Electrically tunable electroluminescence from SiNx-based light-emitting devices

doi:10.1364/OE.20.017359

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Electrically tunable electroluminescence from SiNx-based light-emitting devices

Dongsheng Li, Feng Wang, Deren Yang, and Duanlin Que »View Author Affiliations

Optics Express, Vol. 20, Issue 16, pp. 17359-17366 (2012)
http://dx.doi.org/10.1364/OE.20.017359

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▸ Article Outline
– Abstract
1. Introduction
2. Experimental
3. Results and discussion
4. Conclusion
– References and links

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Abstract

Two obvious Gauss peaks are observed in SiN_x-based light-emitting devices with silver nanoparticles deposited onto the luminous layer, both of which are blue shifted with the increase of injected current. The origin of these two peaks is discussed by means of the changes of their positions, relative intensities, and full width at half maximum. We attribute the blue-shift of both electroluminescence peaks to the improvement of carrier injection as carriers can be injected into higher energy levels along their corresponding band tails, which is also confirmed by the changes of the transport mechanism.

1. Introduction

In recent years, a lot of researches have been devoted to the optoelectronic properties of SiN_x film due to its potential application as a candidate of Si-based light sources [1

N.-M. Park, C.-J. Choi, T.-Y. Seong, and S.-J. Park, “Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride,” Phys. Rev. Lett. 86(7), 1355–1357 (2001). [CrossRef] [PubMed]

–7

R. Huang, D. Q. Wang, H. L. Ding, X. Wang, K. J. Chen, J. Xu, Y. Q. Guo, J. Song, and Z. Y. Ma, “Enhanced electroluminescence from SiN-based multilayer structure by laser crystallization of ultrathin amorphous Si-rich SiN layers,” Opt. Express 18(2), 1144–1150 (2010). [CrossRef] [PubMed]

]. Although the fabrication of SiN_x-based light-emitting devices (LEDs) and their electroluminescence (EL) have been achieved by several groups [4

M. Wang, J. Huang, Z. Yuan, A. Anopchenko, D. Li, D. Yang, and L. Pavesi, “Light emission properties and mechanism of low-temperature prepared amorphous SiN_X film. II. Defect states electroluminescence,” J. Appl. Phys. 104(8), 083505 (2008). [CrossRef]

–9

B.-H. Kim, C.-H. Cho, S.-J. Park, N.-M. Park, and G. Y. Sung, “Ni/Au contact to silicon quantum dot light-emitting diodes for the enhancement of carrier injection and light extraction efficiency,” Appl. Phys. Lett. 89(6), 063509 (2006). [CrossRef]

], with the external quantum efficiency of 10⁻²-10^-4% [4

C. Huh, K.-H. Kim, B. K. Kim, W. Kim, H. Ko, C.-J. Choi, and G. Y. Sung, “Enhancement in light emission efficiency of a silicon nanocrystal light-emitting diode by multiple-luminescent structures,” Adv. Mater. (Deerfield Beach Fla.) 22(44), 5058–5062 (2010). [CrossRef] [PubMed]

], the origin of their EL was not actually clear.

Generally, the EL of SiN_x films ranging from ~1.6 to 3.1 eV was attributed to the recombination between the defect states in SiN_x [4

D. Li, J. Huang, and D. Yang, “Enhanced electroluminescence of silicon-rich silicon nitride light-emitting devices by NH₃ plasma and annealing treatment,” Physica E 41(6), 920–922 (2009). [CrossRef]

,10

J. Warga, R. Li, S. N. Basu, and L. Dal Negro, “Electroluminescence from silicon-rich nitride/silicon superlattice structures,” Appl. Phys. Lett. 93(15), 151116 (2008). [CrossRef]

]. However, there’s no further systematic discussion on the detailed defect energy levels of SiN_x film for the origin of its EL. Besides, the increase of injected current could not only enhance the intensity of EL, but also induce the blue-shift of EL peaks from SiN_x-based LED [4

–6

Z. H. Cen, T. P. Chen, Z. Liu, Y. Liu, L. Ding, M. Yang, J. I. Wong, S. F. Yu, and W. P. Goh, “Electrically tunable white-color electroluminescence from Si-implanted silicon nitride thin film,” Opt. Express 18(19), 20439–20444 (2010). [CrossRef] [PubMed]

,11

B.-H. Kim, C.-H. Cho, J.-S. Mun, M.-K. Kwon, T.-Y. Park, J.-S. Kim, C.-C. Byeon, J. Lee, and S.-J. Park, “Enhancement of the external quantum efficiency of a silicon quantum dot light-emitting diode by localized surface plasmons,” Adv. Mater. (Deerfield Beach Fla.) 20(16), 3100–3104 (2008). [CrossRef]

]. Nevertheless, there’s a lack of interpretation on this blue-shift with the increase of applied voltage/injected current [4

,11

]. The only speculation was that holes in SiN_x might gain higher energy under a higher voltage, which resulted in higher energy photons [4

]. However, this explanation might not be sufficient as there were two Gauss peaks at least in SiN_x-based LED actually due to the asymmetry of the EL spectra observed in those works [4

–6

,11

–13

Z. H. Cen, T. P. Chen, L. Ding, Z. Liu, J. I. Wong, M. Yang, W. P. Goh, and S. Fung, “Influence of implantation dose on electroluminescence from Si-implanted silicon nitride thin films,” Appl. Phys., A Mater. Sci. Process. 104(1), 239–245 (2011). [CrossRef]

]. Cen et al. observed these two distinct EL peaks by implanting Si ions into Si₃N₄ films following a high temperature annealing (1100 °C) for 1 h to recover the implantation damages [6

