Intense electroluminescence (EL) from Tb3+ ions in the Al2O3/Tb2O3 nanolaminate films is achieved in a metal-oxide-semiconductor structure fabricated on silicon, utilizing atomic layer deposition. Precisely controlling of the nanolaminates enables the study on the influence of the atomic Tb layers and the distance between every dopant layers on the EL mechanism. The EL intensity decreases with excessive Tb dopant cycles due to the reduction of optically active Tb3+ ions. Cross-relaxation among adjacent Tb2O3 dopant layers depopulates the excited ions in 5D3 level and contributes to the green EL from 5D4 level, which strongly depends on the Al2O3 sublayer thickness with a critical value of ~3 nm. The 543 nm green EL from Tb3+ ions shows maximum power density of 3.37 mW cm−2 and external quantum efficiency up to 0.73%. Further promotion of efficiency is realized by adopting thicker luminescent layer and Al2O3 cladding layer.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Electroluminescence (EL) from the light-emitting devices based on rare earth (RE)-doped dielectrics and semiconductors has stimulated worldwide interest over the years, aiming at achieving optoelectronic integration on silicon [1,2]. The RE3+ ions incorporated into various hosts emit sharp and wavelength-stable light in a wide spectral range, originating from the intra-4f transitions [3–5]. Moreover, as the RE-related emissions are considerably insensitive to the local environment, there is no stringent requirement on the crystalline quality of host materials. Contemporary optoelectronic devices based on GaInN materials suffer from a steep drop of efficiency when the emission wavelength is designed within the green region, which is well known as “green gap” . In this context, EL from the terbium (Tb)–incorporated films is of special interest because of its intense green emission at ~540 nm, which has already been widely used in organic light emitting devices. So far there have been several reports on the EL from Tb-doped materials on silicon [7,8]. Berencen et al. reported the green EL device based on the Tb-implanted SiNx films on SiO2/Si . Rebohle et al. demonstrated strong EL from the metal-oxide-semiconductor light emitting devices (MOSLEDs) composed of SiO2-Tb2O3-Al2O3 multi-layers with the power efficiency up to 0.15% . However, these prototype devices are far from practical application.
Al2O3 is a technologically important material due to its relatively high refractive index and dielectric constant, which allows well light confinement in the optical waveguide . In addition, the good adhesion to silicon surface and high stability make Al2O3 attractive in the Si microelectronics and optoelectronic applications. Up till now, there are few reports on the EL from the devices with Tb-doped Al2O3, mainly due to the insulating property which impede efficient carrier injection and transport. However, the realization of green EL from Tb3+ ions in the Al2O3 film can be in principle expected, which has great potential in applications such as displays and lighting [12,13]. In this work, we fabricate the efficient Al2O3/Tb2O3 MOSLEDs by atomic layer deposition (ALD). The devices mainly emit green light centered at ~543 nm, together with weak blue emissions, which origin from the impact-excited Tb3+ ions by hot electrons. The EL intensities, decay dynamics and excitation cross sections are studied concerning variations of the Tb dopant cycles and the Al2O3 sublayer thickness, by precisely controlling of the nanolaminate structures utilizing ALD technique. The influence of cross relaxation on the EL emissions originating from different excited states are explored. This work highlights the possibility to use ALD for the dopant design of nanolaminate materials for future development of silicon-based optoelectronic devices from oxides doped with RE ions.
2. Experimental details
The Al2O3/Tb2O3 films were grown on <100>-oriented phosphorous-doped silicon (n-Si) substrates with a resistivity of 2-5 Ω·cm. Prior to the growth, the Si substrates were cleaned through the standard RCA process. The ALD equipment is a 4-in. chamber system (NanoTech Savannah 100, Cambridge). Trimethylaluminum [TMA, Al(CH3)3] and Tb(THD)3 (THD = 2,2,6,6-tetramethyl-3,5-heptanedionate) were used as the metal precursors for Al2O3 and Tb2O3, respectively, while ozone was used as oxidant. During the ALD process, the TMA source was maintained at room temperature, while the Tb precursor and the precursor delivery lines were heated at 190 °C, the substrates were maintained at 350 °C. N2 was used as the carrier and purge gas with a flow rate of 20 sccm. The growth chamber was firstly evacuated to a base pressure of 0.26 Torr. One Al2O3 or Tb2O3 cycle consist of 0.015 s TMA or 2 s Tb(THD)3 pulse, 5 s N2 purge, 1.8 s ozone pulse, and 9 s N2 purge.
