Self-Assembled Growth of InN Microcages and Nanowires by

May 27, 2009 - Abstract. Microcages and nanowires of InN were synthesized by ..... The 0.1 eV shift observed in the new band gap could be attributed t...
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J. Phys. Chem. C 2009, 113, 10967–10974

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Self-Assembled Growth of InN Microcages and Nanowires by Ammonolysis of an Amorphous Precursor and Their Optical Properties Vishal D. Ashok, Tandra Ghoshal, and S. K. De* Department of Materials Science, Indian Association for the CultiVation of Science, Kolkata 700 032, India ReceiVed: February 25, 2009; ReVised Manuscript ReceiVed: April 24, 2009

Microcages and nanowires of InN were synthesized by ammonification of an indium precursor in the temperature range 650-700 °C. The phases of the ammonified products were identified through X-ray diffraction (XRD). Morphology and crystal structure of the samples were determined through field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRTEM) analysis. The thermal behavior of the precursor and synthesized InN powder was investigated using thermogravimetric analysis (TGA). The growth mechanism for the formation of different morphologies at various experimental conditions is explained on the basis of diffusion and vaporsolid process following least action principle. Large blue shift (1-1.5 eV) in the apparent band gap was observed with the increase of reaction time. The contributions of refractive index and the existence of three simultaneous direct transitions at 0.8, 1, 1.4 eV were identified from the optical absorption spectra. Electrical measurements were carried out to obtain a band gap of 1.25 eV in the sample. Introduction Indium nitride (InN) along with other nitrides is a very fascinating material due to its unique properties of low effective mass, high mobility, high saturation velocity, low band gap, and high surface charge accumulation. These novel properties of InN render it as a potential candidate for optoelectronic, photovoltaic, and photonic devices.1 Despite having the potential for a number of applications, InN has been one of the least understood and highly controversial materials due to the difficulty in controlling its preparation arising from its low decomposition temperature, high nitrogen partial pressure, and thermodynamic instability resulting from a higher stability of N-N bond and a relatively weaker In-N bond. It also has a high tendency to form oxides, as the change in Gibb’s free energy per In atom is -334.75 kJ/mol as compared to -86 KJ/ mol for indium nitride formation from NH3 which hinders the synthesis processes.2 InN in nanowires, nanotubes, and particle form has been prepared by wet chemical synthesis, chemical vapor deposition (CVD), metal organic chemical vapor deposition (MOCVD), molecular and chemical beam epitaxy (MBE, CBE) through vapor-solid (VS), and vapor-liquid-solid (VLS) growth mechanisms using expensive substrates and high vacuum technology, making the preparation costly and restricted to thin films.3 It is the most difficult to achieve high quality single crystalline InN as most of the preparation conditions are at high temperature. It is necessary to develop cheaper means to prepare InN nanoforms in large quantity, which may be used in dispersed forms for various novel applications. Such methods include solvothermal,4 which involves unstable, explosive, and highly toxic azides as the nitrogen source and the ammonification of indium and indium oxide, which is another economical and conceptually simple technique.5 Most of the work done on ammonification deals with the structural control using high flow rates, high temperatures, or long times and well-crystallized * Corresponding author. Tel.: 91-33-24734971, fax: 91-33-24732805, e-mail: [email protected].

indium oxide precursors. The advantage of using a highly amorphous precursor and its structural evolution has not been reported. The most controversial issue related to InN is the wide range of subvisible bandgaps (0.7-2 eV) observed in the samples prepared by various methods.1c,6 Several explanations such as the Brustein-Moss effect due to high carrier concentration, the formation of oxides and oxynitrides, and the precipitation of metallic In clusters have been assigned to these variations favoring lower7 and higher band gaps.8-10 These estimated band gaps are devious as the photoluminescence peak observed at 0.7 eV implies a band edge above this value, and the analysis of the absorption spectra were carried out presuming a direct band gap. Hence, it is important to understand the growth and evolution of band gap of InN in order to harness its full potential. In this paper we present the preparation of InN in cage-andwire-like morphology by a simple ammonification of an indium precipitate derived by rapid destabilization of indium in an acid and 2-propanol solution, using ammonium hydroxide. This precursor has the benefit of high Gibbs free energy, being amorphous, and having a homogeneous presence of carbon throughout the precursor that prevents reoxidization during ammonification. An attempt is also made to determine the band gap and the nature of transition from the absorption spectra without any presumption of the nature of the band gap for InN prepared under different conditions. Experimental Section Indium nitride was prepared by simple nitridation of an amorphous and preannealed indium precipitate as a precursor. The precursor was prepared by dissolving indium metal (Aldrich) in 69% HNO3 (Aldrich) solution such that the In:HNO3 molar ratio was greater than 1:3 under ultrasonication. This solution was then diluted by adding into 2-propanol under stirring such that the molar ratio of In:2-propanol was 1:90 and allowed to stir for 30 min. A white precipitate was obtained on slow addition of 25% NH4OH (ammonia solution) to this