,12

Z. H. Cen, T. P. Chen, L. Ding, Y. Liu, J. I. Wong, M. Yang, Z. Liu, W. P. Goh, F. R. Zhu, and S. Fung, “Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions,” Appl. Phys. Lett. 94(4), 041102 (2009). [CrossRef]

,13

]. The blue-shift of EL peaks by increasing the injected current from their devices is originated from the modulation of the relative intensity of these two peaks [6

]. No shifts of these two EL peaks that observed in other groups were obtained from their devices [12

], which may result from its preferable carrier injection level. Obviously, without post treatment to improve its EL performance, the blue-shift of EL peaks might not be obvious in SiN_x film due to its poor carrier injection level and stability [4

,14

F. Wang, D. Li, D. Yang, and D. Que, “Enhancement of light-extraction efficiency of SiN_x light emitting devices through a rough Ag island film,” Appl. Phys. Lett. 100(3), 031113 (2012). [CrossRef]

]. Metal nanoparticles or island film was commonly used to enhance the carrier injection and improve the stability of EL performance of SiN_x-based LED [11

,14

], which would facilitate the observation of this blue-shift phenomenon.

In this work, Ag island film deposited on the SiN_x luminous layer was employed to improve the EL performance of SiN_x-based LED for the facilitation of observing the changes of EL peaks position. SiN_x-based LED without the addition of Ag island film was also fabricated for comparation. The origin of EL peaks and the blue-shift of them with the increase of injected current are interpreted tentatively. The band diagram is also provided on the basis of predecessors' researches and our works. Our work may provide a deep comprehension on the origin of EL from SiN_x-based LED as well as an alternative approach of tuning its EL wavelength.

2. Experimental

SiN_x film of ~50 nm was deposited onto the p/p⁺-Si(100) substrate by plasma enhanced chemical vapor deposition technical. N₂-diluted 10% SiH₄ and NH₃ were used as the reactant gas sources, where the flow rate ratio of them, the r.f. power density, deposition pressure, and substrate temperature were maintained at 10:1, ~1 kW/m², 0.1 Torr, and 400 °C, respectively. Ag island film with the average radii of ~50 nm was embedded between the ITO top electrode and SiN_x matrix for improving the luminescence performance of our devices. This island film deposited by magnetron sputtering was formed via a rapid thermal annealing process (500 °C, 60 s in Ar atmosphere). The detailed fabrication of SiN_x-based LEDs with and without Ag island film has been described in our previous paper [14

The EL signals of SiN_x-based LEDs were recorded by an Acton SpectraPro-2500i monochromator coupled to a photomultiplier tube (PMT) at room temperature. A Keithley 4200 SCS semiconductor parameter analyzer was used for the measurement of current-voltage (I-V) characteristics of our devices.

3. Results and discussion

In Fig. 1 , the EL spectra of SiN_x-based LEDs with and without Ag island film under different injected currents are presented. Two obvious peaks are observed for the device with Ag island film, as shown in Fig. 1(b). We label the peak with shorter wavelength as P1, while the one with longer wavelength as P2. However there is only one peak (labeled as PR) observed clearly in our reference device, as shown in Fig. 1(a), which may result from its poor electron injection as the barrier of electrons on ITO side (3.0 eV) is much higher than that of holes on p-Si side (1.9 eV) [4

]. Furthermore, a blue-shift of about 0.10 eV (from 2.04 to 2.14 eV) is observed in the device without Ag island film, as shown in Fig. 2(a) . This amount of blue-shift is a little smaller than that of P1 (from 2.23 to 2.45 eV) and P2 (from 1.81 to 1.94 eV), as shown in Fig. 2(b) (left). The smaller amount of blue-shift for PR may result from its higher turn-on voltage as well as higher injected current for the measurement of its initial EL signal, as shown in Figs. 2(a) and 2(b). By comparing the integrated area of P1 and P2, I_P1/I_P2, we can get that P1 is dominated at a lower injected current, while they are almost equivalent at a higher injected current, as shown in Fig. 2(b) (right). It stabilizes at about 1.0 with the increase of injected current, eventually. Furthermore, the full width at half maximum (FWHM) of PR and P1 is much larger than that of P2, as shown in Fig. 2(c). The FWHM of P1 decreases gradually with the blue-shift of its EL peak, while that of P2 is completely opposite.

Fig. 1 EL spectra of the SiN_x-based LEDs with and without Ag island film deposited onto the luminous layer, injected by different currents.

Fig. 2 (a) The dependence of EL peak (PR) position on the injected current for the SiN_x-based LED without Ag island film. (b) The dependence of EL peak positions (left) and I_P1/I_P2 (right) on the injected current for the SiN_x-based LED with Ag island film, where I_P1 and I_P2 stand for the integrated area of the Gauss peak1 (P1) and peak2 (P2), respectively. (c) The dependence of full width at half maximum (FWHM) of EL peaks on the injected current for SiN_x-based LEDs with and without Ag island film. (d) Plots of Δλ vs. λ² accompanied with its linear fittings.

An approximate relationship of Δλ≈λ²ΔE_p/hc can be obtained by simply differentiating λ with respect to the photo energy E_p in the Eq. of λ = c/υ = hc/E_p and representing small intervals (Δ) by differentials. A linear relationship between FWHM (Δλ) and the square of the central wavelength (λ²) is obtained for P1 and P2, as shown in Fig. 2(d). The decrease of the FWHM for P1 with the blue-shift of its EL peak can be explained by this reason. For P2, the opposite phenomenon may result from its improved carrier injection due to the significantly enhanced local electromagnetic field surrounding Ag particles [11

,14

–16

P. Cheng, D. Li, and D. Yang, “Influence of substrates in ZnO devices on the surface plasmon enhanced light emission,” Opt. Express 16(12), 8896–8901 (2008). [CrossRef] [PubMed]

], as more carriers can be injected into a higher energy level.

