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Materials Chemistry and Physics 136 (2012) 624e637

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Materials Chemistry and Physics

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Properties of thermally oxidized and nitrided Zr-oxynitride thin film on 4HeSiCin diluted N2O ambient

Yew Hoong Wong, Kuan Yew Cheong*

Energy Efficient & Sustainable Semiconductor Research Group, School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300,Nibong Tebal, Seberang Perai Selatan, Penang, Malaysia

h i g h l i g h t s

< Zr-oxynitride as the gate oxide deposited on 4HeSiC substrate.< Simultaneous oxidation and nitridation of sputtered Zr thin film on 4HeSiC using various concentrations of N2O gas.< Presence of interfacial layer comprised of mixed compounds related to ZreO, ZreN, ZreOeN, SieN, and/or CeN.< The highest electrical breakdown and highest reliability at diluted N2O of 10%.

a r t i c l e i n f o

Article history:Received 13 January 2012Received in revised form18 June 2012Accepted 21 July 2012

Keywords:Thin filmsSputteringOxidationElectrical properties

* Corresponding author. Tel.: þ60 4 599 5259; fax:E-mail address: cheong@eng.usm.my (K.Y. Cheong

0254-0584/$ e see front matter � 2012 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2012.07.035

a b s t r a c t

A systematic investigation on the structural, chemical, and electrical properties of thermally oxidized andnitrided sputtered Zr thin film in various N2O ambient (10e100%) at 500 �C for 15 min to form Zr-oxynitride on 4HeSiC substrate has been carried out. The chemical composition, depth profile anal-ysis, and energy band alignment have been evaluated by X-ray photoelectron spectrometer. Zr-oxynitridelayer and its interfacial layer comprised of compounds related to ZreO, ZreN, ZreOeN, SieN, and/or CeNwere identified. A model related to the oxidation and nitridation mechanism has been suggested.Supportive results related to the model were obtained by energy filtered transmission electronmicroscopy, X-ray diffraction, and Raman analyses. A proposed crystal structure was employed toelucidate the surface roughness and topographies of the samples, which were characterized by atomicforce microscopy. The electrical results revealed that 10% N2O sample has possessed the highestbreakdown field and reliability. This was owing to the confinement of nitrogen-related compounds of ZreOeN and/or ZreN at or near interfacial layer region, smaller grain with finer structure on the surface,the lowest interface trap density, total interface trap density, and effective oxide charge, and highestbarrier height between conduction band edge of oxide and semiconductor.

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1. Introduction

Silicon carbide (SiC) is a potential wide bandgap (WBG) semi-conductor that has huge potential to be used as the next generationmetal-oxide-semiconductor (MOS)-based electronic devices forhigh power, high temperature, and/or high radiation applications[1e3]. One of the main strengths of SiC over other WBG semi-conductors is its ability to grow native oxide (SiO2) [3,4]. None-theless, there are numerous woes associated with the SiO2 thin filmor gate based on SiC, namely high SiO2eSiC interface-trap densityand high electric field [5,6]. In order to overcome these woes,

þ60 4 594 1011.).

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search of an alternative functional high-dielectric constant (k)material based on SiC, acting as a gate dielectric, is of interest.

In this sense, ZrO2 has been identified as a favorable candidateto be integrated as gate dielectric in SiC-based MOS devices due toits high k value of 22e25, large energy bandgap of 5.8e7.8 eV, andeasily stabilized in the form of cubic or tetragonal polymorphs,which may further enhance its effective k value up to 55 [3,7e13].According to literature, this material based on Si substrate haspossessed encouraging and well-behaved characteristics as a gatedielectric [8e11,14e18]. Nevertheless, there were scarce reports onits properties on SiC substrate. Karlsson and co-workers [19] usedchemical vapor deposition technique to grow ZrO2 thin films on4H-SiC(0001) substrate. Heterogeneous layers of decomposedZrO2 were formed after annealing at 1000 �C, with the existenceof undesirable t-ZrO2 remnants, metallic Zr silicide, and Si

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637 625

aggregates. Kurniawan and co-researchers [20] formed ZrO2 bycombination of Zr metal sputtering and thermal oxidation inoxygen ambient. The results indicated that post-annealing of theoxide has yielded a thicker interfacial layer with an increment incapacitance and in dielectric constant as well as reduction ofleakage current density but with lower oxide breakdown field.One of the major factors that affect the overall properties ofa dielectric layer is the interfacial quality between the oxideand SiC.

Extending from the work reported in Refs. [3,9e12], 100% N2Ohas been used to act as the oxidation as well as nitridation sourceto form ZrO2 thin film with an interfacial layer (IL) of Zr-silicateoxynitride (ZrSiON) on Si substrate and to form oxynitrided Zr(ZrON) thin film with an IL comprised of ZrSiON and carbonnitride (CN) on SiC substrate. Formation of these compounds isbasically due to dissociation of the N2O gas into oxygen andnitrogen source that are used for oxidation and nitridationprocesses; in which the oxidation and nitridation rates maydetermine the final products. Though the studies provided highenough effective dielectric constant values (ZrO2/IL/Si:10.58e21.82; ZrON/IL/SiC: 42.32e50.20), however, the recordeddensities of effective oxide charge, slow-trap, and interface-trapwere still considerably high. It is hypothesized that dilution ofN2O may be a possible way to adequately produce a betterquality oxide according to the accumulated knowledge [4,21e26]from thermally nitrided and oxidized Si on SiC to form nitridedSiO2 gate. During this process, nitrogen source may enhanceremoval of carbon that has been accumulated at the interfaceduring thermal oxidation and may improve passivation ofdangling bonds on surface of the semiconductor depending onthe equilibrium rate of oxidation and nitridation that has beenachieved by diluting the oxidation and nitrogen sources [4]. Themost effective gases in achieving these effects are nitrous oxide(N2O) and nitric oxide (NO) [4]. The former gas is more preferabledue to its non-toxic property [9,22,23,27]. Since there is nopertinent investigation has been reported on the effects of N2Oconcentrations on oxidizing and nitriding of sputtered Zr basedon SiC substrate, hence, the main objective of this paper is toreport on the chemical, structural, and electrical properties of Zr-oxynitride thin film produced by this technique on 4HeSiC(0001)substrate.

