CHAPTER 3. Experimental Results of Magnesium oxide (MgO) Thin Films

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1 CHAPTER 3 Experimental Results of Magnesium oxide (MgO) Thin Films

2 Chapter: III Experimental Results of Magnesium oxide (MgO) Thin Films Introduction: In this chapter the results have been given on the experimental studies of magnesium oxide thin films. The experimental details have been elaborated in chapter II. The magnesium oxide thin films were obtained from vacuum evaporated magnesium thin films by thermal oxidation in air. Magnesium thin films were oxidised at three oxidation temperature i. e. 573 K, 623 K and 673 K for 9 min and 18 min oxidation duration. Thin films were studied for three different thicknesses 3, 45 and 6 nm. The comparative study of vapor chopped and nonchopped MgO thin films has been done. The structural, surface morphological properties, optical properties such as optical transmittance, refractive index, band gap and adhesion and stress were studied. The secondary electron emission coefficient was measured for the MgO thin film application for plasma display panel. Optical signal loss was studied by prism coupling method for optical waveguide application. The films were also investigated for the effect of ambient atmosphere exposure for short term and long term exposure durations. For all the investigation 4 to 5 samples were studied per property for the various thickness and oxidation temperatures respectively. 3.2 Crystal structure using X-ray diffraction (XRD): X- ray diffraction (XRD) patterns of vapor chopped () and nonchopped () magnesium oxide thin films oxidized at different oxidation temperatures and duration for different thicknesses are shown in fig. 3.1 to 3.3. The graphs have 97

3 Intensity (a.u.) been plotted as comparison between vapor chopped () and nonchopped () films. Fig. (a) to (d) consists of XRD patterns of 3 nm, thickness figures from e) to h) of 45 nm and i) to l) for 6nm thicknesses respectively. (2) For-9 min (22) B A C A- 673 K B- 623 K C- 573 K Intensity (a.u.) (2) For-9 min (22) A A- 673 K B- 623 K C- 573 K B C θ degrees 2θ degrees a) b) Intensity (a.u.) (2) For 18 min (22) A- 673 K B- 623 K C- 573 K A B Intensity (a.u.) (2) For 18 min (22) A- 673 K B- 623 K C- 573 K A B C C θ (degrees) θ (degrees) c) d) Figure 3.1: XRD patterns of magnesium oxide thin films of thickness 3 nm for various oxidation durations and oxidation temperatures Intensity (a.u.) (2) For 9 min (22) A A- 673 K B- 623 K C- 573 K B Intensity (a.u.) (2) For 9 min (22) A B A- 673 K B- 623 K C- 573 K C C θ degrees 2θ degrees e) 98 f)

4 Intensity (a.u.) (2) For 18 min (22) A- 673 K B- 623 K C- 573 K Intensity (a.u.) (2) For 18 min (22) A- 673 K B- 623 K C- 573 K θ (degrees) θ (degrees) Intensity (a.u.) g) h) Figure 3.2: XRD patterns of magnesium oxide thin films of thickness 45 nm for various oxidation durations and oxidation temperatures (2) For 9 min (22) A- 673 K B- 623 K C- 573 K Intensity (a.u.) (2) For 9 min (22) A B A- 673 K B- 623 K C- 573 K C Intensity (a.u.) θ degrees (2) i) j) for 18 min (22) B A A- 573 K B- 623 K C- 673 K Intensity (a.u.) θ degrees (2) for 18 min (22) B A A- 573 K B- 623 K C- 673 K C C θ (degrees) 2θ (degrees) k) l) Figure 3.3: XRD patterns of magnesium oxide thin films of thickness 6 nm oxidized at 673 K oxidation temperature for 18 min. 99

5 XRD patterns of magnesium oxide thin films showed polycrystalline cubic structure with mainly (2) and (22) plane orientations. The figure show that as far as crystal structure is concerned, the effect of thickness of the MgO thin film is negligible. In figures k) and l) the broad peak between 2 o -3 o corresponding to the hump of amorphous glass substrate has been shown. In the other figure the scale has been selected from 35 degrees for better clarity of the peaks. Vapor chopped and nonchopped magnesium oxide thin films were compared with each other with respect to various oxidation temperature and duration. In all cases, MgO thin films oxidized at 573 K showed amorphous nature, since no peak was observed. The peak intensity increases with decrease in the peak broadening due to increase in oxidation temperature and duration. All the figures showed only (2) and (22) peaks as fundamental characteristic peaks of MgO which were confirmed by the standard JCPDS data file no for copper target. It was observed that, the peaks (2) and (22) observed in vapor chopped films were more intense than those in nonchopped films. Vapor chopped magnesium oxide thin films were more crystalline than the nonchopped thin films. No characteristic peaks of impurity and other phases were observed. The XRD patterns did not showed the presence of magnesium metal, indicating complete oxidation of magnesium metal during magnesium oxide thin films preparation. The effect of thickness variation of MgO thin films was not very promising in the XRD patterns. All the three thicknesses showed only (2) and (22) plane orientations. Cubic structure was observed due to all oxidation temperature and duration for each thickness. The only difference was that a negligible peak broadening and peak intensity enhancement was observed. Vapor chopped and nonchopped thin films showed almost same phases and crystal structure results in each thickness. The crystallite size of vapor chopped and nonchopped magnesium oxide thin film were measured by using Scherrer s formula, d = (.9 x λ) / β Cos Ө 1

