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Bull. Soc. Photogr. Imag. Japan. (2019) Vol. 29 No. 1: 1–5

Original Paper

Fluorescence Characteristics of Gallium Nitride Powders Prepared in the Laboratory
Masashi Iwasaki, Yuki Ueda, Kana Ishii*, Daichi Ozaki**

Abstract:

Fluorescence characteristics of gallium nitride powder samples prepared in our laboratory were examined. The samples processed at 1100 °C
showed band-edge emission and yellow luminescence, but the samples processed at 900 or 1000 °C showed no significant fluorescence. From
the results of XRD analysis, it was confirmed that all these samples had wurtzite gallium nitride structure, and the samples processed at 900
or 1000 °C showed the very low crystallinity. It was suggested that a lot of defects were present in the crystals of these powder samples withlow crystallinity, so that the recombination of excited electrons and holes was promoted to show no significant fluorescence emission. In addition, the sample processed with coexistence of activated carbon at a high ratio showed blue light emission around 425 nm, and it was concluded that a certain amount of carbon was taken into the crystal lattice.

Key words:

Gallium Nitride, Powder, Fluorescence, Crystal Structure, Carbon

1. Introduction
Gallium nitride is III-V compound, and a direct transition typesemiconductor with a band gap of about 3.4 eV. The stable crystal
structure of gallium nitride is hexagonal wurtzite. And a metastablezincblende structure and a rock salt structure under high pressure
have also been reported. All of these structures can be fabricated bychanging the synthesis methods and/or the synthesis conditions.
Since gallium nitride has a large band gap and is a direct transition
type, this material enables to produce a luminescent device emitting
blue light with high luminance.
Gallium nitride is a sublimable material, and its melt (liquid
phase) has not been confirmed. J. Karpiński et al. studied gallium
nitride crystal growth at high pressure1, 2), and concluded that themelting point of gallium nitride at 60 kbar was over 2300 °C. Therefore, crystal synthesis from the liquid phase seems impossible. On
the other hand, in crystal formation from gas phase, it is very difficult to control the crystal nuclei generation, crystal orientation andcrystal size. As a result, most of the synthesized particles become
small or polycrystalline. Due to the crystal grain boundaries in polycrystalline, recombination levels are formed in its band-gap, and thematerial becomes nonradiative. For use as a effective light emitting
device, it is necessary to obtain a single crystal having few crystal
defects. At present, it is ordinal to obtain gallium nitride single crystal by using metal-organic vapor phase epitaxy (MOVPE).
As for powder material, it is mainly obtained by high temperatureprocessing of metallic gallium or gallium oxide powder in an ammonia atmosphere. When using gallium oxide as a starting material, theReceived 27th March, 2019; Accepted 28th June, 2019

reaction starts at the reduction of gallium oxide by hydrogen, which
is formed by thermal decomposition of ammonia, to form gaseous
species, Ga2O, working as a reaction intermediate. Then gaseous
Ga2O reacts with ammonia to produce gallium nitride3). And more,
the attempts to fabricate gallium nitride nanoparticles and/or nanorods using gallium oxide4) and the synthesis of gallium nitride at
low temperature using lithium nitride and gallium oxide5) were reported. And many researchs on gallium nitride powder are carriedout, such as the studies aiming the use as fluorescent display devices6,7) and visible light responsive photocatalyst by GaN-ZnO solid
solution8), and the interest in its physical properties has been increased.
In this study, the fluorescence properties of gallium nitride powder samples prepared in the laboratory under various conditions
were investigated. Some of these lab-made samples were processedat the coexistence of carbon to promote the nitridation, and a part ofcoexisting carbon was incorporated into the gallium nitride crystal
after processing. These incorporated carbon was suggested to affectvarious physical properties of the powder crystal. The fluorescenceemission characteristics of the samples were discussed with preparing conditions and the effects of incorpolated carbon.

2. Experimental
GaN powder sample preparation.
The synthesis of gallium nitride powder starting from gallium oxide is supposed to proceed by the following reactions9, 10). Here, (g)Graduate School of Science and Technology, Kyoto Institute of Technology. Goshokaido-cho Matsugasaki Sakyo-ku Kyoto, 606-8585
Current affiliation: *Fujimoto Chemicals Co. Ltd., **Graduate School of Engineering, Kyoto Univ.

