The Optical Absorption and Photoluminescence Characteristics of Evaporated and IAD Hfo2 Thin Films

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Article The Optical Absorption and Photoluminescence Characteristics of Evaporated and IAD HfO2 Thin Films

Mingdong Kong 1,2, Bincheng Li 1,3,*, Chun Guo 1, Peng Zeng 1, Ming Wei 1 and Wenyan He 1,2

1 Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China; [email protected] (M.K.); [email protected] (C.G.); [email protected] (P.Z.); [email protected] (M.W.); [email protected] (W.H.) 2 Chinese Academy of Sciences, Beijing 100039, China 3 School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China * Correspondence: [email protected]

Received: 4 April 2019; Accepted: 5 May 2019; Published: 8 May 2019

Abstract: HfO2 thin films are extensively applied in optical coatings and microelectronic devices. However, film defects, which are vital to the performance of the thin films, are still under intense investigation. In this work, the absorption, photoluminescence, and crystallization characteristics of HfO2 films prepared by electron-beam evaporation and ion-assisted deposition are investigated in detail. Experimental results showed that high-temperature thermal annealing in air resulted in a reduced absorption coefficient, an increased bandgap width, and an increased degree of crystallization. After thermal annealing, an absorption shoulder near 5.8 eV was caused by excitons in the films, which were independent of oxygen vacancy defects and crystallization. Under 6.4 eV (193 nm) laser excitation, the photoluminescence spectrum showed five emission peaks for HfO2 films both with and without thermal annealing. The emission peak near 4.4 eV was generated by the self-trapped exciton, and the peak near 4.0 eV was related to the OH group in the film. The oxygen vacancy defect-induced absorption of HfO2 films in a broad spectral range significantly increased when HfO2 film was re-annealed in Ar gas after first being annealed in air, while the photoluminescence spectrum showed no significant change, indicating that the emission peaks at 2.3, 2.8, and 3.4 eV were not related to oxygen vacancy defects.

Keywords: HfO2 thin ﬁlms; optical absorption; photoluminescence; electron-beam deposition; ion-assisted deposition (IAD)

1. Introduction

HfO2 is a dielectric material with a wide band-gap, high dielectric constant, high refractive index, and high thermal stability that is widely used in the preparation of optical coatings and microelectronic films [1–3]. For optical thin-film applications, it is widely used in the preparation of optical films with low absorption and high damage threshold for high-power pulsed laser systems [4–8]. For microelectronic applications, HfO2 is the most promising alternative to SiO2 in metal-oxide-semiconductor (MOS) microelectronic devices [9]. Studies have shown that there are various defects in HfO2 films, such as oxygen vacancies and charge traps [9–16]. These defects affect the absorption loss and laser damage threshold of the optical coatings, and the leakage current and carrier mobility of the MOS. The investigation of the defects in HfO2 films is therefore of great significance for the preparation of high-performance laser optical thin films and microelectronic films.

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Defects in HfO2 films are difficult to measure directly. When defects are present, localized states are generated between the valence band and conduction band. When electrons at lower energy levels absorb photon energy, they may jump to higher energy levels, and when electrons at higher energy levels return back to lower energy levels via radiation relaxation, photon emission may occur. Therefore, the optical absorption spectrum and the photoluminescence emission spectrum of HfO2 films can be used to analyze the defect states in the films. There has been some past research studying both the absorption and photoluminescence of HfO2 optical films. Ciapponi et al. found that HfO2 films prepared by electron-beam deposition (EBD) showed significant luminescence, but the luminescence of ion-assisted films could not be distinguished from the uncoated substrate [15]. Papernov et al. studied the optical properties of oxygen vacancies in HfO2 thin films by absorption and luminescence spectroscopy [13]. They only investigated the luminescence for EBD films since no measurable luminescence was detected in the films prepared by ion-beam sputtering (IBS). In this paper, we present the absorption, photoluminescence, and crystallization properties of two kinds of HfO2 films prepared by EBD and ion-assisted deposition (IAD), respectively. The optical absorption and photoluminescence characteristics of the films as deposited and after thermal annealing were investigated in detail in combination with the crystallization of the films, and the causes of the films’ absorption and photoluminescence were analyzed. These results will be very helpful in the preparation of high-performance HfO2 films with low absorption and high damage threshold that are essential for high-power laser applications.