2. Experimental procedures

In this work, 0.8 cm � 0.8 cm diced n-type Si-faced4HeSiC(0001) substrates with 4.09�-off axis, 0.020 U cm of resis-tivity, and 1-mm thick of n-type epitaxial layer doped with nitrogenat concentration of (1e4) � 1016 cm�3, which were commerciallypurchased from CREE Inc. (USA), were used as the starting mate-rials. The substrates were first cleaned using RCA (Radio Corpora-tion of America) cleaning method and completed by dipping theminto a diluted HF solution with a ratio of 1HF:50H2O, aiming toremove any native oxide on the surface of the substrates. By usingan Edwards Auto 500 RF sputtering system, a 5-nm thick Zr filmwas sputtered on the cleaned 4HeSiC substrate. The workingpressure, RF power, inert Ar gas flow, and deposition rate during thesputtering process were configured at 1.2 � 10�7 Torr, 170 W,20 cm3 min�1, and 0.2 nm s�1, respectively. Samples were theninserted into a horizontal tube furnace at atmospheric pressure andheated up from room temperature to 500 �C in an Ar flow ambientand the heating rate was set constant at 10 �C min�1. Once the settemperature was achieved, a set of different concentrations of N2Ogas was introduced, i.e. 10, 30, 70, and 100% N2Omixed with 90, 70,30, and 0% of high purity N2 gas, for duration of 15 min, with a flowrate of 150 ml min�1. Once completed, the furnace was cooled

down to room temperature in an Ar ambient and the samples werewithdrawn from the furnace at room temperature.

Kratos Axis UltraDLD X-ray photoelectron spectrometer witha monochromatic Al-Ka X-ray source (hn ¼ 1486.69 eV) wasemployed to characterize chemical compositional of the producedfilms. The spectrometer was operated at 150Wand a take-off angleof 0� with respect to surface normal. The chemical composition asa function of depth was recorded. In order to perform a depthprofiling analysis, a 5-kV Ar ion etching was used. The pressure ofAr in the analysis chamber was 3 � 10�7 Torr, while the area ofanalysis was 220� 220 mm2. The chemical composition of the filmswas obtained from a combination of survey and narrow scans. Asurvey scan was first performed with a pass energy of 160 eV for9 min to determine the elemental chemical states. The core-levelspectra that had been detected were Zr 3d, Si 2p, O 1s, N 1s, andC 1s. Subsequently, for a narrow scan, a pass energy of 20 eV for5 min was used to scan through the binding-energy range ofinterest. The recorded C 1s peak due to the adventitious carbon-based contaminant on the surface, with respect to the literaturevalue of 284.6 eV [28,29], was used as a reference to compensate forcharging effect of the XPS spectra. Surface charge and linear back-ground were corrected with the assistance of CasaXPS software(version 2.3.15) before deconvolution of the XPS spectra was per-formed. The total concentration of an element (Cx) available in theinvestigated films was calculated based on the following equation[23,30,31]:

Cx ¼ Ix=SxPiIi=Si

� 100 (1)

where, Ix and Iiwere peak intensity of the evaluated element and allother detected elements, respectively. Sx and Si were sensitivityfactor of the respective evaluated element and all other detectableelements. The sensitivity factor is dependent on XPS system. In thiswork, the sensitivity factors for Zr 3d, Si 2p, O 1s, N 1s, and C 1swere2.576, 0.328, 0.78, 0.47, and 0.278, respectively.

For structural characterization, Zeiss Libra 200 energy-filteredtransmission electron microscope (EFTEM) was utilized toexamine the cross-sectional images of the films. Prior to this,a protective layer of resist was deposited on the top most layer ofa sample. Then it was ion-milled by a focused ion beam system.ImageJ software, which gives the accuracy of measurements inthree decimal places, was used to measure the interplanar spacing,d, of the polycrystalline structure obtained from the EFTEM images.In order to identify the crystallinity of the films, X-ray diffraction(XRD) patterns were recorded in a grazing-incident mode, ona PANalytical X’Pert PRO MRD PW3040 diffractometer. Copper (Cu-Ka) radiation was used as the X-ray source, with wavelength (l) of1.5406 nm operating at 40 kV and 30 mA. Jobin Yvon HR 800 UVRaman spectrometer with an Arþ incident beam and wavelength of514.5 nm, were conducted to ascertain the stability of chemicalbonding upon thermal oxidation and nitridation process. By usinga non-contact mode Nano Navi SPI3800N Atomic Force Microscope(AFM), surface topography and roughness of the samples wereanalyzed.

MOS capacitor test structures were fabricated in order to char-acterize electrical properties of the film. A 100-nm thick Al as gateelectrode, with photolithographically defined square area of9 � 10�4 cm2 (0.3 cm � 0.3 cm), was deposited on top of the films.Al film was also deposited on backside of the SiC substrate actingas an Ohmic back contact. A computer-controlled Agilent HP4284LCR meter and HP4155-6C semiconductor parameter analyzer(SPA) were used to carry out high-frequency (1 MHz)capacitanceevoltage (CeV) and currentevoltage (IeV) measure-ments, respectively.

Fig. 1. Atomic percentage of Zr, Si, O, N, and C as a function of etching time for sampleoxidized/nitrided samples in (a) 100%, (b) 70%, (c) 30%, and (d) 10% N2O.

