The Role of the Substrate on Photophysical Propertiesof Highly Ordered 15R-SiC Thin Films

SATYENDRA MOURYA,1,2 JYOTI JAISWAL,1 GAURAV MALIK,1

BRIJESH KUMAR,2 and RAMESH CHANDRA1,3

1.—Thin film Laboratory, Institute Instrumentation Centre, Indian Institute of TechnologyRoorkee, Roorkee 247667, India. 2.—Department of Electronics and Communication Engineering,Indian Institute of Technology Roorkee, Roorkee 247667, India. 3.—e-mail: ramesfic@iitr.ac.in

We report on the structural optimization and photophysical properties ofin situ RF-sputtered single crystalline 15R-SiC thin films deposited on varioussubstrates (ZrO2, MgO, SiC, and Si). The role of the substrates on the struc-tural, electronic, and photodynamic behavior of the grown films have beendemonstrated using x-ray diffraction, photoluminescence (PL) and time-re-solved photoluminescence spectroscopy. The appropriate bonding order andthe presence of native oxide on the surface of the grown samples are confirmedby x-ray photoelectron spectroscopy measurement. A deep-blue PL emissionhas been observed corresponding to the Si-centered defects occurring in thenative oxide. Deconvolution of the PL spectra manifested two decay mecha-nisms corresponding to the radiative recombination. The PL intensity andcarrier lifetime were found to be substrate- dependent which may be ascribedto the variation in the trap-density of the films grown on different substrates.

Key words: Silicon carbide, sputtering, XPS, photoluminescence, time-resolved photoluminescence

INTRODUCTION

Nowadays, the majority of modern photoemission-based devices rely on the photodynamic behavior ofnanostructured semiconductors.1 Thus, under-standing and identifying the luminescence centersin semiconducting thin film systems are of highimportance and also challenging. To date, manyresearch groups in the field of optoelectronics havefocused their attention on wide bandgap semicon-ductors.2–5 Among them, silicon carbide (SiC) hasoutstanding material properties, particularly poly-typism, high thermal conductivity, excellent hard-ness, superplasticity, tunable wide bandgap, highcritical breakdown field, good chemical inertnessand biocompatibility at low cost.6–8 These distinc-tive properties of SiC make it a potential candidatefor technological advancement in wide potential

areas, like high-frequency resonators, bioimagingprobes, microwave absorbers, high-temperature gassensors, supercapacitors, and electron devices.6,9–13

In particular, the optoelectronic applications of SiC-based devices in harsh environments has encour-aged us to study and understand the luminescencephenomenon in SiC thin film systems. Additionally,the tendency of surface passivation (SiOx) of SiC atroom temperature (RT) makes this material uniquefor the development of high-performance optoelec-tronic devices.14

Earlier reports suggested that the study of thephotodynamic behavior of SiC in both bulk as wellas thin films required more attention to explore itsphotoluminescence (PL) properties.15–17 Photoe-mission from various SiC-based nanostructures likenanowires, liquid nanocrystals, nanopowders, andthin films have been investigated by many researchgroups,18–23 with special emphasis on 3C-, 4H-, and6H-polytypes. However, the growth of these poly-types requires high substrate temperatures (1000–

(Received February 22, 2018; accepted May 25, 2018)

Journal of ELECTRONIC MATERIALS

https://doi.org/10.1007/s11664-018-6411-6© 2018 The Minerals, Metals & Materials Society

1400°C), which cause a large residual stress, micro-pin-holes, and cracks in the samples, resulting inpoor-quality films.24 These problems are maleficentfor device applications and, to surmount this, therhombohedral phase of the SiC may be a potentialcandidate. We have recently reported the growth of15R-SiC films at relatively low temperature (800°C)with tunable wide bandgaps (2.8–3.1 eV).25 Up tonow, we have found almost no literature in whichthe deep-blue PL emission and decay process ofin situ RF-sputtered 15R-SiC thin films depositedon different substrates are exhaustively discussed,apart from a few exceptions.26 Since the surfaceenergy of the substrate plays an anchor role incontrolling the nucleation and growth thermody-namics of thin film fabrication process, the deposi-tion of 15R-SiC polytype on a variety of(semiconducting and oxide) substrates and tunabil-ity in its luminescence and structural propertiesmake this study interesting and novel.

