Complete specification

Synthesis and Characterization of Nanocrystalline Gallium Nitride by
Nitridation of Ga-EDTA Complex

Field of the invention:-

The present invention relates to a method to produce gallium nitride nanocrystals. In particular, the invention pertains to a process for obtaining gallium nitride nanocrystals by nitridation of Ga-EDTA complex at low temperature by simple and inexpensive technique.

Objective of the invention:-

The main objectives of the present inventions are simple, inexpensive method and environment friendly.

The second objective of the present invention is the production of GaN nanocrystals at low temperature (500°C).

The third objective of the invention is the synthesis of GaN nanocrystals with an average size of 20 nm.

Background of the invention:-

Gallium nitride (GaN) is a wide band gap (3.4 eV) semiconductor. It has potential application in optoelectronic and electronic devices which are capable of operating at high temperature, high power and in harsh environment. The GaN powders themselves can be used as high quality phosphors.

GaN can be used as a field emission device due to low electron affinity (2.7 - 3.3 eV) compared to carbon nanotubes (CNTs), zinc oxide (ZnO) and Si (3 - 5 eV). Reports on the field emission characteristics of GaN nanowires have already revealed a high emission current density and a long emitter lifetime.

There were many methods to obtain high quality single crystals like silicon (Si), gallium arsenide (GaAs) and indium phosphide (InP) but not gallium nitride (GaN), The non-availability of GaN bulk substrate and thus limits the homoepotaxial growth of GaN.
The growth of GaN on foreign substrates such as sapphire, silicon and silicon carbide (SiC) leads to high dislocations and strain due to large difference in lattice parameter and thermal expansion coefficient between substrate and epitaxial layers. Hence it is necessary to develop GaN substrates for homoepitaxy by fabricating high quality wafers. Ultra fine and highly pure GaN powders are important as they can be used as source material for sublimation growth of bulk GaN single crystals and wafers.

Previously Liu et al reported GaN microcrystals by spray-dry technique using metal - EDTA complex. Metal - EDTA.NH4 complex was synthesized by dissolving the Ga (N03)3.H20 and EDTA and stirred to yield Ga-EDTA.NH4 complex. The Ga-EDTA.NH4 solution was spray dried at a temperature of 160°C and the Ga-EDTA.NH4 complex powder was obtained. Ga-EDTA.NH4 complex was annealed in NH3 ambient at different temperature starting from 1020 to 1100°C and the size of the particle was 400nm due to high temperature annealing.

Jason et al synthesized GaN nanowires by ion implantation technique where a 500nm thick Si02 layer was thermally grown on silicon (100) substrates. Fe2+ ions were implanted into these substrates at energy of 60keV with a dose of 1015 to 10^6 cm-2. The as-implanted samples were then placed into a 1 inch atmospheric quartz tube furnace and annealed at 900°C under 300 seem Ar and 200 seem H2 flow for 30 min to form iron catalyst nanoparticles on the oxide surface. After this GaN nanowires were grown on the iron implanted samples.

Cai et al synthesized GaN nanocrystals by mechanochemical synthesis method. Pure GaaOa powder and pure Mg3N2 powder were sealed in a hardened steel vial with 9.5mm diameter steel balls in a high purity Ar-gas atmosphere. The mass ratio of the reagents to the balls was 1:10. Milling was performed; after milling the black reaction product was annealed in a silica ampule under 10-5 torr vacuum in a tube oven wither at 500°C or 800°C for 5 h. Dilute hydrochloric acid was added to the annealed sample to remove MgO by-product.

Keyan bao synthesized GaN nanorods by solid state reaction method. The preparation of wurtzite GaN nanorods involved two steps: First, hydrothermal synthesis of orthorhombic GaOOH nanorods at 180°C for 12 hours, and second, preparation of GaN nanorods using NaNH2 and the as-prepared orthorhombic GaOOH nanorods as reactants in a stainless steel autoclave at 600°C for 5 hours.