For exploration of the luminescent Al2O3/Tb2O3 film, two different series of devices were fabricated. Firstly, the ALD growth contained 4 Al2O3 cycles, and various Tb2O3 cycles including 1, 2, 4 and 6, in order to investigate the effect of dopant quantity on the EL property. Secondly, the Tb2O3 cycles were fixed at 2, while the cycles of Al2O3 varied from 2 to 100, corresponding to a Al2O3 sublayer of 0.15 to 7.43 nm. The corresponding experimental parameters are summarized in Table 1. The thickness of the films was measured by an ellipsometer with a 632.8 nm He-Ne laser beam at an incident angle of 69.8°. Based on our former results, the growth velocity maintains at around 0.743 Å per cycle for Al2O3, while that for Tb2O3 is slower and susceptible to the surface condition. Thus the growth rate of Al2O3 and the final thickness of the films are used to calculate the Tb2O3 thickness and corresponding Tb doping concentration. The films were subsequently annealed at 800 °C in N2 atmosphere for 1 h to reduce defects and enable Tb activation. X-ray diffraction patterns (D /max 2500/pc, Rigaku, Cu Kα radiation, λ = 1.5406 Å) of the annealed films exhibit no diffraction peaks (not shown herein), indicating the amorphous Al2O3 matrix and no apparent TbOx segregation. Afterwards, ~100 nm TiO2/Al2O3 multilayer films composed of ~1.94 nm Al2O3 and 8.0 nm TiO2 sublayers were deposited by ALD onto the Al2O3/Tb2O3 films as the dielectric layer to enhance the device stability. ~100 nm thick ZnO:Al2O3 films were deposited onto the TiO2/Al2O3 films by ALD, which were lithographically patterned into circular dots with a diameter of 0.5 mm. Then ~100 nm thick Al films were deposited on the backsides of n-Si substrates by heat evaporation, resulting in MOSLEDs with a multi-layer structure of ZnO:Al/TiO2-Al2O3/Al2O3-Tb2O3/Si/Al.
To activate EL from the MOSLEDs, appropriate forward bias was applied with the negative voltage connecting to n-Si substrates. Negative biases feature a lower EL at higher voltages. EL and Current-Voltage (I-V) characteristics were recorded by means of a Keithley 2410 SourceMeter unit. The EL signal was collected by a 0.5 m monochromator and detected by a Si detector connected to a computer controlled Keithley 2010 multimeter. The absolute EL power from the device surface was measured using a calibrated Si detector connected with a picoammeter. Photographic images were taken by a digital camera through the ocular of a microscope. The decay times of the EL emissions were measured by a SR430 multi-channel scaler (Stanford Research Systems) equipped with a waveform generator (DG5072, RIGOL) and a high-voltage amplifier. All measurements were performed at room temperature.
3. Results and discussion
The top-right inset in Fig. 1 presents a schematic diagram manifesting the structure of the luminescent Al2O3/Tb2O3 films fabricated with x Al2O3 sub-cycles and y Tb2O3 sub-cycles. Figure 1(a) shows the EL spectra from the MOSLEDs based on the Al2O3/Tb2O3 films with different Tb dopant cycles. The EL spectra mainly contain several peaks at the wavelengths of 488, 543, 587, and 623 nm, which origin from the intra-4f transitions of 5D4→7FJ (J = 6, 5, 4, and 3, respectively) in Tb3+ ions . The green light is easily seen by naked eyes, as shown in Fig. 1(b). These images were taken by a digital camera from the MOSLEDs in which the Al2O3:Tb2O3 sub-cycle ratio is 4:2, at different injection currents. The whitish color for higher currents is due to the saturation of the camera chip. Moreover, there are much weaker violet and blue EL peaks at 380, 415, 437 and 458 nm which are attributed to the 5D3→7FJ transitions. Note that the EL intensity of these violet and blue peaks are multiplied by 50 for clarity. In comparison, emissions from the 5D3 level monotonically decrease with the Tb doping concentration, while EL from the 5D4 level shows the strongest peak at higher doping concentration (Al2O3:Tb2O3 = 4:2, 2.5 at%). Thus the higher 5D3 level is more susceptible to concentration quenching.