6 where, λ = Wavelength of x-rays = x 1-1 m, β = FWHM of diffraction peak, Ө = Angle corresponding to the peak Crystallite size (nm) Thickness Oxidation Oxidation temperature (K) (nm) duration (min) Table 3.1: Crystallite size of and MgO thin films for different thicknesses. The crystallite size changes with oxidation temperature and duration as well as with respect to thickness of the thin film. It was observed that, crystallite size slightly increases with increase in oxidation temperature and duration with increase in thickness of thin film. Whereas crystallite size decrease were prominent due to vapor chopping. MgO thin films showed lower crystallite size than thin films for all cases Crystallite size of vapor chopped and nonchopped MgO thin films for different oxidation temperature and duration with different thin film thicknesses were measured from XRD. From the table it is seen that, crystallite size increases with increase in thin film thickness as well as oxidation temperature. MgO thin film oxidised at 573 K show amorphous nature. There are no XRD peaks hence unable to measure the crystallite size of these films. 3.3 Surface morphology by scanning electron microscope (SEM): The surface morphological study of magnesium oxide thin film was carried by scanning electron microscope. Fig. 3.4 and 3.5 show the surface morphology of 11

7 MgO thin films for different oxidation temperature and duration for different thicknesses. a) -673 K b) -673 K c) -623 K d) -623 K e) K f) -573 K Figure 3.4: Surface morphology of vapor chopped () and nonchopped () MgO thin films for thickness 6 nm 12

8 The vapor chopped MgO thin films showed a smoother surface morphology than the nonchopped thin films. The vapor chopped films also had granular morphology as compared to fibrulous morphology of the nonchopped films. The films appear more close packed due to vapor chopping. g) -673 K h) -673 K i) K j) -623 K k) K l) -573 K Figure 3.5: Surface morphology of vapor chopped () and nonchopped () MgO thin films for thickness 3 nm 13

9 The thickness variation effect was not so much prominent as compared to vapor chopping effect, so the surface morphological study of lower thickness i. e. 3 nm and maximum thickness 6 nm are given here. It is seen that the grain size of vapor chopped thin films are smaller than those of nonchopped films. The grain size varied due to oxidation temperature variation. The grain size increases with increase in oxidation temperature. The horizontal cross sectional study only provides the information about surface morphology of top most surface of thin film only. Instead of horizontal view vertical cross sectional scanning electron micrographs may provide internal structure of the thin film. Even though horizontal SEM showed smoothed structure of thin film, the film may or may not be uniform internally. It may contain columnar structure. The vertical cross sectional micrographs gives the information about layerwise deposition, void formation, crack formation defect growth, columnar growth within the thin film deposition. Fig. 3.6 shows the vertical cross section of vapor chopped and nonchopped MgO thin films of thickness 6 nm. It showed the vapor chopped thin films have uniform growth with minimum voids whereas nonchopped thin films showed clear columnar growth with larger cracks and voids formation. In this study cross sectional micrographs of 6 nm thickness was more informative than that of the smaller thicknesses, hence SEM of 6 nm is only given here. Crack Columnar growth Figure 3.6: Vertical cross section SEM of vapor chopped () and nonchopped () MgO thin film oxidized at 623 K. 14

10 3.4 Atomic Force Microscopy (AFM): Figure 3.7 shows the AFM images for and MgO films oxidized at various oxidation temperatures for thin film thickness 6nm. It shows that the films are irregular triangular dumbbell shaped with different size and shapes whereas films are spherical in shape of almost same size. It is felt that T can be used to produce state of art engineered nanostructuring from irregular to spherical one. This is one of the interesting results from T (studies on engineered nanostructures by T are under progress). The particle sizes of films are smaller and regular than those in MgO thin films K -673 K K -573 K Figure 3.7: Atomic force micrographs of nonchopped () and vapor chopped () of MgO thin film of thickness 6nm 15

11 Figure 3.8 show the AFM images of vapor chopped and nonchopped magnesium oxide thin films oxidized for different temperature of thickness 3nm. It was observed that surface roughness increases with increase in oxidation temperature. The comparative study between fig. 3.7 and 3.8 gives the information about the effect of thin film thickness and improved grain structure. It was found that, the surface roughness of vapor chopped and nonchopped MgO thin films decreases with increase in thin film thickness. Simultaneously, vapor chopped thin films showed lower surface roughness than the nonchopped thin films for all thickness K -673 K -573 K -573 K Figure 3.8: Atomic force micrographs of nonchopped () and vapour chopped () of MgO thin film of 3 nm for different oxidation temperatures. 16

12 -673 K -673 K -573 K -573 K Figure 3.9: 3-dimensional atomic force micrographs (AFM) of vapor chopped () and nonchopped () MgO thin films of thickness 45nm for different oxidation temperatures. The 3-dimensional atomic force micrographs of vapor chopped and nonchopped MgO thin films are given in fig dimensional micrographs give the exact information about the surface roughness than the 2-dimensional AFM. It 17

13 is clearly observed that, as oxidation temperature increases, surface roughness also increases. The effect of vapor chopping on surface was so prominent. It was clearly seen that, vapor chopping technique reduces the surface roughness. The vapor chopped thin films were smoother than the nonchopped thin films for all oxidation temperature and thicknesses. 3.5 Optical properties: Optical transmittance: As mentioned in chapter II, the optical transmittance was measured by using UV-Visible spectrophotometer. The graph of wavelength (nm) Vs transmittance (%) were plotted for vapor chopped and nonchopped MgO thin films for various oxidation temperatures and durations as shown in fig for different thickness of MgO thin film. The actual values of all the 5 samples of 6 nm thickness are plotted in fig The fig &3.12 gives the average transmittance of the 5 samples studied T % 6 4 T % S1 S2 S3 S4 S λ (nm) The wavelength range nm was selected. It was observed that optical transmittance of and MgO thin film increases with increase in wavelength. It also increased with increase in oxidation temperature and duration whereas decreased with increase in thickness of thin film i. e. as thickness of thin film increased optical transmittance percentage decreased. Vapor chopped thin 2 S1 S2 S3 S4 S λ (nm) Figure.3.1 Scatter diagraph of optical transmittance of vapor chopped and nonchopped MgO thin films 18

