FIG 1
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We have grown SrTiO <sub>3</sub> thin films by rf-sputtering and studied its photoluminescence (PL) property after postannealing treatments. While the as-grown film does not show any PL signal, visible frequency PL emissions are induced by high temperature ( T ≫550 ° C ) annealing. When subsequent low-T (50 ° C ) and long term (≫8 months ) annealin...
Contexts in source publication
Context 1
... 3 STO is a simple cubic perovskite oxide with a wide band gap ͑ E g = 3.2 eV ͒ . The dielectric constant of STO is large ͑ ϳ 300 ͒ and exhibits the quantum paraelectricity at 1,2 low-T. By electron-doping, STO becomes metallic and even superconducting at low temperature. A high mobility electron gas emerges when STO forms a thin film superlat- 3–5 tice with another insulator LaAlO 3 . In technological appli- cation, it is used as ideal substrate for high-T superconductor epitaxial film growth. The use of Nb doped STO as a field- effect transistor and resistance switching memory device has 6 been demonstrated. Photoluminescence ͑ PL ͒ is another important aspect of STO. It has been known for long time that undoped STO shows the low-temperature PL ͑ T Ͻ 110 K ͒ at green ͑ ϳ 540 nm ͒ ͑ Refs. 7 and 8 ͒ and infrared ͑ ϳ 800 nm ͒ 9 10 frequency. Recently D. Kan et al. showed that room- temperature blue-PL can be induced when STO crystal is irradiated by Ar + ion beam. The blue-PL appears also by chemical substitution such as Sr +2 → La +3 or Ti +2 → Nb +3 . 11 In general context PL emission is derived from defect electronic states inside the band gap due to atomic vacancy 12,13 or impurity. In STO, a single vacancy of Sr, Ti, and O can be the defect sources whereas the O-vacancy is most common one. In addition, two O-vacancies can bind together to form a cluster. Depending on the cluster configuration the 14 defect energy changes, O-vacancy may also bind with Sr-or 15 Ti-vacancy in various forms of defect clusters. Therefore, the defect states available in STO can be far more abundant and one should be able to extract much richer PL emissions than previously demonstrated. In this work we have grown highly oriented STO thin films and post annealed them in various thermal environments. Thermal annealing is the most common and easy way to induce vacancy defects in thin films. We find that, depending on the annealing conditions, a series of visible PL peaks can be induced at room temperature. We have deposited STO thin films on thermally oxidized silicon substrates ͑ SiO 2 thickness 200 nm ͒ by sputtering an STO target ͑ 2 inch diameter ͒ . Applied rf power was 100 W and substrate temperature was fixed at 550 ° C. Base pressure of the system was 1 ϫ 10 −6 Torr and working pressure was 1.5 ϫ 10 −2 Torr. An O 2 / Ar flow ratio of 90/10 was maintained during the deposition for 4 h. These as-grown samples were subsequently annealed in either a UHV cham- ber at 10 −7 torr or ambient pressure air for 2 − 6 hours. Four different temperature-pressure conditions used in the thermal treatment are summarized in Table I. Structural properties of the thin film were investigated by high resolution x-ray dif- fraction ͑ XRD ͒ ͑ using Bruker-AXS D8 Discover, Gober mirror ͒ and transmission electron microscope ͑ TEM ͒ . The PL spectra were taken using a PL ͑ HORIBA Fluorolog-3 ͒ equipped with 325 nm excitation laser ͑ power = 30 mW / cm 2 ͒ . Figure 1 shows the XRD data ͑ a ͒ and the TEM image ͑ b ͒ of the as-grown STO film. In Fig. 1 ͑ a ͒ the dominant peak at 32° corresponds to ͑ 110 ͒ reflection of the STO lattice con- 16–21 sistent with other work. Considering the structural fac- tors of each crystalline planes, volume ratio of ͑ 110 ͒ planes was estimated to be ϳ 50 % . Our STO thin films were domi- nantly ͑ 110 ͒ orientation on the SiO 2 surface. Rundqvist et 22 al. reported that Ba x Sr 1− x TiO 3 thin films were textured with a dominant ͑ 110 ͒ orientation on SiO 2 / Si substrates similar to our results. They suggested that such a preferential growth could be due to the lower surface energy of the ͑ 110 ͒ plane. Figure 1 ͑ b ͒ shows that the STO layer ͑ 10 nm thick ͒ is well crystallized and oriented