Figure 2(a) demonstrates the dependence of representative green (543 nm) and blue (437 nm) EL intensities, together with the current densities, on the applied voltages for the MOSLEDs based on different Al2O3/Tb2O3 films. All devices feature the typical I-V characteristic of a MOS structure, which start with a low background current for low electric fields, continues with approximately exponential increase of the current, and ends with a dielectric breakdown . Higher Tb-doping concentration leads to less prominent increase of the current on voltage. The blue emissions are roughly three orders weaker than the green ones, also with a ~20 V larger threshold voltage. After the threshold, both the current densities and EL intensities increase exponentially with the applied voltages until the breakdown. Figure 2(b) shows the dependence of the EL intensities on the current densities for different MOSLEDs. Generally, with the increase of Tb concentration, the EL intensities gradually decrease under the same current density. The threshold currents for the green EL are around 0.1 μA (0.05 mA cm−2), and the blue EL from higher excited state 5D3 starts at higher current of 1-10 μA (0.5-5 mA cm−2). The EL intensities all increase linearly with the current densities except that the green EL peaks begin to saturate above 80 V (50 mA cm−2), while the blue emissions continue to increase. The explanation is that at higher current density (the equivalent of higher electric field), the energy distribution of electrons in the conduction band of Al2O3 shifts to higher energies, which in turn alters the excitation probability in favor of higher energetic states. Green EL with an optical power up to 6.62 μW (power density of 3.37 mW cm−2) was measured from the MOSLED based on the Al2O3/Tb2O3 film with 2.5 at% Tb concentration, at 1.4 mA (0.71 A cm−2) and 88.7 V.
Figure 3(a) illustrates the decay curves of the 543 nm EL emissions from the MOSLEDs with different Al2O3/Tb2O3 films, all of which are nearly in the form of exponential decay function. With the increase of Tb doping concentration, the decay time of the 543 nm emission firstly decreases from 1.14 ms to 0.95 ms, then drastically drops to 0.32 ms for the device with the highest Tb-concentration. These radiative lifetimes are somewhat lower than that of 1.0-1.4 ms for Tb3+ ions in SiO2 matrix . It should be mentioned that the luminescent films are amorphous with no crystalline shown in the X-ray diffraction patterns. Since small agglomerates are formed in the beginning of the film growth in many ALD processes, it is speculated that excessive dopant cycles lead to the growth of Tb clustering in the deposition and annealing, which is responsible for the reduction of the luminescent Tb3+ ions [15,16]. Apparently the Tb-clustering due to excess Tb cycles serves as the main limitation when attempt to enhance the EL intensity by higher doping concentration via thicker Tb layer in the nanolaminate films deposited by ALD.
As mentioned above, the blue emissions from higher 5D3 energy level have a higher threshold voltage than the green ones from the 5D4 level, and the green EL presents saturation while the blue one increases further with the current. Moreover, the intensity ratio between the 543 and 437 nm peaks increases with the voltage up for all the devices. These phenomena indicate that the emissions originate from the direct impact excitation of Tb3+ ions by hot electrons in the conduction band of Al2O3 matrix. The conduction mechanism has been proved to be Poole-Frenkel (P-F) conduction mode, in which the conduction mainly corresponds to the electrical field-enhanced emission of electrons from traps in the dielectric layer [17–19]. In simplicity, the plot of the ln(J/E) versus E1/2 presents linear relationship in the EL-enabling voltages (J is the current density and E is the electric field). As presented in Fig. 3(b), well-defined linearity is established for all the MOSLEDs in the EL-enabling voltages. Therefore, the EL emissions from Al2O3/Tb2O3 are governed by the P-F mechanism: the electrons tunnel and transport into the conduction band of Al2O3 by virtue of the trap states under sufficient electric field. The injected electrons are accelerated by electrical field to reach an equilibrium energy distribution, which afterwards excite the Tb3+ ions through inelastic impaction, the following de-excitation process results in the characteristic EL.
