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Eur. Phys. J. Appl. Phys.
Volume 89, Number 3, March 2020
Article Number 30301
Number of page(s) 11
Section Thin Films
DOI https://doi.org/10.1051/epjap/2020190280
Published online 12 May 2020

© EDP Sciences, 2020

1 Introduction

When the material dimensions are shrinked to nanometer range, the material properties are amazingly altered because of the size of the crystallite and high surface to volume ratio. The optical, electronic and electrical properties are dictated by the surface electronic state of nano materials. This was controlled by high surface to volume ratio. For the development of novel optical, electronic and sensor devices, the fabrication of nanomaterials in the controlled manner are utmost important [1]. New generation development technologies such as high resolution displays requires phosphors that exhibit high luminescence properties and with controlled morphology [2]. Generally, phosphors consist of microcrystalline host which is transparent and an activator, i.e. little amount of impurity atoms are added to the host lattice. Thus, the luminescence phenomenon of a phosphor can be divided into two parts. The first one is the processes related to the host and the second one occurs within and around the activator. In many cases, the impurity ions are responsible for the emission, which,when they generate the particular emission, are called activator ions. Usually, a second kind of impurities known as sensitizers are added when the activator ions exhibit very weak absorption. This sensitizer absorb particular energy, then transfer it to the activators. This process involves energy transport through the luminescent material. Transition or rare earth metal doped phosphors is an important class of phosphors apart from the semiconductor phosphors. In the case of doped phosphors, the centres that exhibit luminescence are shown by transition of rare earth metals. When we make a comparison of doped phosphors and semiconductor nanocrystalline phosphors, we can see that doped phosphors are used in technological applications. This kind of phosphors find applications ranging from old traditional fluorescent lighting to X-ray photography and picture tubes of colour TV and so on. This is because the wavelength of the luminescence of the doped metals seldom changes with confinement of size, doped nanocrystals does not require the strict size control that are required in semiconductor nanocrystalline phosphors. For flat panel displays, phosphors directly influence their life time and brightness. So phosphor quality plays an important role and should be excellent. Since the amount of impurities atoms that are added is comparatively small, phosphor matrix absorbs the excitation energy. Thus, the necessity for the structure of crystalline and composition of the matrix are important.

Several researchers have carried out an extensive research on rare earth activated oxide phosphors in order to enhance the luminescence of phosphors [2]. Some of the phosphor matrices havecations with closed shell electronic configuration. Moreover, their spin momenta and total orbital angular momenta are zero. Rare earth oxides such as La2O3, Y2O3 and Lu2O3 are excellent for phosphor matrices because they exhibit the above said properties. For the visible light, these oxides are transparent, low energy levels are absent and also weaker interaction among the activator ions and matrix. Rare earth doped yttrium oxide [Y2O3] is one of the promising candidate [3]. Table 1 shows the properties of La2O3, Y2O3 and Lu2O3.

Y2O3 exhibits many excellent properties. Its thermal conductivity is high, spectral regions is broad, chemical stability, stark-splitting and comparatively low phonon energies. Thus Y2O3 has been reported as a laser-host materials for a long time [4]. It also has wide band gap energy around 5.67 eV, dielectric constant is high, mechanical strength is good, crystallographic phase stability and nature is optically isotropic. Therefore, it is an important semiconducting material. In addition to this, its thermal stability is high with small phonon energy [380 cm−1]. This property is desirable for radiative transitions among energy levels of transition elements [5]. Yttrium oxide [Y2O3] supports the radiation emission and blocks non-radiation relaxation excitation levels and is considered to be one of the excellent choices for the host material. This is because Y2O3 has vibrational energy which is low and refractive index which is large [1.9]. It possess high melting point [2450 °C], optical band transparency [0.20-8 µm] and high phonon frequency [6]. For the last few years, researchers paid their attention in the fabrication and characterisation of Y2O3 films. This is because of their assurance in transistor gates, metal- insulator-semiconductor [MIS] diodes, dynamic random access memory [DRAM] gate dielectric and metal–oxide- semiconductor [MOS] capacitors [7,8]. Y2O3 has wide range of applications from conventional refractories to advanced latest technologies of ceramic [9]. One of the latest and significant application of Y2O3 is that it has been used as a dielectric layer in the electroluminescent displays [ELDs] and also act as excellent host matrix for transition elements in cathode-luminescent displays (CL) and fluorescent lamps inorder to emit light [7,9].

The growth of phosphors that are nanocrystalline has appeared fundamental because of the reducing size of present display machineries, thus strengthen the interest from the research community [10]. During the past years, many scientists have put their effort to the development of luminescent films by make use of quality oxides. Their main purpose was to find remedies for the technological problems that arises in the flat panel display industry [11]. Luminescent films play a significant role in high resolution devices like electroluminescent devices [ELDs], cathode ray tube [CRTs], that can be used in helmet mounted displays, medical imaging, graphics, thin film electro luminescence (TFEL) panels, gas sensors, remote thermometry, thermos mechanic devices especially field emission displays [FEDs]and Plasma display panels [PDPs] [1214]. In field emission displays electrons are generated by an assemblage of emission tips. They are accelerated towards the screen coated with phosphor that brings the process of cathodoluminescence [CL] [15].