14 films showed higher transmittance than the nonchopped MgO thin films for each thickness as well as for each oxidation temperature and duration. Larger difference in transmittance between and was observed in the lower wavelength range (<55 nm) than the higher range. T % K for 9 min λ (nm) T % 673 for 18 min λ (nm) 623 K for 9 min 623 K for 18 min T % 6 4 T % λ (nm) λ (nm) 573 K for 9 min for 18 min T % T % λ (nm) λ (nm) Figure 3.11: Optical transmittance spectra of vapor chopped () and nonchopped () MgO thin films for thickness 3 nm. 19

15 K For 9 min K For 18 min T % T % λ (nm) λ (nm) K For 9 min K For 18 min 8 8 T % T % T % λ (nm) 673 K For 9 min λ (nm) T % λ (nm) 673 K For 18 min λ (nm) Figure 3.12: Optical transmittance spectra of nonchopped () and vapor chopped () MgO thin films for thickness 45 nm. 11

16 573 K - 9 min 573 K - 18 min T % T % λ (nm) λ (nm) 623 K - 9 min 623 K - 18 min T % 4 T % λ (nm) λ (nm) T % 673 K- 9 min λ (nm) T % 673 K-18 min λ (nm) Figure 3.13: Optical transmittance spectra of nonchopped () and vapor chopped () MgO thin films for thickness 6 nm. 111

17 Fig shows the optical transmittance of vapor chopped and nonchopped MgO thin films of 3 nm thickness. Comparing figures 3.11, 3.12 and It is seen that the transmittance of the film both and beyond 6 nm reaches to a value of ~85% irrespective of the thickness, oxidation temperature and oxidation duration. The films of 6 nm thickness shows a value of ~7 %. In the wavelength less than 6 nm decreasing trend of transmittance as observed being more prominent for films. At 35 nm large thickness and oxidation duration effect are obtained. The transmittance becomes ~4% in the MgO thin film of 6 nm thickness Optical Band gap: There are two types of band gaps, one is semiconductor band gap and other is optical band gap. The semiconductor band gap is the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band. The energy required for an electron to move from the valence band to the conduction band, whereas in optical band gap photons (packet of energy in the form of light waves) are assisting the electrons to move from valence band to conduction band. The optical energy band gap of magnesium oxide thin films was estimated from optical absorption measurement. The optical absorption spectrum was recorded in the wavelength range of 35 nm to 85 nm at room temperature. The equation has been elaborated in chapter I, the band gap was calculated by: α = [α o (hυ-eg) n ] / hυ Where Eg is the separation between bottom of the conduction band and top of the valence band, hυ is the photon energy and n is a constant. The value of n depends on the probability of transition; it takes values as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transition respectively. The optical band gaps of the three thicknesses were studied. Figures 3.14 show typical optical absorption spectra of MgO thin films. The optical absorbance 112

18 was decreased with wavelength and thickness of thin film. thin films showed lower absorbance than nonchopped films nm 45 nm 6 nm nm 45 nm 6 nm Absorbance (αt) Absorbance (αt) λ (nm) λ (nm) Figure 3.14 Absorption spectra of vapor chopped and nonchopped MgO thin films oxidised at 673 K for different thicknesses. In figure 3.15 doted ( ) line denotes the band gap of vapor chopped MgO thin films while continuous single line is for nonchopped thin films. The left hand side bar is for and right hand side bar for. It shows that vapor chopped thin films have higher band gap than the nonchopped. The band gap values are tabulated in table 3.2. The band gap range was in between 2.5 to 3 ev. The band gap increases with increase in oxidation temperature and duration of MgO thin films having thickness 3 nm. The difference in band gap between and films oxidised for 18 min is very less. Oxidation Temperature (K) Band gap (ev) 9 min 18 min 9 min 18 min Table: 3.2 Direct band gap of nonchopped () and vapor chopped () MgO thin films of thickness 3 nm. 113

19 K- 9 min K- 18 min hυ (ev) hυ (ev) 623 K- 9 min 623 K- 18 min hυ (ev) hυ (ev) 673 K- 9 min 673 K- 18 min hυ (ev) Figure 3.15: against hυ for nonchopped () and vapor chopped () MgO thin films of thickness 3 nm hυ (ev) 114

20 673 K- 9 min 673 K-18 min hυ (ev) hυ (ev) 573 K- 9 min 573 K-18 min hυ (ev) hυ (ev) 623 K- 9 min 623 K-18 min hυ (ev) hυ (ev) Figure 3.16: against hυ for nonchopped () and vapor chopped () MgO thin films of thickness 45 nm. 115

21 The band gap of and MgO thin films are in the range the 2.8 to 3 ev. Band gap increases with increase in oxidation temperature and duration. The films show slightly large band gap as compared to films. Oxidation Temperature (K) Band gap (ev) 9 min 18 min 9 min 18 min Table 3.3: Direct band gap of nonchopped () and vapor chopped () MgO thin films of thickness 45 nm The band gap of vapor chopped and nonchopped MgO thin films of 6 nm is shown in fig The band gap range is 2.87 to 3.21 ev. The band gap of vapor chopped MgO thin films were higher than the nonchopped thin films. From table 3.4 it is observed that, band gap of vapor chopped and nonchopped MgO thin films increases with increase in oxidation temperature and duration. Comparing the three thicknesses; it is seen that there is no systematic increase or decreases in band gap with thickness of the MgO thin film. Oxidation Band gap (ev) Temperature (K) 9 min 18 min 9 min 18 min Table 3.4: Direct band gap of nonchopped () and vapor chopped () MgO thin films of thickness 6 nm. 116