By changing the cycle numbers of Al precursor in ALD growth, the Al2O3 sublayer thickness in the nanolaminate film is conveniently adjusted. Figure 4(a) presents the dependence of the 543 nm EL intensities, together with the current densities, on the applied bias voltages for the MOSLEDs based on the Al2O3/Tb2O3 films with different Al2O3 sublayers. The devices feature somewhat diversity in the I-V characteristics. The current density declines with lower Tb2O3 ratio, thus Tb dopant induces defect states into Al2O3 matrix which contributes to the carrier injection. The EL intensity increases with the doping concentration with the maximum at 2.5 at% Tb dopant (Al2O3:Tb2O3 = 4:2), a higher doping is adverse to the EL performance because of the cross-relaxation and concentration quenching.
For clarity, the dependence of the EL decay time and EL intensity from single Tb2O3 dopant layer on the Al2O3 sublayer thickness are compared in Fig. 4(b), for both the 543 nm and 437 nm EL at a constant current of 50 μA (25.5 mA cm−2). In evaluation of the contribution of a single Tb2O3 dopant layer on EL intensity, the green emission tends to decrease gradually as the Al2O3 sublayer thickness decreases and drops significantly with the thickness ranging from 3 nm to 0.15 nm, while the blue one maintains decreasing. The EL decay time of the 543 nm emission is much higher than that of the 437 nm emission, and the former stays nearly constant at 0.7-0.8 ms while the latter keeps decreasing drastically with the Al2O3 thickness decreasing (higher doping concentration), from 110 μs for the 0.1 at% device to less than 1 μs for the 4.5 at% one. Apparently the fast non-radiative decay of 5D3 level is prominent and it leads to a higher average population of 5D4 level as the Al2O3 sublayer thickness decreases. These phenomena verify the cross-relaxation from 5D3 to 5D4 levels when the distance between adjacent Tb layers is short enough to trigger this process among Tb3+ ions, depopulating the excited ions in 5D3 level and contributing to the green EL from the 5D4 level, especially in the 0.5-3 nm region. The prominent decrease of the green EL intensity when the Al2O3 sublayer is thinner than 1 nm should be ascribed to the concentration quenching that diminishing the luminescent Tb3+ ions. Similar effect has been previously reported [10,14]. Since the energy difference between these levels approximately equals that between the 7F0 and 7F6 levels, energy can be exchanged between excited ions in 5D3 state and excite another ion in 7F6 ground state. The cross relaxation process depopulates excited ions from 5D3 state to the 5D4 state, giving rise to a quenching of the 5D3 levels and a simultaneous enhancement for the excitation of the 5D4 levels in the thickness range of Al2O3 from 0.5 nm to 3 nm, leading to the different tendency of EL intensity between the green and blue emissions. The decay time of the 437 nm emission continues to decline while for the 543 nm emission its decay time changes slightly, thus the ions in higher 5D3 level is more susceptible to the concentration quenching effect. This is also in agreement with the EL performance of the first series of devices shown in Figs. 1-3.
In comparison with the aforementioned MOSLEDs, this series of devices have the same Tb dopant cycles with the Al2O3 sublayer changed, and the Tb clustering should be not prominent. The cross-relaxation process occurs when the distance between adjacent Tb layers is short enough to trigger the interaction between excited Tb3+ ions. As the contribution of a single Tb2O3 dopant layer on green EL increases significantly with the thickness of Al2O3 ranging from 0.15 nm to 3 nm due to the cross-relaxation, and tends to saturate afterwards. Considering the possible migration of Tb3+ ions in annealing, this is consistent with the reported value of the critical transfer distance for the coupling RE ions [14,20]. In addition, as the thickness of Al2O3 decrease, there is no sufficient acceleration distance which reduces the average energy of the hot electrons under electric field, resulting in the weaker EL emissions. When the Al2O3 sublayer increases to more than 3 nm, the influence of cross relaxation among adjacent Tb3+ ions gradually declines. Moreover, sufficient acceleration under the electric field and meanwhile phonon scatterings within Al2O3 lattice enable the injected hot electrons to obtain the equilibrium kinetic energy, for which reasons the average EL intensity of a single Tb2O3 dopant layer reaches to a saturation value. From the experimental view, the distance for the prominent impact on the green EL due to cross-relaxation in Tb3+ ions, or the sufficient distance for electron acceleration, is around 3 nm in Al2O3 matrix. It is previously reported that to get an equilibrium distribution with enough hot electrons in the case of Tb3+ in SiO2, high electric fields and a minimum acceleration distance of 10-20 nm are needed [21,22]. By comparison, Al2O3 is superior to SiO2 as the matrix for Tb3+ ions in concern of the downscaling potential. The above-mentioned results also show that ALD supplies a convenient way to modify the dopant structure in the RE-doped nanolaminates. Excess atomic-layer of doped RE is inadvisable due to the clustering and consequent quenching effect. For the sublayer thickness of matrix, it should be sufficient to suppress the cross-relaxation counting against the intended emission and gain sufficient acceleration for the injected electrons.