Comparing thin film phosphors and powders, thin film are found to be more advantages than powders. Thin film phosphors show better adhesion to solid surface, reduced outgassing, better thermal stability and higher lateral resolution from smaller grains [913,15]. However, the brightness and efficiencies are more for bulk powder materials than the thin film phosphors. This hindrance in the case of thin film phosphors are mainly connected with features like (i) substrate material absorbs the generated light (ii) the interaction volume among solid and incident beam is small and (iii) the light that emitted is channelled along transverse axis through internal reflection in a way parallel to the surface in place of vertical emission from the surface [15]. Therefore, it is quite important that in addition to the structural stability at the time of fabrication, the phosphor films that supports the substrate possess better optical properties like low absorption of generated light and high transmittance [15]. Preference goes to thin films because they offer the advantage of scattering of light, capability for developing smaller pixel size, enhancement of resolution and reduction of material waste [13]. For display applications, transparent thin film phosphors are utmost essential. The display performance can be enhanced by make use of quality based phosphors that provide long term stability and enough brightness. A detailed study has been done on the enhancement of luminescence phenomena of rare earth doped oxide phosphors. Here arises the question of why oxides over sulphides? Oxides seems to be stable in high vacuum. Moreover, under the bombardment of electrons, there seems no emission of gas which is corrosive in nature [2,12]. The conventional cathode ray tube red phosphor is an oxy-sulphide. It has got an efficiency of around 13%. Thus oxide based phosphors came out as one of the prospective option for display applications [7,10,13]. Among the phosphors that are oxides, colour emitting phosphors became much more important.

Doping of inorganic materials with rare earth is suitable for several applications that involve light emission. Oxides of rare earth elements activated with Cerium, Thuliumand Terbium and thin film emits the most promising yellow, blue and green phosphors. Such thin film phosphors manifest strong UV and cathode ray excited luminescence. These are essential in display applications and lamp [2,10,13,16]. These kind of materials exhibit excellent applications in lasers and optical amplifiers [10]. Several Lanthanide elements have proved their role in photonic and optoelectronic applications., that range from emitting elements in solid state lasers and in phosphors for displays and colour lamps. Lanthanides have inner [4f] shell which is partially filled. This inner shell is shielded by outer [5s and 5p] orbitals which are completely filled. Because of these shielding, the transitions of intra 4f shell results in optical emissions which are very sharp at a wavelength ranging from ultra violet to infrared [2,10,13,16]. But the transitions of 5D-4f shell results in optical emissions which are broad. Europium has a peculiar property that it exhibits both types of emissions on the basis of their valencies [5D-4f transition for Eu2+ and 4f-4f transition for Eu3+].

Forsooth, Europium doped yttrium oxide has trackdown decades ago. But this is the excellent red phosphor until now. This has been studied in detail because its efficiency is very high around 97%, chemical and thermal stability is high, colour purity [3,9,12], minimized deterioration under applied voltages, precarious components are absent as contradict to sulphide phosphors [8,17], luminescence property is excellent and atmospheric stability is acceptable [2]. In the case of phosphors, which was activated by rare earth elements, overall luminescence can be caused by the effect ofrare earth emission centres and host-lattice emission centres. Europium doped nanocrystalline yttrium oxide was proposed in 1964. This has been used for solid state laser applications because of sharp emission at λ-611 nm of Europium ion activator [11,14,18]. Figure 1 shows the evolution of Y2O3:Eu citation per year in the past decade and Figure 2 shows comparative graph of different rare earth doped Y2O3 phosphors.It has been reported that an extensive study has been done in the case of energy transfer and luminescence properties of Eu3+ doped Y2O3 when compared to other rare earth doped Y2O3.

The luminescence phenomenon has been modified when Eu3+ was incorporated into Y2O3 lattice. This is because the number of emission centres which were capable to generate red light has been formed. It is quite familiar that the luminescence that originated between 4f transitions levels is mainly because of magnetic dipole or electric dipole interactions. When the rare earth ions are incorporated into host lattice, ff transitions of electric dipole becomes partially allowed. This was due to the odd crystal field component. The site symmetry of the host lattice plays an important role in the intensity of electric dipoletransitions. On the otherway around, magnetic dipole transitions are not much influenced by the symmetry of the site, because they are parity allowed. When the rare earth ions occupy sites that exhibit the characteristics of inversion symmetry, transitions within 4fn configuration are forbidden as electric dipole transition. They follow the selection rule ΔJ = 0, +−1 and can exist as magnetic dipole transition. When the rare earth occupy the size where the inversion symmetry is absent, the electric dipole transitions are not forbidden and the selection rule ΔJ = +−2 are hypersensitive. Eu3+ ions have emissionlines from 5D0 level to 7FJ [J = 1,2,3……] of the 4f6 configuration. The emission is very weak in the range of 590–600 nm and strong around 610–630 nm. The former is because of magnetic dipole transition from 5D0 to 7F1 and the latter is because of transition from 5D0 to 7F2 which is electric dipole and is hypersensitiveand is shown in Figure 3.

Nowadays, more and more methods have been developed to fabricate Y2O3:RE nanoparticles. The properties and versatility of the thinfilms can be obtained by selecting proper technique of film deposition. Thin film deposition methods can be broadly classified as either chemical or physical methods. The difference between the chemical and physical thinfilm deposition methods depends upon the method of depositing thin film material on the substrate. In chemical deposition technique, fluid precursor is used which chemically react with the substrate. Since the thin film material is conducted through the fluid precursor, chemical deposition is conformal approaching the substrate without preference to a particular direction. A conformal is an uneven interface with the body and has a constant thickness on horizontal and vertical surfaces. Chemical deposition technique includes Chemical Vapour Deposition (CVD), Plasma Enhanced CVD (PECVD), Atomic Layer Deposition (ALD), Sol-Gel Method, Metal Organic CVD (MOCVD) etc. In physical deposition technique, mechanical or electromechanical methods are used to deposit the thin films on the substrate. Materials to be deposited on the substrate depend upon the temperature, pressure and other physical conditions. In these physical methods, the thin films formed are directional on nature because particles will follow a straight path from the target to the substrate. Physical deposition technique includes Molecular Beam Epitaxy (MBE), Sputtering, Pulsed Laser Deposition (PLD), Cathodic Arc Deposition,Thermal Evaporation Method, Electrohydrodynamic Deposition etc.

