Stability Studies on Nitrogen Doped p-ZnO (NZO) Thin Films Grown by Reactive Magnetron Sputtering

JOURNAL OF DISPLAY TECHNOLOGY, VOL. 9, NO. 9, SEPTEMBER 2013 715

Stability Studies on Nitrogen Doped p-ZnO (NZO)Thin Films Grown by Reactive Magnetron Sputtering

R. V. Muniswami Naidu, Aryasomayajula Subrahmanyam, Arnaud Verger, and Mahaveer Kumar Jain

Abstract—Nitrogen doped ZnO (NZO) thin films, at differentN flow rates have been deposited on glass substrates by pulsedDC reactive magnetron sputtering technique. The effect of N flowrate (1.0 sccm – 3.0 sccm) on the structural, optical, electrical andchemical state of N has been studied. With the effect of N flowrate: the crystallinity of the films decreased, tensile stress is devel-oped, optical transmittance decreased (80% to 60%), conductivitydecreased till 1.5 sccm and films were n-type conducting. At 2.0sccm and 2.5 sccm of N flow rates, NZO thin films showed p-typeconductivity. The changes in the magnitude and type of conduc-tivity have a direct relation with the changes observed in N-chem-ical state in ZnO lattice. p- NZO thin films are electrically unstable;this instability has been explained based on the changes occurredin the N chemical states, resulting from the stress release in NZOlattice.

Index Terms—Nitrogen doped ZnO (NZO), reactive magnetronsputtering, stability of thin films.

I. INTRODUCTION

Z INC OXIDE (ZnO) is an extensively studied semicon-ducting materials in the past 15 years to explore it for

many prospective applications [1]. Due to its high transparency(80% – 90%) in the visible region (400–800 nm) and conduc-tivity (as high as 10 S cm ), it is a good candidate for theapplications in displays, transparent (or invisible) electronics,solar cells, light-emitting diodes (LEDs) with white, blue andgreen emissions, and many more to follow [2]–[5]. Trans-parency in visible range is due to its wide bad gap (3.27 eVat 300 K) and the conductivity is due to the intrinsic defects,unintentional impurities and also one can enhance the conduc-tivity by introducing suitable donor impurities, such as Al, Ga,In etc., [6]–[8].The origin behind the unintentional conductivity in undoped

ZnO is highly debated over the years; a few groups attributingto intrinsic defects and a few others to unintentional impurities[9]. The lack of a reliable, reproducible and stable (RRS) p-ZnO

Manuscript received May 02, 2012; revised December 04, 2012; acceptedJanuary 24, 2013. Date of publication March 7, 2013; date of current versionAugust 30, 2013.R. V. M. Naidu was with the Department of Physics, Indian Institute of Tech-

nology (IIT)Madras, Chennai 600036, India. He is nowwith Saint Gobain GlassIndia Ltd., Chennai, 600008, India (e-mail: [email protected]).A. Subrahmanyam and M. K. Jain are with the Department of Physics,

Indian Institute of Technology (IIT) Madras, Chennai 600036, India (e-mail:[email protected]; [email protected]).A. Verger is with Saint Gobain Recherche, 93303 Aubervilliers Cedex,

France (e-mail:[email protected]).Color versions of one or more of the figures are available online at http://

ieeexplore.ieee.org.Digital Object Identifier 10.1109/JDT.2013.2246540

has hindered its applications in many of the above-mentioneddevices; since many of them deal with p-n junctions and p-typeZnO being the basic requirement. The first break through onp-ZnO came in 1997 [10]. From there many reports followed[11]–[16]. However many of these investigations have not ad-dressed the RRS. Nitrogen (N) incorporation as an acceptor typeimpurity in the place of Oxygen (due to its electronegativityand ionic size compatibility, compared with other group V ele-ments) in ZnO has been extensively studied. Even though manygroups succeeded in making p-ZnO based on N doping, the re-producibility and stability are not well addressed [17].Reproducibility and the stability issues motivated us to study:

the effect of N flow rate on the electrical and physical proper-ties of reactive pulsed dc magnetron sputtered NZO thin filmsand the stability of these properties over a period of time.

