- Email: [email protected]
Channeling study of thermal decomposition of III-N compound semiconductors
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1224–1228 www.elsevier.com/locate/nimb
Channeling study of thermal decomposition of III-N compound semiconductors A. Stonert a,*, K. Pa˛gowska a, R. Ratajczak a, P. Caban b, W. Strupinski b, A. Turos a,b a
Soltan Institute of Nuclear Studies, 05-400 Swierk/Otwock, Hoza 69, 00-680 Warsaw, Poland b Institute of Electronic Materials Technology, 01-919 Warsaw, ul. Wolczynska 133, Poland Received 20 September 2007; received in revised form 6 December 2007 Available online 23 December 2007
Abstract GaN thermal stability is the limiting factor of the growth rate for epitaxially grown films and of the thermal annealing of defects. As a consequence, this issue has been extensively studied for more than one decade. There are, however, substantial differences in the reported kinetics and presumed mechanisms of decomposition, which are primarily related to the reactor design thus, reflecting the complexity of chemical reactions involved. We report here on the use of 1.7 MeV He-ion RBS/channeling for the study of thermal decomposition of MOVPE grown GaN and AlxGa1 xN (x = 0.05–0.5) layers. These layers with thickness of 320 nm were grown on sapphire substrates with 20 nm AlN nucleation layer. Prior to annealing samples were characterized by RBS/channeling, selected samples were also studied by SEM. Thermal treatment was performed in the MOVPE reactor in the temperature range 900–1200 °C in the N2 atmosphere. RBS/channeling analysis provided data on layer thickness, composition and evolution of ingrown defects. GaN decomposition starts at 900 °C and results in the reduction of the layer thickness without observable changes of the film composition. The presence of large density of GaN hillocks on the surface was revealed by SEM after annealing at 1000–1050 °C. Remarkable stability of AlxGa1 xN was observed, this alloy remains unchanged upon annealing at 1200 °C/6 h even for x as low as 0.05. Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Gh; 82.80.Yc Keywords: GaN; Thermal stability; MOVPE; RBS
1. Introduction Semiconductor compounds based on the group III nitrides have great potential for applications in electronic and optoelectronic devices due to their large band-gap and remarkable stability at high temperatures. Because of the lack of a lattice matched substrates thin films of nitride semiconductors are commonly grown in the hexagonal wurzite structure on sapphire single crystals. The detailed understanding of GaN growth is of great interest for further material and device improvements. Because of the great complexity of chemical reactions involved many important
*
Corresponding author. Tel.: +48 22 5532129; fax: +48 22 6213829. E-mail address: [email protected] (A. Stonert).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.12.054
issues in metalorganic vapor phase epitaxy (MOVPE) are still to be elucidated. It has been noted that optimal growth conditions vary from reactor to reactor reflecting the influence of setup design on individual chemical reaction rates. In any case the net growth rate results from the two competing processes, i.e. incorporation of GaN molecules and their decomposition, hence any increase of the decomposition rate will decrease the growth rate [1,2]. The thermal and chemical stability of GaN is also important for understanding of mechanisms of defect reduction in post-grown and post-implantation annealing. Thermal annealing is also used for dopant activation and to improve the crystalline quality after different processing steps. Although GaN decomposition has been extensively studied over last two decades, there are substantial differences in the reported growth kinetics and decomposition
A. Stonert et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1224–1228
rates. Different mechanisms were proposed to explain GaN decomposition [3]. These included decomposition into: gaseous Ga and N: 2GaN(solid) ? 2Ga(gas) + N2(gas), liquid Ga and N: 2GaN(solid) ? 2Ga(liquid) + N2(gas) ? 2Ga(gas), sublimation of GaN as a molecule: GaN(solid) ? GaN(gas). Several experiments were performed that provided evidence of all three reactions, their rates depending on the ambient gas, annealing temperature and film polarity [4– 7]. In this paper, we report on the use of RBS/channeling for the study of GaN thermal stability. Great advantage of this method with respect to the previously applied ones is its simplicity and insensitivity to matrix effects. Random spectra analysis provided data on layer thickness and composition whereas the aligned ones were used for the study of evolution of ingrown defects.
