This study examined the effects of the thickness of Y2O3:Eu3+ phosphor films on quartz substrates coated with two-dimensional (2D) SiO2 square-lattice nanorod photonic crystal layers (PCL) at identical heights on their extraction and absorption efficiency. The photoluminescence (PL) efficiency enhancement ratio decreased exponentially with increasing Y2O3:Eu3+ film thickness. The 2D PCL-assisted Y2O3:Eu3+ film with a thickness (t)=400 nm showed enhancement in the upward and downward PL emission by factors of 6.2 and 8.6, respectively, with respect to those of a conventional flat film. This observation was attributed to diffraction scattering of the excitation and emission light.
© 2008 Optical Society of America
Two-dimensional (2D) photonic crystal layers (PCLs) have been applied to various light emitting devices incorporating thin-film luminescent materials in an attempt to enhance their extraction efficiency [1, 2]. Recently, we fabricated Y2O3:Eu3+ thin-film phosphors (TFPs) on 2D SiO2 nanorod PCLs to enhance the roughness of the interface between the quartz substrates and phosphors. This approach can dramatically enhance the extraction efficiency, which is limited by the total internal reflection to approximately 7.2% in Y2O3:Eu3+ film slabs (refractive index n=1.86 and the fraction of upward escaping light is approx. 1/4n2≈7.2%), [3, 4]. Meanwhile, Ganesh et al. reported significant enhancement in fluorescence from quantum dots on the surface of a 2D PC by engineering the PC to possess leaky eigenmodes at the absorption and emission wavelengths of the quantum dots (QDs) . In agreement with the results for PC light emitting diodes [6, 7] and PC organic light emitting diodes [8, 9], this improvement in extraction and excitation efficiency was attributed to the leaky and/or Bragg scattering produced by the 2D periodic array.
Figure 1 shows a schematic diagram of the structural parameters and light paths of a Y2O3:Eu3+ TFP coated onto a 2D SiO2 PCL-assisted quartz substrate. The modes of light emitted from the conventional and 2D PCL-assisted film phosphors can be divided into upward and downward emitted light modes and guided light modes. Y2O3:Eu3+ film phosphors with various thicknesses, t, were deposited on 2D SiO2 PCL-assisted quartz substrates. The lattice constant, diameter, and height of the nanorods in the PCL arrays patterned on the quartz are denoted as Λ, d, and h, respectively.
Although the introduction of a 2D PCL is an effective method for improving the photoluminescence (PL) efficiency of a TFP, the experimentally obtained enhancement of the PL efficiency is still lower than expected . The following reasons for this mismatch between the expected and obtained enhancement of PL efficiencies have been suggested based on recent reports: (i) the difficulties in controlling the structural parameters of the PCs ; (ii) the difference between the crystallinity of the TFPs on flat substrates and those on 2D PC-assisted substrates ; (iii) the intrinsic optical parameters of the TFPs ; and (iv) the difference between the thermal expansion coefficients of TFPs and substrates after the annealing process .
In addition to these problems, the enhanced PL efficiency can vary with the thickness of the film phosphor. It was reported that the majority of the guided modes are sometimes present in the core of the TFP waveguide. Therefore, they barely overlap with the imaginary extractable region produced by the 2D PCL [12, 13]. Furthermore, when the thickness of the phosphor film is greater than the height of the nanorods for a large film volume, the appearance of the 2D PCL at the interface between the phosphor and air formed by duplicating the morphology of the 2D PCL on the quartz substrate is changed, which can vary the optical characteristics . Chao et al. recently examined the influence of the depth of the photonic crystal and the presence of a bottom PC reflector on the enhancement of the extraction efficiency using theoretical calculations . They concluded that the extraction efficiency degrades with decreasing PC depth and is enhanced when a PC reflector is present on the opposite side with respect to the PC extractor.
Y2O3:Eu3+ TFPs with high PL efficiency is needed to meet the requirements of a variety of applications, such as lighting and flat panel displays . In order to maximize the PL efficiency of Y2O3:Eu3+ thin films, it is essential to understand the effect of varying the thickness of Y2O3:Eu3+ TFPs coated onto 2D SiO2 PCLs on their absorption and extraction efficiency. This study investigated experimentally the change in the absorption and extraction efficiency of identical height PCL-assisted film phosphors as a function of the thickness of the Y2O3:Eu3+ TFP layer to determine what fraction of leaky modes overlap with the excitable and extractable regions produced by 2D SiO2 PCLs with a height (h) of 200 nm, a lattice constant (Λ) of 600 nm and a diameter (d) of 300 nm.
