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Influence of source gas supply sequence on hydride vapor phase epitaxy of AlN on (0001) sapphire substrates
Journal of Crystal Growth 360 (2012) 197–200
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Influence of source gas supply sequence on hydride vapor phase epitaxy of AlN on (0001) sapphire substrates Rie Togashi a,n, Toru Nagashima a,b, Manabu Harada b, Hisashi Murakami a, Yoshinao Kumagai a, Hiroyuki Yanagi b, Akinori Koukitu a a b
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan Tsukuba Research Laboratories, Tokuyama Corporation, 40 Wadai, Tsukuba, Ibaraki 300-4247, Japan
a r t i c l e i n f o
abstract
Available online 17 October 2011
AlN layers were grown on (0001) sapphire substrates by hydride vapor phase epitaxy (HVPE) at 1100 1C with a source gas supply sequence of (1) NH3 preflow or (2) AlCl3 preflow. An Al-polarity AlN layer without inclusion of a N-polarity region was grown when AlCl3 was preflown to the sapphire surface prior to AlN growth, while N- and Al-polarity regions were both present in the same AlN layer when NH3 was preflown, since growth was performed on a nitrided sapphire surface. Compared with the AlN layers grown with NH3 preflow, the Al-polarity AlN layers grown with AlCl3 preflow had improved crystalline structural quality, a low concentration of oxygen impurity, and a photoabsorption edge energy of 6.08 eV, which is close to an ideal value. Therefore, the source gas supply sequence has a significant influence on the growth of AlN layers on (0001) sapphire substrates. Thus, preflow of AlCl3 gas to a sapphire surface prior to AlN growth is a key process for high crystalline quality AlN layer growth with uniform Al-polarity on (0001) sapphire substrates by HVPE. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A3. Hydride vapor phase epitaxy B1. Nitrides B1. Sapphire B2. Semiconducting aluminum compounds
1. Introduction AlN is an attractive substrate material for the fabrication of high-power AlxGa1 xN-based deep ultraviolet (UV) range optoelectronic devices, due to outstanding properties such as deep UV transparency, high thermal conductivity, high breakdown field, and a lattice constant close to that of AlxGa1 xN. Many researchers have attempted to grow AlN layers by vapor phase epitaxy (VPE) using (0001) sapphire as a starting substrate for the preparation of freestanding AlN substrates or AlN templates, because sapphire has good thermal and chemical stability for high-temperature growth conditions, scalability for large area growth, reasonable cost, and deep UV transparency, although the lattice and thermal expansion mismatches between AlN and sapphire are high. We have previously reported the growth of a high crystalline quality and crack-free AlN template layer on a (0001) sapphire substrate at a growth temperature of 1450 1C by hydride VPE (HVPE) [1], which is the commonly used method for the mass-production of GaN wafers [2]. Furthermore, freestanding AlN substrates with Al-polarity have been successfully prepared by self-separation of thick AlN layers grown on (0001) sapphire substrates [3]. However, details for control of the lattice polarity of AlN layers grown on (0001) sapphire, which has a
n
Corresponding author. Tel.: þ81 42 388 7469; fax: þ 81 42 388 7424. E-mail address: [email protected] (R. Togashi).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.10.014
non-polar surface, have not been clarified. Controlling the polarity of grown AlN is a significant obstacle to the realization of high crystalline quality AlN layers on (0001) sapphire. It was reported that lattice polarity of AlN layers grown on (0001) sapphire substrates by metal–organic VPE (MOVPE) is decided by the pre-growth treatment of the sapphire surface. Nitridation of a sapphire surface was performed for the growth of N-polarity AlN layers, while no nitridation was performed for the growth of Al-polarity AlN layers [4,5]. In this study, the influence of the source gas supply sequence on the HVPE growth of AlN layers on a (0001) sapphire substrates was investigated.
