Bright Green Visible Electroluminescence from ...

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Bright Green Visible Electroluminescence from Rare Earth Doped Silicon Rich SiOx T.W. MacElweea, S.E. Hilla, S. Campbella, D. Ducharmea, B. A. Riouxa, and I.D. Caldera, M. Flynnb, J. Wojcikb, S. Gujrathic, and P. Mascherb a

b

Group IV Semiconductor Inc., 400 March Road, Ottawa, ON, K2K 3H4, Canada Centre for Emerging Device Technology, Department of Engineering Physics, McMaster University, Hamilton, Canada c GCM, Lab. René –J.A. Lévesque, Département de Physique, Université de Montréal, Montréal, Canada thickness of ~100nm, and a lower rare earth concentration (up to 0.3at% Tb). Furnace annealing was carried out under flowing argon at temperatures ranging from 900-1200ºC for 30-120 minutes. Characterization of the annealed films is described elsewhere [8]. A transparent top contact was then formed by rf sputtering of indium tin oxide (ITO) from a composite target. Finally, aluminum contact metallization was applied to both the front and back sides of the wafer. The current-voltage characteristics of the device were measured under DC conditions with a HP4145B SPA, and under ac conditions with a HP33120A function generator driving an AVTECH linear power amplifier. Luminescence spectra were captured with an Ocean Optics USB2000 spectrometer operating over the range of 350-850 nm.

Silicon rich silicon oxide was deposited by ECR-PECVD, doped with Er or Tb, and processed into device structures. Electrical measurements were used to characterize conduction mechanisms while spectroscopic electroluminescence provided information on brightness and mechanisms. I.

INTRODUCTION

Among the many physical systems currently under investigation for the generation of light from silicon [1,2], two of the most promising are emission from nanocrystalline silicon embedded in a dielectric matrix [3], and emission from rare earth doped materials [4]. Most investigations have focused on light generation by photoluminescence, particularly infrared luminescence, since the majority of applications have been in telecommunications. However, these materials also emit in the visible spectrum [5], and the combination of nanocrystalline silicon in a silicon oxide matrix with rare earth doping is a promising system for electroluminescent applications [6]. The nanocrystals provide a path for electron conduction and they can efficiently transfer the electronic energy to nearby rare earth ions, which have a much higher solubility in oxide than in silicon itself.

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 − q φ B − qE / πε 0κ  J FP = σ FP E exp  , kT  

EXPERIMENT

We have fabricated visible light emitting structures by depositing in situ (rare earth) doped silicon-rich silicon oxide films onto n+ silicon substrates through electron cyclotron resonance plasma enhanced chemical vapour deposition (ECR-PECVD). Details of film deposition and material characterization have been described elsewhere [7,8]. Subsequently the films were annealed and processed into device structures that could be energized electrically without inhibiting optical emission. The electrical conduction properties of the structures were characterized, along with the electroluminescent (EL) properties, in order to understand the relationships between structure, processing, measurement conditions, and luminescence. All of the Er-doped films were deposited from SiH4, O2, and Er(TMHD)3 onto n+-doped silicon wafers. The film thickness was approximately 80nm, the atomic concentration of silicon ranged from 34-50at%, and the atomic concentration of erbium was varied from 0-10at%. The Tb-doped films were deposited from the same kind of sources (substituting Tb for Er) in a slightly different ECR-PECVD system, but with a

RESULTS

A. Current-Voltage Characteristics A typical I-V curve is shown in Fig. 1. The data are plotted in a form that results in a straight-line dependence for PooleFrenkel (P-F) conduction, in which the current varies as, (1)

where φB is a barrier height, σFP is the P-F conductivity, E is electric field, and κ is the dielectric constant of the material. 1.E-09

Current Density/Field (A/MV-cm)

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Fig. 1. I-V characteristics for DC operation of a Tb doped sample. The film was 93.6nm thick with 1.25% excess Si. The fitted barrier height is 1.15eV.