2. Experimental methods
The 2D SiO2 PCL-assisted Y2O3:Eu3+ film phosphors were fabricated using two successive processes, nanofabrication of the 2D SiO2 PCL-coated quartz (JMC GLASS) and deposition of the Y2O3:Eu3+ film phosphor. The 2D square-lattice rod patterns of SiO2 were generated on the quartz glass substrates by two-step irradiated hologram lithography and reactive ion etching (RIE) . Briefly, a 200 nm thick SiO2 film was coated onto an amorphous quartz glass substrate (JMC GLASS) using plasma enhanced chemical vapor deposition (PECVD), and a 40 nm thick Cr film was then deposited as a hard mask by thermal evaporation. A photoresist (PR) film was spin-coated on top of the Cr-coated SiO2 layer. Square lattice dot patterned PR films fabricated with hologram lithography were used as a mask to obtain the Cr hard mask patterns during the RIE process with Cl2 and O2 being used as the reactive gases. The Cr hard mask patterns were then transferred to the SiO2 films using CF4-based RIE. Two-step irradiated hologram lithography and reactive ion etching (RIE) were then used to generate a 2D square-lattice rod pattern of SiO2 on the quartz glass substrate.
After fabricating each 2D PCL, a Y2O3:Eu3+ thin-film phosphor was then deposited onto the patterned substrate by radio frequency (rf) magnetron sputtering . The sputtering deposition was carried out under the following conditions: pure Ar atmosphere, a background pressure of 5×10-7 Torr, a working pressure of 3.1×10-3 Torr, an rf power of 120 W, Y2O3:Eu3+ pellet targets, and a substrate temperature of 350°. The as-grown Y2O3:Eu3+ films were then annealed for 2 h at 900° in air. These flat Y2O3:Eu3+ thin films with various thicknesses grown under optimum conditions were used as standard phosphor layers in this study to examine the effects the thickness of the film phosphors deposited onto 2D PCLs at identical heights on the extraction efficiency. The thickness of each Y2O3:Eu3+ film phosphor was determined from the cross-sectional view obtained from field emission scanning electron microscopy (FE-SEM). Thicker films can be obtained by increasing the deposition time of the rf magnetron sputtering process. Figure 2 shows the cross-sectional and top views of the four different samples with a film thickness of 200 nm, 400 nm, 2 µm, and 4 µm. The Y2O3:Eu3+ film square lattice array was grown on the PCL-modified glass substrate using a self-organization method . The filling of the nanorod valleys increased with increasing film thickness, resulting in the formation of a partially planarized film over the 2D nanorod PC. Furthermore, the cross-sectional and top views clearly show that with increasing Y2O3:Eu3+ film thickness, there is a constant degree of corrugation formed on the SiO2 and phosphor layers, and a decreasing degree of corrugation formed in the phosphor layer and air. The resulting changes in film thickness satisfactorily match the designed changes in film thickness for identical PCL arrays with constants Λ, d, and h, as shown in Figs. 1 and 2.
The transmitted photoluminescence (PL) spectra of both the upward and downward emissions from the film phosphors was measured in all directions with respect to the surfaces using an integrated sphere (integrated mode) to obtain a systematic comparison of the film phosphors with various thicknesses on the 2D PCL substrates. 254 nm light (SPECTROLINE mercury lamp, MODEL ENF-2400C/FE) was used as the excitation source in a sufficient quantity to excite the full volume of the phosphor film and to illuminate all sides of each sample opposite the one from where the emitted light had been collected . The transmittance ratio between the 2D PCL assisted thin-film phosphor and flat TFP was also measured in all directions with respect to the surfaces using an integrated sphere (integrated mode) at an excitation wavelength of 254nm in order to determine the relative 254 nm absorption intensity of the 2D PCL assisted samples compared with a conventional flat TFP with various thicknesses of TFPs on the substrates.