2. Experimental procedure AlN growth was performed using an atmospheric pressure HVPE system having a horizontal hot-wall quartz glass reactor with a dual-zone electronic furnace. AlCl3 was generated in the upstream region of the reactor (source zone) by the reaction between 6N-grade Al metal and HCl gas introduced over Al metal maintained at 500 1C. AlCl3 and NH3 were separately introduced into the downstream region (growth zone), where an on-axis (0001) sapphire substrate (7.5 5.0 mm2) was placed. Prior to being set in the reactor, the on-axis (0001) sapphire substrate was etched in a hot solution of H3PO4:H2SO4 ¼1:3 for 10 min at 160 1C. A schematic illustration of the substrate temperature and source gas supply sequences of NH3 and AlCl3 are
Temperature (°C)
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R. Togashi et al. / Journal of Crystal Growth 360 (2012) 197–200
1100
12000
NH3
AlCl3
550
10000
NH3 preflow On
AlCl3 NH3
On
FWHM (arcsec)
Time
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6000
(1010)
4000
AlCl3 preflow AlCl3
On
NH3
(0002)
2000 On
0 300
0
300
600
900
1200
Time of preflow (s) Fig. 1. Schematic of the substrate temperature and the supply sequences of NH3 and AlCl3 gases: (a) thermal cleaning of a (0001) sapphire surface, (b) preflow of NH3 or AlCl3 gas over the sapphire surface, and (c) AlN growth by supply of NH3 and AlCl3.
shown in Fig. 1. The sapphire surface was first cleaned in a flowing H2 at 1100 1C for 10 min (step (a)). After this, one of the following source gas supply sequences was employed (step (b)): (1) NH3 preflow and (2) AlCl3 preflow. The growth of AlN layers on the (0001) sapphire substrate was carried out for 30 min at 1100 1C (step (c)). The total thicknesses of the AlN layers were about 3 mm. After AlN growth, the substrate was cooled to room temperature with a supply of NH3 above 550 1C to prevent the decomposition of AlN. The total pressure of the reactor and the input partial pressures of AlCl3 and NH3 were 1.00, 6.67 10 4, and 2.67 10 3 atm, respectively. Thus, the input V/III ratio during growth was fixed at 4. The carrier gas during preflow (step (b)) and AlN growth (step (c)) was a mixture of H2 and N2 (molar fraction of H2 ¼0.29) with a dew point below 110 1C. The total flow was maintained at 2250 sccm during growth. The crystalline structural quality and surface morphology of the grown AlN layers were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The lattice polarity of the AlN layers was determined by wet etching in aqueous KOH solution. Incorporation of impurities in the layers was analyzed by X-ray photoelectron spectroscopy (XPS) depth profile measurements. The optical properties of the layers were also checked using room temperature photoabsorption spectra.
3. Results and discussion Crystalline quality of AlN layers grown with various source gas supply sequences was investigated. Fig. 2 shows the dependence of the full-width at half-maximum (FWHM) values of XRD rocking curves (XRCs) for the symmetric (0002)AlN (tilt distribution) and asymmetric (101¯0)AlN (twist distribution) planes at the time of preflow. The FWHMs of the (0002) plane are almost the same, whereas those of the (101¯0) plane were remarkably improved by a preflow of AlCl3. The FWHM of XRC for (101¯0)AlN showed a minimum (3060 arcsec) when AlN was grown on a sapphire
Fig. 2. FWHM values of XRCs for the symmetric (0002) and asymmetric (101¯0) planes of AlN layers grown on (0001) sapphire substrates as a function of the NH3 or AlCl3 preflow time.
substrate with AlCl3 preflow for 120 s. Therefore, AlCl3 preflow prior to HVPE growth of AlN on (0001) sapphire is quite effective for the growth of higher crystalline quality AlN layers. In addition, the FWHMs of both (0002) and (101¯0) were almost constant with increase of the AlCl3 preflow time. In the atomic layer epitaxy of GaAs by alternative supply of GaCl3 and AsH3, one to two monolayers per cycle were reported to occur due to saturation of GaCl3 adsorption [6]. When the same mechanism was applied to this study, the number of AlCl3 atoms adsorbed on the sapphire surface is saturated, even though the time of AlCl3 supply is increased. Therefore, the AlN layers grown on sapphire substrates with AlCl3 preflow have almost identical FWHMs. On the other hand, deterioration of the (101¯0) FWHM values by NH3 preflow may be due to nitridation of the sapphire surface, since nitridation of sapphire surfaces occur by reaction with NH3 at high temperatures [7]. Wet etching of the grown layers in a KOH solution was performed to investigate the lattice polarity of the grown layers. It can be revealed by KOH etching that a completely inactive surface against etching corresponds to Al-polarity, while an etched surface corresponds to N-polarity. The etching was performed using a 6 mol/l KOH solution at 60 1C for 10 min [8]. Fig. 3 shows bird’s-eye view SEM micrographs of AlN layers grown with NH3 (a,b) or AlCl3 (c,d) preflow for 300 s, observed before (a,c) and after (b,d) KOH etching. Before etching, although the AlN layer grown with NH3 preflow had larger surface roughness than AlCl3 preflow, the sapphire surfaces of both samples were completely covered with AlN layers without pitting. After KOH etching, the AlN layer grown with NH3 preflow was partially etched and the bottom of the remaining part was etched leaving hexagonal pyramids. This result indicates that areas of N- and Al-polarity were both present when an AlN layer was grown with NH3 preflow. In contrast, the top surface of the AlN layer grown with AlCl3 preflow showed no change in morphology after KOH etching (Fig. 3(d)), which indicates that an Al-polarity AlN layer with no incorporation of N-polarity regions was grown, although the starting (0001) sapphire substrate does not have polarity.