Bull. Soc. Photogr. Imag. Japan. Vol. 29 No. 1 (2019)represents a gas phase species and (s) represents a solid phase species.
Hydrogen in formula (1) is generated from the thermal decomposition of ammonia and acts as a reducing agent. Ga2O3(s) is reducedby this hydrogen to form Ga2O(g) , which is a necessary intermediate for the formation of GaN. And the generation of Ga2O(g) is therate-determining step of the overall reaction. The carbothermal reduction process is effective to promote the formation of Ga2O(g).
This process is a heat treatment in which metal oxides are reduced bycoexistence with carbon sources in a nitrogen or argon atmosphere.
For example, the carbothermal reduction process is applied for the
reduction of SiO2 for industrial preparation of AlN11). The reaction
formula for forming Ga2O(g) employing the carbothermal reduction is shown as follows.
Ga2O3(s)+2C(s)Ga2O(g)+2CO(g)(3)The commercially available gallium oxide powder (99.99 %, Dowa
Electronics) was used as gallium source in this study. Some samples
were processed with coexistence of carbon in purpose of promoting
nitridation as mentioned above. As carbon source, activated carbon
powder (Wako Pure Chemical Co.) or polyvinyl alcohol (Wako
Pure Chemical Co., the average molecular weight: 22000) was used.
In case of activated carbon was used, gallium oxide powder together
with activated carbon powder was ground thoroughly till the mixture became uniform. When the activated carbon was used, however,it was supposed that the direct contact area between GaN particles
and the activated carbon particles was very small. Then PVA coatingfor GaN particles before processing was attempted to increase thedirect contact area of gallium oxide and carbon and to promote thereducing reaction by carbon. When PVA was used, gallium oxide
powder was dispersed in PVA aqueous solution to prepare a suspension, and then the suspension was freeze-dried to prepare the PVA
coated gallium oxide particles. In this paper, the coexistence amountof carbon in the starting material is represented by the element ratioof Ga and C. In case of Ga:C = 1:1, the preparation weight ratio ofactivated carbon to Ga2O3 was 12:93.7.
The prepared starting materials were loaded on mullite boats, andnitriding processing was performed with electric tube furnace(ARF-50KC, Asahi-rika Co.) at the temperature range of 900 °C to1100 °C. The heating rate was set at 300 °C/h. For almost all sample
preparations, both NH3 gas and N2 gas were flowed into the system
at 200 ccm at room temperature as the nitrogen source, while thenitriding processing was proceeding. And a few samples were processed under only NH3 gas flow at 200-400 ccm at room temperature. In addition, no nitridation was observed at the processing under only N2 at every temperature in our experiments.
Measurements.
For fluorescence measurement, the powder samples were fixed on
glass plates to be set in spectrofluorometer (FP-6600, JASCO Co.)as the incident angle was 45 degree. The excitation wavelength was

330 nm, and the UV-35 filter was inserted before the detectors. The
fluorescence measurements were performed under room temperature. The XRD measurement for crystal structure analysis was performed by the 2θ/θ method using Rigaku RINT-2500 with Cu
target. Also the thermal gravimetric analysis (TGA) was carried out
with Discovery TGA (TA instrument Co.) to measure the weightloss of the samples caused by carbon burning.

3. Results and Discussion
Figure 1 shows the fluorescence spectra of the samples processedat different temperatures. The starting material of all samples in Fig.
1 was Ga2O3 without carbon. The spectrum of the sample processedat 1100 °C for 3hr showed two emission bands. As shown experimentally by B. Schwenzer et al.12), the band around 375 nm could be
regarded as the band-edge emission (hereafter referred to as BE).
The yellow luminescence near 555 nm (hereafter referred to as YL)
was also observed in the same spectrum. However, neither BE nor
YL were observed at the samples processed both of at 900 °C for 4hr
and at 1000 °C for 3hr. The XRD patterns of the same samples are20

BE : Band-edge Emission

1100 ℃

Intensity

Ga2O3(s)+2H2(g)Ga2O(g)+2H2O(g)(1)
Ga2O(g)+2NH3(g)2GaN(s)+H2O(g)+2H2(g)(2)10
YL : Yellow Luminescence1000 ℃
900 ℃

0

400

500
600
Wavelength /nm

700Fig. 1 F luorescence spectra of the samples processed without coexistence ofcarbon at various temperatures.

30000

(101)
FWHM : 0.12

20000

CPS

2

1100 ℃

10000

FWHM : 0.531000 ℃

FWHM : 0.69

900 ℃

0
10

Ref. GaN

20

30

40

50

2θ/θ

60

70

80

Fig. 2 X
 RD patterns of the samples same as shown in Fig.1. The FWHM
(full width at half maximum) values, measured with (101) peaks
around 37 degree, are shown on the right of the peaks.

