Investigations on the structural and optical properties of sphere- shaped indium nitride (InN)
C. Bagavath1 · L. Nasi2 · J. Kumar1
Received: 25 June 2016 / Accepted: 11 March 2017
© Springer-Verlag Berlin Heidelberg 2017
improve the device performances [4]. Although InN pos- sess unique properties, the study of its nanostructures has not reached the same extent as that of GaN. It is due to the difficulties in the preparation conditions [5] as it has low dissociation temperature, high tendency to form oxides and thermodynamically unstable resulting from a higher stabil- ity of N–N bond and a relatively weaker In–N bond [6].
In the electronic devices, nanodimensions of InN are produced by metal–organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) techniques [7]. Nano- structures of InN have also been realized in different meth- ods, such as low temperature nitridation of LiInO2 using NaNH2 flux [8], reacting Indium halide or Indium sulphide with NaNH2 or lithium nitrite [9, 10], chemically convert- ing organic indium to indium nitrate solution and reacting with ammonium hydroxide at low temperature [11]. It has been already reported in the literature that the decompo- sition temperature is in the range from 773 to 983 K for InN thin films [12, 13]. Schwenzer et al. reported the syn- thesis of InN micro and nanostructures from indium oxide was done in the temperature range of 873–1003 K using ammonolysis method [14]. Although in the past dec- ade, the band gap of InN is ascertained ∼0.7 eV [15], the absorption edges or emission peaks of the InN nanostruc- tures have been reported over a range of 0.7–1.8 eV [16].
Besides the size effect, the electronic property of a mate- rial also depends on the stoichiometric variation and impu- rities presence. The role of oxygen contamination in InN has been a continuous issue. Even a small incorporation of oxygen around 3% would widen the band gap ∼1.9 eV [17]. Hence, efforts have to be taken to minimize the oxy- gen incorporation during the InN growth. In this paper, we report the synthesis of spherical InN micro crystals and nano crystals using a sol–gel assisted growth technique fol- lowed by ammonolysis.
Abstract Indium nitride (InN) sphere-shaped micro crys- tals and nano crystals were made using sol–gel method.
The crystalline size of the samples were calculated using X-ray diffraction, which were found to increase with the increase of nitridation temperature and time. High resolu- tion-transmission electron microscopy images exhibited the distinct sphere shape of InN with different size of micro and nanometers. The calculated band gap of InN spheres using photo luminescence and UV–visible absorption spec- tra, was found to be 1.2 eV. Optical phonon modes of InN were determined from micro-Raman studies.
1 Introduction
Indium nitride (InN) crystalline material has become an integral part in most of the current III-nitride optoelec- tronic devices. The properties of the relatively low electron effective mass [1] and high drift velocity [2, 3] make it as a unique semiconductor among the other nitrides such as gallium nitride (GaN) and aluminum nitride (AlN). InN has given more versatility to the nitride alloys in wavelength tuning from infrared to most part of visible regions in light emitting applications. The performance of III-nitride opto- electronic devices are affected by the material issues such as dislocations and spontaneous polarization. Nanostruc- tures of nitrides are regarded to overcome these issues and
* J. Kumar
1 Crystal Growth Centre, Anna University, Chennai, Tamil Nadu 600025, India
2 IMEM-CNR, Parco Area delleScienze 37/A, 43010 Parma, Italy
2 Experiment
Indium nitride (InN) micro crystals and nano crystals were prepared by sol–gel assisted nitridation method. In the sol–gel process, indium citrate complex have been formed by dissolving In (NO3)3·xH2O in nitric acid along with the citric acid. The solution was stirred with a magnetic stirrer at 353 K for 2 h until it become viscous and then it was dried at 473 K for 4 h. The obtained powder was trans- ferred into molybdenum boat and subjected to nitridation in a quartz tubular furnace. Synthesis was carried out in various temperatures from 873 to 973 K for 4 h under con- stant ammonia flow of 0.4 slm (standard liter per minute) in the reactor. Later, experimentation has been done with the variation of nitridation period from 4 to 8 h. After the completion of nitridation process, the temperature has been carried out within 15 min to room temperature along with constant ammonia flow. This was done to avoid the forma- tion of indium metal droplets. Nitrogen gas of 0.2 slm was allowed to flow into the reactor during the heating process.
