Intense green-yellow electroluminescence from Tb +-implanted silicon-rich silicon nitride / oxide light emitting devices

High optical power density of 0.5 mW/cm2, external quantum efficiency of 0.1%, and population inversion of 7% are reported from Tb+-implanted silicon-rich silicon nitride/oxide light emitting devices. Electrical and electroluminescence mechanisms in these devices were investigated. The excitation cross section for the 543 nm Tb3+ emission was estimated under electrical pumping, resulting in a value of 8.2 × 10−14 cm2, which is one order of magnitude larger than one reported for Tb3+:SiO2 light emitting devices. These results demonstrate the potentiality of Tb+-implanted silicon nitride material for the development of integrated light sources compatible with Si technology.

The incorporation of rare earth (RE) ions in a silicon dioxide (SiO 2 ) matrix sparked the race toward recent routes for developing efficient Si-based light sources compatible with complementary metal-oxide-semiconductor (CMOS) technology.Such light emitters seem an appealing solution to circumvent the microelectronic bottleneck nowadays, for instance, heat dissipation and interconnection problems. 1ther promising approaches toward this aim are Sinanocrystals (Si-ncs) embedded in a SiO 2 matrix (SiO x ) 2 and also co-doped with RE ions. 3 Published works run from those covering the visible range with RE ions, such as Ce 3þ , 4 Eu 3þ , 5 and Tb 3þ , 6 to those only focusing on the near infrared region with Nd 3þ (Ref.7) and Er 3þ (Ref.3) ions.However, little attention has been paid to the electroluminescence (EL) properties of those RE ions embedded either in silicon nitride (Si 3 N 4 ) or silicon-rich silicon nitride (SiN x ) dielectrics.In fact, SiN x -based materials offer considerable advantages over SiO x -based active ones: (i) lower carrier injection barrier at the Si/Si 3 N 4 interface, giving rise to the fabrication of low-voltage light emitting devices (LEDs), 8 (ii) better electrical stability with direct impact on the device operation lifetimes, 9 and (iii) higher effective refractive index for making high quality resonant cavities. 10Also, we have recently demonstrated the suitability of SiN x , in combination with SiO 2 tunnel layers, as promising candidate for the silicon-solid state lighting. 11Thus, an encouraging scenario toward RE-doped SiN x LEDs is envisaged.We are only aware of Er-doped SiN x electroluminescent devices, where the potentiality of this material for the engineering of light emitters at 1.5 lm was reported. 12Therefore, SiN x codoped with RE ions, whose radiative electronic transitions are in the visible range, remains practically unexploited from the electroluminescent viewpoint.Particularly, the most efficient RE ion, 13 namely, Tb 3þ , is an unexplored candidate embedded in SiN x for evaluating its electro-optical properties aimed for the realization of integrated, electrically driven Si-based light emitters.
In the present work, the investigation on the nature of electrical and EL mechanisms in Tb 3þ :SiN x /SiO 2 LEDs is reported.We demonstrate high optical power density as high as 0.5 mW/cm 2 and we estimate the effective Tb 3þ excitation cross section as well as the fraction of inverted Tb 3þ ions.The possibility to obtain integrated Si-based light sources by introducing Tb 3þ ions in a SiN x matrix is established.30 nm-thick SiO 2 and 40 nm-thick SiN x layers were deposited on highly doped n-type Si (100) substrates with resistivity of 1-5 mX cm using plasma enhanced chemical vapor deposition (PECVD).An annealing treatment at 1000 C for 60 min to create the Si-ncs in the silicon nitride layer with 12% of Si excess was performed.The incorporation of Si-ncs was conceived to improve the conductivity and the device operation lifetime. 9Subsequently, the silicon-rich silicon nitride layer was implanted with Tb þ ions, whose implantation energy (75 keV) and dose (1.9 Â 10 15 atoms/cm 2 ) were adjusted to generate a concentration of nearly 1.5% in the depth range of 20-25 nm.The implantation was followed by post annealing procedure at 900 C for 30 min in N 2 atmosphere to mitigate defects caused by implantation.On a reference sample, with identical structure and technological parameters but introducing Er 3þ (a somewhat similar ion in terms of their atomic radii and diffusivity coefficients) instead of Tb 3þ ions, around 93% of the total ions quantity was determined by secondary ion mass spectroscopy profile into the SiN x layer.Likewise, no Er 3þ clusters were detected by energy-filtered transmission electron microscopy images.The preferred gate electrode was a 100 nm transparent indium-tin-oxide (ITO) layer (n-type) deposited by rf sputtering.Different circular areas with sizes ranging from 100 lm to 1200 lm diameter were defined by conventional lithography and fabricated.A cross section scheme of the a) Electronic mail: yberencen@el.ub.edu 0003-6951/2013/103(11)/111102/4/$30.00 obtained devices is shown in Fig. 1(a).Quasi-static currentvoltage characteristics (I-V) were measured using a semiconductor device analyzer (Agilent B1500A) connected to a probe station (Cascade Microtech Summit 11000) with a Faraday cage.The EL spectra were collected from the top of the devices using a cryogenically cooled Princeton Instruments Spec-10-100B/LN charge-coupled device attached to an Acton 2300i grating spectrometer.Timeresolved EL experiments were conducted using an Agilent 8114A pulse generator, whereas the decay EL traces were recorded with a digital GHz oscilloscope connected to a calibrated photomultiplier (R928).A 1 kX resistance in serial connection to back contact of our devices was used for measuring the current passing through it.Both I-V and EL measurements were acquired at room temperature by positively biasing the n-type ITO electrode, as sketched in Fig. 1(a).
