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Growth of Ga- and N- polar gallium nitride layers by metalorganic vapor phase epitaxy on sapphire wafers
ARTICLE IN PRESS
Journal of Crystal Growth 287 (2006) 586–590 www.elsevier.com/locate/jcrysgro
Growth of Ga- and N- polar gallium nitride layers by metalorganic vapor phase epitaxy on sapphire wafers R. Collazo, S. Mita, A. Aleksov, R. Schlesser, Z. Sitar Department of Materials Science and Engineering, North Carolina State University, Campus Box 7919, Raleigh, NC 27695-7919, USA Available online 27 December 2005
Abstract Following an already established polarity control scheme for GaN thin films, we developed a process to simultaneously grow Ga- and N-polarity layers side by side on c-plane sapphire. The simultaneous growth is achieved by properly treating the AlN nucleation/buffer layer and subsequent substrate annealing. During this process, the growth is mass-transfer-limited, permitting the same growth rate for both types of polarity domains. Smooth domains of both polarity types (RMS roughness 1–2 nm) were obtained. r 2005 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 81.05.Ea; 81.15.Gh Keywords: A1. Crystal strcture; A3. Chemical vapor deposition process; A3. Metalorganic chemical vapor deposition; B1. Nitrides; B2. Semiconducting III–V materials
1. Introduction Polar orientation in non-centrosymmetric III-nitride wurtzite semiconductors refers to the crystallographic orientation of the basal plane of the wurtzite lattice with respect to the substrate. The polar structure with three bonds of the III-atom facing towards the substrate is referred to as the +c orientation, while the mirrored structure with three bonds of the III atom facing towards the free surface is referred to as the c orientation. Polar orientation should not be confused with surface termination, as each orientation may be terminated with either one of the species. The wurtzite structure is the lattice type with the highest symmetry that still exhibits spontaneous polarization. The polar orientation determines the direction of the spontaneous polarization vector and, thus, determines the type of charge induced at the surfaces/ interfaces. Along with the piezoelectric polarization, the polarization-induced charge influences the electrical and optical properties of the material. Control of the polar orientation on a macroscopic and microscopic scale is Corresponding author. Tel.: +1 919 515 8965; fax: +1 919 515 3419.
E-mail address: [email protected] (R. Collazo). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.10.080
desirable to exploit the combined properties of both types of orientations. Fig. 1 shows part of the unit cell of a wurtzite III-nitride. The arrow indicates the +c direction of the lattice. The c orientation is obtained by rotating the unit cell by 1801. It is commonly believed that in the case of metalorganic vapor phase epitaxy (MOVPE) growth, the N-rich conditions, together with comparatively high substrate temperatures, always produce +c oriented films, i.e. Ga-polar GaN films. The usually employed nitridation step preceding deposition of a buffer layer forms a thin AlN layer on the sapphire substrate and induces the Ga polar orientation. By contrast, N-polar (c) orientation typically occurs only in films grown by molecular beam epitaxy (MBE). Proper AlN buffer deposition conditions during MBE growth on sapphire substrates allow the possibility to locally determine the lattice polarity. This ability provides new technological opportunities for the design of novel device structures in the III-nitrides family [1]. Nevertheless, the technical difficulties of using MBE as the growth technique have hindered the technological development of this family of devices. Recently, growth of N-polar GaN has been achieved by MOVPE and the possibility of polarity control was suggested [2].
ARTICLE IN PRESS R. Collazo et al. / Journal of Crystal Growth 287 (2006) 586–590
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2. Experimental details
Fig. 1. Partial unit cell of a wurtzite III-nitride semiconductor. The black and gray balls correspond to N and III-atoms, respectively. The arrow indicates the +c direction of the lattice.