3 Results and discussions
InN micro crystals and nano crystals were synthesized by sol–gel assisted growth. The growth mechanism was described in the reactions [1–4] is shown in Fig. 1. This process was divided into four stages.
3.1 Stage 1
Indium nitrate was incorporated into the nitric acid to form a metal nitrate solution is shown in Eq. (1)
(1) In(NO3)3⋅xH2O+HNO3→In3++HNO3+3(NO3)−+xH2O,
3.2 Stage 2
To avoid oxide formation chelating agent (citric acid) was added with indium nitrate solution so that the citric acid ions form complexes with metallic cations as metal citrate complexes. The addition of citric acid results in the forma- tion of coordination complexes or metal complexes. These complexes consists of a central metallic atom or ion sur- rounded by an array of bound molecules or ions that are in turn known as complexing agents. These complexes are called chelating complexes, the formation of such com- plexes is called chelation complexations which is men- tioned in the Eq. (2) [18, 19]
3.3 Stage 3
NH4OH was added to maintain the pH in the range of 2–3 pH and to get a viscous solution of Indium citrate com- plexes Eq. (3).
3.4 Stage 4
Nitridation was carried out at different temperatures and different time. During the nitridation process Eq. (4), due to the reversible metal coordination chemistry cluster-fusion growth mechanism takes place; where several small par- ticles of indium citrate complexes first aggregate and then undergo intra-particle fusion to yield large uniform spheri- cal crystals.
In3++HNO3+3(NO3)−+xH2O+2(C6H8O7)→ (2) [In(C8H5O7)2]+4HNO3+xH2O,
[In(C8H5O7)2]+4HNO3+xH2O+3NH4OH→(NH4)3(3) [In(C8H5O7)2]+4HNO3+xH2O+3OH,
(4) (NH4)3[In(C8H5O7)2] +2NH3�������→Δ InN +2(C6H8O7) +4(NH3)↑,
Fig. 1 Schematic diagram for the formation mechanism of InN sphere-shaped particles
3.5 Fourier transform infrared spectroscopy analysis (FT-IR)
The major absorption peaks around 1756 and 1705 cm−1 in Fig. 2a are attributed to –COOH stretching vibrations [20].
Compared with citric acid these carboxylic-related vibra- tions are observed at 1629 cm−1 [21] and also there has been shift in the C–H vibrations (1390 cm−1) in Fig. 2b.
The characteristic NH3− vibration is found to be present at 825 cm−1. This confirms the formation of Indium citrate complexes.
3.6 Structural analysis (XRD)
Figure 3a shows the XRD pattern of the InN prepared at temperatures 873, 923 and 973 K for period of 4 h. It can be clearly seen that the diffractions of the sample prepared at 873 K indicate the formation of cubic indium oxide.
When the temperature is increased to 923 K, few InN peaks
Fig. 2 shows the FT-IR spectrum of the citric acid and Indium citrate complex
Fig. 3 Powder X-ray diffraction patterns a) 873 K (4 h), ii 923 K (4 h), iii 973 K (4 h), b i 973 K (4 h), ii 973 K (6 h), iii 973 K (8 h) and c (101) peak shifting 2θ value
appeared. At 973 K, intense wurtzite InN diffraction peaks of (101), (103), (201) and (202) found to be emerged along with a single cubic (200) peak. From the diffraction studies it is inferred that at elevated nitridation temperatures, there are diffraction peaks assigned to InN together with those of In2O3 but the intensities of the InN peaks increased with the nitridation temperature. Figure 3b shows the XRD pat- tern of InN synthesized at 973 K for nitridation period of 4, 6 and 8 h. In the increase of nitridation period, there has been huge reduction of indium oxide peaks. As shown in the Fig. 3b, at the duration of 6 h nitridation, the conversion of InN is high as only a small trace of In2O3 peak is noted.
Further increase of nitridation period to 8 h, InN diffrac- tion peaks along with those of metallic indium have been observed. Careful optimization of the experimental condi- tions is essential, as the thermal stability of InN at elevated temperatures is low. The XRD data matched well with the JCPDS #88-2362 for InN, JCPDS #06-0416 for In2O3 and JCPDS # 05-0642 for In.