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637626

3. Results and discussion

3.1. XPS characterization

The atomic compositional depth profiles of Zr, Si, O, N, and C asa function of etching time in all investigated samples are shown inFig. 1(a)e(d). It is observed that at earlier etching time, Zr and Oelements are in dominance with an atomic ratio of about 1:2,implying that the top-most layer of the bulk region of each sampleis covered by a stoichiometric ZrO2. With the increasing etchingtime, the atomic concentrations of both Zr and O elements decreaseand their ratios are deviated from 1:2; until they are totally dis-appeared. Besides, N element is being detected in the bulk region ofall investigated films, indicating that nitrogen has been incorpo-rated into the ZreO network of the film, forming Zr-oxynitride. Theboundary of the oxynitride and SiC is defined by the dotted lines, asdemonstrated in the figure. Beyond this boundary, atomicpercentages of Si and C increase significantly, hence, corroboratingthe appearance of SiC substrate. A mixture of Zr, O, Si, N, and C arebeing observed in between the boundaries of Zr-oxynitride and SiC;indicating the presence of an interfacial layer (IL). In order toidentify chemical compounds of the IL, an analysis by narrow scanof XPS was performed and it will be discussed in the followingparagraphs. The effective thickness of both bulk Zr-oxynitride andIL are dependent on the amount of N2O being supplied during theoxidation and nitridation processes; which is depending on theconcentration of N2O gas. It is observed that the effective thicknessof the bulk Zr-oxynitride and IL is increased when N2O is diluted upto 70%. A reduction in the effective thicknesses for both Zr-oxynitride and IL film is recorded when further dilution of N2Oconcentrations (30 and 10%) are used. This observation is inagreement with EFTEM images, which will be presented in thesubsequent paragraphs. Based on these observations, the followingsuggested oxidation and nitridation model is proposed [9,12]. Zrcations as catalytic ions are able to decompose N2O into N and Oions at temperature of 500 �C. These ions play the oxidizing andnitriding role to transform Zr metal into oxide of Zr (ZrO2) and/oroxynitrided Zr (ZrON). After the decomposition of N2O, twocompeting processes, i.e. oxidation and nitridation, may occursimultaneously. Due to the difference in N2O concentrations (100,70, 30, and 10%) being supplied, the competition between oxidationand nitridationmay vary, with different rates. Therefore, the atomicconcentration and distribution of oxygen and nitrogen in thesamples are varied, thus affecting the formation of O- and N-relatedcompounds in the samples. These compounds may directly and/orindirectly affect the interfacial and electrical properties of a MOS-based device. The distribution profiles of oxygen and nitrogen inall investigated samples have been studied based on the figure.There is an insignificant change in atomic concentration anddistribution of oxygen in the samples with different concentrationsof N2O being supplied. However, from the nitrogen profiles, it isnoted that the distribution in terms of broadness, maximum atomicpercent, and its location vary in each sample, depending on thesupplied amount of N2O. The maximum atomic percentage ofnitrogen decreases as the N2O concentration is reduced (100%: 2.71at%; 70%: 1.87 at%; 30%: 1.24 at%; 10%: 1.08 at%). Besides, it is alsoobserved that nitrogen is distributed towards and confined at the ILregion as the concentration of N2O is reduced. With this observa-tion, it is postulated that by diluting the concentration of N2O; itmay balance the competition between oxidation and nitridation,thus confining the nitrogen at the IL region. As a result, animprovement of the interfacial and electrical properties of theoxide can be obtained as will be shown in the following paragraphs.

Figs. 2[(a)e(d)]e5[(a)e(d)] demonstrate thenarrowscansof eachcore-level spectrum as a function of etching time for various N2O

concentrations (100, 70, 30, and 10%). The measured peaks (dottedlines) were deconvoluted by non-linear GaussianeLorentzian func-tion (solid lines) using CasaXPS software (version 2.3.15). The Zr 3dspectra [Fig. 2(a)e(d)] arewell fitted by ZreO, ZreN, and/or ZreOeNcomponents at their respective binding energies. Zr 3d doublet,

Fig. 2. Evolution of Zr 3d core level XPS spectra as a function of etching time fordifferent N2O concentration samples: (a) 100%, (b) 70%, (c) 30%, and (d) 10%.

Fig. 3. Evolution of Si 2p core level XPS spectra as a function of etching time fordifferent N2O concentration samples: (a) 100%, (b) 70%, (c) 30%, and (d) 10%.

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637 627

Fig. 4. Evolution of O 1s core level XPS spectra as a function of etching time fordifferent N2O concentration samples: (a) 100%, (b) 70%, (c) 30%, and (d) 10%.

Fig. 5. Evolution of N 1s core level XPS spectra as a function of etching time fordifferent N2O concentration samples: (a) 100%, (b) 70%, (c) 30%, and (d) 10%.

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637628

Table 1Possible compounds in the substrate, interfacial layer, and bulk of the investigatedsamples.

Substrate Compounds Concentration of N2O (%)

100 70 30 10

IL SiC O O O OZreO O O O OZreN O O O OZreOeN O O O OSieN O O O OCeN O O O O

Bulk ZreO O O O OZreN O OZreOeN O O O O

Fig. 6. (a) XPS valence band spectra of Zr-oxynitride and IL for all investigated samples.(b) XPS O 1s plasmon loss spectra of Zr-oxynitride and IL for 10% N2O sample.

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637 629

which tallywith 3d5/2 and 3d3/2 spin-orbit split components at 184.4and182.0 eV, respectively, has been detected at the top-most surface(etching time of 0 s) of the bulk region for all investigated samples.This indicates that stoichiometric ZreO (ZrO2) is formed [9,32]. Asthe etching time increases, it is observed that progressive chemicalshift of ZreO peaks towards lower or higher binding energies,depending on the supplied amount of N2O. This is due to the trans-formation of stoichiometric to non- or sub-stoichiometric ZreOtowards a deeper region [9,33,34]. This is observed in all investi-gated films. Apart from ZreO peaks, ZreN (179.0e179.5 eV[9,35e37]) and ZreOeN (180.0e182.8 eV [37e41]) peaks can bedetected in the bulk region towards the IL for all investigatedsamples. This infers that compounds of ZreO, ZreOeN, and/or ZreNare formed in bulk Zr-oxynitride and IL for all investigated samples.As the etching time prolongs towards SiC substrate, the intensity ofZr 3d spectra is decreased and vanished.