Hirata et al., in 1997 came up with a new method namely MOCVD along with pulsed laser deposition technique. Low-pressure MOCVD is acceptable to produce high quality films that exhibit luminescence at a low substrate temperatures [500–700 °C] and high growth rates [1 µm/h]. This technique propound better control over the impurity doping level. Post-deposition annealing treatments in the temperature range 900–1200 °C results in an enhancement of luminescent intensity of the films. The nature of as-prepared laser obtained films was amorphous. When this films are annealed at higher temperature especially above 800 °C, the film exhibit luminescence, which occur in conjunction with crystallisation. The thickness of as prepared film was 500 nm. When it undergo post-annealing at 1000 °C, the film were composed of 15–200 nm grains. When the annealing temperature of the MOCVD film increases, its sharpness and intensity of main emission peak increases that emits pure red colour. This was assigned to the dissemination of activator centres through the film randomly and to enhance crystallization. In short, as synthesized, laser ablated Eu doped Y2O3 films were amorphous and moreover it is not luminescecnt. The luminescence was recovered after post-deposition heat treatment that induces crystallization of the films. In short, MOCVD produced poly crystalline films that have particle size ranging from 0.5 to 1.0 µm while through pulsed laser ablation technique, they obtained films which are smooth and nano-size grains [11]. Table 2 shows the detailed information regarding different thin film elaboration technique and their advantages. It is not possible to predict one method is better than the other. As there are different experimental techniques employed so far for the production of Y2O3:Eu thin films, it is very important to decide which experimental method is best for a specific technological application.

There are various factors on which photoluminescence depends. One of the factors that affect the luminescence of Eu3+ ion in yttrium oxide lattice is the substrate temperature. Mobility of atoms within the bulk and on the surface was determined by the substrate temperature. The mobility of atom increases with increasing substrate temperature. This creates thin films which transform from amorphous to polycrystalline state,then ultimately to a single crystal epitaxial state [19].

Grain size increases with increase in temperature. There occurs direct dependence of grain size on the material strength. In general, materials having smaller grain size exhibit stronger material. For nano phosphors, increase in grain size results in the reduction of surface area which in turn increases the phenomenon of luminescence emission. The reduction of the surface defects and non-radiative rate are the reason behind this emission [13]. When we make a comparison of luminescence spectra of Y2O3:Eu3+ phosphors in as synthesized condition and annealed condition, we infer that the peak intensity of as synthesized sample is very weak. And moreover, there is an increase in the emission intensity when the temperature increases [20]. One of the reason may be the dopant components are well −dispersed inside the host material. Another reason may be the increment in the growth crystallinity [21]. Crystallite growth increases due to high energy obtained from the high annealing temperature [22]. Gu et al. has reported by Aerosol pyrolysis technique that the performance of photoluminescence can be enhance effectively by oxygen vacancies. Same way, Yang et al., in 2009 by make use of sol–gel method has reported the red emission enhancement with the help of oxygen vacancies. In the case of luminescence, these oxygen vacancies usually act as radiative centres [22]. It is also important to note that in the case of as synthesized samples, the magnetic dipole transitions are not perfectly determined. However, when temperature increases, the peaks are clearly seen. The PL yield is very low in the case of as prepared samples. This is because in the case of as synthesized samples,surface to volume ratio of the particles will be high with the speculation of quenching process on the surface. On heat treatment, no change for the line width and position of the emitted wavelength occur.

Film roughness are found to influence the PL response of the films [7]. Nissamudeen et al., in 2009 through Pulsed Laser Deposition Technique [PLD] has reported that when temperature increases, roughness increases. This was assigned to the enhancement of grain size. As a function of annealing temperature, the average roughness and luminescence intensity exhibit similar behaviours. They came up with the result that when the temperature increases from 303 to 1073 K, roughness of the surface also increases which in turn increases the luminescence intensity. The rougher surfaces causes reduction in internal reflections. Thus there occurs an enhancement in the performance of luminescence. The light produced in the film is reflected and transmitted at film substrate and film-air-interfaces. The occurance of scattering at the film air-interface consist of two parts:a diffused scattering part and specular components. There occurs strong dependence of the diffuse scattering part on the roughness of the film surface [13].

In order to study the annealing effect on luminescence properties of the samples, Hrudey et al., in 2016 has fabricated Y2O3:Eu films by a technique called Glancing Angle DepositionTechnique [GLAD]. They kept portions of the film annealed at 850° for 10 hours and also portions without annealing to study the difference. It is evident from the spectra that photoluminescence is more for annealed samples. Because of changes occurs in the composition of film and also crystallinity i.e. the oxygen vacancies that occur in the film are almost filled. They reported an increment in the emission of the film by a factor of 3.9 due to annealing process.