2. Materials and Methods

EBD and IAD are the major techniques for preparing low-absorption HfO2 optical films [16]. Both techniques were employed to prepare the HfO2 film samples in our experiment. The samples were prepared with a Syruspro 1110 coating plant from Leybold Optics, Alzenau, Germany. The raw material for the coating was a high-purity hafnium metal with purity higher than 99.9%, which was oxygenated to form the HfO2 films during the coating process. Two sets of samples were prepared by EBD and IAD, respectively. The starting vacuum pressure for deposition was 3 10 6 mbar. × − The film deposition rate was 0.2 nm/s, and the substrate temperature was 120 ◦C. The O2 flow rate was 45 sccm (standard cubic centimeters per minute). The ion source used for the IAD process was an advanced plasma source (APS, Leybold Optics, Alzenau, Germany) operated with 100 V bias voltage and 50 A beam current. The deposition rate and film thickness were controlled by a quartz crystal. The transmittance and reflectance spectroscopy of the film samples were measured with a Lambda 950 spectrophotometer (Pekin Elmer, Waltham, MA, USA). The photoluminescence (PL) spectra of the film samples were excited by an ArF excimer laser at 193 nm (IndyStar 1000, Coherent, Santa Clara, CA, USA). The laser repetition rate was 100 Hz, and the pulse width was 10.5 ns. The excimer laser beam was approximately normally incident onto the sample surface. PL was collected at a 45◦ detection angle from the incident laser beam with a lens coupling the PL emission into an optical fiber that was connected to the entrance of a high-resolution spectrometer (iHR320, Jobin Yvon, Paris, France, grating grooves: 300–2400 line/mm, spectral resolution: 0.79–0.02 nm) equipped with an intensified charge-coupled device (ICCD). A 193 nm filter was placed in front of the entrance of PL collection optics to avoid interference from the 193 nm laser-excited fluorescence of the optical fiber. The samples were placed inside an adiabatic chamber purged with high-purity N2 gas. The measurements were performed at room temperature and in an N2 atmosphere to avoid the influence of the oxygen absorption of the 193 nm excitation on the PL measurements. The XRD of the film was performed with a X’Pert3 Powder X-ray diffractometer (XRD, PANalytical, Almelo, The Netherlands) with a 2θ scan range of 10◦–70◦ and a step of 0.03◦. Thermal annealing was carried out in a tube furnace with a maximum temperature of 400 ◦C. The furnace temperature was first increased linearly from room temperature to the treatment temperature (400 ◦C) in one hour, then stabilized at 400 ◦C for 1 h, and then decreased back to room temperature (around 25 ◦C). High-purity Ar gas (>99.999%) was introduced into the tube furnace when needed. The substrates used for depositing the HfO2 Coatings 2019, 9, 307 3 of 8

