrier concentration by 10 times relative to undopedCdO film. From transparent conducting oxide point of

A.A. Dakhel / Solar Energy 83 (2009) 934–939 939

view, Dy is sufficiently effective for CdO doping like otherdopants such as In Sn, Sc, and Y.

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