22 2 Nc 573 K- 9 min Nc 573 K- 18 min Vc Vc hυ (ev) hυ (ev) Nc Vc 623 K- 9 min Nc Vc 623 K- 18 min hυ (ev) hυ (ev) Nc Vc 673 K- 9 min Nc Vc 673 K- 18 min hυ (ev) hυ (ev) Figure 3.17: against hυ for nonchopped () and vapor chopped () MgO thin films of thickness 6 nm. 117

23 3.5.3 Refractive index: It this work, vapor chopped and nonchopped magnesium oxide thin films were used as optical waveguide. The refractive index of magnesium oxide thin films was studied for different oxidation temperature and durations. 3 nm, 45nm and 6 nm thickness of thin films were used. The analytical method has been used for the calculation of refractive index n using the following formula [1]. 2 ns T f + n n = T f + ns s ( 1+ R f ) 2 ( 1 R ) where, n = refractive index of the film, n s = refractive index of the substrate T f = transmittance of the film, R f = reflectance of the film f 2 1/ 2 Thin film Thickness (nm) Oxidation Temperature (K) Refractive Index 9 min 18 min 9 min 18 min Table 3.5: Refractive index of nonchopped () and vapor chopped () MgO thin films for different oxidation temperature and durations, with different thicknesses. 118

24 The refractive index values of nonchopped and vapor chopped magnesium oxide thin films oxidized for different oxidation temperatures and duration for different thicknesses is tabulated in Table 3.5. The change in refractive index values due to varied oxidation temperature and duration was observed. It was seen that, refractive index increased due to increased oxidation temperature and duration. The refractive index of MgO thin films increased with increasing thickness of thin film. Table 3.4 shows that refractive index of MgO thin films in the range of for and 1.62 to 1.7 for for 6nm thickness, whereas it was for and for of thickness 45nm and for and for of 3 nm thickness thin film. Vapor chopped films showed lesser refractive index values than those of nonchopped films for all conditions. The refractive index of both vapor chopped and nonchopped thin films are in the range obtained by magnetron sputtering [2]. 3.6 Optical waveguiding properties: For the thin films waveguide applications the optical transmission loss was studied by prism coupling method [3]. The optical signal transmission loss for various oxidation temperatures (573, 623, and 673 K) of vapor chopped and nonchopped magnesium oxide thin film waveguide is shown in fig The effect of film thickness is also plotted in this figure. It was observed that, transmission loss increases with increase in oxidation temperature as well as due to increase in thin film thickness. The films of higher thickness 6 nm showed higher transmission loss. Table 3.6 shows that, signal transmission loss for nonchopped 3 nm thickness thin film in range db/cm, while it becomes , db/cm for 45 and 6 nm respectively. It was , and db/cm for 3, 45 and 6 nm vapor chopped MgO thin films respectively. Vapor chopped MgO thin film showed lower transmission loss as compared to nonchopped thin films. 119

25 Optical Transmission Loss (db/cm) nm 3 nm 45 nm 45 nm 6 nm 6 nm Oxidation temperature (K) Figure 3.18: Optical transition loss of and MgO thin films for different oxidation temperatures (573, 623, and 673 K) and for different thicknesses. Oxidation temperature (K) Transmission Loss (db/cm) 3 nm 45 nm 6 nm Table 3.6: optical transmission loss for vapor chopped and nonchopped magnesium oxide thin films for various oxidation temperatures and thicknesses. Oxidation temperature effect was more prominent then the thickness of deposited MgO thin films. 12

26 3.7 Secondary electron emission coefficient measurements: MgO plays an important role as protecting layer in ac plasma display panel (PDP). The exposure of the protecting layer to the discharge space has an influence on the discharge characteristic and lifetime of PDP. The efficiency of MgO layer was measured by measuring secondary electron emission coefficient. This work was done by using the in house built secondary electron emission coefficient measurement system at IIT Kanpur. The measurement system setup has the similar conditions to those existing in real PDPs [4-6] Y (.65) Y (.1) Y (.12) Y (.2) before plasma cleaning after plasma cleaning -673 K V F (Volt) pxd (Torr.cm) Y (.8) Y (.1) Y (.16) Y (.2) before plasma cleaning after plasma cleaning -673 K V F (Volt) pxd (Torr.cm)

27 35 N C -623 K 3 25 V F (Volt) Y (.1) Y (.2) Y (.23) Y (.3) V f pxd (Torr.cm) V C -623 K 25 V F (Volt) Y (.1) Y (.2) Y (.24) Y (.3) V f 5 pxd (Torr.cm ) N C K V F (Volt) Y (.1 ) Y (.2 ) Y (.2 5 ) Y (.3 3 ) V f p xd (T o rr.c m ) 122