2. Experimental Samples were prepared using MOVPE technique in the Aixtron AX100HTRD reactor at Institute of Electronic Materials Technology, Warsaw. The films were deposited onto 2 in. diameter (0 0 0 1) sapphire wafers. The substrate was heated before the growth in H2 ambient at high temperature. A standard two step deposition process was employed [8,9]. First, after cooling down to the temperature of 500 °C AlN nucleation layer of approximately 20 nm thickness was deposited. Subsequently, the wafer was heated to 1050 °C for deposition of 320 nm thick GaN films. Trimethylgallium and ammonia were used as
1225
precursors with hydrogen as the carrier gas. The produced films were Ga-polar. Annealing of GaN is a difficult process primarily due to its complex decomposition. Moreover, GaN great affinity to oxygen actually precludes thermal treatment in conventional annealing facilities. As shown by Rana et al. [10] even small admixtures of oxygen usually present in high purity gas environment can significantly change the process characteristics. Consequently, the decomposition study has to be carried out in the MOVPE reactor. The reported annealing experiments were carried out in the temperature range 900–1200 °C according to the following scheme: the samples were first heated up to 600 °C under the H2 flow then up to the preset temperature the gas ambient was enriched in ammonia, the H2/NH3 ratio was 7:2. After the preset temperature was reached the anneal gas was switched to N2 for 30 min annealing. Subsequently the system was cooled down to RT in NH3. This annealing scheme has been chosen to comply with the standard growth conditions of GaN templates [11]. Virgin and annealed samples were analyzed with RBS/ channeling using 1.7 MeV 4He-ions at the Institute for Ion Beam Physics and Materials Research Forschungszentrum Dresden, Germany. The Monte Carlo simulation code McChasy was applied for spectra analysis [12,13]. The surface morphology prior and after annealing was observed using scanning electron microscope.
3. Results Fig. 1 shows the portions random spectra originating from the backscattering on Ga atoms for originally 320 nm thick GaN layers subjected to annealing at different
RT 950 oC 1000 oC 1050 oC 1100 oC
1600 1400 1200
Yield
1000 800 600 400 200 0 1000
1100
1200
1300
1400
Energy (keV) Fig. 1. Random RBS spectra for originally 320 nm thick GaN film subjected to 30 min annealing at different temperatures. Only the portion of spectrum due to the scattering by Ga atoms is shown. Solid lines delineate the SIMNRA simulation results for GaN layers of different thickness.
1226
A. Stonert et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1224–1228 Table 1 Estimated layer thickness and roughness as function of the annealing temperature Annealing temperature (°C)
Residual GaN layer thickness (nm)
Layer roughness Dx (nm)
RT 950 1000 1050 1100
320 270 200 150 80a
0 45 78 66 113
a
Fig. 2. SEM micrograph for GaN film annealed at 1050 °C/30 min.
temperatures. The following characteristic features of the spectra should be pointed out: first, for temperatures above 900 °C the width of Ga peak decreases monotonically due to the progressing decomposition of GaN layer; second, equal height of all peaks is a strong indication that GaN decomposition takes place on the surface and does not produce any compositional changes in the layer; third, the rear edge incline decreases with increasing temperature, which is apparently due to increased roughness of the layer. The latter assumption was supported by the SEM observations. SEM micrograph for the sample annealed at
Mean value corresponding to the FWHM of the peak.
1000 °C is shown in Fig. 2. Numerous hillocks on the sample surface of approximately 300 nm diameter at the base were visualized. Similar hillocks with roughly the same density were observed after annealing at 1050 °C. No significant changes of the surface morphology were noticed after annealing at temperatures below 1000 °C. On the other hand, annealing at 1100 °C produces a sponge-like structure and apparently continuous layer does not exists any longer. Hillock formation is responsible for the large decease of the rear edge incline of Ga peak at temperatures exceeding 950 °C. Since SEM micrographs provide only an estimate of hillocks diameter at the base their height can be evaluated from RBS spectra. Assuming that the rear edge of the Ga peak is formed by the convolution of beam straggling and the spread of layer thickness, Dx, one can calculate the later parameter using available simulation codes like McChasy or SIMNRA [14]. It should be pointed out that the accuracy of this estimate depends strongly on the hillock density. Fortunately, for the two temperatures of interest this parameter remains roughly constant and Dx values can be directly compared. Estimated layer thickness and roughness as function of the annealing temperature are listed in Table 1.