The present invention synthesizes GaN particles of average size 20nm at a low temperature through nitridation of Ga-EDTA complex. The GaN nanocrystals are materials for field emission display (FED), vacuum fluorescent display (VFD) and many other applications due to high reliability, low electricity consumption and controllable emission wavelength by doping with various elements.

Description of the prior art:-

No such closest prior art is available to the present invention. However US patent No: 7255844 describe the "Systems and Methods for Synthesis of Gallium Nitride Powders" where high quality GaN powders were produced by combining High purity gallium and high purity Ammonia in a tube reactor under .controlled conditions. The reaction produced a porous gallium melt and to a full reaction yielding high purity crystalline GaN powders with a stoichiometric nitrogen concentration and a hexagonal wurtzite structure.

Whereas, the present invention is suitable to synthesize GaN nanocrystals at low temperature through nitridation of Ga-EDTA complex using simple, inexpensive and environment friendly technique. This technique yielded the GaN crystals with an average size of 20nm.

Summary of the Invention:-

The Gallium Nitride (GaN) nanocrystals were synthesized by nitridation of Ga-EDTA complexes at different temperatures starting from 600 to 900°C. The Ga-EDTA complexes were prepared by contacting GaCb and EDTA at pH of 9 in aqueous solution. X-ray diffraction analysis, Fourier transform infrared spectroscopy and Raman studies revealed that the compound synthesized at 900°C consists of single phase GaN nanocrystals with wurtzite structure.

The change in morphology of the GaN crystals at different temperatures was observed using scanning electron microscopy. The transmission electron microscopy showed the average size of crystalline particle to be ~ 20nm. The room temperature photoluminescence exhibits band-edge emission of GaN at 3.46eV for all the samples. The Nitridation of Ga-EDTA complex method has significant potential for synthesis of GaN nanocrystals as a simple and inexpensive method.

Detailed description of the invention:-

The present invention is a method to produce Gallium Nitride Nanocrystals.

The method consists of a Ga-EDTA.NH4 complex which was prepared from a mixture of GaCla and EDTA.NH4 in aqueous solution at a pH of 9. The solution was stirred for 6 hours and dried in an oven at 70°C. The Ga-EDTA.NH4 complex was taken in an alumina boat and placed inside the quartz reactor and NHS was allowed to react with the complex during the synthesis. The synthesis was carried out for a reaction period of 8 hours at 600, 700, 800 and 900''C. The temperature was brought down to 500°C by nitrogen purging up to room temperature. The reactions for the formation of GaN are:

2GaEDTA.NH4 ► Ga203 + EDTA Fragments
Ga2O3 + 2NH3 ► GaN + 3H2O

Ga-EDTA.NH4 complex decomposes to yield Ga2O3 intermediate which decomposes in the presence of ammonia to form GaN. The Ga2O3 intermediate which is in the nano form cannot be sintered to form bulk Ga203 as the melting point is 1900°C which is high. The main function of EDTA is to provide nano size Ga203, so that it can react with ammonia and convert into nanocrystals of GaN.

Powder X-ray diffraction patterns (XRD) of GaN crystals were recorded using Cu-Ka radiation of wavelength 1.5418 A at a scan speed of l°/min. The morphologies of GaN were studied using LEO stereo scan -440 scanning electron microscopy. Techai-12, FEI transmission electron microscopy (TEM) was used to determine the particle size. Energy dispersive x-ray diffraction analysis (EDAX) indicates the elements present in the synthesized compound. The room temperature photoluminescence (PL) spectrum of GaN samples was recorded using He-Cd Laser (325 run) as the excitation source. Fourier transform infrared spectral analysis (FTIR) of samples was carried out using bruker IFS 66V FT-IR spectrometer by KBr pellet technique. Raman spectra (300-100cm-1) was recorded at room temperature using Renishaw ramascope system -model 1000 with an excitation wavelength of 514.5 nm (Ar+ laser)
Brief Description of the Drawings:-

Fig la shows the XRD pattern of GaN compound synthesized at different temperatures.