To study of the effect of the nitridation temperature and timing, a fine analysis of the position of the prominent InN (101) peak is carried out. It was found that shift in peak were observed towards the higher side of the 2θ value shown in Fig. 3c. The shift in peak value may be due to the incorporation of oxides in the crystals lattices. And it is also observed from the Fig. 3c that for 6 and 8 h of nitridation temperature quality of the crystalline particle is increased. When the nitridation temperature is above 973 K for 8 h, InN with indium metal has been observed at the bottom of the molybdenum boat, because of In–N weak bonds natures the decomposition of InN takes place into indium metal and N2 gas. The synthesis of InN is very sensitive to the nitridation temperature, the reaction time and ammonia flow. The size of the InN micro crystals and nano crystals is calculated using Debey–Scherrer’s formula [22]. The calculated lattice constants matches well with the standard repeated values tabulated in Table 1.
where D is the size of the particle, β is the full width half maximum intensity, λ is the wavelength and θ is the peak (5) D= 0.9𝜆
𝛽cos𝜃,
position. The average crystalline size of InN was found to be increased with respect to nitridation temperature and time is mentioned in Table 1. The increase in the crystal size may be due to the reduction of oxides from the crystal lattice of InN that increases the nucleation and growth rate of InN crystals. The observation of small changes of dif- fracted peak 2θ values and broadening in the peaks is due to the increase or decrease in micro strain.
The shift in the peak position towards the higher side is observed in Fig. 3c. Using the formula below [22], the micro strain values are calculate and tabulated in Table 1.
The lattice parameters of the InN is calculated from the following equation [22]
where a and c are the lattice constant; h, k, and l are the Miller indices; d is the interplanar distance or spacing. Due to different nitridation temperature and time the change in a and c parameters are observed due to the reduction of oxides form the crystal lattice of InN, as shown in Table 2.
The bond length of the In–N has been calculated using the following formula [22]
where a and c are lattice constants
where u is a positional parameter.
The volume of the unit cell for the hexagonal system has been calculated using the following relation [22].
Figure 4a show the relation between crystal size and strain with respect to the nitridation temperature and time. As the nitridation time and temperature is increased, the crystalline size and the micro strain were decreased.
(6)
∈= 𝛽cos𝜃 4 ,
(7) 1∕d2=4∕3((h2+hk+k2))∕a2+l2∕c2,
(8) l=
√ (
a3 3 +
(1 2−u
)2 c2
) ,
(9) u= a2
3c2 +0.25,
(10) V =0.866a2c,
Table 1 Crystallite size and micro strain of different temperature (K) and time (h) growth of InN micro crystals and nano crystals
Different temperature (K)
and time (h) Crystallite size
(nm) Micro
strain × 10−3 lines−2/ m4
923 (4 h) 38 0.906
973 (4 h) 45 0.898
973 (6 h) 47 0.768
973 (8 h) 51 0.643
Table 2 Bond length and cell volume of different temperature (K) and time (h) growth of InN micro crystals and nano crystals
Different tempera-
ture (K) and time (h) Lattice param-
eters (Å) Bond length (l) nm
Cell volume Å3
a c
923 (4 h) 3.681 5.101 0.598 59.85
973 (4 h) 3.651 5.110 0.591 58.98
973 (6 h) 3.486 5.091 0.533 53.57
973(8 h) 3.463 5.061 0.516 52.56
Figure 4b shows the relation between the cell volume and bond length with respect to nitridation time and tempera- ture. The volume of the unit cell and bond length decreases with respect to the increase in nitridation temperature and time, this may be due to the reduction of oxides from the InN crystal lattice. The percentage of InN, In2O3 and In present in the sample for different temperature and time were calculated from XRD is shown in the Table 3.
3.7 Micro-Raman analysis
Raman scattering technique is one of the powerful tool for analyzing the structural and physical properties through the determination of phonon modes. As per group theory, the predicted six phonon modes for the hexagonal InN are Γ =A1+2B1+E1+2E2. The A1 and E1 are polar modes and both Raman and infrared active, whereas the non- polar E2 modes are Raman active only, and the B1 modes are silent. Figure 5 shows the Raman spectrum of the InN micro crystals and nano crystals. The peaks observed at 448, 470, 485 and 592 cm−1 can be assigned to A1 (TO), E1
(TO), E2 (high) and A1 (LO) modes, respectively [23]. The broad peak around 552 cm−1 appeared due to the disorder activated LO (DALO) phonon scattering [24] and also due to the presence of cubic InN [25].