Si 2p spectra [Fig. 3(a)e(d)] were investigated to identify theexistence of other components in the samples. Initially, no peakwasdetected at the earlier etching time for all investigated samples. Asthe etching time increases, SieN and/or SieC are detected atbinding energies of 100.4e101.4 eV [9,23,31] and 99.8 eV [31],respectively, in all investigated samples. The detected SieC peak isoriginated from SiC substrate.

According to the O 1s spectra being analyzed [Fig. 4(a)e(d)], atthe top-most surface (etching time of 0 s) of all investigatedsamples, two peaks can be well fitted in the spectra with theirbinding energies of 530.0 eV [9,42,43] and 531.5e532.0 eV [9,44].These two peaks are assigned to stoichiometric ZreO. Similar to theobservation revealed in Zr 3d spectra (Fig. 2), the O 1s spectra havealso progressive shifted towards lower or higher binding energies,depending on the supplied amount of N2O. This is well matchedwith the earlier claim (Fig. 2) that sub- or non-stoichiometric ZreOis being detected as it moves deeper inside. Beside ZreO peaks,ZreOeN (528.8e531.8 eV [40]) peaks are detectable in the bulkregion towards the IL in all investigated samples. The intensity of O1s spectra decreases and disappears as the etching time is extendedto SiC substrate. These results are in line with the results indicatedin Zr 3d spectra.

Fig. 5(a)e(d) shows the N 1s spectra that are fitted by fourcomponents, i.e. ZreOeN, SieN, CeN, and/or ZreN, with bindingenergies of 399.2 eV [38,39,41], 397.0e397.1 eV [9,23,45], 396.5 eV[46], and 395.9 eV [9,29], respectively. The intensity of these peaksdecreases with the increasing of etching time, until it is totallyfaded away. These fitted peaks are well supported by the analysesobtained from Zr 3d, Si 2p, and O 1s spectra.

A typical narrow scan of C 1s core-level spectra has beeninvestigated (not shown). No peak was detected at the earlieretching time. As the etching time increases until the IL region, threepeaks which are associated with SieC, CeC, and CeN, have beendetected at binding energies of 282.5 eV [23,47], 283.3 eV [23,47],and 284.4 eV [22], respectively. The intensity of SieC, which orig-inated from the SiC substrate, increases as the etching time isincreased. It is in agreement with the results shown in Si 2p spectra.On the other hand, CeC and CeN bonds, which appear in the ILdecreases gradually as the etching time is extended. Based on theanalyses of Zr 3d, Si 2p, O 1s, N 1s, and C 1s spectra, the possiblecompounds being detected in the bulk, interfacial layer, andsubstrate are shown in Table 1.

3.2. Band alignment

By using the same extraction method, as described elsewhere[11], energy band alignment of Zr-oxynitride/IL/SiC system hasbeen evaluated. Typical valence-band spectra of Zr-oxynitride andIL for all investigated samples have been studied [Fig. 6(a)]. The

valence-band edges (Ev) of Zr-oxynitride and IL were approximatedby an intercept of linear extrapolation of amaximumnegative slopenear the edge to the minimum horizontal baseline [11,48]. Thevalence-band offsets (DEv) of Zr-oxynitride and IL with respect toSiC substrate are 1.00� 0.05 eV and 2.50� 0.05 eV, respectively, forall investigated samples. To evaluate conduction-band offset (DEc)of Zr-oxynitride/IL/SiC system, bandgaps (Eg) of Zr-oxynitride and ILwere first deduced fromO 1s plasmon loss spectra [49,50] of the Zr-oxynitride and IL (not shown). The Eg values of Zr-oxynitride and ILextracted from their respective O 1s plasmon loss spectra are pre-sented in Fig. 6(b). As explained earlier, values of Eg were alsoestimated by an intercept of linear extrapolation. The extracted Egvalues of Zr-oxynitride and IL are 4.40e5.00 eV and 8.20e8.50 eV,

Fig. 8. Band alignment of Zr-oxynitride/IL/SiC system. Eg(ZrON) ¼ bandgap of Zr-oxynitride, Eg(IL) ¼ bandgap of IL, Eg(SiC) ¼ bandgap of SiC, DEv(ZrON/SiC) ¼ valenceband offsets of Zr-oxynitride to SiC, DEv(IL/SiC) ¼ valence band offsets of IL to SiC,DEc(ZrON/SiC) ¼ conduction band offset of Zr-oxynitride to SiC, DEc(IL/SiC) ¼ conductionband offset of IL to SiC, DEc(ZrON/IL) ¼ conduction band offset of Zr-oxynitride to IL.

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637630

respectively, with a tolerance of 0.05 eV, dependent on theconcentration of N2O (Fig. 7). By using the following equation(Eq. (2)), conduction-band offset of Zr-oxynitride to IL, DEc(ZrON/IL)for Zr-oxynitride/IL/SiC system can be determined by [51]:

DEcðZrON=ILÞ ¼ EgðILÞ � DEvðIL=SiCÞ þ DEvðZrON=SiCÞ � EgðZrONÞ (2)

where, Eg(ZrON) and Eg(IL) are the bandgaps of Zr-oxynitride and IL,respectively. DEv(ZrON/SiC) and DEv(IL/SiC) are the valence band offsetsof Zr-oxynitride and IL, respectively, with respect to SiC substrate.The calculated values of Eg(ZrON), Eg(IL), DEc(ZrON/SiC), DEc(IL/SiC), andDEc(ZrON/IL), are presented in Fig. 7. The highest value of DEc(ZrON/IL),i.e., 2.30 eV, is attained by sample oxidized/nitrided in 10% N2O(Fig. 7) as compared to others. A schematic of band alignment of Zr-oxynitride/IL/SiC system is illustrated in Fig. 8. The Eg value of SiCsubstrate is obtained from literature [2,52].

