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Thermodynamic analysis of Si doping in GaN
Superlattices and Microstructures 40 (2006) 496–500 www.elsevier.com/locate/superlattices
Thermodynamic analysis of Si doping in GaN I. Halidou ∗ , Z. Benzarti, T. Boufaden, B. El Jani Unit´e de Recherche sur les H´et´eroEpitaxies et Applications (URHEA), Facult´e des Sciences 5000 Monastir, Tunisia Received 20 September 2006; accepted 25 September 2006 Available online 13 November 2006
Abstract Equilibrium calculations of Si-doping in GaN are investigated using the Gemini code. The method of the calculation is based on the minimisation of the Gibbs free energy. Experimental growth conditions are used for the calculation. The variables are the amount of the dopant and the temperature. The results show the formation of a solid Si3 N4 compound with a certain quantity of the input SiH4 , that is the silicon precursor in our MOVPE system. Si3 N4 formation can explain the limitation of Si incorporation and the surface roughening as revealed by MOVPE Si doped layers. c 2006 Elsevier Ltd. All rights reserved.
Keywords: Thermodynamic calculations; MOVPE; GaN; Si doping
1. Introduction Fabrication of nitride devices requires performing a controlled and reproducible n- and p-type doping. Many works have been devoted to studying n-type doping of GaN using silicon (Si), germanium (Ge) and selenium (Se) as dopants [1,2]. Satisfactory electrical properties of Si doped GaN have been reported but still suffer from compensation effects and surface degradation. However, for good ohmic contacts, high electron concentrations as well as smooth surfaces are needed. Therefore, understanding the growth mechanism of Si doping is still of interest. Most of the studies were focused on the effect of the growth parameter (growth temperature, dopant input concentration, growth rate, III/V ratio. . . ) and thus were done empirically. Equilibrium calculations can provide useful information necessary to understand the growth mechanism. Thermodynamic analysis of InGaAlN alloy growth was reported [3]. However, little has been done about Si-doping in GaN growth. ∗ Corresponding author. Tel.: +216 73 500 274; fax: +216 73 500 278.
E-mail address: [email protected] (I. Halidou). c 2006 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2006.09.023
I. Halidou et al. / Superlattices and Microstructures 40 (2006) 496–500
497
Fig. 1. Temperature dependence of equilibrium pressures of species arising from the reaction of the system: 40 µmol TMG, 0.18 mol NH3 , 0.09 mol N2 , and 0.09 mol H2 . The total pressure is 1 atm. Only the most important species are represented for clarity. It should be noted that solid GaN compound disappears above 1200 K.
In this paper, we perform equilibrium calculations of MOVPE Si-doping in GaN growth using SiH4 as Si precursor. The results provide guidelines for understanding the growth mechanism such as the Si incorporation limit and surface roughening. 2. Calculation procedure For the thermodynamic calculations, we have used the GEMINI code provided by Thermodata [4]. We have already successfully used this computer programme to study carbon and vanadium doping of GaAs, etching of GaAs and AlAs in CCl4 + H2 ambient and GaN decomposition under H2 ambient [5–7]. The results showed good agreement with experiment even though MOVPE is a non-equilibrium process. But, a near equilibrium can be reached at the vapour–solid interface. The principle of the calculations is based on the standard minimisation of the Gibbs free energy of a system of compounds. This allows the determination of the system composition when equilibrium is reached. For MOVPE Si-doping in GaN layer growth, the reactants are trimethylgallium (TMG), ammonia (NH3 ), the carrier gas (N2 + H2 ) and SiH4 . The products are all possible compounds that may be formed from a mixture of these reactants for a given temperature. For each compound, the thermodynamic data such as standard enthalpy of formation (1H ◦f (298 K)), standard entropy formation (1S ◦ (298 K)), and the heat capacity as a function of the temperature (C p(T )) are required. Some of the data are provided by Thermodata and the others are taken from Refs. [4,8]. 3. Results and discussion We start from a vapour composition used in our MOVPE growth experiment for undoped GaN layer deposition. The mixture contains: 40 µmol TMG, 0.18 mol NH3 , 0.18 mol H2 and/or N2 . For the calculation, the total pressure is 1 atm. In Fig. 1, the species issuing from this mixture as a function of temperature are shown. It should be noted that the mole concentration of solid
498
I. Halidou et al. / Superlattices and Microstructures 40 (2006) 496–500
Fig. 2. Partial pressure of species containing Si as a function of input mole amount of SiH4 . Saturation occurs above 50 nmol SiH4 due to the formation of solid Si3 N4 indicating thermodynamic limit of Si incorporation.
