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CHAPTER 2

SYNTHESIS AND CHARACTERIZATION OF

NANOCRYSTALLINE GaN BY NITRIDATION OF

Ga-EDTA COMPLEX

2.1 INTRODUCTION

Gallium Nitride (GaN) has potential application in optoelectronic

and electronic devices, which are capable of operating at high temperature,

high power and in harsh environments (Nakamura et al 1997, Morkoc et al

1994). In addition, GaN powders themselves could be used as high quality

phosphors. GaN has low electron affinity value of 2.7-3.3 eV compared to

carbon nanotubes (CNTs), zinc oxide (ZnO) and Si, hence GaN has potential

for field emission devices (Cuntang et al 2009, Yamanaka et al 2002, Nishida

et al 2003). Reports on the field emission characteristics of GaN nanowires

have already revealed a high emission current density and a long emitter

lifetime (Choi et al 1999).

The growth of GaN on foreign substrates such as sapphire, Si, and

silicon carbide (SiC) leads to high dislocations and strain due to large

differences in lattice parameter and thermal expansion coefficient between

substrate and epitaxial layers. It is essential to develop the GaN substrates for

homoepitaxy by inventing a way to produce high quality GaN wafers. The

effort has been made towards the synthesis of GaN powders. The ultra fine

and high pure GaN powders are important because they can be used as the

source material for the sublimation growth of bulk GaN single crystal and

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wafer (Fareed et al 1999, Kallnger et al 2007, Shin et al 2002, Wu et al 2005).

The conventional synthesis methods like nitridation of Ga metal and Ga2S3 by

NH3 results in low yield due to low reaction efficiency (Jhonson et al 1932,

Senthil Kumar et al 2000, Kanie et al 1998). Considerable efforts have been

made to explore suitable synthesis methods for producing uniform size and

low dimensional GaN. The synthesis of nanocrystalline GaN in the pores of

an aerogel matrix is reported (Goodwin et al 1996). GaN nanoparticles with

diameters of 10-40 nm were synthesized in a silica aerogel host matrix by

pyrolysis of dimeric dimethylgallium-diphenylamide, but this method has

complicated experimental procedure. The boron doped silica mesoporous

molecular sieve MCM-41 (Winkler et al 2000, Ying et al 1999) and silica

aerogel (Yang et al 2001) as a matrix was used to confine the synthesis of

nanocrystalline gallium nitride in silica aerogel then heat treated for reducing

the size of the GaN QDs (Murali et al 2000). Synthesis of colloidal GaN by

thermolysis from the molecular gallium azides was reported by Manz et al by

solution pyrolysis from polymeric (Ga(NH)3/2)n in trioctylamine (360 °C,

24 h) and subsequent stabilization by hexadecylamine (Manz et al 2000,

Micic et al 1999), these method also have complicated process. GaN micro-

crystals were catalytically grown using Ni-mesh in direct reaction of gallium

and ammonia, at 1100 °C, the growth rate of the GaN crystals in the presence

of Ni catalyst was almost ten times higher than in the absence of the catalyst,

the use of the catalyst induced the increase of GaN crystal size (Ahn et al

2002).

Many research groups have demonstrated the growth of GaN

powder crystals through various synthetic methods. High pressure solution

growth technique were employed to grow high quality GaN crystals at

relatively low temperatures about 550 °C (Porowski et al 1996). Shibata et al

(1999) reported, GaN microcrystals were synthesized by injecting ammonia

gas into molten gallium under atmospheric pressure at a temperature range of

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850–1100 ºC. The growth of the GaN crystals were also attempted using

various single precursors which contain gallium and nitrogen atoms in their

molecules at relatively low temperature and pressure (Mcmuran et al 1998)

these process are also complicated to maintain high vacuum level during the

synthesis.

Ultrafine GaN nano-crystals were mostly synthesized by

arc-discharge method through the reaction of metal gallium with the mixture

of nitrogen and ammonia gases (Li et al 1997). The growth of high purity

GaN crystal structures was also achieved in a hot-wall tubular furnace via the

reaction of gallium with ammonia and the conversion of gallium oxide

(Balkas et al 1996). Conventionally, polycrystalline GaN can be prepared by

several straight forward methods including heating gallium oxides, gallium

halides, or gallium metal at elevated temperatures (> 750 K) for extended

periods of time (Addaminano et al 1961, Elwel et al 1984). The resulting

materials, obtained from these routes often contain impurities and poor

crystalline nature. Li et al (1966) synthesized GaN powder by the DC arc

plasma method through the reaction of metal gallium (Ga) with the mixture

gas of nitrogen (N2) and ammonia (NH3). Since several methods have been

reported for the synthesis of GaN nanoparticles, each and every method has

certain advantages and disadvantages. Simple, inexpensive, environmentally

friendly and convenient synthesis routes are needed for preparing the GaN

nanoparticles.

