Fig. 3.1 TG/DTG thermogram of the manganese oxalate precursor

Fig. 3.2 Mass spectrometric analysis of magnesium oxalate

350 400 450 500 550 600

20

30

40

50 0.2 M 0.3 M 0.4 M

Film

gro

wth

rate

(nm

/min

.)

Substrate temperature (oC)

Fig. 3.3 Growth rate vs. substrate temperature

Fig. 3.4 XRD patterns of MgO thin films deposited at different substrate temperatures (a) 400˚C (b) 450˚C (c) 500˚C (d) 550˚C (f) 600˚C

400 450 500 550 600

3.3

3.4

3.5

3.6

Density Grain size

Substrate temperature (oC)

Dens

ity (

g/cm

3 )

24

26

28

30

32

34

Grain size (nm

)

Fig. 3.5 Density and grain size variation as a function of substrate temperature

(c)

(b)

(a)

Fig. 3.6 Elemental composition of MgO films prepared at different substrate temperatures: (a) 450˚C, (b) 500˚C and (c) 550˚C

Fig. 3.7 SEM micrographs of MgO films deposited at (a) 450˚C (b) 500˚C (C) 550˚C

(b)

(a)

(c)

Fig. 3.8 3D and 2D AFM images of MgO films deposited at (a&d) 400˚C, (b&e) 450˚C, and (c&f) 500˚C

(c)

(a) (d)

(b) (e)

(f)

Fig. 3.9 Optical transmittance of the MgO films deposited on

quartz substrates at different substrate temperatures

200 400 600 800 1000 1200 1400 16000

20

40

60

80

100

Tran

smitt

ance

(%)

Wavelength (nm)

450oC 500oC 550oC

4.0 4.5 5.0 5.5 6.0 6.5 7.00

20

40

60

80

(αhν

)2 x109 (e

V/cm

)2

hν (eV)

450oC 500oC 550oC

Fig. 3.10 (αhυ)2 vs hυ plot for the MgO films deposited at different substrate temperatures

400 800 1200 1600

1.71

1.72

1.73

1.74

Wavelength (nm)

Ref

ract

ive

inde

x, n

0.03

0.04

0.05

0.06

Extinction co-efficient, k

Fig. 3.11 Variation of refractive index (n) and extinction co-efficient (k) for MgO film deposited at 500˚C

200 300 400 500 600 7000

200

400

600

800

1000

1200Emission

Emission

Excitation248nm 532nm

393nm

Inte

nsity

(a.

u)

Wavelength (nm)

Fig. 3.12 Luminescent spectra of MgO thin films prepared at 500°C

Fig. 3.13 Raman Spectrum of MgO thin film deposited at 500ºC

722

1100

Cou

nts (

a.u)

Fig. 3.14 FT-IR transmission spectra of MgO thin films prepared at different substrate temperature (a) 450˚C (b) 500˚C

Fig. 3.15 Sheet resistance variation as a function of temperature for the MgO films of different thicknesses

350 360 370 3801E8

1E9

1E10

1E11

497 nm 384 nm 280 nm

Shee

t res

ista

nce

(ohm

s/sq

u.)

Temperature (K)

350 360 370 380 3901E-7

1E-6

1E-5

1E-4

1E-3

490 nm 406 nm 301 nm

Con

duct

ivity

(Sc

m-1)

Temperature (K)

Fig. 3.16 Conductivity variations of MgO thin films with temperature

61

CHAPTER – III

PREPARATION AND CHARACTERISATION OF MgO DIELECTRIC FILMS

3.1 INTRODUCTION

Tremendous advances were made in the last few decades in research related to

synthesis, characterization and determination of processing dielectric thin films and

their applications to device fabrication and optimization strategies and concepts.

Besides, alkali halides and many oxides exhibit wide band gaps greater than 6 eV:

MgO, CaO, Al2O3, SiO2, Y2O3 etc. The refractory oxide magnesia finds many

attractive applications in diverse fields. Most of the existing qualities of MgO like

ionic conductivity, thermal and chemical stability make these films to pretend as a

buffer layer for depositing ferro-electric and superconducting films on various

substrates because of the lattice matching of majority of overlaying films. MgO is a

highly ionic crystalline solid, which crystallizes into a rock salt structure. It has fcc

Mg+ and O- sub-lattices, and low energy neutral (100) cleavage planes. The lattice

constant of MgO is 0.421 nm and its refractive index and dielectric constant are 1.736

and 10 respectively.

Moreover, it is very difficult to modify the physical and chemical properties of

binary materials and therefore recently tailoring the physico-chemical properties of

metal oxide thin films have been realized by engaging new semiconductor materials

consisting of multicomponent oxides. Based on this working hypothesis, potential

transparent conducting oxides like; MgIn2O4, ZnxMg(1-x)O, CdIn2O4, CdGa2O4,

ZnGa2O4, Zn2SnO4 Cd2SnO4 and Mg2SnO4 have been studied recently. Among these

ternary oxides, Mg2SnO4 thin film finds application as a new transparent as well as

active electrode in photo-electrochemical solar cells and as sensing element in gas

sensors. In addition, it exhibits a number of superior properties such as low resistivity,

high adhesion, thermal stability, low absorbance in the visible spectral region, and

more compatible to have smooth interface.