The dependence of the EL intensity on the current density can be simulated by the solution of rate equation. At the steady state, the relationship between the EL intensity and the charge flux can be deduced to: IEL = Imax∙[(J⁄e)/(J⁄e + 1⁄στ)], where Imax is the saturated EL intensity, σ is the cross section for excitation, and τ is the luminescence decay time accounting for both radiative and non-radiative decays . The product of στ can be evaluated by fitting the dependence of EL intensity versus J/e. The decay times of the 543 nm emissions of different devices are used for calculation. The excitation cross sections of the 5D4 level as a function of the Tb concentration including the two series of devices are illustrated in Fig. 5(a). In the first series, the excitation cross section keeps decreasing with the increase of Tb concentration, generally consistent with the tendency of EL intensity shown in Figs. 2(a)-2(b). This is mainly ascribed to the Tb-clustering, causing non-radiative de-excitation and the reduction of the luminescent Tb3+ ions at higher doping concentration. In the second series, the excitation cross section increases initially and then drop with the decrease of the thickness of Al2O3 sublayer. The increase again be interpreted by the cross-relaxation from the 5D3 to 5D4 level, proved by the sharp decline of the decay time of the 437 nm emission. With further increase of the Tb dopant, the increasing of interaction between adjacent Tb3+ ions serves as the main limitation and the excitation cross section declines. The device containing 0.5 at% Tb2O3 (1.49 nm Al2O3 sublayer) has the highest excitation cross section of 1.48 × 10−15 cm2. Moreover, even though the two series of devices have somewhat different luminescent layer thickness, they present a similar tendency in the Tb concentration scale, and the calculated values are close, validating the proposed assumption and calculation.
In order to optimize the EL efficiency from the MOSLEDs based on Al2O3/Tb2O3 films, the thickness of luminescent layer is evaluated, together with the effect of cladding Al2O3 layer. By increasing Al2O3/Tb2O3 layer thickness, the EL threshold voltage increases slightly, while the injection current is strongly suppressed by several orders of magnitude. As the maximum of EL intensity is not apparently affected, a thicker Al2O3/Tb2O3 luminescent layer is more preferable in concern of the external efficiency. Moreover, the addition of the cladding layer of Al2O3 further enhances the abovementioned effect on the I-V characteristics, at the expense of lower EL intensity, which should be ascribed to the limited number of hot electrons to excite Tb3+ ions, as the current density can’t reach to a relatively high value beneath the breakdown voltage.
The power efficiency and external quantum efficiency (EQE) versus the current density are compared in Fig. 5(b). The Tb doping concentrations in the luminescent layers are around the optimal value of 2.5 at%. The EL efficiencies increase with the current densities to a maximum, and then falls down at higher current densities. The device with 70 nm Al2O3/Tb2O3 layer presents an EQE of 0.73%, and a power efficiency of 2.4 × 10−4 at ~20 mA cm−2. The obtained EQE and optical power density are about five times higher than that reported in Ref. 9, while the relatively lower power efficiency compared with SiO2:Tb can be mainly ascribed to the higher defect and trap density for which reason the charge transport is dominated by a P-F injection mechanism. Part of electrons are not effective for the Tb3+ excitations as they lost kinetic energy due to the impaction with defects and traps, which also contribute to the non-radiative de-excitations [14,22]. This is verified by the lower excitation cross section. However, it should be mentioned that the working voltages of these Al2O3/Tb2O3 MOSLEDs are 50-80 V, much lower than that of 150-220 V reported in Ref. 10. The efficiencies of the MOSLED with 50 nm Al2O3/Tb2O3 layer are 5-6 times lower than the aforementioned 70 nm one. While the MOSLED with 100 nm Al2O3/Tb2O3 in middle of 60 nm Al2O3 layer presents a maximum EQE of 4.6% and a power efficiency of 7.1 × 10−4 at ~3.3 mA cm−2. Therefore, high external efficiencies can be achieved through further optimization on not only the thickness of the Al2O3/Tb2O3 luminescent layer but also the cladding layer.