R. Togashi et al. / Journal of Crystal Growth 360 (2012) 197–200
199
Fig. 3. Bird’s-eye view SEM micrographs of AlN layers grown with NH3 preflow (a,b) and AlCl3 preflow (c,d) before (a,c) and after (b,d) KOH etching.
The influence of the source gas supply sequence on the incorporation of carbon and oxygen in the AlN surface was studied by means of XPS depth profile measurements. XPS spectra of Al 2p, N 1s, C 1s and O 1s were recorded. The photoemission was excited by a Mg Ka X-ray beam impinging on the AlN surfaces at the angle of 451. Spectra were corrected using the N 1s peak corresponding to Al–N species at a binding energy value of 397.3 eV. Fig. 4 shows atomic concentrations of the elements from the top surface of the AlN layer to a depth of 200 nm. Carbon was observed only at the top surfaces of both AlN layers grown with NH3 preflow and AlCl3 preflow, which was not observed in the AlN layers. It could be explained that the presence of carbon detected at the top surfaces refers to adventitious carbons adsorbed on the AlN layers when the layers were removed from the HVPE reactor and exposed to ambient air after growth. In the case of oxygen, a higher oxygen concentration was also observed at the top surfaces of both AlN layers. It could be explained that the presence of oxygen detected at the top surfaces corresponds to native oxide layers formed on the AlN surfaces, since the surfaces of AlN layers reacted with moisture or oxygen in air after growth. Furthermore, oxygen at approximately 3 at.% was observed even in the AlN layers grown with NH3 preflow, whereas oxygen at 0 to 1 at.%, which corresponds to the noise level, was observed in the Al-polarity AlN layers grown with AlCl3 preflow. Thus, it was found that slightly higher oxygen was incorporated in the AlN layers grown with NH3 preflow than Alpolarity AlN layers grown with AlCl3 preflow. This tendency was in agreement with secondary ion mass spectrometry (SIMS) results reported by Takeuchi et al. [5], which showed that N-polarity AlN samples grown on (0001) sapphire substrates by modified flow-modulation MOVPE had a larger oxygen
concentration than Al-polarity. This can be explained in terms of the easy oxygen adsorption at Al- and N-polarity AlN surfaces. Previously, oxygen adsorption at (0001) Ga- and (0001¯) N-polarity GaN surfaces was studied by Zywietz et al. using density functional theory (DFT) calculations [9]. It was shown that oxygen incorporation is expected to be higher at the N-polarity GaN surface than Ga-polartiy, since oxygen atoms substitute on the N-site in the GaN layers, and the larger number of unsaturated dangling bonds on the N-polarity GaN surfaces makes the surfaces more active to oxygen. Thus, using the same analogy to explain the oxygen adsorption at (0001) Al- and (0001¯) N-polarity AlN surfaces, oxygen atoms prefer to be incorporated into N-polarity AlN layers than Al-polarity layers. Furthermore, the AlN layers grown with NH3 preflow had many defects (Fig. 2), so that oxygen might be easily incorporated at the positions of such defects. In addition, because the AlN layers grown with NH3 preflow had a slightly higher oxygen concentration, XRD y–2y profiles were measured in order to investigate a difference of the c-axis lattice constants between the AlN layers grown with NH3 preflow and AlCl3 preflow. However, shifts of the (0002)AlN diffraction peaks were not observed. Thus, although oxygen is highly incorporated into the AlN layers, the lattice constants of AlN are thought to be not greatly influenced by the oxygen incorporated into the layers. Photoabsorption spectra of the AlN layers were measured at room temperature. The square of the absorption coefficient (a) is plotted in Fig. 5 as a function of the photon energy. The spectrum for the AlN layer grown with AlCl3 preflow shows a very sharp increase above 6 eV. The bandgap energy of the AlN layer grown with AlCl3 preflow was found to be 6.08 eV by extrapolating the sharp increase part of a2, which indicates this AlN layer has good
