Near-IR LEDs fabricated with monodispersed nanocrystallite Si
04/01/2002
by Takehito Yoshida, Nobuyasu Suzuki, Toshiharu Makino, Yuka Yamada, Matsushita Electric Industrial Co. Ltd., Kanagawa, Japan
Overview
Significant research has achieved a near-infrared light-emitting diode using monodispersed silicon nanocrystallites. Engineers synthesized this material by laser ablation and subsequent particle-size classification. The resulting near-infrared emission was sharp and showed a peak above the band-gap region, presumably originating from the spatial quantum confinement effects of carriers. This process is very compatible with today's wafer fabrication technologies and could be used for IC optical interconnects.
Nanostructured silicon (Si) is a candidate material for realizing Si-based light-emitting devices. During the last decade, researchers have studied these materials [1, 2]. Many types of nanostructures have been reported, for example, porous Si formed by liquid phase anodization [3] and dislocation loops formed by ion implantation [4]. These Si nanostructures were mainly formed on and in bulk crystalline Si. Such materials have high percentages of surface-exposed atoms and are extremely sensitive to impurities and damage. Moreover, their properties strongly depend on the mean size of the nanostructure and size distribution.
Pulsed laser ablation (PLA) is a promising method for forming nanocrystallites because it is a clean cold-wall process. PLA can be carried out in a wide range of background gases from high vacuum to several atmospheres of pressure. Background gas conditions can be selected to facilitate gas phase condensation of nanocrystallites. Furthermore, the size of the resulting nanocrystallites can be precisely controlled using a low-pressure differential mobility analyzer (LP-DMA [5, 6]).
In our work, we have synthesized size-controlled Si nanocrystallites — in a processing system that integrates PLA and
LP-DMA — deposited as thin-film active materials for Si-based light-emitting diodes (LEDs).
 Figure 1. A novel integrated process system for synthesizing monodispersed Si nanocrystallites. |
Fabrication process
Our novel integrated process system has three main sections (Fig. 1): a Si nanocrystallite formation chamber, a classification unit, and a codeposition chamber where Si nanocystallite and indium oxide (IN2O3) films are deposited.
Briefly described, this system focuses a second-harmonic Nd:YAG laser beam (λ = 532nm) onto the surface of a p-type single-crystal Si target. During ablation, pure helium (He) is introduced into the formation chamber as a carrier gas; the He is maintained at 7torr using differential evacuation. The carrier gas transports the formed Si nanocrystallites into the LP-DMA, where they are precisely classified using mobility in viscous fluid differences (see "LP-DMA particle classification" on p. 42), and transported to the deposition chamber. There, the size-controlled Si nanocrystallites eject from a nozzle and are deposited as a 20nm thick film. The LED substrates are (100)-oriented lightly doped p-type silicon wafers with a thermal oxide layer into which is patterned (before deposition) an array of electrically isolated 100μm2 squares that become the luminous elements.
Immediately after Si nanocrystallite deposition, we deposit an IN2O3 film by PLA in the same deposition chamber without breaking the vacuum. This is done by focusing an argon-fluoride excimer laser beam (λ = 193nm) onto the surface of a sintered IN2O3 target in a 99% He 1% O2 background gas at 1.5torr. Room temperature PLA deposition produces transparent, stoichiometric, and crystalline IN2O3 thin films. The film is the upper electrode and passivation layer for the active monodispersed Si nanocrystallites. To form contacts with the active regions, we deposit transparent conducting indium-tin-oxide films — the low-resistivity upper electrode (Fig. 2) — by conventional RF sputtering.
 Figure 2. LEDs fabricated with monodispersed Si nanocrystallites. |
In our work, we found that the distribution of the size-controlled Si nanocrystallites was sufficiently sharp to designate them monodispersed (Fig. 3). Both synthesis and characterization of monodispersed Si nanocrystallites <4nm are very difficult because Si is preferentially oxidized and is a low-density element. The measured geometrical mean diameter was 3.8nm (geometrical standard deviation 1.2).
 Figure 3. a) Dark field (left) and high-resolution bright-field (right) TEM images of deposited monodispersed Si nanocrystallites and b) histogram of particle diameters. |
The LEDs
Current-voltage (I-V) analysis of LEDs fabricated with monodispersed Si nanocrystallites revealed a rectifying behavior and the onset of light emission under forward bias at 2.0V. Integrated emission intensity (IE) as a function of injection current (j, Fig. 4) shows a rapid nonlinear increase with increasing j(j: IE ∝ jm, m = 2). This dependence is different from that of conventional minority-carrier injection-type LEDs. One possible mechanism of this emission is impact ionization by hot electron tunneling through the surface oxide layers on the Si nanocrystallites and subsequent radiative recombination. The nonlinearity is attributed to the dependence of impact ionization quantum efficiency on acceleration energies of tunneling hot electrons [7].