3.3. EFTEM analysis

Fig. 9 shows the cross-sectional EFTEM images of films oxidized/nitrided in various concentrations of N2O (100, 70, 30, and 10%).Based on the images shown, the boundaries of bulk, IL, andSiC substrate are clearly defined. Polycrystalline structure of bulkhas been revealed in all investigated samples, as patches offringes can be clearly observed, with interplanar spacing, d, of0.262e0.307 nm. These values weremeasured from the images andit is in line with the d value of ZrO2 [3,9,53,54]. A typical EFTEMimage of two crystalline planes is clearly shown in Fig. 10, with d, of0.266e0.297 nm. The orientation of the two planes will beexplained and discussed in the subsequent sub-section using XRD.According to the XPS measurements, it is found that bulk Zr-oxynitride, which comprised of ZreO, ZreOeN, and/or ZreN, hasbeen formed. Thus, it can be inferred that ZreOeN and ZreN maybe in amorphous structure which embedded in polycrystallineZrO2. The IL of amorphous structure in the images may be consistedof sub- or non-stoichiometric ZreO, ZreOeN, ZreN, SieN, andCeN. As displayed in Fig. 11, the physically thickest Zr-oxynitridefilm of 21.44 nm has been produced by 70% N2O, as compared toother samples (10, 30, and 100% N2O), in which, their physicalthicknesses range between 16.12 and 16.90 nm. As a consequence,the film produced by 70% N2O has the highest effective thickness(Zr-oxynitride þ IL) of 24.10 nm, while the effective thicknesses ofother samples are ranged in 18.76e19.10 nm. On the other hand, thethickness of IL that formed in between the bulk Zr-oxynitride andSiC substrate increases from1.86 to 2.82 nm, as the concentration ofN2O is reduced.

Fig. 7. The calculated values of Eg(ZrON), Eg(IL), DEc(ZrON/SiC), DEc(IL/SiC), and DEc(ZrON/IL) inthe band alignment of Zr-oxynitride/IL/SiC system, as a function of N2O concentration.

3.4. XRD measurements

The close-up XRD patterns of oxidized/nitrided samples atdifferent N2O concentrations within the 2q range of 25�e35� areshown in Fig. 12(a)e(d) in order to identify the characteristic peaksof tetragonal phase of ZrO2 (t-ZrO2). It has been described else-where [3,9] that a very strong intensity of diffraction peak at 35.5�

and a minor peak at 43.2� (not shown) were recorded in allinvestigated samples, in which the peaks are well matched with4HeSiC(004) (ICDD card number 00-022-1317). The featurelesspattern of as-deposited Zr metal on SiC has been verified elsewhere[3,9] that the sample is amorphous structure (not shown). Uponoxidation and nitridation at temperatures of 500 �C in different N2Oconcentrations, crystalline structure is revealed in all investigatedfilms. There are two diffraction peaks at 30.2� and 33.8� that areassociated with t-ZrO2 (011) and (002), respectively. These twopeaks of t-ZrO2 were confirmed by ICDD card numbers, 00-050-1089 and 00-024-1164, respectively. Based on the ICDD files,interplanar spacing of the t-ZrO2 (011) and t-ZrO2 (002) are0.2950 nm and 0.2635 nm, respectively. Therefore, from the XRDmeasurements, it suggests that crystalline t-ZrO2 has been formed.This result is also supported by the measured interplanar spacingobtained from EFTEM images (Figs. 9 and 10). A proposed crystalstructure and its arrangement with respect to the baseline ofsubstrate are illustrated in Fig. 13. The crystal planes of (011) and(002) are shown in the figure. The inclination of the crystal isindicated by maximum (tmax) and minimum (tmin) height of thecrystal with respect to the substrate baseline. By comparing theXRD and XPS results, it can be inferred that the remaining unde-tected compounds by XRD are ZreN, ZreOeN, SieN, and CeN.Therefore, these compounds are believed to be in amorphousstructure.

WilliamsoneHall (WeH) approach was employed to analyzeXRD line broadening, which can be used to determine crystallitesize (D) and microstrains of the Zr-oxynitride films on SiC substrate[55e57]. The WeH approach is utilized by considering that thecombined effects of domain and deformation are simultaneouslyoperative, and these effects result in the final line broadening (b),which is the sum of crystallite size and lattice distortion, as shownbelow:

Fig. 9. Cross-sectional of EFTEM images of the investigated samples in different N2O concentrations: (a) 100%, (b) 70%, (c) 30%, and (d) 10%.

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637 631

b ¼ bgrain size þ blattice distortion (3)

Instrumentation contribution in this relationship is assumed tobe negligible. Therefore, Eq. (3) may be expressed in the followingform:

Fig. 10. A typical cross-sectional of EFTEM images of the 70% N2O sample at lowermagnification. The red circle shows two crystalline planes. (For interpretation of thereferences to colour in this figure legend, the reader is referred to the web version ofthis article.)

bicosqi=l ¼ 1=Dþ 4 3sinqi=l (4)

where, l is the wavelength of the X-rays, qi is the diffraction angle,b is the integral breath (in radius 2q) of the ith Bragg reflectionpositioned at 2qi, and 3is the elastic strain. Fig. 14 shows the WeHplot of Zr-oxynitride films. Since only twoXRD peaks are observablein each sample, there are only two data points available toextrapolate a linear regression of WeH plot for each sample.Therefore, the goodness-of-fit (r2) for the linear regression is 1. The

Fig. 11. Average thicknesses of Zr-oxynitride, IL, and total (Zr-oxynitride þ IL) for theinvestigated samples. Error bars define the maximum and minimum thicknessesobtained.

Fig. 12. Close-up XRD patterns of as deposited Zr on Si and oxidized/nitrided samplesat different temperatures within the 2q range of 25�e35� to clearly show the char-acteristic peaks of tetragonal phase.

Fig. 14. WilliamsoneHall plot of t-ZrO2.

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637632

intercept of the plot with ordinate represents the reciprocal of D,whereas gradient of the line indicates 3, which represents themicrostains in the films [55]. By re-plotting the obtained crystallitesize as a function of N2O concentration (Fig. 15), it is found that thegrain size increases from 156.25 nm to 250.00 nm as the N2Oconcentration is increased. On the other hand, as the N2Oconcentration reduces, microstrains of the films increase (Fig. 15).The measured film thickness and the obtained crystallite size aredissimilar. This could be the reason contributed by the crystalalignment, as shown in Fig. 13, in which the size of crystallitecalculated from (011) and (002) planes were not in the direction oftmax or tmin.