compounds is expressed in partial pressure units (atm) used for gas species. In this manner, we can compare the amount of solid and gas species in equilibrium at a given temperature. The amount of solid species represents the mole numbers condensed to reach the vapour–solid equilibrium per unit volume. It appears from Fig. 1 that the partial pressure of H2 and N2 is constant for the whole temperature range (900–1500 K). The dominant species for gallium and carbon are GaCH3 and CH4 . NH3 has a high equilibrium partial pressure in opposition to NHx species (NH2 and NH). From this calculation, it also appears that solid GaN cannot be deposited above 1200 K and that the GaN gas specie is stable above this temperature. For the Si doping calculation, we add SiH4 with different amounts in the above mixture. The input SiH4 amount lies in the range used in experiment. For clarity, Fig. 2 displays only the partial pressure of the most important compounds with the dopant element (Si) as a function of the input SiH4 mole fraction. The temperature is fixed at 1400 K, a typical value for GaN MOVPE growth and the total pressure is 1 atm. It should be noted that at this temperature, solid GaN formation is excluded as can be seen in Fig. 1. Therefore, the amount of Six Ny species represented in Fig. 2 is exaggerated. Indeed, the absence of solid GaN compound enhances the partial pressure of Ga and N containing species. Keizer et al. [9] proceeded in the same manner for equilibrium calculations of Si-doping in GaAs by MOVPE. However, their results showed a good agreement with the experiment. As can be seen from Fig. 2, SiNH appears to be the dominant specie. The partial pressure of all the species depends linearly on the amount of input SiH4 until a critical value. Indeed, at an input SiH4 of 50 nmol, the partial pressure of the species becomes constant due to the formation of solid Si3 N4 compound. This results in an upper limit for Si incorporation. Another feature that can be observed from Fig. 2 is the absence of gas species such as SiN, Si2 N. . . Sassi et al. [10] reported that the formation of these species requires the presence of NH2 and NH radicals. Since the dissociation of NH3 in the gas phase produces a small amount of these radicals, the probability of the formation of SiN, Si2 N is very low. These authors also showed that the formation of solid Si3 N4 compound is the result of the reaction between NH3 and Si. We have already seen that NH3 has a high equilibrium partial value (Fig. 1). We therefore attribute the formation of SiNH and solid Si3 N4 to a reaction between NH3 and SiH4 . The SiHx species
I. Halidou et al. / Superlattices and Microstructures 40 (2006) 496–500
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Fig. 3. Real time-dependent He–Ne laser reflectometry signal during Si-doping in GaN. The amplitude of the signal is constant during the deposition of the undoped layer. The sudden drop of the signal when the SiH4 flow is turned on is attributed to the formation of solid Si3 N4 as predicted by the thermodynamic calculation.