along the growth direction. For the annealed samples, the XRD and TEM data were identical with the as-grown one without showing any notable change. Figure 2 shows PL spectrum of the annealed samples 1 through 4 and the as-grown samples. In the sample 3 ͑ 750 ° C-760 Torr annealed, Table I ͒ , a prominent PL peak is observed at = 420 nm. It corresponds to the blue- luminescence and is similar to the blue-PL induced by Ar- irradiation ͑ = 420− 430 nm ͒ . In the sample 1, the PL peak appears at = 505 nm whereas in the samples 2 and 4, it is at = 630 nm. These correspond to the green and red-orange emissions, respectively. The as-grown, nonannealed STO does not show any PL signal. We note that single crystal STO close to the ideal stoichiometric ratio ͑ Sr: Ti: O = 1 : 1 : 3 ͒ also shows no PL emission at room temperature. 7,10 The insets in Fig. 2 show the actual PL image recorded with a digital camera during the laser illumination. It is very interesting to note that these PL colors we have induced are in fact the three primary-colors used in the display industry. It is also noteworthy that these colors can be induced in single material STO by the relatively easy processing tech- nique of thermal annealing. We have repeated the complete experimental sequence, film growth → thermal annealing → PL measurement, several times to check the reproducibility of the PL effect. In some cases, the blue- and green-peaks appeared simultaneously in samples 1 and 3. The red-PL in sample 4 had higher reproducibility. To test possible influence of the buffer layer on the PL effect, we have annealed the bare SiO / Si substrate with- out STO film under the same conditions as for the samples 1–4 and measured PL spectra, but no PL signal was seen. It shows that the PL peaks come indeed from the STO layer. What are the defect states that give rise to the PL effects? Single oxygen vacancy makes donor level located 23,24 close the conduction band bottom. A recent LDA+ U cal- culation showed that oxygen vacancies tend to bind together 14 to form two-vacancy clusters. The clustering is accompa- nied by localization of electron at Ti ion. A localized electron level is formed as a result where the level energy is within the gap, about 0.5− 1.5 eV below the conduction band. The oxygen vacancy can bind with Sr vacancy as well where the 14 cluster energy is also within the band gap. If electrons in these in-gap levels recombine with valence band hole, vari- 25 ous visible PL lines will appear as we observed. In attempt to reveal the defect types in our samples, we tried x-ray absorption spectroscopy and Rutherford backscattering spec- trometry ͑ RBS ͒ measurements but could not obtain reliable data due to the ultrathin film thickness. We have kept the Samples 1 and 3 in moisture-free N 2 atmosphere at 50 ° C for more than eight months and remea- sured the PL spectra. After this long term storage, the samples exhibited very interesting changes as shown in Fig. 3. In Sample 3, the original blue peak has evolved into the four distinct peaks ͑ 3–1 and 3–2 ͒ . They are seen in Sample 1 as well that had the green PL peak initially, although the high energy peaks are weak. The four peaks are located at = 392 nm ͑ 3.1 eV ͒ , 461 nm ͑ 2.7 eV ͒ , 555 nm ͑ 2.2 eV ͒ , and 682 nm ͑ 1.8 eV ͒ , respectively, which covers the wide colors from red up to deep blue. The PL emission in these cases appears white in color to naked eye. The four peaks are narrower in widths than the initial blue and green PL peaks. This remarkable PL evolution effect suggests that the defects formed after the high-T short-time annealing are in some metastable states. The long-term, low-T annealing brings them into the energetically stable in-gap states irre- spective of their starting metastable states. If we accept the defect cluster picture, the clusters are in some unstable configurations initially and they evolve into stable clusters during the second annealing stage. Perhaps the vacancies rear- range themselves through thermally assisted migration/ diffusion. Our results suggest that at least four stable in-gap levels exist and contribute to the PL emission. It will be important We thank to S. ...
