Electrical Properties Of Diamond Like Carbon Films In Metal-Carbon-Silicon (MCS) Structure
Reddy, K Siva Sankara
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Amorphous carbon film with Diamond like properties is the subject of intense interest in the past one and half decade. The unusual properties of these diamond like carbon films arise from the preponderance of SP3 tetrahedral bonding of carbon in the film. Depending on the processing technique and the processing conditions used, the structure of the films can range from amorphous carbon to large grain polycrystalline diamond. These deposited amorphous carbon films, which are smooth, may find their use in optoelectronics, in dielectric films and in microelectronics. These films are found to be chemically inhomogeneous(containing SP3 hybridized carbon in a matrix of SP2 hybridized non-graphitic carbon). There is a possibility of using these films as substrates in microelectronics, provided the deposited films are structurally smooth, are chemically homogeneous and are dopable with both types of impurities. A host of other advantages of using diamond like carbon as a substrate material in microelectronics made it a topic of interest to many investigators. This prompted the author to take up investigations on diamond like carbon films from the point of examining the electrical properties of these films and on the possibility of conceiving devices based on these films. This investigation dealt with, sputter deposition of diamond like carbon films and their electrical characteristics in MCS device structures. In this, emphasis is given to the importance of processing parameters involved and the effect of each parameter on the electrical and structural properties of the film. Various substrate treatments were done prior to sputtering and found that the DLC nature of the film exists in all the films but differ from one another in electrical resistivity, in nucleation density and in their adherence to the substrate. Films deposited on substrates treated with low vapour pressure oil resulted in compressive strain in the film and lead to very poor adhesion. The nucleation density increased when the substrates are pretreated with ultrasonic agitation in hard SiC grit. The substrate temperature had a direct impact on the resistivity of the film: resistivity decreases with increase in substrate temperature. The constituents of the plasma modified the structural properties of the film, e.g. the Hydrogen content in the plasma has resulted in increasing the SP3 hybridization content of the film, by acting as SP2- SP2 network terminator. Ultra violet light focused onto the substrate, in general, enhanced the deposition rate. Inclusion of Nitrogen in the plasma substantially increased the conductivity of the material and this is used in doping of the DLC film. The carbon films deposited on silicon are used for electrical characterisation. Deposition of metal electrode on the carbon film lead to the basic (MCS) device structure. The I vs.V characteristics of the MCS structure resemble those of junction diodes. From the I vs.V characteristics at different temperatures, it has been found that the reverse current goes through a maximum, drops back to certain level and once again increases with gradual increase in temperature. This behaviour of the structure with A1 as well as Ag as top electrode materials is explained by the heterojunction formed at the C-pSi interface. The initial increase in the reverse current is dominated by the drift of minority carriers across the depletion width at the reverse biased junction. With increase in temperature, the depletion width reduces to a minimum above a certain temperature, where the diffusion of carriers controls the current across the device. From the constructed energy-band diagram of heterojunction, it is shown that the change in the transport phenomena from drift of minority carriers to diffusion of majority carriers at the junction, introduces a barrier at the critical temperature; This is responsible for the drop in current at the critical temperature. This explains the anomaly of drop in reverse current with increase in temperature. The C vs. v characteristics showed a bell shaped behaviour indicating the presence of two junctions connected back to back. This confirms the type of contact formed at the metal-carbon interface and the type of conductivity of the film, concluding that A1 makes a Schottky contact where as Ag makes an ohmic contact and the deposited film behaves like n-type material. The C vs. V behaviour with temperature is explained by the two types of contacts in the case of Al-GpSi, i.e. Schottky contact at Al-C; and heterojunction at C-pSi interface. These C vs. V and I vs.V changes with temperature are in tune with each other and the model proposed takes care of all the characteristics observed. In case of Ag-GpSi, C vs. V with temperature shows junction like behaviour at elevated temperatures and are explained by the presence of the interface at C-pSi. It has been observed that in some of the carbon films, when an electric field of the order of l06 V/cm is applied, the reflectance of the Aluminium metal dot is increased by 5 times, coupled with a 50 to 100 times increase in the associated capacitance of the MCS structure. The increase in reflectance is explained by considering the film to be inhomogeneous with a matrix of varying dielectric constants (SP3 hybridized carbon in a medium of SP2 bonded carbon). The transformed film, is homogeneous and enhances the reflectance of the Aluminium dot. This is termed as "homogeneity induced smoothness." The transformation of inhomogeneous material to homogeneous material is further confirmed by the Raman spectroscopy, in which the broad peak is converted to a sharp peak changing the FWHM from 93 cm-1 to 4 cm-1 ; denoting the structural order in the film. To the best of our knowledge, this is the first investigation reporting the crystalline nature of the DLC, with structural order and the corresponding FWHM of the Raman peak as low as 4 cm-1. The preparational conditions of the film to get this transformation and the influence of various process parameters are examined. Devices based on Metal-Carbon-Oxide- Silicon (MCOS) structure are realized by thermally grown oxide/sputter deposited oxide on silicon, prior to carbon deposition. These structures showed voltage controlled negative resistance(VCNR) characteristics. The applied voltage and its distribution across the reverse biased junction and across the oxide gives rise to a negative resistance region. With the number of V vs. I characteristics measured, it is observed that the negative resistance region also shifts. This is attributed to the trapped charges in the carbon changing the distribution of applied voltage. This is explained by modifying the energy-band diagram. A concept of the accumalated charges at the oxide barrier filling up the higher energy states in the carbon and silicon, to become hot carriers is used. As long a. more voltage is dropped across the oxide, these hot carriers can surmount the barrier at the reverse biased junction. The flow of these carriers is cut off when the additional voltage is dropped across the reverse biased junction leading to a drop in the current. A further increase in the applied voltage nominally increases the current due to increase in the leakage current. A new hybrid (electrical/optical) read only memory (ROM) element is conceived and the way in which the information can be written and read is discussed. A two terminal negative resistance device using MCOS structure is fabricated and tested for its VCNR property. An analog memory device is proposed using the MCOS structure as gate in an FET. The work reported in this thesis has been divided into nine chapters. The introductory remarks on the importance of the area of research and about the work reported in this thesis are given in chapter one. Chapter two deals with some of the basic concepts related to understand the reported work. In chapter three the research work done by other investigators covering different aspects of this work is reported and some of their investigations are reviewed. Chapter four dealt with the various preparative techniques to deposit films, their structural characterisation, and the experimental work carried out to electrically characterize these films. Chapter five presents the I vs.V & C vs. V analysis and a model to qualitatively explain them. In chapter six field induced transformation phenomena of some of these films and its impact on the reflectance of the metal dot is dealt. Chapter seven consists of the MCOS device structure, its I vs.V characteristics and a model to explain the behaviour. Chapter eight presents the application part of same of the phenomena observed in conceiving a new hybrid ROM element and a two terminal negative resistance device. The concluding ninth chapter itemizes the important results of the work and suggestions to carry forward this work which can open up new vistas in the diamond like carbon film based technology and its applications in microelectronics.