In spite of the progress in the development and application of the various film

deposition techniques, characterization is further necessary to optimize the deposition

parameters and conditions for most of them. Well-defined procedures to prepare

MgO/Mg2SnO4 surfaces of very high quality are of importance in a number of cases

62

of surface physics and materials science. MgO thin films can be prepared by using

variety of techniques. Techniques involved in general are (a) Thermal deposition in

vacuum by resistive heating, electron beam gun or laser beam gun evaporation, etc.

from suitable sources (b) Sputtering of cathode materials in the presence of inert or

active gases either at low or medium pressure (c) Chemical vapor deposition by

pyrolysis and dissociations in vapor phase (d) Chemical depositions including

electrodeposition, anodic oxidation, electroless plating, chemical displacements,

chemical reactions etc. To obtain MgO films with strong preferred orientation and

good quality, however one needs high temperature heat treatment above 450ºC or

MgO single crystal as a source material.

The spray pyrolysis is one of the high temperature processing techniques and

provides large area coatings without high vacuum ambience. So the capital cost and

the production cost of high quality metal oxide thin films are expected to be the

lowest among all the thin film deposition techniques. Furthermore, this technique is

also compatible with mass production system. Also, the coatings produced with the

spray pyrolysis are inherently uniform and the surfaces to volume ratio of the

nanodrops are very large making them very receptive to heat treatment and pyrolysis.

Further, relatively moderate temperature heat treatment is necessary for the growth of

highly oriented thin films on different semiconductor substrates for device

developments.

In this chapter, the preliminary results on the growth of MgO and Mg2SnO4

thin films on quartz substrates using magnesium oxalate and tin oxalate as the starting

precursor in ethanol. The deposition conditions employed in this study enabled us to

deposit thin films of MgO without cracking and inter-diffusion. The structural,

optical, electrical and morphological properties are reported.

3.2 PREPARATION OF MgO THIN FILMS

Spray Pyrolysis involves spraying a solution, usually aqueous/alcoholic of the

constituent atoms of the desired compound onto a heated substrate placed inside the

reaction chamber. Every sprayed droplets reaching the hot substrate surface

undergoes pyrolytic decomposition and forms a single crystallite or a cluster of

crystallites of the product. The other volatile by-products and the excess solvent

escape in the vapor phase. This method is suitable for large area depositions for

certain applications. MgO thin films with superb refractory property and low

63

sputtering rate allow an important application in a plasma display panel. Because of

large PDPs upto 60 inches in size are being developed. Spray pyrolysis method is

suitable for oxide thin films among the available deposition techniques. Many

research groups employed this technique for preparing magnesia thin films. The

detailed preparation and the optimization steps involved in SP technique for the

preparation of high quality MgO and Mg2SnO4 thin films are given in this section.

Prior to deposition, the thermal decomposition of the precursor is prerequisite to

tentatively fix the deposition temperature. Hence a devoted step has been taken to

study the decomposition pattern of the starting precursor magnesium oxalate through

thermal study.

3.2.1 TGA/DTA Studies on Magnesium Oxalate (MgC2O4. 2H2O)

Before thin film formation, thermal analysis was performed on the starting

precursor magnesium salt to fix the substrate temperature range for processing MgO

films. The TG/DTG thermograms are recorded in air atmosphere. TG curve yields

information to determine the weight loss by heating the sample at a given

temperature. From this data, the composition of the compound and the reactions

involved in its decomposition can be determined. From the percentage of total weight,

the theoretical weight loss for the various stages of decomposition can be obtained.

Usually, the horizontal portion indicates the regions where there is no weight change

and from that, the thermal stability of the material can be understood. In addition, the

horizontal levels indicate a definite stoichiometry of the precipitate and from TGA

one can determine the correct drying temperature of precipitates. Alternately,

differential thermo gravimetry (DTG) consists of a record of the temperature at which

decomposition exactly takes place.

Fig. 3.1 shows the typical thermogram of the starting material magnesium

oxalate, which is decomposed in air at atmospheric pressure. Since spray pyrolysis

deposition is carried out in air atmosphere, only the decomposition behavior of the

salts in the recorded entire temperature range (30-600°C) in air atmosphere is

discussed.

It is observed from Fig. 3.1 that the thermal decomposition of hydrated

magnesium oxalate shows two steps with mass losses at 148 and 397 oC of 22.8% and

46.6%. These values correspond with a theoretical mass loss of 24.3 and 48.5% which

64

is in good agreement with the experimental values. It is proposed that the weight loss

steps occur according to the following reactions:

Step 1:

MgC2O4.2H2O → MgC2O4 + 2H2O

Step 2:

MgC2O4 → MgO + CO + CO2

The thermal decomposition of magnesium oxalate occurs via two steps by

conversion of the magnesium oxalate hydrate to the anhydrous compound and then

the decomposition of the oxalate to the magnesium oxide and carbon monoxide and

carbon dioxide. Although there is no evidence for the breakdown of the last step into

two successive steps, it is possible that the magnesium oxalate decomposed to

magnesium carbonate and carbon monoxide with the successive breakdown of the

magnesium carbonate to magnesium oxide and carbon dioxide. The use of vibrational

spectroscopy may assist in the analysis of this reaction. The values for the mass loss

steps are in agreement with the previously published data [1,2]. The thermal analysis

of magnesium oxalate is often obtained by its measurement in mixtures with calcium

or manganese oxalates [3].

