Anomalous band bowing in pulsed laser deposited MgxZn1�xO films

Arpana Agrawal a,n, Tanveer Ahmad Dar a, D.M. Phase b, Pratima Sen a

a Laser Bhawan, School of Physics, Devi Ahilya University, Indore 452001, Indiab UGC-DAE Consortium for Scientific Research, Khandwa Road, Indore 452001, India

a r t i c l e i n f o

Article history:Received 7 June 2013Received in revised form23 August 2013Accepted 30 August 2013Communicated by R. FornariAvailable online 7 September 2013

Keywords:A1. DefectsA1. DopingA1. X-ray diffractionB1. AlloyB1. OxidesB2. Semiconducting II–VI materials

a b s t r a c t

Random variation of band bowing in pulsed laser deposited Mg doped (x¼0.090, 0.147, 0.211, 0.268) ZnOthin films was observed. The X-ray diffraction and ultraviolet–visible spectroscopy data reveal latticerelaxation and increase in band gap as well as disorderness in the samples. The X-ray photoelectronspectroscopy data confirm the presence of magnesium and oxygen interstitials (Mgi and Oi) as well asoxygen vacancies (VO). The randomness of band bowing is attributed to the presence of these defects.

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1. Introduction

Double heterostructures (DHs) are the key materials for makingoptical sources and detectors. In today's optoelectronic devices, thelaser sources and the corresponding detector made of heterostruc-tures are sought at high energies due to their large informationstorage capacity. The challenges faced in making DHs at highenergies are the proper choice of the materials which can meetthe requirements of band gap modulation together with the latticematching. A surge of activities have been going on to investigatesuch materials. The key issue in this regard is to understandstructural, electronic and optical properties of materials whichcan be employed for making the DHs. In view of this, we proposeMgxZn1�xO as an excellent candidate because doping ZnO with Mgcauses increase in the band gap. However, the difficulties encoun-tered in making Mg doped ZnO are: (i) MgO and ZnO have differentpreferred lattice structures, MgO preferentially crystallizes in rock-salt structure while ZnO in hexagonal wurtzite structure. (ii) Mghas lower oxygen affinity due to its smaller electronegativitycompared to Zn which may cause an increase of oxygen vacanciesin Mg doped ZnO. The first difficulty imposes the problem of phasesegregation in MgZnO films. In most of the experimental techni-ques, phase segregation for dopant concentrations Z30% were

reported [1,2]. The phase segregation introduces structural defor-mation which may destroy the linear composition dependence ofthe band gap modification. On the other hand the later difficultypromotes the formation of defect states in the films which stronglyaffect the electronic and optical properties of ZnO films.

As a general case, the composition dependence of the band gap(Eg) of a ternary alloy AxB1�xC is given by [3]

EAxB1�xCg ¼ EBCg ð1�xÞþxEACg �bxð1�xÞ: ð1Þ

Here, b is the nonlinear parameter, known as band bowingparameter which characterizes the degree of deviation fromlinearity. The band bowing occurs due to volume deformation,different electronegativities, structural relaxation and alloy dis-orderness [3]. In MgxZn1�xO films, at lower Mg concentration, thepossibility of phase segregation and lattice deformation is smalland Mg can either occupy position which can suitably favor thegrowth of MgO rocksalt structure or it can occupy Zn site. In eachcase the electronic properties and band bowing will have differentnature. Similar features can occur due to the presence of oxygenvacancies and interstitial oxygen [4]. These vacancies may causelocalized energy levels within the band gap giving rise to the bandtailing effect which occurs below the absorption edge in theabsorption spectra. On the other hand, an increase in Mg dopantconcentration leads to an increase in the band gap [5]. In case ofstructural properties, the oxygen vacancies give rise to latticerelaxation and corresponding increase of c-axis parameter [4]while Mg doping leads to the decrease in c-axis parameter [6].

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Journal of Crystal Growth

0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jcrysgro.2013.08.036

n Corresponding author. Tel.: þ91 731 2762153; fax: þ91 731 2470372.E-mail addresses: agrawal.arpana01@gmail.com,

arpana_agrawal011@yahoo.in (A. Agrawal).

Journal of Crystal Growth 384 (2013) 9–12

So far, in the literature the contributions of oxygen vacancies andMg doping are independently studied and reported.

The present paper aims to study the effect of oxygen vacanciesand magnesium and oxygen interstitials on band bowing para-meter in pulsed laser deposited Mg doped ZnO films (concentra-tion of Mg: 0.090, 0.147, 0.211, 0.268).