Quite interestingly, Gue et al., came up with reason for the enhancement of luminescence intensity on annealing temperature by adopting sol–gel method. They explained that this enhancement was due to multi-phonon non radiative process because of phonon that have high energy rather than crystallite growth. They observed that the peaks are broadened when annealing temperature decreases in Eu3+ doped films. They suggested, broadening may be due to particle size effect. The rare earth ions closed to surface of the particle experience various ligand environment and various crystal fields. Energy levels of these ions and subsequent transitions are somewhat disparate. These may cause in homogeneous broadening of spectra. Reduction of annealing temperature causes reduction in particle size. The reduction of particle size results in increment of surface to volume ratio and defects. This may be the beginning of emission peak broadening. In common, the increment in emission intensity considers the taking away of non radiative processes. Such non-radiative quenching is most probable due to multi-phonon non-radiative process. The Y-O bonds does not cause multi-phonon non-radiative processes. Because Y-O bonds possesses low vibration energy. But CO and OH groups possess high vibration energy and they remain in films. The variation of spectra of Y2O3 films on heat treatment can be revealed systematically from their study of fluorescence. Upon heat treatment, the absorption intensities of CO and OH groups become weaker and ultimately vanishes when annealing temperature reaches 900 °C. Thus, without any doubt, we can say that multiphonon non-radiative processes results in the enhancement of fluorescence intensities with annealing temperatures. The temperature effect on photoluminescence was again confirmed by Nissamudden et al., in 2009 [2]. They showed that grain size increases with enhancement in substrate temperature. When the temperature attains 873 K, agglomeration of nanograins occurs to form larger grains. Quite interestingly, it can be observed that maximum luminescence intensity is observed for this film. Figure 4 shows the scheme of the grain size evolution with the temperature.

Jones et al. [7] in 1997 studied that roughness of the Eu:Y2O3 films has a strong dependence on the luminescence properties of the films. One of the factor on which roughness depends is the oxygen pressure. The films that have different roughness were obtained by making changes in the oxygen pressure during the growth of film. In 1997, Jones et al., has made a comparison on films grown in higher oxygen pressure and lower oxygen pressures. They came to the conclusion that films grown in higher pressure exhibit rougher surfaces than in lower oxygen pressure [7]. For films grown at 600 mTorr oxygen pressure, the route mean square (RMS) roughness of the film, was found to vary from 2 (10−5 Torr) to 71 nm. This indicates an increase in RMS values with an increase in the growth pressure. This was assigned to enhanced particulate formation in the laser induced plume. This is one of the classic feature of high pressure laser ablation. The oxygen pressure increment develops the roughness increment of surface. This in turn results in the PL increment of the films. The enhancement in theluminescence is mainly because of the reduction in internal reflections which was stimulated by rougher surfaces [7]. Nissamudeen et al., in 2009 has also confirmed the dependence of luminescence intensity on the surface roughness and grain size [2].

The role of oxygen that have an impact on the luminescence characteristics of oxide phosphors are not been well studied. In the case of thin film oxide phosphors, the role of oxygen becomes important because of nature of film growth. The film growth is generally recognized when the temperature is high andoxygen pressure is low. In the normal case, when the temperature is high, oxygen − in − diffusion is poorer than oxygen-out-diffusion. As a result, oxygenation is incomplete for the oxide thin films. Thus, inorder to get complete oxygenated films with better properties, oxide films are usually cooled in oxygen pressures which is high. Samples cooled in reduced oxygen pressure exhibit better PL properties were reported by Kumr et al., in 2000 [17]. They made the film grown in 300 mTorr oxygen ambient; one set of films was cooled to room temperature in 2 Torr oxygen at the rate of 10 °C/min. the other set of samples were cooled in vacuum [10−6 Torr] at the rate of 10 °C/min. The third set of films were prepared and cooled in vacuum. The first two films were grown at a temperature of 750 °C. The thickness was same for both the films and was around 0.14 µm. When they compare the two spectra, they found that qualitatively both the films look identical. But CL intensity for films cooled in vacuum is 30% more as compared to films that was cooled in 2 Torr oxygen. These results indicates that post deposition treatment plays an important role in deciding the luminescent property of thin film. Thus a set of Eu doped Y2O3 films were deposited at various temperatures. Then they were cooled in 2 Torr oxygen and in vacuum. Obviously, they showed that PL properties are larger for samples cooled in vacuum than for films cooled in 2 Torr oxygen. Inspired by the result that vacuum cooling gives high quality films, Eu doped Y2O3 films were deposited and also cooled in vacuum. When they compare these films with oxygen cooled films, these films exhibit lower luminescence brightness indicating that Eu doped Y2O3 films deposited in 300 mTorr oxygen and then cooled in vacuum are the supreme among the three class of samples. In short, they confirmed that post deposition oxygen treatment of films plays an inevitable role in the determination of luminescence brightness.

A comparative studies has been made by Jianhua Hao et al., in 2001 [3] regarding Eu doped Y2O3 films annealed in air and in an atmosphere of oxygen. There occurs an increment in the peak intensity and reduction in FWHM when the temperature is increased in an oxygen atmosphere. Moreover, the primary peaks that was annealed in an atmosphere of oxygen seems to be much more sharper and intense than the films annealed in an atmosphere of air at the same temperature. Better crystalline quality is obtained for films annealed in oxygen atmosphere than in air [3].

Hrudey et al., in 2016 [14] has studied how the variation in the background oxygen pressure affect the luminescence properties during the deposition of the film. They did their experiments by GLAD technique and due to some drawbacks they could not compare their results with others especially with PLD deposition films.