films were fused silica and silicon wafers, respectively, with diameters of 25 mm. The film coated Coatingson fused 2019 silica, 9, x FOR was PEER used REVIEW for transmittance and reflectance measurements in the wavelength range3 of 8 of 190–800 nm and for XRD. Considering that fused silica has a strong photoluminescence under and for XRD. Considering that fused silica has a strong photoluminescence under 193 nm laser 193 nm laser irradiation and silicon substrate does not [17], the HfO2 film coated on silicon was used to irradiation and silicon substrate does not [17], the HfO2 film coated on silicon was used to measure measure the PL spectrum in order to avoid the influence of the substrate emission. the PL spectrum in order to avoid the influence of the substrate emission. 3. Results and Discussion 3. Results and Discussion 3.1. Optical Absorption 3.1. Optical Absorption The EBD and IAD HfO2 film samples presented a significantly lower transmission at wavelengths shorterThe than EBD 250 and nm IAD compared HfO2 tofilm bare samples fused-silica presented substrate, a significantly as presented inlower Figure transmission1a, indicating at wavelengthsthat the HfO 2shorterfilms had than a strong250 nm absorption compared atto thebare short fused-silica wavelength substrate, range, as as presented presented in in Figure Figure 11a,b. indicatingAfter thermal that annealingthe HfO2 films at 400 had◦C a in strong air, the absorption transmittance at the in short the shortwavelength wavelength range, range as presented improved in Figurewith decreasing 1b. After absorption.thermal annealing In the meantime,at 400 °C in the air, refractive the transmittance index and in physical the short thickness wavelength of the range films improvedwere calculated with decreasing via a method absorption. proposed In by the Swanepoel meantime, [ 18the]. refractive The EBD andindex IAD and films physical had refractivethickness ofindices the films of 1.93 were and calculated 2.05 at avia wavelength a method propos of 400ed nm by and Swanepoel physical [18]. thicknesses The EBD ofand 252 IAD and films 227 had nm, refractiverespectively. indices After of 1.93 annealing, and 2.05 the at a refractive wavelength indices of 400 increased nm and physical to 1.98 andthicknesses 2.06, and of 252 the and thicknesses 227 nm, respectively.reduced to 245 After and annealing, 226 nm, respectively. the refractive Unlike indice thes idealincreased crystal, to there1.98 and were 2.06, many and mesoscopic the thicknesses voids reducedin both theto 245 EBD and and 226 IAD nm, HfO respectively.2 films. Thermal Unlike the annealing ideal crystal, reduced there the were void many density mesoscopic and volume voids in inthe both film the samples, EBD and increasing IAD HfO the2 films. refractive Thermal index annealing and decreasing reduced the the void physical density thickness and volume of the in films. the filmThe annealing-inducedsamples, increasing changes the refractive in the refractive index and index decreasing and thickness the physical of the IADthickness film were of the smaller films. thanThe annealing-inducedthat of the EBD film. changes On the otherin the hand, refractive the weak index absorption and thickness of the of HfO the2 filmsIAD film at a wavelengthwere smaller greater than thatthan of 250 the nm EBD was film. calculated On the other by the hand, method the weak proposed absorption by Swanepoel of the HfO [182]. films The absorptionat a wavelength coeffi greatercient α thanof a strong250 nm absorption was calculated at a wavelength by the method shorter propos thaned 250 by nm Swanepoel was calculated [18]. The by theabsorption formula coefficient [12]: α of a strong absorption at a wavelength shorter than 250 nm was calculated by the formula [12]: T = (1 R) exp( αd) (1) 𝑇=(1−𝑅− ) exp(−−α𝑑) (1) where T and R are the transmittance and reflecta reflectancence of of the film, film, respectively, and d is the physical thicknessthickness of the film.film. The The relationship relationship between between the the calculated calculated absorption absorption spectrum spectrum presented presented in in Figure 11bb showsshows thatthat thethe IADIAD filmfilm hadhad aa higherhigher absorptionabsorption thanthan thethe EBDEBD film.film. After annealing in air, thethe absorption absorption of the HfO HfO2 filmfilm at at a a photon photon energy energy level level higher higher than than 5.5 5.5 eV eV (225 (225 nm) nm) was was significantly significantly reduced, which could be related to the reduction ofof oxygenoxygen vacancyvacancy defectsdefects inin thethe film.film.

100 30 IAD As-deposited 25 80 IAD Annealing ) EBD As-deposited -1 20 EBD Annealing cm 60 Sub 4 EBD As-deposited 15 EBD Annealing 40 IAD As-deposited 10

Transmission (%) Transmission IAD Annealing

20 (x10 Absorption 5 (a) (b) 0 0 200 300 400 500 600 700 800 5.0 5.5 6.0 6.5 Wavelength (nm) Energy (eV)

Figure 1. ((aa)) The The transmission transmission of of the the prepared prepared HfO HfO22 filmﬁlm samples samples and and the the substrate; substrate; ( (bb)) The The absorption absorption of the the prepared prepared HfO 2 filmﬁlm samples. samples. EBD: EBD: electron-beam electron-beam deposition; deposition; IAD: IAD: ion-assisted ion-assisted deposition; deposition; Sub: Sub: bare fused-silica substrate.

