Hexagonal Nanocrystals into AlGaN Powders Obtained via Pyrolysis from an Organometallic Compound

Hexagonal nanocrystals into Al0.2Ga0.8N and Al0.6Ga0.4N powders via pyrolysis from an organometallic compound, followed by a nitridation process in ammonia flow at 1000 °C for two hours were obtained. X-ray diffraction patterns demonstrated a shift towards greater angles to the right for the AlGaN powders with respect to GaN powders, this shift could indicate the formation of the AlGaN powders. Scanning electron microscopy micrographs showed the obtaining from semi-plates of porous appearance for the Al0.2Ga0.8N powders until well-defined plates for the Al0.6Ga0.4N powders. High resolution transmission electron microscopy micrographs demonstrated the presence of hexagonal nanocrystals into Al0.2Ga0.8N powders with an average crystal size of 10.3 nm, while that for the Al0.6Ga0.4N powders an average crystal size of 9.7 nm was observed. UV-visible spectra showed a transmittance cut-off for the Al0.2Ga0.8N powders of 3.71 eV (334.2 nm) and a transmittance cut-off of 4.53 eV (273.7 nm) for the Al0.6Ga0.4N powders.


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Abstract-Hexagonal nanocrystals into Al0.2Ga0.8Nand Al0.6Ga0.4Npowders via pyrolysis from an organometallic compound, followed by a nitridation process in ammonia flow at 1000 °C for two hours were obtained.X-ray diffraction patterns demonstrated a shift towards greater angles to the right for the AlGaN powders with respect to GaN powders, this shift could indicate the formation of the AlGaN powders.Scanning electron microscopy micrographs showed the obtaining from semi-plates of porous appearance for the Al0.2Ga0.8Npowders until well-defined plates for the Al0.6Ga0.4Npowders.High resolution transmission electron microscopy micrographs demonstrated the presence of hexagonal nanocrystals into Al0.2Ga0.8Npowders with an average crystal size of 10.3 nm, while that for the Al0.6Ga0.4Npowders an average crystal size of 9.7 nm was observed.UV-visible spectra showed a transmittance cut-off for the Al0.2Ga0.8Npowders of 3.71 eV (334.2 nm) and a transmittance cut-off of 4.53 eV (273.7 nm) for the Al0.6Ga0.4Npowders.

I. INTRODUCTION
In nineties the blue laser of solid state obtained by Nakamura, Akasaki and Amano [1], [2], opened the wide way of the III-Nitrides semiconductor materials.This because to its application in the conservation of electrical energy through the replacement of incandescent light bulbs with LED technology bulbs [3].Other important applications of the III-Nitrides are found in solar cells, LED screen, high electron mobility transistors (HEMTs), microwave devices and laser diodes [4]- [7].In the III-Nitrides semiconductors stand out the ternary alloys AlGaN and InGaN due to their band gaps in a range from 3.4 to 6.2 eV.Particularly, the AlGaN alloy can crystallize in the hexagonal structure (wurtzite), which depend on the growth method.However, in the last few years, the AlGaN alloy has stand out due to the possibility of obtain nanocrystals, which could have applications in novel electronic devices for the Published on March 13, 2019.A. M. Herrera, G. García, E. Gastellóu, F. Nieto, C. Morales, E. Rosendo and T. Díaz are with the Centro de Investigación en Dispositivos Semiconductores, Benemérita Universidad Autónoma de Puebla (BUAP), Puebla, Puebla, C.P. 72570, México (e-mail: anhe_26@hotmail.com,godgarcia@yahoo.comerick_gastellou@utpuebla.edu.mx,ruscaballero@yahoo.com.mx,crisomr@yahoo.com.mx,enrique171204@gmail.com, tomas.diaz.be@gmail.com.R. García is with the Departamento de Investigación en Física, Universidad de Sonora (USON), Hermosillo, Sonora, C.P. 83190, México (e-mail: rgarcia@cifus.uson.mx).
The AlGaN ternary alloy is generally obtained via metalorganic chemical vapor deposition (MOCVD), metalorganic vapor phase epitaxy (MOVPE), and molecular beam epitaxy (MBE) [10]- [12].However, other techniques such as ethylene diamine tetra acetic acid (EDTA) complex route, pyrolysis of the metal-organic compound using gallium and aluminum nitrates as precursors, as well as, the pyrolysis of metal amide-imide precursors could synthesize nanocrystals into the AlGaN alloy [13].This work presents the obtaining of hexagonal nanocrystals into AlGaN powders via pyrolysis from an organometallic compound (CH6N4O)3Al-Ga(NO3)3, followed by a nitridation process in NH3 flow at 1000 °C for two hours.The x-ray diffraction patterns (XRD) demonstrated a shift towards greater angles at increase the aluminum composition.Scanning electron microscopy (SEM) micrographs showed changes in the surface morphology at increase the aluminum concentration.Highresolution transmission electron microscopy (HRTEM) micrographs demonstrated the presence of hexagonal nanocrystals into AlGaN powders.Finally, transmittance measurements (%T) at room temperature showed emission energies in the range UV.