10.1021/jp901748p CCC: $40.75  2009 American Chemical Society Published on Web 05/27/2009

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Figure 1. XRD pattern of indium nitride prepared by ammonification of indium precursor at 675 °C for 1 h.

solution under vigorous stirring, which was separated by centrifugation, washed in ethanol, and dried at 60 °C for 24 h (Precursor A). This and another precursor annealed at 200 °C for 30 min (Precursor B) were used for nitridation. Nitridation was carried out in an alumina boat kept in a quartz tube with an inner diameter equal to 24 mm under ammonia flow. The tube was flushed with nitrogen for 10-15 min, and ammonia was allowed to flow into the tube. The flow was set to around 100 sccm, and the temperature was changed from 650 °C, to 675 °C, and to 700 °C for 30 min, 1 h, and 2 h. Black and brown samples were obtained depending on the conditions. For comparison, another precursor (precursor C) was prepared with a similar approach except that the acid solution was diluted using water with a molar ratio of In:H2O at 1:400, hence keeping the same volume ratio. It is noted that the yield of indium nitride was substantially increased under ammonification for the preannealed precursor. The crystal structure and phase of all the samples were investigated with X-ray diffraction (XRD, Bruker axs D8advance). A thermogravimetric analyzer (TGA, TA Instruments SDT-Q600) was used to study the thermal behavior of precursor and the stability of indium nitride. Alumina (Al2O3) was used as reference in these thermal analyses. Morphology and size of the products were investigated by field emission scanning electron microscopy (FESEM, JEOL JSM 6700F) and transmission electron microscopy (TEM, JEOL JEM 2010). UV-visible spectroscopy was carried out in Hitachi U-4100. DC electrical resistance measurements were carried out at different temperatures using a Keithley 2000 multimeter with temperature measurements performed on a Lakeshore 332 temperature controller with a PT100 sensor. Result and Discussion Crystal structure and phase of the products are determined with XRD. It is observed that precursor A is not suitable for the preparation of InN in 1 h except at 675 °C. At lower temperature, 650 °C, indium oxide is the predominant product along with a few peaks of indium nitride. With increasing nitridation temperature and time, a large amount of In metal is formed. The diffraction peaks of all the samples prepared with precursor B are indexed to the wurtzite phase of indium nitride (JCPDS card No. 02-1450). Figure 1 shows one representative XRD pattern of the product prepared by ammonification of precursor B at 675 °C for 1 h. Only a trace of indium oxide is observed for the sample prepared at 650 °C for 30 min. No