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R. Togashi et al. / Journal of Crystal Growth 360 (2012) 197–200
10
60
NH3 preflow Al 2p
50 N 1s
40
NH3 preflow
Atomic concentration (%)
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α2 (x1010 cm-2)
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C 1s
10
5
O 1s
AlCl3 preflow
0 60
AlCl3 preflow Al 2p
50
0 4.0
N 1s
40 30
4.5
5.0 5.5 Photon energy (eV)
6.0
6.5
Fig. 5. Photoabsorption spectra measured at room temperature for AlN layers grown with NH3 preflow and AlCl3 preflow.
20
effective for the growth of high-quality Al-polarity AlN layers on (0001) sapphire substrates.
10 0
C 1s
O 1s
0
Acknowledgments 40
80 Depth (nm)
120
160
Fig. 4. XPS depth profiles for atomic concentrations in the AlN layers grown with NH3 preflow (a) and AlCl3 preflow (b).
crystalline quality. In contrast, the spectrum for the AlN layer grown with NH3 preflow shows a gradual increase above 5 eV, and therefore the bandgap could not be estimated. Thus, it was also revealed that preflow of AlCl3 is a key factor for the growth of high crystalline quality AlN on sapphire substrates.
4. Conclusions The influence of the source gas supply sequence on the HVPE growth of AlN on (0001) sapphire substrates was investigated. When AlN is grown with NH3 preflow, both N- and Al-polarity AlN regions are present in the layer. However, when AlCl3 preflow is used prior to AlN growth, Al-polarity AlN without incorporation of N-polarity regions is grown. The Al-polarity AlN layer grown with AlCl3 preflow has higher crystalline structural quality, a sharp absorption edge with a bandgap energy of 6.08 eV, and lower oxygen concentration than the AlN layer grown with NH3 preflow. Thus, AlCl3 preflow prior to HVPE growth of AlN is quite
This work was supported in part by a Grant-in-Aid for Scientific Research (C) no. 21560009 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] J. Tajima, H. Murakami, Y. Kumagai, K. Takada, A. Koukitu, Journal of Crystal Growth 311 (2009) 2837. [2] K. Motoki, T. Okahisa, N. Matsumoto, M. Matsushima, H. Kimura, H. Kasai, K. Takemoto, K. Uematsu, T. Hirano, M. Nakayama, S. Nakahata, M. Ueno, D. Hara, Y. Kumagai, A. Koukitu, H. Seki, Japanese Journal of Applied Physics 40 (2001) L140. [3] Y. Kumagai, Y. Enatsu, M. Ishizuki, Y. Kubota, J. Tajima, T. Nagashima, H. Murakami, K. Takada, A. Koukitu, Journal of Crystal Growth 312 (2010) 2530. [4] Y. Wu, A. Hanlon, J.F. Kaeding, R. Sharma, R.T. Fini, S. Nakamura, J.S. Speck, Applied Physics Letters 84 (2004) 912. [5] M. Takeuchi, H. Shimizu, R. Kajitani, K. Kawasaki, T. Kinoshita, K. Takada, H. Murakami, Y. Kumagai, A. Koukitu, T. Koyama, S.F. Chichibu, Y. Aoyagi, Journal of Crystal Growth 305 (2007) 360. [6] K. Ishikawa, R. Kobayashi, S. Narahara, F. Hasegawa, Japanese Journal of Applied Physics 31 (1992) 1716. [7] K. Uchida, A. Watanabe, F. Yano, M. Kouguchi, T. Tanaka, S. Minagawa, Journal of Applied Physics 79 (1996) 3487. [8] D. Zhuang, J.H. Edgar, L. Liu, B. Liu, L. Walker, MRS Internet Journal of Nitride Semiconductor Research 7 (2002) 4. [9] T.K. Zywietz, J. Neugebauer, M. Scheffler, Applied Physics Letters 74 (1999) 1695.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Influence of source gas supply sequence on hydride vapor phase epitaxy of AlN on (0001) sapphire substrates Rie Togashi a,n, Toru Nagashima a,b, Manabu Harada b, Hisashi Murakami a, Yoshinao Kumagai a, Hiroyuki Yanagi b, Akinori Koukitu a a b