In summary, intense green EL is obtained from the MOSLEDs based on nanolaminate Al2O3/Tb2O3 films fabricated by ALD. The main green peaks originate from the intra-4f 5D4→7FJ transitions of Tb3+ ions. The green EL intensity benefits from the weaker blue ones because the cross-relaxation from the higher excited levels to the lower ones of Tb3+ ions relieves the concentration quenching of the green emissions, resulting in the optimal doping ratio of around 2.5 at% Tb. The EL is attributed to the impact excitation of the Tb3+ ions by hot electrons via P-F mechanism. The distance for the prominent impact on the green EL due to cross-relaxation among Tb3+ ions and the sufficient acceleration of hot electrons in the conduction band, is determined to be ~3 nm in Al2O3. The excitation cross section for the 543 nm Tb3+ emission is up to 1.48 × 10−15 cm2, while the power density reaches to 3.37 mW cm−2. An EQE of 0.73% and a power efficiency of 2.4 × 10−4 are obtained. Further optimization is achieved by adopting a thicker Al2O3/Tb2O3 luminescent layer and Al2O3 cladding layer, resulting in maximum EQE of 4.6% and a power efficiency of 7.1 × 10−4. These results present that ALD supplies a viable approach to modify the dopant structure in the RE-doped nanolaminates to achieve efficient emissions.
National Natural Science Foundation of China (NSFC) (61674085, 61705114); China Postdoctoral Science Foundation (2017M611154); Program 973 (2013CB632102).
References and links
1. J. H. Kim and P. H. Holloway, “Near-Infrared-Electroluminescent Light-Emitting Planar Optical Sources Based on Gallium Nitride Doped with Rare Earths,” Adv. Mater. 17(1), 91–96 (2005). [CrossRef]
2. C. H. Cheng, Y. C. Lien, C. L. Wu, and G. R. Lin, “Mutlicolor electroluminescent Si quantum dots embedded in SiOx thin film MOSLED with 2.4% external quantum efficiency,” Opt. Express 21(1), 391–403 (2013). [CrossRef] [PubMed]
3. O. Jambois, Y. Berencen, K. Hijazi, M. Wojdak, A. J. Kenyon, F. Gourbilleau, R. Rizk, and B. Garrido, “Current transport and electroluminescence mechanisms in thin SiO2 films containing Si nanocluster-sensitized erbium ions,” J. Appl. Phys. 106(6), 063526 (2009). [CrossRef]
4. C. Zhu, C. Lv, C. Wang, Y. Sha, D. Li, X. Ma, and D. Yang, “Color-tunable electroluminescence from Eu-doped TiO2/p+-Si heterostructured devices: engineering of energy transfer,” Opt. Express 23(3), 2819–2826 (2015). [CrossRef] [PubMed]
5. O. Jambois, F. Gourbilleau, A. J. Kenyon, J. Montserrat, R. Rizk, and B. Garrido, “Towards population inversion of electrically pumped Er ions sensitized by Si nanoclusters,” Opt. Express 18(3), 2230–2235 (2010). [CrossRef] [PubMed]
6. T. Langer, A. Kruse, F. A. Ketzer, A. Schwiegel, L. Hoffmann, H. Jonen, H. Bremers, U. Rossow, and A. Hangleiter, “Origin of the “green gap”: Increasing nonradiative recombination in indium-rich GaInN/GaN quantum well structures,” Phys. Status Solidi., C Curr. Top. Solid State Phys. 8(7–8), 2170–2172 (2011). [CrossRef]
7. C. Zhu, C. Y. Lv, M. M. Jiang, J. W. Zhou, D. S. Li, X. Y. Ma, and D. R. Yang, “Green electroluminescence from Tb4O7 films on silicon: impact excitation of Tb3+ ions by hot carriers,” Appl. Phys. Lett. 108(5), 051113 (2016). [CrossRef]