3.8 Photo luminescence (PL) and UV–visible absorption (UV)
The optical properties of the synthesized InN micro crys- tals and nano crystals are characterized using UV–vis- ible absorption and photoluminescence measurements. The appearances of absorbance was strange: the peak absorp- tion around ~350 nm is may be due to indium oxide. And
Fig. 4 Relations between a crystallite size and strain with different temperature (K) and time (h). b Cell volume and bond length with different temperature (K) and time (h)
Table 3 Calculation of InN, In2O3 and In percentage using XRD pat- tern
Different temperature (K)
and time (h) InN (%) In2O3 (%) In (%)
873 (4 h) – 100 -
923 (4 h) 47 53 –
973 (4 h) 67 33 –
973 (6 h) 98 2 –
973 (8 h) 63 – 37
Fig. 5 Micro-Raman spectra of InN [973 K (6 h)] micro crystals and nano crystals
then a sudden fall in the absorption around ~400 nm and then the absorption increases in the light scattering visible region to the near-IR region is due to the InN particles as shown in Fig. 6b. The (αhυ)2 vs. photon energy has been plotted from the absorption co-efficient value derived from the absorption of dispersed InN nanospheres in ethanol.
The extrapolation of the graph shows the band gap of InN is 1.2 eV shown in inserted Fig. 6b. The observed band gap of InN spheres is greater than the described band gap 0.7 eV [15] but lesser than 1.9 eV [16]. An earlier report by Davydov et al. showed that samples with band gap in the region of 1.7–2.1 eV hold up to 20% of oxygen, which is much higher than samples with narrow band gap [26]. The later higher band gap (1.9 eV) of polycrystalline InN are due to oxygen incorporation. Wu et al. [27] have reported that electron concentration will affect the absorption edge of InN due to Burstein–Moss effect. Figure 6a shows, the band edge emission in photoluminescence spectrum con- firms the band gap is around 1.21 eV. The blue-shift band gap of InN nanospheres can be attributed to oxygen diffu- sion in the InN sample [28]. The emission property of the InN micro crystals and nano crystals has been evaluated using PL spectra at room temperature. The spectrum shows a peak at 1.26 eV and was recorded using 244 nm photo excitation.
3.9 Morphological analysis (HR-TEM and FFT), particle size analysis and EDX analysis
To investigate on the surface morphology and structure of InN [973 K (6 h)] sphere-shaped particles. The synthesized nano powders are found as polycrystalline with crystalline grain varying from tens to two hundreds of nanometers as
indicated by selected area electron diffraction patterns. We used high annular dark field (HAADF) imaging to investi- gate the atomic arrangement in the synthesized InN sphere- shaped particles. Figure 7a–c shows the HR-TEM image of the InN sphere-shaped particles. The surface of InN sphere-shaped particles is smooth. Figure 7d shows high annular dark field (HAADF) imaging of InN sphere-shaped particles and the interplanar distances of 2.67 Å match- ing well with d(101) spacing of wurtzite-type InN and the corresponding fast Fourier transformation (FFT) pattern.
The FFT pattern indicates that the basal plane of the InN sphere-shaped particle is (101). The HR-TEM image shown in Fig. 8, confirmed the spherical morphology particles with three different sizes. The images and particle analy- ses show that the as prepared InN sphere-shaped particle have a different size observed small (8–15 nm), medium (50 nm) and large (200–350 nm) sphere-shaped particles.
Figure 9 shows the energy dispersive X-ray (EDX) spec- trometer analysis of sphere-shaped particles. EDX analysis confirmed both the indium metal and N presence and pres- ence of O peak was may be due to the surface oxidation of the InN particles.