Previously, Liu et al (2007) reported the synthesis of GaN micro

crystals by spray-dry technique at higher temperature using metal-EDTA

complex, which was prepared from the mixture of Gallium Nitrate [Ga(NO3)3.

H2O] and EDTA.NH4 in aqua solutions, but the particles size was reported to

be around 400 nm due to high temperature synthesis of GaN at 1060 ºC. In

order to decrease size of the particle, the present study was planned to

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synthesize GaN at low temperatures. Ga-EDTA complex was prepared by

contacting GaCl3 and EDTA at pH of 9 in aqueous solution. The complex was

then nitridated between 600 and 900 ºC to obtain GaN nanocrystals. This

method has yielded GaN crystals of average size of 20 nm.

2.2 EXPERIMENTAL DETAILS

Ga-EDTA.NH4 complex was prepared from the mixture of GaCl3,

and EDTA.NH4 in aqueous solution at a pH of 9. The solution was stirred for

6 h and dried in an oven at 70 ºC to obtain Ga-EDTA.NH4 complex.

Figure 2.1 shows the experimental setup used for the synthesis of GaN

nanocrystals. The Ga-EDTA.NH4 complex was taken in an alumina boat and

placed inside the quartz reactor and NH3 was allowed to react with the

complex during the synthesis. The synthesis was carried out for a reaction

period of 8 h at 600, 700, 800 and 900 ºC. Then the temperature was brought

down to 500 ºC followed by nitrogen purging up to room temperature.

The following reactions are suggested for the formation of GaN.

2Ga-EDTA·NH4 → Ga2O3 + EDTA fragments (2.1)

Ga2O3 + 2NH3 → 2GaN + 3H2O (2.2)

Ga.EDTA.NH4 complex decomposes to yield Ga2O3 intermediate

compound, which subsequently decomposes in the presence of ammonia to

form GaN. The Ga2O3 though it is in nano-form, cannot be sintered to form

bulk Ga2O3 as the melting point of the Ga2O3 (1900 °C) is rather high. The

main function of EDTA is to provide nano-size Ga2O3, so that it can rapidly

react with ammonia and convert into nanocrystals of GaN.

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Figure 2.1 Experimental setup used for GaN nanocrystals synthesis

Powder X-ray diffraction patterns of GaN crystals were recorded

using Cu Kα radiation of wavelength 1.5418 Å at a scan speed of

1 degree per minute. The morphologies of GaN were studied using LEO

stereoscan-440 scanning electron microscopy. Tecnai-12, FEI, transmission

electron microscopy was used to determine the particle size. Energy

dispersive X-ray analysis revealed the elements present in the synthesized

compound. The room temperature photoluminescence spectrum of the GaN

samples was recorded using He–Cd laser (325 nm) as an excitation source.

Fourier transform infrared spectral analysis of the samples was carried out

using Bruker IFS 66V FT-IR spectrometer by KBr pellet technique. Raman

spectra (300-1000 cm-1) were recorded at room temperature using Renishaw

Ramascope system model 1000 with an excitation wavelength of 514.5 nm

(Ar+ laser).

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2.3 RESULTS AND DISCUSSION

2.3.1 X-ray Diffraction Analysis

Figure 2.2(a) shows the XRD patterns of GaN powder synthesized

at 600, 700, 800 and 900 ºC. The XRD patterns show that the formation of

GaN progresses via intermediate states.

Figure 2.2(a) XRD patterns of GaN compound synthesized at different

temperatures

(a)

22

The XRD patterns of the powder synthesized at 600 ºC shows only

amorphous nature whereas the powders synthesized at 700 and 800 ºC show

GaN phase along with additional phases like beta gallium oxide

(β -Ga2O3) and gallium nitrogen oxide nitrate (2GaONO3.N2O5).