2. Experiment

Predetermined amounts of ZnO (99% pure) and MgO powders(98% pure) are mixed together and grinded for about 6 h, calci-nated, and sintered to form targets (pellets) with different Mgconcentrations (x¼0.090, 0.147, 0.211, 0.268 (in atomic fraction)).These targets were then used to deposit films on glass substrate bypulsed laser deposition (PLD) technique in an ultrahigh vacuumchamber. KrF eximer laser (wavelength¼248 nm) with a fluenceof 2 J/cm2 was used to ablate the target material. The substrateswere ultrasonically cleaned in methanol. The targets were placedat a distance of 5 cm from the substrate. The pulse repetition ratewas set at 10 Hz and the pulse duration was 20 ns. Beforedeposition, the chamber was evacuated at a pressure of 5 nbarwith a turbo molecular pump. Then, ultrapure O2 gas (99.99%) wasintroduced into the chamber. Deposition was carried out in oxygenenvironment maintaining the pressure of 5�10�4 mbar. The filmswere deposited at an optimized substrate temperature of 400 1C.We have directly measured the thickness of MgxZn1�xO film(thickness¼300 nm ) using step profilometer and as all the filmswere grown under same conditions and for same deposition time,we assume that all the samples are of same thickness. It isexpected that the Mg content in MgxZn1�xO films will be largerthan that in the pellets due to the difference of the vapor pressuresbetween ZnO and MgO [2].

3. Result and discussion

Although preferred lattice structure of ZnO (Hexagonal wurt-zite structure) and MgO (cubic rocksalt structure) are not same [7]but the ionic radius of Mg2þ (0.57 Å) is close to that of Zn2þ

(0.60 Å) [8] and therefore one can expect that the ZnO wurtzitephase will be conserved up to certain lowMg concentrations. Fig. 1illustrates the X-ray diffraction (XRD) pattern of pure ZnO and Mg

doped ZnO films. The appearance of only (002) and (004) diffrac-tion peak indicates that the grown MgxZn1�xO films are single-phase, highly c-axis oriented and wurtzite in structure. Apart fromZnO characteristic peaks, no peaks that correspond to either Mg ortheir oxides could be detected, which suggest that the films maynot have any phase segregation or secondary phase formation. Thec-axis lattice parameter is also determined and the values aregiven in Table 1. From Table 1, we find that the incorporation ofMg causes increase in c-axis lattice parameter. This observationcontradicts the earlier reported results [6] where a decrease in c-axis lattice parameter was found to occur due to Mg doping. In oursamples the lattice relaxation may occur due to oxygen vacanciesand oxygen interstitials giving rise to a net increase of c-axisparameter. Thus disorder is mainly caused by oxygen interstitialsand oxygen vacancies. The grain size of pure ZnO film was� 40 nm while that of Mg doped ZnO films was � 25 nm.

The presence of various elements in the grown films wasexamined by X-ray photoelectron spectroscopy (XPS). XPS spectrawere recorded with angle Integrated Photoelectron Beamline onINDUS-I Synchrotron radiation source. Wide survey and detailedscan spectra were recorded and after a Touguard backgroundsubtraction, raw spectra were fitted using Voigt (Gaussian–Lorentzian) peak shape [9].

Fig. 2a shows the XPS spectra of pure ZnO and MgxZn1�xO films(x¼0.211), which clearly signature the presence of Zn, Mg and O inthe grown films. In addition to Zn, Mg and O, the presence ofelemental carbon, which is an unavoidable presence in all airexposed materials, was also identified from the C 1s peak at285 eV. Addition of Mg to ZnO can either replace Zn2þ or formMgO secondary phase or can also be present as Mg interstitials.The MgO secondary phase or Mg interstitial can be identified fromthe core level peaks obtained from XPS spectra. In case of MgO, the2p core level is reported to occur at 50–51 eV while 2p core levelof Mg occurs at 49–50 eV [10]. The Mg 2p core level XPS peak ofMgxZn1�xO film shown in the inset of Fig. 2a is centered at49.54 eV and confirms that Mg occupies interstitial position inthe doped samples. No binding energy shift towards the Mg 2p(corresponding to MgO) between 50 and 51 eV was identifiedwhich is a confirmation of the absence of secondary phase of MgOas corroborated by XRD analysis. Fig. 2b shows the presence ofthree peaks in the O 1s core level spectrum in MgxZn1�xOcentered at 530.44 eV, 532.06 eV and 533.24 eV, respectively. Thelower binding energy peak is dominant and corresponds to O2�

