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Journal of Crystal Growth 310 (2008) 473–476 www.elsevier.com/locate/jcrysgro
Decomposition of InN at high pressures and temperatures and its thermal instability at ambient conditions H. Saitoh, W. Utsumi, K. Aoki Synchrotron Radiation Research Center, Japan Atomic Energy Agency, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan Received 30 May 2007; received in revised form 9 October 2007; accepted 18 October 2007 Communicated by G.B. Stringfellow Available online 23 October 2007
Abstract The pressure–temperature (p–T) relation was determined for the decomposition reaction of InN for a pressure range of 6.7–10.0 GPa and a temperature range of 890–1070 K. The decomposition p–T relation was used to calculate the standard enthalpy of formation of InN at 0.1 MPa and 298 K as 36.3 kJ/mol. Furthermore, this p–T relation was used to estimate the decomposition temperature at ambient pressure as 204 K, which is signiﬁcantly lower than those previously estimated from the decomposition data obtained at relatively low pressures and temperatures, but this value does agree with those determined by calorimetric methods. Moreover, solid InN was conﬁrmed to be metastable under ambient conditions. r 2007 Elsevier B.V. All rights reserved. PACS: 05.70.a Keywords: A1. Phase equilibria; B1. Nitrides; B2. Semiconducting III-V materials
1. Introduction InN based semiconductors are an indispensable compound for manufacturing optoelectronic devices . For example, an InN ternary alloy, InGaN, is often used to fabricate heterostructure-based optoelectronic devices such as LEDs and lasers. However, to improve the performance of these devices and to extend their application ﬁelds, their thermodynamic properties and stability under various conditions should be well established. Unfortunately, even the values for the thermodynamic parameters of pure InN deviate greatly in the literature. For example, the standard enthalpy of formation ranges from 19.2 to 143.5 kJ/ mol. Despite the wide use of InN alloys in device fabrication, thermodynamic properties of pure InN remain unknown. Hahn and Juza initially reported the thermal stability of InN . Using a combustion calorimetric technique, they determined a standard enthalpy of formation of 19.2 kJ/ Corresponding author. Tel.: +81 791 58 2632; fax: +81 791 58 0311.
E-mail address: [email protected]
(H. Saitoh). 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.10.037
mol. The standard enthalpy can be derived from the decomposition pressure–temperature (p–T) relation of InN using the van’t Hoff relation. However, other values, which are in good agreement with each other, have been reported (143.5 kJ/mol , 138.1 kJ/mol , and 131.4 kJ/mol ), but these values are one order of magnitude larger than that determined by the calorimetric method. The discrepancy has raised doubts about the reliability of the combustion calorimetric experiments. Ranade et al.  have used high-temperature oxidative drop solution calorimetry and obtained a formation enthalpy of 28.6 kJ/mol, which is close to the earlier calorimetric values. Table 1 summarizes the standard enthalpy of formation of InN, and clearly shows that the thermodynamic values of InN remain controversial . In the present study, decomposition temperatures were determined for InN at high pressures, ranging from 6.7 to 10.0 GPa. The decomposition temperatures increased to approximately 1000 K, which signiﬁcantly accelerated the decomposition rate. We obtained a decomposition p–T relation free from kinetic restrictions and hence, reliable thermodynamic values. The obtained values are compared
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H. Saitoh et al. / Journal of Crystal Growth 310 (2008) 473–476
Table 1 Summary of experiments on the standard enthalpy of formation of InN Method
Combustion calorimetric technique Decomposition p–T relation Decomposition p–T relation Decomposition p–T relation High-temperature oxidative drop solution calorimetry Decomposition p–T relation
19.2 143.5 138.1 131.4 28.6
    
Electrode Graphite heater Sample Pt-Pt/Rh thermocouple LaCrO3 thermal insulator
The present study
to the literature data and the thermal stability of InN is discussed.
(110) section of a cubic pyrophyllite pressure medium Fig. 1. Schematic diagram of the sample assembly built in the cubic-anvil press.