As HfO2 film is an indirect bandgap material [12,19], the absorption coefficient α related to band edge and the photon energy satisfy the following formula: αh𝑣 = 𝐵h𝑣 −𝐸 (2) where hv is the energy of the incident photon, B is the absorption edge width parameter, and Eg is the band gap. The optical band gap width Eg of the HfO2 film could be determined by the Tauc patterning method, and the results are shown in Figure 2 and Table 1. The bandgap data were consistent with

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As HfO2 ﬁlm is an indirect bandgap material [12,19], the absorption coeﬃcient α related to band edge and the photon energy satisfy the following formula:

2 αhv = B hv Eg (2) − where hv is the energy of the incident photon, B is the absorption edge width parameter, and Eg is the band gap. The optical band gap width E of the HfO film could be determined by the Tauc patterning Coatings 2019, 9, x FOR PEER REVIEW g 2 4 of 8 method, and the results are shown in Figure2 and Table1. The bandgap data were consistent with the reportedreported valuesvalues ofof 5.25–5.85.25–5.8 eV [[14].14]. The smaller bandgap of the IAD filmfilm compared to the EBD filmfilm may be related to the increase of the microsco microscopicpic defects in the film film caused caused by by ion ion bombardment. bombardment. The absorptionabsorption spectrum spectrum of of the the EBD EBD film film after after thermal thermal annealing annealing showed showed a significant a significant shoulder shoulder around 5.8around eV. This5.8 eV. shoulder This shoulder appeared appeared after annealing after annealing in air, indicating in air, indicating that it was that not it was caused not bycaused oxygen by vacancies.oxygen vacancies. Hoppe etHoppe al. showed et al. theshowed same the results same and results thought and it thought was intrinsic it was to intrinsic monoclinic to monoclinic HfO2 [12], andHfO Aarik2 [12], etand al. Aarik indicated et al. itindicated might have it might originated have originated from excitonic from absorption excitonic absorption [19]. [19].

1200 1400 As-deposited As-deposited 1000 Annealing in air 1200 Annealing in air 1/2

1/2 1000 800 eV) eV) -1

-1 800 600 (cm (cm

1/2 600 ) 1/2 ν ν) h 400 h α ( α 400 (

200 200 (a) (b) 0 0 4.5 5.0 5.5 6.0 6.5 4.5 5.0 5.5 6.0 6.5 Energy (eV) Energy (eV) Figure 2. TaucTauc patterning calculated from the absorption spectra of (a)) EBDEBD andand ((b)) IADIAD thinthin ﬁlms.films.

Table 1. The optical bandgap Eg of the HfO2 ﬁlms. Table 1. The optical bandgap Eg of the HfO2 films.

CoatingCoating Method Method As-Deposited As-Deposited (eV) (eV) Annealing Annealing inin AirAir (eV)(eV) Re-Annealing Re-Annealing in in Ar Ar (eV) (eV) EBDEBD 5.565.56 5.66 5.66 5.64 5.64 IADIAD 5.495.49 5.55 5.55 5.54 5.54

3.2. Crystalline Structure

2 A comparison of the crystalline structures ofof thethe HfOHfO2 filmsfilms before and after thermal annealing 2 (as(as shownshown inin FigureFigure3 3)) indicated indicated that that the the EBD EBD film film was was amorphous amorphous and and the the monoclinic-phase monoclinic-phase HfO HfO2 appeared afterafter annealing. annealing. The The as-deposited as-deposited IAD IAD film containedfilm contained a monoclinic a monoclinic phase, and phase, the di ffandraction the peakdiffraction became peak stronger became after stronger annealing. after Accordingannealing. toAccording the Scherrer to the formula, Scherrer the formula, crystal size the Dcrystal(nm) size can beD (nm) evaluated can be as: evaluated as: D𝐷== KKγ/(𝐵/(Bcos cosθθ )) (3)(3) where KK = 0.890.89 is is the the Scherrer Scherrer constant, constant, BB isis the the half-maximum half-maximum width width of of the the diffraction diffraction peak peak (radians), (radians), θ isis thethe didiffractionffraction angle,angle, andand γγ isis thethe X-rayX-ray wavelengthwavelength (0.154056(0.154056 nm).nm). Defects may have contributed to lineline broadening broadening and and thus thus caused caused errors errors when when we used we the used Scherrer the Scherrer formula. formula. For a rough For estimation, a rough weestimation, ignored we the ignored error and the directly error and used directly the Scherrer used the formula Scherrer to calculate formula theto calculate crystallite the sizes crystallite of thin films.sizes of The thin crystallite films. The sizes crystal of thelite EBDsizes andof the IAD EBD HfO and2 filmsIAD HfO after2 films annealing after annealing were 14.90 were and 14.4014.90 and nm, respectively.14.40 nm, respectively. The small The difference small indifference crystallite in sizecrys buttallite the size significant but the disignificantfference indifference the absorption in the shoulderabsorption near shoulder 5.8 eV near after 5.8 annealing eV after annealing for both the for EBDboth andthe EBD IAD and films, IAD as films, well as as thewell fact as thatthe fact no absorptionthat no absorption shoulder shoulder was present was forpresent as-deposited for as-deposited and partially and crystallizedpartially crystallized IAD films, IAD clearly films, indicated clearly thatindicated the absorption that the absorption shoulderwas shoulder not related was not to therelated film to crystallization. the film crystallization.