8. L. Rebohle, J. Lehmann, S. Prucnal, M. Helm, and W. Skorupa, “The electrical and electroluminescence properties of rare earth implanted MOS light emitting devices in the near infrared,” J. Lumin. 132(12), 3151–3153 (2012). [CrossRef]
9. Y. Berencen, R. Wutzler, L. Rebohle, D. Hiller, J. M. Ramirez, J. A. Rodriguez, W. Skorupa, and B. Garrido, “Intense green-yellow electroluminescence from Tb+-implanted silicon-rich silicon nitride/oxide light emitting devices,” Appl. Phys. Lett. 103(11), 111102 (2013). [CrossRef]
10. L. Rebohle, M. Braun, R. Wutzler, B. Liu, J. M. Sun, M. Helm, and W. Skorupa, “Strong electroluminescence from SiO2-Tb2O3-Al2O3 mixed layers fabricated by atomic layer deposition,” Appl. Phys. Lett. 104(25), 251113 (2014). [CrossRef]
11. A. J. Kenyon, “Recent developments in rare-earth doped materials for optoelectronics,” Prog. Quantum Electron. 26(4–5), 225–284 (2002). [CrossRef]
12. Y. Yang, Y. P. Li, C. X. Wang, C. Zhu, C. Y. Lv, X. Y. Ma, and D. R. Yang, “Rare-Earth Doped ZnO Films: A Material Platform to Realize Multicolor and Near-Infrared Electroluminescence,” Adv. Opt. Mater. 2(3), 240–244 (2014). [CrossRef]
13. E. H. Penilla, Y. Kodera, and J. E. Garay, “Blue-Green Emission in Terbium-Doped Alumina (Tb: Al2O3) Transparent Ceramics,” Adv. Funct. Mater. 23(48), 6036–6043 (2013). [CrossRef]
14. J. M. Sun, W. Skorupa, T. Dekorsy, M. Helm, L. Rebohle, and T. Gebel, “Bright green electroluminescence from Tb3+ in silicon metal-oxide-semiconductor devices,” J. Appl. Phys. 97(12), 123513 (2005). [CrossRef]
15. A. N. Nazarov, S. I. Tiagulskyi, I. P. Tyagulskyy, V. S. Lysenko, L. Rebohle, J. Lehmann, S. Prucnal, M. Voelskow, and W. Skorupa, “The effect of rare-earth clustering on charge trapping and electroluminescence in rare-earth implanted metal-oxide-semiconductor light-emitting devices,” J. Appl. Phys. 107(12), 123112 (2010). [CrossRef]
16. Y. Y. Choi, K. S. Sohn, H. D. Park, and S. Y. Choi, “Luminescence and decay behaviors of Tb-doped yttrium silicate,” J. Mater. Res. 16(3), 881–889 (2001). [CrossRef]
17. S. M. Sze, Physics of Semiconductor Devices (Wiley, 1981).
18. S. Cueff, J. M. Ramirez, J. A. Kurvits, Y. Berencen, R. Zia, B. Garrido, R. Rizk, and Ch. Labbe, “Electroluminescence efficiencies of erbium in silicon-based hosts,” Appl. Phys. Lett. 103(19), 191109 (2013). [CrossRef]
19. W. Kim, S. I. Park, Z. P. Zhang, and S. Wong, “Current Conduction Mechanism of Nitrogen-Doped AlOx RRAM,” IEEE Trans. Electron Dev. 61(6), 2158–2163 (2014). [CrossRef]
20. N. Krasutsky and H. W. Moos, “Energy Transfer between the Low-Lying Energy Levels of Pr3+ and Nd3+ in LaCl3,” Phys. Rev. B 8(3), 1010–1020 (1973). [CrossRef]
21. M. V. Fischetti, D. J. DiMaria, S. D. Brorson, T. N. Theis, and J. Kirtley, “Theory of high-field electron transport in silicon dioxide,” Phys. Rev. B Condens. Matter 31(12), 8124–8142 (1985). [CrossRef] [PubMed]
22. L. Rebohle, J. Lehmann, S. Prucnal, J. M. Sun, M. Helm, and W. Skorupa, “Physical limitations of the electroluminescence mechanism in terbium-based light emitters: matrix and layer thickness dependence,” Appl. Phys. B 98(2–3), 439–442 (2010). [CrossRef]
23. S. Coffa, G. Franzò, and F. Priolo, “High efficiency and fast modulation of Er-doped light emitting Si diodes,” Appl. Phys. Lett. 69(14), 2077–2079 (1996). [CrossRef]