The thesis entitled "Preparalioa and Study of DC Electrical Conduction Mechallism ill Plasma Polymerized Thill Films of Tetraelhylortfl(J,\'ilicate~submitted by 1\1IJSTAIU ZAMAN, roll no.: 040214004F, regi,tl'3tion no. session, April 2002, has been ac~ep(ed a~ s3tisfaetol)' in p3rial fulfillment of (the requirement for the degree of MASTER OF RHILOSOPHY (M. Phil.) ill Physics on April 26, 2005. 5,] 4 Plots of current density against inverse absolute temperatures for PPTEOS thin films in ohmic and non-ohmic regions (d = 350. nm).
Isne
Rqfi Uddin and newly appointed lecturers, Department of Physics, BUET for their guidance and inspiration throughout the work. 1 would like to thank the Chairman, Department of Chemistry, Universl/y of Dhaka for allowing me to take the ultrr:violet-visible(Uv-vis)and Ilifrared (lR) spectra.
INTRODUCflON
- Review of Earlier Research Work
- Thesis Layout
They observed that the surface morphology of the PPDP thin films was uniform and without holes. Measurements of the temperature dependence of ohmic and space-charge limited (SCL) currents in thin films of polycrystalline particles of 13-.
FUNDAMENTAL ASPECTS OF POLYMER AND PLASMA POL YMERIZA nON
- Introduction
- Polymers II]
- Plasma polymerization
- Advantages aDd Disadvantages of Plasma Polymers
- Applications ofPlllSma-polymerized Organic-Inorganic Thin Films
Since this technique is used in the preparation of organic thin films that will be investigated in this study, some details about plasma and plasma polymers are documented in the following sections. Because of this multidimensional parameter space of plasma conditions, there is a wide variety of gas discharge plasmas that are used in a wide range of applications. In this process, a low molecular weight molecule (polymer) oceun grows; with the help of plasma energy, which includes accumulated electrons, ions and radicals.
Such polymers also depend on (I) the synthesis of a monomer, (2) the creation of plasma mixtures, (3) the polymerization of the monomer to form a polymer, (4) the cleaning, (5) the application of the polymer. film, and (7) curing the film. However, when a potential difference is applied, the electrons are accelerated by the electric field in front of the cathode and collide with the gas atoms. When the ions and fast atoms from the plasma bombard the cathode, they release not only secondary electrons but also atoms of the cathode materials, which is called sputtering.
In essence, the performance of the glowing disk depends significantly on the role of the cathode dark space. The potential difference applied between the two electrodes is generally not evenly distributed with respect to the cathode and anode, but falls almost completely in the first few millimeters in front of the cathode.
EXPERIMENTAL DETAILS
- Capacitively Conpled Plasma Polymerization Set-up
- Generation of Glow Discharge Plasma in the Laboratory
- Measurement of Thickness of the Thin Films
Fig, 3.2 Schematic diagram of the plasma polymerization system (I high voltage power supply, 2 piraru meters, 3 high voltage cables, 4 gas inlet vahe, 5 measuring head, (, monomer injection valve, 7 flow meter, 8 monomer container, 9 I1Tex glass dome, 10 metal electrodes, 11 electrode stands, 12 gaskets, 13 lower flange, 14 lower flange, 15 copper tube, Hi valve, 17 IiqUld nitrogen lrap, l~ rotary pump, 19 SMLch and 20 variac).substrates are usually held on one of the electrodes for plasma deposition. iii) Pump unit. A vacuum pressure gauge head (Lay bold AG) and a gauge (ThennotronTM 120) from Laybold, Germany, are used to measure the internal pressure of the plasma deposition chamber. v) Input power for plasma generation.
The important feature of glow discharge plasma is the non-equilibrium state of the overall system. A schematic diagram of the multiple beam interferometer together with a typical pattern of Fizeau fringes of a film Slep is shown in Fig.3,5. In these experiments lIlr was used as the primary plasma, and the monomer vapor was sprayed downstream of the pnmary air glow discharge.
CHEMICAL, THERMAL AND OPTICAL
PROPERTJESOF PPTEOS
Elemental Anal}"!is
Introduction
- Experimental procedure
The surface morphology of the plasma polymerized films can be studied using Scanrung Electron Microscopy (SEM). Scanning electron micrographs of the PPTEOS film surface were taken using a Scanning Electron Microscope (XL-30, Philips, Netherlands). Organic functional groups differ greatly from each other in the strength of the bonds and in the mass of the atoms involved.
A molecule absorbs only those fh'quences of light lR that match the vibrations that cause a change in the molecule's dipole moment. With a KBr disc the strength will depend on the amount and homogeneity of the sample dispersed in the KBr powder. Fig, 4,3 shows the IR spectra of mOll{lmer TEOS and PPTEOS, which are represented by M and N respectively.
Differential Thennal Analysis (PTA) Thermogravimetric Analysis (TGA)
- Experimental pr(l-(fllUI'e
- Results and discussion
Chapter 4: Chemical, thermal and optical properties of PPTEOS. SI-O stretching vibrations in S,-OClI;) are observed at I lOS em-' (G), IOOl em-' (H) and 1034 cm-' (l-I') in the TEOS and PPTEOS spectra. TGA is a useful analytical technique for recording the weight loss of the tesl sample as a function of temperature in scanning mode or as a function of time in isothermal mode under different conditions and to examine the kinetics of the physicochemical processes occurring. in the sample. TGA is used to characterize the decomposition and thermal stability of materials. The balance circuit supplies sufficient feedback current to the dnving coil so that the slit returns to the balance position.
