Enhancing Efficiency and Stability of Nanocrystalline Porous Silicon Electroluminescence by Surface Treatments B. Gelloz and N. Koshida Tokyo University of Agriculture and Technology Dpt. of Elec. & Elec. Eng., Tokyo Univ. A&T, Koganei, 2-24-16 Nakacho, Tokyo 184 8588, Japan. Efficient Si-based light-emitting devices with low operating voltage are desirable because of their potential low cost and VLSI-compatibility. Applications include light sources in general, and optical interconnects and displays in particular. Currently, electroluminescence (EL) from nanocrystalline Si (nc-Si) does not fulfill all the necessary requirements for display and interconnects purposes. The external power efficiency (max reported: 0.4 % [1]; needed: >1%) and stability (typical: minutes or hours; needed: >10 5 hrs) should be enhanced. Furthermore the operating voltage (best: >5 V [1,2], typical: >10 V) should be lowered, especially for mobile applications and interconnects (3-5 V). Finally, the brightness also has to be improved (typical: <1cd/m 2 ; needed: >100 cd/m 2 ). The nc-Si layers are formed by anodization of Si in HF solutions followed by electrochemical oxidation performed according to the process described in Ref. 3. This partial oxidation process is very efficient in increasing the EL efficiency of the as-prepared nc-Si layer while preserving most of the original nc-Si passivation by hydrogen atoms. We have studied two nc-Si surface treatments in combination with very thin nc-Si layers in a view to improve all the EL characteristics of nc-Si (operating voltage, efficiency, and stability). The first treatment consists in the deposition of a few nanometer thick amorphous carbon film onto nc-Si, prior to the deposition of the semi-transparent top electrode. The second surface treatment is a thermally activated chemical modification of the nc-Si surface, resulting in the substitution of most hydrogen atoms terminating nc-Si by covalently attached organic groups. Various organic molecules have been considered, such as 1-decene, ethyl-undecylenate or n-caprinaldehyde. Figure 1 shows the current density and the EL intensity as a function of the voltage for two devices, one including a carbon film and one without. It is clear that the incorporation of the film enhances the EL efficiency due to a reduction in current density and an increase in EL intensity. A high value of the power efficiency has been measured (0.34 lm/W) at 3 V for a brightness of 3 cd/m 2 . This is the best performance ever for an LED based on nc- Si in terms of efficiency and operating voltage. The device stability is also enhanced by the carbon film. This is due to the high chemical stability of carbon and Si-C bonds at the nc-Si surface. The capping function of the film is also a factor that should improve the stability. In addition, the carbon film enhances the mechanical and electrical quality of the top contact, resulting in improved reproducibility. Figure 2 shows the current density and the EL intensity as a function of time for a device including a nc- Si layer which surface has been modified using 1-decene. The diode current and EL intensity are very stable. This is because the nc-Si surface is well passivated by covalently bonded organic groups. For comparison, without the organic passivation, the EL intensity drops by about one order of magnitude in 10 min. Therefore, the organic passivation is very effective in enhancing the stability. In summary, two surface treatments (carbon film and nc-Si passivation by organic groups) are very effective in enhancing the efficiency, reproducibility and stability of nc-Si LEDs. Furthermore, record performance has been achieved in term of efficiency and operating voltage. References 1. B. Gelloz and N. Koshida, J. Appl. Phys. 88 (7), 4319 (2000 ). 2. A. Loni, A.J. Simons, T.I. Cox, P.D.J. Calcott and L.T. Canham, Electronics Letters, 31, 1288 (1995) 3. B. Gelloz, T. Nakagawa and N. Koshida, Appl. Phys. Lett. 73, 14, 2021 (1998). -2 -1 0 1 2 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 C included No C CURRENT DENSITY (A/cm 2 ) VOLTAGE (V) 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 EL INTENSITY (Arb. units) Figure 1 : EL intensity and current density versus voltage for a device including a carbon film and a device without any carbon film. 0 5 10 15 20 25 30 35 40 0.00 0.05 0.10 0.15 0.20 CURRENT DENSITY (A/cm 2 ) TIME (min) 0.0 1.0x10 -7 2.0x10 -7 3.0x10 -7 4.0x10 -7 5.0x10 -7 6.0x10 -7 EL INTENSITY (Arb. units) Figure 2 : EL intensity and current density versus time at 4 V for a device including a carbon film. In addition, the surface of nc-Si has been passivated using covalently bonded organic groups (decyl groups in this case). Abs. 1180, 204th Meeting, © 2003 The Electrochemical Society, Inc. View publication stats View publication stats