Ashok et al. indium metal in crystalline form is detected in the XRD pattern, revealing the formation of well-crystallized InN. It is also observed that InN is formed with precursor A at 675 °C for 1 h only with a flow rate greater or equal to 100 ccm. A lower flow rate (50 ccm) led to the formation of shiny metallic indium clusters in the sample, which probably formed because of the reduction of indium precursor at high temperature by the carbon present in it, which came from 2-propanol used in the solvent. To confirm this fact, precursor C is ammonified under the same condition which results in a large amount of white precipitate of indium oxide. This indicates that the carbon in the form of complexes present in the precursor indeed helps in the reduction of the precursor and prevents the formation of indium oxide during ammonification. This also suggests that the flow of ammonia is important in the removal of the carbon and solvent byproducts from the tube or the sample zone, preventing recombination. Hence, the optimum flow rate to synthesize pure InN also depends on the volume of the sample space in the quartz tube. Thermogravimetric analysis (TGA) was carried out for precursor A and indium nitride prepared at 675 °C for 1 h in the temperature range 25-700 °C and 25-1000 °C, respectively, with a heating rate of 15 °C/min. The TGA curve of precursor A shown in Figure 2a indicates gradual weight loss in three steep slopes at 100 °C, 175 °C, and 279 °C. The slope at 100 °C and 175 °C corresponds to weight loss in the form of water and formation of indium hydroxide (In(OH)3) by the decomposition of the organic part (complexes) of the precursor. The slope at 279 °C corresponds to the phase transformation of In(OH)3 to In2O3 which is also observed in the differential thermal analysis (DTA) curve (Figure 2b) with an exothermal peak at 279 °C. The steps between 100 and 175 °C indicate the presence of carbon in the form of organic complexes in the precursor. Thus, it can be concluded that as the hindrance of carbon in the reactions decreased, the yield of indium nitride prepared with precursor B at 650 °C increased. It may also be noted from the DTA that the specific heat of the precursor also changes with annealing which increases its ability to withstand higher temperatures, preventing it from decomposition and desorption, hence increasing the yield at 700 °C. TGA curve of InN (Figure 2c) shows a gradual weight loss up to 400 °C with a broad endothermic nature in the corresponding DTA (Figure 2d). An increase in weight is seen above this temperature due to oxidation of indium nitride along with an increase in the temperature difference between the sample and reference in DTA, signifying the advent of an exothermal process. The DTA graph shows a broad exothermic peak between 560 °C and 620 °C associated with decomposition of InN into In and N2 with a slight shoulder peak around 400 °C corresponding to the oxidation. A secondary kink around 710 °C is observed in the TGA as a result of the difference in the rate of dissociation of InN decreasing the mass and oxidization of the InN increasing the mass. The morphologies of the InN products prepared with precursor B at different temperatures and times are investigated through FESEM. At a lower temperature, 650 °C, porous microstructures having irregular size and shape are obtained for 1 h of reaction time (Figure 3a). The formation of porous structures might be due to the diffusion of ammonia into the precursor. In few places, smaller secondary growths are observed from the porous surfaces, which cover the pores as shown in the inset of Figure 3a. Similar morphology is observed with increasing the reaction time to 2 h.

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Figure 2. Thermal analysis: (a) TGA, (b) DTA for indium precursor A; (c) TGA, (d) DTA for indium nitride prepared by ammonification.

With the increase in ammonification temperature to 675 °C for 30 min, the porous structures tend to twist and are arranged in a different pattern. For the reaction time of 1 h, the porous microstructures acquire a spherical shape with a diameter that varies in the range of 1-2 µm as shown in Figure 3b. As the spheres are largely hollow, the structures can be designated as microcages. The size of the microcages decreases with the increase in ammonification temperature to 700 °C. Completely different morphology is observed with increasing the reaction time to 2 h for both temperatures. A large number of nanowires with a length up to several micrometers is formed. Most of the nanowires have three branches forming “Y” junctions. Some of the InN nanowire Y junctions (INNYJ) have three long branches of several micrometers, while some have two long branches and one short branch (Figure 3c). The diameter at the junction point is larger than that of its branches. There is a uniform size distribution between the branch nanowires having diameters in the range 70-100 nm, but the diameters of each of these nanowires fluctuate by about 30% along the nanowires. Thus, the surfaces of the branch nanowires are not smooth but wavy so that pearl-necklace type morphology appears. The crystal structure and growth direction of the microcages and nanowires are further studied with TEM. Figure 4a shows the TEM image of a microcage prepared at 675 °C for 1 h. It reveals that the microcages are formed by self-assembly of few web-like structures which is a characteristic feature of diffusionlimited growth. The web-like structures are interconnected together with a few wire-like structures protruding from the corners. The cross-section of the wire-like structures are circular, but they do not have uniform diameter along their length. Also, the wires do not have well-defined starting and ending points.