is the result of the dissociation of SiH4 . Finally, it is also seen that from our calculation, no solid Si is formed. This often appears in the Si-doping of GaAs [9]. 4. Comparison with the experiment We have already reported on Si-doping of GaN by MOVPE growth using SiH4 as Si precursor [11,12]. Si was shown to affect significantly the GaN layer morphology. Indeed, upon doping we have observed a decrease of the amplitude of the in situ He–Ne laser reflectometry signal and the oscillations disappear above a critical value of SiH4 flow rate. AFM images of the layers reveal an increase of the surface roughening. Thus the damping of the reflectivity signal is due to surface roughening against SiH4 flow. Our studies also showed a saturation tendency of electron concentration for SiH4 flow of about 45 nmol/min. Similar results were reported in Refs. [13–15]. Liu et al. [15] have reported an electron concentration saturation value of 1.7×1020 cm−3 for SiH4 flow of about 700 nmol/min. The surface roughening upon doping is attributed to the anti-surfactant effect of Si [16,17]. Considering the thermodynamic calculation results, it can be seen that the surface roughening as well as the electron concentration saturation appears for SiH4 amounts in the range where solid Si3 N4 is formed. Therefore, the growth changes upon the Si doping can be attributed to the formation of solid Si3 N4 . To clarify this point of view, we deposited a Si doped GaN layer on a 0.3 µm thick undoped layer. The amount of TMG, NH3 and N2 are the same as that used for the equilibrium calculation. The SiH4 flow was 100 nmol/min, a value which lies in the saturation zone predicted both by experimental and theoretical results. Fig. 3 shows the in situ reflectivity signal record versus the growth time. During the deposition of the undoped GaN layer, the amplitude of the oscillations is constant indicating a 2D smooth surface. When the SiH4 flow was turned on into the MOVPE growth chamber for the growth of the Si-doping GaN layer, the signal drops suddenly and reaches a value below that of the bare sapphire substrate indicating a 3D growth mode (maximum roughness). The formation of an in situ SiN mask by introducing simultaneously NH3 and SiH4 into the MOVPE growth chamber was reported by several groups in the so called “Si/N
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treatment growth method” [18–22]. GaN can not grow on a SiN mask resulting in a 3D growth mode. We thus think that the damping of the reflectivity can be related to a formation of solid Si3 N4 as in MOVPE Si/N treatment growth. Neugebauer [23] reported that under Nrich growth conditions (MOVPE growth), Si segregates to the surface where it can form Si3 N4 islands whereas under Ga-rich conditions (MBE growth) surface segregation does not occur. This explains the morphology differences between the Si-doping in GaN layers grown by MOVPE and MBE. 5. Conclusion Equilibrium calculations, based on standard minimisation of Gibbs free energy, of Si-doping GaN by MOVPE was presented. The results show that Si incorporation is limited by the formation of solid Si3 N4 compound. This compound can explain the Si anti-surfactant effect on the GaN layer during MOVPE processing. A good agreement is obtained between the calculation and the experimental results such as electron concentration saturation and surface roughening upon doping. Equilibrium calculations can therefore be used to predict MOVPE growth mechanisms of GaN doping even though kinetics can take place as MOVPE is a nonequilibrium process. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode, Springer, Berlin, 2000. G.