II. EXPERIMENTAL TECHNIQUES

A. Growth of Thin Films

NZO thin films have been grown on borosilicate glass (BSG)substrates by reactive pulsed dc magnetron sputtering technique(pulsing @ 20 kHz) at 300 K (no intentional substrate heating).Ceramic ZnO target (99.99% purity, procured from M/s GfE,Germany) has been sputtered at 1.9W cm power density (op-timized for growing relatively good crystalline and stoichiom-teric films) in the presence N reactive gas (99.99% purity)plasma. N flow rate has been varied between 1.0 sccm–3.0sccm through a mass flow controller.

B. Characterization of Thin Films

The structural properties have been studied by XRD(Phillips X’ pert Pro- diffractometer with Cu radiation( )) and Raman spectroscopy (Jobin–von Ramanspectrometer, HR800). Optical properties have been investi-gated by UV-Vis. transmittance (300–1000 nm) (Jasco V-570double beam spectrophotometer). Reflectometry technique hasbeen used to obtain the optical constants ( , ) and thickness ( )(Filmetrics F20). Hall effect measurements have been carriedout in van der Pauw geometry on a 1 cm 1 cm square sample(Lakeshore HMS 7604). Low temperature electrical proper-ties have been measured in a closed cycle refrigerator (CCRfrom Janis cryotronics) from 12 K to 300 K ( 1 K). Surfacework function changes have been measured by an in-housebuilt Kelvin probe station. The compositional and chemicalstate analysis has been performed by X-ray photoelectronspectroscopy measurements with Al K photons (1486.6 eV)(Omicron analyzer). C1s at 284.9 eV has been used as reference

1551-319X © 2013 IEEE

716 JOURNAL OF DISPLAY TECHNOLOGY, VOL. 9, NO. 9, SEPTEMBER 2013

Fig. 1. XRD patterns of NZO thin films.

for the charge compensation. The spectral resolution of theinstrument is 0.8 eV.Reproducibility of the results has been ascertained by

growing the films at nearly identical conditions (within theexperimental limitations).

III. RESULTS AND ANALYSIS

A. Structural Analysis: XRD

All the NZO thin films have been found to be polycrystallinein nature with strong -axis orientation, confirmed from (002)peak.With the effect of N flow rate, the peak position shiftedtowards lower angles, confirming the stress development withN incorporation. The stress in NZO thin films has been obtainedfrom bi-axial stress model [18]

(1)

where, is the lattice constant of stress free ZnO powder andis that of the NZO thin film under study. With an increase inN flow, the tensile stress increased; it is attributed to the ionicsize mismatch between N (1.32 Å) and O (1.26 Å).

B. Thickness ( ) and Optical Constants ( , )

Thickness of all the films has been maintained at 200 nm( nm), to exclude the effect of thickness on NZO thin filmproperties.Refractive index decreased (from 2.01 to 1.90) with the N

flow rate and the extinction coefficient increased. These changesare in accordance with the reduced film quality as observed fromXRD and SEM (not shown here).

C. UV-Vis. Transmittance and Optical Band Gaps

The transmittance of NZO thin films decreased with N flowrate (80% to 60%). The reduced transmittance is due to the in-corporation of N in ZnO lattice, which will create interbandstates and thus effectively reducing the transmittance.The optical band gaps have been obtained from Tauc’s rela-

tion [19] for allowed and direct transitions

(2)

Fig. 2. variations and the stress developed in NZO thin films as a functionof N flow rate.

Fig. 3. Refractive index and extinction coefficient variations for NZO thinfilms as a function of N flow rate.

where, is absorption coefficient, is a constant, is the en-ergy of incident photon and is the band gap of the specimenunder study.The optical band gaps have been found to decrease with N

flow rate, from 3.3 eV to 3.1 eV. It is attributed to the createdinterband states with the N incorporation, resulting in the effec-tive reduction of optical band gap of NZO thin films [20].