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For the sample characterized in Fig. 1, the SRSO thickness was measured ellipsometrically to be 93.6 nm and the excess silicon is estimated to be 1.25at% from the index of refraction. Analysis of the I-V data indicates that Poole-Frenkel conduction is dominant with φB ≈1.15 eV. Newly fabricated films that were DC biased (with electron injection either from the substrate or from the ITO) were generally limited to current densities below 100 mA/cm2 before failure. Breakdown occurred along the edges of planar structures due to enhancement of the electric field and charge trapping in this region. It may also arise from inhomogeneity of the film composition that would lead to local electric field concentration. To enable biasing of large structures to high current densities (up to 100A/cm2 peak) and high electric fields, an AC bias scheme was adopted with a sinusoidal drive frequency of 15 kHz. We hypothesize that the overall effect of the trapped oxide charge is reduced using AC bias since electrons are injected from both electrodes and can therefore neutralize any trapped holes and average trapped charges throughout the SRSO film more effectively. The 15 kHz was empirically determined to be the AC frequency that produced the best overall EL performance and ease of measurement. For conduction in the SRSO films, electrons can be injected from either the substrate or ITO interfaces. Under AC bias, there is a small asymmetry observed in current injection, but it diminishes as soon as a small amount of charge is made to flow across the interface. The electric field required to force a constant current through the SRSO film increases with increasing annealing temperature but decreases with increasing excess silicon content as shown in Fig. 2.

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Wavelength (nm ) Fig. 3. EL spectra for (a) Er and (b) Tb vs. drive current. Sample (a) was 88nm thick and 1.04mm square, with 37% Si, 12% C, 3% Er, annealed at 1200°C for 120min; sample (b) was 94nm thick and 0.63mm square, with 34.7% Si and 0.3% Tb, annealed at 1100°C for 30min. The small peaks in (b) represent a series of transitions from the 5D4 level to the 7Fx manifold.

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B. Electroluminescence Characteristics Electroluminescent spectra obtained from an erbium doped sample and a terbium doped sample are shown in Fig. 3. The pixel sizes were similar for both, while the Tb doped sample was approximately 10% thicker. 34%

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2mA 1mA 750µA 500uA 400µA 200µA

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Index of Refraction Fig. 2. Electric field required to sustain a current of 1.5mA/cm2 across a 102nm SRSO film, as a function of the refractive index of the film.

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In the Er doped case, two luminescence sources are operating, with the double peak at 535/545nm arising from Er emission, and the smaller peak at ~650nm from nanocrystal decay. The nanocrystal emission energy of 1.9eV is greater than the 1.6-1.7eV maximum that is usually observed arising from traps at the nanocrystal/SiO2 interface [9]. There is no discernable nanocrystal peak for the Tb doped sample at the 15kHz used to obtain these data, although at lower frequencies it does emerge at 1.65eV, which is typical for silicon nanocrystals in SiO2. On the other hand, the Tb luminescence was very bright, and in fact exceeded the brightness of a conventional green LED measured under the same conditions. Fig. 4 illustrates the dependence of the integrated area under the Tb EL peak on the period of the drive signal. At low frequencies (long periods) the output is approximately constant, but then it peaks around 10µs and falls off for high frequencies. Note that in these AC measurements, because of the exponential nature of Poole-Frenkel conduction, the sample is only excited significantly near the peak of each AC cycle. Therefore the sample is effectively pulsed, where the pulse width varies with the period.

Integrated Signal (a.u.)

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F9/2 transition that couples to phonons and leads to a nonradiative decay path. No such path exists for the green 5D4-7F5 transition in Tb so the decay route is radiative. The higher nanocrystal energy for the Er sample is likely due to the presence of carbon, which passivates the nanocrystals, reducing the incidence of interfacial traps; then the emission energy will more closely represent quantization effects in a Si crystallite with a diameter of a few nm [11].

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We have found that bright electroluminescence is possible from rare earth doped films of silicon rich silicon oxide when an AC bias is applied, rather than DC. For green emission, terbium is much more efficient than erbium; in fact we have been able to obtain electroluminescence from a Tb/SRSO system that is measurably brighter than from a conventional commercial LED. The efficiency of energy transfer from nanocrystals to rare earths is also greater for Tb, since we see no evidence of EL from nanocrystals in Tb doped films.

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Period (µs) Fig. 4. Dependence of integrated EL intensity on the period of the AC drive signal for a Tb doped sample. The peak is at approximately 10µs. IV.