In order to study the origin of these two EL peaks, J-V characteristic for the device with Ag island film was carried out, as shown in Fig. 3(a) . The structure we investigated here is shown in the inset. The threshold voltage (V_th) of the device with Ag island film is around 2.3 V, which is much lower than that of the device with only a single SiN_x layer ~5.5 V [14

]. This voltage is also a little lower than the electron barrier on ITO side (~3.0 eV) [4

,17

J. R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, and A. Stocking, “Organic electroluminescent devices,” Science 273(5277), 884–888 (1996). [CrossRef] [PubMed]

] for the device investigated here due to the significantly enhanced local electromagnetic field surrounding Ag particles [11

,14

–16

P. Cheng, D. Li, and D. Yang, “Influence of substrates in ZnO devices on the surface plasmon enhanced light emission,” Opt. Express 16(12), 8896–8901 (2008). [CrossRef] [PubMed]

]. This enhancement of the local electromagnetic field can improve the carrier injection or/and shorten the transport path of localized carriers. Consequently, we speculate that the carrier injection under low injected current may result from thermal emission. Hence, we fit the J-V curve by Poole-Frenkel (P-F) model for which the carrier injection is thermally activated [18

E. Jacques, L. Pichon, O. Debieu, and F. Gourbilleau, “Electrical behavior of MIS devices based on silicon nanoclusters embedded in SiO_xN_y and SiO₂ films,” Nanoscale Res. Lett. 6(1), 170 (2011). [CrossRef] [PubMed]

], as shown in Fig. 3(b), like being commonly used in SiN_x matrix [4

,19

Y. Yonamoto, Y. Inaba, and N. Akamatsu, “Compositional dependence of trap density and origin in thin silicon oxynitride film investigated using spin dependent Poole–Frenkel current,” Appl. Phys. Lett. 98(23), 232905 (2011). [CrossRef]

Fig. 3 Plots of (a) J-V; (b) Poole-Frenkel (P-F); and (c) Power law (P-L) based on the SiN_x-based LEDs with Ag island film. Inset of (a) is the schematic diagram of SiN_x-based LED structure with Ag island film.

The P-F equation is given by:

J_{PF} = C E exp {- q [ϕ_{B} - {(q E / п ε_{r} ε_{0})}^{1 / 2}] / k T}

(1)

where J_PF, E, ϕ_B, ε_r, and ε₀ stand for the injected current density, the electric field applied on the active layer, the barrier for P-F emission, relative permittivity, and vacuum permittivity, respectively [20

S. M. Sze, “Current transport and maximum dielectric strength of silicon nitride,” J. Appl. Phys. 38(7), 2951–2956 (1967). [CrossRef]

]. The value of ε_r can be extracted from the linear fitting of ln(J/E) vs. E^1/2. Good agreement is achieved by this P-F conduction mechanism under lower voltages (roughly to 1.7-3.5 V) with the fitting ε_r around 10.9, which is consistent with the value for SiN_x films (between the relative permittivity of a-Si ~11.7 and Si₃N₄ ~7.5) [21

D. A. Neamen, Semiconductor Physics and Devices: Basic Principles, 3rd ed. (McGraw-Hill, 2003).

]. Obviously, the value of threshold voltage V_th is in the range of these fitting voltages, which confirms the speculation on the origin of carrier injection under a lower injected current. Therefore, we conclude that the EL of SiN_x-based LEDs under low current resulted from the recombination of electrons and holes injected by thermal emission from ITO electrode and p-type Si substrate, respectively. With the increase of voltage further, the J-V curve is deviated from this P-F mechanism for the inconsistent of fitting ε_r.

Usually, the carrier transport in SiN_x matrix at high electrical fields is dominated by Fowler-Nordheim (F-N) tunneling mechanism [13

], where ln(J/E²) is proportional to 1/E. The potential barrier height ϕ_Si/SiN between the Si substrate and SiN_x matrix can be extracted from the linear fitting of ln(J/E²) vs. 1/E under a higher voltage. We exclude this mechanism for the carrier transport under a higher voltage due to its extremely small fitting value of this ϕ_Si/SiN (<0.1 eV). To determine the carrier injection mechanism under higher voltages for our SiN_x-based LEDs, we transform the J-V curve to the form of lnJ vs. lnE for its distinct representation of power law (J α Eⁿ) [10

J. Warga, R. Li, S. N. Basu, and L. Dal Negro, “Electroluminescence from silicon-rich nitride/silicon superlattice structures,” Appl. Phys. Lett. 93(15), 151116 (2008). [CrossRef]

,18

,22

A. A. Middleton and N. S. Wingreen, “Collective transport in arrays of small metallic dots,” Phys. Rev. Lett. 71(19), 3198–3201 (1993). [CrossRef] [PubMed]

], as shown in Fig. 3(c). By the linear fitting of power law (P-L), we get the fitting parameter n ~4.5 for lower applied voltages (1.1-3.2 V) and n ~2.0 for higher applied voltages (6.0-11.0 V). From the discussion above, we attribute the carrier injection under low voltage to thermal emission as been confirmed by the fitting of P-F mechanism. While for the region with higher applied voltages (6.0-11.0 V), the conduction mechanism can be attributed to space charge limited conduction (SCLC) dominated by carrier transport between discrete trapping levels as the fitting index n ~2.0 [18

,23

W. Chandra and L. K. Ang, “Space charge limited current in a gap combined of free space and solid,” Appl. Phys. Lett. 96(18), 183501 (2010). [CrossRef]