D. Electrical Properties (Room Temperature)

Electrical properties have been measured in van der Pauw ge-ometry [21] on a 1 cm 1 cm square sample. In dots ( 1 mmdiameter) have been pressed on to the four corners of the sam-ples and each and every contact has been checked for the Ohmicnature by tracing – curves.The electrical resistivity has been found to increase and the

electron concentration is decreased in NZO thin films with in-crease in the N flow rate. These changes are attributed to theformation of acceptor levels near the valance band maximum(VBM), which will compensate the donor levels and effectivelyresulting in reduced electron density [22]. At 2.0 sccm and 2.5sccm of N flow rates, NZO thin films showed a positive Hallcoefficient (confirmed by measuring at different magnetic fieldsand excitation currents for several times). The conversion of

NAIDU et al.: STABILITY STUDIES ON NITROGEN DOPED p-ZnO(NZO) THIN FILMS 717

Fig. 4. Transmittance spectra of NZO thin films.

Fig. 5. versus lots of NZO thin films.

Fig. 6. Electrical properties of NZO thin films as a function of N flow rate.

charge carrier type from n-type to p-type at 2.0 sccm and aboveN flow rates is due to the effective domination of acceptorlevels formed by N, compared to the already existing donorlevels.The measured hole concentrations are 6.7 10 cm and

1.7 10 cm for NZO thin films grown at 2.0 sccm and

2.5 sccm, respectively. The reduced hole concentration hasbeen attributed to the pinning of the Fermi level at higher N con-centrations [23], which leads to the formation of compensatingdefects or acceptor passivators. At higher N concentrations, theformation energies for N and NO has been found todecrease, the first one acts as a double donor and the secondone as the acceptor passivator [24] (the atomic % of N indifferent chemical states has been quantified and correlatedwith electrical properties in XPS section). The mobility of NZOthin films has been found to decrease (8.32 cm V s to0.12 cm V s ) with N flow rates (Fig. 6). The decrementin mobility with the N flow rate is attributed to the increasedionized impurity scattering; resulting from the increased Nimpurity levels density. In a doped semiconductor, the mobilitydependence on ionized impurity density is [25]

(3)

where is the density of holes ( for electrons) surrounding theionized impurity, N is ionized impurity concentration, isBoltzmann’s constant and is Planck’s constant. In the presentstudy, N increases with N flow rate and thus effectively re-ducing the mobility. NZO thin films grown at 3.0 sccm andabove N flow rates have been found to be highly resistive (re-liable electrical measurements couldn’t be carried out due to theinstrument limitations).

E. Electrical Properties (Temperature Dependent)

Temperature dependent electrical properties have been foundto provide valuable information on impurity concentrations, im-purity positions in the band gap and the dominant charge carriertransport mechanisms, both qualitatively and quantitatively. Inthe present study, the charge carrier transport properties havebeen measured over a wide temperature range: 12 K–300 K( 1 K).Typical semiconducting behavior has been observed for all

NZO thin films (with negative temperature coefficient of resis-tance). The activation energies corresponding to the band con-duction (i.e., energy needed for the electron to jump from donorlevel to conduction band) has been calculated from the Arrhe-nius’s relation [25] and the fittings are shown in Fig. 7

(4)

where is impurity limited resistivity; is the thermal acti-vation energy for carriers to jump in to either conduction band(for electrons) or valance band (for holes). This activation en-ergy depends on charge carrier concentration and the position ofFermi level in the band gap. The activation energies have beenfound to vary between 68 meV and 258 meV (Fig. 7). The ac-tivation energy for holes (to jump in to valance band) is foundto be 133 meV and 258 meV for NZO thin films grown at 2.0sccm and 2.5 sccm of N flow rates and the enhanced value isin agreement with the reduced effective carrier concentration( , which is in agreement with


Fig. 7. vs. lots of NZO thin films as a function of N flow rate.

i.e., about 0.3 fraction of 6.1 are the holes inthe second case) [22].Temperature dependent carrier concentration also shown

typical semiconducting nature (electron/hole concentrationdecreased with temperature). The activation energies for chargecarriers to jump in to conduction/valance bands and contributefor conduction have been calculated from the following rela-tion, valid only for non-degenerate semiconductors [25] andthe corresponding fittings are shown in Fig. 8