CONCLUSIONS

DISCUSSION

The asymmetry in electrical conduction indicates that the ITO-SRSO interface is not as stable as the Si-SRSO interface and is modified by the passage of current. The asymmetry is due in part to the difference in barrier heights between the SRSO film and the substrate (~3.15eV) or the ITO electrode (~3.85eV depending on the composition and deposition of the ITO). These barriers are much higher than those determined from the I-V data, indicating that the excess silicon and the nanocrystals have formed a sub-band within the band gap of the SiO2 and electrons can tunnel directly into these states. The increase in electric field with annealing temperature may arise from diffusion of excess silicon to the nanocrystals through the Ostwald ripening process. The reduction of the electric field with increasing excess silicon is simply due to the formation of very large closely spaced Si nanocrystals that allow more direct tunneling of electrons from Si nanocrystals. As a result, the effects of temperature and ripening on electron conduction diminish for silicon concentrations above 45at%. At low frequencies the luminescent output is constant since the nanocrystals can easily follow the incoming signal, and the applied power is constant (since the “pulse width” tracks the period). At higher frequencies (where the period exceeds the intrinsic time constant) the nanocrystals are unable to transfer energy fast enough, so more and more pulses are wasted; in addition some non-radiative processes such as Auger recombination may come into play when an energetic electron strikes a nanocrystal that is already excited. The peak at 10µs indicates a time constant of about that magnitude. There are three differences between the Er doped and Tb doped samples. Slightly different deposition systems caused the Er doping to be accompanied by significant carbon (from the organic source material), while negligible carbon was incorporated into the Tb doped film, as seen in Auger spectroscopy. Er doping produced weak EL while Tb doping resulted in bright green EL. Finally, nanocrystal emission occurred at a higher energy for the Er doped samples. It is known in the lighting industry that Tb is preferred over Er for green illumination [10] because the preferred 4S3/2-4I15/2 green transition in Er experiences competition from the 4S3/2-

ACKNOWLEDGMENTS We wish to thank Juan Caballero and Frank Shepherd of the Canadian Photonics Fabrication Centre for the Tb samples, Irwin Sproule of the National Research Council of Canada for Auger analysis, and George Chik for useful discussions. This work was partly funded by the Ontario Centres of Excellence. REFERENCES [1]

G. Reed and A.P. Knights, Silicon Photonics: An Introduction, Wiley: New York, 2004. [2] S. Ossicini, L. Pavesi, and F. Priolo, Light Emitting Silicon for Microphotonics, Springer: New York, 2003. [3] F. Iacona, G. Franzò, and C. Spinella, “Correlation between luminescence and structural properties of Si nanocrystals” J. Appl. Phys., vol. 87, pp. 1295-1303, Feb. 1 2000. [4] L.R. Tessler and A.C. Iñiguez, “Optimization of the as-deposited 1.54 lm photoluminescence intensity in a-SiO x : H〈Er〉” J. Non-Cryst. Sol., vol 266-269, pp. 603-607, 2000. [5] M. Yoshihara, A. Sekiya, T. Morita, K. Ishii, S. Shimoto, S. Sakai and Y. Ohki, “Rare-earth-doped SiO2 films prepared by plasma-enhanced chemical vapour deposition”, J. Phys. D: Appl. Phys. vol. 30, pp. 19081912, 1997. [6] G. Franzò, D. Pacifici, V. Vinciguerra, and F. Priolo, “Er3+ ions–Si nanocrystals interactions and their effects on the luminescence properties”, Appl. Phys. Lett. vol. 76, pp. 2167-2169, Apr. 17 2000. [7] M. Boudreau, M. Boumerzoug, P. Mascher, and P.E. Jessop, “Electron cyclotron resonance chemical vapor deposition of silicon oxynitrides using tris(dimethylamino)silane”, Appl. Phys. Lett. vol. 63, pp. 30143016, Nov. 29 1993. [8] M. Flynn et al., “The impact of the rare-earth precursor on the composition, structure and luminescence of Er-doped silicon-rich silicon oxide films”, this conference, 2006. [9] J. De La Torre, A. Souifi, A. Poncet, C. Busseret, M. Lemiti, G. Bremond, G. Guillot, O. Gonzalez, B. Garrido, J.R. Morante, and C. Bonafos, “Optical properties of silicon nanocrystal LEDs”, Physica E vol. 16, pp. 326-330, 2003. [10] J. Ballato, J.S. Lewis III, and P. Holloway, “Display applications of rare-earth-doped materials”, MRS Bulletin Sept. 1999, pp. 51-56, 1999. [11] S.-Y. Seo, K.-S. Cho, and J.H. Shin, “Intense blue–white luminescence from carbon-doped silicon-rich silicon oxide”, Appl. Phys. Lett. vol. 84, pp. 717-719, Feb. 2 2004.

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