ions in ZnO. The presence of oxygen vacancies (VO) and interstitialoxygen (Oi) can be found in the O 1s peak at 532.06 eV and533.24 eV, respectively. Following the procedure suggested byWagner et al. [10], a rough estimate of the oxygen vacancies andoxygen interstitials is made from our XPS data. We foundOi � 0:085 and VO � 0:247. These estimations clearly indicate alarger contribution of VO compared to that of Oi in the samples.The oxygen interstitials give rise to the occurrence of O–O bonding(� 1:5 Å). The existence of O–Zn–O bond angles reinforces Oi as O2

molecule embedded in ZnO crystal. This defect is reported to be

Fig. 1. X-ray diffraction pattern of MgxZn1�xO films; (a), (b), (c), (d), and (e) showthe pattern with different Mg concentrations (x¼0.0, 0.268, 0.211, 0.147, 0.090),respectively.

Table 1Calculated values of c-axis lattice parameter, energy band gap (Eg), Urbach energy(EU) and the band bowing parameter b for MgxZn1�xO (x¼0.090, 0.147, 0.211, 0.268)films, respectively.

Sample c-parameter(Å)

Eg values(eV)

EU values(meV)

‘b’ value(eV)

ZnO 5.180 3.24 190 –

Mg0.090Zn0.910O 5.158 3.33 200 3.91Mg0.147Zn0.853O 5.170 3.49 218 3.35Mg0.211Zn0.789O 5.173 3.62 369 3.49Mg0.268Zn0.732O 5.176 3.69 387 3.93

A. Agrawal et al. / Journal of Crystal Growth 384 (2013) 9–1210

electrically active and gives rise to acceptor transition levels closeto the conduction band minima (CBM) [11]. It is worthy to notethat VO is one of the most common defects in ZnO and acts as adeep donor with its transition level located around 1 eV belowCBM [11].

The band gap calculations can be made from the ultraviolet–visible (UV–Vis) spectroscopy. We have taken the UV–Vis spectraof the films in transmission mode as shown in Fig. 3. This figure

reveals the formation of highly transparent films (more than 90%)in the entire visible region and near IR region. We have alsonoticed a single absorption edge in all the films that confirm theabsence of phase segregation. In the present analysis of XRD data,we found that the grain size becomes finer for larger Mg dopingconcentration which has lead to better transparency of the films inspite of their same thickness. This observation is consistent withthe earlier reported results of MgZnO films prepared by PLDtechnique [2]. It seems that in terms of transmittance, depositionconditions are important factors for the development of the goodquality alloy thin films. In Mg doped ZnO films, the atomicpositions fluctuate and electrons experience a potential whichdiffers from place to place and causes blurring of the conductionand valence band resulting to the absorption edge profile. The lowenergy region in the UV–Vis spectra below the absorption edgecorresponds to Urbach effect which determines the electronicstates in the valence and the conduction bands. The study ofUrbach tail and Urbach energy can give important information ondisorder. The mechanisms responsible for the Urbach effect arecarrier impurities, carrier–phonon interactions and structuraldisorders. In our samples the contribution of structural disorderis expected to be very small due to the existence of single phase.We therefore ascribe the Urbach effect to the localized impuritystates. We have calculated the Urbach energy EU by relating it tothe absorption coefficient α via the relation [12]

αðhνÞ ¼ α0 expðhν=EU Þ: ð2ÞThe values of EU are given in Table 1. We find that increasing

Mg concentration leads to an increase in the Urbach energy whichreveals an increase in the disorderness.

The effect of Mg doping on band gap is calculated from theexperimental UV–Vis data by plotting ðαhνÞ2 versus photon energyðhνÞ using the relation

αhν¼ Aðhν�EgÞ1=2: ð3ÞEg being the optical band gap in eV. We have determined theoptical band gap by linear extrapolating the spectrum to αhν¼ 0for all the curves and the Mg concentration dependence of theband gap is plotted in Fig. 4. This figure reveals an increase in theband gap with increasing Mg concentration. The cause of such anincrease in band gap can be attributed to lower electronegativityof Mg compared to Zn. On Allred Rochow scale, the electronega-tivity of Zn and Mg is 1.66 and 1.23, respectively. The relationbetween electronaffinity and band gap was discussed by Koffey-berg [13]. An increase in the band gap with decreased electro-negativity of the samples was also reported by Al. Ghamdi [14].Difference of electronegativity in the cations also increases theprobability of formation of defect states and this contribution canbe seen in the Urbach tail of the absorption spectra. In XPS data

Fig. 2. XPS spectra of pure ZnO and Mg0.211Zn0.789O. (a) Wide survey scan of pureZnO and MgxZn1�xO films. Insets show the detailed scan of Mg 1s and Mg 2p corelevel XPS peaks in MgxZn1�xO film; (b) detailed scan of O 1s core level inMgxZn1�xO film. Curve I is a fit on the experimental data of the detailed scan.The peaks of curves II, III and IV were fitted using Voigt peak fit [9]. These peakscorrespond to O2� ions, VO and Oi, respectively.