(5)

where and are the unoccupied and occupiedstate degeneracies, is the activation energy of charge carriersand (the equation holds good forelectrons, being replaced by ).The activation energies have been found to vary between 68

meV and 236 meV, for the p-NZO films grown at 2.0 sccm Nflow rate it is 141 meV, which is in accordance with the valueobtained from Arrhenius’s relation. Similar values have beenreported in N doped p-ZnO thin films in the earlier studies [26].The mobility has been found to decrease continuously as the

temperatures decreased. It could be either due to the ionizedimpurity scattering and/or grain boundary related scattering. Inorder to further confirm the dominant scattering mechanisms,the mobility data has been fitted to the relations, and

where is the grain boundary po-tential (GBP), , are GBP and ionized impurity mediatedmobility terms, contributing to the effective mobility ( ) [25].The corresponding fittings are shown in Fig. 9(a) and (b). The

) versus plots slope varied between 1.3 and 1.8.For ionized carrier scattering alone to be dominated, the slopeshould be 1.5. The variations in the fitted slope value indicatethe dominant scattering mechanism is not only by ionized impu-rities but also GBs. In order to further confirm this, GBP valueshave been calculated by fitting the mobility to the above relationrepresenting GBP. The GBP values varied between 12 meV and46 meV. Increased GBP is due to reduced crystallinity and in-creased porosity (as observed from XRD and SEM studies) ofthe filmwith increase in N flow rate. To conclude, from the mo-

Fig. 8. vs. plots of NZO films as a function of N flowrate.

Fig. 9. (a) Vs fittings of NZO films. (b) versusfittings of NZO films.

bility dependence on temperature, the charge carrier scatteringin N doped ZnO thin films is mainly due to ionized impurityscattering and grain boundary scattering.

F. X-Ray Photoelectron Spectroscopy (XPS) Studies

XPS yields the qualitative and quantitative information onchemical composition and chemical states of elements present


TABLE ICOMPOSITIONAL ANALYSIS OF NZO THIN FILMS

Fig. 10. N 1s core level spectra of NZO thin films.

in NZO thin films. The etching of NZO thin films surface hasbeen carried out by Ar ions for 3 min to remove the surfacecontaminants. The O 1s core level spectra has shown two dis-tinct peaks, one at 530.5 eV, corresponding to O bonded to Zn(Zn-O) and the other one at 532.5 eV, which can be either due tosurface adsorbed or N bonded oxygen (NO or NO ). The Zncore level spectra has shown two characteristic peaks at 1022eV ( ) and 1045 eV ( ), corresponding to oxidized Znin ZnO [27]–[29].The N 1s core level spectra have been curve fitted to three

different regions (Fig. 10), centered at 396.5 eV (atomic N,replacing O: Zn-N), 399.6 eV (N molecule: N-N) and 404.5eV (nitrites: N-O) [29]. The atomic concentrations of elementspresent in the films grown at different N flow rates are shownin Table I.The primary requirement for acceptor level in ZnO isatomic N replacing O. But, when N bonds with N (to make a N )or O (to make nitrites: N-O), the first one acts as donor in ZnObut the second one acts as passivating complex defect for theN related acceptor level by pushing the corresponding acceptorlevel away fromVBM.As theN flow rate increases, N incorpo-rated in ZnO thin films at 1.0 sccm flow rate , totally about 0.87at.% of Nitrogen has been incorporated in two chemical statesZn – N (0.48 at.%) and N – N (0.39 at.%). With increase in Nflow rate, the concentration of N increased but it was found in adifferent chemical state (nitrites: N-O), in addition to the abovetwo chemical states. At 1.0 and 1.5 sccm N corresponding toZn – N (i.e.,N ) and other two chemical states are competingwith each other and it is in accordance with the observed carrier

Fig. 11. Surface work functions of NZO thin films as a function of N flowrate.