N-polar GaN thin films typically feature a rough surface morphology featuring hexagonal pyramids, while the Ga-polar typically has a mirror-like appearance. Mixedpolarity domains, i.e., films containing small domains of both polarities, are difficult to distinguish from the pure polar films, thus surface morphology alone does not provide an unambiguous identification of film polarity. Several techniques have been used to identify the polarity orientations; the easiest, but destructive, technique uses the different chemical reactivity of each polar domain. Ga-polar surfaces are resistant to most chemical etchants except at defects, while N-polar surfaces are reactive to KOH/H2O solutions or KOH/NaOH eutectics [1]. In the same way as the difference in chemical reactivity between +c and c faces affects the etch rate, the growth rate is different as well: Ga-polar surfaces typically grow faster than N-polar surfaces [3]. This feature is not only observed in MOVPE, but also in bulk growth of GaN and AlN. A faster growing Ga-polar surface will overgrow a competing N-polar domain [4]. By contrast, both domains can be deposited at the same rate during MBE growth. In this article, we will briefly describe the process by which we obtain N-polar GaN films grown by MOVPE on sapphire substrates at the same growth rate as the Ga-polar GaN films. Although it is not our intent to introduce models to explain the different results obtained here, we will describe some of the results and explain their consequences, specifically the role of the dilution gas in establishing the same growth rate for both domains.
All the films for these experiments were grown in a vertical, cold-walled, RF-heated, low-pressure MOVPE reactor. The distance between the gas inlet ‘‘shower-head’’ and the susceptor was 5 cm. The reactor base pressure was 1 107 Torr. Triethylgallium (TEG), trimethylaluminum (TMA) and ammonia were used as Ga, Al and N precursors, respectively. In contrast to the traditional process conditions for nitride growth, N2 was used as dilution and carrier gas throughout the growth process. ‘‘Epi-ready’’ sapphire wafers (2 in (0 0 0 1)-oriented) were used as substrates. After the wafer was annealed in vacuum, it was H2 annealed at 1090 1C in order to remove residual contamination and prepare the sapphire surface prior to the growth process. For both types of domains, our standard GaN growth conditions consisted of a TEG flow of 36 mmol/min, an ammonia flow of 0.4 slm and a total flow of 2.15 slm at 20 Torr of total pressure. The growth temperature was maintained at 1030 1C. This condition provided a V/III ratio of 500 at a growth rate of 1.2 mm/h. A nitridation step prior to a low temperature (LT) AlN buffer layer deposition was necessary to control the polarity. The AlN buffer layer thickness was 10–20 nm and was grown at 600 1C. No buffer layer was used to grow N-polar GaN films, while a proper buffer layer anneal after deposition was necessary to obtain Ga-polar GaN films. Further processing details will be published in a later publication. Different flow rates were used to determine the role of nitrogen in promoting the same growth rates for films of both types of polarities. Chemical reactivity with a 3 M KOH/H2O solution was used to unambiguously distinguish between Ga- and N-polar films. N-polar films immediately etched when immersed in the alkaline solution at 65 1C. Crystallographic characterization was performed by acquiring on- and off-axis HRXRD rocking curves using a Philips X’Pert Materials Research Diffractometer with a copper X-ray source operated at 40 kV and 45 mA, and an open slit on the detector side. Atomic force microscopy (AFM) images were acquired with a Digital Intruments Nanoscope in the tapping mode. 3. Results and discussion Under the conditions described in the previous section, we obtained smooth films of both types of polarities. Smooth N-polar films were obtained when the substrate nitridation was around 2 min at 950 1C. Longer or higher temperature nitridation induced the hexagonal surface morphology usually attributed to N-polar films. ‘‘Mirrorlike’’ Ga-polar films were obtained after the LT-AlN buffer layer grown on the nitrided wafer was annealed in ammonia flow for an experimentally determined time and ammonia partial pressure. The use of N2 as a dilution gas seems critical to obtain the results reported here. Sumiya et al. obtained similar
ARTICLE IN PRESS 588
R. Collazo et al. / Journal of Crystal Growth 287 (2006) 586–590
results using H2 as dilution gas but needed longer nitridation and anneal times to get the same effects [2]. It is a prevalent idea that the use of N2 as the only dilution