Context 2
... 3 STO is a simple cubic perovskite oxide with a wide band gap ͑ E g = 3.2 eV ͒ . The dielectric constant of STO is large ͑ ϳ 300 ͒ and exhibits the quantum paraelectricity at 1,2 low-T. By electron-doping, STO becomes metallic and even superconducting at low temperature. A high mobility electron gas emerges when STO forms a thin film superlat- 3–5 tice with another insulator LaAlO 3 . In technological appli- cation, it is used as ideal substrate for high-T superconductor epitaxial film growth. The use of Nb doped STO as a field- effect transistor and resistance switching memory device has 6 been demonstrated. Photoluminescence ͑ PL ͒ is another important aspect of STO. It has been known for long time that undoped STO shows the low-temperature PL ͑ T Ͻ 110 K ͒ at green ͑ ϳ 540 nm ͒ ͑ Refs. 7 and 8 ͒ and infrared ͑ ϳ 800 nm ͒ 9 10 frequency. Recently D. Kan et al. showed that room- temperature blue-PL can be induced when STO crystal is irradiated by Ar + ion beam. The blue-PL appears also by chemical substitution such as Sr +2 → La +3 or Ti +2 → Nb +3 . 11 In general context PL emission is derived from defect electronic states inside the band gap due to atomic vacancy 12,13 or impurity. In STO, a single vacancy of Sr, Ti, and O can be the defect sources whereas the O-vacancy is most common one. In addition, two O-vacancies can bind together to form a cluster. Depending on the cluster configuration the 14 defect energy changes, O-vacancy may also bind with Sr-or 15 Ti-vacancy in various forms of defect clusters. Therefore, the defect states available in STO can be far more abundant and one should be able to extract much richer PL emissions than previously demonstrated. In this work we have grown highly oriented STO thin films and post annealed them in various thermal environments. Thermal annealing is the most common and easy way to induce vacancy defects in thin films. We find that, depending on the annealing conditions, a series of visible PL peaks can be induced at room temperature. We have deposited STO thin films on thermally oxidized silicon substrates ͑ SiO 2 thickness 200 nm ͒ by sputtering an STO target ͑ 2 inch diameter ͒ . Applied rf power was 100 W and substrate temperature was fixed at 550 ° C. Base pressure of the system was 1 ϫ 10 −6 Torr and working pressure was 1.5 ϫ 10 −2 Torr. An O 2 / Ar flow ratio of 90/10 was maintained during the deposition for 4 h. These as-grown samples were subsequently annealed in either a UHV cham- ber at 10 −7 torr or ambient pressure air for 2 − 6 hours. Four different temperature-pressure conditions used in the thermal treatment are summarized in Table I. Structural properties of the thin film were investigated by high resolution x-ray dif- fraction ͑ XRD ͒ ͑ using Bruker-AXS D8 Discover, Gober mirror ͒ and transmission electron microscope ͑ TEM ͒ . The PL spectra were taken using a PL ͑ HORIBA Fluorolog-3 ͒ equipped with 325 nm excitation laser ͑ power = 30 mW / cm 2 ͒ . Figure 1 shows the XRD data ͑ a ͒ and the TEM image ͑ b ͒ of the as-grown STO film. In Fig. 1 ͑ a ͒ the dominant peak at 32° corresponds to ͑ 110 ͒ reflection of the STO lattice con- 16–21 sistent with other work. Considering the structural fac- tors of each crystalline planes, volume ratio of ͑ 110 ͒ planes was estimated to be ϳ 50 % . Our STO thin films were domi- nantly ͑ 110 ͒ orientation on the SiO 2 surface. Rundqvist et 22 al. reported that Ba x Sr 1− x TiO 3 thin films were textured with a dominant ͑ 110 ͒ orientation on SiO 2 / Si substrates similar to our results. They suggested that such a preferential growth could be due to the lower surface energy of the ͑ 110 ͒ plane. Figure 1 ͑ b ͒ shows that the STO layer ͑ 10 nm thick ͒ is well crystallized and oriented along the growth direction. For the annealed samples, the XRD and TEM data were identical with the as-grown one without showing any notable change. Figure 2 shows PL spectrum of the annealed samples 1 through 4 and the as-grown samples. In the sample 3 ͑ 750 ° C-760 Torr annealed, Table I ͒ , a prominent PL peak is observed at = 420 nm. It corresponds to the blue- luminescence and is similar to the blue-PL induced by Ar- irradiation ͑ = 420− 430 nm ͒ . In the sample 1, the PL peak appears at = 505 nm whereas in the samples 2 and 4, it is at = 630 nm. These correspond to the green and red-orange emissions, respectively. The as-grown, nonannealed STO does not show any PL signal. We note that single crystal STO close to the ideal stoichiometric ratio ͑ Sr: Ti: O = 1 : 1 : 3 ͒ also shows no PL emission at room temperature. 