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo 184-8588, Japan Tsukuba Research Laboratories, Tokuyama Corporation, 40 Wadai, Tsukuba, Ibaraki 300-4247, Japan
a r t i c l e i n f o
abstract
Available online 17 October 2011
AlN layers were grown on (0001) sapphire substrates by hydride vapor phase epitaxy (HVPE) at 1100 1C with a source gas supply sequence of (1) NH3 preflow or (2) AlCl3 preflow. An Al-polarity AlN layer without inclusion of a N-polarity region was grown when AlCl3 was preflown to the sapphire surface prior to AlN growth, while N- and Al-polarity regions were both present in the same AlN layer when NH3 was preflown, since growth was performed on a nitrided sapphire surface. Compared with the AlN layers grown with NH3 preflow, the Al-polarity AlN layers grown with AlCl3 preflow had improved crystalline structural quality, a low concentration of oxygen impurity, and a photoabsorption edge energy of 6.08 eV, which is close to an ideal value. Therefore, the source gas supply sequence has a significant influence on the growth of AlN layers on (0001) sapphire substrates. Thus, preflow of AlCl3 gas to a sapphire surface prior to AlN growth is a key process for high crystalline quality AlN layer growth with uniform Al-polarity on (0001) sapphire substrates by HVPE. & 2011 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A3. Hydride vapor phase epitaxy B1. Nitrides B1. Sapphire B2. Semiconducting aluminum compounds
1. Introduction AlN is an attractive substrate material for the fabrication of high-power AlxGa1 xN-based deep ultraviolet (UV) range optoelectronic devices, due to outstanding properties such as deep UV transparency, high thermal conductivity, high breakdown field, and a lattice constant close to that of AlxGa1 xN. Many researchers have attempted to grow AlN layers by vapor phase epitaxy (VPE) using (0001) sapphire as a starting substrate for the preparation of freestanding AlN substrates or AlN templates, because sapphire has good thermal and chemical stability for high-temperature growth conditions, scalability for large area growth, reasonable cost, and deep UV transparency, although the lattice and thermal expansion mismatches between AlN and sapphire are high. We have previously reported the growth of a high crystalline quality and crack-free AlN template layer on a (0001) sapphire substrate at a growth temperature of 1450 1C by hydride VPE (HVPE) [1], which is the commonly used method for the mass-production of GaN wafers [2]. Furthermore, freestanding AlN substrates with Al-polarity have been successfully prepared by self-separation of thick AlN layers grown on (0001) sapphire substrates [3]. However, details for control of the lattice polarity of AlN layers grown on (0001) sapphire, which has a
n
Corresponding author. Tel.: þ81 42 388 7469; fax: þ 81 42 388 7424. E-mail address: [email protected] (R. Togashi).
0022-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2011.10.014
non-polar surface, have not been clarified. Controlling the polarity of grown AlN is a significant obstacle to the realization of high crystalline quality AlN layers on (0001) sapphire. It was reported that lattice polarity of AlN layers grown on (0001) sapphire substrates by metal–organic VPE (MOVPE) is decided by the pre-growth treatment of the sapphire surface. Nitridation of a sapphire surface was performed for the growth of N-polarity AlN layers, while no nitridation was performed for the growth of Al-polarity AlN layers [4,5]. In this study, the influence of the source gas supply sequence on the HVPE growth of AlN layers on a (0001) sapphire substrates was investigated.