In addition, from the DTG plot it is confirmed that the dehydration phenomena

takes at about 148 oC and the dehydrated manganese oxalate decomposed to

manganese oxide by releasing carbon monoxide and carbon dioxide at about 397 oC.

The TGA instrument was coupled to a Balzers (Pfeiffer) mass spectrometer

for gas analysis. Only selected gases were analyzed. Mass spectrometry of the

evolved gases through the thermal treatment of magnesium oxalate is shown in

Fig. 3.2, which confirms that water vapour is evolved over the 136 to 175 oC

temperature range and peaks at 146 oC and that both water and carbon dioxide are

evolved simultaneously over the temperature range 222 to 360 oC. A second carbon

dioxide evolution takes place over the 362 to 444 oC temperature range.

3.2.2 Preparation of MgO Film with Magnesium Oxalate

MgO films with highly preferential orientation were prepared from various

starting precursors in chemical solution deposition technique. Some of the precursors

used are Mg (2,4-pentanedionate)2 [4], magnesium acetylacetonate [5, 6] Mg(tmhd)2

65

[bis(2,2,6,6-tetramethyl-3,5-heptanedionato) magnesium (II)] [7] and magnesium

acetate [8, 9] with ethanol/isopropanol as solvent. In the present work, magnesium

oxalate was used as starting material. Magnesium oxalate (MgC2O4.2H2O) was

dissolved in 100% ethanol for a concentration of 0.1 M. A volume of 50 ml precursor

solution was sprayed for all the depositions.

The precursor solution of MgO was prepared via the sol-gel processing of the

salts in ethanol separately using few drops of an acid catalyst (HCl) to stabilize the

solution. Ethanol was used as the principal solvent and triethylene glycol (TEG) was

used as an additional solvent to facilitate high temperature processing [8]. Precursor

liquid flow rate was controlled using an adjusting valve and moisture filtered air was

used as the carrier gas whose flow rate was monitored using a flow meter. The

substrate was kept on a stainless steel plate and heated to a desired temperature. The

precursor liquid flow rate was adjusted along with the carrier gas pressure to obtain

uniform distribution of micro-sized droplets. The spray deposition parameters were

kept constant for all the precursors, which are summarized in Table 3.1.

Table 3.1

Optimized deposition parameters for the preparation of MgO films

Spray parameters

Values

Substrate

Substrate temperature

Solution feeding rate

Carrier gas pressure

Deposition time

Substrate-nozzle distance

Quartz

500°C

2.5ml/min

0.5 kg/cm2

15 min

28 cm

66

3.2.3 Thickness Measurement

Thickness of the films was calculated from the mass of the substrate alone and

the mass of the substrate deposited material using the gravimetry method. The

standard density of magnesium oxide (3.58 g/cm3) was used for the calculations [10].

The film thickness was also measured with the Stylus thickness profiler. Thickness of

the optimized MgO film measured using gravimetry method was 0.49 μm. The film

thicknesses evaluated using gravimetry method is found to be in good agreement with

the values obtained from the profiler (0.46 μm).

3.2.3.1 Substrate Temperature Effect on Film Thickness

The growth rate of MgO films as a function of substrate temperature was

determined by using the 0.2, 0.3 and 0.4 M magnesium oxalate in ethanol for a carrier

gas pressure of 0.5 kg/cm2 with a fixed flow rate of 2.5 ml/min and the growth rate

variations are shown in Fig. 3.3.

It is observed that the growth rates have a similar trend of variations with the

rise in substrate temperature. In the lower temperature range Ts < 500 oC, the growth

rate of the MgO film increases with substrate temperature and this is because of the

increase in reaction rates of hydrolysis of the magnesium acetate solute molecules.

They reached a maximum at 500 oC and then decrease as the temperature is increased

further because of depletion of reactants. In addition, the rise in substrate temperature

causes an increase in the re-evaporation rate of initial product, leading to diminished

mass transport towards the substrates, which results into decrease in film thickness.

This decrease may also be explained by a lower sticking co-efficient of Mg on the

substrate surface at higher temperatures. This result is consistent with the previous

works on MgO thin films [11, 12].

It is noticed that the growth rate increases with increase in molarity of the

precursor solution. The continuous increase in growth rate with molarity indicates that

the growth rate may be governed by the Mg-containing species. In spray pyrolysis

technique, it is difficult to control the incorporation of O2 into the film during the

growth process because the film grows in atmospheric conditions. So the

stoichiometry is controlled only by the Mg species. The availability of O2 from the

solvent is insufficient for further increase in growth rate in higher molarity precursor

[13]. Consequently, the growth rate decreases with further increase in the molarity of

the precursor solution. In the present case, growth rate of films from 0.4 M precursor

67

solution (42 nm/min) is less than that of 0.3 M precursor solution (48 nm/min) at the

deposition temperature Ts = 500 oC. The yield of the process is limited by the capacity

of the atomizer and therefore for larger concentration of the solutions greater energy

has to be needed to stimulate the process of spray [9].

3.3 STRUCTURAL ANALYSIS

Prepared MgO films were characterized by XRD to obtain the structural

parameters as a function of the deposition parameters. The XRD patterns recorded for

the deposited MgO films revealed only the (200) and (220) peaks for most of the

deposition conditions with (200) as a predominant peak. This is because of the low

crystallization energy requirements for the formation of (200) plane.