II. EXPERIMENTAL PROCEDURE
The obtaining of the AlGaN powders had the same experimental procedure for the Al0.2Ga0.8Nand Al0.6Ga0.4Nalloys, as well as for the GaN powders, which was used as a reference to the different analysis.The procedure of obtaining is described below and a process diagram is shown in Fig. 1.

A. Organometallic compound
The experimental procedure for the synthesis of AlGaN powders with different aluminum concentration (x = 0, 0.2 and 0.6) was made at atmospheric pressure (744 mmHg), using aluminum nitrate (Al(NO3)3), gallium nitrate (Ga(NO3)3) and carbohydrazide (CH6N4O) as reagents.The weight of each reagent was calculated according to the (1), to obtain the amount of the desired polymer: The synthesis via pyrolysis route is based on a reaction by ignition between the carbohydrazide and the nitrates; the reaction wave at high temperatures is propagated through of the heterogeneous mixture due to the formation heat of the product.The nitrates were placed into a teflon beaker Hexagonal Nanocrystals into AlGaN Powders Obtained via Pyrolysis from an Organometallic Compound with 20 ml of HPLC grade toluene, the mixture was magnetically stirred and heated in a hot plate at 111°C, after of fifteen minutes, a homogeneous mixture was obtained.
The carbohydrazide was added in the same conditions, then the process ended when toluene was evaporated and the (CH6N4O)3Al-Ga(NO3)3 organometallic compound was formed.Once obtained the organometallic compound, the teflon beaker was removed from the hot plate to cool at room temperature.

B. Obtaining of AlGaN powders via pyrolysis route
The organometallic compound was removed from the teflon beaker and a sample was selected to perform the pyrolysis (1.5 grams).The process was done using a threezone furnace (Lindberg Blue M).The organometallic compound sample was placed in a high-alumina boat and it was introduced into the furnace.The system was purged three times using a N2 flow (150 sccm) at room temperature to reduce the residual oxygen.When the chamber was oxygen free, it was introduced a N2 flow at 150 sccm, as transport gas of the pyrolysis subproducts.Later the sample was slowly preheated to homogenize the polymer at a temperature close to the pyrolysis temperature [13].The homogenization was made during one hour to prepare the controlled ignition of the organometallic compound.After, the temperature was increased until reaching the pyrolysis temperature (2 °C/min) [13].The formation of white vapors or a slight variation in outlet pressure indicated that the pyrolysis was made.It is important to mention that the pyrolysis temperature was systematically checked by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) in a before work [13].Once the process is finished, the temperature was decreased until reaching the room temperature using a N2 atmosphere (150 sccm).The synthesized material was taken out from the furnace and grinded to obtain the AlGaN powders.The AlGaN powders were introduced again into the furnace for their annealing in NH3 flow (100 sccm) at 1000 °C for two hours, whereupon the NH3 flow was closed and the temperature was decreased to room temperature in N2 atmosphere.Finally, the powders were taken out from the furnace for their characterizations.Table I, shows the pyrolysis temperatures for the Al0.2Ga0.8Nand Al0.6Ga0.4Nalloys, as well as for the GaN powders.

C. Characterizations
The AlGaN and GaN powders were characterized by Xray diffraction patterns (XRD), in a Phillips X´pert MPD equipment with a wavelength (Cu K) of 1.5406 Å.The surface morphology (SEM) of AlGaN and GaN powders were obtained using JEOL JSM-5300 equipment.Highresolution transmission electron microscopy (HRTEM) was obtained in a JEM-ARM200F equipment.Transmittance measurements (%T) at room temperature were performed using a Thermo Scientific Evolution 600 UV-Visible spectrophotometer in a range from 200 to 600 nm with a calibrated target, 99% reflectance standard 60 mm.

III. RESULTS AND DISCUSSION
A. Structural analysis Fig. 2 shows the x-ray diffraction patterns for the Al0.2Ga0.8N,Al0.6Ga0.4N,and GaN powders, obtained via pyrolysis and nitridation of the (CH6N4O)3Al-Ga(NO3)3 organometallic compound.Fig. 2a) shows the peaks related to GaN with the planes orientation (100) and (002), where the peak of higher intensity is located in the plane orientation (101) and is related to hexagonal GaN (all the peaks of the Fig. 2a) are indexed in ICCD card: 01-089-8624).Fig. 2b), shows the x-ray diffraction pattern for Al0.2Ga0.8Npowders, where the peaks correspond to the planes orientation (100), ( 002) and ( 101), which are indexed in ICCD card: 01-080-4096.Fig. 2c) shows the xray diffraction pattern for the Al0.6Ga0.4Npowders.In this figure the peaks are located in the planes orientation (100) and (002), furthermore of a peak in the plane orientation (101), which was indexed in ICCD card: 04-018-2009.All diffraction patterns for the AlGaN powders correspond to a hexagonal structure.In Fig. 2b) and 2c), the x-ray diffraction patterns show a slight shift towards higher angles with respect to the x-ray diffraction pattern of the GaN powders.This shift might indicate the formation of the AlGaN alloy, due to the aluminum incorporation into the GaN lattice.The Al0.2Ga0.8Npowders had a shift of 0.15°(36.85°) to the right, with respect to GaN (36.70°).While the Al0.6Ga0.4Npowders had a shift of 0.39°(37.09°) to the right, with respect to GaN(36.70°).The peaks broadening of the AlGaN powders might be due to the presence of nanocrystals [9].Using the Debye-Scherrer equation and the ICDD PDF-4+ 2018 software, were computed the averages of crystal size of the plane orientation (101) for Al0.2Ga0.8N,Al0.6Ga0.4Nand GaN powders.To GaN powders were found an average crystal size of 21.6 nm, with a lattice constant of a = 3.18 Å and c = 3.18 Å.To Al0.2Ga0.8Npowders was found an average crystal size of 8.9 nm, with a lattice constant of a = 3.17 Å and c = 5.14 Å.Finally, for the Al0.6Ga0.4Npowders was found an average crystal size of 7.9 nm, with a lattice constant of a = 3.14 Å and c = 5.05 Å.