Pulsed laser deposition (PLD) is one of the technique by which we can produce thin film. The PLD technique is an authentic method for the production of oxide thin films. However, appreciative results do not obtained under all experimental conditions. In order to obtain thin films with favourable properties, optimization of various deposition parameters should be done with each type of material [19]. This technique offers a unique method for the evaporation of target materials and also morphology control. Nissamudeen et al. [10,13] has used this technique for the growth of yttriumoxide films. Previously, this method has been used for the growth of phosphor like YAG, ZnO and ZnS. In fact, this technique has not been reported for the growth of Y2O3:Eu films, except by Hirata et al. [7]. Qu et al. described that down conversion process of films synthesized by PLD technique can be modulated by changing the deposition specification such as substrate temperature and oxygen pressure.

In the case of annealed and unannealed samples, the lowest luminescence was obtained in the case of films deposited at oxygen pressure of 1.0 × 10−4 Torr. For the films deposited at oxygen pressure of 4.0 × 10−5 Torr and 2.0 × 10−4 Torr, the luminescence response was similar. The truth is that oxygen pressure between 1.0 × 10−4 Torr and 2.0 × 10−4 Torr given minimum emission level. This is found to be in good relation with experiments done by other groups by using the same technique of pulsed laser deposition. This team has done the experiment by using the GLAD technique. They revealed that films with oxygen pressure greater than 100 mTorr had good luminescence properties than films with oxygen pressure below 10 mTorr. Regrettably, pressures greater than 1 mTorr are not possible with e-beam evaporation. Thus film deposition with electron beam at these pressures and comparison with PLD deposition films are beyond the bounds of possibility.

The difference in the luminescence property of Eudoped Y2O3 films with variousdopant concentrations deposited at various substrate temperatures [under an oxygen partial pressure of 0.002 mbar) and at various oxygen partial pressure [at a substrate temperature of 873 K] was studied by [2] Nissamudeen et al., in 2009. The maximum luminescence was obtained for Li co-doped Y2O3:Eu3+ films with substrate temperature of 873 K which was deposit in different oxygen partial pressure. The maximum peak intensity is observed when the oxygen partial pressure attains 0.002 mbar and after that peak intensity decreases with increase in oxygen partial pressure. When the oxygen partial pressure increases, there will be an increase in the ablated species collision that result in the reduction of kinetic energy and mobility on the surface of the substrates which results in weak crystallanity. It is quite clear that films with good crystallinity can be obtained when the films are deposited at low oxygen partial pressure. This is due to change in concentration of oxygen vacancy and subsequent increase in the atom mobility. With respect to influence of pressure, when pressure decreases, primarily confinement effect hegemonize and film morphology contained nanograins with well-defined boundaries. When oxygen partial pressure further decreases, confinement effect decreases and nanocrystals agglomerate. The photo emission increases with decrease of oxygen partial pressure.The mechanism of oxygen pressure that affect the luminescence properties are summarized in Figure 5.

The luminescence phenomenon has been modified when Eu3+ was incorporated into Y2O3 lattice. This is because the number of emission centres which were capable to generate red light has been formed. P.C.P. Hrudey et al., in 2003 has deposited Europium- doped yttrium oxide films by e-beam evaporation at both normal incidence and glancing incidence. They made the films deposited at two different concentration. One at 4 wt% doping of Europium and the other at 5.6 wt% doping of Europium. Films grown with the higher Europium concentration shows higher luminescence properties than those from the lower Europium concentration. The films with 5.6 wt% of Eu doped Y2O3 exhibit luminescence which was 3-4 times larger than that of the films with 4.0 wt% Eu concentration.

When the amount of dopant changes, the PL intensity changes accordingly but spectral position remain unchanged. Initially, when the dopant concentration increases, the luminescence intensity also increases. Nevertheless, areverse trend has been observed when the dopant concentration exceeds the optimized concentration. For Zeng et al., maximum luminescence was obtained when 4% Eu3+ were doped into Y2O3. Sameway for Pang et al., maximum luminescence was obtained when 6% Eu3+ were doped into Y2O3. Thus it has revealed that dopant concentration is not the only factor that depends on luminescence. Other factors like host material, synthesis process, temperature and particle size are also responsible for the luminescence properties. Because the luminescence characteristics is associated with the exact position of the energy level, cross section, phonon energy of the host material and dielectric constant [23]. Usually there will be an increase in luminescence intensity upto optimized dopant concentration, where the luminescence intensity attain the maximum value. The appearance of more number of luminescence centres that are capable to emit red colour is the reason for higher luminescence intensity. When the dopant ion increases, more number of ions that are excited are transited to the corresponding emitting level [23]. When we increase the dopant concentration beyond the optimized concentration value,the intensity decreases [24]. One of the reason for the reduction in PL intensity may be the leaching out of dopant at higher concentration. Since the concentration quenching occurs between activator ions, the excess incorporation of Europium ion into the host lattice may cause the reduction in luminescence intensity [25]. The quenching process takes place at certain concentrations where the average distance between the emission centres, that contributes to the energy transfer is reduced [26]. An output-limiting effect may be caused by interacting Eu3+ when its concentration is too high. Studies reveals that concentration quenching that occurs beyond the optimized concentration may due to transfer of energy between nearby centres that exhibit luminescence through cross-relaxation [23].The concentration quenching of luminescence at 4 wt% of Eu3+ is shown in Figure 6.