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66 (-111) (-111) (111) (111) 55 (020) (002)

(011) monoclinic HfO (020) (002) (011) (022) monoclinic HfO2

(022) 2

44 IADIAD annealingannealing

33 IADIAD as-depositedas-deposited

Intensity(a. u.) Intensity(a. 2 Intensity(a. u.) Intensity(a. 2

EBDEBD annealingannealing 11

EBDEBD as-depositedas-deposited 00

1010 20 20 30 30 40 40 50 50 60 60 70 70 2θ (°) 2θ (°)

2 FigureFigure 3. 3. TheThe XRD XRD patterns patterns of of the the prepared prepared HfO HfO22 films.films.ﬁlms.

3.3.3.3. PhotoluminescencePhotoluminescence PLPL is is aa sensitivesensitive andand effectiveeffective effective tooltool tool forfor for thethe the defectdefect defect characterizationcharacterization characterization ofof of films.films. films. InIn In ourour our experiment,experiment, experiment, aa 193a193 193 nmnm nm (6.4(6.4 (6.4 eV)eV) eV) excimerexcimer excimer laserlaser laser waswas was usedused used asas as thethe the exex excitationcitationcitation sourcesource for for PL PL measurements.measurements. TheThe laser laser 2 energyenergy density density was was 2 2 mJ/cm mJmJ/cm/cm22.. TheThe measuredmeasured PLPL spectra spectra of of the the as as-depositedas-deposited-deposited and and thermally thermallythermally annealed annealedannealed EBD and IAD HfO films are presented in Figure4. The PL spectra could be decomposed into five PL EBDEBD andand IADIAD HfOHfO222 filmsfilms areare presentedpresented inin FigureFigure 4.4. TheThe PLPL spectraspectra couldcould bebe decomposeddecomposed intointo fivefive PLPL bands,bands, with with corresponding corresponding peakpeak posiposi positionstionstions ofof approximatelyapproximately approximately 2.3,2.3, 2.3, 2.8,2.8, 2.8, 3.4,3.4, 3.4, 4.0,4.0, 4.0, andand and 4.44.4 4.4 eV,eV, eV, respectively,respectively, respectively, asas shown shown in in the the insets insets of of Figure Figure 4 4.4.. TheTheThe PLPLPL bandsbandbandss maymaymay havehavehave originatedoriginatedoriginated fromfrfromom self-trappedself-trappedself-trapped excitons,excitons,excitons, radicals,radicals, defects,defects, etc.etc. etc. ForFor For bothboth both thethe the EBDEBD EBD andand and IADIAD IAD samplessamples samples inin ingeneral,general, general, annealingannealing annealing causedcaused caused increasesincreases increases inin peakinpeak peak intensities.intensities. intensities. TheThe Thepeakpeak peak intensityintensity intensity increaseincrease increase waswas mumu waschch much moremore significant moresignificant significant forfor thethe for EBDEBD the filmfilm EBD thanthan film forfor than thethe IADforIAD the film,film, IAD whichwhich film, whichwaswas believedbelieved was believed toto bebe torelatedrelated be related toto thethe to annealing-inducedannealing-induced the annealing-induced crystallization.crystallization. crystallization. AsAs thethe As theas-as- depositedas-depositeddeposited IADIAD IAD filmfilm film waswas was alreadyalready already partiallypartially partially crystallized,crystallized, crystallized, thethe the annealing-inducedannealing-induced annealing-induced peakpeak peak intensityintensity intensity changechange waswas relatively relatively small. small.