]. The SCLC Eq. is given by [23

W. Chandra and L. K. Ang, “Space charge limited current in a gap combined of free space and solid,” Appl. Phys. Lett. 96(18), 183501 (2010). [CrossRef]

J = 9 ε_{r} ε_{0} μ E^{2} / 8 d

(2)

where μ and d are the carrier drift mobility and the thickness of the active layer, respectively. Hence, we can extract the value of μ ~4*10⁻⁸ cm²/(V*s) for our SiN_x-based LEDs from the intercept of linear fitting of P-L relationship, as shown in Fig. 3(c). This value of μ is almost two orders of magnitude lower than that for SiN_x films measured by T. Güngör et al. [24

T. Güngör and H. Tolunay, “Drift mobility measurements in a-SiN_x: H,” J. Non-Cryst. Solids 282(2–3), 197–202 (2001). [CrossRef]

], which we attribute to a higher N content in the SiN_x films by comparing the values of the optical band gap of SiN_x films investigated in Ref. 24

T. Güngör and H. Tolunay, “Drift mobility measurements in a-SiN_x: H,” J. Non-Cryst. Solids 282(2–3), 197–202 (2001). [CrossRef]

with those of ours [14

]. Besides, there are more dangling bonds in our SiN_x films than those in Ref. 24

T. Güngör and H. Tolunay, “Drift mobility measurements in a-SiN_x: H,” J. Non-Cryst. Solids 282(2–3), 197–202 (2001). [CrossRef]

due to the high temperature annealing as H will be released during this process. More interfacial states produced by these dangling bonds will lower the carrier drift mobility further as carriers will be trapped or scattered [25

T. Shirasawa, K. Hayashi, S. Mizuno, S. Tanaka, K. Nakatsuji, F. Komori, and H. Tochihara, “Epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface,” Phys. Rev. Lett. 98(13), 136105 (2007). [CrossRef] [PubMed]

]. Consequently, we conclude that the electrons (holes) can be injected into higher levels with the increase of applied voltages by the hopping along the valence (conduction) band tails. Meanwhile, we attribute the gradual blue-shift of EL peaks with the enhancement of injection current to the recombination of electrons and holes with higher energy, which result from the deeper injection of carriers into the band tails.

Besides, from the linear fitting shown in Fig. 3(c), the value of V_TFL where the injected current increases significantly for our SiN_x-based LEDs with Ag island films is obtained at ~4.2 V [18

]. Subsequently, the value of the state density N_t for the trapped carriers can be extracted from the expression as follow [18

N_{t} = 2 ε_{r} ε_{0} V_{TFL} / (q d^{2})

(3)

Employing the values of ε_r and V_TFL obtained from the fittings shown above, we get that the density N_t is on the order of 10¹⁸ cm⁻³ magnitude, which corresponds to the defect tail level 0.9-1.1 eV above the valence band of SiN_x as can be deduced from Ref. 26

J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

. This level, corresponding to the tail state composed by N dangling bands ( = N^-), just lies 2.4-2.6 eV underneath the K center (≡Si^-) [26

J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

], which is consistent with the cut-off wavelength for P1, as shown in Fig. 2(b). Meanwhile, the initial energy of EL peak1 (P1) is about 2.2 eV, which is consistent with the width between the K center and the center of = N^- (located at 1.6 eV above the valence band of SiN_x) [26

J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

]. Thus, the EL for P1 is originated from the recombination of the electrons confined in the K center and the holes located at the band tail formed by = N^-, which can also be observed in references 3

G.-R. Lin, Y.-H. Pai, C.-T. Lin, and C.-C. Chen, “Comparison on the electroluminescence of Si-rich SiN_x and SiO_x based light-emitting diodes,” Appl. Phys. Lett. 96(26), 263514 (2010). [CrossRef]

and 5

The changes on the energy of EL peak2 (P2) are also checked, which have the starting and ending values at ~1.8 and 1.9 eV, respectively. This energy range corresponded to the recombination of the electrons lay on the conduction band tail and the holes localized in the center of ≡Si⁰ [26

J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

]. And the energy interval for P2 ~0.1 eV is a reasonable value for the width of conduction band tail. Consequently, the longer wavelength of EL may result from the recombination of electrons and holes located at the conduction band tail and the center of ≡Si⁰, respectively.

Based on the previous fundamental researches on the luminescence mechanisms and the properties for SiN_x-based LEDs [4

,26

J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

] as well as the detailed discussions above, the band gap structure and carriers recombination schematic of the device is drawn in Fig. 4 . Among the defect levels in SiN_x, ≡Si^- and ≡Si⁰ were considered as the electrons and holes trapped levels, respectively, with 80% of carriers localized on its corresponding central site [26

J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

]. While for carriers on the band tails, this confinement was not so strictly due to the larger distribution widths of state density [26

J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

]. As discussed above, electrons can be injected from ITO electrode to the conduction band tail and ≡Si^- center, while for holes from p-Si substrate to = N^- and the center of ≡Si⁰, through thermal emission under lower voltages. Electrons (holes) would prefer to being injected into the center of ≡Si^- (≡Si⁰) for its lower energy, which lead to the weak intensity of EL under low injected current due to the lower carrier injection onto its corresponding band tails. As the holes injection barrier is much lower than that for electrons, the integrated EL intensity for shorter wavelength (I_P1) is much higher than that for the longer one (I_P2) shown in Fig. 2b (right). It also explains the reason of the only one obvious EL peak (P1 or PR) for the device without Ag island films. The larger FWHM shown in Fig. 2(c) results from its larger width of valence-band tail composed by N dangling bands ( = N^-) and Si-Si bonding states (Si-Si), both of which are recognized as the hole trapped levels [25