7,10 The insets in Fig. 2 show the actual PL image recorded with a digital camera during the laser illumination. It is very interesting to note that these PL colors we have induced are in fact the three primary-colors used in the display industry. It is also noteworthy that these colors can be induced in single material STO by the relatively easy processing tech- nique of thermal annealing. We have repeated the complete experimental sequence, film growth → thermal annealing → PL measurement, several times to check the reproducibility of the PL effect. In some cases, the blue- and green-peaks appeared simultaneously in samples 1 and 3. The red-PL in sample 4 had higher reproducibility. To test possible influence of the buffer layer on the PL effect, we have annealed the bare SiO / Si substrate with- out STO film under the same conditions as for the samples 1–4 and measured PL spectra, but no PL signal was seen. It shows that the PL peaks come indeed from the STO layer. What are the defect states that give rise to the PL effects? Single oxygen vacancy makes donor level located 23,24 close the conduction band bottom. A recent LDA+ U cal- culation showed that oxygen vacancies tend to bind together 14 to form two-vacancy clusters. The clustering is accompa- nied by localization of electron at Ti ion. A localized electron level is formed as a result where the level energy is within the gap, about 0.5− 1.5 eV below the conduction band. The oxygen vacancy can bind with Sr vacancy as well where the 14 cluster energy is also within the band gap. If electrons in these in-gap levels recombine with valence band hole, vari- 25 ous visible PL lines will appear as we observed. In attempt to reveal the defect types in our samples, we tried x-ray absorption spectroscopy and Rutherford backscattering spec- trometry ͑ RBS ͒ measurements but could not obtain reliable data due to the ultrathin film thickness. We have kept the Samples 1 and 3 in moisture-free N 2 atmosphere at 50 ° C for more than eight months and remea- sured the PL spectra. After this long term storage, the samples exhibited very interesting changes as shown in Fig. 3. In Sample 3, the original blue peak has evolved into the four distinct peaks ͑ 3–1 and 3–2 ͒ . They are seen in Sample 1 as well that had the green PL peak initially, although the high energy peaks are weak. The four peaks are located at = 392 nm ͑ 3.1 eV ͒ , 461 nm ͑ 2.7 eV ͒ , 555 nm ͑ 2.2 eV ͒ , and 682 nm ͑ 1.8 eV ͒ , respectively, which covers the wide colors from red up to deep blue. The PL emission in these cases appears white in color to naked eye. The four peaks are narrower in widths than the initial blue and green PL peaks. This remarkable PL evolution effect suggests that the defects formed after the high-T short-time annealing are in some metastable states. The long-term, low-T annealing brings them into the energetically stable in-gap states irre- spective of their starting metastable states. If we accept the defect cluster picture, the clusters are in some unstable configurations initially and they evolve into stable clusters during the second annealing stage. Perhaps the vacancies rear- range themselves through thermally assisted migration/ diffusion. Our results suggest that at least four stable in-gap levels exist and contribute to the PL emission. It will be important We thank to S. identify Han, J. the Yu, origins and Y. of Choi these for useful defect discus- ...
Context 3
... 3 STO is a simple cubic perovskite oxide with a wide band gap ͑ E g = 3.2 eV ͒ . The dielectric constant of STO is large ͑ ϳ 300 ͒ and exhibits the quantum paraelectricity at 1,2 low-T. By electron-doping, STO becomes metallic and even superconducting at low temperature. A high mobility electron gas emerges when STO forms a thin film superlat- 3–5 tice with another insulator LaAlO 3 . In technological appli- cation, it is used as ideal substrate for high-T superconductor epitaxial film growth. The use of Nb doped STO as a field- effect transistor and resistance switching memory device has 6 been demonstrated. Photoluminescence ͑ PL ͒ is another important aspect of STO. It has been known for long time that undoped STO shows the low-temperature PL ͑ T Ͻ 110 K ͒ at green ͑ ϳ 540 nm ͒ ͑ Refs. 7 and 8 ͒ and infrared ͑ ϳ 800 nm ͒ 9 10 frequency. Recently D. Kan et al. showed that room- temperature blue-PL can be induced when STO crystal is irradiated by Ar + ion beam. The blue-PL appears also by chemical substitution such as Sr +2 → La +3 or Ti +2 → Nb +3 . 