Thermodynamic calculations also suggest that the synthesis of GaN

proceeded through intermediate products like gallium suboxide (Ga2O)

(Balkas et al 1996). There are several reports on the synthesis of GaN through

intermediate states (Jung et al 2003, Schulz et al 1979, Iwata et al 1996, Cho

et al 2002). The powder synthesized at 900 ºC shows only a single phase of

GaN. The lattice parameters calculated from the XRD data are a = 3.19 Å and

c = 5.19 Å which are in good agreement with reported values (Iwata et al

1996). Figure 2.2 (b) shows the XRD pattern of the compound synthesized at

600 ºC after annealing in nitrogen atmosphere at 900 ºC for a period of 3 h. It

is evident that annealing results in the formation of crystalline GaN with

mixed phases of gallium oxide and gallium oxynitrides. Therefore, direct

synthesis at 900 ºC in NH3 ambient has to be carried out to obtain pure phase

of crystalline GaN.

Lattice parameters a and c were calculated using the formula given

in equation (2.3).

4 2 2 2 2 2/3

ad

hklh hk k l a c

(2.3)

where dhkl is the lattice spacing, h, k and l represents the lattice planes

d value was calculated from the Bragg equation

2dsinθ = nλ (2.4)

where θ represents the diffraction angle,

λ is wavelength of X-ray source (Cu Kα ) and

n is order of the plane.

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Lattice parameters, a and c values were calculated from the (100)

and (002) planes of the XRD pattern.

Figure 2.2(b) XRD pattern of the sample synthesized at 600 ˚C and

subsequently annealed at 900 ˚C in nitrogen ambient

2.3.2 Thermo Gravimetric Analysis

The conversion of gallium-EDTA complex to GaN is accompanied

by weight loss. Figure 2.3 shows the change in sample weight loss (wt %)

against reaction temperature. The change in the wt % with increasing

temperature can be divided into four stages as follows: the first stage of

weight loss below 230 ºC is due to the decomposition of water content in the

Ga-EDTA complex. The second stage of weight loss between 230 and 400 ºC

is due the decomposition of Ga-EDTA complex and desorption of EDTA

organic groups. Third stage of weight loss between 400 and 900 ºC is the

further desorption of organic groups and decomposition of Ga2O3 phase. The

(b)

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fourth stage between 900 and 1200 ºC is may be due to the decomposition of

carbon which arises from the EDTA (Liu et al 2007, Jung et al 2002). These

TGA results are well supported the XRD data that the formation of single

phase GaN at 900 ºC by nitridation of Ga-EDTA complexes in NH3 ambient

via decomposition of intermediated states like Ga2O3 and other organic

groups.

Figure 2.3 Variation of weight loss as a function of Temperature

(TGA Curve)

2.3.3 Scanning Electron Microscopy

The analysis of GaN powder obtained at different temperatures has

shown several morphologies. Platelet-like and needle-like structures were

predominant, although other morphologies with three dimensional growth

tendencies were present. Relatively large crystallites, up to 3 µm in size, with

varied morphology were observed in SEM micrographs. There was significant

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difference in crystal size and shape with respect to the synthesis temperature.

Figure 2.4 shows the SEM images of aggregated ultra fine GaN crystals

synthesized at different temperatures. Figure 2.4a, 2.4b, 2.4c and 2.4d shows

the SEM images of GaN synthesized at 600, 700, 800 and 900 ºC respectively.

The samples synthesized at temperatures of 600, 700 and 800 ºC show mixed

spherical and needle kind of morphologies. This may be due to the presence

of secondary phases because the low temperature synthesis of GaN contains

large amount of oxygen and also the dissociation of NH3 is lower hence the

low temperature synthesized compound contains mixed phases of GaN and

Ga2O3 this is also further confirmed by EDAX, FTIR, PL, and Raman data.

Whereas the single-phase sample synthesized at 900 ºC show only

agglomerated spherical crystallites.

Figure 2.4 SEM images of GaN synthesized at different temperatures

(a) 600 °C, (b) 700 °C, (c) 800 °C and (d) 900 °C

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2.3.4 Transmission Electron Microscopy

TEM image of GaN nanocrystals synthesized at 900 ºC is shown in

Figure 2.5 (a). The image shows that GaN nanocrystals are mostly composed

of uniform spherical particles with several hexagonal shaped nanocrystallites.