In detail, this happens because when the dopant concentration increases, the impurity centres become more closer to one another. If the impurity centres are close to each other, transfer of energy may takes placefrom one excited centre to other centre inspite of being emitted as light. The energy may finally result in light emission, but the transfer increases the occurance of non-radiative transition. If the mismatch in terms of size and valency occurs among the host matrix and impurity, it will be very difficult to incorporate impurity in large amount. Thus the host matrix and luminescent centres are the two important factors that depends on the optimal doping levels.Quenching process takes place when the emitting state loses the energy that are excited through the mechanism of cross-relaxation. This type of relaxation takes place by transfer of resonant energy among two nearby impurity centres.This happens because the impurities centres possess a particular energy-level structure. That means, in the case of two nearby impurity centres, transfer of resonant energy occurs through cross relaxation mechanism in which one of the centre which act as donar exchange a part of its energy that are excited to the nearby centre which act as acceptor [26]. In the case of luminescence process, the photons that are used for excitation must possess energy less than the bandgap of the host matrix. Thus there won't occur the production of electro-hole pairs. Excitation of luminescent centre occurs by make use of a wavelength that lie in the absorption band which will relax non-radiatively after a short while to the 5D0 level then relax radiatively to the ground state by emitting photons that corresponds to particular transitions localised within the Eu3+ itself [26].

Maximum luminescence brightness can be obtained by changing the morphology and increasing the optical volume of thin film phosphors. S.L. Jones et al., [7] has reported film thickness of 1 µm. Moreover, the size of the average powder particle was 5 µm, ranging from 1-10 µm in size. Films with thickness 3 µm yield a luminescence brightness of 22% when compared to the powder. For Eu doped yttrium films, the maximum brightness, obtained is 10% of the brightness of Eu doped Y2O3 powder that was used for the synthesis of target. In 1998, K.G. Cho et al., [15] has studied the luminescence brightness of Eu doped Y2O3 films annealed at 700 and 800 °C as a function of thickness. When they compare the two temperatures, it is clear that PL brightness is higher for films annealed at 800 °C than films grown at 700 °C. The increment in luminescence with increment in film thickness is assumed to obtain from increased volume for photon-solid interaction whereas grain size enhancement and improvement in crystallinity are responsible for higher luminescence of Eu doped Y2O3 films grown at higher temperature. This was again confirmed by Sang Sub Kim et al., in 2003 [8]. In order to synthesize Eu doped Y2O3 films they have chosen the parameters like oxygen pressure of 50 mTorr and substrate temperature of 650 °C. By keeping all the parameters identical except the deposition time, they have prepared the films with four different thickness. The synthesized thickness were 90, 200, 480 and 750 nm. They came to the conclusion that PL intensity increases with increase in film thickness. In 2016, P.C.P. Hrudey [14], et al., has made a comparison on the luminescence properties exhibited by slanted post films and flat films. Flat films exhibit more brightness than the other one. This may be because of the fact that the reference films experience larger interaction volume between the film and the light that is excited than the slanted post films.

Jun Yeol Cho et al., [27] have focused the ways to improve the internal quantum efficiency of thinfilm phosphors. In order to increase the roughness of the interface between the phosphors and the quartz glass substrates, this team has produced Eu doped Y2O3 thin film phosphors on a grooved substrates. Thus they attained an improvement in out- coupling efficiency or extraction efficiency by a factor of 4.7. This enhancement is because of the diffraction scattering which was produced by the 20 periodic nano-array. In fact, it is clear that internal quantum efficiency is an indispensable component in the external efficiency than the extraction efficiency. They prepared 10 layer phosphor films of thickness 620 nm. By increasing the number of coatings, thicker films can be produced. When they increased the number of coatings from 10–30, they obtained the film having 1.7 µm thickness. In all the cases, the films are uniform, smooth and crack free. But films with 30 coating layers or more than that were seemed to reveal macro-sized cracks. The reason may be the difference between the thermal expansion coefficients of the Eu doped Y2O3 films and the sapphire substrate. The luminescence intensity is directly dependent on the number of coating layer and thickness of film. Thus they came to the conclusion that enhancement in the emission volume of Eu doped Y2O3 film phosphors is responsible for the enhancement in the red emission in the case of thicker films. In order to reduce the macro size cracks of thicker films phosphor, efforts such as inclusion of buffer layer and use of crack releasing agent has been chosen.

Phosphors, when they are used in display screens, they need to be patterned. One of the factors that depends on the resolution of flat panel displaying devices is the phosphor screen pattering technology. Thus, screening process has to be done to get high resolution. M.L. Pang et al., in 2003 [12] make use of these patterning method for phosphor screen that include screen printing and electrophoretic deposition etc. This was purely based on photolithography that requires most difficult and expensive equipments. However, researchers paid their attention on non-photolithographic patterning techniques which was collectively called soft-lithography. This technique is more versatile and not much expensive. This can be used for the production of micro meter and sub micrometer size particles. Both patterned and non- patterned films exhibit similar luminescence properties. Thus the entire characterization regarding luminescence was performed on the films which are non-patterned because of their comparatively easy availability. It was shown that when the temperature increases from 500–900 °C, the life time of Eu3+ also increases. The fact behind this is that when the temperature increases, the impurity such as NO3−, CH2− and −OH decreases thereby crystallinity of the film increases. The luminescence quenching of Eu3+ caused by changes in these impurities decreases. This results in PL enhancement and their life time [12].