11 In general context PL emission is derived from defect electronic states inside the band gap due to atomic vacancy 12,13 or impurity. In STO, a single vacancy of Sr, Ti, and O can be the defect sources whereas the O-vacancy is most common one. In addition, two O-vacancies can bind together to form a cluster. Depending on the cluster configuration the 14 defect energy changes, O-vacancy may also bind with Sr-or 15 Ti-vacancy in various forms of defect clusters. Therefore, the defect states available in STO can be far more abundant and one should be able to extract much richer PL emissions than previously demonstrated. In this work we have grown highly oriented STO thin films and post annealed them in various thermal environments. Thermal annealing is the most common and easy way to induce vacancy defects in thin films. We find that, depending on the annealing conditions, a series of visible PL peaks can be induced at room temperature. We have deposited STO thin films on thermally oxidized silicon substrates ͑ SiO 2 thickness 200 nm ͒ by sputtering an STO target ͑ 2 inch diameter ͒ . Applied rf power was 100 W and substrate temperature was fixed at 550 ° C. Base pressure of the system was 1 ϫ 10 −6 Torr and working pressure was 1.5 ϫ 10 −2 Torr. An O 2 / Ar flow ratio of 90/10 was maintained during the deposition for 4 h. These as-grown samples were subsequently annealed in either a UHV cham- ber at 10 −7 torr or ambient pressure air for 2 − 6 hours. Four different temperature-pressure conditions used in the thermal treatment are summarized in Table I. Structural properties of the thin film were investigated by high resolution x-ray dif- fraction ͑ XRD ͒ ͑ using Bruker-AXS D8 Discover, Gober mirror ͒ and transmission electron microscope ͑ TEM ͒ . The PL spectra were taken using a PL ͑ HORIBA Fluorolog-3 ͒ equipped with 325 nm excitation laser ͑ power = 30 mW / cm 2 ͒ . Figure 1 shows the XRD data ͑ a ͒ and the TEM image ͑ b ͒ of the as-grown STO film. In Fig. 1 ͑ a ͒ the dominant peak at 32° corresponds to ͑ 110 ͒ reflection of the STO lattice con- 16–21 sistent with other work. Considering the structural fac- tors of each crystalline planes, volume ratio of ͑ 110 ͒ planes was estimated to be ϳ 50 % . Our STO thin films were domi- nantly ͑ 110 ͒ orientation on the SiO 2 surface. Rundqvist et 22 al. reported that Ba x Sr 1− x TiO 3 thin films were textured with a dominant ͑ 110 ͒ orientation on SiO 2 / Si substrates similar to our results. They suggested that such a preferential growth could be due to the lower surface energy of the ͑ 110 ͒ plane. Figure 1 ͑ b ͒ shows that the STO layer ͑ 10 nm thick ͒ is well crystallized and oriented along the growth direction. For the annealed samples, the XRD and TEM data were identical with the as-grown one without showing any notable change. Figure 2 shows PL spectrum of the annealed samples 1 through 4 and the as-grown samples. In the sample 3 ͑ 750 ° C-760 Torr annealed, Table I ͒ , a prominent PL peak is observed at = 420 nm. It corresponds to the blue- luminescence and is similar to the blue-PL induced by Ar- irradiation ͑ = 420− 430 nm ͒ . In the sample 1, the PL peak appears at = 505 nm whereas in the samples 2 and 4, it is at = 630 nm. These correspond to the green and red-orange emissions, respectively. The as-grown, nonannealed STO does not show any PL signal. We note that single crystal STO close to the ideal stoichiometric ratio ͑ Sr: Ti: O = 1 : 1 : 3 ͒ also shows no PL emission at room temperature. 7,10 The insets in Fig. 2 show the actual PL image recorded with a digital camera during the laser illumination. It is very interesting to note that these PL colors we have induced are in fact the three primary-colors used in the display industry. It is also noteworthy that these colors can be induced in single material STO by the relatively easy processing tech- nique of thermal annealing. We have repeated the complete experimental sequence, film growth → thermal annealing → PL measurement, several times to check the reproducibility of the PL effect. In some cases, the blue- and green-peaks appeared simultaneously in samples 1 and 3. The red-PL in sample 4 had higher reproducibility. To test possible influence of the buffer layer on the PL effect, we have annealed the bare SiO / Si substrate with- out STO film under the same conditions as for the samples 1–4 and measured PL spectra, but no PL signal was seen. It shows that the PL peaks come indeed from the STO layer. What are the defect states that give rise to the PL effects? Single oxygen vacancy makes donor level located 23,24 close the conduction band bottom. A recent LDA+ U cal- culation showed that oxygen vacancies tend to bind together 14 to form two-vacancy clusters. The clustering is accompa- nied by localization of electron at Ti ion. A localized electron level is formed as a result where the level energy is within the gap, about 0.5− 1.5 eV below the conduction band. The oxygen vacancy can bind with Sr vacancy as well where the 14 cluster energy is also within the band gap. If electrons in these in-gap levels recombine with valence band hole, vari- 25 ous visible PL lines will appear as we observed. In attempt to reveal the defect types in our samples, we tried x-ray absorption spectroscopy and Rutherford