3.5. Raman spectroscopy

The Raman spectra of oxidized/nitrided samples at 500 �C indifferent N2O concentrations are displayed in Fig. 16(a)e(d). Twopeaks located at 204 cm�1 and 777 cm�1 are originated from4HeSiC substrate and it can be detected in all investigated samples.For as-deposited Zr metal on SiC substrate, Raman spectrum wasnot detectable except for the two characteristics SiC spectra (notshown) [3,9]. This indicates that the metal layer is an amorphousphase, which is in agreement with XRD results that have beenpresented in Section 3.4. On the other hand, presence of a peak at471 cm�1 and 641 cm�1 are identified to be associated with t-ZrO2and it is recorded in all oxidized/nitrided samples [58]. Based onthis result, it has again confirmed that t-ZrO2 has been formed in allsamples oxidized/nitrided in various N2O concentrations, which isconsistent with the XRD results.

Fig. 13. Illustration of t-ZrO2 crystal structure arrangement with respect to baseline ofsubstrate.

3.6. AFM analysis

Fig. 17 demonstrates the two-dimensional (2D) surface topog-raphies of oxidized/nitrided samples at 500 �C in different N2Oconcentrations recorded on a 3 mm � 3 mm scanned area. Fig. 18displays the corresponding surface root-mean-square (RMS)roughness of these samples. It is observed that the surface topog-raphies of the samples are in wavy form with different waviness,with the surfaceRMSvalue increases from2.45 to 6.11nmwhenN2Ois diluted from 100 to 30%. A slight reduction in the surface RMSvalue is recorded, i.e. 4.79 nm, when N2O is further diluted to 10%.This observation can be related to the proposed crystal structure(Fig.13). A smaller difference between tmax and tmin, i.e. [tmax� tmin]produces smoother surface while a larger value of [tmax � tmin] mayattribute to a rougher surface. It is believed that the surface rough-ness is governed by the orientation and alignment of the tetragonalcrystal structure that is embedded in amorphous phase. As shown inFig. 13, it is expected that tmin would have an almost constant valuewhilst tmax would change when the crystal is collapsed and/orcombined (or agglomerated) that may affect the alignment andarrangementof the crystal. FromtheAFMresult, it is obvious that thealignment and arrangement is affected by the concentration of N2O.This reflects that tmax is the contributor to the surface roughness.When 100% N2O is used, the tetragonal crystal is well aligned on thesurface as shown in Fig.17(a), as a result of the collapse of the crystal,leading to a smaller tmax (smaller [tmax � tmin] value). Therefore,a smoother surface can be attained. As the concentration of N2O isdiluted to 70%, combination of crystal forming a larger grain can beseen in the AFM topography [Fig. 17 (b)], with higher surface

Fig. 15. Relationship of crystallite size and microstrains as a function of N2O concen-tration calculated from WilliamsoneHall plot.

Fig. 16. Raman spectra of oxidized/nitrided samples in various N2O concentrations(10e100%).

Fig. 18. Root-mean-square (RMS) surface roughness of samples oxidized/nitrided invarious N2O concentrations (10e100%).

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637 633

roughness. This may attribute to the increment of tmax. Furtherdiluted the N2O, the produced sample is having a rougher surfacedue to the same explanation but the roughness reduces when thesample was prepared at 10% N2O. For this sample, a smaller grain

Fig. 17. Two-dimensional AFM surface topography of oxidized/nitrided samples at different N2O concentrations: (a) 100%, (b) 70%, (c) 30%, and (d) 10%.

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637634

consisted of finer structure with lower tmax value if comparedwith the tmax value of sample prepared at 30% of N2O, can be seen inFig. 17 (d).

3.7. CeV characteristics

Fig. 19 presents typical high-frequency (1 MHz) CeV curvesmeasured at room temperature for samples oxidized/nitrided invarious N2O concentrations (10e100%), in which the gate bias is bi-directionally swept from �2 to þ10 V. It is noted that the capaci-tance at accumulation level decreases with the dilution of N2O from100 to 10%. This variation can be related to the presence of inter-facial layer sandwiched in between Zr-oxynitride layer and SiCsubstrate, as observed in EFTEM analysis earlier. The thickness of ILreduces as the N2O concentration increases. Based on the review byWilk et al. [7], the overall capacitance of a stacked layer would bestrongly affected by the presence of interfacial layer. Therefore, thiscan be explicated by a double-layer stacking gate model (inset ofFig. 19). The total capacitance (Ctotal) of the stacked layer is relatedby the equation below:

1=Ctotal ¼ 1=CZrON þ 1=CIL (5)

where, CZrON and CIL are the capacitance of Zr-oxynitride andinterfacial layer, respectively. Based on the previous report [8],CZrON and CIL can be individually defined by:

Cox ¼ k 3oA=tox (6)

where, Cox represents capacitance of individual oxide (Zr-oxy-nitride or IL), k is effective dielectric constant, 3o is permittivity offree space (8.85�10�12 F m�1), A is area of capacitor, and tox is totalthickness of Zr-oxynitride or IL. Since A and 3o are constants, hence,CZrON and CIL are proportional to k/tox. Based on this relationship,when thickness of interfacial layer is increased, total oxide capac-itance of the MOS capacitor reduces. Therefore, 100% N2O samplewith the thinnest interfacial layer gives the highest total capaci-tance whilst 10% N2O sample with thickest interfacial layerpossesses the lowest total capacitance.

Using Eq (6), effective dielectric constant (keff) of the oxides (Zr-oxynitride þ IL) was approximated. Cox was obtained from CeVmeasurements and the total oxide thickness was measured fromEFTEM images. The relationship of the keff value and N2O concen-tration is depicted in the inset of Fig. 19. The values obtained in this

Fig. 19. CeV characteristics of MOS capacitors with Zr-oxynitride oxidized/nitrided invarious N2O concentrations (10e100%). The insets show effective dielectric constant ofthe investigated samples deduced from the CeV measurement as a function of N2Oconcentrations (10e100%) and capacitors in series.

work are in the range of 37.38e59.10, which is double the value ofbulk ZrO2 (22e25). As reported by Li et al. [13], the dielectricconstant of ZrO2 on Si substrate ranges in between 7.5 and 55.0.Based on our previous report [10], the dielectric constant of ZrO2 onSi substrate lies in the range of 10.58e21.82. Therefore, the value isvery much dependent on the oxideesubstrate interface character-istics, nature of the substrate, electrode material, and oxide depo-sition method.