Fig lb shows the XRD spectrum of the sample synthesized at 600°C after
annealing at 900°C in nitrogen ambient.

Fig 2 shows the morphologies of GaN nanocrystals

Fig 3 shows the TEM image of GaN nanocrystals synthesized at 900°C

Fig 4 shows the EDAX pattern of GaN powders synthesized at various temperatures

Fig 5 shows the room temperature PL spectrum of synthesized GaN powders at different temperatures.

Fig 6 shows the FTIR spectra of GaN powders synthesized at 600,700,800 and 900''C

Fig 7 shows the Raman Spectra of GaN powder synthesized at 800 and 900°C

Detailed Description of the Drawings:-

Referring to Fig la, the graph was plotted where the intensity (1) was marked in the Y-axis and the degree (2) was marked in the X axis. The XRD pattern shows the formation of GaN via intermediate states. The XRD spectrum of the powder synthesized at 600°C (3) shows only the amorphous nature. The powders synthesized at 700˚C (4) and 800°C (5) show the GaN phase along with additional phases like beta gallium oxide (P-Ga2O3) (6) and gallium nitrogen oxide nitrate (2Ga0N03.N205) (7). Thermodynamic calculations suggest that synthesis of GaN proceeded through intermediate products like gallium sub oxide (GaaOs). The powder synthesized at 900°C (8) shows a single phase GaN.

Referring to Fig lb, the graph was plotted where the intensity (1) was marked in Y-axis and the degree (2) was marked in the X-axis. The XRD spectrum of the compound was synthesized at 600°C after aimealing in nitrogen atmosphere at 900°C for a period of 3 hours. The result showed the formation of crystalline GaN with mixed phases of gallium oxinitrides (6) and gallium oxide (7). Therefore, synthesis at 900°C in NH3 ambient was carried out to obtain pure phase of crystalline GaN (9).

Referring to Fig 2, the SEM images of aggregated ultra fine GaN crystals which were synthesized at temperatures of 600, 700, 800°C showed mixed kind of morphologies which is due to the presence of secondary phases. The single phase sample synthesized at 900°C has only spherical agglomerated crystallites. The GaN (9) nanocrystals are mostly composed of uniform spherical particles with an average size of 20 nm with some hexagonal shaped nanocrystallites observed in TEM is shown in Fig 3.

Referring to Fig 4, the graph was plotted where the counts (1) marked in Y axis and energy (10) was marked in X axis. The EDAX results showed the intensity of carbon and oxygen peak decrease with the increase of synthesis temperature. 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 p-GaaOs and 2Ga0N03.N205 varied with different GaN synthesis temperatures from 600, 700, 800°C. The decomposition of these phases and the formation of GaN at 900°C were observed.

Referring to Fig 5, shows the room temperature Photoluminescence (PL) spectra of the GaN samples were plotted as intensity (1) in Y axis and the Energy [eV] (10) is X-axis. The GaN samples synthesized at different temperature show a band edge emission at 3.46 eV. The PL spectrum showed a mild blue shift indicating the increase in band gap of GaN when compared to bulk emission at 3.40 eV at room temperature. This shows that the increase in band gap of 60 meV is due to finite size effect. A broad band emission at 3.35eV is ascribed to the presence of oxygen in the form of Ga2O3 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. The sample synthesized at 600°C (3) shows amorphous nature which also exhibits band-edge emission. This result shows that the formation of amorphous GaN is possible at low synthesis temperature with additional phases and impurities like carbon and oxygen.