The HRTEM image of a nanowire in the microcage (Figure 4b) reveals the single crystalline nature of the wire with a d-spacing of 3.065 Å corresponding to the (100) plane of wurtzite structure of InN. The corresponding fast Fourier transform (FFT) pattern confirms the growth direction to be (100). Figure 4c shows the TEM image of the nanowire prepared at 675 °C for 2 h. The image reveals that the single nanowire consists of multiple Y-junctions. Figure 4d shows the HRTEM image of the marked region of Figure 4c which suggests that the branched nanowire has single crystalline structure throughout the branch, backbone, and the junction without showing any obvious structural defects. The measured d-spacing of one of the branches is 2.84 Å corresponding to the (002) plane of wurtzite structure of InN. The growth direction of these nanowires is along the (002) direction, which is also confirmed by the corresponding SAED pattern shown in the inset of Figure 4d. It is to be noted that the (002) direction is perpendicular to the (100) direction, which corresponds to the growth direction of the webs in the microcages. Different morphologies of InN at different temperatures and reaction times indicate the involvement of any gaseous In species in the reaction mechanism. The reaction mechanism is quite similar to that suggested by W.-S. Jung et al.5c and L.-W. Yin et al.11 The growth mechanism of different morphologies of InN is shown schematically in Figure 5. At first, the In precursor breaks into smaller particles in the presence of ammonia through pyrolytic decomposition accompanied by a change in the specific heat of the precursor. Murali et al. reported that two modes of kinetics processes are involved in the ammonification of indium oxide, namely diffusion and vapor deposition.5f In this context, the formation of porous structure of InN is also caused by the

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Ashok et al. the building blocks of indium nitride formed in the process of ammonification is considered to follow the trajectories or path where the first-order difference in the kinetic and the potential energy is minimum.14 The temperature of the environment and flow rate gives the kinetic energy of the molecule, and the potential energy comes from the attractive forces of the molecules or chemical potential of the surface of the wurtzite structure. It is known that the wurtzite structure is highly polarized toward the c-axis or the (002) direction; hence, the indium nitride molecule has higher relative potential energy along this direction while along the perpendicular directions has lower potential energy. Hence, at lower kinetic energy, i.e., at lower temperature, the indium nitride molecule moves along the a-b plane and deposit epitaxially along this direction, and at higher kinetic energy or higher temperature they move along the c-axis and deposit along the (002) plane where the molecules gain higher potential energy. The same temperature but greater reaction time induces greater vapor pressure of InN molecules due to desorbtion, which in turn increases their kinetic energy. Thus, the growth direction of InN nanowires prepared for higher reaction time is along the (002) direction, and the flake growth along the (100) direction is in accordance with the least action principle. It may be noted that this explanation does not require the external deposition of the sample. The molecules of the flakes may rearrange themselves to form rod-like structure at higher temperature. The variation in the diameter of the nanowires along their length arises from the fluctuation in the kinetic energy of the depositing or rearranging molecule. The absorption spectrum analysis is carried out for the samples prepared with precursor B at 675 and 700 °C for 1 h and 2 h. The absorption coefficient of a semiconducting crystal is related to its band gap (Eg) by the equation

R ) (K/hν)(hν - Eg)m Figure 3. FESEM images of (a) porous InN prepared at 650 °C for 1 h (inset shows deposition on the pores), (b) InN cage-like structure prepared at 675 °C for 1 h, (c) InN nanowires prepared at 700 °C for 1 h.

diffusion of NH3 into the precursor and desorption of the intermediate products formed in this process. The relatively high ammonification temperature also supports this desorbtion process.12 The third step is the growth of microcage-like structures by diffusion-limited aggregation growth in which a formed particle moves upon the surface randomly and settles at a point, giving rise to nonuniform diameter web-like structures.13 Here, neither a catalyst nor a template is used, and no metallic particles are found at the ends of the InN nanowires. Thus, the growth of InN nanowires prepared for higher reaction time are likely to follow a vapor-solid (VS) process. At higher temperatures and greater reaction time, higher vapor pressure breaks the structures into smaller parts, which rearrange or attached themselves through a vapor-solid process to produce InN nanowires. This attachment process is reasonable, because the surface energy of an individual part is quite high due to larger exposed surface area; thus, they tend to be attached to each other to decrease the surface energies by greatly reducing exposed areas. It is observed that the growth direction of webs in the microcages is along a-b plane or (100) direction while that for nanowires along the c-axis or (002) direction, which are crystallographically mutually perpendicular to each other. This could be understood from the least action principle in which