type from electrical measurements (n-type), due to the compen-sation by intrinsic defects as well acceptor related compensatingdefectsAt 2.0 sccm and 2.5 sccm of N flow rates, the N concentra-

tion, corresponding to Zn-N is higher than the other two chem-ical states by about 0.1 at.% – 0.2 at.%. This effectively leads tothe formation of higher density of acceptor levels, compared toother types of compensating defects and leading to hole domi-nated ZnO and it is in accordance with observed p-type nature.With further increase in N flow rate (3.0 sccm), the effectiveN concentration increased but the compensating defects to ac-ceptor type impurities increased (compensating defects forma-tion energy decreases with an increase in hole concentration)due to pinning of Fermi level. The measured higher N concen-tration from XPS, corresponding to Zn – N (N ) in the filmdeposited at 2.0 sccm compared to 2.5 sccm, which is in accor-dance with higher hole concentrations for the former comparedto latter.

G. Surface Work Function Studies

Fig. 11 shows the variation of surface work function as afunction of N flow rate. As the N flow rate increased, the sur-face work function value increased (from 4.691 eV for undopedZnO thin film) and reached maximum (of 4.982 eV) for film de-posited at N flow rate of 2.0 sccm. The surface work functionchanges are in accordance with the changes measured in carrierconcentration and carrier type measurements [30]. As the N


TABLE IICOMPOSITIONAL ANALYSIS OF P- NZO THIN FILMS AFTER 10 DAYS AND 180 DAYS OF DEPOSITION

Fig. 12. Changes in hole concentration and surface work function of p- NZOthin films (2.0 sccm of N flow rate).

flow rate increased, the electron concentration decreased. It isdue to the formation of acceptor levels. In this case, effectivelythe Fermi level will go down;The corresponding work function value should increase.

NZO films grown at 2.0 sccm showed p-type nature withrelatively higher hole concentrations. The surface work func-tion corresponding to these p-NZO thin films has been foundto be relatively higher, confirming the relative movement ofFermi level towards valance band maximum from the valanceband maximum, which is in accordance with the reduced holeconcentration after 2.0 sccm of N flow rate [22]. With furtherincrease in N flow rate, the surface work function decreased,indicating the Fermi level movement away from the valanceband maximum, which is in accordance with the reduced holeconcentration after 2.0 sccm of N flow rate.

H. Stability of p-Type Nature in p-NZO Thin Films

The p-type nature of ZnO thin filmswas periodically (in an in-terval of 15–20 days) measured from Hall effect measurements.The changes in the carrier concentration for p-type films grownat 2.0 sccm N flow rate are shown in Fig. 12.Both the p- type ZnO films have been found to be stable

with respect to the electrical properties over a period of 35 days(within experimental limitations), thereafter showed measur-able changes in the electrical properties. The hole concentrationdecreased significantly and reached about cm over a pe-riod of 180 days. With further increase in time (about 370 days),

the films became highly resistive and the measurements wereunreliable. Due to the higher resistivities of the aged NZO thinfilms, measuring the accurate carrier type was found to be diffi-cult (instrument limitations). Surface work function values haveshown similar variations in accordance with measured electricalproperties. In order to explain the observed changes, the struc-tural (XRD), optical (UV-VIS transmission), surface work func-tion and chemical state of N in ZnO lattice (by XPS have beenstudied periodically.From XRD, it has been observed that, due to the dopant (N)

incorporation in the grown thin film, the films were under ten-sile stress (discussed in XRD section). It is due to ionic sizemismatch between N and O. The lattice cannot be under stressfor longer times. Lattice has to loose its stress energy by givingit to some other parts of lattice to minimize the energy and be-come stable. In N doped ZnO system, energy cannot be takeneasily by Zn or O but N being a guest element in the lattice (par-ticularly N at O lattice site: N ) it can take the stress energy andmigrate in the lattice, the migration process depends on the sur-rounding chemical atmosphere: surrounding elements and theirco-ordination with migrating element. N has slightly higherformation energy than other forms of N in ZnO lattice, such asN and NO . Over the period of study, the (002) peak po-sition moved towards higher angles. This movement confirmsthe release of tensile stress in ZnO: N lattice. The stress energycould lead to a change in N chemical state and it can be respon-sible for observed changes in the electrical properties and workfunction of p-type ZnO thin films.In order to further investigate the instability of p-type nature