200 RT 1000 oC 1100 oC
Yield
150
100
50
0 1000
1100
1200
1300
140 0
Energy (keV) Fig. 3. Aligned RBS/channeling spectra for originally 320 nm thick GaN film subjected to 30 min annealing at different temperatures. Only the portion of spectrum due to the scattering by Ga atoms is shown.
A. Stonert et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1224–1228
1227
18 RT 1100 oC 1100 oC
Displaced atoms (at.%)
16 14 12 10 8 6 4 2 0 0
50
100
150
200
250
300
Depth (nm) Fig. 4. Depth distributions of displaced lattice atoms due to the dislocation formation as deduced from spectra in Fig. 3.
Fig. 3 shows aligned RBS spectra for the same samples as in Fig. 1. Channeling spectra along c-axis measured for virgin samples revealed a very low channeling minimum yield, Xmin 6 2%, independently of the layer thickness. The large peak in aligned spectrum in the vicinity of 1100 keV is due to defect agglomeration. According to TEM observations these are a tangle of misfit dislocations of different length and type [15]. They are located in the vicinity of interface with only few threading dislocations emerging from the region. For the analysis of channeling spectra the McChasy computer code was applied [13]. Since up to now, there is no possibility to simulate dislocations with the McChasy code simulations of channeling spectra were done assuming defects are randomly displaced atoms. Depth distribution of displaced atoms was included in the input data for the spectrum simulation. The initial distribution was then modified until the best fit to the channeling spectrum was obtained. Best fits to experimental data are shown by solid lines in Fig. 3. The corresponding defect depth distributions are shown in Fig. 4. Upon annealing defect agglomeration moves towards the surface although defect amount and profile remain essentially unchanged until the temperature is reached at which the continuous layer disappears. AlxGa1 xN layers on sapphire with x ranging from 0.05 to 0.5 have also been studied. Remarkable thermal stability of these alloys was noticed. Changes neither of surface morphology nor of layer thickness were observed, even for an alloy of composition so close to GaN as Al0.05Ga0.95N. Epitaxial layer of this composition remained unaffected after annealing at 1200 °C for 6 h. 4. Discussion and conclusions Thermal decomposition of solids is usually composed of several steps. The most important are: thermally induced
bond breaking of molecules, surface migration of released atoms and formation of vapor molecules followed by their transport away from the surface. Products of GaN decomposition exhibit different thermal properties. Surface migration Ga-atom leads typically to the formation of Ga droplets that are quite stable (Ga liquid phase exists from 30 to 2204 °C). Although N atoms are much less mobile than Ga atoms they can easily leave the surface as soon as N2 molecules are formed. The growth of GaN is challenging not only because of the lack of a lattice matched substrate for epitaxy. Since GaN decomposition can take place at temperatures the are about 200 °C lower than typical growth temperatures, the epilayer effective growth rate is equal to the growth rate less the decomposition rate. Since the desorption rate of group III elements is usually much lower than that of group V elements the V/III ratio of more than 100 is needed to ensure the proper epitaxial growth [1]. As a consequence the highest GaN thermal stability upon annealing can obtained in the atmosphere suppressing N-desorption. For this reason in our study annealing in the flowing N2 at the pressure of 100 mbar has been performed. Our results have shown that GaN epitaxial layers annealed in nitrogen flow are thermally stable up to 950 °C. At higher temperatures GaN desorption takes place. SEM analysis revealed important surface development in the form of hillocks. Hillocks are regular, conical shaped of roughly the same size and are of the same composition as the layer. Since the hillock height exceeds 25% of the layer thickness, any change of composition would be noticeable in random spectra. Concerning the nature of this formation one can speculate that they are the remnants of Ga droplets formed during thermal treatment, which have been transformed to GaN due to reaction with NH3 during the cooling down phase. No evolution of the as-grown defects at the layer/substrate
1228
A. Stonert et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1224–1228
interface has been observed up to 1200 °C, the highest temperature used. Acknowledgements We thank A. Piatkowska and M. Romaniec for SEM analysis. This work was supported by the Polish Ministry of Science and Higher Education, Grant No. N51502931/ 1104. RBS/channeling analysis has been carried out in the framework of the EU Program: Access to Large Scale Facilities AIM at the Institute for Ion Beam Physics and Materials Research (IIM), Forschungszentrum Dresden (Germany). References [1] D.D. Koleske, A.E. Wickenden, R.L. Henry, W.J. DeSisto, R.J. Gorman, J. Appl. Phys. 84 (1998) 1998. [2] I. Gherasoiou, S. Nikishin, H. Temkin, J. Appl. Phys. 98 (2005) 053518.