Depletion region is generally observed in the positive bias of allcharacterized capacitors in the CeVmeasurements. In other words,flatband voltage is shifted (DVFB) positively. This indicates thataccumulation of negative effective oxide charges (Qeff) has beencreated in the oxynitride during the oxidation and nitridationprocess [59]. Qeff of the investigated samples was calculated usingthe following equation:

Qeff ¼ ðDVFBÞCox=qA (7)

where, q is the electronic charge. The calculatedQeff for each sampleis presented in Fig. 20. The source of negative Qeff may be ascribedto the existence of acceptor interface traps that acted as fixedcharges when not located in the majority band edge [60]. In thiswork, the lowest Qeff of 1.2 � 1013 cm�2, was obtained by filmoxidized/nitrided in diluted N2O (10%), which was relatively smallas compared to other samples with higher concentration of N2O.This might be attributed to the lesser amount of negatively chargedcompounds as the N2O concentration is reduced, such as ZreOeNand/or ZreN, which only distributed and confined at or nearinterfacial layer region in the samples and did not appear in thebulk layer, as depicted in Figs. 2e5. Besides, it is also proven by thenitrogen distribution profile in Fig. 1, whereby the negative chargecontributor, i.e. nitrogen atoms, are only distributed and confined ator near the interfacial layer region when N2O is diluted to 10%. Inrelation with AFM analysis, 10% N2O sample with a smaller grainconsisted of finer structure may contribute to the lowest Qeff.

Hysteresis of the CeV measurements between forward andreverse bias is noticeable in all investigated samples. As explainedby Tanner et al. [61] and Dimitrijev et al. [62], this phenomenonoccurs due to slow traps located at some distance from the inter-face. The slow trap density (STD) can be indirectly associated withthe hysteresis based on the following equation:

STD ¼ ðDVÞCox=qA (8)

where, DV is the difference between flatband voltages of a hyster-esis curve. Based on the calculated value (Fig. 20), it is identifiedthat film oxidized/nitrided in 10% N2O has the lowest STD

Fig. 20. Effective oxide charge and slow trap density contained in the investigatedsamples as a function of N2O concentrations (10e100%).

Fig. 22. JeE characteristics of investigated samples oxidized/nitrided in various N2Oconcentrations (10e100%).

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637 635

(3.1 � 1012 cm�2). The change in STD as a function of N2Oconcentration possesses the identical trend as Qeff.

Oxynitride-semiconductor interface-trap density (Dit) of theinvestigated samples with respect to energy level of the trap isdepicted in Fig. 21. The Dit valuewas estimated from high frequencyCeV measurements using Terman method that had been per-formed at room temperature [63]:

Dit ¼ �DVg

�Cox=fsqA (9)

where, fs is the surface potential of SiC at a specific gate voltage, Vg.The surface potential of a particular capacitance is taken from anideal MOS capacitor. The gate voltage is then acquired from theexperimental CeV curve of the same capacitance as that of fs. Thesteps are repeated for other data points until a relevant Dit against(Ec � E) curve is attained.

From the Dit curves, sample oxidized/nitrided in 10% N2O givesthe lowest Dit. The value of Dit obtained in this work wasw1013 eV�1 cm�2 at (Ec � E) ¼ 0.15e0.25 eV. This was relativelylower than the Dit obtained for other samples with higherconcentration of N2O [mid 1013e1014 eV�1 cm�2 at(Ec � E) ¼ 0.15e0.25 eV] [20].

To determine the total interface-trap density (Dtotal) of theenergy range (Ec � E), area under the Dit � (Ec � E) plot wascalculated (inset of Fig. 21). The trend of Dtotal as a function of N2Oconcentration is similar to the changes of Qeff and STD. The lowesttotal interface-trap density obtained in this experiment isw3.6 � 1012 cm�2, which is obtained from the film oxidized/nitrided in 10% N2O.

3.8. JeE characteristics

Fig. 22 elucidates the leakage current densityeelectric field(JeE) characteristics of the investigated samples. The JeE plot wastransformed from currentevoltage (IeV) measurement obtainedfrom the computer-controlled SPA system. The E value wasapproximated by [63]:

E ¼ Vg � VFB=tox (10)

In higher N2O concentrations, i.e. 70 and 100%, all characterizedMOS capacitors has revealed a one-step oxide breakdown (EHDB) asbeing shown in the JeE plot, while a two-step oxide breakdown (EB

Fig. 21. Interface-trap density of the investigated samples oxidized/nitrided indifferent N2O concentrations (10e100%). The inset shows total interface-trap density ofthe investigated samples as a function of N2O concentration (10e100%).

and EHDB) is being recorded for all characterized MOS capacitorsoxidized/nitrided in further diluted N2O (10 and 30%). When bothlayers Zr-oxynitride and IL are simultaneous broken down,thus one-step oxide breakdown is recorded (EHDB). If either one(Zr-oxynitride or IL) is pre-mature breakdown, then a two-stepbreaking is being observed [64]. The breakdowns can beexplained as follows. One of the layers may experience an electricalbreakdown at a lower field, which is labeled as EB. Subsequently,another layer would block the carriers. Due to increment of electricfield, concentration of carrier increases until the layer is electricallybroken down at a higher electric field at EHDB. The instantaneousincrement of leakage current density at EB is relatively small whencompared with others and it is defined as soft breakdown. On thecontrary, the instantaneous increment of current density at EHDB islarge and this is considered as hard or permanent breakdown.