When we measured the light emission spectra of our LEDs at room temperature, the dissipation power was 84mW (Fig. 5). In addition, we observed a sharp 0.15eV-wide near-infrared peak at 1.17eV (Fig. 5a). This peak position is higher than the bulk-silicon energy gap of 1.11eV, a result that has not been observed in silicon LEDs thus far [1-4]. We attribute this success (i.e., sharp emission above the band gap) to our sequential processing without exposing the monodispersed Si nanocrystallites to air.
 Figure 4. Integrated emission intensity as a function of forward injection current. |
In some of our other work, LEDs — with monodispersed Si nanocrystallites were exposed to air and thermally oxidized before IN2O3 deposition — showed a broad 0.78eV-wide peak of visible emission at 1.80eV (Fig. 5b). The thermal oxidation was a rapid thermal process at 1000°C for 10 sec in pure oxygen. The visible (i.e., red) emission peak in Fig. 5 decomposes into three Gaussian waveforms at 1.7eV, 2.1eV, and 2.7eV that can be attributed to an Si-SiO2 interface state emission center [8], SiO2 defects [9], and SiO2 neutral oxygen vacancies [10], respectively.
Previously, it was generally concluded that oxidation, high-temperature annealing, or hydrogen passivation was necessary to achieve light emission at room temperature for nanostructured Si. For example, we previously reported that light emission could be obtained only after annealing at >800°C for polydispersed Si nanocrystallites [11]; the light emission spectra from this earlier work had a broad peak around 1.7eV, which was described as an oxygen-related peak (i.e., from Si-SiO2 interface states).
However, the narrow light emission peak obtained in our work reported here can be realized without any oxidation or hydrogenation processing. Therefore, the near-infrared emission above the band gap should originate from spatial quantum confinement effects of carriers in the monodispersed Si nanocrystallites.
Conclusion
We have adopted laser-excited physical vapor growth by PLA combined with aerosol size classification by LP-DMA for the synthesis of monodispersed Si nanocrystallites. These nanocrystallites act as active regions of near-infrared and visible LEDs at room temperature. Our film-deposited Si-based LEDs could prove beneficial for optical interconnect systems in future high-speed, low-power ULSI ICs [12].
Acknowledgments
Development of the integrated process system using the LP-DMA was done in collaboration with T. Seto and his colleagues at the National Institute of Advanced Industrial Science and Technology, Japan.
This work was conducted within the Advanced Photon Processing and Measurement Technologies program of the Ministry of Economy, Trade, and Industry consigned to the R&D Institute for Photonics Engineering from the New Energy and Industrial Technology Development Organization, Japan.
References
1. For example, D. Lockwood, Light Emission in Silicon from Physics to Devices, Academic Press, 1998.
2. A.G. Cullis, L.T. Canham, P.D.J. Calcott, J. Appl. Phys., 82, 909, 1997.
3. K.D. Hirschman et al., Nature, 384, 338, 1996.
4. W.L. Ng et al., Nature, 410, 192, 2001.
5. T. Seto et al., J. Aerosol. Sci., 28, 193, 1997.
6. T. Makino et al., Appl. Phys. A, 69, S243, 1999.
7. C. Chang, C. Hu, R.W. Brodersen, J. Appl. Phys., 57, 302, 1985.
8. Y. Kanemitsu et al., Phys. Rev. B, 48, 4883, 1993.
9. K. S. Min et al., Appl. Phys. Lett., 69, 2033, 1996.
10. R. Tohmon et al., Phys. Rev. Lett., 62, 1388, 1989.
11. T. Yoshida, Y. Yamada, T. Orii, J. Appl. Phys., 83, 5427, 1998.
12. D.A. Miller, Tech. Dig. of International Electron Device Meetings, 343, 1997.
Takehito Yoshida received his doctorate in engineering from the University of Tsukuba. He is research team leader at the Advanced Technology Research Laboratories, Matsushita Electric Industrial Co. Ltd., Tama-ku, Kawasaki, Kanagawa, 214-8501, Japan; ph 81/44-911-6472, fax 81/44-932-4174, e-mail [email protected].
Nobuyasu Suzuki received his masters of engineering from Chiba University. He is a member of the technical staff at Matsushita's Advanced Technology Research Laboratories.
Toshiharu Makino received his doctorate of engineering from the University of Tsukuba. He is a staff member at Matsushita's Advanced Technology Research Laboratories.
Yuka Yamada received her doctorate of engineering from the University of Tsukuba. She is staff researcher at Matsushita's Advanced Technology Research Laboratories.
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LP-DMA particle classification
Within the LP-DMA system (see illustration), charged nanoparticles with an initial broad size distribution enter the classification region through a slit. The sheath gas carries all particles horizontally downward with a uniform velocity. The charged particles are attracted vertically by electrostatic force and drift with each saturated velocity. Drift velocities are proportional to electric field intensities and electrical mobilities. The electrical mobility of nanoparticles depends approximately on particle cross section. Consequently, specific-sized particles reach the bottom slit and are extracted as classified particles.