The available literature has suggested that thefabrication of crystalline SiC films is extremelychallenging because the high temperature requiredto initiate the nucleation process.27–29 In general,chemical vapor deposition is employed to growcrystalline SiC films but this route incorporatesdifferent impurities.30 Another approach is pulsedlaser deposition, which yields highly stoichiometricand ordered films localized in a relatively smallarea. However, photoemission-based microelec-tronic devices require the growth of crystalline anduniform film on a large area for optimum light–matter interaction. To overcome this, RF magnetronsputtering has been pursued as an efficient way togrow large-area high-quality 15R-SiC thin filmsunder optimum growth conditions, because thismethod requires a relatively low substrate temper-ature and results in a lower concentration of latticedefects and a large uniform deposition area at lowcost.25,31

In the present work, we have investigated theradiative centers and associated decay mechanismsof in situ RF-sputtered highly ordered 15R-SiC thinfilm systems. Proper knowledge of these radiativecenters provides an opportunity to understand thePL emissions from 15R-SiC thin films.

EXPERIMENTAL

Highly oriented 15R-SiC thin films were de-posited on various substrates (zirconia: ZrO2, mag-nesium oxide: MgO, silicon carbide: SiC, and silicon:Si) at 800°C by RF magnetron sputtering. A com-mercially procured circular disc-shaped (5 cm diam.,5 mm thick) SiC target of 99.99% purity was used togrow the samples (a schematic of the process is de-picted in Fig. 1). The distance between the substrateand the target was kept to 5 cm. To start with,15 min in an ultrasonic acetone bath and the RadioCorporation of America (RCA) procedure wereadopted to remove impurities from the surface of all

the substrates. A base pressure of 2.66910−7 kPawas maintained in the sputtering chamber (38.1 cmdiam., in-house molybdenum heater) using a rotarypump cascaded with a turbo-molecular pump. Westarted heating the substrate-mounted molybde-num heater and, when the temperature reached800°C, the inert argon (Ar) gas was blown into thesputtering chamber at a flow rate of 20 sccm con-trolled by a mass flow controller. Pre-sputtering oftarget was carried out for 15 min to remove surfacecontaminations. After the deposition, the chamberwas kept in vacuum until the temperature reachedRT. The optimized sputtering parameters are listedin Table I. The 15R-SiC thin films deposited ondifferent substrates, ZrO2, MgO, SiC, and Si,are called hence-forward SiC@ZrO2, SiC@MgO,SiC@SiC, and SiC@Si, respectively.

The structural properties of the films wereexamined by a grazing angle x-ray diffractometer(GAXRD; Bruker, D8 Advance with CuKα (40 kV,40 mA) radiation, λCuKα=1.5406 A, step=0.02°, scanspeed=1 step/s) at RT. To examine the elementalcomposition and electronic structure of the 15R-SiCthin films, x-ray photoelectron spectroscopy (XPS)measurements were performed on a PHI Versaprobe III electron spectrometer with the followingrecipe (Al Kα 1486.708 keV, step size=0.05 eV, en-ergy resolution=0.5 eV, pass energy=55 eV, and abase pressure of 10−8 kPa). An Ar-ion gun was usedto clean and neutralize the specimen surface prior torecording the XPS characteristics. To identify andstudy the radiative recombination centers and PLbehavior of the grown samples, steady-state PLmeasurement was performed on a HITACHI F-4600FL spectrophotometer at RT. A monochromaticradiation of 360 nm from a Xe-lamp was shone onthe samples through an excitation slit of 5 nmwidth. We had placed the samples at an angle of 45°with respect to the incident radiation. The resultingPL signals were collected by a photomultiplier tube(PMT; Hamamatsu H5783) having a monochroma-tor with a slit width of 5 nm. The carrier lifetimeand decay mechanism were investigated by a Hor-iba Jobin-Yvon Florocube time-correlated singlephoton counting system. A light source of 200 kHzwas used to illuminate the samples. The light wasrouted through two f/3.4 monochromator gratings(having adjustable, 0–0.8 cm, bilateral slits; SPEX1681) to spectrally resolve the PL signals. There-after, a PMT operated in single photon countingmode was used for the detection of monochromaticPL signals. A constant fraction discriminator fol-lowed by a time-to-amplitude converter was utilizedto determine the arrival time of a detected photonwith a time resolution of 100 ps, and the resultswere stored in a multichannel analyzer.