(1)

where R is the absorption coefficient, K is a constant, hν is the energy of incident photon, and Eg is the band gap of the material. The value of m determines the nature of transition. It takes a value of 1/2, 3/2, 2, or 3 for allowed direct, forbidden direct, allowed indirect, and forbidden indirect transitions, respectively.15 The value of m is assumed to be 1/2 for InN in order to determine the band gap as reported by various groups.1b,7b,16 Figures 6a and 6b shows the plot of (Rhν)2 vs hν for the sample prepared at 675 °C for 1 h and 2 h, respectively. It is observed that the band gap obtained by this procedure changes from 1 to 1.5 eV with the increase in time of ammonification. The same trend is observed in the case of samples prepared at 700 °C for 1 h and 2 h. Since, the synthesis temperature is well above the decomposition temperature of InN as observed in the TGA analysis, the possibility of nitrogen defects increases with ammonification time. This should lead to a decrease in the band gap due to the Mie resonance resulting from the excessive metallic indium incorporation,8b which is contrary to an increase in band gap observed here. However, indium nitride is known to have a high charge concentration due to high Fermi stabilization energy relative to the conduction band.17 This leads to a significant energy dispersion in refractive index.18 This clearly suggests that the contribution of refractive index must be taken into account to analyze the absorption spectrum of InN. Under the influence of refractive index, eq 1 is modified as

R ) (K/hν)(hν - Eg)m/η

(2)

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Figure 4. (a) TEM image of the InN cage-like structure, (b) HRTEM image of the surface of the cage section (inset: corresponding FFT showing 100 growth direction), (c) InN wire-like structure Y junction, the (d) HRTEM image of the Y junction (inset shows the SAED pattern showing a 001 growth direction).

Figure 5. Schematic representation of the formation of indium nitride porous structures by diffusion of ammonia and expulsion of the desorbed remnants, followed by the formation of cage-like structures involving triangular web-like growth by diffusion-limited growth finally leading to the growth of wire-like structures by a vapor-solid growth mechanism along the (001) direction with an irregular but well-crystallized surface at higher temperature.

where η is the energy dependence of refractive index as pointed by Butcher et al.8a On taking the log, differentiating, and rearranging eq 2 we get

ln(Rhν) + ln(η) ) m ln(hν - Eg) + ln(K)

(3)

d(ln(Rhν))/d(hν) + d(ln(η))/d(hν) ) m/(ν - Eg)

(4) Hence, a plot of d(ln(Rhν))/d(hν) vs hν would show a hyperbolic nature with a discontinuity at hν ) Eg assuming d(ln(η))/d(hν) to be continuous or ln(η) to be differentiable around the band gap.

A plot of (d(ln(Rhν))/d(hν))2 vs hν (Figure 6c and 6d for the sample prepared at 675 °C for 1 h and 2 h, respectively) shows a band gap at 0.92 eV and 1.02 eV for the samples prepared by ammonification at 675 °C for 1 h and 2 h, respectively, determined by the discontinuity appearing as a peak in the graph,15,19 along with a broad feature between 1 and 1.5 eV giving a shoulder peak at 1.5 eV. This broad feature is amplified for the sample ammonified for a longer time. It is observed that the shift in the band gap is substantially reduced, indicating that the refractive index did indeed affect the apparent band gap to a great extent. The 0.1 eV shift observed in the new band gap could be attributed to the Moss-Brustein effect.20,7d In order to determine the nature of transition, m was obtained from the slope of the linear plot of ln(R hν) vs ln(hν - Eg) as shown in eq 3. The nonlinearity might be induced by the

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refractive index term in the equation. Insets of Figures 6c and 6d show ln(Rhν) vs ln(hν - Eg) plot of the corresponding samples. It is observed that the slope is ∼0.58 which corresponds to direct transition for the sample prepared at 675 °C for 1 h, but the sample synthesized for a longer time shows an average value of m ∼ 1.3 which is closer to the forbidden direct transition. This is similar to the observations reported by Butcher et al.8a,9 Hence, it is seen that the nature of transition also changes with time of synthesis. This behavior is understood when the same procedure is applied on indium nitride prepared at 700 °C for 1 h. The plot of (d(ln(Rhν))/d(hν))2 vs hν shows three distinct peaks at 1.42, 1, and 0.854 eV for which the value of m is determined to be 0.6, 0.53, and 0.529, respectively, for the sample prepared at 700 °C for 1 h (Figure 7). All the values of m correspond to the direct transitions. The broad profile is regained for samples prepared for 2 h along with an increase in the value of m. This indicates that the optical absorption actually arises because of the contribution of three direct transitions, which form a degenerate level for samples prepared at 700 °C for 2 h. This might explain the large range observed in the apparent band gap due to the charge accumulation. It is interesting to note that the nature of transition also depends on the degeneracy of the dispersion of absorption coefficient. When the degeneracy is high, the value of exponent m is observed to be close to 1.5. This is as if the excited electron undergoes three simultaneous direct transitions changing the absorption probability as follows