backscattering spec- trometry ͑ RBS ͒ measurements but could not obtain reliable data due to the ultrathin film thickness. We have kept the Samples 1 and 3 in moisture-free N 2 atmosphere at 50 ° C for more than eight months and remea- sured the PL spectra. After this long term storage, the samples exhibited very interesting changes as shown in Fig. 3. In Sample 3, the original blue peak has evolved into the four distinct peaks ͑ 3–1 and 3–2 ͒ . They are seen in Sample 1 as well that had the green PL peak initially, although the high energy peaks are weak. The four peaks are located at = 392 nm ͑ 3.1 eV ͒ , 461 nm ͑ 2.7 eV ͒ , 555 nm ͑ 2.2 eV ͒ , and 682 nm ͑ 1.8 eV ͒ , respectively, which covers the wide colors from red up to deep blue. The PL emission in these cases appears white in color to naked eye. The four peaks are narrower in widths than the initial blue and green PL peaks. This remarkable PL evolution effect suggests that the defects formed after the high-T short-time annealing are in some metastable states. The long-term, low-T annealing brings them into the energetically stable in-gap states irre- spective of their starting metastable states. If we accept the defect cluster picture, the clusters are in some unstable configurations initially and they evolve into stable clusters during the second annealing stage. Perhaps the vacancies rear- range themselves through thermally assisted migration/ diffusion. Our results suggest that at least four stable in-gap levels exist and contribute to the PL emission. It will be important We thank to S. identify Han, J. the Yu, origins and Y. of Choi these for useful defect discus- states through sions. This both work theoretical was supported and experimental by Nuclear investigation. Research and Development In summary, Program we have of the shown Korea that Science rich visible and Engineering PL emissions Foundation can be ͑ KOSEF induced ͒ , in grant STO funded thin films by at the room-temperature Korean govern- by ment thermal ͑ MEST, annealing under Grant treatment No. as 20090067238 follows: an ͒ . initial D.W.K. high-T was annealing supported and by the a subsequent Pioneer Research low-T long-term Center Program annealing. ͑ Grant The three No. 2009-0083007 primary PL ...
Citations
... The origin of the 2.0 eV emission is less clear and maybe qualitatively different from that proposed for the other two bands. This emission has only been distinctly observed in amorphous or heavily disordered STO, obtained after high temperature annealing in vacuum [33] or after light-ion irradiations (H + and C − at 60 keV) [34]. Theoretical calculations reveal that the disorder associated to O-vacancies introduces new electronic levels in the gap as well as modifies the location of the Fermi level. ...
Light emission under MeV hydrogen and oxygen ions in stoichiometric SrTiO3 are identified
at temperatures of 100 K, 170 K and room-temperature. MeV ions predominately deposit
their energies to electrons in SrTiO3 with energy densities orders of magnitude higher than
from UV or x-ray sources but comparable to femtosecond lasers. The ionoluminescence (IL)
spectra can be resolved into three main Gaussian bands at 2.0 eV, 2.5 eV and 2.8 eV, whose relative contributions strongly depend on irradiation temperature, electronic energy loss and irradiation fluence. Two main bands, observed at 2.5 eV and 2.8 eV, are intrinsic and associated with electron–hole recombination in the perfect SrTiO3 lattice. The 2.8 eV band is attributed to recombination of free (conduction) electrons with an in-gap level, possibly related to self-trapped holes. Self-trapped excitons (STEs) are considered suitable candidates for the 2.5 eV emission band, which implies a large energy relaxation in comparison to the intrinsic edge transition. The dynamics of electronic excitation, governs a rapid initial rise of the intensity;
whereas, accumulated irradiation damage (competing non-radiative recombination channels)
accounts for a subsequent intensity decrease. The previously invoked role of isolated oxygen
vacancies for the blue luminescence (2.8 eV) does not appear consistent with the data. An
increasing well-resolved band at 2.0 eV dominates at 170 K and below. It has been only previously observed in heavily strained and amorphous SrTiO3, and is, here, attributed to transitions from d(t2g) conduction band levels to d(eg) levels below the gap. In accordance with ab initio theoretical calculations they are associated to trapped electron states in relaxed Ti3+ centers at an oxygen vacancy within distorted TiO6 octahedra. The mechanism of defect evolution monitored during real-time IL experiments is presented. In conclusion, the light emission data confirm that IL is a useful tool to investigate lattice disorder in irradiated
SrTiO3.