3.3.1 Effect of Substrate Temperature on Crystallization

XRD patterns of MgO thin films prepared for different substrate temperatures

between 400 and 600 oC using 0.3 M metal oxalate precursor is shown in Fig. 3.4.

The other deposition parameters were being kept at the optimum conditions

throughout the experiment. The XRD spectra clearly indicates that the structural

transition above 400 oC is due to the crystallization of the deposited MgO films and

only two orientation are observed along the (200) and (220) directions. Further, it

indicates that the MgO films deposited below 400 oC are amorphous; whereas those

deposited at higher temperature are in crystalline phase, with (200) being the

dominant orientation. At lower substrate temperatures, the deposited films are

amorphous, because the available thermal energy may not be sufficient for the

formation of MgO as revealed from the thermal analysis. At higher temperatures,

surface mobility of the atomic species is dominant and hence produced MgO films are

in the well crystallized form.

Moreover, it is observed that the average grain size increases as the deposition

temperature increases. As the substrate temperature increases, the crystalline quality

of the MgO films improves, as indicted by the fact that the diffraction peak narrows in

the (200) orientation. In addition, the intensity ratio of the peaks shows the dominance

of (200) peak up to the deposition temperature 500 oC and then decreases. An

evaluation of crystalline quality is observed with deposition temperature and three

temperature domains can be distinguished: (1) amorphous films deposited at lower

68

temperature, (2) crystalline films with oriented layers around 500 oC and (3) non-

oriented layers again at higher temperatures above 500 oC.

At the temperature 600 oC, the films are textured with preferential orientation

along the (200) axis. This is mainly due to the migration of molecules on to the

growing surface towards lower nucleation energy configuration. In MgO, (200) is

preferred orientation that has lower nucleation energy. The Mg and O atoms could

easily move to equilibrium atomic sites on the surface and hence the (200) plane of

MgO is energetically stable at higher ‘Ts’ [14, 15].

Experimentally observed interplanar distance ‘d’ is in agreement with the

standard JCPDS values (Card No: 89-7746). The dislocation density, micro strain and

the density variations of MgO films with temperature is given in Table 3.2. The

density of the as-prepared films is calculated using the formula [16]:

D = * * 1.66FW ZV

g/cm3 (3.1)

where, ‘V’ is the volume of the unit cell in Å3, ‘FW’ is the formula weight, and ‘Z’ is

the number of formula units in a unit cell. For MgO, Z = 4.

The density and the grain size are calculated from the diffraction peaks of the

MgO films at different substrate temperatures and their variations as a function of

temperature are shown in Fig. 3.5.

It is evident from figure that the density of the film increases with the

deposition temperature. As the deposition temperature increases, the grain size also

increases and reduces the density of grain boundaries. This in turn increases the

density of the films. As the temperature increases over 500 oC, however, loss of the

sprayed material due to reevaporation of the precursor becomes significant, reducing

the deposition rate. The deposited films at high temperature are therefore denser.

However, the reduction in film thickness reduces the peak intensity but the grain size

is large due to peak narrowing and the unit cell dimension attains almost near the bulk

value (a = 4.219 Å).

69

Table 3.2: X-ray diffraction data of MgO films deposited at different temperatures

Temperature (˚C)

2θ (degree)

d spacing (Å)

Plane (hkl)

Dislocation density (δ)

x 1015 (lines/m2)

Strain (ε) x 10-3

Number of crystallites

(1016/unit area)

Density (g/cm3)

400 41.30

60.91

2.1823

1.5191

(200)

(220)

1.52

6.95

1.02

1.52

2.56

0.25

3.22

450 42.72

61.80

2.1114

1.4991

(200)

(220)

1.143

2.95

0.85

0.97

1.90

7.9

3.537

500 42.79

61.89

2.1104

1.4971

(200)

(220)

1.10

3.86

0.84

1.11

1.32

8.63

3.545

550 42.80

61.94

2.1093

1.4961

(200)

(220)

0.996

4.95

0.79

1.26

1.03

0.11

3.553

600 42.84 2.1090 (200) 0.859 0.74 0.753 3.56

70

3.4 ELEMENTAL COMPOSITION ANALYSIS

It is important to estimate quantitatively the amount of elements present in the

deposited film for the determination of stoichiometry of the film surface. This is most

conveniently done by using a focused probe of x-rays or ion that is scanned over the

surface and the characteristic elemental signals are used to produce an elemental map

of the surface. In the present study, an EDS (Energy Dispersive Spectroscopy)

technique was used to analyze the chemical composition of the prepared MgO film.

The chemical composition of the MgO thin films deposited at various

substrate temperature of 450 oC, 500 oC and 550 oC were extracted from the energy

dispersive x-ray spectra and are shown in Fig. 3.6. Peaks due to the presence of

magnesium and oxygen appeared at the positions with the binding energy 1.27 and

0.54 keV respectively. The peak heights increases as the deposition temperature is

increased showing that the magnesium and oxygen concentration in the films are also

increased. The atomic percentages of the elements present in the films are calculated

and are listed in Table 3.3.