28 K 3 25 V F (Volt) Y (.1) Y (.2) Y (.26) Y (.33) Vf 5 pxd (Torr.cm) 1 15 Figure 3.19: Experimental paschen curves, V f vs (p x d) measured for the nonchopped (a, b, c) and vapor chopped (ac, bc, cc) MgO thin films. Figure 3.19 shows the SEE yield varies with (p x d) values. The plasma cleaning process improves the surface characteristics of MgO layer by removing adsorbed impurities on thin film. Such impurities can affect the firing voltage as well as lifetime of PDP. During the cleaning, firing voltages reduced in both and MgO thin films, but Vf reduction in thin film was higher than the vapor chopped MgO film (Vf before and after cleaning). Oxidation Temperature (K) Secondary electron emission coefficient (γ) Nonchopped Vapor chopped Table 3.7: Secondary electron emission yield of nonchopped and vapor chopped MgO thin films for different temperatures 123

29 The firing voltage decreased in vapor chopped thin films more than nonchopped MgO thin films, while it goes on increasing with oxidation temperature. Table 2 shows the SEE yield values. The SEE γ yield values decreased with increase in oxidation temperature. Vapor chopped MgO films showed higher SEE γ yield and lower firing voltage than nonchopped MgO thin films. 3.8 Mechanical properties: Adhesion: Durability of film is of prime importance in various fields. Adhesion is related to the nature and the strength of the binding forces at interface between the two materials in contact with each other, so that a study of thin film adhesion is of both fundamental and practical interest. Adhesion of thin film is related to the bonding between film-substrate interface contact points and crystallinity of thin film. The concentration of contact points is directly proportional to the adhesion of thin film. The contact point concentration changes due to the surface morphology of thin film; hence adhesion varies with surface morphology of thin film. 7 Adhesion 6 Adhesion ( 1 3 N/m 2 ) Temperature (K) Figure 3.2: Adhesion of Vapor chopped () and Nonchopped () MgO thin films for difference durations (9 min and 18 min) of thin film thickness 6

30 Table 3.8 shows the adhesion of vapor chopped and nonchopped MgO thin films for different oxidation temperature and duration for thickness 3 and 45 nm. The adhesion of MgO thin films of 3 nm thickness was in between x 1 3 N/m 2 for nonchopped thin films while it was between x 1 3 N/m 2. For the film of 45 nm thickness the adhesion was x 1 3 N/m 2 for and x 1 3 N/m 2 for thin films. The graphical presentation of adhesion Vs oxidation temperature for the film of 6 nm thickness is shown in fig It was observed that, adhesion increases with increase in oxidation temperature and duration. The adhesion of vapor chopped MgO thin films was higher than nonchopped thin films for all conditions. Sample to sample variation (error) was larger for films of lower thickness. Thickness (nm) 3 45 Oxidation Adhesion ( x 1 3 N/m 2 ) Temperature (K) 9 min 18 min 9 min 18 min ± ± 5 322±3 346± ± ± 1 247±3 319± ± ± 1 198±2 217± ± ± 3 323±4 47± ± ± 7 31±1 387± ± ± 6 223±8 267±6 Table 3.8: Adhesion and intrinsic stress of MgO thin film oxidized at various temperatures and durations for various thicknesses for 3 nm and 45 nm thickness Stress: The stress of thin film originate due to dislocations, voids, or impurities present in thin film or due to lattice mismatch, difference in thermal expansion of film-substrate interface etc. The existences of such stress have considerable 125

31 importance in the use of thin films in microcircuit and optoelectronics device fabrication technology. The stresses occurs due to defects, microstructural variation, material phase transformation, void formation during deposition or lattice mismatch between thin film-substrate interfaces [7, 8]. Several failure mechanisms due to formation of cracks or voids in thin film associated with the stress have drawn attention in integrated optoelectronic circuit industry. [9]. In this work total stress was measured by interferometric method (Patil, et al., 1997). This method works on the principle of Newton s ring as explained in chapter II. The stress was calculated by measuring the variation in diameter of Newton s ring before and after deposition. The stress was measured by the equation 2 Yh ( K x K S = 6t(1 υ) y ). (1) where, Y = Young s modulus, υ = Poisson s ration, t = film thickness, h =.22 cm (substrate thickness), Kx, Ky = Slope difference of plot nλ/2 Vs radius Newton s ring of before and after deposition. The total stress consist of sum of thermal and intrinsic stress. Thermal stress develops because of the difference between the thermal expansion of the film and the substrate. The thermal stress was calculated by equation of Hussain, 1989 [1]. S th = α α ) Y ( T T ). (2) ( f s f d m where, αf thermal coefficient of film, αs thermal coefficient of substrate, Yf - Young's modulus for the film, Td oxidation temperature, Tm - Temperature at the time of stress measurement. (Room temperature ~ 3 K) The intrinsic stress occurs due to defects, microstructural variation, material phase transformation, voids formation during deposition or lattice mismatch between thin film-substrate interfaces. The intrinsic stress was obtained from equation [1] 126

32 S = S (3) in S th The calculated thermal stress from equation (2) (Hussain, 1989) of vapor chopped and nonchopped MgO thin film is found to be 33.1, 22.4, and x 1 7 N/m 2 for 673 to 573 K oxidation temperature respectively. The vapor chopped and nonchopped thin films were oxidized simultaneously at the same temperature conditions for various oxidation temperatures, so the thermal stress at a certain oxidation temperature remains the same for both vapor chopped and nonchopped thin films, because it mainly depends on the difference between thermal expansion coefficient of the thin film material and substrate material. The changes in total stress might be due to the microstructural variations i.e. due to intrinsic stress originating during the thin film growth. Oxidation Intrinsic Stress ( x 1 7 N/m 2 ) Thickness Temperature (nm) (K) 9 min 18 min 9 min 18 min ±.7 8.4± ± ± ± ± ± ± ± ± ±.2 3.2± ±4 72.2± ± ± ± ± ± ± ± ± ± ±.2 Table 3.9: Adhesion and intrinsic stress of MgO thin film oxidized at various temperatures and durations for various thicknesses. Figure 3.21 shows the graphical presentation of intrinsic stress of and MgO thin films of thickness 6 nm. The table 3.9 gives the information about the intrinsic stress of MgO thin films for various oxidation temperature and durations of 3 and 45 nm thicknesses. 127