RESULTS AND DISCUSSION

Figure 2a shows the x-ray diffraction (XRD)diffractograms of the 15R-SiC thin films deposited

Mourya, Jaiswal, Malik, Kumar, and Chandra

on the Si substrate at 800°C under two differentoperating modes (detector scan with and withoutphi-scan) of the XRD machine. Firstly, an XRDdiffractogram was recorded in detector scan config-uration without considering phi-scan, which re-vealed a rhombohedral structure having a preferredpeak at 2θ�35.37° corresponding to (1 0 10) orien-tation (Ref. JCPDS-ICDD no. 00-002-1042). Anotherpeak appearing at 2θ�56.45° was assigned to the(100) orientation of the Si substrate. Secondly, anXRD diffractogram was recorded in the same con-figuration but followed by a phi-scan at 2θ�35.37°,resulting in single peak centered at 2θ�35.37°(Fig. 2a), which indicates the elimination of the Sisubstrate peak. The preferred orientation (1 0 10) ofthe grown film is ascribed to its minimum surfaceenergy and highest thermodynamic stability.32 Thepractice of eliminating the substrate peak has beentaken into account to record all the further XRDdiffractograms discussed here. From Fig. 2b, it canbe concluded that the sputtering power plays animportant role in improving the crystallinity ofthe15R-SiC thin films. The optimum crystallinity

was found at a deposition power of 150 W. At thisparticular power, the sputtered atoms in the plasmahave enough kinetic energy to grow in the preferredorientation by positioning themselves in thermody-namically favored minimum energy lattice sitesfollowed by an optimum inter-atomic scattering at

Fig. 1. Schematic diagram of synthesis and characterization for 15R-SiC thin films on four different substrates.

Table I. The optimized sputtering parameters usedfor the deposition of the 15R-SiC thin films

Sputtering parameters

Target SiCSubstrates ZrO2, MgO, SiC and SiBase pressure 2.66910−7 kPaWorking pressure 6.66910−4 kPaDeposition rate �15 nm/minDeposition power 150 WDeposition time 120 minSubstrate temperature 800°CTarget–substrate distance 5 cmGas used Ar (20 sccm)

The Role of the Substrate on Photophysical Properties of Highly Ordered 15R-SiC Thin Films

the substrate surface.33,34 Figure 2c depicts thetemperature optimization XRD diffractograms of15R-SiC thin films. The 15R-SiC thin films haveshown an interesting crystallographic dependenceon the deposition temperature, i.e. below 700°C, thefilms were found to be amorphous, turned crys-talline at 700°C and achieved maximum crys-tallinity at 800°C, and further, a decrease incrystallinity was observed with increasing substratetemperature (up to 1100°C). This typical depositiontemperature-dependent growth of the 15R-SiC maybe attributed to the competition between surfacediffusivity and inter-atomic scattering of the sput-tered SiC atoms.35 Initially, an increase in thedeposition temperature (up to 800°C) enhanced thesurface diffusivity of the SiC atoms, which favoredthe growth of the preferred crystallographic orien-tation (1 0 10). Further, the increase in temperatureallowed inter-atomic scattering to dominate due tothe large increment in the kinetic energy of thesputtered atoms, resulting in poor crystallinity.