Fig. 1(b) shows the evolution of the current density (J) with the applied voltage (V) ramped at a constant rate of 50 mV/s from 0 V up to just before device breakdown, which occurs at around 41 V.The constant-current density zone of the curve constitutes a measure of the capacitance of the structure C ¼ J d SðdV=dtÞ À1 , which is around 45 pF, where J d is the displacement current density and S the electrode area.The J-V curve also shows a good concordance with two well-defined conduction mechanisms depending on the voltage values, after the threshold for real current density injection (i.e., for current density typically >J d , V $ 21 V).At moderate voltages, the Poole-Frenkel (PF) mechanism 14 governs the conduction (red solid line), whereas at high voltages the trap-assisted tunneling 15 (TAT) one is predominant (blue dashed dotted line).Hereafter, we will only pay attention to this latter mechanism because the Tb 3þ emission takes place solely under this regime, concretely the EL turn on point is at (31.5 V; 64 lA/cm 2 ) as can be seen in Fig. 1(b).This fact also suggests that the Tb 3þ excitation is primarily generated by impact ionization by hot electrons in the conduction band, rather than, for instance, by energy transfer from Si-ncs to Tb 3þ ions. 3Thus, the electron injection is exclusively limited by the energetic level of the traps situated at the SiO 2 band gap close to the Si/SiO 2 interface.In particular, the trap is energetically placed at around U t ¼ (2.0 6 0.3) eV below the oxide conduction band edge, that is in good agreement with TAT formalism. 15This value was extracted from the experimental data fitting to the TAT relationship given by J TAT / expðÀ4d ox ffiffiffiffiffiffiffiffi 2m Ã p U 3=2 t =3q hV ox Þ, where d ox is the SiO 2 thickness, m Ã is the effective electron mass, U t is the trap energy (in eV) below the oxide conduction band edge, q is the electron charge, h is the reduced Planck's constant, and V ox is the voltage drops in the SiO 2 layer.The V ox relationship with the applied voltage (V) is given by V ox ¼ Vd ox =½d ox þ ðe ox =e SiN x Þd SiN x , where d SiNx is the SiN x thickness, as well as e ox ¼ 3.9 and e SiNx ¼ 7.5 are the relative permittivity of the SiO 2 and SiN x , respectively.This conduction mechanism is related to the intrinsic traps created close to the SiO 2 /Si-substrate interface due to electric stress generated by effects of high electric field. 15The introduction of the SiO 2 layer was ad hoc conceived for boosting the Tb 3þ EL by hot electrons.Indeed, this SiO 2 layer introduces a potential energy step of $1.3 eV at the interface between the SiO 2 and the Si 3 N 4 due to the difference in electron affinities (i.e., SiO 2 ¼ 0.8 eV and Si 3 N 4 ¼ 2.1 eV) which offers the possibility of hot electrons injection into the Tb 3þ :SiN x active layer.In addition, the SiN x /SiO 2 bilayer stack was envisaged for achieving a trade-off between EL efficiency and electrical stability of the devices.
Four emission bands in the visible range under direct current pumping were observed.These bands are related to the radiative electronic transitions of Tb 3þ from 5 D 4 to 7 F 6 , 7 F 5 , 7 F 4 , and 7 F 3 energetic states, 13 respectively, as shown in Fig. 2(a).The line-shape of the measured spectra for different constant current values does also not change (i.e., the emission lines preserve their relative intensities).Moreover, a broad EL band peaking at 465 nm from non-implanted Tb þ reference device was detected.In former published works, this emission was ascribed to Si-related defects in silicon nitride. 16However, this electroluminescence was not clearly observed in the studied devices due to the overlapping with the intense Tb 3þ emission.Fig. 2(b) depicts a linear dependence of the optical power density with respect to the injected current density.An optical power density as high as 0.5 mW/cm 2 is reached at the highest current density, where a light saturation is clearly observed on the graph.This is the largest optical power density value ever reported for Tb þimplanted SiN x LEDs.Moreover, an external quantum efficiency (EQE) of 0.1% is obtained.This EQE value is also the highest ever reported for RE in SiN x matrices, that is comparable with the one obtained for Er 3þ :SRO LEDs. 17trong green-yellow emission coming from the top of the devices is observed to naked eye under daylight conditions as well as very stable over hours.This green-yellow light is ascribed to the main Tb 3þ radiative electronic transition ( 5 D 4 -7 F 5 ) at 543 nm, which is responsible of the 58% of the overall emission in our devices.In the inset of Fig. 2(b), a photograph collected from the emission area of the device by a commercial digital camera demonstrates this latter fact.This emission line is also close to the wavelength of 555 nm, where the human eye sensitivity is maximal. 18Therefore, taking advantage of the high optical power density at the most suitable wavelength, an encouraging scenario can be envisaged for the development of silicon-based solid state lighting.A detailed photometric study, including color quality, color rendering, and luminous efficacy of radiation, is currently in progress in this materials platform.