33 65 Intrinsic stress Intrinsic Stress (x 1 7 N/m 2 ) Temperature (K) Figure 3.21: Intrinsic stress of Vapor chopped () and Nonchopped () MgO thin films for 9 min and 18 min duration of thin film thickness 6 nm. From fig it is seen that, the intrinsic stress increased with increase in oxidation temperature and duration, whereas intrinsic stress decreased with increase in thickness of thin film. The intrinsic stress of vapor chopped MgO thin film was lesser than the nonchopped thin film. The error in measurement is showed by error bar in figure while it is shown by ± factor in table. These readings are the average values of 5 samples. The intrinsic stress was in the range between x 1 7 N/m 2 for nonchopped thin films of 3 nm. While it was x 1 7 N/m 2 for vapor chopped thin films. The intrinsic stress results for the thin films of 45 nm was x 1 7 N/m 2 for and x 1 7 N/m 2 of thin films. 3.9 Air exposure effect study: The tendency to change the material property due to the surrounding ambient is called ageing of that material. Performance of any device depends upon stability of the properties of the material. 128

34 For material air exposure effect study, comparison between fresh and ambient exposed thin film have to be studied. The short term (24 hours) and long term (3 days) ambient exposure was investigated, its effect on the various properties of MgO thin films were studied. The crystal structure, surface morphology, optical and mechanical properties were investigated after exposure to room temperature air ambient for different periods of time Crystal structure: Figure 3.22 shows the X-ray diffraction patterns of the fresh and ambient air exposed (aged) vapor chopped and nonchopped magnesium oxide thin films oxidized at 673 K for 9 min duration for various air exposure durations. The films show, the dominant cubic (2) and (22) phase. Even after the air exposure about 3 days, the crystallographic orientation did not show any significant change. The air exposed vapor chopped thin films showed higher crystallinity than the nonchopped MgO thin films. Intensity (a.u.) (2) (22) Fresh MgO thin films A- Vapour chopped B- Nonchopped A B Intensity (a.u.) (2) Atmospheric ambient exposed (22) A B θ degrees θ degrees Figure 3.22: X-ray diffraction patterns of fresh and ambient air exposed (aged) MgO thin films. 129

35 3.9.2 Surface Morphology: The SEM of vapor chopped () and nonchopped () MgO thin films oxidized for 673 K for 9 min. is shown in fig It is seen that the shiny surface morphology of vapor chopped and nonchopped films having granular structure, turns to dull surface after air exposure for 3 days. The nonchopped MgO thin film showed formation of cracks after air exposure whereas vapor chopped thin film did not develop any cracks. Similar effects were observed for the thin films oxidized at 573 and 623 K also. a) b) c) d) Figure 3.23: SEM of the fresh (a, b) and air exposed (c, d) vapor chopped and nonchopped MgO thin films. 13

36 Although the crystallographic orientation did not show any significant change during air exposure, the surface morphology was seriously altered in both the vapor chopped and nonchopped MgO thin films. The vapor chopping inhibited the crack development in thin films Atomic force micrographs: The fresh and air exposed thin films (for 3 days) AFM images of MgO thin films of thickness 3 is given in fig It was observed that, the scratches occurred on the surface of thin films after 3 days. This effect was also clearly observed in scanning electron micrographs. These scratches disturb the surface morphology of deposited thin films which may be the cause of performance reduction. Figure 3.24: Atomic force micrographs of fresh and 3 days air exposed vapor chopped and nonchopped thin films. 131

37 3.9.4 Optical properties: Optical transmittance Figure.3.25 shows the optical transmission curves of fresh and ambient air exposed vapor chopped and nonchopped magnesium oxide thin films for various duration of exposure. It was seen that the transmittance varied significantly with the ambient exposure time. Fresh vapor chopped and nonchopped thin films showed higher transmittance as compared to air exposed films. The optical transmittance reduction in vapor chopped films was comparatively lower than the nonchopped MgO thin films. The air exposure effects are more prominent in the wavelength range lower than 5nm T.7.6 T fresh 24 hours 1 days 3 days λ (nm) λ (nm) Figure 3.25: Optical transmittance of the fresh and air exposed vapor chopped and nonchopped MgO thin films for various exposure durations Optical band gap Optical band gap of air exposed vapor chopped and nonchopped MgO thin films for different exposure duration shown in fig The observed values of direct band gap are tabulated in Table Due to ambient air exposure the band 132

38 gap decreased. The band gap reduction in nonchopped thin film was higher than vapor chopped. The band gap values of nonchopped films (2. 78 to 2.91 ev) was found lesser than those of vapor chopped films (2.85 to 2.95 ev). The band gap of bulk magnesium oxide is 7.8 ev. Similarly lower band gap as compared to bulk has been reported [11] Fresh (α thυ ) (α thυ) hours 1 days 3 days hυ (ev) hυ (ev) Figure 3.26: against hυ for the fresh and ambient air exposed vapor chopped and nonchopped MgO thin films. Exposure Direct Band Gap period (Day) Fresh Table 3.11: Direct Band Gap of vapor chopped and nonchopped MgO thin films for various ambient exposure durations Refractive index The refractive index values of fresh and ambient air exposed vapor chopped and nonchopped magnesium oxide thin films oxidized at 623K are tabulated in Table 3.1. The refractive index increased due to ambient air exposure. Vapor 133

