Brightness Degradation Controlled by Current Induced Metastable Defect Creation in a-SiC:H Based Light Emitting Diodes
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INTRODUCTION Amorphous silicon carbon based thin film light emitting devices (LEDs) deposited on large substrates by low temperature technologies like Plasma Enhanced Chemical Vapour Deposition (PECVD) are taking on increasing interest. Unfortunately, a-SiC:HI, like a-Si:H, is a metastable material under high current injection.' Though some work has been published on a-SiC:H based LEDs, 2 little information is available about their behaviour under operation, which is of primary importance. It is also important to understand the degradation mechanism of the device. In this paper we present for the first time results about time behaviour and operation under dc and ac bias of LEDs, and make some hypothesis on how the combined effects of forward current flowing and heating, and of reverse bias, act on the final performance of the devices. Defects are shown to recover by annealing. A new result is that the devices presented in this paper emit visible light, for times of the order of 103 seconds, which was not achieved previously.
EXPERIMENTAL LEDs were deposited by PECVD on TCO (tranparent conducting oxide) coated 0.1 cm thick glass substrates, using a standard p-i-n structure. Typical parameters of our red emitting LEDs are shown in Table I. The sample area varied from 0.03 to 0.13 cm2'. Evaporated silver was used as back contact. Total absolute brightness was measured with a calibrated photometric sensor. Electroluminescence (EL) was measured by means of an amplified, factory calibrated silicon 809
Mat. Res. Soc. Symp. Proc. Vol. 377 ©1995 Materials Research Society
detector, under continuous power supply or in ac regime. Ac measurements were made alternatively under current injection or with controlled voltage supply. Jon, Von, indicate the current or voltage values respectively during the pulse duration t4n. Voff indicates the reverse voltage, that can also be zero, during the rest time toff. The duty cycle D is defined as TABLE I - Thickness and electro-optical characteristics of single layers. layer thickness (nm) Eg (eV) Ea(eV) GD (S.cml) 4.0x 10-6 0.38 1.94-2.01 12 p i 80 2.50
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