Ellipsometric and Optical Study of Amorphous Hydrogenated Carbon Films

Abstract

A low-frequency plasma deposition system was used to prepare amorphous hydrogenated carbon (a-C;H) films. The growth energy was varied by changing the power and/or pressure of the plasma. Ellipsometry and optical absorption were used to obtain the optical energy gap, the density of states, and the refractive index. Ion sputtering was used in conjunction with ellipsometry and Auger electron spectroscopy to get absolute sputtering rates. The plasma deposited a-C:H is amorphous with optical energy gap of approximately 2.0-2.4 eV. These a-C:H films have higher density and/or hardness, higher refractive index, and lower optical energy gaps with increasing energy of the particles in the plasma, while the density of states remains unchanged. These results are in agreement and give a fine-tuned positive confirmation to an existing conjecture on the nature of a-C:H films (1).Insulator films on semiconductors have a wide variety of applications, including passivation, insulation, capping, etc. In the case of silicon, the silicon based films SiO2 and Si3N4 have been successfully used for all the required applications. However, the need for high-speed devices and integrated circuits requires the use of Group III-V semiconductor devices and substrates. SiO~ and Si3N4 proved to be inadequate for some of these applications for Group III-V semiconductors. Thus, new types of insulator films for one or more of the required applications are needed. In this study we have chosen a-C:H for the insulator film. Some of the attractive properties of a-C:H are: (i) it is easily prepared as a homogeneous film (2): (it) it is very hard mechanically and chemically: (iii) it has a high breakdown voltage (3) and resistivity: (iv) it has a relatively low interface density of states on Si (4) and InP (3): (v) it has a variable energy gap (1, 5); and (vi) it can be used in metal-insulator-metal (MIM) structures (6).However, not all the desirable properties are achieved in a single sample, i.e., the highest energy gap sample will have a rather low mechanical hardness (1). It was suggested in Ref.(1) that the growth energy environment controls the energy bandgap, the hardness, the hydrogen concentration, and bond type. Thus, it may not be possible to optimize all the desirable parameters simultaneously. This conjecture was made using samples made by different deposition methods having very large variations in growth environment energy. In the present work, we report a study of a-C:H films made by varying the growth energy in a continuous way, i.e., varying the plasma power, and/or pressure, and/or the substrate temperature during deposition. We have used ellipsometry, UV-visible absorption, and Auger electron spectroscopy (AES) profiling for sample characterization. These techniques give us energy gaps, density of state parameters, indexes of refraction, and sputtering rates. We have correlated these results with the growth environment energy in order to check the conjecture of Ref.(1). ExperimentalThe sample preparation will be discussed only very ...

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