Ashok et al. R ) K(hν - Eg1)m(hν - Eg2)m(hν - Eg3)m ) K(hν - Eequ)3m

(5) When the degeneracy is absent (i.e., when the peaks are distinct or the peak is narrower), the nature of the transition tends to become direct. However, the origin of these three levels is unknown. This could arise either from the nonparabolic nature of the conduction band of InN or two types of contribution to the Fermi level as observed by Inushima et al.3a Fritsch et al. showed from pseudopotential calculation the presence of a conduction band minimum at Γ (Γ1, Γ3) point and along the M-L axis of the Brillouin zone.21 A similar approach has been used to calculate the electron mobility and drift velocity of InN by Monte Carlo simulation considering the three lowest conduction band minima.22 These three absorption peaks may correspond to the transitions between the VB to three CB minima at different regions of the Brillouin zone. The determination of the exact band gap of pure InN is very tedious because of the presence of inherent impurities inside it. Electrical resistance as a function of temperature was used to measure the band gap of InN. The temperature was decreased up to 100K from room temperature using liquid nitrogen. The variation of resistance (R) with the temperature (T) is measured for the sample prepared at 700 °C for 1 h. Figure 8 shows the resistance vs temperature curve, which fits well with the Arrhenius relation given by

Figure 6. Apparent band gap determination form (Rhν)2 vs hν plot of InN prepared by ammonilysis at 675 °C for (a) 1 h, (b) 2 h; corresponding refined band gap from (d(ln(Rhν))/d(hν))2 vs hν plot represented by the peak in c and d (insets showing the exponent m represented by the slope of ln(Rhν) vs ln(hν - Eg) plot).

Self-Assembled Growth of InN Microcages and Nanowires

R ) RoeEg/2KT

(6)

where Eg is the band gap, T is the temperature in Kelvin, and K is the Boltzmann constant. This reveals the semiconducting nature of InN. The calculated value of the band gap is 1.244 eV, which is in agreement with the value (1.25 eV) obtained by O. Briot et al.10 It is noted that the value of the band gap corresponds well with the mean value of the band gap observed in the absorption analysis for that sample. In summary, different morphologies such as porous structures, microcages, and nanowires of InN were synthesized by ammonifying an amorphous indium precursor in the temperature range (650-700 °C) for different reaction times ranging from 30 min to 2 h. The indium precursor annealed

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10973 at 200 °C for 30 min was more suitable to synthesize InN than the as-prepared precursor. TGA analysis revealed that InN converted to In2O3 at 400 °C and decomposed in the narrow temperature range of 560-620 °C. The growth mechanism of different morphological features of InN was explained on the basis of diffusion and the vapor-solid process in accordance with the least action principle. Single crystalline nanowire junctions of InN were obtained at higher temperature and higher reaction time. The growth along the (100) plane was observed to be more favorable at low temperature, whereas high temperature favored (002). This was in agreement with the least action principle. The optical band gap varied significantly with reaction time but was independent of the ammonification temperature. A large blue shift (1-1.5 eV) in the apparent band gap instead of a red shift was observed with the increase of reaction time, which is expected due to Mie resonance of the incorporated indium metal produced by the decomposition of InN at high temperature. When the effect of the refractive index was taken into account, the variation in the band gap changed to 0.9-1 eV and two additional direct transitions were observed for samples prepared at 700 °C for 1 h. Hence, the reason behind the shift was investigated to be a combination of degeneracy in three energy levels due to high charge concentration and significant energy dispersion in the refractive index. The value of the band gap obtained by fitting the Arrhenius equation to the resistance vs temperature plot corresponds well with absorption analysis. Acknowledgment. V. D. Ashok and T. Ghoshal acknowledge the Council for Scientific and Industrial Research (CSIR) for providing financial support during the tenure of this work. References and Notes

Figure 7. Plot of (d(ln(Rhν))/d(hν))2 vs hν showing three discontinuity peaks corresponding to three direct transitions as seen from the slopes in the insets.

Figure 8. Resistance vs temperature of indium nitride prepared by ammonification at 700 for 1 h and the corresponding fit to an Arrhenius relation giving an Eg value of 1.245 eV.

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