4 Conclusions
The characteristics of as prepared InN micro crystals and nano crystals have been analyzed above. The results dem- onstrate that the indium nitride micro crystals and nano crystals have been successfully synthesized by the sol–gel assisted growth. The HR-TEM images show that the as prepared InN sphere-shaped particle have a different size observed small (8–15 nm), medium (50 nm) and large
Fig. 6 Photoluminescence spectrum (PL) and plot of (αhυ)2 vs. hυ of InN [973 K (6 h)] micro crystals and nano crystals
Fig. 7 TEM of InN [973 K (6 h)] sphere-shaped particles, a–c HR-TEM image. d HAADF image of the InN sphere-shaped and (FFT) pattern
Fig. 8 a HR-TEM image of the InN [973 K (6 h)] sphere-shaped particles, b size distribution of sphere-shaped particles
(200–350 nm) sphere-shaped particles and the InN micro crystals and nano crystals is crystalline with hexagonal wurtzite structure from XRD and FFT analysis. The UV absorbance spectrum and PL spectrum at room temperature and the bandgap value about 1.26 eV. The InN has promis- ing potential in future nano electronic devices.
Acknowledgements One of the author, Mr. C. Bagavath would like to thank the Government of India, for the financial support through Rajiv Gandhi National Fellowship (RGNF).
References
1. S. Luo, W. Zhao, Z. Zhang, L. Liu, X. Dou, J. Wang, X. Zhao, D. Liu, Y. Gao, L. Song, Y. Yiang, J. Zhou, S. Xie, Synthesis of long indium nitride nanowires with uniform diameters in large quantities. Small 1, 1004 (2005)
2. S.K. O’Lery, B.E. Fortz, M.S. Shur, L.F. Eastman, Steady-state and transient electron transport within bulk wurtzite indium nitride: an updated semi classical three-valley Monte Carlo sim- ulation analysis. Appl. Phys. Lett. 87, 2221031 (2005)
3. S. Strite, H. Morkoc, GaN, AlN and InN: a review, J. Vac. Sci.
Technol. B. 10, 1237 (1992)
4. R. Wang, H.P.T. Nguyen, A.T. Connie, J. Lee, I. Shih, Z. Mi, Color-tunable, phosphor-free InGaN nanowire light-emitting diode arrays monolithically integrated on silicon. Opt. Express 22, 1768 (2014)
5. O. Kryliok, H.J. Park, Y.S. Won, T. Anderson, A. Davydov, I.
Levin, J.A.F.J.H. Kim, Jr, Controlled synthesis of single crystal- line InN nanorods. Nanotechnology 18, 1356061 (2007) 6. S. Krukowski, A. Witek, J. Adamczyk, J. Jun, M. Bockowski, I.
Grzegory, B. Lucznik, G. Nowak, M. Wroblewski, A. Presz, S.
Gierlotka, S. Stelmach, B. Palosz, S. Porowski, P. Zinn, Ther- mal properties of indium nitride. J. Phys. Chem. Solids 59, 289 (1998)
7. A.G. Bhuiyan, A. Hashimoto, A. Yamamoto, Indium nitride (InN): a review on growth, characterization and properties. J.
Appl. Phys. 94, 2779 (2003)
8. A. Miura, T. Takei, N. Kumada, Synthesis of wurtzite-type InN crystals by low temperature nitridation of LilnO2 using NaNH2 flux. J. Cryst. Growth. Des. 12, 4545 (2012)
9. C. Wu, T. Li, L. Lei, S. Hu, Y. Liu, Y. Xie, Indium nitride form indium iodide at low temperatures: synthesis and their optical properties. New J. Chem. 29, 1610 (2005)
10. J. Xiao, Y. Xie, R. Tang, W. Luo, Benzene thermal conversion to nanocrystalline indium nitride from sulfide at low tempera- ture. Inorg. Chem. 42, 107 (2003)
11. M.A. Qaeed, K. Ibrahim, K.M.A. Saron, Q.N. Abdullah, G.N.
Elfadill, S.H. Abud, K.M. Chahrour, The effective role of time in synthesizing InN by chemical method at low temperature. J.