3

Fluorescence Characteristics of Gallium Nitride Powders Prepared in the Laboratory

shown in Fig. 2. The FWHM (full width at half maximum) values,
measured with (101) peaks around 37 degree, are shown on the rightof the peaks. The FWHM value of the sample processed at 1100 °C
was extremely small, and it was supposed to have high crystallinity.
Other samples prepared under various conditions, including thesamples processed with coexistence of carbon, were evaluated at thesame fluorescence and XRD measurements. As the samples processed with coexistence of carbon might have remaining carbon after nitridation, high temperature post-treatment at 600 °C for 3
hours in the air were performed to remove it.
Figure 3 shows the relation between BE, YL emission intensities
and FWHM values. The processing temperatures and starting materials of each sample are indicated in the inner box. The samples A toD, processed at 900 or 1100 °C, showed relatively large FWHM
values, 0.4 to 0.7. In contrast, the samples E to J, processed at1100 °C, showed very small FWHM values, less than 0.2, meaning
to have high crystallinity. And the samples with larger FWHM values showed neither BE nor YL at all. The samples with low crystallinity was considered to have a lot of defects in their crystals, andthese defects worked as recombination centers for photo-generated
electron-hall pair, and then deactivated the excited electrons. These
results suggested that high crystallinity is essential for fluorescenceemission. However, the emission intensities of the samples withhigh crystallinity varied much. Even the samples at the same preparing condition showed various FWHM and emission intensities. So,

to understand the relation between their fluorescence properties andcrystallinity, other factors such as the influences of the particle sizeon FWHM values should be considered.
The fluorescence spectra of the samples processed with coexistence of activated carbon are shown in Fig. 4. The processing condition of these samples was at 1000 °C + 1100 °C. The samples processed at high carbon coexistence ratios, i.e. Ga (in Ga2O3):C = 1:5,
1:10, showed blue luminescence (hereafter referred to as BL) around430 nm. At the same time, it was observed that BE decreased as the10

Intensity

Masashi Iwasaki, Yuki Ueda, Kana Ishii, Daichi Ozaki

Ga:C
1:0
1:1
1:5
1:10

5

0

400

500
600
Wavelength /nm

700Fig. 4 F luorescence spectra of the samples processed with coexistence ofactivated carbon at 1000 °C for 2hr followed by at 1100 °C for 1hr.

Fig. 3 Th e relation between FWHM values and emission intensities of BE (top), YL (bottom) shown by the results of 9 different samples. Nitridation conditions and thestarting materials of each sample are indicated in the inner box, and AC in theconditions means activated carbon.

4Bull. Soc. Photogr. Imag. Japan. Vol. 29 No. 1 (2019)

Fig. 5 Th e energy diagrams for carbon incorporated gallium nitride crystals, proposed by
D. Zhou et al.14). EC : the lowest level of the conduction band, EV : the highest
level of the valence band, and EF : the Fermi level. About other signs representing
the various energy levels, such as VGa, see the text.

Intensity

60

40

Ga:C
1:0
1:0.1

20

1:0.03
1:0.5
1:1
1:3
1:10

0

400

500
600
Wavelength / nm

700Fig. 6 F luorescence spectra of the samples prepared with PVA coated Ga2O3
as starting material at various carbon coexistence ratios, nitridation
condition was at 1000 °C for 2hr followed by at 1100 °C for 1hr.

carbon coexistence ratio increased. J. L. Lyons et al.13) reported thatcarbon atoms replaced the N positions under N-rich conditions, itcaused lowering the Fermi level, then they replaced the Ga positions. In the particles of the samples with relatively large amount ofcarbon, such as processed at Ga:C = 1:5 and 1:10, carbon atoms wereconsidered to replace both N and Ga positions. D. Zhou et al.14)
proposed the mechanism for YL and BL emissions, and Fig. 5 is theillustration diagram from their paper. They attributed YL to theelectron transition from impurities levels, which were created justbelow of conduction band, to Ga vacancies levels, VGa, and the transition energy they showed was 2.2 eV, equivalent to the energy of
564 nm light, and BL to the electron transition from CGa levels to
CN levels, and the value was 2.8 eV, equivalent to 443 nm light.
Here, CGa represents the carbon replaced the gallium position; CN
represents the carbon replaced the nitrogen position; CN-VGa represents the complex of CN and VGa, whose energy level is almost thesame as VGa. As the coexisting carbon concentration becomes larger,
CN is created first and followed by CGa creation. Our results that BL
was observed at the samples of high carbon coexistence ratios wereconsistent with D. Zhou's mechanism. Carbon behaves as an amphoteric impurity in gallium nitride crystal, and it becomes a donor
or an acceptor depending on whether it substitutes for Ga or N

Fig. 7 T
 GA analysis of activated carbon, PVA, and two starting materials,
P1 and P10. The Ga:C (in PVA) ratios of P1 and P10 were 1:1 and
1:10, respectively.