Referring to Fig 6(a), the FTIR spectrum of the samples synthesized at 600°C (3) shows a broad envelop between 2700 and 3700 cm-1 due to the OH stretching water. The bending vibration also confirms that it 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. Fig 6(b), shows the FTIR spectrum of the sample synthesized at 700°C (4). The OH stretch of water gives a very broad envelop with much resolution for its bending vibrations. The peak at 1000 cm-1 is for hydrogen bond modes. Fig 6 (c) shows the FTIR spectrum of sample synthesized at 800°C (5) which is similar in characteristics to that of samples synthesized at 700°C. The water content is less. The peak due to Ga-O vibration shows decrease in intensity. Fig 6(d) shows FTIR spectrum synthesized at 900°C which shows similar features as that of sample synthesized in 800°C, but the water content is still less and the intensity of Ga-O peak is also less. The peak due to GaN appears at 577 cm-1 (13) at 900°C where as at 800°C it appears at 617 cm-1 (14) which may be due to the presence of small quantity of Ga-O content in the sample synthesized at 800°C. The wave number (11) was plotted on X-axis and the transmittance (12) was plotted on Y-axis .

Referring to Fig 7, shows the Raman spectra of the GaN nanocrystals synthesized at 800 and 900 °C. The graph was plotted, where the Raman shift was taken in the X-axis (15) and the intensity (1) was marked in the Y-axis. Five phonon modes corresponding to pure GaN are observed at 419,535,556,568 and 728 cm-1 for samples synthesized at 900°C. The 419 cm-1 phonon mode corresponds to the acoustic overtone and the other four phonon mode correspond to Ai (TO) (16), E1 (TO) (17), E2 (high) (18) and Al(LO) (19) modes respectively. The samples synthesized at 800°C shows E2 (high) and Ai (LO) mode only. As the synthesis temperature decreases the E2 (high) mode broadens and blue shifts indicate that crystalline quality of the nanocrystals increase with temperatures of synthesis.

Claims

We Claim:

1. A gallium nitride nanocrystals comprising of a Ga-EDTA.NH4 complex wherein is prepared from a mixture of GaCl3 and EDTA.NH4 synthesized at different temperatures whereby the characterization of the nanocrystals is recorded.

2. As claimed in claim 1, wherein the mixture of GaCl3 and EDTA.NH4 is prepared in aqueous solution at a pH of 9; stirred for 6 hours and dried in an oven at 70°C.

3. As claimed in claim 1, wherein the Ga-EDTA.NH4 complex is taken in an alumina boat and placed inside the quartz reactor.

4. As claimed in claim 1, wherein the synthesis is carried out at different temperatures such as 600, 700, 800 and 900°C for a reaction period of 8 hours and brought down to 500®C by nitrogen purging up the room temperature.

5. As claimed in claim 1, wherein the EDTA of Ga-EDTA.NH4 complex decomposes to produce nano size Ga2O3 which reacts with ammonia and convert it into nanocrystals of GaN.

6. As claimed in claim 1, wherein the characterization of GaN nanocrystals is recorded by powder X-ray diffraction patterns (XRD) using Cu-Ka radiation of wavelength 1.5418 A at a scan speed of 1°/min.

7. As claimed in claim 1, wherein the characterization of GaN nanocrystals for elements in the synthesized compound by Energy dispersive x-ray diffraction analysis (EDAX).

8. As claimed in claim 1, wherein the characterization of GaN nanocrystals is recorded by Fourier transform infrared spectral analysis (FTIR) using bruker IFS 66V FT-IR spectrometer by KBr pellet technique.

9. As claimed in claim 1, wherein the characterization of GaN nanocrystals is recorded by Raman spectra (300-100cm-1) at room temperature using Renishaw Ramascope system -model 1000 with an excitation wavelength of 514.5 ran (Ar+ laser).

10. As claimed in claim 9, wherein the GaN nanocrystals morphology is studied using LEO stero scan -440 scanning electron microscopy; the Techai-12 FEI transmission electron microscopy (TEM) for determining particle size; room temperature photoluminescence (PL) spectrum of GaN samples was recorded using He-Cd Laser (325 nm) as the excitation source.