20002000 20002000 Annealing (a) Annealing (b) Annealing Annealing (a) As-deposited (b) As-deposited As-deposited As-deposited

15001500 15001500

10001000 10001000 PL Intensity (a.u.) PL Intensity (a.u.) PL Intensity (a.u.) PL Intensity PL Intensity (a.u.) PL Intensity

23456 23456 (a.u.) Intensity PL PL Intensity (a.u.) Intensity PL PL Intensity (a.u.)Intensity PL Energy (eV)

PL Intensity (a.u.)Intensity PL 500 500 500 Energy (eV) 500 23456 23456Energy (eV) Energy (eV)

00 00 2345623456 2345623456 EnergyEnergy (eV) (eV) EnergyEnergy (eV) (eV)

FigureFigure 4.4. PhotoluminescencePhotoluminescencePhotoluminescence (PL) (PL)(PL) spectra spectraspectra of of (ofa ) ( the(aa)) the EBDthe EBDEBD HfO HfO2HfOfilms22 filmsfilms and ( andband) the ((bb IAD)) thethe HfO IADIAD2 films, HfOHfO22excited films,films, excitedbyexcited 193 nm byby 193 laser193 nmnm irradiation. laserlaser irradiation.irradiation. The insets TheThe are insetsinsets the aremulti-bandare thethe multi-bandmulti-band fits. fits.fits.

In recent years, PL of HfO film has been intensively investigated [13,15,19–23]. Aarik et al. [19] InIn recentrecent years,years, PLPL ofof HfOHfO22 filmfilm hashas beenbeen intensivelyintensively investigatedinvestigated [13,15,19–23].[13,15,19–23]. AarikAarik etet al.al. [19][19] assignedassigned the the PL PL band band near near 4.4 4.4 eVeV toto self-trappedself-trapped exex excitons,citons,citons, whichwhich causedcaused thethe absorptionabsorption shouldershoulder nearnear 5.85.8 eV. eV. OurOur absorptionabsorption and and PL PL measurements measurements confir confirmedconfirmedmed the the correlation correlation between between the the 4.4 4.4 eV eV PL PL band band andand thethethe absorptionabsorptionabsorption shoulder shouldershoulder near nearnear 5.8 5.85.8 eV. eV.eV. From FromFrom Figures FiguresFigures1 and 411, and thermaland 4,4, thermalthermal annealing annealingannealing induced induced theinduced obvious thethe obviousabsorptionobvious absorptionabsorption shoulder shouldershoulder near 5.8 eVnearnear and 5.85.8 a eVeV significant andand aa signifsignif increaseicanticant inincreaseincrease PL intensity inin PLPL intensity atintensity 4.4 eV foratat 4.44.4 the eVeV EBD forfor film, thethe EBDwhileEBD film,film, the influencewhilewhile thethe of influenceinfluence annealing ofof onannealingannealing the absorption onon thethe absorption shoulderabsorption near shouldershoulder 5.8 and nearnear the 5.85.8 4.4 and eVand PL thethe band 4.44.4 eVeV of thePLPL bandIADband film ofof thethe was IADIAD much filmfilm less waswas significant. muchmuch lessless For significant.significant. the PL band ForFor nearthethe PLPL 4.0 bandband eV, Rastorguev nearnear 4.04.0 eV,eV, et Rastorguev al.Rastorguev [20] correlated etet al.al. [20] it[20] to correlatedthecorrelated OH radical itit toto the inthe the OHOH film. radicalradical Our inin experimental thethe film.film. OurOur results exexperimentalperimental support thisresultsresults assignment. supportsupport thisthis From assignment.assignment. Figures1 and FromFrom4, FiguresthermalFigures 11 annealing andand 4,4, thermalthermal induced annealingannealing a smaller inducedinduced change aa in smallersmaller the refractive changechange indexinin thethe for refractiverefractive the IAD indexindex film thanforfor thethe for IADIAD the EBDfilmfilm film, which indicated a smaller change in H O content for the IAD film than for the EBD film, as well as thanthan forfor thethe EBDEBD film,film, whichwhich indicatedindicated aa smallersmaller2 changechange inin HH22OO contentcontent forfor thethe IADIAD filmfilm thanthan forfor thethe EBDEBD film,film, asas wellwell asas aa smallersmaller changechange inin PLPL intensityintensity nearnear 4.04.0 eV.eV. OnOn thethe otherother hand,hand, thethe assignmentsassignments forfor thethe 2.3–3.42.3–3.4 eVeV PLPL bandsbands ofof HfOHfO22 filmfilm areare stillstill disputable:disputable: KiiskKiisk etet al.al. [23][23] andand AarikAarik etet al.al. [19][19]

