2.5 Crystallinity control of GaN quantum dots for realization of the band edge emission
2.5.1 Results and discussion
GaN crystallinity control via precursor and reaction pathway optimization.
From the results of the InxGa1-xN and the Eu doped GaN QDs, the crystallinity of the GaN QDs had to enhance for the stabilization of the lattice structure and the realization of the defect free band edge luminescence.1 In this part, we tried for the crystallinity enhancement research for the GaN QDs. As described in Fig. 2.28, the metal nitride had more ionic character than the metal pnictide (P, and As) and the more covalent character than the metal oxide materials.2 Also the Ga metal complex induced the polymerization of the ligand and solvent which included the double bond part.3 From these characters, the Ga oleate with HMDS condition had limitation for optimization of the crystallinity of the GaN QDs.
For improvement of the crystallinity of the GaN, the Ga halide utilized as the Ga precursor and the metal amide used as the nitrogen source for the inducing the acid-base reaction between metal and nitrogen precursor. The dactyl ether used as solvent instead of 1-octadecene (ODE) from double bond free structure and the higher polarity than ODE. The 1-octadecyl amine utilized as surface binding ligand to make up for oleylamine for preventing polymerization from double bond functional group. From above combination, the ionic reaction induced with preventing the polymerization for optimization of the GaN QDs.
The temperature control test progressed under these ionic reaction condition, the results deduced as shown in Fig. 2.29. From room temperature (RT) to 120 ℃ range, the 340 nm photoluminescence peak showed, however the Ga vacancy emission of 500 nm peak emerged at 240 ℃ condition. After the 240 ℃ range, the 320 ℃ temperature condition showed the 360 nm single peak emission with suppression of the Ga vacancy emission. The purified solution image of the Fig. 2.29 served the obvious differences between the 220 ℃ condition and 320 ℃ condition. The 220 ℃ condition showed the dark brown color, which related for the Ga metal complex. The 320 ℃ condition showed the transparent and colorless state.
The GaN and 3.1 eV band gap, which band gap range could not absorb of the visible range light. 1,4 The transparent and colorless state was normal for the GaN case. From these above tendency, the RT to 220 ℃ conditions were complex state of the Ga halide and metal amide,
and the 320 ℃ condition served the GaN QDs from decomposition of the Ga halide and metal amide complex.
For the detail characterization, the TEM utilized as shown in Fig. 2.30. The reaction temperature utilized as test value, which composed for the 40 ℃ to 320 ℃ range. At the 40 ℃ condition, the complex crystal showed from ionic reaction between the GaCl3 and LiNH2 salts. At the 120 ℃ to 220 ℃ condition, the Ga and amide complex melted and growth for the large sized complex as shown in Fig. 2.30. The crystallinity change of these products had singularity point. The product of the 40 ℃ and 120 ℃ range had crystallinity, but the product of the 220 ℃ condition composed for the polymeric complex form. At 320 ℃, the polymeric complex decomposed and changed for the GaN particle forms. The GaN QDs had 7 nm size with crystallinity as shown in Fig. 2.30.
The ligand quantity controlled for optimization of the synthesis condition of the GaN QDs. The Fig. 2.31 described the absorbance and photoluminescence results. The absorbance results showed the 1st excitonic peak at 340 nm position, and the overdose of the ligand quantity induced the blur of the 1st excitonic peak. The photoluminescence peak position showed the 350nm, 377 nm, and 355 nm at 1 : 3, 1 : 6, and 1 : 9 condition of the Ga : octadecylamie (ODA) ratio. From optical characterization, the GaN QDs had band edge emission with Stokes shift.
The TEM characterization utilized at Fig. 2.32 to define the particle size and shape of the various ligand quantity conditions. From the TEM characterization, the synthesis of the GaN QDs effected from the ligand quantity. The Ga : ODA = 1 : 6 condition showed best synthesis condition. The 1 : 3 condition served the GaN particle with polymeric product, and the 1 : 9 condition had large quantity of the polymeric complex than GaN particle product. From these results, the amine based ligand effected for the synthesis pathway of the GaN QDs.
For the crystal structure characterization, the XRD utilized as shown in Fig. 2.33. The XRD peak showed that the GaN QDs had face-centered cubic (FCC) structured GaN.
However, the GaN product composed for two kinds of the lattice constant. For the exact demonstration, the more study needed. The GaCl3 with LiNH2 complex not all converted for the polymeric complex, and then the polymeric complex and the GaCl3 with LiNH2 complex decomposed for the GaN QDs. From these two pathways for the making GaN, the different
From these results, the high crystalline GaN QDs realized from the ionic reaction pathway. The GaN QDs had similar optical properties of the bulk GaN materials as shown above results.
Fig. 2.28 The iconicity differences of the Gallium oxide, Gallium nitride and Gallium arsenide materials.2 (left) The polymeric complex and the GaAs QDs from polymerization of the solvent and the ligand via Ga metal complex.
Fig. 2.29 The photoluminescence result of the Ga-N complex and the GaN QDs. The solution image of the Ga complex rich state and the GaN QDs rich state.
Fig. 2.30 The TEM image of the various reaction temperature conditions. (40 ℃ to 320 ℃, the 320 ℃ condition had reflux of the solvent)
Fig. 2.31 The absorbance and the photoluminescence of the GaN QDs with various ligand quantity condition. (Ligand : octadecyl amine)
Fig. 2.32 The TEM of the GaN QDs with various ligand quantity condition. (Ligand : octadecyl amine)
Fig. 2.33 The XRD result of the GaN QDs. (PDF # : 01-075-2981, 01-088-2364)