... The intensity of this emission decreases rapidly above 60 K and disappears all together beyond 110 K [19][20][21][22]. The luminescence becomes pronounced when oxygen vacancies are incorporated in STO [15,[23][24][25]. Kan et al. have noted that bombardment with 300 eV Ar + ions induces blue (∼420 nm) PL in stoichiometric * rcb@iitk.ac.in single crystals of STO at room temperature, which they attribute to emission from oxygen-vacancy-related defect states [15]. ...
The photoluminescence (PL) spectra of Ar-ion irradiated single crystals of
SrTiO3 (STO) excited by the 325 nm line of a He-Cd laser are compared with
those of pristine crystals, epitaxial films, and amorphous layers of STO at
several temperatures down to 20 K. The 550 eV Ar-beam irradiation activates
three distinctly visible PL peaks: blue (~430 nm), green (~550 nm), and
infrared (~820 nm) at room temperature, making the photoluminescence
multicolored. The abrupt changes in PL properties below ~100 K are discussed in
relation with the antiferrodistortive structural phase transition in SrTiO3
from cubic to tetragonal symmetry, which makes it a direct bandgap
semiconductor. The photoluminescence spectra are also tuned by an electrostatic
gate field in a field-effect transistor geometry. At 20 K, we observed a
maximum increase of ~20% in PL intensity under back gating of SrTiO3.
... [9][10][11][12][13][14][15][16] In undoped-STO, the PL peaks at infrared ($1.5 eV) and green ($2.4 eV) regions were observed below 110 K. Green luminescence (GL) was often considered to be originated from the recombination of excited electrons and holes coupled in a self-trapped exciton (STE) state. 17,18 At whiles, blue luminescence (BL) often appears in annealed STO, 19,20 which also occurred in La and Nb-doped and Ar þ -irradiated STO single crystals. 11,16 And, the blue emission is often accompanied by a spectrally narrow ultraviolet luminescence located at 3.2 eV at low temperatures. ...
... 16,21,22 Up to date, PL dynamics in both pure and electron-doped STO can be described by a model involving in single-carrier trapping, radiative bimolecular recombination, and nonradiative auger recombination. 10,14,22 And in light of this model, it is generally considered that PL emissions may be mainly derived from defect electronic states inside the band gap due to atomic vacancy or impurity, 19,23,24 such as oxygen-vacancy (V O ). However, which exact defects acting as radiative centers are still in debate. ...
SrTiO3 thin films were epitaxially grown on (100) SrTiO3 substrates using molecular beam epitaxy. The temperature for growth of the films was optimized, which was indicated by x-ray diffraction and further confirmed by microstructural characterization. Photoluminescence spectra show that oxygen-vacancy contributes to red and blue luminescence of oxygen-deficient post-annealed films, and a red shift was observed in blue region. On the other hand, ferromagnetism in film form SrTiO3 was observed from 5 K to 400 K and could be further enhanced with decreasing oxygen plasma partial pressure in annealing processes, which might be explained by the theory involving d0 magnetism related to oxygen-vacancy. From the cooperative investigations of optical and magnetic properties, we conclude that intrinsic defects, especially oxygen-vacancy, can induce and enhance luminescence and magnetism in SrTiO3 films.
The present work evaluates the effects of plasma power and oxygen mixing ratios (OMRs) on structural, morphological, optical, and electrical properties of strontium titanate SrTiOx (STO) thin films. STO thin films were grown by magnetron sputtering, and later thermal annealing at 700°C for 1 h was applied to improve film properties. X‐ray diffraction analysis indicated that as‐deposited films have amorphous microstructure independent of deposition conditions. The films deposited at higher OMR values and later annealed also showed amorphous structure while the films deposited at lower OMR value and annealed have nanocrystallinity. In addition, all as‐deposited films were highly transparent (~80%–85%) in the visible spectrum and exhibited well‐defined main absorption edge, while the annealing improved transparency (90%) within the same spectrum. The calculated direct and indirect optical band gaps for films were in the range of 3.60‐4.30 eV as a function of deposition conditions. The refractive index of the films increased with OMRs and the postdeposition annealing. The frequency dependent capacitance measurements at 100 kHz were performed to obtain film dielectric constant values. High dielectric constant values reaching up to 100 were obtained. All STO samples exhibited more than 2.5 μC/cm² charge storage capacity and low dielectric loss (less than 0.07 at 100 kHz). The leakage current density was relatively low (3 × 10⁻⁸Acm⁻² at +0.8 V) indicating that STO films are promising for future dynamic random access memory applications.