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Coatings 2019, 9, x FOR PEER REVIEW 6 of 8 Coatings 2019, 9, x FOR PEER REVIEW 6 of 8 a smaller change in PL intensity near 4.0 eV. On the other hand, the assignments for the 2.3–3.4 eV PL assigned them to oxygen vacancy defects, while Ito [21] studied HfO2 and ZrO2 with different oxygen bandsassigned of HfOthem2 tofilm oxygen are still vacancy disputable: defects, Kiisk while et al.Ito [[21]23] andstudied Aarik HfO et al.2 and [19 ZrO] assigned2 with different them to oxygenoxygen contentsvacancycontents defects,andand concluded concluded while Ito that that [21 these ]these studied PL PL bands bands HfO2 were andwere ZrO independent independent2 with diff erentof of impurities, impurities, oxygencontents oxygen oxygen vacancies, andvacancies, concluded and and interfacethatinterface these defects.defects. PL bands ItIt werewaswas independentsuggessuggestedted thatthat of impurities,thesethese PLPL bandsbands oxygen werewere vacancies, momostst likely andlikely interface duedue toto defects. thethe radiativeradiative It was recombinationsuggestedrecombination that between thesebetween PL localized bandslocalized were states states most at at likelythe the band band due tails. totails. the radiative recombination between localized To investigate the correlation between PL and the oxygen vacancy defects in HfO2 films, the statesTo at investigate the band tails. the correlation between PL and the oxygen vacancy defects in HfO2 films, the sample annealed in air was re-annealed under an Ar gas environment in a tube furnace. The sampleTo investigateannealed thein air correlation was re-annealed between PL under and the an oxygen Ar gas vacancy environment defects inin HfO a tube2 films, furnace. the sample The annealingannealedannealing in processprocess air was waswas re-annealed thethe samesame under as as inin airanair Ar inin ordergasorder environment toto create create onlyonly in a oxygen tubeoxygen furnace. vacancyvacancy The defectsdefects annealing in in thethe process film.film. After re-annealing in Ar gas, the transmittance of the HfO2 film in the range of 190–400 nm was wasAfter the re-annealing same as in airin inAr ordergas, the to create transmittance only oxygen of the vacancy HfO2 defectsfilm in inthe the range film. of After 190–400 re-annealing nm was significantly reduced (Figure 5a). The film absorption (Figure 5b) showed that the oxygen vacancy insignificantly Ar gas, the reduced transmittance (Figure of 5a). the The HfO film2 film absorp in thetion range (Figure of 190–400 5b) showed nm was that significantly the oxygen reducedvacancy defects(Figuredefects 5led leda). to Theto an an film absorption absorption absorption increase increase (Figure over over5b) showedaa wide wide spectral spectral that the range oxygenrange of of vacancy 4.0–6.5 4.0–6.5 eV, defectseV, and and led the the to bandgap bandgap an absorption of of the the filmincreasefilm was was overalso also aslightlyslightly wide spectral reduced reduced range (Table (Table of 4.0–6.51).1). The The eV,changes changes and the of of bandgapthe the 2.3–3.4 2.3–3.4 of eV theeV PL filmPL bands bands was alsowere were slightly very very small, small, reduced as as presented(Tablepresented1). The in in Figure Figure changes 6, 6, ofindicating indicating the 2.3–3.4 that that eV these these PL bands PL PL ba ba werendsnds were verywere small,notnot related related as presented to to oxygen oxygen in Figurevacancy vacancy6, indicatingdefects, defects, in in oppositionthatopposition these PL toto bandsKiiskKiisk wereandand Aarik’s notAarik’s related assumptionsassumptions to oxygen [19, vacancy[19,23].23]. Therefore,Therefore, defects, in theirtheir opposition assignmentsassignments to Kiisk needneed and further Aarik’sfurther investigation.assumptionsinvestigation. [ 19,23]. Therefore, their assignments need further investigation.