-C. Yi, B.W. Wessels, Appl. Phys. Lett. 69 (1996) 3028. A. Koukitu, Y. Kumagai, H. Seki, J. Cryst. Growth 221 (2000) 743. B. Cheynet, J.D. Dubois, A. Rivet, Thermodata/INPG/CNRS, 6, rue du Tour de l’Eau, 38400 Saint Martin d’H`eres Cedex, France. A. Bchetnia, A. Rebey, T. Boufaden, B. El Jani, J. Cryst. Growth 207 (1999) 15. A. Rebey, L. B´eji, B. El Jani, P. Gibart, J. Cryst. Growth 191 (1998) 734. A. Rebey, T. Boufaden, B. El Jani, J. Cryst. Growth 203 (1999) 12. JANAF Thermochemical Tables, 2nd ed. NSRDS-NBS 37, Natl. Bur. Std. US, Washington DC, 1971. L.C. Keizer, X. Tang, R.J.C. Van Meerten, L.J. Giling, J. Cryst. Growth 102 (1990) 667. S. Sassi, J.C. Bureau, K. Chafik, Eur. Phys. J. AP 7 (1999) 207. I. Halidou, Z. Benzarti, Z. Chine, T. Boufaden, B. El Jani, Microelectron. J. 32 (2001) 137. Z. Benzarti, I. Halidou, O. Tottereau, T. Boufaden, B. El Jani, Microelectron. J. 33 (2002) 995. H. Murakami, T. Asaki, H. Amano, K. Hiramatsu, N. Sawaki, I. Akasaki, J. Cryst. Growth 115 (1991) 648. N. Koide, H. Kato, M. Sassa, S. Yamasaki, K. Manabe, M. Hashimoto, H. Amano, K. Hiramatsu, I. Akasaki, J. Cryst. Growth 115 (1991) 639. X. Liu, L. Wang, D.-C. Lu, D. Wang, X. Wang, L. Lin, J. Cryst. Growth 189–190 (1998) 287. S. Tanaka, S. Iwai, Y. Aoyagi, Appl. Phys. Lett. 69 (1996) 4096. K.S. Son, D.G. Kim, H.K. Cho, K. Lee, S. Kim, K. Park, J. Cryst. Growth 261 (2004) 50. E. Frayssinet, B. Beaumont, J.P. Faurie, P. Gibart, Zs. Makkai, B. P´ecz, P. Lefebre, P. Valvin, MRS Internet J. Nitride Semicond. Res. 7 (2002) 8. S. Sakai, T. Wang, Y. Morishima, N. Naoi, J. Cryst. Growth 221 (2000) 334. T. Kunisato, Y. Nomura, H. Ohbo, T. Kano, N. Hayashi, M. Hata, T. Yamaguchi, M. Shono, M. Sawada, A. Ibaraki, Phys. Status Solidi c 0 (2003) 2066. S.E. Park, S.M. Lim, C.R. Lee, C.S. Kim, O. Byungsung, J. Cryst. Growth 249 (2003) 487. T. B¨ottcher, J. Dennemarck, R. Kr¨oger, S. Figge, D. Hommel, Phys. Status Solidi c 0 (2003) 2039. J. Neugebauer, Phys. Status Solidi c 0 (2003) 1651.
Thermodynamic analysis of Si doping in GaN I. Halidou ∗ , Z. Benzarti, T. Boufaden, B. El Jani Unit´e de Recherche sur les H´et´eroEpitaxies et Applications (URHEA), Facult´e des Sciences 5000 Monastir, Tunisia Received 20 September 2006; accepted 25 September 2006 Available online 13 November 2006
Abstract Equilibrium calculations of Si-doping in GaN are investigated using the Gemini code. The method of the calculation is based on the minimisation of the Gibbs free energy. Experimental growth conditions are used for the calculation. The variables are the amount of the dopant and the temperature. The results show the formation of a solid Si3 N4 compound with a certain quantity of the input SiH4 , that is the silicon precursor in our MOVPE system. Si3 N4 formation can explain the limitation of Si incorporation and the surface roughening as revealed by MOVPE Si doped layers. c 2006 Elsevier Ltd. All rights reserved.
Keywords: Thermodynamic calculations; MOVPE; GaN; Si doping
1. Introduction Fabrication of nitride devices requires performing a controlled and reproducible n- and p-type doping. Many works have been devoted to studying n-type doping of GaN using silicon (Si), germanium (Ge) and selenium (Se) as dopants [1,2]. Satisfactory electrical properties of Si doped GaN have been reported but still suffer from compensation effects and surface degradation. However, for good ohmic contacts, high electron concentrations as well as smooth surfaces are needed. Therefore, understanding the growth mechanism of Si doping is still of interest. Most of the studies were focused on the effect of the growth parameter (growth temperature, dopant input concentration, growth rate, III/V ratio. . . ) and thus were done empirically. Equilibrium calculations can provide useful information necessary to understand the growth mechanism. Thermodynamic analysis of InGaAlN alloy growth was reported [3]. However, little has been done about Si-doping in GaN growth. ∗ Corresponding author. Tel.: +216 73 500 274; fax: +216 73 500 278.