The current flowing to the drying coils on the sample side and the current flowing to the driving coil on the reference side are sensed and converted to weighing. In the first region A, the loss of weight may be due to the removal of water, which is not necessarily related to VviJichIS. 34;, with any change in structure and can be attributed to the loss of unconstitutional or adsorbed water and unreacted monomer deposited on the surface of the PPTEOS films and/or to the evolution of hydrogen and low molecular weight hydrocarbon gases.
Ultraviolet-visible (Uv-vis) Optical Absorption Spectroscopic Analysis The optical energy gaps and /he aUowed direct transitions and allowed indirect
- Beer_Lambert Law
This is because the absorption peaks of these transitions fall in an experimentally convenient region of the spectrum mm (200-ROOrun). If the density of states for both band edges is parabolic, then the photon energy dependence of the absorption becomes aowv2I,(vy{hv-Eop,f. It is seen that there is a sharp increase in the absorption in about 315 Ill1l, above 480 nm it decreases absorption slowly.
The opllcal band gap is related to the electronic structure of II malenal, and it is one of the. E.J was determined from the intersection of the linear pari of the curve extrapolated to 7ero(l in the photon energy a.us,. The Tauc parameter, B is a measure of the steepness of the band tail (Urbach region) close of modes .
DC ELECTRICAL PROPERTIES OF PPTEOS
- Introduction
- DC electrical conduction mechanism
- Poole-Fnmkel (PF) mechanism
- Experimental Procedure
A brief discussion of existing theories on the mechanism of dc electrical conduction and the experimental techniques used in current density (IN) voltage measurements is presented in this chapter. The potential energy relative to the 10 Fermi level of the electrode is now given by Due to the reduced image strength of the bamer results and the electrode-limited current obeys the Richardson-SehoUky law L14] and is given by.
The PF effect is the greatest analogue of the Schottky effect at an interface barrier. This value "will be defined on the basis of the exhibited parts of the respective basin in the Inj - Inv curve. An important feature of the tire system is that electrons are being displayed or distributed over the grid.
I MVM
Re'lulls Dod Di'lCunioD
Current density>'-voltDge(J-V) elmBcteristico; PPTEOS of thin films of various thefts at tempera1lZre$ 300, 373 nne. 5,8, Plots of current density versus applied voltage at different temperatures for a PPTEOS thin layer (d=400 nm). Table _ 5.1 Slopes in two stress ranges at different temperatures for samples of different thicknesses.
The voltage dependence of the current density in the region of higher voltage predicts Utal, the current may be due to the SeL unduchon, Schottky or PF mechanism in PPTEOS thin films [15, 16]' To determine the type of conduction mechanism in PPTEOS thin films, the dependence of J on the film thickness, d, for series II of various samples at the wllStanl voltage, 14V, is shown in Fig. It is seen that in the non-ohmic region the slope value satisfies the condition for the SCL mechanism(; as discussed in section 5,2.3 Fig:s show the dependence of J on the inverse absolute temperature, If, for thin PPTEOS ftIms of different thicknesses.
Y Tem erature
Fig.5,13 Plots of current density against inverse absolute temperature for PPTEOS sample in oIme and non-ohmic regions (d =400 nrn). Fig, 5.14 Plots of current density against inverse absolute temperature for PPTEOS sample in ohmJ.can and non-ohmic regions (d =350 nm). Fig.515 Plots of cutTent density against inverse absolute temperature [or PPTEOS sample in ohmic and non-olunic regions (d =300 nm),.
Fig. 5.16 Plots of current density versus m'erse absolute temperature for a PPTEOS sample in ohmic and non-ohmic regions (d = 250 run).
CONCLUSIONS
- Conclusions
- Suggestions for Further work
The hydrogen deficiency and increase in carbon content in PPTEOS may be due to bond breakdown due to the complex reaction of plasma polymerization. The SEM imitation shows a smooth, uniform and pinhole-free surface of the PPTEOS film. The IR research shows that PPTEOS is structurally different from monomer to some extent. The maximum change in the PPTEOS structure has occurred in the temperature range from 446 to 10731 (, where the DTA curve shows the exothenmc peak. In TGA, the weight loss may occur due to thermal breakdown of the PPTEOS structure and expulsion of higher molecular mass. hydrocarbons, oxygen-containing compounds, etc. above 446 K.
In the lower temperature region for respectively 2(ohmic) and 14 Y(non-ohmic) which are respectively 0 46.:r 0.07 eY and 0.43 :I: 0.10 eY for ohmic and non-ohmic region in the higher temperature respectively , This reveals that in the low temperature region l11conduction may be due to hopPlllg of the electrons/ions from lraps and/or sublevels and conduction may be due to movement of carrier between different energy level in the high temperature reglOn,. ,;jngmvestlgations on PPTEOS thin films mll) are performed for elucidation or different properties. The XPS investigation should be performed to see the bonding of different fillers found to be present in chemical states in the PPTEOS thin films.