2. Experimental procedure AlN growth was performed using an atmospheric pressure HVPE system having a horizontal hot-wall quartz glass reactor with a dual-zone electronic furnace. AlCl3 was generated in the upstream region of the reactor (source zone) by the reaction between 6N-grade Al metal and HCl gas introduced over Al metal maintained at 500 1C. AlCl3 and NH3 were separately introduced into the downstream region (growth zone), where an on-axis (0001) sapphire substrate (7.5 5.0 mm2) was placed. Prior to being set in the reactor, the on-axis (0001) sapphire substrate was etched in a hot solution of H3PO4:H2SO4 ¼1:3 for 10 min at 160 1C. A schematic illustration of the substrate temperature and source gas supply sequences of NH3 and AlCl3 are
Temperature (°C)
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R. Togashi et al. / Journal of Crystal Growth 360 (2012) 197–200
1100
12000
NH3
AlCl3
550
10000
NH3 preflow On
AlCl3 NH3
On
FWHM (arcsec)
Time
8000
6000
(1010)
4000
AlCl3 preflow AlCl3
On
NH3
(0002)
2000 On
0 300
0
300
600
900
1200
Time of preflow (s) Fig. 1. Schematic of the substrate temperature and the supply sequences of NH3 and AlCl3 gases: (a) thermal cleaning of a (0001) sapphire surface, (b) preflow of NH3 or AlCl3 gas over the sapphire surface, and (c) AlN growth by supply of NH3 and AlCl3.
shown in Fig. 1. The sapphire surface was first cleaned in a flowing H2 at 1100 1C for 10 min (step (a)). After this, one of the following source gas supply sequences was employed (step (b)): (1) NH3 preflow and (2) AlCl3 preflow. The growth of AlN layers on the (0001) sapphire substrate was carried out for 30 min at 1100 1C (step (c)). The total thicknesses of the AlN layers were about 3 mm. After AlN growth, the substrate was cooled to room temperature with a supply of NH3 above 550 1C to prevent the decomposition of AlN. The total pressure of the reactor and the input partial pressures of AlCl3 and NH3 were 1.00, 6.67 10 4, and 2.67 10 3 atm, respectively. Thus, the input V/III ratio during growth was fixed at 4. The carrier gas during preflow (step (b)) and AlN growth (step (c)) was a mixture of H2 and N2 (molar fraction of H2 ¼0.29) with a dew point below 110 1C. The total flow was maintained at 2250 sccm during growth. The crystalline structural quality and surface morphology of the grown AlN layers were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The lattice polarity of the AlN layers was determined by wet etching in aqueous KOH solution. Incorporation of impurities in the layers was analyzed by X-ray photoelectron spectroscopy (XPS) depth profile measurements. The optical properties of the layers were also checked using room temperature photoabsorption spectra.
3. Results and discussion Crystalline quality of AlN layers grown with various source gas supply sequences was investigated. Fig. 2 shows the dependence of the full-width at half-maximum (FWHM) values of XRD rocking curves (XRCs) for the symmetric (0002)AlN (tilt distribution) and asymmetric (101¯0)AlN (twist distribution) planes at the time of preflow. The FWHMs of the (0002) plane are almost the same, whereas those of the (101¯0) plane were remarkably improved by a preflow of AlCl3. The FWHM of XRC for (101¯0)AlN showed a minimum (3060 arcsec) when AlN was grown on a sapphire
Fig. 2. FWHM values of XRCs for the symmetric (0002) and asymmetric (101¯0) planes of AlN layers grown on (0001) sapphire substrates as a function of the NH3 or AlCl3 preflow time.