[3] D.D. Koleske, A.E. Wickenden, R.L. Henry, J.C. Culbertson, M.E. Twigg, J. Cryst. Growth 223 (2001) 466. [4] D.D. Koleske, A.E. Wickenden, R.L. Henry, M.E. Twigg, J.C. Culbertson, Appl. Phys. Lett. 73 (1998) 2018. [5] M.A. Mastro, O.M. Kryliouk, T.J. Anderson, A. Davydov, A. Shapiro, J. Cryst. Growth 274 (2005) 38. [6] M. Kuball, F. Demangeot, J. Frandon, M.A. Renucci, N. Grandjean, O. Briot, MRS Internet J. Nitride Semicond. Res. 4S1 (1999) G66.28. [7] Akinori Koukitu, Miho Mayumi, Yoshonao Kumagai, J. Cryst. Growth 246 (2002) 230. [8] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Keller, U. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [9] M. Kamp, Opt. Quant. Electron. 32 (2000) 227. [10] M.A. Rana, T. Osipowicz, H.W. Choi, M.B.H. Breese, F. Watt, S.J. Chua, Appl. Phys. A77 (2003) 103. [11] A.E. Wickendeken, D.D. Koleske, R.L. Henry, M.E. Twigg, M. Fatemi, J. Cryst. Growth 260 (2003) 54. [12] A. Dygo, A. Turos, Phys. Rev. B 40 (1989) 7704. [13] L. Nowicki, A. Turos, R. Ratajczak, A. Stonert, F. Garrido, Nucl. Instr. and Meth. B 240 (2005) 277. [14] M. Mayer, Nucl. Instr. and Meth. B 194 (2002) 177. [15] J. Borysiuk, private communication.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 1224–1228 www.elsevier.com/locate/nimb
Channeling study of thermal decomposition of III-N compound semiconductors A. Stonert a,*, K. Pa˛gowska a, R. Ratajczak a, P. Caban b, W. Strupinski b, A. Turos a,b a
Soltan Institute of Nuclear Studies, 05-400 Swierk/Otwock, Hoza 69, 00-680 Warsaw, Poland b Institute of Electronic Materials Technology, 01-919 Warsaw, ul. Wolczynska 133, Poland Received 20 September 2007; received in revised form 6 December 2007 Available online 23 December 2007
Abstract GaN thermal stability is the limiting factor of the growth rate for epitaxially grown films and of the thermal annealing of defects. As a consequence, this issue has been extensively studied for more than one decade. There are, however, substantial differences in the reported kinetics and presumed mechanisms of decomposition, which are primarily related to the reactor design thus, reflecting the complexity of chemical reactions involved. We report here on the use of 1.7 MeV He-ion RBS/channeling for the study of thermal decomposition of MOVPE grown GaN and AlxGa1 xN (x = 0.05–0.5) layers. These layers with thickness of 320 nm were grown on sapphire substrates with 20 nm AlN nucleation layer. Prior to annealing samples were characterized by RBS/channeling, selected samples were also studied by SEM. Thermal treatment was performed in the MOVPE reactor in the temperature range 900–1200 °C in the N2 atmosphere. RBS/channeling analysis provided data on layer thickness, composition and evolution of ingrown defects. GaN decomposition starts at 900 °C and results in the reduction of the layer thickness without observable changes of the film composition. The presence of large density of GaN hillocks on the surface was revealed by SEM after annealing at 1000–1050 °C. Remarkable stability of AlxGa1 xN was observed, this alloy remains unchanged upon annealing at 1200 °C/6 h even for x as low as 0.05. Ó 2007 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Gh; 82.80.Yc Keywords: GaN; Thermal stability; MOVPE; RBS
1. Introduction Semiconductor compounds based on the group III nitrides have great potential for applications in electronic and optoelectronic devices due to their large band-gap and remarkable stability at high temperatures. Because of the lack of a lattice matched substrates thin films of nitride semiconductors are commonly grown in the hexagonal wurzite structure on sapphire single crystals. The detailed understanding of GaN growth is of great interest for further material and device improvements. Because of the great complexity of chemical reactions involved many important
*
Corresponding author. Tel.: +48 22 5532129; fax: +48 22 6213829. E-mail address: [email protected] (A. Stonert).