Table 3.3

Atomic percentages of Mg and O in the deposited MgO films

It is evident that increasing the substrate temperature to 550 oC has created

non-stoichiometry in the film composition. This may be due to the evaporation of Mg

species, before reaching the substrate, resulting Mg deficient films. This result is

already revealed from the thickness measurements and the decrease in peak intensity

of the respective film in XRD analysis. Further, the microanalysis indicates a trace of

5.57 atomic % of carbon present in the film deposited at 400 oC. However, at high

temperature deposited films no trace of carbon contamination is found. Usually in

Temperature

(˚C)

Atomic Percentage Ratio

Mg:O Mg O

450

500

550

53.00

50.47

30.70

49.07

49.00

69.30

1:1.08

1:1.03

1:2.26

71

spray pyrolysis deposition, residual carbon is detected in films and that react with Mg

and form MgCO3, which is thermodynamically stable at low temperature than MgO

as revealed in the thermal decomposition of magnesium oxalate. This is more

consistent with the result reported by other investigators in the synthesis of MgO [17,

18].

3.5 SURFACE MORPHOLOGY OF MgO THIN FILMS

Scanning microscopes are entirely different from optical or electron

microscopes in the sense that they operate with an electron beam or with a small

probe tip, sensing different properties close to atomic resolution in there dimensions.

By scanning this beam or tip across the surface and storing the data, an image is built

up and presented as an image. In the present investigations, scanning electron

microscope (SEM) and Atomic Force microscope (AFM) were used to study the

surface morphology and to determine the various surface morphological properties

such as grain size, roughness etc.

3.5.1 Surface Morphology using SEM Studies

The SEM micrographs of the MgO films deposited at different substrate

temperatures 450 oC, 500 oC and 550 oC are shown in Fig. 3.7. It is observed that the

surface morphologies of the MgO film surfaces are highly dependent on the substrate

temperatures. For the films deposited at 450 oC, the grain sizes is with an average

dimension of about 1.2 μm and are globular aggregate showing an uneven surface.

These aggregates grow in size and uniformly spread all over the surface, with

increasing grain dimension to about 1.2 – 1.8 μm with less roughness at 500 oC. In

550 oC deposited films, the surface morphology becomes once again different

showing both layer and smaller grains due to the growth of grains, and annehelation at

high temperature. This type of surface exhibit high dielectric constant.

3.5.2 Surface Characterization using AFM

Fig. 3.8 a, b and c shows the 3D AFM image of the MgO films spray

deposited at temperatures 450 oC, 500 oC and 550 oC. AFM results show spherical

grains in all the films with varied smoothness. The grain size of MgO films deposited

at lower temperature is smaller than that of the films deposited at higher temperatures.

72

However, at temperatures 450 and 550 oC, the size distribution is non-uniform.

Whereas, in the films deposited at 500 oC uniform grains are observed. The AFM

image gives the rms roughness value, which will be indicating the film surface

smoothness quantitatively. Fig. 3.8 d, e and f show the 2D AFM pictures along with

their respective surface profile. The rms roughness (Rrms) values are 5.62 nm, 7.28 nm

and 9.24 nm for the films deposited at 450 oC, 500 oC and 550 oC respectively.

As depictured in Fig. 3.8, the surface becomes wavier and terraces are

observable as the temperature is increased to 550 oC. Though this high deposition

temperature is enough to make atomic migration along the surface, lower oxygen

fluxes at the surface provides sufficient availability of oxygen for incorporation into

the MgO crystal structure. Hence, more defects are generated by incomplete oxidation

and also oxygen site occupation leading to the surface degrade atom further.

3.6 OPTICAL CHARACTERIZATION

Optical studies are essential for the determination of band structure, absorption

edge, optical band gap energy, refractive index and the nature of materials. Moreover,

in MgO lattice, different F-centers exist depending on the charge of the oxygen

vacancy. It is known that these F-centers give rise to an intense absorption bands in

the UV region. This F-type defect centers are responsible for many of the

luminescence emission. From the photoluminescence spectra, therefore defect centers

can be identified. Further optical characterization through vibrational spectroscopy

has been used to identify the chemical behavior of the thin film surfaces.

3.6.1 Transmittance Spectral Studies on MgO Thin Films

The optical transmittance spectra can be divided into three regions: (i) UV (ii)

Visible and NIR and (iii) IR. In the UV region, the optical absorption depends on

electronic band structure of the material. In the visible region, transmittance is usually

high, and it depends on the material purity and stoichiometry. In the third region,

absorption dominates due to lattice vibration and/or free carrier absorption. All these

informations were extracted from the transmittance spectra of the spray deposited

MgO films.

The effect of substrate temperature on the transmittance properties of MgO

films deposited on quartz substrates was analyzed by taking various transmittance

spectra in the wavelength range of 200 – 1500 nm. Fig. 3.9 shows the transmittance

73

spectra for MgO thin films deposited at substrate temperatures 450, 500 and 550 oC.

The film deposited at 500 oC shows maximum transmittance of about 88% in the

visible region, whereas the film deposited at lower temperature 450 oC shows less

transmittance of about 78%. The increase in optical transmittance with temperature is

attributed to the improved crystallinity of the MgO films grown. At lower temperature,

the grain density is large and hence grain boundary scattering is large which in turn

results a decrease in transmittance. The reported % transmittance values are > 90 %

[19], > 90 % [20] and > 85 % [21] for the films prepared using e-beam evaporation,

reactive sputtering and spray pyrolysis respectively. A high transmittance could be

obtained for the MgO films, which had a stoichiometric Mg/O ratio of 1/1. MgO films

deposited at 500 oC has stoichiometric ratio of Mg/O ~ 1 as revealed from the EDAX

results. The absorption edge shifts towards shorter wavelength, suggesting a widening

of the band gap of MgO films with increasing substrate temperature up to 500 oC.