E-mail address: [email protected] (I. Halidou). c 2006 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2006.09.023
I. Halidou et al. / Superlattices and Microstructures 40 (2006) 496–500
497
Fig. 1. Temperature dependence of equilibrium pressures of species arising from the reaction of the system: 40 µmol TMG, 0.18 mol NH3 , 0.09 mol N2 , and 0.09 mol H2 . The total pressure is 1 atm. Only the most important species are represented for clarity. It should be noted that solid GaN compound disappears above 1200 K.
In this paper, we perform equilibrium calculations of MOVPE Si-doping in GaN growth using SiH4 as Si precursor. The results provide guidelines for understanding the growth mechanism such as the Si incorporation limit and surface roughening. 2. Calculation procedure For the thermodynamic calculations, we have used the GEMINI code provided by Thermodata [4]. We have already successfully used this computer programme to study carbon and vanadium doping of GaAs, etching of GaAs and AlAs in CCl4 + H2 ambient and GaN decomposition under H2 ambient [5–7]. The results showed good agreement with experiment even though MOVPE is a non-equilibrium process. But, a near equilibrium can be reached at the vapour–solid interface. The principle of the calculations is based on the standard minimisation of the Gibbs free energy of a system of compounds. This allows the determination of the system composition when equilibrium is reached. For MOVPE Si-doping in GaN layer growth, the reactants are trimethylgallium (TMG), ammonia (NH3 ), the carrier gas (N2 + H2 ) and SiH4 . The products are all possible compounds that may be formed from a mixture of these reactants for a given temperature. For each compound, the thermodynamic data such as standard enthalpy of formation (1H ◦f (298 K)), standard entropy formation (1S ◦ (298 K)), and the heat capacity as a function of the temperature (C p(T )) are required. Some of the data are provided by Thermodata and the others are taken from Refs. [4,8]. 3. Results and discussion We start from a vapour composition used in our MOVPE growth experiment for undoped GaN layer deposition. The mixture contains: 40 µmol TMG, 0.18 mol NH3 , 0.18 mol H2 and/or N2 . For the calculation, the total pressure is 1 atm. In Fig. 1, the species issuing from this mixture as a function of temperature are shown. It should be noted that the mole concentration of solid
498
I. Halidou et al. / Superlattices and Microstructures 40 (2006) 496–500
Fig. 2. Partial pressure of species containing Si as a function of input mole amount of SiH4 . Saturation occurs above 50 nmol SiH4 due to the formation of solid Si3 N4 indicating thermodynamic limit of Si incorporation.
compounds is expressed in partial pressure units (atm) used for gas species. In this manner, we can compare the amount of solid and gas species in equilibrium at a given temperature. The amount of solid species represents the mole numbers condensed to reach the vapour–solid equilibrium per unit volume. It appears from Fig. 1 that the partial pressure of H2 and N2 is constant for the whole temperature range (900–1500 K). The dominant species for gallium and carbon are GaCH3 and CH4 . NH3 has a high equilibrium partial pressure in opposition to NHx species (NH2 and NH). From this calculation, it also appears that solid GaN cannot be deposited above 1200 K and that the GaN gas specie is stable above this temperature. For the Si doping calculation, we add SiH4 with different amounts in the above mixture. The input SiH4 amount lies in the range used in experiment. For clarity, Fig. 2 displays only the partial pressure of the most important compounds with the dopant element (Si) as a function of the input SiH4 mole fraction. The temperature is fixed at 1400 K, a typical value for GaN MOVPE