0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.12.054
issues in metalorganic vapor phase epitaxy (MOVPE) are still to be elucidated. It has been noted that optimal growth conditions vary from reactor to reactor reflecting the influence of setup design on individual chemical reaction rates. In any case the net growth rate results from the two competing processes, i.e. incorporation of GaN molecules and their decomposition, hence any increase of the decomposition rate will decrease the growth rate [1,2]. The thermal and chemical stability of GaN is also important for understanding of mechanisms of defect reduction in post-grown and post-implantation annealing. Thermal annealing is also used for dopant activation and to improve the crystalline quality after different processing steps. Although GaN decomposition has been extensively studied over last two decades, there are substantial differences in the reported growth kinetics and decomposition
A. Stonert et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1224–1228
rates. Different mechanisms were proposed to explain GaN decomposition [3]. These included decomposition into: gaseous Ga and N: 2GaN(solid) ? 2Ga(gas) + N2(gas), liquid Ga and N: 2GaN(solid) ? 2Ga(liquid) + N2(gas) ? 2Ga(gas), sublimation of GaN as a molecule: GaN(solid) ? GaN(gas). Several experiments were performed that provided evidence of all three reactions, their rates depending on the ambient gas, annealing temperature and film polarity [4– 7]. In this paper, we report on the use of RBS/channeling for the study of GaN thermal stability. Great advantage of this method with respect to the previously applied ones is its simplicity and insensitivity to matrix effects. Random spectra analysis provided data on layer thickness and composition whereas the aligned ones were used for the study of evolution of ingrown defects.
2. Experimental Samples were prepared using MOVPE technique in the Aixtron AX100HTRD reactor at Institute of Electronic Materials Technology, Warsaw. The films were deposited onto 2 in. diameter (0 0 0 1) sapphire wafers. The substrate was heated before the growth in H2 ambient at high temperature. A standard two step deposition process was employed [8,9]. First, after cooling down to the temperature of 500 °C AlN nucleation layer of approximately 20 nm thickness was deposited. Subsequently, the wafer was heated to 1050 °C for deposition of 320 nm thick GaN films. Trimethylgallium and ammonia were used as
1225
precursors with hydrogen as the carrier gas. The produced films were Ga-polar. Annealing of GaN is a difficult process primarily due to its complex decomposition. Moreover, GaN great affinity to oxygen actually precludes thermal treatment in conventional annealing facilities. As shown by Rana et al. [10] even small admixtures of oxygen usually present in high purity gas environment can significantly change the process characteristics. Consequently, the decomposition study has to be carried out in the MOVPE reactor. The reported annealing experiments were carried out in the temperature range 900–1200 °C according to the following scheme: the samples were first heated up to 600 °C under the H2 flow then up to the preset temperature the gas ambient was enriched in ammonia, the H2/NH3 ratio was 7:2. After the preset temperature was reached the anneal gas was switched to N2 for 30 min annealing. Subsequently the system was cooled down to RT in NH3. This annealing scheme has been chosen to comply with the standard growth conditions of GaN templates [11]. Virgin and annealed samples were analyzed with RBS/ channeling using 1.7 MeV 4He-ions at the Institute for Ion Beam Physics and Materials Research Forschungszentrum Dresden, Germany. The Monte Carlo simulation code McChasy was applied for spectra analysis [12,13]. The surface morphology prior and after annealing was observed using scanning electron microscope.
3. Results Fig. 1 shows the portions random spectra originating from the backscattering on Ga atoms for originally 320 nm thick GaN layers subjected to annealing at different
RT 950 oC 1000 oC 1050 oC 1100 oC
1600 1400 1200
Yield
1000 800 600 400 200 0 1000
1100
1200
1300
1400
Energy (keV) Fig. 1. Random RBS spectra for originally 320 nm thick GaN film subjected to 30 min annealing at different temperatures. Only the portion of spectrum due to the scattering by Ga atoms is shown. Solid lines delineate the SIMNRA simulation results for GaN layers of different thickness.