100 100 (a) (a) 16 (b) 16 (b) Re-annealing in Ar 80 Re-annealing in Ar 80 Annealing in Air ) Annealing in Air -1 )

-1 12 12 cm

60 Annealing in Air 4 cm

60 Annealing in Air 4 Re-annealing in Ar Re-annealing in Ar 8 40 8 40 Transimission (%) Transimission (%) 4 20 4 20 Absorption (x10 Absorption (x10

0 0 0200 300 400 500 600 3.54.04.55.05.56.06.50 200 300 400 500 600 3.54.04.55.05.56.06.5 Wavelength (nm) Energy (eV) Wavelength (nm) Energy (eV) Figure 5. The (a) transmission and (b) absorption of EBD HfO2 films before and after annealing under Figure 5. The ( a) transmission and (b) absorption ofof EBDEBD HfOHfO22 filmsﬁlms before and after annealing under

ArArAr gas. gas.gas.

2000 2000 Re-annealing in Ar Re-annealing in Ar Annealing in Air Annealing in Air

1500 1500

1000 1000 PL Intensity (a.u.) Intensity PL PL Intensity (a.u.) Intensity PL 500 500

0 0 23456 23456 Energy (eV) Energy (eV) Figure 6. The PL spectra of the EBD HfO2 film before and after re-annealing in Ar gas, excited by 193 Figure 6. TheThe PL PL spectra spectra of of the the EBD EBD HfO HfO2 film2 ﬁlm before before and and after after re-annealing re-annealing in Ar in Argas, gas, excited excited by 193 by

nm193nm laser nmlaser laser irradiation. irradiation. irradiation. 4. Conclusions 4.4. Conclusions Conclusions In summary, absorption and photoluminescence measurements were employed to investigate InIn summary,summary, absorptionabsorption andand photoluminescencephotoluminescence measurementsmeasurements werewere employedemployed toto investigateinvestigate defectsdefects in in EBD EBD and and IAD IAD HfO HfO22 filmsfilms widely widely employed employed in in optical optical coatings coatings and and microelectronic microelectronic devices, devices, defects in EBD and IAD HfO2 films widely employed in optical coatings and microelectronic devices, which were of great significance for understanding the laser damage mechanism of optical films whichwhich were were of of great great significance significance for for understanding understanding the the laser laser damage damage mechanism mechanism of of optical optical films films for for for high-energy laser applications and the performance improvement of MOS. Experimental results high-energyhigh-energy laserlaser applicationsapplications andand thethe performaperformancence improvementimprovement ofof MOS.MOS. ExperimentalExperimental resultsresults showed that high-temperature thermal annealing in air reduced the absorption of HfO2 films while showed that high-temperature thermal annealing in air reduced the absorption of HfO2 films while re-annealingre-annealing in in Ar Ar gas gas significantly significantly increased increased the the ab absorption,sorption, demonstrating demonstrating that that the the absorption absorption loss loss of the film was closely related to the oxygen vacancy defects. The PL spectra of the HfO2 films showed of the film was closely related to the oxygen vacancy defects. The PL spectra of the HfO2 films showed

Coatings 2019, 9, 307 7 of 8

showed that high-temperature thermal annealing in air reduced the absorption of HfO2 films while re-annealing in Ar gas significantly increased the absorption, demonstrating that the absorption loss of the film was closely related to the oxygen vacancy defects. The PL spectra of the HfO2 films showed five bands at approximately 2.3, 2.8, 3.4, 4.0, and 4.4 eV. The experimental results confirmed the assignments of 4.4 and 4.0 eV PL bands to self-trapped excitons and the OH radical, respectively, but refuted the assignments of 2.3–3.4 eV PL bands to oxygen vacancy defects. Further research on correlation between the photoluminescence and localized states of the film is still needed.

Author Contributions: Conceptualization: M.K. and B.L.; Methodology: M.K. and B.L.; Validation: C.G. and P.Z.; Formal Analysis: M.K. and B.L.; Data Curation: W.H. and M.W.; Original Draft Preparation: M.K.; Review and Editing: B.L. and P.Z. Funding: This research was funded by the National Natural Science Foundation of China, grant number 61805247. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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