Mater. Sci: Mater. Electron. 25, 1376 (2014)
12. J. W. Trainor, K. Rose, Some properties of indium nitride films prepared by reactive evaporation. J. Electron. Mater. 3, 821 (1974)
13. Q. Guo, O. Kato, A. Yoshida, Thermal stability of InN single crystal films. J. Appl. Phys. 73, 7969 (1993)
14. B. Schwenzer, L. Loeffler, R. Seshadri, S. Keller, F.F. Lange, S.P. Denbaars, U.K. Mishra, Preparation of indium nitride micro and nanostructures by ammonolysis of indium oxide. J.
Mater. Chem. 14, 637 (2004)
15. T. Matsuoka, H. Okamota, M. Nakao, H. Harima, E. Kurimoto, Optical bandgap energy of wurtzite InN. App. Phys. Lett. 81, 1246 (2002)
16. M. Kao, R.M. Erasumu, N. Moloto, N.J. Coville, S.D.
Mhlanga, UV-assisted synthesis of indium nitride nano and microstructures. J. Mater. Chem. A 3, 5962 (2015)
17. M. Yoshimoto, H. Yamamoto, W. Hang, H. Harima, J. Saraie, A. Chayahara, Y. Horino, Widening of optical bandgap of polycrystalline InN with a few percent incorporation of oxy- gen. Appl. Phys. Lett. 83, 3480 (2003)
18. S. Ge, B. Wang, J. Lin, L. Zhang, C, N-Co doped InOOH microspheres: one-pot synthesis, growth mechanism and vis- ible light photo catalysis. Cryst. Eng. Commun. 15, 721(2013) 19. A.M.N. Silva, X. Kong, M.C. Parkin, R. Cammack, R.C.
Hider, Iron (III) citrate speciation in aqueous solution. Dalton Trans. 40, 8616 (2009)
20. R.M. Ferreira, M. Motta, A.B. Neto, C.F.O. Graeff, P.N.C.
Filho, F.C. Lavarda, Theoretical investigation of geometric configurations and vibrational spectra in citric acid complexes.
Mater. Res. 17, 550 (2014)
21. A. Hardy, J. D’Haen, M.K.V. Bael, J. Mullens, An aqueous solution–gel citratoperoxo–Ti(IV) precursor: synthesis, gela- tion, thermo-oxidative decomposition and oxide crystalliza- tion. J. Sol. Gel. Sci. Technol. 44, 65 (2007)
Fig. 9 EDX analysis of sphere- shaped particles
22. S. Singhal, J. Kaur, T. Namgyal, R. Sharma, Cu-doped ZnO nan- oparticle: synthesis, structural electrical properties. Phys. B 407, 1223 (2012)
23. O. Briot, B. Maleyre, S. Ruffenach, B. Gil, F. Demangeot, J.
Frandon, Absorption and Raman scattering processes in InN films and dots. J. Cryst. Growth 269, 22 (2004)
24. Z.G. Qian, W.Z. Shen, H. Ogawa, Q.X. Guo, Raman investiga- tions of disorder in InN thin film grown by reactive sputtering on GaAs. J. Appl. Phys. 93, 2643 (2003)
25. P. Bhattacharya, T.K. Sharma, S. Singh, A. Ingale, L.M.
Kukreja, Observation of zincblend phase in InN thin films grown on sapphire by nitrogen plasma-assisted pulsed laser deposition.
J. Cryst. Growth 236, 5 (2002)
26. V.Y. Davydov, A.A. Klochikhin, V.V. Emtsev, D.A. Kurdyu- kov, S.V. Iranov, V.A. Vekshin, F. Bechstedt, J. Furthmuller, J.
Aderhold, J. Graul, A.V. Mudryi, H. Harima, A. Hashimoto, A.
Yamamoto, E.E. Haller, Band gap of hexagonal InN and InGaN alloys. Phys. Stat. Sol. B 234, 787 (2002)
27. J. Wu, When group III-nitrides go infrared: new properties and perspectives. J. Appl. Phys. 106, 0111101 (2009)
28. V.Y. Davydov, A.A. Klochikhin, R.P. Seisyan, V.V. Emtsev, S.V.
Lvanov, F. Bechstedt, J. Furthmuller, H. Harima, A.V. Mudrvi, J.
Aderhold, O. Semchinova, J. Graul, Absorption and emission of hexagonal InN. Evidence of narrow fundamental bandgap. Phys.
Stal. Sol. B. 229, 1 (2003)