1226
A. Stonert et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1224–1228 Table 1 Estimated layer thickness and roughness as function of the annealing temperature Annealing temperature (°C)
Residual GaN layer thickness (nm)
Layer roughness Dx (nm)
RT 950 1000 1050 1100
320 270 200 150 80a
0 45 78 66 113
a
Fig. 2. SEM micrograph for GaN film annealed at 1050 °C/30 min.
temperatures. The following characteristic features of the spectra should be pointed out: first, for temperatures above 900 °C the width of Ga peak decreases monotonically due to the progressing decomposition of GaN layer; second, equal height of all peaks is a strong indication that GaN decomposition takes place on the surface and does not produce any compositional changes in the layer; third, the rear edge incline decreases with increasing temperature, which is apparently due to increased roughness of the layer. The latter assumption was supported by the SEM observations. SEM micrograph for the sample annealed at
Mean value corresponding to the FWHM of the peak.
1000 °C is shown in Fig. 2. Numerous hillocks on the sample surface of approximately 300 nm diameter at the base were visualized. Similar hillocks with roughly the same density were observed after annealing at 1050 °C. No significant changes of the surface morphology were noticed after annealing at temperatures below 1000 °C. On the other hand, annealing at 1100 °C produces a sponge-like structure and apparently continuous layer does not exists any longer. Hillock formation is responsible for the large decease of the rear edge incline of Ga peak at temperatures exceeding 950 °C. Since SEM micrographs provide only an estimate of hillocks diameter at the base their height can be evaluated from RBS spectra. Assuming that the rear edge of the Ga peak is formed by the convolution of beam straggling and the spread of layer thickness, Dx, one can calculate the later parameter using available simulation codes like McChasy or SIMNRA [14]. It should be pointed out that the accuracy of this estimate depends strongly on the hillock density. Fortunately, for the two temperatures of interest this parameter remains roughly constant and Dx values can be directly compared. Estimated layer thickness and roughness as function of the annealing temperature are listed in Table 1.
200 RT 1000 oC 1100 oC
Yield
150
100
50
0 1000
1100
1200
1300
140 0
Energy (keV) Fig. 3. Aligned RBS/channeling spectra for originally 320 nm thick GaN film subjected to 30 min annealing at different temperatures. Only the portion of spectrum due to the scattering by Ga atoms is shown.
A. Stonert et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1224–1228
1227
18 RT 1100 oC 1100 oC
Displaced atoms (at.%)
16 14 12 10 8 6 4 2 0 0
50
100
150
200
250
300
Depth (nm) Fig. 4. Depth distributions of displaced lattice atoms due to the dislocation formation as deduced from spectra in Fig. 3.
Fig. 3 shows aligned RBS spectra for the same samples as in Fig. 1. Channeling spectra along c-axis measured for virgin samples revealed a very low channeling minimum yield, Xmin 6 2%, independently of the layer thickness. The large peak in aligned spectrum in the vicinity of 1100 keV is due to defect agglomeration. According to TEM observations these are a tangle of misfit dislocations of different length and type [15]. They are located in the vicinity of interface with only few threading dislocations emerging from the region. For the analysis of channeling spectra the McChasy computer code was applied [13]. Since up to now, there is no possibility to simulate dislocations with the McChasy code simulations of channeling spectra were done assuming defects are randomly displaced atoms. Depth distribution of displaced atoms was included in the input data for the spectrum simulation. The initial distribution was then modified until the best fit to the channeling spectrum was obtained. Best fits to experimental data are shown by solid lines in Fig. 3. The corresponding defect depth distributions are shown in Fig. 4. Upon annealing defect agglomeration moves towards the surface although defect amount and profile remain essentially unchanged until the temperature is reached at which the continuous layer disappears. AlxGa1 xN layers on sapphire with x ranging from 0.05 to 0.5 have also been studied. Remarkable thermal stability of these alloys was noticed. Changes neither of surface morphology nor of layer thickness were observed, even for an alloy of composition so close to GaN as Al0.05Ga0.95N. Epitaxial layer of this composition remained unaffected after annealing at 1200 °C for 6 h. 4. Discussion and conclusions Thermal decomposition of solids is usually composed of several steps. The most important are: thermally induced