The size distribution of GaN nanocrystals synthesized at 900 ºC is shown in

Figure 2.5 (b).

Figure 2.5(a) TEM image of GaN nanocrystals synthesized at 900 °C

Figure 2.5(b) Particle size distributions for the sample synthesized at 900 °C

The GaN nanocrystalline particles have broad size distribution

range between 12 and 32 nm. The average size of the nanocrystals was 20 nm.

(a)

(b)

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The particle size (diameter) has been estimated using standard software

(Image J) and the distribution of the particles was plotted using frequency

count. Most of the nanoparticles are in hexagonal shape. Published values for

excitonic - Bohr radius of GaN to exhibit quantum confinement effect range

between 10 and 12 nm (Huizhao et al 2008). The results are in the present study

shows very close to reported values.

2.3.5 Energy dispersive X-ray analysis (EDAX)

The EDAX results are shown in Figure 2.6. The intensity of carbon

and oxygen peaks decreases where as Ga peak increases with the increase of

the synthesis temperature.

Figure 2.6 EDAX patterns of GaN powders synthesized at 600, 700, 800

and 900 °C

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This behavior is attributed by complete decomposition of

Ga-EDTA.NH4 complex and desorption of EDTA at high temperature and

formation of other phases like β -Ga2O3 and 2GaONO3.N2O5 that varies with

different GaN synthesis temperatures from 600 to 800 ºC. Further

decomposition of these phases and the formation of GaN at 900 ºC were

observed. This kind of behavior had been explained by Liu et al

(2007).

2.3.6 Photoluminescence

Figure 2.7 shows the room temperature photoluminescence (PL)

spectra of GaN samples. The GaN samples synthesized at different

temperatures show a near band-edge emission at 3.46 eV.

Figure 2.7 Room temperature PL spectra of synthesized GaN powders

at different temperatures

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The PL spectra show a mild blue shift indicating an increase in the

band gap of GaN when compared to bulk emission at 3.4 eV at room

temperature. This clearly illustrates that the increase in band gap is due to

finite size effect. A broad band emission at 3.35 eV is ascribed to the presence

of oxygen in synthesized samples. The intensity of this broad band emission

decreases with increase of synthesis temperature of GaN, suggesting a

decrease in oxygen content for the samples synthesized at higher temperatures.

These results are in good agreement with EDAX results illustrated in

Figure 2.6. The sample synthesized at 600 ºC shows amorphous nature which

also exhibits near band-edge emission. Quantum confinement effect in

amorphous GaN had already been reported in the literature (Yoon et al 2005).

From this result it is clear that the formation of amorphous GaN is possible

even at low synthesis temperature through this route result with additional

phases and impurities like carbon and oxygen.

2.3.7 Fourier Transform Infrared Analysis

In the FTIR spectra of the samples synthesized at 600 ºC, there is a

broad envelop between 2700 and 3700 cm-1 due to the OH stretching of water

shown in Figure 2.8 (a). It is also confirmed by its bending vibration that

gives intense sharp peak at 1630 cm-1. The broad peak between 500 and 700

cm-1 is attributed to GaN and Ga-O vibrations. The FTIR spectrum of the

samples synthesized at 700 ºC is shown in Figure 2.8 (b). The OH stretch of

water gives a very broad envelop without giving much resolution for its

bending vibration. The peak at 1000 cm-1 is assigned to hydrogen bend modes

(Xiao et al 2005).

The FTIR spectrum of the samples synthesized at 700 ºC and

800 ºC as shown in Figure 2.8 (b) and 2.8 (c) displays nearly similar

characteristics. The water content appears to be less. The peak due to Ga-O

vibration also shows a decrease in intensity. The sample synthesized at 900 ºC

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(Figure 2.8 (d)) also shows similar features of Figure 2.8 (c), but the water

content appears to be still less. The intensity of Ga-O peak is also less than

that of Figure 2.8 (c) but the peak due to GaN appears at 577 cm-1, where as in

Figure 2.8 (c) it appears at 617 cm-1 which may be due to the presence of

small quantity of Ga-O content in the sample synthesized at 800 ºC.