gas produces rough GaN films with a mosaic structure [5]. Our results demonstrate that using N2 as the sole dilution gas can yield films of the same quality as obtained by using H2 as the dilution gas. Fig. 2 shows the full-width at halfmaximum (FWHM) of the (00.2), (10.3), (30.2) X-ray rocking curves as a function of the plane inclination angle with respect to the (00.2) on-axis reflection for a 1.35-mmthick Ga-polar GaN film, with an optimized buffer layer anneal step for the reduction of the dislocation density (improvement in the FWHM of the rocking curves). The (00.2) reflection was measured in a symmetric geometry, while the (10.3) and (30.2) reflections were measured in a skew-symmetric geometry. The V/III ratio used during the film growth was 100. Fig. 3 shows the same type of plot for 1.35 mm thick, Ga- and N-polar layers deposited side by side on the same substrate. Rocking curves were measured for each type of domain away from the boundary. Ga-polar domains have a lower FWHM of the on-axis reflection than the N-polar domains, but similar off-axis reflections FWHM. The process is optimized to assure domain purity, i.e., no embedded inversion domains. Fig. 4 shows an AFM images of a Ga-polar 0.6-mm-thick GaN film (a) and a N-polar GaN film of similar thickness (b) grown at a V/III ratio of 500 and using N2 as the
Fig. 2. X-ray rocking curves FWHM of the (00.2), (10.3), (30.2) reflections as a function of the plane inclination angle with respect to the (00.2) on-axis reflection for a 1.35 mm thick Ga-polar GaN film.
Fig. 3. X-ray rocking curves FWHM of the (00.2), (10.3), (30.2) reflections as a function of the plane inclination angle with respect to the (00.2) on-axis reflection for 1.35-mm-thick Ga- and N-polar layers deposited on the same substrate. Rocking curves were measured for each type of domain away from the boundary.
Fig. 4. AFM images of a Ga-polar 0.6-mm-thick GaN film (a) and a Npolar GaN film of similar thickness (b) grown at a V/III ratio of 500 and using N2 as the dilution gas.
ARTICLE IN PRESS R. Collazo et al. / Journal of Crystal Growth 287 (2006) 586–590
dilution gas. The image shows that the films grew in the step-flow mode with randomly distributed pits, with a pit density comparable to reported threading dislocation densities for films of comparable crystalline quality. The surface roughness was estimated to be 0.2 nm root mean square (rms) for the Ga-polar film. The N-polar film surface roughness was estimated to be 2 nm rms, with no hexagonal pyramids on the surface. The growth rate for both types of domains was found to be identical, in contrast to observations by Wu et al., who reported that the Ga-polar grows faster than the N-polar [3]. Although they use MOVPE as their growth technique, they use H2 as their sole dilution gas, in contrast to our process. It has been established that the growth rate is nearly constant at the typical growth temperatures when N2 is used as the sole dilution gas, while under H2 the growth rate decreases with temperature [5]. This suggests that under N2 flow the growth is mass-transfer-limited with a large supersaturation, while under H2 the supersaturation is greatly decreased. Mass-transfer-limited growth is necessary to circumvent the chemical reactivity difference between the two domains and to achieve identical growth rates for the two domains. In order to demonstrate that the growth is mass-transfer-limited using N2 as the dilution gas, it has to be shown that (in the simplest case) the growth rate is limited by the diffusion of the III-atom carrying species through a gas boundary layer formed at the surface of the substrate. In our particular reactor geometry this can be expressed as sffiffiffiffiffi G f i f 0T ¼ , (1) G 0 f 0i f T where G is the growth rate and f i and f T are the III-atom carrying species and total mass flow rate. The equation is referenced to a standard state (O) corresponding to our standard process conditions at a given growth temperature as reported in the experimental section. Fig. 5 demonstrates the linear relationship between the normalized growth rate and the inverse of the square root of the total flow for different growth conditions. Observation of this dependence is a sufficient and necessary condition to show that the growth is mass-transfer-limited. In addition, as suggested by Eq. (1), the growth rate should be independent of the total reactor pressure. This fact is demonstrated in Fig. 6, where the growth rate is plotted as a function of total reactor pressure for a given TEG mass flow rate and total mass flow rate. Consequently, both domains will grow at the same rate, independently of the growth conditions, as long as N2 is used as the dilution gas. Fig. 7 shows a SEM micrograph of the crosssection of Ga- and N-polar layers grown side by side. The region left of the boundary is the Ga-polar domain, while the region right of the boundary is the N-polar domain. No difference in the thickness between the two domains is observed within 30 mm of each side of the boundary.