Figure 2d shows the XRD diffractograms of the15R-SiC thin films grown on four different sub-strates for an optimized power and temperature of150 W and 800°C. All four samples had character-istic peaks appearing at 2θ�35.37°, correspondingto the (1 0 10) plane of the rhombohedral phase ofSiC (Ref. JCPDS-ICDD no. 00-002-1042). Further, a

significant change in the full width at half maxi-mum (FWHM, listed in Table II) was observed withthe change in the substrates, which clearly evi-denced that the film grown on the Si substrate hasthe highest crystallinity. The average crystallitesize (t) of all the representative samples was calcu-lated using Scherrer’s formula,36,37 which is listedin Table II. The highest and smallest t was found forthe samples deposited on the Si and MgO sub-strates, respectively. This may be attributed to thesubstrate-dependent growth of the 15R-SiC thinfilms, as different substrates have different inter-facial energy (i.e. surface energy), which is one ofthe most important parameters to drive the processof nucleation and growth of thin films. Noticeably,an optimum surface energy results in minimumGibb’s free energy, leading to the maximum numberof nucleation sites. To validate this hypothesis, adetailed discussion on the surface energy calcula-tion of all the bare substrates, i.e., ZrO2, MgO, SiC,and Si, using the water contact angle (θw) mea-surement was reported in our recent publication.25

In addition, we calculated the dislocation density,δ, for all the samples using Williamson–Smallmanrelationship38:

d ¼ 1

t2(1)

Fig. 2. XRD diffractograms of the 15R-SiC thin films deposited on the Si substrate with and without phi-scan (a), at different powers (100–200 W)(b), and at different temperatures (600–1100°C) (c). (d) XRD diffractograms of the 15R-SiC deposited on various substrates for an optimizedpower of 150 W and temperature of 800°C.

Mourya, Jaiswal, Malik, Kumar, and Chandra

where t is the average crystallite size. The calcu-lated δ values are listed in Table II and have beenfound to be substrate-dependent. The highest andlowest value of the dislocation density was observedfor the sample grown on MgO and Si substrates,respectively.

Figure 3a shows the XPS survey scan of thesample deposited on the Si substrate indicatingthree characteristic peaks of Si2p (99.4 eV), C1s(284.6 eV), and O1s (532 eV). Figure 3b, c, and dshows the core-level high-resolution (HR)-XPSspectra of all the potential elements present in the15R-SiC thin films. The deconvolution of the HR-XPS spectra was carried out by fitting the Gaussiancurves corresponding to each core element to iden-tify and assign the low- and the high-coordinatedbonds. The core peak of Si2p was deconvoluted inthree significant peaks and assigned as low- andhigh-coordinated Si-C bonds (100.1 eV and101.0 eV) and O-Si-C bonds (101.7 eV), which en-dorsed the formation of Si-C bonds along with theexistence of native oxide. The C1 s peak was fittedinto two prominent peaks at 282.8 eV and 284.2 eV,corresponding to high-coordinated C-Si and C-Cbonds, respectively, which also confirmed the pres-ence of Si-C core bonds in abundance.22 However,the presence of the C-C component may be attrib-uted to the default carbon reference in the XPS fit-ting procedure and adventitious carboncontamination. The narrow scan of O1s was decon-voluted and well matched with the characteristicbinding energy of O-Si-C bonds (531.1 eV) and high-coordinated Si-O bonds (532.4 eV), which againhomologated the presence of Si-centered nativeoxide. In addition, the absence of the Si2p peak at99.2 eV ruled out the termination of Si-face dan-gling bonds into the Si-H surface, which was alsocorroborated by XRD results, as hydrogen incorpo-ration leads to the amorphous nature in the SiCfilms.39 Moreover, the presence of native oxide wasascribed to the oxidation of the Si-face, which plays

a decisive role in the luminescence properties of the15R-SiC thin films., because the Si-centered highconcentration of defects near the SiC–SiO2 inter-faces act as a radiative recombination center andsubstantially contribute to the PL emissions.

Further, we carried out the XPS mapping of theSiC@Si to examine the elemental distribution in thefilm. Prior to recording the elemental mapping, ascanning x-ray image was taken which facilitated usin choosing different geometrical shapes on thesample (point, line, area) for elemental analysis. Asquare (area of 5009500 μm2) shape geometry waschosen to examine the elemental distribution in thefilm. Moreover, Fig. 4a, b, c, and d displays thevarious colors corresponding to the Si (green), C(red), O (blue), and SiC (mixed color), which indi-cates the homogeneous distribution of all the ele-ments.

In addition to the XPS measurement, the Fourier-transform infrared spectroscopy and variable anglespectroscopic ellipsometry, reported in our recentpublication,25 also evidenced the surface passivationof the RF-sputtered 15R-SiC thin films.