B. Electron microscopy
Fig. 3a) shows the SEM micrograph for the GaN powders, whose surface morphology demonstrated the presence of amorphous agglomerated of porous appearance.The porous appearance might be due to the pyrolysis process, which employs nitrates and carbohydrazide as fuel to the reaction.Fig. 3b) shows the SEM micrograph for the Al0.2Ga0.8Npowders.In this figure is possible to observe as the aluminum incorporation begin to form semi-plates, in which the porous appearance still is present.Fig. 3c) shows the SEM micrograph for the Al0.6Ga0.4Npowders, whose surface morphology demonstrated the presence of plates more definite, in which the porous appearance is least.The presence of hexagonal nanocrystals into AlGaN powders was checked by high-resolution transmission electron microscopy (HRTEM).Fig. 4a) shows the HRTEM micrograph for the Al0.2Ga0.8Npowders, in which was demonstrated the presence of hexagonal nanocrystals with an average crystal size of 10.3 nm.In this figure also were observe hexagonal nanocrystals well definite.Fig. 4b) shows the interplanar distance for the HRTEM micrograph of the Fig. 4a), with a value of 2.430 Å.Furthermore, Fig. 4c) shows the electron diffraction pattern of the selected area for the Al0.2Ga0.8Npowders, which could prove the obtaining of AlGaN powders.Fig. 5a) shows the HRTEM micrograph for the Al0.6Ga0.4Npowders.This Figure demonstrated the presence of hexagonal nanocrystals with an average crystal size of 9.7 nm, which indicate that at increase the aluminum concentration, decrease the crystalline size [13].Also, the loss of definition in the hexagonal nanocrystals was observed.These results checked the x-ray diffraction patterns obtained for the AlGaN powders.Fig. 5b) shows the interplanar distance with a value of 2.450 Å, and the Fig. 5c) shows the electron diffraction pattern with a plane orientation (101) for the Al0.6Ga0.4Npowders.C. Transmittance Fig. 6 shows the transmittance spectra (%T) for the GaN, Al0.2Ga0.8N,and Al0.6Ga0.4Npowders performed at room temperature in a range from 200 to 600 nm.Fig. 6 shows a clear absorption edge in the UV region, which changes in a range from 3.42 to 4.53 eV.The transmittance cut-off for the GaN powders is around of 3.42 eV (362.5 nm), which is related to an interval of the direct band.However, at increase the aluminum concentration is observed a transmittance cut-off for the Al0.2Ga0.8Npowders of 3.71 eV (334.2 nm) and a transmittance cut-off of 4.53 eV (273.7 nm) for the Al0.6Ga0.4Npowders [14]- [18].

IV. CONCLUSIONS
Hexagonal nanocrystals of AlGaN were obtained via pyrolysis from a organometallic compound, followed by a nitridation process at 1000 °C for two hours.X-ray diffraction patterns demonstrated a slight shift to the right of 0.15° for the Al0.2Ga0.8Npowders and 0.39° for the Al0.6Ga0.4Npowders, this with respect to the GaN powders.The shift could indicate the formation of the AlGaN powders.SEM micrographs showed the surface morphology of the AlGaN powders, obtaining from semiplates of porous appearance for the Al0.2Ga0.8Npowders until plates more definite for the Al0.6Ga0.4Npowders.HRTEM micrographs demonstrated the presence of hexagonal nanocrystals into AlGaN powders.Hexagonal nanocrystals with an average crystal size of 10.3 nm was observed in the Al0.2Ga0.8Npowders, while an average crystal size of 9.7 nm was observed in the Al0.6Ga0.4Npowders.These results checked that at increase the aluminum concentration decrease the crystalline size, which agree with the XRD analysis.UV-visible spectra showed a transmittance cut-off for the Al0.2Ga0.8Npowders of 3.71 eV (334.2 nm) and a transmittance cut-off of 4.53 eV (273.7 nm) for the Al0.6Ga0.4Npowders, which indicate the obtaining of the AlGaN powders.