2. Experimental procedure
High-pressure and high-temperature experiments were conducted using a multi-anvil high-pressure apparatus. The specimen embedded in a cubic pressure transmission medium was compressed isotropically with six anvils arranged in a cubic symmetry, which were driven with a 180 t hydraulic press. The system was capable of generating pressures up to 12 GPa with tungsten carbide anvils. The sample pressure was estimated from a generated-pressure vs. oil-pressure calibration curve, which was determined in advance by measuring the density of a standard material, NaCl, as a function of oil pressure by in situ X-ray diffraction on the beamline BL14B1, SPring-8. The well established equation-of-state of NaCl allowed the sample pressure to be accurately determined. The estimated uncertainty in pressure determination was within 0.2 GPa. Fig. 1 shows the details of the cell assembly. The pressure medium was a 6 mm edge length cube made of pyrophyllite. A cylindrical hole was drilled in the cube to accommodate a LaCrO3 thermal insulator, a cylindrical carbon heater, and a boron nitride capsule used as a sample container. InN powder (nominal purity 99%, Soekawa Chemical Co.) was compacted into a disk shape (0.8 mm in diameter and 0.4 mm in height) and then placed in the capsule. The sample could be heated up to 2000 1C with the carbon resistance heater. Temperatures were measured with Pt–PtRh (13%) thermocouples, which had uncertainties less than 710 1C at 1000 1C. The sample was initially compressed to the desired pressure and successively heated to a preset temperature at a rate of 200 1C/min. The temperature was maintained for 30 min and then the heated sample was quenched at high pressure by turning off the supplied electric power. After the temperature fell to approximately room temperature, the pressure was gradually released to ambient pressure. Powder X-ray diffraction patterns were taken for the recovered sample to analyze the chemical composition. Decomposition of InN was accompanied by the deposition of indium metal as conﬁrmed by measuring the diffraction peaks from the deposited indium metal. The phase stability
40 50 2θ (degree)
Fig. 2. X-ray powder patterns of recovered samples from (a) 9.9 GPa, 993 K, (b) 9.9 GPa, 1013 K, and (c) scanning electron microscope image of In droplets. Arrows in (b) indicate the Bragg peaks from indium metal.
or decomposition curve of InN was determined by X-ray diffraction measurements for the samples recovered from various pressures and temperatures. 3. Result and discussion Fig. 2 shows representative diffraction patterns taken for the recovered samples. The bottom pattern taken for the sample recovered from 9.9 GPa and 993 K mainly consists of Bragg peaks from wurtzite InN and contains slight peaks due to impurities such as In2O3. However, a signal for decomposition was not detected. Thus, solid InN exists stably at the pressure and temperature condition. The upper pattern, which measured for the sample heated to 1013 K at 9.9 GPa, indicates the presence of diffraction peaks from indium metal as well as the original wurtzite peaks; the InN compound partially decomposes into indium metal and nitrogen gas. The inset of Fig. 2 shows a scanning electron microscope image, which indicates that the decomposed indium metal appears as small droplets with a typical size of a few micrometers in diameter.
ARTICLE IN PRESS H. Saitoh et al. / Journal of Crystal Growth 310 (2008) 473–476
T (K) 1100
1010 Solid InN
109.9 In +
1 N 2 2
ref  ref  ref  ref  present work
1000 / T (K-1) Fig. 3. Stability diagram of InN obtained in the present study. Open and close symbols represent instability and stability points of InN, respectively.
Fig. 3 shows the results of the quench experiments carried out in a pressure range of 6.7–10.0 GPa and a temperature range of 890–1070 K. The decomposition curve is presented with a straight line in this pressure and temperature range. As commonly observed, the decomposition pressure increases with temperature: 9.9 GPa at 1003 K and 10 GPa at 1066 K. A structural transition occurs in the solid phase above 10 GPa. The wurtzite structure begins to transform into the rock salt structure. At ambient temperature, the transition pressure increases to 12.1 GPa. The phase boundary has a negative slope and hence, intersects with the decomposition curve near 10 GPa and 1070 K. Fig. 4 shows the log10(p)1/T plot along with earlier results [3,7–9]. The decomposition pressures obtained in the present experiments show a temperature insensitive behavior, which is located around 10 GPa for the temperature range measured. This behavior contrasts earlier results, which show a rapid decrease in the decomposition pressure as the temperature decreases, especially below 870 K. The standard enthalpy of formation of InN is derived from the decomposition p–T relation. For the decomposition reaction 1 InNðsÞ$InðlÞ þ N2 ðgÞ, 2
the p–T trajectory of the decomposition is described by the van’t Hoff equation: dðln KÞ DH 0f ¼ , dð1=TÞ R
1000 / T (K-1)
where K is the equilibrium constant, T is the temperature, DH 0f is the standard enthalpy of formation of InN, and R is the gas constant. Because nitrogen is the only species in the vapor phase and the partial pressure of indium vapor is
Fig. 4. Comparison of the decomposition p–T relation of InN obtained in the present study with those in the literature. Open and close symbols represent instability and stability points of InN, respectively. Straight line is to guide the eyes.
much lower than the corresponding N2 pressure , the equilibrium constant is given approximately by 1=2
K PN2 .