Figure 5a depicts the RT PL emission spectra ofthe15R-SiC thin films deposited on four differentsubstrates for an excitation wavelength of 360 nm,which exhibit broad-triplet emissions. The line-shape of the PL spectra was observed to be nearlyidentical for all the samples, with a slight relativeshift in the peak positions. This may be attributed tothe substrate-dependent structural and topologicalproperties of the grown films. Further, a substrate-dependent quenching in the PL intensity was found,which may be ascribed to the difference in the dis-location density, δ, of the films deposited on differ-ent substrates. The highest PL intensity wasobserved for SiC@Si due to its smallest δ value, as δis the measure of defect density, which governs thepopulation of various radiative and non-radiativerecombination centers for photoexcited carriers, andmanifests a significant influence on the PL emission

Table II. Average carrier lifetime and structural properties of the 15R-SiC thin films grown on differentsubstrates

Sample t (nm) FWHM (°) δ (1011 line/cm2) PL emission wavelength (nm) τ (ns)

SiC@ZrO2 15.9 0.535 3.95 414 11.58436 6.40471 4.10

SiC@MgO 10.9 0.774 8.41 421 10.78441 5.15469 3.78

SiC@SiC 16.4 0.518 3.71 419 13.18440 6.48466 4.54

SiC@Si 17.7 0.482 3.19 415 14.04436 6.52463 4.56

The Role of the Substrate on Photophysical Properties of Highly Ordered 15R-SiC Thin Films

and decay process. A large value of δ indicates thehigh concentration of defects, which yields a hugenumber of non-radiative recombination centers ascompared to radiative recombination centers forexcited carriers in the decay path. These non-ra-diative recombination centers do not contribute inthe photoemission, while dissipating the absorbed

energy in the form of lattice vibrations, resulting indecreased PL intensity.

It is also evident from Fig. 5a that the PL emis-sion spectra have an intense peak centered on436 nm flanked by two broad shoulders occurring inthe spectral ranges, 414–421 nm and 463–470 nm,which also suggested that different excitonic tran-sitions are strongly overlapped. Therefore, for abetter understanding of the PL mechanism, wedeconvoluted the PL spectra of SiC@Si on a set ofindividual components using the Gaussian functionin terms of the energy of possible radiative transi-tions (shallow and deep traps, surface states and Si/O-related defects) reported in the literature.23,26,40

Figure 5b shows the deconvoluted PL spectra hav-ing 5 sub-peaks, named 1, 2, 3, 4 and 5. The firstthree deconvoluted PL emission peaks (1, 2, and 3)at 2.594 eV (478 nm), 2.655 eV (467 nm), and2.725 eV (455 nm), respectively, were highly red-shifted with respect to the optical bandgap of the15R-SiC thin film,25,41 which confirmed no charac-teristic band-to-band transitions.23 Ma et al. and J.Chen et al. attributed such band-tail emissionbehavior to the radiative recombination of light-ex-cited electron–hole pairs in some localizedstates.42,43 In addition, the inclusion of other poly-types (3C, 2H, 4H, 6H, and 8H) of SiC in the sam-ples may also host these low-energy emissions.However, Raman measurement ruled out such apossibility because no Raman band corresponding tothese additional phases of SiC was observed, as re-ported in our previous work.25 In our consensus, the

Fig. 3. XPS spectra of the 15R-SiC thin film grown on Si substrate: (a) XPS survey scan, (b) HR-XPS of Si2p, (c) C1s, and (d) O1s.

Fig. 4. XPS elemental mapping of the 15R-SiC thin film deposited onthe Si substrate (Color figure online).