in NZO thin films, XPS measurements have been carried out onp-type ZnO thin films, after 180 days. The N1s core level spectra(recorded after 180 days of deposition) showed very significantchanges compared to that recorded after 10 days of depositionand the changes are compared in Fig. 12. Peak at 396.5 eV, cor-responding to Zn-N (N ) decreased in intensity. At the sametime, peak centered around 399.5 eV showed enhancement inintensity. Interestingly for p-type ZnO, grown at 2.0 sccm ofN flow rate, a small peak around 401.3 eV is observed (whichmay be due to N in ZnO N ,). The quantification of N indifferent chemical states has been shown in Table II.

IV. CONCLUSION

The effect of N flow rate on physical, electrical and compo-sitional properties of NZO thin films have been studied. (theformation of compensating defects). p- NZO thin films have


been reproducibly grown for several times to ascertain the re-producibility of the film properties (with in experimental limits).p- NZO thin films have been found to be electrically unstable.The instability of p- type nature in NZO thin films with the ef-fect of ageing has been explained based on stress release mech-anism.NZO films grown at 2.0 sccm of N flow rate have beenfound to be p-type. But, at further higher N flow rates, holeconcentration decreased due to the pinning of the Fermi level

ACKNOWLEDGMENT

The authors acknowledge the help obtained from Dr. S. V. NBhaskara Rao, Dr. S. N. Jha, Dr. D.M. Phase, andMr.Wadiakarfor performing XPS studies on NZO thin films.

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R. V. Muniswami Naidu received the M.Sc..degree in physics from Indian Institue of Technology(IIT)-Guwahati, in 2007, and the Ph.D. degree inthin film physics from Indian Institue of Technology(IIT)-Madras, in 2012.Since 2012, he has been working at Saint Gobain

Glass India Ltd., Chennai, India, as a member ofR&D team. His present interests include the processand product developments related to the anti solarand low emissive coatings.

Aryasomayajula Subrahmanyam received thePh.D. degree from Indian Institute of Technology,Kharagpur, India.He then joined Birla Institute of Technology and

Science, Pilani, India, in 1980 as Lecturer. In 1982,he then moved to Indian Institute of TechnologyMadras, Chennai, where he presently holds theposition of Professor at the Department of Physics.He has established a Semiconductor Laboratory towork on functional metal–oxide thin films. The mainemphasis of the work is to prepare device quality

metal–oxide thin films with lowest energy budgets but with highest perfor-mance in properties. He also has interest in engineering surfaces and interfacesfor devices. He has designed and developed Kelvin Probe for measurement ofsurface work-function in metals and semiconductors. His current interests are: transparent electronics, ultra high density optical memories, high efficiencyand fast response electro-chromics and nano mixed metal oxide photocatalystsfor biomedical applications.


Arnaud Verger received the Eng. & M. Sci. degreein physics from Ecole Polytechnique, France, and thePh.D. degree in quantum physics from the UniversitéPierre et Marie Curie in 2007.Since 2008, he has been a research engineer in

the thin film department of Saint-Gobain Recherche,France. His current research interests focus on thinfilm photovoltaic applications.

Mahaveer Kumar Jain received the Ph.D. degreefrom the Physics Department, Indian Institute ofTechnology, Delhi, India.During his Ph.D., and later as a Post-Doctoral

fellow at University of Kentucky and PennsylvaniaState University, he has worked on various chemicaland physical sensors. Further as Post-Doctoralfellow at CAT Indore, he has worked with trans-parent IR window materials like ZnS grown usingchemical vapor deposition. Subsequently, he joinedIIT Madras, Chennai, India, where he is presently

working as Associate Professor since July 2012. He has supervised 4 Ph.D.students and published about 33 research papers. His research interest isin semiconducting thin films for photovoltaic, biomedical and transparentelectronics applications.

Documents

Stability Studies on Nitrogen Doped p-ZnO (NZO) Thin Films Grown by Reactive Magnetron Sputtering