]. For higher applied voltages, the injection by thermal emission becomes saturated and the SCLC dominated, subsequently. More carriers can be transported onto the higher energy levels of band tails with the increase of the injected current, which lead to the enhancement in EL intensities and the blue-shift of EL peaks, as shown in Fig. 1(a), Figs. 2(a) and 2(b) (left). For the structure with Ag island films on SiN_x, the injection of electrons from ITO side can be improved by the addition of Ag island films due to the significantly enhanced local electromagnetic field surrounding Ag particles [11

,14

–16

P. Cheng, D. Li, and D. Yang, “Influence of substrates in ZnO devices on the surface plasmon enhanced light emission,” Opt. Express 16(12), 8896–8901 (2008). [CrossRef] [PubMed]

]. Meanwhile, holes injected from p-Si substrate have more freedoms on its corresponding valence-band tails than electrons due to the larger width of valence-band tails [26

J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

]. It decreases the relative number of holes contributed to radiative recombination for central wavelength. Consequently, the integrated EL intensities with longer wavelength (I_P2) is comparable with that of P1 at higher voltages as well as the broadening of P2 with its blue-shift, as shown in Fig. 2(b) (right) and Fig. 2(c).

Fig. 4 Band diagram of the SiN_x-based LED under forward bias.

4. Conclusion

In conclusion, two Gauss peaks (P1 and P2) together with their blue-shift have been observed in our SiN_x-based LED with Ag island films. The peak with shorter wavelength ranging from ~2.2-2.5 eV is originated from the recombination of the electrons confined at the K center and the holes located at the band tail formed by = N^-. While, we attribute the one with longer wavelength (~1.8-1.9 eV) to the recombination of the electrons located at conduction band tail and the holes confined in the center of ≡Si⁰. Electrons (holes) are injected by thermal emission from ITO electrode (p-type Si substrate) under lower applied voltages dominated by P-F conduction mechanism. While for higher applied voltages, electrons and holes are transported along their corresponding band tails which is confirmed by the SCLC model. The blue-shift of EL peaks in our SiN_x-based LEDs results from the enhancement of carrier injection that electrons (holes) can be injected into a higher level of the conduction (valence) band tails. Our work provides a deep comprehension on the origin of the EL of SiN_x-based LEDs. The tailoring of EL wavelength of SiN_x-based LEDs can be achieved by the modulation of the density of defect tails state.

Acknowledgment

The authors thank the National Natural Science Foundation of China (No.61176117), the 863 Program (Grant No. 2011AA050517), the foundation of MOST (Grant No. 2008DFR50250), and the Innovation Team Project of Zhejiang Province (No. 2009R50005) for the financial support.