3. Results and discussion
The emission spectra of downward emissions in the inset of Fig. 3 show that almost all of the light emission in these systems emanates from the typical 5D0→7FJ transition of trivalent europium at the C2 sites in the structure. The 2D PCL-assisted TFP showed stronger luminescence under the same excitation conditions, indicating that more light is injected into the phosphor layer or is escaping from the surface of the TFPs. Figure 3 also shows the downward emission PL intensity and film thickness characteristics of the 2D PCL-assisted and conventional Y2O3:Eu3+ thin films. The PL intensities of both the 2D PCL-assisted and conventional Y2O3:Eu3+ thin films increased with increasing Y2O3:Eu3+ film thickness. The enhanced red emission of the thicker Y2O3:Eu3+ films was attributed mainly to their enhanced emission volume. Although the absolute enhancement of the PL intensity that results from the insertion of a 2D nanorod array increases with increasing Y2O3:Eu3+ film thickness, the relative PL enhancement achieved by the insertion of identical 2D nanorod arrays is not constant for the Y2O3:Eu3+ film samples with different thicknesses.
Figure 4 shows the enhancement ratios achieved by the addition of the 2D SiO2 PCL arrays between the substrates and Y2O3:Eu3+ films at various thicknesses (t) including the four points shown in Fig. 2. The dependence of the PL efficiency enhancement ratio on the Y2O3:Eu3+ film thickness (t) coated on PCLs at identical heights is shown in detail in this figure. The enhancement ratio decreases with increasing the film thickness above t=400 nm. An empirical equation for the change in PL efficiency enhancement ratio with the film thickness was obtained numerically by fitting the experimental data above t=400 nm. This equation is an empirical expression for the PL efficiency enhancement ratio (ER) that results from downward scattering occurring at both the interface between the sputter-deposited Y2O3:Eu3+ films and the 2D SiO2 PCLs with h=200 nm, Λ=600 nm, and d=300 nm and the interface between the air and the phosphor films.
A, a, and ER0 are the experimental constants that depend on the structural parameters of the 2D PCLs and the type of film phosphor: A=11.2, a=3.8, and ER0=2.1. This equation shows that the PL efficiency enhancement ratio (ER) decreases exponentially with increasing Y2O3:Eu3+ film thickness. The inset in Fig. 4 also shows cross-section images of the flat and 2D PCL-assisted TFPs with t=400 nm under excitation at 254 nm. These photographs qualitatively confirm the significant enhancement of the upward and downward emissions of the 2D PCL films compared with those of the flat films.
Both the upwards and downwards scattered emission light was coupled out from the phosphor film to the surrounding layer. Moreover, both the upwards and downwards scattered absorption light was coupled to the phosphor film from the surrounding layer or air. The PL enhancement of the downward emission was attributed to the forward scattering of emitted light from the 2D Y2O3:Eu3+/SiO2 PCL and the inward scattering of excitation light to the 2D Y2O3:Eu3+/air PCL . The maximized PL efficiency enhancement ratio is presented for the 400 nm thick Y2O3:Eu3+ phosphor film. In this case, the 400 nm thick phosphor film layer was strongly corrugated by both the bottom and top PCLs after the sputtering process, as shown in Fig. 2(b). The bottom and top PCL has sufficiently fine PCL features to effectively scatter the red emission light and UV excitation light.
The increases in the integrated PL efficiency of the 2D PCL assisted TFPs were determined by a comparison with those of the original flat TFPs. Here, the total enhanced PL efficiency was defined in terms of the combined effects of the enhanced extraction efficiency through 2D PCL scattering of the guided light and the enhanced absorption efficiency by means of 2D PCL scattering of the excitation light . This interrelationship can be expressed using the following equation:
where Δηtot.ext. is the total external efficiency enhancement ratio, ΔηPCLextr. is the extraction efficiency enhancement ratio of 2D PCL scattering, and ΔηPCLabsorp. is the absorption efficiency enhancement ratio that originates from 2D PCL scattering. The two factors affecting the enhanced PL efficiency can be separated i.e. the extraction enhancement and absorption enhancement factors. An integrated transmission or excitation system was used to examine the absorption or photoluminescence (PL) properties of the flat TFPs and the 2D SiO2 PC assisted TFPs. The contribution of the absorption efficiency to the enhanced external efficiency of each of the flat TFPs was determined before examining the effect of the extraction efficiency of the 2D PCLs on Y2O3:Eu3+ TFPs with various film thicknesses on the improvement in the PL efficiency. Figure 5(a) shows the relative absorption intensity of the downward transmitted light at 254 nm of both the flat and 2D PCL assisted TFPs with different Y2O3:Eu3+ TFPs thicknesses. The relative absorption efficiency of the 2D