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Fig. 5. Normalized growth rate as a function of the normalized inverse square root of total flow. A linear relationship is sufficient and necessary to demonstrate that the growth is mass-transfer-limited. The solid line corresponds to the unity line as shown by Eq. (1). The error bars indicate uncertainty in the growth rate.
Fig. 6. GaN film growth rate as a function of total reactor pressure for a TEG mass flow rate of 134 mmol/min and total mass flow rate of 7.2 slm. The error bars indicate the uncertainty in the growth rate.
Micrographs taken further away from the boundary, within each domain, do not show any difference in thickness either.
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mass-transfer-limited, both +c and c polarities can be simultaneously grown on c-plane sapphire in a horizontal manner in a MOVPE process utilizing only N2 as the sole dilution gas. The simultaneous and controlled growth of +c and c polarity regions can be achieved through an appropriate control of the substrate preparation.
Fig. 7. SEM micrograph of the cross-section of a GaN layer near the inversion domain boundary separating a Ga-polar domain (left) from a N-polar domain (right).
4. Conclusions Following the reported polarity control scheme, and the observation that in an N2 atmosphere the growth is
References [1] M. Stutzmann, et al., Phys. Stat. Sol. (B) 228 (2001) 505. [2] M. Sumiya, S. Fuke, MRS Int. J. Nitride Semicond. Res. 9 (2004) 1. [3] F. Wu, M.D. Craven, S.H. Lim, J.S. Speck, J. Appl. Phys. 94 (2003) 942. [4] J. Jasinski, Z. Liliental-Weber, Q.S. Paduano, D.W. Weyburne, Appl. Phys. Lett. 83 (2003) 2811. [5] O. Ambacher, J. Phys. D: Appl. Phys. 31 (1998) 2653.
Journal of Crystal Growth 287 (2006) 586–590 www.elsevier.com/locate/jcrysgro
Growth of Ga- and N- polar gallium nitride layers by metalorganic vapor phase epitaxy on sapphire wafers R. Collazo, S. Mita, A. Aleksov, R. Schlesser, Z. Sitar Department of Materials Science and Engineering, North Carolina State University, Campus Box 7919, Raleigh, NC 27695-7919, USA Available online 27 December 2005
Abstract Following an already established polarity control scheme for GaN thin films, we developed a process to simultaneously grow Ga- and N-polarity layers side by side on c-plane sapphire. The simultaneous growth is achieved by properly treating the AlN nucleation/buffer layer and subsequent substrate annealing. During this process, the growth is mass-transfer-limited, permitting the same growth rate for both types of polarity domains. Smooth domains of both polarity types (RMS roughness 1–2 nm) were obtained. r 2005 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 81.05.Ea; 81.15.Gh Keywords: A1. Crystal strcture; A3. Chemical vapor deposition process; A3. Metalorganic chemical vapor deposition; B1. Nitrides; B2. Semiconducting III–V materials
1. Introduction Polar orientation in non-centrosymmetric III-nitride wurtzite semiconductors refers to the crystallographic orientation of the basal plane of the wurtzite lattice with respect to the substrate. The polar structure with three bonds of the III-atom facing towards the substrate is referred to as the +c orientation, while the mirrored structure with three bonds of the III atom facing towards the free surface is referred to as the c orientation. Polar orientation should not be confused with surface termination, as each orientation may be terminated with either one of the species. The wurtzite structure is the lattice type with the highest symmetry that still exhibits spontaneous polarization. The polar orientation determines the direction of the spontaneous polarization vector and, thus, determines the type of charge induced at the surfaces/ interfaces. Along with the piezoelectric polarization, the polarization-induced charge influences the electrical and optical properties of the material. Control of the polar orientation on a macroscopic and microscopic scale is Corresponding author. Tel.: +1 919 515 8965; fax: +1 919 515 3419.