growth and the total pressure is 1 atm. It should be noted that at this temperature, solid GaN formation is excluded as can be seen in Fig. 1. Therefore, the amount of Six Ny species represented in Fig. 2 is exaggerated. Indeed, the absence of solid GaN compound enhances the partial pressure of Ga and N containing species. Keizer et al. [9] proceeded in the same manner for equilibrium calculations of Si-doping in GaAs by MOVPE. However, their results showed a good agreement with the experiment. As can be seen from Fig. 2, SiNH appears to be the dominant specie. The partial pressure of all the species depends linearly on the amount of input SiH4 until a critical value. Indeed, at an input SiH4 of 50 nmol, the partial pressure of the species becomes constant due to the formation of solid Si3 N4 compound. This results in an upper limit for Si incorporation. Another feature that can be observed from Fig. 2 is the absence of gas species such as SiN, Si2 N. . . Sassi et al. [10] reported that the formation of these species requires the presence of NH2 and NH radicals. Since the dissociation of NH3 in the gas phase produces a small amount of these radicals, the probability of the formation of SiN, Si2 N is very low. These authors also showed that the formation of solid Si3 N4 compound is the result of the reaction between NH3 and Si. We have already seen that NH3 has a high equilibrium partial value (Fig. 1). We therefore attribute the formation of SiNH and solid Si3 N4 to a reaction between NH3 and SiH4 . The SiHx species
I. Halidou et al. / Superlattices and Microstructures 40 (2006) 496–500
499
Fig. 3. Real time-dependent He–Ne laser reflectometry signal during Si-doping in GaN. The amplitude of the signal is constant during the deposition of the undoped layer. The sudden drop of the signal when the SiH4 flow is turned on is attributed to the formation of solid Si3 N4 as predicted by the thermodynamic calculation.
is the result of the dissociation of SiH4 . Finally, it is also seen that from our calculation, no solid Si is formed. This often appears in the Si-doping of GaAs [9]. 4. Comparison with the experiment We have already reported on Si-doping of GaN by MOVPE growth using SiH4 as Si precursor [11,12]. Si was shown to affect significantly the GaN layer morphology. Indeed, upon doping we have observed a decrease of the amplitude of the in situ He–Ne laser reflectometry signal and the oscillations disappear above a critical value of SiH4 flow rate. AFM images of the layers reveal an increase of the surface roughening. Thus the damping of the reflectivity signal is due to surface roughening against SiH4 flow. Our studies also showed a saturation tendency of electron concentration for SiH4 flow of about 45 nmol/min. Similar results were reported in Refs. [13–15]. Liu et al. [15] have reported an electron concentration saturation value of 1.7×1020 cm−3 for SiH4 flow of about 700 nmol/min. The surface roughening upon doping is attributed to the anti-surfactant effect of Si [16,17]. Considering the thermodynamic calculation results, it can be seen that the surface roughening as well as the electron concentration saturation appears for SiH4 amounts in the range where solid Si3 N4 is formed. Therefore, the growth changes upon the Si doping can be attributed to the formation of solid Si3 N4 . To clarify this point of view, we deposited a Si doped GaN layer on a 0.3 µm thick undoped layer. The amount of TMG, NH3 and N2 are the same as that used for the equilibrium calculation. The SiH4 flow was 100 nmol/min, a value which lies in the saturation zone predicted both by experimental and theoretical results. Fig. 3 shows the in situ reflectivity signal record versus the growth time. During the deposition of the undoped GaN layer, the amplitude of the oscillations is constant indicating a 2D smooth surface. When the SiH4 flow was turned on into the MOVPE growth chamber for the growth of the Si-doping GaN layer, the signal drops suddenly and reaches a value below that of the bare sapphire substrate indicating a 3D growth mode (maximum roughness). The formation of an in situ SiN mask by introducing simultaneously NH3 and SiH4 into the MOVPE growth chamber was reported by several groups in the so called “Si/N