Strontium titanate (SrTiO3), a model system with a strongly correlated electronic structure, has attracted much attention recently because of its outstanding physicochemical properties and considerable potentials for technological applications. The capability to control oxygen vacancy profiles and their effect on valence states of cations will increase significantly the functionality of devices based on transition metal oxides. This work presents new insights into the near-infrared luminescence emission of Cr3+ centers in stoichiometric SrTiO3 induced using 3 MeV protons at temperatures of 100 K, 170 K and room temperature. The study covers a wide spectral range, including near-infrared, visible and near-UV regions. Our main purpose is to investigate the role of the oxygen vacancies introduced by energetic charged particles on the shape and yield of induced luminescence spectra, in particular to explore the interplay between the Cr3+ luminescence at 1.55 eV and oxygen disorder. A clear correlation is found between the decay of the Cr luminescence yield during irradiation and the growth of a band at 2.0 eV, well-resolved below 170 K, which has been very recently attributed to d-d transitions of electrons self-trapped as Ti3+ in the close vicinity of oxygen vacancies. This correlation suggests irradiation-induced oxidation of the Cr3+ (Cr3+ → Crn+, n>3) via trapping of irradiation-induced holes, while the partner electrons are self-trapped as Ti3+. These new results provide effective guidelines for further understanding the electronic and photocatalytic behavior of STO:Cr3+.
Visible electroluminescence has been achieved from a device based on the SrTiO3/p+-Si heterostructure, in which the SrTiO3 film is deposited by a sol-gel process on silicon substrate. When applied with sufficient forward bias with the positive voltage connecting to the p+-Si, broad visible emissions peaking at ∼550 nm occur. The self-trapped excitons relevant to oxygen vacancies in SrTiO3 are proposed to be responsible for the visible emissions from the SrTiO3/p+-Si heterostructured device. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
X-ray excited luminescence spectra of strontium titanate are reported over the temperature range from 20 to 300 K. The range includes several crystalline phases, each with different emission spectra. The signals are thermally quenched above ~220 K. There are spectral shifts and intensity changes around the temperatures associated with phase changes and overall there are nominally three spectral emission bands. A remarkable observation is that at fixed lower temperatures the spectra undergo major changes with the energy of the X-rays. A possible cause of the effect is discussed in terms of inner shell excitation from the K shell of the strontium. Comparisons with thermoluminescence spectra from the strontium titanate are reported.
This research shows the influence of the synthesis route in the structural and morphological characteristics as well as in the luminescent properties of doped with europium and pure SrTiO3 (STO) powders prepared by microwave assisted hydrothermal synthesis, MWH, and by the polymeric precursor method, PPM. The XRD at room temperature of the STO powders nominally pure obtained by PPM at 700°C for 3 hours, as well as by the MWH at 190°C by 30 minutes present all the reflection peaks for the cubic perovskite structure (JCPDS-ICDD 35-734). The morphology varies according to the synthesis route. The particles of pure STO obtained by PPM presents morphology in the form of plates and the morphology of the particles synthesized by MWH is spherical with approximately 150 nm. The photoluminescent analysis shows for pure STO wide bands associated with the transition of charge transfer from the titanates group (TiO3)2- that are centered on 450 nm. In both preparation methods the emission bands obtained in the composites spectra were found to be asymmetric and low intense. However, in the case of the STO prepared by the PPM a bigger FWHM of the band can be observed. The excitation of the samples was done using a laser (Coherent Innova) with wavelength of 350 nm.
We report polar nanostructure and electronic transitions in relaxor ferroelectric Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) single crystals around morphotropic phase boundary (MPB) region by variable-temperature (80–800 K) photoluminescence (PL) spectra and low-wavenumber Raman scattering (LWRS). The discontinuous evolution from peak positions and intensity of luminescence emissions can be corresponding to formation of polar nanoclusters and phase transitions. Six emissions have been derived from PL spectra and show obvious characteristics near phase transition temperatures, which indicates that PL spectral measurement is promising in understanding the microcosmic mechanism. The Raman mode at 1145 cm⁻¹ indicates that temperature dependent luminescence phenomena can be modulated by thermal quenching.