Under alternate current pumping, the electrical excitation cross section (r) of the Tb 3þ at 543 nm was calculated by measuring the EL rise time (s rise ) at different excitation square pulses.Fig. 3 shows that the EL decay time (s decay ) remains constant with respect to the charge flux (/), whereas the EL rise time drops as the charge flux increases.This behavior is consistent with a two-level system at low fluxes, where r does not depend of the injected electron flux. 19ence, using the expression 1=s rise ¼ r/ þ 1=s decay , we can deduce the electrical cross section of the Tb 3þ 543 nm emission by a linear fit. 12Prior to that, both EL rise and decay times were well-fitted by a single exponential function.The EL rise time values at different charge fluxes range from 0.5 ms to 0.7 ms, whereas the EL decay time was found to be 0.5 ms, which is close to the one measured under optical pumping in Tb 3þ :SiO 2 films. 20The inset of Fig. 3 depicts the linear relationship between the inverse rise time and charge flux.The charge flux was calculated taking into account the measured current through 1 kX resistance in serial connection to back contact of our devices instead of using the injected current deduced from the applied voltage.Therefore, the Tb 3þ excitation cross section at 543 nm was found to be (8.2 6 0.5) Â 10 À14 cm 2 , that is one order of magnitude larger than one reported for Tb þ -implanted SiO 2 light emitting devices under electrical pumping. 6As a consequence, the probability of Tb 3þ excitation in silicon nitride is higher than the silicon dioxide matrix.
In order to estimate the fraction of inverted Tb 3þ ions (N 2 ) in the main radiative electronic transition ( 5 D 4 -7 F 5 ) at 543 nm, we used the following formula: 17 N 2 ¼ D opt Á s rad = h-Á d, where D opt is the emitted optical power density, s rad is the Tb 3þ radiative lifetime, and h-is the photon energy.Therefore, the maximum emitted optical power density can be calculated considering the following circumstances: (i) the number of introduced Tb 3þ ions is 4.8 Â 10 20 at/cm 3 , which corresponds to a maximum power density value of 4.8 Â 10 20 photons/cm 3 and (ii) a Tb 3þ radiative lifetime of 3.5 ms.This Tb 3þ radiative lifetime value was chosen taking into account that had been reported for two different host matrices, 21,22 which suggests to be not strongly sensitive to the matrix-type.Taking the measured EL decay time value of 0.5 ms would neglect all Tb 3þ ions that are excited, but will relax in a non-radiative way.Then, the total internal emitted power density is 0.2 W/cm 2 .However, the fraction of emitted light able to escape from electrode and collected by our microscope objective with 0.4 of numerical aperture was found to be 4%.The collection angle (23.5 ) related with the numerical aperture of the used microscope objective is lesser than the critical angle (30 ) associated to the total internal reflection phenomena inside the active layer.Additionally, 91% of the emitted light is transmitted at 543 nm through the 100 nm ITO electrode.Therefore, the out-coupling efficiency is around 3.6%.Hence, taking into account these corrections the external optical power density is 7.2 mW/cm 2 .Now, comparing this latter value with the maximum measured optical power density (0.5 mW/cm 2 ) from our device, we get an optical power density ratio of about 7%, which corresponds to the fraction of inverted Tb 3þ ions at steady state.This value is still far from 50% that is the minimum required to achieve lasing action.The precedent calculation had not been reported before for Tb þimplanted SiN x /SiO 2 light emitting devices, even though the results are still lower than those previously published for Er 3þ :SRO-based devices, where 20% of inverted Er 3þ ions was reported. 17n conclusion, Tb þ -implanted SiN x /SiO 2 light emitting devices with optical power density as high as 0.5 mW/cm 2 and EQE of 0.1% were fabricated.The EL mechanism of the devices can be ascribed to impact ionization of the Tb 3þ luminescent centers by hot electrons with cross section of 8.2 Â 10 À14 cm 2 .A Tb 3þ population inversion of 7% under electrical pumping was estimated.The suitability of this materials platform for the realization of integrated Si-based light emitters fully compatible with CMOS technology was demonstrated.

FIG. 1 .
FIG. 1.(a) Cross section scheme of the Tb 3þ :SiN x /SiO 2 light emitting device.(b) J-V characteristic of the device showing both fitting models related with Poole-Frenkel and trap-assisted tunneling conduction mechanisms at moderate and high voltages, respectively.

FIG. 2 .
FIG. 2. (a) Electroluminescence spectra at different constant current values and several Tb 3þ electronic transitions from 5 D 4 to 7 F 6 , 7 F 5 , 7 F 4 , and 7 F 3 , respectively.(b) Optical power density versus current density.Inset shows green-yellow emission at naked eyes.