Figure 2.8 FTIR spectra of GaN powders synthesized at 600, 700, 800

and 900 ºC

The results obtained from the FTIR analysis for GaN nanocrystal

are in close agreement with the previous reports (Xiao et al 2005, Jung et al

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2002, Liu et al 2004, Kisailus et al 2003). The formation of GaN under

ammonia proceeds stepwise via intermediate states like β -Ga2O3 and gallium

oxynitrides, which is also confirmed by XRD. The XRD results already

shown that the formation of impurity phases at low reaction temperatures and

formation of single phase GaN nanocrystals at high temperature.

2.3.8 Raman Analysis

Raman scattering is a standard optical characterization technique

for studying various aspects of solids such as lattice properties, electronic

properties, and magnetic properties. Raman scattering has many advantages

when compared with other spectroscopic techniques: it is in principle

non-destructive, contactless, and requires no special sample preparation

technique such as thinning or polishing. Raman scattering occurs essentially

as a result of modulation of the electronic polarizability induced by various

elementary excitations in solids such as phonons and plasmons. In the case of

Raman scattering by phonons, the scattering efficiency is higher in covalent

crystals than in ionic crystals, because the valence electrons are less localized

and larger fluctuation of the polarizability can be induced by lattice vibration.

From this viewpoint, nitride semiconductors are suitable for Raman scattering

studies, since the chemical bonding is a mixture of covalent and ionic bonding.

Furthermore, nitride semiconductors are generally robust and stand up well to

laser irradiation, which is another advantage of Raman scattering studies.

As described in the previous section, the group-III nitrides

crystallize in hexagonal wurtzite or cubic zinc blende structure. Here, let us

take GaN as an example: in the hexagonal structure, the primitive cell

contains two Ga–N atom pairs, while it contains only one atom pair in the

cubic structure. In both cases, a Ga atom is tetrahedrally surrounded by four N

atoms, and vice versa. The hexagonal and cubic structures differ only in the

stacking sequence of the Ga-N bilayers. The first-order phonon Raman

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scattering is caused by phonons with wave vector k ~ 0 (Γ point) because of a

momentum conservation rule in the light scattering process. In the hexagonal

structure, group theory predicts eight sets of phonon normal modes at the

point, 2A1+2E1+2B1+2E2. Among them, one set of A1 and E modes are

acoustic, while the remaining six modes, A1+E1+2B1+2E2, are optical. The A1

and B1 modes give atomic displacements along the c-axis, while the others, E1

and E2, give atomic displacements perpendicular to the c-axis, Here, the A1

and E1 modes are both Raman and infrared (IR) active, while the two E2

modes are only Raman active, and the two B1 modes are neither Raman nor

IR active (silent modes) (Harmia et al 2002).

Figure 2.9 Raman Spectra of GaN powder synthesized at 800 and 900 ºC

Figure 2.9 shows the Raman spectra of the GaN nanocrystals

synthesized at 800 and 900 ºC. Five phonon modes corresponding to pure

GaN are observed at 419, 535, 556, 568 and 728 cm-1 for samples synthesized

at 900 ºC. The 419 cm-1 phonon mode corresponds to the acoustic overtone,

and the other four phonon modes correspond to A1(TO), E1(TO), E2 (high)

and A1(LO) modes, respectively (Perlin et al 1992, Harima et al 2002).

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The samples synthesized at 800 ºC shows E2 (high) and A1 (LO) mode only.

As the synthesis temperature decreases, the E2 (high) mode broadens and blue

shifts indicating that the crystalline quality of the nanocrystals increases with

temperature. The results are in good agreement with the XRD data.

2.4 SUMMARY

GaN nanocrystals were synthesized at low temperature by a simple

and inexpensive method than the previously reported methods. The lattice

parameters calculated from XRD pattern for the sample synthesized at a

temperature of 900 ºC are well matched with the reported values. GaN

nanocrystals have wurtzite type structure, which has been confirmed by XRD,

FTIR and Raman analysis. The changes in morphology of the synthesized

GaN nanocrystaline powders at different synthesis temperatures are noted,

this is due to the presence of secondary phases in the samples synthesized at

low temperatures. EDAX analysis was performed for all samples synthesized

at different temperatures, the intensity of the carbon and oxygen reduces with

increasing the synthesis temperature. The TEM image shows that the average

size of GaN nanocrystals is ~20 nm. Room temperature PL spectrum of GaN

synthesized at 600 to 900 ºC shows a mild blue shift which has been

explained by the size effect of GaN crystallites.