E-mail address: [email protected] (R. Collazo). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.10.080
desirable to exploit the combined properties of both types of orientations. Fig. 1 shows part of the unit cell of a wurtzite III-nitride. The arrow indicates the +c direction of the lattice. The c orientation is obtained by rotating the unit cell by 1801. It is commonly believed that in the case of metalorganic vapor phase epitaxy (MOVPE) growth, the N-rich conditions, together with comparatively high substrate temperatures, always produce +c oriented films, i.e. Ga-polar GaN films. The usually employed nitridation step preceding deposition of a buffer layer forms a thin AlN layer on the sapphire substrate and induces the Ga polar orientation. By contrast, N-polar (c) orientation typically occurs only in films grown by molecular beam epitaxy (MBE). Proper AlN buffer deposition conditions during MBE growth on sapphire substrates allow the possibility to locally determine the lattice polarity. This ability provides new technological opportunities for the design of novel device structures in the III-nitrides family [1]. Nevertheless, the technical difficulties of using MBE as the growth technique have hindered the technological development of this family of devices. Recently, growth of N-polar GaN has been achieved by MOVPE and the possibility of polarity control was suggested [2].
ARTICLE IN PRESS R. Collazo et al. / Journal of Crystal Growth 287 (2006) 586–590
587
2. Experimental details
Fig. 1. Partial unit cell of a wurtzite III-nitride semiconductor. The black and gray balls correspond to N and III-atoms, respectively. The arrow indicates the +c direction of the lattice.
N-polar GaN thin films typically feature a rough surface morphology featuring hexagonal pyramids, while the Ga-polar typically has a mirror-like appearance. Mixedpolarity domains, i.e., films containing small domains of both polarities, are difficult to distinguish from the pure polar films, thus surface morphology alone does not provide an unambiguous identification of film polarity. Several techniques have been used to identify the polarity orientations; the easiest, but destructive, technique uses the different chemical reactivity of each polar domain. Ga-polar surfaces are resistant to most chemical etchants except at defects, while N-polar surfaces are reactive to KOH/H2O solutions or KOH/NaOH eutectics [1]. In the same way as the difference in chemical reactivity between +c and c faces affects the etch rate, the growth rate is different as well: Ga-polar surfaces typically grow faster than N-polar surfaces [3]. This feature is not only observed in MOVPE, but also in bulk growth of GaN and AlN. A faster growing Ga-polar surface will overgrow a competing N-polar domain [4]. By contrast, both domains can be deposited at the same rate during MBE growth. In this article, we will briefly describe the process by which we obtain N-polar GaN films grown by MOVPE on sapphire substrates at the same growth rate as the Ga-polar GaN films. Although it is not our intent to introduce models to explain the different results obtained here, we will describe some of the results and explain their consequences, specifically the role of the dilution gas in establishing the same growth rate for both domains.