Above 500 oC, due to the evaporation of solvents well above the substrate surface

resulting rough surfaces which in turn lowers the transmittance as seen from

Fig. 3.9. It is known that at high temperatures, the MgO film surface is very rough

compared to other dielectric films such as SiO and MgF2 [22].

For the determination of the optical band gap ‘Eg’ a plot has been drawn

between (αhν)2 against hν (Fig. 3.10). The linearity of the plot in the high-energy

region shows the direct allowed transitions. Extrapolation of linear portions to

abscissa yields the corresponding optical band gap ‘Eg’. The values of the energy gap

obtained for direct allowed transition is less than that of bulk MgO (7.3 eV) material.

However, the film prepared at 500 oC has the maximum optical band gap of 5.82 eV.

The direct allowed transition is found to vary from 5.43 eV to 5.82 eV and the

transition is increased initially from 5.43 eV for the films prepared at 450 oC to 5.82

eV for the films prepared at 500 oC. For the film prepared at high temperature (550 oC), the band gap is less than the values obtained for the films prepared at the

optimized temperature 500 oC.

The observed change of the band gap energy with deposition temperature can

be partially explained on the basis of the density of states model proposed by Mott

and Davis [23]. Accordingly, the width of the localized states near the mobility edges

depend on the degree of disorder and defects present in the materials. The presence of

a high concentration of localized states in the band structure is responsible for the low

74

values of energy gap. The reduction in the number of defects decreases the density of

localized states in the band structure, consequently increasing the optical band gap [24,

25]. Accordingly, the film processed at 500 oC has low density of localized states and

therefore exhibits high optical band gap compared to other films deposited above and

below 500 oC.

Using the transmittance spectra, the values of refractive index (n) and the

extinction co-efficient (k) were calculated. The spectral distribution of refractive

index (n) and extinction co-efficient (k) of MgO films deposited at 500 oC is given in

Fig. 3.11. The refractive index and extinction co-efficient are found to decrease with

increase in wavelength. At higher wavelengths, the refractive index value tends to be

constant. The refractive index of the MgO film grown at 500 oC is 1.714 at 500 nm,

which is comparable to the value for bulk crystal 1.731. The calculated values here

are in good agreement with the reported values [23].

The packing density of the MgO film prepared at 500 oC is calculated using

the calculated values of refractive index ‘n’ and the value of packing density is 0.691

at 500 nm. The relationship between the refractive index and the packing density is

approximately linear as proposed by Kinosita and Nishibori [26].

3.7 PHOTOLUMINESCENCE PROPERTIES OF MgO FILMS

Prepared MgO thin films were excited with the wave length 248 nm and the

emission spectrum due to photoluminescence from F and F+ centers were measured at

room temperature. Decay curves were recorded in the entire wavelength range from

UV to NIR by passing the luminescence light through a monochromater to a

photomultiplier and recorded the signal in digital form. Fig. 3.12 shows the

photoluminescence spectra of MgO films prepared at the optimized deposition

temperature, 500 oC.

In MgO, the absorption peaks of F and F+ centers are at 247 and 251 nm

respectively. Therefore, excitation is given at 248 nm to excite both the centers. The

emission band at 393 and 532 nm remain spectrally distinct. The release of

luminescent energy at 393 nm can be assigned to the ionization of F+ center. The

ejection of an electron results a defect F2+ center (bare oxygen vacancy) and this

recombination on process may be written as,

75

F2+ + e- → F+ + hν (~390 nm)

In general, an intense laser pulse ionizes F to F+ and F+ to F2+ and so the F+

luminescent spectrum is displayed, regardless of the steady state spectrum. The

emission peak band observed at ~532 nm is assigned to the F center. These results are

consistent with many of the earlier results reported [27, 28].

3.8 LASER RAMAN INVESTIGATION ON MgO THIN FILMS

Fig. 3.13 shows the Raman spectrum of MgO thin film deposited at 500 oC. It

is observed that the peaks at 722 cm-1 and 1100 cm-1 are observed in the entire

experimental wavenumber range 500-1500 cm-1. The peak at 722cm-1 is assigned to

the transverse optical longitudinal optical gap (TOLOG) and the second line falls

above TOLOG. The line at 609 cm-1 is not present here. The lowest mode observed at

TOLOG can be explained by the macroscopic theory. However, the lattice theory can

be used to explain the high frequency mode. Since the particle size of the MgO films

prepared at optimized conditions are larger of the order of microns, the line expected

at 609 cm-1 is missing. The other two lines are in agreement with the proposed lattice

theory and also it supports the experimental results given by Bockelmann [29].

3.9 FT-IR STUDIES ON MgO FILMS

Fig. 3.14 shows the FT-IR transmission spectra of MgO films deposited at 450

and 500 oC. The very small peaks observed around 3635 cm-1 is assigned to the

presence of hydrogen bonded hydroxyl group [30-33]. The peak at 1222 cm-1 is

assigned to the deformation band in water. These bands are due to hydration,

originated from the precursor and solvent or from exposure of MgO films to the

atmospheric air. A comparison of the peak intensities of these bands, films deposited

at 500 oC are found to be less hydrated than the films deposited at 450 oC. Two strong

and sharp peaks appeared at 721 cm-1, 406 cm-1 are associated with the longitudinal

and transverse optical phonon modes [34]. The appearance of these two bands in the

spectra is a clear evidence for the presence of highly crystalline MgO film.