The substrate material plays an important role in obtaining high quantity Eu dopd Y2O3 red phosphor thin films. The luminescence brightness of Eu doped Y2O3 films deposited on sapphire substrate obtained by K.G. Cho et al., in 1998 [15] is much more higher than reported by S.L. Jones et al., deposited on silicon substrates. The response of annealed films deposited on sapphire substrate and silicon substrate was confirmed by P.C.P. Hrudey et al., in 2016 [14]. As per their report, films deposited on sapphire substrate showed luminescence 1.5 times more than that of the silicon film. This results seems to be in good agreement with the experimental work done by K.G. Cho et al., [15] and S.L. Jones et al. The same result has been supported by P.C.P. Hrudey et al., in 2016 [14]. The enhanced brightness of Eu doped Y2O3 films deposited on sapphire might be due to enhanced grain size.The difference in grain size can be attributed to the different lattice mismatches between the film and the substrate. The reason for the small grain size of films deposited on Si may be attributed to the misalignment of cubic Y2O3: Eu with Si substrate and due to the incorporation of oxygen impurities which form SiO2 interlayers. Moreover, the grain size looks better in case of sapphire substrates. This may be due to the ease of alignment of cubic Y2O3: Eu with sapphire substrates. Film on sapphire substrate exhibit reduced strain and dislocations. This may be attributed to reduced lattice mismatch, less oxygen incorporation and hence less chances of the formation of oxide layers. The low defect density and high crystalline quality of the Y2O3: Eu films strengthen the oscillators and enhance their optical transitions. Moreover, sapphire substrate exhibit more favourable optical properties than silicon. The red light absorption of sapphire substrate is small as compared to silicon. Thus, a major portion of red light which incidence on the surface of silicon will be absorbed. Moreover, because of smaller refractive index of sapphire (1.77) as compared to Eu:Y2O3(1.93), two different critical angles exit for total internal reflection of red light from sapphire/air (34°) interface and Eu:Y2O3 film/sapphire interface (67°).Total internal reflection is not possible on silicon/Eu:Y2O3 interface because the refractive index of Si (3.91) is larger than that of Eu:Y2O3 [12] at a wavelength of 611 nm. These results indicate that sapphire is abest substrate material for the synthesis of high quality Eu doped Y2O3 red phosphor thin films [15]. In 2009, K.M. Nissamudeen et al., [2] has made a comparison on alumina, silicon and quartz. They clearly showed that films grown on alumina substrate shows more brightness than films on other substrates. This enhanced brightness of Eu doped Y2O3 films on alumina substrates are due to increased grain size and more beneficial optical properties of alumina as compared to quartz and silicon. Silicon substrate absorbs much more red light than alumina substrate. Thus the red light that incident on surface of quartz and silicon may be absorbed. When we look into the ratios of PL intensity between different substrates, it will be 2.2 times for alumina versus quartz, 1.56 times in the case of alumina versus silicon and 1.38 times for quartz versus silicon [2].

However, there arises a basic question in the case of thin film phosphors because of their low brightness when compared to powder phosphors. This is because films suffer from internal reflection [14]. This is known as light piping effect. The one way to reduce theinternal reflection is to increase the surface roughness of the thin film [18]. This can be done by tailoring the surface roughness of the film by make use of various methods. This may include changes in processing conditions and modify the thin film phosphor surface [2,13]. The increase in roughness due to annealing is assigned to the increased grain size [13]. In the case of display devices, the image resolution is closely related to the size of the phosphor particle used. Higher resolution is exhibited by particles with small size. The advancement that takes place in the size reduction of electronic devices needs theninvestigation of material properties that have particle size in the nanometer range. Penetration of electrons into the particle that exhibit luminescence in shallow because of low excitation voltage. This means that the luminescence region is restricted near to the surface of particles. Thus this low excitation voltage supports small particles or large surface area. Thus the small sized particles allows the penetration bylow-voltage electrons for better utilisation of material [13]. Moreover, luminescence properties can be enhanced by the addition of appropriate co-dopant in the phosphor matrix. It has been reported that by make use of small quantities, Li+ ions, Gd3+ ions and Bi3+ ions play a significant role in the enhancement of luminescent efficiency of the phosphors [2].

In 2009, Nissamudden et al., has studied the photoluminescence study of Eu doped Y2O3 and Li+ co-activated Y2O3:Eu3+ thin films. They have reported that for both doped and co-doped Y2O3 films, maximum luminescence intensity has been observed when the substrate temperature was at 873 K. When the substrate temperature was above 737 K, the film that are co-doped with Lithium shows higher luminescence. For lithium co-doped films, photoluminescence becomes almost doubles when the substrate temperature was at 873 K. For this PL enhancement, it can be recommended that introduction of Li+ ions produces oxygen vacancies. This act as a sensitizer for the transfer of energy due to mixing of states that causes charge transfer.

In order to understand the enhancement of luminescence in Li+ co-doped Y2O3:Eu3+ films, they thought about ‘size effect’. If the size of the dopant is larger or smaller than the host lattice, it induces a stress into the lattice. This leads to the broadening of spectra lines inhomogeneously. These disturbances are mainly because of size effect. The ionic radii of Li+, Eu3+ and Y3+ ions are 76.0, 94.7 and 90.0 pm respectively. Comparing these ionic radii, it is clear that Eu3+ and Li+ ions do not occupy these various sites in a systematic way. Thus it is assumed that smaller Li ions are accommodated in C2 siteswith much higher population in the Y2O3 lattice. That means when Li+ ions are introduced into Y2O3:Eu3+, Y3+ is replaced by Li+. Thus in turn defective structures are formed. Thus Eu3+ ions are provided with sites that have more reduced symmetry, and able to lift the parity selection rule. This reduced symmetry sites are responsible for the enhancement ofluminescence characteristics. Origin of this luminescence enhancement is complex. The intensity increment can be due to various factors such as grain size, crystallinity/oscillation strength increment, nano-agglomerations and Li substitution that results in the increment of hole concentration. Substitution of Y3+ site by Li+ ions results in substantial oxygen vacancies.