Fig. 3. Transmission spectra of the grown films; (a), (b), (c), (d), and (e) correspondto MgxZn1�xO with different Mg concentrations (x¼0.0, 0.268, 0.211, 0.147, 0.090),respectively; inset shows the curve of ðαhνÞ2 versus photon energy ðhνÞ.

Fig. 4. Band gap as a function of Mg concentration.

A. Agrawal et al. / Journal of Crystal Growth 384 (2013) 9–12 11

(Fig. 2b), we find significant peak amplitude corresponding tooxygen vacancy. The VO causes redshift of the band gap [4]. Thecontribution of Oi is insignificant. In general, it is difficult toidentify Oi peak due to C¼O or absorbed O states. We considerthat the presence of these defect levels influence the band bowingparameter. Consequently, on one hand the presence of Mg inter-stitials causes increase in the band gap, while on the other hand ,the presence of oxygen vacancies (VO) gives rise to lowering of theband gap. The band gap calculations from the UV–Vis data exhibitan increase in the band gap energy with increasing Mg concentra-tion and shows that Mg has a larger possibility to affect the energyband structure of the film. As a result, we observe a nonlinearincrease in the band gap with increasing Mg concentration (Fig. 4).

The nonlinear increase of the band gap was also found to occurin films prepared by sol–gel method [1] and by combustion flamepyrolysis of solution precursor method [6]. However, in their films,the Mg mole fraction was quite high and the nonlinear depen-dence in their samples occurred at high dopant concentration. Wehave single phase films containing low Mg concentration. Ourobservations along with those of Wei et al. [1] and Kavitha et al. [6]suggest that nonlinear dependence of band gap on concentrationcannot be only ascribed to phase segregation.

In binary and ternary alloy semiconductors (AxB1�xC), thecomposition (x) dependence of lattice parameter and band gaphas been utilized in semiconductor technology since long. Theoptical band gap generally shows a quadratic nonlinearity (Eq. (1)).The nonlinear parameter b in Eq. (1) is termed as the opticalbowing parameter. Band bowing was explained using virtualcrystal approximation model and alloy disorder [15]. Bernardand Zunger [3] ascribed the bowing to volume deformation,charge transfer, structural contribution and disorderness in thesystem. According to Eq. (1), the bowing parameter should becomposition independent. The calculated values of bowing para-meter from our experimental data using Eq. (1) (Table 1) showrandom variation of b with increasing Mg concentrations. Theseobservations suggest that the reason of band bowing is not solelydependent on Mg concentration and some other random variableis expected to contribute in band bowing. The characterization ofour samples shows the presence of oxygen and Mg interstitials (Oi)and oxygen vacancies (VO). In our opinion, Oi and VO are randomlyvarying in our samples and their presence is giving rise to therandom variation of band bowing parameter.

4. Conclusions

We have deposited and characterized pure and Mg doped(x¼0.090, 0.147, 0.211, 0.268) ZnO single phase films using pulsed

laser deposition technique. The following important observationscan be made: (i) the XPS data reveal the presence of oxygenvacancies as well as Mg interstitials. (ii) Increase in Urbach energywith increasing Mg concentration confirms increased disorder inthe samples. (iii) The band gap was found to increase with increasein Mg concentration. Although, the increase in band gap is not ashigh as reported by others and the reason for it has been ascribedto the oxygen vacancies and oxygen interstitials. (iv) Anomalousband bowing observed in the samples is attributed to the mutuallyopposing contributions of oxygen vacancy and Mg interstitials.

Acknowledgments

The authors are thankful to Dr. P.K. Sen from S.G.S.I.T.S, Indore,for helpful discussions, Dr. L.M. Kukreja and his group from RRCAT,Indore, for extending the facility of thin film deposition andDr. M. Gupta of UGC-DAE CSR, Indore, for film characterizations.The financial support provided by UGC-DAE-CSR, Indore and SERB,Government of India, are acknowledged herewith.

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