At high pressures above 0.1 GPa, PN2 is replaced by fugacity f N2 . The fugacity was calculated using an empirical equation, f N2 ¼ f ðp; TÞ, which was developed by Unland et al. . Using Eq. (3), Eq. (2) is rewritten as dðlnðf N2 ÞÞ DH 0f ¼ . R dð1=TÞ
The standard enthalpy of formation can be derived by plotting logðf N2 Þ vs. 1/T on a reasonable approximation; the enthalpy is constant with the temperature variation. An equilibrium temperature for each pressure measured was determined as the mean of the highest annealing temperature without decomposition and the lowest annealing temperature with decomposition. Five equilibrium points were obtained as indicated with crosses in Fig. 3. In Fig. 5, the equilibrium pressures thus determined are plotted as a function of 1/T. Each equilibrium pressure was further converted to the fugacity with the empirical equation and presented as the open circle in Fig. 5. The logðf N2 Þ 1=T relation is described by the solid straight line and its slope yields a standard enthalpy of formation of 36.3 kJ/mol. The decomposition p–T curve is reproduced with the obtained enthalpy as shown with the dashed line, giving an estimated decomposition temperature of 204 K at ambient pressure. The standard enthalpy of formation, 36.3 kJ/mol, shows a large discrepancy from the earlier reported values, which have also been derived from the log(p)1/T curve (see Table 1). This discrepancy is most likely attributed to
ARTICLE IN PRESS H. Saitoh et al. / Journal of Crystal Growth 310 (2008) 473–476
proposed as the metastable range (e.g. the super heating temperature exceeds 500 K at ambient pressure). However, the present results support the calorimetric results and conﬁrm that InN crystal is metastable at ambient conditions.
T (K) 1000
Pressure, Fugacity (Pa)
log10(fN2) - 1/T
The decomposition pressure–temperature relation of InN was determined. The decomposition temperature increased to approximately 1000 K where the decomposition p–T relation free from kinetic restriction was obtained. From this decomposition p–T relation, the standard enthalpy of formation of InN was calculated to be 36.3 kJ/mol. Furthermore, the decomposition p–T curve was reproduced with the obtained enthalpy to give an estimated decomposition temperature of 204 K at ambient pressure. The present results agree with those determined by the calorimetric technique and conﬁrm that the InN crystal is metastable at ambient conditions.
log10(P) - 1/T decomposition curve
1000 / T (K-1) Fig. 5. Calculated logðf N2 Þ 1=T straight line and decomposition curve of InN.
kinetic effects. As shown in Fig. 4, the discrepancy is very large at low pressures and becomes small at high pressures. The decomposition reaction should be an Arrhenius type where the rate is dominated by the ratio of the activation energy and reaction temperature. A low-temperature (i.e. low-pressure) decomposition reaction slowly proceeds and requires superheating beyond the equilibrium temperature to be detected in a limited measuring time. Thus, the decomposition p–T slope measured at low pressures is steep and the slope tends to provide a large exothermic value of the standard enthalpy of formation. The present results, the standard enthalpy of formation and the decomposition temperature at ambient pressure, agree with those determined by the calorimetric technique, and play a key role in establishing the thermodynamic properties of InN. In general, the calorimetric technique is believed to be an effective method to obtain reliable thermodynamic values because it is less affected by kinetic factors. However, the standard enthalpy of formation and ambient-pressure decomposition temperature reported earlier do not seem to be widely accepted. Too small values have been reported for the standard enthalpy of formation and an unusually wide temperature span has been
Acknowledgment This research is supported by a grant-in-aid for Scientiﬁc Research (B) (17360013) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. References  A.G. Bhuiyan, A. Hashimoto, A. Yamamoto, J. Appl. Phys. 94 (2003) 2779.  H. Hahn, R. Juza, Z. Anorg. Allg. Chem. 244 (1940) 111.  J.B. MacChesney, P.M. Bridenbaugh, P.B. O’Connor, Mater. Res. Bull. 5 (1970) 783.  A.M. Vorobev, G.V. Evseeva, L.V. Zenkevich, Russ. J. Phys. Chem. 47 (1973) 1616.  R.D. Jones, K. Rose, J. Phys. Chem. Solids 48 (1987) 587.  M.R. Ranade, F. Tessier, A. Navrotsky, R. Marchand, J. Mater. Res. 16 (2001) 2824.  B. Onderka, J. Unland, R. Schmid-Fetzer, J. Mater. Res. 17 (2002) 3065.  S. Krukowski, A. Witek, J. Adamczyk, J. Jun, M. Bockowski, I. Grzegory, B. Lucznik, G. Nowak, M. Wroblewsk, A. Presz, S. Gierlotka, S. Stelmach, B. Palosz, S. Porowski, P. Zinn, J. Phys. Chem. Solids 59 (1998) 289.  I. Grzegory, S. Krukowski, J. Jun, M. Bockowski, M. Wroblewski, S. Porowski, in: S.C. Schmidt, J.W. Shaner, G.A. Samara, M. Ross (Eds.), High-Pressure Science and Technology, AIP Conf. Proc. 309 (1994) 565.  J. Unland, B. Onderka, A. Davydov, R. Schmid-Fetzer, J. Crystal Growth 256 (2003) 33.