High Reverse Breakdown a-C:H/Si Diodes Manufactured by rf-PECVD
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ABSTRACT
Thin films of hydrogenated amorphous carbon ( a-C:H ) deposited by radio frequency plasmaenhanced chemical vapour deposition ( rf-PECVD ) have been studied for various applications. An interesting property of these films is their high breakdown strength ( 107 Vcm"1 ). This property of a-C:H can be exploited in high breakdown heterostructure diodes or as passivation layers and insulator layers in MIS devices. Reports on the applications of a-C:H/Si diodes exist in the literature. Diodes in which the a-C:H films have been deposited by rf-PECVD, have been reported only once. In this article the diodes produced reportedly failed to exhibit reproducible IV characteristics under high voltage stress. We have investigated the process dependence of structural and electrical properties of rf-PECVD a-C:H films deposited at room temperature from a CH 4/Ar gas mixture (at a pressure of 100 mTorr) using a capacitively coupled rf-PECVD. We observe a clear correlation between the dc-self bias and the rectification ratio of a-C:H/Si heterojunction diodes. Optimised diodes show rectification ratios upto 104 and a stable reverse breakdown voltage, typically around 850 V. I-V and C-V measurements show no evidence of hystersis. Scanning Electron Microscopy was carried out to determine the quality of the films deposited. Micro-Raman analysis was used to estimate the ID/LIG ratio in the films deposited under different dc-self bias.
INTRODUCTION The applications of hydrogenated amorphous carbon (a-C:H) films to electronic devices have been of considerable interest in recent years [1-4]. An interesting property of these films is their high breakdown strength (107 Vcm'). Properties such as a low dielectric constant (which implies higher switching speed), high breakdown strength and deposition at room temperature, make this material useful for cheap, glass compatible technologies and for power electronics. One very widely used class of device that could benefit from these properties of is the p-n diode. The electrical characteristics of p-n junctions are fundamental to the electronics industry. Rectification, isolation, amplification and switching are key operations that involve the use of p-n junctions in electronic circuits. Therefore, fabrication of p-n junction diodes is an appropriate starting point in the development of a semiconductor technology based on amorphous carbon. In order to implement a-C:H films (deposited by rf-PECVD) in heterostructure junctions, it is vital to understand the electrical behaviour of a p-n heterojunction as a function of the growth parameters. The plasma deposition of a-C:H proceeds by ion bombardment which is activated by the negative dc self-bias (VSB) of the driven electrode on which the substrate is placed [5]. The film properties are therefore strongly influenced by the
"Presentaddress:
Department of Engineering, Cambridge University, Cambridge CB2 IPZ, UK
427 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
magnitude of the dc self-bias which is in turn dependant o
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