All the films for these experiments were grown in a vertical, cold-walled, RF-heated, low-pressure MOVPE reactor. The distance between the gas inlet ‘‘shower-head’’ and the susceptor was 5 cm. The reactor base pressure was 1 107 Torr. Triethylgallium (TEG), trimethylaluminum (TMA) and ammonia were used as Ga, Al and N precursors, respectively. In contrast to the traditional process conditions for nitride growth, N2 was used as dilution and carrier gas throughout the growth process. ‘‘Epi-ready’’ sapphire wafers (2 in (0 0 0 1)-oriented) were used as substrates. After the wafer was annealed in vacuum, it was H2 annealed at 1090 1C in order to remove residual contamination and prepare the sapphire surface prior to the growth process. For both types of domains, our standard GaN growth conditions consisted of a TEG flow of 36 mmol/min, an ammonia flow of 0.4 slm and a total flow of 2.15 slm at 20 Torr of total pressure. The growth temperature was maintained at 1030 1C. This condition provided a V/III ratio of 500 at a growth rate of 1.2 mm/h. A nitridation step prior to a low temperature (LT) AlN buffer layer deposition was necessary to control the polarity. The AlN buffer layer thickness was 10–20 nm and was grown at 600 1C. No buffer layer was used to grow N-polar GaN films, while a proper buffer layer anneal after deposition was necessary to obtain Ga-polar GaN films. Further processing details will be published in a later publication. Different flow rates were used to determine the role of nitrogen in promoting the same growth rates for films of both types of polarities. Chemical reactivity with a 3 M KOH/H2O solution was used to unambiguously distinguish between Ga- and N-polar films. N-polar films immediately etched when immersed in the alkaline solution at 65 1C. Crystallographic characterization was performed by acquiring on- and off-axis HRXRD rocking curves using a Philips X’Pert Materials Research Diffractometer with a copper X-ray source operated at 40 kV and 45 mA, and an open slit on the detector side. Atomic force microscopy (AFM) images were acquired with a Digital Intruments Nanoscope in the tapping mode. 3. Results and discussion Under the conditions described in the previous section, we obtained smooth films of both types of polarities. Smooth N-polar films were obtained when the substrate nitridation was around 2 min at 950 1C. Longer or higher temperature nitridation induced the hexagonal surface morphology usually attributed to N-polar films. ‘‘Mirrorlike’’ Ga-polar films were obtained after the LT-AlN buffer layer grown on the nitrided wafer was annealed in ammonia flow for an experimentally determined time and ammonia partial pressure. The use of N2 as a dilution gas seems critical to obtain the results reported here. Sumiya et al. obtained similar
ARTICLE IN PRESS 588
R. Collazo et al. / Journal of Crystal Growth 287 (2006) 586–590
results using H2 as dilution gas but needed longer nitridation and anneal times to get the same effects [2]. It is a prevalent idea that the use of N2 as the only dilution
gas produces rough GaN films with a mosaic structure [5]. Our results demonstrate that using N2 as the sole dilution gas can yield films of the same quality as obtained by using H2 as the dilution gas. Fig. 2 shows the full-width at halfmaximum (FWHM) of the (00.2), (10.3), (30.2) X-ray rocking curves as a function of the plane inclination angle with respect to the (00.2) on-axis reflection for a 1.35-mmthick Ga-polar GaN film, with an optimized buffer layer anneal step for the reduction of the dislocation density (improvement in the FWHM of the rocking curves). The (00.2) reflection was measured in a symmetric geometry, while the (10.3) and (30.2) reflections were measured in a skew-symmetric geometry. The V/III ratio used during the film growth was 100. Fig. 3 shows the same type of plot for 1.35 mm thick, Ga- and N-polar layers deposited side by side on the same substrate. Rocking curves were measured for each type of domain away from the boundary. Ga-polar domains have a lower FWHM of the on-axis reflection than the N-polar domains, but similar off-axis reflections FWHM. The process is optimized to assure domain purity, i.e., no embedded inversion domains. Fig. 4 shows an AFM images of a Ga-polar 0.6-mm-thick GaN film (a) and a N-polar GaN film of similar thickness (b) grown at a V/III ratio of 500 and using N2 as the
Fig. 2. X-ray rocking curves FWHM of the (00.2), (10.3), (30.2) reflections as a function of the plane inclination angle with respect to the (00.2) on-axis reflection for a 1.35 mm thick Ga-polar GaN film.
Fig. 3. X-ray rocking curves FWHM of the (00.2), (10.3), (30.2) reflections as a function of the plane inclination angle with respect to the (00.2) on-axis reflection for 1.35-mm-thick Ga- and N-polar layers deposited on the same substrate. Rocking curves were measured for each type of domain away from the boundary.
Fig. 4. AFM images of a Ga-polar 0.6-mm-thick GaN film (a) and a Npolar GaN film of similar thickness (b) grown at a V/III ratio of 500 and using N2 as the dilution gas.