Reference and links

1.	N.-M. Park, C.-J. Choi, T.-Y. Seong, and S.-J. Park, “Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride,” Phys. Rev. Lett. 86(7), 1355–1357 (2001). [CrossRef] [PubMed]
2.	R. Huang, H. Dong, D. Wang, K. Chen, H. Ding, X. Wang, W. Li, J. Xu, and Z. Ma, “Role of barrier layers in electroluminescence from SiN-based multilayer light-emitting devices,” Appl. Phys. Lett. 92(18), 181106 (2008). [CrossRef]
3.	G.-R. Lin, Y.-H. Pai, C.-T. Lin, and C.-C. Chen, “Comparison on the electroluminescence of Si-rich SiN_x and SiO_x based light-emitting diodes,” Appl. Phys. Lett. 96(26), 263514 (2010). [CrossRef]
4.	M. Wang, J. Huang, Z. Yuan, A. Anopchenko, D. Li, D. Yang, and L. Pavesi, “Light emission properties and mechanism of low-temperature prepared amorphous SiN_X film. II. Defect states electroluminescence,” J. Appl. Phys. 104(8), 083505 (2008). [CrossRef]
5.	D. Li, J. Huang, and D. Yang, “Enhanced electroluminescence of silicon-rich silicon nitride light-emitting devices by NH₃ plasma and annealing treatment,” Physica E 41(6), 920–922 (2009). [CrossRef]
6.	Z. H. Cen, T. P. Chen, Z. Liu, Y. Liu, L. Ding, M. Yang, J. I. Wong, S. F. Yu, and W. P. Goh, “Electrically tunable white-color electroluminescence from Si-implanted silicon nitride thin film,” Opt. Express 18(19), 20439–20444 (2010). [CrossRef] [PubMed]
7.	R. Huang, D. Q. Wang, H. L. Ding, X. Wang, K. J. Chen, J. Xu, Y. Q. Guo, J. Song, and Z. Y. Ma, “Enhanced electroluminescence from SiN-based multilayer structure by laser crystallization of ultrathin amorphous Si-rich SiN layers,” Opt. Express 18(2), 1144–1150 (2010). [CrossRef] [PubMed]
8.	C. Huh, K.-H. Kim, B. K. Kim, W. Kim, H. Ko, C.-J. Choi, and G. Y. Sung, “Enhancement in light emission efficiency of a silicon nanocrystal light-emitting diode by multiple-luminescent structures,” Adv. Mater. (Deerfield Beach Fla.) 22(44), 5058–5062 (2010). [CrossRef] [PubMed]
9.	B.-H. Kim, C.-H. Cho, S.-J. Park, N.-M. Park, and G. Y. Sung, “Ni/Au contact to silicon quantum dot light-emitting diodes for the enhancement of carrier injection and light extraction efficiency,” Appl. Phys. Lett. 89(6), 063509 (2006). [CrossRef]
10.	J. Warga, R. Li, S. N. Basu, and L. Dal Negro, “Electroluminescence from silicon-rich nitride/silicon superlattice structures,” Appl. Phys. Lett. 93(15), 151116 (2008). [CrossRef]
11.	B.-H. Kim, C.-H. Cho, J.-S. Mun, M.-K. Kwon, T.-Y. Park, J.-S. Kim, C.-C. Byeon, J. Lee, and S.-J. Park, “Enhancement of the external quantum efficiency of a silicon quantum dot light-emitting diode by localized surface plasmons,” Adv. Mater. (Deerfield Beach Fla.) 20(16), 3100–3104 (2008). [CrossRef]
12.	Z. H. Cen, T. P. Chen, L. Ding, Y. Liu, J. I. Wong, M. Yang, Z. Liu, W. P. Goh, F. R. Zhu, and S. Fung, “Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions,” Appl. Phys. Lett. 94(4), 041102 (2009). [CrossRef]
13.	Z. H. Cen, T. P. Chen, L. Ding, Z. Liu, J. I. Wong, M. Yang, W. P. Goh, and S. Fung, “Influence of implantation dose on electroluminescence from Si-implanted silicon nitride thin films,” Appl. Phys., A Mater. Sci. Process. 104(1), 239–245 (2011). [CrossRef]
14.	F. Wang, D. Li, D. Yang, and D. Que, “Enhancement of light-extraction efficiency of SiN_x light emitting devices through a rough Ag island film,” Appl. Phys. Lett. 100(3), 031113 (2012). [CrossRef]
15.	W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature 424(6950), 824–830 (2003). [CrossRef] [PubMed]
16.	P. Cheng, D. Li, and D. Yang, “Influence of substrates in ZnO devices on the surface plasmon enhanced light emission,” Opt. Express 16(12), 8896–8901 (2008). [CrossRef] [PubMed]
17.	J. R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, and A. Stocking, “Organic electroluminescent devices,” Science 273(5277), 884–888 (1996). [CrossRef] [PubMed]
18.	E. Jacques, L. Pichon, O. Debieu, and F. Gourbilleau, “Electrical behavior of MIS devices based on silicon nanoclusters embedded in SiO_xN_y and SiO₂ films,” Nanoscale Res. Lett. 6(1), 170 (2011). [CrossRef] [PubMed]
19.	Y. Yonamoto, Y. Inaba, and N. Akamatsu, “Compositional dependence of trap density and origin in thin silicon oxynitride film investigated using spin dependent Poole–Frenkel current,” Appl. Phys. Lett. 98(23), 232905 (2011). [CrossRef]
20.	S. M. Sze, “Current transport and maximum dielectric strength of silicon nitride,” J. Appl. Phys. 38(7), 2951–2956 (1967). [CrossRef]
21.	D. A. Neamen, Semiconductor Physics and Devices: Basic Principles, 3rd ed. (McGraw-Hill, 2003).
22.	A. A. Middleton and N. S. Wingreen, “Collective transport in arrays of small metallic dots,” Phys. Rev. Lett. 71(19), 3198–3201 (1993). [CrossRef] [PubMed]
23.	W. Chandra and L. K. Ang, “Space charge limited current in a gap combined of free space and solid,” Appl. Phys. Lett. 96(18), 183501 (2010). [CrossRef]
24.	T. Güngör and H. Tolunay, “Drift mobility measurements in a-SiN_x: H,” J. Non-Cryst. Solids 282(2–3), 197–202 (2001). [CrossRef]
25.	T. Shirasawa, K. Hayashi, S. Mizuno, S. Tanaka, K. Nakatsuji, F. Komori, and H. Tochihara, “Epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface,” Phys. Rev. Lett. 98(13), 136105 (2007). [CrossRef] [PubMed]
26.	J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett. 44(4), 415–417 (1984). [CrossRef]

OCIS Codes
(230.2090) Optical devices : Electro-optical devices
(300.6280) Spectroscopy : Spectroscopy, fluorescence and luminescence
(310.6860) Thin films : Thin films, optical properties
(350.4600) Other areas of optics : Optical engineering

ToC Category:
Optical Devices

History
Original Manuscript: May 24, 2012
Revised Manuscript: June 25, 2012
Manuscript Accepted: July 10, 2012
Published: July 16, 2012

Citation
Dongsheng Li, Feng Wang, Deren Yang, and Duanlin Que, "Electrically tunable electroluminescence from SiNx-based light-emitting devices," Opt. Express 20, 17359-17366 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-16-17359