During the sintering process, in order to incorporate Eu2O3 completely into Y2O3, Li plays the role of lubricant as suggested by Lopez et al., Because of the enhancement in grain size, grain boundary density is smaller for Li activated Y2O3:Eu3+ films when compared to Y2O3:Eu3+ films. Additionally, grain boundaries are responsible for the light dissipation that was produced inside the film. This reduces the luminescence brightness. Thus Li activated Y2O3:Eu3+ films with smaller grain boundaries betray better luminescence properties [2]. In their study they maintained a strong dependence of luminescence intensity with that of lattice imperfection. This was done by comparing the lattice parameter deviation with luminescence intensity. This enhancement can be connected to more lattice imperfection. The deviation depends on both lithium co-doping and substrate temperature. Deviation is more sensitive in the former case. When lattice imperfection increases, oxygen vacancy also increases. This results in recombination of deep trapped charges and electron. Thus, the intensity peak increases with oxygen vacancy. The variation of PL intensity with substrate temperature is connected to the oxygen vacancy concentration and disorders in the lattice. Thus, it is safer to use host that provides lattice sites with little symmetries.

Nissamudden et al., in 2009 [2] has reported luminescence enhancement by 2.98 times after co-doping with Lithium. Same thing has been reported by Soung-Soo Yi et al., and Jeong et al., They obtained the enhancement by 2.7 times. Apart from Li+, Gd3+ also enhances the luminescence. Jong Seong Bae et al., came up with the result that Gd3+ activated Y2O3:Eu3+ thin film phosphors exhibit luminescence enhancement by 3.1 times. Here, introduction of Gd into Y2O3 act as a self-promoter to obtain good crystallization. Same team has shown luminescence enhancement by co-doping with Gd ions in Li co-activated Y2O3:Eu3+ thin film phosphors. Here brightness of Gd co-doped Li co-activated Y2O3:Eu3+ film increases by 1.6 times as compared to Li co-activated Y2O3:Eu3+ films.

Bismuth is one of the most promising elements, owing to the possibility to be stabilized in different optically active oxidation states [such as 0,+1,+2,+3,+5] whose optical transitions are allowed by the dipole selection rules as a result of the spin-orbit coupling. All the Bi absorption and emission bands are much broader than those of f-f transitions of RE3+ owing to the unshielded outer electrones.These peculiarities give plenty of opportunities for tuning the spectroscopic characteristics of Bi through a proper selection of the host material and therefore for its application as a REs sensitizer. Bi ions have been already demonstrated to act as efficient sensitizer for visible emitting REs [such as Eu, Sm, Tb] for LEDs and for solar spectrum converters.

Adriana Scarangella et al. in 2016 have proposed in his work, the introduction of Bi as an Er sensitizer in a Si-compatible Yttrium oxide thin film. The stabilization of optically active Bi3+ ions in place of Y3+ in the two reticular sites, S6 and C2 of Y2O3 was demonstrated. Due to energy transfer, a great enhancement of Er optical efficiency in the visible wavelength range was achieved, reaching a value as high as 16 times for the S6 symmetric site and as 80 times for the C2 site. Thus their results proved that the Bi introduction as an Er sensitizer in Y2O3 thin film makes this material a good candidate as active medium in Si optical amplifiers.

Table 1

Properties of Lanthanum oxide, Yttrium oxide and Lutetium oxide.

thumbnail Fig. 1

Evolution of Y2O3:Eu citation per year in the past decade.

thumbnail Fig. 2

Comparative graph of different Rare-earth doped Y2O3 phosphor citation in the past decade.

thumbnail Fig. 3

Emission spectra of Eu doped Y2O3 samples.

Table 2

Different thin film elaboration techniques and their advantages.

thumbnail Fig 4

Schematic representation of grain size evolution with temperature.

thumbnail Fig 5

The mechanism of oxygen pressure that affect the luminescence properties.

thumbnail Fig 6

Concentration quenching of luminescence.

Author contribution statement

K. Vini has collected the data, designed the computational framework and analysed the data. K. Vini and K.M.Nissamudeen carried out the implementation. K. Vini and K.M. Nissamudeen wrote the manuscript. K. Vini, K.M. Nissamudeen and C. Aparna conceived the study and were in charge of overall direction and planning.

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Cite this article as: Kalathil Vini, Cheruvathur adukkathayar Aparna, Kavukuzhi Meerasahib Nissamudeen, A brief review on the techniques used for the enhancement of luminescence of red emitting thin film, Eur. Phys. J. Appl. Phys. 89, 30301 (2020)

All Tables

Table 1

Properties of Lanthanum oxide, Yttrium oxide and Lutetium oxide.

Table 2

Different thin film elaboration techniques and their advantages.

All Figures

thumbnail Fig. 1

Evolution of Y2O3:Eu citation per year in the past decade.

In the text
thumbnail Fig. 2

Comparative graph of different Rare-earth doped Y2O3 phosphor citation in the past decade.

In the text
thumbnail Fig. 3

Emission spectra of Eu doped Y2O3 samples.

In the text
thumbnail Fig 4

Schematic representation of grain size evolution with temperature.

In the text
thumbnail Fig 5

The mechanism of oxygen pressure that affect the luminescence properties.

In the text
thumbnail Fig 6

Concentration quenching of luminescence.

In the text

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