The Use of C-V Techniques To Investigate Instability Mechanisms in M-I-S Structures
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The Use of C-V Techniques To Investigate Instability Mechanisms in M-I-S Structures S. Paul∗, W.I Milne and J. Robertson Department of Engineering, Cambridge University, Trumpington Street, Cambridge CB2 1PZ, United Kingdom. Abstract In the field of flat panel displays, the current leading technology is the Active Matrix liquid Crystal Display; this uses a-Si:H based thin film transistors (TFTs) as the switching element in each pixel. However, under gate bias a-Si:H TFTs suffer from instability, as is evidenced by a shift in the gate threshold voltage. The shift in the gate threshold voltage is generally measured from the gate transfer characteristics, after subjecting the TFT to prolonged gate bias. However, a major drawback of this measurement method is that it cannot distinguish whether the shift is caused by the change in the midgap states in the a-Si:H channel or by charge trapping in the gate insulator. In view of this, we have developed a capacitance-voltage (C-V) method to measure the shift in threshold voltage. We employ Metal-Insulator-Semiconductor (MIS) structures to investigate the threshold voltage shift as they are simpler to fabricate than TFTs. We have investigated a large of number Metal/a-Si:H/Si3N4/Si+n structures using our C-V technique. From, the C-V data for the MIS structures, we have found that the relationship between the thermal energy and threshold voltage shift is similar to that reported by Wehrspohn et. al in a-Si:H TFTs (J Appl. Phys, 144, 87, 2000). The a-Si:H and Si3N4 layers were grown using the radiofrequency plasma-enhanced chemical vapour deposition technique.
Introduction Active Matrix liquid Crystal Display, using a-Si:H based thin film transistor (TFT) as the switching element in each pixel is presently the leading technology in the field of flat panel displays for desk top and laptop applications.. However, under gate bias a-Si:H TFTs suffer from instability, as is evidenced by a shift in the gate threshold voltage. Two possible causes of this instability are the creation of midgap states in the a-Si:H channel and charge trapping in the gate insulator [1,2,3,4 ]. The stability of TFTs has been investigated by various workers and gate transfer characteristics of TFTs are commonly used to observe the shift in the threshold voltage. It is however not always appreciated that this voltage shift does not definitely indicate if a change has occurred in the midgap states in a-Si:H or in the charge trapping in the insulator layer or whether both mechanisms have contributed. Such a lack of distinction between the possible sources of the threshold voltage shift leaves room for conjectures about the quality of the active layer in the TFT, as opposed to the insulator layer. In view of this ambiguity we have attempted to develop an alternative method to investigate the instability in the materials required to manufacture TFTs. Our measurement can provide information about the degradation in an active layer as well as in the insulator. There are reports in the literature in which capacita
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