500
I. Halidou et al. / Superlattices and Microstructures 40 (2006) 496–500
treatment growth method” [18–22]. GaN can not grow on a SiN mask resulting in a 3D growth mode. We thus think that the damping of the reflectivity can be related to a formation of solid Si3 N4 as in MOVPE Si/N treatment growth. Neugebauer [23] reported that under Nrich growth conditions (MOVPE growth), Si segregates to the surface where it can form Si3 N4 islands whereas under Ga-rich conditions (MBE growth) surface segregation does not occur. This explains the morphology differences between the Si-doping in GaN layers grown by MOVPE and MBE. 5. Conclusion Equilibrium calculations, based on standard minimisation of Gibbs free energy, of Si-doping GaN by MOVPE was presented. The results show that Si incorporation is limited by the formation of solid Si3 N4 compound. This compound can explain the Si anti-surfactant effect on the GaN layer during MOVPE processing. A good agreement is obtained between the calculation and the experimental results such as electron concentration saturation and surface roughening upon doping. Equilibrium calculations can therefore be used to predict MOVPE growth mechanisms of GaN doping even though kinetics can take place as MOVPE is a nonequilibrium process. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode, Springer, Berlin, 2000. G.-C. Yi, B.W. Wessels, Appl. Phys. Lett. 69 (1996) 3028. A. Koukitu, Y. Kumagai, H. Seki, J. Cryst. Growth 221 (2000) 743. B. Cheynet, J.D. Dubois, A. Rivet, Thermodata/INPG/CNRS, 6, rue du Tour de l’Eau, 38400 Saint Martin d’H`eres Cedex, France. A. Bchetnia, A. Rebey, T. Boufaden, B. El Jani, J. Cryst. Growth 207 (1999) 15. A. Rebey, L. B´eji, B. El Jani, P. Gibart, J. Cryst. Growth 191 (1998) 734. A. Rebey, T. Boufaden, B. El Jani, J. Cryst. Growth 203 (1999) 12. JANAF Thermochemical Tables, 2nd ed. NSRDS-NBS 37, Natl. Bur. Std. US, Washington DC, 1971. L.C. Keizer, X. Tang, R.J.C. Van Meerten, L.J. Giling, J. Cryst. Growth 102 (1990) 667. S. Sassi, J.C. Bureau, K. Chafik, Eur. Phys. J. AP 7 (1999) 207. I. Halidou, Z. Benzarti, Z. Chine, T. Boufaden, B. El Jani, Microelectron. J. 32 (2001) 137. Z. Benzarti, I. Halidou, O. Tottereau, T. Boufaden, B. El Jani, Microelectron. J. 33 (2002) 995. H. Murakami, T. Asaki, H. Amano, K. Hiramatsu, N. Sawaki, I. Akasaki, J. Cryst. Growth 115 (1991) 648. N. Koide, H. Kato, M. Sassa, S. Yamasaki, K. Manabe, M. Hashimoto, H. Amano, K. Hiramatsu, I. Akasaki, J. Cryst. Growth 115 (1991) 639. X. Liu, L. Wang, D.-C. Lu, D. Wang, X. Wang, L. Lin, J. Cryst. Growth 189–190 (1998) 287. S. Tanaka, S. Iwai, Y. Aoyagi, Appl. Phys. Lett. 69 (1996) 4096. K.S. Son, D.G. Kim, H.K. Cho, K. Lee, S. Kim, K. Park, J. Cryst. Growth 261 (2004) 50. E. Frayssinet, B. Beaumont, J.P. Faurie, P. Gibart, Zs. Makkai, B. P´ecz, P. Lefebre, P. Valvin, MRS Internet J. Nitride Semicond. Res. 7 (2002) 8. S. Sakai, T. Wang, Y. Morishima, N. Naoi, J. Cryst. Growth 221 (2000) 334. T. Kunisato, Y. Nomura, H. Ohbo, T. Kano, N. Hayashi, M. Hata, T. Yamaguchi, M. Shono, M. Sawada, A. Ibaraki, Phys. Status Solidi c 0 (2003) 2066. S.E. Park, S.M. Lim, C.R. Lee, C.S. Kim, O. Byungsung, J. Cryst. Growth 249 (2003) 487. T. B¨ottcher, J. Dennemarck, R. Kr¨oger, S. Figge, D. Hommel, Phys. Status Solidi c 0 (2003) 2039. J. Neugebauer, Phys. Status Solidi c 0 (2003) 1651.