bond breaking of molecules, surface migration of released atoms and formation of vapor molecules followed by their transport away from the surface. Products of GaN decomposition exhibit different thermal properties. Surface migration Ga-atom leads typically to the formation of Ga droplets that are quite stable (Ga liquid phase exists from 30 to 2204 °C). Although N atoms are much less mobile than Ga atoms they can easily leave the surface as soon as N2 molecules are formed. The growth of GaN is challenging not only because of the lack of a lattice matched substrate for epitaxy. Since GaN decomposition can take place at temperatures the are about 200 °C lower than typical growth temperatures, the epilayer effective growth rate is equal to the growth rate less the decomposition rate. Since the desorption rate of group III elements is usually much lower than that of group V elements the V/III ratio of more than 100 is needed to ensure the proper epitaxial growth [1]. As a consequence the highest GaN thermal stability upon annealing can obtained in the atmosphere suppressing N-desorption. For this reason in our study annealing in the flowing N2 at the pressure of 100 mbar has been performed. Our results have shown that GaN epitaxial layers annealed in nitrogen flow are thermally stable up to 950 °C. At higher temperatures GaN desorption takes place. SEM analysis revealed important surface development in the form of hillocks. Hillocks are regular, conical shaped of roughly the same size and are of the same composition as the layer. Since the hillock height exceeds 25% of the layer thickness, any change of composition would be noticeable in random spectra. Concerning the nature of this formation one can speculate that they are the remnants of Ga droplets formed during thermal treatment, which have been transformed to GaN due to reaction with NH3 during the cooling down phase. No evolution of the as-grown defects at the layer/substrate
1228
A. Stonert et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 1224–1228
interface has been observed up to 1200 °C, the highest temperature used. Acknowledgements We thank A. Piatkowska and M. Romaniec for SEM analysis. This work was supported by the Polish Ministry of Science and Higher Education, Grant No. N51502931/ 1104. RBS/channeling analysis has been carried out in the framework of the EU Program: Access to Large Scale Facilities AIM at the Institute for Ion Beam Physics and Materials Research (IIM), Forschungszentrum Dresden (Germany). References [1] D.D. Koleske, A.E. Wickenden, R.L. Henry, W.J. DeSisto, R.J. Gorman, J. Appl. Phys. 84 (1998) 1998. [2] I. Gherasoiou, S. Nikishin, H. Temkin, J. Appl. Phys. 98 (2005) 053518.
[3] D.D. Koleske, A.E. Wickenden, R.L. Henry, J.C. Culbertson, M.E. Twigg, J. Cryst. Growth 223 (2001) 466. [4] D.D. Koleske, A.E. Wickenden, R.L. Henry, M.E. Twigg, J.C. Culbertson, Appl. Phys. Lett. 73 (1998) 2018. [5] M.A. Mastro, O.M. Kryliouk, T.J. Anderson, A. Davydov, A. Shapiro, J. Cryst. Growth 274 (2005) 38. [6] M. Kuball, F. Demangeot, J. Frandon, M.A. Renucci, N. Grandjean, O. Briot, MRS Internet J. Nitride Semicond. Res. 4S1 (1999) G66.28. [7] Akinori Koukitu, Miho Mayumi, Yoshonao Kumagai, J. Cryst. Growth 246 (2002) 230. [8] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Keller, U. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [9] M. Kamp, Opt. Quant. Electron. 32 (2000) 227. [10] M.A. Rana, T. Osipowicz, H.W. Choi, M.B.H. Breese, F. Watt, S.J. Chua, Appl. Phys. A77 (2003) 103. [11] A.E. Wickendeken, D.D. Koleske, R.L. Henry, M.E. Twigg, M. Fatemi, J. Cryst. Growth 260 (2003) 54. [12] A. Dygo, A. Turos, Phys. Rev. B 40 (1989) 7704. [13] L. Nowicki, A. Turos, R. Ratajczak, A. Stonert, F. Garrido, Nucl. Instr. and Meth. B 240 (2005) 277. [14] M. Mayer, Nucl. Instr. and Meth. B 194 (2002) 177. [15] J. Borysiuk, private communication.