PCL assisted TFPs was increased by the 2D PCL scattering of UV light, which is consistent with the results reported elsewhere . The absorption enhancement ratio of the 2D PCL assisted Y2O3:Eu3+ TFP was defined as the ratio of the absorption intensity of excitation light (254nm) between the flat TFP and identical TFP on the 2D PCL substrates. Figure 5(b) shows the change in the total PL efficiency enhancement ratio of the downward emitted light from the 2D PCL coated thin film phosphors as a function of the Y2O3:Eu3+ film thicknesses (t), along with the contributions of 2D PCL absorption scattering and extraction scattering to the total PL external efficiency. The figure shows that the almost constant enhancement ratio of the extraction efficiency and the decreased enhancement ratio of the absorption efficiency were obtained as a function of the Y2O3:Eu3+ TFP thickness. The guided modes even in thinner or thicker films were believed to overlap with the PCLs to a similar extent, leading to the constant enhancement of the diffraction efficiency. The constant enhancement ratio of the extraction efficiency is due to the effect of the constant degree of corrugation of the 2D PCL on the extraction efficiency of various Y2O3:Eu3+ TFPs at different thicknesses. This is one of the main causes of the decrease in the downward emission enhancement ratio with increases in the film thickness because a thicker phosphor film loses its ability to scatter excitation light in the downward direction. The decrease in the absorption efficiency enhancement ratio for downward emission with increasing Y2O3:Eu3+ film thickness can be interpreted in terms of the increasing absorption length of the thicker Y2O3:Eu3+ TFPs and the deteriorating degree of the additional absorption capability between the PC scattering region and excitation light. The trend of the absorption factor is the primary reason for the exponential decrease in the external PL efficiency enhancement ratio with increasing film thickness.
Similarly, upward emission enhancement is induced by downward scattering from the Y2O3:Eu3+/SiO2 PCL and upward scattering from the 2D Y2O3:Eu3+/air PCL. The decrease in the PL efficiency enhancement ratio for upward emission with increasing Y2O3:Eu3+ film thickness can also be interpreted in terms of the combination between the almost constant enhancement ratio of the extraction efficiency and exponential trend in the absorption efficiency enhancement ratio with increasing film thickness. The empirically fitted exponential curves for the downward and upward light emissions satisfactorily match the experimental data for the external PL efficiency enhancement ratio (ER) for t>400 nm. However, the trends in the enhancement ratios of both the downward and the upward emitted light deviate from the fitted curves at t<400 nm. These decreases in the observed enhancement ratios are possibly due to the reduced perturbation effects of the 2D PCLs on the extraction and absorption efficiency of thinner phosphor films. As shown in the experimental data for the upward and downward emissions in Fig. 4, the maximum enhancements were obtained when the thickness of the deposited Y2O3:Eu3+ films was 400 nm. The 2D PCL-assisted Y2O3:Eu3+ film at t=400 nm shows an external PL efficiency enhancement in the upward and downward emissions of 6.2 and 8.6, respectively, with respect to those of the conventional flat films. However, the amount of upward emission from the flat film is not the same as its downward emission.
Strong PL enhancement was produced by the photonic crystals at the air and quartz interfaces of Y2O3:Eu3+ films through a combination of the appropriate 2D PCL structure with the optimum deposited film thickness. This ideal extraction and absorption at the phosphor-air and phosphor-substrate interfaces might be due to multi diffraction scattering from both the top and bottom 2D PCL extractors of both guided and excitation light, and the significant overlap between the guided/excited modes and the 2D PCL scattering region. Therefore, when extracting the guided light of a Y2O3:Eu3+ film and injecting the excitation light into a Y2O3:Eu3+ film, the coated phosphor film thickness and the structural parameters of the 2D PCL are the critical factors that determine the degree to which the 2D PCL array enhances the PL emission light from a thin-film phosphor. The extraction enhancement in the 2D PCL-assisted Y2O3:Eu3+ films was almost constant (~2.0) regardless of the film thickness but the excitation enhancement varied significantly with values as high as ~5.0. Therefore, the maximum enhancement of the integrated PL efficiency of 2D PCL-assisted Y2O3:Eu3+ film at t=400 nm in the downward emissions was achieved by combining a 2.1 and 4.1 fold enhancement of excitation and extraction light, respectively.
This study was supported by Grant No. 2007-02397 of the Nano R&D Program and Grant No. R11-2005-048-00000-0 of the Engineering Research Center Program of the Ministry of Science and Technology in Korea.
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