3.10 ELECTRICAL PROPERTIES OF MgO FILMS

The electrical properties of high resistance thin films strongly depend on many

factors and heating the sample modify its conducting nature. Therefore, temperature

76

dependence of electrical conductivity during heat treatment provides information

about the electronic process taking place in the films. The most direct method of

measuring sheet resistance. ‘Rs’ is to prepare a rectangular sample of films and a

potential is applied across the end contacts. Fig. 3.15 shows the sheet resistance

variations as a function of temperature for the MgO films deposited at 500ºC for three

different thicknesses.

At room temperature, the sheet resistance of MgO films are more than 1013

Ω/ , and so the readings were reordered only in the high temperature region between

350 and 385 K. At 350 K, the sheet resistance of 280 nm thick MgO film is 7.48 x

1010 Ω/ and for 497 nm thick film is 6.38 x 108 Ω/ . It is observed that the sheet

resistance decreases gradually with temperature in all the films of different

thicknesses. However, the rate of fall is different and is less in thick film samples. The

conductivity variation with temperature is shown in Fig. 3.16.

Observed electrical conductivity from sheet resistance is found to increase

with film thickness and these variations can be explained on the basis of the change in

the mean free path of the conduction electrons, which is normally very low in an

insulating film such as MgO here. Whenever the specimen becomes thin enough,

collisions with the surface is a significant fraction in the total number of collisions.

Thomson [35] and Fuchs [36] discovered the possibility that the dimension of a

specimen could influence the conductivity. In particular, Sondheimer [37] extended

the work by including mean free path in related theories. All these theories have been

reviewed by Campbell [38] and attained an equation related to film conductivity with

thickness.

Ohmic conduction, whether ionic or electronic gives exponential temperature

dependence, given by:

exp ( )ao

EkT

σ σ= − (3.2)

where, ‘Ea’ is the activation energy of the process.

With extrinsic ionic conduction, mobility of the ion is activated and ‘Ea’ being

the energy of the ion to hop. However, if the electronic conduction is by excitation

into the conduction band, the production of free electrons is activated. Whatever be

the ohmic mechanism, an Arrhenius plot ln (ρ) vs. 1000/T usually exhibits increasing

linear slopes as temperature is raised. It is noted that the activation energy also

depends on the film thickness, which increases as the thickness decreases. This

77

dependence may be due to the combined effect of the following reasons: (i) The

change in the barrier height due to the size of the grains in a polycrystalline film, (ii)

Large density of dislocations (iii) Quantum size effect (iv) Changes in stoichiometry.

The following in the explanation for the conductivity dependence on thickness and

grain size.

According to the charge transfer model [39, 40] a certain number of particles

are charged by gaining or losing electrons at any temperature above 0 K. If the

activation is due to the thermal energy, then at equilibrium the number of charged

particle (n) is related to the total number of particles (N) in the film by the expression:

exp( / )an N E kT= − (3.3)

2

exp eNrkTε

⎛ ⎞−= ⎜ ⎟

⎝ ⎠ (3.4)

Hence, the activation energy 2

aeErε

= due to thermal agitation is inversely

proportional to the average linear dimension ‘r’ of the grains. Since the increase of

film thickness is generally accompanied by an increase of ‘r’, there is lowering of ‘Ea’.

3.11 CONCLUSIONS

A spray pyrolysis set-up has been used for the growth of stoichiometric

magnesium oxide films. Prior to MgO thin film deposition, TGA/DTA studies have

been performed on starting precursor salts to confirm the substrate temperature range.

Magnesium oxalate was chosen as the precursor and was dissolved in ethanol for

MgO film formation. A systematic study on various process parameters has been

carried out to get quality films under optimized deposition conditions. MgO films

prepared by varying the process parameters have been subjected to structural,

morphological, optical and electrical studies.

XRD results revealed polycrystalline nature with cubic fcc structure

possessing (200) and (220) peaks for most of the deposition conditions. Poly

crystalline films were obtained at the deposition temperature of 500 oC.

Elemental compositions of the prepared films have been analyzed with the aid

of EDX technique. The atomic percentage of Mg and O on the MgO surface prepared

at 500 oC is almost 1:1, which shows the perfect stoichiometry of the prepared films.

Surface morphology of the MO films has been investigated using SEM and AFM.

78

SEM micrographs revealed the grain size and shape variation with substrate

temperature. In addition to grain size, the surface roughness values were obtained

from the AFM analysis. Magnesium oxide films prepared at 550 oC shows maximum

transmittance of 88%. Direct allowed transition and a band gap of 5.82 eV was

observed in the optimized films. From the optical spectra, refractive index (1.709),

extinction co-efficient (0.038) and packing density (0.682) were obtained.

The presence of F and F+ centers were confirmed from the PL peak at 532 and

393 nm respectively. In Laser Raman spectrum, peaks at 722 cm-1 and 1100 cm-1 are

present, which are in agreement with the lattice theory of MgO. In FT-IR spectra, two

strong absorption bands at 721 cm-1 and 406 cm-1 were observed, corresponding to the

longitudinal and transverse optical phonon modes of MgO.