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References

N.-M. Park, C.-J. Choi, T.-Y. Seong, and S.-J. Park, “Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride,” Phys. Rev. Lett.86(7), 1355–1357 (2001). [CrossRef] [PubMed]
R. Huang, H. Dong, D. Wang, K. Chen, H. Ding, X. Wang, W. Li, J. Xu, and Z. Ma, “Role of barrier layers in electroluminescence from SiN-based multilayer light-emitting devices,” Appl. Phys. Lett.92(18), 181106 (2008). [CrossRef]
G.-R. Lin, Y.-H. Pai, C.-T. Lin, and C.-C. Chen, “Comparison on the electroluminescence of Si-rich SiNx and SiOx based light-emitting diodes,” Appl. Phys. Lett.96(26), 263514 (2010). [CrossRef]
M. Wang, J. Huang, Z. Yuan, A. Anopchenko, D. Li, D. Yang, and L. Pavesi, “Light emission properties and mechanism of low-temperature prepared amorphous SiNX film. II. Defect states electroluminescence,” J. Appl. Phys.104(8), 083505 (2008). [CrossRef]
D. Li, J. Huang, and D. Yang, “Enhanced electroluminescence of silicon-rich silicon nitride light-emitting devices by NH3 plasma and annealing treatment,” Physica E41(6), 920–922 (2009). [CrossRef]
Z. H. Cen, T. P. Chen, Z. Liu, Y. Liu, L. Ding, M. Yang, J. I. Wong, S. F. Yu, and W. P. Goh, “Electrically tunable white-color electroluminescence from Si-implanted silicon nitride thin film,” Opt. Express18(19), 20439–20444 (2010). [CrossRef] [PubMed]
R. Huang, D. Q. Wang, H. L. Ding, X. Wang, K. J. Chen, J. Xu, Y. Q. Guo, J. Song, and Z. Y. Ma, “Enhanced electroluminescence from SiN-based multilayer structure by laser crystallization of ultrathin amorphous Si-rich SiN layers,” Opt. Express18(2), 1144–1150 (2010). [CrossRef] [PubMed]
C. Huh, K.-H. Kim, B. K. Kim, W. Kim, H. Ko, C.-J. Choi, and G. Y. Sung, “Enhancement in light emission efficiency of a silicon nanocrystal light-emitting diode by multiple-luminescent structures,” Adv. Mater. (Deerfield Beach Fla.)22(44), 5058–5062 (2010). [CrossRef] [PubMed]
B.-H. Kim, C.-H. Cho, S.-J. Park, N.-M. Park, and G. Y. Sung, “Ni/Au contact to silicon quantum dot light-emitting diodes for the enhancement of carrier injection and light extraction efficiency,” Appl. Phys. Lett.89(6), 063509 (2006). [CrossRef]
J. Warga, R. Li, S. N. Basu, and L. Dal Negro, “Electroluminescence from silicon-rich nitride/silicon superlattice structures,” Appl. Phys. Lett.93(15), 151116 (2008). [CrossRef]
B.-H. Kim, C.-H. Cho, J.-S. Mun, M.-K. Kwon, T.-Y. Park, J.-S. Kim, C.-C. Byeon, J. Lee, and S.-J. Park, “Enhancement of the external quantum efficiency of a silicon quantum dot light-emitting diode by localized surface plasmons,” Adv. Mater. (Deerfield Beach Fla.)20(16), 3100–3104 (2008). [CrossRef]
Z. H. Cen, T. P. Chen, L. Ding, Y. Liu, J. I. Wong, M. Yang, Z. Liu, W. P. Goh, F. R. Zhu, and S. Fung, “Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions,” Appl. Phys. Lett.94(4), 041102 (2009). [CrossRef]
Z. H. Cen, T. P. Chen, L. Ding, Z. Liu, J. I. Wong, M. Yang, W. P. Goh, and S. Fung, “Influence of implantation dose on electroluminescence from Si-implanted silicon nitride thin films,” Appl. Phys., A Mater. Sci. Process.104(1), 239–245 (2011). [CrossRef]
F. Wang, D. Li, D. Yang, and D. Que, “Enhancement of light-extraction efficiency of SiNx light emitting devices through a rough Ag island film,” Appl. Phys. Lett.100(3), 031113 (2012). [CrossRef]
W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature424(6950), 824–830 (2003). [CrossRef] [PubMed]
P. Cheng, D. Li, and D. Yang, “Influence of substrates in ZnO devices on the surface plasmon enhanced light emission,” Opt. Express16(12), 8896–8901 (2008). [CrossRef] [PubMed]
J. R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, and A. Stocking, “Organic electroluminescent devices,” Science273(5277), 884–888 (1996). [CrossRef] [PubMed]
E. Jacques, L. Pichon, O. Debieu, and F. Gourbilleau, “Electrical behavior of MIS devices based on silicon nanoclusters embedded in SiOxNy and SiO2 films,” Nanoscale Res. Lett.6(1), 170 (2011). [CrossRef] [PubMed]
Y. Yonamoto, Y. Inaba, and N. Akamatsu, “Compositional dependence of trap density and origin in thin silicon oxynitride film investigated using spin dependent Poole–Frenkel current,” Appl. Phys. Lett.98(23), 232905 (2011). [CrossRef]
S. M. Sze, “Current transport and maximum dielectric strength of silicon nitride,” J. Appl. Phys.38(7), 2951–2956 (1967). [CrossRef]
D. A. Neamen, Semiconductor Physics and Devices: Basic Principles, 3rd ed. (McGraw-Hill, 2003).
A. A. Middleton and N. S. Wingreen, “Collective transport in arrays of small metallic dots,” Phys. Rev. Lett.71(19), 3198–3201 (1993). [CrossRef] [PubMed]
W. Chandra and L. K. Ang, “Space charge limited current in a gap combined of free space and solid,” Appl. Phys. Lett.96(18), 183501 (2010). [CrossRef]
T. Güngör and H. Tolunay, “Drift mobility measurements in a-SiNx: H,” J. Non-Cryst. Solids282(2–3), 197–202 (2001). [CrossRef]
T. Shirasawa, K. Hayashi, S. Mizuno, S. Tanaka, K. Nakatsuji, F. Komori, and H. Tochihara, “Epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface,” Phys. Rev. Lett.98(13), 136105 (2007). [CrossRef] [PubMed]
J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett.44(4), 415–417 (1984). [CrossRef]

Adv. Mater. (Deerfield Beach Fla.)