Of the four investigated samples, sample produced by 10% N2Orecorded the highest EHDB, followed by 30% N2O sample, 100% N2Osample, and the lowest EHDB is demonstrated by the sampleproduced by 70% N2O. Their recorded values are 7.59, 5.72, 5.05,and 4.28 MV cm�1, respectively. This variation can be related to thethickness of bulk oxynitride, interfacial layer, and/or the combinedoxides, as observed in Figs. 9 and 11 by EFTEM analysis. As the N2Oconcentration reduces from 100 to 10%, the interfacial layer growsthicker. This may help to enhance the electrical breakdown field ofthe sample by introducing a two-step oxide breakdown phenom-enon. However, in 70% N2O sample, the thickest bulk oxynitridelayer (21.44 nm) was formed as compared to other samples(16.12e16.90 nm), which may deteriorate the electrical breakdownfield of the sample. Besides, the change of EHDB may be ascribed tothe surface roughness which is governed by the orientation andalignment of the tetragonal crystal structure that is embedded inamorphous phase in the samples (Figs. 17 and 18). In 100% N2O,the smooth surface with well aligned tetragonal crystal may causean easy path for leakage current. When N2O is diluted to 70%,combination of crystal forming a larger grain gives higher surfaceroughness may lead to a lower breakdown field. Although furtherdilution of N2O (30 and 10%) gives rougher surface, however, smallgrain with fine structure on the surface may help to enhance theelectrical breakdown field. Comparatively, 10% N2O sample hasa smaller grain with finer structure on the surface yields a higherbreakdown field than 30% N2O sample. Moreover, the change inEHDB is in the reverse trend of effective dielectric constant (inset ofFig. 19), as a function of N2O concentration. With higher dielectricconstant, a lower EHDB is produced and vice versa. Furthermore, thehighest EHDB in the 10% N2O sample can also be related to the

Fig. 24. FN tunneling linear regression plot [ln(J/E2) � 1/E] of investigated Zr-oxynitride film oxidized/nitrided in various N2O concentrations (10e100%). The insetshows the barrier height values as a function of N2O concentration (10e100%).

Y.H. Wong, K.Y. Cheong / Materials Chemistry and Physics 136 (2012) 624e637636

attainment of the lowest negative Qeff, STD, Dit, and Dtotal. Incomparison, breakdown field recorded in this work is higher thanprevious reported work [20,56,65,66].

Resultant from JeE measurements, time-zero dielectric break-down (TZDB) reliability tests had been performed [67]. A total of 25capacitors were tested and cumulative failure percentage of thebreakdown is presented (Fig. 23). The trend of the reliability plot issimilar to the trend of EHDB (Fig. 22) with sample oxidized/nitridedin 10% N2O gives the highest reliability.

Barrier height (fB) of conduction band edge between SiC andinterfacial layer of the oxide was extracted from FowlereNordheim(FN) tunneling model. FN tunneling is referred to the flow ofelectron through a triangular potential barrier into conductionband of an insulator [63]. The J ascribed to FN tunneling (JFN) can bedefined as below [63]:

JFN ¼ AE2expð�B=EÞ (11)

where,

A ¼�q3=8phfB

�.ðm=moxÞ (12)

and

B ¼h8p

�2moxf

3B

�1=2i.3qh (13)

where, h is Planck’s constant (4.135 � 10�15 eV s), mox is effectiveelectron mass in the oxidized layer, and m is free electron mass. Byreplacing all constants into Eqs (12) and (13), A and B can berewritten as,

A ¼ 1:54� 10�6ðm=moxfBÞ (14)

and

B ¼ 6:83� 107�moxf

3B=m

�1=2(15)

By rearranging Eq (11), it yields

ln�JFN=E

2�

¼ lnðAÞ � B=E (16)

A linear FN plot of ln (JFN/E2) versus 1/E is presented in Fig. 24.The intercept of the plot yields A and gradient of the slope yields B.In order to calculate fB, electron effective mass of high k oxide isassumed to be 0.3m [68,69]. The calculated fB as a function of N2Oconcentration is presented in the inset of Fig. 24. The extracted fB

Fig. 23. Cumulative failure percentage of dielectric breakdown field (EHDB) of theinvestigated samples.

values are ranging from 1.24 to 1.96 eV. The highest value wasattained by 10% N2O sample (1.96 eV). The extracted fB values inthis work are having the same trend as the conduction-band offsets(DEc) extracted from XPS as a function of N2O concentration, withthe highest value achieved by 10% N2O sample (2.30 eV).

4. Conclusions

In this work, structural, chemical, and electrical properties ofsputtered Zr that had been oxidized/nitrided in various N2Oambient (10e100%) to form Zr-oxynitride thin film on 4HeSiCsubstrate were presented. Based on XPS and high resolutionEFTEM results, the oxidized/nitrided layers consisted of a stackedZr-oxynitride layer on an interfacial layer with compounds relatedto ZreO, ZreN, ZreOeN, SieN, and/or CeN were found. A modelrelated to the oxidation and nitridation mechanism has been sug-gested. A polycrystalline with tetragonal structure of ZrO2 had beenidentified by EFTEM, XRD, and Raman analyses, confirming that theremaining compounds are in amorphous structure. Surfaceroughness and topographies of the samples were evaluated by AFMand explained using a proposed crystal structure. Electrical prop-erties of the films were characterized byMOS test structures. It wasshown that 10% N2O sample demonstrated the most encouragingelectrical result, by giving the highest electrical breakdown field of7.59 MV cm�1 at 10�6 A cm�2, with the highest reliability. This isascribed to the several factors, which include confinement ofnitrogen-related compounds of ZreOeN and/or ZreN at or nearinterfacial layer region, smaller grain with finer structure on thesurface, reduction in interface trap density, total interface trapdensity, and effective oxide charge, and increment of barrier heightbetween conduction band edge of the film and the semiconductor.

Acknowledgment

The authors would like to acknowledge USM fellowship, USM-RU-PRGS (8044059), and The Academy Sciences for the Devel-oping World (TWAS) through TWAS-COMSTECH Research Grant(09-105 RG/ENG/AS_C) for the financial support.

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