Mourya, Jaiswal, Malik, Kumar, and Chandra

origin of these three low-energy PL bands (1, 2, and3) may be ascribed to the electron–hole pairrecombination in some localized defect states(shallow traps).23,40 On the other hand, the pho-toemission corresponding to the peaks 4 and 5 at2.987 eV (415 nm), and 2.844 eV (436 nm), respec-tively, (Fig. 5b) was found to be prominent in all thesamples and to have a strong substrate-dependence.These two PL emission peaks lie in the vicinity ofthe optical bandgap of 15R-SiC thin films and oc-cupy the deep-blue region of the electromagneticspectrum. We found in the literature that the originof these deep-blue PL emissions of SiC-basednanostructures is subject to different opin-ions.2,20,23,26,27,44 In this article, we have discussedseveral possible models to accomplish deep-blueemission from the 15R-SiC thin films, i.e, (1) quan-tum confinement effect, (2) inclusion of multiplepolytypes and (3) Si- or O-centered defects in thenative oxide cladding. Here, the deep-blue emissiondoes not correspond to the quantum confinementeffect, as (1) average crystallite size (11–13 nm,Table II) was observed to be larger than the Bohrradius of the SiC,17,22 and (2) the emission peakswere found to be red-shifted with respect to theoptical bandgap of the 15R-SiC thin films.25 Thus,we can disown the quantum confinement effect asan explanation of the deep-blue PL emission. Inaddition, we have already ruled out the inclusion ofmultiple polytypes as an explanation of the deep-blue PL emission on the basis of Raman measure-

ments (reported in our previous work25). Finally, webelieve that the deep-blue emission peaks at 415 nmand 436 nm may be due to the presence of Si-cen-tered defects in the native oxide cladding present onthe 15R-SiC thin films, which is strongly supportedby similar deep-blue PL emissions from Si-basednanostructures in the range of 415–440 nm.20,23,45

However, similar emission energies are also ex-pected from O-related vacancies present in nativeoxide, but this possibility was ruled out by the TRPLmeasurement (discussed in the next paragraph), asthe lifetime associated with the O-centered defectslies in the range of 10−3–10−6 s due to the fact thatthe process involves a triplet–singlet (T1–S0) tran-sition.20 Hence, we can assert that the deep-blueemission peaks are proliferated from the Si-cen-tered defects in the native oxide cladding or at theinterface of SiC/SiO2,

46,47 which is well corroboratedby the XPS results. Further, the PL emission spec-tra displayed in Fig. 5c under different excitationwavelengths (250 nm, 300 nm, and 360 nm) re-vealed that all the peaks appeared at identicalpositions with decreased intensity as the excitationwavelength was increased, which again suggestedthat the defects are the main reason for the visiblefluorescence.

On the other hand, the genesis of the deep-blueemissions at 415 nm and 436 nm were also exam-ined by the lifetime measurement of the associatedcarrier using the TRPL technique. It can be clearlyseen from Fig. 5b that the PL band corresponding to

Fig. 5. Photodynamic behavior of 15R-SiC thin films. (a) Steady-state PL emission spectra of all samples. (b) Deconvoluted PL emission spectraof the sample grown on Si substrate. (c) Steady-state PL emission spectra of the sample grown on the Si substrate at different excitationwavelengths. (d) Fitted TRPL decay curve of all the samples; the inset shows a magnified view of the fitted TRPL curve (Color figure online).

The Role of the Substrate on Photophysical Properties of Highly Ordered 15R-SiC Thin Films

peak 5 at 415 nm (2.987 eV) is very close to theoptical bandgap of 15R-SiC thin films. So, one mayeasily become perplexed regarding the origin of thispeak, i.e., whether it is caused by Si-centered de-fects or originated due to the characteristic band toband recombination of excitons. In order to resolvethis dilemma and obtain a deeper insight into thePL mechanisms, the PL dynamics at selectedwavelengths was measured using TRPL. We ana-lyzed the TRPL signal in terms of a triple expo-nential decay model given by Eq. 2 and the fittedcurve is displayed in Fig. 5d.