substrate with AlCl3 preflow for 120 s. Therefore, AlCl3 preflow prior to HVPE growth of AlN on (0001) sapphire is quite effective for the growth of higher crystalline quality AlN layers. In addition, the FWHMs of both (0002) and (101¯0) were almost constant with increase of the AlCl3 preflow time. In the atomic layer epitaxy of GaAs by alternative supply of GaCl3 and AsH3, one to two monolayers per cycle were reported to occur due to saturation of GaCl3 adsorption [6]. When the same mechanism was applied to this study, the number of AlCl3 atoms adsorbed on the sapphire surface is saturated, even though the time of AlCl3 supply is increased. Therefore, the AlN layers grown on sapphire substrates with AlCl3 preflow have almost identical FWHMs. On the other hand, deterioration of the (101¯0) FWHM values by NH3 preflow may be due to nitridation of the sapphire surface, since nitridation of sapphire surfaces occur by reaction with NH3 at high temperatures [7]. Wet etching of the grown layers in a KOH solution was performed to investigate the lattice polarity of the grown layers. It can be revealed by KOH etching that a completely inactive surface against etching corresponds to Al-polarity, while an etched surface corresponds to N-polarity. The etching was performed using a 6 mol/l KOH solution at 60 1C for 10 min [8]. Fig. 3 shows bird’s-eye view SEM micrographs of AlN layers grown with NH3 (a,b) or AlCl3 (c,d) preflow for 300 s, observed before (a,c) and after (b,d) KOH etching. Before etching, although the AlN layer grown with NH3 preflow had larger surface roughness than AlCl3 preflow, the sapphire surfaces of both samples were completely covered with AlN layers without pitting. After KOH etching, the AlN layer grown with NH3 preflow was partially etched and the bottom of the remaining part was etched leaving hexagonal pyramids. This result indicates that areas of N- and Al-polarity were both present when an AlN layer was grown with NH3 preflow. In contrast, the top surface of the AlN layer grown with AlCl3 preflow showed no change in morphology after KOH etching (Fig. 3(d)), which indicates that an Al-polarity AlN layer with no incorporation of N-polarity regions was grown, although the starting (0001) sapphire substrate does not have polarity.
R. Togashi et al. / Journal of Crystal Growth 360 (2012) 197–200
199
Fig. 3. Bird’s-eye view SEM micrographs of AlN layers grown with NH3 preflow (a,b) and AlCl3 preflow (c,d) before (a,c) and after (b,d) KOH etching.
The influence of the source gas supply sequence on the incorporation of carbon and oxygen in the AlN surface was studied by means of XPS depth profile measurements. XPS spectra of Al 2p, N 1s, C 1s and O 1s were recorded. The photoemission was excited by a Mg Ka X-ray beam impinging on the AlN surfaces at the angle of 451. Spectra were corrected using the N 1s peak corresponding to Al–N species at a binding energy value of 397.3 eV. Fig. 4 shows atomic concentrations of the elements from the top surface of the AlN layer to a depth of 200 nm. Carbon was observed only at the top surfaces of both AlN layers grown with NH3 preflow and AlCl3 preflow, which was not observed in the AlN layers. It could be explained that the presence of carbon detected at the top surfaces refers to adventitious carbons adsorbed on the AlN layers when the layers were removed from the HVPE reactor and exposed to ambient air after growth. In the case of oxygen, a higher oxygen concentration was also observed at the top surfaces of both AlN layers. It could be explained that the presence of oxygen detected at the top surfaces corresponds to native oxide layers formed on the AlN surfaces, since the surfaces of AlN layers reacted with moisture or oxygen in air after growth. Furthermore, oxygen at approximately 3 at.% was observed even in the AlN layers grown with NH3 preflow, whereas oxygen at 0 to 1 at.%, which corresponds to the noise level, was observed in the Al-polarity AlN layers grown with AlCl3 preflow. Thus, it was found that slightly higher oxygen was incorporated in the AlN layers grown with NH3 preflow than Alpolarity AlN layers grown with AlCl3 preflow. This tendency was in agreement with secondary ion mass spectrometry (SIMS) results reported by Takeuchi et al. [5], which showed that N-polarity AlN samples grown on (0001) sapphire substrates by modified flow-modulation MOVPE had a larger oxygen
concentration than Al-polarity. This can be explained in terms of the easy oxygen adsorption at Al- and N-polarity AlN surfaces. Previously, oxygen adsorption at (0001) Ga- and (0001¯) N-polarity GaN surfaces was studied by Zywietz et al. using density functional theory (DFT) calculations [9]. It was shown that oxygen incorporation is expected to be higher at the N-polarity GaN surface than Ga-polartiy, since oxygen atoms substitute on the N-site in the GaN layers, and the larger number of unsaturated dangling bonds on the N-polarity GaN surfaces makes the surfaces more active to oxygen. Thus, using the same analogy to explain the oxygen adsorption at (0001) Al- and (0001¯) N-polarity AlN surfaces, oxygen atoms prefer to be incorporated into N-polarity AlN layers than Al-polarity layers. Furthermore, the AlN layers grown with NH3 preflow had many defects (Fig. 2), so that oxygen might be easily incorporated at the positions of such defects. In addition, because the AlN layers grown with NH3 preflow had a slightly higher oxygen concentration, XRD y–2y profiles were measured in order to investigate a difference of the c-axis lattice constants between the AlN layers grown with NH3 preflow and AlCl3 preflow. However, shifts of the (0002)AlN diffraction peaks were not observed. Thus, although oxygen is highly incorporated into the AlN layers, the lattice constants of AlN are thought to be not greatly influenced by the oxygen incorporated into the layers. Photoabsorption spectra of the AlN layers were measured at room temperature. The square of the absorption coefficient (a) is plotted in Fig. 5 as a function of the photon energy. The spectrum for the AlN layer grown with AlCl3 preflow shows a very sharp increase above 6 eV. The bandgap energy of the AlN layer grown with AlCl3 preflow was found to be 6.08 eV by extrapolating the sharp increase part of a2, which indicates this AlN layer has good
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NH3 preflow Al 2p
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Fig. 5. Photoabsorption spectra measured at room temperature for AlN layers grown with NH3 preflow and AlCl3 preflow.
