Temperature Dependence of Electroluminescence from Nanocrystalline Silicon thin Films
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ABSTRACT Temperature dependence of electroluminescence (EL) from nanocrystalline Si (nc-Si) has been studied in comparison with that of photoluminescence (PL) from the nc-Si. The similarity is found in the temperature dependence of peak shift between EL and PL excited by ultraviolet light. The blue shift in both peaks is also found when the carrier generation rate is increased. The experimental results are indicative that the peak shifts due to the temperature and the generation rate are based on different mechanisms.
INTRODUCTION Nanocrystalline Si is one of the promising materials for the optoelectronic devices [1]. In particular, for the application to light emitting diodes (LEDs), a number of studies on the emission mechanism and the carrier transport mechanism have been performed to improve the performances of LEDs based on porous Si, Si-rich silicon oxide, and plasma-deposited nc-Si [2-5]. We have proposed thin-film light emitting diodes (TFLEDs) using plasma-deposited p-type nc-Si as the light emitting active layer [6,7], because the thin-film devices have many advantages, for example, the possibility of low cost fabrication on a large-area substrate, which has already been realized on amorphous Si (a-Si:H) devices [8]. In the previous reports, we have demonstrated the observation of visible light emission from a nc-Si TFLED with a structure of SnO 2/p-type nc-Si/Al, and analyzed the carrier injection process [7]. In this report, first, the temperature dependence of the peak shift in the EL from the nc-Si TFLED is discussed in comparison with that in the PL from the nc-Si. Second, the dependence of the peak energies in both of the EL and the PL on the carrier generation rate is argued in conjunction with the crystallite size dependence of the PL peak energy.
EXPERIMENTAL The p-type nc-Si thin films were deposited on Sn0 2-coated glass substrates by using a plasma chemical vapor deposition (CVD) method, they were then anodized in an HF aqueous solution. The deposition temperature of 180'C and pressure of 133 Pa were maintained during the deposition. A mixture of SiH4 , B2H6 and H2 gases (SiH 4/B2H6/H2 = 1/0.005/119) was used as a source gas. The fluoride acid was diluted by de-ionized water to 2.4 vol.%. The typical anodizing current density and time were 13 mA/cm2 and 1 min, respectively. Intentional bias illumination was not applied during the anodization. The thicknesses of the nc-Si thin films were set at 1 gtm at deposition, and would be chemically reduced to -0.9 gm by anodization. After the anodization, the aluminum back-contacts were evaporated on the nc-Si thin film with an area of 0.033 cm 2.This completes the fabrication of the TFLEDs. For the EL measurements, the 243 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
TFLEDs were driven by a voltage and/or current source simultaneously monitoring the current and/ or voltage. For the PL measurements, a 325-nm line of a He-Cd laser and a 488-nm line of an Ariion laser were applied from the nc-Si film side (not through the glass
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