C. Huh, K.-H. Kim, B. K. Kim, W. Kim, H. Ko, C.-J. Choi, and G. Y. Sung, “Enhancement in light emission efficiency of a silicon nanocrystal light-emitting diode by multiple-luminescent structures,” Adv. Mater. (Deerfield Beach Fla.)22(44), 5058–5062 (2010). [CrossRef] [PubMed]
B.-H. Kim, C.-H. Cho, J.-S. Mun, M.-K. Kwon, T.-Y. Park, J.-S. Kim, C.-C. Byeon, J. Lee, and S.-J. Park, “Enhancement of the external quantum efficiency of a silicon quantum dot light-emitting diode by localized surface plasmons,” Adv. Mater. (Deerfield Beach Fla.)20(16), 3100–3104 (2008). [CrossRef]

Appl. Phys. Lett.

Z. H. Cen, T. P. Chen, L. Ding, Y. Liu, J. I. Wong, M. Yang, Z. Liu, W. P. Goh, F. R. Zhu, and S. Fung, “Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions,” Appl. Phys. Lett.94(4), 041102 (2009). [CrossRef]
F. Wang, D. Li, D. Yang, and D. Que, “Enhancement of light-extraction efficiency of SiNx light emitting devices through a rough Ag island film,” Appl. Phys. Lett.100(3), 031113 (2012). [CrossRef]
B.-H. Kim, C.-H. Cho, S.-J. Park, N.-M. Park, and G. Y. Sung, “Ni/Au contact to silicon quantum dot light-emitting diodes for the enhancement of carrier injection and light extraction efficiency,” Appl. Phys. Lett.89(6), 063509 (2006). [CrossRef]
J. Warga, R. Li, S. N. Basu, and L. Dal Negro, “Electroluminescence from silicon-rich nitride/silicon superlattice structures,” Appl. Phys. Lett.93(15), 151116 (2008). [CrossRef]
R. Huang, H. Dong, D. Wang, K. Chen, H. Ding, X. Wang, W. Li, J. Xu, and Z. Ma, “Role of barrier layers in electroluminescence from SiN-based multilayer light-emitting devices,” Appl. Phys. Lett.92(18), 181106 (2008). [CrossRef]
G.-R. Lin, Y.-H. Pai, C.-T. Lin, and C.-C. Chen, “Comparison on the electroluminescence of Si-rich SiNx and SiOx based light-emitting diodes,” Appl. Phys. Lett.96(26), 263514 (2010). [CrossRef]
Y. Yonamoto, Y. Inaba, and N. Akamatsu, “Compositional dependence of trap density and origin in thin silicon oxynitride film investigated using spin dependent Poole–Frenkel current,” Appl. Phys. Lett.98(23), 232905 (2011). [CrossRef]
W. Chandra and L. K. Ang, “Space charge limited current in a gap combined of free space and solid,” Appl. Phys. Lett.96(18), 183501 (2010). [CrossRef]
J. Robertson and M. J. Powell, “Gap states in silicon-nitride,” Appl. Phys. Lett.44(4), 415–417 (1984). [CrossRef]

Appl. Phys., A Mater. Sci. Process.

Z. H. Cen, T. P. Chen, L. Ding, Z. Liu, J. I. Wong, M. Yang, W. P. Goh, and S. Fung, “Influence of implantation dose on electroluminescence from Si-implanted silicon nitride thin films,” Appl. Phys., A Mater. Sci. Process.104(1), 239–245 (2011). [CrossRef]

J. Appl. Phys.

M. Wang, J. Huang, Z. Yuan, A. Anopchenko, D. Li, D. Yang, and L. Pavesi, “Light emission properties and mechanism of low-temperature prepared amorphous SiNX film. II. Defect states electroluminescence,” J. Appl. Phys.104(8), 083505 (2008). [CrossRef]
S. M. Sze, “Current transport and maximum dielectric strength of silicon nitride,” J. Appl. Phys.38(7), 2951–2956 (1967). [CrossRef]

J. Non-Cryst. Solids

T. Güngör and H. Tolunay, “Drift mobility measurements in a-SiNx: H,” J. Non-Cryst. Solids282(2–3), 197–202 (2001). [CrossRef]

Nanoscale Res. Lett.

E. Jacques, L. Pichon, O. Debieu, and F. Gourbilleau, “Electrical behavior of MIS devices based on silicon nanoclusters embedded in SiOxNy and SiO2 films,” Nanoscale Res. Lett.6(1), 170 (2011). [CrossRef] [PubMed]

Nature

W. L. Barnes, A. Dereux, and T. W. Ebbesen, “Surface plasmon subwavelength optics,” Nature424(6950), 824–830 (2003). [CrossRef] [PubMed]

Opt. Express

Phys. Rev. Lett.

N.-M. Park, C.-J. Choi, T.-Y. Seong, and S.-J. Park, “Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride,” Phys. Rev. Lett.86(7), 1355–1357 (2001). [CrossRef] [PubMed]
T. Shirasawa, K. Hayashi, S. Mizuno, S. Tanaka, K. Nakatsuji, F. Komori, and H. Tochihara, “Epitaxial silicon oxynitride layer on a 6H-SiC(0001) surface,” Phys. Rev. Lett.98(13), 136105 (2007). [CrossRef] [PubMed]
A. A. Middleton and N. S. Wingreen, “Collective transport in arrays of small metallic dots,” Phys. Rev. Lett.71(19), 3198–3201 (1993). [CrossRef] [PubMed]

Physica E

D. Li, J. Huang, and D. Yang, “Enhanced electroluminescence of silicon-rich silicon nitride light-emitting devices by NH3 plasma and annealing treatment,” Physica E41(6), 920–922 (2009). [CrossRef]

Science

J. R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon, D. Roitman, and A. Stocking, “Organic electroluminescent devices,” Science273(5277), 884–888 (1996). [CrossRef] [PubMed]

Other

D. A. Neamen, Semiconductor Physics and Devices: Basic Principles, 3rd ed. (McGraw-Hill, 2003).

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