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effective for the growth of high-quality Al-polarity AlN layers on (0001) sapphire substrates.
10 0
C 1s
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80 Depth (nm)
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Fig. 4. XPS depth profiles for atomic concentrations in the AlN layers grown with NH3 preflow (a) and AlCl3 preflow (b).
crystalline quality. In contrast, the spectrum for the AlN layer grown with NH3 preflow shows a gradual increase above 5 eV, and therefore the bandgap could not be estimated. Thus, it was also revealed that preflow of AlCl3 is a key factor for the growth of high crystalline quality AlN on sapphire substrates.
4. Conclusions The influence of the source gas supply sequence on the HVPE growth of AlN on (0001) sapphire substrates was investigated. When AlN is grown with NH3 preflow, both N- and Al-polarity AlN regions are present in the layer. However, when AlCl3 preflow is used prior to AlN growth, Al-polarity AlN without incorporation of N-polarity regions is grown. The Al-polarity AlN layer grown with AlCl3 preflow has higher crystalline structural quality, a sharp absorption edge with a bandgap energy of 6.08 eV, and lower oxygen concentration than the AlN layer grown with NH3 preflow. Thus, AlCl3 preflow prior to HVPE growth of AlN is quite
This work was supported in part by a Grant-in-Aid for Scientific Research (C) no. 21560009 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] J. Tajima, H. Murakami, Y. Kumagai, K. Takada, A. Koukitu, Journal of Crystal Growth 311 (2009) 2837. [2] K. Motoki, T. Okahisa, N. Matsumoto, M. Matsushima, H. Kimura, H. Kasai, K. Takemoto, K. Uematsu, T. Hirano, M. Nakayama, S. Nakahata, M. Ueno, D. Hara, Y. Kumagai, A. Koukitu, H. Seki, Japanese Journal of Applied Physics 40 (2001) L140. [3] Y. Kumagai, Y. Enatsu, M. Ishizuki, Y. Kubota, J. Tajima, T. Nagashima, H. Murakami, K. Takada, A. Koukitu, Journal of Crystal Growth 312 (2010) 2530. [4] Y. Wu, A. Hanlon, J.F. Kaeding, R. Sharma, R.T. Fini, S. Nakamura, J.S. Speck, Applied Physics Letters 84 (2004) 912. [5] M. Takeuchi, H. Shimizu, R. Kajitani, K. Kawasaki, T. Kinoshita, K. Takada, H. Murakami, Y. Kumagai, A. Koukitu, T. Koyama, S.F. Chichibu, Y. Aoyagi, Journal of Crystal Growth 305 (2007) 360. [6] K. Ishikawa, R. Kobayashi, S. Narahara, F. Hasegawa, Japanese Journal of Applied Physics 31 (1992) 1716. [7] K. Uchida, A. Watanabe, F. Yano, M. Kouguchi, T. Tanaka, S. Minagawa, Journal of Applied Physics 79 (1996) 3487. [8] D. Zhuang, J.H. Edgar, L. Liu, B. Liu, L. Walker, MRS Internet Journal of Nitride Semiconductor Research 7 (2002) 4. [9] T.K. Zywietz, J. Neugebauer, M. Scheffler, Applied Physics Letters 74 (1999) 1695.