39 chopped films showed lesser increase in refractive index than the nonchopped films. Exposure Refractive Index period (Day) Fresh Table 3.1: Refractive index of vapor chopped and nonchopped MgO thin films for various ambient exposure durations Optical waveguiding properties: The optical signal transmission loss after air exposure of vapor chopped and nonchopped magnesium oxide thin film waveguide for various oxidation temperatures (573, 623, and 673 K) shown in fig Transm ission loss (db/cm ) Transm ission loss (db /cm ) K 623 K 673 K K 623 K 673 K Day (s) Day (s) Figure 3.3: Optical transmission loss variation of vapor chopped () and nonchopped () MgO thin films of 3 nm thickness for different air exposure duration 134

40 The transmission loss increases with ambient air exposure duration. Short term effect was so drastic i.e. in the first 24 hours a drastic enhancement in transmission loss was observed, whereas after this rate of transmission loss reduces and finally became saturated. The signal transmission loss variation due to ageing was lower in vapor chopped MgO thin films than nonchopped films. For higher thicknesses i. e. 45 and 6nm, it was found that the signal transmission loss variation pattern was same. Oxidation temperature Transmission Loss (db/cm) (K) Fresh hours days days days Table 3.12: Optical signal transmission loss of MgO thin film waveguide after aging of 3nm Table 3.12 shows that, the optical signal transmission loss after ageing of MgO thin film for thickness 3 nm. The optical transmission loss was 6.76 db/cm for deposited fresh ( day) nonchopped MgO thin films oxidized at 573 K, after 3 days it reaches to 8.32 db/cm via 7.78, 8.16, 8.24 db/cm loss increments with 1, 1 and 2 days respectively. The same pattern (fresh ( day) to 3 day air exposed) for MgO thin films deposited for 623 and 673 K was in between and db/cm range. The transmission loss for vapor chopped MgO thin films was in between , and db/cm ranges oxidized the films at 573, 623 and 673 K respectively. The optical transmission loss values of all the above cases were found lesser than reported optical transmission loss values (3 db/cm) [12]. 135

41 3.9.6 Secondary electron emission property: Secondary electron emission (SEE) measurement is one of the costly characterization. For the characterization of single sample three days are required, the sample should be kept under vacuum in this period. Magnesium oxide thin film has the maximum change in characteristic properties like optical and mechanical properties within first three days. So instead of studying the separate ageing study of sample, the SEE coefficient was measured before and after plasma cleaning. This process was required to remove the moisture and other absorbed gases from films which mainly causes the ageing of thin films. Fig showed that the change in SEE coefficient of MgO thin films oxidized at 673 K. In case of secondary electron emission coefficient ageing study, the SEE coefficient was increased after plasma cleaning in both and MgO thin films. The nonchopped MgO thin films showed SEE coefficient as.65 before the plasma cleaning whereas vapor chopped MgO thin films showed 1. the SEE coefficient of nonchopped MgO thin films changes form.65 to 1.12 before and after the plasma cleaning, whereas in case vapor chopped MgO thin film it changes from 1 to The increase in SEE coefficient of nonchopped MgO thin film was higher than the vapor chopped thin films. An increase in firing voltage causes the increase in operational cost. The firing voltage reduces due to the plasma cleaning of MgO thin films. The reduction in firing voltage in vapor chopped MgO thin film was higher than the nonchopped films. It was V before the plasma cleaning of nonchopped MgO thin film and reduces up to V after cleaning. Vapor chopped MgO thin films shoed the reduction from 16-2 V to V due to cleaning. Vapor chopped thin films showed lower range of firing voltage before the plasma cleaning as well as after the cleaning than the nonchopped thin films. Due to the coast effective and time consuming characterization the ambient air exposure effect and MgO thin films oxidize at 623 and 573 K was not attempted. The thickness effect study was also not attempted.. 136

42 Y (.65) Y (.1) Y (.12) Y (.2) before plasma cleaning after plasma cleaning -673 K V F (Volt) pxd (Torr.cm) Y (.8) Y (.1) Y (.16) Y (.2) before plasma cleaning after plasma cleaning -673 K V F (Volt) pxd (Torr.cm) 1 15 Figure 3.29: Experimental paschen curves, V f vs (p x d) measured for the plasma cleaning effect of vapor chopped and nonchopped MgO thin films Mechanical Properties: Adhesion Figure 3.27 shows that adhesion of the fresh and ambient air exposed vapor chopped and nonchopped magnesium oxide thin films oxidized at 573, 623 and 137

43 673 K temperatures for various exposure durations. Adhesion of vapor chopped and nonchopped thin films decreased with increase in exposure time. A drastic decrease of adhesion occurs within first 24 hours (short term) for all the temperatures, after that till 3 days (long term) adhesion decreases continuously and finally tends towards saturation. The decrease in adhesion occurs due to absorption of the moisture from air. Adhesion of thin film is related to the bonding between film-substrate interface contact points. The absorbed moisture occupies the pores and reduces the film-substrate interface contact points. Vapor chopped MgO thin films showed lesser ambient air exposure effect on adhesion (from 37.7 to 35.5 x 1 4 N/m 2 ) than nonchopped MgO thin films (from 29.8 to 22.3 x 1 4 N/m 2 ) oxidized at 673 K. Similar vapor chopping effects on thin films oxidized at 573 and 623 K was observed. The adhesion of vapor chopped and nonchopped thin films was drastically decreased within the first 24 hours of ambient air exposure and continued to decrease gradually and finally attaining saturation. Nonchopped thin films showed comparatively larger decrease in adhesion than vapor chopped films and took longer time to saturate. 4 3 A dhe sio n ( 1 3 N /m 2 ) A dhesio n ( 1 3 N /m 2 ) K 623 K 573 K Exposure Duration (Days) K 623 K 573 K Exposure Duration (Days) Figure 3.27: Adhesion of the fresh and ambient air exposed vapor chopped and nonchopped MgO thin films for various exposure durations. 138