Electrical measurements have been performed on MgO films of three different

thicknesses. The electrical conductivity values were found to increase from the values

of 7.48x1010 Ω/ for 280 nm thick films to 6.38x108 Ω/ for 497 nm thick films.

Dependence of conductivity and activation energy with film thickness was confirmed.

79

REFERENCES

[1] E. Wiederholt, V. Fahrney, R. Skrozki, J. Thermal Analysis 35 (1989) 541.

[2] J. Mu, D. D. Perlmutter, Thermochimica Acta 49 (1981) 207.

[3] A. M. M. Gadalla, Thermochimica Acta 74 (1984) 255.

[4] W.J. Desisto, R.L. Henry, J. Crys. Growth, 109 (1991) 314.

[5] X. Yi, W. Wenzhong, Q.Yitai, Y. Li, C. Zhiwen, Surf. Coat. Technol., 82

(1996) 291.

[6] O. Stryckmans, T. Segato, P.H. Duvigneaud, Thin Solid Films, 283 (1996) 17.

[7] S.G. Kim, J.Y. Kim, H.J. Kim, Thin Solid Films, 376 (2000) 110.

[8] S.H. Rhee, Y. Yang, H.S. Choi, J.M. Myoung, K. Kim, Thin Solid Films, 396

(2001) 23.

[9] X. Fu, G. Wu, S. Song, Z. Song, X. Duo, C. Lin, Appl. Surf. Sci., 148 (1999)

223.

[10] J.R. Tesmer, M. Nastasi, “Handbook of Modern Ion Beam Material Analysis”,

Material Research Society, 1995.

[11] W.Y. Hsu, R. Raj, Appl. Phys. Lett., 60 (1992) 3105

[12] O. Michikami, M. Asahi, H. Asano, Jpn. J. Appl. Phys., 28 (1989) L91.

[13] R.N. Ghostagore, J. Electrochem. Soc., 125 (1978) 110.

[14] B.S. Hwak, D.P. Boyd, K. Zhang, A. Erbil, B. Wilkins, Appl. Phys. Lett., 54

(1995) 2542.

[15] J.M. Bian, X.M. Li, T.L. Chen, X.D. Gao, W.D. Yu, Appl. Surf. Sci., 228

(2004) 297.

[16] Antony R. West, “Solid State Chemistry and its Applications”, John Wiley &

Sons, Singapore, 1984.

[17] J.W. Bullond, Z. Xu, M. Menon. J. Crys. Growth, 233 (2001) 389.

[18] A. Bokhimi, O. Accves, T. Navaro, R. Gomez, J. Phys. Chem., 99 (1995)

14403.

[19] C.H. Park, Y.K. Kim, S.H. Lee, W.G. Lee, Y.M. Sung, Thin Solid Films, 366

(2000). 88.

[20] C.H. Park, W.G. Lee, D.H. Kim, H.H. Ha, J.Y. Ryu, Surf. Coat. Technol., 110

(1998) 128.

[21] S.G. Kim, K.H. Choi, J. H. Eon, H.J. Kim, C. Seung, Thin Solid Films, 377-

378 (2000) 694.

[22] Shigeru Baba, Isao Mori, Takeo Nakano, Vacuum, 59 (2000) 531.

80

[23] N.F. Mott, E.A. Davis, “Electronics Process in Non-crystalline Materials”

Claendon Press, Oxford, 1971.

[24] M. Parlak, Thin Solid Films, 322 (1998) 334.

[25] H.T. Ei-Shair, A.E. Bekheet, J. Phys. D: Appl. Phys., 25 (1992) 1122.

[26] K. Kinosita, M. Nishibari, J.Vac. Sci. Technol., 6 (1969) 730.

[27] G.H. Rosenblatt, M.W. Rowe, G.P. Williams Jr., R.T. Williams, Phys. Rev. B.,

39 (1989) 10309.

[28] G.P.Summers, T.M. Wilson, B.T. Jeffries, H.T. Tohver, Y.

Chen,M.M.Abraham, Phys. Rev. B., 27 (1983) 1283.

[29] H.K. Bockelmann, R.G. Schlecht, Phys. Rev. B, 10 (1974) 5225.

[30] L.H. Little, “Infrared Spectra of Absorbed Species”, Academic Press, London,

1966.

[31] G. Socrates, “Infrared Characteristic Group Frequencies”, Wiley, Chichester,

1980.

[32] M.O.Aboelfotoh, K.C. Park, N.A. Parkin, J. Appl. Phys., 48 (1977) 2910.

[33] S. Chakrabarti, D. Ganguli, S. Chaudhuri, Mat. Lett., 57 (2003) 4483.

[34] J.R. Jasperse, A. Kahan, J.N. Plendl, S.S. Mitra, Phys. Rev., 146 (1965) 526.

[35] J.J. Thomson, Proc. Cambridge Phil. Soc., 11 (1901) 120.

[36] K. Fuchs, Proc. Cambridge Phil. Soc., 34 (1938) 100.

[37] E.H. Sondheiner, Advan. Phys., 1 (1952) 1.

[38] C.A. Neugebaur, M.B. Webb, J. Appl. Phys., 33 (1962) 74.

[39] C.A. Neugebaur, M.B. Webb, J. Appl. Phys., 33 (1962) 74.

[40] C.A. Neugebaur, “Physics of thin film”, Academic Press Inc., New York,

Vol.II, 1964.