atom. Therefore, when a large amount of carbon was incorporated ingallium nitride crystal, light emission via impurity levels, such as CGa
and CN, became considerable, so that BL was observed and BE became less.
The fluorescence spectra of the samples processed in coexistencewith carbon by coating of PVA are shown in Fig. 6. The processing
condition was same as in Fig. 4. In the case of PVA coating, BL was
not observed even when the carbon coexistence ratio was high. On
the other hand, similar to the case of activated carbon, BE becameless as the coexistence amount of carbon at nitridation processing
increased.
The weight change of the samples at high temperature post-treatment in the air was measured to confirm the carbon remaining after
nitridation. The samples without carbon or with small amount ofactivated carbon showed no weight change. However, the samples
processed at high carbon coexistence ratios, i.e. Ga:C = 1:5, 1:10,
using activated carbon, showed a certain weight loss caused by burning of remaining carbon. In contrast, in case of PVA coating, allsamples, including the samples processed at high carbon coexistence
ratios, showed no weight loss. This result suggested that PVA waslost while nitridation processing.
Figure 7 shows the results of TGA in nitrogen flow of activatedcarbon and PVA and two starting materials for P1 and P10. The
Ga:C (in PVA) ratios of P1 and P10 were 1:1 and 1:10, respectively.

Masashi Iwasaki, Yuki Ueda, Kana Ishii, Daichi Ozaki

Fluorescence Characteristics of Gallium Nitride Powders Prepared in the Laboratory

Except for activated carbon, the other three showed the similar TG
profiles, showing weight losses at around 280 and 400 °C. The total
weight losses after 600 °C heating in nitrogen were 17.6% for P1,
66.1 % for P10, 7.5% for activated carbon, and 95.5% for PVA. The
value of 17.6% for P1 was close to the weight proportion of PVA in
P1 before TGA measurement, 19%. Also the value of 66.1% for P10
was close to the weight proportion of PVA in P10, 70.1%. The 7.5%
weight loss for activated carbon could be due to the detachment ofadsorbed water. These results suggested that PVA was decomposed
and almost removed at 600 °C in a nitrogen atmosphere, and themost of residues of P1 and P10 were Ga2O3 contained in each starting
material and remaining PVA was a little bit. Yamaguchi et al.15-17)
reported that PVA decomposed thermally at 230 °C in a nitrogen
atmosphere, and the decomposition products obtained were water,
acetaldehyde, crotonaldehyde, benzaldehyde, acetophenone, etc.
As mentioned above, PVA did not work well as a carbon source toexpress BL, but created crystal defects, which worked to deactivate
the photo-generated electrons, and depressed BE and YL as a result.
This crystal defects formation by incorporated carbon, from both of
PVA and activated carbon, is also suggested in Fig. 3. The FWHM
values of samples H, prepared with coexistence of activated carbon,
and samples J, prepared with coexistence of PVA, were somewhat
larger than those of samples G, without carbon. This means that the
crystallinity of samples H and J were lower than that of samples G.
The crystal defects related to incorporated carbon seemed to work as
nonradiative recombination centers.

4. Summary
The gallium nitride powder prepared at 900 or 1000 °C had low
crystallinity, and showed neither band-edge emission nor yellow luminescence. It was suggested that many defects existed in the low
crystallinity powder crystals and these defects acted as nonradiative
recombination centers and that the excited electrons were deactivated. And high crystallinity seemed necessary for fluorescence emission. In case of the coexistence of activated carbon at nitridation
processing, the gallium nitride powder prepared with relatively large
amount of carbon showed blue luminescence. Therefore, it was assumed that carbon was taken into the gallium nitride crystal. However, band-edge emission became less as the coexistence amount ofcarbon at nitridation process increased.
In this study, the relationship between fluorescent luminescenceand crystal properties were investigated. PVA was confirmed to decompose easily at 600 °C in a nitrogen atmosphere, and it did not5

work well as a carbon source to express blue luminescence, BL. In
using activated carbon, the optimization of Ga:C ratios and nitridation conditions are desired to express BL efficiently. Also the othercarbon sources, such as graphite, would be tried for the efficient incorporation of carbon to GaN crystals to express BL. On the other
hand, the relationship between yellow luminescence and gallium
nitride crystal structure was not clear yet, further investigations wereneeded. In the future, GaN materials showing various colors luminescence will be developed by optimizing the incorporation of carbon and controlling blue luminescence and yellow luminescence.

Acknowledgment
The authors express special thanks to Sakai Chemical Industry
Co. Ltd. for providing the sample materials and also important information on gallium nitride powder preparation.

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