I tð Þ ¼ a1 exp � t

s1

� �þ a2 exp � t

s2

� �þ a3 exp � t

s3

� �

(2)

where a1, a2 and a3 are the pre-exponential factorsand τ1, τ2 and τ3 are the lifetimes. The calculatedcarrier lifetimes for all the samples are listed inTable II, which is of the order of 10−9 s and found tobe highest for the sample deposited on the Si sub-strate. This may be attributed to the highest or-dered growth, minimum strain, and lowestdislocation density, indicating inhalation of thesurface states and deeper traps. It can be clearly

seen from Table II that the carrier lifetime changesas a function of the substrate and the extent of δ,which results in the generation of various paths forthe recombination of the charge carriers, leading tothe difference in the PL intensities and TRPL life-times. Additionally, the carrier lifetime of the orderof 10−9 s corresponding to the deep-blue emissiondisowns the possibility of band-to-band excitonicrecombination, because SiC is an indirect bandgapmaterial and a characteristic excitonic recombina-tion would be phonon-assisted to conserve themomentum, causing the carrier lifetime to be in therange of 10−3–10−6 s. Hence, the observed decaymechanism of the deep-blue emission can be wellunderstood according to the bound exciton modelrather than a band-to-band recombination.

Based on our discussion of PL and TRPL spec-troscopy, we have come up with a Jablonski energyband diagram (Fig. 6) depicting the distribution ofthe defect (Si-centred and shallow traps) energy le-vels within the forbidden band and various photonabsorption/emission processes taking place in the15R-SiC thin films. The 15R-SiC film grown on theSi substrate has an indirect energy gap of 3.15 eV.25

Two types of radiative recombination processeswere mainly observed. The first type involves the

Fig. 6. Schematic illustration of the Jablonski energy band diagram of the 15R-SiC thin films showing the different energy levels and defect states(Si-centred and shallow traps) and related light absorption and emission processes.

Mourya, Jaiswal, Malik, Kumar, and Chandra

absorption of light to generate bound excitons be-tween two energy levels of the Si-centred defects,and the observed luminescence occurs in the deep-blue region of the electromagnetic spectrum. Thesecond type involves the electron–hole pair recom-bination in some localized energy states (shallowtraps) present in the film, and the correspondingluminescence occurs in the middle of the visibleregion. Both the observed transitions have a char-acteristic lifetime of the order of a few ns, as cal-culated by TRPL spectroscopy. Simultaneously,some non-radiative emission processes assisted byvibrational relaxation also take place, correspond-ing to the electron transition from the ground statemaximum to the excited state minimum, due toabsorption of the high-energy photon.

CONCLUSIONS

We have come up with an optimized processparameters (sputtering temperature and power) tofabricate highly ordered, single-crystalline 15R-SiCthin films in situ on various substrates (ZrO2, MgO,SiC, and Si). The appropriate bonding order and thepresence of native oxide on the surface of the grownsamples were confirmed by XPS measurements. Therole of the substrates on the structural and photo-dynamic behavior of the 15R-SiC thin films weredemonstrated using XRD, PL and TRPL spec-troscopy. The steady-state PL characteristics re-vealed a substrate-dependent broad-triplet PLemission. The PL emission spectra were observed tobe strongly overlapped with some other sub-bandPL signals and red-shifted with respect to the opti-cal bandgap of the 15R-SiC thin films. The decon-volution of PL spectra for sample SiC@Si resulted ina set of individual components and suggested twodecay mechanisms. The PL components occurring inthe high-wavelength region 500–560 nm were as-cribed to the radiative recombination in shallowtraps. The finding of a deep-blue emission was suc-cessfully discussed with the bound exciton modeland assigned to the Si-centered defects occurring inthe native oxide cladding. No characteristic band-to-band radiative PL emissions were observed, as theSiC is an indirect bandgap material and band-to-band excitonic recombination would cause carrierlifetimes to be in the range of 10−3–10−6 s. However,the TRPL measurement resulted in carrier lifetimesin the range of 10−9 s at all the selected emissionwavelengths corresponding to each sample. Thesefindings open the possibility of designing and fab-ricating SiC-based optical sensors, integrated op-tics, extreme ultraviolet reflectors, phototransistors,photoconductive switches, and photonic and opto-electronic devices for extreme environment appli-cations.

ACKNOWLEDGEMENTS

One of the authors, Satyendra Mourya, would liketo thank the Ministry of Human Resource and

Development (MHRD), Government of India forproviding the financial assistance through GrantNo. MHR02-41-126-429. We would like to acknowl-edge the help of Dr. Bhupender Singh (IIC, IITRoorkee) for discussing the PL and TRPL spec-troscopy results.

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