44 Intrinsic stress Figure 3.28 show the intrinsic stress of the fresh and ambient air exposed vapor chopped and nonchopped magnesium oxide thin films oxidized at different temperatures (at 573, 623 and 673 K) for various exposure duration. Intrinsic stress of vapor chopped and nonchopped thin films decreases with increase in exposure time. The intrinsic stress of vapor chopped and nonchopped thin films was drastically decreased within the first 24 hours of air exposure and continued to decrease gradually and finally attaining saturation. Nonchopped thin films showed comparatively larger decrease in stress than vapor chopped films and took longer time to saturate. The intrinsic stress of vapor chopped MgO thin films oxidized at 673 K becomes 27 x 1 7 N/m 2 from 41.1 x 1 7 N/m 2 after 24 hours and 23.8 x 1 7 N/m 2 after 5 days with 22.1 and 22 x 1 7 N/m 2 after 2 days and a month (3 days) respectively. The same results for nonchopped thin films was found to be 45.6 x 1 7 N/m 2 from 62.9 x 1 7 N/m 2 after 24 hours it became 37.1 x 1 7 N/m 2 and 35.5 x 1 7 N/m 2 after 2 and 3 days respectively. Thin films oxidized at 623 and 573 K also showed reduction in intrinsic stress in the same manner. In strin sic S tress (X 1 7 N /m 2 ) K 623 K 573 K Exposure Duration (Days) Instrinsic Stress (X 1 7 N /m 2 ) K 623 K 573 K Exposure Duration (Days) Figure 3.28: Intrinsic stress of fresh and aged MgO thin films oxidized at different oxidation temperatures. 139

45 3.1 SUMMARY OF SOME IMPORTANT RESULTS The summary of some important results observed from the investigation carried out as follows: 1. The vapor chopped and nonchopped MgO thin films oxidised at 673 and 623 K showed (2) and (22) cubic phases. 2. MgO thin film oxidised at 573 K showed amorphous nature. 3. Vapor chopped thin films showed higher crystallinity than nonchopped MgO thin films. 4. Thickness effect was not so prominent in the crystal structure. 5. Crystallite size increases with increase in oxidation temperature and duration. Thickness increase also causes the same effect. 6. Vapor chopped thin films showed lesser crystallite size. 7. Vapor chopped MgO thin films showed smoother morphology than nonchopped. 8. Surface smoothness deteriorates with increase in oxidation temperature. 9. From AFM it was observed that, surface roughness increases with increase in oxidation temperature. 1. Vapor chopped MgO thin films showed lower surface roughness than nonchopped thin films. 11. Vertical cross section SEM study of vapor chopped and nonchopped MgO thin films showed that, vapor chopped thin films were grown with reduced columnar structure and contained lesser voids formation in deposited thin film than the nonchopped MgO thin films. 12. Optical transmittance increases with increase in oxidation temperature, while it decreases with thin film thickness enhancement. 13. Vapor chopped MgO thin films showed higher transmittance than nonchopped films. 14. Optical transmittance decreases with increase in thin film thickness. 14

46 15. Vapor chopped thin films showed lesser optical absorbance than nonchopped MgO thin films. 16. Optical band gap increase with increase in oxidation temperature and duration. 17. Optical band gap increases with increases in thin film thickness as well as due to vapor chopping. 18. Refractive index increases with increase in oxidation temperature, duration and thin film thickness whereas it decreases due to vapor chopping. 19. Optical transmission loss increases with increase in oxidation temperature as well as thin film thickness. 2. Vapor chopped thin films showed lesser optical transmission loss than nonchopped films. 21. Secondary electron emission coefficient decreases whereas firing voltage increases with increase in oxidation temperature. 22. Vapor chopped thin films showed higher SEE yield and lower firing voltage than nonchopped films. 23. Adhesion of MgO thin films increases with increase in oxidation temperature, duration and thickness of thin film. 24. Vapor chopped thin films showed higher adhesion than nonchopped films. 25. Vapor chopped and nonchopped MgO thin film showed 33.1, 22.4, and x 1 7 N/m 2 thermal stress for 673 to 573 K oxidation temperature respectively. 26. Intrinsic stress increased with oxidation temperature and duration enhancement while it decreased with thin film thickness and due to vapor chopping. 27. No any change was observed in crystal structure due to air exposure. 28. Surface morphology was seriously altered in both the vapor chopped and nonchopped MgO thin films. Crack formation was observed in nonchopped 141

47 thin films while the vapor chopping inhibited the crack development in thin films. 29. The scratches were formed on thin film surfaces due to exposure effect, it was observed by atomic force microscopy. 3. Optical transmittance and band gap decreases due to exposure effect. Vapor chopped thin films showed lesser effect than nonchopped films. 31. Adhesion and intrinsic stress decreases due to exposure effect whereas vapor chopped MgO thin films showed lesser effect. 32. It was observed that, MgO thin films properties were drastically decreased in short term exposure duration and these properties were moved towards the saturation with respect to exposure duration. 33. Secondary electron emission coefficient decreases whereas firing voltage increases due to air exposure. 34. Optical transmission loss increased due to air exposure, vapor chopped thin films showed lesser reduction than nonchopped films. 142