ARTICLE IN PRESS R. Collazo et al. / Journal of Crystal Growth 287 (2006) 586–590
dilution gas. The image shows that the films grew in the step-flow mode with randomly distributed pits, with a pit density comparable to reported threading dislocation densities for films of comparable crystalline quality. The surface roughness was estimated to be 0.2 nm root mean square (rms) for the Ga-polar film. The N-polar film surface roughness was estimated to be 2 nm rms, with no hexagonal pyramids on the surface. The growth rate for both types of domains was found to be identical, in contrast to observations by Wu et al., who reported that the Ga-polar grows faster than the N-polar [3]. Although they use MOVPE as their growth technique, they use H2 as their sole dilution gas, in contrast to our process. It has been established that the growth rate is nearly constant at the typical growth temperatures when N2 is used as the sole dilution gas, while under H2 the growth rate decreases with temperature [5]. This suggests that under N2 flow the growth is mass-transfer-limited with a large supersaturation, while under H2 the supersaturation is greatly decreased. Mass-transfer-limited growth is necessary to circumvent the chemical reactivity difference between the two domains and to achieve identical growth rates for the two domains. In order to demonstrate that the growth is mass-transfer-limited using N2 as the dilution gas, it has to be shown that (in the simplest case) the growth rate is limited by the diffusion of the III-atom carrying species through a gas boundary layer formed at the surface of the substrate. In our particular reactor geometry this can be expressed as sffiffiffiffiffi G f i f 0T ¼ , (1) G 0 f 0i f T where G is the growth rate and f i and f T are the III-atom carrying species and total mass flow rate. The equation is referenced to a standard state (O) corresponding to our standard process conditions at a given growth temperature as reported in the experimental section. Fig. 5 demonstrates the linear relationship between the normalized growth rate and the inverse of the square root of the total flow for different growth conditions. Observation of this dependence is a sufficient and necessary condition to show that the growth is mass-transfer-limited. In addition, as suggested by Eq. (1), the growth rate should be independent of the total reactor pressure. This fact is demonstrated in Fig. 6, where the growth rate is plotted as a function of total reactor pressure for a given TEG mass flow rate and total mass flow rate. Consequently, both domains will grow at the same rate, independently of the growth conditions, as long as N2 is used as the dilution gas. Fig. 7 shows a SEM micrograph of the crosssection of Ga- and N-polar layers grown side by side. The region left of the boundary is the Ga-polar domain, while the region right of the boundary is the N-polar domain. No difference in the thickness between the two domains is observed within 30 mm of each side of the boundary.
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Fig. 5. Normalized growth rate as a function of the normalized inverse square root of total flow. A linear relationship is sufficient and necessary to demonstrate that the growth is mass-transfer-limited. The solid line corresponds to the unity line as shown by Eq. (1). The error bars indicate uncertainty in the growth rate.
Fig. 6. GaN film growth rate as a function of total reactor pressure for a TEG mass flow rate of 134 mmol/min and total mass flow rate of 7.2 slm. The error bars indicate the uncertainty in the growth rate.
Micrographs taken further away from the boundary, within each domain, do not show any difference in thickness either.
ARTICLE IN PRESS 590
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mass-transfer-limited, both +c and c polarities can be simultaneously grown on c-plane sapphire in a horizontal manner in a MOVPE process utilizing only N2 as the sole dilution gas. The simultaneous and controlled growth of +c and c polarity regions can be achieved through an appropriate control of the substrate preparation.
Fig. 7. SEM micrograph of the cross-section of a GaN layer near the inversion domain boundary separating a Ga-polar domain (left) from a N-polar domain (right).
4. Conclusions Following the reported polarity control scheme, and the observation that in an N2 atmosphere the growth is
References [1] M. Stutzmann, et al., Phys. Stat. Sol. (B) 228 (2001) 505. [2] M. Sumiya, S. Fuke, MRS Int. J. Nitride Semicond. Res. 9 (2004) 1. [3] F. Wu, M.D. Craven, S.H. Lim, J.S. Speck, J. Appl. Phys. 94 (2003) 942. [4] J. Jasinski, Z. Liliental-Weber, Q.S. Paduano, D.W. Weyburne, Appl. Phys. Lett. 83 (2003) 2811. [5] O. Ambacher, J. Phys. D: Appl. Phys. 31 (1998) 2653.